Thin films of topological insulators (TI) usually exhibit multiple parallel conduction channels for the transport of electrical current. Beside the topologically protected surface states (TSS), parallel channels may exist, namely the interior of the not-ideally insulating TI film, the interface layer to the substrate, and the substrate itself. To be able to take advantage of the auspicious transport properties of the TSS, the influence of the parasitic parallel channels on the total current transport has to be minimized. Because the conductivity of the interior (bulk) of the thin TI film is difficult to access by measurements, we propose here an approach for calculating the mobile charge carrier concentration in the TI film. To this end, we calculate the near-surface band bending using parameters obtained experimentally from surface-sensitive measurements, namely (gate-dependent) four-point resistance measurements and angle-resolved photoelectron spectroscopy (ARPES). While in most cases another parameter in the calculations, i.e. the concentration of unintentional dopants inside the thin TI film, is unknown, it turns out that in the thin-film limit the band bending is largely independent of the dopant concentration in the film. Thus, a well-founded estimate of the total mobile charge carrier concentration and the conductivity of the interior of the thin TI film proves possible. Since the interface and substrate conductivities can be measured by a four-probe conductance measurement prior to the deposition of the TI film, the total contribution of all parasitic channels, and therefore also the contribution of the vitally important TSS, can be determined reliably.
Even at the lowest accessible temperatures, measurements of the quantum anomalous Hall (QAH) effect have indicated the presence of parasitic dissipative conduction channels. There is no consensus whether parasitic conduction is related to processes in the bulk or along the edges. Here, we approach this problem by comparing transport measurements of Hall bar and Corbino geometry devices fabricated from Cr-doped (BiSb)$_2$Te$_3$. We identify bulk conduction as the dominant source of dissipation at all values of temperature and in-plane electric field. Furthermore, we observe identical breakdown …
The surface conductivity for conduction electrons with a fixed chirality in a topological insulator with impurities scattering is considered. The surface excitations are described by the Weyl Hamiltonian. For a finite chemical potential one projects out the hole band and one obtains a single electronic band with a fixed chirality. One obtains a model of spinless electrons which experience a half vortex when they return to the origin. As a result the conductivity is equivalent to a spinless problem with correlated noise which gives rise to anti-localization. We compute conductivity as a functio…
Electrostatic Control Enables Robust Helical Edge Channel Transport in III-V Quantum Spin Hall Insulators
Julius-Maximilians-Universität Würzburg（维尔茨堡朱利叶斯-马克西米利安大学）；University of British Columbia（不列颠哥伦比亚大学）；Würzburg-Dresden Cluster of Excellence ctd.qmat（维尔茨堡-德累斯顿卓越集群 ctd.qmat）；Laboratoire Charles Coulomb (L2C)（夏尔·库仑实验室 (L2C)）；Université de Montpellier（蒙彼利埃大学）；CNRS（法国国家科学研究中心）；IES（电子与系统研究所）；Institut Universitaire de France（法兰西学院）；École Normale Supérieure, PSL（巴黎高等师范学院，PSL）
When topological insulators possess rotational symmetry their spin lifetime is tied to the scattering time. We show that in anisotropic TIs this tie can be broken and the spin lifetime can be very large. Two different mechanisms can obtain spin conduction over long distances. The first is tuning the Hamiltonian to conserve a spin operator $\cos ϕ\, σ_x + \sin ϕ\, σ_y$, while the second is tuning the Fermi energy to be near a local extremum of the energy dispersion. Both mechanisms can produce persistent spin helices. We report spin lifetimes and spin diffusion equations.
Topological insulators are characterized by specially protected conduction on their outer boundaries. We show that the protected edge conduction exhibited by 2-D topological insulators (and also Chern insulators) is independent of non-magnetic boundary disorder. In particular, the edge states residing inside the bulk gap remain conducting even when edge state inhomogeneities destroy the characteristic linear Dirac relation between energy and momentum. The main effects of boundary disorder on the in-gap states are to decrease the Fermi velocity, increase the density of states, pull the states i…
We study the transport properties of the Kondo insulator SmB$_6$ with a specialized configuration designed to distinguish bulk-dominated conduction from surface-dominated conduction. We find that as the material is cooled below 4 K, it exhibits a crossover from bulk to surface conduction with a fully insulating bulk. We take the robustness and magnitude of the surface conductivity, as is manifest in the literature of SmB$_6$, to be strong evidence for the topological insulator metallic surface states recently predicted for this material.
The lecture introduces a reader to the relatively young field of physics of strongly interacting and disordered 2D electron system, in particular, to the phenomena of the metallic conduction and the apparent metal-insulator transition in 2D. The paper briefly overviews the experimental data on the electron transport, magnetotransport, spin-magnetization in 2D, and on the electron-electron interaction effects.
In this report, we scrutinize the thickness dependent resistivity data from the recent literature on electrical transport measurements in topological insulators. A linear increase in resistivity with increase in thickness is expected in the case of these materials since they have an insulating bulk and conducting surface. However, such a trend is not seen in the resistivity versus thickness data for all the cases examined, except for some samples, where it holds for a narrow range of thickness.
The surface states of a topological insulator in a fine-tuned magnetic field are ideal candidates for realizing a topological metal which is protected against disorder. Its signatures are (1) a conductance plateau in long wires in a finely tuned longitudinal magnetic field and (2) a conductivity which always increases with sample size, and both are independent of disorder strength. We numerically study how these experimental transport signatures are affected by bulk physics in the interior of the topological insulator sample. We show that both signatures of the topological metal are robust aga…
Topological insulators (TIs) have the singular distinction of being electronic insulators while harboring metallic, conductive surfaces. In ordinary materials, defects such as cracks and deformations are barriers to electrical conduction, intuitively making the material more electrically resistive. Peculiarly, 3D TIs should become better conductors when they are cracked because the cracks themselves, which act as conductive topological surfaces, provide additional paths for the electrical current. Significantly, for a TI material, any surface or extended defect harbors such conduction. In this…
Using the Green functions method we study transport properties of surface electrons in topological insulators in the presence of a correlated random exchange field. Such an exchange field may be due to random magnetization with correlated fluctuations. We determine the relaxation time due to scattering from the magnetization fluctuations and from other structural defects. Then we calculate the longitudinal charge conductivity taking into account the contribution due to vertex correction.
A hybrid orbital model for the topological insulator Bi2Se3 is proposed and compared to that of graphene. The existence of a pi-bond trimer on the Se1 layer at the van der Waals gap and on the surface is thought to be responsible for the unusual surface conduction mechanism found in Bi2Se3 as a topological insulator. The three pi-bonds are locked as permanent electric dipoles between the Se1 layers in the van der Waals gap as an attractive force for the bulk as a gapped semiconductor. In contrast, the pi-bond trimers on the surface are unlocked and exhibit two degenerate quantum states, creati…
The electronic orders in magnetic and dielectric materials form the domains with different signs of order parameters. The control of configuration and motion of the domain walls (DWs) enables gigantic, nonvolatile responses against minute external fields, forming the bases of contemporary electronics. As an extension of the DW function concept, we realize the one-dimensional quantized conduction on the magnetic DWs of a topological insulator (TI). The DW of a magnetic TI is predicted to host the chiral edge state (CES) of dissipation-less nature when each magnetic domain is in the quantum anom…
We show how the two-dimensional (2D) topological insulator evolves, by stacking, into a strong or weak topological insulator with different topological indices, proposing a new conjecture that goes beyond an intuitive picture of the crossover from quantum spin Hall to the weak topological insulator. Studying the conductance under different boundary conditions, we demonstrate the existence of two conduction regimes in which conduction happens through either surface- or edge-conduction channels. We show that the two conduction regimes are complementary and exclusive. Conductance maps in the pres…
SmB6 is a strongly correlated mixed-valence Kondo insulator with a newly discovered surface state, proposed to be of non-trivial topological origin. However, the surface state dominates electrical conduction only below T* ~ 4 K limiting its scientific investigation and device application. Here, we report the enhancement of T * in SmB6 under the application of tensile strain. With 0.7% tensile strain we report surface dominated conduction at up to a temperature of 240 K, persisting even after the strain has been removed. This can be explained in the framework of strain-tuned temporal and spatia…
Weak antilocalization (WAL) and linear magnetoresistance (LMR) are two most commonly observed magnetoresistance (MR) phenomena in topological insulators (TIs) and often attributed to the Dirac topological surface states (TSS). However, ambiguities exist because these phenomena could also come from bulk states (often carrying significant conduction in many TIs) and are observable even in non-TI materials. Here, we demonstrate back-gated ambipolar TI field-effect transistors in (Bi0.04Sb0.96)2Te3 thin films grown by molecular beam epitaxy on SrTiO3(111), exhibiting a large carrier density tunabi…
Based on a self-consistent $t$-matrix approximation, we explore the influence of magnetic and nonmagnetic doping on the surface electronic states and conductivity of topological insulators. We show that warping parameter has a crucial impact on the density of states and dc conductivity of the doped surfaces. As the warping strength is increased, the surface density of states at high energies is suppressed and the resonant states induced by impurities in the vicinity of the Dirac point gradually disappear. It is found that nonmagnetic impurities break electron-hole symmetry at low warping stren…
We report the composition and gate voltage induced tuning of transport properties in chemically synthesized Bi2(Te1-xSex)3 nanoribbons. It is found that increasing Se concentration effectively suppresses the bulk carrier transport and induces semiconducting behavior in the temperature dependent resistance of Bi2(Te1-xSex)3 nanoribbons when x is greater than ~10%. In Bi2(Te1-xSex)3 nanoribbons with x ~20%, gate voltage enables ambipolar modulation of resistance (or conductance) in samples with thickness around or larger than 100nm, indicating significantly enhanced contribution in transport fro…
Bulk and surface state contributions to the electrical resistance of single-crystal samples of the topological Kondo insulator compound SmB6 are investigated as a function of crystal thickness and surface charge density, the latter tuned by ionic liquid gating with electrodes patterned in a Corbino disk geometry on a single surface. By separately tuning bulk and surface conduction channels, we show conclusive evidence for a model with an insulating bulk and metallic surface states, with a crossover temperature that depends solely on the relative contributions of each conduction channel. The su…
Electrically tunable interfacial thermal conduction via electronic structure engineering in ${Au}$/$Bi_{1-x}$$Sb_{x}$ topological insulators
The Ohio State University（俄亥俄州立大学）；Korea Institute of Science and Technology (KIST)（韩国科学技术院）；Korea Advanced Institute of Science and Technology (KAIST)（韩国科学技术院）；Pohang University of Science and Technology (POSTECH)（浦项科技大学）；Carnegie Mellon University（卡内基梅隆大学）；E. O. Lawrence Berkeley National Laboratory（劳伦斯伯克利国家实验室）
We report a transport study of exfoliated few monolayer crystals of topological insulator Bi$_2$Se$_3$ in an electric field effect (EFE) geometry. By doping the bulk crystals with Ca, we are able to fabricate devices with sufficiently low bulk carrier density to change the sign of the Hall density with the gate voltage $V_g$. We find that the temperature $T$ and magnetic field dependent transport properties in the vicinity of this $V_g$ can be explained by a bulk channel with activation gap of approximately 50 meV and a relatively high mobility metallic channel that dominates at low $T$. The c…
Using field-angle, temperature, and back-gate-voltage dependence of the weak anti-localization (WAL) and universal conductance fluctuations of thin Bi1.5Sb0.5Te1.7Se1.3 topological-insulator single crystals, in combination with gate-tuned Hall resistivity measurements, we reliably separated the surface conduction of the topological nature from both the bulk conduction and topologically trivial surface conduction. We minimized the bulk conduction in the crystals and back-gate tuned the Fermi level to the topological bottom-surface band while keeping the top surface insensitive to back-gating wi…
Current theories of Kondo insulators employ the interaction of conduction electrons with localized Kramers doublets originating from a tetragonal crystalline environment, yet all Kondo insulators are cubic. Here we develop a theory of cubic topological Kondo insulators involving the interaction of spin quartets with a conduction sea. The spin quartets greatly increase the potential for strong topological insulators, entirely eliminating the weak-topological phases from the diagram. We show that the relevant topological behavior in cubic Kondo insulators can only reside at the lower symmetry X …
We study the effect of electrostatic disorder on the conductivity of a three-dimensional antiferromagnetic insulator (a stack of quantum anomalous Hall layers with staggered magnetization). The phase diagram contains regions where the increase of disorder first causes the appearance of surface conduction (via a topological phase transition), followed by the appearance of bulk conduction (via a metal-insulator transition). The conducting surface states are stabilized by an effective time-reversal symmetry that is broken locally by the disorder but restored on long length scales. A simple self-c…
We report in this Letter our recent low temperature transport results in a Si/SiGe quantum well with moderate peak mobility. An apparent metal-insulating transition is observed. Within a small range of densities near the transition, the conductivity $σ$ displays non-monotonic temperature dependence. After an initial decrease at high temperatures, $σ$ first increases with decreasing temperature T, showing a metallic behavior. As T continues decreasing, a downturn in $σ$ is observed. This downturn shifts to a lower T at higher densities. More interestingly, the downturn temperature shows a power…
$\mathbb Z_2$-Nontrivial Moiré Minibands and Interaction-Driven Quantum Anomalous Hall Insulators in Topological Insulator Based Moiré Heterostructures
We investigated the magnetotransport properties of strained, 80nm thick HgTe layers featuring a high mobility of mu =4x10^5 cm^2/Vs. By means of a top gate the Fermi-energy is tuned from the valence band through the Dirac type surface states into the conduction band. Magnetotransport measurements allow to disentangle the different contributions of conduction band electrons, holes and Dirac electrons to the conductivity. The results are are in line with previous claims that strained HgTe is a topological insulator with a bulk gap of ~15meV and gapless surface states.
In the past few years there has been a surge in the material science engineering in order to synthesize bulk insulating and surface metallic Topological Insulating (TI) materials. This quest is not only theoretically important but also promising from the novel application perspective. The dependence of temperature on resistance (R-T) of a particular sample reveals a plethora of information about the electronic properties especially in a unique sample like TI where there are two components comprising of an insulating bulk and metallic surface states. Depending on the amount of intrinsic doping …
Transport due to spin-helical massless Dirac fermion surface state is of paramount importance to realize various new physical phenomena in topological insulators, ranging from quantum anomalous Hall effect to Majorana fermions. However, one of the most important hallmarks of topological surface states, the Dirac linear band dispersion, has been difficult to reveal directly in transport measurements. Here we report experiments on Bi2Te3 nanoribbon ambipolar field effect devices on high-k SrTiO3 substrates, where we achieve a gate-tuned bulk metal-insulator transition and the topological transpo…
Quantum spin Hall (QSH) insulator with large gap is highly desirable for potential spintronics application. Here we realize electrically tunable QSH insulator with large gap in van der Waals heterobilayer of monolayer transition metal dichalcogenide (TMD) and hexagonal BAs. When the type II band alignment gets inverted in an electric field, the hybridization by interlayer hopping between the spin-valley locked valence band edges in TMD and the BAs conduction band edges leads to a stacking-configuration dependent topological band inversion. In the non-interacting limit, the double spin degenera…
Tunnel transport processes are considered in a square lattice of metallic nanogranules embedded into insulating host to model tunnel conduction in real metal/insulator granular layers. Based on a simple model with three possible charging states ($\pm$, or 0) of a granule and three kinetic processes (creation or recombination of a $\pm$ pair, and charge transfer) between neighbor granules, the mean-field kinetic theory is developed. It describes the interplay between charging energy and temperature and between the applied electric field and the Coulomb fields by the non-compensated charge densi…
Using femtosecond time- and angle- resolved photoemission spectroscopy, we investigated the nonequilibrium dynamics of the topological insulator Bi2Se3. We studied p-type Bi2Se3, in which the metallic Dirac surface state and bulk conduction bands are unoccupied. Optical excitation leads to a meta-stable population at the bulk conduction band edge, which feeds a nonequilibrium population of the surface state persisting for >10ps. This unusually long-lived population of a metallic Dirac surface state with spin texture may present a channel in which to drive transient spin-polarized currents.
We investigate a transition between a two-dimensional topological insulator conduction state, characterized by a conductance $G=2$ (in fundamental units $e^2/h$) and a Chern insulator with $G=1$, induced by polarized magnetic impurities. Two kinds of coupling, ferro and antiferromagnetic, are considered with the electron and hole subbands. We demonstrate that for strong disorder, a phase $G=1$ exists even for ferromagnetic order, in contrast with the prediction of the mean field approximation. This result is supported by direct numerical computations using Landauer transport formula, and by an…
Negative longitudinal magnetoresistance, in the presence of an external magnetic field parallel to the direction of an applied current, has recently been experimentally verified in Weyl semimetals and topological insulators in the bulk conduction limit. The appearance of negative longitudinal magnetoresistance in topological semimetals is understood as an effect of chiral anomaly, whereas it is not well-defined in topological insulators. Another intriguing phenomenon, planar Hall effect - appearance of a transverse voltage in the plane of applied co-planar electric and magnetic fields not perf…
We introduce a new resistance measurement method that is useful in characterizing materials with both surface and bulk conduction, such as three-dimensional topological insulators. The transport geometry for this new resistance measurement configuration consists of one current lead as a closed loop that fully encloses the other current lead on the surface, and two voltage leads that are both placed outside the loop. We show that in the limit where the transport is dominated by the surface conductivity of the material, the four-terminal resistance measured from such a transport geometry is prop…
The recent excitement about Dirac and Weyl fermion systems has renewed interest in magnetotransport properties of multi-carrier systems. However, the complexity of their analysis, even in the simplest two-carrier case, has hampered a good understanding of the underlying phenomena. Here we propose a new analysis scheme for two independent conduction channels, that strongly reduces previous ambiguities and allows to draw robust conclusions. This is demonstrated explicitly for the example of three-dimensional topological insulators. Their temperature and gate voltage-dependent Hall coefficient an…
Planar Hall effect (PHE) in topological insulators (TIs) is discussed as an effect that stems mostly from conduction due to topologically protected surface states. Although surfaces states play a critical role and are of utmost importance in TIs, our present study reflects the need for considering the bulk conduction in understanding PHE in TIs. Here, we demonstrate an enhancement in PHE amplitude by three times by doubling the thickness of Bi$_2$Te$_3$ film on Si (111). The PHE amplitude reaches $\approx$~6 n$Ω$m in 30 quintuple layer (QL) device as compared to $\approx$~2 n$Ω$m in 14 QL. We …
Using the non-equilibrium statistical operator method (NSO), we have investigated the spin transport through the interface in a semiconductor/ferromagnetic insulator hybrid structure. We have analyzed the approximation of effective parameters, when each of the considered subsystems (conduction electrons, magnons, and phonons) is characterized by its effective temperature. We have constructed the macroscopic equations, describing the spin-wave current caused by both resonantly excited spin system of conduction electrons and by an inhomogeneous thermal field in the ferromagnetic insulator.
We propose a mechanism for 1/f-type noise in hopping insulators based on the multi-electron charge redistribution within the specific aggregates of the localized states located in the vicinity of the critical resistors. We predict that the noise with 1/f-type spectrum extends down to practically arbitrarily low frequencies.
We study a current shot noise in a macroscopic insulator based on a two-dimensional electron system in GaAs in a variable range hopping (VRH) regime. At low temperature and in a sufficiently depleted sample a shot noise close to a full Poissonian value is measured. This suggests an observation of a finite-size effect in shot noise in the VRH conduction and demonstrates a possibility of accurate quasiparticle charge measurements in the insulating regime.
The combination of superconductivity and the helical spin-momentum locking at the surface state of a topological insulator (TI) has been predicted to give rise to p-wave superconductivity and Majorana bound states. The superconductivity can be induced by the proximity effect of a an s-wave superconductor (S) into the TI. To probe the superconducting correlations inside the TI, dI/dV spectroscopy has been performed across such S-TI interfaces. Both the alloyed Bi$_{1.5}$Sb$_{0.5}$Te$_{1.7}$Se$_{1.3}$ and the stoichiometric BiSbTeSe$_2$ have been used as three dimensional TI. In the case of Bi$_…
Topological insulators are a novel class of quantum matter with a gapped insulating bulk yet gapless spin helical Dirac fermion conducting surface states. Here, we report local and non-local electrical and magneto transport measurements in dual-gated BiSbTeSe2 thin film topological insulator devices, with conduction dominated by the spatially separated top and bottom surfaces, each hosting a single species of Dirac fermions with independent gate control over the carrier type and density. We observe many intriguing quantum transport phenomena in such a fully-tunable two-species topological Dira…
Ionic liquid gating has become a popular tool for tuning the charge carrier densities of complex oxides. Among these the band insulator SrTiO$_3$ is one of the most extensively studied materials. While experiments have succeeded in inducing (super)conductivity, the process by which ionic liquid gating turns this insulator into a conductor is still under scrutiny. Recent experiments have suggested an electrochemical rather than electrostatic origin of the induced charge carriers. Here, we report experiments probing the time evolution of conduction of SrTiO$_3$ near the glass transition temperat…
The existence of pi-bonds on topological insulator surfaces is found to be closely related to the phenomena of surface conduction and surface band spin polarization. A pi-bond trimer or pi-bond dimer on the surface can form open conjugated systems that are responsible for the unique surface conduction mechanism of a topological insulator. Parity operation in 2D is identified for a pi-bond trimer within a six-fold symmetry coordination and a pi-bond dimer within a four-fold symmetry coordination. Spontaneous 2D parity symmetry breaking is found to be closely related to the theoretically predict…
We report the observation of two-dimensional Shubnikov-de Hass (SdH) oscillations in the topological insulator Tl$_{1-x}$Bi$_{1+x}$Se$_2$. Hall effect measurements exhibited electron-hole inversion in samples with bulk insulating properties. The SdH oscillations accompanying the hole conduction yielded a large surface carrier density of $n_{\rm{s}}=5.1 \times10^{12}$/cm$^2$, with the Landau-level fan diagram exhibiting the $π$ Berry phase. These results showed the electron-hole reversibility around the in-gap Dirac point and the hole conduction on the surface Dirac cone without involving the b…
We report on van der Waals epitaxial growth, materials characterization and magnetotransport experiments in crystalline nanosheets of Bismuth Telluro-Sulfide (BTS). Highly layered, good-quality crystalline nanosheets of BTS are obtained on SiO$_2$ and muscovite mica. Weak-antilocalization (WAL), electron-electron interaction-driven insulating ground state and universal conductance fluctuations are observed in magnetotransport experiments on BTS devices. Temperature, thickness and magnetic field dependence of the transport data indicate the presence of two-dimensional surface states along with …
Thin (6-7 quintuple layer) topological insulator Bi2Se3 quantum dot devices are demonstrated using ultrathin (2~4 quintuple layer) Bi2Se3 regions to realize semiconducting barriers which may be tuned from Ohmic to tunneling conduction via gate voltage. Transport spectroscopy shows Coulomb blockade with large charging energy >5 meV, with additional features implying excited states.
We have utilized time-domain magneto-terahertz spectroscopy to investigate the low frequency optical response of topological insulator Cu$_{0.02}$Bi$_2$Se$_3$ and Bi$_2$Se$_3$ films. With both field and frequency dependence, such experiments give sufficient information to measure the mobility and carrier density of multiple conduction channels simultaneously. We observe sharp cyclotron resonances (CRs) in both materials. The small amount of Cu incorporated into the Cu$_{0.02}$Bi$_2$Se$_3$ induces a true bulk insulator with only a \textit{single} type of conduction with total sheet carrier dens…
We report on the observation of a circular photogalvanic current excited by terahertz (THz) laser radiation in helical edge channels of HgTe-based 2D topological insulators (TIs). The direction of the photocurrent reverses by switching the radiation polarization from right-handed to left-handed one and, for fixed photon helicity, is opposite for the opposite edges. The photocurrent is detected in a wide range of gate voltages. With decreasing the Fermi level below the conduction band bottom, the current emerges, reaches a maximum, decreases, changes its sign close to the charge neutrality poin…
The nature of the Dirac quasiparticles in topological insulators calls for a direct investigation of the electron-phonon scattering at the \emph{surface}. By comparing time-resolved ARPES measurements of the TI Bi_{2}Se_{3} with different probing depths we show that the relaxation dynamics of the electronic temperature of the conduction band is much slower at the surface than in the bulk. This observation suggests that surface phonons are less effective in cooling the electron gas in the conduction band.
Since its discovery as a Kondo insulator 50 years ago, SmB6 recently received a revival of interest due to detection of unexpected quantum oscillations in the insulating state, discovery of disorder-immune bulk transport, and proposals of correlation-driven topological physics. While recent transport results attribute the anomalous low temperature conduction to two-dimensional surface states, important alternatives, such as conduction channel residing in one-dimensional dislocation lines, have not been adequately explored. Here we study SmB6 with scanning microwave impedance microscopy and unc…
Using spin-resolved and angle-resolved photoemission spectroscopy and first-principles calculations, we have identified bulk band inversion and spin polarized surface state evolved from a weak topological insulator (TI) phase in van der Waals materials Nb3XTe6 (X = Si, Ge). The fingerprints of weak TI homologically emerge with hourglass fermions, as multi nodal chains composed by the same pair of valence and conduction bands gapped by spin orbit coupling. The novel topological state, with a pair of valence and conduction bands encoding both weak TI and hourglass semimetal nature, is essential …
The topological insulator is a fundamentally new phase of matter, with the striking property that the conduction of electrons occurs only on its surface, not within the bulk, and that conduction is topologically protected. Topological protection, the total lack of scattering of electron waves by disorder, is perhaps the most fascinating and technologically important aspect of this material: it provides robustness that is otherwise known only for superconductors. However, unlike superconductivity and the quantum Hall effect, which necessitate low temperatures or magnetic fields, the immunity to…
We have studied the carrier transport in two topological insulator (TI) Bi$_{2}$Te$_{3}$ microflakes between 0.3 and 10 K and under applied backgate voltages ($V_{\rm BG}$). Logarithmic temperature dependent resistance corrections due to the two-dimensional electron-electron interaction effect in the presence of weak disorder were observed. The extracted Coulomb screening parameter is negative, which is in accord with the situation of strong spin-orbit scattering as is inherited in the TI materials. In particular, positive magnetoresistances (MRs) in the two-dimensional weak-antilocalization (…
The results of ultra-high vacuum low-temperature scanning-tunneling microscopy (STM) and spectroscopy (STS) of atomically clean (111) surface of the topological insulator Bi$_2$Se$_3$ are presented. We observed several types of new subsurface defects whose location and charge correspond to p-type conduction of grown crystals. The sign of the thermoelectric effect also indicates p-type conduction. STM and STS measurements demonstrate that the chemical potential is always located inside the bulk band gap. We also observed changes in the local density of states in the vicinity of the quintuple la…
We develop a non-equilibrium Green's function formalism to study magnonic spin transport through a strongly anisotropic ferromagnetic insulator contacted by metallic leads. We model the ferromagnetic insulator as a finite-sized one-dimensional spin chain, with metallic contacts at the first and last sites that inject and detect spin in the form of magnons. In the presence of anisotropy, these ferromagnetic magnons become elliptically polarized, and spin conservation is broken. We show that this gives rise to a novel parasitic spin conductance, which becomes dominant at high anisotropy. Moreove…
