Looking for chiral Majoranas: Chiral Majorana modes have been predicted to exist in heterostructures consisting of a quantum anomalous Hall insulator and a superconductor. Kayyalha et al. fabricated more than 30 such samples and used transport measurements to look for signatures of the Majorana modes. The data indicated that the transport signatures previously thought to be associated with Majorana physics could, in their samples, be explained using a more mundane mechanism. Science , this issue p. 64

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.

There is growing interest in using multiterminal Josephson junctions (MTJJs) as a platform to artificially emulate topological phases and to investigate superconducting mechanisms such as multiplet Cooper pairings. Current experimental signatures in MTJJs have led to conflicting interpretations of the salient features. In this work, we report a collaborative experimental and theoretical investigation of graphene-based four-terminal Josephson junctions. We observe resonant features in the differential resistance maps that resemble those ascribed to multiplet Cooper pairings. To understand these features, we model our junctions using a circuit network of resistively and capacitively shunted junctions (RCSJs). We find that the RCSJ model successfully reproduces the observed multiplet features. Therefore, our study suggests that differential resistance measurements alone are insufficient to conclusively distinguish resonant Andreev reflection processes from semiclassical circuit-network effects.

We report on the observation of gate-tunable proximity-induced superconductivity and multiple Andreev reflections (MARs) in a bulk-insulating BiSbTeSe₂ topological insulator nanoribbon (TINR) Josephson junction with superconducting Nb contacts. We observe a gate-tunable critical current (I_C) for gate voltages (V_g) above the charge neutrality point (V_CNP), with I_C as large as 430 nA. We also observe MAR peaks in the differential conductance (dI/dV) versus DC voltage (V_dc) across the junction corresponding to sub-harmonic peaks (at V_dc = V_n = 2Δ_Nb/en, where Δ_Nb is the superconducting gap of the Nb contacts and n is the sub-harmonic order). The sub-harmonic order, n, exhibits a V_g-dependence and reaches n = 13 for V_g = 40 V, indicating the high transparency of the Nb contacts to TINR. Our observations pave the way toward exploring the possibilities of using TINR in topologically protected devices that may host exotic physics such as Majorana fermions.

Nonreciprocal superconducting devices have attracted growing interest in recent years as they potentially enable directional charge transport for applications in superconducting quantum circuits. Specifically, the superconducting diode effect has been explored in two-terminal devices that exhibit superconducting transport in one current direction while showing dissipative transport in the opposite direction. Here, we exploit multiterminal Josephson junctions (MTJJs) to engineer magnetic-field-free nonreciprocity in multiport networks. We show that when treated as a two-port electrical network, a three-terminal Josephson junction (JJ) with an asymmetric graphene region exhibits reconfigurable two-port nonreciprocity. We observe nonreciprocal (reciprocal) transport between superconducting terminals with broken (preserved) spatial mirror symmetry. We explain our observations by considering a circuit network of JJs with different critical currents.