Microwave Absorption Measurements Research Articles

Copper content and superconductivity of Nd 1.85Ce 0.15Cu 1+δO y

The superconducting transition of Nd 1.85Ce 0.15Cu 1+δO y samples was studied as a function of the Cu content and processing temperature through magnetic modulated microwave absorption and dc magnetization measurements. Differential thermal analysis and scanning electron microscopy observations show an indication of a small Cu nonstoichiometry with a maximum Cu excess in the T' structure, δ max ≲ 0.01. Superconducting transitions were observed only for samples with a Cu content 1+ δ ≥ 1.01. This Cu excess seems to be necessary in order to obtain superconductivity in the Nd-Ce-Cu-O system.

The nature of paramagnetic centers and phase transitions in fullerenes

Abstract The paramagnetic center in fullerenes is an ion radical fullerene Ċ60 which may occur as a cation Ċ+ 60 or anion Ċy− 60 of different valence. Concentration of such centers at room temperature was determined using EPR technique. EPR signal was studied from 4 to 300K for fullerene C60 and fullerites MexC60 (Me = K, Rb). Magnetic modulated microwave absorption measurements have been applied to determine Tc for all superconducting MexC60 samples (x = 4,3,2 and ≪1) and finally microwave dielectrometry method were used to the temperature study of the permittivity e′ of the C60 close to its structural fcc-sc phase transition.

The manifestation of vortex-glass state in the microwave absorption of high-T c superconductors

The studies of the flux lattice dynamics via the microwave absorption measurements with the help of ESR spectrometer are presented. For low temperatures the nonlinear dependence of the absorbed power on the input microwave power is discovered. The analysis of this phenomenon in terms of the collective creep theory gives evidence, that the flux lattice is frozen in a vortex-glass state.

Magnetic field dependent microwave absorption in a Sm 2−xCe xCuO 4 Single crystal

We have performed magnetically modulated microwave absorption (MAMMA) measurements on a superconducting Sm 2−xCe xCuO 4 single crystal, with the DC magnetic field parallel to the c-axis. Temperature sweeps of the MAMMA signal show an irreversible behavior after ZFC and FC processes. Similarly, isothermal magnetic field sweeps also show a region of hysteretic behaviour. In both cases, the irreversibility lines are coincident. This work shows that the MAMMA technique is well suited to determine the irreversibility line and can provide new insight into the nature of flux dynamics.

Microwave absorption measurements of the electrical conductivity of small particles

A microwave absorption technique based on cavity perturbation theory is shown to be applicable for electrical conductivity measurements of both a small, single-crystal particle and finely divided powder samples whenσ values fall in either the low (σ<0.1 Ω−1 cm−1) or the intermediate (0.1 ≤σ≤ 100 Ω−1 cm−1) conductivity region. The results here pertain to semiconductors in the latter region. If the skin depth of the material becomes significantly smaller than the sample dimension parallel to theE-field, an appreciable error can be introduced into the calculated conductivity values; however, this discrepancy is eliminated by correcting for the field attenuation associated with the penetration depth of the microwaves. A modification of this approach utilizing the skin depth allows a first-order correction to be applied to powder samples which results in the accurate measurement of absoluteσ values, and results with doped Si powders are compared toσ values obtained from one small single particle using this microwave technique as well as reported DCσ values determined with single crystals. The use of this microwave absorption technique with small particles having high surface/volume ratios, such as catalyst supports and oxide catalysts, under controlled environments can provide fundamental information about adsorption and catalytic processes on such semiconductor surfaces. An application to a ZnO powder demonstrates this capability.

Thermal detection of microwave absorption in high-temperature superconductors.

Nonstationary processes of microwave absorption in a superconducting ceramic sample of Y-Ba-Cu-O have been studied. A method of direct measurement of microwave absorption by detection of the sample-temperature changes versus an external magnetic field has been designed. A thermodynamical description of the studied phenomena is proposed as a generalization of the model of local temperature ${\mathit{T}}^{\mathrm{*}}$ of the Josephson-junction system in granular superconductors. The results are in good agreement with the predictions of the model of weakly linked superconducting clusters presented by Ebner and Stroud.

Comparative investigation of intrinsic Josephson contacts in HTC superconductors by modulated microwave absorption measurements

A quantitative analysis of the temperature dependence of modulated microwave absorption measurements in a sputtered film, a single crystal and a ceramic sample reveals two different types of intrinsic Josephson contacts. By modelling the complete temperature dependence of the microwave absorption signal, we were able to distinguish SNS- and SIS-type junctions. As a result we found SIS-type junctions for the granular (sputtered) layer, whereas the single crystal reveals SNS-type junctions.

Effect of bismuth replacement at barium site in Y-Ba-Cu-O system

X-ray diffraction, electron paramagnetic resonance, microwave absorption and resistivity measurements were carried out on YBa2−x Bi x Cu3O7 (0≤x≤0·5) superconductors in order to study the effect of bismuth on the structural and superconducting properties. A decrease in sintering temperature increased the amount of impurity phase. Transition from an orthorhombic (superconducting) phase to tetragonal structure produced no significant change in EPR zero-field signal at liquid nitrogen temperature (LNT). The non-resonance signal height decreased on higher concentration of bismuth. Sintered YBa2−x Bi x Cu3O7 superconductor had a strong EPR zero-field signal at LNT. We have evaluated particle size from XRD and EPR studies. The average particle size was about 0·4µm.

Measurements of low magnetic field microwave absorption in 110-K superconducting Bi-Sr-Ca-Cu-O thin films

Studies on magnetic-field dependent microwave absorption in Bi(Pb)-Sr-Ca-Cu-O thin films superconducting above 100 K are reported. The films were DC magnetron sputtered on MgO and CaNdAlO/sub 4/ single crystals from Pb-rich targets and post-annealed at high temperatures in air for less than 1 h. The measurements were performed as a function of temperature, DC magnetic field, and the film magnetic history. The results show that the microwave absorption measurements are a very sensitive tool for high-T/sub c/ thin-film characterization, since even small variations in the film phase homogeneity, granularity, or crystalline orientation are clearly visible on the modulated absorption curves. It was found that the films characterized by the lowest absorption exhibited the highest critical current densities.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Effect of Solvent Viscosity on Miscible Flooding

Summary This paper presents experimental results for tertiary miscible displacements in strongly water-wet Berea cores. Floods with both continuous solvent and simultaneous solvent/water injection were studied at ambient conditions. Three solvent/oil systems are compared: heptane (0.4 cp) displacing mineral oil (6 cp), tagged heptane displacing heptane, and tagged mineral oil displacing an equal-viscosity mineral oil (6 cp). Saturation profiles from microwave absorption measurements and production histories are presented. The experimental results show that the manner in which residual oil is mobilized and propagated depends on solvent/water injection ratio, oleic/water-phase viscosity ratio, and fractional-flow hysteresis.

Microwave absorption by single and polycrystals of Bi 2Sr 2CaCu 2O 8+δ

X-band (∼ 9.4 GHz) microwave-absorption measurements have been performed on single and polycrystals of the high- T c superconducting material Bi 2Sr 2CaCu 2O 8+δ in the temperature range 4.2–100 K. The microwave absorption indicated the onset of the transition to the superconducting phase at 89 K, the transition to the superconducting phase being complete at 84 K. Low-field microwave absorptions were recorded at temperatures below 89 K. These absorptions exhibited large anisotropies with respect to the relative orientations of the microwave and external magnetic fields; as well, they exhibited dependence on the orientation of the c-axis of the crystals relative to the external and microwave fields. The magnetic hysteresis, as well as the effect of field cooling on microwave absorption were also investigated and the equilibrium fields, B eq, were determined. In addition, the value of the lower critical field B ⊥ c1 has been estimated. Itis concluded that, although some features of microwave absorptions are common to the various samples investigated, there exist some significant differences in the natures of the microwave absorption by the samples; an explanation based on the existence of isolated superconducting grains and defects has been provided.

ESR study of the electronic properties of the new organic conductors κ-(BEDT-TTF) 2Cu[N(CN) 2]X, X = Br; I

The electron spin resonance data together with AC susceptibility and low field microwave absorption measurements are reported for the two organic conductors κ-(BEDT-TTF) 2Cu[N(CN) 2]X, with X = Br; I. The bromine containing salt is the recently discovered superconductor with T onset c = 11.6 K. In the compound with iodine only traces but not bulk superconductivity were found, with the onset at 9.3 K. Annealing resulted in an increase of the superconducting volume fraction but now with T onset c = 6.9 K. Possible interpretation of the data obtained and the discussion of the electronic structure of these compounds is given. The temperature dependences of the line width and the g-factor are discussed in connection with various mechanisms of interaction of conduction electrons with the lattice.

Modulated microwave absorption in single crystal Bi–Sr–Ca–Cu–O

Modulated microwave absorption measurements at 9.3 GHz were made on the low field absorption observed in the superconducting phase of Bi–Sr–Ca–Cu–O. The line intensity observed with a low modulation amplitude of 50 mOe showed a maximum between 70 K and 80 K and dropped to zero near 95 K. With a modulation amplitude of 10 Oe, an additional broad line absorption was observed above 88 K, peaking in intensity near 95 K, and disappearing above 100 K. Hysteresis, associated with flux trapping, occurred when the maximum field Hmax in a sweep cycle exceeded a critical value H*(T), which varied from about 7 Oe at 59 K to 1 Oe at 82 K. For Hmax &gt; H*, the hysteresis splitting at first varied linearly with (Hmax – H*), later saturating at a value δHmax(T). Above 70 K, the relationship δHmax = b(T0 – T) was obeyed with T0 = 92.3 K and b· = −0.3 Oe/K for H perpendicular and b· = −0.23 Oe/K for H parallel to the c-axis.

Superconductivity related to indium-diffused GaAs

Superconductivity in an indium-diffused GaAs system has been investigated by magnetic-field-modulated microwave absorption and ac susceptibility measurements. These show existence of a superconducting phase in some of the samples. Indium was used to solder GaAs on Mo holder or deposit on the surface of GaAs, following the different heat treatments. This indium-diffused GaAs from surface or back side is most likely the origin of the superconductivity.

Temperature dependent microwave absorption of electrons on helium films

In measurements of the temperature dependent microwave absorption of a two-dimensional electron system on superfluid helium films at a frequency of 9 GHz and between 1.2 and 1.5 K we have found below 1.4 K a broad decrease of the absorption with decreasing temperature, which could be interpreted in terms of a temperature dependent polaron transition or as a weak localization behavior. For a film pretreated with positive ions we observed also maxima between 1.34 and 1.28 K which are interpreted as resonances of electrons in their trapping potential. An extremely sharp decrease below 1.24 K seems to give evidence to a Wigner transition of the 2DEG. A PTFE-FEP copolymer foil ( ϵ = 2.1) of 13 microm thickness served as a substrate for the helium film.

Magnetic-field-dependent microwave absorption in the superconducting state of single crystals of Bi2Sr2CaCu2O8+x

The derivative of the microwave absorption versus DC magnetic field strength is observed near zero field in the superconducting state of single crystals of Bi2Sr2CaCu2O8+x. The absorption intensity decreases as the DC magnetic field is rotated away from the crystallographic c axis, which indicates preferential orientation of weak link current loops parallel to the largest crystal facet. The hysteresis of the low field absorption is also anisotropic because of the anisotropy of flux pinning barriers. Direct microwave absorption measurements at higher fields show a continued increase in absorption above 0.1 T which is associated with flux motion. The temperature, field and orientation dependence of this absorption are reported and interpreted in terms of flux motion. Barriers to flux motion in the surface regions of the crystal are estimated.

Synthesis, reactivity, structural studies and magnetic properties of 4-amino-1,4-dihydro-3-methyl-1,2,4-triazole-5-thione copper complexes

By reaction of CuCl2 with 4-amino-1,4-dihydro-3-methyl-1,2,4-triazole-5-thione (HL) in a 4 : 1 molar ratio and in a 12 mol dm–3 HCl medium, at room temperature, the complex [CuCl2(HL)]·H2O 1 was isolated in an orthorhombic form. With an excess of CuCl2 a polymeric complex [{Cu2Cl4(HL)}n]·nH2O 2 was isolated, while introduction of a bar of metallic copper gave a polymeric mixed-valence CuI–CuII(1 : 2) complex [{Cu3Cl5(OH2)(HL)2}n]·2nH2O 3. Crystals of 1 are orthorhombic, space group Pbca, a= 16.256(1), b= 15.611(2), c= 7.327(1)A and Z= 8. Crystals of 2 are orthorhombic, space group Pbca, a= 12.900(3), b= 12.965(5), c= 14.250(5)A and Z= 8. Crystals of 3 are monoclinic, space group P21/a, a= 34.099(8), b= 8.704(3), c= 7.163(3)A, β= 96.14(2)° and Z= 4. The structures have been solved from diffractometer data by Patterson and Fourier methods and refined by full-matrix least squares to R= 0.0393, 0.0582 and 0.0467 for 1132, 1346 and 3177 observed reflections respectively. In 1 the triazole acts as a chelating ligand through the sulfur and the aminic nitrogen atoms and the co-ordination of CuII is fundamentally square planar. In 2 the ligand acts in a tridentate manner, chelating and bridging through the iminic nitrogen atom. The [Cu2Cl4(HL)] units can be considered as inner salts and are joined in a head-to-tail fashion forming zigzag polymeric chains. In 3 the two independent ligands present different behaviours; one chelates a CuII atom as in 1, while the other acts as a µ3 ligand chelating another CuII atom and bridging through the iminic nitrogen and the sulfur atoms two CuI atoms. The magnetic properties were investigated by measurements of electron paramagnetic resonance microwave absorption. For all compounds S=½ spectra were observed with spectroscopic g constants in the range ca. 2.05–2.20. The resulting single-ion g tensors are typical of a tetragonal elongated copper(II) ion where covalency plays a relevant role due to the presence of the SCNN chelating structure. The magnetic local axes deduced on this basis are strictly related to the copper co-ordination planes (obtained from the structural data) confirming the oxidation state II for all the copper atoms except for the tetrahedral CuI in 3.

Resistive losses at microwave frequencies in granular high-T c superconductors: Flux-creep, flux-flow and Josephson junctions

The resistive losses in a magnetic field H for granular superconductors can be attributed to three different processes: flux-flow, flux-creep and dephasing of Josephson junctions (JJ). The microwave absorption measurements are able to separate the three contributions to the losses, if the ranges of the magnetic field and the temperature are suitably selected. A calculation of the surface-resistance, based on the hypothesis of validity of the Bean model and which qualitatively explains the experimental results is given.

Inter- and intragranular effects in microwave absorption of (Bi, Pb)2Sr2Ca2Cu3Oy

Detailed low DC-field microwave absorption (LFMWA) and AC-susceptibility measurements were performed over a wide temperature range for high-quality bulk and fine powder samples of the (Bi,Pb)2Sr2Ca2Cu3Oy compound. The features observed in fine powders are quite different from those observed in bulk samples. This allows one to make a separation of intragranular from intergranular effects. We find an intragranular critical current density of order 104 A/cm2, which is probably the intrinsic critical current density in the c-direction of the Bi-2223 compound due to the Josephson junctions formed by a repeat spacing of nearly insulating and highly conducting atomic planes along the c-direction of the crystal structure. The corresponding penetration depth λc for fields parallel to the a,b-planes is about 35 μm. Furthermore, we can detect the presence of <3% Bi-2212 defect layers in the Bi-2223 crystallites.

Inter- and intra-grain microwave absorption in YBa2Cu3O7 and Bi2Sr2CaCu2O8 powders

Microwave absorption measurements, detected by a conventional ESR spectrometer, on YBa2Cu3O7 and Bi2Sr2CaCu2O8 powders confirm the distinct features of the signals by the inter- and intra-grain superconductive regions, already recognized in YBa2Cu3O7, but also show interesting differences between the two compounds, related to a much more limited role of the weak links and to a weaker pinning in the Bi-based compound.