36,708 research outputs found
Polaron Crossover in Molecular Solids
An analytical variational method is applied to the molecular Holstein
Hamiltonian in which the dispersive features of the dimension dependent phonon
spectrum are taken into account by a force constant approach. The crossover
between a large and a small size polaron is monitored, in one, two and three
dimensions and for different values of the adiabatic parameter, through the
behavior of the effective mass as a function of the electron-phonon coupling.
By increasing the strength of the inter-molecular forces the crossover becomes
smoother and occurs at higher {\it e-ph} couplings. These effects are more
evident in three dimensions. We show that our Modified Lang-Firsov method
starts to capture the occurence of a polaron self-trapping transition when the
electron energies become of order of the phonon energies. The self-trapping
event persists in the fully adiabatic regime. At the crossover we estimate
polaron effective masses of order times the bare band mass
according to dimensionality and value of the adiabatic parameter. Modified
Lang-Firsov polaron masses are substantially reduced in two and three
dimensions. There is no self-trapping in the antiadiabatic regime.Comment: To be published in J.Phys.:Condensed Matte
Effects of c-axis Hopping in the Interlayer Tunneling Model of High-Tc Layered Cuprates
We consider the interlayer pair-tunneling model for layered cuprates,
including an effective single particle hopping along the c-axis. A
phenomenological suppression of the c-axis hopping matrix element, by the
pseudogap in cuprate superconductors, is incorporated. At optimal doping,
quantities characteristic to the superconducting state, such as the transition
temperature and the superconducting gap are calculated. Results from our
calculations are consistent with the experimental observations with the
noteworthy point that, the superconducting gap as a function of temperature
shows excellent match to the experimental data. Predictions within the model,
regarding T_c variation with interlayer coupling, are natural outcomes which
could be tested further.Comment: Latex file, 18 pages, 4 figures (postscript files included), to
appear in Int. J. Mod. Phys.
Use of bamboo fiber in oil water separation
One of the environmental issues facing the society is the separation of oil from water in emulsions. Oily wastewater enters into the environment through many ways such as oil spill as well as from the industry. Natural fibers are a viable alternative to synthetic fibers in separating oil from the water. The oil physical characteristics and sorbents made from the fiber influences the sorption of oil onto the fiber. This work uses the naturally available bamboo fibers for separation of oil from water. Very high adsorption capacities were obtained for vegetable oil. Furthermore, recovery of oil was also tested and 90% recovery was obtained. Bamboo fiber has thus great advantage in treating oil-water mixture
Comparison of perturbative expansions using different phonon bases for two-site Holstein model
The two-site single-polaron problem is studied within the perturbative
expansions using different standard phonon basis obtained through the Lang
Firsov (LF), modified LF (MLF) and modified LF transformation with squeezed
phonon states (MLFS). The role of these convergent expansions using the above
prescriptions in lowering the energy and in determining the correlation
functions are compared for different values of coupling strength. The
single-electron energy, oscillator wave functions and correlation functions are
calculated for the same system. The applicability of different phonon basis in
different regimes of the coupling strength as well as in different regimes of
hopping are also discussed.Comment: 24 pages (RevTEX), 12 postscript figures, final version accepted in
PRB(2000) Jornal Ref: Phys. Rev. B, 61, 4592-4602 (2000
The pointing errors of geosynchronous satellites
A study of the correlation between cloud motion and wind field was initiated. Cloud heights and displacements were being obtained from a ceilometer and movie pictures, while winds were measured from pilot balloon observations on a near-simultaneous basis. Cloud motion vectors were obtained from time-lapse cloud pictures, using the WINDCO program, for 27, 28 July, 1969, in the Atlantic. The relationship between observed features of cloud clusters and the ambient wind field derived from cloud trajectories on a wide range of space and time scales is discussed
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