Magnetic penetration depth studies of electron-doped cuprate and ultrathin YBCO films.

摘要

Fifteen years after the discovery of superconductivity at 90 K in YBa ₂Cu₃O_7−δ (YBCO), a microscopic theory of cuprate behavior remains elusive. In an attempt to advance basic understanding of the cuprates, this work presents high-precision measurements of the magnetic penetration depth; λ−2(T), in a variety of ultrathin YBCO and electron-doped cuprate films.; Due to the unique properties of the cuprates, fluctuation effects are expected to be significant. An important issue is the effect of thermal fluctuations of the phase of the order parameter on cuprate properties. Theory holds that strong 3D-XY fluctuations observed experimentally over ∼10 K near T_c in clean YBCO crystals are due to weak interlayer coupling. The nature of the observed vortex-pair unbinding transition in measurements of the 2D penetration depth, $l-1\perp$(T) ≡ λ−2( T)d_film, in three ultrathin YBCO films indicates that the films behave as if they are isotropic rather than quasi-two-dimensional. This result agrees with a comparison of experimental results from YBCO and Bi₂Sr₂CaCu₂O₈ samples and from numerical simulations of anisotropic Josephson junction arrays; and challenges the thermal phase fluctuation interpretation of critical behavior near T_c in YBCO.; While it is by now widely accepted that pairing symmetry in the hole-doped cuprates is predominantly d-wave, the nature of pairing symmetry in the electron-doped cuprates is an unresolved and controversial issue. Results from a variety of measurements on nominally optimally doped samples are divided between s- and d-wave conclusions. Measurements of λ−2(T) in numerous Ln _2−xCe_xCuO ₄ (Ln = La or Pr) films at several doping levels, x, reveal a significant qualitative and quantitative change in superconducting behavior near optimal doping. Analysis of the low-T temperature dependence of λ−2(T) suggests that this change may be due to a transition in pairing symmetry from d- to s-wave, a conclusion that would reconcile previous contradictory experimental results. An analysis that deduces a density of states from the experimental superfluid density, however, reveals that there are few states at low energies, consistent with a nodeless gap at all doping levels.