235 research outputs found
Controllable magnetic correlation between two impurities by spin-orbit coupling in graphene
Two magnetic impurities on the edge of a zigzag graphene nanoribbon strongly
interact with each other via indirect coupling, which can be mediated by
conducting carriers. By means of Quantum Monte Carlo (QMC) simulations, we find
that the spin-orbit coupling and the chemical potential in
system can be used to drive the transition of local-spin exchange from
ferromagnetism to anti-ferromagnetism. Since the tunable ranges for
and in graphene are experimentally reachable, we thus open the
possibilities for its device application. The symmetry in spatial distribution
is broken by the vertical and the transversal spin-spin correlations due to the
effect of spin-orbit coupling, leading to the spatial anisotropy of spin
exchange, which distinguish our findings from the case in normal Fermi liquid.Comment: 7 pages, 3 figures and 1 table. This paper has been accepted in
Scientific Report
Discovery of a two-dimensional topological insulator in SiTe
Two-dimensional (2D) topological insulators (TIs), a new state of quantum
matter, are promising for achieving the low-power-consuming electronic devices
owning to the remarkable robustness of their conducting edge states against
backscattering. Currently, the major challenge to further studies and possible
applications is the lack of suitable materials, which should be with high
feasibility of fabrication and sizeable nontrivial gaps. Here, we demonstrate
through first-principles calculations that SiTe 2D crystal is a promising 2D TI
with a sizeable nontrivial gap of 0.220 eV. This material is dynamically and
thermally stable. Most importantly, it could be easily exfoliated from its
three-dimensional superlattice due to the weakly bonded layered structure.
Moreover, strain engineering can effectively control its nontrivial gap and
even induce a topological phase transition. Our results provide a realistic
candidate for experimental explorations and potential applications of 2D TIs
Two-Dimensional Transition Metal Dichalcogenides with a Hexagonal Lattice: Room Temperature Quantum Spin Hall Insulators
So far, several transition metal dichalcogenides (TMDCs) based
two-dimensional (2D) topological insulators (TIs) have been discovered, all of
them based on a tetragonal lattice. However, in 2D crystals, the hexagonal
rather than the tetragonal symmetry is the most common motif. Here, based on
first-principles calculations, we propose a new class of stable 2D TMDCs of
composition MX2 (M=Mo, W, X=S, Se, Te) with a hexagonal lattice. They are all
in the same stability range as other 2D TMDC allotropes that have been
demonstrated experimentally, and they are identified to be practical 2D TIs
with large band gaps ranging from 41 to 198 meV, making them suitable for
applications at room-temperature. Besides, in contrast to tetragonal 2D TMDs,
their hexagonal lattice will greatly facilitate the integration of theses novel
TI states van-der-Waals crystals with other hexagonal or honeycomb materials,
and thus provide a route for 2D-material-based devices for wider nanoelectronic
and spintronic applications. The nontrivial band gaps of both WSe2 and WTe2 2D
crystals are 198 meV, which are larger than that in any previously reported
TMDC-based TIs. These large band gaps entirely stem from the strong spin-orbit
coupling strength within the d orbitals of Mo/W atoms near the Fermi level. Our
findings will significantly broaden the scientific and technological impact of
both 2D TIs and TMDCs
Multiferroic and Ferroic Topological Order in Ligand-Functionalized Germanene and Arsenene
Two-dimensional (2D) materials that exhibit ferroelectric, ferromagnetic, or topological order have been a major focal topic of nanomaterials research in recent years. The latest efforts in this field explore 2D quantum materials that host multiferroic or concurrent ferroic and topological order. We present a computational discovery of multiferroic state with coexisting ferroelectric and ferromagnetic order in recently synthesized CH2OCH3-functionalized germanene. We show that an electric-field-induced rotation of the ligand CH2OCH3 molecule can serve as the driving mechanism to switch the electric polarization of the ligand molecule, while unpassivated Ge p(z) orbits generate ferromagnetism. Our study also reveals coexisting ferroelectric and topological order in ligand-functionalized arsenene, which possesses a switchable electric polarization and a Dirac transport channel. These findings offer insights into the fundamental physics underlying these coexisting quantum orders and open avenues for achieving states of matter with multiferroic or ferroic-topological order in 2D-layered materials for innovative memory or logic device implementations
Two-Dimensional Ferroelastic Topological Insulators in Single-Layer Janus Transition Metal Dichalcogenides MSSe (M=Mo, W)
Two-dimensional topological insulators and two-dimensional materials with
ferroelastic characteristics are intriguing materials and many examples have
been reported both experimentally and theoretically. Here, we present the
combination of both features - a two-dimensional ferroelastic topological
insulator that simultaneously possesses ferroelastic and quantum spin Hall
characteristics. Using first-principles calculations, we demonstrate Janus
single-layer MSSe (M=Mo, W) stable two-dimensional crystals that show the
long-sought ferroelastic topological insulator properties. The material
features low switching barriers and strong ferroelastic signals, beneficial for
applications in nonvolatile memory devices. Moreover, their topological phases
harbor sizeable nontrivial band gaps, which supports the quantum spin Hall
effect. The unique coexistence of excellent ferroelastic and quantum spin Hall
phases in single-layer MSSe provides extraordinary platforms for realizing
multi-purpose and controllable devices
Proximity Enhanced Quantum Spin Hall State in Graphene
Graphene is the first model system of two-dimensional topological insulator
(TI), also known as quantum spin Hall (QSH) insulator. The QSH effect in
graphene, however, has eluded direct experimental detection because of its
extremely small energy gap due to the weak spin-orbit coupling. Here we predict
by ab initio calculations a giant (three orders of magnitude) proximity induced
enhancement of the TI energy gap in the graphene layer that is sandwiched
between thin slabs of Sb2Te3 (or MoTe2). This gap (1.5 meV) is accessible by
existing experimental techniques, and it can be further enhanced by tuning the
interlayer distance via compression. We reveal by a tight-binding study that
the QSH state in graphene is driven by the Kane-Mele interaction in competition
with Kekul\'e deformation and symmetry breaking. The present work identifies a
new family of graphene-based TIs with an observable and controllable bulk
energy gap in the graphene layer, thus opening a new avenue for direct
verification and exploration of the long-sought QSH effect in graphene.Comment: 4 figures in Carbon, 201
Anisotropic Ripple Deformation in Phosphorene
Two-dimensional materials tend to become crumpled according to the
Mermin-Wagner theorem, and the resulting ripple deformation may significantly
influence electronic properties as observed in graphene and MoS2. Here we
unveil by first-principles calculations a new, highly anisotropic ripple
pattern in phosphorene, a monolayer black phosphorus, where compression induced
ripple deformation occurs only along the zigzag direction in the strain range
up to 10%, but not the armchair direction. This direction-selective ripple
deformation mode in phosphorene stems from its puckered structure with coupled
hinge-like bonding configurations and the resulting anisotropic Poisson ratio.
We also construct an analytical model using classical elasticity theory for
ripple deformation in phosphorene under arbitrary strain. The present results
offer new insights into the mechanisms governing the structural and electronic
properties of phosphorene crucial to its device applications.Comment: J. Phys. Chem. Lett. 201
Opening Band Gap without Breaking Lattice Symmetry: A New Route toward Robust Graphene-Based Nanoelectronics
Developing graphene-based nanoelectronics hinges on opening a band gap in the
electronic structure of graphene, which is commonly achieved by breaking the
inversion symmetry of the graphene lattice via an electric field (gate bias) or
asymmetric doping of graphene layers. Here we introduce a new design strategy
that places a bilayer graphene sheet sandwiched between two cladding layers of
materials that possess strong spin-orbit coupling (e.g., Bi2Te3). Our ab initio
and tight-binding calculations show that proximity enhanced spin-orbit coupling
effect opens a large (44 meV) band gap in bilayer graphene without breaking its
lattice symmetry, and the band gap can be effectively tuned by interlayer
stacking pattern and significantly enhanced by interlayer compression. The
feasibility of this quantum-well structure is demonstrated by recent
experimental realization of high-quality heterojunctions between graphene and
Bi2Te3, and this design also conforms to existing fabrication techniques in the
semiconductor industry. The proposed quantum-well structure is expected to be
especially robust since it does not require an external power supply to open
and maintain a band gap, and the cladding layers provide protection against
environmental degradation of the graphene layer in its device applications
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