Browsing by Author "Grüner, George"
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Item Open Access 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, GeorgeThe 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.Item Open Access The electrodynamics of organic superconductors(1993) Dressel, Martin; Degiorgi, Leonardo; Klein, Olivier; Grüner, GeorgeThe 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.Item Open Access Electrodynamics of the organic superconductors κ-(BEDT-TTF)2Cu(NCS)2 and κ-(BEDT-TTF)2Cu[N(CN)2]Br(1994) Dressel, Martin; Klein, Olivier; Grüner, George; Carlson, K. Douglas; Wang, H. Hau; Williams, Jack M.We have performed measurements of the surface impedance in the normal and superconducting states of the title compounds in the millimeter wave-frequency range (1–3 cm-1), and have evaluated the complex conductivity for different crystallographic orientations. Above the transition temperature, the materials behave like metals with a scattering rate of approximately 20 cm-1. In the superconducting state the electrodynamics is in good agreement with calculations based on a BCS ground state: while the penetration depth and the coherence length are anisotropic, the superconducting energy gap shows no indications of line nodes.Item Open Access 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, WolfgangConductivity 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.Item Open Access Field- and frequency dependent transport in the two-dimensional organic conductor α-(BEDT-TTF)2I3(1994) Dressel, Martin; Grüner, George; Pouget, Jean-Paul; Breining, Angela; Schweitzer, DieterWe studied the electrodynamic response of α-(BEDT-TTF)2I3 in a wide range of frequency, covering microwave and millimeter wave frequencies as well as the optical spectral range, and found a frequency dependent conductivity up to 1000 cm-1 in the low temperature phase. This is accompanied be a non-linear transport with a smooth onset at about 10 V /cm. Our X-ray studies show no indication of superstructure reflections and clearly rule out the formation of a charge density wave ground state. The lack of a temperature dependence in the millimeter wave conductivity between 20 K and 100 K makes hopping transport unlikely.Item Open Access Influence of magnetic order on optical properties of the heavy-electron antiferromagnets(1994) Degiorgi, Leonardo; Ott, Hans Rudolf; Grüner, George; Dressel, Martin; Fisk, ZacharyWe 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.Item Open Access Low-temperature microwave surface impedance of the conventional organic superconductor κ-(BEDT-TTF)2Cu(NCS)2(1993) Dressel, Martin; Bruder, S.; Grüner, George; Carlson, K. Douglas; Wang, H. Hau; Williams, Jack M.-Item Open Access Microwave cavity perturbation technique. Part 1, Principles(1993) Klein, Olivier; Donavan, Steve; Dressel, Martin; Grüner, GeorgeThis report reviews the analysis used to extract the complex conductivity of a compound from a microwave cavity perturbation measurement. We intend to present a generalized treatment valid for any spheroidally shaped sample of arbitrary conductivity which is placed at either the electric or magnetic field antinode of the cavity. To begin with, we establish the relationship between the measured parameters and the conductivity for a spherical sample. Next, we extend these results to the case of spheroids; and for the first time, we cover all different configurations that one can possibly use to study an arbitrary conducting sample inside a cavity: in particular, all possible orientations of the sample with respect to the applied field are solved.Item Open Access Microwave cavity perturbation technique. Part 2, Experimental scheme(1993) Donavan, Steve; Klein, Olivier; Dressel, Martin; Holczer, Karoly; Grüner, GeorgeIn this paper, the second in a three part series, we describe an experimental scheme used to measure the electrodynamical response of a material in the millimeter wave range of frequency. In particular, with this technique we can directly evaluate the complex conductivity from a measurement ofboth the bandwidth and characteristic frequency of a resonator containing the specimen. We will describe in detail all the technical improvements achieved which provide the required accuracy.Item Open Access Microwave cavity perturbation technique. Part 3, Applications(1993) Dressel, Martin; Klein, Olivier; Donavan, Steve; Grüner, GeorgeThe resonant cavity perturbation method as described in the preceding two parts of this series is applied to study the electrodynamical properties of different materials in the microwave and millimeter wave spectral range. We briefly discuss the relevant uncertainties which are asociated with the different measurement techniques and we find that employing the amplitude technique it is possible to measure both the width and frequency to nearly the same precision. We then demonstrate the broad range of applicability of this technique by showing results obtained on several different materials, ranging from an insulator to a superconductor. The performance limitations of this technique are discussed in detail.Item Open Access 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.Item Open Access Optical investigation of the electrodynamic response of UPd2Al3(1994) Degiorgi, Leonardo E.; Dressel, Martin; Grüner, George; Wachter, Peter; Sato, Noriaki; Komatsubara, TakemiWe have investigated the electrodynamic response of UPd2Al3 both above and below the antiferromagnetic phase transition at TN = 14 K. At low temperatures, we observe the formation of a low-frequency narrow resonance, which indicates the development of the many-body coherent state. However, we do not fmd any evidence of a gap absorption associated with a spin-density-wave state, which would develop if the state arised as a consequence of a Fermi surface instability.Item Open Access Optical investigations of the electrodynamics of UPd2Al3(1994) Dressel, Martin; Degiorgi, Leonardo E.; Grüner, George; Wachter, Peter; Sato, Noriaki; Komatsubara, Takemi; Uemura, YasutomoWe have investigated the electrodynamic response of UPd2Al3. At low temperatures, we observe the formation of a low-frequency narrow resonance, which indicates the development of the many-body coherent state. We do not find any evidence of a gap absorption associated to a spin density wave state below the antiferromagnetic phase transition at TN = 14 K, which would develop as a consequence of a Fermi surface instability.Item Open Access 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, ZacharyWe 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.