Superconducting study of the intermettalic YNi₂B₂C single crystal and several high-Tc materials

Author

Kyu Jeong Song

Date of Award

8-1999

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Physics

Major Professor

James R. Thompson

Committee Members

T.A. Callcott, M.Breinig, A.J. Pedraza

Abstract

Magnetization studies on a single crystal of YNi₂B₂C superconductor have revealed significant effects of nonlocality in the superconductive state and have displayed a significant angular variation in normal state. The 17 mg crystal was studied at temperatures Τ from above Τ_c(15 K) to 2 ~ 3 K, in magnetic fields Η applied parallel and perpendicular to the (001)-crystal axis, within magnetic fields from zero to Η Η_c2, the upper critical field. The material exhibited little magnetic irreversibility, with a critical current density ~ 10^-4 x J₀, the depairing current density. This nearly reversible behavior has allowed an analysis of its equilibrium properties: the thermodynamic critical field H_c(T), H_c2(T), and the magnetization M(H,T) in both the normal and superconductive states. Near T_c, the equilibrium magnetization M of the clean single crystal of YNi₂B₂C was standard London-like with M α ln(H) Well below T_c however, M is shown to deviate significantly from this simple "local" London predictions, but the behavior is well described by "non-local" London theory, which is a more general theory derived by Kogan et al. [Phys. Rev. B 54, 12386 (1996)]. The non-local analysis yields reasonable values for the nonlocality radius ρ and London penetration depth λ. The T dependence of λ was obtained from both non-local London analysis at low temperatures and a standard local-London analysis near Τ_c. Contrary to the exponential dependence expected for simple s-wave pairing, the nearly Τ³behavior for λ(7) below 10 K seems to give evidence for a more complex, perhaps non-jwave pairing scheme. In addition, the normal state magnetic susceptibility was measured -IVin the temperature regime between 16 K and 295 K in an applied field of 10 kG, for the magnetic field applied parallel or perpendicular to the crystalline (OOl)-direction. The material exhibited a large anisotropy between the two field orientations, particularly in the low temperature regime. Furthermore, according to heat capacity studies of YNi₂B₂C, the material appears to deviate from both weak- and strong-coupling superconductive mechanisms, but agrees relatively well with predictions [C_e,_super∝t³] of a medium-coupling formalism. From magnetization and heat capacity studies, the deduced values of the Ginzburg-Landau parameters κ₁ and κ₂ increase considerably as T decreases. This is consistent with the material's long electronic mean fi-ee path and the observation of non local electrodynamics.

Several features of high temperature superconductors were investigated in complementary work. In studies of the effects of adding elemental Ag to high Τ_c superconducting HgBa₂Cu0_{4+ materials, a series of polycrystalline AgJIgBa2Cu04+j materials (with molar fraction x = 0, 0.05, 0.1, 0.3, and 0.5) were investigated. The processing with Ag at elevated temperatures led to changes in superconducting properties. These are consistently interpreted in terms of the superconducting hole density, calculated from the London penetration depth λ by analysis of the equilibrium magnetization M using standard London theory. The irreversible magnetic properties of these materials are dominated by surface barrier effects and are well described in terms of thermally activated tunneling of pancake vortices through a surface barrier.}

For practical applications, vortex pinning in high-T_c superconducting (HTS) materials is very important. To pin vortices strongly, splayed columnar tracks produced -Vusing a fission process, induced by high energy (GeV) proton irradiation, have been formed in several HTS materials. Overall, the magnetic hysteresis ΔM (α J) of the materials is greatly increased by the splayed columnar defects. The hysteresis or persistent current density first increases with increasing the proton fluence Φ_p, then passes through an optimal proton fluence, and finally decreases at much higher Φ_p. In contrast with ΔM (α J)that is enhanced significantly by the columnar defects, the superconducting transition temperature T_c is suppressed somewhat, with a material-dependent rate. By analyzing the decay rate of J with a time in a Maley analysis, the effective pinning energy U(J) was obtained with both irradiated and unirradiated materials. The net pinning potential barrier of vortices is clearly enhanced by the splayed columnar tracks, from 0.8 GeV proton irradiation. In general, these splayed columnar defects lead to significant enhancements in the vortex pinning effect within the HTS materials investigated.

Recommended Citation

Song, Kyu Jeong, "Superconducting study of the intermettalic YNi₂B₂C single crystal and several high-Tc materials. " PhD diss., University of Tennessee, 1999.
https://trace.tennessee.edu/utk_graddiss/8930