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Tanusri Saha-Dasgupta

Publications and source records attributed to Tanusri Saha-Dasgupta.

At least 19 recordsLinked to original sources
arXiv2026

Thermoelectric properties of Topological Weyl Semimetal Cu$_2$ZnGeTe$_4$

The exploration of topological quantum materials for thermoelectric (TE) applications offers an opportunity to combine nontrivial electronic topology with efficient energy conversion. Topological semimetals (TSMs), including Dirac, Weyl, and nodal-line systems, can exhibit favourable transport properties arising from the coexistence of dispersive and relatively flat bands near the Fermi level (E_F). However, their gapless electronic structure can also suppress thermopower and enhance electronic thermal conductivity, limiting the overall TE performance. Here, we combine first-principles calculations and transport measurements to investigate Cu2ZnGeTe4 as a lattice-tunable platform connecting thermoelectric and topological electronic phases. At the experimentally measured lattice parameters, Cu2ZnGeTe4 is a narrow-gap semiconductor with a calculated band gap of approximately 0.067 eV and a maximum calculated ZT of approximately 1. Experimentally, the compound exhibits p-type semiconducting behavior and low lattice thermal conductivity, yielding a ZT of approximately 0.14 at 623 K and reproducing the calculated temperature-dependent transport trends. Upon lattice expansion, first-principles calculations predict band inversion and a transition to a Weyl-semimetallic phase with Weyl nodes of opposite chirality and topological surface states. Although this phase exhibits enhanced electrical conductivity and a high power factor, its maximum ZT of approximately 0.36 remains lower than that of the semiconducting phase because of concomitant increases in electronic and lattice thermal conductivities. Isovalent Sn substitution at the Ge site reproduces the essential electronic features of the expanded phase, providing a possible chemical route toward the predicted topologically nontrivial state.

cond-mat.mtrl-sci↗
arXiv2026

Dimensional Confinement Driven Scattering Inversion in NaCrTe$_2$

Dimensionality reduction provides a powerful route to tune the electronic and magnetic properties of van der Waals materials, yet its influence on electronic transport remains complex due to competing effects from quantum confinement and modified scattering mechanisms. Here, we investigate this interplay in an antiferromagnetic semiconductor $\text{NaCrTe}_2$ using first-principles calculations combined with the Boltzmann transport equation beyond the constant relaxation time approximation. Our results show that the monolayer limit induces a coupled magnetostructural reconstruction, reducing the band gap from $0.44$ eV (bulk) to $0.15$ eV (monolayer) and significantly enhancing the static dielectric constant. This evolution triggers a fundamental scattering inversion: whereas bulk transport is limited by polar optical phonon (POP) scattering, the monolayer becomes dominated by acoustic deformation potential (ADP) scattering. We show that this crossover originates from the simultaneous suppression of the Fröhlich interaction through enhanced dielectric screening and the amplification of acoustic scattering due to pronounced lattice softening. These results clarify how the interplay between dielectric screening, lattice stiffness, and band topology governs transport in low-dimensional magnetic semiconductors, providing a framework to optimize their electronic performance.

cond-mat.mtrl-sci↗
arXiv2026

Two-orbital $e_g$ model with bond-dependent spin-orbit coupling: A playground for emergent band topology, Kitaev magnetism, and magnetoelectricity

Inspired by the electronic structure of compounds like nickel dihalides Ni$X_2$ ($X$=Cl, Br, I), we propose a low-energy two-orbital $e_g$ model featuring bond-dependent spin-orbit terms, driven by atomic spin-orbit coupling on the ligand $X$. We demonstrate that this model hosts a rich array of phenomena. In the non-interacting band limit, spin-orbit-derived spin-dependent and spin-flip hopping terms produce topological bands with spin-Chern numbers $C_s=\pm 2, \pm 4$, and higher order topological states with fractional corner charges, respectively. In the half-filled Mott insulator limit, we recover a spin-$1$ Hamiltonian with bond-dependent Kitaev exchange interactions. We explore the magnetoelectric effect in this two-orbital model using symmetry-based perspective and microscopic calculations, going beyond the generalized Katsura-Nagaosa-Balatsky theory for the single-orbital case. Our work may be relevant to study of doping, strain, or pressure on Ni$X_2$ and related materials.

cond-mat.str-el↗
arXiv2026

Coexistence of Rashba and Ising Spin-Singlet Pairings in Two-Dimensional IrTe$_{2}$

Symmetry offers a useful approach to unfold the intertwined degrees of freedom. Thus it paves the way to resolve coexisting quantum orders into distinct symmetry sectors. Motivated by the recent observation of superconductivity in nano-flaked IrTe$_2$, we investigate the superconductivity in strain-stabilized two-dimensional (2D) limit of IrTe$_2$ by combining density-functional theory with mean-field solution of spin-fluctuation mediated pairing interaction on a symmetry-constrained ${\bf k}\cdot{\bf p}$ model. The spin-orbit coupled band structure shows $Γ$-centred Fermi sheets with coexistence of band-selective Rashba-like (in-plane) and Ising-like (out-of-plane) superconductivity. Remarkably, the superconducting gaps are odd in spin, orbital, and momentum channels despite the presence of global inversion symmetry. Fermi surface topologies and little-group symmetry enforce distinct irreducible representations to the Rashba and Ising channels, forbidding their mixing. Our findings open up a symmetry-based route to multichannel superconductivity in 2D transition-metal dichalcogenides with unique functionalities.

cond-mat.supr-con↗
arXiv2025

Multiple Dirac Spin-Orbital Liquids in SU(4) Heisenberg Antiferromagnets on the Honeycomb Lattice

We study the strong coupling model of $d^1$ transition metal tri-halides in the large spin-orbit coupled limit. By considering ab-initio-calculation-inspired hierarchy of hopping pathways of these compounds, SU(4) symmetry is found to emerge at multiple points in the parameter space of the hopping parameters. The resultant Dirac spin-orbital liquids, within the parton mean field description, are distinct. The calculated dynamical structure factor fingerprints this distinctive nature, giving rise to observable effects. This opens up a playground for SU(4) Dirac Spin-Orbital liquid in $d^1$ Honeycomb lattice systems.

cond-mat.str-el↗
arXiv2024

Multipolar magnetism in $5d^2$ vacancy-ordered halide double perovskites

Vacancy-ordered halide double perovskites hosting 4d/5d transition metals have emerged as a distinct platform for investigating unconventional magnetism arising out of the interplay of strong atomic spin-orbit coupling (SOC) and Coulomb interactions. Focusing on the $d^2$ system Cs$_2$WCl$_6$, our ab initio electronic structure calculation reveals very narrow electronic bands, fulfilling the necessary condition to realize exotic orders. Using this input, we solve the many-body spin-orbit coupled single-site problem by exact diagonalization and show that the multiplet structure of Cs$_2$WCl$_6$ hosts ground non-Kramers doublets on W, with vanishing dipole moment and a small gap to an excited magnetic triplet. Our work provides the rationale for the observed strong deviation from the classic Kotani behaviour in Cs$_2$WCl$_6$ for the measured temperature dependence of the magnetic moment. The non-Kramers doublets on W exhibit non-zero quadrupolar and octupolar moments, and our calculated two-site exchange supports the dominance of inter-site octupolar exchange over quadrupolar interactions. We predict ferro-octupolar order with a transition temperature $T_c \sim 5$K which may get somewhat suppressed by quantum fluctuations and disorder; this could be tested in future low-temperature experiments.

cond-mat.str-el↗
arXiv2024

Probing octupolar hidden order via Janus impurities

Quantum materials with non-Kramers doublets are a fascinating venue to realize multipolar hidden orders. Impurity probes which break point group symmetries, such as implanted muons or substitutional impurities, split the non-Kramers degeneracy and exhibit a Janus-faced influence in such systems: they can destroy the very order they seek to probe. Here, we explore this duality in cubic osmate double perovskites which are candidates for exotic $d$-orbital octupolar order competing with quadrupolar states. Using {\it ab initio} computations, Landau theory, and Monte Carlo simulations, we show that Janus impurities induce local strain fields, nucleating quadrupolar puddles and suppressing the octupolar $T_c$. At the same time, strains mix the non-Kramers doublet with an excited magnetic triplet, creating parasitic dipole moments which directly expose the hidden octupolar order parameter. Our work unravels this Janus duality in recent impurity nuclear magnetic resonance (NMR) experiments, with important implications for uncovering hidden order in diverse multipolar materials.

cond-mat.str-el↗
arXiv2024

Cu$_2$ZnSiTe$_4$: A potential thermoelectric material with promising electronic transport

Transition metal-based quaternary chalcogenides have gathered immense attention for various renewable energy applications including thermoelectrics (TE). While low-symmetry and complex structure help to achieve low thermal conductivity, the TE power factor and hence the figure of merit (ZT) remains low which hinders to promote these class of materials for future TE applications. Here, we investigated the TE properties of a new system, Cu$_2$ZnSiTe$_4$, with improved electronic transport using first-principles calculation. The presence of heavy chalcogen like Te, helps to achieve a relatively low bandgap (0.58 eV). This, together with unique electronic band topology, leads to a promising value of power-factor of 3.95(n-type) and 3.06(p-type) mWm$^{-1}$K$^{-2}$ at 900 K. Te atoms also play a crucial role in mixing the optical and acoustic phonon branches which, in turn, are responsible for reduced lattice thermal conductivity ($\sim$0.7 Wm$^{-1}$K$^{-1}$ at high temperature). Though the thermal conductivity is not appreciably low, the electronic transport properties (power factor) are quite favorable to yield promising TE figure of merit (ZT $\sim$2.67 (n-type) and $\sim$2.11 (p-type) at 900 K). We propose Cu$_2$ZnSiTe$_4$ to be a potential candidate for TE applications, and believe to attract future experimental/theoretical studies.

cond-mat.mtrl-sci↗
arXiv2023

Proximate Dirac spin liquid in honeycomb lattice $J_1$-$J_3$ XXZ model: Numerical study and application to cobaltates

Recent theoretical and experimental work suggest that the honeycomb cobaltates, initially proposed as candidate Kitaev quantum magnets, are in fact described by a pseudospin-$1/2$ easy-plane spin Hamiltonian with nearest neighbor ferromagnetic (FM) exchange $J_1$ being frustrated by antiferromagnetic third-neighbor exchange $J_3$ and weaker compass anisotropies. Using exact diagonalization and density-matrix renormalization group (DMRG) calculations, we show that this model exhibits FM order at small $J_3/J_1$ and zig-zag (ZZ) order at large $J_3/J_1$, separated by an intermediate phase, which we label as $\widetilde{\mathrm{SL}}$. This $\widetilde{\mathrm{SL}}$ phase is shown to exhibit spin-liquid-like correlations in DMRG, although we cannot preclude weak broken symmetries, e.g. weak Ising type Néel order, given the limits on our explored system sizes. Using a modified parton mean field theory and variational Monte Carlo on Gutzwiller projected wavefunctions, we show that the optimal FM and ZZ orders as well as the intermediate $\widetilde{\mathrm{SL}}$ state are proximate to a `parent' Dirac spin liquid (SL). This Dirac SL is shown to capture the broad continuum in the temperature and magnetic field dependent terahertz spectroscopy of BaCo$_2$(AsO$_4$)$_2$, and the reported low temperature metallic thermal conductivity in Na$_2$Co$_2$TeO$_6$ and BaCo$_2$(AsO$_4$)$_2$ upon incorporating disorder induced broadening.

cond-mat.str-el↗
arXiv2023

Establishing Magnetic Coupling in Spin-crossover-2D Hybrid Nanostructures via Interfacial Charge-transfer Interaction

Despite a clear demonstration of bistability in spin-crossover (SCO) materials, the absence of long-range magnetic order and poor electrical conductivity limit their prospect in spintronic and nanoelectronic applications. Intending to create hybrid devices made of spin-crossover (SCO)-2D architecture, here, we report an easily processable Fe-based SCO nanostructures grown on 2D reduced graphene oxide (rGO). The heterostructure shows enhanced cooperativity due to formation of interfacial charge transfer induced inter-molecular interaction. The spin transition temperature is controlled by tuning the coverage area of SCO nanostructured networks over the 2D surfaces, thus manipulating hysteresis (aka memory) of the heterostructure. The enhanced magnetic coupling of the heterostructure leads to the spontaneous magnetization states with a large coercive field of $\sim$ 3000 Oe. Additionally, the low conductivity of the pristine SCO nanostructures is addressed by encapsulating them on suitable 2D rGO template, enabling detection of magnetic bistable spin states during high-spin/low-spin conductance change. This adds spin functionality in conductance switching for realizing hybrid 2D spintronic devices. Ab-inito calculations, on the experimentally proposed nanostructures, corroborate the enhanced magnetic interaction in the proposed architecture facilitated by interfacial charge transfer and provide insights on the microscopic mechanism.

cond-mat.mtrl-sci↗
arXiv2023

Correlation-driven non-trivial phases in single bi-layer Kagome intermetallics

Bi-layer Kagome compounds provide an exciting playground where the interplay of topology and strong correlations can give rise to exotic phases of matter. Motivated by recent first principles calculation on such systems (Phys. Rev. Lett 125, 026401), reporting stabilization of a Chern metal with topological nearly-flat band close to Fermi level, we build minimal models to study the effect of strong electron-electron interactions on such a Chern metal. Using approriate numerical and analytical techniques, we show that the topologically non-trivial bands present in this system at the Fermi energy can realize fractional Chern insulator states. We further show that if the time-reversal symmetry is restored due to destruction of magnetism by low dimensionality and fluctuation, the system can realize a superconducting phase in the presence of strong local repulsive interactions. Furthermore, we identify an interesting phase transition from the superconducting phase to a correlated metal by tuning nearest-neighbor repulsion. Our study uncovers a rich set of non-trivial phases realizable in this system, and contextualizes the physically meaningful regimes where such phases can be further explored.

cond-mat.str-el↗
arXiv2023

Centrosymmetric-noncentrosymmetric Structural Phase Transition in Quasi one-dimensional compound, (TaSe$_4$)$_3$I

(TaSe$_4$)$_3$I, a compound belonging to the family of quasi-one-dimensional transition-metal tetrachalcogenides, has drawn significant attention due to a recent report on possible coexistence of two antagonistic phenomena, superconductivity and magnetism below 2.5~K (Bera et. al, arXiv:2111.14525). Here, we report a structural phase transition of the trimerized phase at temperature, $T~\simeq$~145~K using Raman scattering, specific heat, and electrical transport measurements. The temperature-dependent single-crystal X-ray diffraction experiments establish the phase transition from a high-temperature centrosymmetric to a low-temperature non-centrosymmetric structure, belonging to the same tetragonal crystal family. The first-principle calculation finds the aforementioned inversion symmetry-breaking structural transition to be driven by the hybridization energy gain due to the off-centric movement of the Ta atoms, which wins over the elastic energy loss.

cond-mat.str-el↗
arXiv2023

Gender Equity in Physics in India: Interventions, Outcomes, and Roadmap

The gender imbalance in physics higher education and advanced professions is a global problem, and India is not an exception. Although the issue has been acknowledged widely, discrimination needs to be recognized as the driving force. The past three years have witnessed initiatives by different gender groups as well as the Government of India in addressing these lacunae. We report various activities, describe interventions, and present statistics indicating improvements achieved. The Gender in Physics Working Group has brought about significant gender reforms in the Indian Physics Association. The working group organized an open discussion on the issue of sexual harassment in physics professions for the first time in 2018. Subsequently, in 2019, GIPWG organized the first-ever national conference on gender issues, Pressing for Progress. The deliberations of the conference culminated in the Hyderabad Charter, a roadmap towards gender equity in India. The Working Group for Gender Equity constituted under the Astronomical Society of India, also played an impactful role. At the government level, notable new initiatives include Gender Advancement through Transforming Institutions and the proposed Science and Technology Innovation Policy for mainstreaming equity and inclusion.

physics.ed-ph↗
arXiv2022

Small moments without long-range magnetic ordering in the zero-temperature ground state of the double-perovskite iridate Ba$_2$YIrO$_6$

The spin-orbit coupled double perovskite iridate Ba$_2$YIrO$_6$ with $d^4$ occupancy of Ir is considered as a candidate material for a non-magnetic $J=0$ ground state. The issue of existence of such a state in Ba$_2$YIrO$_6$ however has opened up intense debates both in experimental and theoretical studies. In this study, we revisit the issue using \textit{ab-initio} density functional combined with dynamical mean-field theory to investigate the magnetic properties of Ba$_2$YIrO$_6$ down to zero temperature. To reach the ground state, a recently developed impurity solver based on tensor-product states working directly at zero temperature is employed. We find that Ba$_2$YIrO$_6$ has a small instantaneous non-zero magnetic moment, both at $T=0$ K as well as at room temperature. We did not observe any evidence of magnetic ordering, not even at $T=0$ K. From the calculated local magnetic susceptibility we see that the quantum fluctuations are very strong and effective in screening the instantaneous moments. This dynamical screening, together with frustration effects in the fcc lattice that can lead to almost degenerate magnetic ground states, prevents any long-range ordering.

cond-mat.str-el↗
arXiv2021

XY magnetism, Kitaev exchange, and long-range frustration in the $J_{\rm eff}=1/2$ honeycomb cobaltates

The quest for Kitaev quantum spin liquids has led to great interest in honeycomb quantum magnets with strong spin-orbit coupling. It has been recently proposed that even Mott insulators with $3d$ transition metal ions, having nominally weak spin-orbit coupling, can realize such exotic physics. Motivated by this, we study the rhombohedral honeycomb cobaltates CoTiO$_3$, BaCo$_2$(PO$_4$)$_2$, and BaCo$_2$(AsO$_4$)$_2$, using $\textit{ab initio}$ density functional theory, which takes into account realistic crystal field distortions and chemical information, in conjunction with exact diagonalization numerics. We show that these Co$^{2+}$ magnets host $j_\text{eff}=1/2$ local moments with highly anisotropic $g$-factors, and we extract their full spin Hamiltonians including longer-range and anisotropic exchange couplings. For CoTiO$_3$, we find a nearest-neighbor easy-plane ferromagnetic $XXZ$ model with additional bond-dependent anisotropies and interlayer exchange, which supports three-dimensional (3D) Dirac nodal line magnons. In contrast, for BaCo$_2$(PO$_4$)$_2$ and BaCo$_2$(AsO$_4$)$_2$, we find a strongly suppressed interlayer coupling, and significant frustration from additional third-neighbor antiferromagnetic exchange mediated by P/As. Such bond-anisotropic $J_1$-$J_3$ spin models can support collinear zig-zag or coplanar spiral ground states; we discuss their dynamical spin correlations which reveal a gapped Goldstone mode, and argue that the effective parameters of these pseudospin-$1/2$ models may be strongly renormalized by coupling to a low energy spin-exciton. Our results call for re-examining proposals for realizing Kitaev spin liquids in the honeycomb cobaltates.

cond-mat.str-el↗
arXiv2021

Effect of geometry on magnetism of Hund's metals: A case study with BaRuO$_3$

In order to explore the effects of structural geometry on properties of correlated metals we investigate the magnetic properties of cubic (3C) and hexagonal (4H) BaRuO$_3$. While the 3C variant of BaRuO$_3$ is ferromagnetic below 60K, the 4H phase does not show any long-range magnetic order, however, there is experimental evidence of short-range antiferromagnetic correlations. Employing a combination of computational tools, namely density-functional theory and dynamical mean-field theory calculations, we probe the origin of contrasting magnetic properties of BaRuO$_3$ in the 3C and 4H structures. Our study reveals that the difference in connectivity of RuO$_6$ octahedra in the two phases results in different Ru-O covalency, which in turn influences substantially the strengths of screened interaction values for Hubbard $U$ and Hund's rule $J$. With estimated $U$ and $J$ values, the 3C phase turns out to be a ferromagnetic metal, while the 4H phase shows paramagnetic behavior with vanishing ordered moments. However, this paramagnetic phase bears signatures of antiferromagnetic correlations, as confirmed by a calculation of the magnetic susceptibility. We find that the 4H phase is found to be at the verge of antiferromagnetic long-range order, which can be stabilized upon slight changes of screened Coulomb parameters $U$ and $J$, opening up the possibility of achieving a rare example of an antiferromagnetic metal.

cond-mat.str-el↗
arXiv2021

Exploring the possible origin of spin reorientation transition in NdCrO$_3$

Spin reorientation transitions and other related magnetic phenomena, which owe their origin to the complex interplay between multiple magnetic sublattices, have long attracted scientific attention both from the perspective of fundamental curiosity and technological applications. In this study, combining first principles calculations together with finite temperature Monte Carlo simulations, we explore the possible origins of reorientation transition of Cr spins in NdCrO$_3$. We construct a NdCrO$_3$ specific magnetic model, consisting of symmetric superexchange interactions between magnetic ions, as well as their magnetic anisotropy. We show that the observed spin reorientation in NdCrO$_3$, arises out of a delicate balance between Nd$-$Cr magnetic exchange interactions, single ion anisotropy of Nd spins, and single ion anisotropy of Cr spins. Moreover, though our model does not take into consideration the effect of anti-symmetric and anisotropic-symmetric magnetic exchanges, the qualitative as well as quantitative agreement of the theoretically derived and the experimentally observed spin-reorientation transition in NdCrO$_3$, confirms the merit of our proposed microscopic model. Our results also propose a hitherto unobserved collective magnetic ordering in Nd sublattice, which is challenging to detect as it is an extreme low temperature phenomena, therefore calls for further investigations.

cond-mat.mtrl-sci↗
arXiv2021

The critical role of stereochemically active lone pair in introducing high temperature ferroelectricity

In this paper a comparative structural, dielectric and magnetic study of two langasite compounds Ba$_3$TeCo$_3$P$_2$O$_{14}$ (absence of lone pair) and Pb$_3$TeCo$_3$P$_2$O$_{14}$ (Pb$^{2+}$ 6$s^2$ lone pair) have been carried out to precisely explore the development of room temperature spontaneous polarization in presence of stereochemically active lone pair. In case of Pb$_3$TeCo$_3$P$_2$O$_{14}$, mixing of both Pb 6$s$ with Pb 6$p$ and O 2$p$ help the lone pair to be stereochemically active. This stereochemically active lone pair brings a large structural distortion within the unit cell and creates a polar geometry, while Ba$_3$TeCo$_3$P$_2$O$_{14}$ compound remains in a nonpolar structure due to the absence of any such effect. Consequently, polarization measurement under varying electric field confirms room temperature ferroelectricity for Pb$_3$TeCo$_3$P$_2$O$_{14}$, which was not the case of Ba$_3$TeCo$_3$P$_2$O$_{14}$. Detailed study was carried out to understand the microscopic mechanism of ferroelectricity which revealed the exciting underlying activity of poler TeO$_6$ octahedral unit as well as Pb-hexagon.

cond-mat.mtrl-sci↗