511 research outputs found
Finite-temperature phase diagram of nonmagnetic impurities in high-temperature superconductors using a d=3 tJ model with quenched disorder
We study a quenched disordered d=3 tJ Hamiltonian with static vacancies as a
model of nonmagnetic impurities in high-Tc materials. Using a position-space
renormalization-group approach, we calculate the evolution of the
finite-temperature phase diagram with impurity concentration p, and find
several features with close experimental parallels: away from half-filling we
see the rapid destruction of a spin-singlet phase (analogous to the
superconducting phase in cuprates) which is eliminated for p > 0.05; in the
same region for these dilute impurity concentrations we observe an enhancement
of antiferromagnetism. The antiferromagnetic phase near half-filling is robust
against impurity addition, and disappears only for p > 0.40.Comment: 5 pages, 4 figures; replaced with published versio
Strongly Asymmetric Tricriticality of Quenched Random-Field Systems
In view of the recently seen dramatic effect of quenched random bonds on
tricritical systems, we have conducted a renormalization-group study on the
effect of quenched random fields on the tricritical phase diagram of the spin-1
Ising model in . We find that random fields convert first-order phase
transitions into second-order, in fact more effectively than random bonds. The
coexistence region is extremely flat, attesting to an unusually small
tricritical exponent ; moreover, an extreme asymmetry of the phase
diagram is very striking. To accomodate this asymmetry, the second-order
boundary exhibits reentrance.Comment: revtex, 4 pages, 2 figs, submitted to PR
d=3 Anisotropic and d=2 tJ Models: Phase Diagrams, Thermodynamic Properties, and Chemical Potential Shift
The anisotropic d=3 tJ model is studied by renormalization-group theory,
yielding the evolution of the system as interplane coupling is varied from the
isotropic three-dimensional to quasi-two-dimensional regimes.
Finite-temperature phase diagrams, chemical potential shifts, and in-plane and
interplane kinetic energies and antiferromagnetic correlations are calculated
for the entire range of electron densities. We find that the novel tau phase,
seen in earlier studies of the isotropic d=3 tJ model, and potentially
corresponding to the superconducting phase in high-T_c materials, persists even
for strong anisotropy. While the tau phase appears at low temperatures at
30-35% hole doping away from =1, at smaller hole dopings we see a complex
lamellar structure of antiferromagnetic and disordered regions, with a
suppressed chemical potential shift, a possible marker of incommensurate
ordering in the form of microscopic stripes. An investigation of the
renormalization-group flows for the isotropic two-dimensional tJ model also
shows a pre-signature of the tau phase, which appears with finite transition
temperatures upon addition of the smallest interplane coupling.Comment: 13 pages, 7 figures; replaced with published versio
Two Superconducting Phases in the d=3 Hubbard Model: Phase Diagram and Specific Heat from Renormalization-Group Calculations
The phase diagram of the d=3 Hubbard model is calculated as a function of
temperature and electron density n_i, in the full range of densities between 0
and 2 electrons per site, using renormalization-group theory. An
antiferromagnetic phase occurs at lower temperatures, at and near the
half-filling density of = 1. The antiferromagnetic phase is unstable to
hole or electron doping of at most 15%, yielding to two distinct "tau" phases:
for large coupling U/t, one such phase occurs between 30-35% hole or electron
doping, and for small to intermediate coupling U/t another such phase occurs
between 10-18% doping. Both tau phases are distinguished by non-zero hole or
electron hopping expectation values at all length scales. Under further doping,
the tau phases yield to hole- or electron-rich disordered phases. We have
calculated the specific heat over the entire phase diagram. The low-temperature
specific heat of the weak-coupling tau phase shows a BCS-type exponential
decay, indicating a gap in the excitation spectrum, and a cusp singularity at
the phase boundary. The strong-coupling tau phase, on the other hand, has
characteristics of BEC-type superconductivity, including a critical exponent
alpha approximately equal to -1, and an additional peak in the specific heat
above the transition temperature indicating pair formation. In the limit of
large Coulomb repulsion, the phase diagram of the tJ model is recovered.Comment: 16 pages, 10 figures; typos in Fig. 2 correcte
Equivalence of the Falicov-Kimball and Brandt-Mielsch forms for the free energy of the infinite-dimensional Falicov-Kimball model
Falicov and Kimball proposed a real-axis form for the free energy of the
Falicov-Kimball model that was modified for the coherent potential
approximation by Plischke. Brandt and Mielsch proposed an imaginary-axis form
for the free energy of the dynamical mean field theory solution of the
Falicov-Kimball model. It has long been known that these two formulae are
numerically equal to each other; an explicit derivation showing this
equivalence is presented here.Comment: 4 pages, 1 figure, typeset with ReVTe
The 3-d Random Field Ising Model at zero temperature
We study numerically the zero temperature Random Field Ising Model on cubic
lattices of various linear sizes in three dimensions. For each random field
configuration we vary the ferromagnetic coupling strength . We find that in
the infinite volume limit the magnetization is discontinuous in . The energy
and its first derivative are continuous. The approch to the thermodynamic
limit is slow, behaving like with for the gaussian
distribution of the random field. We also study the bimodal distribution , and we find similar results for the magnetization but with a
different value of the exponent . This raises the question of the
validity of universality for the random field problem.Comment: 8 pages, 3 PostScript Figure
Simple theory for spin-lattice relaxation in metallic rare earth ferromagnets
The spin-lattice relaxation time is a key quantity both for the
dynamical response of ferromagnets excited by laser pulses and as the speed
limit of magneto-optical recording. Extending the theory for the electron
paramagnetic resonance of magnetic impurities to spin-lattice relaxation in
ferromagnetic rare earths we calculate for Gd and find a value of
48 ps in very good agreement with time-resolved spin-polarized photoemission
experiments. We argue that the time scale for in metals is
essentially given by the spin-orbit induced magnetocrystalline anisotropy
energy.Comment: 18 pages revtex, 5 uuencoded figure
Fermion kinetics in the Falicov-Kimball limit of the three-band Emery model
The three-band Emery model is reduced to a single-particle quantum model of
Falicov-Kimball type, by allowing only up-spins to hop, and forbidding double
occupation by projection. It is used to study the effects of geometric
obstruction on mobile fermions in thermodynamic equilibrium. For low hopping
overlap, there appears a plateau in the entropy, due to charge correlations,
and related to real-space disorder. For large overlap, the equilibrium
thermopower susceptibility remains anomalous, with a sign opposite to the one
predicted from the single-particle density of states. The heat capacity and
non-Fermi liquid response are discussed in the context of similar results in
the literature. All results are obtained by evaluation of an effective
single-particle free-energy operator in closed form. The method to obtain this
operator is described in detail.Comment: New calculations, method explained in detail, 16 pages, 9 figure
Charge-density-wave order parameter of the Falicov-Kimball model in infinite dimensions
In the large-U limit, the Falicov-Kimball model maps onto an effective Ising
model, with an order parameter described by a BCS-like mean-field theory in
infinite dimensions. In the small-U limit, van Dongen and Vollhardt showed that
the order parameter assumes a strange non-BCS-like shape with a sharp reduction
near T approx T_c/2. Here we numerically investigate the crossover between
these two regimes and qualitatively determine the order parameter for a variety
of different values of U. We find the overall behavior of the order parameter
as a function of temperature to be quite anomalous.Comment: (5 pages, 3 figures, typeset with ReVTeX4
Comment on ``Spin Polarization and Magnetic Circular Dichroism in Photoemission from the 2p Core Level of Ferromagnetic Ni''
Although the Ni_4 cluster includes more information regarding the Ni band
structure with respect to the Anderson impurity model, it also favors very
peculiar ground states which are incompatible with a coherent picture of all
dichroism experiments.Comment: 1 page, RevTeX, 1 epsf figur
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