2,368 research outputs found
Melting behavior of large disordered sodium clusters
The melting-like transition in disordered sodium clusters Na_N, with N=92 and
142 is studied by using a first-principles constant-energy molecular dynamics
simulation method. Na_142, whose atoms are distributed in two (surface and
inner) main shells with different radial distances to the center of mass of the
cluster, melts in two steps: the first one, at approx. 130 K, is characterized
by a high intrashell mobility of the atoms, and the second, homogeneous
melting, at approx. 270 K, involves diffusive motion of all the atoms across
the whole cluster volume (both intrashell and intershell displacements are
allowed). On the contrary, the melting of Na_92 proceeds gradually over a very
wide temperature interval, without any abrupt step visible in the thermal or
structural melting indicators. The occurrence of well defined steps in the
melting transition is then shown to be related to the existence of a
distribution of the atoms in shells. Thereby we propose a necessary condition
for a cluster to be considered rigorously amorphouslike (totally disordered),
namely that there are no space regions of the cluster where the local value of
the atomic density is considerably reduced. Na_92 is the only cluster from the
two considered that verifies this condition, so its thermal behavior can be
considered as representative of that expected for amorphous clusters. Na_142,
on the other hand, has a discernible atomic shell structure and should be
considered instead as just partially disordered. The thermal behavior of these
two clusters is also compared to that of icosahedral (totally ordered) sodium
clusters of the same sizes.Comment: LaTeX file. 7 pages with 12 picture
Ultra High Resolution Transmission Electron Microscopy of Matrix Mineral Grains in CM Chondrites: Preaccretionary or Parent Body Aqueous Processing?
CM chondrites are highly hydrated
meteorites associated with a parent asteroid that has
experienced significant aqueous processing. The meteoritic
evidence indicates that these non-differentiated
asteroids are formed by fine-grained minerals embedded
in a nanometric matrix that preserves chemical
clues of the forming environment. So far there are two
hypothesis to explain the presence of hydrated minerals
in the content of CM chondrites: one is based on textural
features in chondrule-rim boundaries [1-3], and
the other ‘preaccretionary’ hypothesis proposes the
incorporation of hydrated phases from the protoplanetary
disk [4-6]. The highly porous structure of these
chondrites is inherited from the diverse materials present
in the protoplanetary disk environment. These
bodies were presumably formed by low relative velocity
encounters that led to the accretion of silicate-rich
chondrules, refractory Ca- and Al-rich inclusions
(CAIs), metal grains, and the fine-grained materials
forming the matrix. Owing to the presence of significant
terrestrial water in meteorite finds [7], here we
have focused on two CM chondrite falls with minimal
terrestrial processing: Murchison and Cold Bokkeveld.
Anhydrous carbonaceous chondrite matrices are usually
represented by highly chemically unequilibrated
samples that contain distinguishable stellar grains.
Other chondrites have experienced hydration and
chemical homogeneization that reveal parent body
processes. We have studied CM chondrites because
these meteorites have experienced variable hydration
levels [8-10]. It is important to study the textural effects
of aqueous alteration in the main minerals to
decipher which steps and environments promote bulk
chemistry changes, and create the distinctive alteration
products. It is thought that aqueous alteration has particularly
played a key role in modifying primordial
bulk chemistry, and homogenizing the isotopic content
of fine-grained matrix materials [7, 11, 12]. Fortunately,
the mineralogy produced by parent-body and terrestrial
aqueous alteration processes is distinctive [5, 11]
A new mechanism of mass protection for fermions
We present a way of protecting a Dirac fermion interacting with a scalar
(Higgs) field from getting a mass from the vacuum. It is obtained through an
implementation of translational symmetry when the theory is formulated with a
momentum cutoff, which forbids the usual Yukawa term. We consider that this
mechanism can help to understand the smallness of neutrino masses without a
tuning of the Yukawa coupling. The prohibition of the Yukawa term for the
neutrino forbids at the same time a gauge coupling between the right-handed
electron and neutrino. We prove that this mechanism can be implemented on the
lattice.Comment: LATTICE99(Higgs,Yukawa,SUSY), 3 page
Ancient Martian Floods in a Plausible Variable Climatic Environment as Revealed from the Sequential Growth of Allan Hills 84001 Carbonate Globules
No abstract available
Almost strictly sign regular rectangular matrices
Almost strictly sign regular matrices are sign regular matrices with a special zero pattern and whose nontrivial minors are nonzero. In this paper we provide several properties of almost strictly sign regular rectangular matrices of maximal rank and analyze their QR factorization
Comparing pivoting strategies for almost strictly sign regular matrices
In this paper some properties of two-determinant pivoting for Neville elimination are presented. In particular, we consider a zero-increasing property and we show an optimal normwise growth factor. Comparisons with other pivoting strategies for Neville elimination and with Gaussian elimination with partial pivoting of almost strictly sign regular matrices are performed. Numerical examples are included
Almost strictly sign regular matrices and Neville elimination with two-determinant pivoting
In 2007 Cortés and Peña introduced a pivoting strategy for the Neville elimination of nonsingular sign regular matrices and called it two-determinant pivoting. Neville elimination has been very useful for obtaining theoretical and practical properties for totally positive (negative) matrices and other related types of matrices. A real matrix is said to be almost strictly sign regular if all its nontrivial minors of the same order have the same strict sign. In this paper, some nice properties related with the application of Neville elimination with two-determinant pivoting strategy to almost strictly sign regular matrices are presented
Wrapped M2/M5 Duality
A microscopic accounting of the entropy of a generic 5D supersymmetric
rotating black hole, arising from wrapped M2-branes in Calabi-Yau compactified
M-theory, is an outstanding unsolved problem. In this paper we consider an
expansion around the zero-entropy, zero-temperature, maximally rotating ground
state for which the angular momentum J_L and graviphoton charge Q are related
by J_L^2=Q^3. At J_L=0 the near horizon geometry is AdS_2 x S^3. As J_L^2 goes
to Q^3 it becomes a singular quotient of AdS_3 x S^2: more precisely, a
quotient of the near horizon geometry of an M5 wrapped on a 4-cycle whose
self-intersection is the 2-cycle associated to the wrapped-M2 black hole. The
singularity of the AdS_3 quotient is identified as the usual one associated to
the zero-temperature limit, suggesting that the (0,4) wrapped-M5 CFT is dual
near maximality to the wrapped-M2 black hole. As evidence for this, the
microscopic (0,4) CFT entropy and the macroscopic rotating black hole entropy
are found to agree to leading order away from maximality.Comment: 10 pages, no figure
Antiferromagnetism in four dimensions: search for non-triviality
We present antiferromagnetism as a mechanism capable of modifying
substantially the phase diagram and the critical behaviour of statistical
mechanical models. This is particularly relevant in four dimensions, due to the
connection between second order transition points and the continuum limit as a
quantum field theory. We study three models with an antiferromagnetic
interaction: the Ising and the O(4) Models with a second neighbour negative
coupling, and the \RP{2} Model. Different conclusions are obtained depending
on the model.Comment: 4 pages LateX. Contribution to Lat9
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