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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

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

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., 5 pages, 5 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

We show, by using a correlated Jastrow wave function and a mapping onto a classical model, that the two-dimensional Mott transition in a simple half-filled one-band model can be unconventional and very similar to the binding-unbinding Kosterlitz-Thouless transition of vortices and anti-vortices, here identified by empty and doubly occupied sites. Within this framework, electrons strongly interact with collective plasmon excitations that induce anomalous critical properties on both sides of the transition. In particular, the insulating phase is characterized by a singular power law behavior in the photoemission spectrum, that can be continuously connected to the fully-projected insulating state, relevant to strongly correlated low-energy models., Comment: Reviewed and enlarged version, 6 pages, 7 figures