Two Dimensional Spin-Polarized Electron Gas at the Oxide Interfaces

The formation of a novel spin-polarized 2D electron gas at the LaMnO$_3$ monolayer embedded in SrMnO$_3$ is predicted from the first-principles density-functional calculations. The La (d) electrons become confined in the direction normal to the interface in the potential well of the La layer, serving as a positively-charged layer of electron donors. These electrons mediate a ferromagnetic alignment of the Mn t$_{2g}$ spins near the interface via the Anderson-Hasegawa double exchange and become, in turn, spin-polarized due to the internal magnetic fields of the Mn moments.Comment: 5 pages, 6 figure

Strain and Electric Field Modulation of the Electronic Structure of Bilayer Graphene

We study how the electronic structure of the bilayer graphene (BLG) is changed by electric field and strain from {\it ab initio} density-functional calculations using the LMTO and the LAPW methods. Both hexagonal and Bernal stacked structures are considered. The BLG is a zero-gap semiconductor like the isolated layer of graphene. We find that while strain alone does not produce a gap in the BLG, an electric field does so in the Bernal structure but not in the hexagonal structure. The topology of the bands leads to Dirac circles with linear dispersion in the case of the hexagonally stacked BLG due to the interpenetration of the Dirac cones, while for the Bernal stacking, the dispersion is quadratic. The size of the Dirac circle increases with the applied electric field, leading to an interesting way of controlling the Fermi surface. The external electric field is screened due to polarization charges between the layers, leading to a reduced size of the band gap and the Dirac circle. The screening is substantial in both cases and diverges for the Bernal structure for small fields as has been noted by earlier authors. As a biproduct of this work, we present the tight-binding parameters for the free-standing single layer graphene as obtained by fitting to the density-functional bands, both with and without the slope constraint for the Dirac cone.Comment: 7 pages, 7 figure

Anatomy of neck configuration in fission decay

The anatomy of neck configuration in the fission decay of Uranium and Thorium isotopes is investigated in a microscopic study using Relativistic mean field theory. The study includes $^{236}U$ and $^{232}Th$ in the valley of stability and exotic neutron rich isotopes $^{250}U$, $^{256}U$, $^{260}U$, $^{240}Th$, $^{250}Th$, $^{256}Th$ likely to play important role in the r-process nucleosynthesis in stellar evolution. Following the static fission path, the neck configurations are generated and their composition in terms of the number of neutrons and protons are obtained showing the progressive rise in the neutron component with the increase of mass number. Strong correlation between the neutron multiplicity in the fission decay and the number of neutrons in the neck is seen. The maximum neutron-proton ratio is about 5 for $^{260}$U and $^{256}$Th suggestive of the break down of liquid-drop picture and inhibition of the fission decay in still heavier isotopes. Neck as precursor of a new mode of fission decay like multi-fragmentation fission may also be inferred from this study.Comment: 16 pages, 5 figures (Accepted

Electronic and Magnetic Structure of the (LaMnO3_3)2n_{2n}/(SrMnO3_3)n_n Superlattices

We study the magnetic structure of the (LaMnO$_3$)$_{2n}$/(SrMnO$_3$)$_n$ superlattices from density-functional calculations. In agreement with the experiments, we find that the magnetism changes with the layer thickness `n'. The reason for the different magnetic structures is shown to be the varying potential barrier across the interface, which controls the leakage of the Mn-e$_g$ electrons from the LMO side to the SMO side. This in turn affects the interfacial magnetism via the carrier-mediated Zener double exchange. For n=1 superlattice, the Mn-e$_g$ electrons are more or less spread over the entire lattice, so that the magnetic behavior is similar to the equivalent alloy compound La$_{2/3}$Sr$_{1/3}$MnO$_3$. For larger n, the e$_g$ electron transfer occurs mostly between the two layers adjacent to the interface, thus leaving the magnetism unchanged and bulk-like away from the interface region.Comment: 5 pages, 5 figure

Synthesis and Application of Nano structured Bi layer YSZ LZ Thermal Barrier Coating

Present work is on synthesis of high purity Nano-structured TBC materials, Lanthanum Zirconate and YSZ. They were prepared via wet chemical routes, starting from the indigenous source minerals such as zircon and monazite available in the beach sand. This is first time that the results of TBC materials synthesis from these base minerals, their purification and a high end application being presented comprehensively. Their characterisation and thermal barrier application on aeroengine components have been presented. The total oxide impurities being critical to the life of the coating, could be controlled within 0.03 per cent by weight. On comparison with other powders it was found that the indigenously synthesised YSZ powder had practically 100 per cent tetragonal prime phase and no monoclinic phase; whereas others had significant amounts of monoclinic phases present in them. Both YSZ and LZ powders were sinter agglomerated at 850 °C to preclude the possibility of any contamination and sieved. APS process was used to realise nano-structured bi-layer coating on the exhaust nozzle parts of an aeroengine. The components were subjected to rapid thermal transients during long accelerated endurance testing, equivalent to 1000 h of engine operations. The coatings also withstood the gas erosion of supersonic combustion products, vibratory loads of 4 g and more than 30000 nozzle actuations similar to aircraft maneuver. The paper also presents a brief review of implications of a nano-structured thermal barrier coating and certain nuances of chemical synthesis which forms the backbone of the strategies for durable coatings

Polar catastrophe, electron leakage, and magnetic ordering at the LaMnO3/SrMnO3 interface

Electronic reconstruction at the polar interface LaMnO3/SrMnO3 (LMO/SMO) (100) resulting from the polar catastrophe is studied from a model Hamiltonian that includes the double and super exchange interactions, the Madelung potential, and the Jahn-Teller coupling terms relevant for the manganites. We show that the polar catastrophe, originating from the alternately charged LMO layers and neutral SMO layers, is quenched by the accumulation of an extra half electron per cell in the interface region as in the case of the LaAlO3/SrTiO3 interface. In addition, the Mn eg electrons leak out from the LMO side to the SMO side, the extent of the leakage being controlled by the interfacial potential barrier and the substrate induced epitaxial strain. The leaked electrons mediate a Zener double exchange, making the layers adjacent to the interface ferromagnetic, while the two bulk materials away from the interface retain their original type A or G antiferromagnetic structures. A half-metallic conduction band results at the interface, sandwiched by the two insulating bulks. We have also studied how the electron leakage and consequently the magnetic ordering are affected by the substrate induced epitaxial strain. Comparisons are made with the results of the density-functional calculations for the (LMO)6/(SMO)4 superlattice.This work was supported by the U. S. Department of Energy through Grant No. DE-FG02-00ER45818

Effects of strain on orbital ordering and magnetism at perovskite oxide interfaces: LaMnO3/SrMnO3

We study how strain affects orbital ordering and magnetism at the interface between SrMnO3 and LaMnO3 from density-functional calculations and interpret the basic results in terms of a three-site Mn-O-Mn model. Magnetic interaction between the Mn atoms is governed by a competition between the antiferromagnetic superexchange of the Mn t2g core spins and the ferromagnetic double exchange of the itinerant eg electrons. While the core electrons are relatively unaffected by the strain, the orbital character of the itinerant electron is strongly affected, which in turn causes a large change in the strength of the ferromagnetic double exchange. The epitaxial strain produces the tetragonal distortion of the MnO6 octahedron, splitting the Mn eg states into x2−y2 and 3z2−1 states, with the former being lower in energy, if the strain is tensile in the plane and opposite if the strain is compressive. For the case of the tensile strain, the resulting higher occupancy of the x2−y2 orbital enhances the in-plane ferromagnetic double exchange owing to the larger electron hopping in the plane, causing at the same time a reduction in the out-of-plane double exchange. This reduction is large enough to be overcome by antiferromagnetic superexchange, which wins to produce a net antiferromagnetic interaction between the out-of-plane Mn atoms. For the case of the in-plane compressive strain, the reverse happens, viz., that the higher occupancy of the 3z2−1 orbital results in the out-of-plane ferromagnetic interaction, while the in-plane magnetic interaction remains antiferromagnetic. Concrete density-functional results are presented for the (LaMnO3)1/(SrMnO3)1 and (LaMnO3)1/ (SrMnO3)3 superlattices for various strain conditions.This work was supported by the U.S. Department of Energy under Grant No. DE-FG02-00ER45818

Electronic and magnetic structure of the (LaMnO3)2n/(SrMnO3)n superlattices

We study the magnetic structure of the (LaMnO3)2n/(SrMnO3)n superlattices from density-functional calculations. In agreement with the experiments, we find that the magnetism changes with the layer thickness n. The reason for the different magnetic structures is shown to be the varying potential barrier across the interface, which controls the leakage of the Mn-eg electrons from the LaMnO3 side to the SrMnO3 side. This in turn affects the interfacial magnetism via the carrier-mediated Zener double exchange. For the n=1 superlattice, the Mn-eg electrons are more or less spread over the entire lattice so that the magnetic behavior is similar to the equivalent alloy compound La2/3Sr1/3MnO3. For larger n, the eg electron transfer occurs mostly between the two layers adjacent to the interface, thus leaving the magnetism unchanged and bulklike away from the interface region.This work was supported by the U.S. Department of Energy under Grant No. DE-FG02-00ER45818. We thank J. W. Freeland for stimulating this work and for valuable discussions