M. R. Beasley

Origin of charge density at LaAlO3 on SrTiO3 heterointerfaces: possibility of intrinsic doping

As discovered by Ohtomo and Hwang, 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.

In situ growth of superconducting MgB2 thin films with preferential orientation by molecular-beam epitaxy

We report the synthesis of superconducting MgB2 thin films grown in situ by molecular-beam epitaxy. Mg-rich fluxes are deposited with B flux by electron-beam evaporation onto c-plane sapphire substrates. The films exhibit c-axis oriented peaks of MgB2, and a full width at half maximum of 3° in their rocking curves. In-plane alignment of MgB2 shows 12-fold symmetry, which is observed by the selected area diffraction pattern in transmission electron microscopy. The MgB2 films show a superconducting transition at 34.5 K with ΔTc<1 K. Even though the residual resistivity of the films is quite high (∼60 μΩu200acm), the normal-state resistivity has a very similar temperature dependence but is five times larger than that of a single crystal, implying that conduction through the whole sample is imperfectly connected or inhomogeneous. Upper critical fields are obtained from measurement of the field dependence of the resistivity. It is estimated that the upper critical field at 20 K is more than 15 T, which is one of t...

Dependence of the electronic structure of SrRuO3 and its degree of correlation on cation off-stoichiometry

We have grown and studied high quality SrRuO{sub 3} films grown by MBE as well as PLD. By changing the oxygen activity during deposition we were able to make SrRuO{sub 3} samples that were stoichiometric (low oxygen activity) or with ruthenium vacancies (high oxygen activity). Samples with strontium vacancies were found impossible to produce since the ruthenium would precipitate out as RuO{sub 2}. The volume of the unit cell of SrRuO{sub 3} becomes larger as more ruthenium vacancies are introduced. The residual resistivity ratio (RRR) and room temperature resistivity were found to systematically depend on the volume of the unit cell and therefore on the amount of ruthenium vacancies. The RRR varied from {approx}30 for stoichiometric samples to less than two for samples that were very ruthenium poor. The room temperature resistivity varied from 190 {micro}{Omega} cm for stoichoimetric samples to over 300 {micro}{Omega} cm for very ruthenium poor samples. UPS spectra show a shift of weight from the coherent peak to the incoherent peak around the Fermi level when samples have more ruthenium vacancies. Core level XPS spectra of the ruthenium 3d lines show a strong screened part in the case of stoichiometric samples. This screened part disappears whenmorexa0» ruthenium vacancies are introduced. Both the UPS and the XPS results are consistent with the view that correlation increases as the amount of ruthenium vacancies increase.«xa0less

Cryogenic scanning Hall-probe microscope with centimeter scan range and submicron resolution

We have constructed a scanning Hall-probe microscope that combines a 1×4cm scan range with 200 nm positioning resolution by coupling stepper motors to high-resolution drivers and reducing gears. The instrument is uniquely suited for efficient magnetic imaging of mesoscopic devices, media, and materials, operating from 4 K to room temperature with fast turn-around time. Its potential for studying dissipation in coated conductors—high-Tc superconducting tapes—is demonstrated via model systems. We image an entire sample of YBa2Cu3O7−δ, then zoom in to individual fluxons. Flux penetration into a single artificial grain boundary is imaged with 4×10−3G∕Hz field resolution and 25μs time resolution by averaging over cycles of ac driving current. Using the resulting magnetic movie, we map out ac power losses.

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

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

Superconducting memory based on ferromagnetism

Answering to the need for dense superconducting memories, the authors propose a memory concept that combines ferromagnetic dots for the storage of the data and Josephson junctions for their readout. Good scalability is expected for large scale integration. Exploratory memory cells have been implemented using 3μm Nb technology and Permalloy dots. Nonvolatile data storage at 300K was demonstrated.

Experimental investigation of electronic properties of buried heterointerfaces 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.

Miniature Nernstian oxygen sensor for deposition and growth environments

A miniature Nernstian-type oxygen sensor employing an ionically conducting stabilized zirconia solid electrolyte and an embedded internal reference electrode is developed and is tested in the temperature range 275–425 °C. It is intended for in situ monitoring of oxygen content in deposition and growth environments. Advantages of this sensor are in situ generation of the Ni/NiO two phase internal reference thin film electrode using a solid-state oxygen Coulometric titration technique, its small size (5 mm×10 mm), flat geometry (<1 mm thick), relatively low operating temperature, and the fact that the entire sensor can be immersed in the gaseous environment with the unknown oxygen content. The latter feature is distinctly different from commercially available Nernstian oxygen sensors, which employ two-compartment geometries. Depending upon temperature, the time constants for the sensor response to changes in oxygen pressure ranged from seconds to minutes, which is exceptionally fast for these moderately low temperatures. The small thermal mass and size of this single-compartment electrochemical sensor make it suitable for in situ and noninvasive real time monitoring of oxygen content during thin-film deposition or bulk growth of complex oxides.

Anomalous double peak structure in superconductor/ferromagnet tunneling density of states

We have experimentally investigated the density of states (DOS) in Nb/Ni (S/F) bilayers as a function of Ni thickness, d(F). Our thinnest samples show the usual DOS peak at +/- Delta(0), whereas intermediate-thickness samples have an anomalous double-peak structure. For thicker samples (d(F) > or =3.5 nm), we see an inverted DOS, which has previously only been reported in superconductor or weak-ferromagnet structures. We analyze the data using the self-consistent nonlinear Usadel equation and find that we are able to quantitatively fit the features at +/- Delta(0) -- in particular the thickness at which the inversion occurs -- only if we include a large amount of spin-orbit scattering in the model. Interestingly, we are unable to reproduce the subgap structure through the addition of any parameter(s). Therefore, the observed anomalous subgap structure represents new physics beyond that contained in the present Usadel theory.

Design and performance of a practical variable-temperature scanning tunneling potentiometry system

We have constructed a scanning tunneling potentiometry system capable of simultaneously mapping the transport-related electrochemical potential of a biased sample along with its surface topography. Combining a novel sample biasing technique with a continuous current-nulling feedback scheme pushes the noise performance of the measurement to its fundamental limit--the Johnson noise of the scanning tunneling microscope (STM) tunnel junction. The resulting 130 nV voltage sensitivity allows us to spatially resolve local potentials at scales down to 2 nm, while maintaining angstrom scale STM imaging, all at scan sizes of up to 15 microm. A millimeter-range two-dimensional coarse positioning stage and the ability to operate from liquid helium to room temperature with a fast turn-around time greatly expand the versatility of the instrument. By performing studies of several model systems, we discuss the implications of various types of surface morphology for potentiometric measurements.