491 research outputs found
Structural phase control of (LaNdSr)CuO thin films by epitaxial growth technique
Epitaxial growth of (LaNdSr)CuO thin films was
studied by pulsed-laser deposition technique on three different substrates,
SrTiO (100), LaSrAlO (001), and YAlO (001). The
(Nd,Sr,Ce)CuO-type structure appears at the initial growth stage on
SrTiO (100) when the film is deposited under the growth conditions
optimized for (La,Sr)CuO. This (Nd,Sr,Ce)CuO-type structure can
be eliminated by increasing the substrate temperature and the laser repetition
frequency. Films on LaSrAlO (001) maintain a LaCuO-type structure
as bulk samples, but those on YAlO (001) show phase separation into
LaCuO- and NdCuO-type structures. Such complicated results are
explained in terms of the competition between lattice misfit and thermodynamic
conditions. Interestingly the films with LaCuO-type structure prepared
on SrTiO and LaSrAlO show different surface structures and transport
properties. The results indicate the possibility of controlling charge stripes
of (LaNdSr)CuO as was demonstrated in
(La,Ba)CuO thin films by Sato et al. (Phys. Rev. B {\bf 62}, R799
(2000)).Comment: 5 pages, 6 EPS figure, accepted for publication in Phys. Rev.
Nuclear Spin-Isospin Correlations, Parity Violation, and the Problem
The strong interaction effects of isospin- and spin-dependent nucleon-nucleon
correlations observed in many-body calculations are interpreted in terms of a
one-pion exchange mechanism. Including such effects in computations of nuclear
parity violating effects leads to enhancements of about 10%. A larger effect
arises from the one-boson exchange nature of the parity non-conserving nucleon-
nucleon interaction, which depends on both weak and strong meson-nucleon
coupling constants. Using values of the latter that are constrained by
nucleon-nucleon phase shifts leads to enhancements of parity violation by
factors close to two. Thus much of previously noticed discrepancies between
weak coupling constants extracted from different experiments can be removed.Comment: 8 pages 2 figures there should have been two figures in v
Glassy nature of stripe ordering in La(1.6-x)Nd(0.4)Sr(x)CuO(4)
We present the results of neutron-scattering studies on various aspects of
crystalline and magnetic structure in single crystals of
La(1.6-x)Nd(0.4)Sr(x)CuO(4) with x=0.12 and 0.15. In particular, we have
reexamined the degree of stripe order in an x=0.12 sample. Measurements of the
width for an elastic magnetic peak show that it saturates at a finite value
below 30 K, corresponding to a spin-spin correlation length of 200 A. A model
calculation indicates that the differing widths of magnetic and (previously
reported) charge-order peaks, together with the lack of commensurability, can
be consistently explained by disorder in the stripe spacing. Above 30 K, the
width of the nominally elastic signal begins to increase. Interpreting the
signal as critical scattering from slowly fluctuating spins, the temperature
dependence of the width is consistent with renormalized classical behavior of a
2-dimensional anisotropic Heisenberg antiferromagnet. Inelastic scattering
measurements show that incommensurate spin excitations survive at and above 50
K, where the elastic signal is neglible. We also report several results related
to the LTO-to-LTT transition.Comment: 13 pp, 2-col. REVTeX, 11 figures embedded with psfig; expanded
discussion of T-dep. of magnetic peak width; version to appear in Phys. Rev.
B (01Jun99
Nab: Measurement Principles, Apparatus and Uncertainties
The Nab collaboration will perform a precise measurement of 'a', the
electron-neutrino correlation parameter, and 'b', the Fierz interference term
in neutron beta decay, in the Fundamental Neutron Physics Beamline at the SNS,
using a novel electric/magnetic field spectrometer and detector design. The
experiment is aiming at the 10^{-3} accuracy level in (Delta a)/a, and will
provide an independent measurement of lambda = G_A/G_V, the ratio of
axial-vector to vector coupling constants of the nucleon. Nab also plans to
perform the first ever measurement of 'b' in neutron decay, which will provide
an independent limit on the tensor weak coupling.Comment: 12 pages, 6 figures, 1 table, talk presented at the International
Workshop on Particle Physics with Slow Neutrons, Grenoble, 29-31 May 2008; to
appear in Nucl. Instrum. Meth. in Physics Research
Hall coefficient of LaYSrCuO () at low temperatures under high magnetic fields
The Hall coefficient in the low-temperature tetragonal phase and the
mid-temperature orthorhombic phase of LaYSrCuO
() single crystals is measured under high magnetic fields up to 9 T
in order to investigate the detailed behavior of the transport properties at
low temperatures in the stripe phase. When the superconductivity is suppressed
by high magnetic fields, the Hall coefficient has negative values in low
temperatures, and the temperature region of the negative values spreads as
increasing magnetic fields. This result indicates that the Hall coefficient in
the stripe phase around is a finite negative value, not zero.Comment: 4 pages, 4 figures. to be published to Physical Review
Strange hadron matter and SU(3) symmetry
We calculate saturation curves for strange hadron matter using recently
constructed baryon-baryon potentials which are constrained by SU(3) symmetry.
All possible interaction channels within the baryon octet (consisting of ,
, , and ) are considered. It is found that a small
fraction in nuclear matter slightly increases binding, but that
larger fractions () rapidly cause a decrease. Charge-neutral
systems, with equal densities for nucleons and cascades, are
only very weakly bound. The dependence of the binding energies on the
strangeness per baryon, , is predicted for various and
systems. The implications of our results in
relativistic heavy-ion collisions and the core of a dense star are discussed.
We also discuss the differences between our results and previous hadron matter
calculations.Comment: 14 pages RevTeX, 7 postscript figure
Mixed-mode fracture characteristics of metal-to-metal adhesively bonded joints: experimental and simulation methods.
Fracture behavior of adhesively bonded joints subjected to mixed-mode (i.e. mode I+II) loading conditions is of importance in many industrial applications. This research therefore aims to characterise the failure behaviour of metal-to-metal (i.e. both aluminium adherends) adhesive joints using the mixed mode bending test (MMB), adapted from ASTM D6671/D6671M standard, along with instrumentation using acoustic emission (AE) sensor. Twenty-four adhesively bonded specimens were prepared using two types of adhesive bond materials (acrylic, cyanoacrylate) with two different bonded area 65% and 100%. To understand the effect of mixed-mode loading conditions on the failure behavior, two different mixity ratios were achieved through the design of the MMB test fixture and tested for each bonded joint. The AE results during mechanical testing shows that the time domain signals were spread over the loading phase with distinct features for different mixity ratios. They successfully identified the moment of adhesive fracture during every test. Also, the fracture behavior of the bonded joints was simulated using virtual crack closure technique (VCCT) method using finite element method to understand the loading dynamics in specimen when considering a combination of various design parameters. In addition, an analytical method (e.g. corrected beam theory or CBT) was used to determine strain energy release rates of each specimen. The results show that both the brittle and ductile specimens exhibited higher energy release rates when mode II proportion of loading was increased during the crack initiation phase. The proposed measurement can be useful to assess the overall structural health of bonded systems
Landau Damping and Coherent Structures in Narrow-Banded 1+1 Deep Water Gravity Waves
We study the nonlinear energy transfer around the peak of the spectrum of
surface gravity waves by taking into account nonhomogeneous effects. In the
narrow-banded approximation the kinetic equation resulting from a
nonhomogeneous wave field is a Vlasov-Poisson type equation which includes at
the same time the random version of the Benjamin-Feir instability and the
Landau damping phenomenon. We analytically derive the values of the Phillips'
constant and the enhancement factor for which the
narrow-banded approximation of the JONSWAP spectrum is unstable. By performing
numerical simulations of the nonlinear Schr\"{o}dinger equation we check the
validity of the prediction of the related kinetic equation. We find that the
effect of Landau damping is to suppress the formation of coherent structures.
The problem of predicting freak waves is briefly discussed.Comment: 4 pages, 3 figure
Systematic Cu-63 NQR studies of the stripe phase in La(1.6-x)Nd(0.4)Sr(x)CuO(4) for 0.07 <= x <= 0.25
We demonstrate that the integrated intensity of Cu-63 nuclear quadrupole
resonance (NQR) in La(1.6-x)Nd(0.4)Sr(x)CuO(4) decreases dramatically below the
charge-stripe ordering temperature T(charge). Comparison with neutron and X-ray
scattering indicates that the wipeout fraction F(T) (i.e. the missing fraction
of the integrated intensity of the NQR signal) represents the charge-stripe
order parameter. The systematic study reveals bulk charge-stripe order
throughout the superconducting region 0.07 <= x <= 0.25. As a function of the
reduced temperature t = T/T(charge), the temperature dependence of F(t) is
sharpest for the hole concentration x=1/8, indicating that x=1/8 is the optimum
concentration for stripe formation.Comment: 10 pages of text and captions, 11 figures in postscript. Final
version, with new data in Fig.
A numerical study of multi-soliton configurations in a doped antiferromagnetic Mott insulator
We evaluate from first principles the self-consistent Hartree-Fock energies
for multi-soliton configurations in a doped, spin-1/2, antiferromagnetic Mott
insulator on a two-dimensional square lattice. We find that nearest-neighbor
Coulomb repulsion stabilizes a regime of charged meron-antimeron vortex soliton
pairs over a region of doping from 0.05 to 0.4 holes per site for intermediate
coupling 3 < U/t <8. This stabilization is mediated through the generation of
``spin-flux'' in the mean-field antiferromagnetic (AFM) background. Holes
cloaked by a meron-vortex in the spin-flux AFM background are charged bosons.
Our static Hartree-Fock calculations provide an upper bound on the energy of a
finite density of charged vortices. This upper bound is lower than the energy
of the corresponding charged stripe configurations. A finite density of charge
carrying vortices is shown to produce a large number of unoccupied electronic
levels in the Mott-Hubbard charge transfer gap. These levels lead to
significant band tailing and a broad mid-infrared band in the optical
absorption spectrum as observed experimentally. At very low doping (below 0.05)
the doping charges create extremely tightly bound meron-antimeron pairs or even
isolated conventional spin-polarons, whereas for very high doping (above 0.4)
the spin background itself becomes unstable to formation of a conventional
Fermi liquid and the spin-flux mean-field is energetically unfavorable. Our
results point to the predominance of a quantum liquid of charged, bosonic,
vortex solitons at intermediate coupling and intermediate doping
concentrations.Comment: 12 pages, 25 figures; added references, modified/eliminated some
figure
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