54 research outputs found
Eigenstate thermalization within isolated spin-chain systems
The thermalization phenomenon and many-body quantum statistical properties
are studied on the example of several observables in isolated spin-chain
systems, both integrable and generic non-integrable ones. While diagonal matrix
elements for non-integrable models comply with the eigenstate thermalization
hypothesis (ETH), the integrable systems show evident deviations and similarity
to properties of noninteracting many-fermion models. The finite-size scaling
reveals that the crossover between two regimes is given by a scale closely
related to the scattering length. Low-frequency off-diagonal matrix elements
related to d.c. transport quantities in a generic system also follow the
behavior analogous to the ETH, however unrelated to the one of diagonal
elements
Real-time broadening of non-equilibrium density profiles and the role of the specific initial-state realization
The real-time broadening of density profiles starting from non-equilibrium
states is at the center of transport in condensed-matter systems and dynamics
in ultracold atomic gases. Initial profiles close to equilibrium are expected
to evolve according to linear response, e.g., as given by the current
correlator evaluated exactly at equilibrium. Significantly off equilibrium,
linear response is expected to break down and even a description in terms of
canonical ensembles is questionable. We unveil that single pure states with
density profiles of maximum amplitude yield a broadening in perfect agreement
with linear response, if the structure of these states involves randomness in
terms of decoherent off-diagonal density-matrix elements. While these states
allow for spin diffusion in the XXZ spin-1/2 chain at large exchange
anisotropies, coherences yield entirely different behavior.Comment: 7 pages, 7 figures, accepted for publication in Phys. Rev.
Spin hydrodynamics in the S = 1/2 anisotropic Heisenberg chain
We study the finite-temperature dynamical spin susceptibility of the
one-dimensional (generalized) anisotropic Heisenberg model within the
hydrodynamic regime of small wave vectors and frequencies. Numerical results
are analyzed using the memory function formalism with the central quantity
being the spin-current decay rate gamma(q,omega). It is shown that in a generic
nonintegrable model the decay rate is finite in the hydrodynamic limit,
consistent with normal spin diffusion modes. On the other hand, in the gapless
integrable model within the XY regime of anisotropy Delta < 1 the behavior is
anomalous with vanishing gamma(q,omega=0) proportional to |q|, in agreement
with dissipationless uniform transport. Furthermore, in the integrable system
the finite-temperature q = 0 dynamical conductivity sigma(q=0,omega) reveals
besides the dissipationless component a regular part with vanishing
sigma_{reg}(q=0,omega to 0) to 0
Production of n-propyl acetate by reactive distillation : experimental and theoretical study
First steps of the development of a catalytic reactive distillation process for the production
of n-propyl acetate based on experiments and simulations are proposed. The kinetics for homogeneously (sulphuric acid) and heterogeneously (Amberlyst 15) catalysed reaction
were investigated and the constants for a pseudo-homogeneous model are presented. Pilot plant experiments were performed using a homogeneous strong acid catalyst in a packed column. A top-column decanter is used to withdraw the aqueous phase and to reflux the organic phase. Simulation results are in good agreement with experimental data. Thermodynamics nonidealities are taken into account using VLE and LLE NRTL interaction parameters. Alcohol conversion and n-propyl acetate purity may be dramatically increased just by adding to the pilot plant a stripping section in an additional column: six different configurations are identified to achieve such a production. The startup is studied in order to determine the best strategy to achieve steady-state conditions. The strong influence of the composition of the initial charging in the decanter can be seen and an initial charging of the two-phase top product leads to the fastest startup
Coherent spin-current oscillations in transverse magnetic fields
We address the coherence of the dynamics of spin-currents with components
transverse to an external magnetic field for the spin-1/2 Heisenberg chain. We
study current autocorrelations at finite temperatures and the real-time
dynamics of currents at zero temperature. Besides a coherent Larmor
oscillation, we find an additional collective oscillation at higher
frequencies, emerging as a coherent many-magnon effect at low temperatures.
Using numerical and analytical methods, we analyze the oscillation frequency
and decay time of this coherent current-mode versus temperature and magnetic
field.Comment: 4 pages, 5 figures (and supplemental material: 4 pages, 6 figures
Spin transport in the XXZ model at high temperatures: Classical dynamics versus quantum S=1/2 autocorrelations
The transport of magnetization is analyzed for the classical Heisenberg chain
at and especially above the isotropic point. To this end, the Hamiltonian
equations of motion are solved numerically for initial states realizing
harmonic-like magnetization profiles of small amplitude and with random phases.
Above the isotropic point, the resulting dynamics is observed to be diffusive
in a hydrodynamic regime starting at comparatively small times and wave
lengths. In particular, hydrodynamic regime and diffusion constant are both
found to be in quantitative agreement with close-to-equilibrium results from
quantum S=1/2 autocorrelations at high temperatures. At the isotropic point,
the resulting dynamics turns out to be non-diffusive at the considered times
and wave lengths.Comment: 6 pages, 5 figures, accepted for publication in Europhys. Let
Transition from diffusive to ballistic dynamics for a class of finite quantum models
The transport of excitation probabilities amongst weakly coupled subunits is
investigated for a class of finite quantum systems. It is demonstrated that the
dynamical behavior of the transported quantity depends on the considered length
scale, e. g., the introduced distinction between diffusive and ballistic
transport appears to be a scale-dependent concept, especially since a
transition from diffusive to ballistic behavior is found in the limit of small
as well as in the limit of large length scales. All these results are derived
by an application of the time-convolutionless projection operator technique and
are verified by the numerical solution of the full time-dependent Schroedinger
equation which is obtained by exact diagonalization for a range of model
parameters.Comment: 4 pages, 5 figures, approved for publication in Physical Review
Letter
Finite size effects on transport coefficients for models of atomic wires coupled to phonons
We consider models of quasi-1-d, planar atomic wires consisting of several,
laterally coupled rows of atoms, with mutually non-interacting electrons. This
electronic wire system is coupled to phonons, corresponding, e.g., to some
substrate. We aim at computing diffusion coefficients in dependence on the wire
widths and the lateral coupling. To this end we firstly construct a numerically
manageable linear collision term for the dynamics of the electronic occupation
numbers by following a certain projection operator approach. By means of this
collision term we set up a linear Boltzmann equation. A formula for extracting
diffusion coefficients from such Boltzmann equations is given. We find in the
regime of a few atomic rows and intermediate lateral coupling a significant and
non-trivial dependence of the diffusion coefficient on both, the width and the
lateral coupling. These results, in principle, suggest the possible
applicability of such atomic wires as electronic devices, such as, e.g.,
switches.Comment: 9 pages, 5 figures, accepted for publication in Eur. Phys. J.
Dephasing-induced diffusive transport in anisotropic Heisenberg model
We study transport properties of anisotropic Heisenberg model in a disordered
magnetic field experiencing dephasing due to external degrees of freedom. In
the absence of dephasing the model can display, depending on parameter values,
the whole range of possible transport regimes: ideal ballistic conduction,
diffusive, or ideal insulating behavior. We show that the presence of dephasing
induces normal diffusive transport in a wide range of parameters. We also
analyze the dependence of spin conductivity on the dephasing strength. In
addition, by analyzing the decay of spin-spin correlation function we discover
a presence of long-range order for finite chain sizes. All our results for a
one-dimensional spin chain at infinite temperature can be equivalently
rephrased for strongly-interacting disordered spinless fermions.Comment: 15 pages, 9 PS figure
From thermal rectifiers to thermoelectric devices
We discuss thermal rectification and thermoelectric energy conversion from
the perspective of nonequilibrium statistical mechanics and dynamical systems
theory. After preliminary considerations on the dynamical foundations of the
phenomenological Fourier law in classical and quantum mechanics, we illustrate
ways to control the phononic heat flow and design thermal diodes. Finally, we
consider the coupled transport of heat and charge and discuss several general
mechanisms for optimizing the figure of merit of thermoelectric efficiency.Comment: 42 pages, 22 figures, review paper, to appear in the Springer Lecture
Notes in Physics volume "Thermal transport in low dimensions: from
statistical physics to nanoscale heat transfer" (S. Lepri ed.
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