223 research outputs found
Electrostatically Controlled Magnetization Rotation in Ferromagnet-Topological Insulator Planar Structures
An approach to the electrostatic control of magnetization
rotation in the hybrid structures composed of topological insulators (TIs) and
adjacent ferromagnetic insulators (FMI) is proposed and studied. The concept is
based on TI electron energy variation with in-plane to put-of plane FMI
magnetization turn. The calculations explicitly expose the effect of free
energy variability in the form of the electrically controlled uniaxial magnetic
anisotropy, which depends on proximate exchange interaction and TI surface
electron density. Combining with inherent anisotropy, the magnetization
rotation from in-plane to out-of-plane direction is shown to be realizable for
1.7~2.7 ns under the electrical variation of TI chemical potential in the range
100 meV around Dirac point. When bias is withdrawn a small signal current
can target the out-of-plane magnetization instable state to the desirable
direction of in-plane easy axis, thus the structure can lay the foundation for
low energy nonvolatile memory prototype
Voltage Control of Electromagnetic Properties in Antiferromagnetic Materials
Dynamic modulation of electromagnetic responses is theoretically examined in
dielectric antiferromagnets. While both magneto-electric and magneto-elastic
coupling can achieve robust electrical control of magnetic anisotropy, the
latter is considered in a bilayer structure with a piezoelectric material.
Numerical calculations based on the frequency-dependent permeability tensor
clearly illustrate that the anisotropy profile in the typical uniaxial or
biaxial antiferromagnets such as NiO and Cr2O3 can be modified sufficiently to
induce a shift in the resonance frequency by as much as tens of percent in the
sub-mm wavelength range (thus, an electrically tunable bandwidth over 10's of
GHz). The polarization of the electromagnetic response is also affected due to
the anisotropic nature of the effect, offering a possibility to encode the
signal. The intrinsic delay in switching may be minimized to the ns level by
using a sufficiently thin antiferromagnets. Application to specific devices
such as a band-pass filter further illustrates the validity of the concept
The cometary composition of a protoplanetary disk as revealed by complex cyanides
Observations of comets and asteroids show that the Solar Nebula that spawned
our planetary system was rich in water and organic molecules. Bombardment
brought these organics to the young Earth's surface, seeding its early
chemistry. Unlike asteroids, comets preserve a nearly pristine record of the
Solar Nebula composition. The presence of cyanides in comets, including 0.01%
of methyl cyanide (CH3CN) with respect to water, is of special interest because
of the importance of C-N bonds for abiotic amino acid synthesis. Comet-like
compositions of simple and complex volatiles are found in protostars, and can
be readily explained by a combination of gas-phase chemistry to form e.g. HCN
and an active ice-phase chemistry on grain surfaces that advances
complexity[3]. Simple volatiles, including water and HCN, have been detected
previously in Solar Nebula analogues - protoplanetary disks around young stars
- indicating that they survive disk formation or are reformed in situ. It has
been hitherto unclear whether the same holds for more complex organic molecules
outside of the Solar Nebula, since recent observations show a dramatic change
in the chemistry at the boundary between nascent envelopes and young disks due
to accretion shocks[8]. Here we report the detection of CH3CN (and HCN and
HC3N) in the protoplanetary disk around the young star MWC 480. We find
abundance ratios of these N-bearing organics in the gas-phase similar to
comets, which suggests an even higher relative abundance of complex cyanides in
the disk ice. This implies that complex organics accompany simpler volatiles in
protoplanetary disks, and that the rich organic chemistry of the Solar Nebula
was not unique.Comment: Definitive version of the manuscript is published in Nature, 520,
7546, 198, 2015. This is the author's versio
A Triple Protostar System Formed via Fragmentation of a Gravitationally Unstable Disk
Binary and multiple star systems are a frequent outcome of the star formation
process, and as a result, almost half of all sun-like stars have at least one
companion star. Theoretical studies indicate that there are two main pathways
that can operate concurrently to form binary/multiple star systems: large scale
fragmentation of turbulent gas cores and filaments or smaller scale
fragmentation of a massive protostellar disk due to gravitational instability.
Observational evidence for turbulent fragmentation on scales of 1000~AU has
recently emerged. Previous evidence for disk fragmentation was limited to
inferences based on the separations of more-evolved pre-main sequence and
protostellar multiple systems. The triple protostar system L1448 IRS3B is an
ideal candidate to search for evidence of disk fragmentation. L1448 IRS3B is in
an early phase of the star formation process, likely less than 150,000 years in
age, and all protostars in the system are separated by 200~AU. Here we
report observations of dust and molecular gas emission that reveal a disk with
spiral structure surrounding the three protostars. Two protostars near the
center of the disk are separated by 61 AU, and a tertiary protostar is
coincident with a spiral arm in the outer disk at a 183 AU separation. The
inferred mass of the central pair of protostellar objects is 1 M,
while the disk surrounding the three protostars has a total mass of 0.30
M_{\sun}. The tertiary protostar itself has a minimum mass of 0.085
M. We demonstrate that the disk around L1448 IRS3B appears susceptible
to disk fragmentation at radii between 150~AU and 320~AU, overlapping with the
location of the tertiary protostar. This is consistent with models for a
protostellar disk that has recently undergone gravitational instability,
spawning one or two companion stars.Comment: Published in Nature on Oct. 27th. 24 pages, 8 figure
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