65 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
Electrically Controlled Reversible Strain Modulation in MoS Field-effect Transistors via an Electro-mechanically Coupled Piezoelectric Thin Film
Strain can efficiently modulate the bandgap and carrier mobilities in
two-dimensional (2D) materials. Conventional mechanical strain-application
methodologies that rely on flexible, patterned or nano-indented substrates are
severely limited by low thermal tolerance, lack of tunability and/or poor
scalability. Here, we leverage the converse piezoelectric effect to
electrically generate and control strain transfer from a piezoelectric thin
film to electro-mechanically coupled ultra-thin 2D MoS. Electrical bias
polarity change across the piezoelectric film tunes the nature of strain
transferred to MoS from compressive 0.23% to tensile 0.14% as
verified through peak shifts in Raman and photoluminescence spectroscopies and
substantiated by density functional theory calculations. The device
architecture, built on a silicon substrate, uniquely integrates an MoS
field-effect transistor on top of a metal-piezoelectric-metal stack enabling
strain modulation of transistor drain current 130, on/off current ratio
150, and mobility 1.19 with high precision, reversibility and
resolution. Large, tunable tensile (1056) and compressive (-1498) strain gauge
factors, easy electrical strain modulation, high thermal tolerance and
substrate compatibility make this technique promising for integration with
silicon-based CMOS and micro-electro-mechanical systems.Comment: Manuscript and Supplementary Informatio
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.
Crossover from 2D ferromagnetic insulator to wide bandgap quantum anomalous Hall insulator in ultra-thin MnBi2Te4
Intrinsic magnetic topological insulators offer low disorder and large
magnetic bandgaps for robust magnetic topological phases operating at higher
temperatures. By controlling the layer thickness, emergent phenomena such as
the Quantum Anomalous Hall (QAH) effect and axion insulator phases have been
realised. These observations occur at temperatures significantly lower than the
Neel temperature of bulk MnBi2Te4, and measurement of the magnetic energy gap
at the Dirac point in ultra-thin MnBi2Te4 has yet to be achieved. Critical to
achieving the promise of this system is a direct measurement of the
layer-dependent energy gap and verifying whether the gap is magnetic in the QAH
phase. Here we utilise temperature dependent angle-resolved photoemission
spectroscopy to study epitaxial ultra-thin MnBi2Te4. We directly observe a
layer dependent crossover from a 2D ferromagnetic insulator with a bandgap
greater than 780 meV in one septuple layer (1 SL) to a QAH insulator with a
large energy gap (>100 meV) at 8 K in 3 and 5 SL MnBi2Te4. The QAH gap is
confirmed to be magnetic in origin, as it abruptly diminishes with increasing
temperature above 8 K. The direct observation of a large magnetic energy gap in
the QAH phase of few-SL MnBi2Te4 is promising for further increasing the
operating temperature of QAH materials
Designing optoelectronic properties by on-surface synthesis: formation and electronic structure of an iron-terpyridine macromolecular complex
Supramolecular chemistry protocols applied on surfaces offer compelling
avenues for atomic scale control over organic-inorganic interface structures.
In this approach, adsorbate-surface interactions and two-dimensional
confinement can lead to morphologies and properties that differ dramatically
from those achieved via conventional synthetic approaches. Here, we describe
the bottom-up, on-surface synthesis of one-dimensional coordination
nanostructures based on an iron (Fe)-terpyridine (tpy) interaction borrowed
from functional metal-organic complexes used in photovoltaic and catalytic
applications. Thermally activated diffusion of sequentially deposited ligands
and metal atoms, and intra-ligand conformational changes, lead to Fe-tpy
coordination and formation of these nanochains. Low-temperature Scanning
Tunneling Microscopy and Density Functional Theory were used to elucidate the
atomic-scale morphology of the system, providing evidence of a linear tri-Fe
linkage between facing, coplanar tpy groups. Scanning Tunneling Spectroscopy
reveals highest occupied orbitals with dominant contributions from states
located at the Fe node, and ligand states that mostly contribute to the lowest
unoccupied orbitals. This electronic structure yields potential for hosting
photo-induced metal-to-ligand charge transfer in the visible/near-infrared. The
formation of this unusual tpy/tri-Fe/tpy coordination motif has not been
observed for wet chemistry synthesis methods, and is mediated by the bottom-up
on-surface approach used here
Electronic bandstructure of in-plane ferroelectric van der Waals
Layered indium selenides () have recently been discovered to
host robust out-of-plane and in-plane ferroelectricity in the and
' phases, respectively. In this work, we utilise angle-resolved
photoelectron spectroscopy to directly measure the electronic bandstructure of
, and compare to hybrid density functional theory (DFT)
calculations. In agreement with DFT, we find the band structure is highly
two-dimensional, with negligible dispersion along the c-axis. Due to n-type
doping we are able to observe the conduction band minima, and directly measure
the minimum indirect (0.97 eV) and direct (1.46 eV) bandgaps. We find the Fermi
surface in the conduction band is characterized by anisotropic electron pockets
with sharp in-plane dispersion about the points, yielding
effective masses of 0.21 along and 0.33 along
. The measured band structure is well supported by hybrid
density functional theory calculations. The highly two-dimensional (2D)
bandstructure with moderate bandgap and small effective mass suggest that
is a potentially useful new van der Waals semiconductor.
This together with its ferroelectricity makes it a viable material for
high-mobility ferroelectric-photovoltaic devices, with applications in
non-volatile memory switching and renewable energy technologies.Comment: 19 pages, 4 + 1 figures; typos corrected;added references; updated
figures & discussion to reflect changes in mode
Quasi-free-standing AA-stacked bilayer graphene induced by calcium intercalation of the graphene-silicon carbide interface
We study quasi-freestanding bilayer graphene on silicon carbide intercalated
by calcium. The intercalation, and subsequent changes to the system, were
investigated by low-energy electron diffraction, angle-resolved photoemission
spectroscopy (ARPES) and density-functional theory (DFT). Calcium is found to
intercalate only at the graphene-SiC interface, completely displacing the
hydrogen terminating SiC. As a consequence, the system becomes highly n-doped.
Comparison to DFT calculations shows that the band dispersion, as determined by
ARPES, deviates from the band structure expected for Bernal-stacked bilayer
graphene. Instead, the electronic structure closely matches AA-stacked bilayer
graphene on Ca-terminated SiC, indicating a spontaneous transition from AB- to
AA-stacked bilayer graphene following calcium intercalation of the underlying
graphene-SiC interface.Comment: 14 pages, 3 figure
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