Aortic dissection: a medical perspective

Over the past 3 years, a greater understanding of predisposing factors and the pathology of the aortic wall has yielded more insights into the pathogenesis, as well as into more accurate diagnostic and preventive methods. This update will discuss: (1) the incidence, definition, and recent classification of aortic dissection; (2) epidemiology, medical, and recent surgical series; (3) the pathogenesis and risk factors; (4) clinical features and evolving noninvasive imaging approaches to diagnosis; and (5) management from a medical perspective.     

Measurement of the g-shift in superfluid3he-B

We have measured the g-shift in superfluid³He-B as a function of temperature above 0.5 mK at a pressure of 1.03 bar, and a frequency of 3.87 MHz. We compare our results with predictions of the quasi-classical and Ginzburg-Landau theories, and present values of the Fermi-liquid parameter F ₂ ^a and the -parameter ₃₄₅.     

Collective modes and sound propagation in a magnetic field in superfluid3He-B

A high-resolution, ultrasonic (12–89 MHz) acoustic impedance technique has been used to investigate the order parameter collective modes in superfluid³He-B over a pressure range of 0–15 bar and in magnetic fields up to 180 mT. In agreement with earlier experiments, theJ=2 real squashing mode has been observed to split into five components in small magnetic fields. However, contrary to earlier theoretical estimates, the Zeeman shifts have been found to become extremely nonlinear as the magnetic field is increased. The extent of this nonlinearity is largest at low pressures and at temperatures close toT _c. In comparison with recent theoretical work, the nonlinear Zeeman shifts may be explained as a result of two effects. First, there is a significant distortion of the B-phase energy gap in large magnetic fields. Second, there is an important coupling between the sameJ _zsubstates of the differentJ modes. In this sense the nonlinear evolution of the real squashing mode constitutes the observation of the Paschen-Back effect in³He-B. A comparison of the observed Zeeman shifts with theoretical expressions has yielded information about particle-particle and particle-hole interaction effects in the superfluid. In the limitT 0 and above a threshold field, the real squashing mode has been found to possess additional structure. TheJ _z=0 substate has been observed to split into a doublet. The separation between the two components of the doublet is of the order of 100–200 kHz and remains independent of the magnetic field. The origin of the doublet may be understood in terms of a recent theory which postulates a texture-dependent collective mode frequency. Further, at extremely small fields the effects due to dispersion of the real squashing modes have been found to be important. The magnitude of the dispersion-induced mode splitting in zero field is found to be consistent with theoretical predictions. TheJ=2 squashing mode has also been studied in the presence of a magnetic field. TheJ _z=0 state of the squashing mode is observed to shift to lower temperatures in a magnetic field. An additional field dependence of the observed acoustic impedance is interpreted as the evolution of theJ _z=–1, –2 states, but appears to be inconsistent with theoretical predictions.     

Musical chronology by seriation

The statistical process of seriation arranges a set of objects along a one-dimensional line so that the distance between each pair of objects reflects the dissimilarity between them. The process has recently been cast into an algorithm which makes it feasible to seriate a few tens of objects efficiently on a personal computer. This algorithm is now used for establishing a chronology of the Troubadours according to certain melodic features in their works. Other musicological uses for seriation are proposed.David Halperin is a Lecturer in the Department of Musicology, Tel Aviv University, specializing in computer-aided analysis of monophonic music. He has published articles on the Ambrosian chant and on the ancient musical notation from Ugarit. His publications include A Segmentation Algorithm and its Application to Medieval Monophonic Music,Musikometrika, 2 (1990) and Towards Deciphering the Ugaritic Musical Notation,Musikometrika, 4 (1992).This is an expanded version of a paper given at the 15th Congress of the International Musicological Society, Madrid, 1992.     

First- and zero-sound velocity and fermi liquid parameter F 2 s in liquid3He determined by a path length modulation technique

We have measured the velocity of first- and zero-sound in liquid³He at 12.6 MHz over the pressure range of 0.6 to 14.5 bar using a path length modulation technique that we have recently developed. From these measurements, the pressure dependent value of the Fermi liquid parameter F ₂ ^s was calculated and found to be larger at low pressure than previously reported. These new values of F ₂ ^s indicate that transverse zero-sound is a propagating mode at all pressures. The new values are important for the interpretation of the frequencies of order parameter collective modes in the superfluid phases. The new acoustic technique permits measurements in regimes of very high attenuation with a sensitivity in phase velocity of about 10 ppm achieved by a feedback arrangement. The sound velocity is thus measured continuously throughout the highly attenuating crossover ( 1) regime, even at the lowest pressures.     

Resistive transition in superconducting films

The consequences of a vortex unbinding picture of two-dimensional superconductivity are worked out. Although there is no true finite-temperature phase transition, dirty superconducting films should display anomalous behavior below the BCS transition temperature and above an effective Kosterlitz-Thouless vortex unbinding temperature. In particular, both the conductivity and fluctuation diamagnetism behave like ₊ ² in this regime, where ₊ is the correlation length calculated by Kosterlitz, _{+ c} exp (B/T – T _c)^1/2. We estimate _c, B, and the vortex unbinding temperature, and determine the nonlinear resistivity below T _c. A recent theory of vortex dynamics, together with a time-dependent Ginzburg-Landau theory, lead to a determination of the frequency-dependent conductivity.This work was supported by the National Science Foundation under Grant No. DMR 77-10210.     

Approximations to the Non-Central t,with Applications

A mildly conservative but very rapid calculation technique is proposed for problems requiring the use of the Non-central t distribution. Examples and a brief table are provided.