Transport,magnetic, and thermal properties of non-centrosymmetric Yb2Co12P7

We report magnetization and specific heat measurements down to 2?K and electrical resistivity down to millikelvin temperatures on polycrystalline samples of the non-centrosymmetric compound Yb₂Co₁₂P₇. In addition to the previously reported ferromagnetic ordering of the cobalt sub-lattice at T _C ?=?136?K we find a magnetic transition below T _M ?=?5?K that is likely associated with ordering of the Yb ions. The broad nature of the specific heat anomaly suggests disordered magnetism and possible short-range correlations well above T_M .     

Thermophysical properties of undercooled liquid Co-Mo alloys

Using electromagnetic levitation in combination with the oscillating drop technique and drop calorimeter method, the surface tensions and specific heats of undercooled liquid Co-10 wt% Mo, Co-26.3 wt% Mo, and Co-37.6 wt% Mo alloys were measured. The containerless state during levitation produces substantial undercoolings up to 223 K (0.13 T _L), 213 K (0.13 T _L) and 110 K (0.07 T _L) respectively for these three alloys. In their respective undercooling ranges, the surface tensions were determined to be 1895 m0.31(T m1744), 1932 m0.33(T m1682), and 1989 m0.34(T m1607) mN m^?1. According to the Butler equation, the surface tensions of these three Co-Mo alloys were also calculated, and the results agree well with the experimental data. The specific heats of these three alloys are determined to be 41.85, 43.75 and 44.92 J mol^?1 K^?1. Based on the determined surface tensions and specific heats, the changes in thermodynamics functions such as enthalpy, entropy and Gibbs free energy are predicted. Furthermore, the crystal nucleation, dendrite growth and Marangoni convection of undercooled Co-Mo alloys are investigated in the light of these measured thermophysical properties.     

Specific heat and latent heat of KMnF3 ferroelastic crystal

The specific heat and the heat flux exchanged by a single crystal of KMnF₃ have been measured simultaneously while cooling the sample at constant rate of 0·06 K/h through the phase transition at T ₀= 186 K. The phase transition is weakly first order and close to a tricritical point. The temperature dependence at T185 K of the excess specific heat and the excess entropy follow very well the predictions of a Landau potential at a tricritical point.     

Si80笼状分子的结构研究

采用基于密度泛函的第一性原理计算方法,构建了Si₈₀笼状分子的模型,并对其结构和电学特性进行了考察. 研究发现,经过结构优化计算,Si₈₀分子从I_h高对称性下降为T_h,但仍然保持较好的笼状结构. 对Si₈₀笼状分子的稳定性、轨道分布和电荷分布等性质进行了分析和讨论. 关键词： 80')" href="#">Si₈₀ 富勒烯 结构     

Magnetic Anisotropy and Hidden Martensitic Transition in V3Si

Magnetization, electrical resistivity and heat capacity have been measured on a single crystal V₃Si in the range of (2-25) K and in magnetic field up to 14 T. A different behavior of magnetization for two orientations of the crystal has been found. In one orientation the magnetization displays a clear ferromagnetic character and below T _c coexistence of ferro-magnetism and superconductivity with a peak-effect in the vicinity of upper critical field H _c2. The specific heat measurements show sharp lambda anomaly corresponding to a transition to superconductive state and an additional anomaly around 15 K when applied field suppresses the superconductivity below this temperature.     

Nb3Sn正常-超导转变时的比热反常

在16.0°K—20.3°K之间测量了Nb₃Sn样品的热容量。Nb₃Sn在临界温度附近的比热跳跃值ΔC=2.21(±5％)焦耳/克分子·度。样品的临界温度T_c=17.88°K,转变宽度ΔT_c≈0.2°K。ΔC值利用热力学关系式确定了Nb₃Sn在0°K时的热力学临界场H₀=5300奥斯特。利用本文的结果和文献上关于热膨胀系数的跳跃值Δα及?T/?P值验证了热力学关系式。扼要地描述了比热测量装置.     

Electrical and thermal properties in CuV2S4

The successive phase transitions of the cubic spinel structure CuV₂S₄ have been studied by the electrical resistivity and the specific heat. In the resistivity and the specific heat two anomalies have been observed at T₁～91 K and T₃≈55 K. The specific heat indicates that the phase transition at T₁ is in the vicinity of a tricritical point, where the resistivity has a small sharp peak. On the other hand, the specific heat shows a step at T₃, where a hysteresis of the resistivity indicates the existence of the first-order phase transition.     

Specific heat of KTiOPO4 within the 80–300 K range

The specific heat of the KTiOPO₄ crystal has been measured with a vacuum adiabatic calorimeter within the 80–300 K range. A peak-shaped anomaly in the specific heat indicating a phase transition has been revealed in the C _p (T) curve at T≅279 K. Numerical integration of smoothened experimental C _p (T) curves yielded the thermodynamic functions of KTiOPO₄, namely, the entropy, enthalpy, and reduced Gibbs energy. The entropy and enthalpy of the observed transition have been determined. Fiz. Tverd. Tela (St. Petersburg) 41, 497–498 (March 1999)     

Specific heat of the elpasolite Pb2MgWO6

The specific heat of Pb₂MgWO₆ has been measured in the temperature interval 83–370 K. An anomaly in the specific heat associated with the phase transition at T ₀=312.8K has been discovered. The thermodynamic parameters of the structural phase transition Fm3m-Pmcn have been determined. Fiz. Tverd. Tela (St. Petersburg) 41, 1686–1688 (September 1999)     

Characteristic features of the magnetic ordering of Dy3+ ions in a monoclinic RbDy(WO4)2 single crystal

The investigation of the specific heat of a RbDy(WO₄)₂ single crystal at temperatures 0.2–2.5 K and in magnetic fields up to 2 T are reported. The temperature dependence of the specific heat near T _N=0.818 K is compared with the predictions for different models. The 2D Ising model describes satisfactorily C(T) below T _N, while for T>T _N none of the theoretical models agree with the behavior of C(T) of RbDy(WO₄)₂. The H-T phase diagram for H∥c is complicated and possesses a triple point, where regions of existence of three magnetic phases converge. The magnetic ordering is analyzed from the standpoint of the Jahn-Teller nature of the structural phase transitions occurring in RbDy(WO₄)₂ at higher temperatures. It is shown that the form of the phase diagram depends on the direction of the vector H, for the general case of an arbitrary direction of H, two phase transitions can occur with increasing field. Fiz. Tverd. Tela (St. Petersburg) 41, 491–496 (March 1999)     

Superconducting properties of the pyrochlore oxide Cd2Re2O7

We report the superconducting properties of the pyrochlore oxide Cd₂Re₂O₇. The bulk superconducting transition temperature T_c is about 1.0 K, and the upper critical field H_c2 determined by the measurement of specific heat under magnetic fields is 0.29 T. The superconducting coherence length is estimated to be 34 nm. Specific heat data measured on single crystals suggest that the superconducting gap of Cd₂Re₂O₇ is nodeless.     

Comment on the paper “Nature of low-temperature phase transitions in CaMn7O12”

We contend that the anomalies observed in the magnetization, specific heat, and thermal expansion of CaMn₇O₁₂ at T _S = 89 K and T _M = 49 K reported in O. Volkova et al., Pis’ma v ZhETF 82, 498 (2005) [JETP Lett. 82, 444 (2005)] and mentioned in Pis’ma v ZhETF 82, 724 (2005) [JETP Lett. 82, 642 (2005)] are both of magnetic origin. The text was submitted by the authors in English.     

Linear birefringence studies of incommensurate systems: II. KFeF4

Abstract

The temperature dependence of the linear birefringence (LB), Δn _c , is measured in the range 5 K ? T ? 500 K on samples of KFeF₄, which originate from hydrothermal, flux and Bridgman growth techniques. Pronounced anomalies are found at the orthorhombic-orthorhomic phase transition at T _c ∽ 400 K. It is weakly discontinuous with a near-tricritical exponent β ∽ 0.2. Weak anomalies near T _i ∽ T _c + (25 … 40 K) seem to indicate a transition into an intermediate incommensurate phase. Its XY-model character is reflected by the critical LB exponent β = 0.8 ± 0.1. A smooth LB anomaly below 200 K is due to 2-dimensional ferromagnetic spin-order.     

Effect of ordering on the structure and specific heat of nonstoichiometric titanium carbide

The experimental results on the change in the crystal structure and specific heat of nonstoichiometric titanium carbide TiC_y (0.5<y<0.65) near disorder-order phase transitions are reported. It is established that at temperatures below 1000 K the ordered phases Ti₂C with cubic and trigonal symmetry and an orthorhombic ordered phase Ti₃C₂ form in titanium carbide by a close-to-first-order phase transition mechanism. The temperatures and heats of order-disorder phase transformations are determined. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 9, 631–637 (10 May 1999)     

Low-temperature anomalies in the specific heat and thermal conductivity of MgB<Subscript>2</Subscript>

The temperature dependences of the specific heat C(T) and thermal conductivity K(T) of MgB₂ were measured at low temperatures and in the neighborhood of T _c . In addition to the well-known superconducting transition at T _c ≈40 K, this compound was found to exhibit anomalous behavior of both the specific heat and thermal conductivity at lower temperatures, T≈10–12 K. Note that the anomalous behavior of C(T) and K(T) is observed in the same temperature region where MgB₂ was found to undergo negative thermal expansion. All the observed low-temperature anomalies are assigned to the existence in MgB₂ of a second group of carriers and its transition to the superconducting state at T_c2≈10?12 K.     

Decrease of soft vibrational modes with increasing density of two-level states in the metallic glass Zr-Rh

Experiments are reported on the specific heat below 1 K of 13 melt quenched ZrRh alloys in the superconducting state (T_c>4.2 K) where electronic degrees of freedom are frozen out. Large differences of the quasilinear (two-level system, TLS) contributions given by λ_TLS occur with no correlation to the rhodium concentration. The coefficient α of the cubic, phonon-type term is found to decrease with the density of TLS for this metallic glass. This is counter-intuitive to the soft-potential model and differs from the relation found for insulating glasses.     

Weak itinerant ferromagnetism and clean superconductivity in Y9Co7

We present low-temperature magnetic properties for a high-quality polycrystalline sample of the first ferromagnetic superconductor Y₉Co₇. The results of susceptibility and magnetization measurements show the coexistence of a weak itinerant ferromagnetic order with a Curie temperature of T_C ? 4.5 K and superconductivity below T_S = 3 K. Several electronic quantities and parameters characterizing the superconducting and normal state are calculated within the Ginzburg–Landau–Abrikosov–Gorkov theory. The data reveal clean limit superconductivity in Y₉Co₇ due to the good chemical purity and structural order of the specimen.     

Investigation of the resistive transition of CaLaBaCu3O7 in magnetic fields up to 10 tesla and pressures up to 46 GPa

Abstract

The onset critical temperature T _co, of CaLaBaCu₃O₇, is measured as a function of pressure by means of a cryogenic diamond anvil cell. We find ?T _co/?p = 0.14 ± 0.02K/GPa. The pressure dependence of the upper critical field B _c2 as a function of pressure is determined for T/T_c , = 0.96. From this we found the corresponding volume dependence of the number of charge carriers to be much smaller than the value derived from Hall effect measurements in YBa₂Cu₃O₇.

Les dérivées par rapport à la pression de la température critique supérieure T_co , (avec ?T _co,??p = 0.14 ± 0.02K/GPa) et du champs critique B _c2 à T/T_c , = 0.96 ont été déterminé á l'aide d'une presse à diamants. Pour le composé CaLaBaCu₃O₇, la variation de la densité de charges en fonction du volume est beaucoup plus faible que celle détermiée par effet Hall dans YBa₂Cu₃O₇.