1,244 research outputs found
Domain walls and the conductivity of mesoscopic ferromagnets
Quantum interference phenomena in the conductivity of mesoscopic ferromagnets
are considered, particularly with regard to the effects of geometric phases
acquired by electrons propagating through regions of spatially varying
magnetization (due, e.g., to magnetic domain walls). Weak localization and
electron-electron interaction quantum corrections to the conductivity and
universal conductance fluctuations are discussed. Experiments are proposed for
multiply-connected geometries that should reveal conductance oscillations with
variations of the profile of the magnetization.Comment: 4 pages, 1 fugure, RevTEX, Submitted to Phys. Rev. Let
The relative importance of electron-electron interactions compared to disorder in the two-dimensional "metallic" state
The effect of substrate bias and surface gate voltage on the low temperature
resistivity of a Si-MOSFET is studied for electron concentrations where the
resistivity increases with increasing temperature. This technique offers two
degrees of freedom for controlling the electron concentration and the device
mobility, thereby providing a means to evaluate the relative importance of
electron-electron interactions and disorder in this so-called ``metallic''
regime. For temperatures well below the Fermi temperature, the data obey a
scaling law where the disorder parameter (), and not the
concentration, appears explicitly. This suggests that interactions, although
present, do not alter the Fermi-liquid properties of the system fundamentally.
Furthermore, this experimental observation is reproduced in results of
calculations based on temperature-dependent screening, in the context of
Drude-Boltzmann theory.Comment: 5 pages, 6 figure
Staggered fermions and chiral symmetry breaking in transverse lattice regulated QED
Staggered fermions are constructed for the transverse lattice regularization
scheme. The weak perturbation theory of transverse lattice non-compact QED is
developed in light-cone gauge, and we argue that for fixed lattice spacing this
theory is ultraviolet finite, order by order in perturbation theory. However,
by calculating the anomalous scaling dimension of the link fields, we find that
the interaction Hamiltonian becomes non-renormalizable for ,
where is the bare (lattice) QED coupling constant. We conjecture that
this is the critical point of the chiral symmetry breaking phase transition in
QED. Non-perturbative chiral symmetry breaking is then studied in the strong
coupling limit. The discrete remnant of chiral symmetry that remains on the
lattice is spontaneously broken, and the ground state to lowest order in the
strong coupling expansion corresponds to the classical ground state of the
two-dimensional spin one-half Heisenberg antiferromagnet.Comment: 30 pages, UFIFT-HEP-92-1
First NuSTAR Limits on Quiet Sun Hard X-Ray Transient Events
We present the first results of a search for transient hard X-ray (HXR)
emission in the quiet solar corona with the \textit{Nuclear Spectroscopic
Telescope Array} (\textit{NuSTAR}) satellite. While \textit{NuSTAR} was
designed as an astrophysics mission, it can observe the Sun above 2~keV with
unprecedented sensitivity due to its pioneering use of focusing optics.
\textit{NuSTAR} first observed quiet Sun regions on 2014 November 1, although
out-of-view active regions contributed a notable amount of background in the
form of single-bounce (unfocused) X-rays. We conducted a search for quiet Sun
transient brightenings on time scales of 100 s and set upper limits on emission
in two energy bands. We set 2.5--4~keV limits on brightenings with time scales
of 100 s, expressed as the temperature T and emission measure EM of a thermal
plasma. We also set 10--20~keV limits on brightenings with time scales of 30,
60, and 100 s, expressed as model-independent photon fluxes. The limits in both
bands are well below previous HXR microflare detections, though not low enough
to detect events of equivalent T and EM as quiet Sun brightenings seen in soft
X-ray observations. We expect future observations during solar minimum to
increase the \textit{NuSTAR} sensitivity by over two orders of magnitude due to
higher instrument livetime and reduced solar background.Comment: 11 pages, 7 figures; accepted for publication in The Astrophysical
Journa
Parafermionic edge zero modes in Z_n-invariant spin chains
A sign of topological order in a gapped one-dimensional quantum chain is the
existence of edge zero modes. These occur in the Z_2-invariant Ising/Majorana
chain, where they can be understood using free-fermion techniques. Here I
discuss their presence in spin chains with Z_n symmetry, and prove that for
appropriate coupling they are exact, even in this strongly interacting system.
These modes are naturally expressed in terms of parafermions, generalizations
of fermions to the Z_n case. I show that parafermionic edge zero modes do not
occur in the usual ferromagnetic and antiferromagnetic cases, but rather only
when the interactions are chiral, so that spatial-parity and time-reversal
symmetries are broken.Comment: 22 pages. v2: small changes, added reference
Massless Scalar QED with Non-minimal Chern Simons Coupling
2+1 dimensional massless scalar QED with scalar
self-coupling is modified by the addition of a non- minimal Chern-Simons term
that couples the dual of the electromagnetic field strength to the covariant
current of the complex scalar field. The theory is shown to be fully one- loop
renormalizable. The one loop effective potential for the scalar field gives
rise to spontaneous symmetry breaking which induces masses for both the scalar
and vector fields. At high temperature there is a symmetry restoring phase
transition.Comment: 18 pages, latex, preprint WIN-93-1
Vortices and domain walls in a Chern-Simons theory with magnetic moment interaction
We study the structure and properties of vortices in a recently proposed
Abelian Maxwell-Chern-Simons model in dimensions. The model which is
described by gauge field interacting with a complex scalar field, includes two
parity and time violating terms: the Chern-Simons and the anomalous magnetic
terms. Self-dual relativistic vortices are discussed in detail. We also find
one dimensional soliton solutions of the domain wall type. The vortices are
correctly described by the domain wall solutions in the large flux limit.Comment: To be published in Phys RevD 23 pages, RevTex, 5 figure
Biogenic cloud nuclei in the central Amazon during the transition from wet to dry season
© Author(s) 2016. This work is distributed under the Creative Commons Attribution 3.0 License. Whitehead, J. D., Darbyshire, E., Brito, J., Barbosa, H. M. J., Crawford, I., Stern, R., Gallagher, M. W., Kaye, P. H., Allan, J. D., Coe, H., Artaxo, P., and McFiggans, G.: Biogenic cloud nuclei in the central Amazon during the transition from wet to dry season, Atmos. Chem. Phys., 16, 9727-9743, doi:10.5194/acp-16-9727-2016, 2016.The Amazon basin is a vast continental area in which atmospheric composition is relatively unaffected by anthropogenic aerosol particles. Understanding the proper- ties of the natural biogenic aerosol particles over the Ama- zon rainforest is key to understanding their inïŹuence on re- gional and global climate. While there have been a number of studies during the wet season, and of biomass burning par- ticles in the dry season, there has been relatively little work on the transition period â the start of the dry season in the absence of biomass burning. As part of the BrazilâUK Net- work for Investigation of Amazonian Atmospheric Composi- tion and Impacts on Climate (BUNIAACIC) project, aerosol measurements, focussing on unpolluted biogenic air masses, were conducted at a remote rainforest site in the central Ama- zon during the transition from wet to dry season in July 2013. This period marks the start of the dry season but before sig- niïŹcant biomass burning occurs in the region. Median particle number concentrations were 266 cmâ3, with size distributions dominated by an accumulation mode of 130â150 nm. During periods of low particle counts, a smaller Aitken mode could also be seen around 80 nm. While the concentrations were similar in magnitude to those seen during the wet season, the size distributions suggest an en- hancement in the accumulation mode compared to the wet season, but not yet to the extent seen later in the dry sea- son, when signiïŹcant biomass burning takes place. Submi- cron nonrefractory aerosol composition, as measured by an aerosol chemical speciation monitor (ACSM), was domi- nated by organic material (around 81 %). Aerosol hygro- scopicity was probed using measurements from a hygro- scopicity tandem differential mobility analyser (HTDMA), and a quasi-monodisperse cloud condensation nuclei counter (CCNc). The hygroscopicity parameter, Îș , was found to be low, ranging from 0.12 for Aitken-mode particles to 0.18 for accumulation-mode particles. This was consistent with pre- vious studies in the region, but lower than similar measure- ments conducted in Borneo, where Îș ranged 0.17â0.37. A wide issue bioaerosol sensor (WIBS-3M) was deployed at ground level to probe the coarse mode, detecting pri- mary biological aerosol by ïŹuorescence (ïŹuorescent biolog- ical aerosol particles, or FBAPs). The mean FBAP number concentration was 400 ± 242 Lâ1; however, this ranged from around 200 Lâ1 during the day to as much as 1200 Lâ1 at night. FBAPs dominated the coarse-mode particles, compris- ing between 55 and 75 % of particles during the day to more than 90 % at night. Non-FBAPs did not show a strong diur- nal pattern. Comparison with previous FBAP measurements above canopy at the same location suggests there is a strong vertical gradient in FBAP concentrations through the canopy. Cluster analysis of the data suggests that FBAPs were dom- inated (around 70 %) by fungal spores. Further, long-term measurements will be required in order to fully examine the seasonal variability and distribution of primary biological aerosol particles through the canopy. This is the ïŹrst time that such a suite of measurements has been deployed at this site to investigate the chemical compo- sition and properties of the biogenic contributions to Ama- zonian aerosol during the transition period from the wet to the dry season, and thus provides a unique comparison to the aerosol properties observed during the wet season in previ- ous similar campaigns. This was also the ïŹrst deployment of a WIBS in the Amazon rainforest to study coarse-mode parti- cles, particularly primary biological aerosol particles, which are likely to play an important role as ice nuclei in the region.Peer reviewe
Local mutations:On the tentative beginnings of molecular oncology in Britain 1980â2000
Popular and scientific accounts of the molecularisation of cancer typically attribute it to advances in laboratory science, particularly molecular geneticists. However, historical research has indicated that clinical expertise input was often vital for advancing such work. The present paper reinforces that view. Looking in detail at British research into the molecular genetics of familial cancers during the 1980s and 1990s, it shows that that research, too, depended on crucial input from family cancer clinics. Moreover, the development of clinical services for familial cancers was in turn shaped by the demands of contributing to molecular genetic research. The paper concludes that accounts of the molecularisation of cancer that suppose a one-way transfer of knowledge and practice from laboratory to clinic misrepresent the complex interactions that were involved in molecularising familial cancers, and that were informed by the particular local and national circumstances in which they took shape
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