[en] The effects of magnetic correlation on the electric properties in the multiferroic materials are studied, where the phase transition temperature of the magnetic subsystem T_m is lower than that of the electric subsystem T_e. A Heisenberg-type Hamiltonian and a transverse Ising model are employed to describe the ferromagnetic and ferroelectric subsystems, respectively. We find that the magnetic correlation can influence the electric properties above the T_m, and magnetic transverse and longitudinal correlations have opposite functions. In the curves of temperature dependence of polarization, kinks appear at T_m which is dominated by the sharp change of decreasing rate of the magnetic correlation. The kinks can be eliminated by an external magnetic field. The magnetic transverse and longitudinal correlations play contrary roles on the manipulation of polarization by the external magnetic field. - Highlights: • Both magnetic longitudinal and transverse correlations can influence the electric subsystem through magnetoelectric (ME) coupling at any temperature. • The magnetic longitudinal and transverse correlations have contrary effects in influencing the phase transition temperature of electric subsystem. • The electric phase transition temperature decrease with the ME coupling strength, while it was not so by mean-field theory. • An external field can make the influence smoother around the transition point, and can enhance the electric polarization. • Magnetic longitudinal and transverse correlations have contrary effects on the manipulation of polarization by magnetic field at temperature above the magnetic phase transition point

[en] In this paper, the dynamics of chaos and the entanglement in triatomic molecular vibrations are investigated. On the classical aspect, we study the chaotic trajectories in the phase space. We employ the linear entropy to examine the dynamical entanglement of the two bonds on the quantum aspect. The correspondence between the classical chaos and the quantum dynamical entanglement is also investigated. As an example, we apply our algebraic model to molecule H₂O. (general)

[en] In this paper, the magnetic and multiferroic properties in the multiferroic material BiMnO_3 are studied. A Heisenberg type Hamiltonian for BiMnO_3 is proposed, in which the nearest and farther neighbors are considered. Thermodynamic quantities such as magnetization and magnetic susceptibility for different magnetic orderings under high pressure or magnetic field are calculated, and the simulation results fit the experimental results. Farther neighboring exchanges can result in the coexistence of the ferromagnetic ordering and certain antiferromagnetic ordering with no centrosymmetry. Our study demonstrates that the BiMnO_3 should be the type-II multiferroic, and the ferromagnetic and ferroelectric orderings could coexist. The magnetic field control of ferroelectric polarization is also studied. The ferroelectric polarization is always suppressed by the external magnetic field. - Highlights: • A Hamiltonian including the nearest and farther neighbors of BiMnO_3 is proposed. • Thermodynamic quantities for different magnetic orderings are calculated. • It is shown that BiMnO_3 should be the type-II multiferroic. • The obtained results fit the experimental results quite well. • The mechanism of magnetic control of polarization is also studied.

[en] The internal energies, including transverse and longitudinal parts, of quantum Heisenberg systems for arbitrary spin S are investigated by the double-time Green's function method. The expressions for ferromagnetic (FM) and antiferromagnetic (AFM) systems are derived when one-component of magnetization is considered with the higher order longitudinal correlation functions being carefully treated. An unexpected result is that around the order–disorder transition points the neighboring spins in a FM (AFM) system are more likely longitudinally antiparallel (parallel) than parallel (antiparallel) to each other for S≤3/2 in spite of the FM (AFM) exchange between the spins. This is attributed to the strong quantum fluctuation of the systems with small S values. We also present the expressions of the internal energies of FM systems when the three-component of magnetizations are considered. - Highlights: • We give the best expressions of the internal energies for Heisenberg magnetic systems. • Around transition temperature, the longitudinal correlation energies for magnetic systems are positive due to strong quantum fluctuation. • A system with smaller spin quantum number has a stronger fluctuation even if there is spontaneous magnetization. • The strong quantum fluctuation cannot be totally suppressed by an external magnetic field. • The expressions of the internal energies when the magnetization has three components are given

[en] The effect of magnetic spin correlation on the thermodynamic properties of Heisenberg ferromagnetic single-walled nanotubes are comprehensively investigated by use of the double-time Green's function method. The influence of temperature, spin quantum number, diameter of the tube, anisotropy strength and external magnetic field to internal energy, free energy, and magnon specific heat are carefully calculated. Compared to the mean field approximation, the consideration of the magnetic correlation effect significantly improves the internal energy values at finite temperature, while it does not so near zero temperature, and this effect is related to the diameter of the tube, anisotropy strength, and spin quantum number. The magnetic correlation effect lowers the internal energy at finite temperature. As a natural consequence of the reduction of the internal energy, the specific heat is reduced, and the free energy is elevated. - Highlights: • Magnon specific heat and free energy of Heisenberg ferromagnetic single-walled nanotubes (HFM-SWNTs) are investigated. • The magnetic correlations effect has a considerable contribution to the thermodynamics properties of HFM-SWNTs. • Magnetic correlation effects are always to lower the internal energy at finite temperature. • At Curie point, magnetic correlation energy is much less than zero. • The peak values of magnon specific heat curves rise and shift right towards higher temperatures with the diameter of tubes, the anisotropy strength, and the spin quantum number rising.

[en] We elucidate the importance of a capping layer on the structural evolution and phase change properties of carbon-doped Ge ₂ Sb ₂ Te ₅ (C-GST) films during heating in air. Both the C-GST films without and with a thin SiO ₂ capping layer (C-GST and C-GST/SiO ₂ ) are deposited for comparison. Large differences are observed between C-GST and C-GST/SiO ₂ films in resistance-temperature, x-ray diffraction, x-ray photoelectron spectroscopy, Raman spectra, data retention capability and optical band gap measurements. In the C-GST film, resistance-temperature measurement reveals an unusual smooth decrease in resistance above 110 °C during heating. X-ray diffraction result has excluded the possibility of phase change in the C-GST film below 170 °C. The x-ray photoelectron spectroscopy experimental result reveals the evolution of Te chemical valence because of the carbon oxidation during heating. Raman spectra further demonstrate that phase changes from an amorphous state to the hexagonal state occur directly during heating in the C-GST film. The quite smooth decrease in resistance is believed to be related with the formation of Te-rich GeTe _4−n Ge_n (n = 0, 1) units above 110 °C in the C-GST film. The oxidation of carbon is harmful to the C-GST phase change properties. (paper)