Debanjan Chowdhury, Brian Swingle, Subir Sachdev
We describe anomalous behavior of the London penetration depth (\lambda_L) across a quantum phase transition between a superconductor and a superconductor coexisting with a Z_2 spin liquid. Our study is motivated by the physics of heavy fermion materials, where it has been argued that the heavy Fermi liquid (FL) phase in a Kondo lattice model of conduction electrons and localized moments can be driven into a fractionalized Fermi liquid (FL*) phase in spatial dimensions d>=2. At low temperatures, both the FL and FL* phases will often be unstable to superconductivity, however, while the FL state forms an ordinary superconductor (SC), the superconducting phase (SC*) of the parent FL* is distinguished by the presence of a Z_2 spin liquid state for the local moments. Because of the relative simplicity of the SC and SC* phases, we propose to study the phase transition between these two phases for signatures of the spin liquid. In particular, we show that the superfluid density is enhanced as we approach this transition from either side. This is in contrast with recent experiments in the vicinity of a magnetic QCP in the iron-pnictides, where a suppression in the superfluid density was observed.
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http://arxiv.org/abs/1303.6288
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