Signatures of Topological Transitions in the Spin Susceptibility of Josephson Junctions

Abstract

We theoretically investigate how the spin susceptibility of a planar Josephson junction is affected when the system transits into the topological superconducting state. We show that the magnetic flux and magnetic field dependence of the spin susceptibility closely maps the phase diagram of the system. In the absence of an external magnetic flux the system can self-tune into the topological superconducting state by minimizing its free energy. Self-tuned topological transitions are accompanied by sharp peaks in the spin susceptibility, which can therefore be use as measurable fingerprints for detecting the topological superconducting state. Away from the peaks, the amplitude of the spin susceptibility can provide qualitative information about the relative size of the topological gap. The signatures in the spin susceptibility are robust, even in junctions with narrow superconducting leads, where critical current minima may no longer serve as an indication of topological phase transitions. Numerical simulations are complemented with simplified analytical models capable of capturing the main features predicted for the spin susceptibility behavior. The predicted results could be particularly relevant for future experiments on realization and detection of the topological superconducting state in planar Josephson junctions.

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“…For example, the fermion parity of the pair can be probed by subjecting the MZM pair to the presence of an out-ofplane flux Φ z . Notably, a recent work [89] discussed the possibility of inferring the topological properties of planar Rashba-Josephson junction [90][91][92][93] by studying the inplane spin susceptibility. While also in our work the magnetic and magnetization fields lie in the plane, we instead propose that signatures of MZMs can be tracked by measuring the out-of-plane orbital component of the magnetization.…”