Microwave Absorption Technique Research Articles

Thin films of the organic superconductor α 1-(BEDT-TTF) 2I 3

α-(BEDT-TTF) 2I 3 is a quasi-two-dimensional organic metal with a metal-insulator phase transition at 135 K. Thermal treatment at about 80° C leads to the metallic system α t-(BEDT-TTF) 2I 3, which becomes superconducting below 8 K. Thin films of the α-phase (between 500 and 3000 A thick) have been evaporated in high vacuum onto several substrates and characterised by means of X-ray diffraction, scanning electron microscopy (SEM), atomic force microscopy (AFM) and a low field microwave absorption technique. Depending on the temperature of the substrate and the evaporation rate, the films exhibit different degrees of microcristallinity, which under certain conditions can be strongly reduced and a completely covering film can be obtained. X-ray diffraction spectra reveal a high orientation with the c-axis perpendicular to the substrate and as well the successful conversion into the α t-phase by tempering. SEM and AFM investigations prove that the conversion takes place without reducing the mechanical quality of the films. Low-field microwave-absorption experiments show that the α t-films become superconducting with an onset at 9 K.

IR and MW Absorption Techniques for Bulk and Surface Recombination Control in High-Quaiity Silicon

ABSTRACTThe carrier recombination rate in high-quality FZ and Cz silicon substrates is studied by contactless infrared and microwave absorption techniques. Different surface treatments covering a wide range of surface recombination velocity have been used for the separation of bulk and surface recombination components and evaluating of the efficiency of passivation. Limitations of effective lifetime approach are analyzed specific for low and high injection level. Sensitivity limits of the techniques for iron contamination are discussed

Remanent microwave absorptin in Y−Ba−Cu−O at low magnetic fields

Polycrystalline samples of YBa2Cu3O7 were studied by the microwave absorption technique at the low magnetic fields in the temperature range from 5 to 80 K. The direct microwave absorption and the field modulated microwave absorption were measured at 77 K. Analysis of the microwave absorption curves indicates that the field penetration into the sample starts from the lowest values of applied magnetic field within the whole temperature range.

On the Recombination Activity of Oxygen Precipitation Related Lattice Defects in Silicon

AbstractThe recombination activity of oxygen precipitation related lattice defects in p- and n-type silicon is studied with photoluminescence (PL) and microwave absorption (MWA) techniques. A direct correlation is observed between the amount of precipitated oxygen and the extended defect density on one hand and the minority carrier lifetime and PL activity on the other hand. The PL analyses show as dominant features in the spectra the Dl and D2 lines. The relative amplitude of the D-lines in the different samples is investigated as a function of the oxygen content, defect density and excitation level. The results are correlated with those of complementary techniques and are interrelated on the basis of Shockley-Read-Hall (SRH) theory.

Magnetically modulated microwave absorption in cuprate superconductors

Magnetically modulated microwave absorption (MMMA) is a highly sensitive microprobe, which is increasingly being used to extract a variety of information about the flux dynamics in the high-transition-temperature superconductors. The correspondence between the information obtained from the MMMA and the directly monitored (without modulation) field-induced microwave absorption techniques is discussed. The validity of some of the commonly held views about the features that characterize the MMMA signal are examined.

EPR and microwave-absorption studies in a new class of superconductors. Quarternary rare-earth transition metal borocarbides YPd 5B 3C 0.35 and YNi 2B 2C

Microwave-absorption techniques have been used for investigating the superconducting behaviour of the recently discovered quarternary rare-earth transition metal borocarbide superconductors. The direct microwave absorption shows a steep drop at 23 K in YPd 5B 3C 0.35. Besides the expected drop in microwave absorption observed in YNi 2B 2C at 15.5 K, there is another drop at ∼ 23 K, the origin of which is not understood at present. We have estimated the value of the lower critical field H c1 (∼ 20 G at 10 K) in the two cases from magnetic-field-modulated low-field microwave-absorption measurements.

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.

Measurements of anisotropic characteristic lengths in YBCO films at microwave frequencies

We used microwave self-resonant and magnetically modulated microwave absorption techniques to measure the London penetration depth, λ, and coherence length, ξ, in the direction parallel and perpendicular to the a–b plane of YBCO films. We found that both λ and ξ were anisotropic; it appeared that the value of λ∥(0) was about 1800 Å and λ⊥(86.5) about 26 000 Å, where λ∥ is the penetration depth for the applied microwave electric field parallel and λ⊥ perpendicular to the film plane (c-axis is perpendicular to the film plane). We deduced ξ∥ to be equal to 129 Å and ξ⊥ 40 Å at 86.5 K. The anisotropy factor γ we determined to be about 3.

Scaling of the anisotropic microwave-absorption peaks across Tc in Y-Ba-Cu-O and Bi-Sr-Ca-Cu-O single crystals.

It has been proposed recently that the magnetic-field-dependent properties of uniaxial superconductors may be mapped onto the isotropic case by the field transformation H\ensuremath{\rightarrow}H (${\mathrm{cos}}^{2}$\ensuremath{\theta}+${\ensuremath{\gamma}}^{\mathrm{\ensuremath{-}}2}$${\mathrm{sin}}^{2}$\ensuremath{\theta}${)}^{1/2}$, where \ensuremath{\gamma} is the anisotropy parameter. Using the magnetic-field-modulated microwave-absorption technique, such a mapping is checked critically for Y-Ba-Cu-O and Bi-Sr-Ca-Cu-O single crystals. For both materials decisive support for the above transformation is found.

Influence of the annealing parameters on the volume fraction of the high- Tc phase in the Bi-Sr-Ca-Cu-O system

The influence of the annealing parameters (time and temperature) on the synthesis of the high critical temperature phase (110 K) of the Bi-Sr-Ca-Cu-O system was investigated using the microwave absorption technique. The experimental results show that, for both annealing temperatures investigated, the volume fraction of the 110 K phase increases with annealing time, goes through a maximum and then starts to drop, approaching zero for very long annealing times. When the annealing temperature is 1148 K, the maximum volume fraction of the 110 K phase is attained after 224 h; for an annealing temperature of 1153 K, the maximum volume fraction is attained after 48 h. The disappearance of the 110 K phase for long annealing times is attributed to the loss of bismuth.

Reactive-ion-etch damage in GaAs processing evaluated by a microwave absorption technique

Through determination of magnetic field-dependent resistivity by electric microwave absorption (EMA), both mobility and carrier concentration of conducting layers in a semiconducting substrate may be evaluated. We have employed this method to study the extent of electric damage produced during standard GaAs plasma processing; various typical heterostructures were subjected to realistic reactive ion etch treatments and the changes in transport properties were examined by EMA. It was seen that predominantly chemical etch processes result in little substrate damage, whereas physical milling produces considerable degradation. Oxynitride etching was noted to be benign only at low voltages, whereas photoresist stripping was not seen to produce any measurable substrate damage.

Magnetically modulated microwave absorption measurement of the penetration depth in a polycrystalline YBa2Cu3O7−x thin film

In previous work, a magnetically modulated microwave absorption (MMMA) technique to measure the Ginzburg–Landau parameter κ=(Λ/ξ) in high-Tc superconductors (bulk samples of YBCO) was described. In this work, how this technique can be applied to the measurement of the London penetration depth Λ is described. Data are reported for the temperature variation of Λ in a well-characterized c-axis-oriented polycrystalline thin film of YBa2Cu3O7−x, prepared by a pulsed laser deposition technique. For low temperatures, Λ(0)≂3640 Å with the microwave electric field oriented parallel to the c axis. The temperature dependence of Λ(0)/Λ(T) can be well described by the two-fluid model. The data are also reasonably fit by a BCS model, however the MMMA technique (in the work here reported) does not have the resolution at low temperatures to obtain strong quantitative evidence for an energy gap Δ (even if it exists).

Water hydrogen bonding: The structure of the water–carbon monoxide complex

Rotational transitions between J≤3 levels within the K=0 manifold have been observed for H2O–CO, HDO–CO, D2O–CO, H2O–13CO, HDO–13CO, and H217O–CO using the molecular beam electric resonance and Fourier transform microwave absorption techniques. ΔMJ=0→1 transitions within the J=1 level were also measured at high electric fields. A tunneling motion which exchanges the equivalent hydrogens gives rise to two states in the H2O and D2O complexes. The spectroscopic parameters for H2O–CO in the spatially symmetric tunneling state are [∼(B0) =2749.130(2)MHz, D0=20.9(2)kHz, and μa=1.055 32(2)D] and in the spatially antisymmetric state are [∼(B0) =2750.508(1)MHz, D0=20.5(1)kHz, and μa=1.033 07(1)D]. Hyperfine structure is resolved for all isotopes. The equilibrium structure of the complex has the heavy atoms approximately collinear. The water is hydrogen bonded to the carbon of CO; however the bond is nonlinear. At equilibrium, the O–H bond of water makes an angle of 11.5° with the a axis of the complex; the C2v axis of water is 64° from the a axis of the complex. The hydrogen bond length is about 2.41 Å. The barrier to exchange of the bound and free hydrogens is determined as 210(20) cm−1 (600 cal/mol) from the dipole moment differences between the symmetric and antisymmetric states. The tunneling proceeds through a saddle point, with C2v structure, with the hydrogen directed towards the CO subunit. The equilibrium tilt away from a linear hydrogen bond is in the direction opposite to the tunneling path.

Substitution of Cu by Fe in the superconducting ceramic oxide EuBa2Cu3Oy: Structure, electrical conductivity, and microwave response

A series of EuBa2(Cu1−x Fe x )3Oy (0.0⩽x⩽0.15) ceramics were prepared and examined by thermogravimetric analysis, X-ray powder diffraction, electrical resistance measurements, and a magnetically modulated microwave absorption technique. The oxygen content (y=6.98±0.03), determined by thermogravimetric analysis in a reducing atmosphere, was independent of the iron concentration forx⩽0.12. The introduction of iron into the EuBa2Cu3O y lattice, via substitution into copper sites, effects a progressive decrease in the superconducting transition temperature with increasing iron concentration. Normal-state conductivity similarly progresses from a low resistivity and negative temperature coefficient behavior, to a semiconducting-like response at the high iron concentrations. X-ray powder diffraction measurements indicate that material containing 6% iron and above is tetragonal, yet it remains superconducting with up to at least 12% iron.

Particle-size and temperature dependence of microwave noise in superconducting YBa2Cu3O7-y: Evidence for random Josephson junctions.

The effects of particle size and temperature on the microwave noise and low-field microwave absorption in YBa/sub 2/Cu/sub 3/O/sub 7-//sub y/ powders have been studied by the magnetically modulated microwave absorption technique. The results can be interpreted in terms of a network of random Josephson junctions in the individual superconducting grains but are not consistent with the thermally activated flux creep model. An estimate of the size of the Josephson junctions is also obtained from the variation of the microwave noise with particle size.

Superconductivity and spin-glass ordering in RBa2 (Cu1−xFex)3Oz: R=Y,Gd; 0≤x≤0.12

Samples of oxygen deficient perovskites, YBa2 (Cu1−x Fex )3Oz and GdBa2 (Cu1−x Fex )3 Oz (z≂7) were prepared with up to 12% Fe substituting for Cu. A novel variation of the microwave absorption technique was employed to establish the superconducting transition in these samples and to show that the reduction in the transition temperature with increasing Fe concentration is faster in the Gd samples than in the Y ones. In both series of samples, the room-temperature Mössbauer spectra reveal two distinct Fe sites corresponding to two local oxygen coordinations, and more significantly the spectra at 4.2 K for specimens with x≥0.03 show that the Fe moments are magnetically ordered, leading to the coexistence of superconductivity and magnetism.

Microwave Conductivity: A New Technique for Gas-Phase Pulse Radiolysis

A new technique involving a combination of microwave absorption techniques and pulse radiolysis has been used to monitor electron processes in irradiated gases. The thermal electron attachment rate constants of various halogen-containing molecules have been measured and compared with the available literature values. The adaption of this technique for the measurement of electron-ion recombination rate constants and thermalization times for these gases is discussed.

Photoelectrochemistry in particulate systems. 6. Electron-transfer reactions of small cadmium sulfide colloids in acetonitrile

Small colloidal CdS particles (particle diameter < 25-42 A) have been prepared in acetonitrile and their absorption and emission properties characterized. A transient bleaching was observed when CdS colloids were subjected to bandgap excitation with a 355-nm laser pulse. The recovery of the bleaching which consisted of at least two components has been attributed to the recombination of trapped charge carriers and to the process of anodic corrosion. The red emission of CdS colloid can be quenched with an electron scavenger such as methylene blue. The quantum yield for the reduction of oxazine and thiazine dyes varied from 0.04 to 0.08 and was dependent on the reduction potential of the dye. The electron-transfer reactions of CdS colloids have been investigated by using nanosecond laser flash photolysis and microwave absorption techniques.

Spatially resolved lifetime measurements in neutron-transmutation-doped polycrystalline silicon

Spatially resolved carrier lifetimes have been measured at the same grain boundary using a series of adjacent samples doped to different levels by neutron transmutation. The samples had dopant concentration levels between 1013 and 1017 atoms/cm3. The carrier lifetime was measured using a contactless microwave absorption technique to monitor the decay of photoconductivity which allowed us to perform either isothermal or isochronal annealing to remove the damage caused by irradiation. An optimum anneal of 30 min at 700 °C resulted in almost full recovery to the lifetime before irradiation. A short laser pulse of wavelength 900 nm was focused to a 0.3-mm-diam spot on the samples. Lifetimes of two different sets of carriers were measured, the bulk carrier lifetime at the center of a grain and the carrier lifetime at a grain boundary. This latter carrier lifetime involves both fast and slow recombination mechanisms. The slow decay mechanism is due to interface states at the grain boundary. The fast recombination lifetime was found to be of the same order as the bulk recombination lifetime (of the order of 1 μs) and the slow one was of the order of 1 ms. Doping levels up to 1016 atoms/cm3 show no significant effect on the measured bulk carrier lifetime.

Photoionization of rare-gas-dimer excited states: Experimental measurements of absolute cross sections forXe2*(0u+,1g,2g)in the ultraviolet (248≤λ≤351nm)

Photoionization of excited states of the xenon dimer (${\mathrm{Xe}}_{2}$) has been observed and absolute ionization cross sections have been measured for several laser wavelengths between 248 and 351 nm. Production of the excimers is accomplished by two-photon ionization of ground-state Xe atoms at 193 nm, followed by formation and the subsequent dissociative recombination of $\mathrm{Xe}_{2}^{}{}_{}{}^{+}$ $1{(\frac{1}{2})}_{u}$ ions, collisional and radiative relaxation of the Xe $6p$ and $6{s}^{\ensuremath{'}}$ manifolds, and formation of low-lying excimer states by three-body collisions. Absolute photoionization cross sections are subsequently determined at 351.1, 337.1, 307.9, 277.0, and 248.4 nm by combining a second rare-gas---halide excimer (or ${\mathrm{N}}_{2}$) laser pulse with a microwave-absorption technique to monitor the absolute photoelectron density (in real time) as a function of the intensity of the second laser pulse. The use of microwave absorption allows for the detection of photoelectrons in the presence of a high-pressure background gas. Laser-induced fluorescence and spontaneous emission studies of the temporal behavior of the populations of all of the Xe $6p$ states as well as the $6{s}^{\ensuremath{'}}{(\frac{1}{2})}_{0}$ and $6s{(\frac{3}{2})}_{1}$ levels confirm that a molecule is being photoionized. The molecular states involved are ${0}_{u}^{+},{1}_{g}$, and ${2}_{g}$ which correlate with the $6s{(\frac{3}{2})}_{1}$ excited level and a $^{1}S_{0}$ (ground-state) atom. The optical transitions associated with each laser wavelength $\ensuremath{\lambda}$ studied appear to be ${\mathrm{Xe}}_{2}^{*}{0}_{u}^{+}\ensuremath{\rightarrow}{\mathrm{Xe}}_{2}^{+} 1{(\frac{3}{2})}_{g} (\ensuremath{\lambda}=248.4 \mathrm{nm})$; ${1}_{g} (or {2}_{g})\ensuremath{\rightarrow}1{(\frac{3}{2})}_{u} (\ensuremath{\lambda}=277.0 \mathrm{nm})$; and ${1}_{g} (or {2}_{g})\ensuremath{\rightarrow}1{(\frac{1}{2})}_{u}$ for $\ensuremath{\lambda}=307.9,337.1, \mathrm{and} 351.1$ nm. The discrepancy between the measured cross sections and the values calculated previously (Lorents, Eckstrom, and Huestis, 1973) from a quantum-defect approach is roughly a factor of 2 at 308 nm but increases rapidly at shorter wavelengths. Also, in contrast to theoretical predictions for the $6s$ atomic Xe excited states in which the photoionization cross section $\ensuremath{\sigma}$ falls monotonically with photon energy, the measured $\ensuremath{\sigma}$ peaks at 7\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}18}$ ${\mathrm{cm}}^{2}$ for $\ensuremath{\lambda}=308$ nm ($\ensuremath{\hbar}\ensuremath{\omega}=4$ eV). The profile of the photoionization cross-section spectrum that is reported here for $\mathrm{Xe}_{2}^{}{}_{}{}^{*}$ is, however, remarkably similar to that calculated by Rescigno et al. [J. Chem. Phys. 68, 5283 (1978)] for the $^{1}\ensuremath{\Sigma}$ excited state of the ${\mathrm{Ar}}_{2}$ excimer ($\mathrm{Ar}_{2}^{}{}_{}{}^{*}$).