68 research outputs found
THE PHASE DIAGRAM FOR THE SINE-GORDON MODEL WITH TWO UMKLAPP TERMS
We study the Landau free energy for a uniaxial ordering, taking into account
two Umklapp terms of comparable strengths (those of the third and fourth
order). Exploring the analogy with the well-known nonintegrable classical
mechanical problem of two mixed nonlinear resonances, we complete the previous
studies of the corresponding phase portrait by calculating numerically periodic
solutions, including those far from the separatrices. It is shown that in the
physical range of parameters only periodic configurations are absolutely
stable. We determine for the first time the complete thermodynamic phase
diagram and show that, in contrast to some earlier claims, the wave number of
the ordering does not pass through the devil's staircase, but through a finite
number of steps which decreases as the amplitudes of the Umklapp terms
increase.Comment: 10 pages, RevTeX 3.0, 5 PostScript figures uuencoded and compressed,
to be published in Phys. Letters
Nuclear Magnetic Relaxation Rate in a Noncentrosymmetric Superconductor
For a noncentrosymmetric superconductor such as CePt3Si, we consider a Cooper
pairing model with a two-component order parameter composed of spin-singlet and
spin-triplet pairing components.
We demonstrate that such a model on a qualitative level accounts for
experimentally observed features of the temperature dependence of the nuclear
spin-lattice relaxation rate 1/T1, namely a peak just below Tc and a line-node
gap behavior at low temperatures.Comment: 4 page
General criteria for the stability of uniaxially ordered states of Incommensurate-Commensurate Systems
Reconsidering the variational procedure for uniaxial systems modeled by
continuous free energy functionals, we derive new general conditions for
thermodynamic extrema. The utility of these conditions is briefly illustrated
on the models for the classes I and II of incommensurate-commensurate systems.Comment: 5 pages, to be published in Phys. Rev. Let
Spectral properties of orbital polarons in Mott insulators
We address the spectral properties of Mott insulators with orbital degrees of
freedom, and investigate cases where the orbital symmetry leads to Ising-like
superexchange in the orbital sector. The paradigm of a hole propagating by its
coupling to quantum fluctuations, known from the spin t-J model, then no longer
applies. We find instead that when one of the two orbital flavors is immobile,
as in the Falicov-Kimball model, trapped orbital polarons coexist with free
hole propagation emerging from the effective three-site hopping in the regime
of large on-site Coulomb interaction U. The spectral functions are found
analytically in this case within the retraceable path approximation in one and
two dimensions. On the contrary, when both of the orbitals are active, as in
the model for electrons in two dimensions, we find propagating
polarons with incoherent scattering dressing the moving hole and renormalizing
the quasiparticle dispersion. Here, the spectral functions, calculated using
the self-consistent Born approximation, are anisotropic and depend on the
orbital flavor. Unbiased conclusions concerning the spectral properties are
established by comparing the above results for the orbital t-J models with
those obtained using the variational cluster approximation or exact
diagonalization for the corresponding Hubbard models. The present work makes
predictions concerning the essential features of photoemission spectra of
certain fluorides and vanadates.Comment: 26 pages, 16 figures; to appear in Physical Review
Conservation of connectivity of model-space effective interactions under a class of similarity transformation
Effective interaction operators usually act on a restricted model space and
give the same energies (for Hamiltonian) and matrix elements (for transition
operators etc.) as those of the original operators between the corresponding
true eigenstates. Various types of effective operators are possible. Those well
defined effective operators have been shown being related to each other by
similarity transformation. Some of the effective operators have been shown to
have connected-diagram expansions. It is shown in this paper that under a class
of very general similarity transformations, the connectivity is conserved. The
similarity transformation between hermitian and non-hermitian
Rayleigh-Schr\"{o}dinger perturbative effective operators is one of such
transformation and hence the connectivity can be deducted from each other.Comment: 12 preprint page
Superfluid Interfaces in Quantum Solids
One scenario for the non-classical moment of inertia of solid He-4 discovered
by Kim and Chan [Nature 427, 225 (2004)] is the superfluidity of
micro-crystallite interfaces. On the basis of the most simple model of a
quantum crystal--the checkerboard lattice solid--we show that the superfluidity
of interfaces between solid domains can exist in a wide range of parameters. At
strong enough inter-particle interaction, a superfluid interface becomes an
insulator via a quantum phase transition. Under the conditions of particle-hole
symmetry, the transition is of the standard U(1) universality class in 3D,
while in 2D the onset of superfluidity is accompanied by the interface
roughening, driven by fractionally charged topological excitations.Comment: 4 revtex4 page
Ising and Bloch domain walls in a two-dimensional parametrically driven Ginzburg-Landau equation model with nonlinearity management
d-wave pairing symmetry in cuprate superconductors
Phase-sensitive tests of pairing symmetry have provided strong evidence for
predominantly d-wave pairing symmetry in both hole- and electron-doped high-Tc
cuprate superconductors. Temperature dependent measurements in YBCO indicate
that the d-wave pairing dominates, with little if any imaginary component, at
all temperatures from 0.5K through Tc. In this article we review some of this
evidence and discuss the implications of the universal d-wave pairing symmetry
in the cuprates.Comment: 4 pages, M2S 2000 conference proceeding
Quantum Interference of Coulomb Interaction and Disorder: Phase Shift of Friedel Oscillations and an Instability of the Fermi Sea
We investigate the influence of interference between Coulomb interaction and
impurity scattering on the static electronic response in
disordered metals to leading order in the effective Coulomb interaction. When
the transport relaxation time is much shorter than the
quasiparticle life time, we find a \mbox{sgn}(2p_F-q)/\sqrt{|2p_F-q|}
divergence of the polarization function at the Fermi surface (). It
causes a phase shift of the Friedel oscillations as well as an enhancement of
their amplitude. Our results are consistent with experiments and may be
relevant for understanding the stability of the amorphous state of certain
alloys against crystallization.Comment: 11 pages, 4 PostScript figures appended as a self-extracting tar
archive; includes output instruction
Interplay of spin density wave and superconductivity with different pairing symmetry
A model study for the coexistence of the spin density wave and
superconductivity is presented. With reference to the recent angle resolved
photo emmission experimental data in high T_c cuprates, presence of the nested
pieces of bands is assumed. The single band Hubbard model, therefore, when
treated within the Hatree-Fock mean field theory leads to a spin density wave
(SDW) ground state. The superconductivity (SC) is assumed to be due to a
generalised attractive potential with a separable form without specifying to
any particular origin. It therefore allows a comparative study of the
coexistence of superconductivity of different order parameter symmetry with the
spin density wave state. We find that the phase diagram, comprising of the
amplitudes of the respective gaps (SC and SDW) Vs. band filling resembles to
that of the high T_c cuprates only when the order parameter of the
superconducting phase has d-wave symmetry. Thermal variation of different order
parameters (e.g, SC and SDW) also show interesting coexistence and reentrance
behaviors that are consistent with experimental observations, specially for the
borocarbides.Comment: 8 pages, 6 figures (postscript attached), Physica C (in press
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