Search for Majorana fermions renewed interest in semiconductor–superconductor interfaces, while a quest for higher-order non-Abelian excitations demands formation of superconducting contacts to materials with fractionalized excitations, such as a two-dimensional electron gas in a fractional quantum Hall regime. Here we report induced superconductivity in high-mobility two-dimensional electron gas in gallium arsenide heterostructures and development of highly transparent semiconductor–superconductor ohmic contacts. Supercurrent with characteristic temperature dependence of a ballistic junction has been observed across 0.6 μm, a regime previously achieved only in point contacts but essential to the formation of well separated non-Abelian states. High critical fields (>16 T) in NbN contacts enables investigation of an interplay between superconductivity and strongly correlated states in a two-dimensional electron gas at high magnetic fields.

Three-dimensional topological insulators (TIs) in proximity with superconductors are expected to exhibit exotic phenomena, such as topological superconductivity (TSC) and Majorana-bound states (MBS), which may have applications in topological quantum computation. In superconductor–TI–superconductor Josephson junctions, the supercurrent versus the phase difference between the superconductors, referred to as the current–phase relation (CPR), reveals important information including the nature of the superconducting transport. Here, we study the induced superconductivity in gate-tunable Josephson junctions (JJs) made from topological insulator BiSbTeSe₂ with superconducting Nb electrodes. We observe highly skewed (non-sinusoidal) CPR in these junctions. The critical current, or the magnitude of the CPR, increases with decreasing temperature down to the lowest accessible temperature (T ~ 20 mK), revealing the existence of low-energy modes in our junctions. The gate dependence shows that close to the Dirac point the CPR becomes less skewed, indicating the transport is more diffusive, most likely due to the presence of electron/hole puddles and charge inhomogeneity. Our experiments provide strong evidence that superconductivity is induced in the highly ballistic topological surface states (TSS) in our gate-tunable TI-based JJs. Furthermore, the measured CPR is in good agreement with the prediction of a model which calculates the phase-dependent eigenstate energies in our system, considering the finite width of the electrodes, as well as the TSS wave functions extending over the entire circumference of the TI.

We report anomalous enhancement of the critical current at low temperatures in gate-tunable Josephson junctions made from topological insulator ${\mathrm{BiSbTeSe}}_2$ nanoribbons with superconducting Nb electrodes. In contrast to conventional junctions, as a function of the decreasing temperature $T$, the increasing critical current $I_c$ exhibits a sharp upturn at a temperature $T_{*}$ around 20% of the junction critical temperature for several different samples and various gate voltages. The $I_c$ vs $T$ demonstrates a short junction behavior for $T>T_{*}$, but crosses over to a long junction behavior for $T<T_{*}$ with an exponential $T$ dependence $I_c\propto \exp (-k_{B}T/\delta )$, where $k_B$ is the Boltzmann constant. The extracted characteristic energy scale $\delta$ is found to be an order of magnitude smaller than the induced superconducting gap of the junction. We attribute the long-junction behavior with such a small $\delta$ to low-energy Andreev bound states arising from winding of the electronic wave function around the circumference of the topological insulator nanoribbon.

The adiabatic temperature changes $\left(\Delta {\text{T}}_{\text{ad}}\right)$ in the vicinity of the Curie and martensitic transition temperatures of ${\text{Ni}}_{50}{\text{Mn}}_{35}{\text{In}}_{15}$ and ${\text{Ni}}_{50}{\text{Mn}}_{35}{\text{In}}_{14}\text{Z}$ ($\text{Z}=\text{Al}$ and Ge) Heusler alloys have been studied using an adiabatic magnetocalorimeter of 250–350 K temperature interval for applied magnetic field changes up to $\Delta \text{H}=1.8\text{T}$ . The largest measured changes were $\Delta {\text{T}}_{\text{ad}}=-2$ and 2 K near the martensitic (first-order) and ferromagnetic (second-order) transitions for $\Delta \text{H}=1.8\text{T}$ , respectively. It was observed that $\left|\Delta {\text{T}}_{\text{ad}}\right|\approx 1\text{K}$ for relatively small field changes $\left(\Delta \text{H}=1\text{T}\right)$ for both types of transitions. The results indicate that these materials should be further explored as potential working materials in magnetic refrigeration applications.

The magnetization, the electrical resistivity, the magnetoresistance, and the Hall resistivity of Ni50Mn35In15 − xSi x ( x = 1.0, 3.0, 4.0) Heusler alloys are studied at T= 80-320 K. The martensitic...