Quantum phase transitions in a charge-coupled Bose-Fermi Anderson model

TL;DR

This paper investigates the interplay of Kondo physics and dissipation in a charge-coupled Bose-Fermi Anderson model, revealing quantum phase transitions and critical behavior influenced by bosonic bath spectral properties.

Contribution

It introduces a detailed analysis of quantum phase transitions in a charge-coupled Bose-Fermi Anderson model, highlighting the effects of bosonic bath spectral density on impurity behavior and critical phenomena.

Findings

Identifies a continuous quantum phase transition at sub-Ohmic bath exponents with hyperscaling and /T scaling.

Shows the transition belongs to the same universality class as the sub-Ohmic spin-boson model.

For Ohmic baths, the transition is of Kosterlitz-Thouless type.

Abstract

We study the competition between Kondo physics and dissipation within an Anderson model of a magnetic impurity level that hybridizes with a metallic host and is also coupled, via the impurity charge, to the displacement of a bosonic bath having a spectral density proportional to \omega^s. As the impurity-bath coupling increases from zero, the effective Coulomb interaction between two electrons in the impurity level is progressively renormalized from its repulsive bare value until it eventually becomes attractive. For weak hybridization, this renormalization in turn produces a crossover from a conventional, spin-sector Kondo effect to a charge Kondo effect. At particle-hole symmetry, and for sub-Ohmic bath exponents 0 < s < 1, further increase of the impurity-bath coupling results in a continuous, zero-temperature transition to a broken-symmetry phase in which the ground-state impurity occupancy \n_d acquires an expectation value <\n_d>_0 \ne 1. The response of the impurity occupancy to a locally applied electric potential features the hyperscaling of critical exponents and \omega/T scaling that are expected at an interacting critical point. The numerical values of the critical exponents suggest that the transition lies in the same universality class as that of the sub-Ohmic spin-boson model. For the Ohmic case s = 1, the transition is instead of Kosterlitz-Thouless type. Away from particle-hole symmetry, the quantum phase transition is replaced by a smooth crossover, but signatures of the symmetric quantum critical point remain in the physical properties at elevated temperatures and/or frequencies.