12,529 research outputs found

### Derivation of quantum master equation with counting fields by monitoring a probe

We show a microscopic derivation of a quantum master equation with counting
terms which describes the electron statistics. A localized spin behaves as a
probe whose precession angle monitors the net electron current by the
magnetic-moment interaction. The probe Hamiltonian is proportional to the
current, and is determined self-consistently for a model of a quantum dot. Then
it turns out that the quantum master equation for the spin-precession contains
the counting terms. As an application, we show the fluctuation theorem for the
electron current.Comment: 7 page

### Fourier's Law from Schroedinger Dynamics

We consider a class of one-dimensional chains of weakly coupled many level
systems. We present a theory which predicts energy diffusion within these
chains for almost all initial states, if some concrete conditions on their
Hamiltonians are met. By numerically solving the time dependent Schroedinger
equation, we verify this prediction. Close to equilibrium we analyze this
behavior in terms of heat conduction and compute the respective coefficient
directly from the theory.Comment: 4 pages, 4 figures, accepted for publication in Phys. Rev. Let

### Two algebraic properties of thermal quantum field theories

We establish the Schlieder and the Borchers property for thermal field
theories. In addition, we provide some information on the commutation and
localization properties of projection operators.Comment: plain tex, 14 page

### Super and Sub-Poissonian photon statistics for single molecule spectroscopy

We investigate the distribution of the number of photons emitted by a single
molecule undergoing a spectral diffusion process and interacting with a
continuous wave laser field. The spectral diffusion is modeled based on a
stochastic approach, in the spirit of the Anderson-Kubo line shape theory.
Using a generating function formalism we solve the generalized optical Bloch
equations, and obtain an exact analytical formula for the line shape and
Mandel's Q parameter. The line shape exhibits well known behaviors, including
motional narrowing when the stochastic modulation is fast, and power
broadening. The Mandel parameter, describing the line shape fluctuations,
exhibits a transition from a Quantum sub-Poissonian behavior in the fast
modulation limit, to a classical super-Poissonian behavior found in the slow
modulation limit. Our result is applicable for weak and strong laser field,
namely for arbitrary Rabi frequency. We show how to choose the Rabi frequency
in such a way that the Quantum sub-Poissonian nature of the emission process
becomes strongest. A lower bound on $Q$ is found, and simple limiting behaviors
are investigated. A non-trivial behavior is obtained in the intermediate
modulation limit, when the time scales for spectral diffusion and the life time
of the excited state, become similar. A comparison is made between our results,
and previous ones derived based on the semi-classical generalized
Wiener--Khintchine theorem.Comment: 14 Phys. Rev style pages, 10 figure

### Steps and dips in the ac conductance and noise of mesoscopic structures

The frequency dependence of the equilibrium ac conductance (or the noise
power spectrum) through a mesoscopic structure is shown to exhibit steps and
dips. The steps, at energies related to the resonances of the structure, are
closely related to the partial Friedel phases of these resonances, thus
allowing a direct measurement of these phases (without interferometry). The
dips in the spectrum are related to a destructive interference in the
absorption of energy by transitions between these resonances, in some
similarity with the Fano effect.Comment: 4 pages, 2 figure

### Generalized Einstein or Green-Kubo relations for active biomolecular transport

For driven Markovian dynamics on a network of (biomolecular) states, the
generalized mobilities, i.e., the response of any current to changes in an
external parameter, are expressed by an integral over an appropriate
current-current correlation function and thus related to the generalized
diffusion constants. As only input, a local detailed balance condition is
required typically even valid for biomolecular systems operating deep in the
non-equilibrium regime.Comment: 4 page

### Electron Magnetic Resonance: The Modified Bloch Equation

We find a modified Bloch equation for the electronic magnetic moment when the
magnetic moment explicitly contains a diamagnetic contribution (a magnetic
field induced magnetic moment arising from the electronic orbital angular
momentum) in addition to the intrinsic magnetic moment of the electron. The
modified Bloch is coupled to equations of motion for the position and momentum
operators. In the presence of static and time varying magnetic field
components, the magnetic moment oscillates out of phase with the magnetic field
and power is absorbed by virtue of the magnetic field induced magnetic moment,
even in the absence of coupling to the environment. We explicitly work out the
spectrum and absorption for the case of a $p$ state electron

### Electronic structure and resistivity of the double exchange model

The double exchange (DE) model with quantum local spins S is studied; an
equation of motion approach is used and decoupling approximations analogous to
Hubbard's are made. Our approximate one-electron Green function G is exact in
the atomic limit of zero bandwidth for all S and band filling n, and as n->0
reduces to a dynamical coherent potential approximation (CPA) due to Kubo; we
regard our approximation as a many-body generalisation of Kubo's CPA. G is
calculated self-consistently for general S in the paramagnetic state and for
S=1/2 in a state of arbitrary magnetization. The electronic structure is
investigated and four bands per spin are obtained centred on the atomic limit
peaks of the spectral function. A resistivity formula appropriate to the model
is derived from the Kubo formula and the paramagnetic state resistivity rho is
calculated; insulating states are correctly obtained at n=0 and n=1 for strong
Hund coupling. Our prediction for rho is much too small to be consistent with
experiments on manganites so we agree with Millis et al that the bare DE model
is inadequate. We show that the agreement with experiment obtained by Furukawa
is due to his use of an unphysical density of states.Comment: 20 pages, 8 figures, submitted to J. Phys.: Condens. Matte

### Sublattice Asymmetric Reductions of Spin Values on Stacked Triangular Lattice Antiferromagnet CsCoBr$_3$

We study the reductions of spin values of the ground state on a stacked
triangular antiferromagnet using the spin-wave approach. We find that the spin
reductions have sublattice asymmetry due to the cancellation of the molecular
field. The sublattice asymmetry qualitatively analyzes the NMR results of
CsCoBr$_3$.Comment: 5pages, 5figure

### Macroscopic Expression Connecting the Rate of Energy Dissipation and Violation of the Fluctuation-Response Relation

A direct connection between the magnitude of the violation of the
fluctuation-response relation (FRR) and the rate of energy dissipation is
presented in terms of field variables of nonequilibrium systems. Here, we
consider the density field of a colloidal suspension either in a relaxation
process or in a nonequilibrium steady state driven by an external field. Using
a path-integral representation of the temporal evolution of the density field,
we find an equality that relates the magnitude of the violation of the FRR for
scalar and vector potentials of the velocity field to the rate of energy
dissipation for the entire system. Our result demonstrates that the violation
of the FRR for field variables captures the entropic component of the
dissipated free energy.Comment: 4 pages, a major reviso

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