[en] The author discusses various experiments conducted in the micro- and millimeter wave spectral range on thin film and single crystal specimens of the high temperature oxide superconductors. For high quality film the surface resistance R_s is, except at low temperatures, due to thermally excited carriers, with extrinsic effects playing only a secondary role. Because of the low loss various passive microwave components, such as resonators, delay lines and filters, with performance far superior to those made of normal metals can be fabricated. The conductivity measured at millimeter wave frequencies displays a peak below T_c. Whether this is due to coherence factors or due to the change of the relaxation rate when the materials enter the superconducting state remains to be seen

[en] Some impurities with an unfilled 3d shell have well-defined magnetic moments when dissolved in simple metals, such as copper, whereas others are nonmagnetic. The resistance minimum phenomenon and its connection with the magnetic impurities has been emphasized. The authors review the studies of dilute alloys of 3d elements in simple hosts and discuss the explanation of the resistance minimum by Kondo. (G.T.H.)

[en] Observations are reported of ac-induced Shapiro-like steps in the dc I-V characteristics of NbSe³. Analysis of the steps in terms of a classical model of charge-density-wave (CDW) transport, mathematically equivalent to the Stewart-McCumber model of Josephson tunnel junctions, indicates a highly coherent sample response, and that coupling phenomena well known in the Josephson literature also occur for CDW transport. (author)

[en] We report conductivity and dielectric-constant measurements on a series of orthorhombic Ta/sub 1-//sub x/Nb/sub x/S₃ alloys, with nominal concentrations between 0% and 0.3%, in the dc to 100-GHz frequency range. The Peierls transition observed at T/sub P/ = 220 K in the pure specimens is smeared and suppressed in the alloys. The pinned--charge-density-wave mode, which occurs at the pinning frequency of ω₀/2π/similar to/5 GHz in the nominally pure specimen, increases with increasing dopant concentration at all temperatures. We analyzed the pinned mode in terms of a harmonic-oscillator response and found that the effective mass m/sup */ and damping constant 1/tau are independent of the impurity concentration. We also discuss the temperature dependence of ω₀ and tau and compare with the available theoretical descriptions of charge-density-wave dynamics

[en] Low field electron spin resonance measurements of the magnetic susceptibility (chi) and absorption linewidth over the temperature range 0.04 to 300 K are reported for quinolinium(TCNQ)₂ into which increased amounts of disorder have been introduced by fast neutron irradiation. It is found that below 20 K, chi = ATsup(-α); A increases linearly with the irradiation dose, but α (= approximately 0.8) is almost independent of it, in agreement with the quasi universal behavior predicted by recent renormalization calculations for a random exchange Heisenberg antiferromagnetic chain. Measurements of the g-shift at 4.2 K range indicate that all of chi is associated with TCNQ chains. These results are discussed in terms of the renormalization calculation of Soos and Bondeson. (author)

[en] The conductivity and 9.1 GHz dielectric constant of neutron irradiated Qn(TCNQ)₂ single crystals are strongly affected by defect concentrations less than 1%. This indicates long range electronic coherence of the order of 100 lattice constants in the non-irradiated material for which the coherence length may still be impurity limited. It is suggested that a collective excitation of electrons is responsible for the temperature dependence of the large dielectric constant. (author)

[en] We investigate the optical conductivity of a half-doped manganite Pr_0.5Ca_0.5MnO₃ as a function of the temperature. Although optical phonon modes correspond to a slightly distorted cubic structure above the charge and orbital ordering T_CO-OO, additional peaks are evidenced at low temperature. These new vibration modes are assigned to changes in Mn-Mn bond angle and length that we interpret as a signature of a cooperative Jahn-Teller distortion associated to charge/orbital ordering. Spectra exhibit a broad mid infrared band which transfers to high energy below T_CO-OO resulting in an optical gap. We argue that this large spectral weight shift is due to freezing of Jahn-Teller polarons with the establishment of the charge ordered CE-type structure

[en] We have used 2.5-MeV protons to produce radiation damage in NbSe₃, in a homogeneous controlled manner. The defect concentration was varied from 10 to 500 ppm in order to study the effects of pinning on the charge-density-wave motion. Measurements were made of the nonlinear conductivity, the threshold electric field, and the dielectric constant and conductivity as a function of frequency in the range 0 --100 MHz. We find that the threshold field and the reciprocal dielectric constant vary linearly with the defect concentration and hence the restoring force

[en] This review focusses on the normal and on the different broken symmetry ground state properties of various metals which have a one- or two-dimensional electronic structure. The oxide, the organic superconductors, and the superconductors based on C₆₀ will be discussed, together with the properties of inorganic and organic compounds which have a linear chain structure, and develop at low temperatures a quasi-one-dimensional charge or spin density wave ground state. (author). 58 refs, 24 figs, 1 tab

[en] The thermoelectric power (TEP) measurements as a function of temperature and concentration of defects are reported on neutron irradiated organic quasi-one-dimensional conductor Qn(TCNQ)₂. The temperature dependence of the TEP suggests a freezing out of the entropy due to short range interactions, while the concentration dependence is shown to be due to the change in configurational entropy. (author)