Nonequilibrium and/or topological superconductivity

Schematic of a hybrid device of X-Y-Z configuration where X, Z are made of topological superconductors (TS) and Y is a normal metal (N). A persistent, oscillating electrical current in the above hybrid device is found in Phys. Rev. B 99, 214514 (2019) even in the absence of any phase or voltage or thermal bias.

Superconductivity (SC) is a quantum behavior at macroscopic length scale (coherence length ~ 100 nm). Two hallmarks of SC are perfect electrical conductivity and ideal diamagnetism. Our understanding of SC comes from the BCS theory, which predicts a fraction of electrons near the Fermi energy below a critical temperature forms bound pairs (Cooper pairs), those build the ground state. The spin state of two electrons in a Cooper pair can be singlet (e.g., most common SCs) or triplet (e.g., topological SCs). Excitations above the SC ground state consist of Bogoliubov quasiparticles which are superpositions of the excitations of negatively charged electrons and positively charged holes. In topological SCs, the Bogoliubov quasiparticles at zero energy turn into Majorana fermions which are their own anti-particle.

Josephson junction, made of two SC electrodes separated by a barrier, is an essential building block of modern quantum electronics and precision measurements. We have investigated dynamics of current-biased SC nanowires shunted with an external resistor, and identified regimes in which quasi-one-dimensional wires can effectively be described by a zero-dimensional circuit model analogous to the resistively and capacitively shunted Josephson junction model. For a Josephson junction made of topological SCs, we have predicted a persistent, oscillating electrical current at the junction even in the absence of any phase or voltage or thermal bias. We have studied nonequilibrium electrical transport through different models of topological SC wire connected two metallic leads and demonstrated topologically trivial zero-bias conductance peak in semiconductor Majorana wires from boundary effects.

M. W. Brenner, D. Roy, N. Shah, and A. Bezryadin, Phys. Rev. B 85, 224507 (2012)

D. Roy, C. J. Bolech, and N. Shah, Phys. Rev. B 86, 094503 (2012)

D. Roy, N. Bondyopadhaya and S. Tewari, Phys. Rev. B 88, 020502(R) (2013)

N. Bondyopadhaya and D. Roy, Phys. Rev. B 99, 214514 (2019)

N. Bondyopadhaya and D. Roy, J. Stat. Phys. 187, 11 (2022)

K. Estake and D. Roy, Phys. Rev. B 111, 085406 (2025)