Optical properties of pyrochlore oxide Pb2Ru2O7−δPb_{2}Ru_{2}O_{7-{\delta}}

We present optical conductivity spectra for $Pb_{2}Ru_{2}O_{7-{\delta}}$ single crystal at different temperatures. Among reported pyrochlore ruthenates, this compound exhibits metallic behavior in a wide temperature range and has the least resistivity. At low frequencies, the optical spectra show typical Drude responses, but with a knee feature around 1000 \cm. Above 20000 \cm, a broad absorption feature is observed. Our analysis suggests that the low frequency responses can be understood from two Drude components arising from the partially filled Ru $t_{2g}$ bands with different plasma frequencies and scattering rates. The high frequency broad absorption may be contributed by two interband transitions: from occupied Ru $t_{2g}$ states to empty $e_{g}$ bands and from the fully filled O 2p bands to unoccupied Ru $t_{2g}$ states.Comment: 4 pages, 6 figure

Magnetism and Optics : From Itinerant to Localized Electrons

IV+151hlm.;24c

Bandwidth Limitations of Nb/AlN/Nb SIS Mixers Around 700 GHz

We study, using niobium-technology, the bandwidth of SIS mixers operating at frequencies close to the energy-gap frequency. Microstriplines of niobium-silicon dioxide-niobium have different properties for the top and bottom superconductor, which we find to depend on the used fabrication process. Replacing the AlOx tunnel barrier by AlN, the bandwidth increases by 53%. The measurements, using a Fourier Transform Spectrometer (FTS) and performed in ambient air, demonstrate that the bandwidth is no longer limited by the tuning circuit but by the atmospheric absorption of radiation. Excellent noise temperatures are found over a full band of 600 to 720 GHz.Kavli Institute of NanosciencesApplied Science

Optical and magneto-optical study of orbital and spin ordering transitions in YVO3

Optical and magneto-optical properties of YVO3 single crystals have been studied in IR, visible, and UV regions. Below 74 K we have observed a large Kerr rotation, comparable to those in ferromagnets. This observation introduces a new class of materials: insulating antiferromagnets with strong magneto-optical Kerr effect, which can have advantages for practical applications. We look into the details of electronic and crystal structure changes. YVO3 was found to undergo two structural phase transitions. Initially, the orthorhombic Pbnm symmetry is lowered below a second-order phase transition at 200 K and recovers below 74 K at a first-order phase transition. Two Mott–Hubbard (MH) bands dominate the electronic spectrum in the visible range, followed by charge-transfer gap in UV. Below the 200 K a transfer of the spectral weight to higher frequencies occurs and a third MH band appears at 3 eV. We discuss the results on the basis of spin and orbital ordering theory, proposed for YVO3.

Racine 'Phaedra'

The optical conductivity of MgB$_2$ has been determined on a dense polycrystalline sample in the spectral range 6 meV - 4.6 eV using a combination of ellipsometric and normal incidence reflectivity measurements. $\sigma_{1}(\omega)$ features a narrow Drude peak with anomalously small plasma frequency (1.4 eV) and a very broad "dome" structure, which comprises the bulk of the low-energy spectral weight. This fact can be reconciled with the results of band structure calculations by assuming that charge carriers from the 2D $\sigma$-bands and the 3D $\pi$-bands have principally different impurity scattering rates and negligible interband scattering. This also explains a surprisingly small correlation between the defect concentration and $T_c$, expected for a two-gap superconductor. The large 3D carrier scattering rate suggests their proximity to the localization limit.Comment: RevTeX, 4 pages, 5 figure