Search results for "Kite"
showing 10 items of 1123 documents
Giant Anomalous Nernst Effect in Noncollinear Antiferromagnetic Mn-based Antiperovskite Nitrides
2020
The anomalous Nernst effect (ANE) - the generation of a transverse electric voltage by a longitudinal heat current in conducting ferromagnets or antiferromagnets - is an appealing approach for thermoelectric power generation in spin caloritronics. The ANE in antiferromagnets is particularly convenient for the fabrication of highly efficient and densely integrated thermopiles as lateral configurations of thermoelectric modules increase the coverage of heat source without suffering from the stray fields that are intrinsic to ferromagnets. In this work, using first-principles calculations together with a group theory analysis, we systematically investigate the spin order-dependent ANE in nonco…
Tetragonal tungsten bronze compounds: relaxor versus mixed ferroelectric-dipole glass behavior.
2011
We demonstrate that recent experimental data (E. Castel et al J.Phys. Cond. Mat. {\bf 21} (2009), 452201) on tungsten bronze compound (TBC) Ba$_2$Pr$_x$Nd$_{1-x}$FeNb$_4$O$_{15}$ can be well explained in our model predicting a crossover from ferroelectric ($x=0$) to orientational (dipole) glass ($x=1$), rather then relaxor, behavior. We show, that since a "classical" perovskite relaxor like Pb(Mn$_{1/3}$ Nb$_{2/3}$)O$_3$ is never a ferroelectric, the presence of ferroelectric hysteresis loops in TBC shows that this substance actually transits from ferroelectric to orientational glass phase with $x$ growth. To describe the above crossover theoretically, we use the simple replica-symmetric so…
Manifestation of dipole-induced disorder in self-assembly of ferroelectric and ferromagnetic nanocubes
2019
The authors thank Marjeta Maˇcek Kržmanc for many useful discussions. The financial support of M-ERA.NET Project Har-vEnPiez (Innovative nano-materials and architectures for integrated piezoelectric energy harvesting applications) is gratefully acknowledged. D.Z. acknowledges the support of the postdoctoral research program at the University of Latvia (Project No. 1.1.1.2/VIAA/1/16/072). The computing time of the LASC cluster was provided by the Institute of Solid State Physics (ISSP).
Controlling thermal conductance using three-dimensional phononic crystals
2021
Controlling thermal transport at the nanoscale is vital for many applications. Previously, it has been shown that this control can be achieved with periodically nanostructured two-dimensional phononic crystals for the case of suspended devices. Here, we show that thermal conductance can also be controlled with three-dimensional phononic crystals, allowing the engineering of the thermal contact of more varied devices without the need for suspension in the future. We show the experimental results obtained at sub-Kelvin temperatures for two different period three-dimensional crystals and for a bulk control structure. The results show that the conductance can be enhanced with the phononic cryst…
Electronic structure of two crystallographic forms ofBaRuO3
2000
Electronic structure calculations have been performed to explain the difference in the electronic properties of two crystallographic forms of ${\mathrm{BaRuO}}_{3}.$ The calculations can explain the qualitatively different resistivities of isoelectronic $4H$- and $9R$-${\mathrm{BaRuO}}_{3}$ below 100 K. The difference in symmetry between the hexagonal four-layer ${\mathrm{BaRuO}}_{3}$ and the rhombohedral nine-layer compound allows the formation of a gap for the later. The electronic structure of these hexagonal perovskites is compared with the more familiar cubic perovskite ${\mathrm{CaRuO}}_{3}.$
Calculations of radiation-induced point defects, polarons and excitons in ferroelectric perovskites
2002
Abstract We review results of our recent large-scale computer simulations of radiation-induced point defects, excitons and polarons in ABO3 perovskite crystals, focusing mostly on KNbO3 and KTaO3 as representative examples. We have calculated the atomic and electronic structure of defects, their optical absorption, defect-induced electron density redistribution, and activation energies for defect migration. The majority of our results were obtained using the quantum-chemical method of the intermediate neglect of differential overlap (INDO) based on the Hartree–Fock formalism, as well as the shell model (SM). The main findings are compared with those obtained by means of ab initio density fu…
Large scale computer modelling of point defects in ABO 3 perovskites
2005
We present results for basic intrinsic defects: F-type electron centers, free and bound electron and hole polarons in ABO3 perovskites. Both one-site (atomic) and two-site (molecular) hole polarons are expected to coexist, characterized by close absorption energies. Shell Model (SM) and intermediate neglect of differential overlap (INDO) calculations of the F center diffusion indicate that the relevant activation energy is quite low, ca. 0.8 eV. Further INDO calculations support the existence of self-trapped electron polarons in PbTiO3, BaTiO3, KNbO3, and KTaO3 crystals. The relevant lattice relaxation energies are typically 0.2 eV, whereas the optical absorption energies are around 0.8 eV.…
Magnetic field tuning of the smart materials domain structure
1998
The investigation of the behavior of the ferroelectric phase transition with magnetic filed tuning and concentration change is highly attractive owing to the possibility to prepare alloying samples and to predict theoretically the parameters of the magnetic field and concentration response at relatively small concentrations. These parameters may be extracted from the equation of states of the perovskite under investigation in the assumption of the linear response. The study of the movement of the paraelectric - ferroelectric interphase boundary in (Ba,Sr)TiO 3 with concentration change and in constant magnetic fields is provided in the framework of the mean-field theory. The analytical solu…
First-Principles Simulation of Substitutional Defects in Perovskites
2000
The results of supercell calculations of electronic structure and related properties of substitutional impurities in perovskite oxides KNbO3 and KTaO3 are discussed. For Fe impurities in KNbO3, the results obtained in the local density approximation (LDA) and in the LDA+U approach (that allows an ad hoc treatment of nonlocality in exchange-correlation) are compared, and different impurity charge configurations are discussed. The study of off-centre Li defects in incipient ferroelectric KTaO3 have been done by the appropriately parametrized Intermediate Neglect of Differential Overlap (INDO) method. The interaction energies of two off-centre impurities in different relative configurations ar…
Theoretical Prediction and Experimental Confirmation of Charge Transfer Vibronic Excitons and Their Phase in ABO3 Perovskite Crystals
2002
AbstractThe current theoretical and experimental knowledge of new polaronic-type excitons in ferroelectric oxides - charge transfer vibronic excitons (CTVE) is discussed. It is shown that quantum chemical Hartree-Fock-type calculations using a semiempirical Intermediate Neglect of Differential Overlap (INDO) method (modified for ionic/partly ionic solids) as well as photoluminescence studies in ferroelectric oxygen-octahedral perovskites confirm the CTVE existence. Our INDO calculations for KTaO3 and KNbO3 have demonstrated that the triplet exciton is a triad centre containing one active O atom and two Ta atoms sitting on the opposite sites from this O atom. The total energy of a system is …