Quenching of Impurity Spins at Cu/CuO Interfaces: An Antiferromagnetic Proximity Effect

It is observed that the magnetoconductance of bilayer films of copper (Cu) and copper monoxide (CuO) has distinct features compared of that of Cu films on conventional band insulator substrates. We analyze the data above 2 K by the theory of weak antilocalization in two-dimensional metals and suggest that spin-flip scatterings by magnetic impurities inside Cu are suppressed in Cu/CuO samples. Plausibly the results imply a proximity effect of antiferromagnetism inside the Cu layer, which can be understood in the framework of Ruderman-Kittel-Kasuya-Yoshida (RKKY) interactions. The data below 1 K, which exhibit slow relaxation reminiscent of spin glass, are consistent with this interpretation.Comment: 6 pages, 4 figures, 2 tables. Added a supplementary materia

What Tc Tells

Superconductivity has continued to be a fascinating phenomenon ever since its discovery in 1911. The magnitude of the transition temperature, Tc, provides valuable insight into the underlying physics. Here we provide select examples of the extensive research that has been done towards understanding Tc, and some cases where further investigation is called for. We believe that searching for new and enhanced Tc's remains a fertile frontier.Comment: Accepted for publication in Physica C, Special Issue on Superconducting Material

Tetragonal CuO: A new end member of the 3d transition metal monoxides

Monoclinic CuO is anomalous both structurally as well as electronically in the 3$d$ transition metal oxide series. All the others have the cubic rock salt structure. Here we report the synthesis and electronic property determination of a tetragonal (elongated rock salt) form of CuO created using an epitaxial thin film deposition approach. In situ photoelectron spectroscopy suggests an enhanced charge transfer gap $\Delta$ with the overall bonding more ionic. As an end member of the 3d transition monoxides, its magnetic properties should be that of a high $T_N$ antiferromagnet

Origin of charge density at LaAlO3-on-SrTiO3 hetero-interfaces; possibility of intrinsic doping

As discovered by Ohtomo et al., a large sheet charge density with high mobility exists at the interface between SrTiO3 and LaAlO3. Based on transport, spectroscopic and oxygen-annealing experiments, we conclude that extrinsic defects in the form of oxygen vacancies introduced by the pulsed laser deposition process used by all researchers to date to make these samples is the source of the large carrier densities. Annealing experiments show a limiting carrier density. We also present a model that explains the high mobility based on carrier redistribution due to an increased dielectric constant.Comment: 14 pages, 3 figures, 1 table; accepted for publication in Phys. Rev. Lett

Superconductivity in zigzag CuO chains

Superconductivity has recently been discovered in Pr$_{2}$Ba$_{4}$Cu$_{7}$O$_{15-\delta}$ with a maximum $T_c$ of about 15K. Since the CuO planes in this material are believed to be insulating, it has been proposed that the superconductivity occurs in the double (or zigzag) CuO chain layer. On phenomenological grounds, we propose a theoretical interpretation of the experimental results in terms of a new phase for the zigzag chain, labelled by C$_1$S$_{3/2}$. This phase has a gap for some of the relative spin and charge modes but no total spin gap, and can have a divergent superconducting susceptibility for repulsive interactions. A microscopic model for the zigzag CuO chain is proposed, and on the basis of density matrix renormalization group (DMRG) and bosonization studies of this model, we adduce evidence that supports our proposal.Comment: 10 pages, 5 figures; Journal-ref. adde

Electronic properties of buried hetero-interfaces of LaAlO3 on SrTiO3

We have made very thin films of LaAlO3 on TiO2 terminated SrTiO3 and have measured the properties of the resulting interface in various ways. Transport measurements show a maximum sheet carrier density of 1016 cm-2 and a mobility around 104 cm2 V-1 s-1. In situ ultraviolet photoelectron spectroscopy (UPS) indicates that for these samples a finite density of states exists at the Fermi level. From the oxygen pressure dependence measured in both transport as well as the UPS, we detail, as reported previously by us, that oxygen vacancies play an important role in the creation of the charge carriers and that these vacancies are introduced by the pulsed laser deposition process used to make the heterointerfaces. Under the conditions studied the effect of LaAlO3 on the carrier density is found to be minimal.Comment: 19 pages, 6 figure

Dielectric-permittivity-driven charge carrier modulation at oxide interfaces

High-quality bilayers of La-doped SrTiO3 (STO) and LaAlO3 (LAO) on SrTiO3 have been grown controlling the location and behavior of the charge carriers by changing the thicknesses of the layers, which are dielectrically mismatched. In this system, the charge carriers are created at the La:SrTiO3/LAO interface and spread out toward the substrate due to the increase in dielectric constant as the temperature is lowered. When the electrons reach the interface of the La:SrTiO3 and the pure STO, they display enhanced mobility in the quantum well at that interface for specific thicknesses

Quasi-1D dynamics and nematic phases in the 2D Emery model

We consider the Emery model of a Cu-O plane of the high temperature superconductors. We show that in a strong-coupling limit, with strong Coulomb repulsions between electrons on nearest-neighbor O sites, the electron-dynamics is strictly one dimensional, and consequently a number of asymptotically exact results can be obtained concerning the electronic structure. In particular, we show that a nematic phase, which spontaneously breaks the point- group symmetry of the square lattice, is stable at low enough temperatures and strong enough coupling.Comment: 8 pages, 5 eps figures; revised manuscript with more detailed discussions; two new figures and three edited figuresedited figures; 14 references; new appendix with a detailed proof of the one-dimensional dynamics of the system in the strong coupling limi