998 research outputs found
Superconductivity in the Kondo lattice model
We study the Kondo lattice model with additional attractive interaction
between the conduction electrons within the dynamical mean-field theory using
the numerical renormalization group to solve the effective quantum impurity
problem. In addition to normal-state and magnetic phases we also allow for the
occurrence of a superconducting phase. In the normal phase we observe a very
sensitive dependence of the low-energy scale on the conduction-electron
interaction. We discuss the dependence of the superconducting transition on the
interplay between attractive interaction and Kondo exchange.Comment: Submitted to ICM 2009 Conference Proceeding
Charge gaps and quasiparticle bands of the ionic Hubbard model
The ionic Hubbard model on a cubic lattice is investigated using analytical
approximations and Wilson's renormalization group for the charge excitation
spectrum. Near the Mott insulating regime, where the Hubbard repulsion starts
to dominate all energies, the formation of correlated bands is described. The
corresponding partial spectral weights and local densities of states show
characteristic features, which compare well with a hybridized-band picture
appropriate for the regime at small , which at half-filling is known as a
band insulator. In particular, a narrow charge gap is obtained at half-filling,
and the distribution of spectral quasi-particle weight reflects the fundamental
hybridization mechanism of the model
An Enhanced Perturbational Study on Spectral Properties of the Anderson Model
The infinite- single impurity Anderson model for rare earth alloys is
examined with a new set of self-consistent coupled integral equations, which
can be embedded in the large expansion scheme ( is the local spin
degeneracy). The finite temperature impurity density of states (DOS) and the
spin-fluctuation spectra are calculated exactly up to the order . The
presented conserving approximation goes well beyond the -approximation
({\em NCA}) and maintains local Fermi-liquid properties down to very low
temperatures. The position of the low lying Abrikosov-Suhl resonance (ASR) in
the impurity DOS is in accordance with Friedel's sum rule. For its shift
toward the chemical potential, compared to the {\em NCA}, can be traced back to
the influence of the vertex corrections. The width and height of the ASR is
governed by the universal low temperature energy scale . Temperature and
degeneracy -dependence of the static magnetic susceptibility is found in
excellent agreement with the Bethe-Ansatz results. Threshold exponents of the
local propagators are discussed. Resonant level regime () and intermediate
valence regime () of the model are thoroughly
investigated as a critical test of the quality of the approximation. Some
applications to the Anderson lattice model are pointed out.Comment: 19 pages, ReVTeX, no figures. 17 Postscript figures available on the
WWW at http://spy.fkp.physik.th-darmstadt.de/~frithjof
Conserving approximations in direct perturbation theory: new semianalytical impurity solvers and their application to general lattice problems
For the treatment of interacting electrons in crystal lattices approximations
based on the picture of effective sites, coupled in a self-consistent fashion,
have proven very useful. Particularly in the presence of strong local
correlations, a local approach to the problem, combining a powerful method for
the short ranged interactions with the lattice propagation part of the
dynamics, determines the quality of results to a large extent. For a
considerable time the non crossing approximation (NCA) in direct perturbation
theory, an approach originally developed by Keiter for the Anderson impurity
model, built a standard for the description of the local dynamics of
interacting electrons. In the last couple of years exact methods like the
numerical renormalization group (NRG) as pioneered by Wilson, have surpassed
this approximation as regarding the description of the low energy regime. We
present an improved approximation level of direct perturbation theory for
finite Coulomb repulsion U, the crossing approximation one (CA1) and discuss
its connections with other generalizations of NCA. CA1 incorporates all
processes up to fourth order in the hybridization strength V in a
self-consistent skeleton expansion, retaining the full energy dependence of the
vertex functions. We reconstruct the local approach to the lattice problem from
the point of view of cumulant perturbation theory in a very general way and
discuss the proper use of impurity solvers for this purpose. Their reliability
can be tested in applications to e.g. the Hubbard model and the
Anderson-lattice model. We point out shortcomings of existing impurity solvers
and improvements gained with CA1 in this context.
This paper is dedicated to the memory of Hellmut Keiter.Comment: 45 pages, 22 figure
Field-induced phase transitions in a Kondo insulator
We study the magnetic-field effect on a Kondo insulator by exploiting the
periodic Anderson model with the Zeeman term. The analysis using dynamical mean
field theory combined with quantum Monte Carlo simulations determines the
detailed phase diagram at finite temperatures. At low temperatures, the
magnetic field drives the Kondo insulator to a transverse antiferromagnetic
phase, which further enters a polarized metallic phase at higher fields. The
antiferromagnetic transition temperature takes a maximum when the Zeeman
energy is nearly equal to the quasi-particle gap. In the paramagnetic phase
above , we find that the electron mass gets largest around the field where
the quasi-particle gap is closed. It is also shown that the induced moment of
conduction electrons changes its direction from antiparallel to parallel to the
field.Comment: 7 pages, 6 figure
Fast two-layer two-photon imaging of neuronal cell populations using an electrically tunable lens
Functional two-photon Ca2+-imaging is a versatile tool to study the dynamics of neuronal populations in brain slices and living animals. However, population imaging is typically restricted to a single two-dimensional image plane. By introducing an electrically tunable lens into the excitation path of a two-photon microscope we were able to realize fast axial focus shifts within 15 ms. The maximum axial scan range was 0.7 mm employing a 40x NA0.8 water immersion objective, plenty for typically required ranges of 0.2–0.3 mm. By combining the axial scanning method with 2D acousto-optic frame scanning and random-access scanning, we measured neuronal population activity of about 40 neurons across two imaging planes separated by 40 μm and achieved scan rates up to 20–30 Hz. The method presented is easily applicable and allows upgrading of existing two-photon microscopes for fast 3D scanning
The emergence of the unmarked: A new perspective on the language-specific function of Broca's Area
A number of neuroimaging studies have implicated an involvement of Broca's area, particularly of the pars opercularis of the left inferior frontal gyrus (IFG), in the processing of complex (permuted) sentences. However, functional interpretations of this region's role range from very general (e.g., in terms of working memory) to highly specific (e.g., as supporting particular types of syntactic operations). A dissociation of these competing accounts is often impossible because in most cases, the language internal complexity of permuted sentence structures is accompanied invariably by increasing costs of a more general cognitive nature (e.g., working memory, task difficulty, and acceptability). We used functional magnetic resonance imaging to explore the precise nature of the pars opercularis activation in the processing of permuted sentences by examining the permutation of pronouns in German. Although clearly involving a permutation operation, sentences with an initial object pronoun behave like simple, subject-initial sentences (e.g., in terms of acceptability) because of a rule stating that pronouns should generally precede non-pro-nominal arguments. The results of the experiment show that in contrast to non-pro-nominal permutations, sentences with a permuted pronoun do not engender enhanced pars opercularis activation. Our findings therefore speak against both language-related working memory and transformation-based accounts of this region's role in sentence comprehension. Rather, we argue that the pars opercularis of the left IFG supports the language-specific linearization of hierarchical linguistic dependencies
Low-energy properties of the Kondo lattice model
We study the zero-temperature properties of the Kondo lattice model within
the dynamical mean-field theory. As impurity solver we use the numerical
renormalization group. We present results for the paramagnetic case showing the
anticipated heavy Fermion physics, including direct evidence for the appearance
of a large Fermi surface for antiferromagnetic exchange interaction. Allowing
for the formation of a Neel state, we observe at finite doping an
antiferromagnetic metal below a critical exchange interaction, which shows a
crossover from a local-moment antiferromagnet with a small Fermi surface for
weak exchange coupling to a heavy-fermion antiferromagnet with a large Fermi
surface for increasing exchange. Including lattice degrees of freedom via an
additional Holstein term we observe a significant suppression of the Kondo
effect, leading to strongly reduced lowenergy scale. For too large
electron-phonon coupling we find a complete collaps of the heavy Fermi liquid
and the formation of polarons.Comment: 11 pages, 7 figure
Inelastic Neutron scattering in CeSi_{2-x}Ga_x ferromagnetic Kondo lattice compounds
Inelastic neutron scattering investigation on ferromagnetic Kondo lattice
compounds belonging to CeSi_{2-x}Ga_{x}, x = 0.7, 1.0 and 1.3, system is
reported. The thermal evolution of the quasielastic response shows that the
Kondo interactions dominate over the RKKY interactions with increase in Ga
concentration from 0.7 to 1.3. This is related to the increase in k-f
hybridization with increasing Ga concentration. The high energy response
indicates the ground state to be split by crystal field in all three compounds.
Using the experimental results we have calculated the crystal field parameters
in all three compounds studied here.Comment: 12 Pages Revtex, 2 eps figures
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