48 research outputs found
Ferroelectricity induced by interatomic magnetic exchange interaction
Multiferroics, where two or more ferroic order parameters coexist, is one of
the hottest fields in condensed matter physics and materials science[1-9].
However, the coexistence of magnetism and conventional ferroelectricity is
physically unfavoured[10]. Recently several remedies have been proposed, e.g.,
improper ferroelectricity induced by specific magnetic[6] or charge orders[2].
Guiding by these theories, currently most research is focused on frustrated
magnets, which usually have complicated magnetic structure and low magnetic
ordering temperature, consequently far from the practical application. Simple
collinear magnets, which can have high magnetic transition temperature, have
never been considered seriously as the candidates for multiferroics. Here, we
argue that actually simple interatomic magnetic exchange interaction already
contains a driving force for ferroelectricity, thus providing a new microscopic
mechanism for the coexistence and strong coupling between ferroelectricity and
magnetism. We demonstrate this mechanism by showing that even the simplest
antiferromagnetic (AFM) insulator MnO, can display a magnetically induced
ferroelectricity under a biaxial strain
Theoretical study of the insulating oxides and nitrides: SiO2, GeO2, Al2O3, Si3N4, and Ge3N4
An extensive theoretical study is performed for wide bandgap crystalline
oxides and nitrides, namely, SiO_{2}, GeO_{2}, Al_{2}O_{3}, Si_{3}N_{4}, and
Ge_{3}N_{4}. Their important polymorphs are considered which are for SiO_{2}:
-quartz, - and -cristobalite and stishovite, for
GeO_{2}: -quartz, and rutile, for Al_{2}O_{3}: -phase, for
Si_{3}N_{4} and Ge_{3}N_{4}: - and -phases. This work
constitutes a comprehensive account of both electronic structure and the
elastic properties of these important insulating oxides and nitrides obtained
with high accuracy based on density functional theory within the local density
approximation. Two different norm-conserving \textit{ab initio}
pseudopotentials have been tested which agree in all respects with the only
exception arising for the elastic properties of rutile GeO_{2}. The agreement
with experimental values, when available, are seen to be highly satisfactory.
The uniformity and the well convergence of this approach enables an unbiased
assessment of important physical parameters within each material and among
different insulating oxide and nitrides. The computed static electric
susceptibilities are observed to display a strong correlation with their mass
densities. There is a marked discrepancy between the considered oxides and
nitrides with the latter having sudden increase of density of states away from
the respective band edges. This is expected to give rise to excessive carrier
scattering which can practically preclude bulk impact ionization process in
Si_{3}N_{4} and Ge_{3}N_{4}.Comment: Published version, 10 pages, 8 figure
Computational Study of the Human Dystrophin Repeats: Interaction Properties and Molecular Dynamics
Dystrophin is a large protein involved in the rare genetic disease Duchenne muscular dystrophy (DMD). It functions as a mechanical linker between the cytoskeleton and the sarcolemma, and is able to resist shear stresses during muscle activity. In all, 75% of the dystrophin molecule consists of a large central rod domain made up of 24 repeat units that share high structural homology with spectrin-like repeats. However, in the absence of any high-resolution structure of these repeats, the molecular basis of dystrophin central domain's functions has not yet been deciphered. In this context, we have performed a computational study of the whole dystrophin central rod domain based on the rational homology modeling of successive and overlapping tandem repeats and the analysis of their surface properties. Each tandem repeat has very specific surface properties that make it unique. However, the repeats share enough electrostatic-surface similarities to be grouped into four separate clusters. Molecular dynamics simulations of four representative tandem repeats reveal specific flexibility or bending properties depending on the repeat sequence. We thus suggest that the dystrophin central rod domain is constituted of seven biologically relevant sub-domains. Our results provide evidence for the role of the dystrophin central rod domain as a scaffold platform with a wide range of surface features and biophysical properties allowing it to interact with its various known partners such as proteins and membrane lipids. This new integrative view is strongly supported by the previous experimental works that investigated the isolated domains and the observed heterogeneity of the severity of dystrophin related pathologies, especially Becker muscular dystrophy
QCD and strongly coupled gauge theories : challenges and perspectives
We highlight the progress, current status, and open challenges of QCD-driven physics, in theory and in experiment. We discuss how the strong interaction is intimately connected to a broad sweep of physical problems, in settings ranging from astrophysics and cosmology to strongly coupled, complex systems in particle and condensed-matter physics, as well as to searches for physics beyond the Standard Model. We also discuss how success in describing the strong interaction impacts other fields, and, in turn, how such subjects can impact studies of the strong interaction. In the course of the work we offer a perspective on the many research streams which flow into and out of QCD, as well as a vision for future developments.Peer reviewe
Combined time- and space-resolved Raman spectrometer for the non-invasive depth profiling of chemical hazards
A time-resolved inverse spatially offset Raman spectrometer was constructed for depth profiling of Raman-active substances under both the lab and the field environments. The system operating principles and performance are discussed along with its advantages relative to traditional continuous wave spatially offset Raman spectrometer. The developed spectrometer uses a combination of space- and time-resolved detection in order to obtain high-quality Raman spectra from substances hidden behind coloured opaque surface layers, such as plastic and garments, with a single measurement. The time-gated spatially offset Raman spectrometer was successfully used to detect concealed explosives and drug precursors under incandescent and fluorescent background light as well as under daylight. The average screening time was 50 s per measurement. The excitation energy requirements were relatively low (20 mW) which makes the probe safe for screening hazardous substances. The unit has been designed with nanosecond laser excitation and gated detection, making it of lower cost and complexity than previous picosecond-based systems, to provide a functional platform for in-line or in-field sensing of chemical substances