A.M. Portis

Radioluminescence in amorphous silica: temperature dependence and relaxation

Abstract Radioluminescence measurements on preform and fiber amorphous silica specimens have been made in the temperature interval 6 to 300 K and wavelength regime 300 to 800 nm. A typical spectrum consists of peaks at ∼550 (main band) and ∼650 (secondary band) nm, which are associated with self-trapped excitons and nonbridging oxygen hole centers of the silica lattice, respectively. Radioluminescence from the main band decays with increasing temperature and becomes undetectable in the 130 to 170 K range, consistent with the decay of self-trapped holes. Assuming the decrease in radioluminescence with temperature to be due to quenching of self-trapped exciton radiative recombination, with a spread in quenching barrier energy, we derive an expression of the form L(T)=L(0)/[1+AT+BT2+CT3], which yields an excellent fit to the experimental data. An observed time-dependent relaxation of radioluminescence is associated with the enhancement of nonradiative recombination.

Microwave surface impedance of granular superconductors

Abstract Microwave surface impedance studies of bulk and film cuprate superconductors are modeled by an intergranular medium that is characterized by an inductor L and a shunt resistor R. Below the crossover frequence ω x = R/L, the surface resistance R s increases as ω 2 and at frequencies above ω x as ω 1/2 . Measurements of R S and estimates of R for high quality epitaxial films, indicate that ω x is in the infrared. The surface resistance R S has been measured as a function of microwave power in bulk ceramics and in films. These studies indicate that R S is finite at arbitrarily low microwave power. Increases in R s with power are associated with the development of an intergranular critical state of microwave flux.

Power-induced switching in YBa2Cu3O7 microwave coplanar resonators

Abstract We have studied the power-induced switching of a number of thin film YBa 2 Cu 3 O 7 (YBCO) coplanar transmission line resonators at 8GHz. Switching occurs at current densities around 10 6 − 10 7 Acm -2 at low T , where it is hysteretic; at higher T it becomes non-hysteretic, and washes out in a narrow temperature range which we believe corresponds to the critical temperature T cj of a single responsible weak link. We develop a thermal model which accounts for all of the observed features of the switching.

A quantitative alternate description of decay in glassy systems applied to the decay in YBaCuO measured indirectly by microwaves

Abstract An alternate decay model for glassy systems is introduced which describes the decay after a field step or during a continuous field scan. The model is applied to data of Muller et al. in BaLaCuO and to microwave signals in YBaCuO registered for different scannig speeds. The resulting mean pinning energy E decreases smoothly towards Tc.

Decay of magnetism in a high-Tc superconductor measured indirectly and interpreted with a glassy quantitative decay model

Abstract A decay model for glassy systems is described which quantitatively includes decay regimes such as exponential, power law and logarithmic, and “changeover” regimes as a function of b = T/ Ē, with Ē as mean energy of an initial Poissonian distribution ≈ exp(- E/Ē) of activation energies E. The model is applied to the decay of critical magnetization in a YBa2Cu3Ox granular sample studied indirectly by measuring the hysteretic signal splitting registered by field modulated microwave technique at different field scanning speeds. The resulting values b-1 = Ē/T seem to decrease linearly when approaching Tc.

Intrinsic trapping sites and ion-lattice coupling parameters of cerium-doped lutetium oxyorthosilicate

Cerium-doped lutetium oxyorthosilicate (LSO:Ce) is an excellent scintillator due to its fast decay time (approximately 40 ns), excellent brightness (> 3X bismuth germanate), and high density (7.4 gm/cm3). However, the luminescence process in this technologically important scintillator is not well understood. Elucidation of electronic traps and their role in scintillator afterglow is lacking and fundamental ion-lattice coupling parameters have not been established. From thermally stimulated luminescence and emission spectra data on several oxyorthosilicates we show the traps to be intrinsic and uniquely associated with the C2/c crystal structure. Temperature dependent optical absorption measurements reveal prominent Gaussian bands at 3.432 eV (peak a), 3.502 eV (peak b), 4.236 eV (peak c) and 4.746 eV (peak d). The second moments are well described by the usual linear coupling model yielding the Huang-Rhys parameter and vibrational quantum energies for each peak. Oscillator strengths of the 4f yields 5d transitions are calculated from Smakulas formula and knowledge of the cerium distribution between the two crystallographically inequivalent sites. From the known correlation between average Ce-ion- ligand distance and oscillator strength we conclude that peak a is correlated with the seven-oxygen-coordinated site, and peaks b, c and d are associated with the six-oxygen- coordinated site.

The Physics of Superconducting Levitation

Phenomenological models for the hysteretic levitation force and vertical magnetic stiffness are presented and compared with experimental results on melt grown ceramics, thin films and sintered pellets of YBa2Cu3O7-δ. Assuming the Bean critical state model the levitation force is calculated as resulting from screening currents. A qualitatively good agreement with experimental results is obtained. The stiffness α is correlated with microscopic pinning properties like Labusch parameter α. In addition the magnetic field dependence of the stiffness exhibits a cross-over from B 1.5 to B 2 which can be understood to arise from flux line lattice effects.

Field-modulated microwave absorption study of Pb-modified BiCaSrCuO

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.

Complex microwave susceptibility of natural squid structures

Abstract Dispersive behavior associated with the sharp periodic absorption lines that have been attributed to fluxon nucleation within Josephson junctions has been observed in single crystals of YBa 2 Cu 3 O 7−δ . Both the absorption and dispersion signals are interpreted with an rf SQUID model that associates the absorption signal with hysteresis and the dispersive signal with constant flux. The observed broadening above a threshold in microwave power of the absorption and dispersion into bands is given a particularly simple interpretation with the rf SQUID model. The observed constancy of the absorption amplitude with increasing rf power may also be interpreted simply on the basis of hysteresis.