Magnetic and high pressure studies in the YPd5B3C3 system

The macroscopic properties of the superconducting phase in the multiphase compound YPd5B3 C.3 have been investigated. The onset of superconductivity was observed at 22.6 K, zero resistance at 21.2 K, the lower critical field Hel at 5 K was determined to be Hel (5) rv 310 Gauss and the compound was found to be an extreme type-II superconductor with the upper critical field in excess of 55000 Gauss at 15 K. From the upper and lower critical field values obtained, several important parameters of the superconducting state were determined at T = 15 K. The Ginzburg-Landau paramater was determined to be ~ > 9 corresponding to a coherence length ~ rv 80A and magnetic penetration depth of 800A. In addition measurements of the superconducting transition temperature Te(P) under purely hydrostatically applied pressure have been carried out. Te(P) of YPd5B3 C.3 decreases linearly with dTe/dP rv -8.814 X 10-5 J

Magnetic properties of the Biâ Srâ CaCuâ Oâ single crystal

The Bi2Sr2CaCu20g single crystal with a superconducting transition temperature equal to 90 ± 2 K was prepared. The irreversibility line of the single crystal for a mgnetic field direction along the c-axis and T* in the ab-plane was determined. The reduced temperature (l - T ) is proportional to H 1.1 for fields below 004 T and proportional to HO.09 for fields above 0.4 T. The zero temperature upper critical field Hc2(0) and coherence length ~ (0) were determined from the magnetization meaurements to be H-lC2=35.9T , H//C2=31.2T, ~c(0)=35.0 A, and ~ab(0)=32.5A,and from the magnetoresistance measurements to be H-lc2 = 134.6T , H//C2=55.5T '~c(0)=38.1 A, and ~ab(0)=2404 A for both directions of the applied magnetic field. The results obtained for Hc2(0) and ~(O) are not reliable due to the rounding that the single crystal exhibits in the magnetization and magnetoresistance curves. The magnetization relaxation of the single crystal was investigated, and was found to be logarithmic in time, and the relaxation rate increases with temperature up to 50 -60 K, then decreases at higher temperatures.

The Effects of Annealing URu2Si2 on the Resistivity and Meissner Effect

The enigmatic heavy fermion URu2Si2, which is the subject of this thesis, has attracted intensive theoretical and experimental research since 1984 when it was firstly reported by Schlabitz et al. at a conference [1]. The previous bulk property measurements clearly showed that one second order phase transition occurs at the Hidden Order temperature THO ≈ 17.5 K and another second order phase transition, the superconducting transition, occurs at Tc ≈ 1 K. Though twenty eight years have passed, the mechanisms behind these two phase transitions are still not clear to researchers. Perfect crystals do not exist. Different kinds of crystal defects can have considerable effects on the crystalline properties. Some of these defects can be eliminated, and hence the crystalline quality improved, by annealing. Previous publications showed that some bulk properties of URu2Si2 exhibited significant differences between as-grown samples and annealed samples. The present study shows that the annealing of URu2Si2 has some considerable effects on the resistivity and the DC magnetization. The effects of annealing on the resistivity are characterized by examining how the Residual Resistivity Ratio (RRR), the fitting parameters to an expression for the temperature dependence of the resistivity, the temperatures of the local maximum and local minimum of the resistivity at the Hidden Order phase transition and the Hidden Order Transition Width ∆THO change after annealing. The plots of one key fitting parameter, the onset temperature of the Hidden Order transition and ∆THO vs RRR are compared with those of Matsuda et al. [2]. Different media used to mount samples have some impact on how effectively the samples are cooled because the media have different thermal conductivity. The DC magnetization around the superconducting transition is presented for one unannealed sample under fields of 25 Oe and 50 Oe and one annealed sample under fields of 0 Oe and 25 Oe. The DC field dependent magnetization of the annealed Sample1-1 shows a typical field dependence of a Type-II superconductor. The lower critical field Hc1 is relatively high, which may be due to flux pinning by the crystal defects.