446 research outputs found
Dynamics of the spin-boson model with a structured environment
We investigate the dynamics of the spin-boson model when the spectral density
of the boson bath shows a resonance at a characteristic frequency but
behaves Ohmically at small frequencies. The time evolution of an initial state
is determined by making use of the mapping onto a system composed of a quantum
mechanical two-state system (TSS) which is coupled to a harmonic oscillator
(HO) with frequency . The HO itself is coupled to an Ohmic environment.
The dynamics is calculated by employing the numerically exact quasiadiabatic
path-integral propagator technique. We find significant new properties compared
to the Ohmic spin-boson model. By reducing the TSS-HO system in the dressed
states picture to a three-level system for the special case at resonance, we
calculate the dephasing rates for the TSS analytically. Finally, we apply our
model to experimentally realized superconducting flux qubits coupled to an
underdamped dc-SQUID detector.Comment: 26 pages, 11 figures, Chemical Physics Special Issue on the
Spin-Boson Problem, ed. by H. Grabert and A. Nitzan, in pres
Charge ratchet from spin flip: space-time symmetry paradox
Traditionally the charge ratchet effect is considered as a consequence of
either the spatial symmetry breaking engineered by asymmetric periodic
potentials, or time asymmetry of the driving fields. Here we demonstrate that
electrically and magnetically driven quantum dissipative systems with
spin-orbit interactions represent an exception from this standard idea. In
contrast to the so far well established belief, a charge ratchet effect appears
when both the periodic potential and driving are symmetric. We show that the
source of this paradoxical charge ratchet mechanism is the coexistence of
quantum dissipation with the spin flip processes induced by spin-orbit
interactions.Comment: 5 pages, 3 figure
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
Driven Tunneling Dynamics: Bloch-Redfield Theory versus Path Integral Approach
In the regime of weak bath coupling and low temperature we demonstrate
numerically for the spin-boson dynamics the equivalence between two widely used
but seemingly different roads of approximation, namely the path integral
approach and the Bloch-Redfield theory. The excellent agreement between these
two methods is corroborated by a novel efficient analytical high-frequency
approach: it well approximates the decay of quantum coherence via a series of
damped coherent oscillations. Moreover, a suitably tuned control field can
selectively enhance or suppress quantum coherence.Comment: 4 pages including 3 figures, submitted for publicatio
Non-collinear single-electron spin-valve transistors
We study interaction effects on transport through a small metallic cluster
connected to two ferromagnetic leads (a single-electron spin-valve transistor)
in the "orthodox model" for the Coulomb blockade. The non-local exchange
between the spin accumulation on the island and the ferromagnetic leads is
shown to affect the transport properties such as the electric current and
spin-transfer torque as a function of the magnetic configuration, gate voltage,
and applied magnetic field.Comment: 4 pages, 3 figure
Basic and translational models of cooperative oncogenesis
No abstract available
Driving-Induced Symmetry Breaking in the Spin-Boson System
A symmetric dissipative two-state system is asymptotically completely
delocalized independent of the initial state. We show that driving-induced
localization at long times can take place when both the bias and tunneling
coupling energy are harmonically modulated. Dynamical symmetry breaking on
average occurs when the driving frequencies are odd multiples of some reference
frequency. This effect is universal, as it is independent of the dissipative
mechanism. Possible candidates for an experimental observation are flux
tunneling in the variable barrier rf SQUID and magnetization tunneling in
magnetic molecular clusters.Comment: 4 pages, 4 figures, to be published in PR
Strong coupling theory for driven tunneling and vibrational relaxation
We investigate on a unified basis tunneling and vibrational relaxation in
driven dissipative multistable systems described by their N lowest lying
unperturbed levels. By use of the discrete variable representation we derive a
set of coupled non-Markovian master equations. We present analytical treatments
that describe the dynamics in the regime of strong system-bath coupling. Our
findings are corroborated by ``ab-initio'' real-time path integral
calculations.Comment: 4 LaTeX pages including 3 figure
Application of double-pulse micro-LIBS 3D compositional mapping to the analysis of ceramics
We developed a new Laser-Induced Breakdown Spectroscopy (LIBS) instrument for 3D compositional mappings of archaeological objects. The system, based on the Modì double-pulse instrument, allows the reconstruction of maps with lateral resolution up to 20 microns and sub-micron depth resolution
Iterative algorithm versus analytic solutions of the parametrically driven dissipative quantum harmonic oscillator
We consider the Brownian motion of a quantum mechanical particle in a
one-dimensional parabolic potential with periodically modulated curvature under
the influence of a thermal heat bath. Analytic expressions for the
time-dependent position and momentum variances are compared with results of an
iterative algorithm, the so-called quasiadiabatic propagator path integral
algorithm (QUAPI). We obtain good agreement over an extended range of
parameters for this spatially continuous quantum system. These findings
indicate the reliability of the algorithm also in cases for which analytic
results may not be available a priori.Comment: 15 pages including 11 figures, one reference added, minor typos
correcte
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