Larry Kevan

Modulated microwave absorption in YBa2Cu3O7−δ single crystal

Abstract The low-field microwave absorption (LFMA) response is investigated by the modulated absorption technique as a function of field modulation, microwave power and temperature in a single crystal of YBa 2 Cu 3 O 7− δ . Periodic oscillations with two different frequencies are observed in the modulated microwave absorption due to magnetic flux quantization. This suggests at least two types of microwave sensitive Josephson junctions present in the single crystal studied. The two different low-field microwave adsorption responses are examined as a function of field modulation, microwave power and temperature. These responses are related to an oxygen deficiency in the single crystals.

Effect of density, magnetic field modulation and magnetic field penetration depth on the microwave absorption properties of YBa2Cu3O7 –δ superconductors

Magnetic field sweep hysteresis of the low-field microwave absorption (LFMA) in YBa2Cu3O7 –δ is stronger for low magnetic field modulation, low temperatures and low microwave power. The field-sweep hysteresis is related to the pinning of magnetic fluxons. Under conditions favouring substantial fluxon pinning, noise-like structures are superimposed on the LFMA signal. The LFMA signal amplitude depends on magnetic flux pinning, the relative contributions of intergranular and intragranular currents, the distribution of Josephson junctions and the variation in the magnetic field penetration depth. The hysteresis behaviour is similar for samples with different densities at low and high modulation amplitudes. However, the LFMA amplitude decreases with increasing density. The effective magnetic field penetration depth is measured by electron paramagnetic resonance line broadening of spin probes adsorbed on the sample surface. The results are discussed in terms of several current models for the LFMA effect.

Low field microwave absorption of a superconducting Bi2Sr2CaCu2Ox crystal

Abstract Magnetic-field-dependent low-field microwave absorption for a Bi2Sr2CaCu2Ox crystal below the superconducting transition temperature was investigated as a function of temperature, magnetic field modulation amplitude and microwave power. The signal from the crystals stored in vacuum is highly anisotropic and shows a strong temperature dependence. The behavior of the signal versus microwave power is unusual. Above 60 K the signal shows a complex signal shape. The complex signal shape is replaced by a broad, structureless signal when the crystal is stored in air for about one month. The complex signal shape is well simulated by a microwave loss mechanism in Josephson junctions involving boundary and microwave-induced currents. The model simulation explains some aspects of the temperature dependence and microwave power dependence of the signal.

Determination of the flux creep time from the microwave absorption in the high-temperature superconductor YBa2Cu3O7−δ

Abstract Absorption of microwave versus magnetic field near zero field by the high-temperature superconductor YBa2Cu3O7−δ shows a large number of equally spaced lines in the superconducting state. The intensity in the progression of these lines is similar to that of Josephson currents which are thought to be induced in such oxide superconductors. There is strong temperature dependence with a peak somewhat below the superconducting transition temperature (Tc) and the lines completely vanish at Tc. The lines are interpreted to arise from magnetic-flux quantization. This interpretation allows determination of the magnetic flux, hopping or creep time of 3.4s in YBa2Cu3O7−δ.

Measurement of magnetic field penetration depth in YBa2Cu3O7−gd single crystals by surface paramagnetic probes

Abstract The applicability of electron spin resonance line broadening of a diphenylpicrylhydrazyl paramagnetic probe for measuring the magnetic field penetration depth into the surface of a YBa 2 Cu 3 O 7−δ single crystal superconductor is studied. The temperature dependence follows a [1−( T / T c ) 4 ] relation where T c is the superconducting transition temperature, and the average magnetic field penetration depth in YBa 2 Cu 3 O 7−δ single crystals is determined to be 6250 A.

Low field microwave absorption of YBa2Cu3O7−δ thin films

Abstract The modulated microwave absorption in YBa 2 Cu 3 O 7−δ thin films was studied as a function of temperature, modulation amplitude, and microwave power. The comparative nature of weak links in YBaCuO thin films, ceramics, and powders is discussed.

Low frequency and low magnetic field effects on the alternating current volume susceptibility of cuprate superconductors

The real and imaginary parts of the alternating current volume susceptibility are simultaneously measured on a cylindrical sample of ceramic superconducting YBa2Cu3O7−δ in the frequency range from 0.5 to 1×103 Hz. A relatively small frequency dependence of the shielding diamagnetism is observed below 2 Hz and above 250 Hz. A logarithmic dependence of the peak loss temperature is observed which is characteristic of an activated process with an activation energy of 2.5 eV. A weak decrease of the shielding signal takes place when the amplitude of the alternating current magnetic field is increased from 0.002 to 0.02 G. Above 0.02 G this shielding signal is field independent. The data are interpreted in terms of flux creep and viscous drag effects.

Influence of sintering temperature and lead content on the formation of the high temperature superconducting phase in Bi2−x PbxSr2Ca2Cu3Oy

Abstract The formation of the high temperature superconducting phase in Bi 2− x Pb x Sr 2 Ca 2 Cu 3 O y ( x = 0, 0.2, 0.4, 0.6, 0.8) with a superconducting transition temperature ( T c near 110 K is studied as a function of sintering temperature and lead content. The optimum range of sintering temperature is based on differential thermal analysis (DTA) results. DTA shows two endothermic peaks near 1135 and 1155 K. The temperature difference between these two peaks varies with lead content and is maximum at x = 0.4. The x = 0.4 sample also shows the highest fraction of high t c phase based on X-ray diffraction (XRD) and resistance measurements. Thus DTA, XRD and resistance measurements all independently support the x = 0.4 composition as having a unique response which seems characteristic of the high t c phase. XRD and DTA results also suggest that the high t c phase is formed by a reaction between the low T c phase and the products of decomposition of Ca 2 PbO 4 which is produced during the synthesis.

Phase Discrimination of High Temperature Superconducting Phases in the BiSrCaCuO System by Microwave Absorption

The formation of the so-called 80 K and 120 K superconducting phases in the BiSrCaCuO system has been investigated by microwave absorption. This material is characterized by an intense low field microwave absorption (LFMA) below the superconducting transition temperature. LFMA is shown to discriminate between the formation of the two high temperature superconducting phases in this system. Various heating stages in the sample preparation are studied

Microwave absorption investigation on the formation of YBa2Cu3O7−σ by a binary metal oxide route involving interaction of BaCuO2 and Y2Cu2O5

Low-field microwave absorption has been used to discriminate between the YBa2Cu3O7−σ high temperature superconductor synthesized by reacting the binary metal oxides, BaCuO2 and Y2Cu2O5, versus synthesis with a single metal oxide route. This discrimination is more clearly seen by microwave absorption than by resistance measurements. Supplementary data was obtained by electron spin resonance in the g=2 region, scanning electron microscopy, thermal analysis and x-ray diffraction techniques. The scanning electron micrographs indicate a more separated granular structure for the material synthesized from the binary oxides. The low field non-resonant microwave absorption, which is characteristic of the superconducting phase, is twice as intense for the binary metal oxide preparation as compared to the single metal oxide preparation. Also, the superconducting transition temperature seems to be several degrees higher for the binary oxide preparation.