Chemical potential pinning near a van Hove singularity in the cuprates

The Fermi energy is shown to be pinned near a van Hove singularity for an extended doping range in the one-band and three-band Hubbard models as a consequence of filling dependent band renormalizations.     

Optical Conductivity in the van Hove Scenario

We calculate the conductivity in the ab plane for a superconductor assumed to have a saddle point in the energy surface due to the logarithmic density of states. The influence of the optical phonons was taken into account. At sufficient high temperatures (corresponding to the normal phase) the conductivity is characterized by a Drude-like behavior.     

Pair-Breaking Effect in a van Hove Superconductor

he pair breaking effect in the critical temperature has been considered. We showed that the decrease in the critical temperature can be described by an Abrikosov-Gorkov-like formula which contains the correction due to the van Hove density of states.     

Spin wave contribution to the specific heat of ferromagnetic terbium at low temperatures

The magnon contribution to the specific heat of ferromagnetic terbium is evaluated and the results are compared with the measurements from 90 K down to absolute zero.     

On the anharmonic contribution to the specific heat of monatomic face-centered cubic crystals

We have presented a method for an exact calculation of the two lowest-order, cubic (F ₃) and quartic (F ₄), perturbation terms in the Helmholtz free energy (F) of an anharmonic crystal in the high temperature limit. The method is applicable to a nearest-neighbor central force model of a monatomic fcc crystal for any phenomenological two-body potential (r). The calculation of F ₃ and F ₄ requires a knowledge of the six dimensionless Brillouin zone (BZ) sums, as a function of a parameter a ₁ depending on the first and second derivatives of (r). These sums are calculated to a high degree of accuracy for a mesh size of 308,000 points in the whole BZ in the range –0.1a ₁+0.1 in steps of 0.02. The linear temperature dependent anharmonic contribution to the specific heat at constant volume, calculated for the elements Pb, Ag, Ni, Cu, Al, Ca, and Sr, from the Morse and Rydberg potentials, is found to be positive in all cases, with the exception of Pb. In this case the Morse potential gives a negative sign. The predictions of theory are in agreement with experiments where the data is available (e.g., Cu, Al, and Pb).     

Transient heat transfer model for normal zone propagation. Part 1 — theory of a bare helium-cooled superconductor

The paper presents a physical model, involving no adjustable parameters of transient heat transfer for normal zone propagation in a composite helium-cooled superconductor. Evaluations are given showing that the main contribution to transient heat flux is made by transverse heat conduction in the coolant. When the zone front passes through a fixed point, the temperature at that point increases drastically, a great temperature gradient appears in coolant which results in transient cooling of the superconductor. The zone propagation equation becomes an integro-differential with a convolution integral. The transient heat transfer intensity is characterized by the introduced dimensionless parameter ?.The problem has been solved analytically for the model with constant coefficients and jump-wise heat release. Formulae have been obtained for the propagation and recovery velocities describing the effect of transient heat transfer (via parameter ?).     

Minimum heat pulse to quench a superconducting magnet

The minimum energy required to quench a fully impregnated superconducting winding has been measured at a constant field of 5 T for various currents. Great care has been taken to match the experimental conditions with those presumed in the minimum propagating zone (MPZ) treatments of the situation. In particular the winding has been designed so that the MPZ is smaller than the heat source thus satisfying the requirement for a point disturbance. The adiabatic requirement has been met with an inductive heating technique in place of the usual resistive heating. As a consequence of these features the minimum quench energies are much smaller than those obtained in previous experiments and agree well with Wilson's theoretical treatment for a point disturbance.     

Response of superconductors to fast heat pulses

We show that for magnets operating close to their short sample limit, the energy density in a fast heat pulse, applied at the edge of the conductor, that is just sufficient to quench the magnet, is typically a small fraction of the reserve enthalpy/cm of the conductor. The minimum energy density needed to quench can be expressed as a simple, and universal function of the physical parameters of the conductor and the operating temperature.     

Determination of effective specific heat of nanofluids

The effective specific heat of several types of nanofluids are measured by transient double hot-wire technique. Sample nanofluids are prepared by suspending 1–5 volume percentages of titanium dioxide (TiO₂), aluminium oxide (A1₂O₃) and aluminium (Al) nanoparticles in various base fluids, such as deionised water, ethylene glycol and engine oil. The effective specific heats of these nanofluids were found to decrease substantially with increased volume fraction of nanoparticles. Besides particle volume fraction, particle materials and base fluids also have influence on the effective specific heat of nanofluids. Except Al/engine oil-based nanofluid, predictions of the effective specific heat of nanofluids by the volume fraction mixture rule-based model showed reasonably good agreement with the experimental results. Based on the calibration results obtained for the base fluids, the measurement error is estimated to be within 2.77%.     

The Gap-to- Tc Ratio of a van Hove Superconductor

We give an analytical expression for the gap-to-T _c ratio (R) of a superconductor with a van Hove singularity in the density of states. Our calculation yields R in very good agreement with the results obtained numerically by S. Ratanaburi et al. [J. Supercond. 9, 485 (1996)].     

Determination of the thermal diffusivity and specific heat using an exponential heat pulse,including heat-loss effects

The one-dimensional heat diffusion equation has been solved analytically for the case of a heat pulse of the form F(t) = exp(–t/)/ applied to the front face of a homogeneous body including the effects of heat loss from the front and back faces. Approximate expressions are presented which yield a simple, accurate technique for the determination of the thermal diffusivity and specific heat, suitable to a wide range of heat-pulse time constant and heat-loss parameters, without recourse to graphical techniques or requiring further computer analysis. A procedure is described for the determination of an effective time constant to allow application of the present results to the case of a nonexponential heat pulse. Experimental results supporting the theoretical analysis are presented for five samples of silicon germanium alloys of various thicknesses, determined using a xenon flash tube heat-pulse exhibiting an exponential dependence. Proper consideration of the experimental heat pulse shape is shown to lead to reliable corrections to the apparent thermal diffusivity, even for relatively long heat-pulse times.Paper presented at the Tenth Symposium on Thermophysical Properties, June 20–23, 1988, Gaithersburg, Maryland, U.S.A.     

An evaluation of specific heat measurement methods using the laser flash technique

Different methods for adapting the laser flash technique to measure simultaneously specific heat have been proposed in the literature. Among them are the coating method, the absorbing disk method, the double-specimen method, the pulse heating-cooling method, and the cavity method. These methods are briefly reviewed, and their merits and demerits are evaluated.     

Gap-to-T c ratio as a function of the Fermi level shift

Within the framework of the BCS theory, the gap-to-T_C ratioR=2‡ ₀/kT_c is evaluated numerically (‡₀ is the energy gap atT = 0 andT> _c is the critical temperature) for a superconductor with a van Hove singularity (vHs) in the density of states as a function of the shifts (δ) of the Fermi level with respect to the vHs. It is found thatR varies asymmetrically with δ and that the variations are strong near δ = 0. Our numerical calculation shows that the largest R’s occur at certain values of δ⊋0.     

NiTi合金热容的测定及传热的研究

使用调制式差示扫描量热器（MDSC）精确测定了Ni-Ti合金的比热容。研究了以不同的速度加热和冷却合金时相变点的变化,并用试验测定的热容值,计算了电流加热的NiTi合金丝的温度变化历程,这将为科学合理地计算加热时合金的温度场提供参考信息。     

Simultaneous measurements of specific heat and total hemispherical emissivity of chromel and alumel by a transient calorimetric technique

Using a transient calorimetric technique, the specific heat and total hemispherical emissivity of chromel and alumel were measured simultaneously in the temperature range 360–760 K. Two types of specimens for each material were prepared. To obtain reliable experimental values of specific heat and total hemispherical emissivity, an expression for the time history of the temperature of the specimens was developed; this expression is accurate over the whole temperature range. An error analysis is made and the uncertainty (the total error) in the values of specific heat and total hemispherical emissivity is estimated to be 3.1% for the well-designed specimens.

     

Stress dependence of specific heat: observations on the TERSA technique

Recently a new method of residual stress measurement, TERSA, was described₁and it was suggested that there was a contribution to the effects reported from the stress dependence of specific heat. In this paper, experimental and theoretical evidence is presented demonstrating that the magnitude of the change of specific heat with stress is too small to make a significant contribution to the change in temperature resulting from a stress increment applied to a laser irradiated object.     

On the relationship between gap edge and specific heat of a superconductor

We critically examine three empirical procedures that have been proposed and used in the literature to extract, from electronic specific heat curves in the superconducting state, a value for the zero temperature energy gap. We do this by calculating, from Eliashberg theory, the electronic specific heat for many superconductors for which the electron-phonon spectral densities are known. Model spectral densities are also considered so as to test, in more extreme cases, the proposed procedures. We conclude that only one case, a modification of an empirical method, yields quantitatively reliable results.     

Low temperature specific heat of Dow Corning Silicon Oil No 704 between 2 and 18 K

The specific heat of Dow Corning Silicon Oil No 704 was measured between 2 and 18 K. Below 5 K it was found to follow a T³ law. Throughout the range it could be fitted with a 5th order polynomial in T.     

Surface contribution to the specific heat of he II films

The surface contribution to the specific heat of saturated and unsaturated He II films has been considered. The temperature dependence of the surface-contributed specific heat is found to depend on the range of temperatures as well as the film thickness under consideration. A comparison with earlier results is made, and it is found that those results are not consistent with the specific heat contributed by the capillary wave modes of the free surface.     

Accurate determination of specific heat at high temperatures using the flash diffusivity method

The flash diffusivity method can be extended, very simply, to measuring simultaneously thermal diffusivity and specific heat and thus obtaining the thermal conductivity directly. This was accomplished by determining the amount of heat absorbed by a sample with a well-known specific heat and then using this to determine the specific heat of any other sample. The key to using this technique was to have identically reproducible surfaces on the standard and the unknowns. This was achieved earlier by sputtering the surfaces of the samples with a thin layer of graphite. However, the accuracy in determining the specific heat was within ±10% and there was considerable scatter in the data. Several improvements in the technique have been made which have improved the accuracy to ±3% and increased the precision. The most important of these changes has been the introduction of a method enabling the samples to be placed in exactly the same position in front of the light source. Also, the control of the thickness and the application of the graphite coating have turned out to be very important. A comparison of specific heats obtained with this improved technique and with results obtained using other techniques has been made for two materials.Paper presented at the Tenth Symposium on Thermophysical Properties, June 20–23, 1988, Gaithersburg, Maryland, U.S.A.