I.F. Schegolev

The role of surface effects in magnetization of high-Tc superconductors

Abstract Experimental data on the field and time dependencies of the isothermal magnetization for Tl and Bi 2212 single crystal are reported. It is shown that, due to some specific features of high-Tc superconductors, Bean-Livingstons surface barrier plays an unusual and important role in the processes of their magnetization. The values of the thermodynamic critical field for both substances are estimated.

Comparative study of Tl-2201 single crystals with Tc = 30 and 110 K by means of X-ray structural analysis and NMR

Abstract The crystal structure of a tetragonal single crystal of Tl-2201 with Tc = 30 K has been determined by the X-ray diffraction technique (a = 3.8717(3), c = 23.224(3) A, space group I4/mmm, Z = 2). The chemical composition is found to be Tl1.90(1)Cu.0.10(1)Ba2CuO6.18(4). Displacements of Tl and O3 atoms from their symmetric sites have been observed. Structural parameters of Tl-2201 cuprates with various Tc have been compared. The superconducting transition temperature is shown to correlate most strongly with the parameter c, with the length of the bond between Cu and apical oxygen as well as with the distance between Ba2+ and the cuprate plane.

Tl NMR study of Tl2Ba2CuOx single crystals with various Tc

Abstract A number of Tl 2 Ba 2 CuO x single crystals with different T c s from 32 to 115 K have been investigated by means of 205 Tl NMR in order to elucidate the possible role of electron spin fluctuations in high- T c superconductivity. In the normal state, the spin part of the Knight shift, K S , has been found to be temperature independent whereas the relaxation time, T 1 obeys T 1 T = a + bT with a increasing and b decreasing as T c is lowered. As in other HTCSs a gap between values of T 1 and K S points to the presence of strong antiferromagnetic correlations. With lowering temperature, T 1 T reaches a minimum value, which is nearly the same for all crystals, at temperatures 1.3 to 1.5 times higher than T c . Below the superconducting transition, in crystals with T c ⩾100 K both K S and T 1 exhibit a gap-like behaviour with a typical Δ/T c ≈3. In crystals with lower T c s a residual K S ( T →0) has been found which is most likely related to nuclei lying inside vortex cores.

Magnetization curves and low critical fields of Tl2Ba2CaCu2Ox single crystals

Abstract Magnetization curves have been measured on Tl2Ba2CaCu2Ox single crystal with Tc=110K in fields both parallel and perpendicular to ab plane. Above 40K nearly reversible behavior of M(H) curves is observed with peaks corresponding to beginning the flux penetration. Hc1 (T) dependence exhibit positive curvature for H//c direction.

Localized moments and spin excitations in the superconducting Tl2Ba2CuO6+x

The presence of localized paramagnetic centers (LPCs) in Tl2Ba2CuO6+x is revealed from the63Cu and205Tl NMR. Concentration of these centers is found growing with oxygen doping, hence they may be responsible for depressingTc. Contribution to the relaxation rate associated with quasiparticle spins has a power temperature dependence far belowTc indicating to the presence of gap zeroes at the Fermi surface. These gap nodes may appear ins-wave pairing case due to the presence of magnetic impurities.

Inhomogeneous 205Tl NMR line broadening in Tl2Ba2CuOx single crystals

Abstract Temperature and angular dependences of 205Tl line width, Δ 1 2 , and transverse relaxation time, τ, have been measured in two crystals of Tl2Ba2CuOx with Tc = 32 and 115 K, respectively. In the normal state both Δ 1 2 and τ have been found to be temperature-independent and not to vary with varying Tc, implying that the size of inhomogeneous broadening is also invariable. Analysis of the line width angular dependence shows that the inhomogeneous broadening originates, most probably, from local distortions associated with Cu substitution for Tl. A conclusion is that variations in Tc are not related to this substitution.

Observation of 2D–3D tansition in Tl2Ba2CaCu2Ox

Abstract Magnetization curves a Tl 2212 single crystal with T c =107 K have been measured in fields perpendicular to the ab -plane and temperatures near T c . Some qualitative changes in M versus H dependences have been observed at a temperature, T 0 ,6.5 K below T c . They are tentatively interpreted as a manifestation of the 2D–3D crossover in the vortex dimensionality. The ordinary anisotropic mass model of GL thoery has been argued to be applicable only at temperatures above T 0 for which the anisotropy parameter, γ, has been estimated to be 20 and the GL parameter K ≈19.

Magnetization process and low critical field of Tl2Ba2CaCu2Ox single crystals

Abstract The penetration field behavior in Tl based 2212 single crystals is reported for H ⊥ ab and H∥ab cases. It is shown that in fields H ⊥ ab Bean-Livingstones surface barrier plays an important role. In the case H∥ab two well different penetration fields are usually observed associated, in some sense, with two different penetration processes. An estimate of the anisotropy factor γ =H ⊥ c 1 /H ∥ c 1 (0) gives γ∼40.

Magnetic susceptibility of Tl2Ba2CuO6+y

The normal state magnetic susceptibility measurements of high quality Tl2Ba2CuO6+y single crystals with various oxygen content have shown that the increasing oxygen content, accompanied by theTc decrease, leads to increase in the number of paramagnetic centers. Thus,Tc decrease with oxygen doping may be explained by spin-flip processes of electron scaterring on additional paramagnetic centers.

Transition from strong to weak coupling regime with lowering Tc in Tl2Ba2CuO6+x

Abstract Using 205 Tl NMR, the changes in electronic properties in the normal and superconducting state of the Tl 2 Ba 2 CuO 6+x compound associated with variation of T c , are studied in samples with T c =30, 100, and 115K. Decreasing T c with oxygen doping is shown to be associated with the growth of the static electron spin susceptibility and with lowering the Δ / T c ratio (Δ is superconductinh gap). This effect indicates to the corresponding weakening of coupling regime.