Universality in the heat capacity of magnetic and superconducting solids for T→0

Experimental heat capacity data of various magnetic and superconducting solids are analyzed empirically. It is observed that in materials in which magnetic and lattice order coexist, the heat capacity approaches zero for T→0 by a single universal power function and not by a sum of two distinguished power functions due to magnetic and lattice subsystem. The observed universal exponent is commonly defined by the lattice degrees of freedom, i.e., by Debye's T3 function. This means that the magnetic heat capacity is not relevant and it contributes to the pre-factor of the T3 function only. Also in the coexistence of superconducting and lattice order, single power function behaviour is observed in the heat capacity for T→0. Only the disordered, i.e., normal conduction electrons give rise to the well-known additive power term ∼T for T→0. This additive term is observed also in the metallic ferromagnets iron, nickel and cobalt. Three universal lattice exponents of ε=2, 3 and 4 are distinguished for T→0. In superconductors, a further exponent of ε≅5 seems to exist asymptotically for T→0. It is furthermore shown that the heat capacity is particularly rich in crossover phenomena. In MnF2 and FeF2, crossover to T3/2 function instead of the common linear T dependence is observed for higher temperatures. In superconducting Al, V and Ga, this crossover is from T5 to T2.