Spintronics in semiconductors, 2D materials and topological insulators

Resume : We will show how to manipulate an individual spin by an RF gate voltage in silicon trigate nanometric field effect transistors at very low temperature. The spin of a single hole localized in the channel of the MOSFET can be rotated with a Rabi frequency of 80MHz by applying an electric RF burst on the front gate of the transistor. Hole spin are sensitive to the electric field thanks to the spin-orbit coupling existing in the valence band of the silicon. The spin orientation is measured using the the Pauli blockade of the current passing through two transistors in series. Echo experiments show dephasing time in the 300 ns range [Maurand et al. Nature Communications 7, Article number: 13575 (2016) doi:10.1038/ncomms13575 ]. We will also show that electron spin in the same type of device can be manipulated by electric RF field (EDSR) even if the spin-orbit coupling is much weaker than for holes [A. Corna et al. in preparation].

Resume : The electrical manipulation of a single electron spin in silicon is challenging but highly desirable in the prospect for an efficient large-scale quantum computer. The challenge originates from the low spin-orbit coupling in the conduction band of silicon; however recent experimental measurements [1] have demonstrated the feasibility of electric dipole spin resonance (EDSR) in a silicon nanowire CMOS device. Here we discuss the physics behind such EDSR experiments in the conduction band of silicon. We highlight, in particular, the interplay between valley-orbit and spin-orbit couplings. We introduce a model for the EDSR, and validate it against realistic tight binding calculations and experimental data. These tight-binding simulations allow us to make predictive simulations on electrically controlled spin quantum bit devices based on 1D channels on a silicon-on-insulator (SOI) substrate. Such devices can achieve Rabi frequencies up to ~80 MHz. We propose a scheme for the manipulation of the electron spin that takes advantage of the versatility of SOI. [1] A. Corna et al., in preparation.

Resume : Heavy holes confined in quantum dots are predicted to be promising candidates for the realization of spin qubits with long coherence times [1]. Here we focus on such heavy hole states confined in Ge hut wires [2]. By tuning the growth density of the latter we can realize a T-like structure between two neighboring wires. Such a structure allows the realization of a charge sensor, which is electrostatically and tunnel coupled to the quantum dot [3], with a charge transfer signal as high as 0.3e. By integrating the T-like structure into a radio-frequency reflectometry setup [4], single shot reflectometry measurements allowing the extraction of holes tunneling times were performed. The extracted tunneling times, shorter than 10µs, pave the way towards projective spin readout measurements [5]. [1] D. V. Bulaev and D. Loss, PRL 95 (2005) 076805; Fischer, J. et. al., PRB 78 (2008), 155329. [2] Watzinger, H. et. al., Nano Lett. 16 (2016) 6879 [3] Morello, A. et. al., PRB 80 (2009), 081307; Mahapatra et. al., Nano Lett 11 (2011), 4376 [4] Schoelkopf, R. J. et. al., Science 280 (1998), 1238 [5] Elzerman, J. M. et. al. Nature 430 (2004), 431; Morello, A. et al. Nature 467 (2010), 687

Resume : Optical spin orientation, i.e. the possibility of optically exciting spin-polarized electrons by means of circularly polarized light, is a powerful tool to explore spin transport and dynamics in semiconductors [1]. So far, the vast majority of optical orientation studies have been focused on direct optical transitions, by exploiting either direct-gap semiconductors such as GaAs, or a ?quasi-direct? material such as Ge. Indeed, despite the long spin lifetimes which make Si a relevant material for spintronic applications [2], only very recently optical orientation cross sections for indirect transitions at the ? minima of the Si band?structure have been calculated [3,4]. We report on the room-temperature optical orientation of spin-polarized electrons at the indirect gap of bulk Si, in the photon energy range comprised between 1.2 and 1.8 eV. The photo-generated spin current is detected by exploiting the inverse spin Hall effect (ISHE) taking place in a Pt layer forming a Schottky barrier with the underlying Si(001) substrate. The photon energy dependence of the ISHE signal is interpreted in the frame of a one-dimensional spin drift?diffusion model, which is used to estimate the electron spin lifetime ?s ? 15 ns. References [1] I. Zuti?, J. Fabian, and S. Das Sarma, Rev. Mod. Phys. 76, 323 (2004) [2] R. Jansen, Nat. Mater. 11, 400 (2012) [3] P. Li and H. Dery, Phys. Rev. Lett. 105, 037204 (2010) [4] J. L. Cheng, J. Rioux, J. Fabian, and J. E. Sipe, Phys. Rev. B 83, 165211 (2011)

Resume : Spin-orbit coupling effects in materials with broken inversion symmetry are responsible for peculiar spin textures. Among others, ferroelectric materials make possible for a non-volatile control of the spin degree of freedom through the electrical inversion of the spin texture, thanks to their reversible spontaneous polarization. Such functionality hold potential for technological applications exploiting spin effects controlled by electric fields. Germanium Telluride stands out as material for spin-orbitronics being a FErroelectric Rashba SemiConductors (FERSCs) [1]. Its ferroelectricity provides a non-volatile state variable able to generate and drive a giant bulk Rashba-type spin-splitting of electronic bands. Finally, GeTe? semiconductivity allows for the realization of spin-based transistors. Here, the ferroelectric control of the Rashba spin texture in GeTe is experimentally proved by Piezoresponse Force Microscopy and Spin and Angular Resolved PhotoEmission Spectroscopy (ref. [2] and advances). Charge-to-spin conversion phenomena in GeTe are investigated by unidirectional spin Hall magnetoresistance [4] in Fe/GeTe heterostructures [3]. This work looks toward non-volatile electric control of spin transport in semiconductors. [1] D. Di Sante et al., Adv. Mater. 25, 509-513 (2013) [2] M. Liebmann, C. Rinaldi et al., Adv. Mater. 28, 560-565 (2016) [3] C. Rinaldi et al., APL Mater. (invited) 4, 032501 (2016) [4] C. O. Avci et al., Nature Phys. 11, 570 (2015)

Resume : The spin field-effect transistor (FET) [1] is a critical vehicle to study injection, manipulation and detection of spin-polarised electrons in a semiconductor. These are essential features for future utilisation of spin-degree of freedom in semiconductors. In particular, the Fe/n-GaAs system has been intensely investigated due to its desirable lattice matching where successful injection and detection of electron spins have been previously reported [2]. However, manipulation of the injected spins via electric or optical gates is still required to create a working spin FET. In this study, we fabricated Fe/n-GaAs non-local spin valves and investigated spin transport using electrical and optical methods. The Fe/n-GaAs structures were deposited using a two-chamber method. Firstly, 2 µm of Si-doped n-GaAs (n=2x1016 cm-3), 15 nm of n->n+ transition layer, and 15 nm of highly-doped n+ layer (n=5x1018 cm-3) were deposited on a semi-insulating GaAs(001) substrate in a Veeco Gen II molecular beam epitaxy (MBE) chamber. 3 nm of Fe and 2 nm of Au layers were deposited by electron-beam evaporation at room temperature, confirming epitaxial (001) growth of Fe. The Fe films, n->n+ transition and n+ GaAs layers were then patterned into a series of 20x(20, 4 and 1) ?m2 rectangular bars using electron-beam lithography, Ar-ion milling and wet-etching. The exposed GaAs surface was passivated with a 100 nm SiO2 layer by plasma-enhanced chemical vapour deposition. Electrical properties of the devices were measured in a vector-magnet He cryostat. Hanle-like Lorentzian peaks were obtained with half-width at half-maximum field values of around 440 mT. According to the formula used in Nam et al. [3] assuming the electron g-factor of -0.44 for n-GaAs, the spin dephasing time was calculated to be around 60 ps for the both 3- and 4-terminal measurements. Our device exhibited a ten-times larger 3-terminal signal, as well as a spin dephasing time about 200-times smaller than the usual values. The large (~1 T) field required to saturate the both 3T and 4T signals indicated possible magnetisation rotation of the Fe injector and detector bars as a possible source of the resistance changes. This was confirmed by pump-probe time-resolved Kerr rotation measurements. The strongest Kerr modulation was observed when the excitation wavelength was 821 nm (1.51 eV), which agreed well with the 1.52 eV GaAs band-gap at 0K [4]. Employing a fit function accounting the exponential decay and the Larmor precession of spins, a spin dephasing time of 2.9 ns and an electron g-factor of -0.43 were estimated. Further optimisation of the Fe/n-GaAs interface has been achieved using non-destructive junction imaging [5]. This work has partially been supported by UK-EPSRC (EP/M02458X/1). [1] S. Datta and B. Das, Appl. Phys. Lett. 56, 665 (1990). [2] X. Lou et al., Nature Phys. 3, 197 (2007). [3] S. H. Nam et al., Appl. Phys. Lett. 109, 122409 (2016). [4] C. Kittel, Introduction to Solid State Physics 6th Ed. (John Wiley, New York, 1986), p. 185. [5] A. Hirohata et al., Nature Commun. 7, 12701 (2016).

Resume : The electron transport through the magnetic tunnel junctions (MTJs) such as Fe|MgO|Fe junctions depends on the relative magnetization in the two ferromagnetic electrodes separated by an insulating layer. This phenomenon is known as tunneling magnetoresistance, which is exploited in magnetoelectronics for making nonvolatile memory elements [1]. Passing a spin-polarized current through the MTJs causes another magnetic phenomenon known as the spin-transfer torque, allowing for magnetization switching in MTJs used for magnetic random-access memories [2]. There have been a number of first-principles, atomistic studies on calculating the tunneling magnetoresistance and spin-transfer torque at zero temperature in the linear response regime [3,4]. Taking into account the effect of phonons on the tunneling magnetoresistance and spin-transfer torque at room temperature beyond the linear response regime is still a challenging task for the first-principles simulations at the atomistic level. In the present study, we develop a general first-principles methodology to calculate the tunneling magnetoresistance and spin-transfer torque at non-zero temperatures, based on density functional theory in combination with the non-equilibrium Green function technique as implemented in the Atomistix ToolKit code [5]. The effect of phonons on the electron tunneling is taken into account using a temperature dependent finite displacement of all atoms, a similar approach has previously proven to provide an accurate description of the temperature dependence of various materials properties [6,7]. Highly-accurate localized atomic orbital basis sets and norm-conserving pseudopotentials, which allow for accurate description of the total energy and electronic structure of magnetic material structures, are adopted to reliably describe the subtle magnetic phenomena. We apply this state-of-art methodology to elucidate the temperature effect on the tunneling magnetoresistance and spin-transfer torque in the Fe|MgO|Fe magnetic tunnel junctions, which are of high relevance for applications. [1] S. S. P. Parkin, C. Kaiser, A. Panchula, P. M. Rice, B. Hughes, M. Samant, and S.-H. Yang, Nature Materials 3, 862 - 867 (2004) [2] M. H. Krydere and C. S. Kim, IEEE Trans. Magn. 45, 3406 (2009). [3] W. H. Butler, X.-G. Zhang, T. C. Schulthess, and J. M. MacLaren, Phys. Rev. B 63, 054416 (2001). [4] P. X. Xu, V. M. Karpan, K. Xia, M. Zwierzycki, I. Marushchenko, and P. J. Kelly, Phys. Rev. B 73, 180402R (2006). [5] ?Atomistix ToolKit version 2016.4?, QuantumWise A/S (www.quantumwise.com). [6] M. Zacharias and F. Giustino, Phys. Rev. B 94, 075125 (2016). [7] T. Gunst et. al. (in preparation).

Resume : Science today is constantly chasing after tunable, nonclassical lightsources such as quantum dots (QDs). Number of effects can be reached due to interaction of QD excitons with the environment e.g. single magnetic dopants. Much better spectral resolution and much higher degree of control over QDs excitons can be achieved by resonant excitation. The goal of this work is to present an optical study of II-VI semiconductor QDs in different systems, optimized for resonant spectroscopy. In order to open possibility of single QD absorption experiments we needed almost transparent samples containing QDs. Since II-VI QDs are typically grown on opaque substrates (e.g., GaAs), we have developed new lift-off technique for II-VI compounds. The second aim of our study is related to the emission wavelength of II-VI quantum dots. II-VI QDs studied so far, exhibit emission wavelength 500 nm < ? < 700 nm. Resonant excitation experiments in this range are challenging, especially in comparison with the ease of use of Ti:sapphire lasers (>700nm). In order to red-shift the QDs, we have grown a series of CdTe QDs in (Cd,Mg)Te barriers with decreasing Mg content. We found that for low Mg content in barrier (below 10%) QD lines are broaden, but for about 14% of Mg in the barrier optical properties of QDs are good and emission is slightly above 700 nm, as required. In our work we combine these two methods in order to achieve the regime of strictly resonant excitation.

Resume : Majorana zero modes have received widespread attention due to their potential to support topologically protected quantum computing. Emerging as zero-energy states in one-dimensional semiconductors with induced superconductivity, Zeeman coupling, and spin-orbit interaction, Majorana modes have been primarily investigated in individual InSb or InAs nanowires, including recently realized epitaxial hybrid nanowires. Tests of non-Abelian statistics of Majoranas involve braiding or interferometric measurement, requiring branched geometries, which are challenging to realizing using nanowire growth. Scaling to large networks using arrays of assembled nanowire also appears difficult. I will present investigations of Majorana zero modes in devices obtained from a two-dimensional heterostructure using top-down lithography and gating. Measurements indicate a hard superconducting gap, ballistic tunneling probes and in-plane critical fields up to 3 T. In the presence of an in-plane magnetic field aligned along the wire, zero energy states robust in field emerge out of coalescing Andreev bound states, indicative of Majorana zero modes. The Majorana peak height and width strongly depend on temperature, demonstrating weak coupling to the leads. Scalable top-down fabrication of high quality Majorana devices readily allows complex geometries and large networks, paving the way toward applications of Majorana devices.

Resume : As demonstrated by our group and by our Warsaw?s co-workers [1-3], wurtzite (wz) compounds possess features attractive for spin-orbitronics, and allowing for spin-charge interconversion via spin-orbit coupling associated with inversion asymmetry and leading to a Rashba field [1,2] and to piezoelectric properties [3]. From antilocalization magnetotransport studies at mK on n-doped wz-GaN:Si epitaxial films, we have determined the Rashba parameter to be ?R = (4.5 ± 1) meV Å [1]. This value shows that in previous studies of electrons adjacent to GaN/(Al,Ga)N interfaces, bulk inversion asymmetry was dominant over structural inversion asymmetry. The comparison between experimental and theoretical values of ?R in a series of wz semiconductors is presented to test relativistic ab initio computation schemes [1]. Spin pumping is an efficient mechanism for the inception of spin current and for its conversion into charge current in non-magnetic metals or semiconductors via spin Hall effects. We have demonstrated the generation of spin current in bilayers Py/n-GaN:Si [2]. For layer thicknesses greater than the spin diffusion length, a condition not met in previous studies on n-ZnO [4], we have found for n-GaN:Si a spin Hall angle ?SH = 3.03 × 10?3, exceeding by one order of magnitude those of other relevant semiconductors, and pointing at wz semiconductors as efficient spin current generators. Work supported by the European Research Council (ERC, #227690) and by the Austrian Science Foundation (FWF, #24471, #26830). [1] W. Stefanowicz et al., Phys. Rev. B 89, 205201 (2014). [2] R. Adhikari et al., Phys. Rev. B 94, 085205 (2016) [3] D. Sztenkiel et al., Nature Commun. 7, 13232 (2016). [4] S. D?Ambrosio et al., Japan. J. Appl. Phys. 54, 093001 (2015).

Resume : Recently it was found that centrosymmetric crystals can show hidden spin polarization due to spin-orbit effects. The total spin polarization of the crystal is zero but strong spin polarization is present on local real space atomic sectors within the crystal. This spin-orbit effect in centrosymmetric materials is called the Rashba and Dresselhaus R-2 and D-2 effects. Here we investigate by first-principles calculations the influence of a surface on these hidden spin polarizations using Bi2Se3 and LaOBiS2 as model systems. Our calculations indicate that these R-2 and D-2 effects disappear and that a net spin Rashba or Dresselhaus polarization appears at the surface. A mechanism that explains these spin-momentum locked surface states is described as a combination of charge redistribution and re-localization of the electronic states onto different real space sectors.

Resume : Topological crystalline insulator Pb1-xSnxSe surface quantum wells (QWs) on Pb1-yEuySe barriers are studied experimentally and theoretically as a function of QW thickness in both the topological and trivial phases at different temperatures and Sn contents. The theoretical tight-binding results are compared with experimental angle resolved photoemission investigations of epitaxial heterostructures grown by molecular beam epitaxy. It is shown that for thin QWs, the interaction between the surface and interface states of the QW layer opens an energy gap in the topological surface states. The Dirac points in the topological phase appear only for QWs with thicknesses exceeding ~24nm. Upon in situ submonolayer Sn deposition on the surface a strong Rashba effect appears in the conduction band which is modeled using the tight-binding approach and recursive Green?s function method to derive the surface spectral density of states of the material. In our calculations we take into account the possibility that Sn covers only partially the surface of the QW. The strong Rashba effect observed in the conduction band was simulated by applying a potential described by Thomas-Fermi screening model, similarly as it was shown for PbSnTe films doped with Bi atoms [1]. Here we find, however, that even without screening potential, a Rashba splitting can be obtained in both valence and conduction bands due to the lack of inversion symmetry. [1] V. Volobuev et al., Adv. Mater. 29, 1604185 (2017).

Resume : I will present some recent results on single and double nanowires with proximity gap hosting Majorana and Para-fermions [1]. Typically, the topological phases are engineered by tuning the magnetic field to the topological threshold value of typically a few Teslas. However, the magnetic field has a detrimental effect on the host superconductor and so it is interesting to search for ways to achieve the topological phase without or with smaller B-fields. A particular way to achieve this goal is to exploit crossed Andreev pairing in a double nanowire setup [1,2,3] which destructively interferes with the direct pairing, and thereby lowers the threshold for the B-field substantially [3]. In re-examining the proximity effect in such finite-size geometries we discovered that the standard procedure of 'integrating out superconductivity' breaks down [2]. I will also present some recent results on hybrid platforms for quantum computing which combine spin qubits in quatum dots with topological qubits on a surface code architecture [4]. [1] J. Klinovaja and D. Loss, PRL 112, 246403 (2014); PRB 90, 045118 (2014). [2] C. Reeg, J. Klinovaja, and D. Loss, arXiv:1701.07107. [3] C. Schrade, M. Thakurathi, C. Reeg, S. Hoffman, J. Klinovaja, and D. Loss, arXiv:1705.09364. [4] S. Hoffman, C. Schrade, J. Klinovaja, and D. Loss. Phys. Rev. B 94, 045316 (2016).

Resume : We report on experimental and theoretical studies of two dimensional HgTe/CdHgTe quantum well structures with the Dirac fermions-like band structure, in view of their application for Quantum Hall Effect resistance standards. We show that due to large Landau levels splitting and high carrier mobility one can reach quantum limit v=1 condition, required for resistance standards, under very competitive conditions (magnetic fields below 1 T). We interpret results by using the band structure calculations on the basis of the eight-band Kane Hamiltonian and show that these conditions can be improved (magnetic field will be further decreased) in the samples with compressively strained HgTe QWs. Our results pave the way towards new competitive resistance standards using HgTe/CdHgTe quantum wells with graphene like band structures. Work partially supported by grants RBFR "15-52-16017 NTSIL_a" and "15-52-16008 NTSIL_a

Resume : Despite strong disorder, the transport of surface Dirac fermions remains quasi-ballistic in narrow nanowires of a 3D topological insulator, as evidenced in Bi2Se3 or Bi2Te3 quantum wires [1,2]. We demonstrate that such a unique behavior for a mesoscopic conductor results from the spin helicity of all quasi-1D surface modes, rather than from the topological nature of a single perfectly-transmitted mode. The weak coupling of spin-helical modes can be revealed by the non-universal behavior of conductance fluctuations [3], and the spin and energy-dependence of transmissions is well captured by both analytical and numerical models. It is further evidenced that, under appropriate conditions, such 3DTI quantum wires could be used not only for ballistic spin transport but also as spin filters. [1] J. Dufouleur et al., Phys. Rev. Lett. 110, 186806 (2013) [2] L.A. Jauregui et al., Nat. Nano. 11, 345 (2016) [3] J. Dufouleur et al., Sci. Rep. 7, 45276 (2017)

Resume : Bismuth exhibits a series of electronic peculiarities that made it the subject of experimental and theoretical interest for decades, in particular in electronic-transport studies [1]. With a lattice structure close to that of graphene and a very large spin-orbit coupling, bismuth may have the potential to be a topological semimetal or semiconductor [2]. In order to explore this possibility, we have grown by molecular beam epitaxy (MBE) very thin films of Bi on Ge(111). Indeed, MBE gives the opportunity to grow metastable allotropic phases of Bi or to induce strain into bulk Bi which may give topological properties to this material [2]. Moreover, the growth of such phase on germanium opens new perspectives to use the electron spin in traditional microelectronics. In this study, we have grown a bismuth wedge (0-15 nm) on Ge(111) by MBE. Using structural characterization (RHEED and x-ray diffraction), we found a critical thickness of ?5 nm below which Bi exhibits an allotropic pseudocubic phase. A careful angle-resolved and spin-resolved photoemission spectroscopy study using synchrotron radiation (ELETTRA, Trieste, Italy) showed that the pseudocubic phase exhibits surface states with a linear band dispersion and a characteristic helical spin texture. Moreover, low temperature magnetotransport measurements demonstrated the 2D character of electron transport into these surface states. We have then investigated the spin-to-charge interconversion at these surface states using 3 different techniques: magneto-optical Kerr effect to probe the spin Hall effect (SHE), inverse SHE using optical spin orientation in the Ge film beneath and finally spin pumping from a ferromagnetic layer grown on top of Bi separated by an Al spacer. We found a clear signature of the strong spin-to-charge interconversion in these surface states. References [1] Y. Fuseya et al., J. Phys. Soc. Jpn. 84, 012001 (2015). [2] I. Aguilera et al., Phys. Rev. B 91, 125129 (2015).

Resume : Topological insulators (TIs) represent a new state of quantum matter which is bulk insulators with metallic surface state that can be described by a single Dirac Fermion. Currently known TI materials can possibly be classified into two families, the HgTe family and the Bi2Se3 family. It has been found that excellent thermoelectric materials can also be topologically nontrivial. Bismuth, antimony and tellurium compounds (Bi/Sb/Te) are known as the best thermoelectric materials for room temperature operation. However, low-dimensional solids such as nanowires (NWs) are a challenge, due to the difficulty of separating surface contributions from the non-conductivity of the bulk. Nanostructured synthesis/growth, doping, compositional tuning or band-gap engineering, via device gating, has not yet completely suppressed bulk conduction in the TIs. 2D nanostructured TIs have a large surface-to-volume ratio that can manifest the conductive surface states and are promising for devices. Electronic transport of nanostructured TI?s exhibit novel quantum effects. Growth of high quality TIs are the main obstacle for the future development of TI based devices. Fabrication of nanowires with high surface to volume ratio can be realized by two methods, chemical vapour transport and electro-deposition. The second method is used in the presented work and allows growth of structures such as p-n junctions, intercalation of magnetic or superconducting dots. We report the synthesis of high quality TI thermoelectric single crystal nanowire (Bi2Te3, Sb2Te3) via electro-deposition (ED). Structural properties of nanowires have been done by X-ray diffraction and crystalline strain observed by Williamson Hall Plot. The morphological properties of nanowires were studied by SEM. Transmission electron microscopy observation and selected area electron diffraction analysis indicate that the nanowires are single-crystalline and grow in a preferred direction of [1 0 0]. ED growth parameters such as substrate, substrate annealing, deposition potential and solution composition have been optimized for growth of mono crystal nanowires of Bi2Te3 and Sb2Te3. Preliminary results show that electronic transport of electrodeposited nanowires is dominated by surface states as testified by weak antilocalization. Keywords: Electro-deposition, topological insulator, nanowires.

Resume : In order to make practical spin devices that function at ambient temperature, there has been a surge in efforts to increase the Curie temperature of dilute magnetic semiconductors (DMSs). Because of the quantum confinement effect on the nanoscale, quantum structures have shown great superiority in enhancing the magnetic properties of DMSs. Adopting molecular-beam epitaxy (MBE) technology, we successfully prepared the self-assemble Co0.06Ge0.094 ferromagnetic semiconductor quantum dots with extremely high Curie temperature above 450 K. Furthermore, electric-field manipulation of ferromagnetism, i.e., hole-mediated effect, is a fascinating property of DMSs. Modulating the electric field of MOS gate, the hole-mediated effect of Co0.06Ge0.094 quantum dots was distinctly observed even at 150k measured by SQUID. Using MEDICI simulation, we found there is a critical condition to realize hole-mediated effect for the first time, which is relative to the concentration of hole and not changed with temperature, called hole-mediated threshold and it is about 6×1018 cm-3.

Resume : Monte Carlo method is one of the most commonly used techniques in the studies of electron relaxation in semiconductors. One can also find application of this method to the study of mono and multilayers as well as nanostructures of graphene. However, in most of the studies, the assumption of nondegenerate electron gas conditions is used. Since, electron gas appears to be degenerate in this material, the effect of the Pauli Principle should be considered. Another important point discussed in the proposed communication is the indiscernibility of two colliding electrons by taking into account the exchange in electron-electron scattering between the electrons of like spin. We have demonstrated that, when electrons are distributed among multiple valleys, the commonly used formulas defining these rates have to be corrected. Our Monte Carlo simulations proceed in the following way: We consider a material with a 'background of electrons' in stationary state. To this system a number of electrons, having nonequilibrium energy and momentum distribution is added and then, we monitor the relaxation of the electron system. Two stages can be distinguished. First, electron-electron scattering restores a Fermi-Dirac shape of the distribution. The 'thermalization' process is very fast (a few fs). Then, the 'cooling' process, governed by phonon scattering, takes place on a much longer timescale. In the present study, we have considered the case when the background electron system is spin-polarized. Various combinations of relative spin polarizations of background and generated electrons are considered and the influence on the dynamics of relaxation process is discussed. Spin polarizations influence electron dynamics in two ways: in electron-electron scattering rates, through the exchange effect, and in Pauli Exclusion Principle, because the region of k-space accessible after a scattering event differs according to spin polarizations.

Resume : Topological insulators (TI) like Bi2Se3 have attracted remarkable consideration because of their strong surface states, which are protected by time-reversal symmetry (TRS) and introduced by strong spin-orbit coupling (SOC). In particular, the interfaces between TI and transition metal ferromagnets represent a powerful platform for the realization of spin-charge conversion processes such as, but not limited to, spin pumping and spin-orbit torques. Uncovering the impact of 3d transition metal (3d-TM) overlayers on the surface states of TI and evaluating the resulting interfacial spin-momentum locking will undoubtedly open promising avenues for the efficient control of spin and charge currents mediated by spin-orbit coupling. In this work, based on density functional theory calculations, we rigorously study the spin-textured configuration of 3d-TM adsorbed on Bi2Se3. This systematic study provides a global description of the interfacial orbital hybridization scheme and its related spin texture, for the whole range of 3d-TM. Our results show that single-ion anisotropy is driven by the spin-polarized of Bi, Se and 3d-TM atoms, and originates from the interaction of 3d-TM magnetic adatom with the topological surface states conducting electrons. The bottom surface of the slab interfaced with vacuum, shows delocalized Bi and Se p-orbitals forming an unperturbed Dirac cone, while the top surface of the slab, interfaced with the 3d-TM overlayer, displays localized p-states with strong spin texture. Indeed, the excess of 3d-TM electronic charge pushes down the top layer Dirac cone towards lower energy, and the distortion on the point of Dirac comes from the hybridization of 3d-TM and TI. Finally, the 3d-TM absorbed on the hollow adsorption site at the top surface display localized d-states that acquire a spin texture through proximity effect with the TI. The strength of the spin texture on the TI and 3d-TM orbitals are reported for the whole range of 3d transition metal series.

Resume : The transition metal dichalcogenides have gained renewed interest due to the successful realization of field-effect transistors and the thickness-dependent, indirect-direct bandgap transition. Additionally, defect engineering is essential to effectively manipulate magnetic property in the diamagnetic 2H-MoS2 for possible spintronics. Particularly, monosulfur vacancies are frequently observed in transmission electron microscope measurements, where the electron beam (80 keV) is lower than the displacement energy of S atoms (90 keV) [Mo atoms (560 keV)]. A prolonged exposure, i.e., an increased electron dose, increases the vacancy concentration and evolves the sulfur defects into line defects. On the basis of these results, the correlation between the electron irradiation-induced defects and the magnetic properties is crucial for the successful integration of MoS2 into possible device. Here, we report that the electron irradiation on the layered MoS2 crystals is found to be an effective and simple method to induce the diamagnetic to ferromagnetic phase transition persisting up to room temperature. The easy axis can be controllable by regulating the electron dose and the acceleration energy. The ferromagnetic states are largely attributed to the strain around the vacancies. This work was supported by Basic Science Research Program (2015R1D1A1A01058332), and National Honor Scientist Program (2010-0020414) through the National Research Foundation of Korea (NRF).

Resume : Last theoretical studies indicate promising results concerning magnetic properties of thin SnS2 layers. This calculations shows long-range ferromagnetic ordering with Curie temperature above room temperature. This effect can be obtained by magnetic dopants (V/Cr/Mn/Fe/Co) which additionally induce strain. The second way is by applying doping with 4d transition metal elements (Nb, Mo, Ru, Rh, Ag, Cd). The syntheses have been carried out using chemical transport in vacuum. Different 4d dopants were used and special attention was paid to Mo dopant as the most promising one. The obtained crystals have been characterized with X-ray diffraction methods, Raman scattering spectroscopy, luminescence, scanning electron microscopy and Atomic force microscopy.

Resume : We carried out Monte Carlo simulations of the pseudo-spin Ising model in 2-dimensional ferroelectric structure. The Metropolis method was employed by studying the size dependence of the dielectric susceptibility and the heat capacity of a two-dimensional ferroelectric system. The observable of the relation between the physical system of the finite size and the infinite system estimates the correlation length. The critical phenomena in the vicinity of the phase transition temperature was considered by the the values of critical exponents in view of the large-scale of lattice.

Resume : Electron spin in semiconductor devices can enable novel devices with a wide variety of potential applications like spin field effect transistors (SpinFETs), considered as a future candidate for high performance computing and memory applications with ultra-low power consumption. Here, we apply finite-element quantum-corrected ensemble Monte Carlo self-consistent device simulations with electron spin to a nanoscale III- V field effect transistor to investigate a spin transport. The simulations include spin as a separate degree of freedom via a spin density matrix. The spin-orbit interaction assumes the D?yakonov-Perel mechanism with two terms: the Dresselhaus, and Rashba Hamiltonians which account for spin-orbit coupling due to bulk (crystal) inversion asymmetry and structural inversion asymmetry respectively. We then investigate the spin dynamics across the channel of a 25nm gate length In0.3Ga0.7As MOSFET. but only the source electrode is ferromagnetic. We vary both the drain and gate biases and apply mechanical strain. The electron spins initially decays as the electrons traverse the channel from the source at approximately 10nm past the gate. However, magnetisation then partially recovers as the electrons approach the drain. Since the drain electrode is non-magnetic, the recovery of the magnetization cannot be attributed to existing polarized carriers inside the drain but must be assumed to be due to partial re-phasing of electron spins resulting in a net magnetization.

Resume : Two-dimensional materials based on transition metal dichalcogenides (TMDs) have gained increasing attention because of their fascinating features in electronic and optical properties. Achievement of layered TMDs with large lateral dimensions is a pre-requisite for an easy implementation into devices and for the exploration of novel physics. Raman, XRD, AFM/MEB, PL, TEM, back gate Ferromagnet deposition, spin pumping, magnetic doping and valley Hall effect combining electrical and optical techniques. In this work, we have employed molecular beam epitaxy for synthetizing large-scale MoSe2 and WSe2 atomically thin films with high uniformity and purity. We have investigated the growth mechanism, chemical interface as well as functionalities of the flat TMD layer on SiO2/Si substrates, which is a primordial substrate for the solid states electronics. The structural analysis has been done by using the scanning transmission electronic microscopy after having transferred the as-grown samples onto Si3N4 grid. Statistically, we have found that the layer grows homogenously over the substrate and presents typical boundary and point defects. In addition, the successful transfer process allowed us to probe the spectroscopic properties of the layer by means of photoluminescence, Raman. Finally, the comprehensive study of structure enabled us to use the large-area and homogenous MoSe2 for studying magneto-transport properties at macroscopic scale as well as spin-to-charge conversion at the 2D interface by employing ferromagnetic resonance driven spin pumping technique. References [1] X. Duan, et al., Chem. Soc. Rev., 44, 8859-8876 (2015)

Resume : The electronic properties of the free standing graphene layer with Ni-adatoms is studied with the density functional theory approach, taking into account the spin-orbit interaction. We focus on the case when the Ni-adatoms form the trigonal lattice above the graphene. The three cases are examined: the Ni-adatoms in ontop, bridge and hollow positions on graphene. We show that the presence of the metallic magnetic layer modifies the band structure of the graphene as well as influences its magnetic state. In the vicinity of the K-point the graphene valence bands states are drag down below the Fermi energy, while the graphene conduction band states are less affected. We show that deformation of the Dirac cone results from the hybridization of the Ni {\it 3d} and C {\it 2p} valence states. The range of the Dirac cone deformation strongly depends on hybridization scale. The presence of the magnetic adatoms generates the magnetic moments on the graphene lattice. We also discuss the influence of the electron-phonon interaction on the electronic properties of the system.

Resume : In order to circumvent the interface states issue in spin injection and develop all-semiconductor devices, GeMn intermetallics constitute attractive materials to fabricate spin-polarized contacts to germanium. Both uniform compounds such as Ge3Mn5 and nanostructured materials such as Ge3Mn5 and Ge-Mn nanocolumns embedded in p-doped germanium [1] have been grown epitaxially on germanium, and exhibit ferromagnetic behavior up to reasonably high temperatures. A key parameter is the spin polarization of the contact, which can be evaluated using Andreev reflection spectroscopy as already applied to (Ga,Mn)As [2]: the conductance-bias spectrum of a junction between the magnetic material and a superconducting layer, is fitted within the Blonder-Tinkham-Klapwijk model [3]. We have grown Ge layers containing GeMn nanocolumns and deposited AlSi layers in the same UHV chamber, with and without an oxide intermediate layer. Tunnel junctions have been fabricated and measured at low temperature (300 mK) under an applied magnetic field. In the samples with the oxide barrier, the current-voltage characteristics exhibit the signature of both the superconducting character of the Al layer and the magnetization of the GeMn structures. The role of the barrier thickness will be discussed. References [1]. M. Jamet et al. Nature Materials 5, 653 (2006). [2] J. G. Braden et al., Phys. Rev. Lett. 91, 056602 (2003). [3] G. E. Blonder, M. Tinkham and T. M. Klapwijk, Phys. Rev. B 25, 4515 (1982).

Resume : Topological insulators (TI) offer a promising route towards efficient electrical control of the magnetization. Here, we investigate the nature and the symmetry of nonequilibrium spin densities at the surface of 3D TI using the linear response theory and their coupling to the magnetization of an adjacent ferromagnetic layer. We highlight the differences of this so-called Dirac spin-orbit torque with respect to other spin-orbit torques, namely spin Hall effect and Rashba torques. We show that the Onsager reciprocal of the spin-orbit torque, i.e. the charge current pumped by magnetization dynamics produces an enhanced anisotropic magnetic damping torque. Finally, we demonstrate numerically using the Landau-Lifshitz-Gilbert equation that the Dirac spin-orbit torque can reverse the magnetization in layers with perpendicular magnetic anisotropy, but is formally less efficient than the torque arising from spin Hall effect.

Resume : Sintering of GaN nanopowders, which are surface-doped with manganese Mn, can be a convenient method to make a new materials form of the dilute magnetic semiconductor DMS. DMS?s are the basis for spintronics ? spin related electronics which utilize both the semiconducting and magnetic properties residing in one material. Herein, described is the high temperature/high pressure sintering of GaN nanopowders surface-functionalized with Mn. The GaN nanopowders were (i) synthesized via the anaerobic synthesis method, (ii) optionally ground using a high energy ball mill, (iii) functionalized with hexane solutions of Mn{N[Si(CH3)3]2}2 as a Mn-source, and (iiii) subjected to ammonolysis. Then, they were sintered at 700-800 ºC under 8 GPa. The products were characterized with XRD, specific surface area SBET, and SEM/EDX. Magnetic properties and Mn-contents were evaluated using a SQUID magnetometer. The investigation confirmed that the key factor determining Mn-content levels before sintering was powder?s specific surface area SBET. The SBET was significantly higher for the ball-milled powders compared with the ones with no mechanical treatment of this kind and the associated Mn-contents mirrored the trend. After sintering, all the ceramics showed even higher levels of the h-GaN-incorporated Mn.

Resume : The NV center in diamond is a qubit that can be initialized, controlled, and read out at room temperature, and it is intensively researched in the context of nanoscale magnetic imaging applications. The main source of decoherence of this qubit is its interaction with the bath of 13C nuclei. We present an extension of calculation of decoherence due to the nuclear bath (using the state-of-the-art Cluster-Correlation Expansion method) to the case of two entangled qubits. When two qubits interact with a partially common bath the decay of entanglement of two-qubit singlet state can be slower or faster than the decay of product of single-qubit coherences, depending on whether the bath is source of correlated/anticorrelated noise, respectively. We have investigated the nature of effective magnetic field noise generated by the bath of nuclear spins coupled by dipolar interaction. Flip-flopping nuclear spin pairs can either accelerate or slow down the two-qubit decoherence, depending on their position and orientation relative to the two qubits. For moderate NV-NV distance (~10 nm) the decoherence acceleration dominates, but at shorter distances the noises experienced by the two qubits become correlated, and decoherence is suppressed. This research is supported by funds of Polish National Science Center (NCN), grants no. DEC-2012/07/B/ST3/03616 and DEC-2015/19/B/ST3/03152.

Resume : Germanium is in the spotlight of research due to its unique optoelectronic properties given by the particular band structure. Its L-valley electrons have a strong spin orbit interaction, which can be harnessed to control their spin degree of freedom. Germanium performs at its best in 2D heterostructures with silicon, where quantum confinement phenomena come into play. This integration opens up novel possibilities and new challenges. In the past years a wealth of information, both experimental and theoretical, has been collected. This led to the fabrication of CMOS-compatible quantum well based on Ge. Specifically, we studied three multiple quantum well (MQW) structures differing in the thickness of the barrier and the well. All the MQWs were fabricated with n-type modulation doping that confines electrons in the well. We observed electron spin resonance of conduction electrons, strong g-factor tunability, and long coherence times, [1] consistent with high carrier mobility and low interface scattering. These results are corroborated by optical measurements of polarization resolved photoluminescence. Such findings marks clearly that large g-factor tunability can coexist with long coherence times. [1] Giorgioni et al., Nat. Commun. 7, 13886 (2016)

Resume : The development of active spintronic devices requires an efficient spin injection into semiconductors, particularly into silicon (Si). However compounds exhibiting both natural impedance match to group-IV semiconductors and high Curie temperature (TC) are noticeably lacking. Mn-doped Ge quantum dots (QDs) grown by self-assembly via Stranski-Krastanov growth mode on Si(001) would be an ideal candidate. Although ferromagnetism has already been observed above room temperature in such a structure [Xiu et al, Nat. Mater. 9, 337 (2010)], these results could not be reproduced up to date [Kassim et al, Appl. Phys. Lett. 101, 242407 (2012)], plausibly due to Ge/Si interdiffusion and Mn diffusion into the Si substrate during growth. We have therefore carried out an exhaustive study of their magnetic, morphological, structural properties by varying both growth temperature and Mn concentration. The first parameter directly influences the QDs size and the Mn-concentration incorporated in the QDs. The second one leads to different dots morphologies and to very different magnetic properties. Interestingly, interdiffusion in (Ge,Mn) QDs is suppressed for a growth temperature below 430°C. SQUID and XMCD measurements reveal the presence of two different ferromagnetic contributions with a Tc of 220K and another one above 300K, which is in both cases much higher than the Tc found in the DMS thin films. A detailed TEM/EDX analysis clarifies the origin of the ferromagnetism.

Resume : SnTe belongs to the family of topological materials classified as topological crystalline insulators (TCI), where topological protection of boundary states (surface states in the case of a bulk material) is due to {110} mirror-plane symmetry of rock-salt crystal structure of these narrow bandgap IV-VI chalcogenides. We have investigated SnTe TCI nanowires (NWs) grown by molecular beam epitaxy (MBE) on graphene/SiC and on silicon substrates. The NWs were crystallized using vapour-liquid-solid (VLS) MBE growth in both Au-catalysed and self-catalysed mode. They crystallize in the rock-salt structure, typical for bulk SnTe, grow along [001] crystalline direction, and are entirely free from stacking-fault defects. The lengths of NWs are in the range of 1 to 3 µm. Typical thickness of shorter NWs is around 50 nm. Occasionally very thin (about 10 nm in cross-section) and long (up to 3 µm) NWs are formed. The selected samples were processed by focused ion beam (FIB) and extensively studied by aberration corrected TEM. Electrical properties of the NWs were studied with local conductivity AFM, in search for contribution of topologically protected surface states of carriers propagating along four {001} NW sidewalls. This work has been supported by the research projects 2014/13/B/ST3/04489, 2014/15/B/ST3/03833, and 2016/21/B/ST5/03411 financed through the National Science Centre (Poland).

Resume : Despite the extensive research efforts devoted to the understanding of spin transport in organic semiconductors, key results, such as the Hanle effect, are still missing [1]. Many of the results in this field were obtained with the organic semiconductor (OS) tris(8-hydroxyquinoline)aluminum (Alq3). We have identified two distinct sets of devices, one with low resistance that shows spin valve magnetoresistance, and one with high resistance, with no magnetoresistance; the set a specific device belongs to does not depend on its thickness. To understand the low resistance devices, that show magnetoresistance and resistive multistability we have expanded on the impurity band model introduced by Yu [2]. In our devices we envisage oxygen to be the impurity and bistability is attributed its migration within the organic semiconductor/AlOx bilayer. These devices can be used as a synapse for neuromorphic computing and can be used in several types of neural networks, from a simple single layer perceptron to spiking neural networks. The presence of magnetoresistance adds a unique tool to effect parallel, selective changes on the weight of synapses. [1] A. Riminucci et al., “Hanle effect missing in a prototypical organic spintronic device,” Appl. Phys. Lett., vol. 102, p. 92407, 2013. [2] Z. G. Yu, “Impurity-band transport in organic spin valves.,” Nat. Commun., vol. 5, p. 4842, Jan. 2014.

Resume : A structure consisting of a lateral semiconducting channel between ferromagnetic (FM) source and drain contacts is a key component of many novel concepts for spin-based devices. The primary example is the Datta-Das spin field effect transistor (sFET) [1], with the transport channel defined within a two-dimensional electron gas (2DEG) confined in a semiconductor structure. In this talk I will present the results of our recent experiments on lateral FM/2DEG/FM devices with a 2DEG channel embedded in the GaAs/AlGaAs interface and with ferromagnetic (Ga,Mn)As/GaAs Esaki diodes as source and drain contacts.[2] I will focus on two aspects crucial for the sFET operation, namely the size of the magnetoresistance (MR) signal and its tunability by an electric gate. Firstly, I will show how the electric field within both source and drain contacts can be employed to boost the spin-to-charge conversion, what leads to very large two-terminal spin valve signals. In our devices, we observed signals on the order of 1 kOhm with the MR ratio reaching up to 80\%. These values are roughly two orders of magnitude larger than reported so far for semiconducting channels. Finally, I will demonstrate how the resistance of the device can be tuned by an electric gate, without resorting to spin orbit interaction, as in the original sFET proposal.[1] Instead, we modulate the MR ratio by affecting the distribution of spins in the channel. [1] S. Datta and B. Das, Appl. Phys. Lett. 56, 665 (1990); [2] M. Oltscher et al., Phys. Rev. Lett. 113, 236602 (2014).

Resume : Many device concepts utilizing spin-polarized carriers for logic applications require the injection of spin-polarized electrons into semiconductor channels. This can be achieved using ferromagnetic electrodes with or without a tunneling oxide barrier. The conductivity of the ferromagnetic metal Mn5Ge3 is comparable to that of highly doped Ge and its Curie temperature of ~ 300 K can be further increased by introducing C. Furthermore, the material can be fabricated by depositing Mn on Ge(111) and subsequent annealing, i.e. by a germanidation process, which could potentially be compatible with complementary metal-oxide semiconductor (CMOS) fabrication processes. Here, we present results on magnetization characterization of Mn5Ge3 on Ge(111) and spin injection measurements.

Resume : Much attention has been recently devoted to C-doped Mn5Ge3 as this compound meets all the requirements for spin-polarized transport and injection into group-IV semiconductors. The common growth process for the Mn5Ge3 thin films is the solid phase epitaxy (SPE) which involves an annealing step to form the Mn5Ge3 phase. However segregation and diffusion of Mn at the Mn5Ge3/Ge interface could severely hinder the efficiency of the spin injection. To avoid these two phenomena we have developed a novel low-temperature epitaxial growth technique that allows to produce samples with unprecedented crystalline quality [M. Petit et al., Thin Solid Films 589, 427 (2015)] and enhanced magnetic properties [C. Dutoit et al., J. Phys. D. Appl. Phys. 49 045001 (2016)]. I-V and C-V electrical measurements have been also performed to check the conduction mechanisms involved at the Mn5Ge3Cx/Ge Schottky contacts [M. Petit et al, J. Phys. D. Appl. Phys 49, 355101 (2016)]: The various parameters of Au/Mn5Ge3C0.6/Ge(111) and Au/ Mn5Ge3C0.6/?-doped Ge(111) Schottky diodes were measured in the temperature range of 30-300 K. The Schottky barrier heights and ideality factors were found to be temperature dependent. We have also shown that the Schottky barrier and the depletion length are significantly decreased by the presence of ?-doped layer, which is very promising for the spin injection.

Resume : Non-local charge carrier injection/detection schemes lie at the very foundation of information manipulation in integrated systems. The next generation electronics may operate on the spin instead of the charge and germanium appears as the best hosting material to develop such spintronics for its compatibility with mainstream silicon technology and long spin lifetime at room temperature. Moreover, the energy proximity between the direct and indirect bandgaps allows for optical spin injection and detection within the telecommunication window. In this presentation, we demonstrate injection of pure spin currents (i.e. with no associated transport of electric charges) in germanium, combined with non-local spin detection blocks at room temperature [1]. Spin injection is performed either electrically through a magnetic tunnel junction (MTJ) or optically, exploiting the ability of lithographed nanostructures to diffuse the light and create an in-plane polarized electron spin population. Pure spin current detection is achieved using either a MTJ or the inverse spin-Hall effect (ISHE) across a Pt stripe. These results broaden the palette of tools available for the realization of opto-spintronic devices. [1] F. Rortais et al., arXiv:1612.09136 (2017)

Resume : Interest in atomically-thin transition metal dichalcogenide (TMD) semiconductors such as MoS2 and WSe2 has exploded in the last few years, driven by the new physics of coupled spin/valley degrees of freedom and their potential for new spintronic and ?valleytronic? devices. Although robust spin and valley degrees of freedom have been inferred from polarized photoluminescence (PL) studies of excitons, PL timescales are necessarily constrained by short-lived (1?30 ps) recombination timescales of excitons. Direct probes of spin and valley dynamics of the resident electrons and holes in n-type or p-type doped TMD monolayers, which may persist long after recombination ceases, are still at a relatively early stage. In this work, we directly measure the coupled spin-valley dynamics of resident electrons and resident holes in n-type and p-type monolayer TMD semiconductors using time-resolved Kerr rotation. Very long relaxation timescales in the nanosecond to microsecond range are observed at low temperatures ? orders of magnitude longer than typical exciton lifetimes. In contrast with III-V or II-VI semiconductors, electron spin relaxation in monolayer MoS2 is found to accelerate rapidly in small transverse magnetic fields. This indicates a novel mechanism of electron spin dephasing in monolayer TMDs that is driven by rapidly-fluctuating internal spin-orbit fields that, in turn, are due to fast electron scattering between the K and K? conduction bands [1]. More recent studies of gated TMD monolayers also allow observation of very long spin/valley relaxation of resident holes, a consequence of spin-valley locking [2]. These studies provide direct insight into the physics underpinning the spin and valley dynamics of resident electrons and holes in 2D TMD semiconductors. [1] L. Yang et al., Nature Physics 11, 830 (2015). [2] P. Dey et al., submitted (2017), arXiv: 1704.05448.

Resume : It is now possible to create atomically thin regions of dopant atoms in silicon patterned with lateral dimensions ranging from the atomic-scale (angstroms) to microns. Such structures are building blocks of quantum devices for physics research and it is anticipated that they will also serve as key components of devices for next generation classical and quantum information processing. Until now, the characteristics of buried dopant nanostructures could only be inferred via indirect and/or destructive techniques, or correlated with the performance of the final electronic device; this severely limits engineering and manufacture of real-world devices based on atomic-scale lithography. We use scanning microwave microscopy (SMM) to image and electronically characterize three-dimensional phosphorus nanostructures fabricated via scanning tunneling microscope-based lithography. Scanning Microwave Microscopy (SMM) combines the nanoscale spatial resolution of an AFM with the broadband electrical measurement capabilities of a VNA. The VNA sends a microwave signal through a conductive AFM-tip and measures the S11 reflection signal from which one can extract the intrinsic electric sample properties like sheet conductivity, dopant concentration and dielectric properties. Due to the capability of the electromagnetic wave to penetrate the surface of the sample under study the technique can be also used to selectively sense sub-surface features [1]. The SMM measurements, which are completely non-destructive and sensitive to as few as 1900 to 4200 densely packed P atoms 4 to 15 nm below a silicon surface, yield electrical and geometric properties in agreement with those obtained from electrical transport and secondary ion mass spectroscopy for un-patterned phosphorus ?-layers containing ~10^13 P atoms [2]. The imaging resolution was 37±1 nm in lateral and 4±1 nm in vertical direction, both values depending on SMM tip size and depth of dopant layers. Additionally, finite element modeling indicates that resolution can be substantially improved using further optimized tips and microwave gradient detection. Our results on three-dimensional dopant structures reveal reduced carrier mobility for shallow dopant layers and suggest that SMM could be applied to aid the development of fabrication processes for surface code quantum computers. [1] G. Gramse, E. Brinciotti et al., Nanotechnology, 26, 13 (2015) [2] G. Gramse et al., Non-destructive imaging of atomically-thin nanostructures buried in silicon. Science Advances (accepted).

Resume : Two dimensional layered films such as graphene and layered inorganic materials are promising for future nanoscale electronics and optics. We also performance of dielectric layer and metal contacts on the performces of field effect tranisistors based on InSe. we discover that carrier scattering from chemical impurities of hydroxyl groups and absorbed water molecules at oxidized dielectric plays a central role in determining the mobilities of 2D layered semiconductors based FETs, and suppression of this carrier scatter can significantly enhance the performance of 2D layered semiconductor devices. Further, we demonstrate high performance multilayer InSe transistors on poly-(methyl methacrylate) (PMMA)/Al2O3 bilayer dielectric with a room-temperature mobility > 1000 cm2V-1s-1, comparable to that of strained-silicon thin-film. The first GaS nanosheet-based photodetectors are demonstrated on both mechanically rigid and flexible substrates. Photocurrent measurements of GaS nanosheet photodetectors made on SiO2/Si substrates and flexible polyethylene terephthalate (PET) substrates exhibit a photo-responsivity at 254nm up to 4.2 AW-1 and 19.2 AW-1, respectively, which exceeds that of graphene, MoS2, or other 2D materials-based devices. Additionally, the linear dynamic range of the devices on SiO2/Si and PET substrates are 97.7dB and 78.73 dB, respectively. Both surpass that of currently-exploited InGaAs photodetectors (66 dB). Further, we have also performed work in GaSe based photodetector have for the first time. GaSe based photodetector showing a fast response of 0.02s, high responsivity of 2.8 AW-1 and high external quantum efficiency of 1367% at 254 nm, which indicates that the two dimensional nanostructure of GaSe is a new promising material for high performance photodetectors. References: [1]J Zhang, W Feng, H Zhang, Z L Wang, H A Calcaterra, B Yeom, P A Hu*, N A Kotov, Nature Comm, 2016, 7,10701 [2]Feng W., Zheng W., Cao W., Hu P A*, Advanced Materials,  2014, 38: 6587-6593 [3]Wang L., Wu, B. Chen,J. Liu, H. Hu P.A*, Liu,Y. Advanced Materials 2014, 26, 1559?1564 [4]J. Zhang, J Li, Z Wang, X. Wang, W Feng, W Zheng, Wenwu C,? P Hu*, Chem.Mater, 2014, 26 (7), 2460?2466. , [5]Hu PA., Wang L., Yoon M., Zhang J. et al. Nanolett. 2013,13, 1649-1654. [6]Hu P.A., Wen Z., Wang L., Tan P., Xiao K., ACS Nano, 2012, 6 (7), 5988?5994.

Resume : 2D MoS2 is a non-centrosymmetric material with a direct energy gap of 1.9 eV which causes a strong photoluminescence with efficient valley and spin control. Also, the reduced dielectric screening in 2 dimensions along with the heavy effective mass of Mo-centered d-electrons contribute to the formation of the quasiparticles viz. excitons along with trions. These quasiparticles are, in particular, important since they open up promises like many-body interactions which are not just fundamentally significant but also enable optoelectronic tunability. So far, the routes to tune the MoS2 quasiparticles have been via conventional methods like application of electric field or by changing the dielectric environment. Tuning these quasiparticles via many body interactions with the substrate itself has not been investigated. This is mainly due to weak interactions of MoS2 with conventional substrates (like SiO2). In this work, we have grown MoS2 directly on SrTiO3, a transition metal oxide well known in ?oxide electronics? for its tunable temperature dependent dielectric constant and rich phonon diagram. Direct CVD growth on these substrates provides us with a strongly coupled system as compared to exfoliation or transfer. Temperature dependent PL measurements show that while the charge neutral exciton does not interact much with SrTiO3, it is possible to selectively tune the lifetime and binding energy of the negatively charged trion by modulating the substrate environment. Electric field dependent luminescence studies on MoS2/SrTiO3 devices which help us manipulate the phonons in SrTiO3 support our hypothesis. Our experiments on various substrates and calculations help us clearly establish the effect of dielectric, temperature and even phonon interactions of SrTiO3 on MoS2. Our results open an unprecedented route to tune quasiparticles in a 2D TMDC through interactions with transition metal oxides for high efficiency valleytronics.

Resume : The two-dimensional character and reduced screening in monolayer transition-metal dichalcogenides (ML-TMDs) lead to the formation of tightly-bound excitons and trions with binding energies orders of magnitude larger than ones in conventional bulk semiconductors. These effects are affected by the unique band structure of TMDs, which in the limit of a single ML become direct bandgap semiconductors with the gap located at the K and K? points in the Brillouin zone. In ML-TMDs, the breaking of inversion symmetry in combination with strong spin-orbit coupling results in a peculiar coupling of the spin- and valley-degrees of freedom, which can be efficiently controlled by optical helicity. At the K and K? valleys, the spin-degeneracy in the valence and conduction bands is lifted and one can distinguish between the bandstructures of tungsten- and molybdenum-based MLs, which differ from each other through the spin ordering in their conduction bands and result in different ground-state excitons. Electromagnetic fields present various opportunities to probe and/or manipulate excitonic effects in ML-TMDs, several of which are discussed here: We show how magnetic proximity effects in magnetic heterostructures can turn dark excitons into bright excitons and vice versa. Moreover, we investigate the effects of in-plane electric fields, leading to an excitonic Stark shift, as well as gate voltages: Notably, we predict gate-induced charge densities to result in the emergence of a low-energy optical sideband in electron-doped WS2 and WSe2, which is conspicuously absent for hole-doping, MoS2 and MoSe2.

Resume : Search for a new two dimensional (2D) systems become very intense in the last years and was initiated by exfoliation of graphene which is characterized by extraordinary mechanical, thermal and electrical properties. Recently a new group of elemental 2D materials has been discovered in 15th group of periodic table and include phosphorene, alpha- and beta-bismuthene and recently synthetized beta-antimonene. It was shown that both alpha- and beta-bismuthene support topologically protected edge states while beta-antimonene is topologically trivial. In this paper we will focus on Van der Waals heterostructure system - growth of alpha-antimonene on top of alpha-bismuthene. It is worth pointing out that this is first report of growing alpha phase of antimonene. Our DFT results clearly suggest topological character of this phase. We will also show coexistence of beta- and alpha-antimonene on alpha-bismuthene. We determined crystallographic structure of both phases using STM and STS. Our experimental results are supported by DFT calculations. This work is supported by the Polish National Science Centre under project DEC- 2015/17/B/ST3/02362.

Resume : Silicene, a honeycomb crystal structures composed of a Si monolayer, combines the advantages of the 2D ultimate-scaled electronics, with its compatibility with industrial processes presently based on silicon technology. Within the framework of the density functional theory and the many-body perturbation theory, by state-of-the-art first principles techniques we computed structural, electronic and magneto-optical properties of free-standing Silicene nanoribbons with zig-zag edges that has been predicted [1] to have an antiferromagnetic ground state, for the possible applications in new conception spintronic devices. After determined the minimal widths at which the nanoribbon geometry is structurally stable, we investigated the dependence of magnetic and electronic properties on the ribbon widths focusing on the short?width range, where confinement effects are expected to be more relevant. We computed the magnetic coupling of ribbon edges as a function of the width to study the stability of ferro- vs. antiferro-magnetic edge states. While, the ferromagnetic Silicene-ribbons have conducting spin-polarized edge states that present the 1D analogous of Dirac cone typical of 2D Silicene layers, the energetically favored state corresponds to anti-ferromagnetic ribbons which are magnetic semiconductors. We simulated the dielectric function and magneto-optical effects as a function of ribbon-widths and role of confined excitons on optical properties. Optical spectra constitute a clear fingerprint of magnetic coupling of the edges. Opportunities and drawback for possible applications of Silicene zig-zag nanoribbons in forthcoming magneto-optical devices are discussed. [1] Y.Ding, Y.Wang Appl.Phys.Lett 104, 083111 (2014).

Resume : The properties of native point defects and impurities in hexagonal boron nitride (h-BN) are studied from first-principles theory. The energetic and kinetic, as well as the electronic and optical properties have been investigated so as to provide a comprehensive picture of the defect chemistry and the impact of defects on fundamental materials properties. It is found that isolated native vacancy and antisite defects have extremely high formation energies, and are unlikely to form under thermodynamic equilibrium for typical growth conditions. These defects have high migration barriers, however, and could form in non-equilibrium concentrations. Interstitial defects can have low formation energies when the Fermi level is near band edge and may form as charge compensating centers. However they are too mobile to be stable, even at room temperature. Interstitial defects can form complexes. Our calculations indicate that the defect chemistry in h-BN is most likely dominated by defects involving carbon, oxygen, and hydrogen impurities; these defects can have low formation energies, and may be present in large concentrations under thermodynamic equilibrium. The dominant impurity-related defect species is determined by growth conditions. Based on the results of this work, several commonly assumed models for deep-level luminescence in h-BN are ruled out, and we are able to make some tentative assignments as to their origins, including the recently observed single-photon emitters.

Resume : Quantum computation has captivated the minds of many for almost two decades. For most of that time, it was seen mostly as an extremely interesting scientific problem. In the last few years, we have entered a new phase as the belief has grown that a large-scale quantum computer may actually be built. Quantum bits encoded in the spin state of individual electrons in silicon quantum dot arrays, have emerged as a highly promising direction. In this talk, I will present progress in our group along three fronts. First, we have achieved universal all-electrical control of two spin qubits in a double quantum dot, in combination with individual single-shot read-out of each qubit [unpublished]. We have begun to test quantum algorithms and other protocols to test and demonstrate integrated device operation. This work builds on our earlier work on all-electrical single-spin manipulation [1]. Second, we have explored coherent coupling of spin qubits at a distance via two routes. In the first approach, the electron spins remain in place and our coupled via an intermediary degree of freedom [2]. In the second approach, spins are shuttled along a quantum dot array, preserving both the spin projection [3] and spin phase [4]. Third, we have developed new concepts and techniques that make quantum dot arrays a credible platform for quantum simulation of the Mott-Hubbard model. As a first demonstration, we map out the transition from Coulomb blockade to collective Coulomb blockade, the finite-size analogue of the Mott insulator transition [5]. When combined, the progress along these various fronts can lead the way to scalable networks of high-fidelity spin qubit registers for computation and simulation. [1] E. Kawakami, T. Jullien, P. Scarlino, D.R. Ward, D.E. Savage, M.G. Lagaly, V.V. Dobrovitski, Mark Friesen, S.N. Coppersmith, M.A. Eriksson and L.M.K. Vandersypen, Gate fidelity and coherence of an electron spin in a Si/SiGe quantum dot with magnet, Proceedings of the National Academy of Science, 113, 11738?11743 (2016) [2] T.A. Baart, T. Fujita, C. Reichl, W. Wegscheider, L.M.K. Vandersypen, Coherent spin-exchange via a quantum mediator, Nature Nanotechnology, 12, 26?30 (2017) [3] T. A. Baart, M. Shafiei, T. Fujita, C. Reichl, W. Wegscheider, L. M. K. Vandersypen, Single-Spin CCD, Nature Nanotechnology 11, 330-334 (2016) [4] T. Fujita, T. A. Baart, C. Reichl, W. Wegscheider, L. M. K. Vandersypen, Coherent shuttle of electron-spin states, npj Q Info, accepted, arXiv:1701.00815 [5] T. Hensgens, T. Fujita, L. Janssen, Xiao Li, C. J. Van Diepen, C. Reichl, W. Wegscheider, S. Das Sarma, L. M. K. Vandersypen, Quantum simulation of a Fermi-Hubbard model using a semiconductor quantum dot array, arXiv:1702.07511

Resume : Germanium is one of the most appealing candidate for spintronic applications, thanks to its compatibility with the Si platform, the long electron spin lifetime and the optical properties matching the conventional telecommunication window. Electrical spin injection schemes have always been exploited to generate spin accumulations and pure spin currents in bulk Ge. However, it is well known that ferromagnetic injection or detection blocks can introduce parasitic effects at the metal/semiconductor interface, which are still under debate. Here, we exploit the spin-Hall effect to generate a uniform pure spin current in an epitaxial n-doped Ge channel and we detect the electrically-induced spin accumulation, transverse to the injected charge current density, with polar magneto-optical Kerr microscopy at low temperature. We show that a large spin density up to 400 ?m?3 can be achieved at the edges of the 100-?m-wide Ge channel for an applied electric field lower than 5 mV/?m. We find that the spin density linearly decreases toward the center of the Ge bar, due to the large spin diffusion length, and such a decay is much slower than the exponential one observed in III?V semiconductors, allowing very large spin accumulations over a length scale of tens of micrometers. We also characterize the electrically-induced spin voltage as a function of the applied bias and temperature, revealing that the spin-to-charge conversion in bulk Ge is preserved up to 120 K.

Resume : Ge is an attractive candidate for transport in novel spintronic architectures, owing to its full compatibility with the CMOS technology, its high bulk mobility, the long spin-relaxation time and the abundance of spin-less isotopes. To assess its potential, we determined the electron spin relaxation time by means of time- and polarization-resolved photoluminescence measurements of quantum wells of pure Ge grown on Si. The time-dependent depolarization of the PL provides a direct means to measure the spin-lattice relaxation. Our findings unveil the pivotal role played by the electron-hole exchange interaction in determining spin-flip processes in low-dimensional Ge heterostructures. Our results were successfully compared with those measured by electron spin resonance, disclosing microsecond long spin-flip times at cryogenic temperatures. Looking ahead, Ge heterostructures can offer a special framework for quantum computation, opening unexplored pathways for future studies of confinement-induced tailoring of the spin properties in group IV semiconductors. This investigation paves the way towards Ge-based spintronics and the exploitation of spin currents in novel transport architectures on Si, such as spin-based interconnects, transistors, and logic.

Resume : The manipulation of the electron spin in semiconductor nanostructures requires the knowledge of the electron g-factor. In this work, we revisit the physics of the electron g-factor in nanostructures of various shape, size, dimensionality (0D-3D) and composition. Our investigation is based on a combination of atomistic and analytical calculations. We show that, for a given compound, the electron g-factors follow a universal law that just depends on the energy gap, in particular along rotational symmetry axes. We demonstrate that the orbital magnetic moment density strongly depends on the shape of the nanostructure but the total (integrated) magnetic moment is independent of the shape and therefore of the electron envelope wavefunction. The physical origin of this non-trivial behavior is explained. We deduce that the bulk component of the g-factor is isotropic and that g-factor anisotropies entirely come from surface effects.