Kondo physics in a dissipative environment

We report nonperturbative results for the interacting quantum-critical behavior in a Bose-Fermi Kondo model describing a spin-1/2 coupled both to a fermionic band with a pseudogap density of states and to a dissipative bosonic bath. The model serves as a paradigm for studying the interplay between Kondo physics and low-energy dissipative modes in strongly correlated systems.Comment: 2 pages, 2 figures. Proceedings of The International Conference on Strongly Correlated Electron Systems (SCES'07), accepted for publication in Physica

Comparison of transcutaneous ultrasound over the right flank with transrectal ultrasonography in the diagnosis of pregnancy in New Zealand dairy herds : a thesis presented in partial fulfillment of the requirements for the degree of Master of Veterinary Science at Massey University

Application of a 3.5 MHz sector transducer over the right flank allows the rapid and clear visualization of bovine pregnancy (ie: fetus, fetal membranes, fetal fluid and/or placentomes). A total of 1736 cows in ten commercial, pasture-based New Zealand dairy herds were examined for pregnancy by transcutaneous ultrasound across the right flank and transrectal ultrasound between 37 and 198 days of gestation. The gold standard was derived from calving records or examination at slaughter. The overall sensitivity of transrectal ultrasound (96.24%) was markedly higher than flank ultrasound (58.55%) and the overall probability of a correct diagnosis of pregnancy status was also significantly higher (p<0.0001). From 155 days of gestation, however, flank ultrasound represented a more accurate method of pregnancy diagnosis and the probability of a correct diagnosis was significantly higher (p<0.0001) after this gestational age. The gestational age of 225 cows from four Spring-calving dairy herds was determined and ultrasound pregnancy test recorded, to determine possible fetal characteristics able to be visualized via transcutaneous ultrasound over the right flank in order to age pregnancy during mid to late gestation. Linear or quadratic equations and curves were formulated from 60 to 198 days of gestation. The fetal characteristics of thoracic diameter, abdominal diameter or umbilical diameter can be used to age pregnancy from 60 days of gestation. Placentome height and length were not significant in the determination of gestational age

Numerical renormalization-group study of the Bose-Fermi Kondo model

We extend the numerical renormalization-group method to Bose-Fermi Kondo models (BFKMs), describing a local moment coupled to a conduction band and a dissipative bosonic bath. We apply the method to the Ising-symmetry BFKM with a bosonic bath spectral function $\eta(\omega)\propto \omega^s$, of interest in connection with heavy-fermion criticality. For $0<s<1$, an interacting critical point, characterized by hyperscaling of exponents and $\omega/T$-scaling, describes a quantum phase transition between Kondo-screened and localized phases. Connection is made to other results for the BFKM and the spin-boson model.Comment: 4 pages, 4 figure

Continuous-Time Monte Carlo study of the pseudogap Bose-Fermi Kondo model

We study the pseudogap Bose-Fermi Anderson model with a continuous-time quantum Monte Carlo (CT-QMC) method. We discuss some delicate aspects of the transformation from this model to the Bose-Fermi Kondo model. We show that the CT-QMC method can be used at sufficiently low temperatures to access the quantum critical properties of these models.Comment: SCES 2010 Proceeding

Magnetic quantum phase transition in an anisotropic Kondo lattice

The quantum phase transition between paramagnetic and antiferromagnetic phases of the Kondo lattice model with Ising anisotropy in the intersite exchange is studied within the framework of extended dynamical mean-field theory. Nonperturbative numerical solutions at zero temperature point to a continuous transition for both two- and three-dimensional magnetism. In the former case, the transition is associated with critical local physics, characterized by a vanishing Kondo scale and by an anomalous exponent in the dynamics close in value to that measured in heavy-fermion CeCu_{5.9}Au_{0.1}.Comment: 4 pages, 3 figures. Version published in Phys. Rev. Let

Critical Kondo destruction and the violation of the quantum-to-classical mapping of quantum criticality

Antiferromagnetic heavy fermion metals close to their quantum critical points display a richness in their physical properties unanticipated by the traditional approach to quantum criticality, which describes the critical properties solely in terms of fluctuations of the order parameter. This has led to the question as to how the Kondo effect gets destroyed as the system undergoes a phase change. In one approach to the problem, Kondo lattice systems are studied through a self-consistent Bose-Fermi Kondo model within the Extended Dynamical Mean Field Theory. The quantum phase transition of the Kondo lattice is thus mapped onto that of a sub-Ohmic Bose-Fermi Kondo model. In the present article we address some aspects of the failure of the standard order-parameter functional for the the Kondo-destroying quantum critical point of the Bose-Fermi Kondo model.Comment: 4 pages, 3 eps figures; published in the Proceedings of the International Conference on Strongly Correlated Systems (SCES) 200

Local moment approach to multi-orbital single impurity Anderson model; application to dynamical mean-field theory

Using a local moment approach of Logan et al. we developed a solver for a multi-orbital single impurity Anderson model. The existence of the local moments is taken from the outset and their values are determined through variational principle by minimizing the corresponding ground state energy. The method is used to solve the dynamical mean-field equations for the multi-orbital Hubbard model. In particular, the Mott-Hubbard metal--insulator transition is addressed within this approach.Comment: 2 pages, 1 figur

Numerical Renormalization Group for Impurity Quantum Phase Transitions: Structure of Critical Fixed Points

The numerical renormalization group method is used to investigate zero temperature phase transitions in quantum impurity systems, in particular in the particle-hole symmetric soft-gap Anderson model. The model displays two stable phases whose fixed points can be built up of non-interacting single-particle states. In contrast, the quantum phase transitions turn out to be described by interacting fixed points, and their excitations cannot be described in terms of free particles. We show that the structure of the many-body spectrum of these critical fixed points can be understood using renormalized perturbation theory close to certain values of the bath exponents which play the role of critical dimensions. Contact is made with perturbative renormalization group calculations for the soft-gap Anderson and Kondo models. A complete description of the quantum critical many-particle spectra is achieved using suitable marginal operators; technically this can be understood as epsilon-expansion for full many-body spectra.Comment: 14 pages, 12 figure

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.Comment: 4 pages, 3 figures, to appear in the proceedings of SCES 07 (the international conference on strongly correlated electron systems 2007