Infrared Conductivity Spectra Research Articles

Origin of the mid-infrared peaks in the optical conductivity of LaMnO3

We calculated the infrared conductivity spectrum of orbitally ordered LaMnO3 in phonon frequency and overtone frequency ranges. We considered orbital exchange, Jahn-Teller electron–phonon coupling, and phonon–phonon coupling. The fundamental excitation of the phonon-coupled orbiton was only Raman active, not infrared active, while its overtone modes were both Raman and infrared active. Our calculations reproduced the small peaks near 1300 cm−1 observed both in Raman scattering and infrared conductivity spectra, as consistent with previous experimental results.

High-frequency dielectric response of polyaniline pellets as nanocomposites of metallic emeraldine salt and dielectric base

Conducting polyaniline (PANI) salt and dielectric PANI base pellets were studied in the infrared (IR) and terahertz (THz) range at room temperature and compared with the IR transmission spectra of powdered samples dispersed in potassium bromide and with those of thin films. The IR reflectivity combined with THz transmission was fitted to calculate the permittivity and IR conductivity spectra. In addition to vibration modes and Drude contribution of the free carriers in the conducting PANI, strong and broad mid-IR absorption was detected and assigned to localized carriers. Additional absorption in the far IR, much stronger in case of the conducting PANI, was assigned to Boson peak. Good agreement between the IR conductivity peaks and those from the absorbance evaluated from the transmission measurements was obtained and differences between the spectra of PANI salt and base are discussed. The dielectric and conductivity spectra of PANI salt were modeled as the spectra of a composite of PANI base and metallic PANI reported by Lee et al., Nature 441, 65 (2006). Effective medium approach using three different models was applied and discussed. The best results, including the semiconductor-like temperature dependence of the low-frequency conductivity, were obtained with the generalized Lichtenecker model. The strong permittivity increase on decreasing frequency below the THz range is discussed and compared with the literature data.

The evolution of electronic structure in few-layer graphene revealed by optical spectroscopy

The massless Dirac spectrum of electrons in single-layer graphene has been thoroughly studied both theoretically and experimentally. Although a subject of considerable theoretical interest, experimental investigations of the richer electronic structure of few-layer graphene (FLG) have been limited. Here we examine FLG graphene crystals with Bernal stacking of layer thicknesses N = 1,2,3,...8 prepared using the mechanical exfoliation technique. For each layer thickness N, infrared conductivity measurements over the spectral range of 0.2-1.0 eV have been performed and reveal a distinctive band structure, with different conductivity peaks present below 0.5 eV and a relatively flat spectrum at higher photon energies. The principal transitions exhibit a systematic energy-scaling behavior with N. These observations are explained within a unified zone-folding scheme that generates the electronic states for all FLG materials from that of the bulk 3D graphite crystal through imposition of appropriate boundary conditions. Using the Kubo formula, we find that the complete infrared conductivity spectra for the different FLG crystals can be reproduced reasonably well within the framework a tight-binding model.

Observation of an Electric-Field-Induced Band Gap in Bilayer Graphene by Infrared Spectroscopy

It has been predicted that application of a strong electric field perpendicular to the plane of bilayer graphene can induce a significant band gap. We have measured the optical conductivity of bilayer graphene with an efficient electrolyte top gate for a photon energy range of 0.2-0.7 eV. We see the emergence of new transitions as a band gap opens. A band gap approaching 200 meV is observed when an electric field approximately 1 V/nm is applied, inducing a carrier density of about 10(13) cm(-2)}. The magnitude of the band gap and the features observed in the infrared conductivity spectra are broadly compatible with calculations within a tight-binding model.

Effect of Metal Substitution in BSCCO Ceramic Superconductors

The fabrication, electrical, and optical properties of ceramic Bi2-xMxSr2Ca2Cu3Oy (where M=Al or Ni and x=0.3) superconductors are described. Resistivity measurements reveal that when Al is partially substituted at the bismuth site in the BiSrCaCuO compound, T c decreases to 77 K and the nickel-substituted compound shows a T c of 70 K, compared to the undoped ceramic BiSrCaCuO samples with T c of 85 K. Infrared reflectance measurements, which cover the 50-4000 cm−1 range, find that the reflectance from the ceramic samples decreases in the metal-doped samples. The observed phonon modes in the infrared conductivity spectra (obtained by Kramers-Kronig analysis) decrease in strength and some features are completely smeared out. Furthermore some of the modes are observed to shift slightly in the frequency on doping, the change in the T c and vibrational modes is attributed to destabilization of 2223 phase.

Optical study on the charge ordering in the organic conductorβ−(ET)2PF6

We report Raman and infrared spectra at room and low temperature on single crystals of $\ensuremath{\beta}\ensuremath{-}(\mathrm{ET}{)}_{2}{\mathrm{PF}}_{6},$ which exhibits a metal-insulator (M-I) transition at 297 K ${(=T}_{M\ensuremath{-}I}).$ Both spectra exhibit drastic changes below ${T}_{M\ensuremath{-}I}.$ By analyzing the $\mathrm{C}=\mathrm{C}$ stretching mode $({\ensuremath{\nu}}_{2})$ appearing in the Raman spectra, we conclude that the charges on ET molecules are disproportionate below ${T}_{M\ensuremath{-}I}$ and evaluate the charge densities on ET molecules. The infrared conductivity spectra demonstrate that the fluctuation of the charge order survives still in the metallic phase above ${T}_{M\ensuremath{-}I}.$ On the basis of a mean-field approximation, we calculate the infrared conductivity spectra in the insulating phase of this salt. The calculated spectra are consistent with those experimentally obtained.

Charge disproportionation and its ordering pattern in θ and α types of BEDT-TTF salts studied by raman and infrared spectroscopies

Raman and infrared conductivity spectra are investigated to reveal two-dimensional patterns of stripe charge-ordering in θ-(BEDT-TTF) 2 RbZn(SCN) 4 and α′-(BEDT-TTF) 2 IBr 2 . Either compounds exhibit similar multiple peak pattern around the C=C stretching region in both Raman and infrared spectra owing to the electron-molecular vibration coupling and charge disproportionation. We discuss the spatial symmetry of the charge-ordering stripes from the group theoretical arguments on the selection rule of these peaks.

ANOMALOUS INFRARED PROPERTIES OF THE OXYGEN DEFICIENT CUPRATE La8-xSrxCu8O20

We present the first infrared conductivity spectra on single crystals of oxygen deficient cuprate La 8- x Sr x Cu 8 O 20. The infrared conductivity for a sample with x = 2.24 behaves as a function of temperature according to a conventional Drude model. On the other hand, the low energy excitations for a sample with x =1.56 are itinerant polarons. Below T =100 K , polarons localize and this effect can be related to the anomalies present in the transport and magnetic properties around this temperature.

Two Interesting Features in the Infrared and Raman Spectra of the 12K Organic Superconductor κ-(ET)2Cu[N(CN)2]Br

Abstract Two of the larger features in the infrared conductivity spectra of χ-(ET)2Cu[N(CN)2]Br are analysed and one is reassigned with the aid of infrared and Raman spectra of isotopically-substituted compounds.

Infrared conductivity in the normal state of cuprate thin films

This paper summarizes the main features of the infrared conductivity spectra in the 500–7000 cm−1 range derived from room-temperature reflectivity measurements performed on a variety of thin films (YBa2Cu3O7, BiSrCaCuO). The conductivity of these samples decreases with frequency as a power law with an exponent ∼0.7, and the scattering rate increases linearly with frequency. A detailed comparison with the conductivity for the marginal Fermi liquid (Abrahams, Littlewood, and Varma) and for the Luttinger liquid (Anderson) is performed. In the former case, the effect of temperature appears crucial even in this energy range (ω≥T) and allows one to achieve satisfactory fits. In the latter case, both the conductivity and the scattering rate may be accounted for by a single parameter.

Infrared conductivity and electron-molecular-vibration coupling in the organic superconductor di

Polarized reflectance spectra of the organic superconductors protonated and deuterated \ensuremath{\kappa}-(BEDT-TTF${)}_{2}$[Cu(NCS${)}_{2}$] (H and D salts) [BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene] were measured over the range from 500 to 28 000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ at room temperature with light polarizations parallel to the crystallographic b and c axes which lie on the two-dimensional conducting plane. Polarized reflectance spectra of the organic superconductor \ensuremath{\beta}-(BEDT-TTF${)}_{2}$${\mathrm{I}}_{3}$ and the organic metal \ensuremath{\beta}''-(BEDT-TTF${)}_{2}$${\mathrm{AuBr}}_{2}$ were also measured in order to discuss the influence of different molecular arrangements and hydrogen-anion contacts on the electronic and vibrational properties of these salts. Frequency-dependent conductivities were calculated by a Kramers-Kronig transformation. By comparison of the infrared conductivity spectra of the H and D salts, the vibrational transitions induced by electron--molecular-vibration (EMV) coupling were clearly distinguished from the carbon-hydrogen bending modes of the BEDT-TTF moiety. A Drude-Lorentz dielectric function was used to evaluate the optical transport parameters and an excitation frequency of the charge-transfer (CT) band superimposed on a plasma-edge-like dispersion which was observed for each compound. The EMV-coupling energies are semiquantitatively estimated to be ca. 70 meV for both the H and D salts from the frequencies of the EMV coupling transition and the CT band in terms of the dimer charge-oscillation model. By use of the coupling energy, various parameters describing the superconducting state were evaluated and discussed on the basis of the BCS theory in a weak-coupling limit. Finally, the magnitudes of hydrogen-anion interaction were estimated from the frequency shifts of the C-H bending modes of the BEDT-TTF moiety.

Optical and infrared properties of tetramethyltetraselenafulvalene [(TMTSF)2X] and tetramethyltetrathiafulvalene [(TMTTF)2X] compounds

The electronic structure of the organic conductors bis-tetramethyltetraselenafulvalene-$X$ [${(\mathrm{TMTSF})}_{2}X$] and bis-tetramethyltetrathiafulvalene-$X$ [${(\mathrm{TMTTF})}_{2}X$] has been investigated by means of polarized optical and infrared reflectance measurements. Analysis of plasma edges in reflectance is used to extract information on transfer integrals. Measurements of infrared reflectance provide information on the energy of charge-transfer processes and on electron-molecular vibration coupling. Far-infrared measurements allow comparison with low-frequency transport properties, and give clues to the transport mechanisms. The main results may be summarized as follows: The ${(\mathrm{TMTSF})}_{2}X$ class of materials has chain-axis transfer integrals of order 0.25 eV at 300 K and 0.28 eV at 30 K. The $b$-axis transfer integral is found to vary from 18 to 24 meV for different $X$. The ${(\mathrm{TMTTF})}_{2}X$ class has a chain-axis transfer integral of the order 0.18-0.20 eV. No $b$-axis plasma edge is observable. The infrared conductivity spectra of the materials consist of a broad electronic band with superimposed vibrational fine structure. The band is centered at 300 ${\mathrm{cm}}^{\ensuremath{-}1}$ in the best ${(\mathrm{TMTSF})}_{2}X$ conductors and at 2200 ${\mathrm{cm}}^{\ensuremath{-}1}$ in ${(\mathrm{TMTTF})}_{2}$P${\mathrm{F}}_{6}$, an organic conductor of moderate conductivity. The electron-molecular vibration coupling constants for TMTSF and TMTTF appear to be qualitatively similar to those of TTF (tetrathiafulvalene). A new feature is the observation of considerable coupling to modes involving methyl groups, suggesting that a sizable charge density is located near these groups. The electronic band in ${(\mathrm{TMTSF})}_{2}$P${\mathrm{F}}_{6}$ sharpens at low temperature, and a pseudogap at 180 ${\mathrm{cm}}^{\ensuremath{-}1}$ is formed at temperatures above the metal-insulator transition. This behavior is discussed in terms of a possible spin-density-wave contribution to the conductivity. The spin-density-wave amplitude is estimated to be $0.02{\ensuremath{\mu}}_{B}$.