12,971 research outputs found
Superconductors are topologically ordered
We revisit a venerable question: what is the nature of the ordering in a
superconductor? We find that the answer is properly that the superconducting
state exhibits topological order in the sense of Wen, i.e. that while it lacks
a local order parameter, it is sensitive to the global topology of the
underlying manifold and exhibits an associated fractionalization of quantum
numbers. We show that this perspective unifies a number of previous
observations on superconductors and their low lying excitations and that this
complex can be elegantly summarized in a purely topological action of the
``'' type and its elementary quantization. On manifolds with boundaries,
the action correctly predicts non-chiral edge states, gapped in general,
but crucial for fractionalization and establishing the ground state degeneracy.
In all of this the role of the physical electromagnetic fields is central. We
also observe that the action describes the topological order in several
other physically distinct systems thus providing an example of topological
universality
Deficiency of RgpG causes major defects in cell division and biofilm formation, and deficiency of LytR-CpsAPsr family proteins leads to accumulation of cell wall antigens in culture medium by Streptococcus mutans
ABSTRACT
Streptococcus mutans
is known to possess rhamnose-glucose polysaccharide (RGP), a major cell wall antigen.
S. mutans
strains deficient in
rgpG
, encoding the first enzyme of the RGP biosynthesis pathway, were constructed by allelic exchange. The
rgpG
deficiency had no effect on growth rate but caused major defects in cell division and altered cell morphology. Unlike the coccoid wild type, the
rgpG
mutant existed primarily in chains of swollen, “squarish” dividing cells. Deficiency of
rgpG
also causes significant reduction in biofilm formation (
P
< 0.01). Double and triple mutants with deficiency in
brpA
and/or
psr
, genes coding for the LytR-CpsA-Psr family proteins BrpA and Psr, which were previously shown to play important roles in cell envelope biogenesis, were constructed using the
rgpG
mutant. There were no major differences in growth rates between the wild-type strain and the
rgpG brpA
and
rgpG psr
double mutants, but the growth rate of the
rgpG brpA psr
triple mutant was reduced drastically (
P
< 0.001). Under transmission electron microscopy, both double mutants resembled the
rgpG
mutant, while the triple mutant existed as giant cells with multiple asymmetric septa. When analyzed by immunoblotting, the
rgpG
mutant displayed major reductions in cell wall antigens compared to the wild type, while little or no signal was detected with the double and triple mutants and the
brpA
and
psr
single mutants. These results suggest that RgpG in
S. mutans
plays a critical role in cell division and biofilm formation and that BrpA and Psr may be responsible for attachment of cell wall antigens to the cell envelope.
IMPORTANCE
Streptococcus mutans
, a major etiological agent of human dental caries, produces rhamnose-glucose polysaccharide (RGP) as the major cell wall antigen. This study provides direct evidence that deficiency of RgpG, the first enzyme of the RGP biosynthesis pathway, caused major defects in cell division and morphology and reduced biofilm formation by
S. mutans
, indicative of a significant role of RGP in cell division and biofilm formation in
S. mutans
. These results are novel not only in
S. mutans
, but also other streptococci that produce RGP. This study also shows that the LytR-CpsA-Psr family proteins BrpA and Psr in
S. mutans
are involved in attachment of RGP and probably other cell wall glycopolymers to the peptidoglycan. In addition, the results also suggest that BrpA and Psr may play a direct role in cell division and biofilm formation in
S. mutans
. This study reveals new potential targets to develop anticaries therapeutics.
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Instant WS2 platelets reorientation of self-adaptive WS2/a-C tribocoating
WS2/a-C nanocomposite coatings were deposited by magnetron co-sputtering using WS2 and graphite targets. The microstructure and triboperformance of the coatings were scrutinized via microscopy (AFM, SEM, FIB, HRTEM), spectroscopy (XRD, XPS) and tribometry. Atomic WS2 platelets are randomly embedded in an amorphous carbon matrix of the as-deposited nanocomposite coating. HRTEM observations of tribofilm/transfer layer reveal that the sliding contact immediately reorients WS2 platelets parallel to the sliding interface and thereby leads to self-adaptive “frictionless” response. The coefficient of friction falls to 0.02 in dry air and reaches 0.10 in humid air, and is reversible as testing atmosphere cycles between dry air and humid air
Sharp and Smooth Boundaries of Quantum Hall Liquids
We study the transition between sharp and smooth density distributions at the
edges of Quantum Hall Liquids in the presence of interactions. We find that,
for strong confining potentials, the edge of a liquid is described by
the Fermi Liquid theory, even in the presence of interactions, a
consequence of the chiral nature of the system. When the edge confining
potential is decreased beyond a point, the edge undergoes a reconstruction and
electrons start to deposit a distance magnetic lengths away from the
initial QH Liquid. Within the Hartree-Fock approximation, a new pair of
branches of gapless edge excitations is generated after the transition. We show
that the transition is controlled by the balance between a long-ranged
repulsive Hartree term and a short-ranged attractive exchange term. Such
transition also occurs for Quantum Dots in the Quantum Hall Regime, and should
be observable in resonant tunneling experiments. Electron tunneling into the
reconstructed edge is also discussed.Comment: 28 pages, REVTeX 3.0, 18 figures available upon request,
cond-mat/yymmnn
A non-abelian spin-liquid in a spin-1 quantum magnet
We study a time-reversal invariant non-abelian spin-liquid state in an symmetric spin quantum magnet on a triangular lattice. The
spin-liquid is obtained by quantum disordering a non-collinear nematic state.
We show that such a spin-liquid cannot be obtained by the standard projective
construction for spin-liquids. We also study phase transition between the
spin-liquid and the non-collinear nematic state and show that it cannot be
described within Landau-Ginzburg- Wilson paradigm.Comment: 4.25 pages, 1 figur
Exact Critical Exponents for Pseudo-Particles in the Kondo Problem
Exact critical exponents of the Green functions for pseudo-fermions and slave
bosons in the SU() Anderson model with are obtained by
using the Bethe ansatz solution and boundary conformal field theory. They are
evaluated exactly for mixed valence systems and Kondo systems with crystalline
fields. The results agree with the prediction of Menge and M\"uller-Hartmann,
which coincide with those of the X-ray problem. Some implication of our results
in one-dimensional chiral systems is also discussed.Comment: 9 pages, no figure
Partition noise and statistics in the fractional quantum Hall effect
A microscopic theory of current partition in fractional quantum Hall liquids,
described by chiral Luttinger liquids, is developed to compute the noise
correlations, using the Keldysh technique. In this Hanbury-Brown and Twiss
geometry, at Laughlin filling factor \nu=1/3, the real time noise correlator
exhibits oscillations which persist over larger time scales than that of an
uncorrelated Hall fluid. The zero frequency noise correlations are negative at
filling factor 1/3 as for bare electrons (anti-bunching), but are strongly
reduced in amplitude. These correlations become positive (bunching) for \nu\leq
1/5, suggesting a tendency towards bosonic behavior.Comment: revised version, curve for time correlations at nu=1/3 adde
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Vitamin A Metabolism by Dendritic Cells Triggers an Antimicrobial Response against Mycobacterium tuberculosis.
Epidemiological evidence correlates low serum vitamin A (retinol) levels with increased susceptibility to active tuberculosis (TB); however, retinol is biologically inactive and must be converted into its bioactive form, all-trans retinoic acid (ATRA). Given that ATRA triggers a Niemann-Pick type C2 (NPC2)-dependent antimicrobial response against Mycobacterium tuberculosis, we investigated the mechanism by which the immune system converts retinol into ATRA at the site of infection. We demonstrate that granulocyte-macrophage colony-stimulating factor (GM-CSF)-derived dendritic cells (DCs), but not macrophages, express enzymes in the vitamin A metabolic pathway, including aldehyde dehydrogenase 1 family, member a2 (ALDH1A2) and short-chain dehydrogenase/reductase family, member 9 (DHRS9), enzymes capable of the two-step conversion of retinol into ATRA, which is subsequently released from the cell. Additionally, mRNA and protein expression levels of ALDH1A2 and DC marker CD1B were lower in tuberculosis lung tissues than in normal lung. The conditioned medium from DCs cultured with retinol stimulated antimicrobial activity from M. tuberculosis-infected macrophages, as well as the expression of NPC2 in monocytes, which was blocked by specific inhibitors, including retinoic acid receptor inhibitor (RARi) or N,N-diethylaminobenzaldehyde (DEAB), an ALDH1A2 inhibitor. These results indicate that metabolism of vitamin A by DCs transactivates macrophage antimicrobial responses.IMPORTANCE Tuberculosis (TB) is the leading cause of death by a single infectious agent worldwide. One factor that contributes to the success of the microbe is the deficiency in immunomodulatory nutrients, such as vitamin A (retinol), which are prevalent in areas where TB is endemic. Clinical trials show that restoration of systemic retinol levels in active TB patients is ineffective in mitigating the disease; however, laboratory studies demonstrate that activation of the vitamin A pathway in Mycobacterium tuberculosis-infected macrophages triggers an antimicrobial response. Therefore, the goal of this study was to determine the link between host retinol levels and retinoic acid-mediated antimicrobial responses against M. tuberculosis By combining established in vitro models with in situ studies of lung tissue from TB patients, this study demonstrates that the innate immune system utilizes transcellular metabolism leading to activation between dendritic cells and macrophages as a means to combat the pathogen
Topological Order and Conformal Quantum Critical Points
We discuss a certain class of two-dimensional quantum systems which exhibit
conventional order and topological order, as well as two-dimensional quantum
critical points separating these phases. All of the ground-state equal-time
correlators of these theories are equal to correlation functions of a local
two-dimensional classical model. The critical points therefore exhibit a
time-independent form of conformal invariance. These theories characterize the
universality classes of two-dimensional quantum dimer models and of quantum
generalizations of the eight-vertex model, as well as Z_2 and non-abelian gauge
theories. The conformal quantum critical points are relatives of the Lifshitz
points of three-dimensional anisotropic classical systems such as smectic
liquid crystals. In particular, the ground-state wave functional of these
quantum Lifshitz points is just the statistical (Gibbs) weight of the ordinary
2D free boson, the 2D Gaussian model. The full phase diagram for the quantum
eight-vertex model exhibits quantum critical lines with continuously-varying
critical exponents separating phases with long-range order from a Z_2
deconfined topologically-ordered liquid phase. We show how similar ideas also
apply to a well-known field theory with non-abelian symmetry, the
strong-coupling limit of 2+1-dimensional Yang-Mills gauge theory with a
Chern-Simons term. The ground state of this theory is relevant for recent
theories of topological quantum computation.Comment: 48 pages. v2: fixed typos, added reference
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