304 research outputs found
Biaxial strain enhanced piezoelectric properties in monolayer g-
Graphite-like carbon nitride (g-) is considered as a
promising candidate for energy materials. In this work, the biaxial strain
(-4\%-4\%) effects on piezoelectric properties of g- monolayer
are studied by density functional theory (DFT). It is found that the increasing
strain can reduce the elastic coefficient -, and increases
piezoelectric stress coefficient , which lead to the enhanced
piezoelectric strain coefficient . Compared to unstrained one, strain
of 4\% can raise the by about 330\%. From -4\% to 4\%, strain can
induce the improved ionic contribution to of g-, and
almost unchanged electronic contribution, which is different from
monolayer (the enhanced electronic contribution and reduced
ionic contribution). To prohibit current leakage, a piezoelectric material
should be a semiconductor, and g- monolayer is always a
semiconductor in considered strain range. Calculated results show that the gap
increases from compressive strain to tensile one. At 4\% strain, the first and
second valence bands cross, which has important effect on transition dipole
moment (TDM). Our works provide a strategy to achieve enhanced piezoelectric
effect of g- monolayer, which gives a useful guidence for
developing efficient energy conversion devices.Comment: 6 pages, 4 figure
Predicted septuple-atomic-layer Janus (M=Mo and W) monolayers with Rashba spin splitting and high electron carrier mobilities
Janus two-dimensional (2D) materials have attracted much attention due to
possessing unique properties caused by their out-of-plane asymmetry, which have
been achieved in many 2D families. In this work, the Janus monolayers are
predicted in new 2D family by means of first-principles
calculations, and of which have been
synthesized in experiment(\textcolor[rgb]{0.00,0.00,1.00}{Science 369, 670-674
(2020)}). The predicted (M=Mo and W) monolayers exhibit
dynamic, thermodynamical and mechanical stability, and they are indirect
band-gap semiconductors. The inclusion of spin-orbit coupling (SOC) gives rise
to the Rashba-type spin splitting, which is observed in the valence bands,
being different from common conduction bands. Calculated results show valley
polarization at the edge of the conduction bands due to SOC together with
inversion symmetry breaking. It is found that (M=Mo and W)
monolayers have high electron mobilities. Both in-plane and much weak
out-of-plane piezoelectric polarizations can be observed, when a uniaxial
strain in the basal plane is applied. The values of piezoelectric strain
coefficient of the Janus (M=Mo and W) monolayers
fall between those of the (M=Mo and W) and
(M=Mo and W) monolayers, as expected. It is proved that
strain can tune the positions of valence band maximum (VBM) and conduction band
minimum (CBM), and enhance the the strength of conduction bands convergence
caused by compressive strain. It is also found that tensile biaxial strain can
enhance of (M=Mo and W) monolayers, and the
compressive strain can improve the (absolute values).Comment: 10 pages, 11 figure
Intrinsic piezoelectric ferromagnetism with large out-of-plane piezoelectric response in Janus monolayer
A two-dimensional (2D) material system with both piezoelectricity and
ferromagnetic (FM) order, referred to as a 2D piezoelectric ferromagnetism
(PFM), may open up unprecedented opportunities for intriguing physics. Inspired
by experimentally synthesized Janus monolayer MoSSe from , in
this work, the Janus monolayer with dynamic,
mechanical and thermal stabilities is predicted, which is constructed from
synthesized ferromagnetic monolayer by replacing the top I
atomic layer with Br atoms. Calculated results show that monolayer
is an intrinsic FM half semiconductor with valence
and conduction bands being fully spin-polarized in the same spin direction.
Furthermore, monolayer possesses a sizable
magnetic anisotropy energy (MAE). By symmetry analysis, it is found that both
in-plane and out-of-plane piezoelectric polarizations can be induced by a
uniaxial strain in the basal plane. The calculated in-plane value of
0.557 pm/V is small. However, more excitingly, the out-of-plane is as
high as 1.138 pm/V, which is obviously higher compared with ones of other 2D
known materials. The strong out of-plane piezoelectricity is highly desirable
for ultrathin piezoelectric devices. Moreover, strain engineering is used to
tune piezoelectricity of monolayer . It is found
that compressive strain can improve the , and tensile strain can
enhance the . A FM order to antiferromagnetic (AFM) order phase
transition can be induced by compressive strain, and the critical point is
about 0.95 strain. That is to say that a 2D piezoelectric antiferromagnetism
(PAFM) can be achieved by compressive strain, and the corresponding
and are 0.677 pm/V and 0.999 pm/V at 0.94 strain, respectively.Comment: 10 pages, 15 figure
Generalized Migdal-Kadanoff Bond-moving Renormalization Recursion Procedure I: Symmetrical Half-length Bond Operation on Translational Invariant Lattices
We report in a series of papers two types of generalized Migdal-Kadanoff
bond-moving renormalization group transformation recursion procedures. In this
first part the symmetrical operation of half length bonds on translational
invariant lattices are considered. As an illustration of their predominance in
application, the procedures are used to study the critical behavior of the
spin-continuous Gaussian model constructed on the triangular lattices. Results
such as the correlation length critical exponents obtained by this means are
found to be in good conformity with the classical results from other studies.Comment: 10 pages, 4 figure
Piezoelectric quantum spin Hall insulator with Rashba spin splitting in Janus monolayer
The realization of multifunctional two-dimensional (2D) materials is
fundamentally intriguing, such as combination of piezoelectricity with
topological insulating phase or ferromagnetism. In this work, a Janus monolayer
is built from 2D family with dynamic,
mechanical and thermal stabilities, which is piezoelectric due to lacking
inversion symmetry. The unstrained monolayer is a narrow
gap normal insulator (NI) with spin orbital coupling (SOC). However, the NI to
topological insulator (TI) phase transition can be induced by the biaxial
strain, and a piezoelectric quantum spin Hall insulator (PQSHI) can be
achieved. More excitingly, the phase transformation point is only about 1.01
tensile strain, and nontrivial band topology can hold until considered 1.16
tensile strain. Moreover, a Rashba spin splitting in the conduction bands can
exit in PQSHI due to the absence of a horizontal mirror symmetry and the
presence of SOC. For monolayer , both in-plane and much
weak out-of-plane piezoelectric polarizations can be induced with a uniaxial
strain applied. The calculated piezoelectric strain coefficients and
of monolayer are -1.865 pm/V and -0.068 pm/V at
1.06 tensile strain as a representative TI. In fact, many PQSHIs can be
realized from 2D family. To confirm that, similar to
, the coexistence of piezoelectricity and topological
orders can be realized by strain (about 1.04 tensile strain) in the
monolayer. Our works suggest that Janus monolayer
is a pure 2D system for PQSHI, enabling future studies
exploring the interplay between piezoelectricity and topological orders, which
can lead to novel applications in electronics and spintronics.Comment: 9 pages,10 figure
Generalized Migdal-Kadanoff Bond-moving Renormalization Recursion Procedure II: Symmetrical Half-length Bond Operation on Fractals
In this second part of the series of two papers we report another type of
generalized Migdal-Kadanoff bond-moving renormalization group transformation
recursion procedures considering symmetrical single bond operations on
fractals. The critical behavior of the spin-continuous Gaussian model
constructed on the Sierpinski gaskets is studied as an example to reveal its
predominance in application. Results obtained by this means are found to be in
good conformity with those obtained from other studies.Comment: 9 pages, 3 figure
Powerful CMD: A Tool for Colour-Magnitude Diagram Studies
We present a new tool for colour-magnitude diagram (CMD) studies,
. This tool is built on the basis of the advanced stellar
population synthesis (ASPS) model, in which single stars, binary stars,
rotating stars, and star formation history have been taken into account. Via
, the distance modulus, colour excess, metallicity, age, binary
fraction, rotating star fraction, and star formation history of star clusters
can be determined simultaneously from observed CMDs. The new tool is tested via
both simulated and real star clusters. Five parameters of clusters NGC6362,
NGC6652, NGC6838 and M67 are determined and compared to other works. It is
shown that this tool is useful for CMD studies, in particular for those with
the data of the Hubble Space Telescope (HST). Moreover, we find that the
inclusion of binaries in theoretical stellar population models may lead to
smaller colour excess compared to the case of single star population models.Comment: Accepted to publish in RA
Coexistence of intrinsic piezoelectricity, ferromagnetism and nontrivial band topology in Li-decorated Janus monolayer with high Curie temperature
Recently, the quantum anomalous Hall (QAH) insulators are predicted in
Lithium-decorated iron-based superconductor monolayer materials (LiFeX (X=S, Se
and Te)) with very high Curie temperature (\textcolor[rgb]{0.00,0.00,1.00}{PRL
125, 086401 (2020)}), which combines the topological and ferromagnetic (FM)
orders. It is interesting and useful to achieve coexistence of intrinsic
piezoelectricity, ferromagnetism and nontrivial band topology in single
two-dimensional (2D) material, namely 2D piezoelectric quantum anomalous hall
insulator (PQAHI). In this work, 2D Janus monolayer is
predict to be a room-temperature PQAHI, which possesses dynamic, mechanical and
thermal stabilities. It is predicted to be a half Dirac semimetal without
spin-orbit coupling (SOC). It is found that the inclusion of SOC opens up a
large nontrivial gap, which means the nontrivial bulk topology (QAH insulator),
confirmed by the calculation of Berry curvature and the presence of two chiral
edge states (Chern number C=2). Calculated results show that monolayer
possesses robust QAH states against biaxial strain and
electronic correlations. Compared to LiFeX, the glide mirror of
disappears, which will induce only out-of-plane
piezoelectric response. The calculated out-of-plane of monolayer
is -0.238 pm/V comparable with ones of other 2D known
materials. Moreover, very high Curie temperature (about 1000 K) is predicted by
using Monte Carlo (MC) simulations, which means that the QAH effect can be
achieved at room temperature in Janus monolayer . Similar
to monolayer , the PQAHI can also be realized in the
Janus monolayer .Comment: 10 pages, 14 figures. arXiv admin note: text overlap with
arXiv:2105.0300
Structure effect on intrinsic piezoelectricity in septuple-atomic-layer (M=Mo and W)
The recently experimentally synthesized monolayer and
(\textcolor[rgb]{0.00,0.00,1.00}{Science 369, 670-674
(2020})) lack inversion symmetry, which allows them to become piezoelectric. In
this work, based on ab initio calculations, we report structure effect on
intrinsic piezoelectricity in septuple-atomic-layer (M=Mo
and W), and six structures ( (=1 to 6)) are considered with the
same space group.It is found that (M=Mo and W) with
(=1 to 6) all are indirect band gap semiconductors. Calculated
results show that and monolayers have
the same structural dependence on piezoelectric strain and stress coefficients
( and ), together with the ionic and electronic contributions
to .Finally, we investigate the intrinsic piezoelectricity of monolayer
(M=Cr, Mo and W; A=Si and Ge; Z=N and P) with and
phases expect , because they all are
semiconductors and their enthalpies of formation between and
phases are very close. The most important result is that monolayer
containing P atom have more stronger piezoelectric
polarization than one including N atom. The largest among
materials is 1.85 pm/V, which is close to the smallest
of 1.65 pm/V in monolayers. For ,
the largest is up to 6.12 pm/V. Among the 22 monolayers,
-, -,
-, - and
- have large , which are greater than or
close to 5 pm/V, a typical value for bulk piezoelectric materials.Comment: 8 pages, 11 figure
Coexistence of intrinsic piezoelectricity and nontrivial band topology in monolayer InXO (X=Se and Te)
The combination of piezoelectricity with other unique properties (like
topological insulating phase and intrinsic ferromagnetism) in two-dimensional
(2D) materials is much worthy of intensive study. In this work, the
piezoelectric properties of 2D topological insulators InXO (X=Se and Te) from
monolayer InX (X=Se and Te) with double-side oxygen functionalization are
studied by density functional theory (DFT). The large piezoelectric strain
coefficients (e.g. =-13.02 pm/V for InSeO and =-9.64 pm/V for
InTeO) are predicted, which are comparable and even higher than ones of many
other familiar 2D materials. Moreover, we propose two strategies to enhance
piezoelectric response of monolayer InXO (X=Se and Te). Firstly, the biaxial
strain (0.94-1.06) is applied, and the (absolute value) is increased
by 53\%/56\% for monolayer InSeO/InTeO at 1.06 strain, which is due to
increased (absolute value) and reduced . In considered
strain range, InXO (X=Se and Te) monolayers are always 2D topological
insulators, which confirm the coexistence of piezoelectricity and nontrivial
band topology. Secondly, a Janus monolayer is designed
by replacing the top Se/Te atomic layer in monolayer InSeO/InTeO with Te/Se
atoms, which is dynamically and mechanically stable. More excitingly, Janus
monolayer is also a 2D topological insulator with
sizeable bulk gap up to 0.158 eV, confirming the coexistence of intrinsic
piezoelectricity and topological nature. The calculated is -9.9 pm/V,
which is in the middle of ones of InSeO and InTeO monolayers.Comment: 10 pages, 11 figure
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