Considerable rise in the stability of combined superconductors doped by intermetallic compounds with an extremely high low-temperature specific heat

The influence of doping intermetallics based on rare-earth elements and ceramics with an extremely high low-temperature specific heat on the stability of combined NbTi superconductors against electromagnetic disturbances is studied experimentally and theoretically. The objects of investigation are standard NbTi conductors (0.85 mm in diameter) in a copper matrix that are soldered to copper wires containing high-specific-heat dopants. CeCu₆, HoCu₂, CeAl₂, and PrB₆ intermetallics (at 4.2 K, their specific heat is on average 150 times that of copper) or Cd₂O₂S ceramics (at 4.2 K, its specific heat is higher than that of copper by 600 times) are introduced into combined superconductor samples in the form of fine powder. The powder is placed into the copper wire either as one thick (0.6 mm in diameter) strand or as 19 thin (0.1 mm in diameter) strands. An undoped reference sample contains a solid conductor. Samples with a transport current placed in an external transverse magnetic field are subjected to longitudinal magnetic disturbances causing pulsed heating of the samples by eddy currents. The disturbance time is varied in a wide range: from 50 μs to 1.2 ms. To compare the critical energy densities of the disturbances in differently shaped samples, a mathematical technique is developed that is based on analytical solution of the equation of electromagnetic diffusion into the sample. It is found that the critical energy density of doped samples is much higher (by several times) than that of the reference sample. Upon direct cooling of the samples by liquid helium in a vertical channel (the most intense heat removal under steady-state conditions), the effect of stability improvement for the doped samples persists. Moreover, it is shown that the high-specific-heat dopants, raising the heat needed to warm the sample, restrict the heat flux into the liquid and thereby increase the energy removed by the coolant during unsteady heat transfer.     

Dimensionality of extremely thin superconducting films

Superconducting fluctuations above the transition temperature are strongly dependent on the dimensionality of the samples. Our experiments on extremely thin quench condensed aluminum films show transitions from zero- to one-, two- and three-dimensional behaviour at an average film thickness of 20, 50 and 100 Å respectively.     

Gasless combustion wave with extremely high excess enthalpy under heat loss conditions

A numerical experiment on investigation of a gasless combustion wave propagating at the limit due to a high excess enthalpy in the heating zone is performed. Under heat loss conditions, waves emerge on the surface of the combustion front, causing changes in the rate of chemical reactions similar to those realized on the burning surface of propellants and explosives.     

Specific heat of the superconducting high pressure phase Ga II

The high pressure modification gallium II, which becomes superconducting near 6.4 K, has for the first time been investigated calorimetrically at about 35 kbar in the temperature range of liquid helium. From the data a Debye temperature _D = 178K and an electronic specific-heat coefficient =1.75 mJ/mole K² are obtained. These values are in good qualitative accord with the increased superconductingT _c with respect to that of Ga I.Work supported by the Deutsche Forschungsgemeinschaft.Dedicated to Prof. Dr. W. Buckel on the occasion of his 60th birthday.     

Effect of plastic deformation of V,Nb and Ta on the superconducting transition temperature and the specific heat coefficients

Specific heat measurements performed between 1.4 and 20 K on bulk and cold-worked V, Nb and Ta superconducting materials are presented. The plastic deformation produces an increase in the superconducting transition temperature T_c, an increase which is relatively less important for Ta than for Nb and less for Nb than for V. An increase is registered for the normal linear coefficient of specific heat γ whereas the Debye temperature decreases slightly. The apparent relation between the increase of T_c and γ suggests qualitatively that the vibrating mobile dislocation contribution is not the only origin of the increase of γ but that a band structure contribution is also to be taken into account.     

Transition temperature and specific heat discontinuity for superconducting alloys containing Kondo impurities

The dependence of the transition temperature and the specific heat discontinuity of a superconductor on the magnetic impurity concentration is calculated selfconsistently using the spin-boson approach for the Kondo problem. The results agree well with experimental dat for the (La, $\text{Ce}$) Al₂ alloy.     

Very low temperature specific heat of crystalline zirconium in normal and superconducting states

Specific heat measurements of zirconium between 0.03 and 1.2 K in both normal and superconducting states are reported. In the normal state a purely linear electronic contribution is observed down to 0.1 K; at lower temperatures there appears the onset of a nuclear hyperfine contribution which is unobservable in the superconducting state within our experimental time scale.     

超导变压器绕组带材抗短路冲击特性研究

为了研究超导变压器绕组带材的抗短路冲击的能力,制备了四组实验样品,用以模拟超导变压器原、副边绕组的运行环境,实验研究了超导带材并联不锈钢带材的抗短路冲击特性。实验结果表明,超导带材并联不锈钢带材后的抗短路冲击能力有明显的提升,这对超导变压器的研发与制作具有重要的应用参考价值。     

Low temperature specific heat of KCl: OH with high OH concentration

We report on measurements of the specific heat (between 0.035 and 3 K) of two KCl crystals doped with 0.5 and 1 at.% OH^-. For the 1 at.% sample, the excess specific heat C due to OH^- varies roughly linearly between 0.035 and 0.6 K. A similar behaviour is also found for 0.5 at.% between 0.2 and 0.6 K while below 0.2 K, C ～ T^1.8. Thus, contrary to previous measurements, a T^{$\text{3}\text{2}$} dependence is not observed. We discuss our results in terms of strain and electric dipolar interactions.     

Effect of high pressure on the superconducting transition temperature of Pd-H

Under hydrostatic pressure a negativedT _c /dp comparable to that of non-transition superconductors has been found for Pd-H. A simple model is discussed, which is able to explain the isotope-effect in Pd-H(D), and which is in agreement with the observed pressure effect.     

Low-temperature specific heat of CeAl2 at high pressures

Abstract

We have obtained new data of the low-temperature specific heat of CeAl₂ under pressure with the aim to complete the phase diagram and to solve an existing discrepancy between results of magnetic and thermodynamic measurements.     

Effect of vortex-antivortex fluctuations on the heat capacity of a type-II superconducting film

The small size vortex-antivortex pairs proliferation in a type-II superconducting film is considered below T(c). The corresponding contribution to the free energy is calculated. It is shown that these fluctuations give the main temperature dependent contribution to the heat capacity of the superconducting film in the sufficiently large interval of temperatures below the transition point.     

Tunneling anomaly in junctions of extremely thin superconducting metal films

The tunneling spectra of extremely thin Al, In and Pb films show an increase of the dynamic resistance in the low bias range. This tunneling anomaly can be explained by size quantization of the electronic states in small particles.     

Effect of high pressure on raman spectra of T1-based superconducting crystals

Abstract

Raman spectra for two members of the high-Tc superconductor family T1_m Ba₂ Ca₂ cuo_6+m were measured at pressure up to 3 0 GPa. It is shown that pressure induces nonuniform deformation of the unit cell. Analysis of Raman shifts indicates the applicability of a Gruneisen model for these high-Tc superconductors. As evidenced by the absence of changes of the Raman spectra, no phase transitions occur in the pressure range studied.     

Theory of specific heat of vortex liquid of high T_c superconductors

Superconducting thermal fluctuation(STF) plays an important role in both thermodynamic and transport properties in the vortex liquid phase of high T_c superconductors.It was widely observed in the vicinity of the critical transition temperature.In the framework of Ginz burg-Landau-La wrence-Doniach theory in magnetic field,a self-consistent analysis of STF including all Landau levels is given.Besides that,we calculate the contribution of STF to specific heat in vortex liquid phase for high T_c cuprate superconductors,and the fitting results are in good agreement with experimental data.     

Specific heat of superconducting Zn nanowires

We present heat capacity measurements on crystalline Zn nanowires with diameters of 230 and 23 nm, bracketing the superconducting coherence length of 155 nm. Transport measurements on superconducting nanowires have found a crossover from three-dimensional to one-dimensional behavior as the wire diameter was reduced below the coherence length. In contrast, the normalized heat capacity peak of the 23 nm Zn nanowires is found to be nearly identical to that of 230 nm wires and bulk Zn, indicating their thermodynamic properties remain three dimensional.     

Porous piezoelectric composites with extremely high reception parameters

The reception parameters Q ₃₃ ^* and Q _h ^* of modified fiber composites based on high-e ₃₃ ^FC /c ₃₃ ^{FC, E} ferroelectric piezoceramics (e ₃₃ ^FC is the piezoelectric constant, c ₃₃ ^{FC, E} is the elastic modulus) are considered as functions of the electromechanical properties and the porosity of the components. Pore configurations at which the values (Q ₃₃ ^* )²≈(Q _h ^* )²≈7×10⁻⁹ Pa⁻¹ for the polymer matrices of 1-0-3 composites are much higher than for conventional ternary composites are analyzed.     

Effect of size on the vibrational specific heat of ultrafine palladium particles

The specific heat of Pd particles smaller than 100 Å was measured between 1.4 and 30 K. The observed enhancement of the vibrational part in comparison with the bulk is discussed in terms of a Bose—Einstein summation of the vibrational modes as proposed by Baltes and Hilf. Satisfactory agreement between experiment and theory is found when size and temperature dependent particle sound velocities are assumed. This involves a considerable reduction of the Debye temperature of the Pd particles comparable with the results for other metals.