Tine Greibe

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.

New Superconducting Sr2CuO4-δ Thin Films Prepared by Molecular Beam Epitaxy

We report the synthesis of new superconducting Sr2CuO4-δ thin films (Tconset~75 K) using molecular beam epitaxy (MBE). This superconductor has a tetragonal structure with an in-plane lattice constant a0~3.79 A and a c-axis lattice constant c0~13.55 A. This c-axis lattice constant is significantly larger than previously reported for this system. Strong oxidation using ozone gas at a low temperature of around 350°C is essential to the realization of phase conversion from the orthorhombic insulating phase (Sr2CuO3) to the tetragonal superconducting phase (Sr2CuO4-δ).

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.

Role of substrates in molecular beam epitaxy growth of superconducting T′-La2−xCexCuO4 films

Abstract We report the molecular beam epitaxy growth of the electron-doped 30-K superconductor, T ′ -La 2− x Ce x CuO 4 . T ′ -La 2− x Ce x CuO 4 with a very limited range of x can be prepared by bulk synthesis using rather complicated techniques. In contrast, this compound is very easy to prepare by thin film synthesis. The key factor in thin film growth to stabilize T ′ -La 2− x Ce x CuO 4 is a low synthesis temperature below 700 °C. The use of appropriate substrates further stabilizes T ′ formation by an epitaxial effect. In this work, we examined various substrates for the growth of T ′ -La 2− x Ce x CuO 4 . Based on the experimental results, we discuss the roles of substrates.