Temperature Dependence Of Hc2 Research Articles

Current conduction and macroscopic quantum interference in BiSrCaCuO screen printed films

Electric current conduction properties of screen printed superconducting BiSrCaCuO films on alumina substrates were investigated. The film contains the low Tc 2212 phase. The temperature dependence of Hc2 and of critical current were determined from the magnetic field dependence of resistance-temperature curves and from current-voltage characteristics. The film cross section dependence of critical current density and change in the shape of current-voltage curves due to the cross section reduction of the film were observed. Quantum interference inherent in the film was demonstrated.

Critical fields of weakly and strongly coupled superconducting multilayers

The authors have compared the parallel (Hc2//) and perpendicular (Hc2 perpendicular to ) upper critical field of superconducting multilayers with strong and weak interlayer coupling (Pb/Cu, Pb/Ge). The temperature dependence of Hc2// reflects the expected dimensionality of the multilayers and can be interpreted in terms of the Takahashi-Tachiki proximity coupling model. In contrast with the strongly coupled Pb/Cu multilayers, weakly coupled Pb/Ge multilayers exhibit a strong thickness dependence of Hc2 perpendicular to . The electronic diffusion coefficient in Pb/Ge, obtained from Hc2 perpendicular to data, is not in agreement with the value calculated from the normal state resistivity. The disagreement reflects the highly inhomogeneous nature of the weakly coupled Pb/Ge multilayers.

Magnetic Properties of High-Tc Bi1.4Pb0.6Ca2Sr2Cu3Oy Superconductor

Magnetization measurements were performed on Bi1.4Pb0.6Ca2Sr2Cu3Oy superconductor with Tc=108 K using a vibrating sample magnetometer in fields up to 18 kOe. The magnetization M(T) is linearly increased in a temperature range of about 15 K below Tc, and the temperature dependence of Hc2 is obtained from the onset of diamagnetism. From the upper critical field slope ∂Hc2/∂T, the zero-temperature upper critical field Hc2(0) is estimated about 61 T. Also, the lower critical field Hc1(0) is 111 Oe using the magnetization measurement M(H). From the magnetic critical fields Hc2 and Hc1, we estimate the superconducting parameters: the coherence length ζ0=23 Å, the penetration depth λ0=2600 Å, the Ginzburg-Landau parameter κ=113 and the jump in the specific heat ΔC/Tc≃2 mJ/molK2.

Magnetization and upper critical fields of LaAl 2 with dilute Gd impurities

Abstract Single- and polycrystalline samples of LaAl2 doped with Gd in the concentration range 0 ⩽ c ⩽ 0.7 at% have been investigated by magnetization and ac susceptibility measurements. The depression of Tc with Gd concentration is in agreement with previous experimental work. Concerning the temperature dependence of Hc2, peaks at intermediate temperatures (∼400 mK) and downward curvatures towards T=0 are observed. The clean limit results can be explained quantitatively in terms of anisotropic Eliashberg theory for the upper critical field, with anisotropy parameters 〈a2〉 = 0.01 for the electron-phonon-coupling and 〈b2〉 = 0.16 for the Fermi velocity. Excellent agreement between theory and experiment can also be achieved for the Gd-doped materials, if spin orientation effects of the Gd atoms are included in the theoretical treatment.

Upper Critical Field of the Anisotropic Organic Superconductors, (TMTSF)2CIO4

The anisotropy of the upper critical field, HC2, of the organic superconductor, (TMTSF)2CIO4 was determined. The principal axes of HC2 were associated with the axes of the lattice. From the temperature dependence of HC2, Pauli (Clogston) limited behavior was apparent when the magnetic field was parallel to the a-axis, which suggests the singlet superconductivity. Coherence lengths of two samples were calculated and found to be strongly dependent on TC.

Upper critical field and related properties of superconducting amorphous alloys Zr-Si

The upper critical fieldH c2 has been measured for the superconducting amorphous alloys Zr100−x Si x withx=13, 15, and 17 over a wide temperature range down to 50 mK. Measurements for related properties, such as the depinning current densityJ p , pinning force densityF p , and flux flow resistivity ρ f , reveal that these systems are well characterized as typically soft and dirty type II superconductors with an extremely lowJ p of 1 A/cm2 andF p of 1.5×104 N/m3 ath (≡H/H c2)=0.5 andt (≡T/T c )=0.5. The temperature dependence ofH c2 is well fitted by a curve of the Werthamer-Helfand-Hohenberg theory with Maki parameter α=1.7−1.5 and spin-orbit interaction parameter λso=1−2.4. These results indicate that the Pauli spin-paramagnetic effect plays an important role in the low-temperature behavior ofH c2 in these low-T c and high-H c2 amorphous superconductors. Corrections of the electron-phonon coupling (λep=0.6) to the Pauli spin-paramagnetic limiting field lead to a reduced importance of the spin-orbit interaction effect, with λso less than 1.0. A discussion is given of the structural inhomogeneities in an amorphous phase, which are sensitively reflected in the strength of the fluxoid line pinning interaction (J p ,F p ) and are intimately related to the nonintrinsic anomaly, observed in many amorphous alloys, thatH c2 depends linearly on temperature even at low temperatures, exceeding the theoretically allowedH c2 values.

Flux flow resistivity and upper critical field in ideal type II amorphous superconductors

Flux flow resistivity ρ f and upper critical fieldH c2 of ideal type II amorphous bulk supercbnductors Zr3Ni and Zr3Rh on both as-quenched and thermally relaxed states have been studied. It is found that thermal annealing does not change the temperature dependence ofH c2 in homogeneous superconductors. The temperature and field dependence of ρ f in all samples studied exhibits a universal scaling relation of the form ρ f /ρ n =f(h, t), where ρ n is the normal state resistivity, andh andt are the reduced field and reduced temperature, respectively. The results are compared with predictions of the time-dependent microscopic theories for bulk superconductors in the dirty limit. In the low-field region (H≪H c2 ) the viscosity coefficient contains both the ordinary (Bardeen-Stephen, Tinkham) and anomalous (Gor'kov-Kopnin) terms. ForH⋍H c2 the results agree qualitatively with the theory of Imai with pair-breaking in the anomalous term. Implications of the present results are discussed.

Superconductivity of the A15 phase in Nb78Ga22−xFex and Nb80Ga20−xFex alloys

en

Anisotropy of the Ginzburg-Landau parameter κ in NbSe2

On small single crystals of 2H NbSe 2 we have measured the temperature and angular dependence ofH c2 as well as the temperature dependence ofH c1 close toT c. Using an effective mass approach to the Ginzburg-Landau equations we have analyzed our data in terms of an anisotropic κ and obtained values for the coherence distances and penetration depths parallel and perpendicular to the layers. The connection between these parameters and normal-state properties is discussed and shown to be more complex than in the isotropic case.