Session Y43: Topological Insulators: Engineered Structures II

Transport studies of a superconductor- InAs/GaSb bilayer junction

We fabricated a superconductor- semiconductor junction, by depositing a superconducting Ta film onto a band inverted InAs/GaSb bilayer. In this talk, we focus on electrical transport studies of this junction as a function of magnetic fields. At Zero magnetic field, the tunneling results show a zero bias conductance peak and this conductance peak survives in a field even up to 2~T. With further increasing magnetic field, the conductance peak eventually becomes a dip above 4~T. Finally, by tuning the front gate, we were able to measure the tunneling conductance when the InAs/GaSb bilayer is in the charge neutrality regime.   

Quantum Hall effect and insulating state near the charge neutrality point in an InAs/GaSb quantum well

We present transport measurements in a gated InAs/GaSb double quantum well (QW) sandwiched between two AlSb barriers. In this system a QW for electrons in InAs and a QW for holes in GaSb coexist next to each other and a hybridization gap is expected to occur. We can tune the transport from electrons to the holes by applying a top gate voltage. In presence of a perpendicular magnetic field, we observe well defined quantum Hall plateaus in both sides. Interestingly, at the charge neutrality point a strong increase in the longitudinal resistivity is observed with increasing perpendicular magnetic field, accompanied by the onset of a non-local resistance of similar magnitude. The co-existence of these two effects is described by a model of counter-propagating and dissipative quantum Hall edge channels, shorted by a residual bulk conductivity.\\[4pt] Reference: Fabrizio Nichele \textit{et al}., arXiv:1308.3128 (2013). Christophe Charpentier \textit{et al}., \textit{Appl. Phys. Lett.} 103, 112102 (2013).   

Low Temperature STM Experiments on Helical Edge States in InAs/GaSb

Inverted InAs/GaSb quantum wells have been recently shown to be a 2D topological insulator hosting robust helical edge states. Attributing to the fact that the hybridized minigap in this system opens at a finite wavevector, the edge states here have a low Fermi velocity V$_F$, and consequently their transport properties may reveal interesting interaction effects. Moreover, the V$_F$ in this system can be continuously tuned by electrostatic gates, providing an experimental knob for tuning the interactions. We report work in progress for STM/STS measurements of edge states in the tunneling regime, where the edge states are exposed at the cleaved edge/UHV interface. Experiments are performed in a 400 mK STM/vector magnet system with in situ sample cleavage and thin film deposition capabilities. Ref. I. Knez, R.-R. Du and G. Sullivan, Phys. Rev. Lett. 107, 136603 (2011); L-.J. Du, I. Knez, G. Sullivan, R-.R. Du, ArXiv:1306.1925 (2013).   

Quantum Anomalous Hall Effect in Magnetically Doped InAs/GaSb Quantum Wells

The quantum anomalous Hall effect has recently been observed experimentally in thin films of Cr doped (Bi,Sb)$_2$Te$_3$ at low temperature ($\sim$ 30mK). In this work, we propose realizing the quantum anomalous Hall effect in more conventional diluted magnetic semiconductors with magnetically doped InAs/GaSb type II quantum wells. Based on a four band model, we find a large increase of the Curie temperature for ferromagnetism due to the band edge singularity in the inverted regime of InAs/GaSb quantum wells. Below the Curie temperature, the quantum anomalous Hall effect is confirmed by the direct calculation of Hall conductance. Remarkably, our calculation based on eight-band Kane model reveals a band gap induced by exchange coupling reaching 10meV. The high sample quality and strong exchange coupling make the magnetically doped InAs/GaSb quantum well a good candidate for the quantum anomalous Hall insulator at high temperature.   

Superconducting proximity effect in inverted InAs/GaSb quantum well structures with Ta electrodes

We report on a systematic study of the proximity effect in top-gated InAs/GaSb quantum wells in contact with a superconducting Ta electrode. We find that the electronic transport across the InAs-Ta interface exhibits distinct zero-bias behavior, either a conductance (dI/dV) peak or dip, depending on the interfacial transparency. For a relatively resistive interface, we observe a dI/dV peak at zero bias, accompanied by two dI/dV dips at high bias voltages, consistent with previous works. When a transparent InAs-Ta interface is achieved, a zero-bias dV/dI dip appears with two coherent-peak-like features forming at bias voltages corresponding to the superconducting gap of Ta. The dI/dV spectra of the transparent InAs-Ta interface at different gate voltages can be fit well using the standard BTK model and the temperature dependence follows a BCS-like behavior. Our work demonstrates the possibility of achieving the highly transparent interfaces in InAs/GaSb hybrid structures, needed for studying the intriguing Andreev bound states in this two-dimensional topological system.   

Imaging Current in Si-doped InAs/GaSb Quantum Wells

Quantum spin hall (QSH) insulators are characterized by current-carrying edges in which single-particle elastic backscattering is forbidden, resulting in a theoretical conductance of e2/h per edge. Various theoretical mechanisms have been proposed to explain why, in devices with edges longer than several microns, the measured resistance is greater than expected. We used a scanning superconducting quantum interference device to image 2D current flow in inverted InAs/GaSb composite quantum wells with edges of tens of microns. We compared wells with Si doping at the InAs/GaSb interface (which acts to suppress residual bulk conductivity) to wells without doping. In the Si-doped samples, we observed that the majority of current flowed along the edge of the device when it was tuned into the bulk gap using a front gate. The current at the edges is consistent with an edge resistance that remained unchanged over a wide range of temperature and gate voltage, even in the presence of bulk conduction. These results set strong limits on candidate mechanisms for edge scattering.   

Observation of Quantum Spin Hall States in InAs/GaSb Bilayers under Broken Time-Reversal Symmetry

Topological insulators (TIs) are a novel class of materials with nontrivial surface or edge states. Time-reversal symmetry (TRS) protected TIs are characterized by the Z2 topological invariant. The fate of the Z2 TIs under broken TRS is a fundamental question in understanding the physics of topological matter but remains largely unanswered. Here we show, a two-dimensional TI is realized in an inverted electron-hole bilayer engineered from InAs/GaSb semiconductors which retains robust helical liquid (HL) edge states under a strong magnetic field. Wide conductance plateaus of 2e2/h value are observed; they persist to 10T applied in-plane field before transitioning to a trivial semimetal. In a perpendicular field up to 35T, broken TRS leads to a spatial separation of the movers in Kramers pair and consequently the intra-pair backscattering phase space vanishes, i.e., the conductance increases from 2e2/h in strong fields manifesting chiral edge transport. We propose a phenomenological phases diagram, where inside the topological gap the HL transfers into a ``canned helical state'' driven by perpendicular fields. Our findings suggest that once established, the HL is remarkably resilient and only undergoes adiabatic deformation under TRS breaking.   

InAs/GaSb quantum wells: quantum spin Hall effect and topological superconductivity

In recent years, topological insulators (TIs) have attracted great attention as a new quantum state of matter. The first experimental 2D TIs were HgTe/CdTe quantum well heterostructures. Recently, another semiconducting system -- the InAs/GaSb quantum well heterostructure -- was shown to be a 2D TI as well. These semiconducting heterojunctions have many advantages compared to HgTe/CdTe systems, including continuously tunable band structure via electric fields and stronger proximity coupling to superconductors. Proximity coupling of a 2D TI and an ordinary superconductor gives rise to one-dimensional topological superconductivity which supports non-local excitations known as Majoranas that can be used for topologically protected quantum computing. We perform empirical tight-binding calculations on these systems, studying the topological phases and their properties. With this knowlegde, we then extend our theory to study the proximity effects when InAs/GaSb quantum wells are coupled to a superconductor.   

Exciton condensation and quantum spin Hall effect in InAs/GaSb bilayers

We study the phase diagram of the inverted InAs/GaSb bilayer quantum wells as a function of tunneling between the layers and spin-orbit coupling. For small tunneling amplitude between the layers, we find that the system is prone to formation of an $s$-wave exciton condensate topologically trivial phase. On the contrary, for large tunneling amplitude, we obtain a topologically non-trivial quantum spin Hall insulator phase with a $p$-wave exciton order parameter, which enhances the hybridization gap and supports edge transport. These topologically distinct insulators are separated by an insulating phase with spontaneously broken time-reversal symmetry. Close to the phase transition between the quantum spin Hall and time-reversal broken phases, the edge transport shows quantized conductance in small samples, whereas in long samples the mean free path associated with the backscattering at the edge is temperature independent, in agreement with recent experiments.   

Localized States and Quantum Spin Hall Effect in Si-Doped InAs/GaSb Quantum Wells

We study localized in-gap states and quantum spin Hall effect in Si-doped InAs/GaSb quantum wells. We propose a model describing donor and/or acceptor impurities to describe Si dopants. This model shows in-gap bound states and wide conductance plateau with the quantized value $2e^{2}/h$ in light dopant concentration, consistent with recent experiments by Du et al.[arXiv: 1306.1925] We predict a conductance dip structure due to backward scattering in the region where the localization length $\xi $ is comparable with the sample width $L_{y} $ but much smaller than the sample length $L_{x} $.   

Pyroelectric Control of Rashba Spin-Split States and Spin-Relaxation Times in a GaN/InN/GaN Topological Insulator

Strong spin-orbit coupling leading to band inversion in bulk is necessary for creation of topological insulator states (TI). Electric field can also be used to invert the band structure. Nitrides in wurtzite phase possess an internal electric field due to spontaneous and piezoelectric polarization which is sufficient to invert the band-ordering of a narrow-gap InN. A TI state exists in a thin-film of InN sandwiched between GaN layers. For a certain quantum well thickness, inversion of bands happen at a threshold value of the polarization field. Polarization fields are controlled by selecting a facet orientation of the quantum well layer determined by the dominant polarization mechanism. Additionally, at a finite k-vector, the Rashba-induced spin-splitting on the surface of this heterostructure is computed. The splitting under a first-order approximation is independent of k-vector and corresponds to the polarization field's contribution to the Rashba coefficient. Finally, the interplay of mechanisms that control spin-relaxation times is used to design a spin transistor. An enhancement in the lifetime of the spin-polarized states under certain growth conditions is observed due to mutual cancelation the Rashba and Dresselhaus splitting to suppress spin-relaxation.   

Spin helical transport from topological surface states and Rashba 2DEG in topological insulators

Topological insulators are an unusual phase of quantum matter with nontrivial spin-momentum-locked gapless topological surface states (TSS) and strong spin-orbit coupled bulk states. Coexistence of parallel conduction channels makes revealing transport signatures of the spin-momentum helical locked TSS difficult. Here, we report the fabrication of spin valve devices from exfoliated topological insulator thin flakes, with two outside nonmagnetic contacts for injecting a DC current together with a middle ferromagnetic (FM) contact for spin detection. By applying an in-plane magnetic field along the easy axis of FM contact, we observe a striking asymmetric magnetoresistance (MR) with a clear hysteresis. Furthermore, the trend of the asymmetric MR can be reversed by reversing the direction of the DC current. Our result is consistent with the current induced spin polarization from TSS, giving the direct transport evidence of spin-momentum-locking of TSS. Furthermore, from more metallic samples due to bulk conduction, we observe a current induced spin polarization opposite to that of TSS but consistent with Rashba 2D electron gas (2DEG) coming from band bending near surface. Our demonstration of spin helical transport opens ways for novel TI-based spintronc devices.   

Correlated zero-bias transport in graphene and 2D topological insulators nanostructures

In recent years, the Kondo effect in graphene [1] and 2D topological insulators (TI) [2] has attracted considerable interest. While an impurity spin in graphene interacts with the Dirac fermions of the lattice, an impurity on the edge of a 2D-TI interacts with the helical edge liquid. Here we first describe the electronic structure of several graphene and 2D-TI model nanostructures, which incorporate a correlated impurity. Then, by combining continuous time quantum Monte Carlo with the Green function transport theory, we discuss how the transport properties are affected by the Kondo effect. Finally, we highlight how the employed method can be combined with density functional theory in the Smeagol code [3] in order to include material specific properties.\\[4pt] [1] V.N.~Kotov {\it et al.}, Rev. Mod. Phys. \textbf{84}, 1067 (2012).\\[0pt] [2] F.~Goth {\it et al.}, Phys. Rev. B \textbf{88}, 075110 (2013).\\[0pt] [3] A.R.~Rocha {\it et al.}, Phys. Rev. B. \textbf{73}, 085414 (2006).   

Magnetically Defined Qubits on 3D Topological Insulators

We explore potentials that break time-reversal symmetry to confine the surface states of 3D topological insulators into quantum wires and quantum dots. A magnetic domain wall on a ferromagnet insulator cap layer provides interfacial states predicted to show the quantum anomalous Hall effect. Here, we show that confinement can also occur at magnetic domain heterostructures, with states extended in the inner domain, as well as interfacial QAHE states at the surrounding domain walls. The proposed geometry allows the isolation of the wire and dot from spurious circumventing surface states. For the quantum dots, we find that highly spin-polarized quantized QAHE states at the dot edge constitute a promising candidate for quantum computing qubits. See [Ferreira and Loss, Phys. Rev. Lett. 111, 106802 (2013)].