We report on transport characteristics of field effect two-dimensional electron gases (2DEGs) in 24 nm wide indium arsenide surface quantum wells. High quality single-subband magnetotransport with clear quantized integer quantum Hall plateaus is observed to filling factor ν = 2 in magnetic fields of up to B = 18 T, at electron densities up to 8 ×1011/cm2. Peak mobility is 11 000 cm2/Vs at 2 ×1012/cm2. Large Rashba spin–orbit coefficients up to 124 meV Å are obtained through weak anti-localization measurements. Proximitized superconductivity is demonstrated in Nb-based superconductor-normal-superconductor (SNS) junctions, yielding 78%–99% interface transparencies from superconducting contacts fabricated ex situ (post-growth), using two commonly used experimental techniques for measuring transparencies. These transparencies are on a par with those reported for epitaxially grown superconductors. These SNS junctions show characteristic voltages IcRn up to 870 μV and critical current densities up to 9.6 μA/μm, among the largest values reported for Nb-InAs SNS devices.

Among superconductor/semiconductor hybrid structures, in situ aluminum (Al) grown on InGaAs/InAs is widely pursued for the experimental realization of Majorana Zero Mode quasiparticles. This is due to the high carrier mobility, low effective mass, and large Landé g-factor of InAs, coupled with the relatively high value of the in-plane critical magnetic field in thin Al films. However, growing a thin, continuous Al layer using the molecular beam epitaxy (MBE) is challenging due to aluminum's high surface mobility and tendency for 3D nucleation on semiconductor surfaces. A study of epitaxial Al thin film growth on In0.75Ga0.25As with MBE is presented, focusing on the effects of the Al growth rate and substrate temperature on the nucleation of Al layers. We find that for low deposition rates, 0.1 and 0.5 Å/s, the growth continues in 3D mode during the deposition of the nominal 100 Å of Al, resulting in isolated Al islands. However, for growth rates of 1.5 Å/s and above, the 3D growth mode quickly transitions into island coalescence, leading to a uniform 2D Al layer. Moreover, this transition is very abrupt, happening over an Al flux increase of less than 1%. We discuss the growth mechanisms explaining these observations. The results give new insights into the kinetics of Al deposition and show that with sufficiently high Al flux, a 2D growth on substrates at close to room temperature can be achieved already within the first few Al monolayers. This eliminates the need for complex cryogenic substrate cooling and paves the way for the development of high-quality superconductor-semiconductor interfaces in standard MBE systems.

We report on transport characteristics of field effect two-dimensional electron gases (2DEGs) in surface indium antimonide quantum wells. The topmost 5 nm of the 30 nm wide quantum well is doped and shown to promote the formation of reliable, low resistance Ohmic contacts to surface InSb 2DEGs. High quality single-subband magnetotransport with clear quantized integer quantum Hall plateaus is observed to filling factor nu = 1 in magnetic fields of up to B = 18 T. We show that the electron density is gate-tunable, reproducible, and stable from pinch-off to 4 x 10(11) cm(-2), and peak mobilities exceed 24 000 cm(2)/Vs. Large Rashba spin-orbit coefficients up to 110meV . angstrom are obtained through weak anti-localization measurements. An effective mass of 0.019m(e) is determined from temperature-dependent magnetoresistance measurements, and a g-factor of 41 at a density of 3.6 x 10(11) cm(-2) is obtained from coincidence measurements in tilted magnetic fields. By comparing two heterostructures with and without a delta-doped layer beneath the quantum well, we find that the carrier density is stable with time when doping in the ternary Al0.1In0.9Sb barrier is not present. Finally, the effect of modulation doping on structural asymmetry between the two heterostructures is characterized.

The research reported in this publication was supported by King Abdullah University of Science & Technology (KAUST) with Grants No. CRF-2015-SENSORS-2708 and No. URF-2015-2549 and by the Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery Grant No. RGPIN-04178, the Ontario Early Researcher Award, and the Canada First Research Excellence Fund.

We conduct a comprehensive study of three different magnetic semiconductors, CrI3, CrBr3, and CrCl3, by incorporating both few-layer and bilayer samples in van der Waals tunnel junctions. We find that the interlayer magnetic ordering, exchange gap, magnetic anisotropy, and magnon excitations evolve systematically with changing halogen atom. By fitting to a spin wave theory that accounts for nearest-neighbor exchange interactions, we are able to further determine a simple spin Hamiltonian describing all three systems. These results extend the 2D magnetism platform to Ising, Heisenberg, and XY spin classes in a single material family. Using magneto-optical measurements, we additionally demonstrate that ferromagnetism can be stabilized down to monolayer in more isotropic CrBr3, with transition temperature still close to that of the bulk.

We describe an experimental protocol to characterize magnetic field dependent microwave losses in superconducting niobium microstrip resonators. Our approach provides a unified view that covers two well-known magnetic field dependent loss mechanisms: quasiparticle generation and vortex motion. We find that quasiparticle generation is the dominant loss mechanism for parallel magnetic fields. For perpendicular fields, the dominant loss mechanism is vortex motion or switches from quasiparticle generation to vortex motion, depending on cooling procedures. In particular, we introduce a plot of the quality factor versus the resonance frequency as a general method for identifying the dominant loss mechanism. We calculate the expected resonance frequency and the quality factor as a function of the magnetic field by modeling the complex resistivity. Key parameters characterizing microwave loss are estimated from comparisons of the observed and expected resonator properties. Based on these key parameters, we find a niobium resonator whose thickness is similar to its penetration depth is the best choice for X-band electron spin resonance applications. Finally, we detect partial release of the Meissner current at the vortex penetration field, suggesting that the interaction between vortices and the Meissner current near the edges is essential to understand the magnetic field dependence of the resonator properties.

The surface states of a topological insulator (TI) can be induced with superconductivity through proximity with a conventional s-wave superconductor (S). To study the coupling between two superconducting TI surfaces, we report the growth and fabrication of vertical Josephson junctions with the topological insulators (Bi0.5Sb0.5)(2)Te-3 sandwiched between Nb electrodes. We observed two Josephson critical currents on the I-V characteristic at 3.5 K, attributed to the bulk Nb and the proximity-induced superconducting (Bi0.5Sb0.5)(2)Te-3 surfaces, respectively. The enhancement of conductance at these two critical currents leads to a bump and a plateau on the differential conductance spectroscopy. By further cooling down to 300 mK, a zero bias conductance peak (ZBCP) appears on the plateau, which is taken as a feature from the unconventional paring at the (Bi0.5Sb0.5)(2)Te-3-Nb interface.

We use both classical magnetotransport and quantum oscillation measurements to study the thickness evolution of the extremely large magnetoresistance (XMR) material and type-II Weyl semimetal candidate gamma-MoTe2, protected from oxidation. We find that the magnetoresistance is systematically suppressed with reduced thickness. This occurs concomitantly with both a decrease in carrier mobility and increase in electron-hole imbalance. We model the two effects separately and conclude that the XMR effect is more sensitive to the former.