48 research outputs found
Tunable Hybridization Between Electronic States of Graphene and Physisorbed Hexacene
Non-covalent functionalization via physisorption of organic molecules
provides a scalable approach for modifying the electronic structure of graphene
while preserving its excellent carrier mobilities. Here we investigated the
physisorption of long-chain acenes, namely, hexacene and its fluorinated
derivative perfluorohexacene, on bilayer graphene for tunable graphene devices
using first principles methods. We find that the adsorption of these molecules
leads to the formation of localized states in the electronic structure of
graphene close to its Fermi level, which could be readily tuned by an external
electric field. The electric field not only creates a variable band gap as
large as 250 meV in bilayer graphene, but also strongly influences the charge
redistribution within the molecule-graphene system. This charge redistribution
is found to be weak enough not to induce strong surface doping, but strong
enough to help preserve the electronic states near the Dirac point of graphene.Comment: 17 pages, 7 figures, supporting informatio
Selective Control of Surface Spin Current in Topological Materials based on Pyrite-type OsX2 (X = Se, Te) Crystals
Topological materials host robust surface states, which could form the basis
for future electronic devices. As such states have spins that are locked to the
momentum, they are of particular interest for spintronic applications.
Understanding spin textures of the surface states of topologically nontrivial
materials, and being able to manipulate their polarization, is therefore
essential if they are to be utilized in future technologies. Here we use
first-principles calculations to show that pyrite-type crystals OsX2 (X= Se,
Te) are a class of topological material that can host surface states with spin
polarization that can be either in-plane or out-of-plane. We show that the
formation of low-energy states with symmetry-protected energy- and
direction-dependent spin textures on the (001) surface of these materials is a
consequence of a transformation from a topologically trivial to nontrivial
state, induced by spin orbit interactions. The unconventional spin textures of
these surface states feature an in-plane to out-of-plane spin polarization
transition in the momentum space protected by local symmetries. Moreover, the
surface spin direction and magnitude can be selectively filtered in specific
energy ranges. Our demonstration of a new class of topological material with
controllable spin textures provide a platform for experimentalists to detect
and exploit unconventional surface spin textures in future spin-based
nanoelectronic devices
Extracting unconventional spin texture in two dimensional topological crystalline insulators via tuning bulk-edge interactions
Tuning the interaction between the bulk and edge states of topological
materials is a powerful tool for manipulating edge transport behavior, opening
up exciting opportunities for novel electronic and spintronic applications.
This approach is particularly suited to topological crystalline insulators
(TCI), a class of topologically nontrivial compounds that are endowed with
multiple degrees of topological protection. In this study, we investigate how
bulk-edge interactions can influence the edge transport in planar bismuthene, a
TCI with metallic edge states protected by in-plane mirror symmetry, using
first principles calculations and symmetrized Wannier tight-binding models. By
exploring the impact of various perturbation effects, such as device size,
substrate potentials, and applied transverse electric field, we examine the
evolution of the electronic structure and edge transport in planar bismuthene.
Our findings demonstrate that the TCI states of planar bismuthene can be
engineered to exhibit either a gapped or conducting unconventional helical spin
texture via a combination of substrate and electric field effects. Furthermore,
under strong electric fields, the edge states can be stabilized through a
delicate control of the bulk-edge interactions. These results open up new
directions for discovering novel spin transport patterns in topological
materials and provide critical insights for the fabrication of topological
spintronic devices.Comment: 23 pages, 8 figure
Near-direct bandgap / type-II pn heterojunction for enhanced ultrafast photodetection and high-performance photovoltaics
PN heterojunctions comprising layered van der Waals (vdW) semiconductors have
been used to demonstrate current rectifiers, photodetectors, and photovoltaic
devices. However, a direct or near-direct bandgap at the heterointerface that
can significantly enhance optical generation, for high light absorbing
few/multi-layer vdW materials, has not yet been shown. In this work, for the
first time, few-layer group-6 transition metal dichalcogenide (TMD) is
shown to form a sizeable (0.7 eV) near-direct bandgap with type-II band
alignment at its interface with the group-7 TMD through density
functional theory calculations. Further, the type-II alignment and
photogeneration across the interlayer bandgap have been experimentally
confirmed through micro-photoluminescence and IR photodetection measurements,
respectively. High optical absorption in few-layer flakes, large conduction and
valence band offsets for efficient electron-hole separation and stacking of
light facing, direct bandgap on top of gate tunable are shown
to result in excellent and tunable photodetection as well as photovoltaic
performance through flake thickness dependent optoelectronic measurements.
Few-layer flakes demonstrate ultrafast response time (5 s) at high
responsivity (3 A/W) and large photocurrent generation and responsivity
enhancement at the heterostructure overlap region (10-100X) for 532 nm laser
illumination. Large open-circuit voltage of 0.64 V and short-circuit current of
2.6 A enables high output electrical power. Finally, long term
air-stability and a facile single contact metal fabrication process makes the
multi-functional few-layer / heterostructure diode
technologically promising for next-generation optoelectronic applications.Comment: Manuscript- 27 pages, 8 figures. Supporting Information- 17 pages, 17
figure
Gate control of Mott metal-insulator transition in a 2D metal-organic framework
Strong electron-electron Coulomb interactions in materials can lead to a vast
range of exotic many-body quantum phenomena, including Mott metal-insulator
transitions, magnetic order, quantum spin liquids, and unconventional
superconductivity. These many-body phases are strongly dependent on band
occupation and can hence be controlled via the chemical potential. Flat
electronic bands in two-dimensional (2D) and layered materials such as the
kagome lattice, enhance strong electronic correlations. Although theoretically
predicted, correlated-electron phases in monolayer 2D metal-organic frameworks
(MOFs) - which benefit from efficient synthesis protocols and tunable
properties - with a kagome structure have not yet been realised experimentally.
Here, we synthesise a 2D kagome MOF comprised of 9,10-dicyanoanthracene
molecules and copper atoms on an atomically thin insulator, monolayer hexagonal
boron nitride (hBN) on Cu(111). Scanning tunnelling microscopy (STM) and
spectroscopy reveal an electronic energy gap of ~200 meV in this MOF,
consistent with dynamical mean-field theory predictions of a Mott insulating
phase. By tuning the electron population of kagome bands, via either
template-induced (via local work function variations of the hBN/Cu(111)
substrate) or tip-induced (via the STM probe) gating, we are able to induce
Mott metal-insulator transitions in the MOF. These findings pave the way for
devices and technologies based on 2D MOFs and on electrostatic control of
many-body quantum phases therein.Comment: 19 pages, 4 figure
Gigantic Anisotropy of Self-Induced Spin-Orbit Torque in Weyl Ferromagnet Co2MnGa
Spin-orbit torque (SOT) is receiving tremendous attention from both
fundamental and application-oriented aspects. Co2MnGa, a Weyl ferromagnet that
is in a class of topological quantum materials, possesses cubic-based high
structural symmetry, the L21 crystal ordering, which should be incapable of
hosting anisotropic SOT in conventional understanding. Here we show the
discovery of a gigantic anisotropy of self-induced SOT in Co2MnGa. The
magnitude of the SOT is comparable to that of heavy metal/ferromagnet bilayer
systems despite the high inversion symmetry of the Co2MnGa structure. More
surprisingly, a sign inversion of the self-induced SOT is observed for
different crystal axes. This finding stems from the interplay of the
topological nature of the electronic states and their strong modulation by
external strain. Our research enriches the understanding of the physics of
self-induced SOT and demonstrates a versatile method for tuning SOT
efficiencies in a wide range of materials for topological and spintronic
devices.Comment: 15pages, 4figures (To appear Nano Lett.