Nobuhisa Kaneko

A UNIVERSAL SCALING RELATION IN HIGH-TEMPERATURE SUPERCONDUCTORS

Since the discovery of superconductivity at elevated temperatures in the copper oxide materials there has been a considerable effort to find universal trends and correlations amongst physical quantities, as a clue to the origin of the superconductivity. One of the earliest patterns that emerged was the linear scaling of the superfluid density (ρs) with the superconducting transition temperature (Tc), which marks the onset of phase coherence. This is referred to as the Uemura relation, and it works reasonably well for the underdoped materials. It does not, however, describe optimally doped (where Tc is a maximum) or overdoped materials. Similarly, an attempt to scale the superfluid density with the d.c. conductivity (σdc) was only partially successful. Here we report a simple scaling relation (ρs∝σdcTc, with σdc measured at approximately Tc) that holds for all tested high-Tc materials. It holds regardless of doping level, nature of dopant (electrons versus holes), crystal structure and type of disorder, and direction (parallel or perpendicular to the copper–oxygen planes).

Discontinuity of the ultrafast electronic response of underdoped superconducting Bi2212 strongly excited by ultrashort light pulses

1Department of Physics, Università Cattolica del Sacro Cuore, Brescia I-25121, Italy. 2Department of Physics, Università degli Studi di Trieste, Trieste I-34127, Italy. 3Department of Applied Physics, Stanford University, CA 94305, USA. 4Stanford Synchrotron Radiation Laboratory, Stanford, CA 94305, USA. 5Sincrotrone Trieste S.C.p.A., Basovizza I-34127, Italy. ✢Present address: Nanoelectronics Research Institute, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8568, Japan. ✢✢Present address: National Metrology Institute of Japan, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8568, Japan.

Charge-transfer excitations in the model superconductor HgBa2CuO4+δ

We report a Cu -edge resonant inelastic x-ray scattering (RIXS) study of charge-transfer excitations in the 2-8 eV range in the structurally simple compound HgBa(2)CuO(4+delta) at optimal doping (T(c)=96.5 K). The spectra exhibit a significant dependence on the incident photon energy which we carefully utilize to resolve a multiplet of weakly dispersive (<0.5 eV) electron-hole excitations, including a mode at 2 eV. The observation of this 2 eV excitation suggests the existence of a remnant charge-transfer gap deep in the superconducting phase. Quite generally, our results, which include additional data for the Mott insulator La(2)CuO(4), demonstrate the importance of exploring the incident photon-energy dependence of the RIXS cross section.

Periodic density-of-states modulations in superconducting (formula presented)

In this paper we show, using scanning tunneling spectroscopy, the existence of static striped density of electronic states in nearly optimally doped Bi 2 Sr 2 CaCu 2 O 8 + δ in zero field. The observed modulation is strongest at roughly half the superconducting gap energy and is aligned with the Cu-O bonds, with a periodicity of four lattice constants, exhibiting features characteristic of a two-dimensional system of line objects. These features also exhibit asymmetries between the two orthogonal directions locally, pointing to a possible broken symmetry state (i.e., a stripe phase). We further show that the density of states modulation manifests itself as a shift of states from above to below the superconducting gap. The fact that a single energy scale (i.e., the gap) appears for both superconductivity and stripes suggests that these two effects have the same microscopic origin.