Deflected light structure in NaNH4SeO4·2H2O crystals

Deflection of light studies in function of temperature in NaNH₄SeO₄·H₂O crystal is presented. At 180?K, this compound undergoes a para-ferroelectric/ferroelastic phase transition of the second order. It changes the symmetry from the orthorhombic symmetry class 222 (space group P2₁2₁2₁) to the monoclinic symmetry class 2 (space group P2₁). A distinct deflection pattern, resulting from the ferroelastic domain structure, occurs in the low temperature phase. The intensity of deflected beams varies considerably with temperature. Detailed studies revealed the structure of deflected spots during cooling–heating cycle. These spots change intensity, become more dispersed and finally split with decreasing temperature. Moreover, a variation of deflection angles was observed. A possible explanation of these phenomena is given in a framework of crystals optics.     

Magnetic transitions in K2MnCl4·2H2O

Results of (dM/dH) measurements on tetrahedral K₂MnCl₄·2H₂O as a function of temperature and magnetic field, are presented. An antiferromagnetic transition along the tetragonal axis is observed at T_N = (3.05±0.05) K. The H-T magnetic phase diagram was completely determined, and shows the usual characteristics of that of a low anisotropy antiferromagnet. The T = 0 critical fields are compatible with the values H_E = (29.2±0.3) kOe and H_A = (5.9±0.6) kOe for the exchange and anisotropy fields.     

Valence fluctuations in La2−xSrxCuO4

Ab-initio electronic band structure calculations are presented for the perovskite La₂CuO₄ and for this material doped with Sr for a supercell of composition La₃SrCu₂O₈. This material is close to the high Tc superconductor La_2−xSr_xCuO₄ discovered recently. The Sr doping gives rise to strong valence fluctuations. We discuss the effect of the valence fluctuations on the stability of the lattice, indicating a small value of U and enhancing the electron-phonon coupling λ, mainly by a mechanism of incipient peroxide formation.     

Zero-field NMRON in54Mn-MnCl2·4H2O

We have observed NMRON lines for⁵⁴Mn doped into MnCl₂.4H₂O corresponding to ordering by a strong hyperfine field B_N=60.95 (6) T modified by a weak quadrupole interaction of strength P=+0.52 (7) MHz. The reduction of S_Z for the manganese ion is larger than expected. Point-charge estimates of P yield values which are positive, but smaller than the experimental value.     

Defect structure of Cu2+ center in K2SO4‒Na2SO4‒ZnSO4 glasses

The spin Hamiltonian parameters (g factors g _//, g _⊥ and hyperfine structure constants A _//, A _⊥) of Cu²⁺ in K₂SO₄?Na₂SO₄?ZnSO₄ glasses are calculated from the high-order perturbation formulas of 3d⁹ ion in tetragonal octahedral sites. The calculated results are in agreement with the observed values. Since the EPR parameters are sensitive to the local structure of a paramagnetic impurity center, the defect structure of Cu²⁺ center in K₂SO₄?Na₂SO₄?ZnSO₄ glasses is estimated. The validity of results is discussed.     

4 T 2→4 A 2 transition in the luminescence spectra of Mn4+ ion in gadolinium gallium garnet at intense laser pumping

The Mn^{4+ 4} T ₂→⁴ A ₂ transition was observed in the luminescence spectrum of Gd₃Ga₅O₁₂:Mn⁴⁺ at intense laser pumping. It is shown that the ⁴ T ₂ → ⁴ A 2 transition becomes more intense than ² E→⁴ A ₂ because of an increase in the role of induced transitions with increasing pump power. This process is most efficient in the region of strongest overlap between the ² E→⁴ A ₂ and ⁴ T ₂→⁴ A ₂ bands, where it leads to strengthening of the zero-phonon line of the latter at 694 nm. It is assumed that GGG:Mn⁴⁺ can be used as an active material in tunable lasers.     

Superionic phase transition in (NH4)2SeO4·2NH4HSeO4 single crystal at 378 K

Abstract

DTA, structural and electric conductivity investigations were made for (NH₄)₄H₂(SeO₄)₃ single crystals. A high-temperature phase transition at 378 K to a superionic phase was found. The phase is characterized by a high electrical conductivity (～4.10^?3 Ω^?1 cm^?1) and a low activation energy (0.11 eV).     

Phase transition in tungsten–bronze Li2Pb2Nd2W2Ti4Nb4O30 ferroelectric

The polycrystalline sample of Li₂Pb₂Nd₂W₂Ti₄Nb₄O₃₀ was prepared by a solid-state reaction technique. Room temperature X-ray structural analysis confirms the formation of a single-phase compound. The morphology of the sintered sample recorded by scanning electron microscope exhibits a uniform grain distribution. Detailed studies of the nature of variation of dielectric constant, tangent loss, and polarization with temperature and frequency confirmed the existence of ferroelectricity in the material at room temperature. The temperature and frequency dependence of impedance parameters (impedance, modulus, etc.) of the material exhibits a strong correlation of its microstructure (i.e., bulk, grain boundary, etc.). Furthermore, the temperature dependence of DC conductivity shows a typical Arrhenius behavior of the material. The nature of variation of pyroelectric coefficient and current with temperature suggests that material has good pyroelectric properties useful for pyroelectric detector.     

4 Ag ↔2 Eg Transition of Mn4+ in Cs2TiF6: MnF6 2-

We report and interpret the ⁴ A_2g ?² E_g absorption, emission, linear dichroism, MCD and MCE of the MnF₆ ^2- octahedra at sites of D_3d symmetry in single crystals of Cs₂TiF₆. The spectra show many sharp features but a simplified calculation using cubic basis functions successfully accounts for all of the major features and much of the detailed structure. The electronic levels are not split by the trigonal field in first order but trigonal distortion splits the three-fold degenerate v ₆(t_2u ) vibration of the octahedron into a two-fold degenerate v ₆(e_u ) and a non-degenerate v ₆(a_1u ) vibration and these two vibrational modes give rise to the features 213 cm^-1 and 246 cm^-1 from the zero-phonon line respectively. Similarly the v ₄(t_1u ) vibration is split into a v ₄(a_2u ) and v ₄(e_u ) vibration and give the features at 313 cm^-1 and 340 cm^-1 respectively.

Several transitions involving two or more quanta of vibration are clearly seen in the emission and MCE spectrum. At an energy shift of greater than 700 cm^-1 from the zero-phonon transition these features are found to consist of progressions in the v ₁(a_1g ) breathing mode and the v ₂(e_g ) two-fold degenerate Jahn-Teller active mode. This structure can again be explained, with one exception, in terms of a calculation which assumes cubic basis functions. The exception is the reversal in sign of the MCE at a frequency v ₆(e_u ) + v ₂(e_g ) compared with that at v ₆(e_u ) which is not fully understood.     

Entanglement in the supermolecular dimer [Mn4]2

This paper investigates the entanglement in the supermolecular dimer [Mn4]2 consisting of a pair of single molecular magnets with antiferromagnetic exchange-coupllng J. The conventional yon Neumann entropy as a function of the exchange-coupling is calculated explicitly for all eigenstates with the quantum number range from M = M1 ＋ M2 = -9 to 0. It is shown that the yon Neumann entropy is not a monotonic function of the coupling strength. However, it is significant that the entropy of entanglement has the maximum values and the minimum values for most eigenstates, which is extremely useful in the quantum computing. It also presents the time-evolution of entanglement from various initial states. The results are useful in the design of devices based on the entanglement of two molecular magnets.     

Thermalcapillary Convection in NaBi（WO4）2 Melt

Surface-tension drive flow in NaBi(WO4)2 melt is visualized by means of the already proved method of high temperature in situ optical observation.The transition phenomena of the melt from steady spatial to oscillatory behaviour in melt are observed.The oscillatory thermocapillary convection is accompanied by synchronous temperature oscillations with the frequency 10Hz.The onset temperature of thermocapillary convection varies with the change of meniscus curvature of the melt free surface,which probably due to the liquid pressure at the surface associated with the curvature of meniscus.The oscillatory frequency is proportional to the temperature.Finally another kind of oscillatory thermocapillary convection with the frequency about 5Hz dominating in the undercooled melt is also observed.     

Picosecond third-harmonic light generation inβ-BaB2O4

The type-II phase-matched third-harmonic light generation in a -BaB₂O₄ crystal is studied experimentally. A passively mode-locked Nd: phosphate glass laser is used as a pump source. At a pump pulse peak intensity ofI ₁₀=5×10¹⁰W/cm² a third-harmonic conversion efficiency of a percent is obtained. A theoretical discussion of phase-matched third-harmonic generation in crystals of the symmetry group of -BaB₂O₄ (trigonal class 3) is given. The effective nonlinear susceptibility _eff for type-II phase-matching is determined.On leave from the Shanghai Institute of Optics and Fine Mechanics, Academia Sinica, Shanghai, P.R. China     

The modification effect in magnetization behaviors for CoFe2O4–p-NiFe2O4 binary ferrofluids

The magnetization curves of CoFe₂O₄ ferrofluids, p-NiFe₂O₄ paramagnetic fluids and CoFe₂O₄–p-NiFe₂O₄ binary ferrofluids, in which the volume fraction of CoFe₂O₄ particles φ _Co is 0.6% and one of p-NiFe₂O₄ particles φ _Ni is 0.2%, 0.4%, 0.6% and 0.8% respectively, prepared by the Massart method, have been measured at room temperature. Comparison of the experimental data from the CoFe₂O₄ ferrofluids with the Langevin theory curves demonstrates a considerable difference between them, but a curve fitted using a model of a gas-like compression (MGC) agrees with the experimental data very well. The experimental results show that the magnetization of the CoFe₂O₄–p-NiFe₂O₄ binary ferrofluid is not a simple summation of the ferrimagnetic CoFe₂O₄ part and the paramagnetic p-NiFe₂O₄ part. From the fitted results, it was found that the saturation magnetization of the CoFe₂O₄ part of the binary ferrofluid depends non-monotonically on the p-NiFe₂O₄ particle volume fraction, and the CoFe₂O₄ part is a stronger “hard” magnet than CoFe₂O₄ in simple ferrofluids. The magnetization behavior of the binary ferrofluids is explained by the modification of the microstructure of CoFe₂O₄ nanoparticle system by the p-NiFe₂O₄ nanoparticle system.     

Sum-frequency generation in a cooledβ-BaB2O4 crystal

Sum-frequency generation in -BaB₂O₄ has been studied by mixing the unpolarized output of an excimer laser pumped dye laser with its second harmonic. The shortest wavelength obtained at 95 K is 195.3 nm. A lower limit of 194.4±0.2 nm at the temperature of liquid helium can be expected.     

Superconductivity in LiTi2O4 Prepared by Hybrid Microwave Method

The well-known superconducting oxide LiTi2 04 has a structural phase transition from spinel to ramsdellite around 900℃. We have successfully obtained the superconducting spinel phase and the non-superconducting ramsdellitephase of LiTi2O4 using a hybrid microwave method. The samples are characterized by x-ray powder diffraction, scanning electron microscopy, and measurements of resistivity and magnetic susceptibility. The results show thatthe low-temperature spinel phase is a superconductor with Tc=13K, while the high-temperature ramsdellite phase is a semiconductor. By comparison between the crystal structures of the spinel and the ramsdellite phases, it is suggested that the geometrical frustration plays an important role in the superconductivity of the spinel LiTi2O4.     

Metal-Insulator Transition in CuIr2（S1-xTex）4

The resistivity and magnetization for the CuIr2（S1-xTex）4 （0≤x≤ 0.10） system are investigated. Compared with the Se doping, the substitution of Te for S leads to stronger suppression on the metal-insulator transition. Vacancies and imperfections in the sample lattice are found to modify the magnetization, which can be qualitatively understood to consist of the Pauli paramagnetism, Landau diamagnetism, Larmor diamagnetism and Curie magnetism contributions.     

Hole superconductivity in Nd2 − zCezCuO4

The reclassification of Nd_2 − zCe_zCuO₄from an electron superconductor to a hole superconductor is suggested by simple valence and electrostatics arguments: In Nd_2 − zCe_zCuO₄, isolated Ce substituting for Nd is not an electron donor; the proposed actual dopant is a defect-complex of Ce and interstitial oxygen which provides holes, making Nd_2 − zCe_zCuO₄a hole superconductor.     

Large deviation estimates in the stochastic quantization ofϕ 2 4

We study in the small noise limit the behaviour of field trajectories for the process constructed by the authors in connection with the stochastic quantization of ₂ ⁴ . Due to the presence of infinite renormalization the usual large deviation techniques do not apply immediately and a new strategy has to be developed. We prove some estimates analogous to the Freidlin-Ventzel inequalities. From these it follows that the field trajectories suitably smeared in space over a scaler ₀ behave, when the noise is small, as the projection on the same scale of a field obeying a regularized stochastic equation with a large cut-off. However the estimates are not uniform in the cut-off and an interesting feature of the problem is that the scale over which the field is smeared determines whether the noise is sufficiently small for the estimates to apply.Laboratoire associé au CNRS UA 280