A. Wittlin

DAMPING MECHANISM OF THE STRONGLY RENORMALIZED C-AXIS PLASMA FREQUENCY IN HIGH-T-C CUPRATES

Abstract We study the charge dynamics of high-Tc superconductors with the electric field perpendicular to the planes, using polarized oblique-incidence reflectometry for thin films of Tl2Ba2Ca2Cu3O10 and normal incidence reflectometry for single crystals of La1.85Sr0.15CuO4. In Tl2Ba2Ca2Cu3O10 we observe no c-axis optical plasmon either above or below Tc. For E//c in La2−xSrxCuO4, no plasmon is observed in the normal state, but as soon as T drops below Tc, a plasma edge in the reflectivity occurs, which moves up to about 2kBTc for T→0. The electronic contribution to σ(ω)≈7ω−1 cm−1 is independent of frequency up to several hundred cm−1, which implies that the optical scattering rate (τc−1) is large compared to the c-axis plasma frequency (ωpc). We prove experimentally that h /τ c h ω pc ⪢ 3.5k E T c , hence the c-axis response is in a universality class different from the dirty (Mattis-Bardeen) limit, and the absence of a plasma edge in the normal state is due to overdamping. Recent experimental data of various groups show that this is a generic feature of high-Tc cuprates. There is no need to assume a large many-body reduction of the ab initio LDA-RPA plasma frequency along the c-axis. Yet the overdamping of the plasmon could be a many-body effect. The ‘confinement’ to the planes as was proposed by Anderson is therefore due to a strong scattering in the c-direction. In the dirty limit only the relatively small amount of oscillator strength in the below-gap region is transferred to the pair-response (the δ-function at zero frequency). As a result the plasma edge in the superconducting state occurs at about 3.4(k B T c h /τ) 0.5 ω pc ⪡3.5k B T c ⪡ h ω pc .

Plasma resonances and vortex dynamics in La2−xSrxCuO4

Abstract We studied the c-axis Josephson plasma edge of the La2−xSrxCuO4 system for various doping levels as a function of external magnetic fields up to 25 T, both in Faraday and Voigt configuration, using broad band Fourier Transform spectrometers and monochromatic mm-wave sources. Only in the underdoped regime we find an appreciable shift of the plasma resonance with applied magnetic field. We compare the experimental results with recent theoretical predictions. In the long wavelength regime we find a new second plasma resonance which disappears in relatively low magnetic fields.

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.

FIR absorption of Zn1−xCrxS

Abstract The energy level structure of Cr2+ in polytypical ZnCrS in applied magnetic fields was studied using far infrared laser and millimeter wave absorption in the energy range 1.4–25 cm−1. The magnetic fields up to 20 T were oriented along the (1 1 1) or (1 1 0) crystal axis. A strong anisotropy of absorption lines is observed. The description of the data is provided by a cubic crystal field model with a static Jahn-Teller distortion included.

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.

Measurement of c-axis optical absorptivity of La1.87Sr0.13CuO4

We have directly measured the c-axis optical absorptivity of a large single crystal of La1.87Sr0.13CuO4 between 9 and 80 cm-1 at 2 K using Fourier transform spectroscopy and an absorbed power technique. We seen an absorptivity onset at 41 cm-1, which is 2 - 3 cm-1 higher than that measured at 6 K with reflectance on the same sample, suggesting that a significant fraction of the electrons remain unpaired at 6 K. The absorption increases with frequency below the onset, increasing from 0.3% at 9 cm-1 to 8% at 40 cm-1. Our data are particularly significant at frequencies below the plasma edge, for which the reflectance is close to unity.

Cyclotron resonance studies of the molecular conductor d2[1,1;0]-(DMe-DCNQI)2Cu

Abstract We have observed cyclotron resonance in the three dimensional organic conductor d 2 [1,1;0]-( DMe - DCNQI ) 2 Cu . An effective mass of (3.35±0.2) m e is obtained, in excellent agreement with the effective mass deduced from de Haas-van Alphen measurements. This result rules out the possibility that electron-electron interactions play an important role in these materials.

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.