Search

It was recently discovered that a conductive, metallic state is formed on the surface of some insulating oxides. Firstly observed on SrTiO$_3$(001), it was then found in other compounds as diverse as anatase TiO$_2$, KTaO$_3$, BaTiO$_3$, ZnO, and also on different surfaces of SrTiO$_3$ (or other oxides) with different symmetries. The spatial extension of the wave function of this electronic state is of only a few atomic layers. Experiments indicate its existence is related to the presence of oxygen vacancies induced at or near the surface of the oxide. In this article we present a simplified model aimed at describing the effect of its small spatial extension on measurements of its 3D electronic structure by angular resolved photoemission spectroscopy (ARPES). For the sake of clarity, we base our discussion on a simple tight binding scheme plus a confining potential that is assumed to be induced by the oxygen vacancies. Our model parameters are, nevertheless, obtained from density functional calculations. With this methodology we can explain from a very simple concept of selective interference the "wobbling", i.e., the photoemission intensity modulation and/or apparent dispersion of the Fermi surface and spectra along the out-of-plane ($k_z$) direction, and the "mixed 2D/3D" characteristics observed in some experiments. We conclude that the critical model parameters for such an effect are the relative strength of the electronic hopping of each band and the height/width aspect ratio of the surface confining potential. By considering recent photoemission measurements under the light of our findings, we can get relevant information on the electronic wave functions and of the nature of the confining potential., Comment: Accepted to Physical Review B

We report the existence of a two-dimensional electron system (2DES) at the $(001)$ surface of ${\mathrm{CaTiO}}_{3}$. Using angle-resolved photoemission spectroscopy, we find a hybridization between the ${d}_{xz}$ and ${d}_{yz}$ orbitals, not observed in the 2DESs at the surfaces of other $A{\mathrm{TiO}}_{3}$ perovskites, e.g., ${\mathrm{SrTiO}}_{3}$ or ${\mathrm{BaTiO}}_{3}$. Based on a comparison of the 2DES properties in these three materials, we show how the electronic structure of the 2DES (bandwidth, orbital energy order, and electron density) is coupled to different typical lattice distortions in perovskites. The orbital hybridization in orthorhombic ${\mathrm{CaTiO}}_{3}$ results from the rotation of the oxygen octahedra, which can also occur at the interface of oxide heterostructures to compensate strain. More generally, the control of the orbital energy order in 2DES by choosing different $A$-site cations in perovskites offers a gateway toward 2DESs in oxide heterostructures beyond ${\mathrm{SrTiO}}_{3}$.

We deduce a model relevant for the anomalous metallic state of Sr doped manganites at low temperatures within the ferromagnetic phase. It provides a natural explanation for several anomalous features observed experimentally, such as the small Drude contribution in optical conductivity, the "pseudo-gap" in the density of states, and the unusual dispersion observed in photoemission.

The nature of the Mott transition in the absence of any symmetry breaking remains a matter of debate. We study the correlation-driven insulator-to-metal transition in the prototypical 3D Mott system GaTa(4)Se(8), as a function of temperature and applied pressure. We report novel experiments on single crystals, which demonstrate that the transition is of first order and follows from the coexistence of two states, one insulating and one metallic, that we toggle with a small bias current. We provide support for our findings by contrasting the experimental data with calculations that combine local density approximation with dynamical mean-field theory, which are in very good agreement.

We introduce a model that accounts for the bipolar resistive switching phenomenom observed in transition metal oxides. It qualitatively describes the electric field-enhanced migration of oxygen vacancies at the nano-scale. The numerical study of the model predicts that strong electric fields develop in the highly resistive dielectric-electrode interfaces, leading to a spatially inhomogeneous oxygen vacancies distribution and a concomitant resistive switching effect. The theoretical results qualitatively reproduce non-trivial resistance hysteresis experiments that we also report, providing key validation to our model., Accepted for publication in Physical Review B, 6 twocolumn pages, 5 figures

Two perovskite-derived materials, CaCu3Mn4O12, have drawn much recent interest due to their magnetoresistive, dielectric, and mafnetoelectronic characteristics. Here we present initial theoretical insights into each of these points, based on first principles, density functional based calculations. Our results predict CCMO to have a spin-asymmetric energy gap, which leads to distinct temperature- and magnetic field-dependent changes in properties, and helps to account for its observed negative magnetoresistivity. We have studied CCTO primarily to gain insight into the exchange coupling in both these compounds, where the conventional superexchange coupling vanishes by symmetry for both nearest and next nearest Cu-Cu neighbors, a consequence of the structure. In CCTO, it is necessary to go 5th Cu-Cu neighbors to obstain a (superexchange) coupling that can provide the coupling necessary to give three dimensional order. Non-superexchange mechanisms may be necessary to describe the magnetic coupling in this structural clss.

Local density functional results are presented for the electron-doped metallochloronitrides A(x)ZrNCl and A(x)HfNCl, A = Li or Na, which superconduct up to 25K. The alkali non-stoichiometry is treated in a virtual crystal approximation. The electronic structure is strongly two dimensional, especially in the conduction band region occupied by the carriers, because the states are formed from the in-plane orbitals d_xy, d_{x^2-y^2} of the metal ion and the p_x, p_y orbitals of the N ion. We predict a change of behavior at a doping level of x=0.3., Comment: To appear in Proc. HTS99 Conf., Miami 1999. Four revtex pages, 5 embedded postscript figures

We review the reasons that make superconductivity unlikely to arise in a ferromagnet. Then, in light of the report by Tallon and collaborators that RuSr2GdCu2O8 becomes superconducting at approximately 35 K which is well below the Curie temperature of 132 K, we consider whether the objections really apply to this compound. Our considerations are supported by local spin density calculations for this compound, which indeed indicate a ferromagnetic RuO2 layer. The Ru moment resides in t_2g orbitals but is characteristic of itinerant magnetism (and is sensitive to choice of exchange-correlation potential and to the atomic positions). Based on the small exchange splitting that is induced in the Cu-O layers, the system seems capable of supporting singlet superconductivity an FFLO-type order parameter and possibly a pi-phase alternation between layers. If instead the pairing is triplet in the RuO2 layers, it can be distinguished by a spin-polarized supercurrent. Either type of superconductivity seems to imply a spontaneous vortex phase if the magnetization is rotated out of the plane.

Superconductivity and ferromagnetism have been believed to be incompatible over any extended temperature range until certain specific examples - RuSr2GdCu2O8 and UGe2- have arisen in the past 2-3 years. The discovery of superconductivity above 8 K in MgCNi3, which is primarily the ferromagnetic element Ni and is strongly exchange-enhanced, provides a probable new and different example. This compound is shown here to be near ferromagmnetism, requiring only hole-doping by 12% substitution of Mg by Na or Li. This system will provide the means to prode coupling, and possible coexistence, of these two forms of collective behavior without the requirement of pressure.