Zhengzhi Sheng

Double-step behavior of critical current vs. magnetic field in Y-, Bi- and Tl-based bulk high-Tc superconductors

Abstract A double step characteristic is observed at 76 K in the transport critical current as a function of magnetic field (10 -4 T to 10 T) in bulk sintered Y-, Bi- and Tl-based high- T c superconducting materials. The low-field, step-like drop in the critical current density J c commences at magnetic fields B between about 0.3 and 2 mT. This is followed by a plateau region of relatively constant critical current extending from about 30 to 300 mT, and then a second drop at fields between about 0.3 and 10 T. These features occur for all three superconductor materials and are interpreted respectively as a self-field/weak-link regime, a remnant percolation path regime and a flux-flow/upper-critical-field regime. The sharpness of the transition of the voltage-current ( V-I characteristic, represented by the transition parameter n (i.e., V ∝ I n ), has a similar double-step shape as a function of magnetic field directly corresponding to the features of the J c ( B ) characteristic.

Synthesis and characterization of HgBa2Can−1CunO2n+2+δ (n = 1, 2, and 3)

Abstract We have successfully prepared the first three members of the mercury-based superconducting compounds Hg Ba2Can−1CunO2n+2+δ, namely Hg1201, Hg1212 and Hg1223 with high purity and very good quality. T he influence of the synthesis parameters is studied in detail. Using the sealed quartz tube method, very simple procedures are found to ensure a 100% reproducibility of nearly 100% pure Hg1201 and 85–90% Hg1212 and Hg1223. Oxygen annealing of the sample Hg1201 at 300°C for 18 h results in an enhancement of its critical temperature up to 97 K. The symmetry of the first and second members is tetragonal with lattice parameters a = 3.8831 (1) A , c = 9.5357 (2) A , and a = 3.8624 (1) A , c = 12.7045 (2) A , respectively. X-ray diffraction lines of Hg1223 can be indexed in a tetragonal cell with a = 3.8564 (1) A and c = 15.8564 (9) A as well as in an orthorhombic cell with lattice parameters a = 5.4537 (1) A , b = 5.4247 (1) A , and c = 15.8505 (7) A .

A new superconducting 1201-type phase (Tl,Cr)Sr2CuO5

Abstract A new 1201-type phase (Tl,Cr)Sr2CuO5 has been synthesized and identified. This phase has a tetragonal unit cell (space group P4/mmm) with a = 3.795 A and c = 8.880 A , and is superconducting at 40–50 K. Cr in (Tl,Cr)Sr2CuO5 plays a similar role to Pb in (Tl,Pb)Sr2CuO5, which is instructive in the search for new Cr-based superconductors.

New 1212-type (Hg, Cr)-based cuprate (Hg1−xCrx)Sr2(Ca1−yYy)Cu2O6+δ

Abstract We have successfully synthesized a new 1212-type (Hg, Cr)-based cuprate (Hg 1− x Cr x )Sr 2 (Ca 1− y Y y )Cu 2 O 6+δ (0.1≤x≤0.5 and 0.1≤ y ≤0.5). According to X-ray diffraction data, the samples with x ≥0.2 and y ≤0.2 are 90–95% pure 1212 phase. The structure of the (Hg, Cr)-1212 compound is tetragonal of space group P4/mmm. Depending on the nominal composition, the lattice parameters are in the order of a =3.8526 A and c =11.8194 A. Refinements of the structure show the presence of two extra oxygen atoms O(3) and O(4) in the (Hg, Cr) layer at the position (1/2,1/2, 0)and(1/2, 0, 0), respectively. The (Hg, Cr)-1212 samples are not superconducting, and show a semiconducting behavior instead. The non-superconductivity of the samples is explained in terms of the concept of mixed Cu 2+ /Cu 3+ valence.

Semiconducting TlSr2RCu2O7 (R=rare earth) and its superconducting derivatives

Semiconducting TlSr2RCu2O7 (R=Pr or Er) with a 1212-type structure has been synthesized in the single-phase form. Partial substitution of Sr2+ for R3+ converts this semiconductor to a 90 K superconductor TlSr2(R1−ySry)Cu2O7. A combination substitution, Sr2+ for R3+ and Pb4+ for Tl3+, leads to the Ca-free 100 K superconductor (Tl, Pb)Sr2(R, Sr)Cu2O7. The results are explained in the framework of the mixed Cu2+/Cu3+ valence.

Low-temperature elastic constants of Y1Ba2Cu3O7

Abstract Using ultrasonic methods, we studied the 4–295 K elastic constants of a “good” Y 1 Ba 2 Cu 3 O 7 polycrystal. We report the bulk, shear, Young moduli and the Poisson ratio. Except for the Poisson ratio, all elastic constants show smooth temperature behavior. Near T c , during cooling, the Poisson ratio decreases irregularly. Focusing on the bulk modulus B , we use simple thermodynamics to show that the harmonic-observed decrement at T =0 corresponds to expectation, but the temperature effect d B /d T is too large by about a factor of two. We attribute the irregular slope to oxygen-atom reordering. Against many reports, our bulk-modulus value is approximately that found in monocrystals or that predicted by an ionic-crystal model calculation.

Optimum preparation and elemental addition for Tl-based 2223 phase Tl2Ba2Ca2Cu3O10−δ

Abstract TlBaCaCuO samples with 95%-pure 2223 phase Tl 2 Ba 2 Ca 2 Cu 3 O 10−gd and 120 K T c can be prepared in a non-closed system from component oxides with a starting composition of 2223 by heating at 895°C for about 48 h in flowing oxygen. With this procedure, elemental addition at 0.2 molar level maintains 2223 phase purity of 75–95% and a T c of 112–120 K for most of the investigated elements (Na, K, Rb, Ti, Zr, Hf, V, Nb, Ta, Mo, W, Zn, Cd, Hg, Se, and Te).

Derivatives of TlSr2CaCu2O7 from elemental substitutions of Pb for Tl, La for Sr, and Y for Ca

Abstract Tl(Sr,La) 2 CaCu 2 O 7 , Tl(Sr,La) 2 (Ca,Y)Cu 2 O 7 , and (Tl,Pb)(Sr,La) 2 (Ca,Y)Cu 2 O 7 were synthesized and characterized. Thus, all derivatives of the 1212-type phase TlSr 2 CaCu 2 O 7 from elemental substitutions, Pb for Tl, La for Sr, or/and Y for Ca, have been synthesized. La and Y play a similar role in the substitutions. Pb is necessary to increase the Tc from 90 K to above 100 K in the double substitutions while it plays a role similar to La and Y in the single substitution. The triple substitution of Pb, La, and Y does not increase Tc to even higher. The optimal average Cu valence is 2.20 ± 0.05, the same for all derivatives.

Crystal structure and Tc of 1212-type cuprate (Tl,Cr)Sr2(Ca,Tl)Cu2O7

Abstract The crystal structure of the 1212-type cuprate (Tl,Cr)Sr 2 (Ca,Tl)Cu 2 O 7 was investigated using X-ray diffraction data and the Rietveld refinement technique. The refinement was performed using the tetragonal symmetry of space group P4/mmm. It was found that Cr atoms entered the partial Tl site while Tl atoms occupied the Ca site. (Tl,Cr) atomes displaced from the origin to (0.101(1), 0, 0), and O(3) shifted from (0.5, 0.5, 0) to (0.626(8), 0.5, 0). The refined lattice parameters were a = 3.8155(1) A and c = 12.0222(5) A . The refined composition was (Tl 0.812 Cr 0.100 Ca 0.088 )Sr 2 (Ca 0.842 Tl 0.158 )Cu 2 O 7 . Cr atoms were most likely to be at a high valence state. The enhancement of T c in the (Tl,Cr)-1212 cuprate is briefly discussed based on the structure parameters by comparison with the un-doped TlSr 2 CaCu 2 O 7 .

Optimum fabrication process and some relevant analysis for the vanadium-lead doubly substituted 2223 superconducting ceramics

Abstract In a previous investigation [2], we have fabricated the superconductor satisfying the nominal stoichiometric composition B2-(x+y)VxPbySr2Ca2Cu3Oq, where x, y cover wide ranges of combinations, based on a particular preheating and sintering procedure. We follow up such work by exploring other solid state sintering processes and have found an optimum method via which we can fabricate very good superconductors with Tcs = 118 K and Tc (R = 0) = 108 K. X-ray analysis indicates that more than 95% of the typical sample belongs to the 2223 structure. All our samples show a paramagnetic — diamagnetic transition within a very small temperature range which is greater than Tc (R = 0) and smaller than Tcs.