On the measurement of microwave absorption of bulk YBaCuO superconductors in X-band (8–12 GHz)

Microwave absorption at the surface of highT _c YBaCUO superconducting sample has been determined in X-band by measuring VSWR. Power reflectivity >98% has been observed in the frequency range of 8·2–10·5 GHz indicating very low absorption at the surface. At some of the frequencies, however, negligible microwave loss has been observed.     

ESR studies of superconducting cuprates: Low magnetic field effects at X-band and 300 MHz

Low field microwave absorption in cuprate superconductors belowT _c is reported in a range of materials using ESR spectrometers operating at X-band and 300 MHz. The radiofrequency apparatus facilitated a detailed study of the 0–100G region where the absorption is most intense and we include here a brief discussion of the effects of incident microwave power and sample temperature on the intensity and position of the signal.     

Development and characterization of nickel-zinc spinel ferrite for microwave absorption at 2·4 GHz

This paper deals with the development and characterization of nickel-zinc spinel ferrite (Ni_(1−δ) Zn _δ Fe₂O₄) for microwave absorption at 2·4 GHz (ISM band). The ferrite powder was prepared by dry attrition and sintering process. Complex permittivity and permeability of the prepared sample have been determined by measuring its scattering parameters with the help of a vector network analyser. The measured parameters have been used to determine its wave absorption properties over a frequency range 2·1–2·6 GHz.     

Magnetic resonance in high-temperature superconductors

Since the discovery of high-temperature superconductors a new microwave absorption has been found. The microwaves are absorbed in the flux-quantized eigenstates so that the absorption is proportional to the Josephson current and hence it varies as the gap of the superconductor. This absorption is found in the electron-paramagnetic resonance configuration. The flux-quantized fields are found in small domains of the size of 10⁻⁶ cm. A giant moment is found to occur. The necessary theory as well as experiments in YBa₂Cu₃O_7−δ type compounds are described. The Cu²⁺ electron-paramagnetic resonance gives an anisotropic exchange narrowed line with anisotropicg-values withg-shift proportional to susceptibility. The symmetry of theg-value also reflects the symmetry of the superconducting gap.     

Magnetic Fieid-lnduced Recovery of Superconductivity in Epitaxial YBa2Cu3O7-x Films at Microwave Frequencies

An anomalous magnetic field-effect has been observed in the microwave surface impedance of epitaxial YBa₂Cu₃O_7−x films on LaAlo₃ substrates. The application of dc and microwave magnetic fields up to 20 mT caused a significant decrease of microwave surface reactanceX _s (≤3%) and resistanceR _s (≤20%) at 8.5 GHz, 77 K and at 87 GHz, 4.2 K. The field effects onX _s andR _s s were found to correlate within the framework of the two-fluid model. The observed effect is attributed to the presence of magnetic impurities in the superconducting films, giving rise to pair breaking. Alignment of the impurity spins suppresses the spin-flip scattering rate and, as a result, leads to a recovery of microwave superconductivity.     

Microwave dielectric properties of 3Li2O–Nb2O5–3TiO2 ceramics with Li2O–V2O5 additions

A new Li₂O–Nb₂O₅–TiO₂ (LNT) ceramic with the Li₂O:Nb₂O₅:TiO₂ mole ratio of 3:1:3 has been investigated. The compound is composed of two phases, the Li₂TiO₃ and “M-phase” solid solution phase. The microwave dielectric ceramic has low sintering temperature (∼1100 °C) and good microwave dielectric properties of a relatively high permittivity (∼51), high Q × f value up to 8700, and small temperature coefficient (∼37 ppm/°C). The low-amount doping of 0.83Li₂O–0.17V₂O₅ (LV) can effectively lower the sintering temperature from 1100 to 900 °C and induce no obvious degradation of the microwave dielectric properties. Typically, the 1 wt.% LV-doped ceramic sintered at 900 °C has better microwave dielectric properties of ε_r = 51.3, Q × f = 7235 GHz, τ _f  = 22 ppm/°C, which suggests that the ceramics can be applied in microwave LTCC devices.     

Melting of Two-Dimensional Electron Crystal on Liquid 3He Induced by Resonance Microwave Absorption

Microwave-resonance induced increase of magneto-resistivity σ _xx ⁻¹ of the surface-state electrons (SSE) on liquid ³He is observed at low temperatures at which SSE crystallize into the Wigner solid (WS). At the resonance, the relative change of σ _xx ⁻¹ of SSE in the solid phase is found to be several times of the corresponding change in the liquid phase measured under similar experimental conditions. We attribute this effect to melting of WS induced by electron heating which accompanies the microwave absorption. σ _xx ⁻¹ is found to increase gradually with microwave power, which is rather different from the sharp resistivity increase induced by direct heating of the substrate in the absence of microwaves. The effect of microwave resonance-induced heating on the sliding of WS from the periodic surface deformation of liquid is also investigated.      

Properties of Mg(Fe<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>)<Subscript>2</Subscript>O<Subscript>4 + δ</Subscript> solid solutions in stable and metastable states

Using pyrohydrolytic synthesis, we have obtained a continuous series of Mg(Fe_{1 − x} Ga _x )₂O_{4 + δ} solid solutions, as checked by X-ray diffraction. The magnetization and coercivity of the samples have been determined from field dependences in applied magnetic fields of ±5 T, and their conductivity has been assessed using current-voltage measurements at 300 K in electric fields from −200 to +200 V. The effective band gap of the solid solutions has been evaluated from their absorption spectra obtained by diffuse reflectance measurements. The optimal composition for spintronic applications is Mg(Fe_0.8Ga_0.2)₂O_{4 + δ}.     

Temperature-dependent integral exciton absorption in semiconducting InP crystals

The temperature dependence of the fundamental absorption edge in free-standing “epitaxial” InP layers has been experimentally studied. The integral exciton absorption coefficient K(T) exhibits an increase at low temperatures, which is explained in terms of the exciton-polariton mechanism of light transfer in semi-conductor crystals with spatial dispersion. A critical temperature (T _c = 200 K), above which the integral absorption becomes constant, has been experimentally determined, and the corresponding critical decay parameter (Γ_c = 0.341 meV), longitudinal-transverse splitting (ħω_LT = 0.175 meV), and oscillator strength of the exciton transition (β = 0.237 × 10⁻⁴) have been calculated. The temperature dependence of the true dissipative decay has been determined.     

Induced of zeeman coherence in an ensemble of cesium atoms interacting with a two-frequency (microwave+rf) magnetic field

Interference between Zeeman states corresponding to the forbidden magnetic-dipole transition ΔF=0, Δm _F=2 is reported in connection with the simultaneous interaction of 6² S _1/2 cesium atoms with a resonant microwave field and an rf field which varies at twice the Larmor frequency and is directed perpendicular to a static magnetic field H ₀=0.2 Oe. Pis’ma Zh. Tekh. Fiz. 24, 89–93 (July 26, 1998)     

Method for microwave radiation enhancement using elongated nanotubes dispersed in a gaseous medium

A new method is proposed to enhance microwave radiation with a wavelength of λ ∼ 1 cm in an active medium comprising elongated conducting nanotubes dispersed in air, which is energy pumped by a nonstationary electric field. For a volume fraction of nanoparticles c ₀ ≈ 10⁻³ in air at atmospheric pressure, the necessary value of the nonstationary pumping electric field (generated by a high-power nanosecond pulsed voltage source) is estimated at ∼ 200 J/m³ and the weak signal gain is estimated at Γ₀ = 0.055 m⁻¹. One possible mechanism of microwave radiation enhancement in a spatial resonator is considered.     

Polarization Effect in ESR of Co Doped CuGeO<Subscript>3</Subscript>

Electron spin resonance of a single crystal of CuGeO₃ doped with 2% of Co has been studied at f = 99 GHz in temperature range 1.8–50 K. Contributions to ESR absorption from Cu²⁺ chains and from Co²⁺ ions were derived. It is found that functions obtained for ESR integrated intensities: Curie-Weiss for Cu²⁺ (χCu ∼ C _Cu/(T + Θ), with Θ = 92 K) and Curie for Co²⁺ (χCo ∼ C _Co/T) are well consistent with temperature dependence of static magnetic susceptibility. Strong dependence of ESR absorption on polarization of oscillating magnetic field was discovered for Co²⁺ contribution. Polarization effect was studied for magnetic field applied along a, b and c directions. Values of g-factors of resonance lines are presented.     

Dielectric constant, magnetic permeability and microwave absorption studies of hot-pressed Ba-CoTi hexaferrite composites in X-band

The microwave absorption, complex permittivity and complex permeability studies of hot-pressed hexaferrite composites prepared with Ba(CoTi)_xFe_12-2xO₁₉ (x = 0.0, 0.2, 0.4, 0.8, and 1.0) were made in the frequency range from 8.0 to 12.4 GHz. The hexaferrite composites with x > 0.0 exhibit significant dispersion in the complex permittivity (ε_r′-jε_r″). However the dispersion in complex permeability (μ_r′-jμ_r″) is not significant and is attributed to the shielding effect of polymer matrix over the ferrite crystallites. The reflection loss has been studied as function of frequency, composition and thickness of absorber. A comparison of reflection loss of hot-pressed ferrite composites with that of normal sintered ferrite composites was made and analyzed. A minimum reflection loss of—24.0 dB is obtained at 9.9 GHz for 2.8 mm thick sample of BaCo_0.4Ti_0.4Fe_11.2O₁₉ hot-pressed hexaferrite composite.     

Synthesis and microwave absorption studies of ferrite paint

A new hexaferrite composition, BaCo _x Si _x+y Fe_12−2x−y O₁₉ (x=0·90;y=0·05), in fine (∼μ size) powder form, is prepared. Ferrite paint formulations are made by dispersing this powder in different weight proportions in an epoxy resin. X-band (8–12 GHz) microwave absorption behaviour of the paint coatings of different thicknesses is studied. Remarkably high value, ≥10 dB, of signal absorption at 9·5 GHz is observed for a coating thickness of 0·60 mm with a ferrite fill factor of 57 wt%. Most of this work, in a poster form, was presented at the 8th AGM of MRSI, BARC, Mumbai, February 10–12, 1997. The paper was adjudged the best for poster paper award     

SmAlO<Subscript>3</Subscript> + Ba(Zn<Subscript>1/2</Subscript>Nb<Subscript>2/3</Subscript>)O<Subscript>3</Subscript>: microwave ceramic composite

The 0.1SmAlO₃ + 0.9Ba(Zn_1/2Nb_2/3)O₃ composite is prepared through the solid state ceramic route. The structure of the materials is studied using X-Ray diffraction analysis. The microstructure is studied using scanning electron microscopy and the elemental composition by energy dispersive spectrometry. The dielectric properties in the radio and microwave frequencies are measured. The photoluminescence property of the sample is also analyzed. The material has dielectric constant (ε _r ) = 37, temperature coefficient of resonant frequency (τ _f ) = +25 ppm/°C and high quality factor. The measured values of ε _r and τ _f are compared with the corresponding theoretical values. The composite is useful in the field of optoelectronics and microwave communication.     

Vortex State Microwave Resistivity in Tl-2212 Thin Films

We present measurements of the field induced changes in the 47 GHz complex resistivity, Δρ~(H, T), in Tl₂Ba₂CaCu₂O_8+x (TBCCO) thin films with T _c ≃ 105 K, prepared on CeO₂ buffered sapphire substrates. At low fields (μ₀ H < 10 mT) a very small irreversible feature is present, suggesting a little role of intergranular phenomena. Above that level Δρ~(H, T exhibits a superlinear dependence with the field, as opposed to the expected (at high frequencies) quasilinear behaviour. We observe a crossover between predominantly imaginary to predominantly real (dissipative) response with increasing temperature and/or field. In addition, we find the clear scaling property Δρ~(H, T = Δρ~[H/H ^* (T)], where the scaling field H ^∗ (T) maps closely the melting field measured in single crystals. We discuss our microwave results in terms of loss of flux lines rigidity.     

Electron paramagnetic resonance studies in the oxides Bi2Sr2Ca1−x Gd x Cu2O y

We have recently reported our results on magnetic susceptibility and microwave absorption studies on Bi₂Sr₂Ca_1−x Gd _x Cu₂O _y and Bi₂Sr₂Ca_1−x Y _x Cu₂O _y . In the present work two important observations of these investigation, viz, the dependence ofT _c on Gdconcentration and the absence of EPR-signals of Gd and Cu, are considered in greater detail. It is suggested that the microwave absorption by fluxon lattice might be playing one of the important and effective pathways for electron spin relaxation resulting in nonobservation of EPR signals.     

Microwave absorption in YBa2Cu3O x granular thin films at the dc magnetic fields

Localization of microwave absorption in YBa₂Cu₃O _x superconducting thin films in the presence of very low dc magnetic field was studied. The granular YBa₂Cu₃O _x thin films of submicrometer thickness had random orientation of grains, were inserted into a microwave cavity for maximum rf magnetic field. The microwave absorption in low dc magnetic field at temperatures just below the critical temperature shows properties which may be ascribed to superconducting grains. At lower temperatures the hysteretic effects and flux-trapping occur in the intergrain regions of the film.     

Low-loss,high-purity (TeO<Subscript>2</Subscript>)<Subscript>0.75</Subscript>(WO<Subscript>3</Subscript>)<Subscript>0.25</Subscript> glass

By melting a mixture of high-purity oxides in a platinum crucible under flowing purified oxygen, we have prepared (TeO₂)_0.75(WO₃)_0.25 glass with a total content of 3d transition metals (Fe, Ni, Co, Cu, Mn, Cr, and V) within 0.4 ppm by weight, a concentration of scattering centers larger than 300 nm in size below 10² cm⁻³, and an absorption coefficient for OH groups (λ ∼ 3 μm) of 0.008 cm⁻¹. The absorption loss in the glass has been determined to be 115 dB/km at λ = 1.06 μm, 86 dB/km at λ = 1.56 μm, and 100 dB/km at λ = 1.97 μm. From reported specific absorptions of impurities in fluorozirconate glasses and the impurity composition of the glass studied here, the absorption loss at λ ∼ 2 μm has been estimated at ≤100 dB/km. The glass has been drawn into a glass-polymer fiber, and the optical loss spectrum of the fiber has been measured.     

Microwave adsorption of core–shell structured Sr(MnTi)<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Fe<Subscript>12−2<Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>19</Subscript>/PANI composites

Polyaniline polymer-coated MnTi-substituted strontium hexaferrite (Sr(MnTi) _x Fe_12−2x O₁₉/PANI, x = 1.0, 1.5, 2.0) composites were synthesized by the oxidative chemical polymerization of aniline in the presence of ammonium peroxydisulfate. The structure and morphologies of the products were characterized by X-ray diffraction, FT-IR, TGA, SEM, and TEM. In the magnetization for the Sr(MnTi) _x Fe_12−2x O₁₉/PANI composites, it was found that the saturation magnetization (M _s) and coercivity (H _c) decreased after polyaniline coating. The composite under an applied magnetic field exhibited hysteretic loops of ferromagnetic behavior, such as high saturation magnetization (M _s = 12.1–1.9 emu/g) and coercivity (H _c = 0.919–0.084 kG). The composite specimens of core–shell Sr(MnTi) _x Fe_12−2x O₁₉/PANI and thermal plastic resin had a band-width microwave absorption due to the reflection losses from −15 to −35 dB at frequencies between 18 and 40 GHz as observed by a high-frequency network analyzer.