arXiv subjects

Stefan Kirchner

Publications and source records attributed to Stefan Kirchner.

60 records · Page 4Linked to original sources

arXiv2007

Bose-Fermi Kondo model with Ising anisotropy: cluster-Monte Carlo approach

The Bose-Fermi Kondo model captures the physics of the destruction of Kondo screening, which is of extensive current interest to the understanding of quantum critical heavy fermion metals. There are presently limited theoretical methods to study the finite temperature properties of the Bose-Fermi Kondo model. Here we provide some of the consistency checks on the cluster-Monte Carlo method, which we have recently applied to the Ising-anisotropic Bose-Fermi Kondo model. We show that the method correctly captures the scaling properties of the Kondo phase, as well as those on approach to the Kondo-destroying quantum critical point. We establish that comparable results are obtained when the Kondo couplings are placed at or away from a Toulouse point.

cond-mat.str-el↗

arXiv2007

Magnetic Single-Electron Transistor as a Tunable Model System for Kondo-Destroying Quantum Criticality

Single-electron transistors attached to ferromagnetic leads can undergo a continuous quantum phase transition as their gate voltage is tuned. The corresponding quantum critical point separates a Fermi liquid phase from a non-Fermi liquid one. Here, we expound on the physical idea proposed earlier. The key physics is the critical destruction of the Kondo effect, which underlies a new class of quantum criticality that has been argued to apply to heavy fermion metals. Its manifestation in the transport properties is studied through an effective Bose-Fermi Kondo model; the bosonic bath, corresponding to the spin waves of the ferromagnetic leads, describes a particular type of sub-Ohmic dissipation. We also present results for general forms of sub-Ohmic dissipative bath, and consider in some detail the case with critical paramagons replacing spin waves. Finally, we discuss some delicate aspects in the theoretical treatment of the effect of a local magnetic field, particularly in connection with the frequently employed Non-Crossing Approximation.

cond-mat.str-el↗

arXiv2005

Quantum Criticality in Ferromagnetic Single-Electron Transistors

Considerable evidence exists for the failure of the traditional theory of quantum critical points (QCPs), pointing to the need to incorporate novel excitations. The destruction of Kondo entanglement and the concomitant critical Kondo effect may underlie these emergent excitations in heavy fermion metals -- a prototype system for quantum criticality -- but the effect remains poorly understood. Here, we show how ferromagnetic single-electron transistors can be used to study this effect. We theoretically demonstrate a gate-voltage induced quantum phase transition. The critical Kondo effect is manifested in a fractional-power-law dependence of the conductance on temperature ($T$). The AC conductance and thermal noise spectrum have related power-law dependences on frequency ($ω$) and, in addition, show an $ω/T$ scaling. Our results imply that the ferromagnetic nanostructure constitutes a realistic model system to elucidate magnetic quantum criticality that is central to the heavy fermions and other bulk materials with non-Fermi liquid behavior.

cond-mat.str-el↗

arXiv2005

Quantum critical properties of the Bose-Fermi Kondo Model in a large-N limit

Studies of non-Fermi liquid properties in heavy fermions have led to the current interest in the Bose-Fermi Kondo model. Here we use a dynamical large-N approach to analyze an SU(N)xSU($κN$) generalization of the model. We establish the existence in this limit of an unstable fixed point when the bosonic bath has a sub-ohmic spectrum ($|ω|^{1-ε} \sgn ω$, with $0<ε<1$). At the quantum critical point, the Kondo scale vanishes and the local spin susceptibility (which is finite on the Kondo side for κ<1) diverges. We also find an ω/T scaling for an extended range (15 decades) of ω/T. This scaling violates (for $ε\ge 1/2$) the expectation of a naive mapping to certain classical models in an extra dimension; it reflects the inherent quantum nature of the critical point.

cond-mat.str-el↗

arXiv2004

Destruction of the Kondo effect in a multi-channel Bose-Fermi Kondo model

We consider the SU(N) x SU(kappa N) generalization of the spin-isotropic Bose-Fermi Kondo model in the limit of large N. There are three fixed points corresponding to a multi-channel non-Fermi liquid phase, a local spin-liquid phase, and a Kondo-destroying quantum critical point (QCP). We show that the QCP has strong similarities with its counterpart in the single-channel model, even though the Kondo phase is very different from the latter. We also discuss the evolution of the dynamical scaling properties away from the QCP.

cond-mat.str-el↗

arXiv2002

Self-consistent Conserving Theory for Quantum Impurity Systems: Renormalization Group Analysis

We review the diagrammatic, conserving theory for quantum impurities with strong on-site repulsion. The method is based on auxiliary particle technique, where Wick's theorem is valid, which opens up the possibility for generalizations to more complicated situations. An analysis in terms of the perturbative renormalization group (RG) shows that on the level of the Conserving T--matrix Approximation the theory correctly describes the RG flow, including the non-scaling of potential scattering terms and the correct Kondo temperature.

cond-mat.str-el↗