One-dimensional (1D) electronic transport and induced superconductivity in semiconductor nanostructures are crucial ingredients to realize topological superconductivity. Our approach for topological superconductivity employs a two-dimensional electron gas (2DEG) formed by an InAs quantum well, cleanly interfaced with a superconductor (epitaxial Al). This epi-Al/InAs quantum well heterostructure is advantageous for fabricating large-scale nano-structures consisting of multiple Majorana zero modes.Here, we demonstrate building-block transport studies using a high-quality epi-Al/InAs 2DEG heterostructure, which could be put together to realize the proposed 1D nanowire-based nano-structures and 2DEG-based networks that could host multiple Majorana zero modes: 1D transport using 1) quantum point contacts and 2) gate-defined quasi-1D channels in the InAs 2DEG as well as induced superconductivity in 3) a ballistic Al-InAs 2DEG-Al Josephson junction. From 1D transport, systematic evolution of conductance plateaus in half-integer conductance quanta are observed as a result of strong spin-orbit coupling in the InAs 2DEG. Large , a product of critical current and normal state resistance from the Josephson junction, indicates that the interface between the epitaxial Al and the InAs 2DEG is highly transparent. Our results of electronic transport studies based on the 2D approach suggest that the epitaxial superconductor/2D semiconductor system is suitable for realizing large-scale nanostructures for quantum computing applications.

We investigate the performance of gate-defined devices fabricated on high mobility InAs metamorphic heterostructures. We find that heterostructures capped with In0.75Ga0.25As often show signs of parallel conduction due to proximity of their surface Fermi level to the conduction band minimum. Here, we introduce a technique that can be used to estimate the density of this surface charge that involves cool-downs from room temperature under gate bias. We have been able to remove the parallel conduction under high positive bias, but achieving full depletion has proven difficult. We find that by using In0.75Al0.25As as the barrier without an In0.75Ga0.25As capping, a drastic reduction in parallel conduction can be achieved. Our studies show that this does not change the transport properties of the quantum well significantly. We achieved full depletion in InAlAs capped heterostructures with non-hysteretic gating response suitable for fabrication of gate-defined mesoscopic devices.Narrow band gap semiconductors such as InAs are of fundamental interest for next generation high-speed electronics due to their unique material properties of small effective mass, large dielectric constant and high room temperature mobility [1,2]. In addition, they possess strong spin orbit interaction and large g-factor which make them an ideal platform for spintronics applications [3,4]. Recently, two-dimensional electron systems (2DESs) confined to InAs layers have become the focus of renewed theoretical and experimental attention partly because of their potential applications in quantum computation [5][6][7]. However, all these applications require precise control of electrostatic potentials and carrier densities using nano-fabricated metallic gates. Unlike widely used GaAs based systems, reliable gating has proven difficult in InAs based systems due to gate leakage and hysteretic behavior. Charge traps and surface Fermi level pinning could drastically affect the device performance. Controlling surface properties in In x Ga 1−x As material has played a crucial role in achieving high quality interfaces for metal oxide semiconductor field effect transistors (MOSFETs) [2].The Fermi level pinning at semiconductor surfaces has been the subject of numerous theoretical and experimental studies [8]. In most semiconductors, such as GaAs, the Fermi level is pinned in the band gap [9]. It is well known that the surface states in case of InAs (100) can result in a two dimensional electron system [10]. The electron accumulation is due to pinning of the Fermi level above the conduction band minimum. The position of the pinning level sensitively depends on the material and the surface treatments [11]. In the case of high indium content InGaAs, the situation is similar to InAs. Experiments on In x Ga 1−x As predicts Schottky barrier height becomes negative, exhibiting an ohmic behavior, for x > 0.85 [12]. In this work, we grow InAs metamorphic heterostructures with an In 0.75 Ga 0.25 As surface layer, see Fig. 1(a). High indium content InGaAs, in this cas...

Current-induced spin-orbit torques in Co 2 FeAl/Pt ultrathin bilayers are studied using a magnetoresistive harmonic response technique, which distinguishes the dampinglike and fieldlike contributions. The presence of a temperature-dependent magnetic proximity effect is observed through the anomalous Hall and anisotropic magnetoresistances, which are enhanced at low temperatures for thin platinum thicknesses. The fieldlike torque efficiency decreases steadily as the temperature is lowered for all Pt thicknesses studied, which we propose is related to the influence of the magnetic proximity effect on the fieldlike torque mechanism.

In an ideal InAs/GaSb bilayer of appropriate dimension in-plane electron and hole bands overlap and hybridize, and a topologically non-trivial, or quantum spin Hall (QSH) insulator, phase is predicted to exist 1 . The in-plane dispersion's potential landscape, however, is subject to microscopic perturbations originating from material imperfections. In this work, the effect of disorder on the electronic structure of InAs/GaSb bilayers was studied by the temperature and magnetic field dependence of the resistance of a dual-gated heterostructures gate-tuned through the inverted to normal gap regimes. Conduction in the inverted (predicted topological) regime was qualitatively similar to behavior in a disordered two-dimensional system. The impact of charged impurities and interface roughness on the formation of topologically protected edge states and an insulating bulk was estimated. The experimental evidence and estimates of disorder in the potential landscape indicated the potential fluctuations in state-of-the-art films are sufficiently strong such that conduction in the predicted topological insulator (TI) regime was dominated by a symplectic metal phase rather than a TI phase. The implications are that future efforts must address disorder in this system and focus must be placed on the reduction of defects and disorder in these heterostructures if a TI regime is to be achieved.

We report on broadband ferromagnetic resonance linewidth measurements performed on epitaxial Heusler thin films. A large and anisotropic two-magnon scattering linewidth broadening is observed for measurements with the magnetization lying in the film plane, while linewidth measurements with the magnetization saturated perpendicular to the sample plane reveal low Gilbert damping constants of (1.5 ± 0.1) × 10 −3 , (1.8 ± 0.2) × 10 −3 , and < 8 × 10 −4 for Co2MnSi/MgO, Co2MnAl/MgO, and Co2FeAl/MgO, respectively. The in-plane measurements are fit to a model combining Gilbert and two-magnon scattering contributions to the linewidth, revealing a characteristic disorder lengthscale of 10-100 nm. arXiv:1909.02738v1 [cond-mat.mtrl-sci]