Browsing by Author "Degiorgi, Leonardo"

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    Charge density wave paraconductivity in K0.3MoO3
    (1994) Gorshunov, Boris P.; Volkov, Alexander; Kozlov, Gennadij V.; Degiorgi, Leonardo; Blank, A.; Csiba, T.; Dressel, Martin; Kim, Yong; Schwartz, A.; Grüner, George
    The dynamic conductivity has been measured in K0.3MnO3 single crystals in the conducting phase (above 180 K), for the first time over a broad frequency range 1-105 cm-1, using a combination of different spectroscopic techniques. Together with a pseudogap, clearly pronounced excitations are observed below 50 cm-1 for E||b, the direction along which the charge density wave develops. We associate these excitations with charge-density-wave fluctuations which exist even at room temperature and give a collective contribution to the conductivity. For the transverse polarization, E|b, a usual single-particle Drude behavior of the conductivity is observed.
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    The electrodynamics of organic superconductors
    (1993) Dressel, Martin; Degiorgi, Leonardo; Klein, Olivier; Grüner, George
    The electrodynamics of two classes of organic superconductors, based on the molecule BEDT-TTF, and on C60, is discussed. For both groups of materials, in the normal state the frequency dependent conductivity is that of a weakly correlated metal (with an interband transition in the inared region). We find that for representative examples of both groups of organic materials the electrodynamics of the superconducting state is in full agreement with s-wave pairing.
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    The electrodynamics of the spin density wave ground state : optical experiments on (TMTSF)2PF6
    (1994) Donavan, Steve; Kim, Yong; Degiorgi, Leonardo; Dressel, Martin; Grüner, George; Wonneberger, Wolfgang
    Conductivity measurements are reported in the organic linear-chain compound (TMTSF)2PF6 , in both the metallic and spin-density-wave states. The components of the complex conductivity were established by measurements in the radio-frequency, micro- and millimeter wave, and infrared spectral ranges. At temperatures above the spin-density-wave transition, a Drude-like metallic behavior was found together with a temperature-independent feature at higher frequencies. An observed Drude scattering rate of 3 cm-1 was found, placing the material well into the clean limit. In the spin-density-wave state, the low field dc resistivity shows an activated behavior similar to a standard semiconductor with a gap value 2Δ/kB≈45 K. The ac response shows a strong frequency dependence, and most importantly, we observe two subgap modes: a very broad one in the radio frequency range, due to internal deformations of the spin density wave, and a narrow mode near 0.1 cm-1, which we interpret as the response of the q=O phason. Furthermore, as expected for a material in the clean limit, we do not see evidence for a single particle gap in the infrared spectral ange. In this paper, we will compare our experimental results with the various models of spindensity-wave dynamics and comment on the current status of the understanding of the dynamical response of spin density waves.
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    Influence of magnetic order on optical properties of the heavy-electron antiferromagnets
    (1994) Degiorgi, Leonardo; Ott, Hans Rudolf; Grüner, George; Dressel, Martin; Fisk, Zachary
    We discuss the influence of antiferromagnetic order on optical properties of several heavy-electron systems. We find different electrodynamic responses of local magnetic moment and itinerant heavy-electron antiferromagnets.
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    Optical evidence of Anderson-Mott localization in FeSi
    (1994) Degiorgi, Leonardo; Hunt, M. B.; Ott, Hans Rudolf; Dressel, Martin; Feenstra, Bokke Johannes; Grüner, George; Fisk, Zachary; Canfield, P.
    We have investigated the electrodynamic response of single-crystalline FeSi over a frequency range from d.c. to 10 hoch 5 cm-1. At low frequencies, we found evidence for an Anderson-Mott localization behaviour of the low-temperature optical conductivity, while at high frequencies the excitation spectrum resembles that of a conventional semiconductor. The missing spectral weight below the gap energy is redistributed around the gap edge, in disagreement with previous claims based on optical measurements.
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    Optical probing of the antiferromagnetic phase transition in the heavy-electron compounds U2Zn17 and UCu5
    (1994) Degiorgi, Leonardo; Ott, Hans Rudolf; Dressel, Martin; Grüner, George; Fisk, Zachary
    We have investigated the complete electrodynamic response of the heavy-electron compounds U2Zn17 and UCu5. Particular emphasis has been devoted to the optical evidence of the antiferromagnetic phase transitions at 9.7 K and 15 K for U2Zn17 and UCu5, respectively. In UCu5, we found an absorption in the far infrared, which is ascribed to excitations across a spin-density-wave-type gap. This feature is absent in U2Zn17. We argue that UCu5 belongs to a characteristically different class of antiferromagnets than U2Zn17 which represents the class of heavy-electron compounds with localized magnetic moments.