CPA density of states and conductivity in a double-exchange system containing impurities

We study density of states and conductivity of the doped double-exchange system, treating interaction of charge carriers both with the localized spins and with the impurities in the coherent potential approximation. It is shown that under appropriate conditions there is a gap between the conduction band and the impurity band in paramagnetic phase, while the density of states is gapless in ferromagnetic phase. This can explain metal-insulator transition frequently observed in manganites and magnetic semiconductors. Activated conductivity in the insulator phase is numerically calculated. Received 13 June 2000 and Received in final form 5 January 2001     

Frustrated superexchange interaction versus orbital order in a LaVO3 crystal

Measurements of magnetic, transport properties, thermal conductivity, and magnetization under pressure as well as neutron diffraction have been made on a single crystal and powder sample of LaVO3. The Néel temperature was found to mark a transition from the phase with both frustrated superexchange interaction and spin-orbit lambdaL.S coupling to the phase where the Jahn-Teller orbital-lattice coupling dominates. The dramatic reduction of absolute entropy in the paramagnetic phase is explained in terms of forming a long-range coherent state due to the interference between frustrated orbits and spins.     

Non-ohmic conductivity of barely localized electrons in three dimensions

We observe a rapid temperature variation and an electric field dependence of the d.c. conductivity in a carefully characterized sample of P doped Si. These results, combined with measurements of Raman scattering, far-infrared absorption and magnetic susceptibility, indicate a large characteristic electronic length (10⁴ times the donor Bohr radius) at a donor concentration just below the critical density of the metal-insulator transition.     

Collapse of coherent quasiparticle states in theta-(BEDT-TTF)2I3 observed by optical spectroscopy

Optical conductivity measurements on the organic metal theta-(BEDT-TTF)2I3 revealed that the system crosses over rapidly from a coherent quasiparticle state to an incoherent state with temperature. Despite the metallic temperature dependence of resistivity, a well-defined Drude peak at low temperatures turns into a far-infrared peak with temperature. The peak energy shifts to higher frequencies and, simultaneously, the spectral weight is transferred to high frequencies beyond the electron band width. These characteristics imply that theta-(BEDT-TTF)2I3, so far believed to be a typical metal, is in fact a strongly correlated electron system with "bad-metal" character.     

Far infrared photoconductivity of TTF-TCNQ

The spectral dependence of the far-infrared photoconductivity of TTF-TCNQ at low temperatures shows an interband transition with an edge at approximately 22 meV. We associate this gap with the d.c. conductivity maximum near 70 K, and suggest that at least two distinct mean-field transition temperatures are important for the observed electrical and magnetic properties of the material.     

Disorder and pseudogap in strongly correlated systems: Phase diagram in the DMFT + Σ approach

The influence of disorder and pseudogap fluctuations on the Mott insulator-metal transition in strongly correlated systems has been studied in the framework of the generalized dynamic mean field theory (DMFT + Σ approach). Using the results of investigations of the density of states (DOS) and optical conductivity, a phase diagram (disorder-Hubbard interaction-temperature) is constructed for the paramagnetic Anderson-Hubbard model, which allows both the effects of strong electron correlations and the influence of strong disorder to be considered. Strong correlations are described using the DMFT, while a strong disorder is described using a generalized self-consistent theory of localization. The DOS and optical conductivity of the paramagnetic Hubbard model have been studied in a pseudogap state caused by antiferromagnetic spin (or charge) short-range order fluctuations with a finite correlation length, which have been modeled by a static Gaussian random field. The effect of a pseudogap on the Mott insulator-metal transition has been studied. It is established that, in both cases, the static Gaussian random field (related to the disorder or pseudogap fluctuations) leads to suppression of the Mott transition, broadening of the coexistence region of the insulator and metal phases, and an increase in the critical temperature at which the coexistence region disappears.     

First-order isostructural Mott transition in highly compressed MnO

We present evidence for an isostructural, first-order Mott transition in MnO at 105+/-5 GPa, based on high-resolution x-ray emission spectroscopy and angle-resolved x-ray diffraction data. The pressure-induced structural and spectral changes provide a coherent picture of MnO phase transitions from paramagnetic B1 to antiferromagnetic distorted B1 at 30 GPa, to paramagnetic B8 at 90 GPa, and to diamagnetic B8 at 105+/-5 GPa. The last is the Mott transition, accompanied by a significant loss of magnetic moment, an approximately 6.6% volume collapse and the insulator-metal transition as demonstrated by recent resistance measurements.     

Coherent diffraction radiation as a source of radiation in far-infrared and terahertz range

The results of measurements of the coherent diffraction radiation yield generated by femtosecond electron beam are presented. It is shown that the simulated and experimental data agreed quite well. The coherent diffraction radiation can be considered as a real candidate to develop a source of radiation in far-infrared and terahertz range.     

Spin-phonon coupling in highly correlated transition-metal monoxides

We report on the optical response of the highly correlated transition-metal monoxides MnO, FeO, CoO and NiO in the far-infrared regime. The main focus is put on spin-phonon coupling effects, which are found to significantly influence the lattice dynamics in the magnetically ordered phase. Measurements have been performed in the ordered and paramagnetic state for temperatures up to 550 K. A clear splitting of the cubic mode accompanying the transition into long range magnetic order can be identified in MnO, CoO and NiO. In the case of FeO it is argued that an anisotropic phonon response seems to be very likely, though it could not be observed directly. The results are compared to recent experimental and theoretical studies in frustrated magnets, which predict the splitting of zone center phonon modes induced by a non-cubic spin-density distribution.     

Crossover behavior of the anomalous Hall effect and anomalous nernst effect in itinerant ferromagnets

The anomalous Hall effect (AHE) and anomalous Nernst effect (ANE) are experimentally investigated in a variety of ferromagnetic metals including pure transition metals, oxides, and chalcogenides, whose resistivities range over 5 orders of magnitude. For these ferromagnets, the transverse conductivity sigma{xy} versus the longitudinal conductivity sigma{xx} shows a crossover behavior with three distinct regimes in accordance qualitatively with a recent unified theory of the intrinsic and extrinsic AHE. We also found that the transverse Peltier coefficient alpha{xy} for the ANE obeys the Mott rule. These results offer a coherent and semiquantitative understanding of the AHE and ANE to an issue of controversy for many decades.     

Novel aspects of charge and lattice dynamics in the hole-doped manganite La0.67Sr0.33MnO3

Previous infrared studies on the hole-doped manganite La_0.67Sr_0.33MnO₃ (LSMO) have analysed its charge dynamics in terms of one type of charge carrier despite evidence of both electron and hole Fermi surfaces. Here, we investigate the charge dynamics of an LSMO film with infrared and optical spectroscopy in order to provide a complete picture of metallic conduction. In the ferromagnetic metallic phase, the low-frequency optical conductivity is best explained by a two-carrier model comprising electrons and holes. The number densities, effective masses and relaxation response of the delocalized electrons and holes are quantified. We discover that only one-third of the doped charges are coherent and contribute to the dc transport. Metallic LSMO cannot be classified as a bad metal at low temperatures because the mean free path of the coherent, mobile charge carriers exceeds the Ioffe–Regel–Mott limit. The incoherent spectral response of the doped charges manifests itself as a broad mid-infrared feature. We also report the first observation of splitting of an infrared-active phonon due to local Jahn–Teller distortion in the vicinity of the thermally driven transition to the nonmetallic, paramagnetic phase in LSMO. This demonstrates that infrared spectroscopy is capable of detecting the presence of local lattice distortions in correlated electron systems.     

Nonisothermal Conductivity of Poly(N-vinylcarbazole)-based Na13-2,9,16,23-Tetramethylamine,Tetracyclohexane Tetrazaporphyrin Composites

Nonisothermal conductivity of poly(N-vinylcarbazole)(PVK)-based Na₁₃-2,9,16,23-tetramethylamine, tetracyclohexane, tetrazaporphyrin composites, varying their additive composition from 50 up to 100 percent (w/w), have been studied. The additive has an ionic character in water solution and a thermal induced paramagnetic state at temperatures higher than 70°C, which improves conductivity in the solid state with respect to its diamagnetic state at lower temperatures. A conductivity peak between 30 and 60°C was detected. It was due to additive-PVK interactions. A sigmoid curve on a plot of lnσ vs. 1/T indicates that an electrical conductivity transition in which a restructurization of ions assisted by the field and temperature took place. Before and after this transition the linearity of the curves is indicative of a thermally activated process for the mechanism of conductivity. The average activation energy, Ea, was in the interval 1.05–1.14 eV.     

Evolution of the 251 cm^-1 infrared phonon mode with temperature in Ba_0.6K_0.4Fe₂As₂

The far-infrared optical reflectivity of an optimally doped Ba1-xKxFe2As2(x =0.4) single crystal is measured from room temperature down to 4 K. We study the temperature dependence of the in-plane infrared-active phonon at 251 cm-1 . This phonon exhibits a symmetric line shape in the optical conductivity, suggesting that the coupling between the phonon and the electronic background is weak. Upon cooling down, the frequency of this phonon continuously increases, following the conventional temperature dependence expected in the absence of a structural or magnetic transition. The intensity of this phonon is temperature independent within the measurement accuracy. These observations indicate that the structural and magnetic phase transition might be completely suppressed by chemical doping in the optimally doped Ba0.6K0.4Fe2As2 compound.     

Phase transition of the |(C6H11)(C2H5)2NH|+(TCNQ)−2 anion radical salt

The TCNQ anion radical salt of diethylcyclohexylammonium undergoes a first-order phase transition at ca.347°K, where sharp discontinuities of the heat capacity, electrical conductivity and paramagnetic susceptibility are observed.     

Non-periodic local motion of silver ions in ßAg3SI from far-infrared conductivity measurements

The frequency dependent far-infrared conductivity of ßAg₃SI, determined by measurement of transmittance and reflectivity, provides dynamic evidence for a non-periodic local motion of the silver ions. This motion takes place in the areas at the centers of the iodine-cube faces where the cationic potential is known to be flat. The experimental far-infrared conductivity is compared with conductivity spectra calculated on the basis of the following simple single-particle models: diffusion on a ring, statistical hopping between nearest-neighbor tetrahedral sites, and Brownian motion in a two-dimensional harmonic potential.     

Simulation of Temperature-Dependent Resistivity for Manganite La1/3Ca2/3MnO3

The resistivity of the heavy-doped La1/3Ca2/3MnO3 （LCMO） is simulated using a random resistor network model, based on a phase separation scenario. The simulated results agree well with the reported experimental data, showing a transition from a charge-disordered （CDO） state embedded with a few ferromagnetic （FM） metallic clusters to a charge-ordered （CO） state, corresponding to the transition from a high-temperature paramagnetic （PM） insulating state to a low-temperature antiferromagnetic （AF） insulating state. Furthermore, we find that the number of AF/CO clusters increases with decreasing temperature, and the clusters start to connect to each other around 250K, which causes percolating in the system. The results further verify that phase separation plays a crucial role in the electrical conductivity of LCMO.     

Low-frequency crossover of the fractional power-Law conductivity in SrRuO3

We combine the results of terahertz time-domain spectroscopy with far-infrared transmission and reflectivity to obtain the conductivity of SrRuO3 over an unprecedented continuous range in frequency, allowing us to characterize the approach to zero frequency as a function of temperature. We show that the conductivity follows a simple phenomenological form, with an analytic structure fundamentally different from that predicted by the standard theory of metals.     

Optical pseudogap from iron states in filled skutterudites AFe4Sb12 (A=Yb, Ca, Ba)

Optical investigations are presented of the filled skutterudites AFe4Sb12 with divalent cations A=Yb, Ca, Ba. For each of these compounds a very similar pseudogap structure in the optical conductivity develops in the far-infrared spectral region at temperatures below 90 K. Highly accurate local-density approximation electronic band structure calculations can consistently explain the origin of the pseudogap structure generated largely by transition metal 3d states. In particular, a 4f-conduction electron hybridization or strong correlations can be ruled out as origin for the pseudogap.     

Far-infrared conductivity measurements of pair breaking in superconducting Nb 0.5 Ti 0.5 N thin films induced by an external magnetic field

We report the complex optical conductivity of a superconducting thin film of Nb 0.5 Ti 0.5 N in an external magnetic field. The field was applied parallel to the film surface and the conductivity extracted from far-infrared transmission and reflection measurements. The real part shows the superconducting gap, which we observe to be suppressed by the applied magnetic field. We compare our results with the pair-breaking theory of Abrikosov and Gor'kov and confirm directly the theory's validity for the optical conductivity.     

Brilliant,coherent far-infrared (THz) synchrotron radiation

A new technology for generating steady state, brilliant, broadband, coherent, far-infrared (FIR) radiation in electron storage rings is presented, suitable for FIR spectroscopy. An FIR power increase of up to 100 000 compared to the normal, incoherent synchrotron radiation in the range of approximately 5 to approximately 40 cm(-1) could be achieved. The source is up to 1000 times more brillant compared to a standard Hg arc lamp. The coherent synchrotron radiation is produced in a "low alpha" optics mode of the synchrotron light source BESSY, by bunch shortening and non-Gaussian bunch deformation.