J.B. Mandal

Scaling of the thermoelectric power in the and systems

The temperature-dependent thermoelectric power S of insulating and metallic Bi-2212 and Tl-2212 samples are scaled to universal master curves. For insulating samples, S passes through a peak at a characteristic temperature , which decreases with increasing carrier concentration. For metallic samples a scaling parameter (= dS/dT at high temperatures) decreases with increasing carrier density. It is argued that is related to the energy needed for activation conduction whereas the change in may be due to increase in with carrier density, where n is the concentration and is the effective mass of the carriers.

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 .

Resistivity anisotropy in the Y-substituted Bi-2212 system

Abstract Undoped (x=0) and yttrium-doped (x=0.05,0.1) single crystals of Bi2Sr2Ca1−xYxCu2O8+y have been grown using flux (KCl+NaCl) and self-flux methods, respectively. Metallic in-plane resistivity (ϱab(T) data are interpreted in terms of Matthies-sens rule: ϱ=m ∗ /(ne 2 τ imp )+m ∗ /(ne 2 τ ph ) . The c-axis resistivity (ϱc(T) and the resistivity anisotropy (ϱc/ϱab) at room temperature are compared with the model proposed by Kumar and Jayannavar.

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, transport and thermal properties of

The structure of has been analysed using the Rietveld technique. The electrical resistivity, Hall constant, thermoelectric power and specific heat of the sample are measured as functions of temperature. for the sample is 51 K and remains unchanged under annealing in a hydrogen + argon atmosphere. The Hall angle shows a dependence with temperature. The thermoelectric power is found to be positive and large and follows a dome-shaped curve with T. The specific-heat anomaly associated with the superconducting transition is broad and small. From the low-temperature specific-heat result the Debye temperature has been estimated to be 234 K.

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.