Superconductivity and field-induced magnetism in Pr2-xCexCuO4 single crystals

We report muon-spin rotation and relaxation (muSR) measurements on single crystals of the electron-doped high-T(c) superconductor Pr2-xCexCuO4. In a zero external magnetic field, superconductivity is found to coexist with dilute Cu spins that are static on the muSR time scale. In an applied field, we observe a mu(+)-Knight shift that is primarily due to the magnetic moment induced on the Pr ions. Below the superconducting transition temperature T(c), an additional source of local magnetic field appears throughout the volume of the sample. This finding is shown to be consistent with field-induced antiferromagnetic ordering of the Cu spins. Measurements of the temperature dependence of the in-plane magnetic penetration depth lambda(ab) in the vortex state are also presented.     

Anomalous behavior of the structural parameters of YBa2Cu4O8 single crystals at the transition to the superconducting state

High-precision x-ray crystallographic studies of YBa₂Cu₄O₈ single crystals (T _c =70 K) are performed at eight temperatures in the interval 20–295 K. It is found that a number of structural parameters exhibit anomalous behavior near the superconducting transition of the crystal. A characteristic effect near the phase transition is the displacement of the O1 bridge atom that joins the Cu1 atom of the cuprate chain to the Cu2 atom of the cuprate plane. The shift of this oxygen toward the Cu2 atom is indicative of a change in the Cu2-O1 chemical bond and of charge transfer to the cuprate plane in the process of the transition of the crystal to the superconducting state. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 7, 502–506 (10 October 1997)     

Nodeless d-wave superconducting pairing due to residual antiferromagnetism in underdoped Pr2-xCexCuO4-delta

We investigate the doping dependence of the penetration depth versus temperature in electron-doped Pr(2-x)Ce(x)CuO(4-delta) using a model which assumes the uniform coexistence of (mean-field) antiferromagnetism and superconductivity. Despite the presence of a d(x2-y2) pairing gap in the underlying spectrum, we find nodeless behavior of the low-T penetration depth in the underdoped case, in accord with experimental results. As doping increases, a linear-in-T behavior of the penetration depth, characteristic of d-wave pairing, emerges as the lower magnetic band crosses the Fermi level and creates a nodal Fermi surface pocket.     

Anisotropy in the length change of gallium single crystals at their superconducting transition

The magnetostriction of gallium single crystals has been measured for the three principal axes between 0.3K and 1.1 K. It is found that the length change at the transition from the superconducting to the normal state is given approximately by (Δl/l)_a = − 3.10⁻⁹, (Δl/l)_b = + 4.4 · 10⁻⁹ and (Δl/l)_c = + 1.8 · 10⁻⁹, for the a, b and c axes at T = 0 K. A short discussion of the results with respect to microscopic theory is presented.     

Disorder-induced inhomogeneities of the superconducting state close to the superconductor-insulator transition

Scanning tunneling spectroscopy at very low temperatures on homogeneously disordered superconducting titanium nitride thin films reveals strong spatial inhomogeneities of the superconducting gap Delta in the density of states. Upon increasing disorder, we observe suppression of the superconducting critical temperature Tc towards zero, enhancement of spatial fluctuations in Delta, and growth of the Delta/Tc ratio. These findings suggest that local superconductivity survives across the disorder-driven superconductor-insulator transition.     

Same superconducting criticality for underdoped and overdoped La2-xSrxCuO4 single crystals

By measuring the superconducting diamagnetic moments for an underdoped and an overdoped La2-xSrxCuO4 single crystal with equal quality and roughly equal transition temperatures, it is found that the underdoped sample has only one transition which corresponds to H(c2), but the overdoped sample has two transitions with the higher one at H(c2). Further investigation reveals the same upper-critical field H(c2) for both samples although the overall charge densities are very different, indicating the possibility of a very direct and detailed equivalence of the superconducting condensation process in the two doping limits. The second transition for the overdoped sample can be understood as the bulk coupling between the superconducting clusters produced by macroscopic phase separation.     

Growth of large superconducting CeCu2Si2single crystals

Large CeCu₂Si₂ single crystals have been grown with the cold boat technique. The transition temperature to the superconducting state is strongly dependent on the copper concentration in the crystal. Annealing under copper atmosphere shifts the transition temperature to higher values.     

Study of the peak effect phenomenon in single crystals of 2H-NbSe2

The weakly pinned single crystals of the hexagonal 2H-NbSe₂ compound have emerged as prototypes for determining and characterizing the phase boundaries of the possible order-disorder transformations in the vortex matter. We present here a status report based on the ac and dc magnetization measurements of the peak effect phenomenon in three crystals of 2H-NbSe₂, in which the critical current densities vary over two orders of magnitude. We sketch the generic vortex phase diagram of a weakly pinned superconductor, which also utilizes theoretical proposals. We also establish the connection between the metastability effects and pinning.