arXiv subjects

B. R. K. Nanda

Publications and source records attributed to B. R. K. Nanda.

At least 55 records · Page 3Linked to original sources

arXiv2020

Facet Dependent Catalytic Activities of Anatase TiO$_2$ for CO$_2$ Adsorption and Conversion

Understanding the atomic-scale interaction mechanism of CO$_2$ and H$_2$O on TiO$_2$ surface is crucial to establish a correlation between the catalytic efficiency with its exposed facet. Here, with the aid of a three-state model, nudged elastic band simulations, and DFT calculations, we examine the chemical restructuring of these molecules during the process of adsorption, coadsorption and conversion on (001) including (1$\times$4)-reconstructed, (010), and (101) facets of anatase TiO$_2$ and thereby, evaluate the step selective reactivity order. In addition, the results reveal the unexplored non-trivialities in the reaction mechanisms. For the most stable (101) facet, we show that the unfavorable carbonate complex formation becomes favorable by switching the reaction from endothermic to exothermic in the presence of water. Further, we find that small binding energy does not necessarily imply physisorption. It can also give rise to chemisorption, where the loss in energy due to repulsive Hartree and Madelung interactions is comparable to the energy gained through the chemical bonding. Such a scenario is demonstrated for the CO$_2$ adsorption on (010) and (101) facets. Though (001) remains the most reactive surface, if it undergoes reconstruction, which happens at ultra-high vacuum and high temperature, the number of active sites is reduced by three-fourths.

cond-mat.mtrl-sci↗

arXiv2020

Fluorine Intercalated Graphene: Formation of a 2D Spin Lattice through Pseudoatomization

A suspended layer made up of ferromagnetically ordered spins could be created between two mono/multilayer graphene through intercalation. Stability and electronic structure studies show that, when fluorine molecules are intercalated between two mono/multilayer graphene, their bonds get stretched enough ($\sim$ 1.9$-$2.0 Å) to weaken their molecular singlet eigenstate. Geometrically, these stretched molecules form a pseudoatomized fluorine layer by maintaining a van der Waals separation of $\sim$ 2.6 Å from the adjacent carbon layers. As there is a significant charge transfer from the adjacent carbon layers to the fluorine layers, a mixture of triplet and doublet states stabilize to induce local spin-moments at each fluorine sites and in turn form a suspended 2D spin lattice. The spins of this lattice align ferromagnetically with nearest neighbour coupling strength as large as $\sim$ 100 meV. Our finite temperature \textit {ab initio} molecular dynamics study reveals that the intercalated system can be stabilized up to a temperature of 100 K with an average magnetic moment of $\sim$ 0.6 $μ_{B}$/F. However, if the graphene layers can be held fixed, the room temperature stability of such a system is feasible.

cond-mat.mes-hall↗

arXiv2020

Defining the Topological Influencers and Predictive Principles to Engineer Band Structure of Halide Perovskites

Complex quantum coupling phenomena of halide perovskites are examined through ab-initio calculations and exact diagonalization of model Hamiltonians to formulate a set of fundamental guiding rules to engineer the bandgap through strain. The bandgap tuning in halides is crucial for photovoltaic applications and for establishing non-trivial electronic states. Using CsSnI$_3$ as the prototype material, we show that in the cubic phase, the bandgap reduces irrespective of the nature of strain. However, for the tetragonal phase, it reduces with tensile strain and increases with compressive strain, while the reverse is the case for the orthorhombic phase. The reduction can give rise to negative bandgap in the cubic and tetragonal phases leading to normal to topological insulator phase transition. Also, these halides tend to form a stability plateau in a space spanned by strain and octahedral rotation. In this plateau, with negligible cost to the total energy, the bandgap can be varied in a range of 1eV. Furthermore, we present a descriptor model for the perovskite to simulate their bandgap with strain and rotation. Analysis of band topology through model Hamiltonians led to the conceptualization of topological influencers that provide a quantitative measure of the contribution of each chemical bonding towards establishing a normal or topological insulator phase. On the technical aspect, we show that a four orbital based basis set (Sn-$\{s,p\}$ for CsSnI$_3$) is sufficient to construct the model Hamiltonian which can explain the electronic structure of each polymorph of halide perovskites.

cond-mat.mtrl-sci↗

arXiv2020

Pressure and Inversion Symmetry Breaking Field Driven First Order Phase Transition and Formation of Dirac Circle in Perovskites

Through model Hamiltonian studies and first-principle electronic structure calculations, we have examined the effect of inversion symmetry breaking (ISB) field and hydrostatic pressure on the band topology of halide perovskites by taking MAPbI$_3$ as a prototype. Our study shows that while hydrostatic pressure induces normal to topological insulator continuous phase transition, the ISB field makes it first order. The pressure smoothly reduces the normal bandgap, and without ISB, the system achieves a gapless state before it produces a non-trivial bandgap with inverted characters. The ISB field does not stabilize the gapless state, and therefore, the discontinuity in the bandgap with pressure gives rise to the first-order transition. Furthermore, in the non-trivial phase, the ISB field forms an invariant surface Dirac circle in the neighbourhood of TRIM, which is first of its kind. The circle is formed due to interpenetration of Dirac cones resembling the band topology of AA-stacked bilayer graphene.

cond-mat.mtrl-sci↗

arXiv2018

Quantum Mechanical Process of Carbonate Complex Formation and Large Scale Anisotropy in the Adsorption Energy of CO$_2$ on Anatase TiO$_2$ (001) Surface

Adsorption of CO$_2$ on a semiconductor surface is a prerequisite for its photocatalytic reduction. Owing to superior photocorrosion resistance, nontoxicity and suitable band edge positions, TiO$_2$ 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 CO$_2$ conversion efficiency on TiO$_2$ surfaces has not been maximized. While anatase TiO$_2$ (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 ~ 89%) of the (001) facet can be synthesized in the presence fluorine ions. Therefore, through density functional calculations we have investigated the CO$_2$ adsorption on TiO$_2$ (001) surface. 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 CO$_2$-TiO$_2$ 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 CO$_2$ primarily determines the BE. A conceptual experiment is devised where the CO$_2$ concentration and flow direction can be controlled to tune the BE within a large window of ~1.5 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 ~0.9 eV.

cond-mat.mtrl-sci↗

arXiv2018

Second Neighbor Electron Hopping and Pressure Induced Topological Quantum Phase Transition in Insulating Cubic Perovskites

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 (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.

cond-mat.mtrl-sci↗

arXiv2018

Giant Exchange Bias in the Single-layered Ruddlesden-Popper Perovskite SrLaCo0.5Mn0.5O4

Exchange bias (EB) as large as ~5.5 kOe is observed in SrLaCo0.5Mn0.5O4 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 suggest 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 sub-nanoscale. 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.

cond-mat.mtrl-sci↗

arXiv2018

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

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.

cond-mat.str-el↗

arXiv2018

Topologically Invariant Double Dirac States in Bismuth based Perovskites: Consequence of Ambivalent Charge States and Covalent Bonding

Bulk and surface electronic structures, calculated using density functional theory and a tight-binding model Hamiltonian, reveal the existence of two topologically invariant (TI) surface states in the family of cubic Bi perovskites (ABiO$_3$; A = Na, K, Rb, Cs, Mg, Ca, Sr and Ba). The two TI states, one lying in the valence band (TI-V) and other lying in the conduction band (TI-C) are formed out of bonding and antibonding states of the Bi-$\{$s,p$\}$ - O-$\{$p$\}$ coordinated covalent interaction. 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 surface energy gaps. The origin of s-p band inversion, necessary to form a TI state, classifies the family of ABiO$_3$ into two. For class-I (A = Na, K, Rb, Cs and Mg) the band inversion, leading to TI-C state, is induced by spin-orbit coupling of the Bi-p states and for class-II (A = Ca, Sr and Ba) the band inversion is induced through weak but sensitive second neighbor Bi-Bi interactions.

cond-mat.mtrl-sci↗

arXiv2017

Universality in the Electronic Structure of 3d Transition Metal Oxides

Electronic structure of strongly correlated transition metal oxides (TMOs) is a complex phenomenon due to competing interaction among the charge, spin, orbital and lattice degrees of freedom. Often individual compounds are examined to explain certain properties associated with these compounds or in rare cases few members of a family are investigated to define a particular trend exhibited by that family. Here, with the objective of generalization, we have investigated the electronic structure of three families of compounds, namely, highly symmetric cubic mono-oxides, symmetry-lowered spinels and asymmetric olivine phosphates, through density functional calculations. From the results we have developed empirical hypotheses involving electron hopping, electron-lattice coupling, Hund's rule coupling, strong correlation and d-band filling. These hypotheses, classified through the point group symmetry of the transition metal - oxygen complexes, can be useful to understand and predict the electronic and magnetic structure of 3d TMOs.

cond-mat.str-el↗

arXiv2017

Effect of Frustrated Exchange Interactions and Spin-half Impurity on the Electronic Structure of Strongly Correlated NiFe$_{2}$O$_{4}$

Spin-polarized density functional calculations, magnetization, and neutron diffraction measurements are carried out to investigate the magnetic exchange interactions and strong correlation effects in Yb substituted inverse spinel nickel ferrite. In the pristine form, the compound is found to be a mixed insulator under the Zaanen-Sawtazsky-Allen classification scheme as it features both charge transfer and Mott insulator mechanism. Estimation of magnetic exchange couplings reveals that both octahedral-octahedral and octahedral-tetrahedral spin-spin interactions are antiferromagnetic which is typical of a spin-frustrated triangular lattice. However, the latter is dominant compared to the former leading to a forced parallel alignment of the spins at the octahedral site which is in agreement with the results of neutron diffraction measurements. The substituent Yb is found to be settled in +3 charge state, as confirmed from the XPS measurements, to behave like a spin-half impurity carried by the localized $f_{z(x^2-y^2)}$ orbital. The impurity $f$ spin significantly weakens the antiferromagnetic coupling with the spins at the tetrahedral site, which explains the experimental observation of fall in Curie temperature with Yb substitution.

cond-mat.mtrl-sci↗

arXiv2017

Metal-Insulator Transition in Ga doped ZnO via Controlled Thickness

We report thickness dependent metal insulator transition in Ga doped ZnO (Ga:ZnO) thin films grown by pulsed laser deposition technique. From the electrical transport measurements, we find that while the thinnest film exhibits a resistivity of 0.05 $Ω$-cm, lying in the insulating regime, the thickest has resistivity of $6.6\times10^{-4}Ω$-cm which shows metallic type of conduction. Our analysis reveals that the Mott's variable range hopping (VRH) model governs the insulating behavior in the thinner Ga:ZnO whereas the 2D weak localization phenomena is appropriate to explain the electron transport in the thicker Ga:ZnO. Magnetoresistance study further confirms the presence of strong localization in 6 nm film while weak localization is observed in 20 nm and above thicker films. From the density functional calculations, it is found that due to surface reconstruction and Ga doping, strong crystalline disorder sets in very thin films to introduce localized states and thereby, restricts the donor electron mobility.

cond-mat.mtrl-sci↗

arXiv2017

Engineering Diffusivity and Operating Voltage in Lithium Iron Phosphate through Transition Metal Doping

Density functional calculations are carried out to understand and tailor the electrochemical profile diffusivity, band gap and open circuit voltage of transition metal doped olivine phosphate $LiFe_{1-x}M_{x}PO_{4}$ (M = V, Cr, Mn, Co and Ni). Diffusion and hence the ionic conductivity is studied by calculating the activation barrier, $V_{act}$, experienced by the diffusing $Li^{+}$ ion. We show that the effect of dopants on diffusion is both site dependent and short ranged and thereby it paves ways for microscopic control of ionic conductivity via selective dopants in this olivine phosphates. Dopants with lower valence electrons (LVE) compared to Fe repel the $Li^{+}$ ion to facilitate its outward diffusion, whereas higher valence electron (HVE) dopants attracts the $Li^{+}$ ion to facilitate the inward diffusion. From the electronic structure calculation we establish that irrespective of the dopant M, except Mn, the band gap is reduced since the M-$d$ states always lie within the pure band gap. Atomically localized $d$ states of HVE dopants lie above the Fermi energy and that of LVE lie below it. Half-filled Mn-$d$ states undergo large spin-exchange split to bury the dopant states in valence and conduction bands of the pristine system and in turn the band gap remains unchanged in $LiFe_{1-x}Mn_{x}PO_{4}$. Baring Mn, the open circuit voltage increases with HVE dopants and decreases with LVE dopants.

cond-mat.mtrl-sci↗

arXiv2017

Microstrain induced deviation from Néel's 1/d behaviour: Size-dependent magnetization in Bi1-xCaxFe1-yTiyO3-delta nanoparticles

Magnetization of antiferromagnetic nanoparticles is known to generally scale up inversely to their diameter (d) according to Néel's model. Here we report a deviation from this conventional linear 1/d dependence, altered significantly by the microstrain, in Ca and Ti substituted BiFeO3 nanoparticles. Magnetic properties of microstrain-controlled Bi1-xCaxFe1-yTiyO3-delta (y = 0 and x = y) nanoparticles are analyzed as a function of their size ranging from 18 nm to 200 nm. A complex interdependence of doping concentration (x or y), annealing temperature (T), microstrain (epsilon) and particle size (d) is established. X-ray diffraction studies reveal a linear variation of microstrain with inverse particle size, 1/d nm-1 (i.e. epsilon.d = 16.5 nm.%). A rapid increase in the saturation magnetization below a critical size dc ~ 35 nm, exhibiting a (1/d)^alpha (alpha ~ 2.6) dependence, is attributed to the influence of microstrain. We propose an empirical formula M is proportional to (1/d)epsilon^beta (beta ~ 1.6) to highlight the contributions from both the size and microstrain towards the total magnetization in the doped systems. The magnetization observed in nanoparticles is thus, a result of competing magnetic contribution from the terminated spin cycloid on the surface and counteracting microstrain present at a given size. Large magnetodielectric response of ~ 9.5 % is observed in spark plasma sintered pellets with optimal size and doping concentration, revealing a strong correlation between magnetic and ferroelectric order parameters.

cond-mat.mtrl-sci↗

arXiv2016

Orbital driven impurity spin effect on the magnetic order of quasi-three dimensional cupric oxide

Density functional calculations are performed to study the magnetic order of the severely distorted square planar cupric oxide (CuO) and local spin disorder in it in the presence of the transition metal impurities M (= Cr, Mn, Fe, Co and Ni). The distortion in the crystal structure, arisen to reduce the band energy by minimizing the covalent interaction, creates two crisscrossing zigzag spin-1/2 chains. From the spin dimer analysis we find that while the spin chain along [10$\bar 1$] has strong Heisenberg type antiferromagnetic coupling (J ~ 127 meV), along [101] it exhibits weak, but robust, ferromagnetic coupling (J ~ 9 meV) mediated by reminiscent p-d covalent interactions. The impurity effect on the magnetic ordering is independent of M and purely orbital driven. If the given spin-state of M is such that the d$_{x^2-y^2}$ orbital is spin-polarized, then the original long-range ordering is maintained. However, if d$_{x^2-y^2}$ orbital is unoccupied, the absence of corresponding covalent interaction breaks the weak ferromagnetic coupling and a spin-flip takes place at the impurity site leading to breakdown of the long range magnetic ordering.

cond-mat.str-el↗

arXiv2016

Intertwined Lattice Deformation and Magnetism in Monovacancy Graphene

Using density functional calculations we have investigated the local spin moment formation and lattice deformation in graphene when an isolated vacancy is created. We predict two competing equilibrium structures: a ground state planar configuration with a saturated local moment of 1.5 $μ_B$, and a metastable non-planar configuration with a vanishing magnetic moment, at a modest energy expense of ~50 meV. Though non-planarity relieves the lattice of vacancy-induced strain, the planar state is energetically favored due to maximally localized defect states (v$σ$, v$π$). In the planar configuration, charge transfer from itinerant (Dirac) states weakens the spin-polarization of v$π$ yielding a fractional moment, which is aligned parallel to the unpaired v$σ$ electron through Hund's coupling. In the non-planar configuration, the absence of orthogonal symmetry allows interaction between v$σ$ and local d$π$ states, to form a hybridized v$σ^\prime$ state. The non-orthogonality also destabilizes the Hund's coupling, and an antiparallel alignment between v$σ$ and v$π$ lowers the energy. The gradual spin reversal of v$π$ with increasing non-planarity opens up the possibility of an intermediate structure with balanced v$π$ spin population. If such a structure is realized under external perturbations, diluted vacancy concentration may lead to v$σ$ based spin-1/2 paramagnetism.

cond-mat.mtrl-sci↗

arXiv2016

Unconventional Magnetism and Band Gap Formation in LiFePO4: Consequence of Polyanion Induced Non-planarity

Oxygen plays a critical role in strongly correlated transition metal oxides as crystal field effect is one of the key factors that determine the degree of localization of the valence d/f states. Based on the localization, a set of conventional mechanisms such as Mott-Hubbard, Charge-transfer and Slater were formulated to explain the antiferromagnetic and insulating (AFI) phenomena in many of these correlated systems. From the case study on LiFePO4, through density-functional calculations, we demonstrate that none of these mechanisms are strictly applicable to explain the AFI behavior when the transition metal oxides have polyanions such as (PO4)3-. The symmetry-lowering of the metal-oxygen complex, to stabilize the polyanion, creates an asymmetric crystal field for d/f states. In LiFePO4 this field creates completely non-degenerate Fe-d states which, with negligible p-d and d-d covalent interactions, become atomically localized to ensure a gap at the Fermi level. Due to large exchange splitting, high spin state is favored and an antiferromagnetic configuration is stabilized. For the prototype LiFePO4, independent electron approximation is good enough to obtain the AFI ground state. Inclusion of additional correlation measures like Hubbard U simply amplifies the gap and therefore LiFePO4 can be preferably called as weakly coupled Mott insulator.

cond-mat.mtrl-sci↗

arXiv2012

Nuclear Tunnelling and Dynamical Jahn-Teller Effect in Graphene with Vacancy

We show that the substitutional vacancy in graphene forms a dynamical Jahn-Teller center. The adiabatic potential surface resulting from the electron-lattice coupling was computed using density-functional methods and subsequently the Schrödinger equation was solved for the nuclear motion. Our calculations show a large tunnelling splitting $3 Γ$ of about 86 cm$^{-1}$. %, which is large as compared to the typical strain splitting. The effect results in a large delocalization of the carbon nuclear wave functions around the vacancy leading to a significant broadening of the Jahn-Teller active $sp^2σ$ electron states. The tunnelling splitting should be observable in electron paramagnetic resonance and two-photon resonance scattering experiments.

cond-mat.mes-hall↗