Flux-line-lattice melting inBi2Sr2

Y. Kopelevich

¹

,

S. Moehlecke

²

,

J. H. S. Torres

³

Abstract

We have performed dc magnetization studies in the c-axis-oriented bulk BizSrzCazCu30&0 compound.The penetration depth A, ,b( T) is extracted from reversible magnetization data M (H) taking into account fluctuations of the vortices. An "irreversibility line" is quantitatively compared with predictions of flux-line-lattice (FLL) melting models. The results strongly suggest that the "irreversibility line" near T, is associated with melting of FLL caused by thermal fluctuations.It is now widely accepted that thermal fluctuations play an important role in the thermodynamic and transport properties of high-temperature supereonductors. The large fluctuations are caused by the high transition temperature T" the short coherence length g, and strong anisotropy of these materials. The fluctuations are responsible for the "anomalous" behavior of reversible magnetization M(H, T). 'Here the magnetization becomes independent of field at some temperature T', which is several degrees below the mean-field transition temperature T,o, i.e. , all M(T} curves for difFerent H cross at the temperature T'. The experimental results have been well described in the model of a threedimensional (3D) XY critical point and in terms of the Ginsburg-Landau fluctuation theory, using the lowest Landau-level approximation.Bulaevskii, Ledvij, and Kogan have shown that in the London regime where the magnetization logarithmically changes with field, the existence of the crossing temperature T* is a result of thermal fluctuations of vortices. They also argue that the superconducting transition temperature Tsc in layered superconductors is the analog of the KosterlitzThouless vortex-antivortex dissociation temperature which lies below T,o. Thus, the resistivity approaches zero at Tsc. For T & Tsc, the motion of dissociated vortex -antivortex pairs give rise to dissipation. Further Kogan et al. ' have demonstrated from magnetization measurements in BizSrzCaCuz08 that vortex fluctuations have a strong influence on the London penetration depth A, (T). On the other hand, strong fiuctuations can lead to melting of the flux-line lattice. " ' In the hightemperature region where the London penetration depth A, ( T)~(1 -T/T, ) '~~, 3D FLL melting models predict a power law for the "melting line" (ML) of the form H~(1 -T/T, ), with a=2. " ' In Josephson-coupled layered superconductor s like Bi-and Tl-based compounds, the thermal vortex fluctuations are enhanced due to weak coupling between CuOz superconducting planes. At the same time, for these strongly anisotropic superconductors, "irreversibility lines" (IL's) near T, have been shown to fit the power-law relationship H;~(1 -T/T, ) with a substantially smaller than 2 (e.g. , Refs. 20 and 21 reported values of an exponent a close to 1), where the T, used is the measured superconducting transition temperature. These results imply that IL in 4o, nH. z 32m A, , b(T) eH k~T 16+k~Tl n ppd apodH v'e the vicinity of T, is not a 3D ML (here we discuss dc IL s which are suitable for comparison with predictions of melting ...

Magnetization and microwave study of superconductingMgB2

“…These data are plotted in figure 2b as [32] where H irr was defined as the field where the initial negative maximum in the magnetization is observed when the magnetization is measured as a function of increasing magnetic field. However, it is less than that observed in the HTSC near T c (1.5 to 2.0) [35][36][37].…”

“…This can be contrasted with the HTSC where a much stronger dependence on the temperature is found [35][36][37].…”

Flux-Line-Lattice Melting and Upper Critical Field of Bi1.65Pb0.35Sr2Ca2Cu3O10+δ Ceramic Samples

“…It is important to mention that the value of c L = 0.29 is within the range expected for high-T c materials, i.e., from 0.1 to 0.5 [33]. Indeed, such a value of c L is in excellent agreement with c L = 0.28 calculated in c-axis oriented bulk Bi-2223 compounds [32].…”

“…Assuming λ ab (0) = 1240 Å [32], and γ = 31 [1,32], a value of the Lindemann parameter c L = 0.29 is obtained for both samples. The other parameters in (6) were H m = 18 T, T m = 46.4(49) K, T c = 105.5(107) K, H ab c2 (0) = 122(136) T, and ξ ab (0) = 91(87) Å, as displayed in Table1 for samples P1 and P6, respectively.…”

“…2 displays the magnetic field phase diagram, H m vs. T , for samples P1 and P6. As has been proposed for BSCCO materials [2,32], the FLL boundary line can be fitted by using the relationship:…”