arXiv subjects

Hang Zheng

Publications and source records attributed to Hang Zheng.

66 records · Page 4Linked to original sources

arXiv2009

Dynamics of a two-level system coupled to a quantum oscillator: Transformed rotating-wave approximation

For studying the dynamics of a two-level system coupled to a quantum oscillator we have presented an analytical approach, the transformed rotating-wave approximation, which takes into account the effect of the counter-rotating terms but still keeps the simple mathematical structure of the ordinary rotating-wave approximation. We have calculated the energy levels of ground and lower-lying excited states, as well as the time-dependent quantum dynamics. It is obvious that the approach is quite simple and can be easily extended to more complicated situation. Besides, the results are compared with the numerically exact ones to show that for weak and intermediate coupling and moderate detuning our analytic calculations are quantitatively in good agreement with the exact ones.

quant-ph↗

arXiv2009

Quantum Monte Carlo study on speckle variation due to photorelaxation of ferroelectric clusters in paraelectric barium titanate

Time-dependent speckle pattern of paraelectric barium titanate observed in a soft x-ray laser pump-probe measurement is theoretically investigated as a correlated optical response to the pump and probe pulses. The scattering probability is calculated based on a model with coupled soft x-ray photon and ferroelectric phonon mode. It is found that the speckle variation is related with the relaxation dynamics of ferroelectric clusters created by the pump pulse. Additionally, critical slowing down of cluster relaxation arises on decreasing temperature towards the paraelectric-ferroelectric transition temperature. Relation between critical slowing down, local dipole fluctuation and crystal structure are revealed by quantum Monte Carlo simulation.

cond-mat.mtrl-sci↗

arXiv2008

Dynamics of ferroelectric nano cluster in BaTiO3 observed as a real time correlation between two soft X-ray laser pulses

We carry out a theoretical investigation to clarify the dynamic property of photo-created nano-sized ferroelectric cluster observed in the paraelectric BaTiO3 as a real time correlation of speckle pattern between two soft X-ray laser pulses, at just above the paraelectric-ferroelectric phase transition temperature. Based on a model with coupled soft X-ray photon and ferroelectric phonon mode, we study the time dependence of scattering probability by using a perturbative expansion approach. The cluster-associated phonon softening as well as central peak effects are well reproduced in the phonon spectral function via quantum Monte Carlo simulation. Besides, it is found that the time dependence of speckle correlation is determined by the relaxation dynamics of ferroelectric clusters. Near the transition point, cluster excitation is stable, leading to a long relaxation time. While, at high temperature, cluster structure is subject to the thermal fluctuation, ending up with a short relaxation time.

cond-mat.mtrl-sci↗

arXiv2007

Non-Markovian disentanglement dynamics of two-qubit system

We investigated the disentanglement dynamics of two-qubit system in Non-Markovian approach. We showed that only the couple strength with the environment near to or less than fine-structure constant 1/137, entanglement appear exponential decay for a certain class of two-qubit entangled state. While the coupling between qubit and the environment is much larger, system always appears the sudden-death of entanglement even in the vacuum environment.

cond-mat.mes-hall↗

arXiv2007

Non-Markovian coherence dynamics of driven spin boson model: damped quantum beat or large amplitude coherence oscillation

The dynamics of driven spin boson model is studied analytically by means of the perturbation approach based on a unitary transformation. We gave the analytical expression for the population difference and coherence of the two level system. The results show that in the weak driven case, the population difference present damped coherent oscillation (single or double frequency) and the frequencies depend on the initial state. The coherence exhibit damped oscillation with Rabi frequency. When driven field is strong enough, the population difference exhibit undamped large-amplitude coherent oscillation. The results easily return to the two extreme cases without dissipation or without periodic driven.

cond-mat.mtrl-sci↗

arXiv2007

Non-markovian dynamics of double quantum dot charge qubit with static bias

The dynamics of charge qubit in double quantum dot coupled to phonons is investigated theoretically. The static bias is considered. By means of the perturbation approach based on unitary transformations, the dynamical tunneling current is obtained explicitly. The biased system displays broken symmetry and a significantly larger coherence-incoherence transition critical point $α_{c}$. We also analyzed the decoherence induced by piezoelectric coupling phonons in detail. The results show that reducing the coupling between system and bath make coherence frequency increase and coherence time prolong. To maintain quantum coherence, applying static bias also is a good means.

cond-mat.mes-hall↗

arXiv2006

Quantum dynamics of the dissipative two-state system coupled with a sub-Ohmic bath

The decoherence of a two-state system coupled with a sub-Ohmic bath is investigated theoretically by means of the perturbation approach based on a unitary transformation. It is shown that the decoherence depends strongly and sensitively on the structure of environment. Nonadiabatic effect is treated through the introduction of a function $ξ_k$ which depends on the boson frequency and renormalized tunneling. The results are as follows:(1) the non-equilibrium correlation function $P(t)$, the dynamical susceptibility $χ''(ω)$ and the equilibrium correlation function $C(t)$ are analytically obtained for $s\leq 1$; (2) the phase diagram of thermodynamic transition shows the delocalized-localized transition point $α_l$ which agrees with exact results and numerical data from the Numerical Renormalization Group; (3) the dynamical transition point $α_c$ between coherent and incoherent phase is explicitly given for the first time. A crossover from the coherent oscillation to incoherent relaxation appears with increasing coupling (for $α> α_c $, the coherent dynamics disappear); (4) the Shiba's relation and sum rule are exactly satisfied when $α\leq α_c $; (5) an underdamping-overdamping transition point $α_c^{*}$ exists in the function $S(ω)$. Consequently, the dynamical phase diagrams in both ohmic and sub-Ohmic case are mapped out. For $Δ\ll ω_c$, the critical couplings ($α_l, α_c$ and $α_c^{*}$) are proportional to $Δ^{1-s}$.

quant-ph↗

arXiv2004

Charge qubit dynamics in a double quantum dot coupled to phonons

The dynamics of charge qubit in a double quantum dot coupled to phonons is investigated theoretically in terms of a perturbation treatment based on a unitary transformation. The dynamical tunneling current is obtained explicitly. The result is compared with the standard perturbation theory at Born-Markov approximation. The decoherence induced by acoustic phonons is analyzed at length. It is shown that the contribution from deformation potential coupling is comparable to that from piezoelectric coupling in small dot size and large tunneling rate case. A possible decoupling mechanism is predicted.

cond-mat.mes-hall↗

arXiv2002

Nonadiabatic approach to dimerization gap and optical absorption coefficient of the Su-Schrieffer-Heeger model

An analytical nonadiabatic approach has been developed to study the dimerization gap and the optical absorption coefficient of the Su-Schrieffer-Heeger model where the electrons interact with dispersive quantum phonons. By investigating quantitatively the effects of quantum phonon fluctuations on the gap order and the optical responses in this system, we show that the dimerization gap is much more reduced by the quantum lattice fluctuations than the optical absorption coefficient is. The calculated optical absorption coefficient and the density of states do not have the inverse-square-root singularity, but have a peak above the gap edge and there exist a significant tail below the peak. The peak of optical absorption spectrum is not directly corresponding to the dimerized gap. Our results of the optical absorption coefficient agree well with those of the experiments in both the shape and the peak position of the optical absorption spectrum.

cond-mat.str-el↗

arXiv2001

Effect of quantum lattice fluctuations on the optical-absorption spectra of halogen-bridged mixed-valence transition-metal complexes

The effect of quantum lattice fluctuations on the optical-absorption spectra in the ground state of halogen-bridged mixed-valence transition-metal linear-chain complexes is studied by using a one-dimensional extended Peierls-Hubbard model. The nonadiabatic effects due to finite phonon frequency $ω_π>0$ are treated through an energy-dependent electron-phonon scattering function $δ(k^{\prime},k)$ introduced by means of an unitary transformation. The calculated optical-absorption spectra do not have the inverse-square-root singularity, but they have a peak above the gap edge and there exists a significant tail below the peak, which are consistent with the optical-absorption coefficient or the optical conductivity measurements of this material.

cond-mat.str-el↗

arXiv2001

Phase diagram and optical conductivity of the one-dimensional spinless Holstein model

The effects of quantum lattice fluctuations on the Peierls transition and the optical conductivity in the one-dimensional Holstein model of spinless fermions have been studied by developing an analytical approach, based on the unitary transformation method. We show that when the electron-phonon coupling constant decreases to a finite critical value the Peierls dimerization is destroyed by the quantum lattice fluctuations. The dimerization gap is much more reduced by the quantum lattice fluctuations than the phonon order parameter. The calculated optical conductivity does not have the inverse-square-root singularity but have a peak above the gap edge and there exists a significant tail below the peak. The peak of optical-conductivity spectrum is not directly corresponding to the dimerized gap. Our results of the phase diagram and the spectral-weight function agree with those of the density matrix renormalization group and the exact diagonalization methods.

cond-mat.str-el↗

arXiv2001

Quantum lattice fluctuations in a model electron-phonon system

An analytical approach, based on the unitary transformation method, has been developed to study the effect of quantum lattice fluctuations on the ground state of a model electron-phonon system. To study nonadiabatic case, the Green's function method is used to implement the perturbation treatment. The phase diagram and the density of states of fermions are obtained. We show that when electron-phonon coupling constant $α^{2}/K$ decreases or phonon frequency $ω_π$ increases the lattice dimerization and the gap in the fermion spectrum decrease gradually. At some critical value the system becomes gapless and the lattice dimerization disappears. The inverse-square-root singularity of the density of states at the gap edge in the adiabatic case disappears because of the nonadiabatic effect, which is consistent with the measurement of optical conductivity in quasi-one-dimensional systems.

cond-mat.str-el↗