42 research outputs found
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
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
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
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
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
A novel superconducting glass state in disordered thin films in Clogston limit
A theory of mesoscopic fluctuations in disordered thin superconducting films
in a parallel magnetic field is developed. At zero temperature, the
superconducting state undergoes a phase transition into a state characterized
by superfluid densities of random signs, instead of a spin polarized disordered
Fermi liquid phase. Consequently, the ground state belongs to the same
universality class as the 2D XY spin glass. As the magnetic field increases
further, mesoscopic pairing states are nucleated in an otherwise homogeneous
spin polarized disordered Fermi liquid. The statistics of these pairing states
is universal depending on the sheet conductance of the 2D film.Comment: Latex, 39 pages, 2 figures included; to appear in Int. J. Mod. Phys.
Interference of a first-order transition with the formation of a spin-Peierls state in alpha'-NaV2O5?
We present results of high-resolution thermal-expansion and specific-heat
measurements on single crystalline alpha'-NaV2O5. We find clear evidence for
two almost degenerate phase transitions associated with the formation of the
dimerized state around 33K: A sharp first-order transition at T1=(33+-0.1)K
slightly below the onset of a second-order transition at T2onset around
(34+-0.1)K. The latter is accompanied by pronounced spontaneous strains. Our
results are consistent with a structural transformation at T1 induced by the
incipient spin-Peierls (SP) order parameter above T2=TSP.Comment: 5 pages, 7 figure
Frustrated kinetic energy, the optical sum rule, and the mechanism of superconductivity
The theory that the change of the electronic kinetic energy in a direction
perpendicular to the CuO-planes in high-temperature superconductors is a
substantial fraction of the condensation energy is examined. It is argued that
the consequences of this theory based on a rigorous -axis conductivity sum
rule are consistent with recent optical and penetration depth measurements.Comment: 4 pages (RevTeX) and 2 eps figure
Collective modes in uniaxial incommensurate-commensurate systems with the real order parameter
The basic Landau model for uniaxial systems of the II class is nonintegrable,
and allows for various stable and metastable periodic configurations, beside
that representing the uniform (or dimerized) ordering. In the present paper we
complete the analysis of this model by performing the second order variational
procedure, and formulating the combined Floquet-Bloch approach to the ensuing
nonstandard linear eigenvalue problem. This approach enables an analytic
derivation of some general conclusions on the stability of particular states,
and on the nature of accompanied collective excitations. Furthermore, we
calculate numerically the spectra of collective modes for all states
participating in the phase diagram, and analyze critical properties of
Goldstone modes at all second order and first order transitions between
disordered, uniform and periodic states. In particular it is shown that the
Goldstone mode softens as the underlying soliton lattice becomes more and more
dilute.Comment: 19 pages, 16 figures, REVTeX, to be published in Journal of Physics
A: Mathematical and Genera