For H‖c-axis, the magnetic field induced broadening of the resistive transitions of high-Tc superconductors (HTS) is shown to depend strongly on the Cu-O layer spacing. For the highly anisotropic HTS, we show experimental evidence that flux motion results from a thermally activated crossover from three dimensional (3D) vortex lines to 2D independent pancake-like vortices in the Cu-O layers, which is intrinsic to the material and occurs when kBT exceeds the Josephson coupling energy of these layers. At low temperatures, however, thermally activated conventional depinning (which can be sample dependent) or melting in the uncoupled 2D Cu-O layers is also required for flux motion. For YBa2Cu3O7, this dimensional crossover does not occur belowHc2, presumably because the conducting Cu-O chains short-circuit the Josephson interlayer coupling, leading to better superconducting properties in a magnetic field. These results show that strong interlayer coupling is a key to finding good alternatives.

Effect of cu-o layer spacing on the magnetic field induced resistive broadening of high-temperature superconductors

Samples containing superconducting HgBa 2 Ca 2 Cu 3 O 8+ x (Hg-1223) (a=3.851 (3) A , c=15.847 (9) A ) as the dominant phase have been investigated by resistivity and magnetization measurements in external magnetic fields up to 50 kOe. The specimens exhibit critical temperatures T c ≈ 133 K, as measured by susceptibility and specific-heat experiments, with typical field-cooling susceptibilities χ FC > 20% of - 1/4 π at low temperatures. In zero field, the electrical resitance is zero below T = 129 K. From magnetization-loop, M ( H ), measurements and the broadening of the resistive transition in external magnetic fields, H , we estimate the location of the irreversibility boundary H irr ( T ) in the magnetic phase diagram of the material. Estimates of the critical-current densities j c in a magnetic field for T = 10 K and 75 K suggest that the intragrain critical-current densities are of comparable order of magnitude to those routinely observed in YBa 2 Cu 3 O 7 at those temperatures.

Magnetization and resistivity study on the 130 K superconductor in the HgBaCaCuO system

Bi-2212 (Bi 2 Sr 2 CaCu 2 O 8 ) and Tl-2212 (Tl 2 Ba 2 CaCu 2 O 8 )-type single crystals have been irradiated by 6.0 GeV Pb ions. The irradiation results in the creation of tubes of amorphized material (latent tracks) which extend throughout the whole thickness of the crystals. Such induced defects act as efficient pinning centers and lead to unusual large changes in the magnetic hysteresis loops. A strong shift of the irreversibility line to high fields and temperatures and a significant charge of its shape are also reported for both compounds, demonstrating that pinning effects have to be taken into account to describe the onset of the FLL motion.

Strong shift of the irreversibility line in bismuth and thallium based 2212 HTSC single crystals irradiated by 6.0 GeV Pb ions

The development results for silver‐sheathed bismuth (2223 phase) superconducting wires in the past two years have suggested that three different fields of application will be feasible in the very near future. The first field is large current conductor application in liquid nitrogen operation. The second is magnet application (relatively low magnetic field below 1 Tesla, such as for semiconductor crystal pulling and magnetic separation) in liquid nitrogen operation. The third is super‐high field application above 20 Tesla in liquid helium operation. In this paper, we will describe the state of the art of high‐Tc superconducting wires using a combination of bismuth high‐Tc phase and powder‐in‐tube technique. Maximum Jc at 77.3 K reached to 53 700 A/cm2 in a zero magnetic field, 42 300 A/cm2 at 0.1 Tesla and 12 000 A/cm2 at 1 Tesla. Prototypes, such as a 1000 A carrying conductor at 77.3 K, a 1000 Gauss coil at 77.3 K and a super‐high field coil at 4.2 K were made and tested successfully.

Development of silver‐sheathed bismuth superconducting wires and their application (invited)

Using magnetization hysteresis loops we have determined the irreversibility line of polycrystalline samples of HgBa2CuO4+δ and Hg0.8Pb0.2Ba2CuO4+δ. The irreversibility line is found at rather high temperatures. It does not show the drastic depression with increasing temperature as is observed in highly anisotropic Bi/Tl‐based CuO superconductors. At 77 K the irreversibility field is around 0.3 T, which is substantially higher than in Bi2Sr2CaCu2O8 and Bi2Sr2Ca2Cu3O10. The advantages of this material for large‐scale applications are outlined.

The irreversibility line of HgBa2CuO4+δ

The temperature dependences of the electrical resistivity, magnetic susceptibility, and specific heat have been measured in ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{Ca}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathit{M}}_{\mathit{x}}$${\mathrm{Cu}}_{2}$${\mathrm{O}}_{8}$, where M=Y, Gd, and Pr, for concentrations 0\ensuremath{\le}x\ensuremath{\le}1.0 and temperatures 1.6 K\ensuremath{\le}T\ensuremath{\le}300 K. All three dopants substitute a trivalent ion for divalent Ca and cause a depression of ${\mathit{T}}_{\mathit{c}}$ observable in the resistivity and susceptibility. Both Pr and Gd retain their free-ion magnetic moment and appear to cause little, if any, magnetic pair breaking. Instead, the dominant suppression mechanism in all three cases is driven by the filling of the Cu 3d hole by the extra electron and the breaking of the Cu 3d\char21{}O 2p hybridization. Consequent localization of Cu 3d holes causes an observable antiferromagnetic ordering of Cu ions within the planes in the case of Y. The evidence for Cu ordering for Gd and Pr is obscured by the large Curie contribution to the magnetic susceptibility due to the rare-earth ions.

Superconducting and magnetic phase boundaries in Bi2Sr2Ca1-xMxCu2O8, with M=Y, Gd, and Pr.

The irreversibility temperatures {ital T}{sub {ital r}}({ital H}) for the {ital c}-axis-oriented powders of pure and alloyed YBa{sub 2}Cu{sub 3}O{sub 7},Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8}, and Bi{sub 2}Sr{sub 2}Ca{sub 2}O{sub 10} were measured for {bold c}{parallel}{bold H}. The results suggest that these temperatures are not the theoretically predicted vortex-lattice melting temperatures nor the vortex-glass-to-liquid transition temperatures.

Irreversibility temperatures in c-axis-oriented powders of YBa2Cu3O7, Bi2Sr2CaCu2O8, and Bi2Sr2Ca2Cu3O10.

The temperature dependence of the specific heat and magnetic susceptibility of single phase (Bi, Pb, Sb) 2 Sr 2 Ca 2 Cu 3 O 10 has been measured. A distinct jump in the specific heat C ( T ) at the superconducting transition temperature T c is obtained with a δ C at T c =6.2±0.2 J/mole K and T c =109 K. This result combined with an estimate of the Sommerfeld constant γ obtained from scaling the temperature independent contribution of the magnetic susceptibility χ( T ) is close to the value of δ C expected assuming a weak coupling BCS theory. The γ-value obtained from δC =1.43 γT c and the temperature independent contribution to χ( T ) is ∼40 mJ/mole K 2 which indicates that the electrons are highly correlated with a band mass ≈10 m 0 where m 0 is the free electron mass. These γ-values and the effective electron mass are very similar to those previously reported for other high- T c superconductors, e.g., YBa 2 Cu 3 O 7 . At low temperatures C ( T ) can be fitted to A / T 2 + γT + βT 3 + C E ( T ) with A =40 mJK/mole, γ⋍0.0 and β =1.76 mJ/mole K 4 and C E ( T ) representing a small Einstein-like contribution to C ( T ) with an Einstein temperature of ∼4 3 K.

Specific heat and magnetic susceptibility of the high-Tc superconductor (Bi, Pb, Sb)2Sr2Ca2Cu3O10

The temperature dependence of the specific heat C(T) has been measured for YBa2(Cu1−xMx)3O7 (YBCO) with M = Zn and Ni and 1.2 λ LT = C(T) T at T = 1.5 K reaching values of 100–150 mJ⧸mol K2 for modest Zn and Ni doping. This γLT dependency for Zn doped YBCO has been associated with a localization of d-holes within the CuO2 planes caused by a disorder induced disruption of hybridization within the planes.

Y. Gao

Papers

Superconducting and magnetic phase boundaries in Bi2Sr2Ca1-xMxCu2O8, with M=Y, Gd, and Pr.

Irreversibility temperatures in c-axis-oriented powders of YBa2Cu3O7, Bi2Sr2CaCu2O8, and Bi2Sr2Ca2Cu3O10.

Specific heat and magnetic susceptibility of the high-Tc superconductor (Bi, Pb, Sb)2Sr2Ca2Cu3O10

Light heavy-fermions in Zn and Ni doped 123 Cu-O superconductors