K. W. Blazey

Microwave Magneto-Surface Impedance of High-Tc Superconductors

Magnetic-field-induced microwave absorption in sintered high-Tc copper oxide superconductors is found to be proportional to the surface area of samples and not to their volume, establishing that absorption takes place near the sample surface. The surface resistance and reactance increase with magnetic field as a result of energy loss through fluxons driven by superconducting currents. When the magnetic field is modulated, absorptive and dispersive signals arise from modulation of the complex surface impedance through processes induced by the changing magnetic fields.

Microwave study of the critical state in high-Tc superconductors

Abstract The signals obtained from microwave absorption at the surface of high-Tc superconducting oxides in a magnetic field depend sensitivily on the amplitude of the magnetic-field modulation and the sign of the field sweep. They are insensitive to the modulation frequency and the rate of the field sweep. These effects are shown to have their origin in the properties of the superconducting critical state with pinning and depinning of fluxons during a modulation cycle.

Electron paramagnetic resonance of Mn4+ in BaTiO3

Abstract The Electron Paramagnetic Resonance (EPR) of Mn 4+ in the rhombohedral low- temperature phase of BaTiO 3 has been observed at 19.3 and 13.0 GHz. The spectra have axial symmetry along 〈111〉 directions. Intensity measurements prove that the Mn 4+ EPR reflects {111} domain orientation. The spin-Hamiltonian constants are g ‖ =1.9968(19), g ‖ =2.0015(5), A ‖ =67.48(5)×10 −4 cm −1 , A ‖ =73.04(15)×10 −4 cm −1 , and D=0.65(1)cm −1 at 4.2 K. The very large D value is more than 20 times that found for the isoelectronic Cr 3+ . The latter ion remains centered in its surrounding octahedral oxygen cage, whereas the Mn 4+ is off-center with similar coordinates as Ti 4+ . This result is deduced from a superposition-model analysis.

Macroscopic Flux Quantization and Microwave Excitation in Single-Crystal YBa2Cu3O7-δ

Series of microwave-absorption lines spaced uniformly in the magnetic field have been observed in single crystals of YBa2Cu3O7-?. As the crystals are rotated, the regularly periodic lines move in field so that the component of the magnetic field along a [110] direction remains constant. Line series appear at successive threshold microwave powers and temperatures, and the lines widen dramatically as either the microwave power or the temperature is increased. The absorptions are believed to arise from the mixing of macroscopic flux states, stabilized by the microwave excitation.

Identification of Fe4+ and Fe5+ charge-transfer photochromic absorption bands in SrTiO3

Optical absorption and the enhancement and bleaching of Fe3+ and Fe5+ electron paramagnetic resonance in SrTiO3: Al measured as a function of wavelength and time show that the photochromic absorption bands are due to electron transfer from O2- valence states to Fe4+ and Fe5+. They occur at 2.09 and 2.82 eV for Fe4+ and at 1.99 and 2.53 eV for Fe5+.

Reduced fluxon viscosity in a Josephson junction

Abstract Fluxon viscosities within weak Josephson junctions are orders of magnitude smaller than in bulk. Highly mobile fluxons within weak junctions, driven by surface microwave currents, are responsible for the large low-field microwave absorption observed in the granular high-temperature superconductors. While typical fluxon displacements in bulk are in the Angstrom range, displacements in weak Josephson junctions are in the micron range.

Oxygen vacancy-transition metal-ion impurity association in SrTiO3

Abstract The progressive reduction of SrTiO3:Mn has been monitored by EPR. The results show the conversion of Mn4+ to Mn2+ and Mn2+ to Mn2+-V0 by oxygen-vacancy capture. The latter defect association takes place with the copious oxygen vacancies produced during high-temperature reduction between 1100 - 1300 K. At still higher reduction temperatures, other defects are formed. Analysis of the temperature dependence of the defect association gives a value of 0.7 eV for the enthalpy of association.

Fluxon nucleation by microwave currents in Josephson junctions

Abstract Regular series of microwave absorption lines, observed in the weakest junctions of superconducting single crystals and irregular particles, result from the nucleation of fluxons. This nucleation has a threshold microwave field H c1J , above which the absorption lines broaden into bands of width H 1 - H c1J . The threshold microwave field is anisotropic with respect to the orientation of the junction and decreases with increasing temperature, going to zero at T c . Fluxon nucleation in Josephson junctions may be the major source of RF loss in zero magnetic field at high power levels.

Spin-glass phase microwave study in high-TC superconductors

Abstract Measurement of low-field microwave absorption in single-crystal YBa 2 Cu 3 O 7−δ shows a series of equally spaced, narrow, anisotropic absorption lines. With the external magnetic field parallel to the twin planes, a typical line spacing is about 200 mOe with a linewidth of about 2 mOe. These lines are associated with the motion of fluxons into the regular array of twin planes. The widely observed absorption in the ceramic is therefore a powder spectrum of many small crystallites with similar line spectra, which are unresolved.

Modulated microwave absorption of YBa2Cu3O7−δ thin films on LiNbO3

Abstract The temperature dependence of the modulated microwave absorption of YBa 2 Cu 3 O 7–δ thin films is shown to exhibit a maximum near T c where the same signals for ceramic samples go to zero. The maximum is very similar to that usually found in the imaginary part of the susceptibility measured at low frequencies due to the intragranular fluxons.