463 research outputs found
More Evidence for a Distribution of Tunnel Splittings in Mn-acetate
In magnetic fields applied parallel to the anisotropy axis, the magnetization
of Mn has been measured in response to a field that is swept back and
forth across the resonances corresponding to steps . The fraction
of molecules remaining in the metastable well after each sweep through the
resonance is inconsistent with expectations for an ensemble of identical
molecules. The data are consistent instead with the presence of a broad
distribution of tunnel splittings. A very good fit is obtained for a Gaussian
distribution of the second-order anisotropy tunneling parameter . We show that dipolar shuffling is a negligible effect which cannot
explain our data.Comment: minor corrections (PACS nos, signs in Fig. 2
Non-Equilibrium Dynamics and Superfluid Ring Excitations in Binary Bose-Einstein Condensates
We revisit a classic study [D. S. Hall {\it et al.}, Phys. Rev. Lett. {\bf
81}, 1539 (1998)] of interpenetrating Bose-Einstein condensates in the
hyperfine states and of Rb and observe striking new non-equilibrium
component separation dynamics in the form of oscillating ring-like structures.
The process of component separation is not significantly damped, a finding that
also contrasts sharply with earlier experimental work, allowing a clean first
look at a collective excitation of a binary superfluid. We further demonstrate
extraordinary quantitative agreement between theoretical and experimental
results using a multi-component mean-field model with key additional features:
the inclusion of atomic losses and the careful characterization of trap
potentials (at the level of a fraction of a percent).Comment: 4 pages, 3 figures (low res.), to appear in PR
Spectroscopic Determination of Chromium(VI) during the Reduction of Chromium(VI) to Chromium(III)
This is the published version. Copyright 1991 Society for Applied SpectroscopyReduction of chromium(VI) to chromium(III) by thiourea between pH 3.0 and 5.5 is a key aspect of the chromium(VI)/thiourea/polyacrylamide gel polymer system used in enhanced oil recovery processes. A method has been developed to determine chromium(VI) concentration during the reduction of chromium(VI) to chromium(III) in this pH range. The reduction reaction is run in the presence of an acetic acid/sodium acetate buffer which reacts with the chromium(III) produced and prevents the formation of a brown precipitate which forms in the absence of the buffer. With interference from the precipitate eliminated, chromium(VI) concentration is determined from the visible absorbance of the reaction mixture and the unique molar absorptivity spectra of the five chromium species present in the reaction mixture. An average error of approximately 1% between known and measured chromium(VI) concentrations was demonstrated over a chromium(VI) concentration range from 0.0005 to 0.0025 M
Abrupt Transition between Thermally-Activated Relaxation and Quantum Tunneling in a Molecular Magnet
We report Hall sensor measurements of the magnetic relaxation of Mn
acetate as a function of magnetic field applied along the easy axis of
magnetization. Data taken at a series of closely-spaced temperatures between
0.24 K and 1.4 K provide strong new evidence for an abrupt ``first-order''
transition between thermally-assisted relaxation and magnetic decay via quantum
tunneling.Comment: 4 pages, including 7 figure
Quantum-classical transition of the escape rate of uniaxial antiferromagnetic particles in an arbitrarily directed field
Quantum-classical escape rate transition has been studied for uniaxial
antiferromagnetic particles with an arbitrarily directed magnetic field. In the
case that the transverse and longitudinal fileds coexist, we calculate the
phase boundary line between first- and second-order transitions, from which
phase diagrams can be obtained. It is shown that the effects of the applied
longitudinal magnetic field on quantum-classical transition vary greatly for
different relative magnitudes of the non-compensation.Comment: to be appeared in Phys. Rev.
Metal-insulator transition in a 2D electron gas: Equivalence of two approaches for determining the critical point
The critical electron density for the metal-insulator transition in a
two-dimensional electron gas can be determined by two distinct methods: (i) a
sign change of the temperature derivative of the resistance, and (ii) vanishing
activation energy and vanishing nonlinearity of current-voltage characteristics
as extrapolated from the insulating side. We find that in zero magnetic field
(but not in the presence of a parallel magnetic field), both methods give
equivalent results, adding support to the existence of a true zero-field
metal-insulator transition.Comment: As publishe
Single and Paired Point Defects in a 2D Wigner Crystal
Using the path-integral Monte Carlo method, we calculate the energy to form
single and pair vacancies and interstitials in a two-dimensional Wigner crystal
of electrons. We confirm that the lowest-lying energy defects of a 2D electron
Wigner crystal are interstitials, with a creation energy roughly 2/3 that of a
vacancy. The formation energy of the defects goes to zero near melting,
suggesting that point defects might mediate the melting process. In addition,
we find that the interaction between defects is strongly attractive, so that
most defects will exist as bound pairs.Comment: 4 pages, 5 encapsulated figure
Magnetic Field Induced Spin Polarization of AlAs Two-dimensional Electrons
Two-dimensional (2D) electrons in an in-plane magnetic field become fully
spin polarized above a field B_P, which we can determine from the in-plane
magnetoresistance. We perform such measurements in modulation-doped AlAs
electron systems, and find that the field B_P increases approximately linearly
with 2D electron density. These results imply that the product |g*|m*, where g*
is the effective g-factor and m* the effective mass, is a constant essentially
independent of density. While the deduced |g*|m* is enhanced relative to its
band value by a factor of ~ 4, we see no indication of its divergence as 2D
density approaches zero. These observations are at odds with results obtained
in Si-MOSFETs, but qualitatively confirm spin polarization studies of 2D GaAs
carriers.Comment: 4 pages, 5 figure
Temperature-Dependence of the Resistivity of a Dilute 2D Electron System in High Parallel Magnetic Field
We report measurements of the resistance of silicon MOSFETs as a function of
temperature in high parallel magnetic fields where the 2D system of electrons
has been shown to be fully spin-polarized. A magnetic field suppresses the
metallic behavior observed in the absence of a magnetic field. In a field of
10.8 T, insulating behavior is found for densities up to n_s approximately 1.35
x 10^{11} cm^{-2} or 1.5 n_c; above this density the resistance is a very weak
function of temperature, varying less than 10% between 0.25 K and 1.90 K. At
low densities the resistance goes to infinity more rapidly as the temperature
is reduced than in zero field and the magnetoresistance diverges as T goes to
0.Comment: 4 pages, including 4 figures. References adde
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