128 research outputs found
Heat flux operator, current conservation and the formal Fourier's law
By revisiting previous definitions of the heat current operator, we show that
one can define a heat current operator that satisfies the continuity equation
for a general Hamiltonian in one dimension. This expression is useful for
studying electronic, phononic and photonic energy flow in linear systems and in
hybrid structures. The definition allows us to deduce the necessary conditions
that result in current conservation for general-statistics systems. The
discrete form of the Fourier's Law of heat conduction naturally emerges in the
present definition
An alternate model for magnetization plateaus in the molecular magnet V_15
Starting from an antiferromagnetic Heisenberg Hamiltonian for the fifteen
spin-1/2 ions in V_15, we construct an effective spin Hamiltonian involving
eight low-lying states (spin-1/2 and spin-3/2) coupled to a phonon bath. We
numerically solve the time-dependent Schrodinger equation of this system, and
obtain the magnetization as a function of temperature in a time-dependent
magnetic field. The magnetization exhibits unusual patterns of hysteresis and
plateaus as the field sweep rate and temperature are varied. The observed
plateaus are not due to quantum tunneling but are a result of thermal
averaging. Our results are in good agreement with recent experimental
observations.Comment: Revtex, 4 pages, 5 eps figure
Macroscopic Quantum Phase Interference in Antiferromagnetic Particles
The tunnel splitting in biaxial antiferromagnetic particles is studied with a
magnetic field applied along the hard anisotropy axis. We observe the
oscillation of tunnel splitting as a function of the magnetic field due to the
quantum phase interference of two tunneling paths of opposite windings. The
oscillation is similar to the recent experimental result with Fe}\textrm{\
molecular clusters.}Comment: 8 pages, 2 postscript figures, to appear in J. Phys.: Condes. Matte
Dynamics of a Quantum Particle in Asymmetric Bistable Potential with Environmental Noise
In this work we analyze the dynamics of a quantum particle subject to an asymmetric bistable potential and interacting with a thermal reservoir. We obtain the time evolution of the population distributions in both energy and position eigenstates of the particle, for different values of the coupling strength with the thermal bath. The calculation is carried out using the Feynman-
Vernon functional under the discrete variable representation
Suppression of decoherence by bath ordering
The dynamics of two coupled spins-1/2 coupled to a spin-bath is studied as an
extended model of the Tessieri-Wilkie Hamiltonian \cite{TWmodel}. The pair of
spins served as an open subsystem were prepared in one of the Bell states and
the bath consisted of some spins-1/2 is in a thermal equilibrium state from the
very beginning. It is found that with the increasing the coupling strength of
the bath spins, the bath forms a resonant antiferromagnetic order. The
polarization correlation between the two spins of the subsystem and the
concurrence are recovered in some extent to the isolated subsystem. This
suppression of the subsystem decoherence may be used to control the quantum
devices in practical applications.Comment: 32 pages, Chinese Physics (accepted
Non-Fermi liquid behavior and Griffiths phase in {\it f}-electron compounds
We study the interplay among disorder, RKKY and Kondo interactions in {\it
f}-electron alloys. We argue that the non-Fermi liquid behavior observed in
these systems is due to the existence of a Griffiths phase close to a quantum
critical point. The existence of this phase provides a unified picture of a
large class of materials. We also propose new experiments that can test these
ideas.Comment: 4 pages, 1 Figure. NEW version of the original manuscript. A single
framework for NFL behavior in different kinds of alloys is presented. Final
version finally allowed to appear on the glorious Physical Review Letter
Study of Spin and Decay-Plane Correlations of W Bosons in the e+e- -> W+W- Process at LEP
Data collected at LEP at centre-of-mass energies \sqrt(s) = 189 - 209 GeV are
used to study correlations of the spin of W bosons using e+e- -> W+W- -> lnqq~
events. Spin correlations are favoured by data, and found to agree with the
Standard Model predictions. In addition, correlations between the W-boson decay
planes are studied in e+e- -> W+W- -> lnqq~ and e+e- -> W+W- -> qq~qq~ events.
Decay-plane correlations, consistent with zero and with the Standard Model
predictions, are measured
Ultrarelativistic sources in nonlinear electrodynamics
The fields of rapidly moving sources are studied within nonlinear
electrodynamics by boosting the fields of sources at rest. As a consequence of
the ultrarelativistic limit the delta-like electromagnetic shock waves are
found. The character of the field within the shock depends on the theory of
nonlinear electrodynamics considered. In particular, we obtain the field of an
ultrarelativistic charge in the Born-Infeld theory.Comment: 10 pages, 3 figure
Measurement of the Cross Section for Open-Beauty Production in Photon-Photon Collisions at LEP
The cross section for open-beauty production in photon-photon collisions is
measured using the whole high-energy and high-luminosity data sample collected
by the L3 detector at LEP. This corresponds to 627/pb of integrated luminosity
for electron-positron centre-of-mass energies from 189GeV to 209GeV. Events
containing b quarks are identified through their semi-leptonic decay into
electrons or muons. The e+e- -> e+e-b b~X cross section is measured within our
fiducial volume and then extrapolated to the full phase space. These results
are found to be in significant excess with respect to Monte Carlo predictions
and next-to-leading order QCD calculations
ВЛИЯНИЕ БАЗОВОГО СОСТАВА И МИКРОСТРУКТУРЫ НИКЕЛЬ–ЦИНКОВЫХ ФЕРРИТОВ НА УРОВЕНЬ ПОГЛОЩЕНИЯ ЭЛЕКТРОМАГНИТНОГО ИЗЛУЧЕНИЯ
Promising absorbing materials include Ni—Zn−ferrites, as they quite intensively absorb electromagnetic waves in the 50 MHz to 1000 MHz frequency range. In this paper we have studied the electromagnetic properties of Ni—Zn ferrite absorbing materials obtained in different technological modes. We propose a model that allows one to evaluate the dielectric constant of the ferrite material depending on the parameters of the microstructure and electrical properties of grain boundaries. Influence of base composition and microstructure on the level of absorption of electromagnetic radiation by Ni—Zn ferrite absorbing materials has been found. An increase in Fe₂O₃ excess to 51 % has been found to shift the frequency interval of electromagnetic radiation absorption towards lower frequencies, and this effect can be explained by an increase in the dielectric and magnetic constants of ferrite. Introduction of excess Fe₂O₃ in step 2 of grinding proved to be more efficient. An increase in the sintering temperature to 1350 °C also provides for a shift of electromagnetic radiation absorption frequency interval towards lower frequencies, which can be explained by an increase of the dielectric and magnetic constants of ferrite and resonance frequency shift of domain walls due to the formation of a coarse−grained structure.Никель−цинковые ферриты относятся к перспективным радиопоглощающим материалам, так как они интенсивно поглощают электромагнитные волны в интервале частот от 50 до 1000 МГц. Проведены исследования электромагнитных свойств Ni—Zn−ферритовых радиопоглощающих материалов, полученных по различным технологическим режимам. Предложена модель, позволяющая оценить диэлектрическую проницаемость ферритового материала в зависимости от параметров микроструктуры и электрофизических свойств границ зерен. Установлено влияние базового состава и микроструктуры на уровень поглощения электромагнитного излучения Ni—Zn− ферритовыми радиопоглощающими материалами. Установлено, что увеличение содержания избытка Fe2O3 до 51 % (мол.) приводит к смещению частотного интервала поглощения электромагнитного излучения в сторону низких частот. Это можно объяснить увеличением магнитной и диэлектрической проницаемостей феррита. Показано, что более эффективно введение избытка Fe2O3 на стадии измельчения синтезированной шихты. Обнаружено, что увеличение температуры спекания до 1350 °С также обеспечивает смещение частотного интервала поглощения электромагнитного излучения в сторону низких частот. Вероятно, это обусловлено увеличением магнитной и диэлектрической проницаемостей феррита и смещением частоты резонанса доменных стенок в результате формирования крупнозернистой структуры
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