1,957 research outputs found

### Nonlinear Optical Properties of Transition Metal Dichalcogenide MX$_2$ (M = Mo, W; X = S, Se) Monolayers and Trilayers from First-principles Calculations

Due to the absence of interlayer coupling and inversion symmetry, transition
metal dichalcogenide (MX$_2$) semiconductor monolayers exhibit novel properties
that are distinctly different from their bulk crystals such as direct optical
band gaps, large band spin splittings, spin-valley coupling, piezoelectric and
nonlinear optical responses, and thus have promising applications in, e.g.,
opto-electronic and spintronic devices. Here we have performed a systematic
first-principles study of the second-order nonlinear optical properties of
MX$_2$ (M = Mo, W; X = S, Se) monolayers and trilayers within the density
functional theory with the generalized gradient approximation plus scissors
correction. We find that all the four MX$_2$ monolayers possess large
second-order optical susceptibility $\chi^{(2)}$ in the optical frequency range
and significant linear electro-optical coefficients in low frequency limit,
thus indicating their potential applications in non-linear optical devices and
electric optical switches. The $\chi^{(2)}$ spectra of the MX$_2$ trilayers are
overall similar to the corresponding MX$_2$ monolayers, {\it albeit} with the
magnitude reduced by roughly a factor of 3. The prominent features in the
$\chi^{(2)}$ spectra of the MX$_2$ multilayers are analyzed in terms of the
underlying band structures and optical dielectric function, and also compared
with available experiments.Comment: references updated, new Figure 2, revised Figure 8 and text improve

### High spin polarization of the anomalous Hall current in Co-based Heusler compounds

Based on first principles density functional calculations of the intrinsic
anomalous and spin Hall conductivities, we predict that the charge Hall current
in Co-based full Heusler compounds Co$_2$XZ (X = Cr and Mn; Z = Al, Si, Ga, Ge,
In and Sn) except Co$_2$CrGa would be almost fully spin-polarized even although
Co$_2$MnAl, Co$_2$MnGa, Co$_2$MnIn and Co$_2$MnSn do not have a half-metallic
band structure. Furthermore, the ratio of the associated spin current to the
charge Hall current is slightly larger than 1.0. This suggests that these
Co-based Heusler compounds, especially Co$_2$MnAl, Co$_2$MnGa and Co$_2$MnIn
which are found to have large anomalous and spin Hall conductivities, might be
called anomalous Hall half-metals and could have valuable applications in
spintronics such as spin valves as well as magnetoresistive and spin-torque
driven nanodevices. These interesting findings are discussed in terms of the
calculated electronic band structures, magnetic moments and also anomalous and
spin Hall conductivities as a function of the Fermi level.Comment: Accepted for publication in New Journal of Physic

### Large anomalous Nernst and spin Nernst effects in noncollinear antiferromagnets Mn$_3X$ ($X$ = Sn, Ge, Ga)

Noncollinear antiferromagnets have recently been attracting considerable
interest partly due to recent surprising discoveries of the anomalous Hall
effect (AHE) in them and partly because they have promising applications in
antiferromagnetic spintronics. Here we study the anomalous Nernst effect (ANE),
a phenomenon having the same origin as the AHE, and also the spin Nernst effect
(SNE) as well as AHE and the spin Hall effect (SHE) in noncollinear
antiferromagnetic Mn$_3X$ ($X$ = Sn, Ge, Ga) within the Berry phase formalism
based on {\it ab initio} relativistic band structure calculations. For
comparison, we also calculate the anomalous Nernst conductivity (ANC) and
anomalous Hall conductivity (AHC) of ferromagnetic iron as well as the spin
Nernst conductivity (SNC) of platinum metal. Remarkably, the calculated ANC at
room temperature (300 K) for all three alloys is huge, being up to 5 times
larger than that of iron. Moreover, the calculated SNC for Mn$_3$Sn and
Mn$_3$Ga is also large, being as large as that of platinum. This suggests that
these anitferromagnets would be useful materials for thermoelectronic devices
and spin caloritronic devices. The calculated ANC of Mn$_3$Sn and iron are in
reasonably good agreement with the very recent experiments. The calculated SNC
of platinum also agrees with the very recent experiments in both sign and
magnitude. The calculated thermoelectric and thermomagnetic properties are
analyzed in terms of the band structures as well as the energy-dependent AHC,
ANC, SNC and spin Hall conductivity via the Mott relations.Comment: Revised version with new Table II, new Figures 3-6, and the corrected
ANC and SNC value

### Topological insulator associated with quantum anomalous Hall phase in ferromagnetic perovskite superlattices

We do a search for topological insulators which are associated with
ferromagnetic ordering and show anomalous quantum Hall effect, among transition
metal oxide superlattices taking the parent compounds as LaAlO3 and SrTiO3.
Among the various superlattices which are studied here, (LaAlO3)10/(LaOsO3)2
exhibits a ferromagnetic ground state with a topologically non-trivial energy
gap when a spin-orbit interaction is turned on. The study of transverse
conductivity shows that the system has quantized Hall conductivity inside the
topological energy gap without applying any external magnetic field. The
ferromagnetic order parameters and the ordering temperature (Tc) have been
estimated by taking a simple Heisenberg model of ferromagnetism.Comment: 6 pages, 6 figure

### Magnetism and Magneto-optical Effects in Bulk and Few-layer CrI$_3$: A Theoretical GGA + U Study

The latest discovery of ferromagnetism in atomically thin films of
semiconductors Cr$_2$Ge$_2$Te$_6$ and CrI$_3$ has unleashed numerous
opportunities for fundamental physics of magnetism in two-dimensional (2D)
limit and also for technological applications based on 2D magnetic materials.
In this paper, we present a comprehensive theoretical study of the magnetic,
electronic, optical and magneto-optical(MO) properties of multilayers
[monolayer(ML), bilayer and trilayer] and bulk CrI$_3$, based on the density
functional theory with the generalized gradient approximation plus on-site
Coulomb repulsion scheme. Interestingly, all the structures are found to be
single-spin ferromagnetic(FM) semiconductors. They all have a large
out-of-plane magnetic anisotropy energy(MAE) of $\sim$0.5 meV/Cr. These large
MAEs suppress transverse spin fluctuations and thus stabilize long-range
magnetic orders at finite temperatures down to the ML limit. They also exhibit
strong MO effects with their Kerr and Faraday rotation angles being comparable
to that of best-known bulk MO materials. The shape and position of the main
features in the optical and MO spectra are found to be nearly
thickness-independent although the magnitude of Kerr rotation angles increases
monotonically with the film thickness. Magnetic transition temperatures
estimated based on calculated exchange coupling parameters, calculated optical
conductivity, MO Kerr and Faraday rotation angles agree quite well with
available experimental data. The calculated MAE as well as optical and MO
properties are analyzed in terms of the calculated orbital-decomposed densities
of states, band state symmetries and dipole selection rules. Our findings of
large out-of-plane MAEs and strong MO effects in these single-spin FM
semiconducting CrI$_3$ ultrathin films suggest that they will find valuable
applications in semiconductor MO and spintronic nanodevices

### Tuning Topological Phase Transitions in Hexagonal Photonic Lattices Made of Triangular Rods

In this paper, we study topological phases in a 2D photonic crystal with
broken time ($\mathcal{T}$) and parity ($\mathcal{P}$) symmetries by performing
calculations of band structures, Berry curvatures, Chern numbers, edge states
and also numerical simulations of light propagation in the edge modes.
Specifically, we consider a hexagonal lattice consisting of triangular
gyromagnetic rods. Here the gyromagnetic material breaks $\mathcal{T}$ symmetry
while the triangular rods breaks $\mathcal{P}$ symmetry. Interestingly, we find
that the crystal could host quantum anomalous Hall (QAH) phases with different
gap Chern numbers ($C_g$) including $|C_g| > 1$ as well as quantum valley Hall
(QVH) phases with contrasting valley Chern numbers ($C_v$), depending on the
orientation of the triangular rods. Furthermore, phase transitions among these
topological phases, such as from QAH to QVH and vice versa, can be engineered
by a simple rotation of the rods. Our band theoretical analyses reveal that the
Dirac nodes at the $K$ and $K'$ valleys in the momentum space are produced and
protected by the mirror symmetry ($m_y$) instead of the $\mathcal{P}$ symmetry,
and they become gapped when either $\mathcal{T}$ or $m_y$ symmetry is broken,
resulting in a QAH or QVH phase, respectively. Moreover, a high Chern number
($C_g = -2$) QAH phase is generated by gapping triply degenerate nodal points
rather than pairs of Dirac points by breaking $\mathcal{T}$ symmetry. Our
proposed photonic crystal thus provides a platform for investigating intriguing
topological phenomena which may be challenging to realize in electronic
systems, and also has promising potentials for device applications in photonics
such as reflection-free one-way waveguides and topological photonic circuits

### Optical left-handed metamaterials made of arrays of upright split-ring pairs

Electromagnetic metamaterials are man-made structures that have novel
properties such as a negative refraction index, not attainable in naturally
occurring materials. Although negative index materials (NIMs) in microwave
frequencies were demonstrated in 2001, it has remained challenging to design
NIMs for optical frequencies especially those with both negative permittivity
and negative permeability [known as left-handed metamaterials (LHMs)]. Here, by
going beyond the traditional concept of the combination of artificial
electronic and magnetic meta-atoms to design NIMs, we propose a novel LHM
composed of an array of simple upright split-ring pairs working in the near
infrared region. Our electromagnetic simulations reveal the underlying
mechanism that the coupling of the two rings can stimulate simultaneously both
the electric and magnetic resonances. The proposed structure has a highest
refractive index of -2, a highest figure of merit of 21, good air-matched
impedance and 180 nm double negative bandwidth, which excel the performances of
many previous proposals. We also numerically demonstrate the negative
refraction of this metamaterial in both the single-layer form and wedge-shaped
lens

### A lower bound of the least signless Laplacian eigenvalue of a graph

Let $G$ be a simple connected graph on $n$ vertices and $m$ edges. In [Linear
Algebra Appl. 435 (2011) 2570-2584], Lima et al. posed the following conjecture
on the least eigenvalue $q_n(G)$ of the signless Laplacian of $G$:
$\displaystyle q_n(G)\ge {2m}/{(n-1)}-n+2$. In this paper we prove a stronger
result: For any graph with $n$ vertices and $m$ edges, we have $\displaystyle
q_n(G)\ge {2m}/{(n-2)}-n+1 (n\ge 6)$

### Cooling of a levitated nanoparticle with digital parametric feedback

The motion control of a levitated nanoparticle plays a central role in
optical levitation for fundamental studies and practical applications. Here, we
presented a digital parametric feedback cooling based on switching between two
trapping laser intensity levels with square wave modulations. The effects of
modulation depth and modulation signal phase on the cooling result were
investigated in detail. With such a digital parametric feedback method, the
centre-of-mass temperature of all three motional degrees of freedom can be
cooled to dozens of milli-Kelvin, which paved the way to fully control the
motion of the levitated nanoparticle with a programmable digital process for
wild applications.Comment: 5 pages, 6 figure

### Advances in quantum dense coding

Quantum dense coding is one of the most important protocols in quantum
communication. It derives from the idea of using quantum resources to boost the
communication capacity and now serves as a key primitive across a variety of
quantum information protocols. Here, we focus on the basic theoretical ideas
behind quantum dense coding, discussing its development history from discrete
and continuous variables to quantum networks, then to its variant protocols and
applications in quantum secure communication. With this basic background in
hand, we then review the main experimental achievements, from photonic qubits
and qudits to optical modes, nuclear magnetic resonance, and atomic systems.
Besides the state of the art, we finally discuss potential future steps.Comment: Accepted by Advanced Quantum Technologies (Invited review

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