2,684 research outputs found
Raising Bi-O bands above the Fermi energy level of hole-doped BiSrCaCuO and other cuprate superconductors
The Fermi surface (FS) of BiSrCaCuO
(Bi2212) predicted by band theory displays Bi-related pockets around the
point, which have never been observed experimentally. We show that
when the effects of hole doping either by substituting Pb for Bi or by adding
excess O in Bi2212 are included, the Bi-O bands are lifted above the Fermi
energy () and the resulting first-principles FS is in remarkable accord
with measurements. With decreasing hole-doping the Bi-O bands drop below
and the system self-dopes below a critical hole concentration. Computations on
other Bi- as well as Tl- and Hg-based compounds indicate that lifting of the
cation-derived band with hole doping is a general property of the electronic
structures of the cuprates.Comment: 4 pages, 4 figures; PRL (2006, in press
Supercooled Water and the Kinetic Glass Transition II: Collective Dynamics
In this article we study in detail the Q-vector dependence of the collective
dynamics in simulated deeply supercooled SPC/E water. The evolution of the
system has been followed for 250 ns at low T, allowing a clear identification
of a two step relaxation process. We present evidence in favor of the use of
the mode coupling theory for supercooled liquid as framework for the
description of the slow alpha-relaxation dynamics in SPC/E water,
notwithstanding the fact that the cage formation in this system is controlled
by the formation of an open network of hydrogen bonds as opposed to packing
constraints, as in the case of simple liquids.Comment: rev-tex + 9 figure
Ethanol Induced Disordering of Pancreatic Acinar Cell Endoplasmic Reticulum: An ER Stress/Defective Unfolded Protein Response Model.
Background & aimsHeavy alcohol drinking is associated with pancreatitis, whereas moderate intake lowers the risk. Mice fed ethanol long term show no pancreas damage unless adaptive/protective responses mediating proteostasis are disrupted. Pancreatic acini synthesize digestive enzymes (largely serine hydrolases) in the endoplasmic reticulum (ER), where perturbations (eg, alcohol consumption) activate adaptive unfolded protein responses orchestrated by spliced X-box binding protein 1 (XBP1). Here, we examined ethanol-induced early structural changes in pancreatic ER proteins.MethodsWild-type and Xbp1+/- mice were fed control and ethanol diets, then tissues were homogenized and fractionated. ER proteins were labeled with a cysteine-reactive probe, isotope-coded affinity tag to obtain a novel pancreatic redox ER proteome. Specific labeling of active serine hydrolases in ER with fluorophosphonate desthiobiotin also was characterized proteomically. Protein structural perturbation by redox changes was evaluated further in molecular dynamic simulations.ResultsEthanol feeding and Xbp1 genetic inhibition altered ER redox balance and destabilized key proteins. Proteomic data and molecular dynamic simulations of Carboxyl ester lipase (Cel), a unique serine hydrolase active within ER, showed an uncoupled disulfide bond involving Cel Cys266, Cel dimerization, ER retention, and complex formation in ethanol-fed, XBP1-deficient mice.ConclusionsResults documented in ethanol-fed mice lacking sufficient spliced XBP1 illustrate consequences of ER stress extended by preventing unfolded protein response from fully restoring pancreatic acinar cell proteostasis during ethanol-induced redox challenge. In this model, orderly protein folding and transport to the secretory pathway were disrupted, and abundant molecules including Cel with perturbed structures were retained in ER, promoting ER stress-related pancreas pathology
Internalization of the Active Subunit of the Aggregatibacter Actinomycetemcomitans Cytolethal Distending Toxin Is Dependent upon Cellugyrin (Synaptogyrin 2), a Host Cell Non-Neuronal Paralog of the Synaptic Vesicle Protein, Synaptogyrin 1
The Aggregatibacter actinomycetemcomitans cytolethal distending toxin (Cdt) is a heterotrimeric AB2 toxin capable of inducing lymphocytes, and other cell types, to undergo cell cycle arrest and apoptosis. Exposure to Cdt results in binding to the cell surface followed by internalization and translocation of the active subunit, CdtB, to intracellular compartments. These events are dependent upon toxin binding to cholesterol in the context of lipid rich membrane microdomains often referred to as lipid rafts. We now demonstrate that, in addition to binding to the plasma membrane of lymphocytes, another early and critical event initiated by Cdt is the translocation of the host cell protein, cellugyrin (synaptogyrin-2) to the same cholesterol-rich microdomains. Furthermore, we demonstrate that cellugyrin is an intracellular binding partner for CdtB as demonstrated by immunoprecipitation. Using CRISPR/cas9 gene editing we established a Jurkat cell line deficient in cellugyrin expression (JurkatCg−); these cells were capable of binding Cdt, but unable to internalize CdtB. Furthermore, JurkatCg− cells were not susceptible to Cdt-induced toxicity; these cells failed to exhibit blockade of the PI-3K signaling pathway, cell cycle arrest or cell death. We propose that cellugyrin plays a critical role in the internalization and translocation of CdtB to critical intracellular target sites. These studies provide critical new insight into the mechanism by which Cdt, and in particular, CdtB is able to induce toxicity
Unconventional transformation of spin Dirac phase across a topological quantum phase transition
The topology of a topological material can be encoded in its surface states.
These surface states can only be removed by a bulk topological quantum phase
transition into a trivial phase. Here we use photoemission spectroscopy to
image the formation of protected surface states in a topological insulator as
we chemically tune the system through a topological transition. Surprisingly,
we discover an exotic spin-momentum locked, gapped surface state in the trivial
phase that shares many important properties with the actual topological surface
state in anticipation of the change of topology. Using a spin-resolved
measurement, we show that apart from a surface band-gap these states develop
spin textures similar to the topological surface states well-before the
transition. Our results offer a general paradigm for understanding how surface
states in topological phases arise and are suggestive for future realizing Weyl
arcs, condensed matter supersymmetry and other fascinating phenomena in the
vicinity of topological quantum criticality.Comment: 20 pages, 5 Figures, Related papers at
http://physics.princeton.edu/zahidhasangroup/index.html, Accepted for
publication in Nature Commun.(2015
A new form of (unexpected) Dirac fermions in the strongly-correlated cerium monopnictides
Discovering Dirac fermions with novel properties has become an important
front in condensed matter and materials sciences. Here, we report the
observation of unusual Dirac fermion states in a strongly-correlated electron
setting, which are uniquely distinct from those of graphene and conventional
topological insulators. In strongly-correlated cerium monopnictides, we find
two sets of highly anisotropic Dirac fermions that interpenetrate each other
with negligible hybridization, and show a peculiar four-fold degeneracy where
their Dirac nodes overlap. Despite the lack of protection by crystalline or
time-reversal symmetries, this four-fold degeneracy is robust across magnetic
phase transitions. Comparison of these experimental findings with our
theoretical calculations suggests that the observed surface Dirac fermions
arise from bulk band inversions at an odd number of high-symmetry points, which
is analogous to the band topology which describes a
-topological phase. Our findings open up an unprecedented and
long-sought-for platform for exploring novel Dirac fermion physics in a
strongly-correlated semimetal
An Analysis of the Environments of FU Orionis Objects with Herschel
We present Herschel-HIFI, SPIRE, and PACS 50-670 {\mu}m imaging and
spectroscopy of six FU Orionis-type objects and candidates (FU Orionis, V1735
Cyg, V1515 Cyg, V1057 Cyg, V1331 Cyg, and HBC 722), ranging in outburst date
from 1936-2010, from the "FOOSH" (FU Orionis Objects Surveyed with Herschel)
program, as well as ancillary results from Spitzer-IRS and the Caltech
Submillimeter Observatory. In their system properties (Lbol, Tbol, line
emission), we find that FUors are in a variety of evolutionary states.
Additionally, some FUors have features of both Class I and II sources: warm
continuum consistent with Class II sources, but rotational line emission
typical of Class I, far higher than Class II sources of similar
mass/luminosity. Combining several classification techniques, we find an
evolutionary sequence consistent with previous mid-IR indicators. We detect [O
I] in every source at luminosities consistent with Class 0/I protostars, much
greater than in Class II disks. We detect transitions of 13CO (J_up of 5 to 8)
around two sources (V1735 Cyg and HBC 722) but attribute them to nearby
protostars. Of the remaining sources, three (FU Ori, V1515 Cyg, and V1331 Cyg)
exhibit only low-lying CO, but one (V1057 Cyg) shows CO up to J = 23 - 22 and
evidence for H2O and OH emission, at strengths typical of protostars rather
than T Tauri stars. Rotational temperatures for "cool" CO components range from
20-81 K, for ~ 10^50 total CO molecules. We detect [C I] and [N II] primarily
as diffuse emission.Comment: 31 pages, 15 figures; accepted to Ap
Super-oblique corrections and non-decoupling of supersymmetry breaking
If supersymmetric partners of the known particles have masses at the
multi-TeV scale, they will not be directly discovered at planned future
colliders and decouple from most observables. However, such superpartners also
induce non-decoupling effects that break the supersymmetric equivalence of
gauge boson couplings and gaugino couplings through supersymmetric
analogues of the oblique corrections. Working within well-motivated theoretical
frameworks, we find that multi-TeV scale supersymmetric particles produce
deviations at the 1-10% level in the ratios . Such effects allow one
to bound the scale of kinematically inaccessible superpartners through
precision measurements of processes involving the accessible superparticles.
Alternatively, if all superpartners are found, significant deviations imply the
existence of highly split exotic supermultiplets.Comment: 18 pages, REVTeX, no figur
High resolution Compton scattering as a Probe of the Fermi surface in the Iron-based superconductor
We have carried out first principles all-electron calculations of the
(001)-projected 2D electron momentum density and the directional Compton
profiles along the [100], [001] and [110] directions in the Fe-based
superconductor LaOFeAs within the framework of the local density approximation.
We identify Fermi surface features in the 2D electron momentum density and the
directional Compton profiles, and discuss issues related to the observation of
these features via Compton scattering experiments.Comment: 4 pages, 3 figure
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