In-Plane Anisotropy of the Penetration Depth inYBa2Cu3O

Abstract

The superfluid response of single-crystal YBa 2 Cu 3 O 72x and YBa 2 Cu 4 O 8 shows a strong anisotropy in the a-b plane as revealed by far infrared spectroscopy. The value of the London penetration depth in both systems is considerably smaller in the direction along the Cu-O chains, suggesting that a substantial portion of the superconducting condensate is on the chains.

Plane-chain coupling in YBa2Cu3O7: temperature dependence of the penetration depth

“…(12) for which only the vicinity of the nodes is relevant : they give a very local information while λ i (0) contains a global one. Therefore we might just take λ i (0) from experiment itself, which gives [9] the currently accepted values of λ a (0) = 1600Å and λ b (0) = 1000Å. This is all the more reasonable as one sees that our result is fairly sensitive to the value of λ i (0) since it comes in the third power.…”

“…We may indeed wonder if the large value of λ c (0) is not due to a weak plane-plane coupling. This possibility has been suggested by Basov et al [9]. In order to explore our model in this direction, we assume that the plane-plane coupling t p is small.…”

“…Other experiments display also a strong ab plane anisotropy for the superconducting properties, such as the magnetic field dependence of the specific heat anomaly at T c of the parent compound LuBa 2 Cu 3 O 7 . Most strikingly the penetration depth has a marked anisotropy [9] since, at zero temperature, the ratio between the superfluid densities λ −2 b (0)/λ −2 a (0) is 2.3, in good agreement with the anisotropy of the normal state resistivity and of the square of the plasma frequency [9]. In Y Ba 2 Cu 4 O 8 , which has an orthorhombic distorsion of 0.8% (that is somewhat less than Y Ba 2 Cu 3 O 7 which has 1.6%) and where there are twice as many chains as in Y Ba 2 Cu 3 O 7 , the superfluid density anisotropy [9] is as high as 6.2 .…”

“…Most strikingly the penetration depth has a marked anisotropy [9] since, at zero temperature, the ratio between the superfluid densities λ −2 b (0)/λ −2 a (0) is 2.3, in good agreement with the anisotropy of the normal state resistivity and of the square of the plasma frequency [9]. In Y Ba 2 Cu 4 O 8 , which has an orthorhombic distorsion of 0.8% (that is somewhat less than Y Ba 2 Cu 3 O 7 which has 1.6%) and where there are twice as many chains as in Y Ba 2 Cu 3 O 7 , the superfluid density anisotropy [9] is as high as 6.2 . In view of this evolution with increasing the number of chains, it is hard to escape the conclusion that the chains are conducting and that it is their direct contribution which is responsible for the superfluid density anisotropy.…”

“…Since these last experiments provide stronger constraints on theory, we will mostly take them for comparison with experimental results ( note that the compound is more precisely Y Ba 2 Cu 3 O 6.95 but we will consider that it does not differ appreciably from Y Ba 2 Cu 3 O 7 ). The T = 0 values of the penetration depths are more difficult to obtain and we will make use of the results of Basov et al [9] for comparison. However experimental agreement is not complete and there might still be some progress in the future on this side, with perhaps some surprises [18].…”