arXiv subjects

Vladimir Koval

Publications and source records attributed to Vladimir Koval.

3 recordsLinked to original sources

arXiv2020

Polar nano-clusters in nominally paraelectric ceramics demonstrating high microwave tunability for wireless communication

Dielectric materials, with high tunability at microwave frequencies, are key components in the design of microwave communication systems. Dense Ba0.6Sr0.4TiO3 (BST) ceramics, with different grain sizes, were prepared in order to optimise the dielectric tunability via polar nano cluster effects. Dielectric permittivity and loss measurements were carried at both high and low frequencies and were supported by results from X-ray powder diffraction, scanning and transmission electron microscopies, Raman spectroscopy and piezoresponse force microscopy. The concentration of polar nano clusters, whose sizes are found to be in the range 20 to 50 nm, and the dielectric tunability increase with increasing grain size. A novel method for measurement of the microwave tunability in bulk dielectrics is presented. The highest tunability of 32% is achieved in ceramics with an average grain size of 10 um. The tunability of BST ceramics with applied DC field is demonstrated in a prototype small resonant antenna.

physics.app-ph↗

arXiv2018

Topological thermal Hall effect driven by fluctuation of spin chirality in frustrated antiferromagnets

By revealing an underlying relation between the Dzyaloshinskii-Moriya interaction (DMI) and the scalar spin chirality, we develop the theory of magnon thermal Hall effects in antiferromagnetic systems. The dynamic fluctuation of the scalar chirality is shown to directly respond to the nontrivial topology of magnon bands. In materials such as the jarosites compounds KFe$_3$(OH)$_6$(SO$_4$)$_2$ and veseignite BaCu$_3$V$_2$O$_8$(OH)$_2$ in the presence of in-plane DMI, the time-reversal symmetry can be broken by the fluctuations of scalar chirality even in the case of coplanar $\mathbf{q}=0$ magnetic configuration. The spin-wave Hamiltonian is influenced by a fictitious magnetic flux determined by the in-plane DMI. Topological magnon bands and corresponding nonzero Chern numbers are presented without the need of a canted non-coplanar magnetic ordering. The canting angle dependence of thermal Hall conductivity is discussed in detail as well. These results offer a clear principle of chirality-driven topological effects in antiferromagnetically coupled systems.

cond-mat.str-el↗

arXiv2018

The origin of grain size effects in Ba(Ti0.96Sn0.04)O3 perovskite ceramics with superior electrical properties

The study of grain size effects in ferroelectric ceramics has attracted great research interest over the last 50 years. Although different theoretical models have been proposed to account for the variation in structure and properties with grain size, the underlying mechanisms are still under debate, creating a significant level of uncertainty in the field. Here, we report the results of a study on the influence of grain size on the structural and physical properties of Ba(Ti0.96Sn0.04)O3, which represents a model perovskite system, where the effects of point defects, stoichiometry imbalance and phase transitions are minimized by Sn substitution. It was found that different microscopic mechanisms are responsible for the various grain size dependences observed. In fine-grained ceramics, high permittivity is due to high domain wall density and polar nanoregions; high d33 in coarse-grained ceramics results from a high degree of domain alignment during poling; large electric field-induced strain in intermediate-grained ceramics is an outcome of a favourable interplay between constraints from grain boundaries and reversible reorientation of non-180 degree domains and polar nanoregions. These paradigms can be regarded as general guidelines for the optimization of specific properties through grain size control.

cond-mat.mtrl-sci↗