Spin-dependent phenomena in semiconductors, 2D materials and topological insulators

Resume : Transitions metal dichalcogenides (TMDs) are direct semiconductors in the atomic monolayer (ML) limit with fascinating optical and spin-valley properties. The strong optical absorption of up to 20 % for a single ML is governed by excitons, electron-hole pairs bound by Coulomb attraction. The optical selection rules for excitonic transitions depend on the spin of valence and conduction states as well as the orientation of the exciton dipole in- or out-of the monolayer plane. Excited exciton states in MoSe2 and MoTe2 monolayers have so far been elusive due to their low oscillator strength and strong inhomogeneous broadening. Here we show that encapsulation in hexagonal boron nitride results in emission line width of the A:1s exciton below 1.5 meV and 3 meV in our MoSe2 and MoTe2 monolayer samples, respectively. This allows us to investigate the excited exciton states by photoluminescence upconversion spectroscopy for both monolayer materials. The excitation laser is tuned into resonance with the A:1s transition and we observe emission of excited exciton states up to 200 meV above the laser energy. We demonstrate bias control of the efficiency of this non-linear optical process. At the origin of upconversion our model calculations suggest an exciton-exciton (Auger) scattering mechanism specific to TMD MLs involving an excited conduction band thus generating high energy excitons with small wave-vectors. Also the role of dark versus bright exciton transitions is discussed.

Resume : Coupled quantum systems are very desirable structures due to potential applications in spintronics and quantum computing. Particularly interesting are coupled objects with different dimensionality like 2D quantum wells (QWs) ?2D structures and 0D quantum dots (QDs) ?0D structures. By combining such different object one can think about unusual combination of physical properties. For example spin relaxation in QWs is very fast but it is rather low in QDs. Therefore structure with coupled QWss and QDs could be used for efficient spin orientation of carriers in QWs and further injection of such polarized carriers to QD where spin will be preserved long time. Polarization of carriers in QDs can be subsequently transfer to magnetic ions and in particular to single magnetic ion in a QD. However, for efficient tunnelingtunnelling, QW energy should be equal to or slightly larger than QDs energy but due to quantum confinement larger objects like QWs exhibit typically higher energy than smaller object like QDs. Moreover, QWs should be not strained too much while QDs are typically formed in strained structure. Therefore it is difficult to find good material combination for realization of coupled QWs and QDs. We propose and realize using molecular beam epitaxy two such systems. In the first system, coupled QWs and QW dots based on (Cd,Mg)Te system, where QW is made of (Cd,Mn,Mg)Te:Mn with low Mg content, barrier is made of (Cd,Mg)Te with high Mg content, and QDs are made of CdTe:Mn. In the second system QW is made of ZnTe, barrier is made of (Zn,Mg)Te, and QDs are made of CdTe or ZnTeOur system is realized using molecular beam epitaxy and investigation are based on optical spectroscopy at low temperatures. Optical spectra confirm that energy of obtained QWs is slightly higher than energy of QDs. Moreover, in a series of samples with various position of QD versus QWs we observe either rapid transfer of energy from QWs to QDs, or efficient luminescence of QWs.

Resume : Spin and photon degrees of freedom have emerged as competing candidates in the quest to extend or even substitute current electronics based on charge transport . Their coupling holds the prospects for a new family of multifunctional, high-performance and energy-efficient devices. Switching the state of light polarization by means of simple turning knobs is yet a crucial roadblock, as current approaches mostly rely on strong external magnetic fields or on bulky phase shifters. Here we demonstrate the dynamic manipulation of the chirality of light at telecom wavelengths. This unique possibility is enrooted in the multivalley nature of the conduction band of Germanium. Ge is indeed appealing, thanks to its compatibility with Silicon microelectronic processing and its quasi-direct behavior. In this work we demonstrate that optical pumping with circularly polarized light suffices to govern the kinetics of spin-polarized carriers and, in turns, the chirality of the radiative recombination without magnetic field control or phase shifter coupling. The demonstration of tunable polarization at the direct gap transition in Ge, although achieved at low temperatures and with low polarization values, can stimulate novel investigations aimed to implement spin-based optoelectronic functionalities directly in a CMOS-compatible platform.

Resume : Doping of semiconductors with magnetic ions results in a number magnetic, transport and optical and magnetooptical properties valuable from the point of view of spintronics and optoelectronics. Typically, the magnetic ions are introduced during the sample fabrication, e. g., in epitaxial or spray pyrolysis growth. In the present work we study magnetooptical properties of thin nonporous ZnO film covered with a semitransparent layer of Fe by sputtering. Thanks to a high surface to volume ratio, the nanoporous film is expected to incorporate Fe ions into its crystal lattice ions due to diffusion during the sample production process. ZnO films thick for 100 nm are deposited onto fused silica substrates by means of DC reactive sputter deposition from a Zn target in an argon?oxygen mixture. Next, 25 nm of iron is sputtered on the film. Photoluminescence is measured at T = 1.8 K, in magnetic field of up to 10 T applied in Faraday configuration. Two emission maxima observed in the near band-gap spectral region are related to bound exciton at around 3.36 eV and to donor acceptor pairs (DAP) at around 3.32 eV. In the case of Fe covered ZnO the DAP maximum dominates, while in the case of a reference, as grown ZnO sample the excitonic one is stronger. Upon application of external magnetic field the emission of the Fe covered layers increases in both polarizations and shows a tendency to saturation. In sigma+ polarization the increase is much stronger (up to two-fold) than in sigma- polarization. For reference ZnO layers, the emission gets weaker in sigma- and stronger in sigma+ polarization, with a linear dependence on the field and no saturation. The above observations are in agreement with what was observed previously for semimagnetic (Zn,Fe)O layers produced by spray pyrolysis [1]. This indicates that doping of the nanoporous ZnO by diffusion effects is feasible. The presented doping method is likely to work also with other magnetic ions than Fe and provides an alternative for well established, previously used ones. [1] J. Papierska et al., Physical Review B 94, 224414 (2016).

Resume : Materials stages of spin transport are expanding from conventional metallic and semiconducting bulk materials to exotic systems, such as two-dimensional electron gases (2DEGs), low dimensional materials like graphene and transition metal dichalcogenides (TMDs), topological insulators (TIs), and artificial low dimensional materials. In fact, huge attention has been collected to spin transport and spin conversion in graphene and TMDs, and spin detection in TIs is also the most significant topic in condensed matter physics. Our group is focusing on spin physics in 2DEGs [1], graphene [2] and TIs [3,4], and in this presentation, I introduce our recent success on experimental demonstration of successful room temperature spin transport in 2DEGs formed in LaAlO3/SrTiO3 [1]. The spin relaxation length at room temperature is estimated to be ca. 300 nm from the gap length dependence. Calculations using two different theoretical models supported the claim. If time permits, I also introduce all-electric spin conversion with reciprocity in TIs [4] and/or continuous and gigantic modulation of spin-orbit coupling in ultrathin Pt, as an artificial low dimensional system, by an external electric field [5]. [1] R. Ohshima, M. Shiraishi et al., Nature Mater. 16, 609 (2017). [2] S. Dushenko, M. Shiraishi et al., PRL116, 166102 (2016). [3] Yu. Ando, M. Shiraishi et al., Nano Lett. 14, 6226 (2014). [4] Yu. Ando, M. Shiraishi et al., in preparation. [5] S. Dushenko, M. Shiraishi et al., submitted.

Resume : Current induced spin-orbit torques (SOTs) provide a new way to manipulate the magnetization. We find that a full sign reversal of SOTs occurs as the oxygen bonding increases in Pt/CoFeB/MgO, which evidences an interfacial SOT mechanism [1]. We show enhanced current induced SOTs from multilayers such as Co/Pd [2] and ferrimagnetic CoGd systems [3]. SOTs in a topological insulator Bi2Se3 [4,5] as well as an oxide heterostructure SrTiO3/LaAlO3 [6,7] show the largest SOTs obtained to date, which generate strong spin currents to switch the magnetization. Time resolved SOT spin dynamics is first measured [8], and oscillatory SOT switching induced by field-like torques is observed [9]. In order to probe spin textures, we utilize the bilinear magnetoelectric resistance [10], and Lorentz transmission electron microscopy [11] for imaging of chiral spin textures, skyrmions in an exchange-coupled Co/Pd multilayer. Finally, we discuss the generation of THz for heavy metal/ferromagnet structures using SOTs [12]. 1. X. Qiu et al., Nat. Nanotechnol. 10, 333 (2015) 2. M. Jamali et al., Phys. Rev. Lett. 111, 246602 (2013) 3. R. Mishra et al., Phys. Rev. Lett. 118, 167201 (2017) 4. Y. Wang et al., Phys. Rev. Lett. 114, 257202 (2015) 5. Y. Wang et al., Nat. Commun. 8, 1364 (2017) 6. K. Narayanapillai et al., Appl. Phys. Lett. 105, 162405 (2014) 7. Y. Wang et al., Nano Lett. 17, 7659 (2017) 8. J. Yoon et al., Sci. Adv. 3, e1603099 (2017) 9. J. Lee et al., Commun. Phys. 1, 2 (2018) 10. P. He et al., Nat. Phys. 14, 495 (2018) 11. S. Pollard et al., Nat. Commun. 8, 14761 (2017) 12. Y. Wu et al., Adv. Mat. 29, 1603031 (2017)

Resume : A plethora of exciting properties have been observed at the oxide heterointerface between LaAlO3 and SrTiO3. Most prominently it has been demonstrated that the confined electron gas formed at the interface features e.g. gate-tunable superconductivity, ferromagnetism and a sizeable Rashba spin-orbit coupling. This Rashba spin-orbit coupling allows a considerable charge/spin interconversion to take place at this particular inter-face as previously demonstrated by spin pumping ferromagnetic resonance experiments from a NiFe contact. Moreover, the efficiency of this charge-to-spin conversion was amazingly even demonstrated to be gatable. Transport experiments of patterned LaAlO3/SrTiO3 devices have also allowed detection and quantification of this charge-to-spin conversion through non-local resistance measurements. Here, we perform such Hanle measurements of the non-local resistance in patterned amorphous-LaAlO3/SrTiO3 devices. Specifically, we investigate the temperature dependence of the non-local resistance as well as its dependence with magnetic field direction. From these measurements, we extract the spin Hall angle and characteristic spin diffusion length at 2 K to be 0.17 and 750 nm, respectively.

Resume : In heavy metal (HM) elements exhibiting significant spin-orbit coupling, the charge current (Jc) may be converted into the spin current (Js), which may exert a torque on the magnetization in the adjacent ferromagnet (FM) due to spin Hall effect. Furthemore, due to magnetoresistance effect present in such HM/FM bilayers, osillations in the resistance driven by microwave current give rise to the generation of DC mixing voltage (Vmix) proportional to the spin-orbit torque (SOT) [1]. In this work, the SOT effect in bilayers containing W or Pt metals and CoFeB or Co ferromagnetic layers is investigated. Vmix vs. magnetic field dependence is measured in W(0-10)/CoFeB(5) and CoFeB(5)/Pt(0-10) (thicknesses in nm) 100 × 20 µm stripes supplied with a microwave signal of a frequency between 4 and 12 GHz. The obtained experimental dependencies are modelled using the sum of symmetric Lorentzian component originating from damping-like SOT and asymmetric Lorentzian component being a sum of the Oersted field, field-like SOT and interface contribution [2]. As a result, spin Hall angle, mixing conductance on HM/FM interface and spin diffusion length in the investigated bilayer systems are determined. The project is supported by National Science Centre, Poland, Grant No. 2015/17/D/ST3/00500 and 2016/23/B/ST3/01430. [1] L. Liu et al. PRL 106, 036601 (2011) [2] M. Cecot et al. Sci. Rep. 7, 968 (2017)

Resume : Majorana bound states have been shown to be good candidate for the development of a topological quantum computer. These quasiparticles can be engineered in a hybrid system consisting of a high spin coupling III-V nanomaterial interfaced with a s-wave superconductor, such as Al. On the materials side, most progress in the field has relied until now on high quality vapor liquid solid growth of free-standing III-V nanowires functionalized at a later stage by ex-situ deposited Al. Advanced proposed device designs require a more scalable approach enabling easy formation of complex connected wire networks. Here I will report on the progress of our team in using selective area epitaxy in a molecular beam epitaxy (MBE) reactor to grow both III-V semiconductor nanowire networks and a high quality epitaxial s-wave metal superconductor. First, the general physics context and current state of the art will be introduced, with highlight of the challenges and advantages of the selective area epitaxy/MBE approach with respect to other growth techniques, geometries and growth mechanisms. Then we will show how to map the selectivity window enabling successful growth of the III-V semiconductor nanowires. Then, fundamental knowledge grounded in growth understanding and crystallography will be applied to obtain reproducible, high yield advanced high spin-orbit III-V nanowire/superconductor networks. The materials properties and quality are characterized by scanning and transmission electr

Resume : Intriguing phenomena such as tunable ferromagnetism [1] and topological transition [2] have been theoretically predicted in III-VI layered semiconductors. However, experimental studies on spin related transport is very limited in these systems. Magnetoconductance (MC) at low temperature was measured to investigate spin-related transport affected by spin-orbit interaction (SOI) in III-VI n-type layered GaSe films. Results reveal that MC shows weak antilocalization (WAL). Its temperature and gate voltage dependences reveal that the dominant spin relaxation is governed by the D?yakonov-Perel? mechanism associated with the Rashba SOI. The estimated Rashba SOI strength in GaSe is much stronger than that of III-V compound GaAs quantum wells, although the energy gap and spin split-off band in GaSe closely resemble those in GaAs. The angle dependence of WAL amplitude in the in-plane magnetic field direction is almost isotropic. This isotropy indicates that the strength of the Dresselhaus SOI is negligible compared with the Rashba SOI strength. The SOI effect in n-GaSe thin films differs greatly from those of III-V compound semiconductors [3]. [1] T. Cao, Z. Li, and S. Louie, Phys. Rev. Lett. 114, 23602 (2015). [2] Z. Zhu, Y. Cheng, and U. Schwingenschloegl, Phys. Rev. Lett. 108,266805 (2012). [3] S. Takasuna, J. Shogai, M. Kohda, Y. Oyama, and J. Nitta, Phys. Rev. B 96, 161303 (R) (2017).

Resume : Top-down exfoliation from bulk transition metal dichalcogenides like MoS2 usually leads to micron-sized flakes. We have recently developed an alternative growth method of two-dimensional transition metal diselenides (TMDS) from multilayers down to a single layer based on the Van der Waals (VdW) epitaxy. In the VdW epitaxy, the TMDS is grown either on a passivated surface with a very low density of dangling bonds (it is then called quasi-VdW epitaxy) or on a layered VdW substrate. The basic concept of this growth method relies on the very weak interaction between the epilayer and the substrate in order to largely release the constraint of lattice matching. Therefore it leads to the formation of fully relaxed TMDS layers. Moreover, this technique allows for the growth of uniform layers over centimeter scale surfaces making it compatible with the development of a large scale 2D electronics based on these materials. In this presentation, we will present our recent results on three different topics relying on the VdW epitaxy: (i) the epitaxy of multi and monolayers of WSe2 on a mica substrate either non-intentionally doped, p-type doped (with Nb) or magnetically doped (with Mn), (ii) the full characterization of the layers using Raman and photoemission spectroscopies, x-ray diffraction and SQUID magnetometry and (iii) their transfer onto a SiO2/Si substrate to study magnetotransport properties. In particular, we find a ferromagnetic order in Mn-doped WSe2 layers at low temperature adding a new member to the recently discovered 2D ferromagnets family [1,2,3]. References: [1] C. Gong et al., Nature 546, 265 (2017). [2] B. Huang et al., Nature 546, 270 (2017). [3] M. Bonilla et al., Nature Nanotech. 13, 289 (2018).

Resume : Spin-charge interconversion (SCI) phenomena have attracted a large interest in nowadays spintronics. Here, we investigate SCI in ultrathin Bi films epitaxially grown on Ge(111) as a function of the Bi thickness t. We use x-ray diffraction and scanning tunneling microscopy to obtain a clear picture of the morphology and crystallography of the system. Through spin- and angle-resolved photoemission we show that spin-polarized surface states crossing the Fermi level are present. Then, we directly probe the charge-to-spin conversion by detecting with magneto-optical Kerr effect the electrically-induced spin accumulation in Bi, and the spin-to-charge conversion by generating a spin current in the system with either optical or ferromagnetic resonance driven spin injection. We recover large SCI signals in the thickness range between 1 and 3 nm, characterized by the presence of small Bi nanocrystals, whereas the conversion efficiency drastically decreases as t increases, when the Bi islands start to percolate. Since bulk SCI is small, the conversion is mainly related to the Rashba-Edelstein effect associated with electron transport in the spin-polarized surface states. We explain the observed thickness dependence of SCI by reminding that the Bi conductivity can be strongly affected by quantum confinement effects. In the high confinement conditions realized in the Bi nanoislands obtained between 1 and 3 nm, the bulk resistivity is high enough to electrically disentangle the upper and lower Bi surfaces, which otherwise would give rise to opposite contributions to SCI conversion that tends to cancel out. Our results indicate that quantum size effects might be exploited as a tool to tune SCI and investigate a very rich spin-physics.

Resume : We show room-temperature spin transport in both n-type and p-type germanium (Ge). First, we detect pure spin current transport in n-Ge using four-terminal nonlocal magnetoresistance measurements in lateral spin-valve (LSV) devices with Heusler-alloy/Ge Schottky-tunnel contacts [1-3], where the values of the resistance area product (RA) are less than 0.5 k?µm2. Next, using the same LSV devices, we demonstrate the detection of two-terminal local spin transport up to room temperature. Finally, we introduce methods for detecting spin transport even for p-Ge by using vertically stacked Ge/Heusler-alloy structures with Schottky-tunnel contacts [4,5]. K.H appreciates good collaborative research with Prof. K. Sawano, Prof. V. Lazarov, and the colleagues of our group in Osaka University. This work was partially supported by JSPS/MEXT KAKENHI (No. 16H02333, No. 17H06120, No. 17H06832, and No. 26103003). [1] Y. Fujita et al., Phys. Rev. Appl. 8, 014007 (2017). [2] M. Yamada et al., Phys. Rev. B 95, 161304(R) (2017). [3] M. Yamada et al., Appl. Phys. Exp. 10, 093001 (2017). [4] M. Kawano et al., Appl. Phys. Lett. 109, 022406 (2016). [5] M. Kawano et al., Phys. Rev. Mater. 1, 034604 (2017).

Resume : The maturity of Si-based technology provides compelling motivation to integrate the spin functionality in novel Si-based devices. Recently, we demonstrated the creation of a giant spin accumulation in Si up to room temperature in nonlocal (NL) devices with Fe/MgO tunnel contacts [1]. In the latter study, spin transport was studied in the low bias regime, where it can be described as purely diffusive using the standard theory for spin injection and diffusion. In the high bias regime, however, the presence of an electric field (E) in the Si channel causes spin drift that can strongly affect spin transport and spin accumulation voltages in Si [2,3]. Here, we study the spin transport in NL devices with a heavily-doped n-type Si channel in the presence of E in the high bias regime and quantify the effect of spin drift on the NL spin signals. The epitaxial Fe/MgO magnetic tunnel contacts were deposited by molecular beam epitaxy on a SOI substrate having a 70 nm-thick n-type Si(001) channel with a phosphorous concentration of 2.4 × 10^19 cm-3. The NL structures consist of two central ferromagnetic electrodes (FM1 and FM2) separated by a spacing d and two outer non-magnetic Au/Ti reference contacts. To study the spin-drift effect in the NL geometry, we use two different measurement configurations called NL1 and NL2, in which the direction of the electric field E is reversed, while the detector remains unbiased. To vary the strength of the electric field, we vary the current in the Si channel by either changing the bias across the injector, or by keeping the tunnel current density constant and changing the size of the injector tunnel contact. We first measure the NL spin-valve and Hanle signals in the low bias regime and extract a spin lifetime of 15 ns, a spin-diffusion length of 1.7 m, and a tunnel spin polarization of Fe/MgO of 42 % at 60 K. We then perform NL spin-valve measurements using both NL1 and NL2 schemes and different injector sizes at various biases. We demonstrate a clear modulation of the spin signal and the spin-transport length as a function of E. Using the spin-drift-diffusion model, we establish a simple and reliable method to accurately quantify spin drift and obtain the relevant spin-transport parameters as a function of the electric field or the bias current. Notably, we are able to enhance the spin-transport length by a factor of 3 and the magnitude of the spin signal up to 250 % at 60 K with a moderate electric field under the nonlocal detector of ~ -300 V/cm for spin injection condition. These findings show that spin drift is beneficial for spintronic purposes as it provides enhanced spin-transport length compared to the spin-diffusive regime. References: [1] A. Spiesser et al., Phys. Rev. Appl. 8, 064023 (2017), [2] Z.G. Yu and M. E. Flatte, Phys. Rev. B 66, 235302 (2002), [3] M. Kameno et al., Appl. Phys. Lett. 104, 092409 (2014).

Resume : SiGe alloys have been utilized in the field of Si-CMOS technologies. However, there is almost no information on spin transport properties in SiGe alloys. In this study, we demonstrate reliable spin transport (i.e., pure spin current transport) in SiGe-based lateral spin-valve (LSV) devices with Schottky-tunnel contacts. By using molecular beam epitaxy (MBE), an n-Si0.1Ge0.9 spin-transport layer (n ~ 5x10^18 cm^-3) was grown on a Ge/Si(111) virtual substrate. Then, we grew a Co2FeAl0.5Si0.5 (CFAS) ferromagnetic layer on the Si0.1Ge0.9 layer as a spin injector and detector. Finally, we fabricated LSV devices with n-Si0.1Ge0.9 spin-transport channel. Using four-terminal nonlocal measurements, we observed evident nonlocal magnetoresistance (?RNL) curves and Hanle-effect curves at 50 K, indicating reliable pure spin current transport in a SiGe alloy. We also obtained a spin diffusion length (?SiGe) of ~ 0.5 µm at 50 K by analyzing d dependence of ?RNL and Hanle-effect curves [1]. Because of the Ge-rich SiGe alloy (Si0.1Ge0.9), the value of ?SiGe was consistent with that of pure Ge at low temperatures [2]. We also observed two-terminal magnetoresistance in the SiGe-based LSV devices for the first time. This work was partially supported by JSPS/MEXT KAKENHI (No. 16H02333, No. 17H06120, No. 17H06832, and No. 26103003). [1] T. Naito et al., Appl. Phys. Express 11, 053006 (2018). [2] M. Yamada et al., Phys. Rev. B 95, 161304(R) (2017).

Resume : The rising interest of solid state community toward two-dimensional (2D) materials is driven by the capability of ab initio simulations to predict structural and electronic properties of these mono-layered compounds which have potential application in new 2D electronic devices ranging from molecular sensors to ultimate-scaled spintronic junctions. On the basis of first principles calculations we proposed and investigated a novel 2D material with honeycomb structure composed of SiGe random alloy with different Ge concentration, focusing our study on the structural, elastic, electronic and magnetic properties. We found that both with and without H passivation of dangling bonds, the lattice parameter of 2D-SiGe random alloy varies according to Vegard's law as a function of the concentration, making this material suitable as "seed ribbon" (i.e. the 2D analogous of conventional 3D substrates) in 2D lateral growth, therefore allowing the tuning of electronic properties of silicene and germanene (or of their H-passivized counterpart) in lateral 2D heterostructures, recently proposed in Ref.[1]. In particular, for partial H passivation, we found that this compound presents half-metallic properties with fully spin polarization at the Fermi energy, thus making this new 2D material a promising candidate for spin injection in 2D junctions and heterostructures. Further, for its tunable lattice and electronic properties, the H-passivized 2D-SiGe random alloy provides a excellent template also as 2D substrate for "more traditional" vertical 2D heterostructures, paving the way toward a ultra-scaled 2D electronics. [1] A. Debernardi, L. Marchetti, Ab initio simulations of pseudomorphic silicene and germanene bidimensional heterostructures, Phys. Rev. B., 93, 245426 (2016).

Resume : Recent experimental investigations have confirmed the possibility to synthesize and exploit polytypism in group IV nanowires. Indeed, while the crystal structure of Si and Ge nanowires (NWs) at standard conditions usually takes a well-defined cubic-diamond phase (as for their bulk counterparts), in the last few years several experimental observations of NWs exhibiting other phases - i.e. the hexagonal-diamond one - have been reported [1-2]. Driven by this promising evidence, here I will discuss recent first-principles calculations of the electronic and optical properties of hexagonal-diamond and cubic-diamond Si and Ge NWs as well as their homojunctions [3-4]. I will outline how a change in the crystal phase can strongly modify the electronic structure and optical response of the NW inducing novel and fascinating properties. Furthermore, I will show that, in the case of homojunctions, playing on crystal phase, size and length of the junction is an efficient tool to modulate band offsets and electron-hole separations. [1] S. Assali et al., Nano Lett. 15, 8062-8069 (2015) [2] J. Tang et al., Nanoscale, 9, 8113-8118 (2017) [3] M. Amato et al., Nano Lett. 16, 5694-5700 (2016) [4] T. Kaewmaraya et al., J. Phys. Chem. C 121, 5820-5828 (2017)

Resume : Energy dissipation has become the major challenge for the present electronics. Recent advances of physics involving topology in spintronics have made possible many low dissipation device possibilities. A large spin orbit coupling (SOC) is also shown to give rise to topological insulator with spin momentum lock and a large spin-orbit torque (SOT). Quantum anomalous Hall was also demonstrated with magnetic-doped topological insulators. Likewise, topological transition can be used to control spin textures. As the result of these and other related efforts, a field of topological spintronics now emerges via interfacing topological insulators with other materials. [1, 2] Furthermore, topological material heterostructures composed of, e.g., antiferromagnets and superconnectors are shown to offer additional new physics. I will discuss some of these exciting physics and potentials. In particular, our recent experimental results have shown the convincing evidence of several different quantized conductance plateaus (2, 1, and 0.5 e2/h) for the one-dimensional chiral Majorana fermion at the proper topological phase transitions using a quantum-anomalous-Hall/superconductor heterostructure. The half-integer quantized conductance plateau gives a firm signature in the quest of searching the elusive Majorana fermion for the first time in 80 years. Similar quantization was also recently demonstrated in an InSb nanowire/superconductor structure. [3] Our finding offers a new direction for[1]. Q.L. He, L Pan, ..., K. L. Wang, arXiv:1606.05712, (18 June, 2016). [2]. Q.L. He, L. Pan, ..., K. L. Wang, Science, 357, 294-299 (21 Jul 2017). [3]. H. Zhang, ..., L. P. Kouwenhoven, Nature, 556, 74-79 (05 April 2018)

Resume : Electron quantum optics aims at reproduce concepts and ideas of ordinary quantum optics using single-electrons excitations instead of photons. By means of gate voltages connected to Lorentzian-shaped time-dependent drive, it is possible to excite single-electron states along the edge states of two-dimensional topological insulators, where the spin of electrons is locked to their direction of motion, thus envisaging the excitation of single-electron states with a definite spin, which is an appealing scenario in relation to spintronics and quantum information processing. In our work, we connect a topological bar to two single-electron sources as described and we put a very thin superconducting gate in the middle of the bar. We focus on the role played by Rashba interactions on the edge states of topological insulators, illustrating peculiar processes that arise at the superconductors due to the Rashba coupling itself. In particular, we provide some experimental signatures of the strength of this coupling and of electron-electron interactions by studying current and current-current correlators.

Resume : Topological insulators (TI) have gained much interest in the field of spintronics for the generation and the detection of pure spin currents. Indeed, three-dimensional TI are predicted to host exotic properties like topologically protected surface states (TSS), which show Dirac-like band dispersion and strong spin-momentum locking [1]. Although the transport in TIs grown on insulator has been widely studied, TI films grown on semiconducting substrate are still challenging because of the parallel conduction canal. In this work, we report the growth of single crystalline Bi2Se3 thin film on slightly p-doped Ge (111) by molecular beam epitaxy. Germanium is an optically active material with a relatively long spin diffusion length (lsf ? 10 ?m) and it is compatible with the current Si technology platform. We carried out low temperature and high field magnetotransport measurements on micro-fabricated Hall bars, our results highlight a 2D transport in TSS through the weak-antilocalisation (WAL) signature. We propose an innovative method to probe the spin-to-charge conversion of TIs by taking advantages of Ge optical properties. We designed microdevices where pure spin currents with in-plane spin polarization are generated by optical spin orientation by scanning a laser beam at the edge of Pt bar [2,3]. Spin currents are then detected in a non-local geometry by the inverse Rashba-Edelstein effect in a Bi2Se3 bar at room temperature. These results open a new way of investigating the spin properties of topological insulators. [1] M. Z. Hasan and C. L. Kane, Rev. Mod. Phys. 82, 3045 (2010) [2] C. Zucchetti and al, Phys. Rev. B 96, 014403 (2017) [3] F. Bottegoni and al, Nature Materials volume 13, pages 790?795 (2014)

Resume : The lifetime of magnetic states and the mechanism for thermally activated transitions from one magnetic state to another can be analyzed in terms of the shape of the energy surface, i.e. the energy as a function of the angles defining the orientation of the magnetic moments. Minima on the energy surface correspond to stable or metastable magnetic states and can represent parallel, antiparallel or, more generally, non-collinear arrangements of the magnetic moments. Harmonic transition state theory has been developed and a method for finding minimum energy paths and first order saddle points for magnetic transitions [1,2]. An expression for the pre-exponential factor in the rate expression is obtained from the Landau?Lifshitz?Gilbert equation for spin dynamics. An application of this rate theory to nanoscale Fe islands and full Fe overlayer on W(110) has revealed how the transition mechanism and rate depend on island shape/size and presence of magnetic tip, and led to reinterpretation of experimental observations [3,4]. Calculations for one and two ring elements of kagome spin ice also show close agreement with experimental measurements but give a pre-exponential factor that is three orders of magnitude smaller than has previously been assumed [5]. More recently, this methodology has been used to study ferromagnetic and antiferromagnetic skyrmions which have been proposed as building blocks of future memory devices [6,7]. The first steps in the extension of the rate theory to include tunneling of the magnetic moments have been taken [8]. [1] P.F. Bessarab, V.M. Uzdin and H. Jónsson, Phys. Rev. B. 85, 184409 (2012). [2] P.F. Bessarab, V.M. Uzdin and H. Jónsson, Computer Physics Communications 196, 335 (2015). [3] P.F. Bessarab, V.M. Uzdin and H. Jónsson, Phys. Rev. Letters 110, 020604 (2013). [4] A. Ivanov, P.F. Bessarab, V.M. Uzdin and H. Jónsson, Nanoscale 9, 13320 (2017). [5] S. Liashko, V.M. Uzdin and H. Jónsson, New Journal of Physics 19, 113008 (2017). [6] I. Lobanov, H. Jónsson and V. M. Uzdin. Phys. Rev. B 94, 174418 (2016). [7] V. M. Uzdin, M. N. Potkina, I. S. Lobanov, P. F. Bessarab, H. Jónsson, Journal of Magnetism and Magnetic Materials 459, 236 (2018). [8] S. Vlasov, P. F. Bessarab, V. M. Uzdin and H. Jónsson, Nanosystems: Physics, Chemistry, Mathematics 8, 746 (2017).

Resume : The quasi 2D electron system (q2DES) that forms at the surface of SrTiO3 (STO) capped with an ultrathin metallic layer is starting to gain attention from the oxide electronics community [1,2]. Unlike for the celebrated LAO/STO system, this approach allows the formation of a q2DEG through the deposition of a simple metal at room-temperature, and the q2DEG properties can be dramatically tuned depending on the metal used. Additionally, just like for LAO/STO [3], the Rashba spin-orbit coupling present at the STO surface may be exploited to interconvert spin and charge currents through the direct and inverse Edelstein effect, which presents exciting advantages for future spintronic devices [4,5]. In this presentation, we will show experiments performed on the q2DES formed in NiFe/(metal)/STO heterostructures. We investigate the nature of the inverse Edelstein effect (responsible for the spin-to-charge conversion) through a combination of spin pumping, magnetotransport, XPS, ARPES and theory. In particular, we will explore the highly-doped regime where topological states have been predicted at special points in the band structure [6]. There, spin-charge conversion should reach extremely high values. This work received support from the ERC Consolidator Grant #615759 "MINT" [1] T.C. Rödel et al., Adv. Mater. 28, 1976 (2016) [2] D.C. Vaz et al., Adv. Mater. 29, 1700486 (2017) [3] E. Lesne, D.C. Vaz et al., Nat. Mater. 15, 1261 (2016) [4] J. Varignon et al., Nat. Phys. 14, 322 (2018) [5] S. Manipatruni et al., Nat. Phys. 14, 338 (2018) [6] M. Vivek et al., Phys. Rev. B 95, 165117 (2017)

Resume : Graphene is an ideal medium for long-distance spin communication in future spintronic technologies. We demonstrated a high spintronic performance in CVD graphene on SiO2/Si substrate at room temperature. We observed a long distance spin transport over 16 µm and spin lifetimes up to 1.2 ns in large area CVD graphene at room temperature [1]. Hexagonal boron nitride (h-BN) is an insulating tunnel barrier that has potential for efficient spin polarized tunneling from ferromagnets. We demonstrate the spin filtering effect in cobalt|few layer h-BN|graphene junctions leading to a large negative spin polarization up to 65 % in graphene at room temperature [2,3]. Two-dimensional (2D) crystals offer a unique platform due to their remarkable and contrasting spintronic properties, such as weak spin?orbit coupling (SOC) in graphene and strong SOC in molybdenum disulfide (MoS2). Here we combined graphene and MoS2 in a van der Waals heterostructure (vdWh) to demonstrate the electric gate control of the spin current and spin lifetime at room temperature [4]. Topological insulators (TIs) are a new class of quantum materials that exhibit a current-induced spin polarization due to spin-momentum locking (SML) of massless Dirac Fermions in their surface states. Recently, experiments were performed to detect and utilize the spin polarized surface currents in 3D TIs by using ferromagnetic contacts [5, 6]. References [1] Long distance spin communication in chemical vapour deposited graphene; MV Kamalakar, G. Chris, A Dankert, SP Dash; Nature Communication, 6, 6766 (2015). [2] Enhanced Tunnel Spin Injection into Graphene using CVD Hexagonal Boron Nitride; MV Kamalakar, A Dankert, J Bergsten, T Ive, SP Dash; Scientific Reports, 4: 61446 (2014) [3] Inversion of spin signal and spin filtering in ferromagnet| hexagonal boron nitride-graphene van der Waals heterostructures; MV Kamalakar, A Dankert, P. Kelly, SP Dash; Scientific Reports, 6, 21168 (2016). [4] Electrical gate control of spin current in van der Waals heterostructures at room temperature; A Dankert, SP Dash; Nature Communication 8, 16093 (2017). [5] Room Temperature Electrical Detection of Spin Polarized Currents in Topological Insulators; A. Dankert, J. Geurs, M.V. Kamalakar, S. Charpentier, S.P. Dash; Nano Letters, 15, 7976 (2015). [6] Surface Dominated Magnetoresistance in Topological Insulators, A. Dankert, P. Bhaskar, D. Khokhriakov, I. H. Rodrigues, M.V. Kamalakar, S. Charpentier, I. Garate, S. P. Dash; Phys. Rev. B, 97, 125414 (2018).

Resume : Graphene (Gr) is an ideal material for long distance spin transport. It has been proven that by placing a transition metal dichalcogenide (TMD) on top, the spin transport in Gr can be electrically modulated [1]. Furthermore, the TMD imprints its strong spin-orbit coupling into the Gr by proximity giving rise to an anisotropic spin relaxation between out-of-plane and in-plane spins. This has been experimentally observed in Gr/WS2 and Gr/MoSe2 heterostructures [2,3]. In these reports, the anisotropy was calculated from a complex study of the in-plane and the out-of-plane spin precessions via non-local Hanle measurements. Here we demonstrate a similar effect in a different heterostructure: Gr/MoS2. We fabricated Gr-based lateral spin valve with MoS2 placed on top of multilayer Gr in between ferromagnetic Co electrodes. Then, we performed non-local Hanle measurements by applying in-plane and out-of-plane magnetic fields. We found that, unlike the earlier reports that analyse the spin precession regimes at low magnetic fields, the spin lifetime anisotropy can be calculated by a simple model that compares the non-local Hanle resistances measured at large magnetic fields that saturate the Co magnetization. The calculated spin lifetime anisotropy is lower in our device compared to the earlier reports[2,3] due to the multilayer nature of our Gr. [1] Yan et al., Nat. Comm. 7, 13372 (2016). [2] Benitez et al., Nat. Phys. 14, 303 (2017). [3] Ghiasi et al., Nano Lett. 17, 12 , 7528 (2017).

Resume : The recent discovery of graphene, and other 2D materials, has opened novel exciting opportunities in terms of functionalities and performances for spintronics devices. While to date, it is mainly graphene properties for efficient spin transport which have been put forward, we will present here experimental results on another avenue for 2D materials in spintronics. We will show that a thin graphene passivation layer, directly integrated by low temperature catalyzed chemical vapor deposition (CVD), can prevent the oxidation of a ferromagnet [1]. This in turn enables the use of novel humide/ambient low-cost processes for spintronics devices, which would usually lead to oxidation during the fabrication and thus a quenching of the spintronic performances. We will illustrate this property by demonstrating the use of ozone based ALD processes to fabricate efficient spin valves protected with graphene [2]. Importantly, the use of graphene on ferromagnets allows to preserve a highly surface sensitive spin current polarizer/analyzer behavior and adds new enhanced spin ?ltering property [1][2]. Finally, we will present results concerning another 2D material isomorph to graphene: the atomically thin insulator hexagonal boron nitride (h-BN). Characterizations of complete spin valves making use of monolayer h-BN tunnel barriers grown by CVD will be presented [3]. These di?erent experiments unveil promising uses of 2D materials for spintronics [4]. References [1] Dlubak et al. ACS Nano 6 (2012) 10930 & Weatherup et al. ACS Nano 6 (2012) 9996 [2] Martin et al. ACS Nano 8 (2014) 7890 & Appl. Phys. Lett.107 (2015) 012408 [3] Piquemal-Banci et al. Appl. Phys. Lett. 108 (2016) 102404 [4] Review: Piquemal-Banci et al. J. Phys. D : Appl. Phys. 50 (2017) 203002

Resume : Ferromagnetic order in two-dimensional (2D) crystals is of great significance for fundamental studies and applications in spintronics. Recent experiments have revealed that intrinsic ferromagnetism occurs in 2D crystals of Cr2Ge2Te6 and CrI3, while graphene and group-VI dichalcogenide monolayers acquire large exchange splitting when integrated with nanomagnets. In this talk, I will present a quantum theory of charge carrier transport in topologically nontrivial 2D materials combining strong spin-orbit and magnetic exchange interactions. I will show that the strong coupling of spin and pseudospin degrees of freedom in such systems manifests in a distinctive gate voltage dependence of the transverse transport coefficients, including a reversal of anomalous Hall currents approaching the topological gap, which remains robust against disorder and hence is a forerunner of the quantum anomalous Hall regime. Moreover, a novel skew scattering mechanism - resulting from the skyrmionic spin texture of the magnetized 2D Dirac bands - is shown to cause a net spin accumulation at the sample boundaries even for spin-transparent disorder. The novel magnetic spin Hall effect is readily tunable by a gate voltage and opens novel opportunities for the control of spin currents in 2D Dirac heterostructures.