61 research outputs found
Topological phonon analysis of the 2D buckled honeycomb lattice: an application to real materials
By means of group theory, topological quantum chemistry, first-principles and
Monte Carlo calculations, we analyze the topology of the 2D buckled honeycomb
lattice phonon spectra. Taking the pure crystal structure as an input, we show
that eleven distinct phases are possible, five of which necessarily have
non-trivial topology according to topological quantum chemistry. Another four
of them are also identified as topological using Wilson loops in an analytical
model that includes all the symmetry allowed force constants up to third
nearest neighbors, making a total of nine topological phases. We then compute
the ab initio phonon spectra for the two-dimensional crystals of Si, Ge, P, As
and Sb in this structure and construct its phase diagram. Despite the large
proportion of topological phases found in the analytical model, all of the
crystals lie in a trivial phase. By analyzing the force constants space using
Monte Carlo calculations, we elucidate why topological phonon phases are
physically difficult to realize in real materials with this crystal structure
Higher-Order Topological Insulators
Three-dimensional topological (crystalline) insulators are materials with an
insulating bulk, but conducting surface states which are topologically
protected by time-reversal (or spatial) symmetries. Here, we extend the notion
of three-dimensional topological insulators to systems that host no gapless
surface states, but exhibit topologically protected gapless hinge states. Their
topological character is protected by spatio-temporal symmetries, of which we
present two cases: (1) Chiral higher-order topological insulators protected by
the combination of time-reversal and a four-fold rotation symmetry. Their hinge
states are chiral modes and the bulk topology is -classified. (2)
Helical higher-order topological insulators protected by time-reversal and
mirror symmetries. Their hinge states come in Kramers pairs and the bulk
topology is -classified. We provide the topological invariants for
both cases. Furthermore we show that SnTe as well as surface-modified
BiTeI, BiSe, and BiTe are helical higher-order topological insulators and
propose a realistic experimental setup to detect the hinge states.Comment: 8 pages (4 figures) and 16 pages supplemental material (7 figures
Electronic properties, correlated topology and Green's function zeros
There is extensive current interest about electronic topology in correlated
settings. In strongly correlated systems, contours of Green's function zeros
may develop in frequency-momentum space, and their role in correlated topology
has increasingly been recognized. However, whether and how the zeros contribute
to electronic properties is a matter of uncertainty. Here we address the issue
in an exactly solvable model for Mott insulator. We show that the Green's
function zeros contribute to several physically measurable correlation
functions, in a way that does not run into inconsistencies. In particular, the
physical properties remain robust to chemical potential variations up to the
Mott gap as it should be based on general considerations. Our work sets the
stage for further understandings on the rich interplay among topology, symmetry
and strong correlations.Comment: 15 pages, 3 figure
Monopole-like orbital-momentum locking and the induced orbital transport in topological chiral semimetals
The interplay between chirality and topology nurtures many exotic electronic
properties. For instance, topological chiral semimetals display multifold
chiral fermions that manifest nontrivial topological charge and spin texture.
They are an ideal playground for exploring chirality-driven exotic physical
phenomena. In this work, we reveal a monopole-like orbital-momentum locking
texture on the three-dimensional Fermi surfaces of topological chiral
semimetals with B20 structures (e.g., RhSi and PdGa). This orbital texture
enables a large orbital Hall effect (OHE) and a giant orbital magnetoelectric
(OME) effect in the presence of current flow. Different enantiomers exhibit the
same OHE which can be converted to the spin Hall effect by spin-orbit coupling
in materials. In contrast, the OME effect is chirality-dependent and much
larger than its spin counterpart. Our work reveals the crucial role of orbital
texture for understanding OHE and OME effects in topological chiral semimetals
and paves the path for applications in orbitronics, spintronics, and enantiomer
recognition.Comment: 23 pages, 5 figure
Large anomalous Hall, Nernst effect and topological phases in the 3d-4d/5d based oxide double perovskites
Magnetism and spin-orbit coupling are two fundamental and interconnected properties of oxide materials, that can give rise to various topological transport phenomena, including anomalous Hall and anomalous Nernst effects. These transport responses can be significantly enhanced by designing an electronic structure with a large Berry curvature. In this context, rocksalt-ordered double perovskites (DP), denoted as A2BB’O6, with two distinct transition metal sites are very powerful platforms for exploration and research. In this work, we present a comprehensive study based on the intrinsic anomalous transport in cubic and tetragonal stable DP compounds with 3d-4d/5d elements. Our findings reveal that certain DP compounds show a large anomalous Hall effect, displaying topological band crossings in the proximity of the Fermi energy
Double crystallographic groups and their representations on the Bilbao Crystallographic Server
A new section of databases and programs devoted to double crystallographic
groups (point and space groups) has been implemented in the Bilbao
Crystallographic Server (http://www.cryst.ehu.es). The double crystallographic
groups are required in the study of physical systems whose Hamiltonian includes
spin-dependent terms. In the symmetry analysis of such systems, instead of the
irreducible representations of the space groups, it is necessary to consider
the single- and double-valued irreducible representations of the double space
groups. The new section includes databases of symmetry operations (DGENPOS) and
of irreducible representations of the double (point and space) groups
(REPRESENTATIONS DPG and REPRESENTATIONS DSG). The tool DCOMPATIBILITY
RELATIONS provides compatibility relations between the irreducible
representations of double space groups at different k-vectors of the Brillouin
zone when there is a group-subgroup relation between the corresponding little
groups. The program DSITESYM implements the so-called site-symmetry approach,
which establishes symmetry relations between localized and extended crystal
states, using representations of the double groups. As an application of this
approach, the program BANDREP calculates the band representations and the
elementary band representations induced from any Wyckoff position of any of the
230 double space groups, giving information about the properties of these
bands. Recently, the results of BANDREP have been extensively applied in the
description and the search of topological insulators.Comment: 32 pages, 20 figures. Two extra figures and minor typo mistakes
fixed. Published versio
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