Browsing by Author "Lin, CT"
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- ItemElectronic and phononic Raman scattering in detwinned YBa2Cu3O6.95 and Y0.85Ca0.15Ba2Cu3O6.95: s-wave admixture to the d(x)(2)-y(2)-wave order parameter(America Physical Society, 2009-08-01) Bakr, M; Schnyder, AP; Klam, L; Manske, D; Lin, CT; Keimer, B; Cardona, M; Ulrich, CInelastic light (Raman) scattering has been used to study electronic excitations and phonon anomalies in detwinned, slightly overdoped YBa2Cu3O6.95 and moderately overdoped Y0.85Ca0.15Ba2Cu3O6.95 single crystals. In both samples modifications of the electronic pair-breaking peaks when interchanging the a and b axis were observed. The lineshapes of several phonon modes involving plane and apical oxygen vibrations exhibit pronounced anisotropies with respect to the incident and scattered light-field configurations. Based on a theoretical model that takes both electronic and phononic contributions to the Raman spectra into account, we attribute the anisotropy of the superconductivity-induced changes in the phonon lineshapes to a small s-wave admixture to the d(x)(2)-y(2) pair wave function. Our theory allows us to disentangle the electronic Raman signal from the phononic part and to identify corresponding interference terms. We argue that the Raman spectra are consistent with an s-wave admixture with an upper limit of 20%. © 2009, American Physical Society.
- ItemHighly anisotropic anomaly in the dispersion of the copper-oxygen bond-bending phonon in superconducting YBa2Cu3O7 from inelastic neutron scattering(American Physical Society, 2011-10-19) Raichle, M; Reznik, D; Lamago, D; Heid, R; Li, Y; Bakr, M; Ulrich, C; Hinkov, V; Hradil, K; Lin, CT; Keimer, BMotivated by predictions of a substantial contribution of the "buckling'' vibration of the CuO2 layers to d-wave superconductivity in the cuprates, we have performed an inelastic neutron scattering study of this phonon in an array of untwinned crystals of YBa2Cu3O7. The data reveal a pronounced softening of the phonon at the in-plane wave vector q = (0, 0.3) upon cooling below similar to 105 K, but no corresponding anomaly at q = (0.3,0). Based on the observed in-plane anisotropy, we argue that the electron-phonon interaction responsible for this anomaly supports an electronic instability associated with a uniaxial charge-density modulation and does not mediate d-wave superconductivity. © 2011, American Physical Society.
- ItemPhonon anomalies in pure and underdoped R1-xKxFe2As2 (R=Ba, Sr) investigated by raman light scattering(American Physical Society, 2009-08) Rahlenbeck, M; Sun, GL; Sun, DL; Lin, CT; Keimer, B; Ulrich, CWe present a detailed temperature-dependent Raman light scattering study of optical phonons in Ba1-xKxFe2As2 (x similar to 0.28, superconducting T-c similar to 29 K), Sr1-xKxFe2As2 (x similar to 0.15, T-c similar to 29 K), and nonsuperconducting BaFe2As2 single crystals. In all samples we observe a strong continuous narrowing of the Raman-active Fe and As vibrations upon cooling below the spin-density wave transition T-s. We attribute this effect to the opening of the spin-density wave gap. The electron-phonon linewidths inferred from these data greatly exceed the predictions of ab initio density-functional calculations without spin polarization, which may imply that local magnetic moments survive well above T-s. A first-order structural transition accompanying the spin-density wave transition induces discontinuous jumps in the phonon frequencies. These anomalies are increasingly suppressed for higher potassium concentrations. We also observe subtle phonon anomalies at the superconducting transition temperature T-c with a behavior qualitatively similar to that in the cuprate superconductors. © 2009, American Physical Society