498 research outputs found
Stoichiometry control of the electronic properties of the LaAlO_3/SrTiO_3 heterointerface
We investigate the effect of the laser parameters of pulsed laser deposition
on the film stoichiometry and electronic properties of LaAlO_3/SrTiO_3 (001)
heterostructures. The La/Al ratio in the LaAlO_3 films was varied over a wide
range from 0.88 to 1.15, and was found to have a strong effect on the interface
conductivity. In particular, the carrier density is modulated over more than
two orders of magnitude. The film lattice expansion, caused by cation
vacancies, is found to be the important functional parameter. These results can
be understood to arise from the variations in the electrostatic boundary
conditions, and their resolution, with stoichiometry.Comment: 4 pages, 3 figures, submitted for publicatio
Scanning SQUID Susceptometry of a paramagnetic superconductor
Scanning SQUID susceptometry images the local magnetization and
susceptibility of a sample. By accurately modeling the SQUID signal we can
determine the physical properties such as the penetration depth and
permeability of superconducting samples. We calculate the scanning SQUID
susceptometry signal for a superconducting slab of arbitrary thickness with
isotropic London penetration depth, on a non-superconducting substrate, where
both slab and substrate can have a paramagnetic response that is linear in the
applied field. We derive analytical approximations to our general expression in
a number of limits. Using our results, we fit experimental susceptibility data
as a function of the sample-sensor spacing for three samples: 1) delta-doped
SrTiO3, which has a predominantly diamagnetic response, 2) a thin film of
LaNiO3, which has a predominantly paramagnetic response, and 3) a
two-dimensional electron layer (2-DEL) at a SrTiO3/AlAlO3 interface, which
exhibits both types of response. These formulas will allow the determination of
the concentrations of paramagnetic spins and superconducting carriers from fits
to scanning SQUID susceptibility measurements.Comment: 11 pages, 13 figure
Optically tuned dimensionality crossover in photocarrier-doped SrTiO: onset of weak localization
We report magnetotransport properties of photogenerated electrons in undoped
SrTiO single crystals under ultraviolet illumination down to 2 K. By tuning
the light intensity, the steady state carrier density can be controlled, while
tuning the wavelength controls the effective electronic thickness by modulating
the optical penetration depth. At short wavelengths, when the sheet conductance
is close to the two-dimensional Mott minimum conductivity we have observed
critical behavior characteristic of weak localization. Negative
magnetoresistance at low magnetic field is highly anisotropic, indicating
quasi-two-dimensional electronic transport. The high mobility of photogenerated
electrons in SrTiO allows continuous tuning of the effective electronic
dimensionality by photoexcitation.Comment: 7 pages, 7 figure
The computerized derivation of rate equations for enzyme reactions on the basis of the pseudo-steady-state assumption and the rapid-equilibrium assumption
Critical thickness for ferromagnetism in LaAlO3/SrTiO3 heterostructures
In heterostructures of LaAlO3 (LAO) and SrTiO3 (STO), two nonmagnetic
insulators, various forms of magnetism have been observed [1-7], which may [8,
9] or may not [10] arise from interface charge carriers that migrate from the
LAO to the interface in an electronic reconstruction [11]. We image the
magnetic landscape [5] in a series of n-type samples of varying LAO thickness.
We find ferromagnetic patches that appear only above a critical thickness,
similar to that for conductivity [12]. Consequently we conclude that an
interface reconstruction is necessary for the formation of magnetism. We
observe no change in ferromagnetism with gate voltage, and detect
ferromagnetism in a non-conducting p-type sample, indicating that the carriers
at the interface do not need to be itinerant to generate magnetism. The fact
that the ferromagnetism appears in isolated patches whose density varies
greatly between samples strongly suggests that disorder or local strain induce
magnetism in a population of the interface carriers
Built-in and induced polarization across LaAlO/SrTiO heterojunctions
Ionic crystals terminated at oppositely charged polar surfaces are inherently
unstable and expected to undergo surface reconstructions to maintain
electrostatic stability. Essentially, an electric field that arises between
oppositely charged atomic planes gives rise to a built-in potential that
diverges with thickness. In ultra thin film form however the polar crystals are
expected to remain stable without necessitating surface reconstructions, yet
the built-in potential has eluded observation. Here we present evidence of a
built-in potential across polar \lao ~thin films grown on \sto ~substrates, a
system well known for the electron gas that forms at the interface. By
performing electron tunneling measurements between the electron gas and a
metallic gate on \lao ~we measure a built-in electric field across \lao ~of 93
meV/\AA. Additionally, capacitance measurements reveal the presence of an
induced dipole moment near the interface in \sto, illuminating a unique
property of \sto ~substrates. We forsee use of the ionic built-in potential as
an additional tuning parameter in both existing and novel device architectures,
especially as atomic control of oxide interfaces gains widespread momentum.Comment: 6 pages, 4 figures. Submitted to Nature physics on May 1st, 201
Micro/nanostructural properties of peri-implant jaw bones: a human cadaver study
PURPOSE: Many points concerning the structure of osseointegration and the surrounding jaw bone remain unclear, and its optimal histological form has yet to be identified. The aim of this study was to clarify the structural characteristics of peri-implant jaw bone on the micro- and nano-scales by quantitatively evaluating bone quality. METHODS: Five samples of human mandibular bone containing dental implants and one dentate sample that had been in place for some years while the donors were still alive were collected. Bulk staining was performed, and 100-μm-thick polished specimens were prepared. The osteon distributions in peri-implant bone and mandibular cortical bone were measured, after which alignment analysis of biological apatite (BAp) crystallites and anisotropy analysis of collagen fiber orientation using second-harmonic generation imaging were carried out. RESULTS: Osteons in the vicinity of the implant body ran parallel to it. In the cortical bone at the base of the mandible, however, most osteons were oriented mesiodistally. The preferential alignment of BAp crystallites was generally consistent with osteon orientation. The orientation of collagen fibers in peri-implant jaw bone resembled the concentric rings seen in normal cortical bone, but there were also fibers that ran orthogonally across these concentric fibers. CONCLUSIONS: These results suggest that the mechanical strain imposed by implants causes the growth of cortical bone-like bone in areas that would normally consist of cancellous bone around the implants, and that its structural characteristics are optimized for the load environment of the peri-implant jaw bone
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