Faculty Publications
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Item Effect of global monopole on the microscopic structure of rn-ads black hole(Springer Science and Business Media Deutschland GmbH info@springer-sbm.com, 2020) Ahmed Rizwan, C.L.A.; Naveena Kumar, A.; Ananthram, K.S.In this work we study the microscopic structure of RN-AdS Black hole with a monopole. We utilize the connection between the thermodynamic Ruppeiner geometry and the nature of underlying microscopic interaction. The behaviour of the Ruppeiner invariant scalar is analysed in the extended space. The sign of the curvature scalar is the measure of the strength of attractive or repulsive statistical interaction. It is observed that the monopole parameter influences the microscopic structure of the black hole. © Springer Nature Singapore Pte Ltd 2020.Item Charge-transfer interface of insulating metal-organic frameworks with metallic conduction(Nature Research, 2022) Sindhu, P.; Ananthram, K.S.; Jain, A.; Tarafder, K.; Ballav, N.Downsizing materials into hetero-structured thin film configurations is an important avenue to capture various interfacial phenomena. Metallic conduction at the interfaces of insulating transition metal oxides and organic molecules are notable examples, though, it remained elusive in the domain of coordination polymers including metal-organic frameworks (MOFs). MOFs are comprised of metal centers connected to organic linkers with an extended coordination geometry and potential void space. Poor orbitals overlap often makes these crystalline solids electrical insulators. Herein, we have fabricated hetero-structured thin film of a Mott and a band insulating MOFs via layer-by-layer method. Electrical transport measurements across the thin film evidenced an interfacial metallic conduction. The origin of such an unusual observation was understood by the first-principles density functional theory calculations; specifically, Bader charge analysis revealed significant accumulation and percolation of charge across the interface. We anticipate similar interfacial effects in other rationally designed hetero-structured thin films of MOFs. © 2022, The Author(s).Item Ag Nanoparticles-Induced Metallic Conductivity in Thin Films of 2D Metal-Organic Framework Cu3(HHTP)2(American Chemical Society, 2023) Saha, S.; Ananthram, K.S.; Hassan, N.; Ugale, A.; Tarafder, K.; Ballav, N.Two-dimensional (2D) metal-organic frameworks (MOFs) are usually associated with higher electrical conductivity and charge carrier mobility when compared with 3D MOFs. However, attaining metallic conduction in such systems through synthetic or postsynthetic modifications is extremely challenging. Herein, we present the fabrication of thin films of a 2D MOF, Cu3(HHTP)2 (HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene), decorated with silver nanoparticles (AgNPs) exhibiting significant conductivity enhancement at room temperature. Variable-temperature electrical transport measurements across the low-temperature (200 K) to high-temperature (373 K) regime evidenced metallic conduction. Interestingly, thin films of a 3D MOF, CuTCNQ (TCNQ = 7,7,8,8-tetracyanoquinodimethane), upon decoration with AgNPs, disclosed a converse trend. The origin of such distinctive observations on AgNPs@Cu3(HHTP)2 and AgNPs@CuTCNQ systems was comprehended by using first-principles density functional theory (DFT) calculations and attributed to an interfacial electronic effect. Our work sheds new light on rationally designing synthetic modifications in thin films of MOFs to tune the electrical transport property. © 2023 American Chemical Society.Item Insulator-to-metal-like transition in thin films of a biological metal-organic framework(Nature Research, 2023) Sindhu, P.; Ananthram, K.S.; Jain, A.; Tarafder, K.; Ballav, N.Temperature-induced insulator-to-metal transitions (IMTs) where the electrical resistivity can be altered by over tens of orders of magnitude are most often accompanied by structural phase transition in the system. Here, we demonstrate an insulator-to-metal-like transition (IMLT) at 333 K in thin films of a biological metal-organic framework (bio-MOF) which was generated upon an extended coordination of the cystine (dimer of amino acid cysteine) ligand with cupric ion (spin-1/2 system) – without appreciable change in the structure. Bio-MOFs are crystalline porous solids and a subclass of conventional MOFs where physiological functionalities of bio-molecular ligands along with the structural diversity can primarily be utilized for various biomedical applications. MOFs are usually electrical insulators (so as our expectation with bio-MOFs) and can be bestowed with reasonable electrical conductivity by the design. This discovery of electronically driven IMLT opens new opportunities for bio-MOFs, to emerge as strongly correlated reticular materials with thin film device functionalities. © 2023, The Author(s).Item First Principles Studies of Topological Insulating Behavior in Lanthanum-Monopnictides and their Heterostructures(John Wiley and Sons Inc, 2024) Ananthram, K.S.; Suneetha, N.; Tarafder, K.The topological insulating behavior of LaX (X = Bi, Sb, As, and P) and their heterostructure combinations are investigated. The system's bulk-band-inversion is analyzed, and the (Formula presented.) -topological indexes are calculated using LDA and mBJLDA exchange functionals. Strong topological insulating behavior in LaBi is confirmed from bulk band inversion. Linearly dispersive Dirac-cone in the (001)-surface band structure is observed. The other LaX binaries also exhibit bulk-band-inversion under LDA+SOC settings but disappear after including the mBJ potential. The robustness of the surface Dirac cone is tested by estimating the surface band structure under uniaxial strain. The result shows that Dirac-cone in the (001)-surface is unaltered only for the LaBi system and destroyed in all other three binaries, concluding the topologically trivial nature of LaP, LaSb, and LaAs. The investigation is further extended to study the multilayer LaX1/LaX2 systems composed of an alternate stacking of two different LaX binaries. The findings suggest that the multilayer structures exhibit topological insulating behavior only when LaBi is present in the system. Bulk-band-inversion and surface Dirac-cone structures in LaBi/LaAs, LaBi/LaSb, and LaBi/LaP multilayers are observed. Furthermore, the surface Dirac cones remain unchanged under the influence of TR-preserving perturbation, confirming these systems possess strong topological insulating character. © 2023 Wiley-VCH GmbH.Item Anticipation of Large Intrinsic Spin Hall Conductivity in Mercury Chalcogenides: A First-Principles Study(John Wiley and Sons Inc, 2024) Suneetha, N.; Ananthram, K.S.; Tarafder, K.The report carried out detailed first-principle calculations of Mercury chalcogenides (HgX; X = Te, Se and S) using density functional theory, verifying the bulk band inversion property with different exchange-correlation functionals. The Wannier function method is used to study the non-trivial topology of HgX systems, spin Berry curvature, and intrinsic spin Hall conductivity. Quantized intrinsic spin Hall conductivity is observed in the HgX systems. Large intrinsic spin Hall conductivity is found in the systems due to a strong spin Berry curvature accumulation near the triply degenerate points in the Brillouin zone. Calculation shows that the intrinsic spin Hall conductivity for all three HgX systems has stable plateaus, with Mercury Telluride having a maximum width of up to 1.05 eV. The maximum intrinsic spin Hall conductivity of –931 (Formula presented.) /e ((Formula presented.)) is obtained in mercury sulfide, higher than the reported values for spin Hall conductivity and the plateau width in typical topological insulators such as (Formula presented.), (Formula presented.), and (Formula presented.) as well as in transition metal pnictides (TaX, X = As, P and N) and transition metal iridates. © 2024 Wiley-VCH GmbH.Item An Intricate Balance of Ionicity and Covalency: Metal-Like Conduction in All-Inorganic Halide Double Perovskite Cs2AgSbCl6(American Chemical Society, 2025) Kalyani, M.; Ananthram, K.S.; Saha, S.; Ninawe, P.; Tarafder, K.; Ballav, N.Halide perovskites have recently evolved as attractive materials with enormous technological significance due to synthetic control over the structure-property relationship. Halide perovskites are often realized to be either electrical insulators or semiconductors. We present an unusual metal-like conduction (thermally deactivated) in a Pb-free all-inorganic halide double perovskite, Cs2AgSbCl6. The experimental results were understood using density functional theory studies, combined with molecular dynamics simulations and electron localization function calculations, revealing retention of the predominant ionicity of the Ag-Cl bond and an increase in the covalency of the Sb-Cl bond at an elevated temperature, which resulted in a significant change of the electronic band structure, including the density of states, thereby exhibiting an intricate balance of ionicity and covalency. A significant modulation of the electrical conductivity (more than 3 orders of magnitude) without any noticeable structural change will stimulate the investigation of hitherto unknown electronic phase transitions in halide double perovskites. Additionally, light-induced unidirectional rectification of current in Cs2AgSbCl6 was ascribed to a dynamic internal polarization effect. © 2025 American Chemical Society.Item Phonon dynamics with collective spin excitations and orbital ordering in quasi-two-dimensional La1.4Sr1.6Mn2 O7(American Physical Society, 2025) Mekap, S.; Kumar, B.; Ananthram, K.S.; Tarafder, K.; Ghosh, S.; Roy, A.This paper discusses the coupling between spin-wave-mediated magnetic interactions and lattice degrees of freedom in bilayer La1.4Sr1.6Mn2O7 (BL-LSMO-0.3). The thermal evolution of the Raman shift, as obtained from micro-Raman spectroscopic measurements over a wide temperature range, reveals the evolution of phonon dynamics while the system undergoes a balance between double and superexchange spin-spin interactions through spin-wave modulation in different magnetic phases. The evolution of relative magnetoelastic coupling strengths for nearest-neighbor ferromagnetic and next-nearest-neighbor antiferromagnetic superexchange interactions in the high-temperature canted antiferromagnetic phases are estimated. We have also demonstrated the existence of orbital ordering and its correlation with spin-phonon dynamics in the canted antiferromagnetic phases of BL-LSMO-0.3 from the detailed analysis of the high-wave-number orbiton-related phonon modes. Our study further reveals the absence of charge ordering in the compound. The complex coupling of lattice distortion, orbital ordering, the nearest- and next-nearest-neighbor spin-spin interactions in determining the canted spin states of the system is discussed. © 2025 American Physical Society.Item Thermally-driven conformational twist in organic azobenzene linker activates molecular doping effect in thin films of lanthanide MOFs(Royal Society of Chemistry, 2025) Bhoi, U.; Kalyani, M.; Ananthram, K.S.; Saha, S.; Acharya, A.; Hassan, N.; Raj, M.; Tarafder, K.; Ballav, N.Azobenzene-based photo-switchable molecules have shown significant potential in stimuli-responsive systems, especially when incorporated into metal–organic frameworks (MOFs). This study reports thin films of lanthanide-based metal–organic frameworks (Ln-MOFs) with 4,4?-azobenzene dicarboxylic acid (H2ADA) as the organic linker – Tb-ADA, Eu-ADA, and Gd-ADA – using an electrodeposition method. Upon heating to 400 K, a reversible structural transition was observed via variable temperature grazing-incidence X-ray diffraction (GIXRD) and Raman spectroscopy, not due to trans–cis isomerization but rather a thermally-induced conformational twist of the ADA linker. Density functional theory (DFT) combined with molecular dynamics (MD) simulations supports this interpretation, revealing high-energy atropisomeric states stabilized by MOF confinement. Molecular doping of these films with 7,7,8,8-tetracyanoquinodimethane (TCNQ) significantly enhanced their electrical conductivity, increasing by two orders of magnitude at 400 K. This enhancement is attributed to improved ?–? stacking and charge-transfer interactions facilitated by the conformational twist. Temperature-dependent X-ray photoelectron spectroscopy (XPS) confirmed redox activity in TCNQ@Tb-ADA films, showing reversible conversion between Tb(iii) and Tb(iv), with back electron transfer at 400 K restoring Tb(iii). These findings introduce a new mechanism of thermally-driven conformational switching in MOFs and open avenues for developing responsive electronic materials based on azobenzene linkers. This journal is © The Royal Society of Chemistry, 2025Item Synergistic Photoconductivity and Ultralow Thermal Conductivity upon Stabilizing Iron(III)-tris(2,2?-bipyridine) in a Two-Dimensional Haloargentate Network(American Chemical Society, 2025) Jose, T.M.; Hassan, N.; Ananthram, K.S.; Kalyani, M.; Tarafder, K.; Ballav, N.Crystalline organic–inorganic halometallate hybrids have emerged as promising materials for optoelectronic applications due to their structural diversity and tunable properties. We report a three-dimensional (3D) hybrid organic–inorganic crystal?[Fe(bpy)3]2Ag6Br11·NO3(bpy = 2,2? bipyridine)?consisting of two-dimensional (2D) Ag(I)-based (Ag6Br11)n5n–anionic sheets, zero-dimensional (0D) [Fe(bpy)3]3+complexes (acting as the structure-directing agent), and interlayer disordered NO3–anions. Specifically, the thermodynamically unstable cation [Fe(bpy)3]3+is stabilized under ambient conditions by the two-dimensional (2D) inorganic anionic scaffold. The crystal exhibits strong ligand-supported argentophilic interactions (Ag···Ag bond distance of 2.98 Å), forming an extended (Ag6Br11)n5n–network, and displays broad UV–visible absorption with a band gap of 1.90 eV. Remarkably, this organic–inorganic hybrid shows a ?103-fold increase in photocurrent under 532 nm light illumination. Density functional theory calculations provided the mechanistic insights, and such a remarkable photoconductivity is attributed to an efficient charge delocalization and inorganic-to-organic charge transfer. Additionally, the crystal exhibits an ultralow thermal conductivity over a broad temperature range (?0.3 W/m·K; 300–400 K), making it an excellent candidate for heat management applications. © 2025 American Chemical Society