5 research outputs found
Switchable Anomalous Hall Effects in Polar-Stacked 2D Antiferromagnet MnBi<sub>2</sub>Te<sub>4</sub>
van der Waals (vdW) assembly of two-dimensional
(2D) materials
allows polar layer stacking to realize novel properties switchable
by the induced electric polarization. Here, based on symmetry analyses
and density-functional calculations, we explore the emergence of the
anomalous Hall effect (AHE) in antiferromagnetic MnBi2Te4 films assembled by polar layer stacking. We demonstrate that
breaking PÌ‚TÌ‚ symmetry in an MnBi2Te4 bilayer produces a magnetoelectric effect and
a spontaneous AHE switchable by electric polarization. We find that
reversible polarization at one of the interfaces in a three-layer
MnBi2Te4 film drives a metal–insulator
transition, as well as switching between the AHE and quantum AHE (QAHE).
Finally, we predict that engineering interlayer polarization in a
three-layer MnBi2Te4 film allows converting
MnBi2Te4 from a trivial insulator to a Chern
insulator. Overall, our work emphasizes the topological properties
in 2D vdW antiferromagnets induced by polar layer stacking, which
do not exist in a bulk material
Multiferroic Materials Based on Organic Transition-Metal Molecular Nanowires
We report on the density functional theory aided design
of a variety
of organic ferroelectric and multiferroic materials by functionalizing
crystallized transition-metal molecular sandwich nanowires with chemical
groups such as −F, −Cl, −CN, −NO<sub>2</sub>, O, and −OH. Such functionalized polar wires exhibit
molecular reorientation in response to an electric field. Ferroelectric
polarizations as large as 23.0 μC/cm<sup>2</sup> are predicted
in crystals based on fully hydroxylized sandwich nanowires. Furthermore,
we find that organic nanowires formed by sandwiching transition-metal
atoms in croconic and rhodizonic acids, dihydroxybenzoquinone, dichloro-dihydroxy-<i>p</i>-benzoquinone, or benzene decorated by −COOH groups
exhibit ordered magnetic moments, leading to a multiferroic organometallic
crystal. When crystallized through hydrogen bonds, the microscopic
molecular reorientation translates into a switchable polarization
through proton transfer. A giant interface magnetoelectric response
that is orders of magnitude greater than previously reported for conventional
oxide heterostructure interfaces is predicted
Solid-State Synapse Based on Magnetoelectrically Coupled Memristor
Brain-inspired computing
architectures attempt to emulate the computations performed in the
neurons and the synapses in the human brain. Memristors with continuously
tunable resistances are ideal building blocks for artificial synapses.
Through investigating the memristor behaviors in a La<sub>0.7</sub>Sr<sub>0.3</sub>MnO<sub>3</sub>/BaTiO<sub>3</sub>/La<sub>0.7</sub>Sr<sub>0.3</sub>MnO<sub>3</sub> multiferroic tunnel junction, it
was found that the ferroelectric domain dynamics characteristics are
influenced by the relative magnetization alignment of the electrodes,
and the interfacial spin polarization is manipulated continuously
by ferroelectric domain reversal, enriching our understanding of the
magnetoelectric coupling fundamentally. This creates a functionality
that not only the resistance of the memristor but also the synaptic
plasticity form can be further manipulated, as demonstrated by the
spike-timing-dependent plasticity investigations. Density functional
theory calculations are carried out to describe the obtained magnetoelectric
coupling, which is probably related to the Mn–Ti intermixing
at the interfaces. The multiple and controllable plasticity characteristic
in a single artificial synapse, to resemble the synaptic morphological
alteration property in a biological synapse, will be conducive to
the development of artificial intelligence
Imprint Control of BaTiO<sub>3</sub> Thin Films via Chemically Induced Surface Polarization Pinning
Surface-adsorbed polar molecules
can significantly alter the ferroelectric properties of oxide thin
films. Thus, fundamental understanding and controlling the effect
of surface adsorbates are crucial for the implementation of ferroelectric
thin film devices, such as ferroelectric tunnel junctions. Herein,
we report an imprint control of BaTiO<sub>3</sub> (BTO) thin films
by chemically induced surface polarization pinning in the top few
atomic layers of the water-exposed BTO films. Our studies based on
synchrotron X-ray scattering and coherent Bragg rod analysis demonstrate
that the chemically induced surface polarization is not switchable
but reduces the polarization imprint and improves the bistability
of ferroelectric phase in BTO tunnel junctions. We conclude that the
chemical treatment of ferroelectric thin films with polar molecules
may serve as a simple yet powerful strategy to enhance functional
properties of ferroelectric tunnel junctions for their practical applications
Surface Electronic Structure of Hybrid Organo Lead Bromide Perovskite Single Crystals
The electronic structure and band
dispersion of methylammonium
lead bromide, CH<sub>3</sub>NH<sub>3</sub>PbBr<sub>3</sub>, has been
investigated through a combination of angle-resolved photoemission
spectroscopy (ARPES) and inverse photoemission spectroscopy (IPES),
as well as theoretical modeling based on density functional theory.
The experimental band structures are consistent with the density functional
calculations. The results demonstrate the presence of a dispersive
valence band in MAPbBr<sub>3</sub> that peaks at the MÌ… point
of the surface Brillouin zone. The results also indicate that the
surface termination of the CH<sub>3</sub>NH<sub>3</sub>PbBr<sub>3</sub> is the methylammonium bromide (CH<sub>3</sub>NH<sub>3</sub>Br) layer.
We find our results support models that predict a heavier hole effective
mass in the region of −0.23 to −0.26 m<sub>e</sub>,
along the Γ̅ (surface Brillouin center) to M̅ point
of the surface Brillouin zone. The surface appears to be n-type as
a result of an excess of lead in the surface region