T. Kopp

Superconducting interfaces between insulating oxides.

At interfaces between complex oxides, electronic systems with unusual electronic properties can be generated. We report on superconductivity in the electron gas formed at the interface between two insulating dielectric perovskite oxides, LaAlO3 and SrTiO3. The behavior of the electron gas is that of a two-dimensional superconductor, confined to a thin sheet at the interface. The superconducting transition temperature of ≅ 200 millikelvin provides a strict upper limit to the thickness of the superconducting layer of ≅ 10 nanometers.

Very Large Capacitance Enhancement in a Two-Dimensional Electron System

Electron correlation effects at the interface of two metal oxides lead to a lower chemical potential and enhance capacitance. Increases in the gate capacitance of field-effect transistor structures allow the production of lower-power devices that are compatible with higher clock rates, driving the race for developing high-κ dielectrics. However, many-body effects in an electronic system can also enhance capacitance. Onto the electron system that forms at the LaAlO3/SrTiO3 interface, we fabricated top-gate electrodes that can fully deplete the interface of all mobile electrons. Near depletion, we found a greater than 40% enhancement of the gate capacitance. Using an electric-field penetration measurement method, we show that this capacitance originates from a negative compressibility of the interface electron system. Capacitance enhancement exists at room temperature and arises at low electron densities, in which disorder is strong and the in-plane conductance is much smaller than the quantum conductance.Novel electronic systems forming at oxide interfaces comprise a class of new materials with a wide array of potential applications. A high mobility electron system forms at the LaAlO3/SrTiO3 interface and, strikingly, both superconducts and displays indications of hysteretic magnetoresistance[1– 5]. An essential step for device applications is establishing the ability to vary the electronic conductivity of the electron system by means of a gate. We have fabricated metallic top gates above a conductive interface to vary the electron density at the interface. By monitoring capacitance and electric field penetration, we are able to tune the charge carrier density and establish that we can completely deplete the metallic interface with small voltages. Moreover, at low carrier densities, the capacitance is significantly enhanced beyond the geometric capacitance for the structure. In the same low density region, the metallic interface overscreens an external electric field. We attribute these observations to a negative compressibility of the electronic system at the interface. Similar phenomena have been observed previously in semiconducting two-dimensional electronic systems [6–12]. The observed compressibility result is consistent with the interface containing a system of mobile electrons in two dimensions [13, 15].

Oxygen vacancies at titanate interfaces: Two-dimensional magnetism and orbital reconstruction

We show that oxygen vacancies at titanate interfaces induce a complex multiorbital reconstruction which involves a lowering of the local symmetry and an inversion of t2g and eg orbitals resulting in the occupation of the eg orbitals of Ti atoms neighboring the O vacancy. The orbital reconstruction depends strongly on the clustering of O vacancies and can be accompanied by a magnetic splitting between the local eg orbitals with lobes directed towards the vacancy and interface dxy orbitals. The reconstruction generates a two-dimensional interface magnetic state not observed in bulk SrTiO3. Using generalized gradient approximation with intra-atomic Coulomb repulsion (GGA + U ), we find that this magnetic state is common for titanate surfaces and interfaces.

Two-dimensional electron liquid state at LaAlO 3 -SrTiO 3 interfaces

Using tunneling spectroscopy we have measured the spectral density of states of the mobile, two-dimensional electron system generated at the LaAlO3-SrTiO3 interface. As shown by the density of states the interface electron system differs qualitatively, first, from the electron systems of the materials defining the interface and, second, from the two-dimensional electron gases formed at interfaces between conventional semiconductors.

How grain boundaries limit supercurrents in high-temperature superconductors

S. Graser, 2, ∗ P. J. Hirschfeld, T. Kopp, R. Gutser, B. M. Andersen, and J. Mannhart Center for Electronic Correlations and Magnetism, Institute of Physics, University of Augsburg, D-86135 Augsburg, Germany Department of Physics, University of Florida, Gainesville, FL 32611, USA Niels Bohr Institute, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark (Dated: December 21, 2009)

Calculation of the capacitances of conductors: Perspectives for the optimization of electronic devices

The equation describing the capacitance of capacitors is determined. It is shown that by optimizing the material of the conducting electrodes, the capacitance of capacitors reaching the quantum regime can be substantially enhanced or reduced. Dielectric capacitors with negative total capacitances are suggested and their properties analyzed. Resulting perspectives to enhance the performance of electronic devices are discussed.

Observation of Two-Magnon Bound States in the Two-Leg Ladders of (Ca, La)14Cu24O41

Phonon-assisted two-magnon absorption is studied in the spin- 1/2 two-leg ladders of (Ca,La)(14)Cu(24)O(41) for E parallel c (legs) and E parallel a (rungs). We verify the theoretically predicted existence of two-magnon singlet bound states, which give rise to peaks at approximately equal to 2140 and 2800 cm(-1). The two-magnon continuum is observed at approximately equal to 4000 cm(-1). Two different theoretical approaches (Jordan-Wigner fermions and perturbation theory) describe the data very well for J parallel approximately equal to 1020-1100 cm(-1), J parallel/J perpendicular approximately equal to 1-1.2. At high energies, the magnetic contribution to sigma(omega) is strikingly similar in the ladders and in the undoped high-T(c) cuprates, which emphasizes the importance of strong quantum fluctuations in the latter.

Origin of magnetic interactions in Ca 3 Co 2 O 6

We investigate the microscopic origin of the ferromagnetic and antiferromagnetic spin exchange couplings in the quasi-one-dimensional cobalt compound Ca 3 Co 2 O 6 . In particular, we establish a local model which stabilizesa ferromagnetic alignment of the S = 2 spins on the cobalt sites with trigonal prismatic symmetry, for a sufficiently strong Hunds rule coupling on the cobalt ions. The exchange is mediated through an S = 0 cobalt ion at the octahedral sites of the chain structure. We present a strong coupling evaluation of the Heisenberg coupling between the S = 2 Co spins on a separate chain. The chains are coupled antiferromagnetically through super-superexchange via short O-O bonds.

Suppression of the ferromagnetic state in LaCoO 3 films by rhombohedral distortion

Epitaxially strained

Polaronic excitations in colossal magnetoresistance manganite films

{\text{LaCoO}}_{3}