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Abstract: The plane-wave pseudo-potential method within the framework of first-principles is used to investigate the structural and elastic properties of Mg2Si in its low pressure phase (Fm-3m) and intermediate pressure phase (Pnma). The high-pressure lattice constants, the elastic constants, the elastic moduli and the anisotropy factors of the anti-cotunnite Mg2Si are presented and discussed. The results show that our system is mechanically stable. The reversible phase transition from anti-fluorite to anti-cotunnite structure is successfully reproduced through the quasi-harmonic Debye model. The phase boundary can be described as P=4.06826−6.95×10−3 T+5.08838×10−5 T 2−4.24073×10−8 T 3. To complete the fundamental characteristics of these compounds we have analysed the thermodynamic properties such as thermal expansion, bulk modulus, isochoric heat capacity and Debye temperature in a pressure range 0–21GPa and a temperature range 0–1200K. The obtained results tend to support the experimental data when available. Therefore, the present results indicate that the combination of first-principles and quasi-harmonic approximations is an efficient scheme to simulate the high-temperature behaviours of semiconductors like Mg2Si. [Copyright &y& Elsevier]

Abstract: We study the effect of the hydrostatic pressure on the phonons in single-walled carbon nanotubes (SWNTs) in a magnetic field. We calculate the magnetic moments of the phonons using a functional integral technique, and find that the phonons in SWNTs undergo a pressure-induced phase transition from the paramagnetic phase to the diamagnetic phase under hydrostatic pressure 2GPa. We explain the mechanism of generating this phase transition. [Copyright &y& Elsevier]

Abstract: First-principles calculations of the crystal structure and the elastic properties of RuB2 have been carried out with the plane-wave pseudopotential density functional theory method. The calculated values are in very good agreement with experimental data as well as with some of the existing model calculations. The elastic constants cij , the aggregate elastic moduli (B, G, E), Poisson''s ratio, and the elastic anisotropy with pressure have been investigated. Through the quasi-harmonic Debye model considering the phonon effects, the isothermal bulk modulus, the thermal expansions, Grüneisen parameters, and Debye temperatures depending on the temperature and pressure are obtained in the whole pressure range from 0 to 60GPa and temperature range from 0 to 1100K as well as compared to available data. [Copyright &y& Elsevier]

Abstract: First-principles calculations of the crystal structures (wurtzite (WZ) and rocksalt (RS)) and phase transition of AlN have been carried out with the plane-wave pseudopotential density functional theory (DFT) method. The calculated values (for crystal structures) are in agreement with available experimental value and other calculated data. Through the quasi-harmonic Debye model combined with the first-principles theory, the thermodynamic properties of different phases under high temperature and high pressure are investigated. The phase transition from WZ structure to RS structure occurs at the pressure of 15.0GPa, which agrees well with experimental value. The evaluated equilibrium volume using this model agrees with the values obtained from ab intio and from experiment. The temperature and pressure dependence of quantities such as the equation of state (EOS), the isothermal bulk modulus, the heat capacity, and the thermal expansion are successfully obtained. [Copyright &y& Elsevier]

Abstract: The transition phase of PtN from zincblende (ZB) structure to rocksalt (RS) structure is investigated by ab initio plane-wave pseudopotential density functional theory method, and the thermodynamic properties of the ZB and RS structures under high pressure and temperature are obtained through the quasi-harmonic Debye model. The transition phase from the ZB structure to the RS structure occurs at the pressure of 18.2GPa, which agrees well with other calculated values. Moreover, the dependences of the relative volume V/V 0 on the pressure P, the Debye temperature Θ and heat capacity CV on the pressure P, together with the heat capacity CV on the temperature T are also successfully obtained. [Copyright &y& Elsevier]

Abstract: The equations of state (EOS) and other thermodynamic properties of TiB2 are investigated using ab initio plane-wave pseudopotential density functional theory method. The obtained results are in good agreement with the experimental measured data and other theoretical calculated ones. Through the quasi-harmonic Debye model, in which the phononic effects are considered, the dependences of relative volume V/V 0 on pressure P, cell volume V on temperature T, and Debye temperature Θ and specific heat C V on pressure P are successfully obtained. [Copyright &y& Elsevier]

Abstract: We study the atomic-displacement fluctuation in a graphite sheet under a microwave field by a modified functional integral approach. Our calculations show that the fluctuation continuously increases in some time ranges if the field strength and the frequency satisfy proper conditions. Since the atomic-displacement fluctuation is proportional to the phonon population number, this phenomenon may be considered as a phonon amplification effect led by a hopping electron–phonon coupling. [Copyright &y& Elsevier]

Abstract: In the second-order Raman scattering process, the laser fields can stimulate the phonon excitation, and change the atomic displacement fluctuation. In this paper, we calculate the quantum fluctuation of the atomic displacement in the crystal with a single impurity, and discuss the effect of the laser fields and the impurity on the fluctuation. [Copyright &y& Elsevier]

Abstract: We investigate the spin fluctuation in a two-dimensional antiferromagnet with a single impurity. The impurity plays the role of modulating the effective coupling between the spins. This system can realize the squeezed states under the alternate magnetic field like the perfect two-dimensional system. We calculate the 0-wavevector fluctuation of the spin system, and obtain the conditions of generating the squeezed states. [Copyright &y& Elsevier]

Abstract: The time evolution of the displacement-field fluctuation in the quantum Hall smectic phase is investigated in this work. It is found that a weak laser field can manipulate the fluctuation of the displacement field (or Goldstone modes), and even realize the squeezed states under certain conditions. The Goldstone-mode squeezed states allow a reduction in the quantum fluctuation of the stripe transverse displacements to below the vacuum fluctuation level. We discuss the effect of the Goldstone-mode squeezed states on the macroscopically measurable quantity. [Copyright &y& Elsevier]

Abstract: The time-evolution of the spin-component fluctuation in a two-dimensional system is studied by a simple quantization scheme of the xy model. The alternate magnetic fields on the two-dimensional plane can effectively modulate the spin fluctuation, and even enable the system to enter into the squeezed states that the fluctuation is below the level of the vacuum fluctuation. The effect of the quantum fluctuation on the magnetic susceptibility of the two-dimensional system is discussed. [Copyright &y& Elsevier]

We study the magnon-squeezed states generated by second-Raman scattering. The magnon-squeezed states allow a reduction in the spin-component fluctuation to below the level of the vacuum fluctuation. As the laser fields change the symmetry of the spin system, this leads to the anisotropy of the spin-fluctuation distribution. We evaluate the fluctuation of the spin component and discuss the anomalies of the spin-fluctuation distribution. [Copyright &y& Elsevier]

The elementary excitations of the spin waves, magnons, can generate the squeezing states in an antiferromagnet, which allow a reduction in the quantum fluctuation of the spin components to below the zero-point quantum noise level of the vacuum states. Here we calculate the spin-component fluctuation of an antiferromagnet in a laser field, and discuss how to modulate the squeezing states by the external electric field. [Copyright &y& Elsevier]

The magnon squeezing states in some magnetic crystals allow a reduction in the quantum fluctuations of the spin component to below the zero-point quantum noise level of the coherent magnon states. It is known that there are the magnon squeezing states in an antiferromagnet. However, their generating mechanism is not suitable for the ferromagnet. In this paper, we discuss the possibility of generating the magnon squeezing states in a ferromagnet, and discuss the effect of the crystal field on the magnon squeezing states. [Copyright &y& Elsevier]

Abstract: We study the effect of strain on the exchange interaction between two magnetic impurities in graphene. We calculate the exchange integrals between the impurities using a Green function technique. The calculation results indicate that strain may remarkably enhance the exchange interaction between the impurities within small distance. We explain the micromechanism of generating this phenomenon. [Copyright &y& Elsevier]

Abstract: A magnetic field not only changes the electronic structure in graphene but also affects the phonon excitations via the electron–phonon interaction and even enables the phonons to generate magnetism. In this paper, we evaluate the magnetic moment of phonons in graphene using a generating-functional technique. The calculation results indicate that the phonon magnetic moment exists only in a weak magnetic field. The step-like change of the magnetic moment with the magnetic field reflects a macroscopic quantum effect. [Copyright &y& Elsevier]

Abstract: We have investigated structural and elastic properties of PtN2 under high pressures using norm-conserving pseudopotentials within the local density approximation (LDA) in the frame of density-functional theory. Calculated results of PtN2 are in agreement with experimental and available theoretical values. The a/a 0, V/V 0, ductility/brittleness, elastic constants Cij , shear modulus C′, bulk modulus B, shear modulus G, Young''s modulus E, Poisson''s ratio σ and anisotropy factor A as a function of applied pressure are presented. Through the quasi-harmonic Debye model, we also study thermodynamic properties of PtN2. The thermal expansion versus temperature and pressure, thermodynamic parameters X (X=Debye temperature or specific heat) with varying pressure P, and heat capacity of PtN2 at various pressures and temperatures are estimated. [Copyright &y& Elsevier]

Herein, we have performed extensive swarm-intelligence structures searching simulations on the KSi x (x = 4, 5, 6, 7, 8) compounds within the pressure range from 0 to 50 GPa. Seven crystal structures of potassium silicides are proposed, and the dynamic stability of these phases were explored in the corresponding pressure range. KSi 4 was predicted to be stable, and it was found to have two phases, named C 2 /m-a and C 2 /m - b , respectively. As for KSi 5 , two thermodynamically stable phases were determined as C 2 /m and Imma , respectively. Above 1.33 GPa, the proposed monoclinic phase with C 2 /m is energetically favorable. With the increasing pressure, our proposed new Imma phase becomes energetically favorable at 16 GPa. For KSi 6 , it was determined that has the following space groups, Cmcm , C 2 /m and P 6/ m. These findings provide insights for helping advance our understanding of the KSi x compounds under high pressure. • This work systematically explored the high-pressure open-framework crystal structures of KSi x (x = 4 ∼ 8) compounds. • Seven crystal structures of potassium silicides were proposed. • The thermal stability, dynamical stability and electronic of K-Si system were performed. [ABSTRACT FROM AUTHOR]