W.H. Lee

Superconductivity in the Ni based ternary carbide LaNiC2

Abstract Electrical-resistivity, magnetic-susceptibility and specific-heat data reveal that the bulk phase transition at 2.63 ∼ 2.86 K (10 ∼ 90% values) in LaNiC 2 is to a superconducting ground state. The normal-state specific-heat data can be fitted to the expression C n = γT + βT 3 (3 K T γ = 7.83 mJ/mol K 2 and β = 0.0635 mJ/mol K 4 , resulting in Debye temperature θ D = 496 K. A special emphasis is on the specific heat in the superconducting state below 1.6 K; C s is not exponential but has a power-law form: C s ≈ 3.5( γT c )( T / T c ) 3 . According to the observed specific-heat data, we suggest that LaNiC 2 belongs to a nonconventional BCS superconductor. In addition, LaNiC 2 is the first nickel-based ternary-carbide superconductor.

Formation mechanism of 110 K high-Tc superconducting phase in the Ca- and Cu-rich Bi-Pb-Sr-Ca-Cu-O system

Abstract The starting compositions of Bi 1.7 Pb 0.4 Sr 2− x Ca 2+ x Cu 3.6 O y ( x ⩾0) have been studied to illustrate the formation mechanism of 110 K superconducting phase. It was proposed that a reactive liquid-phase-aided sintering process is responsible for the promotion of 110 K phase in the Pb-doped Bi-Sr-Ca-Cu-O system. The Ca-rich stoichiometry containing more Ca 2 PbO 4 in calcined powder would result in more reactive liquid phase during the sintering period. The acceleration of the formation of 110 K phase and large grain growth were observed. A nearly monophasic 110 K superconducting phase can be easily obtained within 20 h when appropriate stoichiometry and sintering conditions were met.

High transport critical currents and flux jumps in bulk YBa2Cu3Ox superconductors

A preferentially oriented bulk YBa2Cu3Ox superconductor was prepared by the liquid phase method. A continuous dc current carrying capacity exceeding 120 A with current density higher than 37 300 A/cm2 at 77 K has been obtained. The high current effects causing fracture of the sample were studied in relation to the thermal diffusion properties. In magnetization measurements, flux jumps were observed for currents in the sample a‐b plane below the temperature of about 20 K. The jumps are more closely spaced with decreasing temperature or increasing field sweep rates.

Formation mechanism study of REBa2Cu4O8+δ (RE=Y and Dy) accelerated by nitric acid

An accelerated formation of REBa2Cu4O8+δ (RE=Y and Dy) was observed as a suitable amount of nitric acid was added. This provides an excellent route for preparing high‐ purity REBa2Cu4O8+δ in ∼ 30 h. X‐ray diffraction and thermal analyses (TGA/DTA) were conducted to reveal the formation mechanism. The addition of nitric acid did not change the reaction routes but formed more Ba2Cu3O5 which might play an important role for the formation of REBa2Cu4O8+δ. Magnetization data confirmed the Tc’s of 80 and 75 K for as‐prepared Y‐124 and Dy‐124, respectively.

Changes in doping state of Tl(Sr2−xRx) CaCu2O7−δ (R = Sm, Eu, Dy) system

Abstract Static magnetization and electrical resistivity measurements on 30 samples in the three series Tl(Sr2−xRx)CaCu2O7−δ (R = Sm, Eu, Dy, 0.1 ≤ x ≤ 1.0) reflect the smooth transformation of the doping state from overdoped to underdoped regime with the R-doping level in each series. The optimal superconducting critical temperature Tc value around 80 K occurs for the sample with x ∼ 0.4 for R = Sm, Dy and with x ∼ 0.5 for R = Eu. The resistivity of the nonsuperconducting samples with x = 0.9 and 1.0 follows the 3-dimensional Mott variable-range-hopping (VRH) dependence at temperatures below ∼ 100 K.

Preparation of high purity 110 K phase in the (Bi, Pb)-Sr-Ca-Cu-O superconductor system using a solution method

Abstract A solution method has been demonstrated to purify the 110 K superconducting phase in the (Bi, Pb)-Sr-Ca-Cu-O system. Oxalic acid was used to chelate metal cations from the nitrates. The amount of chelating agent impacts the cation absorption in the precursor. The higher purity 110 K superconducting phase can be obtained in the shortest time and at the lowest sintering temperature. X-ray diffraction, resistivity and magnetization measurements all indicated positive evidence. The process can be expedited when the stoichiometry was enriched by calcium and copper. The preparation technique is considered valuable to purify multi-phase superconductors for mass production.

Accelerated formation of the three Cu‐O layered Tl‐(Pb,Bi)‐Sr‐Ca‐Cu‐O compound

Bulk samples containing nearly single phase of the three Cu‐O layered compound in the Tl‐Pb(Bi)‐Sr‐Ca‐Cu‐O system have been readily synthesized. By partially substituting Ca for Sr, an accelerated formation of the desired three Cu‐O layered phase was observed when the Ca/Sr ratio reached up to 2.4/1.6. The phase purity was clearly evidenced by x‐ray diffraction and magnetization measurement. The synthesis is valuable for further application interests.

Magnetic and thermal instabilities in a textured YBa2Cu3Ox superconductor with high transport Jc

Abstract Magnetization hysteresis measurements on a melt-textured YBa 2 Cu 3 O x superconductor have been carried out. Saw-toothed flux jumps occurred more frequently when the relaxation time (the stable-field-time interval during measurement) was reduced, or the field sweep rate was increased. The jump-starting field H FJ was found to be related with the relaxation time in a linear form. A large amount of trapped flux due to flux jumps was observed when the direction of the sweeping field was reversed and illustrated using an adiabatic critical state model. An evaluation of the temperature variation from flux jumping has been made by comparing the hysteresis loops in some particular field at various temperatures. It is estimated that the thermal pulses induced by flux jumps at 5 K can cause an instantaneous temperature rising of the sample as high as 32 K, a value about 30 times higher than that of the conventional type-II materials. An intergranular effect of the field-dependent magnetization was also discussed.

Superconductivity in the series (La1−xThx)NiC2 (0 ≤ x ≤ 0.8)

Abstract We present the room-temperature lattice parameters, the d.c. magnetization data and the electrical-resistivity between 1.8 and 10 K for a superconducting series of nine polycrystalline (La 1− x Th x )NiC 2 samples with x = 0−0.8. The variation of a , b and ν lattice parameters of these substitutional intermetallic samples obeys the Vegards law, which is an indication of the systematic substitution of La 3+ ions by Th 4+ ions. As determined by the electrical and magnetic measurements, the superconducting phase transition temperature T c increases with Th concentration up to x = 0.5 ( T c = 7.9 K), then decreases.

Effects of Ce on the crystal structure and superconductivity in TlSr2CaCu2O7−δ

Abstract The partial substitution of Sr or Ca in TlSr 2 CaCu 2 O 7−δ (Tl 1 : 2 : 1 : 2) with the rare-earth element Ce has the advantage to stabilize the tetragonal phase with space group P4/mmm and shows a prominent deviation from Vegards law for the changes in the a , c and v parameters. The transitions ranging from non-superconducting metal to superconductor to insulator are displayed in both the Tl(Sr 2− x Ce x )CaCu 2 O 7 (0⩽ x ⩽0.6) and TlSr 2 (Ca 1− y Ce y )Cu 2 O 7−δ ) (0⩽ y ⩽0.7) series. As expected from the lattice-parameter data, the T c value and the superconducting volume fraction increase with increasing Ce concentration up to x = y = 0.35 ( T c = 62 K), then decrease. In the insulating region, the metal-insulator (M-I) transition appears near the composition of x = y = 0.6. The resistivity of the sample with x = 0.6 or y = 0.7 does follow the three-dimensional Mott variable-range-hopping (VRH) dependence in the temperature below ∼ 100 K.