Modified London Penetration Depth in Oxide Superconductors

London penetration depth AL was calculated for superconductors having the lifetime broadened Bardeen-Cooper-Schrieffer (BCS) density of states.With the increase of gap states, .1Lincreases and the temperature dependence deviates from the ordinary BCS form.The model gives a T2 dependence of AL in the case of weak-coupling BCS.The results are discussed referring some experimental results of magnetic field penetration depth of oxide superconductors.KEY WORDS : Field Penetration Depth/ Superconductor/ Discoveries of new superconductors having higher transition temperature TT have been rewritten by pioneers1-3) very quickly since Bednorz and Miiller° found the superconductivity with TT=40 K in La-Sr-Cu-O system.Surprisingly all the new superconductors are Cu-0 based materials and have the superlattice structure.However, the fundamental mechanisms in the new system have not been uncovered.Although the direct probe for the pairing mechanism is the gap parameter d, clear-cut consensus has not been established yet.As a matter of fact, there is a big puzzle between the infrared') and tunnel') measurements of 4. The former suggests the weak coupling BCS, contrarily the latter appeals the strong coupling BCS.The magnetic field penetration depths 2(T) which have been reported from several groups are also controversial.Muon-spin relaxation measurements for YBa2Cu3Ox (YBCO) gives 2(T) = 2(0) (1 -(T/ Te)4) -112, being similar to an ordinary BCS superconductor.''').On the other hand, 2(T) determined from the susceptibility measurements with powdered specimens deviates from the above relation.Cooper et al.9) derived a T2 law for YBCO.However, Ishida and Mazaki reported a Tl law for YBCO10) and a T2 law for La2_xSrx Cu04 (LSCO).11)As a possible explanation for the discrepancy among these experimental values of 2, we report here the calculations of the London penetration depth 2L in the framework of the BCS theory, where we use the broadened density of states," and attempt to find how the temperature dependence of 2L alters with the broadening parameter.Generally speaking, the London penetration depth is valid for the local limit materials which have a very short coherence length e compared to the 2. In the case of YBCO and LSCO, the reported values of e/2 are much less than * rw