Abstract
A single-spin qubit placed near the surface of a conductor acquires an additional contribution to its $1/T_1$ relaxation rate due to magnetic noise created by electric current fluctuations in the material. We analyze this technique as a wireless probe of superconductivity in atomically thin two dimensional materials. At temperatures $T \lesssim T_c$, the dominant contribution to the qubit relaxation rate is due to transverse electric current fluctuations arising from quasiparticle excitations. We demonstrate that this method enables detection of metal-to-superconductor transitions, as well as investigation of the symmetry of the superconducting gap function, through the noise scaling with temperature. We show that scaling of the noise with sample-probe distance provides a window into the non-local quasi-static conductivity of superconductors, both clean and disordered. At low temperatures the quasiparticle fluctuations get suppressed, yet the noise can be substantial due to resonant contributions from collective longitudinal modes, such as plasmons in monolayers and Josephson plasmons in bilayers. Potential experimental implications are discussed.
Highlights
A superconductor is a phase of matter characterized by the dissipation-free flow of electrical current owing to the intrinsic quantum coherence between electron pairs [1]
At temperatures T Tc, the dominant contribution to the qubit relaxation rate is due to transverse electric current fluctuations arising from quasiparticle excitations. We demonstrate that this method enables detection of metal-to-superconductor transitions, as well as investigation of the symmetry of the superconducting gap function, through the noise scaling with temperature
Our calculations are valid away from the critical regime, where critical current fluctuations might lead to an appreciable enhancement of noise, facilitating easier experimental detection
Summary
A superconductor is a phase of matter characterized by the dissipation-free flow of electrical current owing to the intrinsic quantum coherence between electron pairs [1]. Atomically thin superconductors and require local magnetometry [13,14] This calls for new experimental probes which can be used to diagnose the onset of superconductivity and elucidate the pairing symmetries in 2D materials. The decay rate (1/T1), studied as a function of experimentally tunable parameters such as qubit-probe distance z0, probe frequency , and temperature T , furnishes valuable information about the nature of superconductivity in the 2D sample. The scaling of the noise with probe-qubit distance can be used to study the nonlocal conductivity in the quasistatic limit (q = 0, → 0), a regime complementary to existing probes (such as dc transport, THz spectroscopy, etc.) We elucidate these distinct scaling regimes in both clean and disordered superconductors, carefully accounting for additional modifications arising from superflow. We illustrate that deep in the superconducting phase the noise
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