Tricritical phenomena at the gamma-->alpha transition in Ce0.9-xLaxTh0.1 alloys

The gamma-->alpha isostructural transition in the Ce0.9-xLaxTh0.1 system is measured as a function of La alloying using specific heat, magnetic susceptibility, resistivity, thermal expansivity or striction measurements. A line of discontinuous transitions, as indicated by the change in volume, decreases exponentially from 118 K to close to 0 K with increasing La doping, and the transition changes from being first-order to continuous at a critical concentration, x(c) approximately 0.14. At the tricritical point, the coefficient of the linear T term in the specific heat gamma and the magnetic susceptibility increase rapidly near x(c) and approach large values at x=0.35 signifying that a heavy Fermi-liquid state evolves at large doping. The Wilson ratio reaches a value above 2 for a narrow range of concentrations near x(c), where the specific heat and susceptibility vary most rapidly with the doping concentration.     

Pressure-induced phase transition(s) in KMnF3 and the importance of the excess volume for phase transitions in perovskite structures

We report a pressure-dependent investigation of KMnF(3) by x-ray diffraction up to 30 GPa. The results are discussed in the framework of Landau theory and in relation to the isostructural phase transition in SrTiO(3). The phase transition temperature near 186 K in KMnF(3) shifts to room temperature at a critical pressure of P(c) = 3.4 GPa; the pressure dependence of the transition point follows ΔP(c)/ΔT(c) = 0.0315 GPa K(-1). The transition becomes second order under high pressure, close to the tricritical point. The phase transition is determined by the rotation of MnF(6) octahedra with their simultaneous expansion along the rotation axis. The rotation angle was found to increase to 10.5° at 24 GPa. An additional anomaly was observed at higher pressure around 25 GPa, suggesting a further phase transition.     

Effect of nonmesogenic impurities on the order of the nematic to smectic-A phase transition in liquid crystals

By means of adiabatic scanning calorimetry, we have studied the effect of the nonmesogenic solutes cyclohexane (CH) and biphenyl (BP) on the nematic-smectic-A phase transition in the liquid crystal octylcyanobiphenyl (8CB). For all concentrations of BP studied, the transition remains second-order. For 8CB+CH, however, crossover from second-order to first-order is observed at a tricritical point of the mole fraction x of CH around 0.0460. The difference between the two systems and the crossover is explained in terms of a mean-field free energy density expression including coupling terms of x with the nematic and smectic-A order parameters.     

The application of the differential operator method to the Blume-Emery-Griffiths model

Within an effective-field approximation, general expressions for evaluating the second-order phase transition line and the tricritical point of the anisotropic Blume-Emery-Griffiths model are obtained by the use of the differential operator technique. The phase diagrams and the behavior of the tricritical point are investigated numerically for the honeycomb lattice (z = 3) and square lattice (z = 4). We find a new disordered phase which may correspond to the staggered quadrupolar phase predicted in the Monte Carlo simulation, when some conditions are satisfied. The phase diagrams for z = 3 and z = 4 systems exhibit a reentrant behavior for positive values of the uniaxial anisotropy parameter. The change of the tricritical point with the value of the reduced biquadratic parameter is also studied for the system with z = 3.     

On the nature of the γ-α transition in cerium

In 1964 Davis and Adams established that the large increase of the thermal expansion and compressibility in the critical region of the γ-to α-Ce phase transition occurs predominantly in the α phase. This provides strong evidence that a tricritical point is realized in Ce. This also means that the aforementioned transition is not isomorphic and that α-Ce should have a distorted fcc structure. A careful examination of Jayaraman’s data (1965) shows that a second-order transition line continues beyond the tricritical point to the vicinity of a triple point on the melting curve. The phase boundary with the tricritical point and the minimum of the melting curve are reconstructed within the framework of Landau theory. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 2, 111–117 (25 January 1998) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Observation of the color-suppressed decay B( 0)-->D(0)pi(0)

We report the first observation of color-suppressed B( 0)-->D(0)pi(0), D(*0)pi(0), D0eta, and D0omega decays, and evidence for B( 0)-->D(*0)eta and D(*0)omega. The branching fractions are B(B( 0)-->D0pi(0)) = (3.1 +/- 0.4 +/- 0.5)x10(-4), B(B( 0) -->D(*0)pi(0)) = (2.7(+0.8+0.5)(-0.7-0.6))x10(-4), B(B( 0) --> D0eta) = (1.4(+0.5)(-0.4) +/- 0.3)x10(-4), B(B( 0) --> D0omega) = (1.8 +/- 0.5(+0.4)(-0.3))x10(-4), and we set 90% confidence level upper limits of B(B( 0) --> D(*0)eta)<4.6 x 10(-4) and B(B( 0)-->D(*0)omega)<7.9 x 10(-4). The analysis is based on a data sample of 21.3 fb(-1) collected at the Upsilon(4S) resonance by the Belle detector at the KEKB e(+)e(-) collider.     

A systematic approach to the calculation of tricritical temperature with application to the nematic-smecticA phase transition

Using the Landau expansion equations have been found that allow for systematic calculations of tricritical temperature for a given molecular model. The theory has been applied to nematic-smecticA (NA) phase transition of liquid crystals. It has been shown exactly that the NA tricritical temperature depends only on the couplings between the two lowest order translational order parameters and the orientational degrees of freedom. Numerical results of Mayer and Lubensky for the NA tricritical point have been derived exactly. Also a stability condition of the obtained solution has been discussed.Alexander von Humboldt Foundation Fellow, 1985–1986     

Behavior of the antiferromagnetic phase transition near the fermion condensation quantum phase transition in YbRh2Si2

Low-temperature specific-heat measurements on YbRh₂Si₂ at the second order antiferromagnetic (AF) phase transition reveal a sharp peak at TN=72 mK. The corresponding critical exponent α turns out to be α=0.38, which differs significantly from that obtained within the framework of the fluctuation theory of second order phase transitions based on the scale invariance, where α?0.1. We show that under the application of magnetic field the curve of the second order AF phase transitions passes into a curve of the first order ones at the tricritical point leading to a violation of the critical universality of the fluctuation theory. This change of the phase transition is generated by the fermion condensation quantum phase transition. Near the tricritical point the Landau theory of second order phase transitions is applicable and gives α?1/2. We demonstrate that this value of α is in good agreement with the specific-heat measurements.     

Coulomb interaction and first-order superconductor-insulator transition

The superconductor-insulator transition (SIT) in regular arrays of Josephson junctions is studied at low temperatures. We derived an imaginary time Ginzburg-Landau-type action properly describing the Coulomb interaction. The renormalization group analysis at zero temperature T=0 in the space dimensionality d=3 shows that the SIT is always of the first order. At finite T, a tricritical point separates the lines of the first- and second-order phase transitions. The same conclusion holds for d=2 if the mutual capacitance is larger than the distance between junctions.     

The McMillan parameter as an effective field variable for the nematic–smectic tricritical point in NO.M compounds

The dependence of the coupling of orientational and translational order parameters on the nematic-smectic A tricritical point (TCP), observed through birefringence experiments in the pure compounds and in the binary mixtures in the homologous series of N(p-n-alkyloxybenzylidene)p-n-alkylanilines, results in a mean field tricritical exponent. The McMillan parameter, M = T _NA/T _NI (where T _NA and T _NI are the smectic A—nematic (AN) and the nematic—isotropic (NI) phase transition temperatures respectively), is used as an effective and relevant field variable in analysing the TCP. The observed linear dependence of the jump in the orientational order parameter with new density variables in the tricritical region is found to be consistent with the results obtained from both the Landau mean field and the decoupled mean field models. The width of the mean field tricritical region is discussed in terms of the McMillan parameter. Further, the McMillan parameter is observed to be characteristic of chemical composition at the NA tricritical point.     

Correlation between superfluid density and T(C) of underdoped YBa2Cu3O6+x near the superconductor-insulator transition

We report measurements of the ab-plane superfluid density n(s) (magnetic penetration depth lambda) of heavily underdoped films of YBa2Cu3O6+x, with T(C)'s from 6 to 50 K. We find the characteristic length for vortex unbinding transition equal to the film thickness, suggesting strongly coupled CuO2 layers. At the lowest dopings, T(C) is as much as 5 times larger than the upper limit set by the 2D Kosterlitz-Thouless-Berezinskii transition temperature calculated for individual CuO2 bilayers. Our main finding is that T(C) is not proportional to n(s)(0); instead, we find T(C) proportional to ns(1/2.3+/-0.4). This conflicts with a popular point of view that quasi-2D thermal phase fluctuations determine the transition temperature.     

Cu掺杂对高熵合金FeCrVTi相结构的影响

基于CALPHAD方法,建立了高熵合金FeCrVTiCux(x=0-1)的热力学计算模型,对Cu掺杂的高熵合金FeCrVTi相结构和屈服强度进行了讨论分析. 研究发现当x=0时,室温下FeCrVTi的相结构为BCC-A2、LAV-C14和NITI2相. 在815-1380K温度区域,该体系发生有序-无序LAVES->BCC-A2转变. 当x增加到0.2时,室温下FeCrVTiCu0.2的 LAV-C14相消失,出现了LAVES和HCP-A3相. 当x增加到0.4时,室温下FeCrVTiCu0.4出现FCC-A1相. 600K温度时,随Cu含量的不断增加FeCrVTiCux在x=0.0-1.0之间HCP-A3相成分呈线性增长,其相含量最大值位于x=1.0的位置. Cu的掺杂提高了高熵合金FeCrVTi的室温塑性和高温强度,使得FeCrVTi的室温屈服强度高于FeCrVTiCu,在600K温度下,FeCrVTiCu的屈服强度高于FeCrVTi.     

An exactly soluble model with tricritical points for structural-phase transitions

We present and study a lattice-dynamical model whose static and dynamic properties can be described exactly for all dimensionsd≧3 (d an integer) and which, in addition, exhibits tricritical points. For certain model parameters, the tricritical behaviour is found to be identical to that of the spherical model. By changing the model parameters continuously however, the transition suddenly becomes of first order at a tricritical point (TCP). The order parameter and the susceptibility are given explicitly ford≧3. The tricritical exponents are Gaussian. The critical dynamics is also discussed.     

Observation of the hc(1P1) state of charmonium

The h(c)((1)P(1)) state of charmonium has been observed in the reaction psi(2S) --> pi(0)h(c) --> (gammagamma)(gammaeta(c)) using 3.08 x10(6) psi(2S) decays recorded in the CLEO detector. Data have been analyzed both for the inclusive reaction, where the decay products of the eta(c) are not identified, and for exclusive reactions, in which eta(c) decays are reconstructed in seven hadronic decay channels. We find M(h(c)) = 3524.4 +/- 0.6 +/- 0.4 MeV which corresponds to a hyperfine splitting DeltaM(hf)(1P) triple-bond pi(0)h(c)) x B(h(c) --> gammaeta(c)) = (4.0 +/- 0.8 +/- 0.7) x 10(-4).     

Temperature dependence of the field induced magnetization reorientation in dzialoshinsky-moriya type weak ferromagnets

Using a Hamiltonian consisting of isotropic, anisotropic, and antisymmetric exchange terms plus a Zeeman term, the temperature dependence of the weak ferromagnetic-spin-flop phase transition is calculated in the molecuar field approximation. For all temperatures less than the tricritical one, the phase transition is of first order. The stability limits of the two phases and the thermodynamic phase boundary are calculated for this temperature region. For temperatures greater than the tricritical one, the second order transition boundary between the phases is calculated. A set of equations precisely defining the tricritical point in the field-temperature plane is derived and results are presented for the location of this point as a function of the parameters of the system. The methods developed in this work are general and can be applied directly to similar problems.     

Tricritical and Critical Exponents in Microcanonical Ensemble of Systems with Long-Range Interactions

We explore the tricritical points and the critical lines of both Blume-Emery-Griffiths and Ising model within long-range interactions in the microcanonical ensemble. For K=K_MTP, the tricritical exponents take the values β=1/4, 1=γ^-≠γ⁺=1/2 and 0=α^-≠α⁺=-1/2, which disagree with classical (mean field) values. When K>K_MTP, the phase transition becomes second order and the critical exponents have classical values except close to the canonical tricritical parameters (K_CTP), where the values of the critical expoents become β=1/2, 1=γ^-≠γ⁺=2 and 0=α⁺≠α⁺=1.     

Magnetic fluids in an external field

Within mean field approximation we investigate the phase diagrams of magnetic fluids in presence of a magnetic field. In a finite field the magnetic phase transition is absent, but instead a line of first order liquid-liquid transitions ending in a critical point occurs for a magnetic interaction, which is sufficiently strong. Varying the magnetic field these critical points extend from the tricritical point at H=0 to a critical endpoint. For a fluid with Ising spins we calculate the critical lines and several tricritical exponents analytically. For Heisenberg fluids we obtain the phase diagrams from a numerical solution of the mean field equations of state. Received 20 March 1998     

Phase Transition for a Mixed Spin-1/2 and Spin-sB System with a Transverse Crystal Field

The critical behaviors of a mixed spin-1/2 and spin-s_B Ising system with a transverse crystal field are studied by use of the effective-field theory with correlations. The effect of the transverse crystal field on transition temperatures is investigated numerically for the honeycomb (z=3) and square (z=4) lattices. The results show that there is no tricritical point for the system.     

Critical behavior in the vicinity of nematic to smectic A (N–SmA) phase transition of two polar–polar binary liquid crystalline systems

Phase diagram, critical behavior and order of the nematic (N)–smectic A (SmA) phase transition of two polar–polar binary systems (i) 4-n-heptyloxy-4′-cyanobiphenyl (7OCB) and 4-n-octyloxy-4′-cyanobiphenyl (8OCB); (ii) 4-n-octyloxy-4′-cyanobiphenyl (8OCB) and 4-n-nonyloxy-4′-cyanobiphenyl (9OCB) by means of a high-resolution temperature scanning measurement of birefringence have been reported in this work. A simple power law analysis has been adopted to extract the specific heat critical exponent (α′) at N–SmA transition from birefringence data. The α′ for N–SmA transition indicates a uniform crossover behavior and has appeared to be non-universal in nature. With increasing concentration of the higher homologues for both the binary systems, the N–SmA transition reveals a strong tendency to be driven towards the tricritical nature. The 3D-XY limit (i.e. α′ = ?0.007) for N–SmA transition reaches at the concentration x_8OCB = 0.28 corresponding to the McMillan ratio 0.914, whereas the tricritical point has been found to appear near x_9OCB = 1.0 corresponding to McMillan ratio 0.992.     

Direct observation of the strange b baryon Xib-

We report the first direct observation of the strange b baryon Xi(b)- (Xi(b)+). We reconstruct the decay Xi(b)- -->J/psiXi-, with J/psi-->mu+mu-, and Xi--->Lambdapi--->ppi-pi- in pp collisions at square root of s =1.96 TeV. Using 1.3 fb(-1) of data collected by the D0 detector, we observe 15.2 +/- 4.4(stat)(-0.4)(+1.9)(syst) Xi(b)- candidates at a mass of 5.774 +/- 0.011(stat) +/- 0.015(syst) GeV. The significance of the observed signal is 5.5 sigma, equivalent to a probability of 3.3 x 10(-8) of it arising from a background fluctuation. Normalizing to the decay Lambda(b)-->J/psiLambda, we measure the relative rate sigma(Xi(b-) x B(Xi)b})- -->J/psiXi-)/sigma(Lambda(b)) x B(Lambda(b)-->J/psiLambda) = 0.28+/-0.09(stat)(-0.08)(+0.09)(syst).