Specific heat of type-II superconductors in magnetic fields nearT _c

Abstract

The GLAG theory predicts that the value of the specific heatC _M in the mixed state just below the transition to the normal state, is a function of the Ginzburg—Landau (GL) parameter κ, even for vanishingly small magnetic fields(T≈T _c ).C _M is thus different from the specific heatC _S in the pure superconducting state(H=0) for all κ except κ≅1.9. Experimental results of the authors generally confirm this paradoxical prediction which is not in contradiction with the thermodynamics of phase transitions. To experimentally clarify this behavior close toT _c , Nb₈₀Mo₂₀(κ≅4) and Pb₉₈In₂(κ≅0.8) specimens are studied in fields much smaller than those usually applied (down to17 Oe). A new method for measuring specific heat in decreasing as well as increasing temperature is introduced. Very good agreement with the theory is found for fields higher than ∼120 Oe while a deviation is observed close toT _c for lower fields where values of the Abrikosov lattice parameter would become larger than 1 µ. The sense and the magnitude of the deviation are such thatC _M andC _S would be equal for fields of 20 Oe and below. Possible explanations are discussed. It is suggested that a fundamental modification of the vortex state structure occurs in small fields close toT _c .