Percolation Phenomena in Low and High Density Systems

We consider the 2D quenched–disordered q–state Potts ferromagnets and show that in the translation invariant measure, averaged over the disorder, at self–dual points any amalgamation of q?1 species will fail to percolate despite an overall (high) density of 1?q ^?1. Further, in the dilute bond version of these systems, if the system is just above threshold, then throughout the low temperature phase there is percolation of a single species despite a correspondingly small density. Finally, we demonstrate both phenomena in a single model by considering a “perturbation” of the dilute model that has a self–dual point. We also demonstrate that these phenomena occur, by a similar mechanism, in a simple coloring model invented by O. Häggström.     

Dynamical and Geometric Phases of Exciton Emission in a Semiconductor Microcavity

By using of the invariant theory, we have studied the phase of exciton emission in a semiconductor microcavity, the dynamical and geometric phases are presented respectively. The Aharonov-Anandan phase is also obtained in the case of cyclical evolution.     

Exciton warming in III–V semiconductors and microcavities

We present a time resolved experiment in which we dynamically tailor the occupation and temperature of a photogenerated exciton distribution in QWs by excitation with two delayed picosecond pulses. The modification of the excitonic distribution results in ultrafast changes in the PL dynamics. Our experimental results are well accounted by a quasiequilibrium thermodynamical model, which includes the occupation and momentum distribution of the excitons. We use this model and the two-pulse experimental technique to study the polariton dynamics in InGaAs-based microcavities in the strong coupling regime. In particular, we demonstrate that resonantly injected upper polaritons mainly relax to the lower polariton branch via scattering to large momentum polariton states, producing the warming of the polariton reservoir.     

Exciton effects on the refractive index and optical absorption in wurtzite quantum wells via a fractional-dimensional space approach

The optical refractive index changes and absorption coefficients of quantum wells (QWs) are theoretically investigated with considering exciton effects within the framework of the fractional-dimensional space approach. The exciton wave functions and bound energies are obtained as a function of spatial dimensionality, and the dimension increases with the well width increasing. Then optical properties are obtained by using the compact-density matrix approach and an iterative method. Numerical results are presented for wurtzite ZnO/Mg_xZn_1−xO QWs. The calculated results show that the changes of refractive index and absorption coefficients are greatly enhanced due to the quantum confinement of exciton. And the smaller the QW width (dimension) is, the larger influence of exciton on the optical properties will be. Furthermore, the exciton effects make the resonant peaks move to a lower energy. In addition, the optical properties are related to the QW width, the incident optical intensity and carrier density.     

High-pressure phase transitions of Mg2Ge and Mg2Sn: First-principles calculations

Phase transitions of the anti-fluorite compounds Mg₂Ge and Mg₂Sn under high pressure were investigated using the first-principles plane-wave method within the pseudopotential and generalized gradient approximations. The calculated results show that Mg₂Ge and Mg₂Sn undergo two first-order phase transitions at high pressure and the sequence of the pressure-induced phase transitions is from the anti-fluorite to the anti-cotunnite, and then to the Ni₂In-type structure. The high pressure behaviors of Mg₂Ge and Mg₂Sn are similar to Mg₂Si and the isostructural alkali-metal oxide Li₂O. Moreover, the electronic and optical properties of both the anti-fluorite and the high-pressure phases are presented.     

Theory of Phase Separation in Finite-Lifetime Systems

We study the phase separation dynamics in finite lifetime systems. Particles created by the external fields are unstable and decay within the finite lifetime. Effects of the particle lifetime and the interparticle interaction for the phase separation dynamics are discussed. In our theory, the formulation is based on the coarse-grained lattice gas model. As a result, effects of the finite lifetime are that the phase separation is restrained and that the finite size of domains remain finally at t = X .     

Exciton eigenstates and biexciton interaction energies in a spherical core/shell semiconductor hetero-nano structure

The exciton eigenstates and biexciton interaction energies in a spherical core–shell hetero-nano structure in the type II and the quasi-type II carrier localization regimes have been analyzed. For the analysis, we have evaluated the electron–hole overlap integral, the binding energy of exciton ground state, and the interaction energy of bi-exciton ground state in the structure. In the evaluation, the first order perturbation approach has been employed, where the direct Coulomb interaction energy, the surface polarization energy and the dielectric solvation energy are included. Our results show that the exciton eigenenergies and exciton–exciton interaction energy strongly depend on the choice of materials on which both the dielectric constants and the electron and hole effective masses rely.     

湍流大气中光束的相位不连续点数密度

基于Rytov近似和几何光学近似条件下,导出了对数振幅导数方差的解析表达式,证明了该参量主要取决于湍流内尺度、Rytov方差以及Fresnel尺寸大小.在此基础上,给出了光束相位不连续点数密度的修正表达式,分析了相位不连续点数密度随上述湍流参量变化的情况.分析表明在Rytov方差小于1的湍流条件下,不连续点数密度数随Rytov方差的增大而增大,而随湍流内尺度和Fresnel尺寸的增大而减小.     

Pressure controllable phase transition in MoTe2 by the interlayer band occupancy

High pressure can effectively control the phase transition of MoTe₂ in experiment, but the mechanism is still unclear. In this work, we show by first-principles calculations that the phase transition is suppressed and $1T^{\prime }$ phase becomes more stable under high pressure, which originates from the pressure-induced change of the interlayer band occupancies near the Fermi energy. Specifically, the interlayer states of $1T^{\prime }$ phase tend to be fully occupied under high pressure, while they keep partially occupied for the $T_d$ phase. The increase of the band occupancies makes the $1T^{\prime }$ phase more favorable in energy and prevents the structure changing from $1T^{\prime }$ to $T_d$ phase. Moreover, we also analyze the superconductivity under high pressure based on BCS theory by calculating the density of states and phonon spectra. Our results may shed some light on understanding the relationship between the interlayer band occupancy and crystal stability of MoTe₂ under high pressures.     

Exciton binding energy and excitonic absorption spectra in a parabolic quantum wire under transverse electric field

The effects of transverse electric field on the energy levels of electron and heavy hole, exciton binding energy and excitonic absorption spectra of GaAs parabolic quantum wire are theoretically investigated in detail. The results indicate that the electron and hole energy levels, exciton binding energy, excitonic absorption coefficient and absorption energy becomes smaller with the increase of electric field. That is more significant at the condition of weaker parabolic confinement potential. The phenomena can be explained by the separation of overlap integral of the electron and hole at the ground states.     

Exciton states and interband optical transitions in wurtzite InGaN/GaN quantum dot nanowire heterostructures

Within the framework of the effective-mass approximation, the exciton states and interband optical transitions in In_xGa_1−xN/GaN strained quantum dot (QD) nanowire heterostructures are investigated using a variational method, in which the important built-in electric field (BEF) effects, dielectric-constant mismatch and three-dimensional confinement of the electron and hole in In_xGa_1−xN QDs are considered. We find that the strong BEF gives rise to an obvious reduction of the effective band gap of QDs and leads to a remarkable electron-hole spatial separation. The BEF, QD height and radius, and dielectric mismatch effects have a significant influence on exciton binding energy, electron interband optical transitions, and the exciton oscillator strength.     

High-Density Excitons in a Diluted Magnetic Semiconductor: Cd 1 − x Mn x Te

Time-resolved magneto photoluminescence in a diluted magnetic semiconductor Cd 0.9 Mn 0.1 Te has been carried out with varying exciton density from 10 14 to 10 19 cm m 3 . The reduction of the Zeeman shift and that of the magnetic polaron energy was found under strong photoexcitation. The spectral feature is interpreted in terms of the heating of the manganese spin subsystem. Polarization dependence of the spin heating is observed for the first time, revealing the contribution of the spin flip between excitons and magnetic ions to the heating process.     

Solid and Liquid AgI at High Pressure and High Temperature: A X-ray Absorption Spectroscopy Study

We report about recent X-ray absorption spectroscopy (XAS) measurements on solid and liquid AgI under high pressure. The structural behaviour of AgI has been investigated to pressures P～4.3 GPa at room temperature and to P～1.8 GPa at 1100 K. The high temperature/high pressure conditions have been obtained by means of a large-volume press of the Paris-Edinburgh type, coupled with a 10 mm boron/epoxy biconical gasket. The absorption spectra have been collected in transmission mode, both at the K-edge of Ag and I, and the samples have been characterized in situ by energy scanning X-ray diffraction at fixed angles. Our XAS results for solid AgI are compatible with previous X-ray diffraction measurements. For liquid AgI, we observe a slight change in the intensity and a shift in the frequency of the XAS oscillations with respect to what obtained in the case of the ambient pressure liquid.     

High Energy Density Physics Studies at the Facility for Antiprotons and Ion Research: the HEDgeHOB Collaboration

The forthcoming F acility for A ntiprotons and I on R esearch (FAIR) at Darmstadt, is going to be a unique accelerator facility that will deliver high quality, strongly bunched, well focused, intense beams of heavy ions that will lead to unprecedented specific power deposition in solid matter. This will generate macroscopic samples of H igh E nergy D ensity (HED) matter with fairly uniform physical conditions. These samples can be used to study the thermophysical and transport properties of HED matter. Extensive theoretical work has been carried out over the past decade to design numerous dedicated experiments to study HED physics at the FAIR, which has provided the basis for the HEDgeHOB ( H igh E nergy D ensity Matter Ge nerated by H eavy I o n B eams) scientific proposal. This work is still in progress as the feasibility studies for more experimental schemes are being carried out. Another, very important research area that will benefit tremendously from the FAIR facility, is the production of radioactive beams. A superconducting fragment separator, Super–FRS is being designed for the production and separation of rare radioactive isotopes. Unlike the HED targets, the Super–FRS production target should not be destroyed or damaged by the beam, but should remain intact during the long experimental campaign. However, the high level of specific power deposited in the production target by the high intensity ion beam at FAIR, could cause serious problems to the target survival. These HED issues related to the Super–FRS production target are also discussed in the present paper (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Density functional theory study of 3R- and 2H-CuAlO2 in tensile stress

We report a direct computational result of a phase transformation from the 3R phase to the 2H phase in CuAlO₂ with the application of tensile stress using the first-principles density functional theory calculations. The calculations of enthalpy variation with tensile stress indicates the 3R-to-2H phase transformation is expected to occur around −26.0 GPa. As the applied tensile stress increases, the independent elastic constants of 3R- and 2H-CuAlO₂ show the presences of mechanical instability at −27.5 and −27.6 GPa, which are possibly related with the ideal tensile strength.     

Hierarchic Natures and Dynamics of Photoinduced Structural Phase Transition in Non-Degenerate Charge Density Wave States Via Successive Photoexcitations

We investigate the adiabatic and dynamical natures of the lattice relaxation of excitons in strongly coupled electron-phonon (e-ph) systems using the extended Peierls-Hubbard model, so as to clarify the possible mechanisms of the photoinduced structural phase transition (PISPT) via multi-photon. Focusing on the growth process of relaxed domains that is induced by multi-photoexcitation, we calculate the adiabatic potential energy surfaces relevant to the nonlinear lattice relaxations of excitons in this process. Calculated potentials lead to an essential model of a multi-stepwise potential-crossing (MSPC) system that is composed of many displaced harmonic oscillators as an elementary process of the domain growth in the strongly coupled e-ph systems. We also investigate the dynamical natures in such MSPC systems calculating the time-developments the excited wave packet in this system using the density operator. It is concluded from calculated results that the system possibly develops from the lowest-energy potential state to the higher ones by the effect of the photoexcitations followed by the lattice relaxations.     

Exciton states and optical transitions in InGaN/GaN quantum dot nanowire heterostructures: Strong built-in electric field and dielectric mismatch effects

Considering the strong built-in electric field (BEF), dielectric-constant mismatch and 3D confinement of the electron and hole, the exciton states and interband optical transitions in [0 0 0 1]-oriented Ga-rich wurtzite In_xGa_1−xN/GaN strained quantum dot (QD) nanowire heterostructures are investigated theoretically using a variational approach under the effective mass approximation. We find that the strong BEF gives rise to an obvious reduction of the effective band gap of QDs and leads to a remarkable electron-hole spatial separation. The BEF, QD height and radius, and dielectric mismatch effects have a significant influence on exciton binding energy, electron interband optical transitions, and the radiative decay time. Our calculations show that the radiative decay time of the redshifted transitions is large and increases almost exponentially when the QD height increases, which is in good agreement with the previous experimental and theoretical results.     

High pressure lattice dynamics, dielectric and thermodynamic properties of SrO

Using density functional theory and density functional perturbation theory we have studied the effects of hydrostatic pressure on lattice dynamics, dielectric and thermodynamic properties of the rocksalt (NaCl) and CsCl phases of SrO. The stability of the NiAs type structure, experimentally confirmed to be stable in BaO, is also investigated. Studying the lattice dynamics of the NaCl and CsCl phases at various pressures, in the range of the phase stability, we have found the lattice dynamical instabilities which govern the phase transitions between NaCl and CsCl phases with increasing and decreasing pressure. By monitoring the behaviour of the found soft modes, we have calculated the transition pressures upon compression and decompression of SrO crystal. Lattice dynamics calculations reveal that the rocksalt and CsCl structures are unstable with respect to the soft transversal acoustic modes at single points of the Brillouin zone, which points to the fact that the transitions are of displacive type. Responses to electric fields and thermodynamic properties at high pressures are also given and discussed. All our results are in a good agreement with experimental data where applicable.     

Observation of the Cinnabar Phase in ZnSe at High Pressure

In this paper we describe the results of an energy dispersive X-ray diffraction experiment carried out in the ZnSe 1 m x Te x alloy and pure ZnSe under high pressure. In the downstroke the cinnabar phase is observed between the rocksalt and the zincblende phases. The analysis of the whole series of compositions ( x =0, 0.05, 0.1 and 0.2) enables us to establish its lattice parameters in ZnSe ( a =3.785 + and c =8.844 + at 10.5 GPa). The X-ray diffraction pattern simulation suggests that the internal parameters u and v are close to 0.5, indicating that the cinnabar phase in ZnSe is similar to that observed in GaAs and ZnTe. The cinnabar's stability range decreases as the Te content is reduced.     

金属钨三键多核金属有机配合物高压拉曼及红外光谱研究

研究了金属钨三键多核金属有机配合物 [Cl(CO) 2 (DPPE)WCC6 H4CCC6 H4NC]2 ReCl(CO) 3(DPPE =bis(diphenylphosphino)ethane,(Ph) 2 PCH2 CH2 P(Ph) 2 )不同压力下的拉曼和红外光谱 (最高压力约 5 0kbar)。配合物在 3 0kbar存在一压力诱导二级相变。金属钨三键伸缩振动ν(WC)压力相关 (dν/dp)在低压相区 ( 0 5 9cm-1·(kbar) -1)与高压相区 ( 0 0 4cm-1·(kbar) -1)差别很大 (约 15倍 )。在高压下 ,从基团Cl(CO) 2 (DPPE)WC中的金属钨W向COπ 轨道的π 反馈明显增强 ,WC的键强度减弱 ,力常数减小和ν(WC)降低 ;此效应与压力缩短WC键 ,增大力常数和增加ν(WC)的效应互相竞争。