59 research outputs found
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
Point-contact tunneling spectroscopy measurement of CuTiSe: disorder-enhanced Coulomb effects
We performed point-contact spectroscopy tunneling measurements on
CuTiSe bulk with and at temperatures ranging from
K and observe a suppression in the density of states around zero-bias
that we attribute to enhanced Coulomb interactions due to disorder. We find
that the correlation gap associated with this suppression is related to the
zero-temperature resistivity. We use our results to estimate the disorder-free
transition temperature and find that the clean limit is close to the
experimentally observed .Comment: 4 pages, 4 figure
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STANFORD IN-SITU HIGH RATE YBCO PROCESS: TRANSFER TO METAL TAPES AND PROCESS SCALE UP
Executive Summary The materials science understanding of high rate low cost processes for Coated Conductor will benefit the application to power utilities for low loss energy transportation and power generation as well for DOD applications. The research in this program investigated several materials processing approaches that are new and original, and are not being investigated elsewhere. This work added to the understanding of the material science of high rate PVD growth of HTSC YBCO assisted by a liquid phase. A new process discovered uses amorphous glassy precursors which can be made at high rate under flexible conditions of temperature and oxygen, and later brought to conditions of oxygen partial pressure and temperature for rapid conversion to YBCO superconductor. Good critical current densities were found, but further effort is needed to optimize the vortex pinning using known artificial inclusions. A new discovery of the physics and materials science of vortex pinning in the HTSC system using Sm in place of Y came at growth at unusually low oxygen pressure resulting in clusters of a low or non superconducting phase within the nominal high temperature phase. The driving force for this during growth is new physics, perhaps due to the low oxygen. This has the potential for high current in large magnetic fields at low cost, applicable to motors, generators and transformers. The technical demands of this project were the motivation for the development of instrumentation that could be essential to eventual process scale up. These include atomic absorption based on tunable diode lasers for remote monitoring and control of evaporation sources (developed under DARPA support), and the utility of Fourier Transform Infrared Reflectivity (FTIR) for aid in the synthesis of complex thin film materials (purchased by a DURIP-AFOSR grant)
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 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 with the overall bonding more ionic. As
an end member of the 3d transition monoxides, its magnetic properties should be
that of a high antiferromagnet
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
Dependence of electronic structure of SrRuO3 and the degree of correlation on cation off-stoichiometry
We have grown and studied high quality SrRuO3 films grown by MBE as well as
PLD. By changing the oxygen activity during deposition we were able to make
SrRuO3 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 RuO2. The
volume of the unit cell of SrRuO3 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 ~30 for
stoichiometric samples to less than two for samples that were very ruthenium
poor. The room temperature resistivity varied from 190 microOhm cm for
stoichoimetric samples to over 300 microOhm 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 when 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.Comment: 21 pages, 5 figures, submitted to Physical Review
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