18,970 research outputs found
Magnetic spin moment reduction in photoexcited ferromagnets through exchange interaction quenching: Beyond the rigid band approximation
The exchange interaction among electrons is one of the most fundamental
quantum mechanical interactions in nature and underlies any magnetic phenomena
from ferromagnetic ordering to magnetic storage. The current technology is
built upon a thermal or magnetic field, but a frontier is emerging to directly
control magnetism using ultrashort laser pulses. However, little is known about
the fate of the exchange interaction. Here we report unambiguously that
photoexcitation is capable of quenching the exchange interaction in all three
ferromagnetic metals. The entire process starts with a small number of
photoexcited electrons which build up a new and self-destructive potential that
collapses the system into a new state with a reduced exchange splitting. The
spin moment reduction follows a Bloch-like law as , where is
the absorbed photon energy and is a scaling exponent. A good agreement
is found between the experimental and our theoretical results. Our findings may
have a broader implication for dynamic electron correlation effects in
laser-excited iron-based superconductors, iron borate, rare-earth
orthoferrites, hematites and rare-earth transition metal alloys.Comment: 16 pages, 3 figures, one supplementary material fil
Generating high-order optical and spin harmonics from ferromagnetic monolayers
High-order harmonic generation (HHG) in solids has entered a new phase of
intensive research, with envisioned band-structure mapping on an ultrashort
time scale. This partly benefits from a flurry of new HHG materials discovered,
but so far has missed an important group. HHG in magnetic materials should have
profound impact on future magnetic storage technology advances. Here we
introduce and demonstrate HHG in ferromagnetic monolayers. We find that HHG
carries spin information and sensitively depends on the relativistic spin-orbit
coupling; and if they are dispersed into the crystal momentum space,
harmonics originating from real transitions can be -resolved and carry
the band structure information. Geometrically, the HHG signal is sensitive to
spatial orientations of monolayers. Different from the optical counterpart, the
spin HHG, though probably weak, only appears at even orders, a consequence of
SU(2) symmetry. Our findings open an unexplored frontier -- magneto-high-order
harmonic generation.Comment: 19 pages, 4 figure
Strong and nearly 100 spin-polarized second-harmonic generation from ferrimagnet MnRuGa
Second-harmonic generation (SHG) has emerged as a promising tool for
detecting electronic and magnetic structures in noncentrosymmetric materials,
but 100 spin-polarized SHG has not been reported. In this work, we
demonstrate nearly 100 spin-polarized SHG from half-metallic ferrimagnet
MnRuGa. A band gap in the spin-down channel suppresses SHG, so the
spin-up channel contributes nearly all the signal, as large as 3614 pm/V about
10 times larger than that of GaAs. In the spin-up channel,
is dominated by the large intraband current in three highly dispersed bands
near the Fermi level. With the spin-orbit coupling (SOC), the reduced magnetic
point group allows additional SHG components, where the interband contribution
is enhanced. Our finding is important as it predicts a large and complete
spin-polarized SHG in a all-optical spin switching ferrimagnet. This opens the
door for future applications
A Formalization of The Natural Gradient Method for General Similarity Measures
In optimization, the natural gradient method is well-known for likelihood
maximization. The method uses the Kullback-Leibler divergence, corresponding
infinitesimally to the Fisher-Rao metric, which is pulled back to the parameter
space of a family of probability distributions. This way, gradients with
respect to the parameters respect the Fisher-Rao geometry of the space of
distributions, which might differ vastly from the standard Euclidean geometry
of the parameter space, often leading to faster convergence. However, when
minimizing an arbitrary similarity measure between distributions, it is
generally unclear which metric to use. We provide a general framework that,
given a similarity measure, derives a metric for the natural gradient. We then
discuss connections between the natural gradient method and multiple other
optimization techniques in the literature. Finally, we provide computations of
the formal natural gradient to show overlap with well-known cases and to
compute natural gradients in novel frameworks
Extended Calculations of Spectroscopic Data: Energy Levels, Lifetimes and Transition rates for O-like ions from Cr XVII to Zn XXIII
Employing two state-of-the-art methods, multiconfiguration
Dirac--Hartree--Fock and second-order many-body perturbation theory, the
excitation energies and lifetimes for the lowest 200 states of the ,
, , , , , , , and configurations, and multipole (electric
dipole (E1), magnetic dipole (M1), and electric quadrupole (E2)) transition
rates, line strengths, and oscillator strengths among these states are
calculated for each O-like ion from Cr XVII to Zn XXIII. Our two data sets are
compared with the NIST and CHIANTI compiled values, and previous calculations.
The data are accurate enough for identification and deblending of new emission
lines from the sun and other astrophysical sources. The amount of data of high
accuracy is significantly increased for the states of several O-like
ions of astrophysics interest, where experimental data are very scarce
Non-Abelian toplogical superconductors from topological semimetals and related systems under superconducting proximity effect
Non-Abelian toplogical superconductors are characterized by the existence of
{zero-energy} Majorana fermions bound in the quantized vortices. This is a
consequence of the nontrivial bulk topology characterized by an {\em odd} Chern
number. It is found that in topological semimetals with a single two-bands
crossing point all the gapped superconductors are non-Abelian ones. Such a
property is generalized to related but more generic systems which will be
useful in the search of non-Abelian superconductors and Majorana fermions
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