The quantum compass chain in a transverse magnetic field

We study the magnetic behaviors of a spin-1/2 quantum compass chain (QCC) in a transverse magnetic field, by means of the analytical spinless fermion approach and numerical Lanczos method. In the absence of the magnetic field, the phase diagram is divided into four gapped regions. To determine what happens by applying a transverse magnetic field, using the spinless fermion approach, critical fields are obtained as a function of exchanges. Our analytical results show, the field-induced effects depend on in which one of the four regions the system is. In two regions of the phase diagram, the Ising-type phase transition happens in a finite field. In another region, we have identified two quantum phase transitions (QPT)s in the ground state magnetic phase diagram. These quantum phase transitions belong to the universality class of the commensurate-incommensurate phase transition. We also present a detailed numerical analysis of the low energy spectrum and the ground state magnetic phase diagram. In particular, we show that the intermediate state (h _c1 < h < h _c2) is gapful, describing the spin-flop phase.     

Magnetic field effects on a classical anisotropic spin-glass model

The simple model of a classical spin-glass proposed by van Hemmen [1] is extended to the case of a generalS Ising spin and anisotropy energy. Due to its simplicity, the model is amenable to an exact solution and in the case of spin-1 the phase diagram is similar to that of the Sherrington-Kirkpatrick model. ForS=1 and for certain values of the anisotropy the phase diagram in the (H-T) plane shows continuous and discontinuous transitions, a critical point within the condensed phase, and a field-induced mixed phase in addition to the paramagnetic and spin-glass phases.Work partially supported by CNPq and FINEP (Brazilian Agencies). Part of this work was presented at the International Conference of Magnetism, San Francisco, 1985     

The ferroelectric phase transition in tridymite type BaAL2O4 studied by electron microscopy

The paraelectric-ferroelectric (PE-FE) phase transition in stuffed tridymite BaAl₂O₄ was studied in situ by transmission electron microscopy. Electron diffraction revealed that the PE and FE phases have hexagonal symmetry. The PE-FE phase transition is accompanied by a doubling of the cell dimensions in the a-b plane. The transition is reversible, takes place over a wide temperature range (400–670 K.) and the interfaces related to the transition have a fluctuating character. The crystal structure of the high temperature PE phase was determined by high resolution electron microscopy. The structures of the PE phase (space group P6₃22, a≈ 5.22 Å, c ≈ 8.8 Å) and of the FE phase (space group P6₃, a= 10.4469(1)Å, c = 8.7927(1)Å) differ mainly by the configuration of the Al-O strings oriented along the c-axis. In the PE phase all the strings are equivalent whereas straight and corrugated strings alternate in an ordered manner in the FE phase resulting in doubling of the a and b cell parameters. Translation and orientation domains due to the decrease of the translation and point symmetry were frequently observed.     

Ferroelectric phase transitions of the 0.32PIN-0.345PMN-0.335PT single crystals studied by temperature-dependent Raman spectroscopy,dielectric and ferroelectric performance

The ferroelectric phase transition characteristics of the 0.32Pb(In_1/2Nb_1/2)O₃-0.345Pb(Mg_1/3Nb_2/3)O₃-0.335PbTiO₃ (0.32PIN-0.345PMN-0.335PT) single crystals were studied by the temperature-dependent Raman spectroscopy and some electrical properties. Ferroelectric monoclinic phase was confirmed at room temperature by the numbers of the Raman modes. Successive ferroelectric phase transitions, i.e. ferroelectric monoclinic phase to ferroelectric tetragonal phase transition (FE_M-FE_T) and ferroelectric tetragonal phase to paraelectric cubic phase transition (FE_T-P_C), are evidenced by the anomalies of Raman modes line width, peaks intensity and their ratios around T_M-T and T_C/T_m temperatures. The temperature dependent permittivity derivative ξ = d?/dT not only provides further evidence of the successive ferroelectric phase transitions, but also demonstrates the second-order transition characteristic of the FE_M-FE_T phase transition and the first-order transition feature of the FE_T-P_C phase transition. The FE_M-FE_T phase transition is also confirmed by the abnormal narrowing of the P-E loops, decrease of the P_r and E_c values, and extremums of the pyroelectric performance.     

一维 Frenkel-Kontorova(FK)模型原子链的相变研究

文章基于一维Frenkel-Kontorova模型,研究了边界原子的初始速度对原子链运动状态的影响,数值模拟结果表明:当v1时,原子处于"振荡区",当v₁2时,原子处于"混沌区",当v>v₂时,原子处于 "均匀区". 同时我们发现临界速度v₁和v₂随原子数目和垫底势高度的变化而变化. 关键词： Frenkel-Kontorova(FK)模型 相变     

Uniform uniaxial anisotropic systems with coupled vector order parameters

Phase transitions in magnets, described by two coupled, m-component vector order parameters, having uniform uniaxial anisotropies are studied. Using a phenomenological model, it is shown that when both order parameters are anisotropic, phase transitions are always second order, in either the uniaxial or the (m???1)-isotropic phase. This is contrary to the isotropic case of two coupled order parameters, for which phase transitions are fluctuation-induced first order. The transitions are still continuous into the m-isotropic phase even when the only anisotropic order parameter is the one with the lowest mean-field critical temperature. New discontinuous transitions still occur in either the uniaxial or the (m???1)-isotropic phase, when the only anisotropic order parameter has the highest mean-field critical temperature.     

Electrons with correlated hopping interaction in one dimension

The 1D system of correlated electrons characterized in addition to the usual on-site (U) and nearest-neighbour (V) repulsion by a correlated-hopping term (t*) is considered. The ground state phase diagram is studied within the framework of the weak-coupling continuum-limit approach. At filling v the effective interaction originating from the correlated-hopping term which appears in the continuum-limit theory is given by t* cos (πv). Being repulsive for v < 1/2 and attractive for v > 1/2, this interaction leads to a characteristic band-filling dependence of the phase diagram of the system. For v ≠ 1/2, depending on the relation between the bare values of the coupling constants (U, V, t* cos (π v)) and, hence, on the band-filling, the system shows three different phases in the infrared limit: a Luttinger metal, a nonmagnetic metal and a singlet superconducting phase. For v = 1/2, the correlated-hopping interaction is dynamically trivial, leading only to a renormalization of the oneelectron hopping amplitude and the phase diagram of the model coincides with that of the extended Hubbard model.     

Different structural states and root-mean-square atomic displacements in CuPd alloys

Results of X-ray diffraction studies onCu-39 at.%Pd andCu-36 at.%Pd are discussed. Root-mean-square (rms) atomic displacements in an ordered B2 phase and a disordered A1 phase are determined at various temperatures. The data obtained are indicative of an anomalous state of the crystalline structure of the disordered A1 phase prior to phase transitions both in the high-temperature range of its existence and in the low-temperature metastable range. During the stress-induced phase transformation, the rms atomic displacements are found to be larger in the A1 product phase, containg abundant defects, than in the parent B2 phase. Tomsk State University of Architecture and Building. Siberian Physicotechnical Institute. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 40–46, July 1999.     

Availability of the thermodynamics and weak cosmic censorship conjecture for a charged AdS black hole in the large dimension limit

The thermodynamics and the weak cosmic censorship conjecture (WCCC) in a high dimensional RN ? AdS_d+?1 black hole with energy-momentum relation are investigated by absorbing a charged particle in the phase space. In the RN ? AdS_d+?1 space-time, the cosmological constant Λ is treated as a thermodynamic pressure and its conjugate quantity as a thermodynamic volume. We use the energy-momentum relation of the absorbed particle to discuss the thermodynamics of the RN ? AdS_d+?1 black hole and to prove the WCCC in the phase space. Based on this assumption, we find that the first law and the second law of thermodynamics are satisfied in normal phase space. On the other hand, in the extend phase space, the first law is satisfied and the second law is violated. Then we study the WCCC in the phase space, we find that the WCCC is satisfied for an extreme black and a near-extreme black hole in the normal phase space. In the extend phase space, the WCCC is satisfied for an extreme black hole and unidentified for a near-extreme black hole.

     

Si含量对高锰硅化合物相组成及热电性能的影响研究

采用高频感应熔融、退火结合放电等离子烧结方法制备高锰硅(HMS)化合物MnSi1.70+x(x=0,0.05,0.1,0.15),系统研究了Si含量变化对材料相组成、微结构和热电性能的影响规律.结果表明,当x0.1时,样品由HMS和贫Si的MnSi金属相两相组成,随着Si含量x的增加,MnSi相相对含量减小;当x=0.1时,所得样品为单相HMS化合物;当x0.1时,样品由HMS和过量Si两相组成.随着x的增加,由于样品中高电导的金属相MnSi含量逐渐减少,样品的电导率逐渐下降,而Seebeck系数随之增加.室温下样品载流子浓度和有效质量随x增大逐渐减小,而迁移率逐渐增加.MnSi和Si杂相与HMS相比均为高热导相,因此当x=0.1时,由于样品为单相HMS,从而表现出最低热导率和最高ZT值.MnSi1.80样品在800K时热导率最小值达到2.25W·m-1K-1,并在850K处获得最大ZT值(0.45).     

An order-by-order construction of low-temperature phase diagrams for classical lattice systems

An inductive algorithm is presented for the construction of phase diagrams by means of the low-temperature expansion technique. First the phase diagram is studied in the set of formal series. In each step, properties of this phase diagram are related to extremal elements of some family of convex sets. Approximations of the phase diagram in orderN are obtained by truncating all formal series at theNth term.This paper was presented at the Trebon, Czechoslovakia, Symposium September 1–6, 1986.     

High pressure studies on α-cristobalite form of Al0.5Ga0.5PO4: hydrostatic versus non-hydrostatic conditions

High pressure angle-dispersive X-ray diffraction investigations have been carried out on α-cristobalite form of Al_0.5Ga_0.5PO₄. Our investigations show that the structural stability of this phase under high pressure depends on the nature of pressure conditions in the diamond anvil cell. Under hydrostatic pressure conditions using neon as a pressure transmitting medium, ambient orthorhombic C222₁ phase transforms to orthorhombic Cmcm phase at 4.9?GPa. The high pressure Cmcm phase remains stable up to the highest pressure in the experiment, i.e. 19?GPa. The values of bulk modulus for C222₁ and Cmcm phases are 19(2) and 126(4)?GPa, respectively. In contrast to this, under non-hydrostatic pressure conditions, transformation of ambient C222₁ phase to Cmcm phase has not observed up to 17.4?GPa. Instead, a new monoclinic phase P2₁ is observed which contains layers of six coordinated Al/Ga ions separated by less dense five coordinated Al/Ga ions.     

新型BCN化合物的结构表征和相转变

将石墨和六方氮化硼(h-BN)混合粉球磨120h形成的非晶B-C-N粉在4.5GPa，1600K等温退火45min. XRD，TEM和Raman散射测量结果表明，高压合成的产物由晶格常数为a₁=0.2551nm，c₁=0.6716nm的六方Ⅰ相和a₂=1.2360nm，c₂=0.8570nm的六方Ⅱ相组成，其中六方Ⅱ相为B-C-N 新相. 在室温该新相在1279，1368，1398cm^-1出现三个特征Raman峰. 变温Raman测量结果表明，在测量温度T=93K时，样品中的主要相为六方Ⅰ相，随着温度的升高，六方Ⅰ相逐渐向六方Ⅱ相转变，当T>473K时，六方Ⅰ相完全转变成六方Ⅱ相. 当温度从673K降到93K过程中，样品又从六方Ⅱ相逐渐变回到六方Ⅰ相. 对这一相变的机理进行了讨论. 关键词： B-C-N 机械球磨 高温高压 相转变     

Solidification microstructure of as-cast Mg–Zn–Y alloys

Composites consisting of icosahedral (i) phase and ductile α-Mg can be fabricated by controlling the alloy composition. With increasing Mg content, the primary solidification phase changed from the i phase to the α-Mg phase and single eutectic structure can be obtained at a composition of Mg₇₂Zn_23.5Y_3.5. The i phase showed a variation in structural order from the well-ordered icosahedral phase to the 1/1 rhombohedra1 approximant with lattice constants a=27.2 Å and α = 63.43°. The structural change in the i phase can be explained by microdomain formation due to compositional change during solidification. Annealing treatment improves the structural order of the i phase by homogenization. By controlling the alloy composition, a single eutectic structure consisting of a hard i phase and a ductile α-Mg phase could be obtained.     

Symmetry aspect of the phase transitions in boracites

The phase transitions in boracites are analysed by using the group-theoretical formulation of the Landau theory of phase transitions. It is shown that the orthorhombic, monoclinic and trigonal phase transitions could be induced by the same irreducible representation of the space groupT _d ⁵ with the star determined by the wave vectork=1/2(b ₁+b ₂). The corresponding free energy function is constructed and the symmetry of normal modes is discussed.The authors thank Dr. V. Janovec of the Institute of Physics for valuable remarks to this paper.     

Dynamics of Multiple Scalar Fields Model of Dark Energy with Spatial Curvature

The dynamical system of multiple scalar fields in FRW universe with different spatial curvature have been analyzed in this paper. In the radiation-dominated phase, the constant curvature factor k does not work on the cosmic dynamical behaviors, including the scaling solution, energy density parameter and equation-of-state parameter. These aspects are affected by curvature factor k in the matter-dominated phase. In the special scalar field-dominated phase, the energy density parameter normalization restricts the Universe is spatial flat and the curvature factor k is not present in the dynamics. In this paper, the Universe is closed in the matter-dominated phase, and flat in the scalar field-dominated phase. The spatial flatness and the w _ϕ =−1 in the third phase are coincide with the current observations.     

Phase transitions in RbxK1−x LiSO4 mixed crystals

Structural phase transitions in Rb_xK_1−x LiSO₄ mixed crystals (with x varying from 0 to 1) have been studied from the melting point to liquid-nitrogen temperature. Calorimetric (DTA and DSM) data, birefringence and optical polarization measurements were used to construct the full phase (T-x) diagram. It has been established that crystals of most compositions (x⩽0.95) grow in the hexagonal-trigonal KLiSO₄ structure. Replacement of K by the larger Rb atom results in a considerable increase of the region of existence of the P31c phase and expulsion of the high-temperature hexagonal phase. Fiz. Tverd. Tela (St. Petersburg) 40, 1341–1344 (July 1998)     

Distortion of alpha-uranium structure in praseodymium metal to 311 GPA

It is generally observed that the rare earth metals adapt an orthorhombic alpha-uranium (α-U) structure at high pressures following the delocalization of 4f shell under compression. We examine the stability of the α-U structure in praseodymium metal at ultrahigh pressures of 313 GPa (volume compression V/V ₀?=?0.343) in a diamond anvil cell at room temperature. X-ray diffraction data show a transformation from the α-U structure to a primitive orthorhombic P2₁2₁2₁ phase at 147±5 GPa, which is characterized by the anisotropic compressibility of various crystallographic axes. This anisotropic compressibility leads to an interesting situation when the b-axis and the c-axis of the orthorhombic phase become nearly equal above 260 GPa and the structure can be regarded as a pseudo-tetragonal phase. Our present study shows that the 4f band metal Pr does not adapt a body centred tetragonal phase as predicted by theory, but instead novel crystallographic phases are observed at extreme compressions. The present results have a broader impact on the stability of the α-U phase in a variety of f-band systems at high pressures.     

Interrelationship between superprotonic conductivity and strain in CsHXO4

In order to clarify the mechanism of superprotonic conductivity (SPC) for the hydrogen-bonded compound CsHXO₄ (X=S, Se), we have investigated the elastic properties of CsHXO₄ with various transition temperatures (T_c), at which CsHXO₄ exhibits a structural phase transition from a low-temperature monoclinic phase with low protonic conductivity (phase II) to a high-temperature SPC phase (phase I). It was found that, in CsHS_xSe_1 − xO₄ (x=0–1), the transition temperature T_c decreases in proportion to the square of spontaneous strain b in the a_II–b_II plane of phase II. This result indicates that the spontaneous strain in the a_II–b_II plane plays an important role in the appearance of SPC in CsHXO₄. Moreover, thermodynamic analyses suggest that proton migration in phase I is induced by the release of the strain energy. From these results, it is deduced that the phase transition temperature in CsHXO₄ is determined by the competition between the kinetic energy of proton and the released-strain energy.     

Electronic,mechanical, phase transition,and thermo-physical properties of TMC (TM = V,Nb, and Ta): high pressure ab initio study

The structural, electronic, mechanical, phase transition, and thermo-physical properties of refractory carbides, viz. VC, NbC, and TaC have been computed in stable B1 and high pressure B2 phases by means of two different ab initio calculations using pseudo- and full-potential schemes. These materials have mixed covalent-, metallic-, and ionic-type bonding. The calculations of elastic constants show the mechanical stability of these materials in B1 phase only. The brittle nature and anisotropy is observed in these materials in B1 phase. Non-central forces are present in both the phases. Elastic wave velocities and Debye temperature have also been calculated. The present results on structural, phase transition, elastic, and other properties are in reasonably good agreement with the available experimental and theoretical data. The calculations in high pressure phase need experimental verification.