1,589 research outputs found
TaIrTe4 a ternary Type-II Weyl semi-metal
In metallic condensed matter systems two different types of Weyl fermions can
in principle emerge, with either a vanishing (type-I) or with a finite
(type-II) density of states at the Weyl node energy. So far only WTe2 and MoTe2
were predicted to be type-II Weyl semi-metals. Here we identify TaIrTe4 as a
third member of this family of topological semi-metals. TaIrTe4 has the
attractive feature that it hosts only four well-separated Weyl points, the
minimum imposed by symmetry. Moreover, the resulting topological surface states
- Fermi arcs connecting Weyl nodes of opposite chirality - extend to about 1/3
of the surface Brillouin zone. This large momentum-space separation is very
favorable for detecting the Fermi arcs spectroscopically and in transport
experiments
Structural classification of quasi-one-dimensional ternary nitrides
This review focuses on the crystal structural features of ternary (mixed-metal) quasi-one-dimensional nitrides i.e., nitrides containing (cation-³⁻) coordination polyhedra sharing either corners, edges, or faces, arranged in linear chains, and intercalated by a counter ion. The current relevance of these nitrides, and of quasi-one-dimensional compounds in general, lies in the fact that they are closely related to the pure one-dimensional systems (i.e., nanowires), which are vastly researched for their amazing properties closely related to their low dimensionality. A number of these properties were firstly discovered in quasi-one-dimensional compounds, highlighting the importance of expanding knowledge and research in this area. Furthermore, unlike oxides, nitrides and other non-oxide compounds are less developed, hence more difficult to categorise into structural classes that can then be related to other classes of compounds, leading to a fuller picture of structure–properties relationship. Within this context, this review aims to categorise and describe a number of ternary (mixed-metal) quasi-one-dimensional nitrides according to their structural features, specifically, the polyhedra forming the one-dimensional chains
Symmetry Decomposition of Potentials with Channels
We discuss the symmetry decomposition of the average density of states for
the two dimensional potential and its three dimensional
generalisation . In both problems, the energetically
accessible phase space is non-compact due to the existence of infinite channels
along the axes. It is known that in two dimensions the phase space volume is
infinite in these channels thus yielding non-standard forms for the average
density of states. Here we show that the channels also result in the symmetry
decomposition having a much stronger effect than in potentials without
channels, leading to terms which are essentially leading order. We verify these
results numerically and also observe a peculiar numerical effect which we
associate with the channels. In three dimensions, the volume of phase space is
finite and the symmetry decomposition follows more closely that for generic
potentials --- however there are still non-generic effects related to some of
the group elements
Charge order in Fe2OBO3: An LSDA+U study
Charge ordering in the low-temperature monoclinic structure of iron oxoborate
(Fe2OBO3) is investigated using the local spin density approximation (LSDA)+U
method. While the difference between t_{2g} minority occupancies of Fe^{2+} and
Fe^{3+} cations is large and gives direct evidence for charge ordering, the
static "screening" is so effective that the total 3d charge separation is
rather small. The occupied Fe^{2+} and Fe^{3+} cations are ordered alternately
within the chain which is infinite along the a-direction. The charge order
obtained by LSDA+U is consistent with observed enlargement of the \beta angle.
An analysis of the exchange interaction parameters demonstrates the
predominance of the interribbon exchange interactions which determine the whole
L-type ferrimagnetic spin structure.Comment: 7 pages, 8 figure
Spectral curves and the mass of hyperbolic monopoles
The moduli spaces of hyperbolic monopoles are naturally fibred by the
monopole mass, and this leads to a nontrivial mass dependence of the
holomorphic data (spectral curves, rational maps, holomorphic spheres)
associated to hyperbolic multi-monopoles. In this paper, we obtain an explicit
description of this dependence for general hyperbolic monopoles of magnetic
charge two. In addition, we show how to compute the monopole mass of higher
charge spectral curves with tetrahedral and octahedral symmetries. Spectral
curves of euclidean monopoles are recovered from our results via an
infinite-mass limit.Comment: 43 pages, LaTeX, 3 figure
Complexity in surfaces of densest packings for families of polyhedra
Packings of hard polyhedra have been studied for centuries due to their
mathematical aesthetic and more recently for their applications in fields such
as nanoscience, granular and colloidal matter, and biology. In all these
fields, particle shape is important for structure and properties, especially
upon crowding. Here, we explore packing as a function of shape. By combining
simulations and analytic calculations, we study three 2-parameter families of
hard polyhedra and report an extensive and systematic analysis of the densest
packings of more than 55,000 convex shapes. The three families have the
symmetries of triangle groups (icosahedral, octahedral, tetrahedral) and
interpolate between various symmetric solids (Platonic, Archimedean, Catalan).
We find that optimal (maximum) packing density surfaces that reveal unexpected
richness and complexity, containing as many as 130 different structures within
a single family. Our results demonstrate the utility of thinking of shape not
as a static property of an object in the context of packings, but rather as but
one point in a higher dimensional shape space whose neighbors in that space may
have identical or markedly different packings. Finally, we present and
interpret our packing results in a consistent and generally applicable way by
proposing a method to distinguish regions of packings and classify types of
transitions between them.Comment: 16 pages, 8 figure
SU(N) Monopoles and Platonic Symmetry
We discuss the ADHMN construction for SU(N) monopoles and show that a
particular simplification arises in studying charge N-1 monopoles with minimal
symmetry breaking. Using this we construct families of tetrahedrally symmetric
SU(4) and SU(5) monopoles. In the moduli space approximation, the SU(4)
one-parameter family describes a novel dynamics where the monopoles never
separate, but rather, a tetrahedron deforms to its dual. We find a
two-parameter family of SU(5) tetrahedral monopoles and compute some geodesics
in this submanifold numerically. The dynamics is rich, with the monopoles
scattering either once or twice through octahedrally symmetric configurations.Comment: 14pp, RevTex, two figures made of six Postscript files. To appear in
the Journal of Mathematical Physic
Nematic phases and the breaking of double symmetries
In this paper we present a phase classification of (effectively)
two-dimensional non-Abelian nematics, obtained using the Hopf symmetry breaking
formalism. In this formalism one exploits the underlying double symmetry which
treats both ordinary and topological modes on equal footing, i.e. as
representations of a single (non-Abelian) Hopf symmetry. The method that exists
in the literature (and is developed in a paper published in parallel) allows
for a full classification of defect mediated as well as ordinary symmetry
breaking patterns and a description of the resulting confinement and/or
liberation phenomena. After a summary of the formalism, we determine the double
symmetries for tetrahedral, octahedral and icosahedral nematics and their
representations. Subsequently the breaking patterns which follow from the
formation of admissible defect condensates are analyzed systematically. This
leads to a host of new (quantum and classical) nematic phases. Our result
consists of a listing of condensates, with the corresponding intermediate
residual symmetry algebra and the symmetry algebra characterizing the effective
``low energy'' theory of surviving unconfined and liberated degrees of freedom
in the broken phase. The results suggest that the formalism is applicable to a
wide variety of two dimensional quantum fluids, crystals and liquid crystals.Comment: 17 pages, 2 figures, correction to table VII, updated reference
Topological insulators and thermoelectric materials
Topological insulators (TIs) are a new quantum state of matter which have
gapless surface states inside the bulk energy gap. Starting with the discovery
of two dimensional TIs, the HgTe-based quantum wells, many new topological
materials have been theoretically predicted and experimentally observed.
Currently known TI materials can possibly be classified into two families, the
HgTe family and the Bi2Se family. The signatures found in the electronic
structure of a TI also cause these materials to be excellent thermoelectric
materials. On the other hand, excellent thermoelectric materials can be also
topologically trivial. Here we present a short introduction to topological
insulators and thermoelectrics, and give examples of compound classes were both
good thermoelectric properties and topological insulators can be found.Comment: Phys. Status Solidi RRL, accepte
Excited state baryon spectroscopy from lattice QCD
We present a calculation of the Nucleon and Delta excited state spectrum on
dynamical anisotropic clover lattices. A method for operator construction is
introduced that allows for the reliable identification of the continuum spins
of baryon states, overcoming the reduced symmetry of the cubic lattice. Using
this method, we are able to determine a spectrum of single-particle states for
spins up to and including J = 7/2, of both parities, the first time this has
been achieved in a lattice calculation. We find a spectrum of states
identifiable as admixtures of SU(6) x O(3) representations and a counting of
levels that is consistent with the non-relativistic constituent quark
model. This dense spectrum is incompatible with quark-diquark model solutions
to the "missing resonance problem" and shows no signs of parity doubling of
states.Comment: 29 pages, 18 figure
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