arXiv subjects

Gergely Zarand

Publications and source records attributed to Gergely Zarand.

49 records · Page 3Linked to original sources

arXiv2003

Quantum Magnetic Impurities in Magnetically Ordered Systems

We discuss the problem of a spin 1/2 impurity immersed in a spin S magnetically ordered background. We show that the problem maps onto a generalization of the dissipative two level system (DTLS) with two independent heat baths, associated with the Goldstone modes of the magnet, that couple to different components of the impurity spin operator. Using analytical perturbative renormalization group (RG) methods and accurate numerical renormalization group (NRG) we show that contrary to other dissipative models there is quantum frustration of decoherence and quasi-scaling even in the strong coupling regime. We make predictions for the behavior of the impurity magnetic susceptibility that can be measured in nuclear magnetic resonance (NMR) experiments. Our results may also have relevance to quantum computation.

cond-mat.str-el↗

arXiv2003

Kondo effect and spin filtering in triangular artificial atoms

We study strongly correlated states in triangular artificial atoms. Symmetry-driven orbital degeneracy of the single particle states can give rise to an SU(4) Kondo state with entangled orbital and spin degrees of freedom, and a characteristic phase shift $δ= π/4$. Upon application of a Zeeman field, a purely orbital Kondo state is formed with somewhat smaller Kondo temperature and a fully polarized current through the device. The Kondo temperatures are in the measurable range. The triangular atom also provides a tool to systematically study the singlet-triplet transitions observed in recent experiments.

cond-mat.mes-hall↗

arXiv2002

Quantum phase transitions in the Bose-Fermi Kondo model

We study quantum phase transitions in the Bose-Fermi Kondo problem, where a local spin is coupled to independent bosonic and fermionic degrees of freedom. Applying a second order expansion in the anomalous dimension of the Bose field we analyze the various non-trivial fixed points of this model. We show that anisotropy in the couplings is relevant at the SU(2) invariant non Fermi liquid fixed points studied earlier and thus the quantum phase transition is usually governed by XY or Ising-type fixed points. We furthermore derive an exact result that relates the anomalous exponent of the Bose field to that of the susceptibility at any finite coupling fixed point. Implications on the dynamical mean field approach to locally quantum critical phase transitions are also discussed.

cond-mat.str-el↗

arXiv2002

Kondo effect and STM spectra through ferromagnetic nanoclusters

Motivated by recent scanning tunneling microscope (STM) experiments on cobalt clusters adsorbed on single wall metallic nanotubes [Odom {\em et al.}, Science {\bf 290}, 1549 (2000)], we study theoretically the size dependence of STM spectra and spin-flip scattering of electrons from finite size ferromagnetic clusters adsorbed on metallic surfaces. We study two models of nanometer size ferromagnets: (i) An itinerant model with delocalized s, p and d electrons and (ii) a local moment model with both localized d-level spins and delocalized cluster electrons. The effective exchange coupling between the spin of the cluster and the conduction electrons of the metallic substrate depends on the specific details of the single particle density of states on the cluster. The calculated Kondo coupling is inversely proportional to the total spin of the ferromagnetic cluster in both models and thus the Kondo temperature is rapidly suppressed as the size of the cluster increases. Mesoscopic fluctuations in the charging energies and magnetization of nanoclusters can lead to large fluctuations in the Kondo temperatures and a very asymmetric voltage dependence of the STM spectra. We compare our results to the experiments.

cond-mat.str-el↗

arXiv2001

Magnetic Semiconductors are Frustrated Ferromagnets

Starting from microscopic and symmetry considerations, we derive the Hamiltonian describing the exchange interaction between the localized Mn spins and the valence band holes in $Ga_{1-x}Mn_x As$. We find that due to the strong spin-orbit coupling in the valence band, this exchange interaction has a rather complex structure and generates a highly anisotropic effective interaction between the Mn spins. The corresponding ground state has a finite ferromagnetic magnetization but is intrinsically spin-disordered even at zero temperature.

cond-mat.str-el↗

arXiv2000

Few comments on preprint cond-mat/0007430 by Aleiner et al

A few comments are made on the recent preprint cond-mat/0007430 by Aleiner et al. We show that the results presented there heavily rely on the assumption of perfect electron-hole symmetry breaking and once we drop this assumption we find a Kondo temperature in the experimentally accessible range.

cond-mat.str-el↗

arXiv2000

Is the Multichannel Kondo Model Appropriate to Describe the Single Electron Transistor?

We investigate the low-temperature dynamics of single electron boxes and transistors close to their degeneracy point using renormalization group methods. We show that intermode scattering is a relevant perturbation and always drives the system to the two-channel Kondo fixed point, where the two channels correspond to the real spins of the conduction electrons. However, the crossover temperature T^*, below which Matveev's two-channel Kondo scenario [K.A. Matveev, Phys. Rev. B 51, 1743 (1995)] develops decreases exponentially with the number of conduction modes in the tunneling junctions and is extremely small in most cases. Above T^* the 'infinite channel model' of Ref.{onlinecite{Schoen}} turns out to be a rather good approximation. We discuss the experimental limitations and suggest a new experimental setup to observe the multichannel Kondo behavior.

cond-mat.str-el↗

arXiv2000

Toward a systematic 1/d expansion: Two particle properties

We present a procedure to calculate 1/d corrections to the two-particle properties around the infinite dimensional dynamical mean field limit. Our method is based on a modified version of the scheme of Ref. onlinecite{SchillerIngersent}}. To test our method we study the Hubbard model at half filling within the fluctuation exchange approximation (FLEX), a selfconsistent generalization of iterative perturbation theory. Apart from the inherent unstabilities of FLEX, our method is stable and results in causal solutions. We find that 1/d corrections to the local approximation are relatively small in the Hubbard model.

cond-mat.str-el↗

arXiv1999

Crystal Field Triplets: A New Route to Non-Fermi Liquid Physics

A model for crystal field triplet ground states on rare earth or actinide ions with dipolar and quadrupolar couplings to conduction electrons is studied for the first time with renormalization group methods. The quadrupolar coupling leads to a new nontrivial, non-Fermi liquid fixed point, which survives in an intermediate valence Anderson model. The calculated magnetic susceptibility displays one parameter scaling, going as $T^{-α}$ ($α\approx 0.4$) at intermediate temperatures, reminiscent of the non-Fermi liquid alloy UCu_{5-x}Pd_x.

cond-mat.str-el↗

arXiv1999

Self-organized criticality in the hysteresis of the Sherrington - Kirkpatrick model

We study hysteretic phenomena in random ferromagnets. We argue that the angle dependent magnetostatic (dipolar) terms introduce frustration and long range interactions in these systems. This makes it plausible that the Sherrington - Kirkpatrick model may be able to capture some of the relevant physics of these systems. We use scaling arguments, replica calculations and large scale numerical simulations to characterize the hysteresis of the zero temperature SK model. By constructing the distribution functions of the avalanche sizes, magnetization jumps and local fields, we conclude that the system exhibits self-organized criticality everywhere on the hysteresis loop.

cond-mat.dis-nn↗

arXiv1998

Simple Bosonization Solution of the 2-channel Kondo Model: I. Analytical Calculation of Finite-Size Crossover Spectrum

We present in detail a simple, exact solution of the anisotropic 2-channel Kondo (2CK) model at its Toulouse point. We reduce the model to a quadratic resonant-level model by generalizing the bosonization-refermionization approach of Emery and Kivelson to finite system size, but improve their method in two ways: firstly, we construct all boson fields and Klein factors explicitly in terms of the model's original fermion operators $c_{k σj}$, and secondly we clarify explicitly how the Klein factors needed when refermionizing act on the original Fock space. This enables us to explicitly follow the adiabatic evolution of the 2CK model's free-fermion states to its exact eigenstates, found by simply diagonalizing the resonant-level model for arbitrary magnetic fields and spin-flip coupling strengths. In this way we obtain an {\em analytic} description of the cross-over from the free to the non-Fermi-liquid fixed point. At the latter, it is remarkably simple to recover the conformal field theory results for the finite-size spectrum (implying a direct proof of Affleck and Ludwig's fusion hypothesis). By analyzing the finite-size spectrum, we directly obtain the operator content of the 2CK fixed point and the dimension of various relevant and irrelevant perturbations. Our method can easily be generalized to include various symmetry-breaking perturbations. Furthermore it establishes instructive connections between different renormalization group schemes such as poor man's scaling, Anderson-Yuval type scaling, the numerical renormalization group and finite-size scaling.

cond-mat.str-el↗

arXiv1998

Finite-Size Bosonization of 2-Channel Kondo Model: a Bridge between Numerical Renormalization Group and Conformal Field Theory

We generalize Emery and Kivelson's (EK) bosonization-refermionization treatment of the 2-channel Kondo model to finite system size and on the EK-line analytically construct its exact eigenstates and finite-size spectrum. The latter crosses over to conformal field theory's (CFT) universal non-Fermi-liquid spectrum (and yields the most-relevant operators' dimensions), and further to a Fermi-liquid spectrum in a finite magnetic field. Our approach elucidates the relation between bosonization, scaling techniques, the numerical renormalization group (NRG) and CFT. All CFT's Green's functions are recovered with remarkable ease from the model's scattering states.

cond-mat.str-el↗

arXiv1998

Solution of the Multichannel Coqblin-Schrieffer Impurity Model and Application to Multi-Level systems

A complete Bethe Ansatz solution of the SU(N)xSU(f) Coqblin Schrieffer model and a detailed analysis of some physical applications of the model are given. As in the usual multichannel Kondo model a variety of Fermi liquid and non-Fermi liquid (NFL) fixed points is found, whose nature depends on the impurity representation, $μ$. For $μ=f$ we find a Fermi liquid fixed point, with the impurity spin completely screened. For $f>μ$ the impurity is overscreened and the model has NFL properties. The form the NFL behavior takes depends on the N and f: for $N\le f$ the specific heat and the susceptibility are dominated by the NFL contributions, for N>f the leading contributions are Fermi-liquid like and the NFL behavior can be seen only to subleading order, while for N=f the behavior is marginal. We also analyze the possibility of physical realizations. We show by a detailed renormalization group and 1/f analysis that the tunneling N-state problem can be mapped into the SU(N)xSU(f) exchange model, and discuss the subtle differences between the two models. As another physical realization we suggest a double quantum dot structure that can be described by means of an SU(3)xSU(2) model if the parameters of the dots are tuned appropriately.

cond-mat.str-el↗