M. Grüninger

Global and local measures of the intrinsic Josephson coupling in Tl2Ba2CuO6 as a test of the interlayer tunnelling model

One leading candidate theory of high-temperature superconductivity in the copper oxide systems is the interlayer tunnelling (ILT) mechanism. In this model, superconductivity is created by tunnelling of electron pairs between the copper oxide planes — contrasting with other models in which superconductivity first arises by electron pairing within each plane. The ILT model predicts that the superconducting condensation energy is approximately equal to the gain in kinetic energy of the electron pairs due to tunnelling. Both these energies can be determined independently, providing a quantitative test of the model. The gain in kinetic energy of the electron pairs is related to the interlayer plasma frequency, ωJ, of electron pair oscillations, which can be measured using infrared spectroscopy. Direct imaging of magnetic flux vortices also provides a test, which is performed here on the same samples. In the high-temperature superconductor Tl2Ba2CuO6, both the sample-averaging optical probe and the local vortex imaging give a consistent value of ωJ ≈ 28 cm−1 which, when combined with the condensation energy, produces a discrepancy of at least an order of magnitude with deductions based on the ILT model.

Direct Two-Magnon Optical Absorption in α'-NaV2O5

We investigated the temperature-dependent optical conductivity of NaV2O5 in the energy range 4 meV-4 eV. The intensities and the polarization dependence of the detected electronic excitations give a direct indication for a broken-parity electronic ground-state and for a non-centrosymmetric crystal structure of the system in the high-temperature phase. A direct two-magnon optical absorption process, proposed in this Letter, is in quantitative agreement with the optical data. By analyzing the optically allowed phonons at various temperatures above and below the phase transition, we conclude that a second-order change to a larger unit cell takes place below 34 K.

Mid-infrared absorption in YBa2Cu3O6: Failure of spin-wave theory in undoped cuprates

The optical conductivity sigma (omega) of undoped YBa2Cu3O6 is studied in detail in the mid-infrared range. Substitutions on all but the Ba site are used to identify the prominent absorption processes at 2800 and 3800 cm^-1. Experimental evidence for bimagnon-plus-phonon absorption is collected. A more critical analysis of the lineshape and the spectral weight reveals the limits of this approach. Although phonon-2-magnon multiple scattering seems to reproduce the lineshape, the necessary coupling is unrealistically large. The strong increase of high frequency spectral weight with increasing temperature makes the failure of spin-wave theory even more evident.

Orbital superexchange and crystal field simultaneously at play in YVO3: resonant inelastic x-ray scattering at the V L edge and the O K edge

We report on the observation of orbital excitations in YVO3 by means of resonant inelastic x-ray scattering (RIXS) at energies across the vanadium L-3 and oxygen K absorption edges. At the V L3 edge, we are able to resolve the full spectrum of orbital excitations up to 5 eV. In order to unravel the effect of superexchange interactions and the crystal field on the orbital excitations, we analyzed the energy and temperature dependence of the intra-t(2g) excitations at 0.1-0.2 eV in detail. While these results suggest a dominant influence of the crystal field, peak shifts of about 13-20 meV observed as a function of the transferred momentum q parallel to a reflect a finite dispersion of the orbital excitations. This is puzzling since theoretical models based on superexchange interactions predict a dispersion only for q parallel to c. Furthermore, we demonstrate that RIXS at the O K edge is very sensitive to intersite excitations. At the O K edge, we observe excitations across the Mott-Hubbard gap and an additional feature at 0.4 eV, which we attribute to two-orbiton scattering, i.e., an exchange of orbitals between adjacent sites. Altogether, our results indicate that both superexchange interactions and the crystal field are important for a quantitative understanding of the orbital excitations in YVO3.

Far- and mid-infrared spectrum of YBa2Cu3O6.0 in high magnetic fields

The far- and mid-infrared spectrum of antiferromagnetic YBa2Cu3O6.0 was investigated by infrared transmission measurements (→k∥c-axis) in high magnetic fields up to 16.5Tesla at T=1K. A peak at 1436cm−1 which previously was assigned to the excitation of single optical magnons did not show a measurable shift with magnetic field. In the far-infrared, no signature of acoustic magnon absorption has been observed in the magneto transmittance. However, at 83 cm−1 the sixth phonon of symmetry Eu in YBa2 Cu3 O6.0 has been found.

Probing the mid-infrared spectrum of YBa2Cu3O6.0 with high magnetic fields and Zink doping

The mid-infrared phonon and spin-wave spectrum of antiferromagnetic YBa2Cu3O6.0 was investigated by infrared transmission measurements (k→‖c-axis) atT=4K. Peaks at 178 meV, 346 meV and 470 meV were previously interpreted as excitations of single magnons of the optical branch and of bimagnons, respectively. Infrared measurements in high magnetic fields up to 16.5 Tesla and on samples doped with 5% of Zink have been performed to query this interpretation.