2,163 research outputs found
Magic-angle helical trilayer graphene
We propose helical trilayer graphene (HTG), a helical structure featuring
identical rotation angles between three consecutive
layers of graphene, as a unique and experimentally accessible platform for
realizing exotic correlated topological states of matter. While nominally
forming a supermoir\'e (or moir\'e-of-moir\'e) structure, we show that HTG
locally relaxes into large regions of a periodic single-moir\'e structure in
which is broken, giving rise to flat topological bands carrying
valley-Chern numbers . These bands feature near-ideal quantum
geometry and are isolated from remote bands by a large gap
meV, making HTG a promising platform for
experimental realization of correlated topological states such as integer and
fractional quantum anomalous Hall states in and bands
Hard and soft spectral states of ULXs
I discuss some differences between the observed spectral states of
ultraluminous X-ray sources (ULXs) and the canonical scheme of spectral states
defined in Galactic black holes. The standard interpretation of ULXs with a
curved spectrum, or a moderately steep power-law with soft excess and
high-energy downturn, is that they are an extension of the very high state, up
to luminosities ~ 1 to 3 L_{Edd}. Two competing models are Comptonization in a
warm corona, and slim disk; I suggest bulk motion Comptonization in the
radiatively-driven outflow as another possibility. The interpretation of ULXs
with a hard power-law spectrum is more problematic. Some of them remain in that
state over a large range of luminosities; others switch directly to a curved
state without going through a canonical high/soft state. I suggest that those
ULXs are in a high/hard state not seen in Galactic black holes; that state may
overlap with the low/hard state at lower accretion rates, and extend all the
way to Eddington accretion rates. If some black holes can reach Eddington
accretion rates without switching to a standard-disk-dominated state, it is
also possible that they never quench their steady jets.Comment: 6 pages, accepted for publication in the Astronomische Nachrichten,
to appear in the proceedings of the conference "Ultra-Luminous X-ray sources
and Middle Weight Black Holes" (Madrid, May 24-26, 2010
The hard to soft spectral transition in LMXBs - affected by recondensation of gas into an inner disk
Soft and hard spectral states of X-ray transient sources reflect two modes of
accretion, accretion via a geometrically thin, optically thick disk or an
advection-dominated accretion flow (ADAF). The luminosity at transition between
these two states seems to vary from source to source, or even for the same
source during different outbursts, as observed for GX 339-4. We investigate how
the existence of an inner weak disk in the hard state affects the transition
luminosity. We evaluate the structure of the corona above an outer truncated
disk and the resulting disk evaporation rate for different irradiation. In some
cases, recent observations of X-ray transients indicate the presence of an
inner cool disk during the hard state. Such a disk can remain during quiescence
after the last outburst as long as the luminosity does not drop to very low
values (10^-4 to 10^-3 of the Eddington luminosity). Consequently, as part of
the matter accretes via the inner disk, the hard irradiation is reduced. The
hard irradiation is further reduced, occulted and partly reflected by the inner
disk. This leads to a hard-soft transition at a lower luminosity if an inner
disk exists below the ADAF. This seems to be supported by observations for GX
339-4.Comment: 9 pages, 4 figures, accepted for publication in Astronomy and
Astrophysic
Helical trilayer graphene: a moir\'e platform for strongly-interacting topological bands
Quantum geometry of electronic wavefunctions results in fascinating
topological phenomena. A prominent example is the intrinsic anomalous Hall
effect (AHE) in which a Hall voltage arises in the absence of an applied
magnetic field. The AHE requires a coexistence of Berry curvature and
spontaneous time-reversal symmetry breaking. These conditions can be realized
in two-dimensional moir\'e systems with broken -inversion symmetry
() that host flat electronic bands. Here, we explore helical trilayer
graphene (HTG), three graphene layers twisted sequentially by the same angle
forming two misoriented moir\'e patterns. Although HTG is globally
-symmetric, surprisingly we observe clear signatures of topological
bands. At a magic angle , we uncover a
robust phase diagram of correlated and magnetic states using magnetotransport
measurements. Lattice relaxation leads to large periodic domains in which
is broken on the moir\'e scale. Each domain harbors flat topological
bands with valley-contrasting Chern numbers . We find correlated
states at integer electron fillings per moir\'e unit cell and
fractional fillings with the AHE arising at and .
At , a time-reversal symmetric phase appears beyond a critical electric
displacement field, indicating a topological phase transition. Finally,
hysteresis upon sweeping points to first-order phase transitions across a
spatial mosaic of Chern domains separated by a network of topological gapless
edge states. We establish HTG as an important platform that realizes ideal
conditions for exploring strongly interacting topological phases and, due to
its emergent moir\'e-scale symmetries, demonstrates a novel way to engineer
topology
Internal alignment and position resolution of the silicon tracker of DAMPE determined with orbit data
The DArk Matter Particle Explorer (DAMPE) is a space-borne particle detector
designed to probe electrons and gamma-rays in the few GeV to 10 TeV energy
range, as well as cosmic-ray proton and nuclei components between 10 GeV and
100 TeV. The silicon-tungsten tracker-converter is a crucial component of
DAMPE. It allows the direction of incoming photons converting into
electron-positron pairs to be estimated, and the trajectory and charge (Z) of
cosmic-ray particles to be identified. It consists of 768 silicon micro-strip
sensors assembled in 6 double layers with a total active area of 6.6 m.
Silicon planes are interleaved with three layers of tungsten plates, resulting
in about one radiation length of material in the tracker. Internal alignment
parameters of the tracker have been determined on orbit, with non-showering
protons and helium nuclei. We describe the alignment procedure and present the
position resolution and alignment stability measurements
Heterogeneity of melanoma cell responses to sleep apnea-derived plasma exosomes and to intermittent hypoxia
Obstructive sleep apnea (OSA) is associated with increased cutaneous melanoma incidence and adverse outcomes. Exosomes are secreted by most cells, and play a role in OSA-associated tumor progression and metastasis. We aimed to study the effects of plasma exosomes from OSA patients before and after adherent treatment with continuous positive airway pressure (CPAP) on melanoma cells lines, and also to identify exosomal miRNAs from melanoma cells exposed to intermittent hypoxia (IH) or normoxia. Plasma-derived exosomes were isolated from moderate-to-severe OSA patients before (V1) and after (V2) adherent CPAP treatment for one year. Exosomes were co-incubated with three3 different melanoma cell lines (CRL 1424; CRL 1619; CRL 1675) that are characterized by genotypes involving different mutations in BRAF, STK11, CDKN2A, and PTEN genes to assess the effect of exosomes on cell proliferation and migration, as well as on pAMK activity in the presence or absence of a chemical activator. Subsequently, CRL-1424 and CRL-1675 cells were exposed to intermittent hypoxia (IH) and normoxia, and exosomal miRNAs were identified followed by GO and KEG pathways and gene networks. The exosomes from these IH-exposed melanoma cells were also administered to THP1 macrophages to examine changes in M1 and M2 polarity markers. Plasma exosomes from V1 increased CRL-1424 melanoma cell proliferation and migration compared to V2, but not the other two cell lines. Exposure to CRL-1424 exosomes reduced pAMPK/tAMPK in V1 compared to V2, and treatment with AMPK activator reversed the effects. Unique exosomal miRNAs profiles were identified for CRL-1424 and CRL-1675 in IH compared to normoxia, with six miRNAs being regulated and several KEGG pathways were identified. Two M1 markers (CXCL10 and IL6) were significantly increased in monocytes when treated with exosomes from IH-exposed CRL-1424 and CRL-1625 cells. Our findings suggest that exosomes from untreated OSA patients increase CRL-1424 melanoma malignant properties, an effect that is not observed in two other melanoma cell lines. Exosomal cargo from CRL-1424 cells showed a unique miRNA signature compared to CRL-1675 cells after IH exposures, suggesting that melanoma cells are differentially susceptible to IH, even if they retain similar effects on immune cell polarity. It is postulated that mutations in STK-11 gene encoding for the serine/threonine kinase family that acts as a tumor suppressor may underlie susceptibility to IH-induced metabolic dysfunction, as illustrated by CRL-1424 cells. © 2021 by the authors. Licensee MDPI, Basel, Switzerland
Heterogeneity of Melanoma Cell Responses to Sleep Apnea-Derived Plasma Exosomes and to Intermittent Hypoxia
Obstructive sleep apnea (OSA) is associated with increased cutaneous melanoma incidence and adverse outcomes. Exosomes are secreted by most cells, and play a role in OSA-associated tumor progression and metastasis. We aimed to study the effects of plasma exosomes from OSA patients before and after adherent treatment with continuous positive airway pressure (CPAP) on melanoma cells lines, and also to identify exosomal miRNAs from melanoma cells exposed to intermittent hypoxia (IH) or normoxia. Plasma-derived exosomes were isolated from moderate-tosevere OSA patients before (V1) and after (V2) adherent CPAP treatment for one year. Exosomes were co-incubated with three3 different melanoma cell lines (CRL 1424; CRL 1619; CRL 1675) that are characterized by genotypes involving different mutations in BRAF, STK11, CDKN2A, and PTEN genes to assess the effect of exosomes on cell proliferation and migration, as well as on pAMK activity in the presence or absence of a chemical activator. Subsequently, CRL-1424 and CRL-1675 cells were exposed to intermittent hypoxia (IH) and normoxia, and exosomal miRNAs were identified followed by GO and KEG pathways and gene networks. The exosomes from these IH-exposed melanoma cells were also administered to THP1 macrophages to examine changes in M1 and M2 polarity markers. Plasma exosomes from V1 increased CRL-1424 melanoma cell proliferation and migration compared to V2, but not the other two cell lines. Exposure to CRL-1424 exosomes reduced
pAMPK/tAMPK in V1 compared to V2, and treatment with AMPK activator reversed the effects. Unique exosomal miRNAs profiles were identified for CRL-1424 and CRL-1675 in IH compared to normoxia, with six miRNAs being regulated and several KEGG pathways were identified. Two M1 markers (CXCL10 and IL6) were significantly increased in monocytes when treated with exosomes from IH-exposed CRL-1424 and CRL-1625 cells. Our findings suggest that exosomes from untreated OSA patients increase CRL-1424 melanoma malignant properties, an effect that is not observed in two other melanoma cell lines. Exosomal cargo from CRL-1424 cells showed a unique miRNA signature compared to CRL-1675 cells after IH exposures, suggesting that melanoma cells are differentially susceptible to IH, even if they retain similar effects on immune cell polarity. It is postulated that mutations in STK-11 gene encoding for the serine/threonine kinase family that acts as a tumor suppressor may underlie susceptibility to IH-induced metabolic dysfunction, as illustrated by CRL1424 cells
Genome of Crucihimalaya himalaica, a close relative of Arabidopsis, shows ecological adaptation to high altitude
Crucihimalaya himalaica is a close relative of Arabidopsis with typical Qinghai–Tibet Plateau (QTP) distribution. Here, by combining short- and long-read sequencing technologies, we provide a de novo genome sequence of C. himalaica. Our results suggest that the quick uplifting of the QTP coincided with the expansion of repeat elements. Gene families showing dramatic contractions and expansions, as well as genes showing clear signs of natural selection, were likely responsible for C. himalaica’s specific adaptation to the harsh environment of the QTP. We also show that the transition to self-pollination of C. himalaica might have enabled its occupation of the QTP. This study provides insights into how plants might adapt to extreme environmental conditions
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