2,091 research outputs found
Antiferromagnetic crystalline topological insulators
The gapless surface Dirac cone of time reversal invariant topological
insulators is protected by time reversal symmetry due to the Kramers' theorem.
Spin degree of freedom is usually required since Kramers' theorem only
guarantees double degeneracy for spinful fermions, but not for spinless
fermions. In this paper, we present an antiferromagnetic spinless model, which
breaks time reversal symmetry. Similar to time reversal invariant topological
insulators, this model possesses a topologically non-trivial phase with a
single surface Dirac cone, which is protected by the combination of time
reversal and translation operation. Our results show that in magnetic crystals,
a single Dirac cone can exist on the surface even without any spin degree of
freedom and spin-orbit coupling.Comment: 5 pages, 3 figure
Topological non-symmorphic crystalline insulators
In this work, we identify a new class of Z2 topological insulator protected
by non-symmorphic crystalline symmetry, dubbed a "topological non-symmorphic
crystalline insulator". We construct a concrete tight-binding model with the
non-symmorphic space group pmg and confirm the topological nature of this model
by calculating topological surface states and defining a Z2 topological
invariant. Based on the projective representation theory, we extend our
discussion to other non-symmorphic space groups that allows to host topological
non-symmorphic crystalline insulators.Comment: 7 pages, 5 figure
Crystalline Symmetry-Protected Majorana Mode in Number-Conserving Dirac Semimetal Nanowires
One of the cornerstones for topological quantum computations is the Majorana
zero mode, which has been intensively searched in fractional quantum Hall
systems and topological superconductors. Several recent works suggest that such
an exotic mode can also exist in a one-dimensional (1D) interacting double-wire
setup even without long-range superconductivity. A notable instability in these
proposals comes from interchannel single-particle tunneling that spoils the
topological ground state degeneracy. Here we show that a 1D Dirac semimetal
(DSM) nanowire is an ideal number-conserving platform to realize such Majorana
physics. By inserting magnetic flux, a DSM nanowire is driven into a 1D
crystalline-symmetry-protected semimetallic phase. Interaction enables the
emergence of boundary Majorana zero modes, which is robust as a result of
crystalline symmetry protection. We also explore several experimental
consequences of Majorana signals.Comment: 32 pages, 6 figure
Topological invariants for three dimensional Dirac semimetals and four dimensional topological rotational insulators
Dirac semimetal is a class of semi-metallic phase protected by certain types
of crystalline symmetries, and its low-energy effective Hamiltonian is
described by Dirac equations in three dimensions (3D). Despite of various
theoretical studies, theories that describe the topological nature of Dirac
semimetals have not been well established. In this work, we define a
topological invariant for 3D Dirac semimetals by establishing a mapping between
a 3D Dirac semimetal and a topological crystalline insulator in four dimension
(4D). We demonstrate this scheme by constructing a tight-binding model for 4D
topological crystalline insulators that are protected by rotational symmetry. A
new type of topological invariant, "rotational Chern number", is shown to
characterize the topology of this system. As a consequence of the rotational
Chern number, gapless Dirac points are found on the 3D surface of this 4D
system. For a slab with two surfaces, we find that the corresponding low-energy
effective theory of two surface states can be directly mapped to that of a 3D
Dirac semimetal, suggesting that topological nature of 3D Dirac semimetals can
be characterized by rotational Chern number which is defined in 4D. Our scheme
provides a new systematic approach to extract topological nature for
topological semimetal phases.Comment: 12 pages, 3 figure
Piezoelectricity and Topological Quantum Phase Transitions in Two-Dimensional Spin-Orbit Coupled Crystals with Time-Reversal Symmetry
Finding new physical responses that signal topological quantum phase
transitions is of both theoretical and experimental importance. Here, we
demonstrate that the piezoelectric response can change discontinuously across a
topological quantum phase transition in two-dimensional time-reversal invariant
systems with spin-orbit coupling, thus serving as a direct probe of the
transition. We study all gap closing cases for all 7 plane groups that allow
non-vanishing piezoelectricity and find that any gap closing with 1 fine-tuning
parameter between two gapped states changes either the invariant or the
locally stable valley Chern number. The jump of the piezoelectric response is
found to exist for all these transitions, and we propose the HgTe/CdTe quantum
well and BaMnSb as two potential experimental platforms. Our work provides
a general theoretical framework to classify topological quantum phase
transitions and reveals their ubiquitous relation to the piezoelectric
response.Comment: Close to the published versio
Interacting topological phases in thin films of topological mirror Kondo insulators
We study the interaction effects on thin films of topological mirror Kondo
insulators (TMKI), where the strong interaction is expected to play an
important role. Our study has led to the following results: (1) We identify a
rich phase diagram of non-interacting TMKI with different mirror Chern numbers
in the monolayer and bilayer thin films; (2) We obtain the phase diagram with
interaction and identify the regimes of interaction parameters to mimic bosonic
symmetry protected topological phases with either gapless bosonic modes or
spontaneous mirror symmetry breaking at the boundary; (3) For the spontaneous
mirror symmetry breaking boundary, we also study various domain-wall defects
between different mirror symmetry breaking order parameters at the boundary.
Our results reveal that the thin film TMKI serves as an intriguing platform for
the experimental studies of interacting topological phases.Comment: 11 pages, 4 figure
Spin Susceptibility, Upper Critical Field and Disorder Effect in Superconductors with Singlet-Quintet Mixing
Recently, a new pairing state with the mixing between s-wave singlet channel
and isotropic d-wave quintet channel induced by centrosymmetric spin-orbit
coupling has been theoretically proposed in the superconducting materials with
electrons. In this work, we derive the expressions of the
zero-temperature spin susceptibility, the upper critical field close to the
zero-field critical temperature and the critical temperature with weak
random non-magnetic disorders for the singlet-quintet mixed state based on the
Luttinger model. Our study revealed the following features of the
singlet-quintet mixing. (1) The zero-temperature spin susceptibility remains
zero for the singlet-quintet mixed state if only the centrosymmetric spin-orbit
coupling is taken into account, and will deviate from zero when the
non-centrosymmetric spin-orbit coupling is introduced. (2) The singlet-quintet
mixing can help enhance the upper critical field roughly because it can
increase . (3) Although the quintet channel is generally suppressed by the
non-magnetic disorder scattering, we find the strong mixing between singlet and
quintet channels can help to stabilize the quintet channel. As a result, we
still find a sizable quintet component mixed into the singlet channel in the
presence of weak random non-magnetic disorders. Our work provides the guidance
for future experiments on spin susceptibility and upper critical field of the
singlet-quintet mixed superconducting states, and illustrates the stability of
the singlet-quintet mixing against the weak random non-magnetic disorder.Comment: 23 pages and 6 figure
Classification of topological crystalline insulators based on representation theory
Topological crystalline insulators define a new class of topological
insulator phases with gapless surface states protected by crystalline
symmetries. In this work, we present a general theory to classify topological
crystalline insulator phases based on the representation theory of space
groups. Our approach is to directly identify possible nontrivial surface states
in a semi-infinite system with a specific surface, of which the symmetry
property can be described by 17 two-dimensional space groups. We reproduce the
existing results of topological crystalline insulators, such as mirror Chern
insulators in the or groups, topological insulators in the
, and groups, and topological nonsymmorphic crystalline
insulators in the and groups. Aside from these existing results, we
also obtain the following new results: (1) there are two integer mirror Chern
numbers () in the group but only one () in the
or group for both the spinless and spinful cases; (2) for the
() groups, there is no topological classification in the spinless case but
() classifications in the spinful case; (3) we
show how topological crystalline insulator phase in the group is related
to that in the group; (4) we identify topological classification of the
, , and for the spinful case; (5) we find topological
non-symmorphic crystalline insulators also existing in and groups,
which exhibit new features compared to those in and groups. We
emphasize the importance of the irreducible representations for the states at
some specific high-symmetry momenta in the classification of topological
crystalline phases. Our theory can serve as a guide for the search of
topological crystalline insulator phases in realistic materials
Electrically tunable spin polarization of chiral edge modes in a quantum anomalous Hall insulator
In the quantum anomalous Hall effect, chiral edge modes are expected to
conduct spin polarized current without dissipation and thus hold great promise
for future electronics and spintronics with low energy consumption. However,
spin polarization of chiral edge modes has never been established in
experiments. In this work, we theoretically study spin polarization of chiral
edge modes in the quantum anomalous Hall effect, based on both the effective
model and more realistic tight-binding model constructed from the first
principles calculations. We find that spin polarization can be manipulated by
tuning either a local gate voltage or the Fermi energy. We also propose to
extract spin information of chiral edge modes by contacting the quantum
anomalous Hall insulator to a ferromagnetic (FM) lead. The establishment of
spin polarization of chiral edge modes, as well as the manipulation and
detection in a fully electrical manner, will pave the way to the applications
of the quantum anomalous Hall effect in spintronics.Comment: 12 pages, 10 figure
In-plane Magnetization Induced Quantum Anomalous Hall Effect
In a two-dimensional electron gas, the quantized Hall conductance can be
induced by a strong magnetic field, known as the quantum Hall effect, and it
can also result from the strong exchange coupling of magnetic ions, dubbed as
the "quantum anomalous Hall effect". The quantum Hall effect requires the
out-of-plane magnetic field, and similarly, it is commonly believed that the
magnetization should be out-of-plane for the quantum anomalous Hall effect. In
the present work, we find this condition is not necessary and predict that the
quantum anomalous Hall effect can also be induced by the purely in-plane
magnetization in two realistic systems, including BiTe thin film with
magnetic doping and HgMnTe quantum wells with shear strains, when all the
reflection symmetries are broken. An experimental setup is proposed to confirm
this effect, the observation of which will pave the way to search for the
quantum anomalous Hall effect in a wider range of materials.Comment: 7 pages, 3 figure
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