Showing papers by "Zhifeng Ren published in 1996"

Pairing Symmetry in Single-Layer Tetragonal Tl2Ba2CuOβ+δ Superconductors

Abstract: A high-resolution scanning superconducting quantum interference device microscopy study of tetragonal single-layer Tl 2 Ba 2 CuO β+δ films, deposited on tricrystal SrTiO 3 substrates, demonstrates the effect of spontaneously generated half flux quanta. This observation shows that in addition to YBa 2 Cu 3 O 7 , the order parameter symmetry in Tl 2 Ba 2 CuO β+δ is consistent with that of a d x 2 - y 2 pair state. This result also rules out any bilayer or twinning effects and any pairing that is incompatible with the fourfold rotational symmetry as in the Tl 2 Ba 2 CuO β+δ superconducting system.

Superconducting epitaxial Tl2Ba2CuO6 + δ thin films on SrTiO3 with tetragonal lattice and continuously adjustable critical temperature

Abstract: Epitaxial Tl 2 Ba 2 CuO 6 + ° (Tl-2201) thin films on single crytalline SrTiO 3 substrate have been synthesized by RF magnetron sputtering followed by a two-step annealing process. X-ray diffraction data show that the film is c -axis oriented with full width at half maximum of rocking curve (FWHM) of 0.265° and alignment in the ab -plane. The zero-resistance T c of the same film can be increased or decreased continually and reversibly in the 11–80 K range by annealing in argon or air, with no change in tetragonal lattice symmetry.

Vortex-state resistance near parallel orientation in layered superconductors.

Abstract: Angular studies of in-plane resistivity in the mixed-state of layered superconductors reveal a local maximum for magnetic field oriented parallel to the quasi-two-dimensional layers. The peak is observed in single crystals of ${\mathrm{Tl}}_{2}$${\mathrm{Ba}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8}$, ${\mathrm{TlBa}}_{2}$${\mathrm{Ca}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{10}$, ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8}$, and (BEDT-TTF${)}_{2}$Cu(NCS${)}_{2}$, and so appears to be a general phenomenon of layered superconductors. Attendant with this peak is a local minimum in the critical current density at the parallel orientation. Weak interlayer coupling and fluctuations are suggested to provide for the nucleation of noninteracting point vortices with free in-plane motion in the parallel field, yielding excess dissipation. \textcopyright{} 1996 The American Physical Society.

Composition and phase development of epitaxial superconducting Tl0.78Bi0.22Sr1.6Ba0.4Ca2Cu3O9−δ thin films by laser ablation and post annealing

Abstract: For the formation of epitaxial superconducting Tl 0.78 Bi 0.22 Sr 1.6 Ba 0.4 Ca 2 Cu 3 O 9−δ ((Tl,Bi)-l223) thin films, the dependence of phase development on the chemical state of the source materials, deposition distance from source to substrate, and O 2 pressure during deposition by laser ablation and post annealing was studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). It was found necessary to have thallium in the precursor film and that a reacted source target is better than the unreacted source for obtaining (Tl,Bi)-1223 phase-pure films. Better (Tl,Bi)-1223 films were obtained at a deposition distance of 44 mm from source to substrate than at a distance of 61 mm over a range of O2 pressure from 5.7 to 31.3 mTorr The best films prepared under the optimal conditions exhibited transport J c close to 3 × 10 6 A/cm 2 at 77 K and zero field.

Scanning SQUID microscope tests of the symmetry of the high-T c gap

Abstract: Direct observations of the half-integer flux quantum effect in rings, disks, and blanket films of the high-Tc cuprate superconductorsY Ba 2 Cu 3O 7−δ (YBCO),GdBa 2 Cu 3O 7−δ (GdBCO),Tl 2 Ba 2 CuO 6+δ (Tl2201), andBi 2 Sr 2 CaCu 2O 8+δ (BSCCO) with three tricrystal substrate geometries show that their gaps have sign changes consistent with d-wave symmetry. The blanket geometry gives the Josephson penetration depth directly, and provides a means for symmetry tests without patterning. Tl2201 is a tetragonal, single-layer, cuprate without chains. When combined with photoemission results, our BSCCO results imply that the gap followsd x 2−y2 symmetry closely. Asymmetric 45 degree bicrystal grain boundary samples show spontaneous magnetization, as expected for d-wave superconductors with interface roughness.

Half-integer flux quantum effect in cuprate superconductors – a probe of pairing symmetry

Abstract: Based on macroscopic quantum coherence effects arising from pair tunneling and flux quantization, a series of tricrystal experiments have been designed and carried out to test the order parameter symmetry in high-Tc cuprate superconductors. By using a scanning SQUID microscope, we have directly and non-invasively observed the spontaneously generated half-integer flux quantum effect in controlled-orientation tricrystal cuprate superconducting systems. The presence or absence of the half-integer flux quantum effect as a function of the tricrystal geometry allows us to prove that the order parameter symmetry in the YBCO and Tl2201 systems is consistent with that of the dx2−y2 pair state.