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We demonstrate the formation of orbital and spin Hall effects (OHE/SHE) in the 1T phase of non-magnetic transition metal dichalcogenides. With the aid of density functional theory calculations and model Hamiltonian studies on MX$_2$ (M = Pt, Pd and X = S, Se, and Te), we show an intrinsic orbital Hall conductivity ($\sim 10^3 \hbar /e\ \Omega^{-1}cm^{-1}$) , which primarily emerges due to the orbital texture around the valleys in the momentum space. The robust spin-orbit coupling in these systems induces a sizable SHE out of OHE. Furthermore, to resemble the typical experimental setups, where the magnetic overlayers produce a proximity magnetic field, we examine the effect of magnetic field on OHE and SHE and showed that the latter can be doubled in these class of compounds. With a giant OHE and tunable SHE, the 1T-TMDs are promising candidates for spin and orbital driven quantum devices such as SOT-MRAM, spin nano-oscillators, spin logic devices etc., and to carry out spin-charge conversion experiments for fundamental research.

Recent discovery of superconductivity at a transition temperature of $73$K in the doped layered compound Ba$_{2}$CuO$_{3+x}$ for $x\sim 0.2$ has generated a lot of interest. Experiments in this alternately stacked oxygen octahedral and chain layered structure reveal that a compression of the octahedra causes the Cu- {$d_{z^{2}}$} orbital to lie above the Cu- {$d_{x^2-y^2}$} orbital unlike in the well-known cuprate superconducting materials. Our first-principle calculations and low-energy Hamiltonian studies on the $x$ = 0.25 system reveal that this energy ordering results in the formation of $d_{z^2}$ dominated electron pockets. The strong nesting in the Fermi pockets leads to an AFM spin fluctuation mediated $d_{xy}$ wave superconducting state dominated by pairing among the $d_{z^{2}}$ orbitals. This is in contrast to the cuprate superconductors (e.g., YBCO) where both electron and hole pockets exist and the superconducting state with B$_{1g}$ symmetry is formed by the $d_{x^2-y^2}$ orbital electrons. Unlike the earlier reports, we find that inter-layer hybridization has an important contribution to the low-energy band structure and formation of the unconventional superconducting state., Comment: 13 pages, 12 figures

Like single perovskites, halide double perovskites (HDP) have truly emerged as efficient optoelectronic materials since they display superior stability and are free of toxicity. However, challenges still exist due to either wide and indirect bandgaps or parity-forbidden transitions in many of them. The lack of understanding in chemical bonding and the formation of parity-driven valence and conduction band edge states have hindered the design of optoelectronically efficient HDPs. In this study, we have developed a theoretical workflow using a multi-integrated approach involving ab-initio density functional theory (DFT) calculations, model Hamiltonian studies, and molecular orbital picture leading to momentum matrix element (MME) estimation. This workflow gives us detailed insight into chemical bonding and parity-driven optical transition between edge states. In the process, we have developed a band-projected molecular orbital picture (B-MOP) connecting free atomic orbital states obtained at the Hartree-Fock level and orbital-resolved DFT bands. From the B-MOP, we show that the nearest neighbor cation-anion interaction determines the position of atom-resolved band states, while the second neighbor cation-cation interactions determine the shape and width of band dispersion and, thereby, MME. The latter is critical to quantify the optical absorption coefficient. Considering both B-MOP and MME, we demonstrate a mechanism of tailoring bandgap and optical absorptions through chemical doping at the cation sites. Furthermore, the cause of bandgap bowing, a common occurrence in doped HDPs, is explained by ascribing it to chemical effect and structural distortion., Comment: 31 pages, 19 figures

We examine the origin of the formation of narrow bands in LK-99 (Pb$_{9}$Cu(PO$_4$)$_6$O) and the parent compound without the Cu doping using density functional theory calculations and model Hamiltonian studies. Explicit analytical expressions are given for a nearest-neighbor tight-binding (TB) Hamiltonian in the momentum space for both the parent and the LK-99 compound, which can serve as an effective model to study various quantum phenomena including superconductivity. The parent material is an insulator with the buckle oxygen atom on the stacked triangular lattice forming the topmost bands, well-separated from the remaining oxygen band manifold. The $C_3$ symmetry-driven two-band TB model describes these two bands quite well. These bands survive in the Cu-doped, LK-99, though with drastically altered band dispersion due to the Cu-O interaction. A similar two-band model involving the Cu $xz$ and $yz$ orbitals broadly describes the top two valence bands of LK-99. However, the band dispersions of both the Cu and O bands are much better described by the four-band TB model incorporating the Cu-O interactions on the buckled honeycomb lattice. We comment on the possible mechanisms of superconductivity in LK-99. even though the actual T$_c$ may be much smaller than reported, and suggest that interstitial Cu clusters leading to broad bands might have a role to play, Comment: 8 pages, 6 figures

As a topological Dirac semimetal with controllable spin-orbit coupling and conductivity, PtSe$_2$, a transition-metal dichalcogenide, is a promising material for several applications from optoelectric to sensors. However, its potential for spintronics applications is yet to be explored. In this work, we demonstrate that PtSe$_{2}$/Ni$_{80}$Fe$_{20}$ heterostructure can generate a large damping-like current-induced spin-orbit torques (SOT), despite the absence of spin-splitting in bulk PtSe$_{2}$. The efficiency of charge-to-spin conversion is found to be $(-0.1 \pm 0.02)$~nm$^{-1}$ in PtSe$_{2}$/Ni$_{80}$Fe$_{20}$, which is three times that of the control sample, Ni$_{80}$Fe$_{20}$/Pt. Our band structure calculations show that the SOT due to the PtSe$_2$ arises from an unexpectedly large spin splitting in the interfacial region of PtSe$_2$ introduced by the proximity magnetic field of the Ni$_{80}$Fe$_{20}$ layer. Our results open up the possibilities of using large-area PtSe$_{2}$ for energy-efficient nanoscale devices by utilizing the proximity-induced SOT., Comment: 18 pages, 4 figures

Vacancy-ordered antifluorite materials (A$_2$BX$_6$) are garnering renewed attention as novel magnetic states driven by spin-orbit coupling (SOC) can be realized in them. In this work, by pursuing density functional theory calculations and model studies, we analyze the ground state electronic and magnetic structure of face-centered cubic (fcc) antifluorites K$_2$ReCl$_6$ (KReC, 5$d^3$), K$_2$OsCl$_6$ (KOsC, 5$d^4$), and K$_2$IrCl$_6$ (KIrC, 5$d^5$). We find that KReC stabilizes in the high-spin $S$ = 3/2 state instead of the expected pseudo-spin $J_{eff}$ = 3/2 state. The former occurs due to large exchange-splitting as compared to the SOC strength. On the contrary, the KOsC stabilizes in broken $J_{eff}$ = 0 ($S$ = 1) simple Mott insulating state while KIrC stabilizes in $J_{eff}$ = 1/2 spin-orbit-assisted Mott insulating state. The presence of an isolated metal-chloride octahedron makes these antifluorites weakly coupled magnetic systems with the nearest and next-nearest-neighbor spin-exchange parameters ($J_1$ and $J_2$) are of the order of 1 meV. For KReC and KOsC, the $J_1$ and $J_2$ are estimated to be antiferromagnetic and ferromagnetic, which leads to a Type-I antiferromagnetic ground state, whereas for KIrC, both $J_1$ and $J_2$ are antiferromagnetic, hence, it stabilizes with a Type-III antiferromagnetic state. Interestingly, in their equilibrium structure, these antifluorites possess large magnetic anisotropy energy (0.6-4 meV/transition metal), which is at least one-to-two orders higher than traditional MAE materials like transition metals and multilayers formed out of them. Moreover, with epitaxial tensile/compressive strain, the MAE enhances by one order, becoming giant for KOsC (20-40 meV/Os).

5$d$ transition metal oxides host a variety of exotic phases due to the comparable strength of Coulomb repulsion and spin-orbit coupling. Herein, by pursuing density-functional studies on a delta-doped quasi-two-dimensional iridate Sr$_3$Ir$_2$O$_7$, where a single SrO layer is replaced by LaO layer, we predict the formation of a spin-orbital entangled two-dimensional electron gas (2DEG) which is sharply confined on two IrO$_2$ layers close to the LaO layer. In this bilayer crystal structure, an existing potential well is further augmented with the inclusion of positively charged LaO layer which results in confining the extra valence electron made available by the La$^{3+}$ ion. The confined electron is bound along crystal $a$ direction and is highly mobile in the $bc$ plane. From the band structure point of view, now the existing half-filled $J_{eff}$ = 1/2 states are further electron doped to destroy the antiferromagnetic Mott insulating state of IrO$_2$ layers near to the delta-doped layer. This leads to partially occupied Ir upper-Hubbard subbands which host the spin-orbital entangled 2DEG. The IrO$_2$ layers far away from the interface remain insulating and preserve the collinear G-type magnetic ordering of pristine Sr$_3$Ir$_2$O$_7$. The conductivity tensors calculated using semi-classical Boltzmann theory at room temperature reveal that the 2DEG exhibits large electrical conductivity of the order of 10$^{19}$.

Here we present a theoretical analysis (applicable to all twist angles of TBG) of band dispersion and density of states in TBG relating evolution of flat band and Van-Hove singularities with evolution of interlayer coupling in TBG. A simple tight binding Hamiltonian with environment dependent interlayer hopping and incorporated with internal configuration of carbon atoms inside a supercell is used to calculate band dispersion and density of states in TBG. Various Hamiltonian parameters and functional form of interlayer hopping applicable to a wide range of twist angles in TBG is estimated by fitting calculated dispersion and density of states with available experimentally observed dispersion and density of states in Graphene, AB-stacked bilayer graphene and some TBG systems. Computationally obtained band dispersion reveal that flat band in TBG occurs very close to Dirac point of graphene and only along linear dimension of two-dimensional wave vector space connecting two closest Dirac points of two graphene layers of TBG., Comment: 14 pages, 6 figures

The state-of-the-art defect engineering techniques have paved the way to realize novel quantum phases out of pristine materials. Here, through density-functional calculations and model studies, we show that the chain-doped monolayer transition metal dichalcogenides (TMDs), where M atoms on a single the zigzag chains are replaced by a higher-valence transition-metal element M$^\prime$ (MX$_2$/M$^\prime$), exhibit one-dimensional (1D) bands. These 1D bands, occurring in the fundamental gap of the pristine material, are dispersive along the doped chain but are strongly confined along the lateral direction. This confinement occurs as the bare potential of the dopant chain formed by the positively charged M$^\prime$ ions resembles the potential well of a uniformly charged wire. These bands could show novel 1D physics, including a new type of Tomonaga-Luttinger liquid behavior, multi-orbital Mott insulator physics, and an unusual optical absorption, due to the simultaneous presence of the spin-orbit coupling, strong correlation, multiple orbitals, Rashba spin splitting, and broken symmetry. For the half-filled 1D bands, we find, quite surprisingly, a broadening of the 1D bands due to correlation, as opposed to the expected band narrowing. This is interpreted to be due to multiple orbitals forming the single Hubbard band at different points of the Brillouin zone. Furthermore, due to the presence of an intrinsic electric field along the lateral direction, the 1D bands are Rashba spin-split and provide a new mechanism for tuning the valley dependent optical transitions.

A-site ordered perovskites, CaCu$_3$B$_4$O$_{12}$, which are derivatives of conventional ABO$_3$ perovskites, exhibit varying electronic and magnetic properties. With the objective of examining the role of Cu in this work, we have studied CaCu$_3$Ti$_4$O$_{12}$ and CaCu$_3$Zr$_4$O$_{12}$ and presented the cause of the crystallization of A-site ordered perovskite from conventional ABO$_3$ perovskite and the underlying mechanism leading to the stabilization of non-trivial and experimentally estabilished G-type antiferromagnetic (G-AFM) ordering in these systems. The first-principles electronic structure calculations supplemented with phonon studies show that the formation of A-site ordered perovskite is driven by Jahn-Teller distortion of the CuO$_{12}$ icosahedron. The crystal orbital Hamiltonian population analysis and magnetic exchange interactions estimated using spin dimer analysis infers that the nearest and next-nearest-neighbor interactions (J$_1$ and J$_2$) are direct and weakly ferromagnetic whereas the third-neighbor interaction (J$_3$) is unusually strong and antiferromagnetic driven by indirect superexchange mechanism. The structural geometry reveals that stabilization of G-AFM requires J$_1$ $<$ 2J$_2$, J$_1$ $<$ 2J$_3$. The experimental and theoretical values of Neel Temperature agrees well for $U$ $\approx$ 7 eV, highlighting the role of strong correlation. The magnetic ordering is found to be robust against pressure and strain.

Double perovskite-based magnets wherein frustration and competition between emergent degrees of freedom are at play can lead to novel electronic and magnetic phenomena. Herein, we report the electronic structure and magnetic properties of an ordered double perovskite material Ho2CoMnO6. In the double perovskite with general class A2BB'O6, the octahedral B and B'-site has a distinct crystallographic site. The Rietveld refinement of XRD data reveals that Ho2CoMnO6 crystallizes in the monoclinic P21/n space group. The X-ray photoelectron spectroscopy confirms the charge state of cations present in this material. The temperature dependence of magnetization and specific heat exhibit a long-range ferromagnetic ordering at Tc ~ 76 K owing to the presence of super exchange interaction between Co2+ and Mn4+ moments. Furthermore, the magnetization isotherm at 5 K shows a hysteresis curve that confirms ferromagnetic behavior of this double perovskite. We observed a re-entrant glassy state in the intermediate temperature regime, which is attributed to inherent anti-site disorder and competing interactions. A large magnetocaloric effect has been observed much below the ferromagnetic transition temperature. The temperature-dependent Raman spectroscopy studies support the presence of spin-phonon coupling and short-range order above Tc in this double perovskite. The stabilization of magnetic ordering and charge states is further analyzed through electronic structure calculations. The latter also infers the compound to be a narrow band gap insulator with the gap arising between the lower and upper Hubbard Co-d subbands. Our results demonstrate that anti-site disorder and complex 3d-4f exchange interactions in the spin-lattice account for the observed electronic and magnetic properties in this promising double perovskite material.

Optical trapping of magnetic Fe-oxide particles is notoriously difficult due to their high refractive indices, not to mention high absorptivity at the trapping infra-red wavelengths. We synthesize Fe co-doped NaYF4:Yb,Er ferromagnetic upconversion particles that not only have refractive indices conducive for optical trapping, but also heat less than Haematite particles at off-resonant wavelengths. These particles are hexagonal shaped with dimensions of the order of 3 micrometer and also bear high coercivity of 20 mT and saturation magnetisation of 1 Am^2/kg. This enables simultaneous use of optical trapping and magnetic forces to generate micro-manipulation in all the six degrees of freedom of a rigid body. We also show that these particles heat significantly when illuminated on absorption resonance at 975 nm while emitting visible light with possible implications for fluorescence microscopy and photothermal therapy of cancer cells., Comment: 18 pages, 10 figures

The 5$d$ transition metal oxides, in particular iridates, host novel electronic and magnetic phases due to the interplay between onsite Coulomb repulsion ($U$) and spin-orbit coupling (SOC). The reduced dimensionality brings another degree of freedom to increase the functionality of these systems. By taking the example of ultrathin films of SrIrO$_3$,theoretically, we demonstrate that confinement led localization can introduce large magnetic anisotropy energy (MAE) in the range 2-7 meV/Ir which is one to two orders higher than that of the traditional MAE compounds formed out of transition metals and their multilayers. Furthermore, in the weak correlation limit, tailored terminations can yield multiple Dirac states across a large energy window of 2 eV around the Fermi energy which is a rare phenomena in correlated oxides and upon experimental realization it will give rise to unique transport properties with excitation and doping.

Density functional calculations are carried out to study the symmetry and substitution-driven electronic phase transition in BaPb$_{1-x}$Sn$_x$O$_3$. Two end members BaSnO$_3$ and BaPbO$_3$, are found to be insulating and metallic, respectively. In the latter case, the metallicity arises with the presence of an electron pocket, formed by Pb-s dominated conduction band edge, and a hole pocket formed O-p dominated valence bands. While electron carriers are found to be highly mobile, the hole carriers are localized. Our study reveals that an insulating phase can be realized in the metallic cubic BaPbO$_3$ in three ways in order to explore optoelectronic properties. Firstly, by lowering the symmetry of the lattice to monoclinic through rotation and tilting of the PbO$_6$ octahedra. Secondly, by hydrostatic pressure, and thirdly by alloying with Sn substitution. The presence of soft phonon modes implies the plausibility of symmetry lowering structural transitions. Furthermore, unlike the earlier reports, we find that Sn substituted BaPbO$_3$ cannot exhibits topological insulator phase due to absence of the band inversion., Comment: 11 pages, 12 figures

Halide perovskites are perceived to be the promising class of materials for optoelectronics, spinorbitronics and topological electronics due to presence of strong spin-orbit coupling and polarized field. Here, we develop a Hamiltonian using quasi-degenerate perturbation theory to describe polarized field driven band structures and unique topological phases such as Dirac semimetallic rings for the universal class of Halide perovskites including both inorganic and hybrid members. The spin textures obtained through this theoretical model captures minute effects of polarization and hence can be used as a modern tool to determine the polarized field directions which have significant influence on spinorbitronics. The analysis brings out the concepts of hybrid spin textures intermixing the effects of valence and conduction bands under topological quantum phase transitions, and spin texture binaries where alignment of the spins are reversed between domains in the momentum space with sharp boundaries. The results are validated through density functional calculations., Comment: 11 pages, 10 figures

The $4d$ and $5d$ transition metal oxides have become important members of the emerging quantum materials family due to competition between onsite Coulomb repulsion ($U$) and spin-orbit coupling (SOC). Specifically, the systems with $d^5$ electronic configuration in an octahedral environment are found to be capable of posessing invariant semimetallic state and perturbations can lead to diverse magnetic phases. In this work, by formulating a multi-band Hubbard model and performing SOC tunable DFT+$U$ calculations on a prototype SrIrO$_3$ and extending the analysis to other iso-structural and isovalent compounds, we present eight possible electronic and magnetic configurations in the $U$-SOC phase diagram that can be observed in the family of low-spin $d^5$ perovskites. They include the protected Dirac semimetal state, metal and insulator regimes, collinear and noncollinear spin ordering. The latter is explained through connecting hopping interactions to the rotation and tilting of the octahedra as observed in GdFeO$_3$. Presence of several soft phase boundaries makes the family of $d^5$ perovskites an ideal platform to study electronic and magnetic phase transitions under external stimuli.

The halide perovskites have truly emerged as efficient optoelectronic materials and show the promise of exhibiting nontrivial topological phases. Since the bandgap is the deterministic factor for these quantum phases, here we present a comprehensive electronic structure study using first-principle methods by considering nine inorganic halide perovskites CsBX$_3$ (B = Ge, Sn, Pb; X = Cl, Br, I) in their three structural polymorphs (cubic, tetragonal and orthorhombic). A series of exchange-correlations (XC) functionals are examined towards accurate estimation of the bandgap. Furthermore, while thirteen orbitals are active in constructing the valence and conduction band spectrum, here we establish that a four orbital based minimal basis set is sufficient to build the Slater-Koster tight-binding model (SK-TB), which is capable of reproducing the bulk and surface electronic structure in the vicinity of the Fermi level. Therefore, like the Wannier based TB model, the presented SK-TB model can also be considered as an efficient tool to examine the bulk and surface electronic structure of halide family of compounds. As estimated by comparing the model study and DFT band structure, the dominant electron coupling strengths are found to be nearly independent of XC functionals, which further establishes the utility of the SK-TB model., Comment: 12 pages, 10 figures

Oxide double perovskites wherein octahedra formed by both 3d elements and sp-based heavy elements give rise to unconventional magnetic ordering and correlated quantum phenomena crucial for futuristic applications. Here, by carrying out experimental and first principles investigations, we present the electronic structure and magnetic phases of Ba2MnTeO6, where Mn^2+ ions with S = 5/2 spins constitute a perfect triangular lattice. The magnetic susceptibility reveals a large Curie- Weiss temperature -152 K suggesting the presence of strong antiferromagnetic interactions between Mn^2+ moments in the spin lattice. A phase transition at 20 K is revealed by magnetic susceptibility and specific heat which is attributed to the presence of a sizeable inter-plane interactions. Below the transition temperature, the specific heat data show antiferromagnetic magnon excitations with a gap of 1.4 K. Furthermore, muon spin-relaxation reveals the presence of static internal fields in the ordered state and provides strong evidence of short-range spin correlations for T > TN. The DFT+U calculations and spin-dimer analysis infer that Heisenberg interactions govern the inter and intra-layer spin-frustrations in this perovskite. The inter and intra-layer exchange interactions are of comparable strengths (J1 = 4.6 K, J2 = 0.92 J1). However, a weak third nearest-neighbor ferromagnetic inter-layer interaction exists (J3=-0.04 J1) due to double-exchange interaction via the linear path Mn-O-Te-O-Mn. The combined effect of J2 and J3 interactions stabilizes a three dimensional long-range magnetic ordering in this frustrated magnet.

We propose the design of low strained and energetically favourable mono and bilayer graphene overlayer on anatase TiO$_2$ (001) surface and examined the electronic structure of the interface with the aid of first principle calculations. In the absence of hybridization between surface TiO$_2$ and graphene states, dipolar fluctuations govern the minor charge transfer across the interface. As a result, both the substrate and the overlayer retain their pristine electronic structure. The interface with the monolayer graphene retains its gapless linear band dispersion irrespective of the induced epitaxial strain. The potential gradient opens up a few meV bandgap in the case of Bernal stacking and strengthens the interpenetration of the Dirac cones in the case of hexagonal stacking of the bilayer graphene. The difference between the macroscopic average potential of the TiO$_2$ and graphene layer(s) in the heterostructure lies in the range 3 to 3.13 eV, which is very close to the TiO$_2$ bandgap ($\sim$ 3.2 eV). Therefore, the proposed heterostructure will exhibit enhanced photo-induced charge transfer and the graphene component will serve as a visible light sensitizer., Comment: 31 pages, 14 figures

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., Comment: 33 pages, 11 figures

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 {\AA}) 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 {\AA} 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 $\mu_{B}$/F. However, if the graphene layers can be held fixed, the room temperature stability of such a system is feasible., Comment: 16 pages, 18 figures

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.

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.

Artificial confinement of electrons by tailoring the layer thickness has turned out to be a powerful tool to harness control over competing phases in nano-layers of complex oxides. We investigate the effect of dimensionality on transport properties of $d$-electron based heavy-fermion metal CaCu$_{3}$Ru$_{4}$O$_{12}$. Transport behavior evolves from metallic to localized regime upon reducing thickness and a metal insulator transition is observed below 3 nm film thickness for which sheet resistance crosses $h/e^{2} \sim 25~$k$\Omega$, the quantum resistance in 2D. Magnetotransport study reveals a strong interplay between inelastic and spin-orbit scattering lengths upon reducing thickness, which results in weak antilocalization (WAL) to weak localization (WL) crossover in magnetoconductance., Comment: 10 pages, 4 figures

The disadvantage of BaBiO$_3$ of not being a topological insulator despite having symmetry protected Dirac state is overcome by shifting the Fermi level (E$_F$) via fluorination. The DFT calculations reveal that the fluorination neither affects the spin-orbit coupling nor the parity of the states, but it acts as a perfect electron donor to shift the E$_F$. We find that 33 % fluorination is sufficient to shift the E$_F$ by $\sim$ 2 eV so that the invariant Dirac state lies on it to make BaBiO$_2$F a topological insulator. The fluorinated cubic compound can be experimentally synthesized as the phonon studies predict dynamical stability above $\sim$ 500 K. Furthermore, the Dirac states are found to be invariant against the low-temperature phase lattice distortion which makes the structure monoclinic. The results carry practical significance as they open up the possibility of converting the family of superconducting oxides, ABiO$_3$ (A = Na, K, Cs, Ba, Sr, Ca), to real topological insulator through appropriate fluorination.

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., Comment: 13 pages, 13 figures

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., Comment: 6 figures, Accepted in Physical Review B

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., Comment: 22 pages, 11 figures

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., Comment: 10 pages, 12 figures

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., Comment: 20 pages,23 figures

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., Comment: 17 pages, 14 figures. Changes made on V2 (version-2): Minor revision of the text and an additional information is added in the appendix

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 $\Omega$-cm, lying in the insulating regime, the thickest has resistivity of $6.6\times10^{-4}\Omega$-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., Comment: 9 pages, 7 figures

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., Comment: 11 pages, 8 figures

Magnetization of antiferromagnetic nanoparticles is known to generally scale up inversely to their diameter (d) according to N\'eel'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., Comment: 44 pages, 15 figures

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., Comment: 15 Pages, 4 Figures (This is the updated version of our previous submission.)

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., Comment: 12 pages, 4 figures

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 $\mu_B$, 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$\sigma$, v$\pi$). In the planar configuration, charge transfer from itinerant (Dirac) states weakens the spin-polarization of v$\pi$ yielding a fractional moment, which is aligned parallel to the unpaired v$\sigma$ electron through Hund's coupling. In the non-planar configuration, the absence of orthogonal symmetry allows interaction between v$\sigma$ and local d$\pi$ states, to form a hybridized v$\sigma^\prime$ state. The non-orthogonality also destabilizes the Hund's coupling, and an antiparallel alignment between v$\sigma$ and v$\pi$ lowers the energy. The gradual spin reversal of v$\pi$ with increasing non-planarity opens up the possibility of an intermediate structure with balanced v$\pi$ spin population. If such a structure is realized under external perturbations, diluted vacancy concentration may lead to v$\sigma$ based spin-1/2 paramagnetism., Comment: Published version - URL http://link.aps.org/doi/10.1103/PhysRevB.93.165403

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., Comment: Sci. Rep. 6, 19573

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 Schr\"odinger equation was solved for the nuclear motion. Our calculations show a large tunnelling splitting $3 \Gamma$ 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 $sp^2\sigma$ electron states. The tunnelling splitting should be observable in electron paramagnetic resonance and two-photon resonance scattering experiments., Comment: 5 pages, 4 figures

We study the electronic structure of graphene with a single substitutional vacancy using a combination of the density-functional, tight-binding, and impurity Green's function approaches. Density functional studies are performed with the all-electron spin-polarized linear augmented plane wave (LAPW) method. The three $sp^2 \sigma$ dangling bonds adjacent to the vacancy introduce localized states (V$\sigma$) in the mid-gap region, which split due to the crystal field and a Jahn-Teller distortion, while the $p_z \pi$ states introduce a sharp resonance state (V$\pi$) in the band structure. For a planar structure, symmetry strictly forbids hybridization between the $\sigma$ and the $\pi$ states, so that these bands are clearly identifiable in the calculated band structure. As for the magnetic moment of the vacancy, the Hund's-rule coupling aligns the spins of the four localized V$\sigma_1 \uparrow \downarrow$, V$\sigma_2 \uparrow $, and the V$\pi \uparrow$ electrons resulting in a S=1 state, with a magnetic moment of $2 \mu_B$, which is reduced by about $0.3 \mu_B$ due to the anti-ferromagnetic spin-polarization of the $\pi$ band itinerant states in the vicinity of the vacancy. This results in the net magnetic moment of $1.7 \mu_B$. Using the Lippmann-Schwinger equation, we reproduce the well-known $\sim 1/r$ decay of the localized V$\pi$ wave function with distance and in addition find an interference term coming from the two Dirac points, previously unnoticed in the literature. The long-range nature of the V$\pi$ wave function is a unique feature of the graphene vacancy and we suggest that this may be one of the reasons for the widely varying relaxed structures and magnetic moments reported from the supercell band calculations in the literature., Comment: 24 pages, 15 figures. Accepted for publication in New Journal of Physics

From a mean-field solution of the Hubbard-Holstein model, we show that a rich variety of different electronic phases can result at the interface between two polar materials such as LaAlO$_3$/SrTiO$_3$. Depending on the strengths of the various competing interactions, viz., the electronic kinetic energy, electron-phonon interaction, Coulomb energy, and electronic screening strength, the electrons could (i) either be strongly confined to the interface forming a 2D metallic or an insulating phase, (ii) spread deeper into the bulk making a 3D phase, or (iii) become localized at individual sites forming a Jahn-Teller polaronic phase. In the polaronic phase, the Coulomb interaction could lead to unpaired electrons resulting in magnetic Kondo centers. Under appropriate conditions, electronic phase separation may also occur resulting in the coexistence of metallic and insulating regions at the interface., Comment: 7 pages, 10 figures