Charge dynamics in the half-metallic ferromagnet CrO\u3csub\u3e2\u3c/sub\u3e

Infrared spectroscopy is used to investigate the electronic structure and charge carrier relaxation in crystalline films of CrO2 which is the simplest of all half-metallic ferromagnets. Chromium dioxide is a bad metal at room temperature but it has a remarkably low residual resistivity (\u3c5 \u3eμΩ cm) despite the small spectral weight associated with free carrier absorption. The infrared measurements show that low residual resistivity is due to the collapse of the scattering rate at ω\u3c2000 \u3ecm-1. The blocking of the relaxation channels at low v and T can be attributed to the unique electronic structure of a half-metallic ferromagnet. In contrast to other ferromagnetic oxides, the intraband spectral weight is constant below the Curie temperature

On the peak in the far-infrared conductivity of strongly anisotropic cuprates

We investigate the far-infrared and submillimeter-wave conductivity of electron-doped La_(2-x)Ce_xCuO_4 tilted 1 degree off from the ab-plane. The effective conductivity measured for this tilt angle reveals an intensive peak at finite frequency (\nu ~ 50 cm{-1}) due to a mixing of the in-plane and out-of-plane responses. The peak disappears for the pure in-plane response and transforms to the Drude-like contribution. Comparative analysis of the mixed and the in-plane contributions allows to extract the c-axis conductivity which shows a Josephson plasma resonance at 11.7 cm{-1} in the superconducting state.Comment: 4 pages, 4 figures include

User interface development and metadata considerations for the Atmospheric Radiation Measurement (ARM) archive

This paper will discuss user interface development and the structure and use of metadata for the Atmospheric Radiation Measurement (ARM) Archive. The ARM Archive, located at Oak Ridge National Laboratory (ORNL) in Oak Ridge, Tennessee, is the data repository for the U.S. Department of Energy's (DOE's) ARM Project. After a short description of the ARM Project and the ARM Archive's role, we will consider the philosophy and goals, constraints, and prototype implementation of the user interface for the archive. We will also describe the metadata that are stored at the archive and support the user interface

Non-Drude Optical Conductivity of (III,Mn)V Ferromagnetic Semiconductors

We present a numerical model study of the zero-temperature infrared optical properties of (III,Mn)V diluted magnetic semiconductors. Our calculations demonstrate the importance of treating disorder and interaction effects simultaneously in modelling these materials. We find that the conductivity has no clear Drude peak, that it has a broadened inter-band peak near 220 meV, and that oscillator weight is shifted to higher frequencies by stronger disorder. These results are in good qualitative agreement with recent thin film absorption measurements. We use our numerical findings to discuss the use of f-sum rules evaluated by integrating optical absorption data for accurate carrier-density estimates.Comment: 7 pages, 3 figure

Sum rules and electrodynamics of high-Tc cuprates in the pseudogap state

We explore connections between the electronic density of states (DOS) in a conducting system and the frequency dependence of the scattering rate $1/\tau(\omega)$ inferred from infrared spectroscopy. We show that changes in the DOS upon the development of energy gaps can be reliably tracked through the examination of the $1/\tau(\omega)$ spectra using the sum rules discussed in the text. Applying this analysis to the charge dynamics in high-$T_c$ cuprates we found radically different trends in the evolution of the DOS in the pseudogap state and in the superconducting state.Comment: 4 pages, 3 figure

Interplane Transport and Superfluid Density in Layered Superconductors

We report on generic trends in the behavior of the interlayer penetration depth $\lambda_c$ of several different classes of quasi two-dimensional superconductors including cuprates, Sr$_2$RuO$_4$, transition metal dichalcogenides and organic materials of the $(BEDT-TTF)_2X$-series. Analysis of these trends reveals two distinct patterns in the scaling between the values of $\lambda_c$ and the magnitude of the DC conductivity: one realized in the systems with a Fermi liquid (FL) ground state and the other seen in systems with a marked deviation from the FL response. The latter pattern is found primarily in under-doped cuprates and indicates a dramatic enhancement (factor $\simeq 10^2$) of the energy scale $\Omega_C$ associated with the formation of the condensate compared to the data for the FL materials. We discuss implications of these results for the understanding of pairing in high-$T_c$ cuprates.Comment: 4 pages, 2 figure

Infrared magneto-optical properties of (III,Mn)V ferromagetic semiconductors

We present a theoretical study of the infrared magneto-optical properties of ferromagnetic (III,Mn)V semiconductors. Our analysis combines the kinetic exchange model for (III,Mn)V ferromagnetism with Kubo linear response theory and Born approximation estimates for the effect of disorder on the valence band quasiparticles. We predict a prominent feature in the ac-Hall conductivity at a frequency that varies over the range from 200 to 400 meV, depending on Mn and carrier densities, and is associated with transitions between heavy-hole and light-hole bands. In its zero frequency limit, our Hall conductivity reduces to the $\vec k$-space Berry's phase value predicted by a recent theory of the anomalous Hall effect that is able to account quantitatively for experiment. We compute theoretical estimates for magnetic circular dichroism, Faraday rotation, and Kerr effect parameters as a function of Mn concentration and free carrier density. The mid-infrared response feature is present in each of these magneto-optical effects.Comment: 11 pages, 5 figure

Single-Band Model for Diluted Magnetic Semiconductors: Dynamical and Transport Properties and Relevance of Clustered States

Dynamical and transport properties of a simple single-band spin-fermion lattice model for (III,Mn)V diluted magnetic semiconductors (DMS) is here discussed using Monte Carlo simulations. This effort is a continuation of previous work (G. Alvarez, Phys. Rev. Lett. 89, 277202 (2002)) where the static properties of the model were studied. The present results support the view that the relevant regime of J/t (standard notation) is that of intermediate coupling, where carriers are only partially trapped near Mn spins, and locally ordered regions (clusters) are present above the Curie temperature T_C. This conclusion is based on the calculation of the resistivity vs. temperature, that shows a soft metal to insulator transition near T_C, as well on the analysis of the density-of-states and optical conductivity. In addition, in the clustered regime a large magnetoresistance is observed in simulations. Formal analogies between DMS and manganites are also discussed.Comment: Revtex4, 20 figures. References updated, minor changes to figures and tex

An infrared probe of the insulator-to-metal transition in GaMnAs and GaBeAs

We report infrared studies of the insulator-to-metal transition (IMT) in GaAs doped with either magnetic (Mn) or non-magnetic acceptors (Be). We observe a resonance with a natural assignment to impurity states in the insulating regime of Ga$_{1-x}$Mn$_x$As, which persists across the IMT to the highest doping (16%). Beyond the IMT boundary, behavior combining insulating and metallic trends also persists to the highest Mn doping. Be doped samples however, display conventional metallicity just above the critical IMT concentration, with features indicative of transport within the host valence band

Universal scaling in the dynamical conductivity of heavy fermion Ce and Yb compounds

Dynamical conductivity spectra s(w) have been measured for a diverse range of heavy-fermion (HF) Ce and Yb compounds. A characteristic excitation peak has been observed in the mid-infrared region of s(w) for all the compounds, and has been analyzed in terms of a simple model based on conduction (c)-f electron hybridized band. A universal scaling is found between the observed peak energies and the estimated c-f hybridization strengths of these HF compounds. This scaling demonstrates that the model of c-f hybridized band can generally and quantitatively describe the charge excitation spectra of a wide range of HF compounds.Comment: 5 pages, 1 table, 3 figures, to appear in J. Phys. Soc. Jpn. 76 (2007