Prof Rok Žitko

Variational solution of the superconducting single-impurity Anderson model

Quantum impurity models, such as Kondo and Anderson Hamiltonians, describe a localized degree of freedom coupled to a continuum of states. These paradigmatic models find many applications in condensed-matter and statistical physics. While detailed numerical solutions can be obtained using sophisticated tools such as Wilson's numerical renormalization group, variational solutions can provide direct physical insight into the nature of the ground state. Variational wavefunctions have long been known for the Kondo and Anderson models with a normal-state bath (Yosida, Varma Ansätze) as well as the Kondo model with a superconducting bath (Soda, Matsuura, Nagaoka Ansatz), but not for the Anderson model case. I will present a recently proposed set of variational wavefunctions for this model and discuss the nature of the solution as a function of the U/Delta (charge repulsion over superconducting gap) ratio. At U~2Delta the behaviour is governed by band-edge singularity physics leading to fractional exponent (2/3) in the subgap energy versus the hybridization strength Gamma. This anomalous behaviour could observed in superconducting quantum devices.