Showing papers by "Birabar Nanda published in 2018"

Second-neighbor electron hopping and pressure induced topological quantum phase transition in insulating cubic perovskites

Abstract: Perovskite structure is one of the five symmetry families suitable for exhibiting topological insulator phase. However, none of the halides and oxides stabilizing in this structure exhibit the same. Through density functional calculations on cubic perovskites (${\mathrm{CsSnX}}_{3}$; X = Cl, Br, and I), we predict a band insulator--Dirac semimetal--topological insulator phase transition with uniform compression. With the aid of a Slater-Koster tight binding Hamiltonian, we show that, apart from the valence electron count, the band topology of these perovksites is determined by five parameters involving electron hopping among the Sn-{$s,p$} orbitals. These parameters monotonically increase with pressure to gradually transform the positive band gap to a negative one and thereby enable the quantum phase transition. The universality of the mechanism of phase transition is established by examining the band topology of Bi based oxide perovskites. Dynamical stability of the halides against pressure strengthens the experimental relevance.

Topologically invariant double Dirac states in bismuth-based perovskites: Consequence of ambivalent charge states and covalent bonding

Abstract: Density functional calculations and model tight-binding Hamiltonian studies are carried out to examine the bulk and surface electronic structure of the largely unexplored perovskite family of $A{\mathrm{BiO}}_{3}$, where $A$ is a group I-II element. From the study, we reveal the existence of two TI states, one in valence band (V-TI) and the other in conduction band (C-TI), as the universal feature of $A{\mathrm{BiO}}_{3}$. The V-TI and C-TI are, respectively, born out of bonding and antibonding states caused by Bi-${\mathrm{s},\mathrm{p}}$-O-${\mathrm{p}}$ coordinated covalent interactions. Further, we outline a classification scheme in this family where one class follows spin orbit coupling and the other follows the second neighbor Bi-Bi hybridization to induce s-p band inversion for the realization of C-TI states. Below a certain critical thickness of the film, which varies with $A$, TI states of top and bottom surfaces couple to destroy the Dirac type linear dispersion and consequently to open narrow surface energy gaps.

Giant exchange bias in the single-layered Ruddlesden-Popper perovskite SrLaC o 0.5 M n 0.5 O 4

Abstract: Exchange bias (EB) as large as \ensuremath{\sim}5.5 kOe is observed in $\mathrm{SrLaC}{\mathrm{o}}_{0.5}\mathrm{M}{\mathrm{n}}_{0.5}{\mathrm{O}}_{4}$, which is the highest ever found in any layered transition-metal oxides including Ruddlesden-Popper series. Neutron-diffraction measurement rules out long-range magnetic ordering and together with dc magnetic measurements suggests formation of short-range magnetic domains. Ac magnetic susceptibility, magnetic memory effect, and magnetic training effect confirm the system to be a cluster spin glass. By carrying out density functional calculations on several model configurations, we propose that EB is originated at the boundary between Mn-rich antiferromagnetic and Co-rich ferromagnetic domains at the subnanoscale. Reversal of magnetization axis on the Co side alters the magnetic coupling between the interfacial Mn and Co spins, which leads to EB. Our analysis infers that the presence of competing magnetic interactions is sufficient to induce exchange bias and thereby a wide range of materials exhibiting giant EB can be engineered for designing novel magnetic memory devices.

Oxygen vacancy induced photoconductivity enhancement in Bi 1-x Ca x FeO 3-δ nanoparticle ceramics: A combined experimental and theoretical study

Abstract: Based on experimental and density functional studies, we show that tailoring of oxygen vacancies (OV) leads to large scale enhancement of photoconductivity in BiFeO3 (BFO). The OV concentration is increased by substituting an aliovalent cation Ca2+ at Bi3+ sites in the BFO structure. Furthermore, the OV concentration at the disordered grain boundaries can be increased by reducing the particle size. Photoconductivity studies carried out on spark plasma sintered Bi1-xCaxFeO3-δ ceramics show four orders of enhancement for x = 0.1. Temperature dependent Nyquist plots depict a clear decrease in impedance with increasing Ca2+ concentration which signifies the role of OV. A significant reduction in photoconductivity by 2 to 4 orders and a large increase in impedance of the air-annealed (AA) nanocrystalline ceramics suggest that OV at the grain boundaries primarily control the photocurrent. In fact, activation energy for AA samples (0.5 to 1.4 eV) is larger than the as-prepared (AP) samples (0.1 to 0.5 eV). Therefore, the room temperature J-V characteristics under 1 sun illumination show 2–4 orders more current density for AP samples. Density-functional calculations reveal that, while the defect states due to bulk OV are nearly flat, degenerate, and discrete, the defect states due to surface OV are non-degenerate and interact with the surface dangling states to become dispersive. With large vacancy concentration, they form a defect band that enables a continuous transition of charge carriers leading to significant enhancement in the photoconductivity. These studies reveal the importance of tailoring the microstructural features as well as the composition-tailored properties to achieve large short circuit current in perovskite oxide based solar cells.Based on experimental and density functional studies, we show that tailoring of oxygen vacancies (OV) leads to large scale enhancement of photoconductivity in BiFeO3 (BFO). The OV concentration is increased by substituting an aliovalent cation Ca2+ at Bi3+ sites in the BFO structure. Furthermore, the OV concentration at the disordered grain boundaries can be increased by reducing the particle size. Photoconductivity studies carried out on spark plasma sintered Bi1-xCaxFeO3-δ ceramics show four orders of enhancement for x = 0.1. Temperature dependent Nyquist plots depict a clear decrease in impedance with increasing Ca2+ concentration which signifies the role of OV. A significant reduction in photoconductivity by 2 to 4 orders and a large increase in impedance of the air-annealed (AA) nanocrystalline ceramics suggest that OV at the grain boundaries primarily control the photocurrent. In fact, activation energy for AA samples (0.5 to 1.4 eV) is larger than the as-prepared (AP) samples (0.1 to 0.5 eV). There...

Quantum well structure of a double perovskite superlattice and formation of a spin-polarized two-dimensional electron gas

Abstract: Layered oxide heterostructures are the new routes to tailor desired electronic and magnetic phases emerging from competing interactions involving strong correlation, orbital hopping, tunnelling and lattice coupling phenomena. Here, we propose a half-metal/insulator superlattice that intrinsically forms spin-polarized two-dimensional electron gas (2DEG) following a mechanism very different from the widely reported 2DEG at the single perovskite polar interfaces. From DFT$+U$ study on Sr$_2$FeMoO$_6$/La$_2$CoMnO$_6$ (001) superlattice, we find that a periodic quantum well is created along [001] which breaks the three-fold $t_{2g}$ degeneracy to separate the doubly degenerate $xz$ and $yz$ states from the planar $xy$ state. In the spin-down channel, the dual effect of quantum confinement and strong correlation localizes the degenerate states, whereas the dispersive $xy$ state forms the 2DEG which is robust against perturbations to the superlattice symmetry. The spin-up channel retains the bulk insulating. Both spin polarization and orbital polarization make the superlattice ideal for spintronic and orbitronic applications. The suggested 2DEG mechanism widens the scope of fabricating next generation of oxide heterostructures.

Quantum-mechanical process of carbonate complex formation and large-scale anisotropy in the adsorption energy of C O 2 on anatase Ti O 2 (001) surface

Abstract: Adsorption of $\mathrm{C}{\mathrm{O}}_{2}$ on a semiconductor surface is a prerequisite for its photocatalytic reduction. Owing to superior photocorrosion resistance, nontoxicity, and suitable band-edge positions, $\mathrm{Ti}{\mathrm{O}}_{2}$ is considered to be the most efficient photocatalyst for facilitating redox reactions. However, due to the absence of adequate understanding of the mechanism of adsorption, the $\mathrm{C}{\mathrm{O}}_{2}$ conversion efficiency on $\mathrm{Ti}{\mathrm{O}}_{2}$ surfaces has not been maximized. While anatase $\mathrm{Ti}{\mathrm{O}}_{2}$ (101) is the most stable facet, the (001) surface is more reactive, and it has been experimentally shown that the stability can be reversed and a larger percentage (up to $\ensuremath{\sim}89%)$ of the (001) facet can be synthesized in the presence fluorine ions. Therefore, through density functional calculations we have investigated the $\mathrm{C}{\mathrm{O}}_{2}$ adsorption on $\mathrm{Ti}{\mathrm{O}}_{2}$ (001) surfaces. We have developed a three-state quantum-mechanical model that explains the mechanism of chemisorption, leading to the formation of a tridentate carbonate complex. The electronic structure analysis reveals that the $\mathrm{C}{\mathrm{O}}_{2}\text{\ensuremath{-}}\mathrm{Ti}{\mathrm{O}}_{2}$ interaction at the surface is uniaxial and long ranged, which gives rise to anisotropy in binding energy (BE). It negates the widely perceived one-to-one correspondence between coverage and BE and infers that the spatial distribution of $\mathrm{C}{\mathrm{O}}_{2}$ primarily determines the BE. A conceptual experiment is devised where the $\mathrm{C}{\mathrm{O}}_{2}$ concentration and flow direction can be controlled to tune the BE within a large window of $\ensuremath{\sim}1.5\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$. The experiment also reveals that a maximum of $50%$ coverage can be achieved for chemisorption. In the presence of water, the activated carbonate complex forms a bicarbonate complex by overcoming a potential barrier of $\ensuremath{\sim}0.9\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$.