Semi-Classical Dynamics in Quantum Spin Systems

We consider two limiting regimes, the large-spin and the mean-field limit, for the dynamical evolution of quantum spin systems. We prove that, in these limits, the time evolution of a class of quantum spin systems is determined by a corresponding Hamiltonian dynamics of classical spins. This result can be viewed as a Egorov-type theorem. We extend our results to the thermodynamic limit of lattice spin systems and continuum domains of infinite size, and we study the time evolution of coherent spin states in these limiting regimes.     

胆绿素二甲酯金属锌配合物超快激发态动力学研究

胆绿素(BV)及其二甲酯(BVE)是一种生物内源色素,他们在溶液中具有极弱的荧光,量子产率小于0.01%。然而,随着锌离子的加入,情况发生了变化. BVE-Zn²⁺配合物的荧光强度大大增强,荧光QY可提高到∽5%. 本文研究了BVE-Zn²⁺络合物在乙醇、正丙醇和二甲基亚砜溶液中的超快激发态动力学,以揭示其荧光增强的机理. 结果表明,BVE能与锌离子在溶液中形成1:1的稳定配合物,BVE分子结构在复合物中较为刚性,能量上更稳定. 利用皮秒时间分辨荧光光谱和飞秒瞬态吸收光谱技术,发现BVE-Zn²⁺配合物在二甲基亚砜中具有较小的非辐射速率常数,这是其高荧光量子产率关键原因.     

PPV衍生物的激发态动力学特性研究

利用共振飞秒光学克尔效应研究了C10－PPV溶液的非线性光学响应。实验结果表明，在共振激发条件下，C10－PPV挑学克尔效应信号表现为快速上升和随后的驰豫过程，其驰豫过程由两部分组成，一个200fs的快过程和一个400ps的慢过程。通过比较C10－PPV和掺杂C60后的C10－PPV荧光光谱，400ps的慢过程可归属为聚合物链内单重态激子在不同共轭段间的迁移过程。     

Dynamics of a Quantum Damped Oscillator with Relaxation

The quantum problem of a nonlinear oscillator interacting with the field of harmonic oscillators with continuously distributed frequencies is examined. The Heisenberg representation is used. It is demonstrated that after elimination of field variables from the equation, the oscillator dynamics is described by an integro-differential operator equation. It is proved that in the general nonlinear case, an exact solution for the quantum oscillator demonstrates dissipative behavior under certain limitations on the relaxation kernel.     

Dynamics of Spin I=3/2 under Spin-Locking Conditions in an Ordered Environment

We have derived approximate analytic solutions to the master equation describing the evolution of the spin I=3/2 density operator in the presence of a radio-frequency (RF) field and both static and fluctuating quadrupolar interactions. Spectra resulting from Fourier transformation of the evolutions of the on-resonance spin-locked magnetization into the various coherences display two satellite pairs and, in some cases, a central line. The central line is generally trimodal, consisting of a narrow component related to a slowly relaxing mode and two broad components pertaining to two faster relaxing modes. The rates of the fast modes are sensitive to slow molecular motion. Neither the amplitude nor the width of the narrow component is affected by the magnitude of the static coupling, whereas the corresponding features of the broad components depend in a rather complicated manner on the spin-lock field strength and static quadrupolar interaction. Under certain experimental conditions, the dependencies of the amplitudes on the dynamics are seen to vanish and the relaxation rates reduce to relatively simple expressions. One of the promising emerging features is the fact that the evolutions into the selectively detected quadrupolar spin polarization order and the rank-two double-quantum coherence do not exhibit a slowly relaxing mode and are particularly sensitive to slow molecular motion. Furthermore, these coherences can only be excited in the presence of a static coupling and this makes it possible to discern nuclei in anisotropic from those in isotropic environment. The feasibility of the spin-lock pulse sequences with limited RF power and a nonvanishing average electric field gradient has been demonstrated through experiments on sodium in a dense lyotropic DNA liquid crystal.     

2-羟基吡啶S1态超快动力学

本文利用飞秒时间分辨的光电子成像技术研究了2-羟基吡啶分子在276.9∽250.0 nm波长范围的紫外光激发条件下S₁态的衰变动力学. 根据衰变动力学的泵浦波长相关性,提出了一种衰变机制. 在276.9 nm泵浦波长下,S₁态主要通过系间穿越到更低的三重态衰变. 在264.0 nm泵浦波长下,系间穿越到三重态和内转换到基态的通道同时存在. 在250.0 nm泵浦波长下,内转换到基态过程占据主导.     

Nonlinear Optical Dynamics and High Reflectance of a Monolayer of Three-Level Quantum Emitters with a Doublet in the Excited State

Journal of Experimental and Theoretical Physics - We study theoretically the nonlinear optical response of a monolayer of regularly arranged three-level identical quantum emitters with a doublet in...     

Dimensional Crossover of Magnetic Transition in Diluted Ising Spin Nets Probed by Muon Spin Relaxation

The magnetic ordering process of Ising spins on diluted square lattice was studied by muon spin relaxation using model compounds Rb₂Co _c Mg_1−c F₄. Muon relaxation shows an anomaly at a remarkably higher temperature T _N ^μSR than the transition temperature determined by neutron Bragg scattering T _N ^ND near the percolation threshold for square lattice (c _p=0.593). The difference between the two temperatures amounts to 50% of T _N ^ND just above c _p. The field cooling effect of DC magnetic susceptibility is appreciable below T _N ^ND while the temperature of the anomaly in AC susceptibility approaches to T _N ^μSR as the frequency is increased. It was concluded that there is a crossover from two-dimensional ordering at T _N ^μSR to three-dimensional ordering at T _N ^ND but the two-dimensional order between T _N ^μSR and T _N ^ND has slow fluctuations due to the fractal structure with a plenty of weak links. This revised version was published online in September 2006 with corrections to the Cover Date.     

Spin Fluctuations and a Spin-Fluctuation Phase Transition in the Magnetically Ordered Phase of Manganese Monosilicide

JETP Letters - A manganese monosilicide (MnSi) single crystal has been studied by the high-frequency (60 GHz) electron paramagnetic resonance method. The most currently detailed temperature...     

Spin Relaxation of Electrons in Single InAs Quantum Dots

By using polarization-resolved photoluminescence spectra, we study the electron spin relaxation in single InAs quantum dots （QDs） with the configuration of positively charged excitons X＋ （one electron, two holes）. The spin relaxation rate of the hot electrons increases with the increasing energy of exciting photons. For electrons localized in QDs the spin relaxation is induced by hyperfine interaction with the nuclei. A rapid decrease of polarization degree with increasing temperature suggests that the spin relaxation mechanisms are mainly changed from the hyperfine interaction with nuclei into an electron-hole exchange interaction.     

移动势阱下一维自旋链上量子态的传输

本文从无序能够抑制量子态的传输作为启发,验证了外加势垒的低穿透性,提出了利用外加移动势阱将量子态限制在势阱底部进行量子态传输的方案,并利用有限方势阱,余弦曲线势阱,V型势阱和抛物线曲线势阱进行了讨论和比较。可以将自旋链看做是与一定常衰减系数γ相关的衰减通道,并讨论了衰减系数和磁场强度的关系。在一定移动势阱条件下,可以得到很低的衰减系数,量子态能够进行较远距离的传输,并且在无序存在的情况下具有很强的稳定性。     

Many-Spin Entanglement in Multiple Quantum NMR with a Dipolar Ordered Initial State

Journal of Experimental and Theoretical Physics - Many-spin entanglement is investigated in a gas of spin-carrying molecules (atoms) in a nanopore under NMR conditions with a dipolar ordered...     

抛物量子线中极化子的激发态性质

研究抛物型量子线中极化子激发态的性质。采用线性组合算符法和Pekar变分法,导出了抛物量子线中强耦合和弱耦合两种情况下极化子的基态能量、第一内部激发能量、激发能和振动频率随量子线的约束强度的变化关系。数值计算结果表明:抛物型量子线中极化子的基态能量、第一内部激发能量、激发能和振动频率都随量子线约束强度的增强而增大,体现了量子线受限强度越大,相应的极化子的能量也越大。     

Zero-Variance of Perturbation Hamiltonian Density in Perturbed Spin Systems

Journal of Statistical Physics - We study effects of perturbation Hamiltonian to quantum spin systems which can include quenched disorder. Model-independent inequalities are derived, using an...     

Self-Averaging of Perturbation Hamiltonian Density in Perturbed Spin Systems

Journal of Statistical Physics - It is shown that the variance of a perturbation Hamiltonian density vanishes in the infinite-volume limit of perturbed spin systems with quenched disorder. This is...     

Relaxation of Vibrationally Excited States in Solid Binary Carbonate-Sulfate Systems

Physics of the Solid State - The processes of molecular relaxation in solid binary carbonate-sulfate systems, such as Li2CO3-Li2SO4, Na2CO3-Na2SO4, K2CO3-K2SO4, have been studied by Raman...     

Dynamics of Einstein-Podolsky-Rosen Steering in Quantum Spin Environment

We explore the dynamics of Einstein-Podolsky-Rosen steering (EPR), measured by steering robustness, for two spin-qubits coupled to a general XY spin chain environment. The evolution process is numerically investigated in the vicinity of critical point for the spin environment. The results show an obvious suppression of steering robustness when the environment undergoes a quantum phase transition. The scaling behavior for the dynamics of EPR steering is also revealed and analyzed around the critical point. Furthermore, we find that steering robustness power (i.e. the average value of steering robustness within a certain time) can indicate the quantum criticality of the environment directly.

     

Ultrafast Relaxation Dynamics of the Optical Nonlinearity in Nanometric Gold Particles

Measurements of the resonantly enhanced, third-order nonlinear optical properties of gold nanostructures exhibiting reduced charge-carrier mobility in three dimensions were performed with a number of ultrafast nonlinear optical techniques. The size of the particles investigated was varied between 5 and 40 nm. The magnitude of the nonlinear susceptibility is of the order of 5·10^–16 m²V^–2 at resonance and an order of magnitude lower off-resonance. The response time of the nonlinearity is found to be extremely fast and could not be resolved in the experiments undertaken here. The only statement that can be made in this regard is that the phase relaxation time is of the order of or less than 20 fs while the energy relaxation time is of the order of or less than 75 fs.     

Quantum State Transfer in Engineered Spin Chain under Influence of Spatially Distributed Environment

It has been shown that a quantum state could be perfectly transferred via a spin chain with engineered ``always-on interaction". In this paper, we study a more realistic problem for such a quantum state transfer (QST) protocol, how the efficacy of QST is reduced by the quantum decoherence induced by a spatially distributed environment. Here, the environment is universally modeled as a bath of fermions located in different positions. By making use of the irreducible tensor method in angular momentum theory, we investigate the effect of environment on the efficiency of QST for both cases at zero and finite temperatures. We not onlyshow the generic exponential decay of QST efficiency as the number of sites increase, but also find some counterintuitive effect, the QST can be enhanced as temperature increases in some cases.