15 research outputs found
Enhancement of pair correlation in a one-dimensional hybridization model
We propose an integrable model of one-dimensional (1D) interacting electrons
coupled with the local orbitals arrayed periodically in the chain. Since the
local orbitals are introduced in a way that double occupation is forbidden, the
model keeps the main feature of the periodic Anderson model with an interacting
host. For the attractive interaction, it is found that the local orbitals
enhance the effective mass of the Cooper-pair-like singlets and also the pair
correlation in the ground state. However, the persistent current is depressed
in this case. For the repulsive interaction case, the Hamiltonian is
non-Hermitian but allows Cooper pair solutions with small momenta, which are
induced by the hybridization between the extended state and the local orbitals.Comment: 11 page revtex, no figur
Evolution From Insulator (x=0.003) To Metal (x=1) Of The Eu 2+ Local Environment In Ca 1-xeu Xb 6
The local environment of Eu2+ (4 f7, S=72) in Ca1-x Eux B6 (0.003≤x≤1.00) is studied by means of electron spin resonance (ESR). For x≲0.07 the resonances have Lorentzian line shape, indicating an insulating environment for the Eu2+ ions. For x≳0.07, the lines broaden and become Dysonian in shape, suggesting a change to metallic environment for the Eu2+ ions, anticipating the semimetallic character of EuB6. The broadening is attributed to a spin-flip scattering relaxation process due to the exchange interaction between conduction and Eu2+ 4f electrons. High field ESR measurements for x≳0.30 reveal narrower and anisotropic linewidths, which are attributed to magnetic polarons and Fermi surface effects, respectively. © 2005 American Institute of Physics.9710Young, D.P., (1999) Nature (London), 397, p. 412Zhitomirsky, M.E., (1999) Nature (London), 402, p. 251Tromp, H.J., (2000) Phys. Rev. Lett., 87, p. 016401Massidda, S., Continenza, A., De Pascale, T.M., Monnier, R., (1997) Z. Phys. B: Condens. Matter, 102, p. 83Urbano, R.R., (2002) Phys. Rev. B, 65, p. 180407Bennett, M.C., (2004) Phys. Rev. B, 69, p. 132407Urbano, R.R., Pagliuso, P.G., Rettori, C., Oseroff, S.B., Sarrao, J.L., Schlottmann, P., Fisk, Z., (2004) Phys. Rev. B, 70, p. 140401Pake, G.E., Purcell, E.M., (1948) Phys. Rev., 74, p. 1184Bloembergen, N., (1952) J. Appl. Phys., 23, p. 1383Feher, G., Kip, A.F., (1955) Phys. Rev., 98, p. 337Dyson, F.J., (1955) Phys. Rev., 98, p. 349Sperlich, G., Jansen, K., (1974) Solid State Commun., 15, p. 1105Essam, J.W., (1972) Phase Transitions and Critical Phenomena, 2, p. 197. , Academic, LondonSchlottmann, P., Hellberg, C.S., (1996) J. Appl. Phys., 79, p. 6414Fisk, Z., (1979) J. Appl. Phys., 50, p. 1911Goodrich, R.G., Harrison, N., Vuillemin, J.J., Tekul, A., Hall, D.W., Fisk, Z., Young, D., Sarrao, J., (1998) Phys. Rev. B, 58, p. 14896Rhyee, J.-S., Cho, B.K., Ri, H.-C., (2003) Phys. Rev. B, 67, p. 125102Wigger, G.A., Beeli, C., Felder, E., Ott, H.R., Bianchi, A.D., Fisk, Z., (2004) Phys. Rev. Lett., 93, p. 14720
Critical exponents of a multicomponent anisotropic t-J model in one dimension
A recently presented anisotropic generalization of the multicomponent
supersymmetric model in one dimension is investigated. This model of
fermions with general spin- is solved by Bethe ansatz for the ground state
and the low-lying excitations. Due to the anisotropy of the interaction the
model possesses massive modes and one single gapless excitation. The
physical properties indicate the existence of Cooper-type multiplets of
fermions with finite binding energy. The critical behaviour is described by a
conformal field theory with continuously varying exponents depending on
the particle density. There are two distinct regimes of the phase diagram with
dominating density-density and multiplet-multiplet correlations, respectively.
The effective mass of the charge carriers is calculated. In comparison to the
limit of isotropic interactions the mass is strongly enhanced in general.Comment: 10 pages, 3 Postscript figures appended as uuencoded compressed
tar-file to appear in Z. Phys. B, preprint Cologne-94-474
How model sets can be determined by their two-point and three-point correlations
We show that real model sets with real internal spaces are determined, up to
translation and changes of density zero by their two- and three-point
correlations. We also show that there exist pairs of real (even one
dimensional) aperiodic model sets with internal spaces that are products of
real spaces and finite cyclic groups whose two- and three-point correlations
are identical but which are not related by either translation or inversion of
their windows. All these examples are pure point diffractive.
Placed in the context of ergodic uniformly discrete point processes, the
result is that real point processes of model sets based on real internal
windows are determined by their second and third moments.Comment: 19 page
Dynamics of Tunneling Centers in Metallic Systems
Dynamics of tunneling centers (TC) in metallic systems is studied, using the
technique of bosonization. The interaction of the TC with the conduction
electrons of the metal involves two processes, namely, the screening of the TC
by electrons, and the so-called electron assisted tunneling. The presence of
the latter process leads to a different form of the renormalized tunneling
frequency of the TC, and the tunneling motion is damped with a temperature
dependent relaxation rate. As the temperature is lowered, the relaxation rate
per temperature shows a steep rise as opposed to that in the absence of
electron assisted process. It is expected that this behavior should be observed
at very low temperatures in a careful experiment. The present work thus tries
to go beyond the existing work on the {\it dynamics} of a two-level system in
metals, by treating the electron assisted process.Comment: REVTeX twocolumn format, 5 pages, two PostScript figures available on
request. Preprint # : imsc 94/3
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.Comment: 13 pages, 9 figures, some references added/correcte
X-boson cumulant approach to the periodic Anderson model
The Periodic Anderson Model (PAM) can be studied in the infinite U limit by
employing the Hubbard X operators to project out the unwanted states. We have
already studied this problem employing the cumulant expansion with the
hybridization as perturbation, but the probability conservation of the local
states (completeness) is not usually satisfied when partial expansions like the
Chain Approximation (CHA) are employed. Here we treat the problem by a
technique inspired in the mean field approximation of Coleman's slave-bosons
method, and we obtain a description that avoids the unwanted phase transition
that appears in the mean-field slave-boson method both when the chemical
potential is greater than the localized level Ef at low temperatures (T) and
for all parameters at intermediate T.Comment: Submited to Physical Review B 14 pages, 17 eps figures inserted in
the tex
Theory of the first-order isostructural valence phase transitions in mixed valence compounds YbIn_{x}Ag_{1-x}Cu_{4}
For describing the first-order isostructural valence phase transition in
mixed valence compounds we develop a new approach based on the lattice Anderson
model. We take into account the Coulomb interaction between localized f and
conduction band electrons and two mechanisms of electron-lattice coupling. One
is related to the volume dependence of the hybridization. The other is related
to local deformations produced by f- shell size fluctuations accompanying
valence fluctuations. The large f -state degeneracy allows us to use the 1/N
expansion method. Within the model we develop a mean-field theory for the
first-order valence phase transition in YbInCu_{4}. It is shown that the
Coulomb interaction enhances the exchange interaction between f and conduction
band electron spins and is the driving force of the phase transition. A
comparison between the theoretical calculations and experimental measurements
of the valence change, susceptibility, specific heat, entropy, elastic
constants and volume change in YbInCu_{4} and YbAgCu_{4} are presented, and a
good quantitative agreement is found. On the basis of the model we describe the
evolution from the first-order valence phase transition to the continuous
transition into the heavy-fermion ground state in the series of compounds
YbIn_{1-x}Ag_{x}Cu_{4}. The effect of pressure on physical properties of
YbInCu_{4} is studied and the H-T phase diagram is found.Comment: 17 pages RevTeX, 9 Postscript figures, to be submitted to Phys.Rev.