169 research outputs found
Destruction of long-range antiferromagnetic order by hole doping
We study the renormalization of the staggered magnetization of a
two-dimensional antiferromagnet as a function of hole doping, in the framework
of the t-J model. It is shown that the motion of holes generates decay of spin
waves into ''particle-hole'' pairs, which causes the destruction of the
long-range magnetic order at a small hole concentration. This effect is mainly
determined by the coherent motion of holes. The value obtained for the critical
hole concentration, of a few percent, is consistent with experimental data for
the doped copper oxide high-Tc superconductors.Comment: 12 pages, 2 figure
Phase diagram of a Bose gas near a wide Feshbach resonance
In this paper, we study the phase diagram of a homogeneous Bose gas with a
repulsive interaction near a wide Feshbach resonance at zero temperature. The
Bose-Einstein-condensation (BEC) state of atoms is a metastable state. When the
scattering length exceeds a critical value depending on the atom density
, , the molecular excitation energy is imaginary and the atomic
BEC state is dynamically unstable against molecule formation. The BEC state of
diatomic molecules has lower energy, where the atomic excitation is gapped and
the molecular excitation is gapless. However when the scattering length is
above another critical value, , the molecular BEC state becomes a
unstable coherent mixture of atoms and molecules. In both BEC states, the
binding energy of diatomic molecules is reduced due to the many-body effect.Comment: 5 pages, 4 figure
Theory of the density fluctuation spectrum of strongly correlated electrons
The density response function of the two-dimensional
model is studied starting from a mixed gauge formulation of the slave boson
approach. Our results for are in remarkable agreement with exact
diagonalization studies, and provide a natural explanation of the anomalous
features in the density response in terms of the spin polaron nature of the
charge carriers. In particular we have identified unexplained low energy
structures in the diagonalization data as arising from the coherent polaron
motion of holes in a spin liquid.Comment: 4 pages with 4 figures, to be published in Physical Review B (RC
Normal Fermi Liquid Behavior of Quasiholes in the Spin-Polaron Model for Copper Oxides
Based on the t-J model and the self-consistent Born approximation, the
damping of quasiparticle hole states near the Fermi surface is calculated in a
low doping regime. Renormalization of spin-wave excitations due to hole doping
is taken into account. The damping is shown to be described by a familiar form
characteristic of the 2-dimensional
Fermi liquid, in contrast with the earlier statement reported by Li and Gong
[Phys. Rev. B {\bf 51}, 6343 (1995)] on the marginal Fermi liquid behavior of
quasiholes
Magnetic Properties of Weakly Doped Antiferromagnets
We study the spin excitations and the transverse susceptibility of a
two-dimensional antiferromagnet doped with a small concentration of holes in
the t-J model. The motion of holes generates a renormalization of the magnetic
properties. The Green's functions are calculated in the self-consistent Born
approximation. It is shown that the long-wavelength spin waves are
significantly softened and the shorter-wavelength spin waves become strongly
damped as the doping increases. The spin wave velocity is reduced by the
coherent motion of holes, and not increased as has been claimed elsewhere. The
transverse susceptibility is found to increase considerably with doping, also
as a result of coherent hole motion. Our results are in agreement with
experimental data for the doped copper oxide superconductors.Comment: 20 page
Cumulant approach to weakly doped antiferromagnets
We present a new approach to static and dynamical properties of holes and
spins in weakly doped antiferromagnets in two dimensions. The calculations are
based on a recently introduced cumulant approach to ground--state properties of
correlated electronic systems. The present method allows to evaluate hole and
spin--wave dispersion relations by considering hole or spin excitations of the
ground state. Usually, these dispersions are found from time--dependent
correlation functions. To demonstrate the ability of the approach we first
derive the dispersion relation for the lowest single hole excitation at
half--filling. However, the main purpose of this paper is to focus on the
mutual influence of mobile holes and spin waves in the weakly doped system. It
is shown that low-energy spin excitations strongly admix to the ground--state.
The coupling of spin waves and holes leads to a strong suppression of the
staggered magnetization which can not be explained by a simple rigid--band
picture for the hole quasiparticles. Also the experimentally observed doping
dependence of the spin--wave excitation energies can be understood within our
formalism.Comment: REVTEX, 25 pages, 7 figures (EPS), to be published in Phys. Rev.
Jahn-Teller effect and Electron correlation in manganites
Jahn-Teller (JT) effect both in the absence and presence of the strong
Coulomb correlation is studied theoretically focusing on the reduction of the kinetic energy gain which is directly related to the spin wave
stiffness. Without the Coulomb interaction, the perturbative analysis gives
depending on the electron number
[: electron-phonon(el-ph) coupling constant, : mass of the oxygen atom,
: frequency of the phonon]. Although there occurs many channels of
the JT el-ph interaction in the multiband system, the final results of roughly scales with the density of states at the Fermi energy. In the limit
of strong electron correlation, the magnitude of the orbital polarization
saturate and the relevant degrees of freedom are the direction (phase) of it.
An effective action is derived for the phase variable including the effect of
the JT interaction. In this limit, JT interaction is {\it{enhanced}} compared
with the non-interacting case, and is given by the lattice
relaxation energy for the localized electrons, although the electrons
remains itinerant. Discussion on experiments are given based on these
theoretical results.Comment: 24 pages, 7 figure
Interplay between superconductivity and flux phase in the t-J model
We study the phase diagram of the t-J model using a mean field type
approximation within the Baym-Kadanoff perturbation expansion for Hubbard
-operators. The line separating the normal state from a d-wave flux or
bond-order state starts near optimal doping at T=0 and rises quickly with
decreasing doping. The transition temperature for d-wave
superconductivity increases monotonically in the overdoped region towards
optimal doping. Near optimaldoping a strong competition between the two d-wave
order parameters sets in leading to a strong suppression of in the
underdoped region. Treating for simplicity the flux phase as commensurate the
superconducting and flux phases coexist in the underdoped region below ,
whereas a pure flux phase exists above with a pseudo-gap of d-wave
symmetry in the excitation spectrum. We also find that incommensurate
charge-density-wave ground states due to Coulomb interactions do not modify
strongly the above phase diagram near the superconducting phase, at least, as
long as the latter exists at all.Comment: 15 pages revtex, 8 postscript figures include
Density of states for dirty d-wave superconductors: A unified and dual approach for different types of disorder
A two-parameter field theoretical representation is given of a 2-dimensional
dirty d-wave superconductor that interpolates between the Gaussian limit of
uncorrelated weak disorder and the unitary limit of a dilute concentration of
resonant scatterers. It is argued that a duality holds between these two
regimes from which follows that a linearly vanishing density of states in the
Gaussian limit transforms into a diverging one in the unitary limit arbitrarily
close to the Fermi energy
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