58,833 research outputs found
Theory of Spin-Split Cyclotron Resonance in the Extreme Quantum Limit
We present an interpretation of recent cyclotron resonance experiments on the
two-dimensional electron gas in GaAs/AlGaAs heterostructures. We show that the
observed dependence of the resonance spectrum on Landau level occupancy and
temperature arises from the interplay of three factors: spin-splitting of the
cyclotron frequency; thermal population of the two spin states; and coupling of
the resonances for each spin orientation by Coulomb interactions. In addition,
we derive an -sum rule which allows spin polarisation to be determined
directly from resonance spectra.Comment: 10 pages (revtex 3.0) + 4 uuencoded postscript figure
Low Temperature Superfluid Response of High-Tc Superconductors
We have reviewed our theoretical and experimental results of the low
temperature superfluid response function of high temperature superconductors
(HTSC). In clean high-Tc materials the in-plane superfluid density rho_s^{ab}
varies linearly with temperature. The slope of this linear T term is found to
scale approximately with 1/Tc which, according to the weak coupling BCS theory
for a d-wave superconductor, implies that the gap amplitude scales
approximately with Tc. A T^5 behavior of the out-of-plane superfluid density
rho_s^c for clean tetragonal HTSC was predicted and observed experimentally in
the single layer Hg-compound HgBa_2CuO_{4+delta}. In other tetragonal high-Tc
compounds with relatively high anisotropy, such as Hg_2Ba_2Ca_2Cu_3O_{8+delta},
rho_s^c varies as T^2 due to disorder effects. In optimally doped
YBa_2Cu_3O_{7-delta}, rho_s^c varies linearly with temperature at low
temperatures, but in underdoped YBa_2Cu_3O_{7-delta}, rho_s^c varies as T^2 at
low temperatures; these results are consistent with our theoretical
calculations.Comment: 26 pages, 8 figure
Optical sum rules that relate to the potential energy of strongly correlated systems
A class of sum rules for inelastic light scattering is developed. We show
that the first moment of the non-resonant response provides information about
the potential energy in strongly correlated systems. The polarization
dependence of the sum rules provide information about the electronic
excitations in different regions of the Brillouin zone. We determine the sum
rule for the Falicov-Kimball model, which possesses a metal-insulator
transition, and compare our results to the light scattering experiments in
SmB_6.Comment: (5 pages, 3 figures, typeset in ReVTeX
Structural and electrostatic effects at the surfaces of size- and charge-selected aqueous nanodrops.
The effects of ion charge, polarity and size on the surface morphology of size-selected aqueous nanodrops containing a single ion and up to 550 water molecules are investigated with infrared photodissociation (IRPD) spectroscopy and theory. IRPD spectra of M(H2O) n where M = La3+, Ca2+, Na+, Li+, I-, SO42- and supporting molecular dynamics simulations indicate that strong interactions between multiply charged ions and water molecules can disrupt optimal hydrogen bonding (H-bonding) at the nanodrop surface. The IRPD spectra also reveal that "free" OH stretching frequencies of surface-bound water molecules are highly sensitive to the ion's identity and the OH bond's local H-bond environment. The measured frequency shifts are qualitatively reproduced by a computationally inexpensive point-charge model that shows the frequency shifts are consistent with a Stark shift from the ion's electric field. For multiply charged cations, pronounced Stark shifting is observed for clusters containing ∼100 or fewer water molecules. This is attributed to ion-induced solvent patterning that extends to the nanodrop surface, and serves as a spectroscopic signature for a cation's ability to influence the H-bond network of water located remotely from the ion. The Stark shifts measured for the larger nanodrops are extrapolated to infinite dilution to obtain the free OH stretching frequency of a surface-bound water molecule at the bulk air-water interface (3696.5-3701.0 cm-1), well within the relatively wide range of values obtained from SFG measurements. These cluster measurements also indicate that surface curvature effects can influence the free OH stretching frequency, and that even nanodrops without an ion have a surface potential that depends on cluster size
The use of the LANDSAT data collection system and imagery in reservoir management and operation
The author has identified the following significant results. An increase in the data collection system's (DCS) ability to function in the flood control mission with no additional manpower was demonstrated during the storms which struck New England during April and May of 1975 and August 1976. It was found that for this watershed, creditable flood hydrographs could be generated from DCS data. It was concluded that an ideal DCS for reservoir regulation would draw features from LANDSAT and GOES. MSS grayscale computer printout and a USGS topographic map were compared, yielding an optimum computer classification map of the wetland areas of the Merrimack River estuary. A classification accuracy of 75% was obtained for the wetlands unit, taking into account the misclassified and the unclassified pixels. The MSS band 7 grayscale printouts of the Franklin Falls reservoir showed good agreement to USGS topographic maps in total area of water depicted at the low water reservoir stage and at the maximum inundation level. Preliminary analysis of the LANDSAT digital data using the GISS computer algorithms showed that the radiance of snow cover/vegetation varied from approximately 20 mW/sq cm sr in nonvegetated areas to less than 4 mW/sq cm sr for densely covered forested area
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