1,216 research outputs found
Uhlmann curvature in dissipative phase transitions
We study the mean Uhlmann curvature in fermionic systems undergoing a
dissipative driven phase transition. We consider a paradigmatic class of
lattice fermion systems in non-equilibrium steady-state of an open system with
local reservoirs, which are characterised by a Gaussian fermionic steady state.
In the thermodynamical limit, in systems with translational invariance we show
that a singular behaviour of the Uhlmann curvature represents a sufficient
criterion for criticalities, in the sense of diverging correlation length, and
it is not otherwise sensitive to the closure of the Liouvillian dissipative
gap. In finite size systems, we show that the scaling behaviour of the mean
Uhlmann curvature maps faithfully the phase diagram, and a relation to the
dissipative gap is put forward. We argue that the mean Uhlmann phase can shade
light upon the nature of non equilibrium steady state criticality in particular
with regard to the role played by quantum vs classical fluctuations.Comment: 5 pages, 3 figures with appendix of 10 pages, 1 figur
Switching times in long-overlap Josephson junctions subject to thermal fluctuations and non-Gaussian noise sources
We investigate the superconducting lifetime of long current-biased Josephson
junctions, in the presence of Gaussian and non-Gaussian noise sources. In
particular, we analyze the dynamics of a Josephson junction as a function of
the noise signal intensity, for different values of the parameters of the
system and external driving currents. We find that the mean lifetime of the
superconductive state is characterized by nonmonotonic behavior as a function
of noise intensity, driving frequency and junction length. We observe that
these nonmonotonic behaviours are connected with the dynamics of the junction
phase string during the switching towards the resistive state. An important
role is played by the formation and propagation of solitons, with two different
dynamical regimes characterizing the dynamics of the phase string. Our analysis
allows to evidence the effects of different bias current densities, that is a
simple spatially homogeneous distribution and a more realistic inhomogeneous
distribution with high current values at the junction edges. Stochastic
resonant activation, noise enhanced stability and temporary trapping phenomena
are observed in the system investigated.Comment: 16 pages, 9 figures, Physical Review B, in pres
Heisenberg Uncertainty Relation in Quantum Liouville Equation
We consider the quantum Liouville equation and give a characterization of the solutions which satisfy the Heisenberg uncertainty relation. We analyze three cases. Initially we consider a particular solution of the quantum Liouville equation: the Wigner transformf(x,v,t) of a generic solutionÏ(x;t) of the Schrödinger equation. We give a representation ofÏ(x,t) by the Hermite functions. We show that the values of the variances ofxandvcalculated by using the Wigner functionf(x,v,t) coincide, respectively, with the variances of position operatorX^and conjugate momentum operatorP^obtained using the wave functionÏ(x,t). Then we consider the Fourier transform of the density matrixÏ(z,y,t) =Ïâ(z,t)Ï(y,t). We find again that the variances ofxandvobtained by usingÏ(z,y,t) are respectively equal to the variances ofX^andP^calculated inÏ(x,t). Finally we introduce the matrixâ„AnnâČ(t)â„and we show that a generic square-integrable functiong(x,v,t) can be written as Fourier transform of a density matrix, provided that the matrixâ„AnnâČ(t)â„is diagonalizable
Quantum resonant activation
Quantum resonant activation is investigated for the archetype setup of an
externally driven two-state (spin-boson) system subjected to strong dissipation
by means of both analytical and extensive numerical calculations. The
phenomenon of resonant activation emerges in the presence of either randomly
fluctuating or deterministic periodically varying driving fields. Addressing
the incoherent regime, a characteristic minimum emerges in the mean first
passage time to reach an absorbing neighboring state whenever the intrinsic
time scale of the modulation matches the characteristic time scale of the
system dynamics. For the case of deterministic periodic driving, the first
passage time probability density function (pdf) displays a complex,
multi-peaked behavior, which depends crucially on the details of initial phase,
frequency, and strength of the driving. As an interesting feature we find that
the mean first passage time enters the resonant activation regime at a critical
frequency which depends very weakly on the strength of the driving.
Moreover, we provide the relation between the first passage time pdf and the
statistics of residence times.Comment: 14 pages, 13 figure
Quantum dissipative dynamics of a bistable system in the sub-Ohmic to super-Ohmic regime
We investigate the quantum dynamics of a multilevel bistable system coupled
to a bosonic heat bath beyond the perturbative regime. We consider different
spectral densities of the bath, in the transition from sub-Ohmic to super-Ohmic
dissipation, and different cutoff frequencies. The study is carried out by
using the real-time path integral approach of the Feynman-Vernon influence
functional. We find that, in the crossover dynamical regime characterized by
damped \emph{intrawell} oscillations and incoherent tunneling, the short time
behavior and the time scales of the relaxation starting from a nonequilibrium
initial condition depend nontrivially on the spectral properties of the heat
bath.Comment: 16 pages, 7 figure
Effects of L\'evy noise on the dynamics of sine-Gordon solitons in long Josephson junctions
We numerically investigate the generation of solitons in current-biased long
Josephson junctions in relation to the superconducting lifetime and the voltage
drop across the device. The dynamics of the junction is modelled with a
sine-Gordon equation driven by an oscillating field and subject to an external
non-Gaussian noise. A wide range of -stable L\'evy distributions is
considered as noise source, with varying stability index and asymmetry
parameter . In junctions longer than a critical length, the mean
switching time (MST) from superconductive to the resistive state assumes a
values independent of the device length. Here, we demonstrate that such a value
is directly related to the mean density of solitons which move into or from the
washboard potential minimum corresponding to the initial superconductive state.
Moreover, we observe: (i) a connection between the total mean soliton density
and the mean potential difference across the junction; (ii) an inverse behavior
of the mean voltage in comparison with the MST, with varying the junction
length; (iii) evidences of non-monotonic behaviors, such as stochastic resonant
activation and noise enhanced stability, of MST versus the driving frequency
and noise intensity for different values of and ; (iv) finally,
these non-monotonic behaviors are found to be related to the mean density of
solitons formed along the junction.Comment: 24 pages, 8 figures, submitted to J. Stat. Mech.: Theory Exp. arXiv
admin note: text overlap with arXiv:1406.481
Lifetime of the superconductive state in short and long Josephson junctions
We study the transient statistical properties of short and long Josephson
junctions under the influence of thermal and correlated fluctuations. In
particular, we investigate the lifetime of the superconductive metastable state
finding the presence of noise induced phenomena. For short Josephson junctions
we investigate the lifetime as a function both of the frequency of the current
driving signal and the noise intensity and we find how these noise-induced
effects are modified by the presence of a correlated noise source. For long
Josephson junctions we integrate numerically the sine-Gordon equation
calculating the lifetime as a function of the length of the junction both for
inhomogeneous and homogeneous bias current distributions. We obtain a
nonmonotonic behavior of the lifetime as a function of the frequency of the
current driving signal and the correlation time of the noise. Moreover we find
two maxima in the nonmonotonic behaviour of the mean escape time as a function
of the correlated noise intensity.Comment: 12 pages, 9 figure
Chemical Absorption by Aqueous Solution of Ammonia
Carbon capture is proposed as a viable way of exploiting the fossil resources for power plants and industrial processes. The post-combustion capture by chemical absorption in amine aqueous solutions has been in use in chemical and petrochemical areas for decades. As an alternative, the absorption in aqueous ammonia has received great attention recently. The carbon capture by aqueous ammonia is based on the conventional absorption-regeneration scheme applied to the ternary system CO2âNH3âH2O. It can be implemented in a chilled and a cooled process, depending upon the temperatures in the absorber and, hence, the precipitation of salts. The process simulation can be conducted in two manners: the equilibrium and the rate-based approaches. The specific heat duty is as low as 3.0, for the cooled process, and 2.2 MJ/kgCO2, for the chilled one. Moreover, the index SPECCA is as low as 2.6, for the cooled, and 2.9 MJ/kgCO2, for the chilled one. The overall energy performances from the simulations in the rate-based approach, compared against those in the equilibrium approach, result only slightly penalized. From an economic perspective, the carbon capture via chemical absorption by aqueous ammonia is a feasible retrofitting solution, yielding a cost of electricity of 82.4 âŹ/MWhe and of avoided CO2 of 38.6 âŹ/tCO2 for the chilled process
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