64 research outputs found
New Symmetries in Crystals and Handed Structures
For over a century, the structure of materials has been described by a
combination of rotations, rotation-inversions and translational symmetries. By
recognizing the reversal of static structural rotations between clockwise and
counterclockwise directions as a distinct symmetry operation, here we show that
there are many more structural symmetries than are currently recognized in
right- or left-handed handed helices, spirals, and in antidistorted structures
composed equally of rotations of both handedness. For example, though a helix
or spiral cannot possess conventional mirror or inversion symmetries, they can
possess them in combination with the rotation reversal symmetry. Similarly, we
show that many antidistorted perovskites possess twice the number of symmetry
elements as conventionally identified. These new symmetries predict new forms
for "roto" properties that relate to static rotations, such as rotoelectricity,
piezorotation, and rotomagnetism. They also enable symmetry-based search for
new phenomena, such as multiferroicity involving a coupling of spins, electric
polarization and static rotations. This work is relevant to structure-property
relationships in all material structures with static rotations such as
minerals, polymers, proteins, and engineered structures.Comment: 15 Pages, 4 figures, 3 Tables; Fig. 2b has error
Grain size-dependent magnetic and electric properties in nanosized YMnO3 multiferroic ceramics
Magnetic and electric properties are investigated for the nanosized YMnO3 samples with different grain sizes (25 nm to 200 nm) synthesized by a modified Pechini method. It shows that magnetic and electric properties are strongly dependent on the grain size. The magnetic characterization indicates that with increasing grain size, the antiferromagnetic (AFM) transition temperature increases from 52 to 74 K. A corresponding shift of the dielectric anomaly is observed, indicating a strong correlation between the electric polarization and the magnetic ordering. Further analysis suggests that the rising of AFM transition temperature with increasing grain size should be from the structural origin, in which the strength of AFM interaction as well as the electrical polarization is dependent on the in-plane lattice parameters. Furthermore, among all samples, the sample with grain size of 95 nm is found to have the smallest leakage current density (< 1 μA/cm2)
Chirality of Matter Shows Up via Spin Excitations
Right- and left-handed circularly polarized light interact differently with
electronic charges in chiral materials. This asymmetry generates the natural
circular dichroism and gyrotropy, also known as the optical activity. Here we
demonstrate that optical activity is not a privilege of the electronic charge
excitations but it can also emerge for the spin excitations in magnetic matter.
The square-lattice antiferromagnet BaCoGeO offers an ideal arena to
test this idea, since it can be transformed to a chiral form by application of
external magnetic fields. As a direct proof of the field-induced chiral state,
we observed large optical activity when the light is in resonance with spin
excitations at sub-terahertz frequencies. In addition, we found that the
magnetochiral effect, the absorption difference for the light beams propagating
parallel and anti-parallel to the applied magnetic field, has an exceptionally
large amplitude close to 100%. All these features are ascribed to the
magnetoelectric nature of spin excitations as they interact both with the
electric and magnetic components of light
Electrical half-wave rectification at ferroelectric domain walls
Ferroelectric domain walls represent multifunctional 2D-elements with great
potential for novel device paradigms at the nanoscale. Improper ferroelectrics
display particularly promising types of domain walls, which, due to their
unique robustness, are the ideal template for imposing specific electronic
behavior. Chemical doping, for instance, induces p- or n-type characteristics
and electric fields reversibly switch between resistive and conductive
domain-wall states. Here, we demonstrate diode-like conversion of
alternating-current (AC) into direct-current (DC) output based on neutral
180 domain walls in improper ferroelectric ErMnO. By combining
scanning probe and dielectric spectroscopy, we show that the rectification
occurs for frequencies at which the domain walls are fixed to their equilibrium
position. The practical frequency regime and magnitude of the output is
controlled by the bulk conductivity. Using density functional theory we
attribute the transport behavior at the neutral walls to an accumulation of
oxygen defects. Our study reveals domain walls acting as 2D half-wave
rectifiers, extending domain-wall-based nanoelectronic applications into the
realm of AC technology
Value of risk scores in the decision to palliate patients with ruptured abdominal aortic aneurysm
Background: The aim of this study was to develop a 48-h mortality risk score, which included morphology data, for patients with ruptured abdominal aortic aneurysm presenting to an emergency department, and to assess its predictive accuracy and clinical effectiveness in triaging patients to immediate aneurysm repair, transfer or palliative care. Methods: Data from patients in the IMPROVE (Immediate Management of the Patient With Ruptured Aneurysm: Open Versus Endovascular Repair) randomized trial were used to develop the risk score. Variables considered included age, sex, haemodynamic markers and aortic morphology. Backwards selection was used to identify relevant predictors. Predictive performance was assessed using calibration plots and the C-statistic. Validation of the newly developed and other previously published scores was conducted in four external populations. The net benefit of treating patients based on a risk threshold compared with treating none was quantified. Results: Data from 536 patients in the IMPROVE trial were included. The final variables retained were age, sex, haemoglobin level, serum creatinine level, systolic BP, aortic neck length and angle, and acute myocardial ischaemia. The discrimination of the score for 48-h mortality in the IMPROVE data was reasonable (C-statistic 0·710, 95 per cent c.i. 0·659 to 0·760), but varied in external populations (from 0·652 to 0·761). The new score outperformed other published risk scores in some, but not all, populations. An 8 (95 per cent c.i. 5 to 11) per cent improvement in the C-statistic was estimated compared with using age alone. Conclusion: The assessed risk scores did not have sufficient accuracy to enable potentially life-saving decisions to be made regarding intervention. Focus should therefore shift to offering repair to more patients and reducing non-intervention rates, while respecting the wishes of the patient and family
Perturbed angular correlations investigations on YMnO3 multiferroic manganite
International audienceThe Perturbed Angular Correlation (PAC) technique was applied to study the yttrium local environment in YMnO3 multiferroic manganite. The electric field gradients (EFG) at the Y site have been measured as function of temperature, covering both ferroelectric and magnetic transitions. The results were compared with point charge model (PCM) calculations. The experimental results show two different EFG distributions for all temperatures. Only one can be directly attributed to the yttrium crystalline site in the hexagonal structure
- …