Krishna Chaitanya Pitike

Bio: Krishna Chaitanya Pitike is an academic researcher from Oak Ridge National Laboratory. The author has contributed to research in topics: Dielectric & Perovskite (structure). The author has an hindex of 10, co-authored 24 publications receiving 270 citations. Previous affiliations of Krishna Chaitanya Pitike include University of Connecticut.

Towards an accurate description of perovskite ferroelectrics: exchange and correlation effects

Abstract: Using the van der Waals density functional with C09 exchange (vdW-DF-C09), which has been applied to describing a wide range of dispersion-bound systems, we explore the physical properties of prototypical ABO3 bulk ferroelectric oxides. Surprisingly, vdW-DF-C09 provides a superior description of experimental values for lattice constants, polarization and bulk moduli, exhibiting similar accuracy to the modified Perdew-Burke-Erzenhoff functional which was designed specifically for bulk solids (PBEsol). The relative performance of vdW-DF-C09 is strongly linked to the form of the exchange enhancement factor which, like PBEsol, tends to behave like the gradient expansion approximation for small reduced gradients. These results suggest the general-purpose nature of the class of vdW-DF functionals, with particular consequences for predicting material functionality across dense and sparse matter regimes.

Phase-field model for dielectric breakdown in solids

Abstract: Using an analogy between dielectric breakdown and fracture of solids, this paper develops a phase field model for the electric damage initiation and propagation in dielectric solids during breakdown. Instead of explicitly tracing the growth of a conductive channel, the model introduces a continuous phase field to characterize the degree of damage, and the conductive channel is represented by a localized region of fully damaged material. Similar as in the classic theory of fracture mechanics, an energetic criterion is taken: The conductive channel will grow only if the electrostatic energy released per unit length of the channel is greater than that dissipated through damage. Such an approach circumvents the detailed analysis on the complex microscopic processes near the tip of a conductive channel and provides a means of quantitatively predicting breakdown phenomena in materials, composites, and devices. This model is implemented into a finite-element code, and several numerical examples are solved. With randomly distributed defects, the model recovers the inverse power relation between breakdown strength and sample thickness. Finally, the effect of the layered structure in a breakdown-resistant laminate is demonstrated through a numerical example.

Probing the Local Site Disorder and Distortion in Pyrochlore High-Entropy Oxides.

Abstract: High-entropy oxides (HEOs) have attracted great interest in diverse fields because of their inherent opportunities to tailor and combine materials functionalities. The control of local order/disorder in the class is by extension a grand challenge toward realizing their vast potential. Here we report the first examples of pyrochlore HEOs with five M-site cations, for Nd2M2O7, in which the local structure has been investigated by neutron diffraction and pair distribution function (PDF) analysis. The average structure of the pyrochlores is found to be orthorhombic Imma, in agreement with radius-ratio rules governing the structural archetype. The computed PDFs from density functional theory relaxed special quasirandom structure models are compared with real space PDFs in this work to evaluate M-site order/disorder. Reverse Monte Carlo combined with ab initio molecular dynamics and Metropolis Monte Carlo simulations demonstrates that Nd2(Ta0.2Sc0.2Sn0.2Hf0.2Zr0.2)2O7 is synthesized with its M-site local to nanoscale order highly randomized/disordered, while Nd2(Ti0.2Nb0.2Sn0.2Hf0.2Zr0.2)2O7+x exhibits a strong distortion of the TiO6 octahedron and small degree of Ti chemical short-range order (SRO) on the subnanometer scale. Calculations suggest that this may be intrinsic, energetically favored SRO rather than due to sample processing. These results offer an important demonstration that the engineered variation of participating ions in HEOs, even among those with very similar radii, provides richly diverse opportunities to control local order/disorder motifs-and therefore materials properties for future designs. This work also hints at the exquisite level of detail that may be needed in computational and experimental data analysis to guide structure-property tuning in the emerging HEO materials class.

Room-temperature relaxor ferroelectricity and photovoltaic effects in tin titanate directly deposited on a silicon substrate

Abstract: Tin titanate ($\mathrm{SnTi}{\mathrm{O}}_{3}$) has been notoriously impossible to prepare as a thin-film ferroelectric, probably because high-temperature annealing converts much of the $\mathrm{S}{\mathrm{n}}^{2+}$ to $\mathrm{S}{\mathrm{n}}^{4+}$. In the present paper, we show two things: first, perovskite phase $\mathrm{SnTi}{\mathrm{O}}_{3}$ can be prepared by atomic-layer deposition directly onto $p$-type Si substrates; and second, these films exhibit ferroelectric switching at room temperature, with $p$-type Si acting as electrodes. X-ray diffraction measurements reveal that the film is single-phase, preferred-orientation ferroelectric perovskite $\mathrm{SnTi}{\mathrm{O}}_{3}$. Our films showed well-saturated, square, and repeatable hysteresis loops of around $3\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{C}/\mathrm{c}{\mathrm{m}}^{2}$ remnant polarization at room temperature, as detected by out-of-plane polarization versus electric field and field cycling measurements. Furthermore, photovoltaic and photoferroelectricity were found in $\mathrm{Pt}/\mathrm{SnTi}{\mathrm{O}}_{3}/\mathrm{Si}/\mathrm{SnTi}{\mathrm{O}}_{3}/\mathrm{Pt}$ heterostructures, the properties of which can be tuned through band-gap engineering by strain according to first-principles calculations. This is a lead-free room-temperature ferroelectric oxide of potential device application.

Electrical switching of an antiferromagnet

Abstract: Manipulating a stubborn magnet Spintronics is an alternative to conventional electronics, based on using the electron's spin rather than its charge. Spintronic devices, such as magnetic memory, have traditionally used ferromagnetic materials to encode the 1's and 0's of the binary code. A weakness of this approach—that strong magnetic fields can erase the encoded information—could be avoided by using antiferromagnets instead of ferromagnets. But manipulating the magnetic ordering of antiferromagnets is tricky. Now, Wadley et al. have found a way (see the Perspective by Marrows). Running currents along specific directions in the thin films of the antiferromagnetic compound CuMnAs reoriented the magnetic domains in the material. Science, this issue p. 587; see also p. 558 Transport and optical measurements are used to demonstrate the switching of domains in the antiferromagnetic compound CuMnAs. [Also see Perspective by Marrows] Antiferromagnets are hard to control by external magnetic fields because of the alternating directions of magnetic moments on individual atoms and the resulting zero net magnetization. However, relativistic quantum mechanics allows for generating current-induced internal fields whose sign alternates with the periodicity of the antiferromagnetic lattice. Using these fields, which couple strongly to the antiferromagnetic order, we demonstrate room-temperature electrical switching between stable configurations in antiferromagnetic CuMnAs thin-film devices by applied current with magnitudes of order 106 ampere per square centimeter. Electrical writing is combined in our solid-state memory with electrical readout and the stored magnetic state is insensitive to and produces no external magnetic field perturbations, which illustrates the unique merits of antiferromagnets for spintronics.

Composition 构好图，出好片

Abstract: Balsamodendron mukul (Guggul): The Oleo-gum-resin exuded by this tree has been regarded as a sovereign remedy in ancient medicine. It is a bitter stomachic and carminative and improves digestion. It is quickly absorbed and is excreted by skin, mucous membranes and kidneys and in the course of excretion it disinfects their secretions improves the function and stimulates the activity of the respective organs. It has marked antiseptic properties.

First-principles calculations of electronic structure and spectra of strongly correlated systems: the LDA+U method

Abstract: The generalization of the Local Density Approximation (LDA) method for the systems with strong Coulomb correlations is presented which gives a correct description of the Mott insulators. The LDA+U method is based on the model hamiltonian approach and allows to take into account the non-sphericity of the Coulomb and exchange interactions. parameters. Orbital-dependent LDA+U potential gives correct orbital polarization and corresponding Jahn-Teller distortion. To calculate the spectra of the strongly correlated systems the impurity Anderson model should be solved with a many-electron trial wave function. All parameters of the many-electron hamiltonian are taken from LDA+U calculations. The method was applied to NiO and has shown good agreement with experimental photoemission spectra and with the oxygen Kα X-ray emission spectrum.

High-entropy ceramics: Present status, challenges, and a look forward

Abstract: High-entropy ceramics (HECs) are solid solutions of inorganic compounds with one or more Wyckoff sites shared by equal or near-equal atomic ratios of multi-principal elements. Although in the infant stage, the emerging of this new family of materials has brought new opportunities for material design and property tailoring. Distinct from metals, the diversity in crystal structure and electronic structure of ceramics provides huge space for properties tuning through band structure engineering and phonon engineering. Aside from strengthening, hardening, and low thermal conductivity that have already been found in high-entropy alloys, new properties like colossal dielectric constant, super ionic conductivity, severe anisotropic thermal expansion coefficient, strong electromagnetic wave absorption, etc., have been discovered in HECs. As a response to the rapid development in this nascent field, this article gives a comprehensive review on the structure features, theoretical methods for stability and property prediction, processing routes, novel properties, and prospective applications of HECs. The challenges on processing, characterization, and property predictions are also emphasized. Finally, future directions for new material exploration, novel processing, fundamental understanding, in-depth characterization, and database assessments are given.

A Continuum Phase Field Model for Fracture

Abstract: A variational free-discontinuity formulation of brittle fracture was given by Francfortand Marigo [1], where the total energy is minimized with respect to the crackgeometry and the displacement field simultaneously. The entire evolution of cracksincluding their initiation and branching is determined by this minimization principlerequiring no further criterion. However, a direct numerical discretization of themodel faces considerable difficulties as the displacement field is discontinuous inthe presence of cracks.