A.K. Rajarajan

Nuclear magnetic resonance studies of thallium in a superconducting phase of the TlBaCaCuO system

Abstract Nuclear magnetic resonance measurements of 203 Tl and 205 Tl nuclei have been performed in a nearly single phase sample Tl 2 Ba 2 Ca 2 Cu 3 O 10 both in the normal as well as in the superconducting phase. The resonance shift δ ( = + 0.30) is small and largely temperature independent above and below the superconducting transition temperature (T c = 104K, measured inductively). The smallness of the shift sugg- ests that the Tl-derived s-band is filled completely. The magnitude of the relaxation time T 1 , that has been measured as a function of temperature, strongly suggests a low density of states at the Fermi level. T 1 is substantially large in the superconducting state as compared to that in the normal state.

Simultaneous doping at the chain- and plane-sites of Cu IN YBa2Cu3O7−y

Abstract The effect of simultaneous substitution of Ga 3+ and Zn 2+ in YBa 2 Cu 3 O 7−y has been studied with respect to its superconducting transition temperature and structural parameters. The result indicates the preferential substitution of Zn and Ga at plane and chain sites of Cu respectively as well as the creation of local magnetic moments at the respective sites due to slowing down of spin and charge fluctuations. The mutual coupling of the moments at the chain- and plane-sites gives rise to enhanced depression of T c .

Cu(1)-nuclear quadrupole resonance in REBa2Cu3O6 (RE=Rare earth)

Abstract We have measured 63,65Cu(1)-nuclear quadrupole resonance as a function of temperature in a number of REBa2Cu3O6 materials with RE=rare earth atoms such as Nd, Sm, Eu, Gd, Dy, Ho, Er and Y. The results are discussed in the light of the fact that the major contribution to the Cu(1)-nuclear relaxation rate arises due to the fluctuation of the RE-spins. In the superconducting O7-systems, formation of Cooper pairs, below Tc, modifies the fluctuation spectrum of the RE-spins. On the other hand, in the insulating O6-materials, there is no such effect. The fluctuation spectrum of the RE spins, and therefore, the nuclear relaxation rate, in O6-materials remains essentially unchanged in the investigated interval of temperature.

Nuclear quadrupole resonance studies in Bi2Sr2CaCu2Ox

Abstract Nuclear quadrupole resonance measurements at 4.2K in Bi 2 Sr 2 CaCu 2 O x ( T c = 70K ) are reported. A broad resonance extending from 18 MHz to 30 MHz is observed. Relaxation times T 1 and T 2 measured at 22.5MHz are 30 msec. and 120 μsec. respectively.

Non-isovalent substitution of Ba2+ in superconducting YBa2Cu3O7-y

We report the structural, magnetic and superconducting properties of YBa 2−x M x Cu 3 O 7 (M=La 3+ or K + ). Single-phase compounds form over a wide range of x in YBa 2−x M x Cu 3 O 7 and up to x = 0.2 in YBa 2−x M x Cu 3 O 7 . A substituent concentration of 15% triggers an orthorhombic to tetragonal phase15 transition in both systems. In spite of the associated disorder in the Cu(1)-0(1) chains, such a transition does not affect T c drastically. T c gets systematically depressed with increasing x. A temperature-dependent paramagnetic response is observed in YBa 2−x M x Cu 3 O 7 between 90 and 300K.

Nuclear magnetic resonance of Tl in superconducting TlBa2-xLaxCuO5-y

Results of nuclear magnetic resonance measurements on two superconducting materials, namely, TlBa1.8La0.2CuO5-y and TlBa1.5La0.5CuO5-y (Tc=45 K and 32 K respectively) are reported. The most significant finding of the present work is that the nuclear relaxation rate deviates from Korringa behaviour at a temperature much higher than the superconducting transition temperature Tc. An increase in the linewidth is observed below Tc which is due to the formation of the vortex lattice.

Cooperative effect of Zn and Ga doping in YBa2Cu3O7

The creation of local magnetic moments at the plane and chain Cu-sites in YBa2Cu3O7 due to incorporation of Ga3+ and Zn2+ has been studied with respect to the superconducting transition temperature and structural parameters of the compound. The result indicates that the local moments are created due to slowing down of spin and charge fluctuations in the planes and chains respectively due to doping.

Studies of the superconducting behaviour of TlBa2−xLaxCuO5−y

Abstract The superconducting behaviour of the system TlBa 2− x La x CuO 5− y has been investigated. Samples prepared by the procedure described in this paper show a decrease in T c with increasing lanthanum concentration. This is in contrast to the trend reported by other authors [15, 19]. This difference in behaviour was explained in terms of the oxygen deficiency y in the parent compound TlBa 2 CuO 5− y .

Nuclear quadrople resonance studies in Ga-doped YBa2(Cu1−xGax)3O7-δ ( δ ⋍ 0.05)

Abstract NQR measurements on YBa2(Cu1−xGax)3O7−δ ( δ ⋍ 0.05) at 4.2K are reported. Resonance lines become broader with the introduction of Ga-ions suggesting a distribution of the electric field gradient (EFG) at the Cu-sites. EFG at both the sites is affected though Ga-ions occupy only the chain sites. Spin-lattice relaxation time reduces sharply with the increase of x which suggests strongly that certain copper atoms acquire magnetic moment as a consequence of introducing non magnetic gallium atoms.

Superconductivity in TmBa2Cu3O9-x

Superconductivity has been observed in a Tm-Ba-Cu-O system with Tc=89 K. Even though the predominant phase on this system was found to be TmBa2Cu3O9-x, this compound could not be synthesised as a single phase. Magnetisation measurements indicated that at least 8% of the sample was superconducting. At 10 K, the lower critical field was found to be 1.5 kG.