The paper analyzes the characteristics of the microwave (MW) response of FeSe1-xTex films based on the results of measuring the impedance properties of the films in the X-band for two orientations of the film in the MW magnetic field, perpendicular and parallel. The analysis of the temperature dependence of the microwave response of a film with a perpendicular orientation (in which the peculiarity of the response is manifested) was carried out by means of physical considerations, taking into account also the results of the research of this superconductor by other authors using the ARPES technique and tunneling spectroscopy. It was concluded that with perpendicular orientation, two competing mechanisms of MW energy dissipation in the film can occur, one of which leads to the increase in energy dissipation caused by magnetic vortices with an MW field, and the other to its decrease due to the emergence of Majorana bound states with zero energy

Microwave (MW) surface impedance Z(s)=R-s+iX(s) of the recently discovered high-T-c iron-based superconductor (Li0.84Fe0.16)OHFe0.98Se single crystal was measured for the first time. The special X-band "hot finger" resonator allowed carrying out measurements when temperature of the sample under test can be varied from 2K to 90K and in the same time temperature of the resonator remained constant (4.2K). The unusual and important inequality R-s= T-c=42K was found. The possible reason of the observed feature is considered. The temperature dependence of the quasiparticle sigma(1) and superfluid sigma(2) conductivities and structure of energy gap are analyzed. Experiments show the temperature dependence of the field penetration depth as T-n with n=2.8 at T

The paper presents preliminary results on the analysis of the applicability of microwave measurement techniques for study the properties of superconductors in normal state using dielectric resonators excited with whispering gallery modes (WGM). The expressions for microwave surface impedance for a normal conductor are derived. The experimental technique for measuring the surface impedance of superconductors in a normal state using mm-wave sapphire WGM resonator is proposed and described. The reliability of the proposed approach is demonstrated measuring the response of a resonator with copper samples is presented.

The paper presents a review of main results obtained by the authors in the process of microwave (MW) investigations on unconventional superconductors and developments of MW devices based on cuprate high-temperature superconductors (HTS) carried out over the course of the past 10-15 years. Experimental investigations were carried out by the methods of impedance measurements of superconductor samples. For this purpose the authors have developed two measuring techniques in the mm wavelength range: on the base of quasioptical sapphire resonators and using a feature of the polarized wave p-field reflection from the superconductor surface at a grazing angle of incidence. Epitaxial films of the YBa2Cu3O7–δ cuprate superconductor and Fe-containing superconductors in the form of Ba(Fe0.926Co0.074)2As2 pnictide single crystals and FeSexTe1–x (x = 0.5 and 0.7) chalcogenide epitaxial films have been investigated. The results of the MW response of electrodynamic structures with the investigated samples were used as a base for finding the complex conductivity including the fluctuation one. In general, the results obtained confirm the scenario of the slit function d-wave symmetry for cuprate superconductors and s-wave symmetry for Fe-based superconductors. However, further investigations are required as there is a number of features and effects, namely: abnormal frequency dependence of the residual surface resistance in YBa2Cu3O7–δ in the form of ω3./2, increase of the quasi-particle conductivity with temperature decreasing after the critical one and avalanche-like transition from the superconducting state to the strongly dissipative state in the coplanar transmission line. New MW devices based on cuprate HTS films in the mm wave range 1) quasioptical sapphire resonator with radial slit and HTS end walls for investigation of Fe-based superconductors in the mm wave range: 1) quasioptical sapphire resonator with a radial slit and HTS end walls for investigation of Fe-based superconductors in the form of small (1-2 mm in the a-b plane) samples; 2) quasioptical planar resonator; 3) band-pass filter with an E-plane insertion in the cross-type waveguide. A possibility of noncontact testing the homogeneity properties of massive superconductors at room temperature is shown. The temperature dependence of the complex conductivity of YBa2Cu3O7-(Ba (Fe0.926Co0.074)2As2 and FeSexTe1-x (x = 0.5 and 0.7) superconductors as well as the corresponding physical quantities have been obtained, that enables one to judge about the evidence of relevant scenarios of the wave symmetry in the slit function of investigated superconductors. However, a number of revealed features and effects require further study. A possibility of developing large-scale MW HTS-based devices operating at frequencies up to 40 GHz was experimentally confirmed.

The microwave characterization technique for the investigation of liquids of small volumes is presented. The technique is developed on the basis of a high-quality whispering gallery mode quartz resonator with a capillary placed in the hole of the resonator in the axial direction. The optimal inner capillary diameter was determined by experimental data obtained in Ka-band for the aqueous solution of glucose filling. The measurements of the bovine serum albumin and lactalbumin solutions confirm the applicability of the technique for studies of the biological liquids.

Antioxidants play a crucial role in the life sciences, as the regulators of biochemical reactions. We studied the dielectric properties of the low-molecular weight antioxidant specific biomarkers sodium ascorbate and glutathione in solutions of different concentrations. The biomarkers are multifunctional metabolites relevant to the reactive oxygen species (ROS) scavenging system of cells. The newly developed high-Q microwave whispering-gallery-mode (WGM) dielectric resonator based technique was applied. The technique allows investigation of liquids of nanoliter volumes filled in microfluidic channel within several milliseconds. The revealed peculiarities in the dependence of permittivity on concentrations of the sodium ascorbate and glutathione solutions are explained by differences in relaxation times and loses introduced by molecules of different shapes. We suggest that this novel approach offers the potential for the detection and characterization of ROS-relevant biomarkers with millisecond-time resolution.

The response of a sapphire whispering gallery mode (WGM) resonator to a single-layer graphene film was studied in the millimeter wave band (frequency of about 40 GHz) at different distances of graphene from the resonator. In the resonator, the HE141δ WGM was excited, in which the longitudinal component of the electric field is predominant. Based on the fitting results of both the response measurement and the numerical simulation of the resonator, the conductivity value was obtained for a known film thickness. The conductivity of our CVD-grown and transferred graphene was found to be (1.02 ± 0.06) × 106 S/m. This deviates slightly from the values obtained through our DC conductivity measurements, reflecting the real parameters of the graphene material after transfer from copper to a quartz substrate. A significant difference was demonstrated between the conductivity values obtained by the fitting procedure and those calculated using the perturbation method. In explanation for the discrepancy, we propose a possible inapplicability of the perturbation method for the cases of both the resonator and mode polarization used in this work. The results of this work show that a WGM resonator technique allows contactless exploration of graphene parameters, such as conductivity or sheet resistance, in the millimeter wave band.

We substantiate the earlier proposed hypothesis about the nature of an anomaly previously observed in the temperature dependence of microwave losses of a cavity resonator containing a thin superconducting film of FeSe1-xTex placed perpendicular to the magnetic field of the resonator. An experiment with a film oriented parallel to the field shows the absence of the anomaly. The appearance of the anomaly for T ≤ Tc in the case of the perpendicular orientation of the film is explained by a change in the field orientation and distribution in the film with changing temperature. Comparison of the experimental data with the theoretical model that does not take into account the field evolution in the film shows that the model is applicable to microwave impedance studies of films with a perpendicular orientation in the range from low temperatures to ∼2Tc/3. The expressions for the effective surface impedance of superconducting films in the N-state are also obtained, which are required for constructing the temperature dependence of the effective impedance in the temperature range including Tc.

The subject and purpose of the work. A review of the main results obtained by the authors in the process of microwave (MW) research of non-conventional superconductors and the development of MW devices based on cuprate high-temperature superconductors (HTS) over the past 10–15 years is presented. Methods and methodology of work. Experimental studies were carried out by the methods of impedance measurements of superconducting samples. To this end, the authors developed two measurement techniques in the mm wavelength range: based on quasi-optical sapphire resonators and using the feature of reflection of a p-polarized wave from the surface of a superconductor at grazing angles of incidence. The results of the work.. The epitaxial films of the cuprate YBa2Cu3O7–d superconductor and Fe-containing superconductors in the form of single crystals of pnictide Ba (Fe0.926Co0.074)2As2 and epitaxial films of chalcogenide FeSexTe1–x (x = 0.5 and 0.7) are investigated. The results of the MW response of the electrodynamic structures with the samples under study served as the basis for finding the complex conductivity, including fluctuation one, and physical quantities related to it. In general, the results obtained confirm the scenario of d-wave symmetry of the gap function for cuprate superconductors and s±-wave symmetry for Fe superconductors. However, a number of detected features and effects, namely, an unusual frequency in the formddependence of the residual surface resistance in YBa2Cu3O7–d in the form of w3./2, the growth of quasiparticle conductivity with decreasing temperature, starting from the critical one, as well as the avalanche-like transition from the superconducting to the strongly dissipative state in the nonlinear coplanar transmission line, require further study. New MW devices based on cuprate HTS films in the mm wavelength range have been developed and created: 1) a quasi-optical sapphire resonator with a radial gap and HTS end walls for studying Fe – superconductors in the form of small (1–2 mm in the plane a–b) samples; 2) planar quasi-optical resonator; 3) a band-pass filter with an E-plane insert in a cross-shaped waveguide. The possibility of contactless testing at room temperature of the homo-geneity of the properties of massive superconductors is also shown. Conclusion. The temperature dependence of the complex conductivity of YBa2Cu3O7–d, Ba (Fe0.926Co0.074)2As2 and FeSexTe1–x (x = 0.5 and 0.7) superconductors and physical quantities related with it was obtained, which allows us to judge the confirmation of the corresponding wave symmetry scenarios of gap function function in the investigated superconductors. However, a number of detected features and effects require further study. The previously expressed assessment of the possibility of creating passive HTS-based MW devices with operating frequencies up to 40 GHz has been experimentally confirmed.

We present the accurate microwave characterization technique for investigation of liquids of nanoliters volumes. This technique is an improved modification of the previously developed approach based on a sapphire whispering gallery mode (WGM) resonator. It is based on a high-quality quartz WGM resonator with microfluidic chip. Experimental data are obtained in Ka-band for the aqueous solution of glucose studied down to the concentration equal to concentration of glucose in human blood. The detection limit of the technique for low molecular weight substances was found to be less than their concentration in real biological samples.

This work presents the results of an accurate real-time microwave characterization of low-molecular-weight antioxidant biomarkers in low-volume solutions using a high-Q whispering gallery-mode dielectric resonator with a microfluidic channel. Sodium ascorbate and glutathione were characterized as the most significant non-enzymatic biomarkers of oxidative stress with respect to the reactive oxygen species scavenging system of cells. Microwave characterization was performed using a high-Q whispering-gallery-mode dielectric resonator with a microfluidic channel. The experiments demonstrated a fast response with high sensitivity in detecting sodium ascorbate and glutathione of different concentrations in biological solutions of small nanoliter volumes at a resonance frequency of about 35.5 GHz. These results indicate great potential for applying real-time microwave whispering-gallery-mode biosensing techniques in the characterization of low-molecular-weight antioxidant-specific biomarkers, which could be used in the laboratory and clinical biomedical research.

We review the main results of the development of whispering gallery mode (WGM) resonators and their unique applications due to their quasi-optical functionality. Several types of advanced WGM resonators are proposed by the authors. The theoretical results are described for the resonators with an analytical solution of the electromagnetic problems. Special emphasis is given to the interaction of moving charged particles and waves of cylindrical resonators. Important aspects are described concerning the developed sapphire resonators, for which an exact solution can only be found by using specially designed computer program products. A separate section of the paper is devoted to application aspects of the WGM resonators. In particular, it describes advanced solutions for overcoming the problems of measuring the small microwave (MW) surface impedance of unconventional superconductors in the form of large-area thin films and small samples under study. In addition, a demonstration of accurate complex permittivity measurements of small volumes of lossy liquids is provided. Special emphasis is given to highly stable MW signal sources, namely Ka-band transistor-based feedback oscillator and solid-state maser WGM oscillators. Recently obtained results are presented of experimental studies of the auto-oscillatory system developed on the basis of the WGM resonator with relativistic electron beam.

A new type of planar high-Tc superconductor (HTS) resonator, proposed by the authors, is described. Its feature is excitement with whispering gallery modes (WGM) unlike the known microstrip resonators excited with the lower modes. The resonant structure of two-planar disks and two microstrip lines was simulated and made on the basis of a DyBa 2 Cu 3 O 7-δ superconductor epitaxial film of 600-nm thickness using photolithography technique. The film was deposited on MgO single crystal substrate of thickness h = 0.5 mm. Microwave response in a form of Q-factor and resonant frequency depending on temperature was measured in Kand Ka-bands. The obtained results show higher values of Q-factor of WGM microstrip HTS resonators in comparison with the known HTS microstrip resonators with lower modes. In the authors' opinion, these resonators open prospects for development of new planar microwave devices and techniques. The response of HTS disk resonators obtained earlier and about six months later at approximately the same conditions showed the lowering Q-factor, which may be explained by the degradation of HTS film properties and requires further study.

WGM resonators can be used for studies of condensed matter, especially for the study of the dielectric liquids properties. However, the electromagnetic properties of the real resonator structures are not well understood. The microwave response of quasioptical radially-two-layered dielectric resonator in the form of a cylindrical ring sandwiched between two flat conducting endplates is studied. Usually real resonator has an additional layer between the leucosapphire ring and a conducting surface, that eliminates a possibility of the rigorous solution for the corresponding electromagnetic problem. The leucosapphire resonator is studied with numerical modeling methods and experimental measurements in the Ka-band. A good agreement between the experimentally measured and calculated values of frequencies and Q-factors is achieved. It is shown that the resonator allows the measurement of the complex permittivity of solutions with an error of about 3 %.

In this paper, the idea of a band-pass filter (BPF) with HTS E-plane insert in the cross waveguide is implemented in Ka-band. The advantage of this approach is to solve the problem of mounting HTS insert into the body of the metal waveguide in order to ensure good electrical contact. A three-step method for the synthesis and design of such a filter is proposed and implemented. The method includes synthesis of the normalized prototype filter, sequentially iterative synthesis of filter geometry, and the final synthesis with optimization using numerical electrodynamic analysis of the entire geometry. It is shown that for the narrowband BPF, resonance windows in the E-plane insert should be performed with accuracy better than +/- 0.2 mu m. The characteristics of such filters are expected better or not worse than their counterparts on the basis of rectangular waveguides, but at less stringent conditions of fastening HTS insert in the body of the waveguide. For fabricating the BPF, epitaxial HTS YBa2Cu3O7-delta films sputtered on both sides of MgO single crystal dielectric substrate of 0.5 mm thickness were used. The difference between losses in the produced filter and its simulated sample is explained with insufficient accuracy of making resonance windows in the HTS insert.

In the present work, the measurement cell for investigation of small-volume liquids in sub-THz range is proposed. The proposed measurement cell is based on the whispering-gallery mode quartz resonator covered with the plastic layer comprising the microfluidic channel and allows obtaining the complex permittivity of the liquid filling the microfluidic channel. Investigation of the resonator structure was carried out experimentally and using the numerical model of the structure. It is shown that the resonator structure can be used as a measurement cell with additional calibration procedure.

This paper presents the results of the designing Ka-band band-pass filter with HTS E-plane insert in the cross waveguide. Mainly electromagnetic features of the proposed filter are presented. The approach to the design of the filter is used based on the results of the characteristics analysis of two coupled resonators in a chain of multiple coupled resonators formed by E-plane insert in the cross waveguide.

We present the measurement technique for studying the electromagnetic properties of small-volume water solutions of bioliquids in sub-THz band. The measurement cell is based on the quartz resonator covered by the plastic layer with the microfluidic channel filled with the liquid under test. The experimental and numerical studies of the resonator with the microfluidic channel filled with liquids with known properties were carried out to show the suitability of the resonator structure for the permittivity measurements. The resonator structure with the microfluidic channel filled with glucose and albumine water solutions was studied experimentally. The solution permittivity values can be obtained by solving of the inverse electromagnetic problem.

Whispering gallery mode resonator unit for low phase-noise oscillators was designed and tested at room and low temperatures. Use of the E-plane filter and optimization of the unit of coupling the resonator with microstrip transmission line allowed us to separate the operating EH1111 mode for the purpose of steady operation of the oscillator at 29.4 GHz. The obtained dependence of the resonator external Q-factor on the resonator height over a microstrip transmission line allows foretelling the transmission coefficient depending on the resonator height over a microstrip line that gives the chance to simplify process of the oscillator tuning, especially at low temperatures.