Resistance maximum across the spin glass to Kondo transition

In a recent theory of the noise model of alloys like AuFe a singular point at zero temperature was found to separate a spin glass phase at high concentrations and a Kondo phase at low concentrations. Despite this there is a resistance maximum in both “phases”, although of different characters. In the present letter a relation is given between the temperature of the maximum, T_m, the noise temperature, Δ_c, and the Kondo temperature, T_K. This extends a previously given expression, that is only valid in the spin glass limit Δ_c >> T_K, across the transition at Δ_c = T_K into the Kondo phase and values of Δ_c less than T_K.     

The entropy change on melting of simple substances

It is shown that the volume independent component, R 1n 2, of the entropy of melting, ΔS_m, for argon persists, unchanged, some distance into the fluid phase as a configurational entropy. For argon, sodium, and caesium in its normal melting regime ΔS_m = Rln 2 + αB_TδV_m, where ΔV_m is the volume change on melting and αB_T is the temperature independent product of the thermal expansion coefficient and the isothermal bulk modulus.     

Mathematical Characterization of Thermo-reversible Phase Transitions of Agarose Gels

The thermal phase transition temperatures of high (HMP) and low melting point (LMP) agarose gels were investigated by using UV–vis spectroscopy techniques. Transmitted light intensities from the gel samples with different agarose concentrations were monitored during the heating (gel-sol) and cooling (sol–gel) processes. It was observed that the transition temperatures, T_m, defined as the location of the maximum of the first derivative of the sigmoidal transition paths obtained from the UV–vis technique, slightly increased by increasing the agarose concentration in both the HMP and LMP samples. Here, we express the phase transitions of the agar-water system, as a representative of reversible physical gels, in terms of a modified Susceptible-Infected-Susceptible epidemic model whose solutions are the well-known 5-point sigmoidal curves. The gel point is hard to determine experimentally and various computational techniques are used for its characterization. Based on previous work, we locate the gel point, T₀, of sol-gel and gel-sol transitions in terms of the horizontal shift in the sigmoidal transition curve. For the gel-sol transition (heating), T₀ is greater than T_m, i.e. later in time, and the difference between T₀ and T_m is reduced as the agarose content increases. For the sol-gel transition (cooling), T₀ is again greater than T_m, but it is earlier in time for all agarose contents and moves forward in time and gets closer to T_m as the agarose content increases.     

Raman analyses of co‐phasing and hysteresis behaviors in V2O3 thin film

We present the studies of the phase transition behaviors of V₂O₃ thin film using temperature‐dependent Raman scattering spectroscopy. Our results show that in both the cooling and heating processes of V₂O₃ thin film, the phase transition occurs gradually but not suddenly, contrary to that in single crystal. The coexistence of both the metal and insulator phases with co‐phasing ΔT_c larger than 30 K is observed in both the cooling and heating processes. We discuss that this large co‐phasing ΔT_c should be distinguished with the large hysteresis ΔT_h reported in nanostructures. In addition, our discussions indicate that co‐phasing ΔT_c and hysteresis ΔT_h would be mainly correlated with stress and defect states in sample, respectively. Furthermore, our Raman analyses suggest that stress would also induce phase transitions in V₂O₃, and the stress (pressure)‐induced phase transitions would behave differently comparing with the temperature‐induced transitions under normal pressure. Copyright © 2012 John Wiley & Sons, Ltd.     

Microstructure evolution of undercooled Fe-Co-Cu alloys

Rapid solidification of undercooled Fe-Co-Cu alloys was investigated by means of fluxing purification and cyclic superheating technique. A transition in microstructure from dendrites to phase-separation occurred above a phase-separation undercooling ΔT_sep. When ΔT<ΔT_sep, dendrite was observed, the trunks were rich in Fe and Co, while Cu was rich at inter-dendrites. However, the phase-separated microstructure was obtained once ΔT>ΔT_sep, with a large sphere of L1 phase located almost at the center of the sample and enwrapped by L2 phase. ΔT_sep was 222, 88 and 45 K for Fe₅₀Co₃₀Cu₂₀, Fe₂₅Co₂₅Cu₅₀ and Fe₁₅Co₁₅Cu₇₀ alloys in this work, respectively. It was investigated that L1 phase solidified before L2 phase after liquid separation and followed different ways.     

Specific Features of the Domain Structure of BaTiO<Subscript>3</Subscript> Crystals during Thermal Heating and Cooling

This paper presents the results of the study of the domain structure of barium titanate crystals in a wide temperature range including the Curie point (T_C) using the polarization-optical method in the reflected light and the force microscopy of the piezoelectric response. It is shown that a new a–c domain structure forms during cyclic heating of the crystal above T_C and subsequent cooling to the ferroelectric phase. The role of uncompensated charges appeared on the crystal surface during the phase transition and their influence on the formation of the domain structure during cooling are discussed.     

On the effect of heating and cooling rates on the melting and crystallization of metal nanoclusters

The effect of heating and cooling rates on melting (T_m) and crystallization (T_c) temperatures of metal nanoclusters is investigated in terms of the isothermal molecular dynamics. We report on the results obtained for nickel nanoclusters, although analogous results were also obtained for gold and aluminum nanoclusters. It is found that T_m increases, while T_c decreases with increasing heating and cooling rates, both T_m and T_c tending to the same value for heating and cooling rates tending to zero. The results indicate that the hysteresis of melting and crystallization of nanoparticles must be completely due to nonequilibrium conditions of heating and cooling. The transition of Ni nanoclusters to the amorphous state begins at very high cooling rates exceeding 10 TK/s.     

High pressure study of BaPb0.7Bi0.3O3 films

The resistance R, the superconducting transition temperature T_c and the energy gap Δ(T) have been measured on the BaPb_0.7Bi_0.3O₃ films up to 14 kbar. We have found that up to 14 kbar: (1) pressure suppresses T_c and Δ(T) while enhances R, (2) the value of 2Δ(0)/kT_c is 3.8±0.1, independent of pressure, and (3) the Δ(T)/Δ(0) varies with T/T_c in a BCS fashion but only for T/T_c<0.75 and independent of pressure. The results show that BaPb_1?xBi_xO₃ is a weak-coupling superconductor, but fail to provide information about the cause for the high T_c of the compound.     

Theory of high-T c superconductivity based on the fermion-condensation quantum phase transition

A theory of high-temperature superconductivity based on the combination of the fermion-condensation quantum phase transition and the conventional theory of superconductivity is presented. This theory describes maximum values of the superconducting gap, which can be as big as Δ₁～0.1ε _F , with ε _F being the Fermi level. We show that the critical temperature 2T _c ?Δ₁. If the pseudogap exists above T _c , then 2T*?Δ₁ and T* is the temperature at which the pseudogap vanishes. A discontinuity in the specific heat at T _c is calculated. The transition from conventional superconductors to high-T _c ones as a function of the doping level is investigated. The single-particle excitations and their lineshape are also considered     

On the tricritical point in MnSi under high pressures

The magnetic susceptibility of a MnSi single crystal is measured in the region of the ferromagnetic phase transition under pressures up to 0.8 GPa in compressed helium. It is found that the tricritical point on the phase-transition curve corresponds to a much lower pressure and a considerably higher temperature (P _tr ≈ 0.355 GPa and T _tr ≈ 25.2 K) than was reported earlier (P _tr ≈ 1.2 GPa and T _tr ≈ 12 K). New results impose certain limitations on theoretical analysis of tricritical phenomena in MnSi.     

DSC study of superionic phase transition in (NH4)4H2(SeO4)3 single crystals

DSC and complex impedance studies of the protonic conductor (NH₄)₄H₂(SeO₄)₃, which undergoes a superionic phase transition of first order at T_s = 378 K show that the activation energy of ionic conductivity d(lg σ)/dt and the ordering enthalpy ΔC_p of the crystal are proportional: d(lg σ)/dT = XΔC_p/RT_s + const, as found for MAg₄I₅ crystals undergoing a second-order superionic phase transition. Thus the short-range order environment of the species involved in fast-ion transport plays the main role in the superionic phase transition. This is also supported by the value of the entropy change at T_s, ΔS = 43 J/mole·K. A new metastable phase was found to be induced on heating the (NH₄)₄H₂(SeO₄)₃ crystal above T_s.     

Infrared spectroscopy of solids

For materials like small gap semiconductors or intercalated layered compounds the general form of the complex dielectric function is: ϵ(ω) = ϵ_x + Δϵ_inter + Δϵ_intra + Δϵ_ph, where ϵ_∞is the high frequency dielectric constant due to all interband transitions except the uppermost valence band and the lowest conduction band, Δϵ_inter is the contribution to the dielectric constant due to this two bands in particular, Δϵ_intra is the contribution due to intraband free carrier transitions and Δϵ_ph is the contribution due to lattice vibrations. The contribution from transitions between valence and conduction bands to the imaginary part of the dielectric function Δϵ_inter(ω,T) for a narrow gap material can be readily calculated in the random phase approximation (RPA) formalism. The real part Δϵ_inter is obtained by performing the Kramers-Kronig inversion on the expression Δϵ_inter. Dielectric function of HgTe between 8 and 300 K is discussed. The interband contribution to the complex dielectric function in a layered intercalation compound is also examined. Pure graphite, first and second stage compounds are treated as an example. Reflectivity and magnetoreflectivity spectra simultaneously determining the plasma and the cyclotron frequencies, allow one to measure the free carrier density, hence the Fermi level, and the effective mass of the carriers. The variation of the effective mass as a function of the position of the Fermi level traces the energy bands dispersion relation. An example of such investigations is given for PbSe layered materials like Bi₂Se₃ are also studied by infrared reflectivity spectroscopy. Intercalation of such materials increases the free carrier population which consequently moves the Fermi level up in the conduction band. Analysis of reflectivity spectra allows an accurate determination of the free carrier concentration and gives a useful tool for the investigation of atom insertion in layered materials. Recent experiments on the intercalation of Li in a certain number of layered materials will be presented. In the frame of the classical theory of independent harmonic oscillators, the phonon contribution to the dielectric function is given by the sum of transverse modes for each oscillator with the corresponding damping parameters and oscillator strength. The complex dielectric function can then be written as a set of separate equations for the real and imaginary parts of the wave-number-dependent dielectric function. In the spectral region when phonon and plasmon frequencies may coincide a strong plasmon-phonon coupling will be experienced. In a simple model with one LO and one TO frequency, one expects two singularities at the two maxima of the function 1m −ϵ⁻¹; representing longitudinal modes. The frequencies generally labeled ω₊ and ω_- correspond to longitudinal oscillations with the lattice and electron plasma vibrating, respectively, in phase and 180° out of phase. In small gap materials the situation is more complex. Because of the particular band structure, the contribution Δϵ_inter(ω) must be included. In some cases it also becomes necessary to include additional oscillators with strong polar character corresponding to a particular defect or to additional vibrations. The implications of all these fundamental concepts in the investigation of high Tc materials is discussed and examples given.     

On the nature of a new phase transition in α-MnS

Magnetic-susceptibility and X-ray-diffraction data of polycrystalline and oriented single crystal α-MnS reveal a new phase transition at T_tr = 131 K below the Néel temperature T_N = 148 K. The phase transition is characterized by an abrupt inversion of the rhombohedral distortion of the f.c.c. lattice along [111]. At T_tr there is a discontinuous change in the susceptibility of single crystals.     

A new, lead free, family of perovskites with a diffuse phase transition: NaNbO3-based solid solutions

(1−x)NaNbO₃-(x)ABO₃ perovskite solid solutions belonging to group II according to the Krainik classification [Izv. Akad. Nauk SSSR, Ser. Phys. 28 (1964) 643] exhibit a dramatic diffusion of the dielectric permittivity ε′ maximum and relaxor-type behavior when the second component concentration exceeds a threshold value x₀. The concentration phase transition to this relaxor-like phase is abrupt (of the first order kind) that is seen from the step in the dependence of the ε′(T) maximum temperature, T_m, on x. Some relaxor-like properties appear even at x<x₀ in the course of cooling while disappear during the course of heating. Due to this fact and because of coupling of the antiferroelectric (AFE) and ferroelectric (FE) order parameters a giant (up to 100 K) temperature hysteresis of ε′(T) arises at AFE-AFE first order phase transition. The T_m values of all the known NaNbO₃-ABO₃ relaxor-type compositions are well below the room temperature and the dielectric permittivity maximal values, ε′_m, are much lower than in the case of Pb-containing relaxors. However both T_m and ε′_m values can be increased substantially by Li or K-doping leading to the formation of NaNbO₃-ABO₃-LiNbO₃ (KNbO₃) solid solutions.     

Phases,phase transitions and excitons of the superionic conductor AgI in the temperature range between 4.2 K and 761 K detected by diffuse reflectance spectrometry

The crystalline phase transformations, at atmospheric pressure, of the hexagonal, wurtzite type, β-silver iodide powder were studied by spectrometry of the visible, diffusely remitted light. By increasing the temperature of β-AgI from room temperature, the superionic phase may be reached in two modes. One through the β-AgI phase followed by a pretransition β′-AgI phase, the other through a β″-AgI phase only. β-AgI was transformed, by a stochastic process, into β″-AgI which, by stepwise cooling, passed into the intermediate δ-AgI phase. Maintaining the stepwise cooling, below room temperature, δ-AgI transformed itself into a temperature sensitive ϵ(T)-AgI phase which eventually resulted in the ϵ-AgI phase. Slow continuous heating retransformed ϵ-AgI into ϵ(T)-AgI which by cooling gave a ζ-AgI phase, stable down to liquid helium temperature. The original β-AgI, cooled stepwise, transformed itself, at 279 K, by a first order transition, into the sphalerite type γ-AgI. The known α, β and γ phases as well as the new β′, β″, β‴, ϵ and ζ phases were characterized by their thermal activation energies of photon absorption as well as, at sufficiently low temperatures, by their exciton emission and absorption spectra. Stochastic processes played an important role in the formation particularly of the new phases.     

Phase transition and structure of (C3N2H5)2SbF5 single crystal

New crystal of the formula (C₃N₂H₅)₂SbF₅ was obtained and characterized with DSC, DTA, TGA, structural and dielectric studies. DSC and dielectric studies revealed a structural phase transition of the first order at 216 K on cooling and 220 K on heating. The entropy of the transition ΔS equal to 11.5 J/mol·K gives evidence that the phase transition is order-disorder type. X-ray studies showed that transition undergoes from orthorhombic phase I with a space group of Pmmn to monoclinic phase II with a space group P2₁/m. The phase transition is proposed to be ferroelastic type. The molecular mechanism of the phase transition is related to ordering of imidazolium cations in phase II that are disordered in phase I.     

Effect of grain boundaries on paraconductivity of YBa2Cu3Ox

To investigate the effect of grain boundaries on paraconductivity of YBa₂Cu₃O_x, melt-textured and c-axis oriented thin films with controlled grain boundaries (superconducting transition width, ΔT, varying between 0.54 and 2.85 K) were prepared, and dc-conductivity has been measured as a function of temperature. In the logarithmic plots of excess-conductivity (Δσ) and reduced temperature (?), starting from low values of ?, we have observed three different regions namely critical region, mean field region and short wave fluctuation region. A correlation is observed between the range of critical region and ΔT, which is found to increase with ΔT. While for ΔT values smaller than 2.5 K only static critical region is observed, for higher ΔTs both static and dynamic critical regions are observed. In the mean field region a crossover from 3D to 2D was observed for all the samples. At ? values larger than 0.24, the excess-conductivity decreased sharply as ?⁻³, which suggested the existence of the short wave fluctuations.     

Fragmentation phase transition in atomic clusters IV

Within the micro-canonical ensemble it is well possible to identify phase-transitions in small systems. The consequences for the understanding of phase transitions in general are discussed by studying three realistic examples. We present micro-canonical calculations of the fragmentation phase transition in Na-, K-, and Fe- clusters of N = 200 to 3000 atoms at a constant pressure of 1 atm. The transition is clearly of first order with a backbending micro-canonical caloric curve T _P (E, V (E,P)) = {?S(E, V (E,P))/?E _P}^?1. From the Maxwell construction of β_P (E/N,P) = 1/T _P one can simultaneously determine the transition temperature T _tr, the specific latent heat q _lat, and the specific entropy-loss Δs _surf linked to the creation of intra-phase surfaces. T _trΔs _surf*N/(4πr _ws ² N _eff ^2/3 ) = γ gives the surface tension γ. Here 4πr _ws ² N _eff ^2/3 = ΣN _i*4πr _ws ² m _i ^2/3 is the combined surface area of all fragments with a mass m _i ≥ 2 and multiplicity N _i. All these characteristic parameters are for ～1000 atoms similar to their experimentally known bulk values. This finding shows clearly that within micro-canonical thermodynamics phase transitions can unambiguously be determined without invoking the thermodynamic limit. However, one has carefully to distinguish observables which are defined for each phase-space point, like the values of the conserved quantities, from thermodynamic quantities like temperature, pressure, chemical potential, and also the concept of pure phases, which refer to the volume of the energy shell of the N-body phase-space and thus do not refer to a single phase-space point. At the same time we present here the first successful microscopic calculation of the surface tension in liquid sodium, potassium, and iron at a constant pressure of 1 atm.     

The heat capacity of the layer compounds (CH3CH2CH2NH3)2MnCl4 and (CH3CH2CH2NH3)2CdCl4 from 10 K TO 300 K

The heat capacity of the layer compounds tetrachlorobis (n-propylammonium) manganese II and tetrachlorobis (n-propylammonium) cadmium II, (CH₃CH₂CH₂NH₃)₂MnCl₄ and (CH₃CH₂CH₂NH₃)₂CdCl₄ respectively, has been measured over the temperature range 10 K ?T ? 300 K.Two known structural phase transitions were observed for the Mn compound in this temperature region: at T = 112.8 ± 0.1 K (ΔH_t= 586 ± 2 J mol^?1; ΔS_t = 5.47 ± 0.02 J K^?1mol^?1) and at T =164.3 ± (ΔH_t = 496 ± 7 J mol^?1; ΔS_t =3.29 ± 0.05 J K^?1mol^?1). The lower transition is known to be from a monoclinic structure to a tetragonal structure, while the upper is from the tetragonal phase to an orthorhombic one. From comparison with the results for the corresponding methyl Mn compound it is deduced that the lower transition primarily involves changes in H-bonding while the upper transition involves motion in the propyl chain.A new structural phase transition was observed in the Cd compound at T= 105.5 ± 0.1 K (ΔH_t= 1472.3 ± 0.1 J mol^?1; ΔS_t = 13.956 ± 0.001 J K^?1mol^?1), in addition to two transitions that have been observed previously by other techniques. The higher of these transitions(T = 178.7 ± 0.3 K; ΔH_t = 982 ± 4 J mol^?1 ΔS_t = 6.16 ± 0.02 J K^? mol^?1) is known to be between two orthorhombic structures, while the structural changes at the lower transition (T= 156.8 ± 0.2 K; ΔH_t = 598 ± 5 J mol^?1, ΔS_t = 3.85 ± 0.03 J K^?1 mol^?1) and at the new transition are not known. It is proposed that these two transitions correspond respectively to the tetragonal to orthorhombic and monoclinic to tetragonal transitions in the propyl Mn compounds.In addition to the structural phase transitions (CH₃CH₂CH₂NH₃)₂MnCl₄ magnetically orders at t? 130 K. The magnetic contribution to the heat capacity is deduced from the heat capacity of the corresponding diamagnetic Cd compound and is of the form expected for a quasi 2-dimensional Heisenberg antiferromagnet.     

A.C. specific heat of terbium and dysprosium

The dependence of a.c. specific heat measurements upon the temperature modulation amplitude ΔT has confirmed the first-order nature of the antiferromagnetic-ferromagnetic transition in single crystal terbium (T_C～221K) and polycrystalline dysprosium (T_C～85K). No peak in a.c. specific heat is obtained for 2ΔT less than the temperature hysteresis at the transition. By comparison, the peaks expected in a.c. specific heat were observed at the higher-order antiferromagnetic-paramagnetic transition in both materials for ΔT amplitudes as low as～40 mK.