214 research outputs found
A Unified Stochastic Formulation of Dissipative Quantum Dynamics. I. Generalized Hierarchical Equations
We extend a standard stochastic theory to study open quantum systems coupled
to generic quantum environments including the three fundamental classes of
noninteracting particles: bosons, fermions and spins. In this unified
stochastic approach, the generalized stochastic Liouville equation (SLE)
formally captures the exact quantum dissipations when noise variables with
appropriate statistics for different bath models are applied. Anharmonic
effects of a non-Gaussian bath are precisely encoded in the bath multi-time
correlation functions that noise variables have to satisfy. Staring from the
SLE, we devise a family of generalized hierarchical equations by averaging out
the noise variables and expand bath multi-time correlation functions in a
complete basis of orthonormal functions. The general hiearchical equations
constitute systems of linear equations that provide numerically exact
simulations of quantum dynamics. For bosonic bath models, our general
hierarchical equation of motion reduces exactly to an extended version of
hierarchical equation of motion which allows efficient simulation for arbitrary
spectral densities and temperature regimes. Similar efficiency and exibility
can be achieved for the fermionic bath models within our formalism. The spin
bath models can be simulated with two complementary approaches in the presetn
formalism. (I) They can be viewed as an example of non-Gaussian bath models and
be directly handled with the general hierarchical equation approach given their
multi-time correlation functions. (II) Alterantively, each bath spin can be
first mapped onto a pair of fermions and be treated as fermionic environments
within the present formalism.Comment: 31 pages, 2 figure
Quantum transport in d-dimensional lattices
We prove analytically that both fermionic and bosonic uniform d-dimensional
lattices can be reduced to a set of independent one-dimensional modes. This
reduction leads to the conclusion that the dynamics in uniform fermionic and
bosonic lattices is always ballistic. By the use of the Jordan-Wigner
transformation we extend our analysis to spin lattices, proving the existence
of both ballistic and non-ballistic subspaces in any dimension and for any
system size. We then relate the nature of transport with the number of
excitations in the spin lattice, indicating that a single excitation propagates
always ballistically and that the non-ballistic behavior of uniform spin
lattices is a consequence of the interaction between different excitations.Comment: 13 pages, 5 figure
Forster resonance energy transfer, absorption and emission spectra in multichromophoric systems: III. Exact stochastic path integral evaluation
A numerically exact path integral treatment of the absorption and emission
spectra of open quantum systems is presented that requires only the
straightforward solution of a stochastic differential equation. The approach
converges rapidly enabling the calculation of spectra of large excitonic
systems across the complete range of system parameters and for arbitrary bath
spectral densities. With the numerically exact absorption and emission
operators one can also immediately compute energy transfer rates using the
multi-chromophoric Forster resonant energy transfer formalism. Benchmark
calculations on the emission spectra of two level systems are presented
demonstrating the efficacy of the stochastic approach. This is followed by
calculations of the energy transfer rates between two weakly coupled dimer
systems as a function of temperature and system-bath coupling strength. It is
shown that the recently developed hybrid cumulant expansion is the only
perturbative method capable of generating uniformly reliable energy transfer
rates and spectra across a broad range of system parameters.Comment: 20 pages, 4 figure
Nonequilibrium Energy Transfer at Nanoscale: A Unified Theory from Weak to Strong Coupling
We investigate the microscopic mechanism of quantum energy transfer in the
nonequilibrium spin-boson model. By developing a nonequilibrium
polaron-transformed Redfield equation based on fluctuation decoupling, we
dissect the energy transfer into multi-boson associated processes with even or
odd parity. Based on this, we analytically evaluate the energy flux, which
smoothly bridges the transfer dynamics from the weak spin-boson coupling regime
to the strong-coupling one. Our analysis explains previous limiting predictions
and provides a unified interpretation of several observations, including
coherence-enhanced heat flux and absence of negative differential thermal
conductance in the nonequilibrium spin-boson model. The results may find wide
applications for the energy and information control in nanodevices.Comment: 11 pages, 4 figure
Interfacial thermal transport with strong system-bath coupling: A phonon delocalization effect
We study the effect of system-bath coupling strength on quantum thermal
transport through the interface of two weakly coupled anharmonic molecular
chains using quantum self-consistent phonon approach. The heat current shows a
resonant to bi-resonant transition due to the variations in the interfacial
coupling and temperature, which is attributed to the delocalization of phonon
modes. Delocalization occurs only in the strong system-bath coupling regime and
we utilize it to model a thermal rectifier whose ratio can be non-monotonically
tuned not only with the intrinsic system parameters but also with the external
temperature.Comment: 7 pages, 7 figure
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