Excitonic lasing in semiconductor quantum wires

Direct experimental evidences for excitonic lasing is obtained in optically pumped V-groove quantum wire structures. We demonstrate that laser emission at a temperature of 10 K arises from a population inversion of localized excitons within the inhomogenously-broadened luminescence line. At the lasing threshold, we estimate a maximum exciton density of about 1.8 105cm-1.Comment: 11 pages, 4 figures, submitted to Phys. Rev.

Electronic Structure and Charge Dynamics of Huesler Alloy Fe2TiSn Probed by Infrared and Optical Spectroscopy

We report on the electrodynamics of a Heusler alloy Fe2TiSn probed over four decades in energy: from the far infrared to the ultraviolet. Our results do not support the suggestion of Kondo-lattice behavior inferred from specific heat measurements. Instead, we find a conventional Drude-like response of free carriers, with two additional absorption bands centered at around 0.1 and 0.87 eV. The latter feature can be interpreted as excitations across a pseudogap, in accord with band structure calculations.Comment: 3 pages, 4 figure

Dynamics of disordered heavy Fermion systems

Dynamics of the disordered heavy Fermion model of Dobrosavljevic et al. are calculated using an expression for the spectral function of the Anderson model which is consistent with quantum Monte Carlo results. We compute the self-energy for three distributions of Kondo scales including the distribution of Bernal et al. for UCu{5-x}Pd{x}. The corresponding low temperature optical conductivity shows a low-frequency pseudogap, a negative optical mass enhancement, and a linear in frequency transport scattering rate, consistent with results in Y{1-x}U{x}Pd{3} and UCu{5-x}Pd{x}.Comment: 5 pages, LaTeX and 4 PS figure

Effective-Medium Theory for the Normal State in Orientationally Disordered Fullerides

An effective-medium theory for studying the electronic structure of the orientationally disordered A3C60 fullerides is developed and applied to study various normal-state properties. The theory is based on a cluster-Bethe-lattice method in which the disordered medium is modelled by a three-band Bethe lattice, into which we embed a molecular cluster whose scattering properties are treated exactly. Various single-particle properties and the frequency-dependent conductivity are calculated in this model, and comparison is made with numerical calculations for disordered lattices, and with experiment.Comment: 12 pages + 2 figures, REVTeX 3.

Optical Properties of Heavy Fermion Systems with SDW Order

The dynamical conductivity $\sigma (\omega)$, reflectivity $R(\omega)$, and tunneling density of states $N(\omega)$ of strongly correlated systems (like heavy fermions) with a spin-density wave (SDW) magnetic order are studied as a function of impurity scattering rate and temperature. The theory is generalized to include strong coupling effects in the SDW order. The results are discussed in the light of optical experiments on heavy-fermion SDW materials. With some modifications the proposed theory is applicable also to heavy fermions with localized antiferromagnetic (LAF) order.Comment: 9 pages, 10 figure

Electronic correlations in iron-pnictide superconductors and beyond; what can we learn from optics

The Coulomb repulsion, impeding electrons' motion, has an important impact on the charge dynamics. It mainly causes a reduction of the effective metallic Drude weight (proportional to the so-called optical kinetic energy), encountered in the optical conductivity, with respect to the expectation within the nearly-free electron limit (defining the so-called band kinetic energy), as evinced from band-structure theory. In principle, the ratio between the optical and band kinetic energy allows defining the degree of electronic correlations. Through spectral weight arguments based on the excitation spectrum, we provide an experimental tool, free from any theoretical or band-structure based assumptions, in order to estimate the degree of electronic correlations in several systems. We first address the novel iron-pnictide superconductors, which serve to set the stage for our approach. We then revisit a large variety of materials, ranging from superconductors, to Kondo-like systems as well as materials close to the Mott-insulating state. As comparison we also tackle materials, where the electron-phonon coupling dominates. We establish a direct relationship between the strength of interaction and the resulting reduction of the optical kinetic energy of the itinerant charge carriers

Theoretical Investigation of Optical Conductivity in Ba [Fe(1-x)Co(x)]2 As2

We report on theoretical calculations of the optical conductivity of Ba [Fe(1-x)Co(x)]2 As2, as obtained from density functional theory within the full potential LAPW method. A thorough comparison with experiment shows that we are able to reproduce most of the observed experimental features, in particular a magnetic peak located at about 0.2 eV which we ascribe to antiferromagnetic ordered magnetic stripes. We also predict a large in-plane anisotropy of this feature, which agrees very well with measurements on detwinned crystals. The effect of Co doping as well as the dependence of plasma frequency on the magnetic order is also investigated

Impurity effects on optical response in a finite band electronic system coupled to phonons

The concepts, which have traditionally been useful in understanding the effects of the electron--phonon interaction in optical spectroscopy, are based on insights obtained within the infinite electronic band approximation and no longer apply in finite band metals. Impurity and phonon contributions to electron scattering are not additive and the apparent strength of the coupling to the phonon degrees of freedom is substantially reduced with increased elastic scattering. The optical mass renormalization changes sign with increasing frequency and the optical scattering rate never reaches its high frequency quasiparticle value which itself is also reduced below its infinite band value

Mitochondrial aminoacyl‐trna synthetase and disease: The yeast contribution for functional analysis of novel variants

In most eukaryotes, mitochondrial protein synthesis is essential for oxidative phosphorylation (OXPHOS) as some subunits of the respiratory chain complexes are encoded by the mitochondrial DNA (mtDNA). Mutations affecting the mitochondrial translation apparatus have been identified as a major cause of mitochondrial diseases. These mutations include either heteroplasmic mtDNA mutations in genes encoding for the mitochondrial rRNA (mtrRNA) and tRNAs (mttRNAs) or mutations in nuclear genes encoding ribosomal proteins, initiation, elongation and termination factors, tRNA‐modifying enzymes, and aminoacyl‐tRNA synthetases (mtARSs). Aminoacyl‐tRNA synthetases (ARSs) catalyze the attachment of specific amino acids to their cognate tRNAs. Differently from most mttRNAs, which are encoded by mitochondrial genome, mtARSs are encoded by nuclear genes and then imported into the mitochondria after translation in the cytosol. Due to the extensive use of next‐generation sequencing (NGS), an increasing number of mtARSs variants associated with large clinical heterogeneity have been identified in recent years. Being most of these variants private or sporadic, it is crucial to assess their causative role in the disease by functional analysis in model systems. This review will focus on the contributions of the yeast Saccharomyces cerevisiae in the functional validation of mutations found in mtARSs genes associated with human disorders

Optical investigation of the charge-density-wave phase transitions in NbSe3NbSe_{3}

We have measured the optical reflectivity $R(\omega)$ of the quasi one-dimensional conductor $NbSe_{3}$ from the far infrared up to the ultraviolet between 10 and 300 $K$ using light polarized along and normal to the chain axis. We find a depletion of the optical conductivity with decreasing temperature for both polarizations in the mid to far-infrared region. This leads to a redistribution of spectral weight from low to high energies due to partial gapping of the Fermi surface below the charge-density-wave transitions at 145 K and 59 K. We deduce the bulk magnitudes of the CDW gaps and discuss the scattering of ungapped free charge carriers and the role of fluctuations effects