44 research outputs found
Coupled spin and valley physics in monolayers of MoS2 and other group-VI dichalcogenides
We show that inversion symmetry breaking together with spin-orbit coupling
leads to coupled spin and valley physics in monolayers of MoS2 and other
group-VI dichalcogenides, making possible controls of spin and valley in these
2D materials. The spin-valley coupling at the valence band edges suppresses
spin and valley relaxation, as flip of each index alone is forbidden by the
valley contrasting spin splitting. Valley Hall and spin Hall effects coexist in
both electron-doped and hole-doped systems. Optical interband transitions have
frequency-dependent polarization selection rules which allow selective
photoexcitation of carriers with various combination of valley and spin
indices. Photo-induced spin Hall and valley Hall effects can generate long
lived spin and valley accumulations on sample boundaries. The physics discussed
here provides a route towards the integration of valleytronics and spintronics
in multi-valley materials with strong spin-orbit coupling and inversion
symmetry breaking.Comment: published versio
Large magneto-optical Kerr effect in noncollinear antiferromagnets Mn ( = Rh, Ir, or Pt)
Magneto-optical Kerr effect, normally found in magnetic materials with
nonzero magnetization such as ferromagnets and ferrimagnets, has been known for
more than a century. Here, using first-principles density functional theory, we
demonstrate large magneto-optical Kerr effect in high temperature noncollinear
antiferromagnets Mn ( = Rh, Ir, or Pt), in contrast to usual wisdom.
The calculated Kerr rotation angles are large, being comparable to that of
transition metal magnets such as bcc Fe. The large Kerr rotation angles and
ellipticities are found to originate from the lifting of the band
double-degeneracy due to the absence of spatial symmetry in the Mn
noncollinear antiferromagnets which together with the time-reversal symmetry
would preserve the Kramers theorem. Our results indicate that Mn would
provide a rare material platform for exploration of subtle magneto-optical
phenomena in noncollinear magnetic materials without net magnetization
Intrinsic spin Hall effect in monolayers of group-VI dichalcogenides: A first-principles study
Using first-principles calculations within density functional theory, we
investigate the intrinsic spin Hall effect in monolayers of group-VI
transition-metal dichalcogenides MX2 (M = Mo, W and X = S, Se). MX2 monolayers
are direct band-gap semiconductors with two degenerate valleys located at the
corners of the hexagonal Brillouin zone. Because of the inversion symmetry
breaking and the strong spin-orbit coupling, charge carriers in opposite
valleys carry opposite Berry curvature and spin moment, giving rise to both a
valley- and a spin-Hall effect. The intrinsic spin Hall conductivity (ISHC) in
p-doped samples is found to be much larger than the ISHC in n-doped samples due
to the large spin-splitting at the valence band maximum. We also show that the
ISHC in inversion-symmetric bulk dichalcogenides is an order of magnitude
smaller compared to monolayers. Our result demonstrates monolayer
dichalcogenides as an ideal platform for the integration of valleytronics and
spintronics.Comment: published version (7 pages, 6 figures
Half-Heusler Topological Insulators: A First-Principle Study with the Tran-Blaha Modified Becke-Johnson Density Functional
We systematically investigate the topological band structures of half-Heusler
compounds using first-principles calculations. The modified Becke-Johnson
exchange potential together with local density approximation for the
correlation potential (MBJLDA) has been used here to obtain accurate band
inversion strength and band order. Our results show that a large number of
half-Heusler compounds are candidates for three-dimensional topological
insulators. The difference between band structures obtained using the local
density approximation (LDA) and MBJLDA potential is also discussed.Comment: 5 figures, 1 tabl
Topological magneto-optical effects and their quantization in noncoplanar antiferromagnets
Reflecting the fundamental interactions of polarized light with magnetic
matter, magneto-optical effects are well known since more than a century. The
emergence of these phenomena is commonly attributed to the interplay between
exchange splitting and spin-orbit coupling in the electronic structure of
magnets. Using theoretical arguments, we demonstrate that topological
magneto-optical effects can arise in noncoplanar antiferromagnets due to the
finite scalar spin chirality, without any reference to exchange splitting or
spin-orbit coupling. We propose spectral integrals of certain magneto-optical
quantities that uncover the unique topological nature of the discovered effect.
We also find that the Kerr and Faraday rotation angles can be quantized in
insulating topological antiferromagnets in the low-frequency limit, owing to
nontrivial global properties that manifest in quantum topological
magneto-optical effects. Although the predicted topological and quantum
topological magneto-optical effects are fundamentally distinct from
conventional light-matter interactions, they can be measured by readily
available experimental techniques.Comment: 10 pages, 5 figure