Superconductivity in an extreme strange metal

Abstract

Some of the highest-transition-temperature superconductors across various materials classes exhibit linear-in-temperature ‘strange metal’ or ‘Planckian’ electrical resistivities in their normal state. It is thus believed by many that this behavior holds the key to unlock the secrets of high-temperature superconductivity. However, these materials typically display complex phase diagrams governed by various competing energy scales, making an unambiguous identification of the physics at play difficult. Here we use electrical resistivity measurements into the micro-Kelvin regime to discover superconductivity condensing out of an extreme strange metal state—with linear resistivity over 3.5 orders of magnitude in temperature. We propose that the Cooper pairing is mediated by the modes associated with a recently evidenced dynamical charge localization–delocalization transition, a mechanism that may well be pertinent also in other strange metal superconductors.

Nuclear-order-induced quantum criticality and heavy-fermion superconductivity at ultra-low temperatures in YbRh$_2$Si$_2$

“…The observation of a weak but distinct superconducting shielding signal at a temperature as high as 10 mK suggests the formation of insulated random islands with emergent A-phase order and superconductivity. Upon cooling, the shielding signal increases almost linearly in temperature, indicating a growth of the islands which eventually percolate at T ≈ 6.5 mK, as inferred from recent electrical-resistivity results by [2]. These authors also report on two superconducting regions, the first one below 4 mT, with the highest Tc occurring at B = 0, and a second 'dome-shaped' regime with much lower Tc at 4 mT < B < 60 mT.…”

“…2B, as motivated by the theory of 'critical quasiparticles' [28] (which, however, is inapt to explain the measured asymptotic linear-in-T dependence of the resistivity at the unconventional QCP in YbRh 2 Si 2 ). Most importantly, though, for all samples studied so far, the asymptotic (T → 0) T -dependence of ρ(T ) registered at B = B N is found to be linear, see also [2]. The linearin-T coefficient, A , can be converted to a linear-in-T coefficient in the scattering rate, 1/τ , in a procedure based on a Drude analysis: the latter is found to be much smaller than what appears in a Planckian form when a 'background' heavy-fermion value is used for the effective mass [2,29].…”

“…Most importantly, though, for all samples studied so far, the asymptotic (T → 0) T -dependence of ρ(T ) registered at B = B N is found to be linear, see also [2]. The linearin-T coefficient, A , can be converted to a linear-in-T coefficient in the scattering rate, 1/τ , in a procedure based on a Drude analysis: the latter is found to be much smaller than what appears in a Planckian form when a 'background' heavy-fermion value is used for the effective mass [2,29]. One of the important techniques that provides new insight into such correlation-driven phenomena is scanning tunneling spectroscopy (STS) with its unique ability to give local, atomically resolved information that relates to the one-particle Green's function [30].…”

Spin-singlet and spin-triplet pairing correlations in antiferromagnetically coupled Kondo systems

“…Discussion and Summary: Our findings shed light on the recent experiment in YbRh 2 Si 2 near its field-driven QCP [14]. YbRh 2 Si 2 has antiferromagnetic order at the ambient conditions.…”

“…Recently, superconductivity has been discovered at ultra-low temperatures in the canonical quantum critical heavy fermion metal, YbRh 2 Si 2 , which exhibits AF order and a field-induced QCP [9]. In addition to the superconducting phase at zero field, which has been interpreted as arising from the intrinsic electronic quantum criticality unmasked by the hyperfine coupling to nuclear spins [13], a new superconducting phase occurs near the critical magnetic field [14]. The latter appears to have spin-triplet pairing, raising a profound question of how antiferromagnetic correlations can drive spin-triplet pairing in quantum critical Kondo systems.…”