7,445 research outputs found
First- and Second Order Phase Transitions in the Holstein-Hubbard Model
We investigate metal-insulator transitions in the Holstein-Hubbard model as a
function of the on-site electron-electron interaction U and the electron-phonon
coupling g. We use several different numerical methods to calculate the phase
diagram, the results of which are in excellent agreement. When the
electron-electron interaction U is dominant the transition is to a
Mott-insulator; when the electron-phonon interaction dominates, the transition
is to a localised bipolaronic state. In the former case, the transition is
always found to be second order. This is in contrast to the transition to the
bipolaronic state, which is clearly first order for larger values of U. We also
present results for the quasiparticle weight and the double-occupancy as
function of U and g.Comment: 6 pages, 5 figure
Antisymmetrized molecular dynamics of wave packets with stochastic incorporation of Vlasov equation
On the basis of the antisymmetrized molecular dynamics (AMD) of wave packets
for the quantum system, a novel model (called AMD-V) is constructed by the
stochastic incorporation of the diffusion and the deformation of wave packets
which is calculated by Vlasov equation without any restriction on the one-body
distribution. In other words, the stochastic branching process in molecular
dynamics is formulated so that the instantaneous time evolution of the averaged
one-body distribution is essentially equivalent to the solution of Vlasov
equation. Furthermore, as usual molecular dynamics, AMD-V keeps the many-body
correlation and can naturally describe the fluctuation among many channels of
the reaction. It is demonstrated that the newly introduced process of AMD-V has
drastic effects in heavy ion collisions of 40Ca + 40Ca at 35 MeV/nucleon,
especially on the fragmentation mechanism, and AMD-V reproduces the
fragmentation data very well. Discussions are given on the interrelation among
the frameworks of AMD, AMD-V and other microscopic models developed for the
nuclear dynamics.Comment: 26 pages, LaTeX with revtex and epsf, embedded postscript figure
Antisymmetrized molecular dynamics with quantum branching processes for collisions of heavy nuclei
Antisymmetrized molecular dynamics (AMD) with quantum branching processes is
reformulated so that it can be applicable to the collisions of heavy nuclei
such as Au + Au multifragmentation reactions. The quantum branching process due
to the wave packet diffusion effect is treated as a random term in a
Langevin-type equation of motion, whose numerical treatment is much easier than
the method of the previous papers. Furthermore a new approximation formula,
called the triple-loop approximation, is introduced in order to evaluate the
Hamiltonian in the equation of motion with much less computation time than the
exact calculation. A calculation is performed for the Au + Au central
collisions at 150 MeV/nucleon. The result shows that AMD almost reproduces the
copious fragment formation in this reaction.Comment: 24 pages, 5 figures embedde
Using single quantum states as spin filters to study spin polarization in ferromagnets
By measuring electron tunneling between a ferromagnet and individual energy
levels in an aluminum quantum dot, we show how spin-resolved quantum states can
be used as filters to determine spin-dependent tunneling rates. We also observe
magnetic-field-dependent shifts in the magnet's electrochemical potential
relative to the dot's energy levels. The shifts vary between samples and are
generally smaller than expected from the magnet's spin-polarized density of
states. We suggest that they are affected by field-dependent charge
redistribution at the magnetic interface.Comment: 4 pages, 1 color figur
Response of convection electric fields in the magnetosphere to IMF orientation change
[1] The transient response of convection electric fields in the inner magnetosphere to southward turning of the interplanetary magnetic field (IMF) is investigated using inâsitu electric field observations by the CRRES and Akebono spacecraft. Electric fields earthward of the inner edge of the electron plasma sheet show quick responses simultaneously with change in ionospheric electric fields, which indicates the arrival of the first signal related to southward turning. A coordinated observation of the electric field by the CRRES and Akebono spacecraft separated by 5 RE reveals a simultaneous increase in the dawnâdusk electric field in a wide region of the inner magnetosphere. A quick response associated with the southward turning of the IMF is also identified in inâsitu magnetic fields. It indicates that the southward turning of the IMF initiates simultaneous (less than 1 min) enhancements of ionospheric electric fields, convection electric fields in the inner magnetosphere, and the ring or tail current and region 2 FACs. In contrast, a quick response of convection electric fields is not identified in the electron plasma sheet. A statistical study using 161 events of IMF orientation change in 1991 confirms a prompt response within 5 min for 80% of events earthward of the electron plasma sheet, while a large time lag of more than 30 min is identified in electric fields in the electron plasma sheet. The remarkable difference in the response of electric fields indicates that electric fields in the electron plasma sheet are weakened by high conductance in the magnetically conjugated auroral ionosphere.https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2009JA014277https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2009JA014277Published versio
Compatibility of localized wave packets and unrestricted single particle dynamics for cluster formation in nuclear collisions
Antisymmetrized molecular dynamics with quantum branching is generalized so
as to allow finite time duration of the unrestricted coherent mean field
propagation which is followed by the decoherence into wave packets. In this new
model, the wave packet shrinking by the mean field propagation is respected as
well as the diffusion, so that it predicts a one-body dynamics similar to that
in mean field models. The shrinking effect is expected to change the diffusion
property of nucleons in nuclear matter and the global one-body dynamics. The
central \xenon+\tin collisions at 50 MeV/nucleon are calculated by the models
with and without shrinking, and it is shown that the inclusion of the wave
packet shrinking has a large effect on the multifragmentation in a big
expanding system with a moderate expansion velocity.Comment: 16 pages, 7 figure
Torsion-induced persistent current in a twisted quantum ring
We describe the effects of geometric torsion on the coherent motion of
electrons along a thin twisted quantum ring. The geometric torsion inherent in
the quantum ring triggers a quantum phase shift in the electrons' eigenstates,
thereby resulting in a torsion-induced persistent current that flows along the
twisted quantum ring. The physical conditions required for detecting the
current flow are discussed.Comment: 9 pages, 3 figure
Thermomechanical force application
The present work conducted in Summer 1987 continues investigations on Thermal Components for 1.8 K Space Cryogenics (Grant NAG 1-412 of 1986). The topics addressed are plug characterization efforts in a small pore size regime of sintered metal plugs, characterization in the nonlinear regime, temperature profiles in a heat supply unit for a fountain effect pump and modeling efforts
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