John A. Skinta

Evidence for a transition in the pairing symmetry of the electron-doped cuprates La(2-x)Ce(x)CuO(4-y) and Pr(2-x)Ce(x)CuO(4-y).

We present measurements of the magnetic penetration depth, lambda(-2)(T), in Pr(2-x)Ce(x)CuO(4-y) and La(2-x)Ce(x)CuO(4-y) films at three Ce doping levels, x, near optimal. Optimal and overdoped films are qualitatively and quantitatively different from underdoped films. For example, lambda(-2)(0) decreases rapidly with underdoping but is roughly constant above optimal doping. Also, lambda(-2)(T) at low T is exponential at optimal and overdoping but is quadratic at underdoping. In light of other studies that suggest both d- and s-wave pairing symmetry in nominal optimally doped samples, our results are evidence for a transition from d- to s-wave pairing near optimal doping.

Evidence for a nodeless gap from the superfluid density of optimally doped Pr(1.855)Ce(0.145)CuO(4-y) films.

We present measurements of the ab-plane magnetic penetration depth, lambda(T), in five optimally doped Pr(1.855)Ce(0.145)CuO(4-y) films for 1.6 K< or =T < or =T(c) approximately 24 K. Low resistivities, high superfluid densities n(s)(T) proportional, variant lambda(-2)(T), high T(c)s, and small transition widths are reproducible and indicative of excellent film quality. For all five films, lambda(-2)(T)/lambda(-2)(0) at low T is well fitted by an exponential temperature dependence with a gap, Delta(min), of 0.85k(B)T(c). This behavior is consistent with a nodeless gap and is incompatible with d-wave superconductivity.

Magnetic penetration depth measurements of Pr2-xCexCuO4-delta films on buffered substrates: evidence for a nodeless gap.

We report measurements of the inverse squared magnetic penetration depth, lambda(-2)(T), in Pr(2-x)Ce(x)CuO(4-delta) (0.115< or =x < or =0.152) superconducting films grown on SrTiO3 (001) substrates coated with a buffer layer of insulating Pr2CuO4. lambda(-2)(0), T(c), and normal-state resistivities of these films indicate that they are clean and homogeneous. Over a wide range of Ce doping, 0.124< or =x < or =0.144, lambda(-2)(T) at low T is flat: it changes by less than 0.15% over a factor of 3 change in T, indicating a gap in the superconducting density of states. Fits to the first 5% decrease in lambda(-2)(T) produce values of the minimum superconducting gap in the range of 0.29< or =Delta(min)/k(B)T(c)< or =1.01.

Effect of the pseudogap on the temperature dependence of the magnetic penetration depth in YBCO films

Abstract We have measured the magnetic penetration depth, λ ( T ), of YBCO films from optimal to several underdoped states. Films do not degrade under repeated de- and re-oxygenation. Our deoxygenation procedure permits very small incremental reductions in oxygen content. The two-coil measurement apparatus accurately detects small changes in λ(0). At moderate underdoping, the low- T slope of λ −2 is essentially independent of doping. The data are well described by a simple model in which the pseudogapped portions of the Fermi surface do not contribute to electronic properties, and the pseudogapped portions grow with underdoping.

Superconducting Density of States from the Magnetic Penetration Depth of Electron-Doped Cuprates La2−xCexCuO4−y and Pr2−xCexCuO4−y

From measurements of the magnetic penetration depth, λ(T), from 1.6 K to Tc in films of electron-doped cuprates La2−xCexCuO4−y and Pr2−xCexCuO4−y we obtain the normalized density of states, Ns(E) at T=0 by using a simple model. In this framework, the flat behavior of λ−2(T) at low T implies Ns(E) is small, possibly gapped, at low energies. The upward curvature in λ−2(T) near Tc seen in overdoped films implies that superfluid comes from an anomalously small energy band within about 3kBTc of the Fermi surface.

Nanostructures in high-temperature superconductors

Correlation between the nanoscopic disorder in the nanostructures in the a-b planes of high Tc superconductors and the macroscopic properties has been studied. It has been found that nanostructures govern the temperature dependence of the transport and magnetic properties such as: the critical current density Jc(T), the superfluid density ns(T)varies direct as1/(lambda) ab2(T) and the normalized magnetic relaxation rate S(T). They also determine magnetic flux pinning: the dependence of the energy barrier on the current density Ueff(J). Irradiation with heavy ions affects the above properties through the change of the disorder in the nanostructure array. The studies have been performed on YBCO and TlBCCO thin films.