3,811 research outputs found
Laser-induced topological transitions in phosphorene with inversion symmetry
Recent ab initio calculations and experiments reported
insulating-semimetallic phase transitions in multilayer phosphorene under a
perpendicular dc field, pressure or doping, as a possible route to realize
topological phases. In this work, we show that even a monolayer phosphorene may
undergo Lifshitz transitions toward semimetallic and topological insulating
phases, provided it is rapidly driven by in-plane time-periodic laser fields.
Based on a four-orbital tight-binding description, we give an
inversion-symmetry-based prescription in order to apprehend the topology of the
photon-renormalized band structure, up to the second order in the
high-frequency limit. Apart from the initial band insulating behavior, two
additional phases are thus identified. A semimetallic phase with massless Dirac
electrons may be induced by linear polarized fields, whereas elliptic polarized
fields are likely to drive the material into an anomalous quantum Hall phase.Comment: Includes Supplemental Materia
Dynamical and Reversible Control of Topological Spin Textures
Recent observations of topological spin textures brought spintronics one step
closer to new magnetic memories. Nevertheless, the existence of Skyrmions, as
well as their stabilization, require very specific intrinsic magnetic
properties which are usually fixed in magnets. Here we address the possibility
to dynamically control their intrinsic magnetic interactions by varying the
strength of a high-frequency laser field. It is shown that drastic changes can
be induced in the antiferromagnetic exchange interactions and the latter can
even be reversed to become ferromagnetic, provided the direct exchange is
already non-negligible in equilibrium as predicted, for example, in Si doped
with C, Sn, or Pb adatoms. In the presence of Dzyaloshinskii-Moriya
interactions, this enables us to tune features of ferromagnetic Skyrmions such
as their radius, making them easier to stabilize. Alternatively, such
topological spin textures can occur in frustrated triangular lattices. Then, we
demonstrate that a high-frequency laser field can induce dynamical frustration
in antiferromagnets, where the degree of frustration can subsequently be tuned
suitably to drive the material toward a Skyrmionic phase
Excitonic Instability and Pseudogap Formation in Nodal Line Semimetal ZrSiS
Electron correlation effects are studied in ZrSiS using a combination of
first-principles and model approaches. We show that basic electronic properties
of ZrSiS can be described within a two-dimensional lattice model of two nested
square lattices. High degree of electron-hole symmetry characteristic for ZrSiS
is one of the key features of this model. Having determined model parameters
from first-principles calculations, we then explicitly take electron-electron
interactions into account and show that at moderately low temperatures ZrSiS
exhibits excitonic instability, leading to the formation of a pseudogap in the
electronic spectrum. The results can be understood in terms of
Coulomb-interaction-assisted pairing of electrons and holes reminiscent to that
of an excitonic insulator. Our finding allows us to provide a physical
interpretation to the unusual mass enhancement of charge carriers in ZrSiS
recently observed experimentally.Comment: 6 pages, 4 figures. Final versio
Effective Heisenberg model and exchange interaction for strongly correlated systems
We consider the extended Hubbard model and introduce a corresponding
Heisenberg-like problem written in terms of spin operators. The derived
formalism is reminiscent of Anderson's idea of the effective exchange
interaction and takes into account nonlocal correlation effects. The results
for the exchange interaction and magnetic susceptibility are expressed in terms
of single-particle quantities, which can be obtained efficiently in realistic
calculations of multiband systems. In the strongly spin-polarized limit, when
the local magnetic moment is well-defined, the exchange interaction reduces to
a standard expression of the density functional theory that has been
successfully used in practical calculations of magnetic properties of real
materials.Comment: Accepted to Physical Review Letter
Search for Non-Triggered Gamma Ray Bursts in the BATSE Continuous Records: Preliminary Results
We present preliminary results of an off-line search for non-triggered
gamma-ray bursts (GRBs) in the BATSE daily records for about 5.7 years of
observations. We found more GRB-like events than the yield of the similar
search of Kommers et al. (1998) and extended the Log N - log P distribution
down to 0.1 ph cm s. The indication of a turnover of the
log N - log P at a small P is not confirmed: the distribution is straight at
1.5 decades with the power law index -.6 and cannot be fitted with a standard
candle cosmological model.Comment: 4 pages, LaTeX, to appear in Proceedings "Gamma Ray Bursts in the
Afterglow Era", Rome, November 1998, A&AS, 199
- …