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We report anomalous enhancement of the critical current at low temperaturesin gate-tunable Josephson junctions made from topological insulatorBiSbTeSe$_2$ nanoribbons with superconducting Nb electrodes. In contrast toconventional junctions, as a function of the decreasing temperature $T$, theincreasing critical current $I_c$ exhibits a sharp upturn at a temperature$T_*$ around 20$\%$ of the junction critical temperatures for several differentsamples and various gate voltages. The $I_c$ vs. $T$ demonstrates a shortjunction behavior for $T>T_*$, but crosses over to a long junction behavior for$T<T_*$ with an exponential $T$-dependence $I_c \propto \exp\big(-k_B T/\delta\big)$, where $k_B$ is the Boltzmann constant. The extracted characteristicenergy-scale $\delta$ is found to be an order of magnitude smaller than theinduced superconducting gap of the junction. We attribute the long-junctionbehavior with such a small $\delta$ to low-energy Andreev bound states (ABS)arising from winding of the electronic wavefunction around the circumference ofthe topological insulator nanoribbon (TINR). Our TINR-based Josephson junctionswith low-energy ABS are promising for future topologically protected devicesthat may host exotic phenomena such as Majorana fermions.

physical review letters

Anomalous low-temperature enhancement of supercurrent in  topological-insulator nanoribbon Josephson junctions: evidence for low-energy  Andreev bound states