A.M. Gerrits

FIR-spectroscopy of the FI-SDW excitation spectrum in a quasi-1D organic conductor

Abstract The anisotropic quasi one-dimensional conductor (TMTSF) 2 X (X = ClO 4 , PF 6 ) exhibits a cascade of field-induced spin density wave (FI-SDW) phase transitions. In this contribution, we present a magneto-optical study of the far infrared excitation spectrum of (TMTSF) 2 ClO 4 as a function of magnetic field and temperature covering the low field part of the phase diagram of this system. The observed transition temperatures of the various FI-SDW subphases follow closely the established phase diagram. Our results show that the magnitude of the SDW gap depends neither on magnetic field nor on temperature. In addition we find evidence for the existence of a collective mode excitation in the SDW state, indicating a relatively strong pinning to the lattice. Further indication of a large electron lattice coupling is given by the strong phonon renormalisation in the SDW state.

Josephson Plasma Edge in La1.85sr0.15cuo4

The far-infrared dielectric c-axis response for La 1.85 Sr 0.15 CuO 4 shows a pronounced plasma edge below the BCS superconductive gap frequency. The low frequency of this plasmon was previously interpreted as an indication for anomalous mass enhancement for the c-axis transport. However, a natural explanation for this low-frequency plasmon can be given in terms of Josephson plasma oscillations of the order parameter phase difference between adjacent planes in the layered high-T c materials. In this contribution we concentrate on the effect of a large magnetic field parallel to the planes of the superconducting layers. We will demonstrate that the spectroscopic measurement of the plasma frequency and damping as a function of magnetic field yields direct information about the c-axis critical current density and the vortex dynamics for this configuration. In particular, we find for La 1.85 Sr 0.15 CuO 4 that the vortices for B parallel to the planes are not pure Josephson vortices but rather in between the Abrikosov and Josephson limits

Far-infrared studies of the metal-insulator transition in PrNiO3 and NdNiO3

Abstract We present infrared reflectivity measurements of the rare-earth nickelates PrNiO 3 and NdNiO 3 between 30 and 5000 cm −1 at temperatures between 15 and 300 K. We observe a dramatic change of the spectra from metallic-like to nearly insulating in a narrow range of temperatures below the metal-insulator transition.

Josephson plasma oscillations in La1.85Sr0.15CuO4

Abstract We present new data on the c -axis far infrared reflection spectrum of La 1.85 Sr 0.15 CuO 4 . The observed reflectance edge below T c is interpreted in a Josephson plasma oscillation model, which provides a novel spectroscopic way to determine the critical current density. Furthermore, we show that the presence of a small absorption peak near the zero crossing of the dielectric function can be explained in an effective medium calculation.

Infrared investigation of field induced SDW formation in bechgaard salts.

Summary form only given. We report on microwave and far infrared spectroscopic measurements on (TMTSF)/sub 2/ClO/sub 4/ taken at various magnetic fields and temperatures, covering a substantial part of the phase diagram of this quasi one-dimensional organic conductor. The data taken at monochromatic energies versus magnetic field at various temperatures show steps in the optical signal, which follow closely the established phase diagram. Spectroscopic measurements reveal a SDW gap at 14 cm/sup -1/ which is equal in all subphases and independent of magnetic field and temperature. Moreover, we found a sharp mode at 2.4 cm/sup -1/ which can be identified as a strongly pinned, collective excitation of the condensate. Finally, we observed two phonon features at 17 cm/sup -1/ and at 31 cm/sup -1/ which renormalise when the SDW state is entered. The latter two observations indicate an unexpected, strong coupling between the condensate and the underlying lattice.

Far infrared reflectance of YBa2Cu3O7−δ at high magnetic fields

Abstract We report on low temperature far infrared reflection measurements on thin film YBa 2 Cu 3 O 7 at magnetic fields up to 15.5 Tesla. Our measurements show for the superconductive state a clear edge in the reflectance near 400 cm −1 , which develops into a distinct kink if the temperature is lowered to 1.2 K. We observed practically no influence of the magnetic field on the infrared reflectance of YBa 2 Cu 3 O 7 . In particular we find no field induced shift of the 400 cm −1 edge, as would be expected for the effect if pairbreaking or Zeeman splitting on a single gap structure. The absence of distinct field-dependent features indicates a very anisotropic gap in the ab plane.

Far-infrared ellipsometry of electrons and phonons in La1.87Sr0.13CuO4

Abstract We measured the complex dielectric function from 30 to 680 cm −1 along the c axis of a large, high quality single crystal of La 1.87 Sr 0.13 CuO 4 at temperatures between 6 and 300K with a far-infrared Fourier-ellipsometer. We obtain the complex dielectric function directly, without Kramers-Kronig analysis. In agreement with findings of Tamasaku et al . [PRL 69, 1455 (1989)], our data indicate an almost complete blockade of c axis single-particle transport. We present accurate optical phonon data and discuss upper limits of the low temperature far-infrared c axis ac conductivity. We find an anomalously large strength of the 235 cm −1 optical phonon and discuss its possible origin.

Optical properties of incommensurately modulated calaverite, AuTe2

The optical reflection spectrum of incommensurately modulated calaverite is presented. Using a Kramers-Kronig analysis the dielectric function is derived from the reflection spectrum. The results for the imaginary part of the dielectric function are qualitatively discussed in terms of absorption by phonon, free electron and interband scattering. The phonon contribution clearly shows the influence of the incommensurability by the activity of phonons with k not=0. The free electron scattering can be understood in terms of a simple Drude model. The interband scattering is found at a lower frequency than predicted from electronic band structure calculations.