Publications (4)11.06 Total impact

Article: Manual for the Flexible DMNRG code
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ABSTRACT: Quantum impurity models describe interactions between some local degrees of freedom and a continuum of noninteracting fermionic or bosonic states. The investigation of quantum impurity models is a starting point towards the understanding of more complex strongly correlated systems, but quantum impurity models also provide the description of various correlated mesoscopic structures, biological and chemical processes, atomic physics and describe phenomena such as dissipation or dephasing. Prototypes of these models are the Anderson impurity model, or the single and multichannel Kondo models. The numerical renormalization group method (NRG) proposed by Wilson in mid 70's has been used in its original form for a longtime as one of the most accurate and powerful methods to deal with quatum impurity problems. Recently, a number of new developments took place: First, a spectral sumconserving density matrix NRG approach (DMNRG) has been developed, which has also been generalized for nonAbelian symmetries. In this manual we introduce some of the basic concepts of the NRG method and present recently developed Flexible DMNRG code. This code uses userdefined nonAbelian symmetries dynamically, computes spectral functions, expectation values of local operators for userdefined impurity models. The code can also use a uniform density of states as well as a userdefined density of states. The current version of the code assumes fermionic bath's and it uses any number of U(1), SU(2) charge SU(2) or Z(2) symmetries. The Flexible DMNRG code can be downloaded from http://www.phy.bme.hu/~dmnrg  [Show abstract] [Hide abstract]
ABSTRACT: We generalize the spectral sum rule preserving density matrix numerical renormalization group (DMNRG) method in such a way that it can make use of an arbitrary number of not necessarily Abelian, local symmetries present in the quantum impurity system. We illustrate the benefits of using nonAbelian symmetries by the example of calculations for the Tmatrix of the twochannel Kondo model in the presence of magnetic field, for which conventional NRG methods produce large errors and/or take a long runtime. Comment: 12 pages, 6 figures, PRB format  [Show abstract] [Hide abstract]
ABSTRACT: Dynamical correlations of various local operators are studied in the spinhalf twochannel Kondo (2CK) model in the presence of channel anisotropy or external magnetic field. A conformal field theorybased scaling approach is used to predict the analytic properties of various spectral functions in the vicinity of the twochannel Kondo fixed point. These analytical results compare well with highly accurate density matrix numerical renormalization group results. The universal crossover functions interpolating between channelanisotropy or magnetic fieldinduced Fermi liquid regimes and the twochannel Kondo, nonFermi liquid regimes are determined numerically. The boundaries of the real 2CK scaling regime are found to be rather restricted, and to depend both on the type of the perturbation and on the specific operator whose correlation function is studied. In a small magnetic field, a universal resonance is observed in the local fermion's spectral function. The dominant superconducting instability appears in the composite superconducting channel. Comment: 20 pages, 24 figures, PRB format  [Show abstract] [Hide abstract]
ABSTRACT: We study finitefrequency transport properties of the doubledot system recently constructed to observe the twochannel Kondo effect [R. M. Potok et al., Nature 446, 167 (2007)]. We derive an analytical expression for the frequencydependent linear conductance of this device in the Kondo regime. We show how the features characteristic of the 2channel Kondo quantum critical point emerge in this quantity, which we compute using the results of conformal field theory as well as numerical renormalization group methods. We determine the universal crossover functions describing nonFermi liquid vs. Fermi liquid crossovers and also investigate the effects of a finite magnetic field.
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Institutions

20072008

Budapest University of Technology and Economics
 Department of Theoretical Physics
Budapest, Budapest fovaros, Hungary
