On the peak in the far-infrared conductivity of strongly anisotropic cuprates

We investigate the far-infrared and submillimeter-wave conductivity of electron-doped La_(2-x)Ce_xCuO_4 tilted 1 degree off from the ab-plane. The effective conductivity measured for this tilt angle reveals an intensive peak at finite frequency (\nu ~ 50 cm{-1}) due to a mixing of the in-plane and out-of-plane responses. The peak disappears for the pure in-plane response and transforms to the Drude-like contribution. Comparative analysis of the mixed and the in-plane contributions allows to extract the c-axis conductivity which shows a Josephson plasma resonance at 11.7 cm{-1} in the superconducting state.Comment: 4 pages, 4 figures include

Charge transfer fluctuation, d−d-wave superconductivity, and the B1gB_{1g} Raman phonon in the Cuprates: A detailed analysis

The Raman spectrum of the $B_{1g}$ phonon in the superconducting cuprate materials is investigated theoretically in detail in both the normal and superconducting phases, and is contrasted with that of the $A_{1g}$ phonon. A mechanism involving the charge transfer fluctuation between the two oxygen ions in the CuO$_2$ plane coupled to the crystal field perpendicular to the plane is discussed and the resulting electron-phonon coupling is evaluated. Depending on the symmetry of the phonon the weight of different parts of the Fermi surface in the coupling is different. This provides the opportunity to obtain information on the superconducting gap function at certain parts of the Fermi surface. The lineshape of the phonon is then analyzed in detail both in the normal and superconducting states. The Fano lineshape is calculated in the normal state and the change of the linewidth with temperature below T$_{c}$ is investigated for a $d_{x^{2}-y^{2}}$ pairing symmetry. Excellent agreement is obtained for the $B_{1g}$ phonon lineshape in YBa$_{2}$Cu$_{3}$O$_{7}$. These experiments, however, can not distinguish between $d_{x^{2}-y^{2}}$ and a highly anisotropic $s$-wave pairing.Comment: Revtex, 21 pages + 4 postscript figures appended, tp

Temperature dependence of the EPR linewidth of Yb3+ - ions in Y0.99Yb0.01Ba2Cu3OX compounds: Evidence for an anomaly near TC

Electron paramagnetic resonance experiments on doped Yb3+ ions in YBaCuO compounds with different oxygen contents have been made. We have observed the strong temperature dependence of the EPR linewidth in the all investigated samples caused by the Raman processes of spin-lattice relaxation. The spin-lattice relaxation rate anomaly revealed near TC in the superconducting species can be assigned to the phonon density spectrum changesComment: 10 pages, 4 figures Renewed versio

Small and large polarons in nickelates, manganites, and cuprates

By comparing the optical conductivities of La_{1.67}Sr_{0.33}NiO_{4} (LSNO), Sr_{1.5}La_{0.5}MnO_4 (SLMO), Nd_2CuO_{4-y} (NCO), and Nd_{1.96}Ce_{0.04}CuO_{4} (NCCO), we have identified a peculiar behavior of polarons in this cuprate family. While in LSNO and SLMO small polarons localize into ordered structures below a transition temperature, in those cuprates the polarons appear to be large, and at low T their binding energy decreases. This reflects into an increase of the polaron radius, which may trigger coherent transport.Comment: File latex, 15 p. incl. 4 Figs. epsf, to appear on the Journal of Superconductivity - Proc. "Stripes 1996" - Roma Dec 199

Charge Ordering and Phase Competition in the Layered Perovskite Lasr2mn2o7

Charge-lattice fluctuations are observed in the layered perovskite manganite LaSr2Mn2O7 by Raman spectroscopy as high as 340 K and with decreasing temperature they become static and form a charge ordered (CO) phase below TCO=210 K. In the static regime, superlattice reflections are observed through neutron and x-ray diffraction with a propagation vector (h+1/4,k-1/4,l). Crystallographic analysis of the CO state demonstrates that the degree of charge and orbital ordering in this manganite is weaker than the charge ordering in three dimensional perovskite manganites. A TN=170K a type-A antiferromagnetism (AF) develops and competes with the charge ordering, that eventually melts below T*=100K. High resolution diffraction measurements suggest that that CO- and AF-states do not coincide within the same region in the material but rather co-exist as separate phases. The transition to type-A antiferromagnetism at lower temperatures is characterized by the competition between these two phases.Comment: 9 pages, 6 figure

Anisotropic conductivity of Nd_{1.85}Ce_{0.15}CuO_{4-\delta} films at submillimeter wavelengths

The anisotropic conductivity of thin Nd$_{1.85}$Ce$_{0.15}$CuO$_{4-\delta}$ films was measured in the frequency range 8 cm$^{-1}<\nu <$ 40 cm$^{-1}$ and for temperatures 4 K $<T<300$ K. A tilted sample geometry allowed to extract both, in-plane and c-axis properties. The in-plane quasiparticle scattering rate remains unchanged as the sample becomes superconducting. The temperature dependence of the in-plane conductivity is reasonably well described using the Born limit for a d-wave superconductor. Below T_{{\rm C}%} the c-axis dielectric constant $\epsilon_{1c}$ changes sign at the screened c-axis plasma frequency. The temperature dependence of the c-axis conductivity closely follows the linear in T behavior within the plane.Comment: 4 pages, 4 figure

Raman light scattering study and microstructural analysis of epitaxial films of the electron-doped superconductor La_{2-x}Ce_{x}CuO_{4}

We present a detailed temperature-dependent Raman light scattering study of optical phonons in molecular-beam-epitaxy-grown films of the electron-doped superconductor La_{2-x}Ce_{x}CuO_{4} close to optimal doping (x ~ 0.08, T_c = 29 K and x ~ 0.1, T_c = 27 K). The main focus of this work is a detailed characterization and microstructural analysis of the films. Based on micro-Raman spectroscopy in combination with x-ray diffraction, energy-dispersive x-ray analysis, and scanning electron microscopy, some of the observed phonon modes can be attributed to micron-sized inclusions of Cu_{2}O. In the slightly underdoped film (x ~ 0.08), both the Cu_{2}O modes and others that can be assigned to the La_{2-x}Ce_{x}CuO_{4} matrix show pronounced softening and narrowing upon cooling below T ~ T_c. Based on control measurements on commercial Cu_{2}O powders and on a comparison to prior Raman scattering studies of other high-temperature superconductors, we speculate that proximity effects at La_{2-x}Ce_{x}CuO_{4}/Cu_{2}O interfaces may be responsible for these anomalies. Experiments on the slightly overdoped La_{2-x}Ce_{x}CuO_{4} film (x ~ 0.1) did not reveal comparable phonon anomalies.Comment: 7 pages, 8 figure

Anomalous Self-Energy Effects of the B_1g Phonon in Y_{1-x}(Pr,Ca)_xBa_2Cu_3O_7 Films

In Raman spectra of cuprate superconductors the gap shows up both directly, via a redistribution of the electronic background, the so-called "2Delta peaks", and indirectly, e.g. via the renormalization of phononic excitations. We use a model that allows us to study the redistribution and the related phonon self-energy effects simultaneously. We apply this model to the B_1g phonon of Y_{1-x}(Pr,Ca)_xBa_2Cu_3O_7 films, where Pr or Ca substitution enables us to investigate under- and overdoped samples. While various self-energy effects can be explained by the strength and energy of the 2\Delta peaks, anomalies remain. We discuss possible origins of these anomalies.Comment: 6 pages including 4 figure

Polaronic optical absorption in electron-doped and hole-doped cuprates

Polaronic features similar to those previously observed in the photoinduced spectra of cuprates have been detected in the reflectivity spectra of chemically doped parent compounds of high-critical-temperature superconductors, both $n$-type and $p$-type. In Nd$_2$CuO$_{4-y}$ these features, whose intensities depend both on doping and temperature, include local vibrational modes in the far infrared and a broad band centered at $\sim$ 1000 cm$^{-1}$. The latter band is produced by the overtones of two (or three) local modes and is well described in terms of a small-polaron model, with a binding energy of about 500 cm$^{-1}$. Most of the above infrared features are shown to survive in the metallic phase of Nd$_{2-x}$Ce$_x$Cu0$_{4-y}$, Bi$_2$Sr$_2$CuO$_6$, and YBa$_2$Cu$_3$O$_{7-y}$, where they appear as extra-Drude peaks. The occurrence of polarons is attributed to local modes strongly coupled to carriers, as shown by a comparison with tunneling results.Comment: File latex, 31 p., submitted to Physical Review B. Figures may be faxed upon reques

Systematics of two-component superconductivity in YBa2Cu3O6.95YBa_{2}Cu_{3}O_{6.95} from microwave measurements of high quality single crystals

Systematic microwave surface impedance measurements of YBCO single crystals grown in $BaZrO_3$ crucibles reveal new properties that are not directly seen in similar measurements of other YBCO samples. Two key observations obtained from complex conductivity are: a new normal conductivity peak at around 80K and additional pairing below 65K. High pressure oxygenation of one of the crystals still yields the same results ruling out any effect of macroscopic segregation of O-deficient regions. A single complex order parameter cannot describe these data, and the results suggest at least two superconducting components. Comparisons with model calculations done for various decoupled two-component scenarios (i.e. s+d, d+d) are presented. Systematics of three single crystals show that the 80K quasiparticle peak is correlated with the normal state inelastic scattering rate. Close to Tc, the data follow a mean-field behavior. Overall, our results strongly suggest the presence of multiple pairing temperature and energy scales in $YBa_{2}Cu_{3}O_{6.95}$.Comment: 14 pages, 2-column, Revtex, 5 embedded postscript figures, uses graphicx. Postscript version also available at http://sagar.physics.neu.edu/preprints.htm