1,983 research outputs found
Frustration of freezing in a two dimensional hard-core fluid due to particle shape anisotropy
The freezing mechanism suggested for a fluid composed of hard disks [Huerta
et al., Phys. Rev. E, 2006, 74, 061106] is used here to probe the
fluid-to-solid transition in a hard-dumbbell fluid composed of overlapping hard
disks with a variable length between disk centers. Analyzing the trends in the
shape of second maximum of the radial distribution function of the planar
hard-dumbbell fluid it has been found that the type of transition could be
sensitive to the length of hard-dumbbell molecules. From the Monte
Carlo simulations data we show that if a hard-dumbbell length does not exceed
15% of the disk diameter, the fluid-to-solid transition scenario follows the
case of a hard-disk fluid, i.e., the isotropic hard-dumbbell fluid experiences
freezing. However, for a hard-dumbbell length larger than 15% of disk diameter,
there is evidence that fluid-to-solid transition may change to continuous
transition, i.e., such an isotropic hard-dumbbell fluid will avoid freezing.Comment: 9 pages, 7 figure
How different Fermi surface maps emerge in photoemission from Bi2212
We report angle-resolved photoemission spectra (ARPES) from the Fermi energy
() over a large area of the () plane using 21.2 eV and 32 eV
photons in two distinct polarizations from an optimally doped single crystal of
BiSrCaCuO (Bi2212), together with extensive
first-principles simulations of the ARPES intensities. The results display a
wide-ranging level of accord between theory and experiment and clarify how
myriad Fermi surface (FS) maps emerge in ARPES under various experimental
conditions. The energy and polarization dependences of the ARPES matrix element
help disentangle primary contributions to the spectrum due to the pristine
lattice from those arising from modulations of the underlying tetragonal
symmetry and provide a route for separating closely placed FS sheets in low
dimensional materials.Comment: submitted to PR
PRÁCTICAS DE CONSERVACIÓN Y RESTAURACIÓN DE SUELOS EN CLIMAS HÚMEDOS
The National Forest Commission (Comisión Nacional Forestal, CONAFOR) in México provides backing for conservation and restoration in forest matters, participating in the formulation of plans and programs, applying the sustainable forest development policy. The conservation and restoration practices of soils are actions that contribute to decreasing their degradation, primarily erosion, and to increasing water capture, and promote, in addition, payment of workdays in the communities where the studies are performed. The mountain mesophyll forest (MMF) is an ecosystem that presents 19.2 % of degradation (with at least some type of degradation). The CONAFOR also supports carrying out practices torecover the tree cover. Thus, during the period of 2013 to 2015, a total of 417 projects were performed on a surface of 5,746 hectares in México.La Comisión Nacional Forestal (CONAFOR) en México, proporciona apoyos para la conservación y restauración en materia forestal, participando en la formulación de planes y programas, aplicando la política de desarrollo forestal sustentable. Las prácticas de conservación y restauración de suelos son acciones que contribuyen a disminuir su degradación, principalmente la erosión, e incrementar la captación de agua, y promueven, además, el pago de jornales en las comunidades donde se realizan los trabajos. El bosque mesófilo de montaña (BMM), es un ecosistema que presenta 19.2% de degradación (con al menos algún tipo de degradación). La CONAFOR también apoya a la realización de prácticas para recuperar la cobertura arbórea. Así, durante el periodo 2013 a 2015, se ejecutaron un total de 417 proyectos en una superficie de 5,746 hectáreas en BMM de Méxic
Mixing Effects in the Crystallization of Supercooled Quantum Binary Liquids
By means of Raman spectroscopy of liquid microjets we have investigated the
crystallization process of supercooled quantum liquid mixtures composed of
parahydrogen (pH) diluted with small amounts of up to 5\% of either neon or
orthodeuterium (oD), and of oD diluted with either Ne or pH. We
show that the introduction of Ne impurities affects the crystallization
kinetics in both the pH-Ne and oD-Ne mixtures in terms of a significant
reduction of the crystal growth rate, similarly to what found in our previous
work on supercooled pH-oD liquid mixtures [M. K\"uhnel et {\it al.},
Phys. Rev. B \textbf{89}, 180506(R) (2014)]. Our experimental results, in
combination with path-integral simulations of the supercooled liquid mixtures,
suggest in particular a correlation between the measured growth rates and the
ratio of the effective particle sizes originating from quantum delocalization
effects. We further show that the crystalline structure of the mixture is also
affected to a large extent by the presence of the Ne impurities, which likely
initiate the freezing process through the formation of Ne crystallites.Comment: 19 pages, 7 figures, submitted to J. Chem. Phy
Vacuum Polarization and Dynamical Chiral Symmetry Breaking: Phase Diagram of QED with Four-Fermion Contact Interaction
We study chiral symmetry breaking for fundamental charged fermions coupled
electromagnetically to photons with the inclusion of four-fermion contact
self-interaction term. We employ multiplicatively renormalizable models for the
photon dressing function and the electron-photon vertex which minimally ensures
mass anomalous dimension = 1. Vacuum polarization screens the interaction
strength. Consequently, the pattern of dynamical mass generation for fermions
is characterized by a critical number of massless fermion flavors above which
chiral symmetry is restored. This effect is in diametrical opposition to the
existence of criticality for the minimum interaction strength necessary to
break chiral symmetry dynamically. The presence of virtual fermions dictates
the nature of phase transition. Miransky scaling laws for the electromagnetic
interaction strength and the four-fermion coupling, observed for quenched QED,
are replaced by a mean-field power law behavior corresponding to a second order
phase transition. These results are derived analytically by employing the
bifurcation analysis, and are later confirmed numerically by solving the
original non-linearized gap equation. A three dimensional critical surface is
drawn to clearly depict the interplay of the relative strengths of interactions
and number of flavors to separate the two phases. We also compute the
beta-function and observe that it has ultraviolet fixed point. The power law
part of the momentum dependence, describing the mass function, reproduces the
quenched limit trivially. We also comment on the continuum limit and the
triviality of QED.Comment: 9 pages, 10 figure
Giant alkali-metal-induced lattice relaxation as the driving force of the insulating phase of alkali-metal/Si(111):B
Ab initio density-functional theory calculations, photoemission spectroscopy (PES), scanning tunneling microscopy, and spectroscopy (STM, STS) have been used to solve the 2√3 x 2√3R30 surface reconstruction observed previously by LEED on 0.5 ML K/Si:B. A large K-induced vertical lattice relaxation occurring only for 3/4 of Si adatoms is shown to quantitatively explain both the chemical shift of 1.14 eV and the ratio 1/3 measured on the two distinct B 1s core levels. A gap is observed between valence and conduction surface bands by ARPES and STS which is shown to have mainly a Si-B character. Finally, the calculated STM images agree with our experimental results. This work solves the controversy about the origin of the insulating ground state of alkali-metal/Si(111):B semiconducting interfaces which were believed previously to be related to many-body effectsThis work has received the financial support of the French ANR SURMOTT program (ANR-09-BLAN- 0210-01) and the Spanish MICIIN under Project No. FIS2010-1604
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