Low-energy Conduction Research Articles

Synthesis and photophysical properties of ruthenium(II) charge transfer sensitizers containing 4,4′-dicarboxy-2,2′-biquinoline and 5,8-dicarboxy-6,7-dihydro-dibenzo[1,10]-phenanthroline

Ruthenium (II) complexes of the type cis-Ru(H 2dcbiq) 2X 2 and cis-Ru(H 2dcdhph) 2X 2, where H 2dcbiq=4,4′-dicarboxy-2,2′-biquinoline, H 2dcdhph=5,8-dicarboxy-6,7-dihydro-dibenzo[1,10]-phenanthroline, and X=Cl −, NCS − or CN −, have been synthesized and spectroscopically characterized. The resulting complexes show a broad and intense metal-to-ligand charge transfer (MLCT) band in the visible region with a peak between 580 and 700 nm and are emissive at room temperature. The ground-state first p K a value of cis-Ru(H 2dcbiq) 2(NCS) 2 ( 2) was determined to be 2.9 by the spectrophotometric method. Photoelectrochemical measurements show that all dyes, when anchored to a nanocrystalline TiO 2 film electrode, present low light-harvesting efficiencies due to inefficient driving force for electron injection into the conduction band of TiO 2 from their lower energy MLCT band. The photoelectrochemical performance of 2 was also investigated on a number of oxide semiconductor thin films such as Nb 2O 5, ZnO, SnO 2 and In 2O 3. The results show that a high value of short-circuit photocurrent ( J sc) is observed for the semiconductors having a low-energy conduction band potential (SnO 2 and In 2O 3). In the dye 2-sensitized TiO 2 film, the absorbed photon-to-current conversion efficiency (APCE) spectrum shows an absorption band selective electron injection yield, while a wavelength independent electron injection yield is observed when dye 2 is anchored to SnO 2. These results indicate that the lowest excited MLCT state is energetically favorable for electron injection into the conduction band of SnO 2 but not for TiO 2.

Optical Study of Metal-Insulator Transition in the Ferromagnetic Ground State of La1-xSrxMnO3

Optical spectra for the ferromagnetic ground states have been investigated for single crystals of perovskite-type manganese oxides La 1- x Sr x MnO 3 with finely varying hole-doping concentration ( x ) around the metal-insulator (MI) phase boundary ( x c ∼ 0.16). For such a fully spin-polarized state, orbital degree of freedom may play an important role in the MI transition, which is anticipated to be figured out in terms of optical spectroscopy. In the ferromagnetic ground state, the low-energy optical conductivity spectra are observed to vary from Drude-like (typically for x =0.4) to incoherent broad feature (for x < 0.2) with decreasing x . The spectral-weight analysis shows that the kinetic energy of conduction electron scarcely changes with change of x when viewed on a large energy scale (about 1 eV) whereas the coherent response on a low energy scale (∼0.1 eV) is critically suppressed toward the insulating phase. Orbital correlation and its coupling with lattice dynamics are likely responsible for such an anomalous variation of charge dynamics. Far-infrared pseudo gap structure observed in the ferromagnetic metallic phase of x =0.16 on the verge of MI transition suggests the charge localization affected by randomness and non-local interaction in the course of orbital and charge ordering.

Electronic phase diagram of the two-leg ladder compound Sr 14− xCa xCu 24O 41 as investigated by the transport property measurements

We study the charge transport properties of the two-legged ladder compound, Sr 14− x Ca x Cu 24O 41, with various Ca concentrations. For 0 ≤ x ≤ 3, the charge dynamics of this compound is characterized by the strong suppression in the low-energy optical conductivity (σ(ω)) with decreasing temperature, which is not observed for higher x. The decrease in σ(ω) is responsible for the insulating behavior in these regions and may be associated with the ordering of hole carriers within the ladders. This possible charge ordering is suppressed by applying high pressure and the system becomes metallic for x ∼ 0, although the metallic state is distinct from that for higher Ca region where the superconducting ground state is realized under high pressure. It is considered that the charge ordered state observed for x = 0 is another ground state in this system which competes with superconductivity, and that the Ca substitution destabilizes the charge ordered state.

Itinerant ferromagnetism in the narrow band limit

It is shown that in the narrow band, strong interaction limit the paramagnetic state of an itinerant ferromagnet is described by the disordered local moment state. As a result, the Curie temperature is orders of magnitude lower than what is expected from the large exchange splitting of the spin bands. An approximate analysis has also been carried out for the partially ordered state, and the result explains the temperature evolvement of the magnetic contributions to the resistivity and low-energy optical conductivity of CrO 2.

Low-energy conductivity ofPF6-doped polypyrrole

In this paper we present a set of temperature-dependent reflectivity and dc conductivity measurements on a series of ${\mathrm{PF}}_{6}$-doped polypyrrole samples differing only in their synthesis temperatures, spanning the metal-insulator transition. From the reflectivity we have derived optical constants, taking particular care that they are Kramers-Kronig consistent. We have analyzed our data in the light of two proposed models for the structure of conducting polymers, homogeneous and inhomogeneous disorder, and find that the observed decrease in the low-energy ac conductivity can be accounted for by a simple generalization of the heterogeneous disorder model used to explain the dc conductivity. The homogeneous localization-modified Drude model fails to satisfactorily model the ac conductivity at low energy.

Optical conductivity of manganites: Crossover from Jahn-Teller small polaron to coherent transport in the ferromagnetic state

We report on the optical properties of the hole-doped manganites Nd_{0.7}Sr _{0.3}MnO_{3}, La_{0.7}Ca_{0.3}MnO_{3}, and La_{0.7}Sr_{0.3}MnO_{3}. The low-energy optical conductivity in the paramagnetic-insulating state of these materials is characterized by a broad maximum near 1 eV. This feature shifts to lower energy and grows in optical oscillator strength as the temperature is lowered into the ferromagnetic state. It remains identifiable well below Tc and transforms eventually into a Drude-like response. This optical behavior and the activated transport in the paramagnetic state of these materials are consistent with a Jahn-Teller small polaron. The optical spectra and oscillator strength changes compare well with models that include both double exchange and the dynamic Jahn-Teller effect in the description of the electronic structure.

Ab initio calculations of the electronic structure and optical properties of ferroelectric tetragonal

The electronic structure, charge distribution, effective charge, and charge transfer in ferroelectric tetragonal are carefully studied using a local density functional potential and a self-consistent ab initio LCAO (linear combination of atomic orbitals) method. It is shown that the band gap and low-energy conduction band can be calculated with a reasonable accuracy when the ab initio LCAO method is used with an optimum basis set of atomic orbitals. The calculated optical spectrum, band gap, and effective mass of , obtained from the calculated electronic structure, are in good agreement with experimental results.

A 1- and 2-D 19F MAS NMR Study of Fluoride-Ion Mobility in α PbF2

One- and two-dimensional 19F high-speed MAS NMR are used to probe motion of the fluorine sublattices in the fluoride-ion conductors α PbF2 and potassium fluoride-doped α PbF2. The two crystallographic sites F(1) and F(2) are resolved in the 19F spectrum of the pure material and are assigned on the basis of their 207Pb−19F J-coupling. A resonance from fluoride-ions jumping rapidly between the two sites is also observed above 120 °C, which increases in intensity as the temperature is raised. The resonance from the mobile fluoride-ions is observed at room temperature for the α PbF2 sample containing potassium impurities and for samples that have been intentionally doped with KF by direct reaction of KF and PbF2. The correlation times of the rigid and mobile fluoride-ions in these samples differ by more than 2 orders of magnitude, and 2-D magnetization exchange methods show that the exchange between these two sets of fluoride-ions is negligible. The vacancies produced by potassium doping appear to remain closely associated with the potassium defects at low temperatures, and the mobile fluoride-ions at these temperatures are assigned to fluoride-ions near the potassium defects. In contrast, in the pure, or more uniformly potassium-doped, materials, the vacancies are more uniformly distributed over the solid, resulting in spectra with a narrower range of correlation times for fluoride-ion motion. Finally, a low activation energy conduction pathway between F(1) and F(2) sites along the y-axis is proposed to rationalize the rapid F(1)↔F(2) fluoride-ion diffusion.

From Tomonaga-Luttinger to Fermi liquid in transport through a tunneling barrier

Finite length of a one channel wire results in crossover from a Tomonaga-Luttinger to Fermi liquid behavior with lowering energy scale. In condition that voltage drop $(V)$ mostly occurs across a tunnel barrier inside the wire we found coefficients of temperature/voltage expansion of low energy conductance as a function of constant of interaction, right and left traversal times. At higher voltage the finite length contribution exhibits oscillations related to both traversal times and becomes a slowly decaying correction to the scale-invariant $V^{1/g-1}$ dependence of the conductance.

Infrared response of ordered polarons in layered perovskites.

We report on the infrared absorption spectra of three oxides, where charged superlattices have been recently observed in diffraction experiments. In ${\mathrm{La}}_{1.67}$${\mathrm{Sr}}_{0.33}$Ni${\mathrm{O}}_{4}$, polaron localization is found to suppress the low-energy conductivity through the opening of a gap and to split the ${E}_{2u}\ensuremath{-}{A}_{2u}$ vibrational manifold of the oxygen octahedra. Similar effects are detected in ${\mathrm{Sr}}_{1.5}$${\mathrm{La}}_{0.5}$Mn${\mathrm{O}}_{4}$ and in ${\mathrm{La}}_{2}\mathrm{Ni}{\mathrm{O}}_{4+y}$, with peculiar differences related to the type of charge ordering.

Influence of oxygen-ordering kinetics on Raman and optical response in YBa2Cu3O6.4.

Kinetics of the optical and Raman response in YBa_2Cu_3O_{6.4} were studied during room temperature annealing following heat treatment. The superconducting T_c, dc resistivity, and low-energy optical conductivity recover slowly, implying a long relaxation time for the carrier density. Short relaxation times are observed for the B_{1g} Raman scattering -- magnetic, continuum, and phonon -- and the charge transfer band. Monte Carlo simulations suggest that these two relaxation rates are related to two length scales corresponding to local oxygen ordering (fast) and long chain and twin formation (slow).

Charge dynamics of Ce-based compounds: Connection between the mixed valent and Kondo-insulator states.

The reflectivities of the mixed-valent compounds Ce${\mathrm{Sn}}_{3}$ and Ce${\mathrm{Pd}}_{3}$ have been measured to obtain the optical conductivity as a function of temperature. Both compounds show a renormalization of the low-energy Drude conductivity at low temperatures. In addition, for the low-carrier-density compound Ce${\mathrm{Pd}}_{3}$, but not for high-carrier-density Ce${\mathrm{Sn}}_{3}$, substantial spectral weight lost from the conductivity below 1000 ${\mathrm{cm}}^{\ensuremath{-}1}$ reappears in the frequency range 2000-10 000 ${\mathrm{cm}}^{\ensuremath{-}1}$, indicating that Ce${\mathrm{Pd}}_{3}$ can be considered a lightly doped Kondo insulator.

Conductivity in quasicrystals via hierarchically variable-range hopping.

This paper takes the structure example of the AlPdMn quasicrystal, as determined from diffraction data, to derive useful self-similarity rules in consistency with composition and atomic valence constraints. These rules are then accounted for to explain inelastic-neutron-scattering data and thermal conductivity behavior. Basically, three regimes can be identified: extended low-energy phonon conductivity at low temperature following roughly a ${\mathit{T}}^{2}$ power law which transforms into a plateau of constant conductivity and, at higher temperature, a phonon assisted localized-state hopping mechanism with a ${\mathit{T}}^{3/2}$ power law and hierarchically distributed hopping distances. Electrical conductivity is also briefly analyzed along the same approach: accordingly, pure perfect quasicrystals should show a \ensuremath{\sigma}(T)\ensuremath{\propto}T behavior with deviation in ${\mathit{T}}^{0.5}$ at low temperature and in ${\mathit{T}}^{3/2}$ at high temperature coming from quasiperiodicity breaking. \textcopyright{} 1996 The American Physical Society.

Impact ionization and light emission in InAlAs/InGaAs heterostructure field-effect transistors

We present measurements on impact ionization effects, real space transfer of holes and electrons, and light emission occurring in n-channel InAlAs/InGaGs heterostructure Field-Effect Transistors based on InP operated at high electric fields and at different temperatures. The channel electrons heated by the lateral electric field give rise to impact ionization and light emission. By comparing the electrical characteristics and the integrated light intensity in different energy ranges and at different temperatures, we were able to identify two main different light emission mechanisms: conduction to conduction-band transitions for low energy photons and conduction to valence-band transitions for high energy photons. The correlation between the gate current and the light intensity allowed us to separately evaluate the electron and hole components of the gate current. >

Effects of Carrier-Doping on Optical Conductivity in HighTcCopper Oxides

The optical conductivity in high T c Cu oxides is theoretically examined by diagonalizing exactly the Hamiltonian of a Cu 4 O 13 cluster and using the Kubo formula. We show how the optical conductivity caused by the charge transfer from Cu to O ions in the insulator is depressed and how the low-energy conductivity of the Drude type emerges when electron- and hole-carriers are introduced into the CuO 2 plane.

Kinetics of Dehydrogenation of 2-Propanol on La2−xSrxCuO4−y

Abstract The catalytic properties of mixed oxides La2−xSrxCuO4−y (0.0≤x≤1.0 and y=non-stoichiometry) having K2NiF4 structure were investigated with regard to the effect of changes in the lattice parameters, the oxidation state of copper and oxygen nonstoichiometry in order to correlate the catalytic activity with physicochemical properties. The decomposition of 2-propanol was studied by pulse and flow reactor techniques. The flow reactor data was analyzed by statistical methods to deduce the mechanism on the basis of Langmuir–Hinshelwood and Hougen–Watson models. The reaction followed the first order kinetics with dual site surface reaction as the rate controlling step. The kinetic and thermodynamic parameters obtained from flow reactor data are comparable to those obtained from pulse reactor studies. The higher catalytic activity as indicated by low activation energy and higher conductivity value of La1.8Sr0.2CuO3.96 among the series have been attributed to the presence of higher concentration of Cu(III).

Experimental photoemission results on the low-energy conduction bands of silicon.

A l'aide des differentes raies d'un laser a ions krypton on mesure les courbes de distribution d'energie d'electrons photoemis pres de la bande interdite, a partir de surfaces de Si(100) dope de type p, a 90 K. Dans le regime U.V., la raie a 3,1 eV semble due aux electrons accumules au bord et au minimum de la bande de conduction lors du processus de thermalisation

Electron spectroscopy of conduction bands in graphite

The dispersion of certain graphite conduction bands, lying up to 22 eV above the Fermi level, has been determined from secondary electron features appearing in synchrotron radiation excited angle-resolved photoemission data obtained for a single crystal of natural graphite. Such high energy conduction bands have not been observed with inverse photoemission. In addition, constant-final-state photoemission spectroscopy has been used to determine the position of the interlayer state in graphite. The results highlight discrepancies between photoemission and secondary electron emission and inverse photoemission over the position of low energy conduction bands in graphite.

Enhanced far-infrared absorption inCePd3andYbCu2Si2. III. Comparison of a resonant-scattering model with experiment

We present a model calculation for the low-energy (~10-meV) optical conductivity of valence-fluctuation compounds for the case of resonant scattering of conduction electrons from a scattering level lying within 10 meV of the Fermi energy. The model is characterized by an enhancement of the optical absorption in the far infrared over that of a normal metal, and it agrees reasonably well with the measured low-temperature optical conductivity of ${\mathrm{CePd}}_{3}$ and ${\mathrm{YbCu}}_{2}$${\mathrm{Si}}_{2}$ with suitable choices of the model parameters. In the case of ${\mathrm{CePd}}_{3}$, in which optic phonons are observed, the effects of including the optic-phonon modes in the model calculations are also discussed.

Solution grown mercuric iodide crystals as low-energy gamma-ray conduction type detectors

Single crystals of mercuric iodide (HgI2) of detector quality have been grown using the solution-growth method. The solvents used are dimethylsulphoxide (DMSO) and KI-acetone. Purification of HgI2, growth techniques and detector fabrication are discussed. A resolution of 8 keV (FWHM) at 23 keV has been obtained at room temperature for crystals grown from the KI-acetone system.