G. Quirion

Microwave absorption of the high‐Tc superconductor YBa2Cu3O7

Using a resonant‐cavity perturbation technique, the microwave absorption at 16.8 GHz of the high‐Tc compound YBa2Cu3O7 is measured as a function of temperature (4–300 K) and magnetic field (0–7 T). At zero field the microwave loss shows a rapid decrease below 93 K, the onset of the superconducting transition. An important absorption, which is a decreasing function of temperature, is observed in the superconducting state.When a magnetic field is applied, the transition is broadened and shifted toward lower temperatures: an upper‐critical field Hc2(T=0)∼ 150 T is obtained via extrapolation of the data. In the superconducting state, the absorption is found to be highly dependent on the magnetic‐field intensity. For magnetic fields above 500 Oe, the absorption is a slowly increasing function and it is understood in terms of the temperature behavior of Hc2 ; for H<200 Oe, the absorption increases very rapidly with field and it is believed to be correlated to Hc1 and to the anisotropic character of the structure.

Microwave absorption of aligned crystalline grains of YBa2Cu3O7−x

We investigate here the anisotropic magnetic and electric properties of aligned crystalline grains of YBa2Cu3O7−x. The microwave absorption is indeed consistent with the expected temperature behaviors of the resistivity in the normal state along the c axis and in the a‐b plane. In the superconducting state, both the absorptions in pure rf electric and magnetic fields are high compared to the conventional superconductors; for the magnetic case activation energies are deduced and the dispersion is related to the diamagnetic susceptibility. These results are discussed qualitatively in relation to similar data obtained on single crystals and ceramic samples.

Microwave transport properties of Y2BaCuO5−x

Abstract The temperature behavior (4–300 K) of the microwave dielectric constant and conductivity confirms the semiconducting character of the green phase Y2BaCuO5-x compound. At low temperatures (T

Microwave dielectric properties of Cu(tatbp)I

Abstract Cu(tatbp)I (tatbp = triazatetrabenzporphyrinato) is a quasi-one-dimensional molecular conductor that incorporates in its structure a dense array of S = 1 2 moments localized on a metal spine Cu+2 sites and interacting strongly with charge carriers on the p-π molecular orbitals of the tatbp aromatic ring. The microwave dielectric properties have been measured at 13 GHz between 2 and 300K. The a.c. conductivity is in agreement with the d.c. measurement, and its magnitude is similar to that found in other good molecular conductors. The conductivity increases from a room temperature value of σ ∼ 3 × 10 2 Ω −1 cm −1 to a maximum of ca. 7 × 102Ω−1cm−1 at T ∼ 90K; it decreases by almost three orders of magnitude at lower temperatures, but remains practically constant below 6K at σ ∼ 1Ω −1 cm −1 . In the same range the dielectric constant decreases with temperature, the slope being less pronounced below 7K. This temperature behaviour of both properties is not believed to be associated with a normal metal-insulator or semiconductor transition but to be related to the presence of local moments on the metal spine.

ac Impurity hopping conducting in p-InSb

Abstract We report the first measurement of the ac conductivity of p-InSb at low temperatures in the frequency range 10 −10 10 Hz. The conductivity has been determined from the gathering of three sets of data, the dc conductivity curve, the conductivity data obtained from an ultrasonic experiment and the microwave conductivity. Frequency effects have only been observed in the hopping conduction regime. It was found that the ac conductivity varies as ω ( S =0.75). The results are discussed in relation with the single-phonon hopping theory.

Local moment scattering in the one-dimensional molecular conductors CuxNi1-x (Phthalocyaninato) I

Abstract CuxNi1-x (pc)I (pc=phthalocyaninato) are molecular metals whose conducting stacks incorporate a one-dimensional chain of local moments (Cu2+, 1 2 ) strongly coupled to the conduction electrons. We report here the thermoelectric power and the microwave conductivity (17 GHz) of these solid solutions as a function of temperature. The microwave conductivity being metallic at room temperature suddenly drops and shows a large positive magnetoconductivity down to low temperature. These effects are discussed in terms of magnetic scattering of the free carriers by the local moments Cu2+ ( S = 1 2 . At lower temperatures (T

Ultrasonic Velocity and Electrical Conductivity in p-InSb at Low Temperatures and High Magnetic Fields

The piezoelectric interaction in semiconductors connects the ultrasonic velocity and attenuation variations to the electrical conductivity [1]. The ultrasonic techniques offer then the possibility to measure the ac conductivity in a frequency range which is generally hardly accessible for conventional techniques. The ultrasonic investigation of p-InSb at low temperatures is susceptible to give very interesting and important information on the conduction regimes at low temperatures (4–20 K) [2]. We are particularly interested on how the activation and hopping regimes are affected by high magnetic fields and relatively high frequencies. We have thus measured in the low — temperature range the ultrasonic velocity in p-InSb samples having different majority carrier concentration. The results have given the opportunity to evaluate with a good precision the frequency and magnetic field effects on the low-temperature conduction regimes.