86 research outputs found
Evolution of Raman G and G'(2D) Modes in Folded Graphene Layers
Bernal- and non-Bernal-stacked graphene layers have been systematically
studied by Raman imaging and spectroscopy. Two dominant Raman modes, G and G'
(or 2D) of folded graphene layers exhibit three types of spectral features when
interlayer lattice mismatches, defined by a rotational angle varies. Among
these folded graphene layers, the most interesting one is the folded graphene
layers that present an extremely strong G mode enhanced by a twist-induced Van
Hove singularity. The evolution of Raman G and G' modes of such folded graphene
layers are probed by changing the excitation photon energies. For the first
time, doublet splitting of the G' mode in folded double-layer (1 + 1) and of
the G mode in folded tetra-layer (2 + 2) graphene are clearly observed and
discussed. The G' mode splitting in folded double-layer graphene is attributed
to the coexistence of inner and outer scattering processes and the trigonal
warping effect as well as further downwards bending of the inner dispersion
branch at visible excitation energy. While the two peaks of the G mode in
folded tetra-layer graphene are assigned to Raman-active mode (E2g) and lattice
mismatch activated infrared-active mode (E1u), which is further verified by the
temperature-dependent Raman measurements. Our study provides a summary and
thorough understanding of Raman spectra of Bernal- and non-Bernal-stacked
graphene layers and further demonstrates the versatility of Raman spectroscopy
for exploiting electronic band structures of graphene layers.Comment: 29 pages, 10 figure
Stacking sequence determines Raman intensities of observed interlayer shear modes in 2D layered materials - A general bond polarizability model
2D layered materials have recently attracted tremendous interest due to their
fascinating properties and potential applications. The interlayer interactions
are much weaker than the intralayer bonds, allowing the as-synthesized
materials to exhibit different stacking sequences (e.g. ABAB, ABCABC), leading
to different physical properties. Here, we show that regardless of the space
group of the 2D material, the Raman frequencies of the interlayer shear modes
observed under the typical configuration blue shift for AB stacked materials,
and red shift for ABC stacked materials, as the number of layers increases. Our
predictions are made using an intuitive bond polarizability model which shows
that stacking sequence plays a key role in determining which interlayer shear
modes lead to the largest change in polarizability (Raman intensity); the modes
with the largest Raman intensity determining the frequency trends. We present
direct evidence for these conclusions by studying the Raman modes in few layer
graphene, MoS2, MoSe2, WSe2 and Bi2Se3, using both first principles
calculations and Raman spectroscopy. This study sheds light on the influence of
stacking sequence on the Raman intensities of intrinsic interlayer modes in 2D
layered materials in general, and leads to a practical way of identifying the
stacking sequence in these materials.Comment: 30 pages, 8 figure
Magnetic Oscillation of Optical Phonon in ABA- and ABC-Stacked Trilayer Graphene
We present a comparative measurement of the G-peak oscillations of phonon
frequency, Raman intensity and linewidth in the Magneto-Raman scattering of
optical E2g phonons in mechanically exfoliated ABA- and ABC-stacked trilayer
graphene (TLG). Whereas in ABA-stacked TLG, we observe magnetophonon
oscillations consistent with single-bilayer chiral band doublets, the features
are flat for ABC-stacked TLG up to magnetic fields of 9 T. This suppression can
be attributed to the enhancement of band chirality that compactifies the
spectrum of Landau levels and modifies the magnetophonon resonance properties.
The drastically different coupling behaviour between the electronic excitations
and the E2g phonons in ABA- and ABC-stacked TLG reflects their different
electronic band structures and the electronic Landau level transitions and thus
can be another way to determine the stacking orders and to probe the
stacking-order-dependent electronic structures. In addition, the sensitivity of
the magneto-Raman scattering to the particular stacking order in few layers
graphene highlights the important role of interlayer coupling in modifying the
optical response properties in van der Waals layered materials.Comment: 25 pages, 6 figure
On the Stochastic Gradient Descent and Inverse Variance-flatness Relation in Artificial Neural Networks
Stochastic gradient descent (SGD), a widely used algorithm in deep-learning
neural networks has attracted continuing studies for the theoretical principles
behind its success. A recent work uncovered a generic inverse variance-flatness
(IVF) relation between the variance of neural weights and the landscape
flatness of loss function near solutions under SGD [Feng & Tu, PNAS 118,0027
(2021)]. To investigate this seemly violation of statistical principle, we
deploy a stochastic decomposition to analyze the dynamical properties of SGD.
The method constructs the true "energy" function which can be used by Boltzmann
distribution. The new energy differs from the usual cost function and explains
the IVF relation under SGD. We further verify the scaling relation identified
in Feng's work. Our approach may bridge the gap between the classical
statistical mechanics and the emerging discipline of artificial intelligence,
with potential for better algorithm to the latter
Topology, Vorticity and Limit Cycle in a Stabilized Kuramoto-Sivashinsky Equation
A noisy stabilized Kuramoto-Sivashinsky equation is analyzed by stochastic
decomposition. For values of control parameter for which periodic stationary
patterns exist, the dynamics can be decomposed into diffusive and transverse
parts which act on a stochastic potential. The relative positions of stationary
states in the stochastic global potential landscape can be obtained from the
topology spanned by the low-lying eigenmodes which inter-connect them.
Numerical simulations confirm the predicted landscape. The transverse component
also predicts a universal class of vortex like circulations around fixed
points. These drive nonlinear drifting and limit cycle motion of the underlying
periodic structure in certain regions of parameter space. Our findings might be
relevant in studies of other nonlinear systems such as deep learning neural
networks.Comment: Main body: 16 pages, 3 figures; Supplementary: 14 pages, 6 figure
Stacking Dependent Optical Conductivity of Bilayer Graphene
The optical conductivities of graphene layers are strongly dependent on their
stacking orders. Our first-principle calculations show that while the optical
conductivities of single layer graphene (SLG) and bilayer graphene (BLG) with
Bernal stacking are almost frequency independent in the visible region, the
optical conductivity of twisted bilayer graphene (TBG) is frequency dependent,
giving rise to additional absorption features due to the band folding effect.
Experimentally, we obtain from contrast spectra the optical conductivity
profiles of BLG with different stacking geometries. Some TBG samples show
additional features in their conductivity spectra in full agreement with our
calculation results, while a few samples give universal conductivity values
similar to that of SLG. We propose those variations of optical conductivity
spectra of TBG samples originate from the difference between the commensurate
and incommensurate stackings. Our results reveal that the optical conductivity
measurements of graphene layers indeed provide an efficient way to select
graphene films with desirable electronic and optical properties, which would
great help the future application of those large scale misoriented graphene
films in photonic devices.Comment: 20 pages, 5 figures, accepted by ACS Nan
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