234 research outputs found
Fragile topology in line-graph lattices with two, three, or four gapped flat bands
The geometric properties of a lattice can have profound consequences on its
band spectrum. For example, symmetry constraints and geometric frustration can
give rise to topologicially nontrivial and dispersionless bands, respectively.
Line-graph lattices are a perfect example of both of these features: their
lowest energy bands are perfectly flat, and here we develop a formalism to
connect some of their geometric properties with the presence or absence of
fragile topology in their flat bands. This theoretical work will enable
experimental studies of fragile topology in several types of line-graph
lattices, most naturally suited to superconducting circuits.Comment: 8+25 pages, 3+19 figures, 2+3 table
Spin-Orbit-Induced Topological Flat Bands in Line and Split Graphs of Bipartite Lattices
Topological flat bands, such as the band in twisted bilayer graphene, are
becoming a promising platform to study topics such as correlation physics,
superconductivity, and transport. In this work, we introduce a generic approach
to construct two-dimensional (2D) topological quasi-flat bands from line graphs
and split graphs of bipartite lattices. A line graph or split graph of a
bipartite lattice exhibits a set of flat bands and a set of dispersive bands.
The flat band connects to the dispersive bands through a degenerate state at
some momentum. We find that, with spin-orbit coupling (SOC), the flat band
becomes quasi-flat and gapped from the dispersive bands. By studying a series
of specific line graphs and split graphs of bipartite lattices, we find that
(i) if the flat band (without SOC) has inversion or symmetry and is
non-degenerate, then the resulting quasi-flat band must be topologically
nontrivial, and (ii) if the flat band (without SOC) is degenerate, then there
exists an SOC potential such that the resulting quasi-flat band is
topologically nontrivial. This generic mechanism serves as a paradigm for
finding topological quasi-flat bands in 2D crystalline materials and
meta-materials
Measurements of the Correlation Function of a Microwave Frequency Single Photon Source
At optical frequencies the radiation produced by a source, such as a laser, a
black body or a single photon source, is frequently characterized by analyzing
the temporal correlations of emitted photons using single photon counters. At
microwave frequencies, however, there are no efficient single photon counters
yet. Instead, well developed linear amplifiers allow for efficient measurement
of the amplitude of an electromagnetic field. Here, we demonstrate how the
properties of a microwave single photon source can be characterized using
correlation measurements of the emitted radiation with such detectors. We also
demonstrate the cooling of a thermal field stored in a cavity, an effect which
we detect using a cross-correlation measurement of the radiation emitted at the
two ends of the cavity.Comment: 5 pages, 4 figure
H_2 emission arises outside photodissociation regions in ultra-luminous infrared galaxies
Ultra-luminous infrared galaxies are among the most luminous objects in the
local universe and are thought to be powered by intense star formation. It has
been shown that in these objects the rotational spectral lines of molecular
hydrogen observed at mid-infrared wavelengths are not affected by dust
obscuration, leaving unresolved the source of excitation of this emission. Here
I report an analysis of archival Spitzer Space Telescope data on ultra-luminous
infrared galaxies and demonstrate that star formation regions are buried inside
optically thick clouds of gas and dust, so that dust obscuration affects
star-formation indicators but not molecular hydrogen. I thereby establish that
the emission of H_2 is not co-spatial with the buried starburst activity and
originates outside the obscured regions. This is rather surprising in light of
the standard view that H_2 emission is directly associated with star-formation
activity. Instead, I propose that H_2 emission in these objects traces shocks
in the surrounding material, which are in turn excited by interactions with
nearby galaxies, and that powerful large-scale shocks cooling by means of H_2
emission may be much more common than previously thought. In the early
universe, a boost in H_2 emission by this process may speed up the cooling of
matter as it collapsed to form the first stars and galaxies and would make
these first structures more readily observable.Comment: Main text and supplemental information, 21 pages including 6 figures,
2 table
The magnetic nature of disk accretion onto black holes
Although disk accretion onto compact objects - white dwarfs, neutron stars,
and black holes - is central to much of high energy astrophysics, the
mechanisms which enable this process have remained observationally elusive.
Accretion disks must transfer angular momentum for matter to travel radially
inward onto the compact object. Internal viscosity from magnetic processes and
disk winds can in principle both transfer angular momentum, but hitherto we
lacked evidence that either occurs. Here we report that an X-ray-absorbing wind
discovered in an observation of the stellar-mass black hole binary GRO J1655-40
must be powered by a magnetic process that can also drive accretion through the
disk. Detailed spectral analysis and modeling of the wind shows that it can
only be powered by pressure generated by magnetic viscosity internal to the
disk or magnetocentrifugal forces. This result demonstrates that disk accretion
onto black holes is a fundamentally magnetic process.Comment: 15 pages, 2 color figures, accepted for publication in Nature.
Supplemental materials may be obtained by clicking
http://www.astro.lsa.umich.edu/~jonmm/nature1655.p
Optical vs. infrared studies of dusty galaxies and AGN: (I) Nebular emission lines
Optical nebular emission lines are commonly used to estimate the star
formation rate of galaxies and the black hole accretion rate of their central
active nucleus. The accuracy of the conversion from line strengths to physical
properties depends upon the accuracy to which the lines can be corrected for
dust attenuation. For studies of single galaxies with normal amounts of dust,
most dust corrections result in the same derived properties within the errors.
However, for statistical studies of populations of galaxies, or for studies of
galaxies with higher dust contents such as might be found in some classes of
"transition" galaxies, significant uncertainty arises from the dust attenuation
correction. We compare the strength of the predominantly unobscured mid-IR
[NeII]15.5um + [NeIII]12.8um emission lines to the optical H alpha emission
lines in four samples of galaxies: (i) ordinary star forming galaxies, (ii)
optically selected dusty galaxies, (iii) ULIRGs, (iv) Seyfert 2 galaxies. We
show that a single dust attenuation curve applied to all samples can correct H
alpha emission for dust attenuation to a factor better than 2. Similarly, we
compare mid-IR [OIV] and optical [OIII] luminosities to find that [OIII] can be
corrected to a factor better than 3. This shows that the total dust attenuation
suffered by the AGN narrow line region is not significantly different to that
suffered by the starforming HII regions in the galaxy. We provide explicit dust
attenuation corrections, together with errors, for [OII], [OIII] and H alpha.
The best-fit average attenuation curve is slightly greyer than the Milky-Way
extinction law, indicating either that external galaxies have slightly
different typical dust properties to the Milky Way, or that there is a
significant contribution from scattering. Finally, we uncover an intriguing
correlation between Silicate absorption and Balmer decrement.Comment: 19 pages, 9 figures, accepted for publication in MNRA
Purified native and recombinant human alpha lymphotoxin [tumor necrosis factor (TNF)-beta] induces inflammatory reactions in normal skin
These studies report findings that demonstrate that human alpha lymphotoxin (LT) induces local, visible, and microscopic inflammatory reactions in normal skin. Skin sites in rabbits, when inoculated with a single injection of native or recombinant human alpha lymphotoxin, demonstrated erythema, swelling, and warmth within 5 hr. Erythema peaked between 24 and 48 hr had resolved by 72 hr. Histologic studies of skin sites injected with native LT revealed polymorphonuclear neutrophil (PMN) infiltration and edema beginning as early as 3 hr posttreatment. Individual skin sites that received three daily injections of native LT exhibited persistent erythema and swelling. Palpable induration was evident 24 hr after the second injection in the series. Histologic examination revealed the presence of many PMNs with associated focal dermal destruction, in the form of microabscesses, and scattered mononuclear cells. In contrast, control materials and recombinant human tumor necrosis factor (TNF-alpha) did not induce visible skin reactions in the rabbit. Several additional controls excluded endotoxin as being the agent responsible for the inflammatory skin reactions observed. The ability of LT to induce inflammation may have a role in its antitumor activity and it may be an important endogenous mediator in other immunologic reactions
The development of a protoplanetary disk from its natal envelope
Class 0 protostars, the youngest type of young stellar objects, show many signs of rapid development from their initial, spheroidal configurations, and therefore are studied intensively for details of the formation of protoplanetary disks within protostellar envelopes. At millimetre wavelengths, kinematic signatures of collapse have been observed in several such protostars, through observations of molecular lines that probe their outer envelopes. It has been suggested that one or more components of the proto-multiple system NGC 1333-IRAS 4 (refs 1, 2) may display signs of an embedded region that is warmer and denser than the bulk of the envelope(3,4). Here we report observations that reveal details of the core on Solar System dimensions. We detect in NGC 1333-IRAS 4B a rich emission spectrum of H2O, at wavelengths 20-37 mu m, which indicates an origin in extremely dense, warm gas. We can model the emission as infall from a protostellar envelope onto the surface of a deeply embedded, dense disk, and therefore see the development of a protoplanetary disk. This is the only example of mid-infrared water emission from a sample of 30 class 0 objects, perhaps arising from a favourable orientation; alternatively, this may be an early and short-lived stage in the evolution of a protoplanetary disk.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/62894/1/nature06087.pd
Dusty Planetary Systems
Extensive photometric stellar surveys show that many main sequence stars show
emission at infrared and longer wavelengths that is in excess of the stellar
photosphere; this emission is thought to arise from circumstellar dust. The
presence of dust disks is confirmed by spatially resolved imaging at infrared
to millimeter wavelengths (tracing the dust thermal emission), and at optical
to near infrared wavelengths (tracing the dust scattered light). Because the
expected lifetime of these dust particles is much shorter than the age of the
stars (>10 Myr), it is inferred that this solid material not primordial, i.e.
the remaining from the placental cloud of gas and dust where the star was born,
but instead is replenished by dust-producing planetesimals. These planetesimals
are analogous to the asteroids, comets and Kuiper Belt objects (KBOs) in our
Solar system that produce the interplanetary dust that gives rise to the
zodiacal light (tracing the inner component of the Solar system debris disk).
The presence of these "debris disks" around stars with a wide range of masses,
luminosities, and metallicities, with and without binary companions, is
evidence that planetesimal formation is a robust process that can take place
under a wide range of conditions. This chapter is divided in two parts. Part I
discusses how the study of the Solar system debris disk and the study of debris
disks around other stars can help us learn about the formation, evolution and
diversity of planetary systems by shedding light on the frequency and timing of
planetesimal formation, the location and physical properties of the
planetesimals, the presence of long-period planets, and the dynamical and
collisional evolution of the system. Part II reviews the physical processes
that affect dust particles in the gas-free environment of a debris disk and
their effect on the dust particle size and spatial distribution.Comment: 68 pages, 25 figures. To be published in "Solar and Planetary
Systems" (P. Kalas and L. French, Eds.), Volume 3 of the series "Planets,
Stars and Stellar Systems" (T.D. Oswalt, Editor-in-chief), Springer 201
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