938 research outputs found
Superconductivity in higher titanium oxides
Recent renewal of the highest transition temperature in a conventional
superconductor of the sulfer hydride attracts much attention to exploring
simple compounds with the lighter elements, situated in unconventional
conditions. We report the discovery of superconductivity in simple oxides of
Ti4O7 and g-Ti3O5 in a thin-film form having deliberately tuned epitaxial
structures and off-stoichiometry. These higher titanium oxides join in a class
of simple-oxide superconductors, and g-Ti3O5 now holds the highest
superconducting transition temperature of 7.1 kelvin among them. The mechanism
behind the superconductivity is discussed on the basis of electrical
measurements and theoretical predictions. We conclude that superconductivity
arises from unstabilized bipolaronic insulating states.Comment: 25 pages, 4 figures in main text, 14 pages, 11 figures in
supplemental informatio
Hole Transport in p-Type ZnO
A two-band model involving the A- and B-valence bands was adopted to analyze
the temperature dependent Hall effect measured on N-doped \textit{p}-type ZnO.
The hole transport characteristics (mobilities, and effective Hall factor) are
calculated using the ``relaxation time approximation'' as a function of
temperature. It is shown that the lattice scattering by the acoustic
deformation potential is dominant. In the calculation of the scattering rate
for ionized impurity mechanism, the activation energy of 100 or 170 meV is used
at different compensation ratios between donor and acceptor concentrations. The
theoretical Hall mobility at acceptor concentration of
cm is about 70 cmVs with the activation energy of 100 meV
and the compensation ratio of 0.8 at 300 K. We also found that the compensation
ratios conspicuously affected the Hall mobilities.Comment: 5page, 5 figures, accepted for publication in Jpn. J. Appl. Phy
Gallium concentration dependence of room-temperature near-bandedge luminescence in n-type ZnO:Ga
We investigated the optical properties of epitaxial \textit{n}-type ZnO films
grown on lattice-matched ScAlMgO substrates. As the Ga doping concentration
increased up to cm, the absorption edge showed a
systematic blueshift, consistent with the Burstein-Moss effect. A bright
near-bandedge photoluminescence (PL) could be observed even at room
temperature, the intensity of which increased monotonically as the doping
concentration was increased except for the highest doping level. It indicates
that nonradiative transitions dominate at a low doping density. Both a Stokes
shift and broadening in the PL band are monotonically increasing functions of
donor concentration, which was explained in terms of potential fluctuations
caused by the random distribution of donor impurities.Comment: accepted for publication for Applied Physics Letters 4 figure
Why Some Interfaces Cannot be Sharp
A central goal of modern materials physics and nanoscience is control of
materials and their interfaces to atomic dimensions. For interfaces between
polar and non-polar layers, this goal is thwarted by a polar catastrophe that
forces an interfacial reconstruction. In traditional semiconductors this
reconstruction is achieved by an atomic disordering and stoichiometry change at
the interface, but in multivalent oxides a new option is available: if the
electrons can move, the atoms don`t have to. Using atomic-scale electron energy
loss spectroscopy we find that there is a fundamental asymmetry between
ionically and electronically compensated interfaces, both in interfacial
sharpness and carrier density. This suggests a general strategy to design sharp
interfaces, remove interfacial screening charges, control the band offset, and
hence dramatically improving the performance of oxide devices.Comment: 12 pages of text, 6 figure
Monte-Carlo simulation of localization dynamics of excitons in ZnO and CdZnO quantum well structures
Localization dynamics of excitons was studied for ZnO/MgZnO and CdZnO/MgZnO
quantum wells (QW). The experimental photoluminescence (PL) and absorption data
were compared with the results of Monte Carlo simulation in which the excitonic
hopping was modeled. The temperature-dependent PL linewidth and Stokes shift
were found to be in a qualitatively reasonable agreement with the hopping
model, with accounting for an additional inhomogeneous broadening for the case
of linewidth. The density of localized states used in the simulation for the
CdZnO QW was consistent with the absorption spectrum taken at 5 K.Comment: 4 figures, to appear in J. Appl. Phy
Photoemission study of TiO2/VO2 interfaces
We have measured photoemission spectra of two kinds of TiO-capped VO
thin films, namely, that with rutile-type TiO (r-TiO/VO) and that
with amorphous TiO (a-TiO/VO) capping layers. Below the
Metal-insulator transition temperature of the VO thin films, K,
metallic states were not observed for the interfaces with TiO, in contrast
with the interfaces between the band insulator SrTiO and the Mott insulator
LaTiO in spite of the fact that both TiO and SrTiO are band
insulators with electronic configurations and both VO and LaTiO
are Mott insulators with electronic configurations. We discuss possible
origins of this difference and suggest the importance of the polarity
discontinuity of the interfaces. Stronger incoherent part was observed in
r-TiO/VO than in a-TiO/VO, suggesting Ti-V atomic diffusion due
to the higher deposition temperature for r-TiO/VO.Comment: 5 pages, 6 figure
Analysis on reflection spectra in strained ZnO thin films
Thin films of laser molecular-beam epitaxy grown ZnO films were studied with
respect to their optical properties. 4-K reflectivity was used to analyze
various samples grown at different biaxial in-plane strain. The spectra show
two structures at 3.37 eV corresponding to the A-free exciton transition and at
3.38 eV corresponding to the B-free exciton transition. Theoretical
reflectivity spectra were calculated using the spatial dispersion model. Thus,
the transverse energies, the longitudinal transversal splitting (ELT,), the
oscillator strengths, and the damping parameters were determined for both the
A- and B-free excitons of ZnO. As a rough trend, the strain dependence of the
energy E_LT for the A-excitons is characterized by a negatively-peaking
behavior with a minimum around the zero strain, while ELT for the B-excitons is
an increasing function of the strain field values.Comment: 4 pages, 2 figures, 1 table, conference: ICMAT2005 (Singapore), to
appear in an issue of J. Cryst. Growt
Tailoring a two-dimensional electron gas at the LaAlO3/SrTiO3 (001) interface by epitaxial strain
Recently a metallic state was discovered at the interface between insulating
oxides, most notably LaAlO3 and SrTiO3. Properties of this two-dimensional
electron gas (2DEG) have attracted significant interest due to its potential
applications in nanoelectronics. Control over this carrier density and mobility
of the 2DEG is essential for applications of these novel systems, and may be
achieved by epitaxial strain. However, despite the rich nature of strain
effects on oxide materials properties, such as ferroelectricity, magnetism, and
superconductivity, the relationship between the strain and electrical
properties of the 2DEG at the LaAlO3/SrTiO3 heterointerface remains largely
unexplored. Here, we use different lattice constant single crystal substrates
to produce LaAlO3/SrTiO3 interfaces with controlled levels of biaxial epitaxial
strain. We have found that tensile strained SrTiO3 destroys the conducting
2DEG, while compressively strained SrTiO3 retains the 2DEG, but with a carrier
concentration reduced in comparison to the unstrained LaAlO3/SrTiO3 interface.
We have also found that the critical LaAlO3 overlayer thickness for 2DEG
formation increases with SrTiO3 compressive strain. Our first-principles
calculations suggest that a strain-induced electric polarization in the SrTiO3
layer is responsible for this behavior. It is directed away from the interface
and hence creates a negative polarization charge opposing that of the polar
LaAlO3 layer. This both increases the critical thickness of the LaAlO3 layer,
and reduces carrier concentration above the critical thickness, in agreement
with our experimental results. Our findings suggest that epitaxial strain can
be used to tailor 2DEGs properties of the LaAlO3/SrTiO3 heterointerface
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