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B. R. K. Nanda

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

At least 37 records · Page 2Linked to original sources
arXiv2023

Formation of spin-orbital entangled 2D electron gas in layer delta-doped bilayer iridate La$_δ$Sr$_3$Ir$_2$O$_7$

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$^{19}$.

cond-mat.mtrl-sci↗
arXiv2023

Unique $d_{xy}$ Superconducting State in the Cuprate Member Ba$_{2}$CuO$_{3.25}$

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.

cond-mat.supr-con↗
arXiv2023

Electronic structure and optoelectronic properties of halide double perovskites: Fundamental insights and design of a theoretical workflow

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.

cond-mat.mtrl-sci↗
arXiv2023

Effective tight-binding Hamiltonian for the low-energy electronic structure of the Cu-doped lead apatite and the parent compound

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

cond-mat.supr-con↗
arXiv2023

Magnetic Proximity induced efficient charge-to-spin conversion in large area PtSe$_{2}$/Ni$_{80}$Fe$_{20}$ heterostructures

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.

cond-mat.mtrl-sci↗
arXiv2023

Breakdown of $J_{eff}$ = 0 and $J_{eff}$ = 3/2 states and existence of large magnetic anisotropy energy in vacancy ordered 5$d$ antifluorites: K$_2$ReCl$_6$, K$_2$OsCl$_6$, and K$_2$IrCl$_6$

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).

cond-mat.str-el↗
arXiv2023

Electron confinement in chain-doped TMDs: A platform for spin-orbit coupled 1D physics

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.

cond-mat.str-el↗
arXiv2022

Electronic structure and magnetic properties of 3d-4f double perovskite material

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.

cond-mat.mtrl-sci↗
arXiv2022

Stabilization of A-site ordered perovskites and formation of spin-half antiferromagnetic lattice: CaCu$_3$Ti$_4$O$_{12}$ and CaCu$_3$Zr$_4$O$_{12}$

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.

cond-mat.str-el↗
arXiv2022

Simultaneous optical trapping and magnetic micromanipulation of ferromagnetic iron-doped upconversion microparticles in six degrees of freedom

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.

physics.optics↗
arXiv2022

Induction of Large Magnetic Anisotropy Energy and Formation of Multiple Dirac States in SrIrO$_3$ Films: Role of correlation and spin-orbit coupling

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.

cond-mat.str-el↗
arXiv2022

Exploration of trivial and non-trivial electronic phases and of collinear and non-collinear magnetic phases in low-spin d$^5$ perovskites

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.

cond-mat.str-el↗
arXiv2022

Spin Texture as Polarization Fingerprint of Halide Perovskites

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.

cond-mat.mtrl-sci↗
arXiv2021

Feeble Metallicity and Robust Semiconducting Regime in Structurally Sensitive Ba(Pb, Sn)O$_3$ Alloys

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.

cond-mat.mtrl-sci↗
arXiv2021

A Generic Slater-Koster Description of the Electronic Structure of Centrosymmetric Halide Perovskites

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.

cond-mat.mtrl-sci↗
arXiv2020

Development of short and long-range magnetic order in the double perovskite based frustrated triangular lattice antiferromagnet Ba2MnTeO6

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.

cond-mat.str-el↗
arXiv2020

Electronic Structure of Graphene/TiO$_2$ Interface: Design and Functional Perspectives

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.

cond-mat.mtrl-sci↗
arXiv2020

Evidence for weak antilocalization-weak localization crossover and metal-insulator transition in CaCu$_{3}$Ru$_{4}$O$_{12}$ thin films

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$Ω$, 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.

cond-mat.str-el↗