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Register a new userHighly skewed current-phase relation in superconductor-topological insulator-superconductor Josephson junctions
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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.
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Recently, a quantum anomalous Hall insulator (QAHI)/superconductor heterostructure has been realized and shows half-quantized conductance plateaus in two-terminal conductance measurements [Q. L. He textit{et al.}, Science {bf357}, 294 (2017)]. The half-quantized conductance plateaus are considered as a solid evidence of chiral Majorana edge modes. However, there is a strong debate over the origin of the half-quantized conductance plateaus. In this work, we propose a Josephson junction based on the QAHI/superconductor heterostructure to identify the existence of chiral Majorana edge modes. We find that the critical Josephson current dramatically increases to a peak value when a half-quantized conductance plateau $sigma_{12}=e^2/2h$ is showing up for the $N=1$ chiral topological superconductor phase with a single chiral Majorana mode. Furthermore, we show that the critical Josephson current of the $N=1$ chiral topological superconductor exhibits an $h/e$-period oscillation and is robust to disorder, in contrast to the behaviors of conventional two-dimensional electron gas systems. We also estimate experimentally relevant parameters and believe that the supercurrent can be observed in experiments.
We report the first experimental observation of the two-node thickness dependence of the critical current in Josephson junctions with a ferromagnetic interlayer. Vanishings of the critical current correspond to transitions into pi-state and back into conventional 0-state. The experimental data allow to extract the superconducting order parameter oscillation period and the pair decay length in the ferromagnet. We develope a theoretical approach based on Usadel equations, which takes into account the spin-flip scattering. Results of numerical calculations are in good agreement with the experimental data.
Since the discovery of superconductivity in MgB2 considerable progress has been made in determining the physical properties of the material, which are promising for bulk conductors. Tunneling studies show that the material is reasonably isotropic and has a well-developed s-wave energy gap (∆), implying that electronic devices based on MgB2 could operate close to 30K. Although a number of groups have reported the formation of thin films by post-reaction of precursors, heterostructure growth is likely to require considerable technological development, making single-layer device structures of most immediate interest. MgB2 is unlike the cuprate superconductors in that grain boundaries do not form good Josephson junctions, and although a SQUID based on MgB2 nanobridges has been fabricated, the nanobridges themselves do not show junction-like properties. Here we report the successful creation of planar MgB2 junctions by localised ion damage in thin films. The critical current (IC) of these devices is strongly modulated by applied microwave radiation and magnetic field. The product of the critical current and normal state resistance (ICRN) is remarkably high, implying a potential for very high frequency applications.
Josephson junctions with topological insulator weak links can host low energy Andreev bound states giving rise to a current phase relation that deviates from sinusoidal behaviour. Of particular interest are zero energy Majorana bound states that form at a phase difference of $pi$. Here we report on interferometry studies of Josephson junctions and superconducting quantum interference devices (SQUIDs) incorporating topological insulator weak links. We find that the nodes in single junction diffraction patterns and SQUID oscillations are lifted and independent of chemical potential. At high temperatures, the SQUID oscillations revert to conventional behaviour, ruling out asymmetry. The node lifting of the SQUID oscillations is consistent with low energy Andreev bound states exhibiting a nonsinusoidal current phase relation, coexisting with states possessing a conventional sinusoidal current phase relation. However, the finite nodal currents in the single junction diffraction pattern suggest an anomalous contribution to the supercurrent possibly carried by Majorana bound states, although we also consider the possibility of inhomogeneity.
We study a tunnel junction consisting of two thin-film s-wave superconductors separated by a thin, insulating barrier in the presence of misaligned in-plane exchange fields. We find an interesting interplay between the superconducting phase difference and the relative orientation of the exchange fields, manifested in the Josephson current across the junction. Specifically, this may be written $I_text{J}^text{C} = (I_0+I_m ~ cosphi) sinDeltatheta$, where I_0 and I_m are constants, and $phi$ is the relative orientation of the exchange fields while $Deltatheta$ is the superconducting phase difference. Similar results have recently been obtained in other S/I/S junctions coexisting with helimagnetic or ferromagnetic order. We calculate the superconducting order parameter self-consistently, and investigate quantitatively the effect which the misaligned exchange fields constitute on the Josephson current, to see if I_m may have an appreciable effect on the Josephson current. It is found that I_0 and I_m become comparable in magnitude at sufficiently low temperatures and fields close to the critical value, in agreement with previous work. From our analytical results, it then follows that the Josephson current in the present system may be controlled in a well-defined manner by a rotation of the exchange fields on both sides of the junction. We discuss a possible experimental realization of this proposition.
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