Fate of vortex-synchronized state in oscillator networks with node defects.

We investigate synchronization behaviors of a Kuramoto oscillator network with a two-dimensional square-lattice configuration. We show that the oscillator network can reach a phase-locking vortex synchronized state in the long time limit starting from random initial oscillator phases sampled according to the von Mises distribution characterized by a zero mean and a finite concentration parameter. We further reveal that the stability of the vortex synchronized state is sensitive to the presence of local node defects, in contrast to the usual knowledge that oscillator networks should exhibit robustness against local perturbations.

Synchronization of Van der Pol oscillators in a thermal bath.

The phenomenon of synchronization in self-sustained systems has been successfully illuminated in many fields, ranging from biology to electrical engineering. To date, the majority of theoretical studies on synchronization focus on isolated self-sustained systems, leaving the effects of surrounding environments less touched due to the lack of appropriate descriptions. Here we derive a generalized Langevin equation that governs the dynamics of open classical Van der Pol (VdP) oscillators immersed in a common thermal bath with arbitrary memory time and subsumes an existing equation for memoryless bath as a special limit.

Crossover from hopping to band-like transport in crystalline organic semiconductors: The effect of shallow traps.

We show a crossover from coherent to incoherent behavior of charge transport in crystalline organic semiconductors by considering the effect of shallow traps within the dynamical disorder model. The mixed quantum-classical system is treated by the Ehrenfest dynamics method complementing with instantaneous decoherence corrections and energy relaxation, which has been shown to properly make the system close to equilibrium. The shallow traps, which are incorporated by a static diagonal disorder, are shown to play a central role in the crossover.

Frequency-dependent current noise in quantum heat transfer: A unified polaron calculation.

To investigate frequency-dependent current noise (FDCN) in open quantum systems at steady states, we present a theory which combines Markovian quantum master equations with a finite time full counting statistics. Our formulation of the FDCN generalizes previous zero-frequency expressions and can be viewed as an application of MacDonald's formula for electron transport to heat transfer. As a demonstration, we consider the paradigmatic example of quantum heat transfer in the context of a non-equilibrium spin-boson model.

Energy transfer in the nonequilibrium spin-boson model: From weak to strong coupling.

To explore energy transfer in the nonequilibrium spin-boson model (NESB) from weak to strong system-bath coupling regimes, we propose a polaron-transformed nonequilibrium Green's function (NEGF) method. By combining the polaron transformation, we are able to treat the system-bath coupling nonperturbatively, thus in direct contrast to conventionally used NEGF methods which take the system-bath coupling as a perturbation. The Majorana-fermion representation is further utilized to evaluate terms in the Dyson series.

Polaron dynamics with off-diagonal coupling: beyond the Ehrenfest approximation.

Treated traditionally by the Ehrenfest approximation, the dynamics of a one-dimensional molecular crystal model with off-diagonal exciton-phonon coupling is investigated in this work using the Dirac-Frenkel time-dependent variational principle with the multi-DAnsatz. It is shown that the Ehrenfest method is equivalent to our variational method with the single DAnsatz, and with the multi-DAnsatz, the accuracy of our simulated dynamics is significantly enhanced in comparison with the semi-classical Ehrenfest dynamics. The multi-DAnsatz is able to capture numerically accurate exciton momentum probability and help clarify the relation between the exciton momentum redistribution and the exciton energy relaxation.

Drift of charge carriers in crystalline organic semiconductors.

We investigate the direct-current response of crystalline organic semiconductors in the presence of finite external electric fields by the quantum-classical Ehrenfest dynamics complemented with instantaneous decoherence corrections (IDC). The IDC is carried out in the real-space representation with the energy-dependent reweighing factors to account for both intermolecular decoherence and energy relaxation by which conduction occurs. In this way, both the diffusion and drift motion of charge carriers are described in a unified framework.

Decoherence and energy relaxation in the quantum-classical dynamics for charge transport in organic semiconducting crystals: An instantaneous decoherence correction approach.

We explore an instantaneous decoherence correction (IDC) approach for the decoherence and energy relaxation in the quantum-classical dynamics of charge transport in organic semiconducting crystals. These effects, originating from environmental fluctuations, are essential ingredients of the carrier dynamics. The IDC is carried out by measurement-like operations in the adiabatic representation.

Renormalized phonons in nonlinear lattices: A variational approach.

We propose a variational approach to study renormalized phonons in momentum-conserving nonlinear lattices with either symmetric or asymmetric potentials. To investigate the influence of pressure for phonon properties, we derive an inequality which provides both the lower and upper bound of the Gibbs free energy as the associated variational principle. This inequality is a direct extension to the Gibbs-Bogoliubov inequality.

Thermoelectric effect in Aharonov-Bohm structures.

The thermoelectric effects of a single Aharonov-Bohm (SAB) ring and coupled double Aharonov-Bohm (DAB) rings have been investigated on a theoretical basis, taking into account the contributions of both electrons and phonons to the transport process by using the nonequilibrium Green's function technique. The thermoelectric figure of merit of the coupled DAB rings cannot be predicted directly by combining the values of two SAB ring systems due to the contribution of electron-phonon interaction to coupling between the two sites connecting the rings. We find that thermoelectric efficiency can be optimized by modulating the phases of the magnetic flux threading the two rings.

Dynamics of the sub-Ohmic spin-boson model: a comparison of three numerical approaches.

Dynamics of the sub-Ohmic spin-boson model is examined using three numerical approaches, namely the Dirac-Frenkel time-dependent variation with the Davydov D(1) ansatz, the adaptive time-dependent density matrix renormalization group method within the representation of orthogonal polynomials, and a perturbative approach based on a unitary transformation. In order to probe the validity regimes of the three approaches, we study the dynamics of a qubit coupled to a bosonic bath with and without a local field. Comparison of the up-state population evolution shows that the three approaches are in agreement in the weak-coupling regime but exhibit marked differences when the coupling strength is large.

Monte Carlo simulation based on dynamic disorder model in organic semiconductors: from coherent to incoherent transport.

The dynamic disorder model for charge carrier transport in organic semiconductors has been extensively studied in recent years. Although it is successful on determining the value of bandlike mobility in the organic crystalline materials, the incoherent hopping, the typical transport characteristic in amorphous molecular semiconductors, cannot be described. In this work, the decoherence process is taken into account via a phenomenological parameter, say, decoherence time, and the projective and Monte Carlo method are applied for this model to determine the waiting time and thus the diffusion coefficient.

Dissipative dynamics of charged polarons in organic molecules.

Within a non-adiabatic dynamical method, we simulate charged polaron motion and dissociation in an organic molecule in the presence of dissipation. The dissipation, represented by damping, is introduced to investigate the influence of temperature on the mobility of carriers. We find that the velocity of the polaron has an inversely linear dependence on the damping, and hence the relationship between mobility and temperature is a power law, which is in agreement with the corresponding experiments.

Spin-flip processes of polarons by magnetic impurities in conjugated polymers.

By employing an adaptive time-dependent density-matrix-renormalization-group method, the spin-flip process of polarons is investigated in a polymer chain with magnetic impurities. Being driven by an external electric field, a polaron carrying both spin 1/2 and charge +/-e moves at a constant speed in the polymer chain. When the polaron passes through a specific site, which couples to a magnetic impurity via spin-exchange interaction, a spin-flip process is observed if its spin is antiparallel to the impurity spin.

Correlation effects on the dynamics of bipolarons in nondegenerate conjugated polymers.

By employing an adaptive time-dependent density-matrix-renormalization-group method, we investigate the dynamics of a charged bipolaron in the presence of both electron-phonon and electron-electron interactions. We use a Su-Schrieffer-Heeger model modified to include electron-electron interactions via a Hubbard Hamiltonian, a Brazovskii-Kirova symmetry-breaking term, and an external electric field. Our results show that the velocity of the bipolaron increases first and then decreases with the increasing of the on-site Coulomb interaction, U.

Normal modes of a small bilayer system of binary classical charged particles trapped in a parabolic confinement potential.

The normal modes and the melting character of a bilayer system consisting of binary charged particles with different charge and/or different mass, interacting through a Coulomb potential and confined in a parabolic trap are investigated. The normal mode spectrum is discussed as a function of the charge ratio (CR) and mass ratio (MR) of the two kinds of charged particles as well as the interlayer separation. We show that the dependence of the normal modes on the excited states can be tuned by varying the CR, the MR, and the interlayer distance.

Concurrence and fidelity of a Bose-Fermi mixture in a one-dimensional optical lattice.

We study the ground-state fidelity and entanglement of a Bose-Fermi mixture loaded in a one-dimensional optical lattice. It is found that the fidelity is able to signal quantum phase transitions between the Luttinger liquid phase, the density-wave phase, and the phase separation state of the system, and the concurrence, as a measure of the entanglement, can be used to signal the transition between the density-wave phase and the Ising phase.

[Diagnosis on the preoperative staging of rectal cancer by magnetic resonance imaging with water-bag in rectum].

Objective: To evaluate the accuracy of preoperative magnetic resonance imaging (MRI) with water-bag in rectum in prediction of pathological staging of rectal cancer.

Methods: Clinical data of 19 patients with rectal carcinoma assessed by MRI with water-bag in rectum for tumour (T) and mesorectal nodal (N) staging were analyzed retrospectively. Preoperative MRI assessment was compared with postoperative histopathological findings.

Switching and nonswitching phases of photomechanical molecules in dissipative environments.

We study the effects of dissipation on photoisomerization (PI). The result suggests the existence of two types of environment depending on whether it entangles with the molecule. With entanglement there is a quantum phase transition between a state where PI persists, to a state where PI is quenched by the environment.

Phonon effects on spin-charge separation in one dimension.

Phonon effects on spin-charge separation in one dimension are investigated through the calculation of one-electron spectral functions in terms of the recently developed cluster perturbation theory together with an optimized phonon approach. It is found that the retardation effect due to the finiteness of phonon frequency suppresses the spin-charge separation and eventually makes it invisible in the spectral function. By comparing our results with experimental data of TTF-TCNQ, it is observed that the electron-phonon interaction must be taken into account when interpreting the angle-resolved photoemission spectroscopy data.