305 research outputs found
Transient dynamics of a one-dimensional Holstein polaron under the influence of an external electric field
Following the Dirac-Frenkel time-dependent variational principle, transient
dynamics of a one-dimensional Holstein polaron with diagonal and off-diagonal
exciton-phonon coupling in an external electric field is studied by employing
the multi-D {\it Ansatz}, also known as a superposition of the usual
Davydov D trial states. Resultant polaron dynamics has significantly
enhanced accuracy, and is in perfect agreement with that derived from the
hierarchy equations of motion method. Starting from an initial broad wave
packet, the exciton undergoes typical Bloch oscillations. Adding weak
exciton-phonon coupling leads to a broadened exciton wave packet and a reduced
current amplitude. Using a narrow wave packet as the initial state, the bare
exciton oscillates in a symmetric breathing mode, but the symmetry is easily
broken by weak coupling to phonons, resulting in a non-zero exciton current.
For both scenarios, temporal periodicity is unchanged by exciton-phonon
coupling. In particular, at variance with the case of an infinite linear chain,
no steady state is found in a finite-sized ring within the anti-adiabatic
regime. For strong diagonal coupling, the multi- {\it Anstaz} is found
to be highly accurate, and the phonon confinement gives rise to exciton
localization and decay of the Bloch oscillations
Variational approach to time-dependent fluorescence of a driven qubit
We employ the Dirac-Frenkel variational principle and multiple Davydov ansatz
to study time-dependent fluorescence spectra of a driven qubit in the weak- to
strong qubit-reservoir coupling regimes, where both the Rabi frequency and
spontaneous decay rate are comparable to the transition frequency of the qubit.
Our method agrees well with the time-local master-equation approach in the
weak-coupling regime, and offers a flexible way to compute the spectra from the
bosonic dynamics instead of two-time correlation functions. While the
perturbative master equation breaks down in the strong-coupling regime, our
method actually becomes more accurate due to the use of bosonic coherent states
under certain conditions. We show that the counter-rotating coupling between
the qubit and the reservoir has considerable contributions to the photon number
dynamics and the spectra under strong driving conditions even though the
coupling is moderately weak. The time-dependent spectra are found to be
generally asymmetric, a feature that is derived from photon number dynamics. In
addition, it is shown that the spectral profiles can be dramatically different
from the Mollow triplet due to strong dissipation and/or multiphoton processes
associated with the strong driving. Our formalism provides a unique perspective
to interpret time-dependent spectra.Comment: 19 pages, 8 figure
Ground state properties of sub-Ohmic spin-boson model with simultaneous diagonal and off-diagonal coupling
By employing the variational approach, density matrix renormalization group
(DMRG), exact diagonalization as well as symmetry and mean-field analyses, the
ground state properties of the two-bath spin boson model with simultaneous
diagonal and off-diagonal coupling are systematically studied in the sub-Ohmic
regime. A novel quantum phase transition from a doubly degenerate "localized
phase" to the other doubly degenerate "delocalized phase" is uncovered. Via the
multi-D1 ansatz as the variational wave function, transition points are
determined accurately, consistent with the results from DMRG and exact
diagonalization. An effective spatial dimension is then
estimated, which is found to be compatible with the mean-field prediction.
Furthermore, the quantum phase transition is inferred to be of first order for
the baths described by a continuous spectral density function. In the case of
single mode, however, the transition is softened.Comment: revised version after the paper is publishe
Finite-temperature time-dependent variation with multiple Davydov states
The Dirac-Frenkel time-dependent variational approach with Davydov Ans\"atze
is a sophisticated, yet efficient technique to obtain an acuurate solution to
many-body Schr\"odinger equations for energy and charge transfer dy- namics in
molecular aggregates and light-harvesting complexes. We extend this variational
approach to finite temperatures dynamics of the spin-boson model by adopting a
Monte Carlo importance sampling method. In or- der to demonstrate the
applicability of this approach, we compare real-time quantum dynamics of the
spin-boson model calculated with that from numerically exact iterative
quasiadiabatic propagator path integral (QUAPI) technique. The comparison shows
that our variational approach with the single Davydov Ans\"atze is in excellent
agreement with the QUAPI method at high temperatures, while the two differ at
low temperatures. Accuracy in dynamics calculations employing a multitude of
Davydov trial states is found to improve substantially over the single Davydov
Ansatz, especially at low temperatures. At a moderate computational cost, our
variational approach with the multiple Davydov Ansatz is shown to provide
accurate spin-boson dynamics over a wide range of temperatures and bath
spectral densities.Comment: 8 pages, 3 figure
Research on Energy Response Characteristics of Rock under Harmonic Vibro-Impacting Drilling
Open access via Springer Compact Agreement The support of National Natural Science Foundation of China (No. 51704074) and Youth Science Foundation of Heilongjiang Province (No. QC2018049) are gratefully acknowledged. The work is also supported by Talent Cultivation Foundation (No. SCXHB201703; No. ts26180119; No. td26180141) and Youth Science Foundation (No. 2019QNL-07) of Northeast Petroleum University.Peer reviewedPublisher PD
Empirical Assessment and Comparison of Educational Efficiency between Major Countries across the World
Education is a fundamental factor to enhance a country’s comprehensive national strength and international competitiveness. Recently, several governments have been attracting investments in educational sectors in contemplation of meliorating a country’s overall strength. This study empirically assesses and compares the educational efficiency of 29 major countries across the world using panel data for 2010–2016 by employing data envelopment analysis (DEA) and the super-slacks-based measure (super-SBM) model at the static level combined with the Malmquist index (MI) to investigate educational efficiency at the dynamic level. The results indicate, inter alia, huge average education efficiency differences existed among the studied countries, the highest being Japan (3.2845) and lowest Norway (0.4137), there are differences in the bias of technological progress among the studied countries during the sample period and technological progress directly affects the sustainability of educational efficiency, the growth rate of total factor productivity (TFP) index has been reduced in 2010–2013 but increased in 2014–2016 and techno-logical progress has been the dominant factor influencing the rise of the education TFP index. Based on the results, this study identifies the merits and drawbacks of education efficiency across the sample countries and presents relevant recommendations to promote investment in the education sector and human capital
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