B. Bandyopadhyay

A new mercury-based high-Tc cuprate Hg0.7V0.3Sr2−xLaxCuO4+δ

Abstract A new barium-free mercury-based superconductor Hg 0.7 V 0.3 Sr 2− x La x CuO 4+δ ( x = 0.1–0.8) has been synthesized. T c onset of this compound is ∼50 K and does not vary appreciably with x . While the lanthanum-free ( x = 0) sample contains predominantly various impurity phases, samples with x = 0.2 and 0.3 are almost single phase. X-ray diffraction analysis reveals that the crystal structure has a tetragonal symmetry with lattice parameters a=3.8133–3.8266 A and c = 8.7361–8.7944 A . A neutron powder diffraction (NPD) study on the x = 0.2 sample shows that the O(3) site is four-fold disordered in the HgO plane having a total occupancy ( δ ) ∼ 0.8. The variation of the resistivity with temperature shows a metallic character for all the samples. The as-prepared samples are in the overdoped region which has been established by thermoelectric-power measurements and annealing treatments. T c is found to increase with CuO(1) and CuO(2) bond distances whereas it is inversely related to (1/ ϱ 300 ) d ϱ /d T .

Impurity substitution in bismuth and thallium cuprates: suppression of Tc and estimation of pseudogap

Abstract Suppression of T c in bilayer bismuth and thallium cuprates by substitution of Co impurities at Cu sites were taken for examination. T c suppression data on differently doped Bi2212 and Tl2212 were analysed within the unitary pair-breaking formalism due to Abrikosov and Gorkov, by fitting data points to a phenomenological relation valid for weak coupling d-wave superconductors. Values of the pseudogap magnitude at each doping were thereby estimated within a “Fermi-level density of states suppression” picture. Pseudogap magnitude from our estimation was observed to have a correspondence with a related characteristic temperature T * obtained by thermoelectric power measurements. Effects of pseudogap on the density of states were studied by calculating the susceptibility, which shows a broad peak at high temperature. This peak feature in susceptibility is indicative of an unusual metallic state, which could be explored further by other systematic measurements.

Neutron diffraction structural study of 1201 superconductor (Hg0.7Cr0.3)Sr2CuOδ

Rietveld analysis of powder neutron diffraction pattern recorded on Sr-based 1201-phase superconductor (Tc=53 K), (Hg0.7Cr0.3)Sr2CuO4+δ shows that the oxygen atoms at O(3) site are disordered atx=y=0.3712. This fourfold split site has a total combined occupancy of ∼80% and is the only occupied site in the (Hg, Cr)O plane.

Effect of Tl doping on the structural and physical properties of HgBa2CuO4+δ superconductor

Abstract Nearly single phase superconducting (Hg 1− x Tl x )Ba 2 CuO 4+ δ samples with T c onset up to 86 K have been synthesised for 0.1≤ x ≤0.5. The crystal structure of these samples is determined by X-ray Rietveld refinement. The crystal belongs to a tetragonal system with a space group P 4/ mmm . A highly distorted octahedral coordination is proposed for thallium with four short Tl–O bonds and two long Tl–O bonds. Tl–O axial bond decreases with the increase of Tl concentration ( x ). This observation has support from Raman results. From resistivity and TEP data normal state gap temperature is found for x =0.1 sample which is in the overdoped region whereas this is absent in materials with Tl concentration x >0.1. T c – p phase diagram of Hg-1201 shows maximum T c for hole concentration p =0.16 and the hole concentration range Δ p (0.29) over which superconductivity occurs is higher than that in other high- T c systems (0.22).

Raman study of the Hg0.7Cr0.3Sr2CuO4+δ superconductors

Abstract The local environment of the apical oxygens (O A ) in the Sr-substituted mercury-based superconductor

Structural distortions and extension of superconducting region in the Tc-p phase diagram of HgBa2CuO4+δ

Abstract In monolayer Hg-based high- T c materials, the T c – p phase diagram follows a relationship different from that followed by many other high- T c materials. It has been observed that an extension of the superconducting region takes place in the overdoped region and a buckling of the Cu–O–Cu bond on the copper–oxide plane is introduced when overdoped materials are made by substituting thallium in the place of mercury. Connection between this buckling and the extension of superconducting region is investigated, and an interpretation of the extension proposed in the light of a quantum paraelectric model.

Two dimensional superconducting behavior of Hg0.7Cr0.3Sr2CuO4+δ

Abstract We have measured magnetization of grain-aligned H 0.7 Cr 0.3 Sr 2 CuO 4+δ high- T c superconductors with high magnetic fields perpendicular to the CuO 2 planes. Analyses based on fluctuation magnetization models show that this compound has a highly two-dimensional character, or weak coupling between the CuO 2 planes. This is against the previous belief of stronger flux pinning due to a shorter in this compound than in the pristine HgBa 2 CuO 4+δ . The highly two-dimensional character is also confirmed by the good 2D scaling behavior of high-field magnetization around T c ( H ).

Raman and FT-ir studies on (Hg, V)(La,Sr)2CuO4+δ, (Hg,Cr)Sr2CuO4+δ and (Hg,V)Ba2CuO4+δ superconductors

Abstract Room temperature Raman and ir spectra of several doped Hg-1201 samples are presented. Assignments of different modes are made on the basis of existing knowledge.

: a new series of mercury-based 1212 superconductors

A new superconductor with up to 80 K has been isolated for . Rietveld refinements of the structure are carried out with a tetragonal structure of space group P4/mmm. Electrical resistivity and magnetic susceptibility of the as-prepared and oxygen-annealed samples are measured as a function of temperature. The room-temperature thermoelectric power is found to be positive and large, a characteristic of underdoped samples.

Low temperature synthesis of superconducting HgBa2CuO4+δ and its deterioration to non-superconducting phase studied by X-ray and IR methods

Abstract The superconductivity in HgBa 2 CuO 4+δ and its deterioration in the non-superconducting state have been studied by x-ray, resistivity and IR measurements.