Observation by resonant angle-resolved photoemission of a critical thickness for 2-dimensional electron gas formation in SrTiO3_3 embedded in GdTiO3_3

For certain conditions of layer thickness, the interface between GdTiO$_3$ (GTO) and SrTiO$_3$ (STO) in multilayer samples has been found to form a two-dimensional electron gas (2DEG) with very interesting properties including high mobilities and ferromagnetism. We have here studied two trilayer samples of the form [2 nm GTO/1.0 or 1.5 unit cells STO/10 nm GTO] as grown on (001) (LaAlO$_3$)$_{0.3}$(Sr$_2$AlTaO$_6$)$_{0.7}$ (LSAT), with the STO layer thicknesses being at what has been suggested is the critical thickness for 2DEG formation. We have studied these with Ti-resonant angle-resolved (ARPES) and angle-integrated photoemission and find that the spectral feature in the spectra associated with the 2DEG is present in the 1.5 unit cell sample, but not in the 1.0 unit cell sample. We also observe through core-level spectra additional states in Ti and Sr, with the strength of a low-binding-energy state for Sr being associated with the appearance of the 2DEG, and we suggest it to have an origin in final-state core-hole screening.Comment: 12 pages, 4 figure

Energetic, spatial and momentum character of a buried interface: the two-dimensional electron gas between two metal oxides

The interfaces between two condensed phases often exhibit emergent physical properties that can lead to new physics and novel device applications, and are the subject of intense study in many disciplines. We here apply novel experimental and theoretical techniques to the characterization of one such interesting interface system: the two-dimensional electron gas (2DEG) formed in multilayers consisting of SrTiO$_3$ (STO) and GdTiO$_3$ (GTO). This system has been the subject of multiple studies recently and shown to exhibit very high carrier charge densities and ferromagnetic effects, among other intriguing properties. We have studied a 2DEG-forming multilayer of the form [6 unit cells STO/3 unit cells of GTO]$_{20}$ using a unique array of photoemission techniques including soft and hard x-ray excitation, soft x-ray angle-resolved photoemission, core-level spectroscopy, resonant excitation, and standing-wave effects, as well as theoretical calculations of the electronic structure at several levels and of the actual photoemission process. Standing-wave measurements below and above a strong resonance have been introduced as a powerful method for studying the 2DEG depth distribution. We have thus characterized the spatial and momentum properties of this 2DEG with unprecedented detail, determining via depth-distribution measurements that it is spread throughout the 6 u.c. layer of STO, and measuring the momentum dispersion of its states. The experimental results are supported in several ways by theory, leading to a much more complete picture of the nature of this 2DEG, and suggesting that oxygen vacancies are not the origin of it. Similar multi-technique photoemission studies of such states at buried interfaces, combined with comparable theory, will be a very fruitful future approach for exploring and modifying the fascinating world of buried-interface physics and chemistry.Comment: 34 pages, 10 figure

Prospects and applications near ferroelectric quantum phase transitions : a key issues review

The emergence of complex and fascinating states of quantum matter in the neighborhood of zero temperature phase transitions suggests that such quantum phenomena should be studied in a variety of settings. Advanced technologies of the future may be fabricated from materials where the cooperative behavior of charge, spin and current can be manipulated at cryogenic temperatures. The progagating lattice dynamics of displacive ferroelectrics make them appealing for the study of quantum critical phenomena that is characterized by both space- and time-dependent quantities. In this Key Issues article we aim to provide a self-contained overview of ferroelectrics near quantum phase transitions. Unlike most magnetic cases, the ferroelectric quantum critical point can be tuned experimentally to reside at, above or below its upper critical dimension; this feature allows for detailed interplay between experiment and theory using both scaling and self-consistent field models. Additional degrees of freedom like charge and spin can be added and characterized systematically. Satellite memories, electrocaloric cooling and low-loss phased-array radar are among possible applications of low-temperature ferroelectrics. We end with open questions for future research that include textured polarization states and unusual forms of superconductivity that remain to be understood theoretically.PostprintPeer reviewe

Low-dimensional Mott material: Transport in ultrathin epitaxial LaNiO3 films

Electrical resistivity and magnetotransport are explored for thin (3-30 nm), epitaxial LaNiO3 films. Films were grown on three different substrates to obtain LaNiO3 films that are coherently strained, with different signs and magnitude of film strain. It is shown that d-band transport is inhibited as the layers progress from compression to tension. The Hall coefficient is "holelike." Increasing tensile strain causes the film resistivity to increase, causing strong localization to appear below a critical thickness.open11125125sciescopu

Epitaxial SrTiO3 films with electron mobilities exceeding 30,000 cm2V-1s-1

The study of quantum phenomena in semiconductors requires epitaxial structures with exceptionally high charge-carrier mobilities(1). Furthermore, low-temperature mobilities are highly sensitive probes of the quality of epitaxial layers, because they are limited by impurity and defect scattering. Unlike many other complex oxides, electron-doped SrTiO3 single crystals show high (similar to 10(4) cm(2) V(-1)s(-1)) electron mobilities at low temperatures. High-mobility, epitaxial heterostructures with SrTiO3 have recently attracted attention for thermoelectric applications(2), field-induced superconductivity(3) and two-dimensional (2D) interface conductivity(4). Epitaxial SrTiO3 thin films are often deposited by energetic techniques, such as pulsed laser deposition. Electron mobilities in such films are lower than those of single crystals(5). In semiconductor physics, molecular beam epitaxy (MBE) is widely established as the deposition method that produces the highest mobility structures(1,6,7). It is a low-energetic, high-purity technique that allows for low defect densities and precise control over doping concentrations and location. Here, we demonstrate controlled doping of epitaxial SrTiO3 layers grown by MBE. Electron mobilities in these films exceed those of single crystals. At low temperatures, the films show Shubnikov-de Haas oscillations. These high-mobility SrTiO3 films allow for the study of the intrinsic physics of SrTiO3 and can serve as building blocks for high-mobility oxide heterostructures.X11223207sciescopu