66 research outputs found
Tuning gaps and phases of a two-subband system in a quantizing magnetic field
In this work we study the properties of a two-subband quasi-two-dimensional
electron system in a strong magnetic field when the electron filling factor is
equal to four. When the cyclotron energy is close to the intersubband splitting
the system can be mapped onto a four-level electron system with an effective
filling factor of two. The ground state is either a ferromagnetic state or a
spin-singlet state, depending on the values of the inter-level splitting and
Zeeman energy. The boundaries between these phases are strongly influenced by
the inter-electron interaction. A significant exchange-mediated enhancement of
the excitation gap results in the suppression of the electron-phonon
interaction. The rate of absorption of non-equilibrium phonons is calculated as
a function of Zeeman energy and inter-subband splitting. The phonon absorption
rate has two peaks as a function of intersubband splitting and has a step-like
structure as a function of Zeeman energy
Two-phonon scattering of magnetorotons in fractional quantum Hall liquids
We study the phonon-assisted process of dissociation of a magnetoroton, in a
fractional quantum Hall liquid, into an unbound pair of quasiparticles. Whilst
the dissociation is forbidden to first order in the electron-phonon
interaction, it can occur as a two-phonon process. Depending on the value of
final separation between the quasiparticles, the dissociation is either a
single event involving absorption of one phonon and emission of another phonon
of similar energy, or a two-phonon diffusion of a quasiexciton in momentum
space. The dependence of the magnetoroton dissociation time on the filling
factor of the incompressible liquid is found.Comment: 4 pages, no figure
1/f noise in a dilute GaAs two-dimensional hole system in the insulating phase
We have measured the resistance and the 1/f resistance noise of a
two-dimensional low density hole system in a high mobility GaAs quantum well at
low temperature. At densities lower than the metal-insulator transition one,
the temperature dependence of the resistance is either power-like or simply
activated. The noise decreases when the temperature or the density increase.
These results contradict the standard description of independent particles in
the strong localization regime. On the contrary, they agree with the
percolation picture suggested by higher density results. The physical nature of
the system could be a mixture of a conducting and an insulating phase. We
compare our results with those of composite thin films.Comment: 4 pages, 3 figures; to appear in Physica E (EP2DS-16 proceedings
An atomic carbon source for high temperature molecular beam epitaxy of graphene
We report the use of a novel atomic carbon source for the molecular beam epitaxy (MBE) of graphene layers on hBN flakes and on sapphire wafers at substrate growth temperatures of ~1400 °C. The source produces a flux of predominantly atomic carbon, which diffuses through the walls of a Joule-heated tantalum tube filled with graphite powder. We demonstrate deposition of carbon on sapphire with carbon deposition rates up to 12 nm/h. Atomic force microscopy measurements reveal the formation of hexagonal moiré patterns when graphene monolayers are grown on hBN flakes. The Raman spectra of the graphene layers grown on hBN and sapphire with the sublimation carbon source and the atomic carbon source are similar, whilst the nature of the carbon aggregates is different - graphitic with the sublimation carbon source and amorphous with the atomic carbon source. At MBE growth temperatures we observe etching of the sapphire wafer surface by the flux from the atomic carbon source, which we have not observed in the MBE growth of graphene with the sublimation carbon source
Mixed States of Composite Fermions Carrying Two and Four Vortices
There now exists preliminary experimental evidence for some fractions, such
as = 4/11 and 5/13, that do not belong to any of the sequences
, and being integers. We propose that these states
are mixed states of composite fermions of different flavors, for example,
composite fermions carrying two and four vortices. We also obtain an estimate
of the lowest-excitation dispersion curve as well as the transport gap; the
gaps for 4/11 are smaller than those for 1/3 by approximately a factor of 50.Comment: Accepted for PRB rapid communication (scheduled to appear in Nov 15,
2000 issue
Physics in the Real Universe: Time and Spacetime
The Block Universe idea, representing spacetime as a fixed whole, suggests
the flow of time is an illusion: the entire universe just is, with no special
meaning attached to the present time. This view is however based on
time-reversible microphysical laws and does not represent macro-physical
behaviour and the development of emergent complex systems, including life,
which do indeed exist in the real universe. When these are taken into account,
the unchanging block universe view of spacetime is best replaced by an evolving
block universe which extends as time evolves, with the potential of the future
continually becoming the certainty of the past. However this time evolution is
not related to any preferred surfaces in spacetime; rather it is associated
with the evolution of proper time along families of world linesComment: 28 pages, including 9 Figures. Major revision in response to referee
comment
Direct band-gap crossover in epitaxial monolayer boron nitride
Hexagonal boron nitride is a large band-gap insulating material which complements the electronic and optical properties of graphene and the transition metal dichalcogenides. However, the intrinsic optical properties of monolayer boron nitride remain largely unexplored. In particular, the theoretically expected crossover to a direct-gap in the limit of the single monolayer is presently not con_rmed experimentally. Here, in contrast to the technique of exfoliating few-layer 2D hexagonal boron nitride, we exploit the scalable approach of high-temperature molecular beam epitaxy to grow high-quality monolayer boron nitride on graphite substrates. We combine deep-ultraviolet photoluminescence and reectance spectroscopy with atomic force microscopy to reveal the presence of a direct gap of energy 6.1 eV in the single atomic layers, thus con_rming a crossover to direct gap in the monolayer limit
Multi-species sociology of the body
The human body has become a central focus in sociology. Such work has centred largely on the human body and its significance in social contexts. This article draws on sociological understandings of human embodiment, especially the idea of the ‘body as a project’, to facilitate a multi-species understanding of bodies and their entanglements. Conceptualising the body as a project has provided sociological insights into the scientific and technological innovations that are designed to improve health and delay death. Nonhuman animals are entangled in these efforts, though their presence is often occluded. By examining notions of body masks, body regimes and body options, which are well established in sociological thinking about the body, this article seeks to prompt consideration of how to utilise theories of the body to examine human–nonhuman animal entanglements in order to establish a multi-species sociology of the body
The amyloid precursor protein controls PIKfyve function
While the Amyloid Precursor Protein (APP) plays a central role in Alzheimer's disease, its cellular function still remains largely unclear. It was our goal to establish APP function which will provide insights into APP's implication in Alzheimer's disease. Using our recently developed proteo-liposome assay we established the interactome of APP's intracellular domain (known as AICD), thereby identifying novel APP interactors that provide mechanistic insights into APP function. By combining biochemical, cell biological and genetic approaches we validated the functional significance of one of these novel interactors. Here we show that APP binds the PIKfyve complex, an essential kinase for the synthesis of the endosomal phosphoinositide phosphatidylinositol-3,5-bisphosphate. This signalling lipid plays a crucial role in endosomal homeostasis and receptor sorting. Loss of PIKfyve function by mutation causes profound neurodegeneration in mammals. Using C. elegans genetics we demonstrate that APP functionally cooperates with PIKfyve in vivo. This regulation is required for maintaining endosomal and neuronal function. Our findings establish an unexpected role for APP in the regulation of endosomal phosphoinositide metabolism with dramatic consequences for endosomal biology and important implications for our understanding of Alzheimer's disease
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