C.T. Lin

Intense paramagnon excitations in a large family of high-temperature superconductors

In the copper oxide superconductors, spin fluctuations might be involved in the electronic pairing mechanism. The case for such magnetically mediated superconductivity is now strengthened by the discovery of high-energy magnetic excitations that are not affected by chemical doping levels within several cuprates.

Two-dimensional geometry of spin excitations in the high-transition-temperature superconductor YBa2Cu3O6+x.

The fundamental building block of the copper oxide superconductors is a Cu4O4 square plaquette. The plaquettes in most of these materials are slightly distorted to form a rectangular lattice, for which an influential theory predicts that high-transition-temperature (high-Tc) superconductivity is nucleated in ‘stripes’ aligned along one of the axes. This theory received strong support from experiments that indicated a one-dimensional character for the magnetic excitations in the high-Tc material YBa2Cu3O6.6 (ref. 4). Here we report neutron scattering data on ‘untwinned’ YBa2Cu3O6+x crystals, in which the orientation of the rectangular lattice is maintained throughout the entire volume. Contrary to the earlier claim, we demonstrate that the geometry of the magnetic fluctuations is two-dimensional. Rigid stripe arrays therefore appear to be ruled out over a wide range of doping levels in YBa2Cu3O6+x, but the data may be consistent with liquid-crystalline stripe order. The debate about stripes has therefore been reopened.

Resonant Spin Excitation in an Overdoped High Temperature Superconductor

An inelastic neutron scattering study of overdoped Bi(2)Sr(2)CaCu(2)O(8+delta) ( T(c) = 83 K) has revealed a resonant spin excitation in the superconducting state. The mode energy is E(res) = 38.0 meV, significantly lower than in optimally doped Bi(2)Sr(2)CaCu(2)O(8+delta) ( T(c) = 91 K, E(res) = 42.4 meV). This observation, which indicates a constant ratio E(res)/k(B)T(c) approximately 5.4, helps resolve a long-standing controversy about the origin of the resonant spin excitation in high temperature superconductors.

Observation of a d -wave nodal liquid in highly underdoped Bi 2 Sr 2 CaCu 2 O 8+ δ

High-temperature superconductivity in the cuprates arises when charge carriers are added to an insulator. Between these states lies the so-called nodal liquid at low temperature. Photoemission spectroscopy suggests that superconductivity evolves smoothly from this nodal-liquid state.

The dielectric function of AlSb from 1.4 to 5.8 eV determined by spectroscopic ellipsometry

We have prepared AlSb substrates for optical measurements by chemomechanical polishing and etching. The quality of the surface was investigated with optical and electron microscopy and by Raman scattering and ellipsometry. We have measured the pseudodielectric function 〈e〉(ω) of AlSb in the 1.4–5.8 eV photon‐energy region with a spectroscopic ellipsometer. A peak value of 〈e2〉=24.6 at 4 eV was reached. We list the refractive index, the reflectivity, and the absorption coefficient, and obtain the critical point parameters at 300 K. Finally, we fit the index of refraction of AlSb at low photon energies with a semiempirical model.

Growth of large and untwinned single crystals of YBCO

Abstract Large (up to 5×4×1.5 mm3) single crystals of YBa2Cu3O7−σ with perfect orthorhombic morphology were successfully grown from favourable flux composition in alumina crucibles. The number of nucleation centres was reduced by using smooth inner wall crucibles and a temperature oscillation method. The crystals could be isolated from the CuO flux by tipping the crucible over while remaining in the furnace at 970°C. By applying a uniaxial stress at the annealing temperature, both macro- and micro-twin domains in the thick crystals could be removed and a single untwinned domain was obtained. The structures of bith twin- and single-domain crystals were characterized by means of high resolution electron microscopy. We also report the magnetic property of the as-grown single crystals.

Evidence for two separate energy gaps in underdoped high-temperature cuprate superconductors from broadband infrared ellipsometry

Li Yu, D. Munzar, A.V. Boris, P. Yordanov, J. Chaloupka, Th. Wolf, C.T. Lin, B. Keimer, and C. Bernhard 1.) University of Fribourg, Department of Physics and Fribourg, Center for Nanomaterials (FriMat), Chemin du Musee 3, CH-1700 Fribourg, Switzerland 2.) Max-Planck-Institute for Solid State Research, Heisenbergstrasse 1, D-70569 Stuttgart, Germany 3.) Institute of Condensed Matter Physics, Masaryk University, Kotlárská 2, CZ-61137 Brno, Czech Republic 4.) Forschungszentrum Karlsruhe, IFP, D-76021 Karlsruhe, Germany (Dated: February 5, 2008)

Dispersive spin excitations in highly overdoped cuprates revealed by resonant inelastic x-ray scattering

Using resonant inelastic x-ray scattering (RIXS) at the Cu

Quantum criticality in electron-doped BaFe2−xNixAs2

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Carrier-Concentration Dependence of the Pseudogap Ground State of Superconducting Bi(2)Sr2-xLaxCuO6+delta Revealed by Cu-63,Cu-65-Nuclear Magnetic Resonance in Very High Magnetic Fields

-absorption edge, we have observed intense, dispersive spin excitations in highly overdoped Tl