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1.
R. Jursa S. Wermbter G. Czycholl 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,12(3):387-391
We study the influence of the short-ranged Hubbard correlation U between the conduction electrons on the Cooper pair formation in normal (s-wave) superconductors. The Coulomb correlation is considered within the standard second order perturbation theory, which
becomes exact in the weak coupling limit but goes beyond the simple Hartree-Fock treatment by yielding a finite lifetime of
the quasiparticles at finite temperature. An attractive pairing interaction V, which may be mediated by the standard electron-phonon mechanism, is considered between nearest neighbor sites. A critical
value for the attractive interaction is required to obtain a superconducting state. For finite temperature a gapless superconductivity is obtained due to the finite lifetime of the quasiparticles, i.e. the Coulomb correlation has a pair-breaking influence. The energy gap and depend very sensitively on U, V and band filling n and develop a maximum away from half filling as function of n. The ratio varies with n, being higher than the BCS value near half filling and reaching the BCS value for lower n.
Received 17 February 1999 相似文献
2.
Y. Kohori Y. Yamato Y. Iwamoto T. Kohara 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,18(4):601-604
We have carried out 115In nuclear quadrupole resonance (NQR) measurements in CeRhIn5. At ambient pressure, CeRhIn5 undergoes an antiferromagnetic AF phase transition at K. The 115In NQR spectrum has shown the appearance of a small internal field in the direction perpendicular to the tetragonal c-axis. With application of a hydrostatic pressure, the AF state is suppressed and the superconductivity appears just above
the critical pressure (P
= 17 kbar). The nuclear spin lattice relaxation rate 1/T1 of 115In measured at P
= 27 kbar indicates the occurrence of the superconductivity in the nearly AF region. In the superconducting state, 1/T1 has no Hebel-Slichter coherence peak just below of 2 K and has a power law T-dependence (T3) down to 300 mK. This is consistent with anisotropic superconductivity, with line nodes in the superconducting energy gap:
non-s-wave superconductivity occurs in CeRhIn5.
Received 5 July 2000 相似文献
3.
S. De Toro Arias X. Waintal J.-L. Pichard 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,10(1):149-158
The interplay between the quantum interferences responsible for one particle localization over a length L1, and the partial dephasing induced by a local interaction of strength U with another particle leading to partial delocalization over a length L
2
> L
1
, is illustrated by a study of the motion of two particles put close to each other at the initial time. Localization is reached
in two steps. First, before the time t1 necessary to propagate over L1, the interaction slows down the ballistic motion. On the contrary, after t1 the interaction favors a very slow delocalization, characterized by a spreading of the center of mass, until L2 is reached. This slow motion is related to the absence of quantum chaos in this one dimensional model, the interaction being
only able to induce weaker chaos with critical spectral statistics. Under appropriate initial conditions, the motion remains
invariant under the duality transformation mapping the behavior at small U onto the behavior at large U.
Received 24 August 1998 相似文献
4.
R. Gonczarek M. Krzyzosiak A. Gonczarek 《The European Physical Journal B - Condensed Matter and Complex Systems》2008,61(3):299-308
In this paper we find and present on diagrams in the coordinates of η=2t1/t0 (the ratio of the second and the first nearest
neighbor hopping integrals) and n (the carrier concentration) the
areas of stability for the superconducting spin-singlet s-
and d-wave and the spin-triplet p-wave order
parameters hatching out during the phase transition from the normal
to the superconducting phase. The diagrams are obtained for an
anisotropic two-dimensional superconducting system with a relatively
wide partially-filled conduction band. We study a tight-binding
model with an attractive nearest neighbor interaction with the
amplitude V1, and the on-site interaction (with the amplitude
V0) taken either as repulsive or attractive. The problem of the
coexistence of the s-, p- and d-wave order
parameters is addressed and solved for chosen values of the ratio
V0/V1. A possible island of stability of the d-wave
order parameter in the s-wave order parameter environment for
a relatively strong on-site interaction is revealed. The triple
points, around which the s-, d-, and p-wave
order parameters coexist, are localized on diagrams. It is shown
that results of the calculations performed for the two-dimensional
tight-binding band model are dissimilar with some obtained within
the BCS-type approximation. 相似文献
5.
W. Fettes I. Morgenstern 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,9(4):635-639
We perform numerical simulations of the Hubbard model using the projector Quantum Monte Carlo method. A novel approach for
finite size scaling is discussed. We obtain evidence in favor of d-wave superconductivity in the repulsive Hubbard model. For U=4, is roughly estimated as K.
Received 8 September 1998 相似文献
6.
7.
G.I. Japaridze S. Sarkar 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,28(2):139-144
A one-dimensional model of interacting electrons with on-site U, nearest-neighbor V, and pair-hopping interaction W is studied at half-filling using the continuum limit field theory approach. The ground state phase diagram is obtained for
a wide range of coupling constants. In addition to the insulating spin-density wave (SDW) and charge-density wave (CDW) phases
for large U and V, respectively, we identify a bond-charge-density-wave (BCDW) phase W < 0, | U - 2V| < | 2W| and a bond-spin-density-wave (BSDW) for W > 0, | U - 2V| < W. The possibility of bond-located ordering results from the site-off-diagonal nature of the pair-hopping term and is a special
feature of the half-filled band case. The BCDW phase corresponding to an enhanced Peierls instability in the system. The BdSDW
is an unconventional insulating magnetic phase, characterized by a gapless spin excitation spectrum and a staggered magnetization
located on bonds between sites. The general ground state phase diagram including insulating, metallic, and superconducting
phases is discussed. A transition to the η-superconducting phase at | U - 2V| ≪ 2t?W is briefly discussed.
Received 20 February 2002 / Received in final form 11 April 2002 Published online 19 July 2002 相似文献
8.
A numerical method is described for evaluating transverse spin correlations in the random phase approximation. Quantum spin-fluctuation
corrections to sublattice magnetization are evaluated for the antiferromagnetic ground state of the half-filled Hubbard model
in two and three dimensions in the whole U/t range. Extension to the case of defects in the AF is also discussed for spin vacancies and low-U impurities. In the limit, the vacancy-induced enhancement in the spin fluctuation correction is obtained for the spin-vacancy problem in two
dimensions, for vacancy concentration up to the percolation threshold. For low-U impurities, the overall spin fluctuation correction is found to be strongly suppressed, although surprisingly spin fluctuations
are locally enhanced at the low-U sites.
Received 27 April 1998 and Received in final form 13 August 1998 相似文献
9.
10.
M. Martínez J.-L. Pichard 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,30(1):93-100
The two dimensional crossover from independent particle towards collective motion is studied using 2 polarized electrons (spinless
fermions) interacting via a U/r Coulomb repulsion in a L×L square lattice with periodic boundary conditions and nearest neighbor hopping t. Three regimes characterize the ground state when U/t increases. Firstly, when the fluctuation Δr of the spacing r between the two particles is larger than the lattice spacing a, there is a scaling length L
0 = π2(t/U) such that the relative fluctuation Δr/〈r〉 is a universal function of the dimensionless ratio L/L
0, up to finite size corrections of order L-2. L < L
0 and L > L
0 are respectively the limits of the free particle Fermi motion and of the correlated motion of a Wigner molecule. Secondly,
when U/t exceeds a threshold U
*(L)/t, Δr becomes smaller than a, giving rise to a correlated lattice regime where the previous scaling breaks down and analytical expansions in powers of
t/U become valid. A weak random potential reduces the scaling length and favors the correlated motion.
Received 28 March 2002 Published online 19 November 2002 相似文献
11.
The article by Villain [Z. Phys. B — Condensed Matter33, 31 (1979)] is discussed and a modified magnetic phase diagram is suggested for the spinel system (AB2O4) in which theA andB sites are partially (or completely) occupied by magnetic atoms. This diagram takes into account the antiferromagnetic exchange interactionsJ
AA,J
BB andJ
AB between nearest neighbor cations of various types. Regions of paramagnetic, antiferromagnetic, ferrimagnetic and possible spin glass behaviour are indicated on the diagram.Supported by the National Science Foundation under Grant ISP-80-11451 相似文献
12.
J.E. Espinosa O. Navarro M. Avignon 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,18(1):9-16
A real-space method has been introduced to study the pairing problem within the generalized Hubbard Hamiltonian. This method
includes the bond-charge interaction term as an extension of the previously proposed mapping method [1] for the Hubbard model.
The generalization of the method is based on mapping the correlated many-body problem onto an equivalent site- and bond-impurity
tight-binding one in a higher dimensional space, where the problem can be solved exactly. In a one-dimensional lattice, we
analyzed the three particle correlation by calculating the binding energy at the ground state, using different values of the
bond-charge, the on-site (U) and the nearest-neighbor (V) interactions. A pairing asymmetry is found between electrons and holes for the generalized hopping amplitude, where the
hole pairing is not always easier than the electron case. For some special values of the hopping parameters and for all kinds
of interactions in the Hubbard Hamiltonian, an analytical solution is obtained.
Received 21 January 2000 and Received in final form 18 July 2000 相似文献
13.
The perfect-nesting instability towards antiferromagnetism of the Hubbard model is suppressed by next-nearest neighbor hopping t′. The asymptotic behavior of the critical coupling Uc(t′) at small t′ is calculated in dimensions d = 2,3, ∞ using Hartree theory; this yields the exact result at least in d > 2. The order of the transition is also determined. A region of stability of a metallic antiferromagnetic phase in d = 3 is identified. 相似文献
14.
R. Zeyher A. Greco 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,6(4):473-485
The feasibility of a perturbation expansion for Green's functions of the t-J model directly in terms of X-operators is demonstrated using the Baym-Kadanoff functional method. As an application we derive explicit expressions for
the kernel of the linearized equation for the superconducting order parameter in leading order of a 1/N expansion. The linearized equation is solved numerically on a square lattice taking instantaneous and retarded contributions
into account. Classifying the order parameter according to irreducible representations of the point group C4v of the square lattice and according to even or odd parity in frequency we find that a reasonably strong instability occurs
only for even frequency pairing with d-wavelike symmetry. The corresponding transition temperature Tc is where t is the nearest-neighbor hopping integral. The underlying effective interaction consists of an attractive, instantaneous term
and a retarded term due to charge and spin fluctuations. The latter is weakly attractive at low frequencies below ,strongly repulsive up to and attractive towards even higher energies. Tc increases with decreasing doping until a d-wavelike bond-order wave instability is encountered near optimal doping at for J=0.3. Tc is essentially linear in J and rather insensitive to an additional second-nearest neighbor hopping integral t'. A rather striking property of Tc is that it is hardly affected by the soft mode associated with the bond-order wave instability or by the Van Hove singularity
in the case with second-nearest neighbor hopping. This unique feature reflects the fact that the solution of the gap equation
involves momenta far away from the Fermi surface (due to the instantaneous term) and many frequencies (due to the retarded
term) so that singular properties in momentum or frequency are averaged out very effectively.
Received: 16 June 1998 / Accepted: 14 July 1998 相似文献
15.
The density-matrix renormalization group (DMRG) technique is used to study the ground-state properties of the one-dimensional half-filled Hubbard model with on-site (nearest-neighbor) repulsive interaction U (V) and nearest-neighbor hopping t. We calculate the static spin structure factor to consider the spin degrees of freedom. We notice a striking difference of the static spin structure factor among the spin-density-wave, charge-density-wave (CDW), and bond-order-wave (BOW) phases. Based on the results, we identify the BOW-CDW transition at small (large) U value as continuous (of first order). We also calculate the double occupancy to consider the charge degrees of freedom. For large U, the double occupancy show a discontinuous jump at the BOW-CDW critical point and it implies first-order transition. With decreasing U, the jump becomes smaller and vanishes at the tricritical point Ut≈5.961t. This value is close to our previous estimation Ut=5.89t obtained with other quantities. Consequently, the results of static spin structure factor and double occupancy support the accuracy of our ground-state phase diagram. 相似文献
16.
本文应用了Kirkwood方法去计算面心立方体的固溶体AB3的自由能。在这个方法中,自由能被表为(kT)-1的幂级数。我们的计算一直算到了(kT)-4的系数。如果称原子排列的秩为S,称忽略O(kT)-n的自由能为Fn,那末Fn与S的关系对于不同的n(n=2,3,4,5)是极不同的。事实上,F3,F5和S=0处始终为极小,使理论中看不到超点阵的结论。这说明自由能F对(kT)-1的展开的级数收敛极慢。将F表为η≡e(-(VAA+VBB-2VAB)/kT)-1的级数(式中VAA,VBB,VAB代表最近邻AA,BB,AB对的作用能)而称忽略O(ηn)的自由能为Fn′那末F2′,F3′依然不给我们超点阵的结论,但由F4′,F5′我们非但获得了超点阵,并也看到了S的突变及固溶体的潜热。F4′,F5′是极相似的,使我们相信它们近似于真正的F。 相似文献
17.
S. C. Bradley R. Bulla A. C. Hewson G -M. Zhang 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,11(4):535-550
The O(3) symmetric Anderson model is an example of a system which has a stable low energy marginal Fermi liquid fixed point
for a certain choice of parameters. It is also exactly equivalent, in the large U limit, to a localized model which describes the spin degrees of freedom of the linear dispersion two channel Kondo model.
We first use an argument based on conformal field theory to establish this precise equivalence with the two channel model.
We then use the numerical renormalization group (NRG) approach to calculate both one-electron and two-electron response functions
for a range of values of the interaction strength U. We compare the behaviours about the marginal Fermi liquid and Fermi liquid fixed points and interpret the results in terms
of a renormalized Majorana fermion picture of the elementary excitations. In the marginal Fermi liquid case the spectral densities
of all the Majorana fermion modes display a dependence on the lowest energy scale, and in addition the zero Majorana mode has a delta function contribution. The weight
of this delta function is studied as a function of the interaction U and is found to decrease exponentially with U for large U. Using the equivalence with the two channel Kondo model in the large U limit, we deduce the dynamical spin susceptibility of the two channel Kondo model over the full frequency range. We use renormalized
perturbation theory to interpret the results and to calculate the coefficient of the ln divergence found in the low frequency behaviour of the T=0 dynamic susceptibility.
Received 29 January 1999 相似文献
18.
S. Haddad R. Bennaceur 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,11(2):227-233
By means of perturbative renormalization approach we study the effect of relevant umklapp process on dimensional crossover
caused by interladder one particle hopping in weakly coupled two-leg Hubbard ladders with a half filled-band. We found that a crossover takes place at a finite value
which increases as the amplitude of umklapp process increases. For the system undergoes a phase transition to the spin density wave phase (SDW) via the two particle hopping process, while for the system undergoes a crossover to the two dimensional Fermi liquid phase via one particle hopping process.
Received 25 December 1998 相似文献
19.
M.B. Zölfl Th. Maier Th. Pruschke J. Keller 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,13(1):47-53
We study one-particle spectra and the electronic band-structure of a CuO
2
-plane within the three-band Hubbard model. The Dynamical Mean-Field Theory (DMFT) is used to solve the many particle problem.
The calculations show that the optical gap is given by excitations from the lower Hubbard band into the so called Zhang-Rice singlet band. The optical gap turns out to be considerably smaller than the bare charge transfer energy () for a typical set of parameters, which is in agreement with experiment. We also investigate the dependence of the shape
of the Fermi surface on the different hopping parameters t
CuO and t
OO. A value t
OO
/
t
CuO
>0 leads to a Fermi surface surrounding the M point.
Received 21 September 1998 and Received in final form 8 June 1999 相似文献
20.
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