8,458 research outputs found
Anomalous magnetoresistance peak in (110) GaAs two-dimensional holes: Evidence for Landau-level spin-index anticrossings
We measure an anomalous magnetoresistance peak within the lowest Landau level
(nu = 1) minimum of a two-dimensional hole system on (110) GaAs.
Self-consistent calculations of the valence band mixing show that the two
lowest spin-index Landau levels anticross in a perpendicular magnetic field B
consistent with where the experimental peak is measured, Bp. The temperature
dependence of the anomalous peak height is interpreted as an activated behavior
across this anticrossing gap. Calculations of the spin polarization in the
lowest Landau levels predict a rapid switch from about -3/2 to +3/2 spin at the
anticrossing. The peak position Bp is shown to be affected by the confinement
electrostatics, and the utility of a tunable anticrossing position for
spintronics applications is discussed.Comment: 4 pages, 4 figure
Spin Density Matrix of Spin-3/2 Hole Systems
For hole systems with an effective spin j=3/2, we present an invariant
decomposition of the spin density matrix that can be interpreted as a multipole
expansion. The charge density corresponds to the monopole moment and the spin
polarization due to a magnetic field corresponds to a dipole moment while heavy
hole-light hole splitting can be interpreted as a quadrupole moment. For quasi
two-dimensional hole systems in the presence of an in-plane magnetic field B
the spin polarization is a higher-order effect that is typically much smaller
than one even if the minority spin subband is completely depopulated. On the
other hand, the field B can induce a substantial octupole moment which is a
unique feature of j=3/2 hole systems.Comment: 8 pages, 1 figure, 3 table
Coherent optical transfer of Feshbach molecules to a lower vibrational state
Using the technique of stimulated Raman adiabatic passage (STIRAP) we have
coherently transferred ultracold 87Rb2 Feshbach molecules into a more deeply
bound vibrational quantum level. Our measurements indicate a high transfer
efficiency of up to 87%. As the molecules are held in an optical lattice with
not more than a single molecule per lattice site, inelastic collisions between
the molecules are suppressed and we observe long molecular lifetimes of about 1
s. Using STIRAP we have created quantum superpositions of the two molecular
states and tested their coherence interferometrically. These results represent
an important step towards Bose-Einstein condensation (BEC) of molecules in the
vibrational ground state.Comment: 4 pages, 5 figure
Determination of Gd concentration profile in UO2-Gd2O3 fuel pellets
A transversal mapping of the Gd concentration was measured in UO2-Gd2O3
nuclear fuel pellets by electron paramagnetic resonance spectroscopy (EPR). The
quantification was made from the comparison with a Gd2O3 reference sample. The
nominal concentration in the pellets is UO2: 7.5 % Gd2O3. A concentration
gradient was found, which indicates that the Gd2O3 amount diminishes towards
the edges of the pellets. The concentration varies from (9.3 +/- 0.5)% in the
center to (5.8 +/- 0.3)% in one of the edges. The method was found to be
particularly suitable for the precise mapping of the distribution of Gd3+ ions
in the UO2 matrix.Comment: 10 pages, 5 figures, 2 tables. Submitted to Journal of Nuclear
Material
Invariant expansion for the trigonal band structure of graphene
We present a symmetry analysis of the trigonal band structure in graphene,
elucidating the transformational properties of the underlying basis functions
and the crucial role of time-reversal invariance. Group theory is used to
derive an invariant expansion of the Hamiltonian for electron states near the K
points of the graphene Brillouin zone. Besides yielding the characteristic
k-linear dispersion and higher-order corrections to it, this approach enables
the systematic incorporation of all terms arising from external electric and
magnetic fields, strain, and spin-orbit coupling up to any desired order.
Several new contributions are found, in addition to reproducing results
obtained previously within tight-binding calculations. Physical ramifications
of these new terms are discussed.Comment: 10 pages, 1 figure; expanded version with more details and additional
result
Pancreatic lipases: evolutionary intermediates in a positional change of catalytic carboxylates?
Comparison of the fold of lipases from Geotrichum candidum and from human pancreas identified a high degree of similarity which was not expected on the basis of their amino acid sequences. Although both enzymes utilize a serine protease-like catalytic triad, they differ in the topological position of the acid. We speculate that these proteins are evolutionarily related and that the pancreatic lipase is an evolutionary intermediate in the pathway of migration of the catalytic acid to a new position within the fold
Shuttle time and frequency transfer experiment
A proposed space shuttle experiment to demonstrate techniques for global high precision comparison of clocks and primary frequency standards is described. The experiment, using transmitted microwave and pulsed laser signals, compared a hydrogen maser clock onboard the space shuttle with a clock in a ground station in order to demonstrate time transfer with accuracies of 1 nsec or better and frequency comparison at the 10 to the -14th power accuracy level
V-V Bond-Length Fluctuations in Vox
We report a significantly stronger suppression of the phonon contribution to
the thermal conductivity in VOx than can be accounted for by disorder of the 16
% atomic vacancies present in VO. Since the transition from localized to
itinerant electronic behavior is first-order and has been shown to be
characterized by bond-length fluctuations in several transition-metal oxides
with the perovskite structure, we propose that cooperative V-V bond-length
fluctuations play a role in VO similar to the M-O bond-length fluctuations in
the perovskites. This model is able to account for the strong suppression of
the thermal conductivity, the existence of a pseudogap confirmed by
thermoelectric power, an anomalously large Debye-Waller factor, the temperature
dependence of the magnetic susceptibility, and the inability to order atomic
vacancies in VO.Comment: 5 pages, 5 figure
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