J. C. Irwin

Evidence for Two Superconducting Gaps in MgB2

We have measured the Raman spectra of polycrystalline MgB2 from 25 to 1200 cm(-1). A superconductivity-induced redistribution in the electronic Raman continuum was observed. Two pair-breaking peaks appear in the spectra, suggesting the presence of two superconducting gaps. The measured spectra were analyzed using a quasi-two-dimensional model in which two s-wave superconducting gaps open on two sheets of Fermi surface. For the gap values we have obtained Delta(1) = 22 cm(-1) ( 2.7 meV) and Delta(2) = 50 cm(-1) ( 6.2 meV). Our results suggest that a conventional phonon-mediated pairing mechanism occurs in the planar boron sigma bands and is responsible for the superconductivity of MgB2.

Raman scattering from cupric oxide

Abstract Raman scattering experiments have been carried out on Cupric Oxide (CuO) throughout the temperature range from 15K to 300K. The three Raman active modes predicted by group theory have been identified and their room temperature frequencies determined to be 298, 345 and 632 cm −1 . A new mode appears in the spectra (240 cm −1 at 15K) as the sample is cooled below the Neel Temperature (TN = 225± 5K of CuO. Based upon its temperature dependence this mode is tentatively assigned to scattering from a low-lying electronic excitation. Another mode (218 cm −1 at 300K) softens and disappears on cooling the sample below 200K. The results are discussed in terms of data obtained from recent neutron and Mossbauer studies on CuO.

Raman scattering from single crystals of cupric oxide

Abstract Raman scattering experiments have been carried out on single crystals of cupric oxide (CuO) in the temperature range from 15 K to 300 K. The symmetry of the three first order Raman active phonons has been determined. A broad peak occurs at approximately 1100 cm −1 and is assigned to multiphonon scattering on the basis of the observed temperature dependence. Another peak appears in the Raman spectrum as the temperature of the sample is lowered below the Neel temperature. The frequency, intensity and linewidth of this peak have been studied as a function of temperature. The frequency of this peak cannot be reconciled with the known spin wave spectrum of CuO and thus, on the basis of the measured temperature dependence, it has been assigned to scattering from a magnetic exciton. The Raman spectrum also contains a background continuum which is weakly peaked at about 2000 cm −1 and the possible origin of this scattering is discussed. The observed optical and magnetic properties of CuO are compared, where possible, to results that have been obtained for the high- T c cuprates.

Doping Dependence of the Pseudogap in La(2-x)Sr(x)CuO(4)

We report the results of Raman scattering experiments on single crystals of

ELECTRON-PHONON INTERACTIONS OF RAMAN ACTIVE PHONONS IN YBA2CU3O7-Y

{\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}

Determination of current and flux distribution in squares of thin-film high-temperature superconductors

that span the range from underdoped

Raman scattering from YBaCuO high Tc-superconductors

(x=0.10)

The oxygen isotope effect in superconducting YBa2Cu3O7−δ

to overdoped

Characterization of films prepared by laser ablation of precursor targets and ex situ thalliation

(x=0.22).

Low energy Raman continua of YBa2Cu3Ox tight-binding model implications

The spectra are consistent with the existence of a strong anisotropic quasiparticle interaction that results in a normal state depletion of spectral weight from regions of the Fermi surface located near the zone axes. The strength of the interaction decreases rapidly with increasing hole concentration and the spectral evidence for the pseudogap vanishes when the optimum doping level is reached. The results suggest that the pseudogap and superconducting gap arise from different mechanisms.