Anderson localization in Hubbard ladders

The effect of a weak random potential on two-leg Hubbard ladders is investigated. The random potential is shown to induce Anderson localization except for attractive enough interactions, supressing completely d-wave superconductivity. These localization effects remain very strong even for many ladders coupled by Josephson coupling. Both dc and ac conductivities and localization lengths are obtained. Consequences for the superconducting ladder compound Sr$_x$Ca$_{14-x}$Cu$_{24}$O$_{41+\delta}$ are discussed.Comment: 2 Pages, 2 Figures, Uses espcrc2.sty (included); Proceedings of the SCES98 conference, July 1998 Paris, France; To be published in Physica

Disordered Quantum smectics

We study the impurity pinning of the Quantum Hall (QH) smectic state arising in two dimensional electron systems in high Landau levels. We use replicas and a Gaussian Variational method to deal with the disorder. The pinned quantum smectic exhibits very anisotropic behaviour, with density correlations along the direction of the stripes manifesting a Bragg-Glass type behaviour i.e., quasi long range order whereas those in the transverse direction are infra red divergent. We calculate the dynamical conductivity along the stripe direction and find a wavelength dependent pinning peak.Comment: 5 pages, one eps figure, revtex

Magnetostriction in an array of spin chains under magnetic field

We consider an array of XX spin-1/2 chains coupled to acoustic phonons and placed in a magnetic field. Treating the phonons in the mean field approximation, we show that this system presents a first order transition as a function of the magnetic field between a partially magnetized distorted state and the fully polarized undistorted state at low temperature. This behavior results from the magnetostriction of the coupled chain system. A dip in the elastic constant of the material near the saturation field along with an anomaly in the magnetic susceptibility is predicted. We also predict the contraction of the material as the magnetic field is reduced (positive magnetostriction) and the reciprocal effect i.e. a decrease of magnetization under applied pressure. At higher temperature, the first order transition is replaced by a crossover. However, the anomalies in the susceptibilities in the system near the saturation field are still present. We discuss the relevance of our analysis in relation to recent experiments on spin-1/2 chain and ladder materials in strong magnetic fields.Comment: 23 pages, Revtex 4, more detailed discussion of the connection with Bose-Einstein condensation. Discussion of long range ordering by interladder exchange, comparison with mean field theory. Some extra references adde

Response functions in multicomponent Luttinger liquids

We derive an analytic expression for the zero temperature Fourier transform of the density-density correlation function of a multicomponent Luttinger liquid with different velocities. By employing Schwinger identity and a generalized Feynman identity exact integral expressions are derived, and approximate analytical forms are given for frequencies close to each component singularity. We find power-like singularities and compute the corresponding exponents. Numerical results are shown for the case of three components.Comment: 16 pages, two EPS figure

Competition of disorder and interchain hopping in a two-chain Hubbard model

We study the interplay of Anderson localization and interaction in a two chain Hubbard ladder allowing for arbitrary ratio of disorder strength to interchain coupling. We obtain three different types of spin gapped localized phases depending on the strength of disorder: a pinned $4k_F$ Charge Density Wave (CDW) for weak disorder, a pinned $2k_F$ CDW$^\pi$ for intermediate disorder and two independently pinned single chain $2k_F$ CDW for strong disorder. Confinement of electrons can be obtained as a result of strong disorder or strong attraction. We give the full phase diagram as a function of disorder, interaction strength and interchain hopping. We also study the influence of interchain hopping on localization length and show that localization is enhanced by a small interchain hopping but suppressed by a large interchain hopping.Comment: 12 pages, uses svjour and svepj macros, 8 EPS figure

Raman Scattering cross section of Spin Ladders

The Raman scattering spectra from magnetic excitations in an antiferromagnetic spin-1/2 two leg ladder is investigated for weak and strong interladder coupling. In the first case, a cusp in the Raman intensityis obtained at a frequency twice the gap. In the second case, a peak at twice the gap replaces the cusp. We discuss the relevance of our calculation to recent experiments on CaV2O5 and Sr14Cu24O41.Comment: RevTeX, 4 pages, 2 EPS Figure

Competition of superfluidity and density waves in one-dimensional Bose-Fermi mixtures

We study a mixture of one-dimensional bosons and spinless fermions at incommensurate filling using phenomenological bosonization and Green's functions techniques. We derive the relation between the parameters of the microscopic Hamiltonian and macroscopic observables. Galilean invariance results in extra constraints for the current current interactions. We obtain the exact exponents for the various response functions, and show that superfluid fluctuations are enhanced by the effective boson-fermion density-density interaction and suppressed by the effective boson-fermion current-current interaction. In the case of a bosonized model with purely density-density interaction, when the effective boson-fermion density-density interaction is weak enough, the superfluid exponent of the bosons has a non-monotonous variation with the ratio of the fermion velocity to the boson velocity. By contrast, density-wave exponent and the exponent for fermionic superfluidity are monotonous functions of the velocity ratio.Comment: 9 pages, RevTeX 4, 1 EPS figur