共查询到20条相似文献,搜索用时 15 毫秒
1.
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
2.
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
3.
M. S. Figueira R. Franco 《The European Physical Journal B - Condensed Matter and Complex Systems》2007,58(1):1-10
In this paper we study the low temperature (T) properties of the Kondo insulator FeSi within the X-boson approach. We show
that the ground state of the FeSi is metallic and highly correlated with a large effective mass; the low temperature contributions
to the specific heat and the resistivity are of the Fermi-liquid type. The low temperature properties are governed by a reentrant
transition into a metallic state, that occurs when the chemical potential crosses the gap and enters the conduction band,
generating a metallic ground state. The movement of the chemical potential is due to the strong correlations present in the
system. We consider the low temperature regime of the Kondo insulator FeSi, where the hybridization gap is completely open.
In this situation we identify the two characteristic temperatures: the coherence temperature T0 and the Kondo temperature TKL. In the range T < T0, we identify a regime characterized by the formation of coherent states and Fermi-liquid behavior of the low temperature
properties; in the range TKL > T > T0, we identify a regime characterized by an activation energy. Within the X-boson approach we study those low temperature regimes
although we do not try to adjust parameters to recover the experimental energy scales. 相似文献
4.
5.
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
6.
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
7.
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
8.
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
9.
10.
T. Sommer 《The European Physical Journal B - Condensed Matter and Complex Systems》2006,52(3):315-327
The one-dimensional Kondo lattice model is investigated by means of Wegner's
flow equation method. The renormalization procedure leads to an effective
Hamiltonian which describes a free one-dimensional electron gas and a
Heisenberg chain. The localised spins of the effective model are coupled by
the well-known RKKY interaction. They are treated within a Schwinger boson
mean field theory which permits the calculation of static and dynamic
correlation functions. In the regime of small interaction strength static
expectation values agree well with the expected Luttinger liquid
behaviour. The parameter Kρ of the Luttinger liquid theory is estimated
and compared to recent results from density matrix renormalization group
studies. 相似文献
11.
P. Bonville M. Rams K. Królas J.-P. Sanchez P. C. Canfield O. Trovarelli C. Geibel 《The European Physical Journal B - Condensed Matter and Complex Systems》2007,55(1):77-84
We have examined the magnetic properties of the heavy electron compounds
YbAgGe and YbPtIn by 170Yb M?ssbauer spectroscopy down to 0.1 K, and
the crystal field properties of YbAgGe by Perturbed Angular Correlations
(PAC) measurements up to 900 K. In YbAgGe, we show that each of the two
magnetically ordered phases below 0.8 K involves a specific incommensurate
modulation of the Yb moment. An analysis of existing low temperature specific
heat data suggests the
persistence of fluctuations of the correlated Yb spins down to 0.1 K.
The PAC data allow to discriminate among proposed Yb3+ crystal field
level schemes. In YbPtIn, we show that the low temperature magnetic order
phase has an antiferro-para structure, where zero moment Yb ions coexist with
large moment ones, and that a 90° moment reorientation occurs at
1.4 K. 相似文献
12.
G. Uimin Y. Kudasov P. Fulde A. Ovchinnikov 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,16(2):241-244
We discuss the effects of an applied magnetic field on the low-energy excitations in the low temperature phase of Yb4As3. We show also why the magnetic interaction of the Yb3+ ions is nearly of an isotropic Heisenberg spin-1/2 type. A small anisotropy due to an intrachain dipolar interaction leads
to the opening of a gap when a magnetic field is applied. The model agrees with available experimental data. Simple experiments
are suggested in order to further test the present theory.
Received 3 February 1999 相似文献
13.
S.G. Magalh aes A.A. Schmidt A. Theumann B. Coqblin 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,30(4):419-425
The Kondo lattice model has been analyzed in the presence of a random inter-site interaction among localized spins with non
zero mean J0 and standard deviation J. Following the same framework previously introduced by us, the problem is formulated in the path integral formalism where
the spin operators are expressed as bilinear combinations of Grassmann fields. The static approximation and the replica symmetry
ansatz have allowed us to solve the problem at a mean field level. The resulting phase diagram displays several phase transitions
among a ferromagnetically ordered region,a spin glass one, a mixed phase and a Kondo state depending on J0, J and its relation with the Kondo interaction coupling JK. These results could be used to address part of the experimental data for the CeNi
1 - x
Cu
x compound, when x ⩽ 0.8.
Received 24 June 2002 Published online 31 December 2002 相似文献
14.
R. Franco J. Silva-Valencia M. S. Figueira 《The European Physical Journal B - Condensed Matter and Complex Systems》2009,67(2):159-167
In this paper we address the problem of the spectral weight transfer in Kondo insulators (KI). We employ the X-boson approach
for the periodic Anderson model, in the U →∞ limit. We calculate the two energy gaps of the system analytically: the indirect
gap, Δind = Eg ≃ Emir, present in the density of states, and the direct one Δdir, associated with the minimum energy necessary to produce inter-band transitions. We find that the optical behavior of the
system is governed by two energy scales: one of low frequency, characterized by Eg ≃ Emir, in the mid-infrared region (MIR), which is a reminiscent of the heavy fermion Emir peak, that appears in Kondo insulators as a broad maximum in the MIR region and that controls the low temperature transport
properties, the gap opening in optical conductivity and the formation of the Drude peak, at ω = 0, in the intermediate temperature
range. The other energy scale appears at high frequencies, and is characterized by the direct gap Δdir. According to our results, this peak controls the anomalous redistribution of spectral weight in the optical conductivity.
We apply the theory in order to study the Kondo insulator FeSi, and we calculate the optical conductivity of the system and
the spectral weight transfer in the optical conductivity. 相似文献
15.
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
16.
D. Meyer W. Nolting 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,18(3):385-395
We compare different approximation schemes for investigating ferromagnetism in the periodic Anderson model. The use of several
approximations allows for a detailed analysis of the implications of the respective methods, and also of the mechanisms driving
the ferromagnetic transition. For the Kondo limit, our results confirm a previously proposed mechanism leading to ferromagnetic
order, namely an RKKY exchange mediated via the formation of Kondo screening clouds in the conduction band. The contrary case is found in the intermediate-valence regime.
Here, the bandshift correction ensuring a correct high-energy expansion of the self-energy is essential. Inclusion of damping
effects reduces stability of the ferromagnetic phase.
Received 5 June 2000 and Received in final form 3 August 2000 相似文献
17.
C. Gilles P. Bonville K.K.W. Wong S. Mann 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,17(3):417-427
The magnetic behaviour of nanoparticles of antiferromagnetic artificial ferritin has been investigated by 57Fe M?ssbauer absorption spectroscopy and magnetization measurements, in the temperature range 2.5-250 K and with magnetic
fields up to 7 T. Samples containing nanoparticles with an average number of 57Fe atoms ranging from 400 to 2 500 were studied. By analysing the magnetic susceptibility and zero field M?ssbauer data, the
anisotropy energy per unit volume is found, in agreement with some of the earlier studies, to have a value typical for ferric
oxides, i.e. a few 105 ergs/cm3. By comparing the results of the two experimental methods at higher fields, we show that, contrary to what is currently assumed,
the uncompensated magnetization of the ferritin cores in the superparamagnetic regime does not follow a Langevin law. For
magnetic fields below the spin-flop field, we propose an approximate law for the field and temperature variation of the uncompensated
magnetization, which was early evoked by Néel but has so far never been applied to real antiferromagnetic systems. More generally,
this approach should apply to randomly oriented antiferromagnetic nanoparticles systems with weak uncompensated moments.
Received 20 January 2000 相似文献
18.
V.E. Smith D.E. Logan H.R. Krishnamurthy 《The European Physical Journal B - Condensed Matter and Complex Systems》2003,32(1):49-63
The symmetric periodic Anderson model is well known to capture the essential physics of Kondo insulator materials. Within
the framework of dynamical mean-field theory, we develop a local moment approach to its single-particle dynamics in the paramagnetic
phase. The approach is intrinsically non-perturbative, encompasses all energy scales and interaction strengths, and satisfies
the low-energy dictates of Fermi liquid theory. It captures in particular the strong coupling behaviour and exponentially
small quasiparticle scales characteristic of the Kondo lattice regime, as well as simple perturbative behaviour in weak coupling.
Particular emphasis is naturally given to strong coupling dynamics, where the resultant clean separation of energy scales
enables the scaling behaviour of single-particle spectra to be obtained.
Received 19 December 2002 Published online 14 March 2003 相似文献
19.
R. Calemczuk J. Riera D. Poilblanc J.-P. Boucher G. Chaboussant L. Lévy O. Piovesana 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,7(2):171-174
Specific heat (CV) measurements in the spin-1/2 Cu2(C2H12N2)2Cl4 system under a magnetic field up to H
=8.25 T are reported and compared to the results of numerical calculations based on the 2-leg antiferromagnetic Heisenberg ladder.
While the temperature dependences of both the susceptibility and the low-field specific heat are accurately reproduced by
this model, deviations are observed above the critical field HC1 at which the spin gap closes. In this Quantum High Field phase, the contribution of the low-energy quantum fluctuations are
stronger than in the Heisenberg ladder model. We argue that this enhancement can be attributed to dynamical lattice fluctuations.
Finally, we show that such a Heisenberg ladder, for H
>
H
C1, is unstable, when coupled to the 3D lattice, against a lattice distortion. These results provide an alternative explanation
for the observed low temperature ( K-0.8 K) phase (previously interpreted as a 3D magnetic ordering) as a new type of incommensurate gapped state.
Received: 23 July 1998 / Accepted: 24 August 1998 相似文献
20.
I. Souza Azevedo E. Baggio Saitovitch M. Hücker B. Büchner H. Micklitz 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,6(3):313-315
57Fe M?ssbauer effect studies of La1.65Eu0.20Sr0.15CuO4 doped with 0.5 at% 57Fe performed in the temperature region 300 K >
T
> 4.2 K give an onset temperature for magnetic ordering of K. This temperature practically is the same as that found in Nd doped La2-xSrxCuO4. It indicates that the magnetic ordering temperature in the LTT phase of rare earth (RE) doped La2-xSrxCuO4 is independent of the RE moment. The direction of the 57Fe magnetic moment in the magnetically ordered state is within the CuO2 plane, while it has been found to be parallel to the c-axis in Nd doped La2-xSrxCuO4.
Received: 23 June 1998 / Accepted: 14 July 1998 相似文献