Field Modulated Microwave Absorption Research Articles

Flux-pinning effects in field-modulated microwave absorption of YBa 2Cu 3O 7−δ thin films

It is shown that the shape of the field-modulated microwave-absorption signal in the mixed state of type-II superconductors reveals the field dependence of the flux-pinning parameters. Experimental data are presented for thin films of YBa 2Cu 3O 7−δ grown by the pulsed laser deposition (PLD) technique.

Magnetically modulated microwave absorption and harmonic generation in YBa 2Cu 3O 7 thin films

We have performed a comparative study of field-modulated microwave absorption of YBa 2Cu 3O 7 textured and epitaxial thin films using an EPR spectrometer. The Fourier analysis of the signal obtained from microwave absorption of the films has been investigated as a function of temperature and AC magnetic field amplitude. The response is found to be essentially non-linear. We show that the model of Portis et al. based on the existence of a critical state, with pinning and depinning of vortices during a modulation cycle, can account for the observed non-linear absorption signals. The response at the excitation frequency observed close to T c is discussed in terms of flux diffusion.

Comparative study of the irreversibility line and of harmonic generation in field modulated microwave absorption on YBa 2Cu 3O 7 thin films

Magnetic properties and field modulated microwave absorption have been studied on epitaxial YBa 2Cu 3O 7 thin films prepared by MOCVD. Possible correlations between the irreversibility line and the properties of harmonics of the modulation field frequency in the reflected microwave power are discussed.

Surface resistance of grain-aligned YBa2Cu3Ox bulk: Evidence for two kinds of weak link.

We have measured the magnetic-field and temperature dependence of the surface resistance ${\mathit{R}}_{\mathit{s}}$ of grain-aligned ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{\mathit{x}}$. Experiments were conducted on ab-plane- and ac-plane-oriented samples, with the dc magnetic field parallel and perpendicular to the sample surface. We explain the nonlinear behavior of ${\mathit{R}}_{\mathit{s}}$ at low field using a model with an array of Josephson weak links and the linear dependence on magnetic field at high field using an oscillatory-vortex-motion-loss model. The model allows a quantitative fit to the experimental data using the lower critical field of the grain, the entry field ${\mathit{H}}_{\mathit{c}1\mathit{J}}$ of weak links, the junction decoupling parameter ${\mathit{H}}_{\mathit{d}}$, and the vortex viscosity as fitting parameters. Two different sets of parameters ${\mathit{H}}_{\mathit{d}}$ and ${\mathit{H}}_{\mathit{c}1\mathit{J}}$ were used to fit the data, suggesting that in grain-aligned ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{\mathit{x}}$ bulk material at least two kinds of weak link are responsible for low-field losses. An estimation of the Josephson-junction area was obtained from field-modulated microwave absorption. In addition, thermal-excitation and radiation-activation mechanisms for rf losses are compared.

Measurements of surface resistance of grain-aligned bulk material as a function of DC magnetic field; weak link study

The temperature and DC magnetic field dependences of the surface resistance of grain-aligned YBa/sub 2/Cu/sub 3/O/sub x/ bulk material were measured at 9.5 and 75 GHz. Experiments were conducted on a-b-plane- and a-c-plane-oriented sample surfaces. A nonlinear increase of R/sub s/(H/sub dc/) at low fields, which turns into a linear dependence on H at fields larger than H/sub c1/, was observed and explained. A model that allows a quantitative fit using as parameters the lower critical field of the weak links or the grain, the decoupling parameter, and the slope alpha was employed. Two different sets of these parameters must be used to fit all of the data, indicating the existence of two kinds of weak links. The parameter values indicate that there exist inter- and intragrain weak links that contain Josephson fluxons, and a bulk crystal region that contains Abrikosov fluxons. The field-modulated microwave absorption method was used to identify intergrain weak links with lower critical fields <10/sup -4/ T. The correlation between microstructure of the samples and the angular dependence of the field modulated microwave absorption lines is discussed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Investigation of the magnetic field-induced microwave absorption across Tc in single crystals of YBa 2Cu 3O 7−δ and BiSrCaCuO : Anisotropy and magnetic field dependence

The magnetic field-modulated microwave absorption near T c has been studied in high-quality single crystals of YBa 2Cu 3O 7− x ( T c ≈ 90 K) and in BiSrCaCuO ( T c ≈ 85 K) as a function of magnetic field B < 1.3 T and orientation. Sharp microwave absorption peaks were observed when sweeping the temperature through T c from above. The analysis of the peaks using Tinkham's model for fluxon losses gives values for the pinning strength of A ≈ 1000 T and A ≈ 3 T for the YBaCuO and BSCCO crystals, respectively. The anisotropy values obtained from the orientation dependence of the peaks were ϵ = 5 and ϵ = 40−60 for these two compounds, respectively. The results are also discussed with respect to the B-T vortex phase diagram. A marked difference between the YBaCuO and BSCCO compounds is the presence of a low-temperature absorption signal with a different orientation dependence, observed only in the BSCCO compound, in addition to the signals just below T c observed in both compounds.

Shock wave synthesis and processing of high temperature superconductors with defect microstructures

The tetragonal (T c=55 K) and orthorhombic (non-superconducting) Tl2Ba2CuO6 (Tl 2201) phases have been synthesized by aca 20 GPa planar shock wave in a microsecond time frame. Because of local rapid quench rates (up to 106 K/s) defects related to metastable phases are frozen in the sample. In the 2201 phase the predominant defect identified by high resolution lattice imaging corresponds to a Tl-Ba-Cu-Tl-Cu (Tl 1212) phase which is metastable but can be synthesized via a low temperature reaction. Defects of this type may account for the enhanced flux trapping observed in the material using field modulated microwave absorption. Attempts at shock synthesizing the more complex Tl2Ba2CaCu2O8 and Tl2Ba2Ca2Cu3O10 phases are also discussed. Shock processing pre-synthesized YBa2Cu3O7 powder in the radial geometry followed by an O2 anneal at 890 C produces near-theoretical density cylinders that sustain inter-grain critical currents at zero field of 1350 and 750 amps/cm2 at 60 and 77 K respectively. The magnetic field dependence of the critical current and the magnitude of flux-pinning in this material relative to sintered pellets are enhanced by a factor of 2–3. Preliminary investigations of the defect microstructure of this material are discussed.

Microwave absorption, surface resistance and magnetization: Comparative studies of YBa 2Cu 3O7 thin films

We report the results of comparative studies of field modulated microwave absorption, surface resistance and remanent magnetization performed on YBa 2Cu 3O 7 thin films prepared by rf sputtering, laser ablation and electrophoresis. The microwave absorption results are related to those of remanent magnetization. The role of flux penetration at planar defects or grain boundaries is discussed in relation with the film granularity. A correlation between a small grain size and an enhanced surface resistance is shown.

Field-modulated microwave absorption in granular superconductors: First and second harmonic signals

Abstract Field-modulated microwave absorption in granular superconductors is analyzed within the weak link model. The signals are calculated for various modulation amplitudes, both for the first and the second harmonic detection. The calculation signal shapes are in good agreement with experimental results.

Field modulated microwave absorption in YBa2Cu3O7−X thin films

We report investigations on the field modulated non resonant microwave absorption in the superconducting state of textured YBa2Cu3O7−x thin films. The temperature dependence of the modulated signal shows irregular structures below Tc which are shifted towards lower temperature with increasing the amplitude of the modulation field. The modulated signal shows, as a function of the applied field, a rather regular pattern of narrow lines. These results are ascribed to a distribution of weak intergrain Josephson links.

Microwave absorption and intermittency in textured Y 1Ba 2Cu 3O 7−x thin films

We report simultaneous measurements of direct and field-modulated microwave absorption in textured Y 1Ba 2Cu 3O 7−x thin film and ceramic samples. The two techniques are shown to monitor two different characteristic onset temperatures in the superconducting system. Below the critical transition temperature T c, the modulated signal shows noise-like fluctuations whose onset temperature T ∗ decreases logarithmically with the amplitude of the modulation field. These results are ascribed to a distribution of weak Josephson links. Intermittent very low frequency microwave absorption jumps of constant amplitude are observed at fixed temperatures, and are similar to the predicted behaviour of an ideal Josephson junction shunted with a capacitor and a resistor.

Analysis of superconducting transitions in c ≈ 31 and c ≈ 37Å BSCCO systems

Abstract The relation between the 110K transition and the c≈37 A structure in BSCCO system is examined. The superconducting properties of several samples showing the c ≈ 31 and the c ≈ 37 A structures are studied by means of electrical resistivity and field-modulated microwave absorption.

Field-modulated microwave absorption study of Pb-modified Bi Ca Sr Cu O

Abstract High-resolution transmission-electron-microscopy (HRTEM) has established that samples of nominal composition Bi 2 Sr 2 Ca 2 Cu 4 O y prepared by solid-state reaction are of mixed phase. These samples are mostly composed of the 115 K polytypoid structure with the 80 K polytypoid structure found at grain-boundaries and at planar defects within grains. The partial replacement of Bi by Pb leads to a single phase of the 115 K polytypoid as determined from HRTEM. Low-magnetic-field modulated microwave absorption in the absence of Pb yields absorption onsets associated with transitions around 115 K and around 80 K. The addition of Pb eliminates the lower temperature transition. This observation is consistent with the HRTEM observations and supports the general view that microwave absorption arises primarily from grain boundaries and planar defects.

Field modulated microwave absorption in high T/sub c/ superconductors

Modified electron spin resonance (ESR) techniques were used to measure low-field (<200 Oe) microwave absorption of high-T/sub c/ cuprates YBa/sub 2/Cu/sub 3/O/sub 7-x/. A series of experiments on polycrystalline and single-crystal samples in which the angle between the DC and the modulating magnetic fields was varied showed that the microwave absorption obeys results predicted from thermodynamic considerations of fluxoids. The absorption takes place in the normal regions created by flux penetration at low fields in these samples. The change in magnetically modulated microwave absorption on passing through T/sub c/ is explained by the decrease in absorption that occurs when part of the sample becomes superconducting and by the rapid variation of the penetration depth near T/sub c/. The technique is an extremely sensitive method of detecting superconductivity in very small samples and for studying the superconducting state. Possible applications such as microwave attenuators, magnetic memories, and gaussmeters are discussed.