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 $a$ exceeds a critical value depending on the atom density $n$, $na^3>0.035$, 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, $na^3>0.0164$, 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 $N(q,\omega)$ of the two-dimensional $t-J$ model is studied starting from a mixed gauge formulation of the slave boson approach. Our results for $N(q, \omega)$ 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 $\text{Im}\Sigma({\bf k}^{\prime},\epsilon)\propto (\epsilon^{2}/ \epsilon_{F})\ln(\epsilon/ \epsilon_{F})$ 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 $\Delta K$ of the kinetic energy gain which is directly related to the spin wave stiffness. Without the Coulomb interaction, the perturbative analysis gives $\Delta K / (g^2/M\Omega^2) \cong 0.05-0.13$ depending on the electron number [$g$: electron-phonon(el-ph) coupling constant, $M$: mass of the oxygen atom, $\Omega$: frequency of the phonon]. Although there occurs many channels of the JT el-ph interaction in the multiband system, the final results of $\Delta K$ 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 $\Delta K$ is given by the lattice relaxation energy $E_{L}$ 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 $X$-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 $T_c$ 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 $T_c$ in the underdoped region. Treating for simplicity the flux phase as commensurate the superconducting and flux phases coexist in the underdoped region below $T_c$, whereas a pure flux phase exists above $T_c$ 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

Charge Transfer from Regularized Symmetry-Adapted Perturbation Theory

16 pages, 16 figure