Topological materials and disorder: vital or fatal?

Resume : This unusual band structure of topological semi-metals is expected to introduce exotic phenomena from relativistic physics into condensed matter experiments, such as the famous axial anomaly and the appearance of quasiparticle chirality as a conserved quantity. We search for distinct signatures of topological charge carriers in mesoscale crystalline samples where the quasiparticle mean free path is comparable to the sample size. These microstructures are fabricated from high quality crystals using Focused Ion Beam (FIB) machining. FIB milling involves sample irradiation with Ga-ions at 60keV, and as any cutting technique introduces damage and disorder at the surface. Commonly, such surface defect layers extend about 20nm into the bulk and are generally considered disadvantageous. Yet when crystals of the monopnictide family of Weyl semi-metals, (Nb,Ta)(As,P), are FIB-machined, nature surprises us with a peculiar mechanism inducing robust superconductivity in these microstructures. The origin of the superconductivity is a natural self-enrichment of Nb, or respectively Ta, on the surface due to the vastly different sputter yield compared to As or P. This is a result of the sizable energetic difference for atoms to leave the solid body, which is also reflected in their very different temperature of sublimation. We present ion beam treatment as an reliable and effective approach to define proximity-effect induced superconducting Weyl structures on the mesoscale. References: M.D. Bachmann et al., Science Advances 3, e1602983 (2017)

Resume : Recently a large negative longitudinal (parallel to the magnetic field) magnetoresistance was observed in Weyl and Dirac semimetals. It is believed to be related to the chiral anomaly associated with topological electron band structure of these materials. We show that in a certain range of parameters such a phenomenon can also exist in conventional centrosymmetric and time reversal conductors, lacking topological protection of the electron spectrum and the chiral anomaly. We also discuss the magnetic field enhancement of the longitudinal components of the thermal conductivity and thermoelectric tensors.

Resume : The emergence of Weyl (massless) fermions in TaAs, due to its electronic band structure, is indicative of a new state of matter in the condensed phase that is of great interest for fundamental physics and possibly new applications. To date, however, the issue of crystal defects and their effect on the electronic energy states and hence the presence of Weyl fermions in TaAs has been largely overlooked. Determining defect structures and assigning physical effects to particular defects is challenging to accomplish experimentally but is an area in which first-principles calculations can offer crucial information. Here we present the results of density functional theory calculations on the intrinsic defect properties of TaAs. We investigate further how the point defects influence the Fermi surface. We demonstrate that there is a thermodynamical driving force to form defects related to different stoichiometries, and determine the phase stabilities as a function of the elemental chemical potentials. Our results show that Weyl fermions can only be observed under specific conditions, and that a small change in the balance of defect concentrations will cause these particles to vanish. We therefore provide key new insights into the physics of Weyl semimetals via their defect chemistry. Our calculations are in excellent agreement with available experiment.

Resume : Materials possessing two-dimensional Dirac states may revolutionize electronics and optoelectronic devices. Topological insulators are among the top most on the list of potential contenders. Their unique properties spanning from high surface carrier mobilities to immunity to non-magnetic defects and impurities have been of intense attraction to a number of disciplines. In many of the proposed applications, involving topological insulators light pulses can be employed to control excited charge or spin currents. The efficiency of such devices will depend on the ability to control the lifetime and population of excited photo-carriers and on the ability to manipulate charge balance between electrons and holes at and near the surface. Using angle- and femtosecond time-resolved photoelectron spectroscopy, we explore the out-of-equilibrium dynamics of surface fermions in the ternary topological system Bi2Te2Se. We show that the presence of localized trap states at the surface is one of the key material parameters undergirding the unusually long relaxation time of photo-excited Dirac electrons lying in the projected band gap of the bulk insulating pristine compound. Doping this ternary compound with Sn substituting on Bi sites, while desirably increasing the resistivity at low temperatures decreases decay times of the excited homologous Dirac electrons.

Resume : Topological crystalline insulators (TCIs) are topological materials where the existence of surface Dirac states is guaranteed by crystal symmetries. This protection mechanism promises a rich phenomenology in response to crystal perturbations. In my presentation, I will report on the discovery of robust 1D spin-polarized channels naturally emerging at TCI surfaces once translational invariance is broken [1]. I will illustrate how 1D channels can be easily obtained in the prototypical TCI Pb1−xSnxSe without the need of any sophisticated preparation technique. In particular, by correlating topographic and electronic structure information, I will show that 1D states naturally emerge at step edges consisting of an odd number of atomic layers, where translational invariance is broken, while even step edges maintain translational symmetry and are featureless. By systematically acting on the crystals stoichiometry, I will demonstrate how these 1D states are directly linked to the existence of a topologically non-trivial bulk band structure. A minimal toy model and realistic tight-binding calculations allow to identify them as spin-polarized at bands connecting two Dirac points. Finally, I will show how, contrary to 1D topological states known so far, their protection mechanisms result in a striking robustness to defects, strong magnetic fields, and elevated temperature.

Resume : The “double Dirac cone” 2D topological interface states found on the (001) faces of topological crystalline insulators such as Pb1-xSnxSe feature degeneracies located away from time reversal invariant momenta, and are a manifestation of both mirror symmetry protection and valley interactions. Similar shifted degeneracies in the case of 1D interface states have been highlighted as a potential basis for a topological transistor, but realizing such a device will require a detailed understanding of the intervalley physics involved. In addition, the operation of this or similar devices outside of ultra-high vacuum will require encapsulation, and the consequences of this for the topological interface state must be understood. Here we address both topics for the case of 2D surface states using angle-resolved photoemission spectroscopy and tight binding calculation. We examine bulk Pb1-xSnxSe (001) crystals overgrown with PbSe, realizing trivial/topological heterostructures. We demonstrate that the valley interaction which splits the two Dirac cones is extremely sensitive to atomic-scale details of the TCI-normal interface and the heterostructure surface, exhibiting non-monotonic changes as PbSe deposition proceeds. This includes an apparent total collapse of the splitting for sub-monolayer coverage, eliminating the Lifshitz transition. For a large overlayer thickness we observe quantized PbSe states, which may reflect a novel symmetry confinement at the buried topological interface.

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 : The research on three-dimensional topological insulators requires access to high quality bulk crystals. In this communication we report results of growth experiments and characterization of quaternary topological insulators, which are expected to offer lower bulk conductivity than their binary end members. The Bi2-xSbxTe3-ySey single crystals were grown by horizontal Bridgman method. The characterization was performed using X-ray technique, transmission electron microscopy, electric transport measurements and angle-resolved photoemission spectroscopy (ARPES). The X-ray diffraction measurements indicated rhombohedral structure and did not show presence of a foreign phase. This was confirmed also by transmission electron microscopy. The technique also showed that the outermost and the middle atomic layers within a quintuplet of Bi2-xSbxTe3-ySey were of mixed Se and Te character while the neighboring inner layers were composed of randomly distributed Bi and Sb. The crystals were p-type at high temperatures while at lower temperatures, below about 120 K, we observed change of the conductivity to the n-type. The effective low-temperature Hall concentration (measured on thick samples) was as low as ~ 10^15 cm^-3 with effective Hall mobility reaching ~ 10 000 cm^2/(Vs). ARPES studies showed gapless Dirac cone surface states with the Dirac point touching the top of the Bi2-xSbxTe3-ySey valence band. The results show high quality of the obtained crystals and clear presence of the Dirac cone surface states. The effects of doping and the change of Bi1-xSbxTe3-ySey stoichiometry on the topological surface states can be thus investigated. We would like to acknowledge National Science Centre, Poland grant No. 2016/21/B/ST3/02565.

Resume : Topological insulators (TIs) are materials which are called new quantum phase, because their specific properties resulting from characteristic electronic structure and strong spin-orbital coupling instead of similar chemical composition. These materials are insulating in the bulk but have gapless surface states (SS) which are characterized by linear dispersion and are protected against scattering by time-reversal symmetry[1,2]. Unfortunately, there is several obstacles to overcome to observe their most interesting properties. Many phenomena and properties are hard to follow due to covering of the effects of surface electrons by volume electrons. There are a few ways to overcome this difficulty. One is doping with compensating elements or controlling the stoichiometry[3]. The other way is controlling the geometry and changing surface to volume ratio, which decreases bulk carriers contribution. And therefore nanostructures with topological insulators are good candidates for studies surface states[4]. Another problem is the sensitivity of the surface of TI on ambient atmosphere. There are several studies confirming material aging what impair transport properties of SS. The solution is obtaining heterostructures, where topological insulator?s surface would be protected by different trivial material. Understanding of physics of the TIs and influence of the vicinity of other materials on SS are fundamental for applications like spintronic devices and quantum computing but also for development of physics. So far two main connections between TIs and other materials are studied: TI ferromagnets or magnetic impurities [5,6,7] (Impact of magnetic impurities on the energy gap opening) and TIs superconductors (Exotic particles ? Majorana fermions, observed on the interfaces)[1,2]. In this work we are showing materials containing TIs obtained through novel method based on the new approach. We decided to manufacture eutectic materials in which one of the phases would have topological insulator properties. Eutectics are materials that are mixture of two solid state phases obtained from homogenous liquid in temperature much lower than melting points of individual components. Such materials could crystallize in various structures (globular, fibrous, lamellar)[8] but because typical 3D topological insulators (Bi2Te3, Bi2Se3, Sb2Te¬3¬) have layered structure, eutectics containing TIs are supposed to form rather lamellar texture. This would result in the creation of numerous interfaces, what should strengthen signals come from SS. This approach would also enable us to study properties of interfaces between TIs and metals, insulators, ferromagnets and superconductors. Here we present first obtained eutectic materials based on Bi2Te3, BiTeI and Sb2Te3 and their characterization by structural, optical and electronic structure measurements. References: (1) Fu L. and Kane C. L., Phys. Rev. B 2007, 76. (2) Xiao-Liang Qi, Shou-Cheng Zhang, Reviews Of Modern Physics 2011, 83, 1057 (3) Hruban A., Strzelecka S., Materna A., Wo?o? A., Jurkiewicz-Wegner E., Piersa M.; Or?owski W., Dalecki W., Kami?ska M., Romaniec M.; J. Cryst. Growth 2014, 407, 63-67 (4) Kong D. , Cui Y., Nature Chemistry 2011, 3, 845 (5) Ji H., Cava R. J. et al., Phys. Rev. B 2012, 85, 165313 (6) Kandala A. et al., Appl. Phys. Lett. 2013, 103, 202409 (7) Fan Y., Wang K. L., Nature Materials 2014, 13, 699 (8) Pawlak D. A., Turczynski S.; Gajc M.; Kolodziejak K.; Diduszko R.; Rozniatowski K.; Smalc J.; Vendik I. Adv. Funct. Mater. 2010, 7, 1116-1124.

Resume : Doping with magnetic atoms of topological insulators (TI) attracts great interest due to predictions of novel electronic states emerging from interaction of spin-locked surface states with magnetic moments. Measurements of Anomalous Hall Effect (ANH) and magnetization are the most common methods used to explore those effects. At sufficiently high doping/substitution level ferromagnetism emerges as testified by hysteresis magnetization and ANH. Angular Resolved Photoemission Spectroscopy (ARPES) provides evidence of the gap opening in the Dirac-type surface states. Nanoscale disorder in the ferromagnetism can influence for spatial arrangements of Dirac-mass gap. Also, the spatial disorder due to random distribution of dopant atoms results in the magnetic domain formation. This possibility should be taken into account in the analysis of the electronic transport and magnetization measurements. In this study we use magneto-optical imaging in order to explore spatially resolved magnetization of Mn doped Bi2Te3 and BiSbTe3. We find that the re-magnetization process induces the formation of the elongated magnetic domains. Dynamics of domain motion is investigated as the function of temperature and the external magnetic field history. We would like to acknowledge National Science Centre, Poland grant No. 2016/21/B/ST3/02565

Resume : Anomalous Hall effects in magnetic materials have been of an important problem both for scientific interests and for spintronics applications of materials. Magnetic skyrmions, namely particlelike magnetic excitations with swirling magnetic texture at the boundary, give rise to the so-called topological Hall effect (THE), which is the unconventional Hall effect of the conduction electrons caused by the real-space Berry curvature from to the magnetic texture. Surface states of topological insulators (TIs), on the other hand, show the quantum anomalous Hall effect if they are magnetically doped, due to the momentum-space Berry curvature from the Dirac surface state. This talk introduces our recent work on the interplay between skyrmions and TI surfaces, especially their effect on the electron transport. We focus on the interface between a TI and a non-centrosymmetric magnetic insulator (MI) with skyrmions on the top, and take into account the exchange interaction between the electron spin on the TI and the local magnetization in the MI. We calculate the transport coefficients with the Boltzmann transport theory, by estimating the electron scattering rate at a skyrmion. The system reveals a finite anomalous Hall conductivity due to the scattering skewness by a skyrmion, arising from the geometric phase acquired through the electron transmission process at the skyrmion boundary. This mechanism is distinct from that of the THE in non-topological magnets.

Resume : We have performed magnetotransport studies of HgTe/Cd0.67Hg0.33Te quantum well (QW) under high hydrostatic pressures, up to 5.8 kbar. 7.1 nm wide QW has been grown by MBE and modulation doped with In. Low-temperature transport measurements have been carried out on Hall-bar device of 3 mm size, without any gate electrode. Prior to pressing, the sample shows quantum Hall effect, with electron concentration n = 5*10^11 cm^-2 and Hall mobility µ= 2.7 m^2/Vs. Application of pressure decreases the electron mobility by about 100 times, indicating inducing of severe disorder into the quantum well. Surprisingly, it also causes strong nonlocal resistance, which is clear evidence of edge transport [1]. Furthermore, the nonlocal signal vanishes in high magnetic fields, at values decreasing with pressure. We interpret our results in terms of magnetic field- and pressure-induced transition of the two-dimensional topological insulator into band insulator state [2]. It is supported by theoretical calculations based on eight-band k·p approximation [3], well reproducing our results. We also discuss possible formation of disorder-induced Anderson topological insulator state [4] in our system. Work partially supported by grants RBFR "15-52-16017 NTSIL_a" and "15-52-16008 NTSIL_a References 1. A. Roth, et al., Science 325, 294 (2009). 2. E.Y. Ma et al. Nat. Commun. 6, 7252 (2015). 3. S. S. Krishtopenko, et al., Phys. Rev. B 94, 245402 (2016). 4. Jian Li, et al., Phys. Rev. Lett. 102, 136806 (2009).

Resume : Since the breakthrough report that indicated the existence of quantized spin Hall resistance in HgTe quantum wells, it has been unclear which mechanism accounts for a relatively short topological protection length in two dimensional topological insulators. It was argued that backscattering in these systems originates from either disorder-induced charge puddles or intrinsic edge reconstruction. Our results obtained for HgTe/(Hg,Cd)Te:In quantum wells reveal a large dispersion in resistance values at a given channel length, which points out to an extrinsic origin of topological protection breaking. We have drawn the above conclusion by studying two types of microstructures (equipped with local or global gate electrode) in the regime in which negative gate voltage deplete the quantum well from electrons. In this range, non-local resistance vanishes at about 4 T, in agreement with theoretical expectations. We employ both quantum simulations (Kwant package) and a classical approach (a finite element method for a non-uniform current distribution) in order to evaluate channel resistance for various 4 probe configurations and channel lengths from 2 to 20 μm at 1.8 K. While for the channel length of Lch = 2 μm, the resistances R is close to the expected value RK= h/e2, at Lch = 10-20 μm, R varies between 10 and 28RK. The research was partially supported by National Center of Science in Poland (Decision No. 2015/17/N/ST3/02314), by grants RBFR “15-52-16017 NTSL_a” and 15 -52-16008 NTSIL_a”. Partial support by LIA TERAMIR is appreciated.

Resume : Recently the phenomenon of negative magnetoresistance (MR) is attracting renewed interest due to its occurence in Weyl semimetals because of the chiral anomaly. In other systems a large MR typically relates to magnetism. In this talk a novel mechanism leading to a large negative MR will be presented that is based not on magnetism, but on disorder. In the newly synthesized bulk-insulating topological insulator material TlBi$_x$Sb$_{1-x}$Te$_2$ we find a suppression of the resistivity by up to 98 \% in 14 T at low temperature. From transport data and numerical simulations, this gigantic negative MR is understood by a percolation of charge puddles formed in the disordered bulk.

Resume : Proximity-induced magnetic effects on the surface Dirac spectra of topological insulators are investigated by scanning tunneling spectroscopic (STS) and transport studies of MBE-grown bilayer structures consisting of either pure Bi2Se3 on top of Cr-doped Bi2Se3 or pure (Bi0.5Sb0.5)2Te3 on top of Cr-doped (Bi0.5Sb0.5)2Te3. For thickness of the top Bi2Se3 or (Bi0.5Sb0.5)2Te3 layer equal to or smaller than 3 quintuple layers (QLs), a surface spectral gap opens up below a characteristic temperature (T_c)^{2D}, which is much higher than the bulk Curie temperature (T_c)^{3D} determined from the anomalous Hall effect (AHE). The gap of magnetic Bi2Se3-bilayers is significantly more inhomogeneous than that of magnetic (Bi0.5Sb0.5)2Te3-bilayers, and the mean value and spatial homogeneity of \Delta for both systems increase with increasing c-axis magnetic field (H) and increasing Cr doping level (x). The (T_c)^{3D} values from AHE data are comparable in both bilayer systems with the same Cr-doping level, whereas hysteretic longitudinal and Hall resistivity vs. H loops are only observed in the magnetic (Bi0.5Sb0.5)2Te3-bilayers at T < < (T_c)^{3D}. In addition, spatially localized sharp spectral resonances are found in the magnetic Bi2Se3-bilayers at T < (T_c)^{2D}. These resonances are long-lived at H = 0 and are attributed to isolated magnetic impurity-induced topological spin textures of the surface Dirac fermions. In contrast, no resonances are observed in magnetic (Bi0.5Sb0.5)2Te3-bilayers. These findings suggest that disorder in magnetism is responsible for the disparity of (T_c)^{2D} and (T_c)^{3D}, the appearance of QAHE in Cr-doped (Bi1-xSbx)2Te3 only at T < < (T_c)^{3D}, and the absence of QAHE in Cr-doped Bi2Se3.

Resume : Recently, almost all physics domains have witnessed the emergence of quantum topological phases. Among them, two-dimensional topological insulators have attracted huge attention due to their possible application in spintronics. These systems host within their bulk gap helical edge states that are protected against backscattering. If one takes a flake of these materials, the helical states form loops and give rise to large orbital magnetic moments. In this presentation, we will theoretically show that edge states are not the only states with large magnetic moments, but band states (in the bulk energy bands) can host even larger ones. However, the two types of states have a drastically different behavior against disorder. We will emphasis the properties related to this difference and show that magnetic moments in those systems can be used as a probe to topology. We will also discuss how the strength of a non-trivial gap against disorder not only depends on its bandwidth, but on the overall bandstructure of the considered system. This feature is clearly observed thanks to magnetic moments behavior. The presence of disorder, usually seen as a disadvantage, can help probe the non-trivial to trivial topological transition. Moreover, it can be engineered so that only edge states magnetic moments survive.

Resume : Topological insulators are bulk insulators supporting metallic surface states. These surface states are often referred to as ?topologically protected? and this protection has several aspects. The first is the protection that guarantees the very existence of the metallic states. It is derived from the bulk band structure and topology: The bulk band structure of a topological insulator requires the existence of a gap-closing surface state, in contrast to a conventional insulator where metallic surface states are merely a coincidence. A different kind of protection applies to scattering of the surface state electrons by defects or phonons. While the spin texture of the state ensures the absence of direct back scattering, other scattering processes are less affected and dissipationless transport cannot be achieved for a two-dimensional surface state. This talk discusses the scattering of surface state electrons by different types of ?defects?: static defects, phonons and the surface as such, as this can lead to band bending and the existence of two-dimensional electron gas states, giving ries to further scattering channels. Experimental results are reported, primarily from angle-resolved photoemission spectroscopy.

Resume : Cadmium arsenide has recently been identified as a 3D Dirac semimetal in which two Dirac nodes separated near the Gamma point. While the available ARPES data [1] show these cones extend over several hundreds of meV, experiments such as STM [2] imply that the energy range of Dirac cones is an order of magnitude smaller. I will show our recent optical reflectivity data taken on cadmium arsenide in a broad range of photon energies and magnetic fields up to 33 T [3]. We clearly observe square root of field dependence of cyclotron resonance absorption in high magnetic fields, which is the hallmark of massless carriers. However, a closer look at our data tells us that the massless particles we observe are not Dirac-like. Using a model developed in the past [4] to understand our data, we infer that the massless carriers in Cd3As2 are so-called Kane electrons. While Dirac particles likely appear in cadmium arsenide, their energy scale is significantly smaller than probed in our far and mid infrared experiments. The Fermi level in Cd3As2 is never below 80 meV in any of the available samples, and its position is essentially given by impurity doping. This means that defects in Cd3As2 are detrimental to the observation of the topological Dirac states. [1] Z. Liu et al., Nature Mater. 13, 677 (2014); M. Neupane et al., Nature Comm. 5, 3786 (2014); S. Borisenko et al, Phys. Rev. Lett. 113, 027603 (2014). [2] S. Jeon et al., Nature Mater. 13, 851 (2014). [3] A. Akrap et al., Phys. Rev. Lett 117, 136401 (2016). [4] J. Bodnar, in Proc. III Conf. Narrow-Gap Semiconductors, p. 311. (Elsevier, 1977).

Resume : Heralded as one of the key elements for next generation spintronics devices and next generation electronic, topological insulators (Tls) are now step by step envisioned as charge-to-spin current conversion gates or as Dirac fermions based nanometer scale Schottky diodes. However, reduced to few nanometers, Tls layers exhibit a profound modification of the electronic structure and the consequence of this quantum size efect on the fundamental carriers and phonons ultrafast dynamics has been poorly investigated so far. With the use of MBE system we have grown step-like sample with thicknesses of particular Bi2Te3 layers ranging from 4 to 10 nm. The sample was grown on mica substrate in the co-deposition mode as described in our previous work [2]. Thanks to a complete study of high quality molecular beam epitaxy grown nanolayers, we report the existence of a critical thickness of around ~ 6 nm, below which reduction of the carrier relaxation time by a factor of ten is found in comparison to bulk Bi2Te3. The femtosecond pump-probe optical spectroscopy revealed the drastic evolution of the carriers and phonons dynamics which is probably related to an important electron-phonon coupling evolution caused by the quantum confinement. These properties have to be taken into account for future Tls-based spintronic devices. This work was supported by Research Grant NCN 2016/21/B/ST5/02531. [1] M.Z. Hasan and C.L. Kane, Rev. Mod. Phys., 82, 3045–3067 (2010) [2] R. Rapacz, K. Balin, M. Wojtyniak, J. Szade, Nanoscale 7(38), 16034-16038 (2015) [3] M.Weis, K.Balin, J.Szade, P.Ruello, Physical Review B 92,014301 (2015)