6,422 research outputs found
Interplay of magnetic and structural transitions in Fe-based pnictide superconductors
The interplay between the structural and magnetic phase transitions occurring
in the Fe-based pnictide superconductors is studied within a Ginzburg-Landau
approach. We show that the magnetoelastic coupling between the corresponding
order parameters is behind the salient features observed in the phase diagram
of these systems. This naturally explains the coincidence of transition
temperatures observed in some cases as well as the character (first or
second-order) of the transitions. We also show that magnetoelastic coupling is
the key ingredient determining the collinearity of the magnetic ordering, and
we propose an experimental criterion to distinguish between a pure elastic from
a spin-nematic-driven structural transition.Comment: 5 pages, 3 figures. v2: Fig. 1 improved, references added
Graph Theory Data for Topological Quantum Chemistry
Topological phases of noninteracting particles are distinguished by global
properties of their band structure and eigenfunctions in momentum space. On the
other hand, group theory as conventionally applied to solid-state physics
focuses only on properties which are local (at high symmetry points, lines, and
planes) in the Brillouin zone. To bridge this gap, we have previously [B.
Bradlyn et al., Nature 547, 298--305 (2017)] mapped the problem of constructing
global band structures out of local data to a graph construction problem. In
this paper, we provide the explicit data and formulate the necessary algorithms
to produce all topologically distinct graphs. Furthermore, we show how to apply
these algorithms to certain "elementary" band structures highlighted in the
aforementioned reference, and so identified and tabulated all orbital types and
lattices that can give rise to topologically disconnected band structures.
Finally, we show how to use the newly developed BANDREP program on the Bilbao
Crystallographic Server to access the results of our computation.Comment: v1: 29 Pages, 13 Figures. Explains how to access the data presented
in arXiv:1703.02050 v2: Accepted version. References updated, figures
improve
Building Blocks of Topological Quantum Chemistry: Elementary Band Representations
The link between chemical orbitals described by local degrees of freedom and
band theory, which is defined in momentum space, was proposed by Zak several
decades ago for spinless systems with and without time-reversal in his theory
of "elementary" band representations. In Nature 547, 298-305 (2017), we
introduced the generalization of this theory to the experimentally relevant
situation of spin-orbit coupled systems with time-reversal symmetry and proved
that all bands that do not transform as band representations are topological.
Here, we give the full details of this construction. We prove that elementary
band representations are either connected as bands in the Brillouin zone and
are described by localized Wannier orbitals respecting the symmetries of the
lattice (including time-reversal when applicable), or, if disconnected,
describe topological insulators. We then show how to generate a band
representation from a particular Wyckoff position and determine which Wyckoff
positions generate elementary band representations for all space groups. This
theory applies to spinful and spinless systems, in all dimensions, with and
without time reversal. We introduce a homotopic notion of equivalence and show
that it results in a finer classification of topological phases than approaches
based only on the symmetry of wavefunctions at special points in the Brillouin
zone. Utilizing a mapping of the band connectivity into a graph theory problem,
which we introduced in Nature 547, 298-305 (2017), we show in companion papers
which Wyckoff positions can generate disconnected elementary band
representations, furnishing a natural avenue for a systematic materials search.Comment: 15+9 pages, 4 figures; v2: minor corrections; v3: updated references
(published version
Topological quantum chemistry
The past decade's apparent success in predicting and experimentally
discovering distinct classes of topological insulators (TIs) and semimetals
masks a fundamental shortcoming: out of 200,000 stoichiometric compounds extant
in material databases, only several hundred of them are topologically
nontrivial. Are TIs that esoteric, or does this reflect a fundamental problem
with the current piecemeal approach to finding them? To address this, we
propose a new and complete electronic band theory that highlights the link
between topology and local chemical bonding, and combines this with the
conventional band theory of electrons. Topological Quantum Chemistry is a
description of the universal global properties of all possible band structures
and materials, comprised of a graph theoretical description of momentum space
and a dual group theoretical description in real space. We classify the
possible band structures for all 230 crystal symmetry groups that arise from
local atomic orbitals, and show which are topologically nontrivial. We show how
our topological band theory sheds new light on known TIs, and demonstrate the
power of our method to predict a plethora of new TIs.Comment: v1: 8 pages + 40 pages supplemenetary material. Previously submitted
v2: ~ Published version. 11 pages + 79 pages supplementary material.
Descriptions of the data used in this paper can be found in arXiv:1706.08529
and arXiv:1706.09272. All data can be accessed via the Bilbao
Crystallographic Server (http://cryst.ehu.es). Two additional papers
elaborating on the general theory currently in pre
Band Connectivity for Topological Quantum Chemistry: Band Structures As A Graph Theory Problem
The conventional theory of solids is well suited to describing band
structures locally near isolated points in momentum space, but struggles to
capture the full, global picture necessary for understanding topological
phenomena. In part of a recent paper [B. Bradlyn et al., Nature 547, 298
(2017)], we have introduced the way to overcome this difficulty by formulating
the problem of sewing together many disconnected local "k-dot-p" band
structures across the Brillouin zone in terms of graph theory. In the current
manuscript we give the details of our full theoretical construction. We show
that crystal symmetries strongly constrain the allowed connectivities of energy
bands, and we employ graph-theoretic techniques such as graph connectivity to
enumerate all the solutions to these constraints. The tools of graph theory
allow us to identify disconnected groups of bands in these solutions, and so
identify topologically distinct insulating phases.Comment: 19 pages. Companion paper to arXiv:1703.02050 and arXiv:1706.08529
v2: Accepted version, minor typos corrected and references added. Now
19+epsilon page
Electroexcitation of the Roper resonance from CLAS data
The helicity amplitudes of the electroexcitation of the Roper resonance on
proton are extracted at 1.7 < Q2 < 4.2 GeV2 from recent high precision
JLab-CLAS cross sections data and longitudinally polarized beam asymmetry for
pi+ electroproduction on protons. The analysis is made using two approaches,
dispersion relations and unitary isobar model, which give consistent results.
It is found that the transverse helicity amplitude for the gamma* p -->
P11(1440) transition, which is large and negative at Q2=0, becomes large and
positive at Q2 ~ 2 GeV2, and then drops slowly with Q2. Longitudinal helicity
amplitude, that was previously found from CLAS data as large and positive at
Q2=0.4,0.65 GeV2, drops with Q2. These results rule out the presentation of
P11(1440) as a 3qG hybrid state, and provide strong evidence in favor of this
resonance as a first radial excitation of the 3q ground state.Comment: 3 pages, 2 figures, Talk on the Workshop on "The Physics of Excited
Nucleons", Bonn, Germany, October 200
Una tarde con Ramón. Entrevista a Ramón Valdés
Aquesta entrevista al Catedràtic Emèrit de la UAB, Dr. Ramón Valdés del Toro, la van realitzar l'11 de juliol de 2006 un grup d'alumnes de l'assignatura Etnologia Regional que es va impartir el curs 2005-06 a la Llicenciatura d'Antropologia Social d'aquesta universitat. L'entrevista forma part d'un projecte més ampli vinculat a l'assignatura dirigit per la professora que l'imparteix, Montserrat Clua i Fainé.Esta entrevista al Catedrático Emérito de la UAB, Dr. Ramón Valdés del Toro, fue realizada el 11 de julio de 2006 por un grupo de alumnas de la asignatura Etnología Regional que se impartió el curso 2005-06 en la Licenciatura de Antropología Social de dicha universidad. La entrevista forma parte de un proyecto más amplio vinculado a la asignatura y dirigido por su profesora, Montserrat Clua i Fainé
The contribution of 211 particles to the mechanical reinforcement mechanism of 123 superconducting single domains
Hardness and fracture toughness of Dy-123 single-domains were studied by
Vickers micro-indentation. A significant anisotropy of the mechanical
properties was observed. Hardness tests give higher values when performed in
(001) planes rather than in planes parallel to the c-axis. Moreover cracks
pattern around the indentation follows preferential orientation in planes
parallel to the c-axis whereas a classical ''four-cracks'' pattern is observed
in the (001) planes. It has been possible to show the crucial role played by
the 211-particles in the deviating mechanism of cracks and the relevance of the
211-particle distribution high homogeneity in the material.Comment: 14 pages, including 5 figures and 1 Table. submitted to Supercond.
Sci. Techno
Anaerobic Thermophilic Colonization of Porous Support
Biofilm development in an open-pore sintered glass material (SIRAN) was studied using a laboratory-scale anaerobic fixed-film bioreactor under thermophilic conditions.
The startup and performance of that bioreactor, operating on distillery waste water feed (vinasses), were also studied.
Results obtained indicated that stepped organic loading during initial bioreactor start-up reduced the periods of adaptation in the colonization process and micro-organism attachment and biofilm formation was accelerated by the surface characteristics of
the carrier. The results obtained by operating with stepped organic loading (& = 2.0 kg m–3 d–1 COD) over a period of 70 days suggest that a stable operation of the process (90% COD removal) and high density of biomass immobilized on the support (20 kgVSatt per m3 SIRAN) is achieved. Epifluorescence microscopy demonstrated that, initially, attached growth was developed in crevices, where biomass was protected from shear forces and, finally, SIRAN was completely covered and biofilm developed on the entire SIRAN particles
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