Abstract
We propose a method to probe and control the interactions between an ensemble of magnetic impurities in a superconductor via microwave radiation. Our method relies upon the presence of sub-gap Yu-Shiba-Rusinov (YSR) states associated with the impurities. Depending on the sign of the detuning, radiation generates either a ferro- or antiferromagnetic contribution to the exchange interaction. This contribution can bias the statistics of the random exchange constants stemming from the RKKY interaction. Moreover, by measuring the microwave response at the YSR resonance, one gains information about the magnetic order of the impurities. To this end, we estimate the absorption coefficient as well as the achievable strength of the microwave-induced YSR-interactions using off-resonant radiation. The ability to utilize microwave fields to both probe and control impurity spins in a superconducting host may open new paths to studying metallic spin glasses.
Highlights
The nature of interactions between magnetic impurities embedded in a metallic host gives rise to an intriguing state of matter: a spin glass [1]
We have shown that the electronic subgap states hosted by magnetic impurities in a superconductor provide a way to access magnetic order
Keeping the transverse size w of the bridge thinner than the London length ensures that the supercurrent is approximately uniform and couples to the magnetic moments in the full volume
Summary
The nature of interactions between magnetic impurities embedded in a metallic host gives rise to an intriguing state of matter: a spin glass [1]. The efforts to understand the resulting low-temperature spin glass phase and the corresponding phase transition have led to the introduction of several important concepts in condensed-matter physics, including the Edwards-Anderson [5] and functional [6] order parameters These efforts have motivated an ever-expanding tool set of quantum control techniques aimed at directly controlling the interactions between magnetic impurities. A pair of YSR states separated by distances ξYSR creates√a discrete-energy state for an electron pair where ξYSR = ξ /( − EYSR ) is the characteristic length scale of a YSR state To this end, at low temperatures, the subgap absorption results from a process in which a microwave photon transfers a Cooper pair from the condensate onto the pair of YSR states, leading to an absorption line centered at ω = 2EYSR/h. One enters the phase of gapless superconductivity, see Ref. [32] and the references therein
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