600 research outputs found
Quantum criticality at cryogenic melting of polar bubble lattices
Quantum fluctuations (QFs) caused by zero-point phonon vibrations (ZPPVs) are
known to prevent the occurrence of polar phases in bulk incipient
ferroelectrics down to 0K1-3. On the other hand, little is known about the
effects of QFs on the recently discovered topological patterns in ferroelectric
nanostructures4-9. Here, by using an atomistic effective Hamiltonian within
classical Monte Carlo (CMC) and path integral quantum Monte Carlo
(PI-QMC)1,3,10,11, we unveil how QFs affect the topology of several dipolar
phases in ultrathin Pb(Zr0.4Ti0.6)O3 (PZT) films. In particular, our PI-QMC
simulations show that the ZPPVs do not suppress polar patterns but rather
stabilize the labyrinth4, bimeron5 and bubble phases12,13 within a wider range
of bias field magnitudes. Moreover, we reveal that quantum fluctuations induce
a quantum critical point (QCP) separating a hexagonal bubble lattice from a
liquid-like state characterized by spontaneous motion, creation and
annihilation of polar bubbles at cryogenic temperatures. Finally, we show that
the discovered quantum melting is associated with anomalous physical response,
as, e.g., demonstrated by a negative longitudinal piezoelectric coefficient.Comment: Nature communication, accepted, 21 pages, 4 Fig
Elastic and magnetic effects on the infrared phonon spectra of MnF2
We measured the temperature dependent infrared reflectivity spectra of MnF2
between 4 K and room temperature. We show that the phonon spectrum undergoes a
strong renormalization at TN. The ab-initio calculation we performed on this
compound accurately predict the magnitude and the direction of the phonon
parameters changes across the antiferromagnetic transition, showing that they
are mainly induced by the magnetic order. In this material, we found that the
dielectric constant is mostly from phonon origin. The large change in the
lattice parameters with temperature seen by X-ray diffraction as well as the
A2u phonon softening below TN indicate that magnetic order induced distortions
in MnF2 are compatible with the ferroelectric instabilities observed in TiO2,
FeF2 and other rutile-type fluorides. This study also shows the anomalous
temperature evolution of the lower energy Eu mode in the paramagnetic phase,
which can be compared to that of the B1g one seen by Raman spectroscopy in many
isostructural materials. This was interpreted as being a precursor of a phase
transition from rutile to CaCl2 structure which was observed under pressure in
ZnF2.Comment: 8 pages, 8 figures, updated version accepted in PR
Giant electrocaloric response in the prototypical Pb(Mg,Nb)O relaxor ferroelectric from atomistic simulations
An atomistic effective Hamiltonian is used to investigate electrocaloric (EC)
effects of Pb(MgNb)O (PMN) relaxor ferroelectrics in its
ergodic regime, and subject to electric fields applied along the pseudocubic
[111] direction. Such Hamiltonian qualitatively reproduces (i) the electric
field-versus-temperature phase diagram, including the existence of a critical
point where first-order and second-order transitions meet each other; and (ii)
a giant EC response near such critical point. It also reveals that such giant
response around this critical point is microscopically induced by field-induced
percolation of polar nanoregions. Moreover, it is also found that, for any
temperature above the critical point, the EC coefficient-versus-electric field
curve adopts a maximum (and thus larger electrocaloric response too), that can
be well described by the general Landau-like model proposed in [Jiang et al,
Phys. Rev. B 96, 014114 (2017)] and that is further correlated with specific
microscopic features related to dipoles lying along different rhombohedral
directions. Furthermore, for temperatures being at least 40 K higher than the
critical temperature, the (electric field, temperature) line associated with
this maximal EC coefficient is below both the Widom line and the line
representing percolation of polar nanoregions.Comment: 6 pages, 5 figure
Six state molecular revolver mounted on a rigid platform
The rotation of entire molecules or large moieties happens at 100 ps time scales and the transition process itself is experimentally inaccessible to scanning probe techniques. However, the reversible switching of a molecule between more than two metastable states allows to assign a rotational switching direction. Rotational switching is a phenomenon that is particularly interesting with regard to possible applications in molecular motors. In this work, single tetraphenylmethane molecules deposited on a Au(111) surface were studied in a low temperature scanning tunneling microscope (STM). These molecules comprise rotational axes mounted on a tripodal sulfur-anchored stand and with the STM tip, we were able to induce transitions between six rotational states of the molecular motif. We were able to identify critical parameters for the onset of rotational switching and to characterize the influence of the local environment. The subtle difference between fcc and hcp stacking and the rotational state of neighboring molecules clearly influence the population of the rotational states
Ferroelectric Solitons Crafted in Epitaxial Bismuth Ferrite Superlattices
In ferroelectrics, complex interactions among various degrees of freedom
enable the condensation of topologically protected polarization textures. Known
as ferroelectric solitons, these particle-like structures represent a new class
of materials with promise for beyond CMOS technologies due to their ultrafine
size and sensitivity to external stimuli. Such polarization textures have
scarcely been reported in multiferroics. Here, we report a range of soliton
topologies in bismuth ferrite strontium titanate superlattices. High-resolution
piezoresponse force microscopy and Cs-corrected high-angle annular dark-field
scanning transmission electron microscopy reveal a zoo of topologies, and
polarization displacement mapping of planar specimens reveals center-convergent
and divergent topological defects as small as 3 nm. Phase field simulations
verify that some of these topologies can be classed as bimerons, with a
topological charge of plus and minus one, and first-principles-based effective
Hamiltonian computations show that the co-existence of such structures can lead
to non-integer topological charges, a first observation in a BiFeO3-based
system. Our results open new opportunities in multiferroic topotronics
Biallelic mutations in valyl-tRNA synthetase gene VARS are associated with a progressive neurodevelopmental epileptic encephalopathy.
Aminoacyl-tRNA synthetases (ARSs) function to transfer amino acids to cognate tRNA molecules, which are required for protein translation. To date, biallelic mutations in 31 ARS genes are known to cause recessive, early-onset severe multi-organ diseases. VARS encodes the only known valine cytoplasmic-localized aminoacyl-tRNA synthetase. Here, we report seven patients from five unrelated families with five different biallelic missense variants in VARS. Subjects present with a range of global developmental delay, epileptic encephalopathy and primary or progressive microcephaly. Longitudinal assessment demonstrates progressive cortical atrophy and white matter volume loss. Variants map to the VARS tRNA binding domain and adjacent to the anticodon domain, and disrupt highly conserved residues. Patient primary cells show intact VARS protein but reduced enzymatic activity, suggesting partial loss of function. The implication of VARS in pediatric neurodegeneration broadens the spectrum of human diseases due to mutations in tRNA synthetase genes
Nanometric moiré stripes on the surface of Bi2Se3 topological insulator
Mismatch between adjacent atomic layers in low-dimensional materials, generating moiré patterns, has recently emerged as a suitable method to tune electronic properties by inducing strong electron correlations and generating novel phenomena. Beyond graphene, van der Waals structures such as three-dimensional (3D) topological insulators (TIs) appear as ideal candidates for the study of these phenomena due to the weak coupling between layers. Here we discover and investigate the origin of 1D moiré stripes on the surface of Bi2Se3TI thin films and nanobelts. Scanning tunneling microscopy and high-resolution transmission electron microscopy reveal a unidirectional strained top layer, in the range 14-25%, with respect to the relaxed bulk structure, which cannot be ascribed to the mismatch with the substrate lattice but rather to strain induced by a specific growth mechanism. The 1D stripes are characterized by a spatial modulation of the local density of states, which is strongly enhanced compared to the bulk system. Density functional theory calculations confirm the experimental findings, showing that the TI surface Dirac cone is preserved in the 1D moiré stripes, as expected from the topology, though with a heavily renormalized Fermi velocity that also changes between the top and valley of the stripes. The strongly enhanced density of surface states in the TI 1D moiré superstructure can be instrumental in promoting strong correlations in the topological surface states, which can be responsible for surface magnetism and topological superconductivity
Magnetoresistance through a single molecule
The use of single molecules to design electronic devices is an extremely
challenging and fundamentally different approach to further downsizing
electronic circuits. Two-terminal molecular devices such as diodes were first
predicted [1] and, more recently, measured experimentally [2]. The addition of
a gate then enabled the study of molecular transistors [3-5]. In general terms,
in order to increase data processing capabilities, one may not only consider
the electron's charge but also its spin [6,7]. This concept has been pioneered
in giant magnetoresistance (GMR) junctions that consist of thin metallic films
[8,9]. Spin transport across molecules, i.e. Molecular Spintronics remains,
however, a challenging endeavor. As an important first step in this field, we
have performed an experimental and theoretical study on spin transport across a
molecular GMR junction consisting of two ferromagnetic electrodes bridged by a
single hydrogen phthalocyanine (H2Pc) molecule. We observe that even though
H2Pc in itself is nonmagnetic, incorporating it into a molecular junction can
enhance the magnetoresistance by one order of magnitude to 52%.Comment: To appear in Nature Nanotechnology. Present version is the first
submission to Nature Nanotechnology, from May 18th, 201
- …