3,913 research outputs found
Fast magnetization reversal of nanoclusters in resonator
An effective method for ultrafast magnetization reversal of nanoclusters is
suggested. The method is based on coupling a nanocluster to a resonant electric
circuit. This coupling causes the appearance of a magnetic feedback field
acting on the cluster, which drastically shortens the magnetization reversal
time. The influence of the resonator properties, nanocluster parameters, and
external fields on the magnetization dynamics and reversal time is analyzed.
The magnetization reversal time can be made many orders shorter than the
natural relaxation time. The reversal is studied for both the cases of a single
nanocluster as well as for the system of many nanoclusters interacting through
dipole forces.Comment: latex file, 21 pages, 7 figure
Is the equivalence for the response of static scalar sources in the Schwarzschild and Rindler spacetimes valid only in four dimensions?
It was shown recently that in four dimensions scalar sources with fixed
proper acceleration minimally coupled to a massless Klein-Gordon field lead to
the same responses when they are (i) uniformly accelerated in Minkowski
spacetime (in the inertial vacuum) and (ii) static in the Schwarzschild
spacetime (in the Unruh vacuum). Here we show that this equivalence is broken
if the spacetime dimension is more than four.Comment: 4 pages, 1 figur
Fulde-Ferrell-Larkin-Ovchinnikov state in a perpendicular field of quasi two-dimensional CeCoIn5
A Fulde-Ferrell-Larkin-Ovchinnkov (FFLO) state was previously reported in the
quasi-2D heavy fermion CeCoIn5 when a magnetic field was applied parallel to
the ab-plane. Here, we conduct 115^In NMR studies of this material in a
PERPENDICULAR field, and provide strong evidence for FFLO in this case as well.
Although the topology of the phase transition lines in the H-T phase diagram is
identical for both configurations, there are several remarkable differences
between them. Compared to H//ab, the FFLO region for H perpendicular to the
ab-plane shows a sizable decrease, and the critical field separating the FFLO
and non-FFLO superconducting states almost ceases to have a temperature
dependence. Moreover, directing H perpendicular to the ab-plane results in a
notable change in the quasiparticle excitation spectrum within the planar node
associated with the FFLO transition.Comment: 5 pages, 3 figure
Accurate numerical verification of the instanton method for macroscopic quantum tunneling: dynamics of phase slips
Instanton methods, in which imaginary-time evolution gives the tunneling
rate, have been widely used for studying quantum tunneling in various contexts.
Nevertheless, how accurate instanton methods are for the problems of
macroscopic quantum tunneling (MQT) still remains unclear because of lack of
their direct comparison with exact time evolution of the many-body Schroedinger
equation. Here, we verify instanton methods applied to coherent MQT.
Specifically applying the quasi-exact numerical method of time-evolving block
decimation to the system of bosons in a ring lattice, we directly simulate the
real-time quantum dynamics of supercurrents, where a coherent oscillation
between two macroscopically distinct current states occurs due to MQT. The
tunneling rate extracted from the coherent oscillation is compared with that
given by the instanton method. We show that the error is within 10% when the
effective Planck's constant is sufficiently small. We also discuss phase slip
dynamics associated with the coherent oscillations.Comment: 19 pages, 14 figures, 1 tabl
Nonpolar resistance switching of metal/binary-transition-metal oxides/metal sandwiches: homogeneous/inhomogeneous transition of current distribution
Exotic features of a metal/oxide/metal (MOM) sandwich, which will be the
basis for a drastically innovative nonvolatile memory device, is brought to
light from a physical point of view. Here the insulator is one of the
ubiquitous and classic binary-transition-metal oxides (TMO), such as Fe2O3,
NiO, and CoO. The sandwich exhibits a resistance that reversibly switches
between two states: one is a highly resistive off-state and the other is a
conductive on-state. Several distinct features were universally observed in
these binary TMO sandwiches: namely, nonpolar switching, non-volatile threshold
switching, and current--voltage duality. From the systematic sample-size
dependence of the resistance in on- and off-states, we conclude that the
resistance switching is due to the homogeneous/inhomogeneous transition of the
current distribution at the interface.Comment: 7 pages, 5 figures, REVTeX4, submitted to Phys. Rev. B (Feb. 23,
2007). If you can't download a PDF file of this manscript, an alternative one
can be found on the author's website: http://staff.aist.go.jp/i.inoue
Finite Black Hole Entropy and String Theory
An accelerating observer sees a thermal bath of radiation at the Hawking
temperature which is proportional to the acceleration. Also, in string theory
there is a Hagedorn temperature beyond which one cannot go without an infinite
amount of energy. Several authors have shown that in the context of Hawking
radiation a limiting temperature for string theory leads to a limiting
acceleration, which for a black hole implies a minimum distance from the
horizon for an observer to remain stationary. We argue that this effectively
introduces a cutoff in Rindler space or the Schwarzschild geometry inside of
which accelerations would exceed this maximum value. Furthermore, this natural
cutoff in turn allows one to define a finite entropy for Rindler space or a
black hole as all divergences were occurring on the horizon. In all cases if a
particular relationship exists between Newton's constant and the string tension
then the entropy of the string modes agrees with the Bekenstein-Hawking
formula.Comment: 17 pages, 1 figure, Florida Preprint UFIFT-HEP-94-0
Enhancement and suppression of tunneling by controlling symmetries of a potential barrier
We present a class of 2D systems which shows a counterintuitive property that
contradicts a semi classical intuition: A 2D quantum particle "prefers"
tunneling through a barrier rather than traveling above it. Viewing the one
particle 2D system as the system of two 1D particles, it is demonstrated that
this effect occurs due to a specific symmetry of the barrier that forces
excitations of the interparticle degree of freedom that, in turn, leads to the
appearance of an effective potential barrier even though there is no "real"
barrier. This phenomenon cannot exist in 1D.Comment: 10 pages and 7 figure
Magnetocaloric effect and magnetization in a Ni-Mn-Ga Heusler alloy in the vicinity of magnetostructural transition
The magnetic and thermodynamic properties of a Ni2.19Mn0.81Ga alloy with
coupled magnetic and structural (martensitic) phase transitions were studied
experimentally and theoretically. The magnetocaloric effect was measured by a
direct method in magnetic fields 0-26 kOe at temperatures close to the
magnetostructural transition temperature. For theoretical description of the
alloy properties near the magnetostructural transition a statistical model is
suggested, that takes into account the coexistence of martensite and austenite
domains in the vicinity of martensite transformation point.Comment: presented at ICM-2003, to appear in JMM
Decoherence, tunneling and noise-activation in a double-potential well at high and zero temperature
We study the effects of the environment on tunneling in an open system
described by a static double-well potential. We describe the evolution of a
quantum state localized in one of the minima of the potential at , both in
the limits of high and zero environment temperature. We show that the evolution
of the system can be summarized in terms of three main physical phenomena,
namely decoherence, quantum tunneling and noise-induced activation, and we
obtain analytical estimates for the corresponding time-scales. These analytical
predictions are confirmed by large-scale numerical simulations, providing a
detailed picture of the main stages of the evolution and of the relevant
dynamical processes.Comment: Version to appear in Phys. Rev. E. 15 pages, 12 figures (low quality
due to upload size limitations). Good quality figures in a pdf file can be
downloaded from http://www.df.uba.ar/users/lombardo/tunne
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