Field-Effect Devices Utilizing LaAlO3_3-SrTiO3_3 Interfaces

Using LaAlO$_3$-SrTiO$_3$ bilayers, we have fabricated field-effect devices that utilize the two-dimensional electron liquid generated at the bilayers' {\textit n}-type interfaces as drain-source channels and the LaAlO$_3$ layers as gate dielectrics. With gate voltages well below 1\,V, the devices are characterized by voltage gain and current gain. The devices were operated at temperatures up to 100\,{\deg}C.Comment: to be published in Applied Physics Letter

Emerging magnetism and electronic phase separation at titanate interfaces

The emergence of magnetism in otherwise nonmagnetic compounds and its underlying mechanisms have become the subject of intense research. Here we demonstrate that the nonmagnetic oxygen vacancies are responsible for an unconventional magnetic state common for titanate interfaces and surfaces. Using an effective multiorbital modelling, we find that the presence of localized vacancies leads to an interplay of ferromagnetic order in the itinerant t2g band and complex magnetic oscillations in the orbitally-reconstructed eg-band, which can be tuned by gate fields at oxide interfaces. The magnetic phase diagram includes highly fragmented regions of stable and phase-separated magnetic states forming beyond nonzero critical defect concentrations.Comment: 5 pages, 4 figure

How Large is the Intrinsic Flux Noise of a Magnetic Flux Quantum, of Half a Flux Quantum and of a Vortex-Free Superconductor?

This article addresses the question whether the magnetic flux of stationary vortices or of half flux quanta generated by frustrated superconducting rings is noisy. It is found that the flux noise generated intrinsically by a superconductor is, in good approximation, not enhanced by stationary vortices. Half flux quanta generated by $\pi$-rings are characterized by considerably larger noise.Comment: 11 pages, 3 figures. in: A. Bussmann-Holder, H. Keller (Eds.) High Tc Superconductors and Related Transition Metal Oxides, Springer, 237-242; also to be published in: Journal of Superconductivity (2007

Alex M\"uller, the High-Tc Field-Effect Transistor and Electric-Field Gated Quantum Materials

Alex M\"uller and Georg Bednorz are widely recognized for their trailblazing discovery of high-temperature superconductivity and their groundbreaking research on SrTiO3. In comparison, their substantial contributions to inventing the high-Tc superconducting field-effect transistor remain relatively unknown. Nevertheless, their efforts were crucial in developing the electric field effect into a valuable tool for studying a broad spectrum of complex materials. This article provides a brief overview of these developments and of the current status in this field, with a particular focus on Alex M\"uller's visionary role in advancing the field following the discovery of high-temperature superconductivity.Comment: Contribution to a planned memorial issue for K.A. M\"uller by Physica

Quartett formation at (100)/(110)-interfaces of d-wave superconductors

Across a faceted (100)/(110) interface between two d-wave-superconductors the structure of the superconducting order parameter leads to an alternating sign of the local Josephson coupling. Describing the Cooper pair motion along and across the interface by a one-dimensional boson lattice model, we show that a small attractive interaction between the bosons boosts boson binding at the interface -- a phenomenon, which is intimately tied to the staggered sequence of 0- and Pi-junction contacts along the interface. We connect this finding to the recently observed h/4e oscillations in (100)/(110) SQUIDS of cuprate superconductors.Comment: 13 pages, 2 figure

Large Negative Electronic Compressibility of LaAlO3-SrTiO3 Interfaces with Ultrathin LaAlO3 Layers

A two-dimensional electron liquid is formed at the n-type interface between SrTiO3 and LaAlO3. Here we report on Kelvin probe microscopy measurements of the electronic compressibility of this electron system. The electronic compressibility is found to be negative for carrier densities of \approx10^13/cm^2. At even smaller densities, a metal-to-insulator transition occurs. These local measurements corroborate earlier measurements of the electronic compressibility of LaAlO3-SrTiO3 interfaces obtained by measuring the capacitance of macroscopic metal-LaAlO3-SrTiO3 capacitors

Chemical control of orbital polarization in artificially structured transition-metal oxides: La2NiXO6 (X=B, Al, Ga, In) from first principles

The application of modern layer-by-layer growth techniques to transition-metal oxide materials raises the possibility of creating new classes of materials with rationally designed correlated electron properties. An important step toward this goal is the demonstration that electronic structure can be controlled by atomic composition. In compounds with partially occupied transition-metal d shells, one important aspect of the electronic structure is the relative occupancy of different d orbitals. Previous work has established that strain and quantum confinement can be used to influence orbital occupancy. In this paper we demonstrate a different modality for orbital control in transition-metal oxide heterostructures, using density-functional band calculations supplemented by a tight-binding analysis to show that the choice of nontransition-metal counterion X in transition-metal oxide heterostructures composed of alternating LaNiO3 and LaXO3 units strongly affects orbital occupancy, changing the magnitude and in some cases the sign of the orbital polarization