Preparation of ternary lead molybdenum sulfide with high superconducting transition temperature

The high field superconducting compound PbMo₆S₈ has been prepared by an isothermal chemical vapour transport reaction at 1273 K. The reaction product was single phase PbMo₆S₈ with lattice parameters of $\text{a}{}_0\text{= 6.545}\text{A}\text{?}$, and α = 89° 12′ and has been found to transform to the superconducting state at a midpoint T_c of 14.7 K.     

Influence of composition on the superconducting transition temperature of alloys with the A-15 structure

Superconducting transition temperatures are presented for a wide range of compositions within the A-15 phase of the NbAl system. The maximum T_c of 19.0 K clearly occurs for a composition greater than that of stoichiometric Nb₃Al. This observation, together with a modern calculation by Shen, suggests that the important controlling factor for the maximum T_c in the A-15 class of materials is the approach to the “$\text{λ ? 2}$ limit” rather than structural instabilities or the stoichiometric composition.     

The concentration dependence of the ferroelectric transition temperature of the solid solution of TSCC and TSCB

The concentration dependence of the ferroelectric transition temperature T_c of single crystals of the solid solution tris-sarcosine calcium chloride_1?x bromide_x is studied experimentally by measurement of the static dielectric constant ?_b and described by the empirical relation $\text{T}{}_{\mathrm{c}}\text{(X) = T}{}_{\mathrm{c}}\text{(0) [1?(X/X}{}_{\mathrm{c}}\text{)]}\text{1}\text{2}$. The Ising model with transverse field and the model of coupled anharmonic oscillators are used to explain the concentration dependence of the transition temperature T_c.     

Hydrodynamics of an n-component phonon system

The dynamic properties of an n-component phonon system in d dimensions, which serves as a model for a structural phase transition of second order, are investigated. The symmetry group of the hamiltonian is the group of orthogonal transformations $\text{O}$(n). For n ≥ 2 a continuous symmetry is broken for T<T_c, where T_c is the transition temperature. We derive the hydrodynamic equations for the generators of this group, the $\text{1}\text{2}\text{n (n ? 1)}$ angular-momentum variables. Besides the usual hydrodynamics of a phonon system, there are $\text{1}\text{2}\text{n (n ? 1)}$ additional independent diffusive modes for T > T_c. In the ordered phase we find 2 (n ? 1) propagating modes with linear dispersion and quadratic damping. Formally the hydrodynamics is similar in the isotropic Heisenberg ferromagnet (n = 2) or the isotropic antiferromagnet (n ≥ 3). The relaxing modes for T < T_c require special care. We study the critical dynamics by means of the dynamical scaling hypothesis and by a mode-coupling calculation, both of which give the critical dynamical exponent $\text{z =}\text{1}\text{2}\text{d}$. The results are compared with the 1/n expansion. It is shown that for large n there is a non-asymptotic region characterized by an effective exponent $\text{z}\text{?}\text{= φ/2ν}$, where φ is the crossover exponent for a uniaxial perturbation, and ν the critical exponent of the correlation length.     

Superconducting—normal phase boundaries of (La,Th)Ce systems

The dependence of the superconducting transition temperature T_c on Ce impurity concentration n is reported for various ($\text{La, Th}$)Ce systems. These results document for the first time the evolution of the superconducting— normal phase boundary (T_c vs n curve) as an impurity undergoes magnetic- nonmagnetic transition which for Ce impurities in ($\text{La, Th}$)Ce systems proceeds with increasing Th concentration. Observed for only the second time, re-entrant T_c vs n curves are reported for two La-rich La, Th host compositions.     

Low temperature structural distortion in the high Tc Chevrel-phase superconductors PbMo6S8 and SnMo6S8

High-resolution powder neutron diffraction data reveal that, at temperatures below 100 K, oxygen-free, high-T_c samples of PbMo₆S₈ and SnMo₆S₈ exhibit a small structural distortion from the R$\text{3}$ rhombohedral space group. The data cannot be refined in a simple P$\text{1}$ triclinic space group as previously reported for EuMo₆S₈ and BaMo₆S₈. Thus, a supercell ordering is suggested.     

Chaleur spécifique et transformation martensitique dans le système Nb1−xSnx

Experimental data on the superconducting temperature, and low temperature specific heat, together with X-ray and neutron investigations are reported for two sets of Nb_xSn_1?x, samples with the A15 type structure. The first one, intended for a study of the effect of stoichiometry, consists of cast alloys with lattice constants ranging from 5.273 to 5.289 Å, γ's from 4.9 to $\text{9}\text{mJ}\text{K}{}^2$ g-at and T_c's from 6 to 17.9 K. The second one consists of nearly stoichiometric sintered alloys with lattice constants ranging from 5.288 to 5.291 Å, γ's from 5.6 to 13.5 $\text{mJ}\text{K}{}^2$ g-at and T_c's close to 18 K. The martensitic transformation ratio has been determined by neutron diffraction in the latter set and the values 32, 77 and 95% were obtained. The analysis of the specific heat jumps at T_c shows that the superconducting T_c of the tetragonal phase is 17.9 K and both T_c and γ are reduced in the cubic phase.Phonon information inferred from specific heat data are indicative of a structure with a typical energy of 9 meV in the density of states. The agreement with spectra obtained by inelastic neutron scattering is good.     

Critical slowing down of fluctuations in squaric acid (H2C4O4) at the phase transition observed by 13C spin-lattice relaxation

Spin lattice relaxation T₁ of naturally abundant ¹³C nuclei in squaric acid was measured close to the antiferroelectric-paraelectric phase transition temperature T_c = 373 K. A rapid increase in $\text{1}\text{T}{}_1$ is observed close to T_c coming from above, which follows the power law $\text{1}\text{T}{}_1\text{～ ε}{}^{\mathrm{?1.4}}$ where $\text{ε =}\text{(T ? T}{}_{\mathrm{c}}\text{)}\text{T}{}_{\mathrm{c}}$. This behaviour is explained on the basis of the two-dimensional character of the fluctuations.     

On the validity of the Stoner-Wohlfarth model for FeNi invar alloys

After having recalled that the Arrot plots, i.e. M³ linear function of $\text{H}\text{M}$ are not a characteristic feature of weak ferromagnetism, we present experimental data on FeNi invar alloys near T_c that may seem difficult to be explained in the framework of this model.     

Effects of dipolar forces on critical sound attenuation in uniaxial systems

Critical sound attenuation in uniaxial dipolar systems is investigated above and below T_c by using the dynamic renormalization-group analysis for an elastically coupled Ising-type model at d = 4. We predict a logarithmic crossover of the divergence of sound damping as $\text{t =}\text{|T ? T}{}_{\mathrm{c}}\text{|}\text{T}{}_{\mathrm{c}}\text{→ 0}$ and derive a universal, crossover-independent ratio R = 12.984|lnt| between the relaxation and the fluctuation damping below T_c. The effects of elastic anisotropy on the phase transition are discussed.     

Torque magnetometer measurements of the temperature dependence of induced anisotropy energy and of saturation magnetization in amorphous Fe40Ni40P14B6

Induced anisotropy energy K_u and saturation magnetization M have been measured as a function of temperature T between 77 K and T_c on co$\text{n}$venient specimens of amorphous Fe₄₀Ni₄₀P₁₄B₆ by means of a torque magnetometer. The M vs. T curve is found to follow an anomalously low behaviour with respect to crystalline materials. The exper$\text{i}$mental K_u vs. T curve is well fitted by a M² law, also different from higher power laws typical of crystalline alloys.     

A light scattering investigation of the incommensurate transition in BaMnF4

The results of a Brillouin scattering investigation of BaMnF₄ near its incommensurate structural phase transition at T_i=247 K are reported. We have observed an anomalous dynamic central peak in the polarized (bb) spectrum, with $\text{q}\text{6}\text{c}$. This feature is visible only very near T_i, and possesses a width which is strongly wavevector dependent. The indicated behavior is Λ=D_tq², with D_t=0.14 ± 0.02 cm²sec^-1. It is identified as the result of a coupling between the LA phonon and the soft mode, where the latter contains a relaxing self energy due to a linear coupling to a diffusive process, possibly entropy fluctuations. The narrow central peak is not visible for $\text{q}\text{6}\text{a}$. In addition, we report an anomalous dispersion (～4%), in the LA mode propagating along $\text{c}$ (V_cc) near T_i.     

Phase transition and phonon-drag conductivity in quasi-unidimensional systems

We investigate the properties of a quasi-unidimensional system which exhibits Peierls instability at T_c(μ), where μ is the Fermi energy as measured from the middle of the conduction band. T_c(μ) decreases as 6μ6 increases. The phonon-drag diagrams contribute to fluctuation conductivity, which is proportional to $\text{(T ? T}{}_{\mathrm{c}}\text{)}{}^{\mathrm{<mtext>?</mtext><mtext>1</mtext><mtext>2</mtext>}}$, if |μ| > 1.056 T_c(0). The Kohn Anomalies do not occur at 2k_F unless μ = 0 or |μ| ? T.     

Superconducting properties of ZnCr and ZnMn alloys

Measurements on the superconducting critical temperature T_c and critical field, H_c, of $\text{Zn}$Cr and $\text{Zn}$Mn alloys, down to 0.037°K are presented.The variation of T_c with increasing concentration depends strongly on T_cp/θ, with θ a characteristic temperature, while effects of H_c are similar to previously studied alloys.     

The frequency dependence of the ac susceptibility of the PdMn spin glass

We have measured the frequency dependence of the ac susceptibility _X(T) for three alloys of $\text{Pd}$Mn in the spin-glass regime. A strong dependence of _X(T) on frequency is observed near and below the freezing temperature in contrast to the archetypal $\text{Cu}$Mn spin glass. We discuss possible reasons for these differences.     

Primordial two-component maximally symmetric inflation

We propose a two-component inflation model, based on maximally symmetric supergravity, where the scales of reheating and the inflation potential at the origin are decoupled. This is possible because of the second-order phase transition from SU(5) to SU(3)×SU(2)×U(1) that takes place when φ?φ_c<φ₀, when φ₀?O(M) is the value of the inflation at the global minimum, and leads to a reheating temperature T_R?(10¹⁵–10¹⁶) GeV. This makes it possible to generate baryon asymmetry in the conventional way without any conflict with experimental data on proton lifetime. The mass of the gravitinos is $\text{m}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{?10}{}^{12}\text{GeV}$, thus avoiding the gravitino problem. Monopoles are diluted by residual inflation in the broken phase below the cosmological bounds if φ_c?0.3M.     

Ferromagnetic ordering in the MMnF6 series (M = Ni,Cu, Zn,Pd)

Neutron diffraction experiments performed on MMnF₆ compounds (M = Ni, Cu, Zn, Pd) have shown that both NiMnF₆ and ZnMnF₆ crystallize in the rhombohedral R$\text{3}$ space group with a cationic ordering, whereas PdMnF₆ shows cationic disordering (R$\text{3}$c, a = 5.46 Å, α = 54.45°). CuMnF₆ has a monoclinic distortion derived from the MnF₃ structural type (a = 8.57 Å, b = 4.85 Å, c = 13.46 Å, β = 92.12°). The magnetic properties can be correlated with the structure: the R$\text{3}$ ordered compounds show a ferromagnetic behavior (T_C(NiMnF₆) = 39 K; T_C(ZnMnF₆) = 9.5 K; T_C(CdMnF₆) = 8 K), while the other two have an antiferromagnetic ordering.     

Effects of composition on the upper critical field of V-Ga

Measurements of the temperature dependence of the upper critical field, H_c2(T), for a series of V_100?xGa_x materials are presented for 20.5 ≤ × ≤ 29.6. Fits of the data to conventional theory for a paramagnetically limited, dirty, type II superconductor show: 1) a maximum in T_c and H_c2(0) for x ? 25; 2) a constant ( for x ≤ 25; 3) a slowly increasing value of λ_so with increasing x up to x ～ 25; and 4) good agreement with stoichiometric ordered and thermally disordered V₃Ga. Above $\text{x ? 25}$ broader transitions are observed. For x = 25, T_c = 15.3 K, (, λ_so = 0.3 and H_c2(0) = 23.4 tesla. The effects of inclusion of strong-coupling in the theory are discussed briefly.     

Critical behaviour of electrical resistivity in 5 binary liquid systems

For five binary liquid systems CS₂+CH₃CN, CS₂+CH₃NO₂, CS₂+(CH₃CO)₂O, C₆H₁₂+(CH₃CO)₂O, n-C₇H₁₆+(CH₃CO)₂O, the electrical resistance has been measured near the critical solution temperatures. The behaviour is universal. Below T_c, the conductivities of the two phases follow ∣σ₁?σ₂∣～?^β, where ?=∣T?T_c∣T_c with β≈0.35. In the one phase region $\text{d}\text{R}\text{d}\text{T}\text{～?}{}^{\mathrm{-b}}$ with b≈0.35±0.1 and $\text{d}\text{R}\text{d}\text{T}$ is positive in some cases and negative in others.