Anomalous doping dependence of the fluctuation-induced diama

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Anomalous doping dependence of the ?uctuation-induced diamagnetism in superconductors of YBCO dfce8ea9d1f34693daef3e26scialfari 1,A.Rigamonti 1L.Romano’2,P.Tedesco 1,A.Varlamov 3and D.Embriaco 41Department of Physics ”A.Volta”and Unita’INFM,University of Pavia,Via Bassi n.6,Pavia,I-27100(Italy)2Department of Physics and Unita’INFM,University of Parma,Parco Area delle Scienze n 7A,Parma,I-43100(Italy)3Unita’INFM ”Tor Vergata”,Department STFE,University of Roma Tor Vergata 110,Roma,I-00133(Italy)4Department of Physics and Unita’INFM,University of Pisa,Piazza Torricelli 2,Pisa,I-56126(Italy)Abstract SQUID magnetization measurements in oriented pow-ders of Y 1?x Ca x Ba 2Cu 3O y ,with x ranging from 0to 0.2,for y ≈6.1and y ≈6.97,have been performed in order to study the doping dependence of the ?uctuating diamagnetism above the superconducting transition tempera-ture T c .While for optimally doped compounds the diamagnetic susceptibility and the magnetization curves ?M fl (T =const )vs.H are rather well jus-ti?ed on the basis of an anisotropic Ginzburg-Landau (GL)functional,in

underdoped and overdoped regimes an anomalous diamagnetism is observed,with a large enhancement with respect to the GL scenario.Furthermore the shape of magnetization curves di?ers strongly from the one derived in that scheme.The anomalies are discussed in terms of phase ?uctuations of the order parameter in a layered system of vortices and in the assumption of

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charge inhomogeneities inducing local,non percolating,superconducting re-

gions with T(loc)

c higher than the resistive transition temperature T c.The

susceptibility displays activated temperature behavior,a mark characteristic of the vortex-antivortex description,while history dependent magnetization, with relaxation after zero-?eld cooling,is consistent with the hypothesis of superconducting droplets in the normal state.Thus the theoretical picture consistently accounts for most experimental?ndings.

PACS:74.40.+k,74.70.Vy,74.25.Ha

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I.INTRODUCTION

A variety of experiments1points out that the small coherence length,reduced carrier density,high transition temperature T c and marked anisotropy of cuprate superconductors cause strong enhancement of superconducting?uctuations(SF).In contrast to conventional superconductors,in cuprates the transition region is considerably smeared by SF which can be detected in a wide temperature range,up to10-15K.The formation of the?uctuation Cooper pairs above T c results in the appearance of a Langevin-type diamagnetic contribution to the magnetization?M fl(T,H),existing side by side to the paramagnetic contribution from fermionic carriers.

Since the size of?uctuating pairsξ(T)grows when T approaches the transition temper-ature T c,M fl(T,H)should diverge near the transition for any small?xed magnetic?eld, being equal zero for H=0.On the other hand it is evident that very strong magnetic?elds, comparable to H c2(0),must suppress SF.Therefore the isothermal magnetization curve M fl(T=const,H)has to exhibit an upturn.This upturn can be quantitatively described in the framework of the exactly solvable,for any magnetic?eld,zero-dimensional model2 (superconducting granula with the size?ξ(T))or by means of cumbersome microscopic treatment accounting for the short-wavelength?uctuation contribution in the3D case.3 The experiments on conventional BCS superconductors show that the magnetization is quenched for?elds as low as～10?2H c2(0)(see Ref.2).The value of the upturn?eld H up in the magnetization curves can be considered inversely proportional to the coherence length.2,4 This explains why in optimally doped high-temperature superconductors,the Ginzburg-Landau(GL)picture works pretty well.Here the coherence length is so short that the quenching of?uctuating magnetization on increasing the magnetic?eld has not yet been observed.

The?uctuating magnetization of layered superconductors in the vicinity of the transition temperature and for H?H c2(0),when the contribution of short-wavelength?uctuations3is negligible,can be theoretically described5?7in the framework of the GL scheme with the

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Lawrence-Doniach Hamiltonian.8?10The?uctuating diamagnetism(FD)turns out to be a complicated nonlinear function of temperature and magnetic?eld and cannot be factorized on these variables.An important role in FD is played by the degree of anisotropy of the electronic spectrum.All these aspect of FD have been found to occur in optimally doped YBCO.7?11Also scaling arguments12were found13,14rather well obeyed in this compound.

In underdoped YBCO,instead,marked deviations from the behaviour expected in the framework of GL approaches have been detected.A?rst qualitative claim in this regard goes back to Kanoda et al.,15who noticed that in oxygen de?cient YBCO the FD in small ?elds was dfce8ea9d1f34693daef3e26ter on,novel features of FD in underdoped compounds have been reported8,16?20.In particular,in underdoped YBCO at T c?63K marked enhancement of the susceptibility for?xed(T?T c)in a?eld of0.02Tesla was detected above T c(0), with magnetization curves strongly di?erent from the ones in optimally doped YBCO.16,18 Magnetization curves have been subsequently reported19in underdoped La1.9Sr0.1CuO4 (LASCO).For the moment we only mention that the magnetization curves reported by Carballeira et al.19in underdoped LASCO,although indicating?eld-a?ected?uctuation-induced diamagnetism,do not exhibit the upturn with the magnetic?eld as th …… 此处隐藏：27039字，全部文档内容请下载后查看。喜欢就下载吧 ……