A.A. Martin

Phonon Self-Energy Effects in High-Temperature Superconductors

The coupling of phonons to electronic excitations in high-temperature superconductors (HTS) results in a rich variety of self-energy effects. In particular, those phonons that strongly interact with electrons occupying states near the Fermi surface are very sensitive to the opening of the superconducting gap. This is usually reflected in appreciable changes of the phonon renormalization (self-energy) around and below Tc. In addition, when both, the phonons and the pair-breaking excitations show significant Raman activity, the Raman transition amplitudes interfere in a way that also changes the phonon intensity when crossing Tc. We illustrate these effects with recent Raman data for (Cu, C)Ba2Ca3Cu4O10 and NdBa2Cu3O7.

Crystal field effect on the f-levels of R1+xBa2−xCu3O6+δ (R=Sm, Nd)

Abstract Infrared (IR) spectroscopy has been used to observe the crystal field (CF) levels of the five lowest J multiplets of Sm3+ in regular D4h symmetry sites in Sm1+xBa2−xCu3O6+δ. From a fit to these levels, we have calculated a set of the CF parameters. Doublet features in the IR spectra have been tentatively associated with the anisotropic Sm–Cu exchange interaction. Theoretical analysis, using the superposition model, as well as density functional based ab initio calculations of the CF interaction have indicated that some of the IR bands arise from the Sm3+ ions occupying the C4v symmetry Ba sites. The magnetic susceptibility of single-crystalline REBa2Cu3O6 (Re=Nd, Sm) has been measured and compared with calculations using the phenomenological CF parameters.

Optical study of RE1+xBa2-xCu3O6 (RE = Nd, Sm) and YBa2Cu3O6 in the mid infrared range

Abstract:Infrared reflectance, and transmission measurements as well as Raman scattering have been used to study the RE1+xBa2-xCu3O6 (RE = Nd, Sm) and YBa2Cu3O6 absorption bands in the 1100-1500 cm-1 infrared range as a function of temperature and beam polarization. In addition to two-phonon absorption between 1100 and 1170 cm-1, we observe excitations around 1400 cm-1, occurring in oxygen rich enclosures within the samples, and assign them to an excitation involving two-phonons plus the 270 cm-1 local mode related to Cu-O broken chains. Thus, the previously reported possible magnetic origin of the 1436 cm-1 sharp absorption band in YBa2Cu3O6 is contested.

Optical Study of Crystal-Field Excitation in (R)Ba2Cu3O7–δ Single Crystals

A large number of high-temperature superconductors (HTS) contain rare-earth (R) ions with 4f-electron multiplets which are split by the crystal field (CF) of the neighboring ligands and, in many cases, their energy differences are in the range of optical phonons. When these two excitations (phonons and CF) have the same symmetry and are close in energy they can couple to each other, leading to a doublet structure at low temperature. Since the R ions are located between the CuO2 superconducting planes, this crystal-field–phonon coupling can be used as a probe to investigate superconductivity related phenomena. In this paper, we discuss the coupling of phonons to CF excitations in (R)Ba2Cu3O7–δ and demonstrate that Raman and infrared spectroscopy can be successfully used as a novel tool for CF studies in HTS, since they are not limited to low energy transfer and can be applied to study small single crystals.

On the origin of the A

The electronic Raman scattering has been investigated in optimally oxygen doped YBa

_{1g}

_{2}

and B

Cu

_{1g}

_{3}

electronic Raman scattering peaks in the superconducting state of YBa

O

_{2}

_{7-\delta}