Theoretical and experimental investigations of collective microwave phenomena in solids Semiannual status report, 1 Oct. - 31 Dec. 1969

Theoretical and experimental investigations of collective microwave phenomena in solid

Theoretical and experimental investigations of collective microwave phenomena in solids

Methods of generating microwave shear waves efficiently were studied. A new technique of mode conversion from a longitudinal wave to a shear wave was investigated and virtually 100% conversion efficiency could be obtained. Several aspects of Gunn oscillators were studied. One involved the detailed study of domains in Gunn oscillators using long samples of bulk gallium arsenide. Several new techniques were devised for measuring the properties of the domains. A fundamental theory of the efficiency of a Gunn oscillator was worked out. A computer program was devised to study the transient effect of domains passing through Gunn diodes. Some of the first planar epitaxial Gunn oscillators were made using liquid epitaxial material grown by liquid phase techniques on semi-insulating substrate. A new theory which predicted the properties of RF signals propagating in a thin film layer of GaAs was developed. The theory of the operation of a traveling wave amplifier is described

Theoretical and experimental investigations of collective microwave phenomena in solids Semiannual status, 1 Apr. - 30 Sep. 1969

Microwave amplification in high resistivity gallium arsenide

Positronium signature in organic liquid scintillators for neutrino experiments

Electron anti-neutrinos are commonly detected in liquid scintillator experiments via inverse beta decay, by looking at the coincidence between the reaction products, neutron and positron. Prior to positron annihilation, an electron-positron pair may form an orthopositronium (o-Ps) state, with a mean life of a few ns. Even if the o-Ps decay is speeded up by spin flip or pick off effects, it may introduce distortions in the photon emission time distribution, crucial for position reconstruction and pulse shape discrimination algorithms in anti-neutrino experiments. Reversing the problem, the o-Ps induced time distortion represents a new signature for tagging anti-neutrinos in liquid scintillator. In this paper, we report the results of measurements of the o-Ps formation probability and lifetime, for the most used solvents for organic liquid scintillators in neutrino physics (pseudocumene, linear alkyl benzene, phenylxylylethane, and dodecane). We characterize also a mixture of pseudocumene +1.5 g/l of 2,5-diphenyloxazole, a fluor acting as wavelength shifter. In the second part of the paper, we demonstrate that the o-Ps induced distortion of the scintillation photon emission time distributions represent an optimal signature for tagging positrons on an event by event basis, potentially enhancing the anti-neutrino detection.Comment: 6 pages, 9 figure

Ultrasonic Measurements of Inhomogenous Stress Fields

An acoustic technique for measuring inhomogeneous stress fields in externally loaded solids is described. This method requires a measurement of transit time of a longitudinal acoustic wave through a stressed thin metal specimen using a small diameter water-coupled acoustic transducer. The transducer is mechanically scanned over the surface of the sample by a computer controlled system to take stress field contour plots. Samples investigated include an aluminum plate with a centrol hole, a double edge-notched panel and a single edge-notched panel. In addition to measuring stress fields, the nondestructive determination of stress intensity factors is also discussed

An Electronic Model for CoO2CoO_2 layer based systems: Chiral RVB metal and Superconductivity

Takada et al. have reported superconductivity in layered Na__x CoO_2.yH_2O ($T_c \approx5 K$) and more recently Wen et al. in $A_xCoO_{2+\delta}$ ($A = Na,K$)(\tc$\approx~31 K$). We model a reference neutral \cob layer as an orbitally non-degenerate spin-\half antiferromagnetic Mott insulator on a triangular lattice and Na__x CoO_2.yH_2O and $A_xCoO_{2+\delta}$ as electron doped Mott insulators described by a t-J model. It is suggested that at optimal doping chiral spin fluctuations enhanced by the dopant dynamics leads to a d-wave superconducting state. A chiral RVB metal, a PT violating state with condensed RVB gauge fields, with a possible weak ferromagnetism and low temperature p-wave superconductivity are also suggested at higher dopings.Comment: 4 pages of LaTex file, 6 figures in eps files. Typos and minor corrections mad

Enhanced Polarized Emission from the One-Parsec-Scale Hotspot of 3C 84 as a Result of the Interaction with Clumpy Ambient Medium

We present Very Long Baseline Array polarimetric observations of the innermost jet of 3C$\sim$84 (NGC$\sim$1275) at 43$\sim$GHz. A significant polarized emission is detected at the hotspot of the innermost re-started jet, which is located $\sim$1 pc south from the radio core. While the previous report presented a hotspot at the southern end of the western limb, the hotspot location has been moved to the southern end of the eastern limb. Faraday rotation is detected within an entire bandwidth of the 43-GHz band. The measured rotation measure (RM) is at most (6.3$\pm$1.9)$\times10^{5}$$\sim$rad$\sim$m$^{-2}$ and might be slightly time variable on the timescale of a month by a factor of a few. Our measured RM and the RM previously reported by the CARMA and SMA observations cannot be consistently explained by the spherical accretion flow with a power-law profile. We propose that a clumpy/inhomogeneous ambient medium is responsible for the observed rotation measure. Using equipartition magnetic field, we derive the electron density of $2\times10^{4}$$\sim$cm$^{-3}$. Such an electron density is consistent with the cloud of narrow line emission region around the central engine. We also discuss the magnetic field configuration from black hole scale to pc scale and the origin of low polarization.Comment: 8 pages, 8 figures, accepted for publication in Ap

On the dependence of the spectral parameters on the observational conditions in homogeneous time dependent models of the TeV blazars

Most of current models of TeV blazars emission assume a Synchrotron Self-Compton mechanism where relativistic particles emit both synchrotron radiation and Inverse Compton photons. For sake of simplicity, these models usually consider only steady state emission. The spectral features are thus only related to the shape of the particle distribution, and do not depend on the timing of observations. In this letter, we study the effect of, firstly, the lag between the beginning of the injection of the fresh particles and the trigger of the observation, and secondly, of a finite injection duration. We illustrate these effects considering an analytical time-dependent model of the synchrotron emission by a monoenergetic distribution of leptons. We point out that the spectral shape can be in fact very dependent on observational conditions if the particle injection term is time-dependent, particularly taking into account the effect of the time averaging procedure on the final shape of the SED. Consequences on the acceleration process are also discussed.Comment: Letter to Editor, accepted for publication in A&

A quest for frustration driven distortion in Y2Mo2O7

We investigated the nature of the freezing in the geometrically frustrated Heisenberg spin-glass Y2Mo2O7 by measuring the temperature dependence of the static internal magnetic field distribution above the spin-glass temperature, Tg, using the muSR technique. The evolution of the field distribution cannot be explained by changes in the spin susceptibility alone and suggests a lattice deformation. This possibility is addressed by numerical simulations of the Heisenberg Hamiltonian with magneto-elastic coupling at T>0.Comment: 5 pages 4 figures. Accepted for publication in PR