X. K. Chen

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.

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

Low energy Raman continua of YBa2Cu3Ox tight-binding model implications

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

Electronic Raman scattering in optimally doped La2-xSrxCuO4

that span the range from underdoped

Imaging EGFR distribution using Surface Enhanced Raman Spectroscopy

(x=0.10)

Raman investigation of Pb-substituted Bi2Sr2Cu6+δ single crystals

to overdoped

Electronic Raman scattering of superconducting YBa2Cu3O y single crystals

(x=0.22).

Raman scattering investigation of the superconducting gap anisotropy in La2−xSrxCuO4

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.

The intrinsic ⅛ lock-in improper ferroelectric phase in thiourea: dielectric susceptibility and hysteresis loop analysis

The low-frequency (ω≤1200 cm-1) Raman continua of YBa2Cu3Ox single crystals have been measured in both the A1g and B1g scattering geometries. The A1g continuum is found to be independent of oxygen concentration x for 6.93<x<7.00, but the spectral distribution in the B1g continuum depends sensitively on x in this same region. The spectra were also calculated by assuming that the Raman continua arise predominantly from scattering from the electronic excitations in the copper-oxygen planes, and that the band structure for the planes is reasonably reproduced by a two-dimensional tight-binding model. If a superconducting gap function with d-wave symmetry is adopted, the calculated results for A1g symmetry are found to be in good agreement with the measured A1g spectrum. In contrast, however, the spectrum calculated in B1g symmetry is inconsistent with the observed spectral distribution. It is thus suggested that there must be additional sources, for example scattering from spin fluctuations, that contributed to the B1g continuum.

Hole concentration and phonon renormalization of the 340-cm(-1) B-1g mode in 2% Ca-doped YBa2Cu3Oy (6.76 <= y <= 7.00)

Abstract The low energy electronic Raman continua of a La 1.83 Sr 0.17 CuO 4 single crystal have been investigated in the x ′ y ′( B 1 g ), x y ( B 2 g ), x x ( A 1 g + B 1 g ) and x ′ x ′( A 1 g + B 2 g ) scattering geometries. When the sample is cooled below the critical temperature T c a superconductivity induced renormalization of the continua occurs and gap-like peaks are observed in all geometries. The superconducting gap is anisotropic with maxima located near the k x and k y axes. The maximum magnitude of the gap is given by ω ( B 1 g )=2 Δ max =7.7 k T c , which is similar to values found in other optimally doped cuprates. The intensities in the low frequency portions of the B 1 g and B 2 g continua increase as ω 3 and ω respectively, a behaviour that is consistent with the existence of nodes along the diagonal | k x |=| k y | directions in the reciprocal lattice. The relative peak frequencies and intensities in the different scattering channels are also similar to those found in other cuprates. No spectral features have been identified that would enable one to distinguish between compounds having a single CuO 2 layer in the unit cell and those having two closely spaced layers in a unit cell. The peak energies, ω ( B 1 g ) and ω ( B 2 g ), reach their maximum value very quickly when the sample is cooled below T c , indicating that the gap opens very rapidly. Finally, the results have been compared to the predictions of the conventional model of light scattering from superconductors and differences are discussed.