Search

In recent years, quantum computing has evolved as an exciting frontier, with the development of numerous algorithms dedicated to constructing quantum circuits that adeptly represent quantum many-body states. However, this domain remains in its early stages and requires further refinement to understand better the effective construction of highly-entangled quantum states within quantum circuits. Here, we demonstrate that quantum many-body states can be universally represented using a quantum circuit comprising multi-qubit gates. Furthermore, we evaluate the efficiency of a quantum circuit constructed with two-qubit gates in quench dynamics for the transverse-field Ising model. In this specific model, despite the initial state being classical without entanglement, it undergoes long-time evolution, eventually leading to a highly-entangled quantum state. Our results reveal that a diamond-shaped quantum circuit, designed to approximate the multi-qubit gate-based quantum circuit, remarkably excels in accurately representing the long-time dynamics of the system. Moreover, the diamond-shaped circuit follows the volume law behavior in entanglement entropy, offering a significant advantage over alternative quantum circuit constructions employing two-qubit gates., Comment: 15 pages, 13 figures

We study the superfluid density of hard-core bosons on quasi-one-dimensional lattices using the quantum Monte Carlo method. Because of phase slippage, the superfluid density drops quickly to zero at finite temperatures with increasing the system length $\ell$ and the superfluid transition temperature is zero in one spatial dimension and also in quasi-one dimension in the limit of $\ell\rightarrow\infty$. We calculate the superfluid density of a model where no phase slippage is allowed and show that the superfluid density remains finite at finite temperatures even in the one-dimensional limit. We also discuss how finite superfluid density can be observed in a quasi-one-dimensional system using a torsional oscillator., Comment: 8 pages, 6figures

We adapt the block-Lanczos density-matrix renormalization-group technique to study the spin transport in a spin chain coupled to two non-interacting fermionic leads. As an example, we consider leads described by two-dimensional tight-binding models on a square lattice. Although the simulations are carried out using a chain representation of the leads, observables in the original two-dimensional lattice can be calculated by reversing the block-Lanczos transformation. This is demonstrated for leads with Rashba spin-orbit coupling., Comment: 6 pages, 6 figures, final version

The perovskite SrIrO3 is an exotic narrow-band metal owing to a confluence of the strengths of the spin-orbit coupling (SOC) and the electron-electron correlations. It has been proposed that topological and magnetic insulating phases can be achieved by tuning the SOC, Hubbard interactions, and/or lattice symmetry. Here, we report that the substitution of nonmagnetic, isovalent Sn4+ for Ir4+ in the SrIr1-xSnxO3 perovskites synthesized under high pressure leads to a metal-insulator transition to an antiferromagnetic (AF) phase at TN > 225 K. The continuous change of the cell volume as detected by x-ray diffraction and the lamda-shape transition of the specific heat on cooling through TN demonstrate that the metal-insulator transition is of second-order. Neutron powder diffraction results indicate that the Sn substitution enlarges an octahedral-site distortion that reduces the SOC relative to the spin-spin exchange interaction and results in the type-G AF spin ordering below TN. Measurement of high-temperature magnetic susceptibility shows the evolution of magnetic coupling in the paramagnetic phase typical of weak itinerant-electron magnetism in the Sn-substituted samples. A reduced structural symmetry in the magnetically ordered phase leads to an electron gap opening at the Brillouin zone boundary below TN in the same way as proposed by Slater., Comment: 15 pages, 4 figures

In this study, we systematically investigate 3D momentum($\hbar k$)-resolved electronic structures of Ruddlesden-Popper-type iridium oxides Sr$_{n+1}$Ir$_n$O$_{3n+1}$ using soft-x-ray (SX) angle-resolved photoemission spectroscopy (ARPES). Our results provide direct evidence of an insulator-to-metal transition that occurs upon increasing the dimensionality of the IrO$_2$-plane structure. This transition occurs when the spin-orbit-coupled $j_{\rm eff}$=1/2 band changes its behavior in the dispersion relation and moves across the Fermi energy. In addition, an emerging band along the $\Gamma$(0,0,0)-R($\pi$,$\pi$,$\pi$) direction is found to play a crucial role in the metallic characteristics of SrIrO$_3$. By scanning the photon energy over 350 eV, we reveal the 3D Fermi surface in SrIrO$_3$ and $k_z$-dependent oscillations of photoelectron intensity in Sr$_3$Ir$_2$O$_7$. In contrast to previously reported results obtained using low-energy photons, folded bands derived from lattice distortions and/or magnetic ordering make significantly weak (but finite) contributions to the $k$-resolved photoemission spectrum. At the first glance, this leads to the ambiguous result that the observed $k$-space topology is consistent with the unfolded Brillouin zone (BZ) picture derived from a non-realistic simple square or cubic Ir lattice. Through careful analysis, we determine that a superposition of the folded and unfolded band structures has been observed in the ARPES spectra obtained using photons in both ultraviolet and SX regions. To corroborate the physics deduced using low-energy ARPES studies, we propose to utilize SX-ARPES as a powerful complementary technique, as this method surveys more than one whole BZ and provides a panoramic view of electronic structures., Comment: 16 pages, 12 figures, 4 tables

We theoretically study the magnetization inside a normal metal induced in an s-wave superconductor/ferromagnetic metal/normal metal/ferromagnetic metal/s-wave superconductor (S/F1/N/F2/S) Josephson junction. Using quasiclassical Green's function method, we show that the magnetization becomes finite inside N. The origin of this magnetization is due to odd-frequency spin-triplet Cooper pairs formed by electrons of equal and opposite spins, which are induced by proximity effect in the S/F1/N/F2/S junction. We find that the magnetization M(d,q) in N can be decomposed into two parts, M(d,q)=MI(d)+MII(d,q), where q is the superconducting phase difference between two Ss and d is the thickness of N. MI(d) exists generally in S/F junctions, while MII(d,q) carries all q dependence and represents the fingerprint of phase coherence between two Ss in Josephson junctions. The q dependence thus allows us to control the magnetization in N by tuning q for a fixed d. We show that MI(d) weakly decreases with increasing d, while the q dependent magnetization MII(d,q) rapidly decays with d. Moreover, we find that the time-averaged magnetization exhibits discontinuous peak at each resonance DC voltage Vn=nhw_S/2e(n: integer) when DC voltage V as well as AC voltage v_ac(t) with frequency w_S are both applied to the S/F1/N/F2/S junction. This is because MII(d,q) oscillates generally in time t (AC magnetization) with dq/dt=2e[V+v_ac(t)]/h and thus =0, but can be converted into the time-independent DC magnetization for DC voltage at Vn. We also discuss that the magnetization induced in N can be measurably large in realistic systems. Therefore, the measurement of the induced magnetization serves as an alternative way to detect the phase coherence between two Ss in Josephson junctions. Our results also provide a basic concept for tunable magnetization in superconducting spintronics devices., Comment: 17 pages, 12 figures, to be published in Phys. Rev. B

We theoretically study spin current through ferromagnet (F) in a Josephson junction composed of s-wave superconductors and two layers of ferromagnets. Using quasiclassical theory, we show that the long-range spin current can be driven by the superconducting phase difference without voltage drop. The origin of this spin current is due to spin-triplet Cooper pairs (STCs) formed by electrons of equal-spin, which are induced by proximity effect inside the F. We find that the spin current carried by the STCs exhibits long-range propagation in the F even where the Josephson charge current is practically zero. We also show that this spin current persists over a remarkably longer distance than the ordinary spin current carried by spin polarized conduction electrons in the F. Our results thus indicate the promising potential of Josephson junctions based on multilayer ferromagnets for spintronics applications with long-range propagating spin current., Comment: 5 pages, 3 figures, to be published in Phys. Rev. Lett

The observation of a two dimensional electron gas (2DEG) (1, 2), superconductivity (3, 4), magnetic effects (5) and electronic phase separation (6-8) at the interfaces of insulating oxides, especially LaAlO3/SrTiO3, has further enhanced the potential of complex oxides for novel electronics. The occurrence of the 2DEG is strongly believed to be driven by the polarization discontinuity (9) at the interface between the two oxides. In this scenario, the crystal orientation plays an important role and no conductivity would be expected for e.g., the interface between LaAlO3 and (110)-oriented SrTiO3, which should not have a polarization discontinuity (10, 11). Here, we report the observation of unexpected conductivity at the LaAlO3/SrTiO3 interface prepared on (110)-oriented SrTiO3. The conductivity was further found to be strongly anisotropic, with the ratio of the conductance along the different directions parallel to the substrate surface showing a remarkable dependence on the oxygen pressure during deposition. The conductance and its anisotropy are discussed based on the atomic structure at the interface, as revealed by Scanning Transmission Electron Microscopy (STEM) and further supported by density functional theory (DFT) calculations., Comment: 17 pages, 5 figures

We propose a spin Hall device to induce a large spin Hall effect in a superconductor/normal metal (SN) junction. The side jump and skew scattering mechanisms are both taken into account to calculate the extrinsic spin Hall conductivity in the normal metal. We find that both contributions are anomalously enhanced when the voltage between the superconductor and the normal metal approaches to the superconducting gap. This enhancement is attributed to the resonant increase of the density of states in the normal metal at the Fermi level. Our results demonstrate a novel way to control and amplify the spin Hall conductivity by applying an external dc electric field, suggesting that a SN junction has a potential application for a spintronic device with a large spin Hall effect., Comment: 5 pages, 4 figures, To be published as a Rapid Communication in Physical Review B

Electric-field controlled exchange bias in a heterostructure composed of the ferromagnetic manganite La0.7Sr0.3MO3 and the ferroelectric antiferromagnetic BiFeO3 has recently been demonstrated experimentally. By means of a microscopic model Hamiltonian we provide a possible explanation of the origin of this magnetoelectric coupling. We find, in agreement with experimental results, a net ferromagnetic moment at the BiFeO3 interface. The induced ferromagnetic moment is the result of the competition between the e_g-electrons double exchange and the t_2g-spins antiferromagnetic superexchange that dominate in bulk BiFeO3. The balance of these simultaneous ferromagnetic and antiferromagnetic tendencies is strongly affected by the interfacial electronic charge density which, in turn, can be controlled by the BiFeO3 ferroelectric polarization., Comment: 4 pages, including 4 figures

A spin-tunnel-junction based on manganites, with La$_{1-x}$Sr$_x$MnO$_3$ (LSMO) as ferromagnetic metallic electrodes and the undoped parent compound LaMnO$_3$ (LMO) as insulating barrier, is here theoretically discussed using double exchange model Hamiltonians and numerical techniques. For an even number of LMO layers, the ground state is shown to have anti-parallel LSMO magnetic moments. This highly resistive, but fragile, state is easily destabilized by small magnetic fields, which orient the LSMO moments in the direction of the field. The magnetoresistance associated with this transition is very large, according to Monte Carlo and Density Matrix Renormalization Group studies. The influence of temperature, the case of an odd number of LMO layers, and the differences between LMO and SrTiO$_3$ as barriers are also addressed. General trends are discussed., Comment: 14 pages, 18 figures

In this manuscript, recent theoretical investigations by the authors in the area of oxide multilayers are briefly reviewed. The calculations were carried out using model Hamiltonians and a variety of non-perturbative techniques. Moreover, new results are also included here. They correspond to the generation of a metallic state by mixing insulators in a multilayer geometry, using the Hubbard and Double Exchange models. For the latter, the resulting metallic state is also ferromagnetic. This illustrates how electron or hole doping via transfer of charge in multilayers can lead to the study of phase diagrams of transition metal oxides in the clean limit. Currently, these phase diagrams are much affected by the disordering standard chemical doping procedure, which introduces quenched disorder in the material., Comment: 14 pages, 9 figures. Invited article for a special issue of JPCM on Metal Oxide Thin Films; minor changes in the text

The electron doping of undoped high-$T_c$ cuprates via the transfer of charge from manganites (or other oxides) using heterostructure geometries is here theoretically discussed. This possibility is mainly addressed via a detailed analysis of photoemission and diffusion voltage experiments, which locate the Fermi level of manganites above the bottom of the upper Hubbard band of some cuprate parent compounds. A diagram with the relative location of Fermi levels and gaps for several oxides is presented. The procedure discussed here is generic, allowing for the qualitative prediction of the charge flow direction at several oxide interfaces. The addition of electrons to antiferromagnetic Cu oxides may lead to a superconducting state at the interface with minimal quenched disorder. Model calculations using static and dynamical mean-field theory, supplemented by a Poisson equation formalism to address charge redistribution at the interface, support this view. The magnetic state of the manganites could be antiferromagnetic or ferromagnetic. The former is better to induce superconductivity than the latter, since the spin-polarized charge transfer will be detrimental to singlet superconductivity. It is concluded that in spite of the robust Hubbard gaps, the electron doping of undoped cuprates at interfaces appears possible, and its realization may open an exciting area of research in oxide heterostructures., Comment: 12 pages, 9 figures

Motivated by recent photoemission experiments on cuprates, the low-lying excitations of a strongly correlated superconducting state are studied numerically. It is observed that along the nodal direction these low-lying one-particle excitations show a linear momentum dependence for a wide range of excitation energies and, thus, they do not present a kink-like structure. The nodal Fermi velocity $v_{\rm F}$, as well as other observables, are systematically evaluated directly from the calculated dispersions, and they are found to compare well with experiments. It is argued that the parameter dependence of $v_{\rm F}$ is quantitatively explained by a simple picture of a renormalized Fermi velocity., Comment: 5 pages, 4 figures, to be published in Phys. Rev. Lett

Using a recently developed variational quantum Monte Carlo method, magnetic properties of high-$T_{\rm C}$ superconductors are studied at zero temperature ($T$), by numerical simulations on the 2D t-J model. Our focus here is to explore the difference in the properties of $p$-type and $n$-type cuprates as a function of the carrier concentrations close to half filling. As observed experimentally, it is found that the antiferromagnetically ordered phase persists even for a small, yet finite amount of carrier doping, and that this phase is more robust in the electron doped case., Comment: 2 pages, 2 figures

The unconventional low-lying spin excitations, recently observed in neutron scattering experiments on ${\rm Cs_2 Cu Cl_4}$, are explained with a spin liquid wave function. The dispersion relation as well as the wave vector of the incommensurate spin correlations are well reproduced within a projected BCS wave function with gapless and fractionalized spin-1/2 excitations around the nodes of the BCS gap function. The proposed wave function is shown to be very accurate for one-dimensional spin-1/2 systems, and remains similarly accurate in the two-dimensional model corresponding to ${\rm Cs_2 Cu Cl_4}$, thus representing a good ansatz for describing spin fractionalization in two dimensions., Comment: 5 pages, 4 figures. To appear in Phys. Rev. Lett

A comprehensive analysis of half-doped manganites is presented using Monte Carlo simulations applied to the double-exchange model with cooperative Jahn-Teller lattice distortions in two dimensions. A variety of novel results are reported: (i) The phase diagram is established in the lambda-J_ AF plane, with lambda the electron-phonon coupling and J_AF the antiferromagnetic exchange between classical t_2g spins. The results include standard phases, such as the CE-insulating and FM-metallic regimes, but they also unveil novel states, such as a ferromagnetic charge-ordered (CO) orbital-ordered phase compatible with recent experimental results by Loudon et al. (ii) For realistic couplings, it was observed that the charge disproportionation delta of the CO phase is far from the widely accepted extreme limit delta=0.5 of a 3+/4+ charge separation. A far smaller delta appears more realistic, in agreement with recent experiments by Garcia et al. and Daoud-Aladine it et al. (iii) Colossal magnetoresistance (CMR) effects are found in calculations of cluster resistances using the Landauer formalism. This occurs near the ubiquitous first-order phase transitions between the insulating and metallic states. (iv) The CE-state is found to be (very sensitive to disorder) since its long-range order rapidly disappears when quenched-disorder is introduced, contrary to the FM state which is more robust. This is also in qualitative agreement with recent experiments by Akahoshi et al. and Nakajima et al. (v) The phase diagram in the half-doped (electron doping) regime is briefly discussed as well. A charge-ordered state is found which is the analog of the x=0.5 CE phase. It contains a 3+/2+ charge arrangement at large lambda., Comment: 14 pages, 18 figures, submitted to PRB. Correction Ref. 11 (04/02/2003). Correction page 4, right column ("it was not presented...") (05/20/2003). Fig. 2 replaced, some new text added (06/27/2003). Changes in Ref.[7] and [24] (07/08/2003)

We introduce a path to a possibly new class of magnetic materials whose properties are determined entirely by the presence of a low concentration of specific point defects in their crystal structure. Using model Hamiltonian and ab-initio band structure methods we demonstrate that even large band gap nonmagnetic materials as simple as CaO can exhibit extraordinary properties like half metallic ferromagnetism upon introducing a small concentration of Ca vacancies. We show that such defects will initially bind the introduced charge carriers at neighboring sites and depending on the internal symmetry of the clusters formed by neighboring sites form "local" magnetic moments which for concentrations as low as 3% transform this non-magnetic insulator into a half metallic ferromagnet., Comment: 4 pages, 6 figures

The optical conductivity and Drude weight of a Spin-Fermion model for cuprates are studied as a function of electronic density and temperature. This model develops stripes and robust D-wave pairing correlation upon hole doping, and it has the advantage that it can be numerically simulated without sign problems. Both static and dynamical information can be obtained. In this work it was possible to analyze up to 12x12 site clusters at low temperatures ranging between 0.01t and 0.1t (between 50K and 500K for a hopping amplitude $t \sim 0.5 eV$). As the temperature is reduced, spectral weight is transferred from high to low frequencies in agreement with the behavior observed experimentally. Varying the hole density, the Drude weight has a maximum at the optimal doping for the model, i.e., at the density where the pairing correlations are stronger. It was also observed that the inverse of the Drude weight, roughly proportional to the resistivity, decreases linearly with the temperature at optimal doping, and it is abruptly reduced when robust pairing correlations develop upon further reducing the temperature. The behavior and general form of the optical conductivity are found to be in good agreement with experimental results for the cuprates. Our results also establish the Spin-Fermion model for cuprates as a reasonable alternative to the t-J model, which is much more difficult to study accurately., Comment: 5 pages, 5 figures

Pairing Correlations are studied numerically in the hole-doped spin-fermion model for cuprates. Simulations performed on up to 12x12 clusters provide robust indications of D-wave superconductivity away from half-filling. The pairing correlations are the strongest in the direction perpendicular to the dynamic stripe-like inhomogeneities that appear in the ground state at some densities. An optimal doping, where the correlations reach a maximum value, was observed at about 25% doping, in qualitative agreement with hight T_{c} cuprates' experiments. On the other hand, pairing correlations are suppressed by static stripe inhomogeneities., Comment: 4 pages, 4 figures

Using numerical techniques we study the spectral function $A(k,\omega)$ of a spin-fermion model for cuprates in the regime where magnetic and charge domains (stripes) are developed upon hole-doping. From $A(k,\omega)$ we study the electronic dynamics and determine the Fermi Surface (FS), which is compared with angular resolved photoemission results for $La_{2-x}Sr_xCuO_2$. A pseudogap is observed in the density of states at the chemical potential for all finite dopings. The striped ground state appears to be metallic in this model since there is finite spectral weight at the chemical potential, but the electronic hopping seems to be stronger perpendicular to the stripes rather than along them. The band structure is not rigid, contrary to the behavior found in mean-field studies, and changes with doping. Both mid-gap (stripe induced) and valence band states determine the FS. For vertical (horizontal) stripes, a clear FS appears close to $(\pi,0)$ $((0,\pi))$, while no FS is observed close to $(0,\pi)$ $((\pi,0))$. Along the diagonal direction the spectral function shows a clear quasi-particle peak close to (0,0), but its weight is reduced as the chemical potential is approached. A weak FS develops along this direction as the system is doped., Comment: 10 pages, 13 figures

We provide numerical evidence that a spin-flux phase exists as a ground state of the Kondo lattice model with classical local spins on a square lattice. This state manifests itself as a double-Q magnetic order in the classical spins with spin density at both $(0,\pi)$ and $(\pi,0)$ and further exhibits fermionic spin currents around an elementary plaquette of the square lattice. We examine the spin-wave spectrum of this phase. We further discus an extension to a face centered cubic (FCC) lattice where a spin-flux phase may also exist. On the FCC lattice the spin-flux phase manifests itself as a triple-Q magnetically ordered state and may exist in $\gamma$-Mn alloys., Comment: 5 pages, 3 figures

This paper is an informal review of the present status of the computational studies of models for manganites that recently reported the existence of phase separation at low temperatures (S. Yunoki et al., Phys. Rev. Lett. 80, 845 (1998) and cond-mat/9807149). Results for both the one-orbital and two-orbital models, the latter with Jahn-Teller phonons, are included. A large list of experimental results discussed in the paper are compatible with the existence of charge-inhomogeneities in the manganites, which may be an important ingredient for the explanation of the CMR effect in these compounds., Comment: 18 pages, Revtex, with 8 figures embedded. To appear in proceedings of the workshop ``Physics of Manganites'', Michigan State University, July 26--29, 1998, eds. by T. A. Kaplan and S. D. Mahanti, Plenum Publishing Corporation

The influence of a nearest-neighbor Coulomb repulsion of strength V on the properties of the Ferromagnetic Kondo model is analyzed using computational techniques. The Hamiltonian studied here is defined on a chain using localized S=1/2 spins, and one orbital per site. Special emphasis is given to the influence of the Coulomb repulsion on the regions of phase separation recently discovered in this family of models, as well as on the double-exchange-induced ferromagnetic ground state. When phase separation dominates at V=0, the Coulomb interaction breaks the large domains of the two competing phases into small ``islands'' of one phase embedded into the other. This is in agreement with several experimental results, as discussed in the text. Vestiges of the original phase separation regime are found in the spin structure-factor as incommensurate peaks, even at large values of V. In the ferromagnetic regime close to density $n=0.5$, the Coulomb interaction induces tendencies to charge ordering without altering the fully polarized character of the state. This regime of ``charge- ordered ferromagnetism'' may be related with experimental observations of a similar phase by C. H. Chen and S-W. Cheong (Phys. Rev. Lett. 76, 4042 (1996)). Our results reinforce the recently introduction notion (see e.g., S. Yunoki et al., Phys. Rev. Lett. 80, 845 (1998)) that in realistic models for manganites analyzed with unbiased many-body techniques, the ground state properties arise from a competition between ferromagnetism and phase-separation/charge-ordering tendencies., Comment: 11 pages with 14 ps figures embedded in the text

The two-orbital Kondo model with classical Jahn-Teller phonons is studied using Monte Carlo techniques. The observed phase diagram is rich, and includes a novel regime of phase separation induced by the orbital degrees of freedom. Experimental consequences of our results are discussed. In addition, the optical conductivity $\sigma(\omega)$ of the model is presented. It is shown to have several similarities with experimental measurements for manganites., Comment: Accepted for publication in Phys. Rev. Letters, revised version of the paper, 5 pages, Revtex, with 4 figures embedded

The phase diagram of the ferromagnetic Kondo model for manganites is investigated using computational techniques. In clusters of dimensions 1 and 2, Monte Carlo simulations in the limit where the localized spins are classical show a rich low temperature phase diagram with three dominant regions: (i) a ferromagnetic phase, (ii) phase separation between hole-poor antiferromagnetic and hole-rich ferromagnetic domains, and (iii) a phase with incommensurate spin correlations. Possible experimental consequences of the regime of phase separation are discussed. Studies using the Lanczos algorithm and the Density Matrix Renormalization Group method applied to chains with localized spin 1/2 (with and without Coulombic repulsion for the mobile electrons) and spin 3/2 degrees of freedom give results in excellent agreement with those in the spin localized classical limit. The Dynamical Mean Field ($D=\infty$) approximation was also applied to the same model. At large Hund coupling phase separation and ferromagnetism were identified, again in good agreement with results in low dimensions. In addition, a Monte Carlo study of spin correlations allowed us to estimate the critical temperature for ferromagnetism $T_c^{FM}$ in 3 dimensional clusters. It is concluded that $T_c^{FM}$ is compatible with current experimental results., Comment: 20pages, 27 embedded eps figures in the text

The Kondo lattice Hamiltonian with ferromagnetic Hund's coupling as a model for manganites is investigated. The classical limit for the spin of the (localized) $t_{2g}$ electrons is analyzed on lattices of dimension 1,2,3 and $\infty$ using several numerical methods. The phase diagram at low temperature is presented. A regime is identified where phase separation occurs between hole undoped antiferromagnetic and hole-rich ferromagnetic regions. Experimental consequences of this novel regime are discussed. Regions of incommensurate spin correlations have also been found. Estimations of the critical temperature in 3D are compatible with experiments., Comment: Accepted in Phys. Rev. Letters

The effects of a long range electronic potential on a one dimensional commensurate Charge Density Wave (CDW) state are investigated. Using numerical techniques it is shown that a transition to a metallic ground state is reached as the range of the electron-electron repulsion increases. In this metallic state, the optical conductivity exhibits a large Drude weight. Possible interpretations of our results are discussed., Comment: 5 pages, Revtex, minor misprints corrected and a reference to earlier work by V. Emery and C. Noguera added