A.Ya. Kirichenko

Giant magnetic-field changes in radio-frequency absorption in La0:67Sr0:33MnO3 near the Curie temperature

Abstract The DC transport properties of and the radio-frequency (RF) wave absorption (at 2.525 MHz ) in a sample of La0.67Sr0.33MnO3 prepared by floating-zone method are measured. The Curie temperature, Tc, of the sample is about 374 K . Giant temperature and magnetic-field variations in RF absorption are found in the vicinity of Tc. Relative change of the RF absorption in magnetic field (magnetoabsorption) is about 67% in field 2.1 kOe and about 55% in field 1 kOe . This giant magnetoabsorption effect can be used to develop RF devices controlled by temperature and low magnetic field. A weak temperature dependence of magnetoabsorption for the sample studied in the range from room temperature to about 350 K makes it especially attractive for practical use. The RF study supplemented with transport, magnetoresistive and magnetic measurements enables us to discuss the optimal properties of manganite samples for observation of giant magnetoabsorption in low field.

Quasioptical dielectric resonators in millimeter-wave band experiments

New results of theoretical and experimental investigations of non-uniform dielectric resonators with the higher order azimuthal waves (whispering gallery modes) have been presented. Their application for study of various materials (dielectric, high-Tc superconductor) and for generation obtaining of millimeter wave oscillations has been considered.

High-frequency response to millimeter wave irradiation of YBaCuO thin film and ceramic

Abstract Microwave (35 GHz) and radiowave (9 MHz) responses of an YBaCuO thin film and a ceramic to millimeter (mm) wave irradiation (31.5 GHz) have been studied by means of a quasioptical dielectric resonator with whispering gallery modes and an inductive technique at micro- and radiowaves, respectively. The responses are shown to have a mixed nature including a sufficiently strong non-bolometric component. Relaxation of the surface resistance in time after the irradiation removal obeys the logarithmic law implying the nucleation and flux creep of vortices induced by the irradiation is a mechanism of the response at temperatures 3–10 K below the critical temperature T c . Dependence of the microwave surface resistance R s mw on the mm wave pump amplitude H ω is well described by Halbritters theory of vortex motion inside weak links. A correlation between dependences of the radiowave (rw) response on H ω with that of R s mw has been found. Thus the mechanism of rw-response is believed to arise from intergranular Josephson couplings. The latter conclusion is further confirmed by a comparison of the pump power dependence of the rw-response with that of conventional DC-response found for granular HTSC in other recent experiments on the response to the subgap radiation.

Microwave power dependent surface resistance of YBaCuO superconductors: material quality linkage

Three types of dependences of microwave (36 GHz) surface resistance on microwave magnetic field amplitude have been found for YBaCuO samples of various qualities. For the poor-quality samples is proportional to at low fields; then it changes to linear versus and, finally, saturates at large . The intermediate-quality samples demonstrate a threshold character of the nonlinearity with a jump-like growth of at field amplitudes higher than some characteristic value. Finally, the best samples exhibit an dependence which is initially quadratic with and then grows faster with the microwave field. The results are analysed in the framework of various models for the nonlinear surface impedance of type II superconductors.

Auto-oscillation system based on dielectric resonator with “Whispering gallery” modes

A new method of microwave generation in a system with high-Q quasi-optical cylindrical dielectric resonator (CDR) excited by an azimuthal-periodic electron beam current is proposed. Characteristic parameters of a cylindrical fluoroplastic CDR have been determined. Data on microwave generation in a system based on a CDR with whispering-gallery eigenmodes excited by a relativistic azimuthal-periodic electron beam current are presented. Electromagnetic radiation of this system was measured using a detector for the 8-mm wavelength range.

The resonant method for water solution complex permittivity measurement

The possibility of the resonant method application for water solution permittivity measurement in 54-78 GHz frequency range is described. The main attention was given to water solutions with small impurity concentrations, for which the standard microwave methods are low efficient. It is shown, that physical mechanism of resonant absorption is based on the eigen. oscillation excited in the capillary with water solution.

Dielectric constant characterization of large-area substrates in millimeter wave band

A possibility of the dielectric constantε′ measurement for substrates with permittivityε=ε′+iε″ without an essential restriction on their area has been shown experimentally. The method uses frequency measurement of quasioptical dielectric resonator (QDR) with two slots oriented along the QDR radius with a dielectric substrate in one of them. Taking QDR of teflon in 8mm waveband as an example it is found that measurable values ofε′ can ran up 15ε′q, whereε′q is the QDR material permittivity. Absolute error of the measurements is determined by an accuracy with which the permittivity of calibrated (standard) samples is known. The relative measuring error is determined by the accuracy of the QDR frequency measurement and can be quite a small. As an example the method is demonstrated forLaAlO3 single crystals.

Giant radio-frequency magnetoabsorption effect in the cobaltite ceramic La0.5Sr0.5CoO3

The DC transport properties of and the radio-frequency (RF) wave absorption (at 1.33 MHz) in a ceramic sample of La0.5Sr0.5CoO3−δ are measured. The Curie temperature Tc of the sample is about 250 K. A giant negative magnetoabsorption effect is found. In the vicinity of Tc, the absolute value of the magnetoabsorption is about 38% in the rather low magnetic field 2.1 kOe. This differs drastically from the measured DC magnetoresistance (MR) δ(H)=[R(0)−R(H)]/R(0) which is a mere 0.26% near Tc in the same field and increases to about 2.15% in H=20 kOe. The phenomenon can be understood taking into account that the magnetoabsorption is determined by influence of magnetic field on the conductivity and the magnetic permeability, while the MR is determined solely by the former. The magnetoabsorption effect can be used to develop RF devices controlled by magnetic field and temperature.

Microwave and radiowave surface resistance of high-quality YBaCuO ceramic Nonlinear aspects

Abstract An unusual microwave magnetic-field amplitude H ω dependence of the microwave (35 GHz), surface resistance R s mw and radiowave (9 MHz) response on the microwave irradiation of an YBa 2 Cu 3 O 7−δ ceramic has been found. The observed behavior is consistent with the model of Sridhar for the nonlinear high-frequency response of a planar superconductor in the current-induced critical state. A good agreement of the experimental dependence of R s on H ω with Gurevichs theory of the nonlinear dynamics of vortices in high- J c overdamped Josephson contacts has been also observed. The radiowave response ΔR s rf = R s rf ( H ω ) − R s rf (0) dependence on H ω measured simultaneously with R s mw ( H ω ) was found to obey the same law. Temperature and DC magnetic field dependences of R s in both frequency ranges have also been investigated. The temperature dependence of the critical current density has been extracted from the radiowave absorption. Possible physical origins of the observed nonlinear effects are briefly discussed.

Frequency tuning of Gunn diode self-oscillator stabilized by quasi-optical dielectric resonator

The capability of mechanical frequency tuning of Gunn diode self-oscillator by the both metal and dielectric pin inserted into the partially shielded quasi-optical dielectric resonator is considered. The resonators eigen frequencies obtained theoretically and experimentally are compared with oscillation frequencies