46 research outputs found
Thermodynamics of a subensemble of a canonical ensemble
Two approaches to describe the thermodynamics of a subsystem that interacts
with a thermal bath are considered. Within the first approach, the mean system
energy is identified with the expectation value of the system
Hamiltonian, which is evaluated with respect to the overall (system+bath)
equilibrium distribution. Within the second approach, the system partition
function is considered as the fundamental quantity, which is postulated
to be the ratio of the overall (system+bath) and the bath partition functions,
and the standard thermodynamic relation is used to
obtain the mean system energy. % (, is the
Boltzmann constant, %and is the temperature). Employing both classical and
quantum mechanical treatments, the advantages and shortcomings of the two
approaches are analyzed in detail for various different systems. It is shown
that already within classical mechanics both approaches predict significantly
different results for thermodynamic quantities provided the system-bath
interaction is not bilinear or the system of interest consists of more than a
single particle. Based on the results, it is concluded that the first approach
is superior
Thermal Schrödinger Equation: Efficient Tool for Simulation of Many-Body Quantum Dynamics at Finite Temperature
Simulation of Quantum Dynamics of Excitonic Systems at Finite Temperature: An efficient method based on Thermo Field Dynamics
A model for dynamical solvent control of molecular junction electronic properties
Experimental measurements of electron transport properties of molecular
junctions are often performed in solvents. Solvent-molecule coupling and
physical properties of the solvent can be used as the external stimulus to
control electric current through a molecule. In this paper, we propose a model,
which includes dynamical effects of solvent-molecule interaction in the
non-equilibrium Green's function calculations of electric current. The solvent
is considered as a macroscopic dipole moment that reorients stochastically and
interacts with the electrons tunnelling through the molecular junction. The
Keldysh-Kadanoff-Baym equations for electronic Green's functions are solved in
time-domain with subsequent averaging over random realisations of rotational
variables using Furutsu-Novikov method for exact closure of infinite hierarchy
of stochastic correlation functions. The developed theory requires the use of
wide-band approximation as well as classical treatment of solvent degrees of
freedom. The theory is applied to a model molecular junction. It is
demonstrated that not only electrostatic interaction between molecular junction
and solvent but also solvent viscosity can be used to control electrical
properties of the junction. Aligning of the rotating dipole moment breaks
particle-hole symmetry of the transmission favouring either hole or electron
transport channels depending upon the aligning potential
Hierarchical Equations-of-Motion Method for Momentum System-Bath Coupling
[Image: see text] For a broad class of quantum models of practical interest, we demonstrate that the Hamiltonian of the system nonlinearly coupled to a harmonic bath through the system and bath coordinates can be equivalently mapped into the Hamiltonian of the system bilinearly coupled to the bath through the system and bath momenta. We show that the Hamiltonian with bilinear system–bath momentum coupling can be treated by the hierarchical equations-of-motion (HEOM) method and present the corresponding proof-of-principle simulations. The developed methodology creates the opportunity to scrutinize a new family of nonlinear quantum systems by the numerically accurate HEOM method
Dissipative Landau-Zener transitions in a three-level bow-tie model: accurate dynamics with the Davydov multi-D2 Ansatz
We investigate Landau-Zener (LZ) transitions in the three-level bow-tie model
(3L-BTM) in a dissipative environment by using the numerically accurate method
of multiple Davydov D2 Ansatze. We first consider the 3L-TBM coupled to a
single harmonic mode, study evolutions of the transition probabilities for
selected values of the model parameters, and interpret the obtained results
with the aid of the energy diagram method. We then explore the 3L-TBM coupled
to a boson bath. Our simulations demonstrate that sub-Ohmic, Ohmic and
super-Ohmic boson baths have substantially different influences on the 3L-BTM
dynamics, which cannot be grasped by the standard phenomenological Markovian
single-rate descriptions. We also describe novel bath-induced phenomena which
are absent in two-level LZ systems.Comment: 11 pages, 7 figure
Exciton Dynamics and Time-Resolved Fluorescence in Nanocavity-Integrated Monolayers of Transition-Metal Dichalcogenides
We have developed an ab-initio-based fully-quantum numerically-accurate
methodology for the simulation of the exciton dynamics and time- and
frequency-resolved fluorescence spectra of the cavity-controlled
two-dimensional materials at finite temperature and applied this methodology to
the single-layer WSe2 system. This allowed us to establish dynamical and
spectroscopic signatures of the polaronic and polaritonic effects as well as
uncover their characteristic timescales in the relevant range of temperatures
First-passage time theory of activated rate chemical processes in electronic molecular junctions
Confined nanoscale spaces, electric fields and tunneling currents make the
molecular electronic junction an experimental device for the discovery of new,
out-of-equilibrium chemical reactions. Reaction-rate theory for
current-activated chemical reactions is developed by combining a Keldysh
nonequilibrium Green's functions treatment of electrons, Fokker-Planck
description of the reaction coordinate, and Kramers' first-passage time
calculations. The NEGF provide an adiabatic potential as well as a diffusion
coefficient and temperature with local dependence on the reaction coordinate.
Van Kampen's Fokker-Planck equation, which describes a Brownian particle moving
in an external potential in an inhomogeneous medium with a position-dependent
friction and diffusion coefficient, is used to obtain an analytic expression
for the first-passage time. The theory is applied to several transport
scenarios: a molecular junction with a single, reaction coordinate dependent
molecular orbital, and a model diatomic molecular junction. We demonstrate the
natural emergence of Landauer's blowtorch effect as a result of the interplay
between the configuration dependent viscosity and diffusion coefficients. The
resultant localized heating in conjunction with the bond-deformation due to
current-induced forces are shown to be the determining factors when considering
chemical reaction rates; each of which result from highly tunable parameters
within the system