441 research outputs found
Molecular orbital polarization in Na2Ti2Sb2O: microscopic route to metal-metal transition without spontaneous symmetry breaking
Ordered phases such as charge- and spin-density wave state accompany either
full or partial gapping of Fermi surface (FS) leading a metal-insulator or
metal-metal transition (MMT). However, there are examples of MMT without any
signatures of symmetry breaking. One example is NaTiSbO, where a
partial gapping of FS is observed but a density wave ordering has not been
found. Here we propose a microscopic mechanism of such a MMT which occurs due
to a momentum dependent spin-orbit coupled molecular orbital polarization.
Since a molecular orbital polarization is present due to a small spin-orbit
coupling of Ti, there is no spontaneous symmetry breaking involved. However, a
sharp increase of polarization happens above a critical electron interaction
which gaps out the orbtial FS and reduces the density of states
significantly, while the rest of FS associated with Sb orbtials is almost
intact across MMT. Experimental implications to test our proposal and
applications to other systems are also discussed.Comment: 5 pages, 3 figure
Crystal structure and magnetism in -RuCl3: an ab-initio study
-RuCl has been proposed recently as an excellent playground for
exploring Kitaev physics on a two-dimensional (2D) honeycomb lattice. However,
structural clarification of the compound has not been completed, which is
crucial in understanding the physics of this system. Here, using {\it
ab-initio} electronic structure calculations, we study a full three dimensional
(3D) structure of -RuCl including the effects of spin-orbit
coupling (SOC) and electronic correlations. Three major results are as follows;
i) SOC suppresses dimerization of Ru atoms, which exists in other Ru compounds
such as isostructural LiRuO, and making the honeycomb closer to an
ideal one. ii) The nearest-neighbor Kitaev exchange interaction between the
=1/2 pseudospin depends strongly on the Ru-Ru distance and the Cl
position, originating from the nature of the edge-sharing geometry. iii) The
optimized 3D structure without electronic correlations has space
group symmetry independent of SOC, but including electronic correlation changes
the optimized 3D structure to either or within 0.1 meV per
formula unit (f.u.) energy difference. The reported structure is also
close in energy. The interlayer spin exchange coupling is a few percent of
in-plane spin exchange terms, confirming -RuCl is close to a 2D
system. We further suggest how to increase the Kitaev term via tensile strain,
which sheds new light in realizing Kitaev spin liquid phase in this system.Comment: 10 pages, 10 figures, and 4 table
Topological crystalline semimetals in non-symmorphic lattices
Numerous efforts have been devoted to reveal exotic semimetallic phases with
topologically non-trivial bulk and/or surface states in materials with strong
spin-orbit coupling. In particular, semimetals with nodal line Fermi surface
(FS) exhibit novel properties, and searching for candidate materials becomes an
interesting research direction. Here we provide a generic condition for a
four-fold degenerate nodal line FS in non-symmorphic crystals with inversion
and time-reversal symmetry (TRS). When there are two glide planes or screw axes
perpendicular to each other, a pair of Bloch bands related by non-symmorphic
symmetry become degenerate on a Brillouin Zone (BZ) boundary. There are two
pairs of such bands, and they disperse in a way that the partners of two pairs
are exchanged on other BZ boundaries. This enforces a nodal line FS on a BZ
boundary plane protected by non-symmorphic symmetries. When TRS is broken,
four-fold degenerate Dirac points or Weyl ring FS could occur depending on a
direction of the magnetic field. On a certain surface double helical surface
states exist, which become double Ferm arcs as TRS is broken.Comment: 6 pages, 4 figure
Topological edge states in single layers of honeycomb materials with strong spin-orbit coupling
We study possible edge states in single layers of honeycomb materials such as
-RuCl and AIrO (A=Li, Na) with strong spin-orbit coupling
(SOC). These two dimensional systems exhibit linearly dispersing
one-dimensional (1D) edge states when their 1D boundary forms a zig-zag shape.
Using an effective tight-binding model based on first principles band structure
calculations including Hubbard U and SOC, we find degenerate edge states at the
zone center and zone boundary. The roles of chiral symmetry and time-reversal
symmetry are presented. The implications to experimental signatures and the
effects of disorder are also discussed.Comment: 5 pages, 2 tables, 4 figure
Kitaev magnetism in honeycomb RuCl3 with intermediate spin-orbit coupling
Intensive studies of the interplay between spin-orbit coupling (SOC) and
electronic correlations in transition metal compounds have recently been
undertaken. In particular, = 1/2 bands on a honeycomb lattice
provide a pathway to realize Kitaev's exactly solvable spin model. However,
since current wisdom requires strong atomic SOC to make
bands, studies have been limited to iridium oxides. Contrary to this
expectation, we demonstrate how Kitaev interactions arise in 4-orbital
honeycomb -RuCl, despite having significantly weaker SOC than the
iridium oxides, via assistance from electron correlations. A strong coupling
spin model for these correlation-assisted = 1/2 bands is derived,
in which large antiferromagnetic Kitaev interactions emerge along with
ferromagnetic Heisenberg interactions. Our analyses suggest that the ground
state is a zigzag-ordered phase lying close to the antiferromagnetic Kitaev
spin liquid. Experimental implications for angle resolved photoemission
spectroscopy, neutron scattering, and optical conductivities are discussed.Comment: 8 pages, 5 figures, Accepted in Phys. Rev. B Rapid communication
Magnetic Orders Proximal to the Kitaev Limit in Frustrated Triangular Systems: Application to BaIrTiO
Frustrated transition metal compounds in which spin-orbit coupling (SOC) and
electron correlation work together have attracted much attention recently. In
the case of 5 transition metals, where SOC is large,
bands near the Fermi level are thought to encompass the essential physics of
the material, potentially leading to a concrete realization of exotic magnetic
phases such as the Kitaev spin liquid. Here we derive a spin model on a
triangular lattice based on pseudospins that interact via
antiferromagnetic Heisenberg () and Kitaev () exchanges, and crucially,
an anisotropic exchange. Our classical analysis of the spin model
reveals that, in addition to small regions of 120, /
dual- vortex crystal and nematic phases, the stripy and
ferromagnetic phases dominate the -- phase diagram. We apply our
model to the 5 transition metal compound, BaIrTiO, in which the
Ir ions form layered two-dimensional triangular lattices. We compute the
band structure and nearest-neighbor hopping parameters using ab-initio
calculations. By combining our ab-initio and classical analyses, we predict
that BaIrTiO has a stripy ordered magnetic ground state.Comment: 8 pages, 5 figure
Surface States of Perovskite Iridates AIrO; Signatures of Topological Crystalline Metal with Nontrivial Index
There have been increasing efforts in realizing topological metallic phases
with nontrivial surface states. It was suggested that orthorhombic perovskite
iridates are classified as a topological crystalline metal (TCM) with flat
surface states protected by lattice symmetries. Here we perform
first-principles electronic structure calculations for epitaxially stabilized
orthorhombic perovskite iridates. Remarkably, two different types of
topological surface states are found depending on surface directions. On side
surfaces, flat surface states protected by lattice symmetries emerge,
manifesting the topological crystalline character. On the top surface, on the
other hand, an unexpected Dirac cone appears, indicating surface states
protected by a time-reversal symmetry, which is confirmed by the presence of a
nontrivial topological index. These results suggest that the
orthorhombic iridates are unique systems exhibiting both lattice- and
global-symmetry-protected topological phases and surface states. Transitions to
weak and strong topological insulators and implications of surface states in
light of angle resolved photoemission spectroscopy are also discussed.Comment: 7 pages, 7 figure
Bound states in the continuum accompanied by avoided crossings in leaky-mode photonic lattices
When two nonorthogonal resonances are coupled to the same radiation channel,
avoided crossing arises and a bound state in the continuum (BIC) appears in
parametric space. This paper presents numerical and analytical results on the
properties of avoided crossing and BIC due to the coupled guided-mode
resonances in one-dimensional leaky-mode photonic lattices with slab geometry.
In symmetric photonic lattices with up-down mirror symmetry, Friedrich-Wintgen
BICs with infinite lifetime are accompanied by avoided crossings due to the
coupling between two guided modes with the same transverse parity. In
asymmetric photonic lattices with broken up-down mirror symmetry, quasi-BICs
with finite lifetime appear with avoided crossings because radiating waves from
different modes cannot be completely eliminated. We also show that
unidirectional-BICs are accompanied by avoided crossings due to guided-mode
resonances with different transverse parities in asymmetric photonic lattices.
The Q factor of a unidirectional-BIC is finite, but its radiation power in the
upward or downward direction is significantly smaller than that in the opposite
direction. Our results may be helpful in engineering BICs and avoided crossings
in diverse photonic systems that support leaky modes
Anisotropic metallic metasurface superlattices supporting Fano resonances and bound states in the continuum
A perfect metal film with a periodic arrangement of cut-through slits, an
anisotropic metallic metamaterial film, mimics a dielectric slab and supports
guided electromagnetic waves in the direction perpendicular to the slits. Here,
we introduce metallic metasurface superlattices that include multiple slits in
a period, and demonstrate that the superlattices support the Fano resonances
and bound states in the continuum. The number of Fano resonances and bound
states depend on the number of slits in a period of superlattices. The metallic
metasurface superlattices provide new mechanisms to manipulate electromagnetic
waves, ranging from microwave to far-infrared wavelengths, where a conventional
metal can be considered as a perfect electric conductor
Metasurfaces with bound states in the continuum enabled by eliminating first Fourier harmonic component in lattice parameters
Conventional photonic lattices, such as metamaterials and photonic crystals,
exhibit various interesting physical properties that are attributed to periodic
modulations in lattice parameters. In this study, we introduce novel types of
photonic lattices, namely Fourier-component-engineered metasurfaces, that do
not possess the first Fourier harmonic component in the lattice parameters. We
demonstrate that these metasurfaces support the continuous high- bound
states near second stop bands. The concept of engineering Fourier harmonic
components in periodic modulations provides a new method to manipulate
electromagnetic waves in artificial periodic structures
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