Anisotropy Of Hc2 Research Articles

Investigation of Gd1-xPrxBa2Cu3O7-y single crystals. Transition from magnetic superconductor to magnetic insulator

Superconducting critical magnetic fields Hc2(T), magnetoresistance and the Hall effect have been investigated for Gd1-xPrxBa2Cu3O7-y single crystals grown in Pt crucibles. Several essential differences of the properties of the system, based on nonmagnetic Y3+, were found. The anisotropy of Hc2(T) was found to be ∼ 6, i.e. two times higher than for the (YPr)-123 system. Highly anisotropic negative magnetoresistance and non-linear dependence of the Hall voltage on magnetic field were observed at low temperatures.

D+s wave superconductivity; clean, Zn and other impurity substituted high Tc cuprates

The orthorhombicity within the the layers of high Tc cuprates naturally introduces anisotropy in the electron interaction. This anisotropy creates a d+s pairing in the weak-coupling limit and for a cylindrical Fermi surface, enhancements of Tc and Δ(0)/Tc, the appearance of two neighboring peaks in the density of states and an a−b anisotropy in the upper critical field Hc2. In the presence of nonmagnetic impurities and in the vicinity of the critical concentration where Tc vanishes, there is a gap opening in the d+s case showing a tendency to become more s like. The low temperature thermal conductivity reflects this anomaly and also exhibits an a−b anisotropy. These features should be testable.

Superconductivity and crystalline orientation in V/Cr-multilayers

Abstract We investigated the superconductivity of V/Cr-multilayers grown epitaxially on MgO(001) and Al2O3(1120) plate. The preferred orientation of V/Cr is (001) and (110) for MgO and Al2O3, respectively. Thickness of V-layer is fixed at d v =300 A and that of Cr-layer is varied from 20 to 80A. Critical temperatures of (001)V/Cr are lower than those of (110)V/Cr. Dimensional crossover in the parallel critical fields Hc2|(T) is observed for d Cr =20–60 A V/Cr on Al2O3 and d Cr =20 A V/Cr on MgO. Stronger anisotropy of Hc2| in 2D region is shown in the samples grown on MgO.

Anisotropy of Jc, Hc2 and flux pinning in c-axis oriented epitaxial YBa 2Cu 3O 7 thin film

Anisotropic properties of orientation dependence of J c and H c2 for a c-axis oriented epitaxial YBa 2Cu 3O 7 thin film have been studied in three kinds of configurations of magnetic field H , transport current I and c-axis in H up to 12 T. In the case of H ⊥ c-axis it is found that at the temperature ∼ 77 K and in the low field region ( H⩽3 T), no obvious Lorentz force type influence on J c is observed, while when H is increased to ∼ 7 T, J c is influenced by the Lorentz force rather distinctly. On the other hand, in the case that H is oriented between the c-axis and a-b plane, J c is sensitive to whether H is perpendicular to I or not, but it is quite sensitive to the angle ϕ between H and c-axis, in particular, in the case of ϕ > 60°. Further, at each angle ϕ, H- and T- dependences of J c show that J c is not simply affected by the Lorentz force, but through the action of the flux creep type effect.

Anisotropy of upper critical field in the (110) t and (001) t planes for single-crystal La 1.86Sr 0.14CuO 4

Anisotropy of the upper critical field HC2 has been investigated in detail for single-crystal La1.86Sr0.14CuO4 grown by the TSFZ method. When magnetic fields are applied in the (110)t plane, large anisotropy of Hc2, H⊥c2/H″c2 ≳ 10, is observed and the angular dependence is well expressed by the effective mass model suitable for anisotropic 3-dimensional superconductors. When magnetic fields are applied in the (001)t plane, namely in the CuO2 plane, small but clear anisotropy of Hc2 with 4-fold symmetry is found. This is not attributed to the anisotropy of Hc2 or the critical current due to the twin structure in the orthorhombic phase, but seems due to the anisotropy originating in the electronic state in the CuO2 plane.

Critical parameters of YBa2Cu3O7- δ and Bi2Sr2CaCu2Ox single crystals under pressure

The effect of hydrostatic pressure on the superconducting transition temperature Tc and the anisotropy of the critical field of irreversibility Hc2 for YBa2Cu3O7- delta and Bi2Sr2CaCu2Ox single crystals was investigated by an inductive technique. The critical parameters increase under pressure. The anisotropy of Hc2 at constant temperature decreases for YBa2Cu3O7- delta and increases for Bi2Sr2CaCu2Ox.

Superconducting properties of Mo/Si multilayer films

Multilayered Mo/Si thin films have been prepared by dual rf-magnetron sputtering technique, and the superconducting transition temperature Tc and the upper critical field Hc2 have been examined by resistance measurements. X-ray diffractometry shows that for the modulation wavelength λ shorter than 3 nm both Mo and Si sublayers are amorphous, while for λ longer than 3 nm, crystalline bcc structures appear in Mo sublayers. The λ dependence of Tc of the multilayer films with equal sublayer thickness shows that Tc takes a maximum value of 6.1 K for λ=2.3 nm and approaches the Tc value of amorphous Mo56Si44 in the small λ limit. The superconductivity of the multilayers has been confirmed to result from amorphous MoSi phase formed in the interfacial region of the sublayers. The thickness of the superconducting amorphous layer is estimated to be about 3 nm from the λ dependence of Tc and from the result of the anisotropy of Hc2 . Keeping the Mo sublayer thickness less than 2.4 nm to suppress the appearance of the crystalline Mo and increasing the Si sublayer thickness to reduce the interlayer Josephson coupling, an ideal quasi-two-dimensional superconductor with dHc2∥ /dT as high as 180 kOe/K has been realized.

Upper critical field anisotropy in the Chevrel phase compound PbMo6S8

The anisotropy of the upper critical field,H c2, was measured on deoxygenated single crystals of the Chevrel phase compound PbMo6S8. The superconducting transition temperature,T c, was determined with the applied field pointing in various directions in the hexagonal $$(2\overline 1 0)$$ plane. At a fixed value of the external field,T c is an oscillating function of direction. Since theH c2 versusT curve is linear in the field region we probed, the shape of the anisotropy inT c at constantH is the same as the anisotropy ofH c2 at constantT. The data reflect the low-temperature symmetry of the crystal structure. Two-fold symmetry was observed in the $$(2\overline 1 0)$$ plane. The data were fitted to the effective mass model with uniaxial symmetry. The anisotropy ofH c2 is more complex than that previously observed by us in Cu1.8Mo6S8. We relate this observation to the structural differences of these compounds.

Anisotropy of the upper critical field in the magnetic heavy-fermion superconductor URu2Si2

Measurements have been performed of the upper critical field Hc2 anisotropy in the magnetic heavy-fermion superconductor URu2Si2. The dHc2/dT value is constant within 5% when H is rotated in the basal plane, whereas ‖dHc2/dT‖ decreases by about 35% for H rotated by 20°–30° out of the basal plane. Contrary to CeCu2Si2 the Hc2 anisotropy in URu2Si2 is enhanced as temperature decreases below Tc. This disparity can be attributed to the different strengths of paramagnetic and spin-orbit effects responsible for the suppression of superconductivity by a magnetic field in CeCu2Si2 and URu2Si2.

第３の高温超電導体 Ｂｉ基およびＴｌ基酸化物超電導体の出現と実用化への課題

New high-Tc oxide compounds, whose superconducting transition Tc's exceed 100K, were discovered in Bi-Sr-Ca-Cu-O (BSCCO) and Tl-Ba-Ca-Cu-O (TBCCO) systems. The stoichiometric composition of the high-Tc phase was found to be Bi2Sr2Ca2Cu3Ox and Tl2Ba2Ca2Cu3Ox. Upper critical field Hc2's of these compounds were evaluated for bulk sintered specimens. The Hc2 (T=0K) defined at the midpoint of the resistive transitions are 160T and 220T for BSCCO and TBCCO high-Tc phases, respectively. High values of critical current density Jc at 77K have been already attained for sputtered BSCCO and TBCCO, while the Jc of the bulk specimens still remain less than 104A/cm2 at 77K and 0T. The parameters restricted the Jc in the bulk polycrystalline materials are considered to be (1) anisotropy in Hc2 and Jc, (2) weak link between grains, (3) weak pinning force, and (4) flux creep. The effect of each parameters is discussed with the experimental results of Jc and magnetic measurements.

スパッタ法によるＮｂＮ薄膜の製作と高磁界超電導特性に関する研究

NbN films with B1 structure have been investigated on their superconducting critical temperature Tc, upper critical field Hc2 and field dependent critical current density Jc. NbN has been formed on quartz and sapphire substrates heated up to a temperature between 200 and 700°C in an argon-nitrogen atmosphere by rf sputtering from a Nb disk target. The film thickness ranges from 1, 500 to 6, 000A. The Tc of films is strongly influenced by one of deposition parameters, the nitrogen partial pressure PN2. With increasing PN2 from 1m Torr, the Tc rapidly increases. The maximum Tc in excess of 16K is obtained around PN2=4m Torr. A further increase of PN2 provides a slight decrease of Tc and causes an appreciable increase of the normal state resistivity ρn which is closely related to the Hc2 of films. Most films deposited at PN2 higher than 4m Torr show a single B1 phase and exhibit a strong anisotropy of Hc2. The upper critical fields perpendicular to the film plane (Hc2⊥) are lamer than those parallel (Hc2_??_). Ratios of Hc2_??_ to Hc2⊥ are in the range from -0.72 to -0.85 and are smaller for films deposited on quartz. Both kinds of films deposited on sapphire and quartz show similar relationships between Hc2 and ρn. The Hc2 increases with ρn in a range of below 500μΩcm. High values of Hc2 are obtained in films with ρn above 500μΩcm and with Tc of 14-15K. The maximum values of Hc2⊥ and Hc2_??_ are to -30T and -25T, respectively. NbN films with a ρn of -1, 000μΩcm maintain high Jc in fields higher than 10T. The highest values of Jc at 15T reach 1.0×105A/cm2 for the parallel field direction and 5×105A/cm2 for the perpendicular field direction.

Critical current of Y1Ba2Cu3O7 in strong applied fields

Critical currents of the new high-temperature superconductor Y1Ba2Cu3O7 have been measured in applied fields of up to 7 T and for temperatures down to 70 K. We find that the critical current is drastically reduced by the application of magnetic fields much smaller than the upper critical field of the samples, Hc2. This anomalous behavior might be due to very weak pinning, or to a very strong anisotropy of Hc2. Hc2 is found to follow a linear temperature dependence that however extrapolates to a critical temperature higher than that measured directly. This might result from the existence of a percolative structure, or from the presence of a small volume fraction of high critical temperature, high critical field regions.

Magnetization of Superconducting LaAl2 Single Crystals

Magnetization and critical field measurements were carried out on a high-purity LaAl2 single crystal (RRR = 830) in the temperature range from 80 mK to the transition temperature (Tc=3.301 K). The material was identified to be a type-II superconductor with κ(Tc)=0.75±0.01 and µ0Hc2(0)=92.04 mT. As expected from the magnitude of κ, evidence for a first-order phase transition at Hcl is found. Experiments on the anisotropy of Hc2 in the (100)-plane show a field modulation of about 1% at 2 K. A complete evaluation of superconductive parameters will be presented.

The superconducting Kondo lattice: CeCu2Si2

In the heavy fermion superconductor CeCu2Si2 the angular dependences of the upper critical field HC2 were measured. A three-dimensional picture of the HC2 anisotropy has been obtained. The dHC2/dT value is constant within 8% when H is rotated in the basal plane, whereas dHC2/dT is decreased by a factor of 2 if H is deviated by 5–7° from that plane. These data are analysed within the framework of symmetry classification of possible superconducting classes.

Investigation of UBe13 and other beryllium and uranium compounds in normal and superconducting states

The Hall voltageUxy(H), magnetoresistance ρ(H), magnetization in the superconducting stateM(H), and the second superconducting critical magnetic field of UBe13 have been investigated. It is found thatUxy (H) is not linear, but has a maximum nearH=70 kOe atT=1.9 K. When the temperature rises from 1.9 to 100 K, the Hall coefficientR(H→0) decreases about 20 times from 2.5×10−10 to 1.35×10−11 Ω-cm/Oe. The rise of the magnetic field causes a strong decrease of the electrical resistance: atT=1.7 K, ρ(0)/ρ(210kOe)⋍5.8. From theHc2 investigation for single crystals it can be seen that near Tc the anisotropy ofHc2 is absent (it becomes visible only upon loweringT to 0.9Tc). TheM\((H)_{T< T_c } \) is similar to the typical dependence for type II superconductors. The thermodynamic critical fieldHc(0)≈1 kG, the Ginzburg-Landau parameter x ≈ 60, and the penetration depth λ≈4.10−5 cm. The effects of alloying a small quantity of iron and of neutron irradiation are investigated. The effects of alloying with other compounds of U and Be are also discussed. It is possible to suppose that UBe13 has two groups of carriers, “heavy” and “light,” with the anomalous properties of UBe13 caused by the specificity of the interaction between these two groups of carriers.

Anisotropic magnetic and superconducting properties of single crystals of RERh4B4 compounds

We report the synthesis of small single crystals of several RERh4B4 compounds where RE=Sm, Gd, Tb, Dy, Ho, Er, and Tm using a flux-growth technique. Measurements of the ac electrical resistance R in zero and applied magnetic fields and dc magnetic susceptibility are reported. The upper critical magnetic field Hc2(T) determined from the resistance measurements for RE=Sm, Er, and Tm are also presented. Considerable anisotropy of Hc2(T) is observed for Er and Tm, while very little is found for Sm.

Superconductive properties of vanadium and their impurity dependence

We report on a comprehensive study of the magnetic properties of the super-conductor vanadium. Starting from a high-purity sample (RRR=1900), we use magnetization measurements to evaluate the primary superconductive parameters of vanadium and their impurity dependence up to α⋍3.1. For the clean limit we findT c0 =5.47 K and κ 0 =0.78. From the impurity dependence of the transition temperature an rms gap anisotropy of ≈16% is obtained. Furthermore, for Ginzburg-Landau parameters ≲1.5 an attractive flux line interaction is found. The experiments on Hc2 anisotropy show results comparable to those for niobium, i.e., a slow decrease of the anisotropy coefficient a4 with increasing impurity parameter and a much faster decrease of the 1=6 component. A detailed analysis of these results awaits further progress of theory.

Anisotropy of Hc2 in the Chevrel phases

The anisotropy of the upper critical field of PbMo 6 S 8 , PbMo 6 Se 8 and SnMo 6 Se 8 has been measured. All three compounds show an anisotropy of about 20 %, with the maximum H c2 value when H is perpendicular to the ternary axis. The effective mass model with the mass ratio ε 2 ≈ 0.69 fits well our results.

Anisotropy of Hc2 in Rb 0.2WO 3

The T c and H c 2 of hexagonal single crystals Rb 0.2WO 3 were measured. Upper critical field rotation data in the plane perpendicular to the c-axis showed a large anisotropy with a 60° periodicity. Rotation in a plane containing the c-axis showed an even larger anisotropy having 180° symmetry.

Impurity Effects in the Hc2 Anisotropy of Cubic Superconductors

AbstractFor cubic type II superconductors the Hc2 anisotropy is evaluated in the non‐local regime, i.e., for metals with moderate impurity content at arbitrary temperatures below Tc. In particular it is shown that the contributions of Hc2 anisotropy originating from anisotropic electron‐electron coupling and those resulting from Fermi surface anisotropies differ in their impurity and temperature dependences.