486 research outputs found
Ionic high-pressure form of elemental boron
Boron is an element of fascinating chemical complexity. Controversies have
shrouded this element since its discovery was announced in 1808: the new
'element' turned out to be a compound containing less than 60-70 percent of
boron, and it was not until 1909 that 99-percent pure boron was obtained. And
although we now know of at least 16 polymorphs, the stable phase of boron is
not yet experimentally established even at ambient conditions. Boron's
complexities arise from frustration: situated between metals and insulators in
the periodic table, boron has only three valence electrons, which would favour
metallicity, but they are sufficiently localized that insulating states emerge.
However, this subtle balance between metallic and insulating states is easily
shifted by pressure, temperature and impurities. Here we report the results of
high-pressure experiments and ab initio evolutionary crystal structure
predictions that explore the structural stability of boron under pressure and,
strikingly, reveal a partially ionic high-pressure boron phase. This new phase
is stable between 19 and 89 GPa, can be quenched to ambient conditions, and has
a hitherto unknown structure (space group Pnnm, 28 atoms in the unit cell)
consisting of icosahedral B12 clusters and B2 pairs in a NaCl-type arrangement.
We find that the ionicity of the phase affects its electronic bandgap, infrared
adsorption and dielectric constants, and that it arises from the different
electronic properties of the B2 pairs and B12 clusters and the resultant charge
transfer between them.Comment: Published in Nature 453, 863-867 (2009
Gravity waves and the LHC: Towards high-scale inflation with low-energy SUSY
It has been argued that rather generic features of string-inspired
inflationary theories with low-energy supersymmetry (SUSY) make it difficult to
achieve inflation with a Hubble scale H > m_{3/2}, where m_{3/2} is the
gravitino mass in the SUSY-breaking vacuum state. We present a class of
string-inspired supergravity realizations of chaotic inflation where a simple,
dynamical mechanism yields hierarchically small scales of post-inflationary
supersymmetry breaking. Within these toy models we can easily achieve small
ratios between m_{3/2} and the Hubble scale of inflation. This is possible
because the expectation value of the superpotential relaxes from large to
small values during the course of inflation. However, our toy models do not
provide a reasonable fit to cosmological data if one sets the SUSY-breaking
scale to m_{3/2} < TeV. Our work is a small step towards relieving the apparent
tension between high-scale inflation and low-scale supersymmetry breaking in
string compactifications.Comment: 21+1 pages, 5 figures, LaTeX, v2: added references, v3: very minor
changes, version to appear in JHE
Matrix theory origins of non-geometric fluxes
We explore the origins of non-geometric fluxes within the context of M theory
described as a matrix model. Building upon compactifications of Matrix theory
on non-commutative tori and twisted tori, we formulate the conditions which
describe compactifications with non-geometric fluxes. These turn out to be
related to certain deformations of tori with non-commutative and
non-associative structures on their phase space. Quantization of flux appears
as a natural consequence of the framework and leads to the resolution of
non-associativity at the level of the unitary operators. The quantum-mechanical
nature of the model bestows an important role on the phase space. In
particular, the geometric and non-geometric fluxes exchange their properties
when going from position space to momentum space thus providing a duality among
the two. Moreover, the operations which connect solutions with different fluxes
are described and their relation to T-duality is discussed. Finally, we provide
some insights on the effective gauge theories obtained from these matrix
compactifications.Comment: 1+31 pages, reference list update
Is Our Universe Natural?
It goes without saying that we are stuck with the universe we have.
Nevertheless, we would like to go beyond simply describing our observed
universe, and try to understand why it is that way rather than some other way.
Physicists and cosmologists have been exploring increasingly ambitious ideas
that attempt to explain why certain features of our universe aren't as
surprising as they might first appear.Comment: Invited review for Nature, 11 page
Conformal Quivers and Melting Molecules
Quiver quantum mechanics describes the low energy dynamics of a system of
wrapped D-branes. It captures several aspects of single and multicentered BPS
black hole geometries in four-dimensional supergravity such
as the presence of bound states and an exponential growth of microstates. The
Coulomb branch of an Abelian three node quiver is obtained by integrating out
the massive strings connecting the D-particles. It allows for a scaling regime
corresponding to a deep AdS throat on the gravity side. In this scaling
regime, the Coulomb branch is shown to be an invariant
multi-particle superconformal quantum mechanics. Finally, we integrate out the
strings at finite temperature---rather than in their ground state---and show
how the Coulomb branch `melts' into the Higgs branch at high enough
temperatures. For scaling solutions the melting occurs for arbitrarily small
temperatures, whereas bound states can be metastable and thus long lived.
Throughout the paper, we discuss how far the analogy between the quiver model
and the gravity picture, particularly within the AdS throat, can be taken.Comment: 49 pages, 16 figure
PET probe-guided surgery: applications and clinical protocol
This is an Open Access article distributed under the terms of the Creative Commons Attribution Licens
Some geometric invariants from resolutions of Hilbert modules
The model theory of Sz.-Nagy and Foias for contractions was reformulated in [15, chapter 3]. The existence of a unique minimal unitary dilation amounts to the exis-tence of a Silov resolution for contractive Hilbert modulesM over the disc algebra A(D) along with the fact that any two minimal Silov resolutions are isomorphic
A direct physical interaction between Nanog and Sox2 regulates embryonic stem cell self-renewal
Embryonic stem (ES) cell self-renewal efficiency is determined by the Nanog protein level. However, the protein partners of Nanog that function to direct self-renewal are unclear. Here, we identify a Nanog interactome of over 130 proteins including transcription factors, chromatin modifying complexes, phosphorylation and ubiquitination enzymes, basal transcriptional machinery members, and RNA processing factors. Sox2 was identified as a robust interacting partner of Nanog. The purified Nanog–Sox2 complex identified a DNA recognition sequence present in multiple overlapping Nanog/Sox2 ChIP-Seq data sets. The Nanog tryptophan repeat region is necessary and sufficient for interaction with Sox2, with tryptophan residues required. In Sox2, tyrosine to alanine mutations within a triple-repeat motif (S X T/S Y) abrogates the Nanog–Sox2 interaction, alters expression of genes associated with the Nanog-Sox2 cognate sequence, and reduces the ability of Sox2 to rescue ES cell differentiation induced by endogenous Sox2 deletion. Substitution of the tyrosines with phenylalanine rescues both the Sox2–Nanog interaction and efficient self-renewal. These results suggest that aromatic stacking of Nanog tryptophans and Sox2 tyrosines mediates an interaction central to ES cell self-renewal
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