6,056 research outputs found
Impurity relaxation mechanism for dynamic magnetization reversal in a single domain grain
The interaction of coherent magnetization rotation with a system of two-level
impurities is studied. Two different, but not contradictory mechanisms, the
`slow-relaxing ion' and the `fast-relaxing ion' are utilized to derive a system
of integro-differential equations for the magnetization. In the case that the
impurity relaxation rate is much greater than the magnetization precession
frequency, these equations can be written in the form of the Landau-Lifshitz
equation with damping. Thus the damping parameter can be directly calculated
from these microscopic impurity relaxation processes
Adsorption and two-body recombination of atomic hydrogen on He-He mixture films
We present the first systematic measurement of the binding energy of
hydrogen atoms to the surface of saturated He-He mixture films.
is found to decrease almost linearly from 1.14(1) K down to 0.39(1) K, when the
population of the ground surface state of He grows from zero to
cm, yielding the value K cm
for the mean-field parameter of H-He interaction in 2D. The experiments
were carried out with overall He concentrations ranging from 0.1 ppm to 5 %
as well as with commercial and isotopically purified He at temperatures
70...400 mK. Measuring by ESR the rate constants and for
second-order recombination of hydrogen atoms in hyperfine states and we
find the ratio to be independent of the He content and to
grow with temperature.Comment: 4 pages, 4 figures, all zipped in a sigle file. Submitted to Phys.
Rev. Let
Overtones of Isoscalar Giant Resonances in medium-heavy and heavy nuclei
A semi-microscopic approach based on both the
continum-random-phase-approximation (CRPA) method and a phenomenological
treatment of the spreading effect is extended and applied to describe the main
properties (particle-hole strength distribution, energy-dependent transition
density, partial direct-nucleon-decay branching ratios) of the isoscalar giant
dipole, second monopole, and second quadrupole resonances. Abilities of the
approach are checked by description of gross properties of the main-tone
resonances. Calculation results obtained for the resonances in a few singly-
and doubly-closed-shell nuclei are compared with available experimental data.Comment: 12 pages, 14 figures, submitted to Phys. Rev.
Higher order effective low-energy theories
Three well-known perturbative approaches to deriving low-energy effective
theories, the degenerate Brillouin-Wigner perturbation theory (projection
method), the canonical transformation, and the resolvent methods are compared.
We use the Hubbard model as an example to show how, to fourth order in hopping
t, all methods lead to the same effective theory, namely the t-J model with
ring exchange and various correlated hoppings. We emphasize subtle technical
difficulties that make such a derivation less trivial to carry out for orders
higher than second. We also show that in higher orders, different approaches
can lead to seemingly different forms for the low-energy Hamiltonian. All of
these forms are equivalent since they are connected by an additional unitary
transformation whose generator is given explicitly. The importance of
transforming the operators is emphasized and the equivalence of their
transformed structure within the different approaches is also demonstrated.Comment: 14 pages, no figure
Enhanced shot noise in resonant tunnelling via interacting localised states
In a variety of mesoscopic systems shot noise is seen to be suppressed in
comparison with its Poisson value. In this work we observe a considerable
enhancement of shot noise in the case of resonant tunnelling via localised
states. We present a model of correlated transport through two localised states
which provides both a qualitative and quantitative description of this effect.Comment: 4 pages, 4 figure
Two-dimensional atom trapping in field-induced adiabatic potentials
We show how to create a novel two-dimensional trap for ultracold atoms from a conventional magnetic trap. We achieve this by utilizing rf-induced adiabatic potentials to enhance the trapping potential in one direction. We demonstrate the loading process and discuss the experimental conditions under which it might be possible to prepare a 2D Bose condensate. A scheme for the preparation of coherent matterwave bubbles is also discussed
Time scales of epidemic spread and risk perception on adaptive networks
Incorporating dynamic contact networks and delayed awareness into a contagion
model with memory, we study the spreading patterns of infectious diseases in
connected populations. It is found that the spread of an infectious disease is
not only related to the past exposures of an individual to the infected but
also to the time scales of risk perception reflected in the social network
adaptation. The epidemic threshold is found to decrease with the rise
of the time scale parameter s and the memory length T, they satisfy the
equation .
Both the lifetime of the epidemic and the topological property of the evolved
network are considered. The standard deviation of the degree
distribution increases with the rise of the absorbing time , a power-law
relation is found
Bose-Einstein condensation of magnons under incoherent pumping
Bose-Einstein condensation in a gas of magnons pumped by an incoherent
pumping source is experimentally studied at room temperature. We demonstrate
that the condensation can be achieved in a gas of bosons under conditions of
incoherent pumping. Moreover, we show the critical transition point is almost
independent of the frequency spectrum of the pumping source and is solely
determined by the density of magnons. The electromagnetic power radiated by the
magnon condensate was found to scale quadratically with the pumping power,
which is in accordance with the theory of Bose-Einstein condensation in magnon
gases
Partial Schauder estimates for second-order elliptic and parabolic equations
We establish Schauder estimates for both divergence and non-divergence form
second-order elliptic and parabolic equations involving H\"older semi-norms not
with respect to all, but only with respect to some of the independent
variables.Comment: CVPDE, accepted (2010)
Fermi-liquid behaviour of the low-density 2D hole gas in GaAs/AlGaAs heterostructure at large values of r_s
We examine the validity of the Fermi-liquid description of the dilute 2D hole
gas in the crossover from 'metallic'-to-'insulating' behaviour of R(T).It has
been established that, at r_s as large as 29, negative magnetoresistance does
exist and is well described by weak localisation. The dephasing time extracted
from the magnetoresistance is dominated by the T^2 -term due to Landau
scattering in the clean limit. The effect of hole-hole interactions, however,
is suppressed when compared with the theory for small r_s.Comment: 4 pages ReVTeX, 4 ps figure
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