K.W. Wong

Synthesis and neutron powder diffraction study of the superconductor HgBa2Ca2Cu3O8 + δ by Tl substitution

Substitution of T1 for Hg was performed in the Hg based 1223 phase HgBa2Ca2Cu3Os + ~ ( Tc = 135 K), resulting in an increase of the superconducting transition temperature to 138 K for samples with a nominal composition of Hgo.sTlo.2Ba2Ca2Cu3Os ÷ ~. The crystal structure of this solid solution has been investigated by neutron powder diffraction techniques at room temperature and at 10 K. The compound has the same crystal structure as Hg-1223 with the space group symmetry P4/mmm and lattice parameters a = 3.8489 ( 1 ), c = 15.816 ( 1 ) ,~. Rietveld analysis results indicate that Hg is partially replaced by T1, and the oxygen content, & is 0.33. The lattice-parameter changes resulting from the T1 substitution are too small to account for the T~ change by mimicking the effect of pressure. No phase transition occurs down to 10 K.

Tc enhancement of HgBa2Ca2Cu3O8+δ by Tl substitution

Abstract A substitution of Tl for Hg was performed in the Hg-based 1223 phase HgBa 2 Ca 2 Cu 3 O 8+δ , resulting in an increase of the superconducting transition temperature to 138 K for the sample with a composition of Hg 0.8 Tl 0.2 Ba 2 Ca 2 Cu 3 O 8+δ . Thus partial Tl substitution for Hg is suggested as a means of enhancing the oxygen content in the oxygen deficient Hg 1223 structured compound.

Thick film of HgBa2Ca2Cu3O8+δ via the sol-gel technique

Abstract We have prepared superconducting HgBa 2 Ca 2 Cu 3 O 8+δ thick films on polycrystalline MgO substrate via the sol-gel technique together with a slow annealing method. A precursor film of Ba 2 Ca 2 Cu 3 O 7 was annealed with a bulk mixture of unreacted HgBa 2 Ca 2 Cu 3 O 8+δ and a precursor, Ba 2 Ca 2 Cu 3 O 7 , pellet that was employed to control the HgO vapor pressure. Our films show an onset superconducting transition temperature of 140 K and a zero resistance temperature of 130 K after oxygen annealing. The high T c thick films are consistently reproducible.

Effects of fluorine substitution in superconducting Tl2Ba2CuO6±δ

Abstract The fluorine substituted thallium based 2 2 0 1 phase samples were fabricated with starting compositions 0.95Tl 2 O 3 + 0.1SnO 2 + (2− x )BaO 2 + x BaF 2 + CuO. Mossbauer and X-ray spectra were used as a study of the role played by fluorine. All samples show a near 2 2 0 1 pure phase. The best fluorine substituted sample has T c (onset) of 100 K and the T c (R = 0) of 97 K. The Mossbauer absorption area varies, parallel to T c , with the fluorine content and reaches a maximum absorption near x = 0.6, which indicates that fluorine could have replaced oxygen in the crystal lattice sites and thus does not rule out the presence of a phonon superconducting mechanism.

Possible competing superconducting and insulating mechanisms in semiconductors and semimetals

Abstract Superconducting and excitonic insulating phase transitions in semimetals and semiconductors are studied. It is suggested that in some materials with a many valley type band structure, excitonic insulating transition may compete with the superconducting transition and therefore these materials may exhibit a kind of “mixed” transition. A few materials are proposed which may show either the excitonic insulating or the mixed transition at low temperature and under pressure.

HALL RESISTIVITY SIGN REVERSAL IN SUPERCONDUCTOR'S MIXED STATE

Abstract In this Letter, we propose a mixed-charge model to explain the sign reversals of Hall resistivity for p-type high T c superconductors in the mixed state. The applied field vs temperature boundaries separating positive and negative Hall resistivities would be suppressed in both the underdoped and overdoped regimes, and the suppression is more severe in the overdoped regime. Although flux pinning damps the magnitude of Hall resistivity, from both positive and negative sides, it does not change either the temperature or the applied field values where the Hall resistivity changes sign.

Fluorine doped high Tc cuprate TlSr2CaCu2O7

Superconducting samples with the starting composition of 12 Tl2O3+ 2SrO + (1 − p)CaO + pCaF2 + 2CuO, where p = 14, 12, and 1, were prepared. The participation of fluorine in the starting composition converts the 1 2 1 2 structured non-superconducting TlSr2CaCu2O7 to a superconductor. The doping of F appears to reduce the hole carrier concentration of the parent material. This experimental result may provide the possible evidence of anion substitution in high Tc superconductors.

Thick films of the Tl-based superconductor fabricated via the sol-gel technique

Abstract Superconducting thick films of Tl-based cuprate were grown via the sol-gel technique with a two-step method: deposition of a Tl-free precursor film followed by Tl-vapor annealing. A zero resistance superconducting transition temperature ( T c ) of 120 K on polycrystalline MgO substrates for Tl-1223 films and of 124 K on (100) LaAlO 3 substrates and polycrystalline MgO substrates for Tl-2223 films was obtained. The intragranular critical current density at 5 K of these films is greater than 10 6 A/cm 2 for Tl-1223 and Tl-2223. The microstructure of these films was examined with X-ray diffraction and scanning electron microscopy.

Flux noise, vortex glass transition, and irreversibility line in ceramic Tl2Ba2Ca2Cu3O10−δ high-Tc superconductors

Abstract The flux noise and irreversibility line in ceramic Tl 2 Ba 2 Ca 2 Cu 3 O 10− δ high- T c superconductors have been measured by a DC SQUID, and the vortex glass transition has been detected by the standard four-terminal I vs. V measurement. In the low magnetic field region (0 H ≤1000 Oe), it is found that the flux-noise peak coincides with the vortex glass phase transition temperature T g . However, there is an obvious difference in temperature between the irreversibility temperature T irr and T g . T irr is always higher than T g . Our experimental results indicate that the irreversibility line is a dynamical boundary rather than a phase boundary.

New Tl 2201 (Tl, Cr)2Sr2CuO6±δ phase material

Abstract Evidence of the existence of the strontium analog of Tl2Ba2CuO6, new HTSC material which bears the formula (Tl,Cr)2Sr2CuO6±δ is obtained. This phase has a transition temperature of 76 K.