Specific Heat ΔC Research Articles

Superconducting fluctuations of the specific heat in the short wavelength fluctuation regime

Abstract Effects of the short wavelength fluctuations on the specific heat ΔC in a superconductor are studied within the scheme of the Aslamazov-Larkin (AL) theory. By imposing the momentum cutoff in the fluctuation spectrum, formulae for ΔC are derived in a layered system as well as in n -dimensional ( n D) systems ( n =1, 2 and 3). Formulae under a total-energy cutoff condition are also presented. It is found that ΔC is significantly suppressed at high-reduced temperatures in a similar manner as in the paraconductivity. Our formulae in 3D and layered systems may be better fitted to experiments on YBa2Cu3O7-δ crystals in a wider temperature region than the standard AL formulae without any cutoff effect.

The glass transition of bulk metallic glasses studied by real-time diffraction in transmission using high-energy synchrotron radiation

The isentropic glass transition Tg in metallic glasses can be studied by calorimetric techniques which show an endothermic event corresponding to a specific heat ΔC p difference between the glassy and supercooled liquid states. In this work we show that isostructural thermal expansion coefficient α th is easily obtained by diffraction data hence an isochoric T g should be detectable by diffraction. Real-time diffraction studies of phenomena occurring on a time scale of τ t can only be effective if acquisition time τ a < < τ t . The problem was that near T g , metallic glasses are unstable and crystallize during real-time diffraction experiments. This limitation does not occur in diffraction experiments on bulk metallic glasses with large supercooled regions ΔT = T x - T g using high energy, high intensity monochromatic light from synchrotron sources that allow τ a << τ t . Under such conditions, the variation of α th , the volume coefficient of thermal expansion has been measured for the first time by diffraction near T g for a number of bulk metallic glasses. A clear Δα th has been detected in the same glass transition temperature range of ΔC p for Pd-based metal-metalloid bulk glasses but not in Zr-based metal-metal type bulk glasses where Δα th occurs well above the calorimetric T g .

Investigation of Kondo impurity — Kondo lattice transition in Ce xLa 1− xRu 2

The Kondo impurity — Kondo lattice (KI-KL) transition in the intermetallic compounds Ce x La 1− x Ru 2 is induced by variation of cerium concentration x. It allows to observe the change of magnetic state of the system which involves Kondo effect, intermediate valence fluctuations phenomena and formation of heavy-fermion state. This problem affects also the coexistence and interplay of superconductivity and magnetism, that will be especially investigated. We have measured specific heat down to 4.5 K, susceptibility down to 1 K, resistivity down to 0.6 K in magnetic field up to 2 T and X-ray photoabsorption spectra for x = 0, 0.25, 0.5, 0.75, 1. It is shown that with increasing x system goes to an intermediate valence state in which the Kondo effect is suppressed. We propose to interpretations of T c behaviour as a function of x: at first we use the concurrence between the Kondo and the RKKY interactions; second, we take into account the variation of the density of states at the Fermi level versus x and the value of the discontinuity in specific heat ΔC = 1.85 γ( T C) T C in the CeRu 2 sample.

Calorimetric observation of the metal-insulator phase transition in α-(BEDT-TTF) 2I 3

The quasi-2D ‘organic’ conductor α-(BEDT-TTF) 2I 3 undergoes a metal to insulator transition near 135 K. We have observed for the first time in the specific heat a sharp, hysteretic, first order phase transition associated with this metal-insulator transition. We find a lower bound on the entropy change ΔS of 94% (±20%) RIn2 per mole (2.4 mJ/g-K), corresponding to a lower bound for the latent heat of 0.32 J/g. Extrapolating from below and above the transition, we estimate the change in the specific heat ΔC at the transition to be ≈ 1.4 μJ/mole K (3.2 mJ/g-K). The observed behavior differs significantly from both Peierls type and large Coulomb repulsion Mott-Hubbard type metal-insulator transitions in quasi-1D organic conductors.

Thermodynamic properties of liquid Na-polyvalent metal alloys under high pressure

The EMF of liquid NaIn and NaBi systems has been measured a as function of temperature and pressure in detail by using an electrochemical cell with sodium β-alumina as the electrolyte. We have obtained the associated thermodynamic quantities of mixing such as free energy δG, entropy ΔS, enthalpy ΔH, excess specific heat ΔC p, excess volume ΔV, compressibility χ T and concentration-concentration fluctuation in the long wavelength limit S cc(O).

Crystal-field effects and f-derived specific heats in heavy-fermion compounds

We discuss experimetal results of calorimetry on heavy-fermion compounds (HFC) with special regard to the f-derived specific heat ΔC. Attention is given to the extraction of Δ C and the uncertainties implied therein. The discussion includes some Ce-based 4f systems but concentrates on U-based 5f HFC. In four of these (UBe 13, UPt 3, URu 2Si 2, U 2Zn 17) distinct anomalies in Δ C have recently been found in the temperature range between 20 and 100 K which we think are likely to be caused by crystal-field effects thus indicating strong 5f localization.

Effect of crosslink density on physical ageing of epoxy networks

Physical ageing of polypropylene oxide/DGEBA networks with different crosslink densities was investigated using the small-strain stress relaxation technique in simple extension. The effects of crosslink density on the glass transition temperature, T g, and the change in specific heat ΔC p at T g were measured using a differential scanning calorimeter (d.s.c.) in heating. Although we observed an increase in T g as crosslink density increased, contrary to other studies of crosslinked polymers, the ΔC p did not change as the crosslink density changed. Ageing was studied at several temperatures below T g after quenching from above T g. It was possible to superimpose the stress relaxation curves at different ageing times, temperatures and crosslink densities to form a single master curve, demonstrating the applicability of a time-ageing time-temperature-crosslink density superposition principle to this type of network. Under all ageing conditions, the double logarithmic shift rate was found to decrease with increasing crosslink density while being independent of temperature for a given network. Furthermore, at temperatures of 10 and 5°C below T g, we were able to age the network glasses into structural equilibrium, thus obtaining t∗, the time required to reach structural equilibrium. At a constant temperature ΔT below T g, we observed increases in t∗ as the crosslink density increased.

Thermodynamic properties of liquid Na-VIB alloys

The thermodynamic quantities of mixing such as free energy ΔG, entropy ΔS and enthalpy ΔH of liquid Na-IVb alloys are discussed in view of formation of chemical complex. From these, the excess specific heat ΔC p as a function of temperature, which is resulted from the dissociation of chemical complex, is particularly concerned. The discrepancies in the compound-forming tendencies among liquid NaPb, NaSn and LiPb are also discussed.

A disordered ising model with internal degrees of freedom

A solvable Ising-system withnn-interactions as a basic system is considered. This system is assumed to undergo a phase transition so that the thermal quantityq=Σ 02/N, which is not necessarily the order parameter of the basic system, is different from zero (q≠0) forT<Tc. The whole system consists of the basis {Si} and an equal number of additional spins ηi which are coupled linearly with the basic spinsSi through an exchange constantJi which is distributed at random. The pairs of spinsSi−ηi can be interpreted as clusters with an internal degree of freedom. The specific heat Δc and the susceptibility Δχ of the internal degree of freedom can be separated exactly and show typical spin glass properties: the specific heat Δc is an analytic function over the whole temperature range and the susceptibility Δχ shows a cusp at the transition temperatureTc of the basic system.

Neon impurity effects in the specific heat of solid D2

Measurements have been made of the specific heat of solid deuterium with neon impurity concentrations of between 1 and 4%. In addition, the para-D2 concentration of the samples studied has been varied between 0.9 and 6%. An excess specific heat ΔC was observed above approximately 1.4 K which was proportional to the Ne concentration and independent of the para-D2 concentration. ΔC is attributed to resonance modes involving the heavy neon atoms randomly distributed throughout the host lattice. The values of ΔC are qualitatively in general agreement with classical theory, with the quantitative differences due to the quantum nature of the solid.

A macroscopic calculation of the specific heats of a vacancy

AbstractThe constant pressure specific heat Δc, associated with the temperature variation of the formation enthalpy of a vacancy, is calculated from macroscopic properties of the solid. A similar calculation of the corresponding “constant volume” specific heat Δc shows that its absolute value is equal to a few tenths of k. The value of Δc is positive whereas that of Δc is negative, in agreement with recent considerations of Gilder and Lazarus.

Spin-glass with short-range interaction in a magnetic field

The properties of spin-glasses with nonmagnetic defects are considered in the vicinity of the “transition point” in a finite magnetic field, by means of a percolational approach. It is shown that the dependence of the magnetic specific heat δC(h) = C(h)−C(o) or of the magnetic susceptibility ∂M(h)/∂h on field and temperature allows us to define the distribution function of finite clusters with respect to their size. In addition to general formulas resulting from the scaling laws, more detailed formulas are obtained for two model distributions: (a) the Bethe lattice distribution, (b) a special model distribution with arbitrary indices.

Toward Simple Powers of T in the Kondo Effect?

Experimental evidence is presented which strongly suggests that simple power laws govern the temperature dependence of ground-state properties of dilute magnetic alloys exhibiting the Kondo effect. The systems Cu–Fe, Au–V, Pd–Cr, and Pt–Cr are discussed to show that in most cases the impurity specific heat ΔC∝T and the temperature-dependent part of the electrical resistivity Δρ∝−T2, provided the concentration of impurities is low enough. If this concentration is increased above some critical value, the temperature dependences become more complicated.