Transport properties of bulk and thin-film MgB2 superconductors:: effects of preparation conditions

Two sets of bulk superconducting samples of MgB2 were synthesized and analyzed by X-ray diffraction technique. The first set was prepared by annealing in vacuum, whereas samples belonging to the second one were annealed in Ar atmosphere. The resistive critical temperature of the superconducting transition, T-c, for bulk samples was with one exception found to be in the range 34-39 K depending on the preparation conditions. Temperature, T, dependences of resistivity, rho, and thermoelectric power coefficient, S, were measured. The curves rho(T) were well approximated by the polynomial rho(0) + rho(3)T(m), where 2.5 less than or equal to m less than or equal to 3. The values of rho(T-c) differed substantially for various annealing conditions. The quantity S was linear in T at low temperatures and tended to 9-14 muV/K at room T. Thin-films of MgB2 were prepared by pulsed laser deposition method using stoichiometric bulk MgB2 as a target and MgO and LaAlO3 substrates. The T-dependences of the resistance, R, and S(T) for films were measured and found similar to those of bulk samples. The behavior of R(T) was studied in the external magnetic fields B up to 8 T. The largest applied fields strongly suppressed the superconducting transition and moderately enhanced its width. (C) 2003 Elsevier B.V. All rights reserved.