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1.
A. Hübsch K. W. Becker 《The European Physical Journal B - Condensed Matter and Complex Systems》2006,52(3):345-353
A very rich phase diagram has recently been found in CeCu2Si2 from
high pressure experiments where, in particular, a transition between an
intermediate valence configuration and an integral valent heavy fermion
state has been observed. We show that such a valence transition can be
understood in the framework of the periodic Anderson model. In particular,
our results show a breakdown of a mixed-valence state which is accompanied
by a drastic change in the f occupation in agreement with
experiment. This valence transition can possibly be interpreted as a
collapse of the large Fermi surface of the heavy fermion state which
incorporates not only the conduction electrons but also the localized
f electrons. The theoretical approach used in this paper
is based on the novel projector-based renormalization method (PRM). With
respect to the periodic Anderson model, the method was before only employed
in combination with the basic approximations of the well-known slave-boson
mean-field theory. In this paper, the PRM treatment is performed in a more
sophisticated manner where both mixed as well as integral valent solutions
have been obtained. Furthermore, we argue that the presented PRM approach
might be a promising starting point to study the competing interactions in
CeCu2Si2 and related compounds. 相似文献
2.
V.Yu. Irkhin A.V. Zarubin 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,16(3):463-468
The Kondo divergences owing to interaction of current carriers with local moments in highly correlated electron systems are
considered within the Hubbard and s-d exchange models with infinitely strong on-site interaction, the many-electron Hubbard representation being used. The picture
of density of states containing a peak at the Fermi level is obtained. Various forms of the self-consistent approximation
are used. The problem of the violation of analytical properties of the Green's function is discussed. Smearing of the “Kondo”
peak owing to spin dynamics and finite temperatures is investigated.
Received 25 November 1999 and Received in final form 31 January 2000 相似文献
3.
A. K. Zhuravlev V. Yu. Irkhin M. I. Katsnelson 《The European Physical Journal B - Condensed Matter and Complex Systems》2007,55(4):377-382
Using the numerical renormalization group method, the dependences on
temperature of the magnetic susceptibility χ(T) and specific heat C(T)
are obtained for the single-impurity Anderson model with inclusion of
d-f the Coulomb interaction. It is shown that the exciton effects
caused by this effect (charge fluctuations) can significantly change the
behaviour of C(T) in comparison with the standard Anderson model at
moderately low temperatures, whereas the behaviour of χ(T) remains nearly
universal. The ground-state and temperature-dependent renormalizations of the
effective hybridization parameter and f-level position caused by the
d-f interaction are calculated, and satisfactory agreement with the
Hartree-Fock approximation is derived. 相似文献
4.
S. Pairault D. Sénéchal A.-M.S. Tremblay 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,16(1):85-105
The strong-coupling perturbation theory of the Hubbard model is presented and carried out to order (t/U)5 for the one-particle Green function in arbitrary dimension. The spectral weight is expressed as a Jacobi continued fraction and compared with new Monte-Carlo data of the one-dimensional, half-filled Hubbard
model. Different regimes (insulator, conductor and short-range antiferromagnet) are identified in the temperature-hopping
integral (T,t) plane. This work completes a first paper on the subject (Phys. Rev. Lett. 80, 5389 (1998)) by providing details on diagrammatic rules and higher-order results. In addition, the non half-filled case,
infinite resummations of diagrams and the double occupancy are discussed. Various tests of the method are also presented.
Received 25 October 1999 相似文献
5.
B. Andraka R. Pietri S.G. Thomas G.R. Stewart E.-W. Scheidt T. Schreiner 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,12(1):55-59
A comprehensive study of the relationship between the electronic specific heat coefficient () and the temperature square coefficient (A) of the electrical resistivity for a single, cubic, heavy fermion alloy system, UPt5-xAux is presented. In this alloy system, whose low temperature properties are consistent with the Fermi-liquid behavior, varies by more than a factor of 10 while the corresponding A coefficient changes by a factor larger than 200. A tracks changes in fairly well, but , postulated to have a universal value for heavy fermions, is not constant and varies from about 10-6 (x = 0, 0.5) to 10-5
cm (mol K/mJ)2 (x
> 1.1), thus from a value typical of transition metals to that characteristic of other heavy fermion compounds. We have found a
correlation between and magnetic characteristics such as the paramagnetic Curie-Weiss temperature and the low temperature magnetic susceptibility
divided by .
Received 29 January 1999 相似文献
6.
F.F. Assaad M. Imada 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,10(4):595-598
The dispersion relation of a doped hole in the half-filled 2D Hubbard model is shown to follow a law around the and points in the Brillouin zone. Upon addition of pair-hopping processes this dispersion relation is unstable towards a law. The above follows from T=0 Quantum Monte-Carlo calculations of the single particle spectral function on lattices. We discuss finite dopings and argue that the added term restores coherence to charge dynamics and drives the system
towards a d
x2 - y2
superconductor.
Received 22 March 1999 相似文献
7.
H. A. Razafimandimby D. Randrianasoloharisoa A. Rakotomahevitra J. C. Parlebas 《The European Physical Journal B - Condensed Matter and Complex Systems》2009,68(4):495-500
In Rare-Earth (RE) systems, direct overlapping of 4f orbitals plays a role if the distance between RE ions is sufficiently
small. Especially, such a situation happens under pressure for RE metals and RE compounds. Furthermore, in Actinide systems,
5f orbitals are more extended than the previously considered 4f orbitals. Therefore, in this case, direct overlapping of 5f
orbitals should be taken into account even without applying pressure. In the present work, using Standard Basis Operators,
we study direct overlapping effect of f orbitals for valence fluctuating materials in Kondo regime. An important result within
our present model is that direct f orbital overlapping reduces f electron effective mass. 相似文献
8.
P. Schlottmann A.A. Zvyagin 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,5(3):325-335
We consider the one-dimensional t
-
J model, which consists of electrons with spin S on a lattice with nearest neighbor hopping t constrained by the excluded multiple occupancy of the lattice sites and spin-exchange J between neighboring sites. The model is integrable at the supersymmetric point, J
=
t. Without spoiling the integrability we introduce an Anderson-like impurity of spin S (degenerate Anderson model in the limit), which interacts with the correlated conduction states of the host. The lattice model is defined by the scattering
matrices via the Quantum Inverse Scattering Method. We discuss the general form of the interaction Hamiltonian between the impurity and
the itinerant electrons on the lattice and explicitly construct it in the continuum limit. The discrete Bethe ansatz equations
diagonalizing the host with impurity are derived, and the thermodynamic Bethe ansatz equations are obtained using the string
hypothesis for arbitrary band filling as a function of temperature and external magnetic field. The properties of the impurity
depend on one coupling parameter related to the Kondo exchange coupling. The impurity can localize up to one itinerant electron
and has in general mixed valent properties. Groundstate properties of the impurity, such as the energy, valence, magnetic
susceptibility and the specific heat coefficient, are discussed. In the integer valent limit the model reduces to a Coqblin-Schrieffer impurity.
Received: 31 December 1997 / Accepted: 17 March 1998 相似文献
9.
Th. Maier M. Jarrell Th. Pruschke J. Keller 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,13(4):613-624
We develop a Non-Crossing Approximation (NCA) for the effective cluster problem of the recently developed Dynamical Cluster
Approximation (DCA). The DCA technique includes short-ranged correlations by mapping the lattice problem onto a self-consistently
embedded periodic cluster of size . It is a fully causal and systematic approximation to the full lattice problem, with corrections in two dimensions. The NCA we develop is a systematic approximation with corrections . The method will be discussed in detail and results for the one-particle properties of the Hubbard model are shown. Near
half filling, the spectra display pronounced features including a pseudogap and non-Fermi-liquid behavior due to short-ranged
antiferromagnetic correlations.
Received 16 June 1999 相似文献
10.
Ph. Brune A.P. Kampf 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,18(2):241-245
We apply a diagrammatic expansion method around the atomic limit () for the U-t-t
' Hubbard model at half filling and finite temperature by means of a continued fraction representation of the one-particle
Green's function. From the analysis of the spectral function we find an energy dispersion relation with a modulation of the energy gap in the insulating phase. This anisotropy is compared with experimental ARPES results on insulating
cuprates.
Received 18 May 2000 and Received in final form 9 August 2000 相似文献
11.
P. Bonville E. Vincent E. Bauer 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,8(3):363-369
We present measurements in the YbCu5-xAlx series, down to the 50 mK range, using 170Yb M?ssbauer absorption spectroscopy and magnetisation measurements. In this series, the hybridisation between the Yb 4
f electrons and the conduction electrons is known to decrease as the Al content x increases. We apply the variational solution of the impurity Kondo problem to the interpretation of our data. We show that
the Kondo temperature can be derived from the measured 4
f quadrupole moment and, for the magnetically ordered compounds (), we obtain the exchange energy as a function of the Al content. Our findings are in general agreement with Doniach's model
describing the onset of magnetic ordering according to the relative values of the Kondo and exchange energy scales.
Received 16 April 1998 相似文献
12.
J.I. Espeso J. García Soldevilla J.A. Blanco J. Rodríguez Fernández J.C. Gómez Sal M.T. Fernández Díaz 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,18(4):625-632
The ground state of the strongly correlated electron CeNi1-xCux compounds has been investigated by means of neutron scattering experiments. Thus, magnetic diffraction was performed for
compounds showing long-range magnetic order (x
> 0.2). An evolution from a collinear ferromagnetic structure for x
=0.6 to a simple antiferromagnetic one for CeCu takes place through some more complex magnetic structures for intermediate compositions.
The magnetic moments are continuously reduced when the Ni content increases reflecting the progressive enhancement of the
Kondo screening. The large reduction found for x
=0.6 compound is discussed and the existence of a spin glass like component of the magnetic moment cannot be discarded. From the
quasielastic spectra, we have obtained the Kondo temperatures which are close to the magnetic ordering ones. The quasielastic
line-width evolves from a linear temperature dependence to a T
1/2
behaviour when approaching the non-magnetic limit. Then, this system provides an interesting example for the evolution of
unstable 4
f shell relaxation regimes when modifying the hybridisation strength.
Received 22 May 2000 相似文献
13.
M.A. Continentino 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,13(1):31-35
Recently we have shown that a one-parameter scaling, , describes the physical behavior of several heavy fermions in a region of their phase diagram. In this paper we fully characterize
this region, obtaining the uniform susceptibility, the resistivity and the specific heat in terms of the coherence temperature
. This allows for an explicit evaluation of the Wilson and the Kadowaki-Woods ratios in this regime. These quantities turn
out to be independent of the distance to the quantum critical point (QCP). The theory of the one-parameter scaling corresponds to a local interacting model. Although
spatial correlations are irrelevant in this case, time fluctuations are critically correlated as a consequence of the quantum
character of the transition.
Received 23 December 1998 and Received in final form 10 June 1999 相似文献
14.
T. Saikawa A. Ferraz 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,20(1):65-74
A precursor effect on the Fermi surface in the two-dimensional Hubbard model at finite temperatures near the antiferromagnetic
instability is studied using three different itinerant approaches: the second order perturbation theory, the paramagnon theory
(PT), and the two-particle self-consistent (TPSC) approach. In general, at finite temperature, the Fermi surface of the interacting
electron systems is not sharply defined due to the broadening effects of the self-energy. In order to take account of those
effects we consider the single-particle spectral function A(, 0) at the Fermi level, to describe the counterpart of the Fermi surface at T = 0. We find that the Fermi surface is destroyed close to the pseudogap regime due to the spin-fluctuation effects in both
PT and TPSC approaches. Moreover, the top of the effective valence band is located around = (π/2,π/2) in agreement with earlier investigations on the single-hole motion in the antiferromagnetic background. A crossover
behavior from the Fermi-liquid regime to the pseudogap regime is observed in the electron concentration dependence of the
spectral function and the self-energy.
Received 8 September 2000 and Received in final form 20 December 2000 相似文献
15.
M. Potthoff W. Nolting 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,8(4):555-568
The correlation-driven transition from a paramagnetic metal to a paramagnetic Mott-Hubbard insulator is studied within the
half-filled Hubbard model for a thin-film geometry. We consider simple-cubic films with different low-index surfaces and film
thickness d ranging from d=1 (two-dimensional) up to d=8. Using the dynamical mean-field theory, the lattice (film) problem is self-consistently mapped onto a set of d single-impurity Anderson models which are indirectly coupled via the respective baths of conduction electrons. The impurity models are solved at zero temperature using the exact-diagonalization
algorithm. We investigate the layer and thickness dependence of the electronic structure in the low-energy regime. Effects
due to the finite film thickness are found to be the more pronounced the lower is the film-surface coordination number. For
the comparatively open sc(111) geometry we find a strong layer dependence of the quasi-particle weight while it is much less
pronounced for the sc(110) and the sc(100) film geometries. For a given geometry and thickness d there is a unique critical interaction strength U
c2
(d) at which all effective masses diverge and there is a unique strength U
c1
(d) where the insulating solution disappears. U
c2
(d) and U
c1
(d) gradually increase with increasing thickness eventually approaching their bulk values. A simple analytical argument explains
the complete geometry and thickness dependence of Uc2. Uc1 is found to scale linearly with Uc2.
Received 19 August 1998 相似文献
16.
T. Saikawa A. Ferraz 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,3(1):17-22
Dynamical properties of the spin and charge response functions in the doped two-dimensional Hubbard model are calculated by
taking into account the drastic separation of the single-particle spectral function into the low-energy coherent and high-energy
incoherent parts due to the strong Coulomb interaction. We show that this evolution of the electronic states is the origin
of the broad and structureless feature in the charge response function. In the weak coupling regime the low-energy enhancement
of the spin excitation is produced which can be explained within the random phase approximation. However, for the larger interaction
close to the antiferromagnetic Stoner condition, the low-energy intensity of the spin excitation is suppressed.
Received: 25 September 1997 / Revised: 19 December 1997 / Accepted: 9 January 1998 相似文献
17.
P. Nozières 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,4(4):447-457
The so called exhaustion problem occurs when few electrons have to screen many spins in a metal with magnetic impurities.
A singlet Fermi liquid ground state is possible only if all impurities are “isotropized” in such a way as to suppress their
entropy. That takes a time and the corresponding energy limits the Fermi liquid range. The present note explores that issue of time and energy scales, and it concludes that is much smaller than the single impurity Kondo temperature. Similarly the relevant energy scale is proportional to the number
of electrons. Recent results on the Mott metal insulator transition in infinite dimension are reconsidered in the light of
these results: controversies in that respect are shown to reduce to a simple physical question, with no firm answer as to
now.
Received: 5 May 1998 / Received in final form and Accepted: 29 July 1998 相似文献
18.
M. Capone M. Grilli W. Stephan 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,11(4):551-557
We investigate polaron formation in a many-electron system in the presence of a local repulsion sufficiently strong to prevent
local-bipolaron formation. Specifically, we consider a Hubbard-Holstein model of interacting electrons coupled to dispersionless
phonons of frequency . Numerically solving the model in a small one-dimensional cluster, we find that in the nearly adiabatic case , the necessary and sufficient condition for the polaronic regime to occur is that the energy gain in the atomic (i.e., extremely localized) regime overcomes the energy of the purely electronic system . In the antiadiabatic case, , polaron formation is instead driven by the condition of a large ionic displacement (g being the electron-phonon coupling). Dynamical properties of the model in the weak and moderately strong coupling regimes
are also analyzed.
Received 15 February 1999 相似文献
19.
H. von Löhneysen A. Neubert T. Pietrus A. Schröder O. Stockert U. Tutsch M. Loewenhaupt A. Rosch P. Wölfle 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,5(3):447-455
We report on extensive elastic neutron scattering to determine the wave vector of the magnetic order in CeCu6-xAux single crystals for x
> 0.1. For all values of x investigated (0.2, 0.3, 0.5, 1.0) we find long-range incommensurate antiferromagnetic order with an ordering vector (0.625
0.275) for x
=0.2, nearly unchanged for x
=0.3, and (0.59) for x
=0.5, staying roughly the same for x
=1.0. In addition, short-range correlations are observed for x
=0.2, reminiscent of those found previously for x
=0.1. The ordered magnetic moment is found to increase rapidly for small x, and more slowly for the larger x values. The increase of the specific-heat anomaly at the ordering temperature with x is in qualitative accord with this behavior. Finally, data of the electrical resistivity for current flow along the three
crystallographic directions are presented, showing a clear signature of the magnetic order. A theoretical interpretation of
the interplay of magnetic order and transport in terms of (i) the partial suppression of the Kondo effect by the staggered
magnetization and (ii) the anisotropic band structure induced by the staggered field is shown to account well for the data,
provided the ordering vector is close to 2
k
F, where k
F is a typical Fermi momentum.
Received: 20 February 1998 / Accepted: 17 April 1998 相似文献
20.
L. Proville S. Aubry 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,15(3):405-421
We study the effective mass of the bipolarons and essentially the possibility to get both light and strongly bound bipolarons
in the Holstein-Hubbard model and some variations in the vicinity of the adiabatic limit. Several approaches to investigate
the quantum mobility of polarons and bipolarons are proposed for this model. First, the quantum fluctuations are treated as
perturbations of the mean-field (or adiabatic) approximation of the electron-phonon coupling in order to calculate the bipolaron
bands. It is found that the bipolaron mass generally remains very large except in the vicinity of the triple point of the
phase diagram (see [1]), where the bipolarons have several degenerate configurations at the adiabatic limit (single site (S0),
two sites (S1) and quadrisinglet (QS)), while the polarons are much lighter. This degeneracy reduces the bipolaron mass significantly.
Next we improve this result by variational methods (modified Toyozawa Exponential Ansatz or TEA) valid for larger quantum
perturbations away from the adiabatic limit. We first test this new method for the single polaron. We find that the triple
point of the phase diagram is washed out by the lattice quantum fluctuations which thus suppress the light bipolarons. Further
improvements of the method by hybridization of several TEA states do not change this conclusion. Next we show that some model
variations, for example a phonon dispersion may increase the stability of the (QS) bipolaron against the quantum lattice fluctuations.
We show that the triple point of the phase diagram may be stable to quantum lattice fluctuations and a very sharp mass reduction
may occur, leading to bipolaron masses of the order of 100 bare electronic mass for realistic parameters. Thus we argue that
such very light bipolarons could condense as a superconducting state at relatively high temperature when their interactions
are not too large, that is, their density is small enough. This effect might be relevant for understanding the origin of the
high superconductivity of doped cuprates far enough from half filling.
Received 15 September 1999 相似文献