The magnetic properties of superconducting alloys. part i

Thermal conductivity (κ) and thermopower (S) of two series of high-temperature superconductors (HTSCs) with a general formula Sm1+xBa2−xCu3Oy have been measured. Both series originate from the same sample of Tc=90.4 K and extend to the nonsuperconducting, antiferromagnetic phases. The first series is of a 1:2:3 cation stoichiometry (x=0) and variable oxygen content (6.11<y<6.85), the second one is a series of solid solutions with a variable Sm/Ba ratio (0<x<0.6). For a solid solution series, regular changes in transport properties were observed with decreasing Tc: a gradual decrease of κ and increase of S at all temperatures, while the type of temperature dependences of respective quantities remained still characteristic for HTSCs. Therefore, the last, nonsuperconducting sample from this series may be used as a reference for estimation of the lattice thermal conductivity for superconducting samples. As it turned out, the stoichiometric series is different. The thermal transport properties of the most oxygen deficient sample (y=6.11) resembles a classical, poorly conducting material more than a HTSC. The κ(T) dependence for this sample reveals a broad maximum around 210 K. The S(T) curve for this sample follows a 1/T dependence like for a nondegenerate semiconductor with the activation energy of ∼0.3 eV. This value agrees well with that estimated from the ρ(T) dependence. The dc magnetization measurements carried out for the nonsuperconducting samples suggest that the origin of the difference in their thermal transport properties is not magnetic.

Thermal transport in Sm1+xBa2−xCu3Oy solid solution

The electric and magnetic measurements have been carried out on a polycrystalline single phase SmBa2Cu3O7 sample. The normal state susceptibility of it consists of a temperature-dependent Curie-Weiss paramagnetic term with an effective Bohr magneton number peff=1.1 and the Curie-Weiss temperature θ=38 K and temperature-independent terms: the Van Vleck and Pauli paramagnetism and the ionic core and Landau diagmagnetism. The Pauli susceptibility derived from the temperature-independent normal-state susceptibility of YBa2Cu3O7 is so large that it is due to the strong electron-electron interactions among conduction electrons. We deduce the low critical field Hc1(1.5 K), which is 580 Oe and the critical magnetization current density which is of the order of 104 A/cm2 at 5 kOe and decreases with increasing applied field from the magnetization hysteresis loop of SmBa2Cu3O7 for fields up to 60 kOe and temperature at 1.5 K. Some magnetic and superconductive parameters such as Hc(0), kGL, χ, λ, etc. are also given by using the values of the low and upper critical fields and the energy gap within the Ginzburg-Landau theory.

X.S. Chen

Papers

Magnetic characteristics of normal and superconductive states in smba2cu3o7