157 research outputs found
Interplay between Kitaev interaction and single ion anisotropy in ferromagnetic CrI and CrGeTe monolayers
Magnetic anisotropy is crucially important for the stabilization of
two-dimensional (2D) magnetism, which is rare in nature but highly desirable in
spintronics and for advancing fundamental knowledge. Recent works on CrI
and CrGeTe monolayers not only led to observations of the long-time-sought
2D ferromagnetism, but also revealed distinct magnetic anisotropy in the two
systems, namely Ising behavior for CrI versus Heisenberg behavior for
CrGeTe. Such magnetic difference strongly contrasts with structural and
electronic similarities of these two materials, and understanding it at a
microscopic scale should be of large benefits. Here, first-principles
calculations are performed and analyzed to develop a simple Hamiltonian, to
investigate magnetic anisotropy of CrI and CrGeTe monolayers. The
anisotropic exchange coupling in both systems is surprisingly determined to be
of Kitaev-type. Moreover, the interplay between this Kitaev interaction and
single ion anisotropy (SIA) is found to naturally explain the different
magnetic behaviors of CrI and CrGeTe. Finally, both the Kitaev
interaction and SIA are further found to be induced by spin-orbit coupling of
the heavy ligands (I of CrI or Te of CrGeTe) rather than the commonly
believed 3d magnetic Cr ions
Room Temperature Quantum Spin Hall Insulators with a Buckled Square Lattice
Two-dimensional (2D) topological insulators (TIs), also known as quantum spin
Hall (QSH) insulators, are excellent candidates for coherent spin transport
related applications because the edge states of 2D TIs are robust against
nonmagnetic impurities since the only available backscattering channel is
forbidden. Currently, most known 2D TIs are based on a hexagonal (specifically,
honeycomb) lattice. Here, we propose that there exists the quantum spin Hall
effect (QSHE) in a buckled square lattice. Through performing global structure
optimization, we predict a new three-layer quasi-2D (Q2D) structure which has
the lowest energy among all structures with the thickness less than 6.0 {\AA}
for the BiF system. It is identified to be a Q2D TI with a large band gap (0.69
eV). The electronic states of the Q2D BiF system near the Fermi level are
mainly contributed by the middle Bi square lattice, which are sandwiched by two
inert BiF2 layers. This is beneficial since the interaction between a substrate
and the Q2D material may not change the topological properties of the system,
as we demonstrate in the case of the NaF substrate. Finally, we come up with a
new tight-binding model for a two-orbital system with the buckled square
lattice to explain the low-energy physics of the Q2D BiF material. Our study
not only predicts a QSH insulator for realistic room temperature applications,
but also provides a new lattice system for engineering topological states such
as quantum anomalous Hall effect.Comment: 17pages, 4 figures Accepted by nano letter
First Principles Study of Adsorption of on Al Surface with Hybrid Functionals
Adsorption of molecule on Al surface has been a long standing puzzle
for the first principles calculation. We have studied the adsorption of
molecule on the Al(111) surface using hybrid functionals. In contrast to the
previous LDA/GGA, the present calculations with hybrid functionals successfully
predict that molecule can be absorbed on the Al(111) surface with a
barrier around 0.20.4 eV, which is in good agreement with
experiments. Our calculations predict that the LUMO of molecule is
higher than the Fermi level of the Al(111) surface, which is responsible for
the barrier of the adsorption.Comment: 14 pages, 5 figure
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