889 research outputs found
Fragile topology in line-graph lattices with two, three, or four gapped flat bands
The geometric properties of a lattice can have profound consequences on its
band spectrum. For example, symmetry constraints and geometric frustration can
give rise to topologicially nontrivial and dispersionless bands, respectively.
Line-graph lattices are a perfect example of both of these features: their
lowest energy bands are perfectly flat, and here we develop a formalism to
connect some of their geometric properties with the presence or absence of
fragile topology in their flat bands. This theoretical work will enable
experimental studies of fragile topology in several types of line-graph
lattices, most naturally suited to superconducting circuits.Comment: 8+25 pages, 3+19 figures, 2+3 table
Second-order Band Topology in Antiferromagnetic (MnBiTe)(BiTe) Films
The existence of fractionally quantized topological corner states serves as a
key indicator for two-dimensional second-order topological insulators (SOTIs),
yet has not been experimentally observed in realistic materials. Here, based on
first-principles calculations and symmetry arguments, we propose a strategy for
achieving SOTI phases with in-gap corner states in
(MnBiTe)(BiTe) films with antiferromagnetic (AFM) order.
Starting from the prototypical AFM MnBiTe bilayer, we show by an
effective lattice model that such SOTI phase originate from the interplay
between intrinsic spin-orbital coupling and interlayer AFM exchange
interactions. Furthermore, we demonstrate that the nontrivial corner states are
linked to rotation topological invariants under three-fold rotation symmetry
, resulting in -symmetric SOTIs with corner charges fractionally
quantized to (mod ). Due to the great recent
achievements in (MnBiTe)(BiTe) systems, our results
providing reliable material candidates for experimentally accessible AFM
higher-order band topology would draw intense attentions.Comment: 6 pages, 4 figure
Fully spin-polarized nodal loop semimetals in alkaline-metal monochalcogenide monolayers
Topological semimetals in ferromagnetic materials have attracted enormous
attention due to the potential applications in spintronics. Using the
first-principles density functional theory together with an effective lattice
model, here we present a new family of topological semimetals with a fully
spin-polarized nodal loop in alkaline-metal monochalcogenide ( = Li,
Na, K, Rb, Cs; = S, Se, Te) monolayers. The half-metallic ferromagnetism
can be established in monolayers, in which one nodal loop formed by two
crossing bands with the same spin components is found at the Fermi energy. This
nodal loop half-metal survives even when considering the spin-orbit coupling
owing to the symmetry protection provided by the mirror
plane. The quantum anomalous Hall state and Weyl-like semimetal in this system
can be also achieved by rotating the spin from the out-of-plane to the in-plane
direction. The monolayers hosting rich topological phases thus offer an
excellent materials platform for realizing the advanced spintronics concepts
Catalogue of topological phonon materials
Phonons play a crucial role in many properties of solid state systems, such
as thermal and electrical conductivity, neutron scattering and associated
effects or superconductivity. Hence, it is expected that topological phonons
will also lead to rich and unconventional physics and the search of materials
hosting topological phonons becomes a priority in the field. In electronic
crystalline materials, a large part of the topological properties of Bloch
states can be indicated by their symmetry eigenvalues in reciprocal space. This
has been adapted to the high-throughput calculations of topological materials,
and more than half of the stoichiometric materials on the databases are found
to be topological insulators or semi-metals. Based on the existing phonon
materials databases, here we have performed the first catalogue of topological
phonon bands for more than ten thousand three-dimensional crystalline
materials. Using topological quantum chemistry, we calculate the band
representations, compatibility relations, and band topologies of each isolated
set of phonon bands for the materials in the phonon databases. We have also
calculated the real space invariants for all the topologically trivial bands
and classified them as atomic and obstructed atomic bands. In particular,
surface phonon modes (dispersion) are calculated on different cleavage planes
for all the materials. Remarkably, we select more than one thousand "ideal"
non-trivial phonon materials to fascinate the future experimental studies. All
the data-sets obtained in the the high-throughput calculations are used to
build a Topological Phonon Database.Comment: 8+535 pages, 187 figures, 21 tables. The Topological Phonon Database
is available at https://www.topologicalquantumchemistry.com/topophonons or
https://www.topologicalquantumchemistry.fr/topophonon
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