A critical Mach number for electron injection in collisionless shocks

Electron acceleration in collisionless shocks with arbitrary magnetic field orientations is discussed. It is shown that the injection of thermal electrons into diffusive shock acceleration process is achieved by an electron beam with a loss-cone in velocity space that is reflected back upstream from the shock through shock drift acceleration mechanism. The electron beam is able to excite whistler waves which can scatter the energetic electrons themselves when the Alfven Mach number of the shock is sufficiently high. A critical Mach number for the electron injection is obtained as a function of upstream parameters. The application to supernova remnant shocks is discussed.Comment: 4 pages, 2 figure, accepted for publication in Physical Review Letter

Non-collinear magnetism in Al-Mn topologically disordered systems

We have performed the first ab-initio calculations of a possible complex non-collinear magnetic structure in aluminium-rich Al-Mn liquids within the real-space tight-binding LMTO method. In our previous work we predicted the existence of large magnetic moments in Al-Mn liquids [A.M. Bratkovsky, A.V. Smirnov, D. N. Manh, and A. Pasturel, \prb {\bf 52}, 3056 (1995)] which has been very recently confirmed experimentally. Our present calculations show that there is a strong tendency for the moments on Mn to have a non-collinear (random) order retaining their large value of about 3~$\mu_B$. The d-electrons on Mn demonstrate a pronounced non-rigid band behaviour which cannot be reproduced within a simple Stoner picture. The origin of the magnetism in these systems is a topological disorder which drives the moments formation and frustrates their directions in the liquid phase.Comment: 10 pages, RevTex 3.0, 24kb. 3 PS figures available on request from [email protected] The work has been presented at ERC ``Electronic Structire of Solids'' (Lunteren, The Netherlands, 9-14 September 1995

Electronic Orders Induced by Kondo Effect in Non-Kramers f-Electron Systems

This paper clarifies the microscopic nature of the staggered scalar order, which is specific to even number of f electrons per site. In such systems, crystalline electric field (CEF) can make a singlet ground state. As exchange interaction with conduction electrons increases, the CEF singlet at each site gives way to Kondo singlets. The collective Kondo singlets are identified with itinerant states that form energy bands. Near the boundary of itinerant and localized states, a new type of electronic order appears with staggered Kondo and CEF singlets. We present a phenomenological three-state model that qualitatively reproduces the characteristic phase diagram, which have been obtained numerically with use of the continuous-time quantum Monte Carlo combined with the dynamical mean-field theory. The scalar order observed in PrFe_4P_{12} is ascribed to this staggered order accompanying charge density wave (CDW) of conduction electrons. Accurate photoemission and tunneling spectroscopy should be able to probe sharp peaks below and above the Fermi level in the ordered phase.Comment: 7 pages, 8 figure

X-Ray Study of the Outer Region of Abell 2142 with Suzaku

We observed outer regions of a bright cluster of galaxies A2142 with Suzaku. Temperature and brightness structures were measured out to the virial radius ($r_{200}$) with good sensitivity. We confirmed the temperature drop from 9 keV around the cluster center to about 3.5 keV at $r_{200}$, with the density profile well approximated by the $\beta$ model with $\beta = 0.85$. Within 0.4\r_{200}, the entropy profile agrees with $r^{1.1}$, as predicted by the accretion shock model. The entropy slope becomes flatter in the outer region and negative around $r_{200}$. These features suggest that the intracluster medium in the outer region is out of thermal equilibrium. Since the relaxation timescale of electron-ion Coulomb collision is expected to be longer than the elapsed time after shock heating at $r_{200}$, one plausible reason of the low entropy is the low electron temperature compared to that of ions. Other possible explanations would be gas clumpiness, turbulence and bulk motions of ICM\@. We also searched for a warm-hot intergalactic medium around $r_{200}$ and set an upper limit on the oxygen line intensity. Assuming a line-of-sight depth of 2 Mpc and oxygen abundance of 0.1 solar, the upper limit of an overdensity is calculated to be 280 or 380, depending on the foreground assumption.Comment: 14 pages, 8 figure

Massive Dirac fermions and the zero field quantum Hall effect

Through an explicit calculation for a Lagrangian in quantum electrodynamics in (2+1)-space--time dimensions (QED$_3$), making use of the relativistic Kubo formula, we demonstrate that the filling factor accompanying the quantized electrical conductivity for massive Dirac fermions of a single species in two spatial dimensions is a half (in natural units) when time reversal and parity symmetries of the Lagrangian are explicitly broken by the fermion mass term. We then discuss the most general form of the QED$_3$ Lagrangian, both for irreducible and reducible representations of the Dirac matrices in the plane, with emphasis on the appearance of a Chern-Simons term. We also identify the value of the filling factor with a zero field quantum Hall effect (QHE).Comment: 15 pages. Accepted in Jour. Phys.

The role of TBX22 in craniofacial development

Cleft lip and/or cleft palate are a heterogeneous group of disorders that rank among the commonest birth defects known, affecting 1 in 700 births worldwide. The underlying cause is poorly understood, with a complex interaction of genes and environmental factors being implicated. Nevertheless, several important genetic causes have been identified, including that of X-linked cleft palate and ankyloglossia (CPX). CPX is a semi-dominant condition caused by mutations in TBX22 which encodes a T-box containing transcription factor. TBX22/Tbx22 is highly conserved and expressed in the developing palatal shelves as well as at the base of the tongue, medial and lateral nasal prominences and periocular mesenchyme in both human and mouse embryos. This project set out to better understand the functional role of TBX22 using Tbx22 null mouse model that is characterised by overt or submucous cleft palate, ankyloglossia and choanal atresia. Microarray analysis of E13.5 palatal shelves dissected from wild type and Tbx22 null mice revealed a global upregulation of muscle genes such as myosin and muscle actin in the null palatal shelves. Key myogenic regulatory factors MyoD and myogenin were moderately upregulated. Increased expression was independently confirmed using real-time PCR. In vitro analysis in a mammalian cell line using luciferase reporter assays and chromatin immunoprecipitation showed that TBX22 could repress the MyoD promoter and was capable of interacting with its promoter regions. This may provide a link between lack of Tbx22 and upregulation of muscle markers. These results support a hypothesis that MyoD is a possible direct target gene of TBX22. In addition, decreased cell proliferation in the Tbx22 null palatal shelves was observed, along with reduced expression of Cyclin D2. This indicates that TBX22 has a role in the regulation of cell proliferation during palate development as well as a previously identified role in osteoblast differentiation and maturation