369 research outputs found
Engineering the magnetic anisotropy of atomic-scale nanostructure under electric field
Atomic-scale magnetic nanostructures are promising candidates for future
information processing devices. Utilizing external electric field to manipulate
their magnetic properties is an especially thrilling project. Here, by careful
identifying different contributions of each atomic orbital to the magnetic
anisotropy energy (MAE) of the ferromagnetic metal films, we argue that it is
possible to engineer both the MAE and the magnetic response to the electric
field of atomic-scale magnetic nanostructures. Taking the iron monolayer as a
matrix, we propose several interesting iron nanostructures with dramatically
different magnetic properties. Such nanostructures could exhibit strong
magnetoelectric effect. Our work may open a new avenue to the artificial design
of electrically controlled magnetic devices
Giant dielectric difference in chiral asymmetric bilayers
Twistronics rooted in the twist operation towards bilayer van der Waals
crystals is of both theoretical and technological importance. The realize of
the correlated electronic behaviors under this operation encourages enormous
effort to the research on magic-angle systems which possess sensitive response
to the external field. Here, a giant dielectric difference between 30 plus or
minus degree twist case is observed in a typical magnetic system 2H-VSe2
bilayer. It is shown that due to the structural inversion asymmetry in its
monolayer, the different stacking of the two cases corresponds to the two kind
of valley polarized states: interlayer ferrovalley and interlayer
antiferrovalley. Further investigations reveal that such different dielectric
response between the two states stems from the different Fermi wave vectors
coupled to the electric field. More interestingly, we even obtain the selective
circularly polarized optical absorption by tuning the interlayer twist. These
findings open an appealing route toward functional 2D materials design for
electric and optical devices
Concepts of Ferrovalley Material and Anomalous Valley Hall Effect
Valleytronics rooted in the valley degree of freedom is of both theoretical
and technological importance as it offers additional opportunities for
information storage and electronic, magnetic and optical switches. In analogy
to ferroelectric materials with spontaneous charge polarization in electronics,
as well as ferromagnetic materials with spontaneous spin polarization in
spintronics, here we introduce a new member of ferroic-family, i.e. a
ferrovalley material with spontaneous valley polarization. Combining a two-band
kp model with first-principles calculations, we show that 2H-VSe2 monolayer,
where the spin-orbit coupling coexists with the intrinsic exchange interaction
of transition-metal-d electrons, is such a room-temperature ferrovalley
material. We further predict that such system could demonstrate many
distinctive properties, for example, chirality-dependent optical band gap and
more interestingly, anomalous valley Hall effect. On account of the latter, a
series of functional devices based on ferrovalley materials, such as
valley-based nonvolatile random access memory, valley filter, are contemplated
for valleytronic applications.Comment: 6 pages,5 figure
Electrically tunable polarizer based on two-dimensional orthorhombic ferrovalley materials
The concept of ferrovalley materials has been proposed very recently. The
existence of spontaneous valley polarization, resulting from ferromagnetism, in
such hexagonal two-dimensional materials makes nonvolatile valleytronic
applications realizable. Here, we introduce a new member of ferrovalley family
with orthorhombic lattice, i.e. monolayer group-IV monochalcogenides (GIVMs),
in which the intrinsic valley polarization originates from ferroelectricity,
instead of ferromagnetism. Combining the group theory analysis and
first-principles calculations, we demonstrate that, different from the
valley-selective circular dichroism in hexagonal lattice, linearly polarized
optical selectivity for valleys exists in the new type of ferrovalley
materials. On account of the distinctive property, a prototype of electrically
tunable polarizer is realized. In the ferrovalley-based polarizer, a laser beam
can be optionally polarized in x- or y-direction, depending on the ferrovalley
state controlled by external electric fields. Such a device can be further
optimized to emit circularly polarized radiation with specific chirality and to
realize the tunability for operating wavelength. Therefore, we show that
two-dimensional orthorhombic ferrovalley materials are the promising candidates
to provide an advantageous platform to realize the polarizer driven by electric
means, which is of great importance in extending the practical applications of
valleytronics
Concept of the half-valley-metal and quantum anomalous valley Hall effect
Valley, the energy extrema in the electronic band structure at momentum
space, is regarded as a new degree of freedom of electrons, in addition to
charge and spin. The studies focused on valley degree of freedom now form an
emerging field of condensed matter physics, i.e. valleytronics, whose
development is exactly following that of spintronics which focuses on the spin
degree of freedom. Here, in analogy to half-metals in spintronics with one spin
channel is conducting whereas the other is insulating, we propose the concept
of half-valley-metal, in which conduction electrons are intrinsically 100%
valley polarized, as well as 100% spin-polarized even when spin-orbit
interactions are considered. Combining first-principles calculations with
two-band kp model, the physical mechanism to form the half-valley-metal is
illuminated. Taking the ferrovalley H-FeCl2 monolayer with strong exchange
interaction as an example, we find that the strong electron correlation effect
can induce the ferrovalley to half-valley-metal transition. Due to the
valley-dependent optical selection rules, such system could be transparent to,
e.g., left-circularly polarized light, yet the right-circularly polarized light
will be reflected, which can in turn be used as a crucial method to detect
half-valley-metal state. In addition, we find that in the so obtained
half-valley-metal state, the conduction valley demonstrates Dirac cone-like
linear energy dispersion. Interestingly, with the increase of the correlation
effect, the system becomes insulating again with all valleys follow same
optical selection rule. We confirm that in this specific case, the valence
bands, which consist of single spin, possess non-zero Chern number and
consequently intrinsic quantum anomalous valley Hall effect emerges. Our
findings open an appealing route toward functional 2D materials design of
valleytronics
Manipulation of the large Rashba spin splitting in polar two-dimensional transition metal dichalcogenides
Transition metal dichalcogenide (TMD) monolayers MXY (M=Mo, W, X(not equal
to)Y=S, Se, Te) are two-dimensional polar semiconductors. Setting WSeTe
monolayer as an example and using density functional theory calculations, we
investigate the manipulation of Rashba spin orbit coupling (SOC) in the MXY
monolayer. It is found that the intrinsic out-of-plane electric field due to
the mirror symmetry breaking induces the large Rashba spin splitting around the
Gamma point, which, however, can be easily tuned by applying the in-plane
biaxial strain. Through a relatively small strain (from -2% to 2%), a large
tunability (from around -50% to 50%) of Rashba SOC can be obtained due to the
modified orbital overlap, which can in turn modulate the intrinsic electric
field. The orbital selective external potential method further confirms the
significance of the orbital overlap between W-dz2 and Se-pz in Rashba SOC. In
addition, we also explore the influence of the external electric field on
Rashba SOC in the WSeTe monolayer, which is less effective than strain. The
large Rashba spin splitting, together with the valley spin splitting in MXY
monolayers may make a special contribution to semiconductor spintronics and
valleytronics
Emergent exotic chirality dependent dielectricity in magnetic twisted bilayer system
Twisted van der Waals bilayers provide an ideal platform to study the
electron correlation in solids. Of particular interest is the 30 degree twisted
bilayer honeycomb lattice system, which possesses an incommensurate Moire
pattern and uncommon electronic behaviors may appear due to the absence of
phase coherence. Such system is extremely sensitive to further twist and many
intriguing phenomena will occur. In this work, we show that due to the twist
induced spatial inhomogeneity of interlayer coupling, there emerges an U(1)
gauge field in magnetic transition-metal dichalcogenides (TMD) bilayers.
Interestingly, for further twist near 30 degree, the induced gauge field could
form a chirality dependent real-space skyrmion pattern, or magnetic charge.
Moreover, such twist also induces the topology dependent electronic
polarization of the bilayer system through the nonzero flux of the real-space
Berry curvature. Further analysis proves that the antiferromagnetically coupled
twisted bilayer system is indeed also antiferroelectric! When an external
electric field is applied to break the potential balance between layers, there
will emerge novel magnetoelectric coupling and exotic chirality dependent
dielectricity. Such findings not only enrich our understanding on Moire
systems, but also open an appealing route toward functional 2D materials design
for electronic, optical and even energy storage devices
X-Band deflecting cavity design for ultra-short bunch length measurement of SXFEL at SINAP
For the development of the X-ray Free Electron Lasers test facility (SXFEL)
at SINAP, ultra-short bunch is the crucial requirement for excellent lasing
performance. It's a big challenge for deflecting cavity to measure the length
of ultra-short bunch, and higher deflecting gradient is required for higher
measurement resolution. X-band travelling wave deflecting structure has
features of higher deflecting voltage and compact structure, which is good
performance at ultra-short bunch length measurement. In this paper, a new
X-band deflecting structure has been designed operated at HEM11-2pi/3 mode. For
suppressing the polarization of deflecting plane of the HEM11 mode, two
symmetrical caves are added on the cavity wall to separate two polarized modes.
More details of design and simulation results are presented in this paper
Magnetic Ordering Induced Giant Optical Property Change in Tetragonal BiFeO3
Magnetic ordering, as one of the most important characteristics in magnetic
materials, could have significant influence on the band structure, spin
dependent transport, and other important properties of materials. Its
measurement, especially for the case of antiferromagnetic ordering, however, is
generally difficult to be achieved. Here we demonstrate the feasibility of
magnetic ordering detection using a noncontact and nondestructive optical
method. Taking the compressive strained tetragonal BiFeO3 (BFO) as an example
and combining density functional theory calculations with the minimal one-band
tight-binding models, we find that when BFO changes from C1-type
antiferromagnetic (AFM) phase to G-type AFM phase, the top of valance band
shifts from the Z point to {\Gamma} point, which makes the original direct band
gap become indirect. This can be explained by the two-center Slater-Koster
parameters using the Harrison approach. The impact of magnetic ordering on
energy band dispersion dramatically changes the optical properties of
tetragonal BFO. For the linear ones, the energy shift of the optical band gap
could be as large as 0.4 eV. As for the nonlinear ones, the change is even
larger. The second-harmonic generation coefficient d33 of G-AFM becomes more
than 13 times smaller than that of C1-type AFM case. Finally, we propose a
practical way to distin-guish the C1- and G-type AFM of BFO using the optical
method, which might be of great importance in next-generation information
storage technologies and widens the potential application of BFO to optical
switch
Exotic dielectric behaviors induced by pseudo-spin texture in magnetic twisted bilayer
Twisted van der Waals bilayers provide an ideal platform to study the
electron correlation in solids. Of particular interest is the 30 degree twisted
bilayer honeycomb lattice system, which possesses an incommensurate moire
pattern and uncommon electronic behaviors may appear due to the absence of
phase coherence. Such system is extremely sensitive to further twist and many
intriguing phenomena will occur. In this work, based on first-principles
calculations we show that, for further twist near 30 degree, there could induce
dramatically different dielectric behaviors of electron between left and right
twisted cases. Specifically, it is found that the left and right twists show
suppressed and amplified dielectric response under vertical electric field,
respectively. Further analysis demonstrate that such exotic dielectric property
can be attributed to the stacking dependent charge redistribution due to twist,
which forms twist-dependent pseudospin textures. We will show that such
pseudospin textures are robust under small electric field. As a result, for the
right twisted case, there is almost no electric dipole formation exceeding the
monolayer thickness when the electric field is applied. Whereas for the left
case, the system could even demonstrate negative susceptibility, i.e. the
induced polarization is opposite to the applied field, which is very rare in
the nature. Such findings not only enrich our understanding on moire systems
but also open an appealing route toward functional 2D materials design for
electronic, optical and even energy storage devices.Comment: arXiv admin note: substantial text overlap with arXiv:1912.0843
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