2,110 research outputs found
Emergence of Bulk CsCl Structure in (CsCl)nCs+ Cluster Ions
The emergence of CsCl bulk structure in (CsCl)nCs+ cluster ions is
investigated using a mixed quantum-mechanical/semiempirical theoretical
approach. We find that rhombic dodecahedral fragments (with bulk CsCl symmetry)
are more stable than rock-salt fragments after the completion of the fifth
rhombic dodecahedral atomic shell. From this size (n=184) on, a new set of
magic numbers should appear in the experimental mass spectra. We also propose
another experimental test for this transition, which explicitely involves the
electronic structure of the cluster. Finally, we perform more detailed
calculations in the size range n=31--33, where recent experimental
investigations have found indications of the presence of rhombic dodecahedral
(CsCl)32Cs+ isomers in the cluster beams.Comment: LaTeX file. 6 pages and 4 pictures. Accepted for publication in Phys.
Rev.
Structures and Stabilities of Doubly-charged (MgO)nMg2+ (n=1-29) Cluster Ions
Ab initio perturbed ion plus polarization calculations are reported for
doubly-charged nonstoichiometric (MgO)nMg2+ (n=1-29) cluster ions. We consider
a large number of isomers with full relaxations of the geometries, and add the
correlation correction to the Hartree-Fock energies for all cluster sizes. The
polarization contribution is included at a semiempirical level also for all
cluster sizes. Comparison is made with theoretical results for neutral (MgO)n
clusters and singly-charged alkali-halide cluster ions. Our method is also
compared to phenomenological pair potential models in order to asses their
reliability for calculations on small ionic systems. The large
coordination-dependent polarizabilities of oxide anions favor the formation of
surface sites, and thus bulklike structures begin to dominate only after n=24.
The relative stabilities of the cluster ions against evaporation of a MgO
molecule show variations that are in excellent agreement with the experimental
abundance spectra.Comment: Final version accepted in Journal of Chemical Physics; 8 pages plus 8
figures (6 GIFs and 2 PSs). The main difference with respect to the original
submission is the inclusion of coordination-dependent polarizabilities for
oxide anions. That results in substantial changes in the result
Coupling Nitrogen Vacancy Centers in Diamond to Superconducting Flux Qubits
We propose a method to achieve coherent coupling between Nitrogen-vacancy
(NV) centers in diamond and superconducting (SC) flux qubits. The resulting
coupling can be used to create a coherent interaction between the spin states
of distant NV centers mediated by the flux qubit. Furthermore, the magnetic
coupling can be used to achieve a coherent transfer of quantum information
between the flux qubit and an ensemble of NV centers. This enables a long-term
memory for a SC quantum processor and possibly an interface between SC qubits
and light.Comment: Accepted in Phys. Rev. Lett. Updated text and Supplementary Material
adde
IMF and [Na/Fe] abundance ratios from optical and NIR Spectral Features in Early-type Galaxies
We present a joint analysis of the four most prominent sodium-sensitive
features (NaD, NaI8190, NaI1.14, and NaI2.21), in the optical and Near-Infrared
spectral range, of two nearby, massive (sigma~300km/s), early-type galaxies
(named XSG1 and XSG2). Our analysis relies on deep VLT/X-Shooter long-slit
spectra, along with newly developed stellar population models, allowing for
[Na/Fe] variations, up to 1.2dex, over a wide range of age, total metallicity,
and IMF slope. The new models show that the response of the Na-dependent
spectral indices to [Na/Fe] is stronger when the IMF is bottom heavier. For the
first time, we are able to match all four Na features in the central regions of
massive early-type galaxies, finding an overabundance of [Na/Fe], in the range
0.5-0.7dex, and a bottom-heavy IMF. Therefore, individual abundance variations
cannot be fully responsible for the trends of gravity-sensitive indices,
strengthening the case towards a non-universal IMF. Given current limitations
of theoretical atmosphere models, our [Na/Fe] estimates should be taken as
upper limits. For XSG1, where line strengths are measured out to 0.8Re, the
radial trend of [Na/Fe] is similar to [Mg/Fe] and [C/Fe], being constant out to
0.5Re, and decreasing by 0.2-0.3dex at 0.8Re, without any clear correlation
with local metallicity. Such a result seems to be in contrast with the
predicted increase of Na nucleosynthetic yields from AGB stars and TypeII SNe.
For XSG1, the Na-inferred IMF radial profile is consistent, within the errors,
with that derived from TiO features and the Wing-Ford band, presented in a
recent paper.Comment: 22 pages, 8 figure, accepted for publication in MNRAS. The new
Na-enhanced models will be available soon at http://miles.iac.es
The puzzling interpretation of NIR indices: The case of NaI2.21
We present a detailed study of the Na I line strength index centered in the
-band at , {\AA} (NaI2.21 hereafter) relying on different samples of
early-type galaxies. Consistent with previous studies, we find that the
observed line strength indices cannot be fit by state-of-art scaled-solar
stellar population models, even using our newly developed models in the NIR.
The models clearly underestimate the large NaI2.21 values measured for most
early-type galaxies. However, we develop a Na-enhanced version of our newly
developed models in the NIR, which - together with the effect of a bottom-heavy
initial mass function - yield NaI2.21 indices in the range of the observations.
Therefore, we suggest a scenario in which the combined effect of [Na/Fe]
enhancement and a bottom-heavy initial mass function are mainly responsible for
the large NaI2.21 indices observed for most early-type galaxies. To a smaller
extent, also [C/Fe] enhancement might contribute to the large observed NaI2.21
values.Comment: 13 pages, 4 figures, accepted for publication in MNRA
Magnetic Field Probing of an SU(4) Kondo Resonance in a Single Atom Transistor
Semiconductor nano-devices have been scaled to the level that transport can
be dominated by a single dopant atom. In the strong coupling case a Kondo
effect is observed when one electron is bound to the atom. Here, we report on
the spin as well as orbital Kondo ground state. We experimentally as well than
theoretically show how we can tune a symmetry transition from a SU(4) ground
state, a many body state that forms a spin as well as orbital singlet by
virtual exchange with the leads, to a pure SU(2) orbital ground state, as a
function of magnetic field. The small size and the s-like orbital symmetry of
the ground state of the dopant, make it a model system in which the magnetic
field only couples to the spin degree of freedom and allows for observation of
this SU(4) to SU(2) transition.Comment: 12 pages, 10 figures, accepted for publication in Physical Review
Letter
Hormigón ligero autocompactante con fibras para rehabilitación de forjados
The slabs of some buildings in Barcelona are formed by unidirectional beams, with a ceramic arch in between, which are filled with broken pottery or construction waste. These structures often present problems such as displacement of the tiles arranged over it due to the lack of stiffness of the filling material. This supposes a risk to the user and could also cause durability problems. In order to rehabilitate it, a lightweight self-compacting concrete reinforced with fibres (HLACF) has been designed to be used as a filling material, improving the stiffness of the structure.
This paper presents a structural analysis of a standard case and the results of an experimental campaign. The concrete showed a density of 1665 kg/m3, a slump flow of 605 mm and a compressive strength of 22.3 MPa, at 28 days. These results are in agreement with the requirements, overcoming common lightweight concrete segregation problems.Los forjados de ciertos edificios del ensanche de Barcelona, formados por viguetas unidireccionales con un revoltón de cerámica entre ellas y un relleno posterior (material cerámico y residuos de construcción), suelen presentar problemas de movimientos y despegues de las baldosas situadas en la parte superior, con el consiguiente riesgo para el usuario, aparte de los problemas de durabilidad asociados. Para rehabilitar esas estructuras se ha diseñado un hormigón ligero autocompactante con fibras (HLACF), como relleno de modo que mejore la rigidez a la estructura.
El artículo presenta el análisis estructural de una solución tipo así como los resultados de una campaña experimental realizada. Como resultado se obtiene un hormigón de densidad de 1.665 kg/m3, escurrimiento de 605 mm y resistencia a compresión de 22,3 MPa, a los 28 días, que cumple con los requisitos y significa superar problemas de segregación previsibles para este tipo de hormigones
A new calibration guideline for worm and worm-gear rolling testers
The evaluation and calibration of gear testers are considered as a key point to ensure the obtained results in gear metrology. Although ISO TC 60 working group has developed standards in this regard, the fact is that for the time being, there are not specific international standards for gear rolling tests. In this work, a periodical calibration guideline for gear rolling testers, particularly for worm gear transmission, is proposed, allowing the reduction of possible error sources in the measurement process. A series of tasks distributed over time are suggested in this work to maintain the accuracy of the gear rolling test machines
Caracterización de los morteros de relleno usados en diferentes túneles españoles
The main objective of this paper is to compare typical backfill mortars used in Spanish tunnels to fill the annular void left between the lining and the ground by the TBM. Initially, a new experimental program is outlined using material corresponding to 6 mixes from 4 tunnels. The results obtained indicate a considerable difference in the density and in the rheological properties of the mixes tested. According to the estimations performed, this leads to a difference of up to 67% on the potency required from the pumps to inject the material. Furthermore, a correlation between the fine content and the rheological properties of the mix was observed. This correlation may be a practical tool to control and modify the performance of the mortars directly in the worksite.El objetivo principal del presente estudio es llevar a cabo la comparación de las dosificaciones de mortero de relleno empleadas en algunos de los grandes túneles españoles para rellenar el hueco anular dejado entre el terreno y el extradós de las dovelas. Inicialmente se hace una nueva propuesta experimental usando la composición y los materiales correspondientes a 6 dosificaciones usadas en 4 túneles. Los resultados obtenidos indican diferencias significativas en cuanto a la densidad, a la consistencia y a las propiedades reológicas. De acuerdo con las estimaciones realizadas, ello se traduce en diferencias de hasta un 67% en la potencia requerida del sistema de bombas de la tuneladora para inyectar el material. Por otro lado, se refleja una correlación entre el contenido de finos de la mezcla y las propiedades reológicas. Esa correlación puede servir para controlar y modificar dichas propiedades de manera fácil y rápida a pie de obra
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