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3 results on '"Simmonds, M. B."'

1. Observation of ferromagnetism above 900 K in Cr-GaN and Cr-AlN

Author

Liu, H. X., Wu, Stephen Y., Singh, R. K., Gu, Lin, Smith, David J., Dilley, N. R., Montes, L., Simmonds, M. B., and Newman, N.

Subjects
Condensed Matter - Materials Science
Abstract

We report the observation of ferromagnetism at over 900K in Cr-GaN and Cr-AlN thin films. The saturation magnetization moments in our best films of Cr-GaN and Cr-AlN at low temperatures are 0.42 and 0.6 u_B/Cr atom, respectively, indicating that 14% and 20%, of the Cr atoms, respectively, are magnetically active. While Cr-AlN is highly resistive, Cr-GaN exhibits thermally activated conduction that follows the exponential law expected for variable range hopping between localized states. Hall measurements on a Cr-GaN sample indicate a mobility of 0.06 cm^2/V.s, which falls in the range characteristic of hopping conduction, and a free carrier density (1.4E20/cm^3), which is similar in magnitude to the measured magnetically-active Cr concentration (4.9E19/cm^3). A large negative magnetoresistance is attributed to scattering from loose spins associated with non-ferromagnetic impurities. The results indicate that ferromagnetism in Cr-GaN and Cr-AlN can be attributed to the double exchange mechanism as a result of hopping between near-midgap substitutional Cr impurity bands., Comment: 14 pages, 4 figures, submitted to APL

Published
2004
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2. 3 KHz SQUID Receiver Development.

Author

S H E CORP SAN DIEGO CA, Simmonds,M B, Fertig,W A, Giffard,R P, S H E CORP SAN DIEGO CA, Simmonds,M B, Fertig,W A, and Giffard,R P

Abstract

We have designed and tested a model for a 3 kHz receiver using a superconducting antenna circuit and a SQUID (Superconducting QUantum Interference Device) amplifier. Q factors as high as 6400 have been demonstrated for the resonant antenna, giving it an extremely low intrinsic noise voltage. We have anlayzed and solved various problems of matching between antenna and SQUID, as well as measuring noise, contributed by materials in the surrounding cryostat. We have been able to use feedback from the SQUID amplifier into the antenna to effectively broaden the 3 dB bandwidth to 1200 Hz without raising the noise level of the system significantly. By scaling the results obtained with our model receiver to an antenna volume of .022 M-cubed we have been able to estimate that a noise level of 2.4 x 10 to the -17th power T/(sq.rt. Hz) could be obtained in a 100 Hz bandwidth. (Author)

Published
1978

3. Doppler examination of superior vena caval flow for the detection of acute cardiac rejection.

Author

Simmonds MB, Lythall DA, Slorach C, Ilsley CD, Mitchell AG, and Yacoub MH

Subjects
Acute Disease, Biopsy, Blood Flow Velocity physiology, Endocardium pathology, Female, Heart Transplantation immunology, Humans, Male, Middle Aged, Mitral Valve diagnostic imaging, Myocardium pathology, Sensitivity and Specificity, Systole physiology, Tricuspid Valve diagnostic imaging, Echocardiography, Doppler, Graft Rejection diagnostic imaging, Heart Transplantation diagnostic imaging, Vena Cava, Superior diagnostic imaging
Abstract

Background: Doppler echocardiographic studies have previously documented abnormalities of mitral flow during acute rejection similar to those seen in patients with "restrictive" physiology. As central venous flow is known to be abnormal in such patients, it was proposed that examination of superior vena caval flow with Doppler echocardiography might be useful for the detection of acute cardiac rejection., Methods and Results: Thirty orthotopic cardiac transplant patients, 15 of whom had acute cardiac rejection diagnosed by endomyocardial biopsy, were studied within 36 hours of biopsy. Superior vena caval Doppler flow velocities as well as mitral and tricuspid flow velocities were recorded using a Hewlett-Packard Sonos 500/1000 echocardiograph system. Examinations were performed blinded to the biopsy result. Mitral and tricuspid peak early flow velocities in the nonrejector group were similar to those seen in normal subjects (mitral, 70 +/- 5 cm/sec; tricuspid midexpiratory apnea, 50 +/- 11 cm/sec). Superior vena caval flow was abnormal with 13 of 15 patients demonstrating a biphasic pattern of forward flow with dominant diastolic flow. In the 15 patients with acute cardiac rejection, both mitral and tricuspid flow velocities developed a "restrictive"-type pattern with increased peak early flow velocities (mitral, 89 +/- 24 cm/sec; tricuspid midexpiratory apnea, 63 +/- 19 cm/sec; p < or = 0.05 versus nonrejectors) and decreased mitral early flow-velocity deceleration times (rejectors, 97 +/- 26 msec; nonrejectors, 144 +/- 41 msec; p < or = 0.05). The pattern of superior vena caval flow became markedly abnormal with a virtually complete loss of forward systolic flow (rejectors, 4.4 +/- 6.6 cm/sec; nonrejectors, 26.1 +/- 8.8 cm/sec at midexpiratory apnea; p < or = 0.0001). In 10 of 15 patients, systolic forward flow was absent. If acute rejection was defined as forward systolic flow < or = 17 cm/sec, then sensitivity was 100%, specificity was 80%, and predictive accuracy was 90%., Conclusions: During acute cardiac rejection, forward systolic superior vena caval flow is markedly diminished compared with nonrejectors. This is accompanied by other Doppler echocardiographic features consistent with the development of "restrictive" physiology. It is postulated that the loss of forward systolic flow in the superior vena cava is due to diminished long-axis shortening of the right ventricle associated with acute cardiac rejection.

Published
1992

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