903 research outputs found
Ideal Strength of Doped Graphene
While the mechanical distortions change the electronic properties of graphene
significantly, the effects of electronic manipulation on its mechanical
properties have not been known. Using first-principles calculation methods, we
show that, when graphene expands isotropically under equibiaxial strain, both
the electron and hole doping can maintain or improve its ideal strength
slightly and enhance the critical breaking strain dramatically. Contrary to the
isotropic expansions, the electron doping decreases the ideal strength as well
as critical strain of uniaxially strained graphene while the hole doping
increases the both. Distinct failure mechanisms depending on type of strains
are shown to be origins of the different doping induced mechanical stabilities.
Our findings may resolve a contradiction between recent experimental and
theoretical results on the strength of graphene.Comment: published version; two more references are adde
Moir\'{e} phonons in the twisted bilayer graphene
We study the in-plane acoustic phonons in twisted bilayer graphenes using the
effective continuum approach. We calculate the phonon modes by solving the
continuum equation of motion for infinitesimal vibration around the static
relaxed state with triangular domain structure. We find that the moir\'{e}
interlayer potential only affects the in-plane asymmetric modes, where the
original linear dispersion is broken down into miniphonon bands separated by
gaps, while the in-plane symmetric modes with their linear dispersion are
hardly affected. The phonon wave functions of asymmetric modes are regarded as
collective vibrations of the domain-wall network, and the low-energy phonon
band structure can be qualitatively described by an effective moir\'{e}-scale
lattice model.Comment: 10 pages, 7 figure
A new family of two-dimensional crystals: open-framework TX (T=C, Si, Ge, Sn; X=O, S, Se, Te) compounds with tetrahedral bonding
To accelerate development of innovative materials, their modelings and
predictions with useful functionalities are of vital importance. Here, based on
a recently developed crystal structure prediction method, we find a new family
of stable two-dimensional crystals with an open-channel tetrahedral bonding
network, rendering a potential for electronic and energy applications. The
proposed structural prototype with a space group of Cmme hosts at least
thirteen different freestanding TX compounds with group IV (T=C, Si, Ge,
Sn) and VI (X=O, S, Se, Te) elements. Moreover, the proposed materials display
diverse electronic properties ranging from direct band gap semiconductor to
topological insulator at their pristine forms, which are further tunable by
mechanical strain
Quasiparticle energy bands and Fermi surfaces of monolayer NbSe
A quasiparticle band structure of a single layer 2H-NbSe is reported by
using first-principles calculation. We show that a self-energy correction
increases the width of a partially occupied band and alters its Fermi surface
shape when comparing those using conventional mean-field calculation methods.
Owing to a broken inversion symmetry in the trigonal prismatic single layer
structure, the spin-orbit interaction is included and its impact on the Fermi
surface and quasiparticle energy bands are discussed. We also calculate the
doping dependent static susceptibilities from the band structures obtained by
the mean-field calculation as well as calculation with and without
spin-orbit interactions. A complete tight-binding model is constructed within
the three-band third nearest neighbour hoppings and is shown to reproduce our
quasiparticle energy bands and Fermi surface very well. Considering
variations of the Fermi surface shapes depending on self-energy corrections and
spin-orbit interactions, we discuss the formations of charge density wave (CDW)
with different dielectric environments and their implications on recent
controversial experimental results on CDW transition temperatures.Comment: 13 pages, 14 figure
Scattering Theory Approach to Inelastic Transport in Nanoscale Systems
We present a scattering-state description for the non-equilibrium
multichannel charge transport in the presence of electron-vibration couplings.
It is based on an expansion of scattering orders of eigenchannel states.
Examining charge transitions between scattering states, we clarifies competing
inelastic and elastic scattering processes, and compare with the interpretation
based on the non-equilibrium Green's functions formalism. We also derive a
general expression for conductance variations in single-channel systems. It
provides a comprehensive picture for the variation including the well-known
result, the 0.5 rule, from the aspect of interplay between elastic and
inelastic scattering processes.Comment: 8 pages, 5 figure
Poisson's Ratio of Layered Two-dimensional Crystals
We present first-principles calculations of elastic properties of
multilayered two-dimensional crystals such as graphene, h-BN and 2H-MoS2 which
shows that their Poisson's ratios along out-of-plane direction are negative,
near zero and positive, respectively, spanning all possibilities for sign of
the ratios. While the in-plane Poisson's ratios are all positive regardless of
their disparate electronic and structural properties, the characteristic
interlayer interactions as well as layer stacking structures are shown to
determine the sign of their out-of-plane ratios. Thorough investigation of
elastic properties as a function of the number of layers for each system is
also provided, highlighting their intertwined nature between elastic and
electronic properties.Comment: 11 pages, 3 figure
Atomically flat two-dimensional silicon crystals with versatile electronic properties
Silicon (Si) is one of the most extensively studied materials owing to its
significance to semiconductor science and technology. While efforts to find a
new three-dimensional (3D) Si crystal with unusual properties have made some
progress, its two-dimensional (2D) phases have not yet been explored as much.
Here, based on a newly developed systematic materials searching
strategy, we report a series of novel 2D Si crystals with unprecedented
structural and electronic properties. The new structures exhibit perfectly
planar outermost surface layers of a distorted hexagonal network with their
thicknesses varying with the atomic arrangement inside. Dramatic changes in
electronic properties ranging from semimetal to semiconducting with indirect
energy gaps and even to one with direct energy gaps are realized by varying
thickness as well as by surface oxidation. Our predicted 2D Si crystals with
flat surfaces and tunable electronic properties will shed light on the
development of silicon-based 2D electronics technology
Reentrant Quantum Spin Hall States in Charge Density Wave Phase of Doped Single-Layer Transition Metal Dichalcogenides
Using first-principles calculation methods, we reveal a series of phase
transitions as a function of electron doping in single-layer 1T-MoTe and
1T-WTe exhibiting quantum spin Hall (QSH) edge states without doping. As
increasing doping, we show that a phonon mediated superconducting phase first
realizes and is followed by a charge density wave (CDW) phase with a
nonsymmorphic lattice symmetry. The newly found CDW phase exhibits Dirac or
Weyl energy bands with a spin-orbit coupling in case of a fractional band
filling and re-enters into topological insulating phase with fully filled
bands. The robust resurgence of QSH state coexisting with the CDW phase is
shown to originate from band inversions induced by the nonsymmorphic lattice
distortion through the strong electron-phonon interaction, thus suggesting a
realization of various interfacial states between superconducting, density wave
and topological states on a two-dimensional crystal only by doping.Comment: 14 pages, 12 figure
Optical absorption of twisted bilayer graphene with interlayer potential asymmetry
We investigate the band structure and the optical absorption spectrum of
twisted bilayer graphenes with changing interlayer bias and Fermi energy
simultaneously. We show that the interlayer bias lifts the degeneracy of the
superlattice Dirac point, while the amount of the Dirac point shift is
significantly suppressed in small rotation angles, and even becomes opposite to
the applied bias. We calculate the optical absorption spectrum in various
asymmetric potentials and Fermi energies, and associate the characteristic
spectral features with the band structure. The spectroscopic features are
highly sensitive to the interlayer bias and the Fermi energy, and widely
tunable by the external field effect.Comment: 11 pages, 7 figure
Dynamic Jahn-Teller Mechanism of Superconductivity in MgB
We propose a novel mechanism of superconductivity in MgB based on the
dynamic electronic structure of the -orbitals coupled with
phonons. A nonconventional superconducting state is found to arise frome
lectron-phonon interactions in the presence of additional pairing channels made
available by the dynamic Jahn-Teller effects. A partially broken pseudo-spin
symmetry in this Jahn-Teller system, together with two-phonon exchange pairing,
naturally gives rise to two distinct gaps both of which are basically isotropic
in the (, ) space. Important experimental observations including high
and the anomalous specific heat are explained using this theory
- β¦