11,527 research outputs found
Decay of correlations in the dissipative two-state system
We study the equilibrium correlation function of the polaron-dressed
tunnelling operator in the dissipative two-state system and compare the
asymptoptic dynamics with that of the position correlations. For an Ohmic
spectral density with the damping strength , the correlation functions
are obtained in analytic form for all times at any and any bias. For ,
the asymptotic dynamics is found by using a diagrammatic approach within a
Coulomb gas representation. At T=0, the tunnelling or coherence correlations
drop as , whereas the position correlations show universal decay
. The former decay law is a signature of unscreened attractive
charge-charge interactions, while the latter is due to unscreened dipole-dipole
interactions.Comment: 5 pages, 5 figures, to be published in Europhys. Let
Identification of Coulomb blockade and macroscopic quantum tunneling by noise
The effects of Macroscopic Quantum Tunneling (MQT) and Coulomb Blockade (CB)
in Josephson junctions are of considerable significance both for the
manifestations of quantum mechanics on the macroscopic scale and potential
technological applications. These two complementary effects are shown to be
clearly distinguishable from the associated noise spectra. The current noise is
determined exactly and a rather sharp crossover between flux noise in the MQT
and charge noise in the CB regions is found as the applied voltage is changed.
Related results hold for the voltage noise in current-biased junctions.Comment: 6 pages, 3 figures, epl.cls include
Electron transfer in the nonadiabatic regime: Crossover from quantum-mechanical to classical behaviour
We study nonadiabatic electron transfer within the biased spin-boson model.
We calculate the incoherent transfer rate in analytic form at all temperatures
for a power law form of the spectral density of the solvent coupling. In the
Ohmic case, we present the exact low temperature corrections to the zero
temperature rate for arbitrarily large bias energies between the two redox
sites. Both for Ohmic and non-Ohmic coupling, we give the rate in the entire
regime extending from zero temperature, where the rate depends significantly on
the detailed spectral behaviour, via the crossover region, up to the classical
regime. For low temperatures, the rate shows characteristic quantum features,
in particular the shift of the rate maximum to a bias value below the
reorganization energy, and the asymmetry of the rate around the maximum. We
study in detail the gradual extinction of the quantum features as temperature
is increased.Comment: 17 pages, 4 figures, to be published in Chem. Phy
Duality Relation for Quantum Ratchets
A duality relation between the long-time dynamics of a quantum Brownian
particle in a tilted ratchet potential and a driven dissipative tight-binding
model is reported. It relates a situation of weak dissipation in one model to
strong dissipation in the other one, and vice versa. We apply this duality
relation to investigate transport and rectification in ratchet potentials: From
the linear mobility we infer ground-state delocalization for weak dissipation.
We report reversals induced by adiabatic driving and temperature in the ratchet
current and its dependence on the potential shape.Comment: Modified content, corrected typo
Damped bounces of an isolated perfect quantum gas
The issue of the thermalization of an isolated quantum system is addressed by
considering a perfect gas confined by gravity and initially trapped above a
certain height. As we are interested in the behavior of truly isolated systems,
we assume the gas is in a pure state of macroscopically well-defined energy. We
show that, in general, for single-particle distributions, such a state is
strictly equivalent to the microcanonical mixed state at the same energy. We
derive an expression for the time-dependent gas density which depends on the
initial gas state only via a few thermodynamic parameters. Though we consider
non-interacting particles, the density relaxes into an asymptotic profile, but
which is not the thermal equilibrium one determined by the gas energy and
particle number
Ultraslow quantum dynamics in a sub-Ohmic heat bath
We show that the low-frequency modes of a sub-Ohmic bosonic heat bath
generate an effective dynamical asymmetry for an intrinsically symmetric
quantum spin -1/2. An initially fully polarized spin first decays towards a
quasiequilibrium determined by the dynamical asymmetry, thereby showing
coherent damped oscillations on the (fast) time scale of the spin splitting. On
top of this, the dynamical asymmetry itself decays on an ultraslow time scale
and vanishes asymptotically since the global equilibrium phase is symmetric. We
quantitatively study the nature of the initial fast decay to the
quasiequilibrium and discuss the features of ultraslow dynamics of the
quasiequilibrium itself. The dynamical asymmetry is more pronounced for smaller
values of the sub-Ohmic exponent and for lower temperatures, which emphasizes
the quantum many-body nature of the effect. The symmetry breaking is related to
the dynamic crossover between coherent and overdamped relaxation of the spin
polarization and is not connected to the localization quantum phase transition.
In addition to this delocalized phase, we identify a novel phase which is
characterized by damped coherent oscillations in the localized phase. This
allows for a sketch of the zero-temperature phase diagram of the sub-Ohmic
spin-boson model with four distinct phases.Comment: published version (minor changes), 8 pages, 5 figure
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