P. Maselli

INFRARED ABSORPTION FROM CHARGE DENSITY WAVES IN MAGNETIC MANGANITES

The infrared absorption of charge density waves coupled to a magnetic background is first observed in two manganites La{1-x}Ca{x}MnO{3} with x = 0.5 and x = 0.67. In both cases a BCS-like gap 2 Delta (T), which for x=0.5 follows the hysteretic ferro-antiferromagnetic transition, fully opens at a finite T{0} < T{Neel}, with 2 Delta(T{0})/kT{c} close to 5. These results may also explain the unusual coexistence of charge ordering and ferromagnetism in La{0.5}Ca{0.5}MnO{3}.

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 {ital n} type and {ital p} type. In Nd{sub 2}CuO{sub 4{minus}{ital 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 {approximately} 1000 cm{sup {minus}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{sup {minus}1}. Most of the above infrared features are shown to survive in the metallic phase of Nd{sub 2{minus}{ital x}}Ce{sub {ital x}}CuO{sub 4{minus}{ital y}}, Bi{sub 2}Sr{sub 2}CuO{sub 6}, and YBa{sub 2}Cu{sub 3}O{sub 7{minus}{ital 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. {copyright} {ital 1996 The American Physical Society.}

Observation of Charge-Density-Wave Excitations in Manganites

In the optical conductivity of four different manganites with commensurate charge order (CO), strong peaks appear in the meV range below the ordering temperature T_{CO}. They are similar to those reported for one-dimensional charge density waves (CDW) and are assigned to pinned phasons. The peaks and their overtones allow one to obtain, for La_{1-n/8}Ca_{n/8}MnO_{3} with n=5, 6, the electron-phonon coupling, the effective mass of the CO system, and its contribution to the dielectric constant. These results support a description of the CO in La-Ca manganites in terms of moderately weak coupling and of the CDW theory.

Evolution of a Polaron Band through the Phase Diagram ofNd2−xCexCuO4−y

The evolution with doping and temperature of the polaronic absorption in a family of high-T

Polaron imprints in the infrared spectra of Nd2CuO4−y

_c

Infrared optical properties of perovskite substrates for high-Tc superconducting films

superconductors is first followed here across the whole phase diagram. The polaron band softens with increasing doping, is still present in the superconducting phase, and vanishes in the overdoped metallic phase. These results point toward the existence of polaron aggregates at low temperature and may provide an experimental ground for the increasing number of theoretical descriptions of many-polaron systems.

Charge-localization effects in the infrared transmittance of layered perovskites

Abstract In addition to four Eu phonons, Nd2CuO4-y single crystals show far-infrared reflectivity peaks which depend on temperature and y. They provide evidence for the insurgence of local modes induced by doping. The so called d- or J-band at ∼ 1000 cm-1 can be partially resolved, and shown to be produced by combination bands of those local modes. These results provide direct evidence for polaron formation in this compound.

Analysis of translational band observed in gaseous and liquid Kr–Ar mixtures

Abstract Perovskite-like compounds like LaAlO 3 , LaGaO 3 , and NdGaO 3 have been recently proposed as substrates for high- T c superconducting films with optimum mechanical and dielectric properties. Measurements of room temperature and 25 K reflectance have been performed from 100 to 20 000 cm -1 in these compounds. The complex dielectric functions ~ϵ, as well as phonon mode parameters, have been obtained by a best fitting procedure in terms of Lorentzian oscillators. The use of these compounds in the study of the optical properties of thin films is discussed.

Fano effect in the a-b plane of Nd{sub 1.96}Ce{sub 0.04}CuO{sub 4+y}: Evidence of phonon interaction with a polaronic background

In a previous study it has been shown that the formation of charged superlattices in Ni- and Mn-based perovskites opens a gap in their infrared polaronic background. This signature of charge localization is here used to compare the behavior of the polaronic carriers in Sr2MnO4, La2NiO4+y, Nd2CuO4−y and Nd1.85Ce0.15CuO4−y. It is shown that, as the temperature decreases, the carriers tend to localize in the nickelate and the manganite, to slightly increase their mobility in both cuprates.

COMPLEX REFRACTIVE INDEX OF MgO IN THE FAR INFRARED FROM TRANSMISSION AND REFLECTION MEASUREMENTS

The translational absorption spectrum of Kr–Ar gaseous and liquid mixtures, arising from the collision‐induced dipole moment, has been measured in the 13–300 cm−1 frequency region. From the translational band obtained in the gaseous phase we have derived the intensity and the range ρo of the induced dipole moment. The high frequency part of the spectrum observed in the gaseous phase is similar in shape to that observed in the liquid phase. The low frequency part of the spectrum obtained in the latter case is dominated by the intercollisional interference effect and is discussed in terms of a simple model for the motion of the impurities in the liquid. We have also derived the value of the mean square force 〈F2〉 acting on the impurity dissolved in the liquid matrix.