arXiv subjects

Shishir Kumar Pandey

Publications and source records attributed to Shishir Kumar Pandey.

18 recordsLinked to original sources

arXiv2026

Signature of Magnon-Raman Phonon-Polariton Condensation in a Cavity via Transverse Pumping

Polaritons---hybrid light-matter quasiparticles---provide a versatile platform for dynamically controlling a wide range of condensed matter systems. While conventional polaritonic platforms rely on direct dipole coupling, controlling dipole-forbidden or Raman-active lattice and spin excitations remains challenging due to optical selection rules and the limitations of THz cavities. Here, we propose a theoretical platform for realizing a continuous-wave magnon--Raman phonon--polariton condensate. By embedding a magnetic medium hosting strongly coupled magnon and Raman-active phonon (MRP) modes inside an optical microcavity under continuous-wave transverse laser pumping, we derive the stationary-state phase diagram and demonstrate the emerging signature of distinct MRP condensation phases under critical conditions. Furthermore, since the bare magnon frequency is tunable via an external magnetic field, we show that the transverse pump frequency and the external magnetic field-dependent magnon simultaneously as highly flexible control parameters for exploring and controlling macroscopic quantum phenomena at the interface of cavity quantum optics, lattice dynamics, and quantum magnetism.

physics.optics↗

arXiv2025

Deep Learning-Based Quantum Transport Simulations in Two-Dimensional Materials

Two-dimensional (2D) materials exhibit a wide range of electronic properties that make them promising candidates for next-generation nanoelectronic devices. Accurate prediction of their quantum transport behavior is therefore of both fundamental and technological importance. While density functional theory (DFT) combined with the non-equilibrium Green$'$s function (NEGF) formalism provides reliable insights, its high computational cost limits applications to large-scale or high-throughput studies. Here we present DeePTB-NEGF, a framework that combines a deep learning-based tight-binding Hamiltonians derived learned directly from first-principles calculations (DeePTB) with efficient quantum transport simulations implemented in the DPNEGF package. To validate the method, we apply it to three prototypical 2D materials: graphene, hexagonal boron nitride (h-BN), and MoS$_2$. The resulting band structures and transmission spectra show excellent agreement with conventional DFT-NEGF results, while achieving orders-of-magnitude improvement in efficiency. These results highlight the capability of DeePTB-NEGF to enable accurate and efficient quantum transport simulations, thereby opening avenues for large-scale exploration and device design in 2D materials.

cond-mat.mtrl-sci↗

arXiv2025

Signatures of emergent surface states across a displacive topological phase transition in Bi$_4$I$_4$

Topological phase transitions involving crystalline symmetry breaking provide a fertile ground to explore the interplay between symmetry, topology, and emergent quantum phenomena. Recently discovered quasi-one-dimensional topological material, Bi$_4$I$_4$, has been predicted to host topologically non-trivial gapless surfaces at high temperature, which undergo a finite temperature phase transition to a low temperature gapped phase. Here we present experimental signatures of this room temperature phase transition from a high-temperature $β$-phase with a surface state to a gapped $α$-phase hosting hinge states. Using real-space current mapping and resistance fluctuation spectroscopy, we identify signatures of a displacive topological phase transition mediated by a first-order thermodynamic structural change. Near the emergence of $β$-phase, we observe pronounced telegraphic noise, indicating fluctuating phase domains with topological surface states. The spatially resolved current map reveals electron transport via the gapless surface states in the $β$-phase, which vanishes upon transitioning to the $α$-phase with localized conduction channels (or hinge modes). Our experimental results, supported by first principles estimates and effective theory of a topological displacive phase transition, establish Bi$_4$I$_4$ as a candidate material showing intricate interplay of classical thermodynamic phase transitions with topological quantum phenomena.

cond-mat.str-el↗

arXiv2025

Deep Neural Network for Phonon-Assisted Optical Spectra in Semiconductors

Ab initio based accurate simulation of phonon-assisted optical spectra of semiconductors at finite temperatures remains a formidable challenge, as it requires large supercells for phonon sampling and computationally expensive high-accuracy exchange-correlation (XC) functionals. In this work, we present an efficient approach that combines deep learning tight-binding and potential models to address this challenge with ab initio fidelity. By leveraging molecular dynamics for atomic configuration sampling and deep learning-enabled rapid Hamiltonian evaluation, our approach enables large-scale simulations of temperature-dependent optical properties using advanced XC functionals (HSE, SCAN). Demonstrated on silicon and gallium arsenide across temperature 100-400 K, the method accurately captures phonon-induced bandgap renormalization and indirect/direct absorption processes which are in excellent agreement with experimental findings over five orders of magnitude. This work establishes a pathway for high-throughput investigation of electron-phonon coupled phenomena in complex materials, overcoming traditional computational limitations arising from large supercell used with computationally expensive XC-functionals.

cond-mat.mtrl-sci↗

arXiv2025

Half-metallicity and wandering axis ferromagnetism in Fe$_2$Ti$_{1-x}$Mn$_x$Sn (0 $< x \leq$ 0.3) Heusler Alloys

We investigate the effect of Mn substitution in Fe$_2$Ti$_{1-x}$Mn$_x$Sn on electronic structure and magnetic and electrical transport properties. The spin-polarized density of states calculations using density-functional theory (DFT) yields a half-metallic ground state in Mn-rich compositions. Localized magnetic moments at Mn sites interacting through the cloud of conduction electrons formed by Fe and Ti atoms are also predicted. Electrical resistivity and magneto-transport measurements reveal a Kondo-like ground state at low temperatures and a peculiar linear negative temperature coefficient of resistance in the high-temperature regime with a predominant electron-phonon scattering mechanism. Analysis of room temperature powder X-ray diffraction data reveals a highly ordered L2$_1$ structure and reduction of antisite disorder upon Mn substitution. The temperature-dependent magnetization measurements reveal distinct features indicative of weak anisotropy in the system. Isothermal magnetization measured as a function of the applied field helps identify the unique magnetic ground state of the half-metallic Fe$_2$Ti$_{1-x}$Mn$_x$Sn composition as a ferromagnet with a wandering axis that distinctively orients in the direction of the applied magnetic field. The measurement of X-ray absorption fine structure (XAFS) reveals that the random anisotropy arises due to the local lattice distortion around Mn atoms in the prepared compositions. Our findings thus provide a new perspective for studying the mechanism of half-metallicity and associated magnetic order in Heuslers.

cond-mat.mtrl-sci↗

arXiv2025

Learning local equivariant representations for quantum operators

Predicting quantum operator matrices such as Hamiltonian, overlap, and density matrices in the density functional theory (DFT) framework is crucial for material science. Current methods often focus on individual operators and struggle with efficiency and scalability for large systems. Here we introduce a novel deep learning model, SLEM (strictly localized equivariant message-passing) for predicting multiple quantum operators, that achieves state-of-the-art accuracy while dramatically improving computational efficiency. SLEM's key innovation is its strict locality-based design for equivariant representations of quantum tensors while preserving physical symmetries. This enables complex many-body dependency without expanding the effective receptive field, leading to superior data efficiency and transferability. Using an innovative SO(2) convolution and invariant overlap parameterization, SLEM reduces the computational complexity of high-order tensor products and is therefore capable of handling systems requiring the $f$ and $g$ orbitals in their basis sets. We demonstrate SLEM's capabilities across diverse 2D and 3D materials, achieving high accuracy even with limited training data. SLEM's design facilitates efficient parallelization, potentially extending DFT simulations to systems with device-level sizes, opening new possibilities for large-scale quantum simulations and high-throughput materials discovery.

cond-mat.mtrl-sci↗

arXiv2024

Deep learning tight-binding approach for large-scale electronic simulations at finite temperatures with $ab$ $initio$ accuracy

Simulating electronic behavior in materials and devices with realistic large system sizes remains a formidable task within the $ab$ $initio$ framework due to its computational intensity. Here we show DeePTB, an efficient deep learning-based tight-binding approach with $ab$ $initio$ accuracy to address this issue. By training on structural data and corresponding $ab$ $initio$ eigenvalues, the DeePTB model can efficiently predict tight-binding Hamiltonians for unseen structures, enabling efficient simulations of large-size systems under external perturbations such as finite temperatures and strain. This capability is vital for semiconductor band gap engineering and materials design. When combined with molecular dynamics, DeePTB facilitates efficient and accurate finite-temperature simulations of both atomic and electronic behavior simultaneously. This is demonstrated by computing the temperature-dependent electronic properties of a gallium phosphide system with $10^6$ atoms. The availability of DeePTB bridges the gap between accuracy and scalability in electronic simulations, potentially advancing materials science and related fields by enabling large-scale electronic structure calculations.

cond-mat.mtrl-sci↗

arXiv2024

Terahertz crystal electric field transitions in a Kondo-lattice antiferromagnet

Hybridization between the localized f-electrons and the delocalized conduction electrons together with the crystal electric field (CEF) play a determinant role in governing the many-body ground state of a correlated-electron system. Here, we investigate the low-energy CEF states in CeAg_2Ge_2, a prototype Kondo-lattice antiferromagnet where Kondo correlation is found to exist within the antiferromagnetic phase. Using time-domain THz reflection spectroscopy, we show the first direct evidence of two low-energy CEF transitions at 0.6 THz (2.5 meV) and 2.1 THz (8.7 meV). The presence of low-frequency infrared-active phonon modes further manifests as a Fano-modified lineshape of the 2.1 THz CEF conductivity peak. The temporal spectral weights obtained directly from the THz time traces, in addition, corroborate the corresponding CEF temperature scales of the compound.

cond-mat.str-el↗

arXiv2024

Pitfalls of Exchange-Correlation Functionals in Descriptions of Magnetism: Cautionary Tale of the FeRh Alloy

The magnetic ground state of FeRh is highly sensitive towards the lattice constant. This, in addition to partially filled d-shells of Fe and Rh, posed a significant challenge for Density Functional Theory (DFT) calculations in the past. Here, we have investigated the performance of various exchange-correlation (XC) functionals within the DFT formalism for this challenging binary alloy. We have employed Local Spin Density Approximation (LSDA), various Generalized Gradient Approximations (GGAs), and newly developed Strongly Constrained and Appropriately Normed (SCAN) meta-GGA functional. Our results show the limitations of any single functional in capturing the intricate interplay of structural, electronic, and magnetic properties in FeRh. While SCAN can accurately describe some magnetic features and phonon dispersion, it significantly overestimates the Fe-Fe magnetic interactions, leading to an unreasonable magnetic ordering temperature. Conversely, the Perdew-Burke-Ernzerhof (PBE) GGA exhibits the opposite behavior. These findings highlight the challenges in simulating materials with partially filled $d$-shells using DFT, underscoring the crucial need for developing a versatile XC functional that can effectively account for the multifaceted nature of such systems.

cond-mat.mtrl-sci↗

arXiv2023

Computational exploration of a viable route to Kitaev-quantum spin liquid phase in OsCl$_3$

In this computational study, we explore a viable route to access the Kitaev-Quantum Spin Liquid (QSL) state in recently synthesized monolayer of a so-called spin-orbit assisted Mott insulator OsCl$_3$. In addition to other magnetic ground states in different regions, the small $J_\text{H}$/$U$ region of our Hubbard $U$--Hund's $J_\text{H}$ quantum phase diagram, obtained by combining second-order perturbation and pseudo-Fermion renormalization group calculations, hosts Kitaev-QSL phase. Only Kitaev interaction of a smaller magnitude is obtained in this region. Negligibly small farther neighbor interactions appear as a distinct feature of monolayer \os, suggesting this material to be a better candidate for the exploration of possible Kitaev-QSL state than earlier proposed materials. Insights from our study might be useful to probe magnetic phase transitions by purportedly manipulating $U$ and $J_\text{H}$ with epitaxial strain in advanced crystal growth techniques.

cond-mat.str-el↗

arXiv2022

Emergence of bond-dependent highly anisotropic magnetic interactions in Sr$_4$RhO$_6$: a theoretical study

The quantum spin liquid states as a natural ground state of the Kitaev model has led to a quest for new materials candidates hosting Kitaev physics. Yet, there are very few material candidates in this category. Using a combination of $ab$ $initio$ and model Hamiltonian methods, we propose that Ruddlesden-Popper compound Sr$_4$RhO$_6$ belongs to this category. With a tight-binding model and exact diagonalization approach, we show that despite substantial trigonal-like distortion, the electronic and magnetic properties of Sr$_4$RhO$_6$ can be well described in terms of pseudo-spin = 1/2 states. Magnetic interactions among pseudo-spins, estimated using the second-order perturbation method are highly bond-dependent anisotropic in nature with two particularly noticeable features, antiferromagnetic Kitaev and Dzyaloshinskii-Moriya interactions. The gaped spin-wave spectra of Sr$_4$RhO$_6$ obtained with linear spin-wave theory is consistent with the underlying magnetic frustration. Additional analysis of the role of individual or a particular combination of magnetic interactions reveals that the spin-wave spectra of Sr$_4$RhO$_6$ is a combined effect of the highly anisotropic interactions and a relatively simpler minimal model may not be plausible in the current case. The crucial insights about coupling between the local structural features and magnetic properties of Sr$_4$RhO$_6$ obtained in this study may be helpful for future studies belonging to this class.

cond-mat.str-el↗

arXiv2022

A computational method to estimate spin-orbital interaction strength in solid state systems

Spin-orbit coupling (SOC) drives interesting and non-trivial phenomena in solid state physics, ranging from topological to magnetic to transport properties. Thorough study of such phenomena often require effective models where SOC term is explicitly included. However, estimation of SOC strength for such models mostly depend on the spectroscopy experiments which can only provide a rough estimate. In this work, we provide a simple yet effective computational approach to estimate the on-site SOC strength using a combination of the $ab$ $initio$ and tight-binding calculations. We demonstrate the wider applicability and high sensitivity of our method considering materials with varying SOC strengths and the number of SOC active ions. The estimated SOC strengths agree well with the proposed values in literature lending support to our methodology. This simplistic approach can readily be applied to a wide range of materials.

cond-mat.mtrl-sci↗

arXiv2022

Spin interaction and magnetism in cobaltate Kitaev candidate materials: an $ab$ $initio$ and model Hamiltonian approach

In the quest for materials hosting Kitaev spin liquids, much of the efforts have been focused on the fourth- and fifth-row transition metal compounds, which are spin-orbit coupling assisted Mott insulators. Here, we study the structural and magnetic properties of 3$d$ transition metal oxides, Na$_2$Co$_2$TeO$_6$ and Na$_3$Co$_2$SbO$_6$. The partial occupancy of sodium in former compound is addressed using a cluster expansion, and a honeycomb lattice of sodiums is found to be energetically favored. Starting from the \textit{ab initio} band structures, a many-body second order perturbation theory leads to a pseudospin-$\frac{1}{2}$ Hamiltonian with estimated magnetic interactions. We show that the experimentally observed zigzag magnetic state is stabilized only when the first neighbor Kitaev coupling dominates over the Heisenberg term, both of which are highly suppressed due to presence of $e_g$ orbitals. A third neighbor Heisenberg interaction is found dominant in both these compounds. We also present a phase diagram for Na$_2$Co$_2$TeO$_6$ by varying the electron-electron and spin-orbit interactions. The computed spin excitation spectra are found to capture essential features of recent experimental magnon spectrum, lending support to our results.

cond-mat.str-el↗

arXiv2022

Charge doping to flat AgF2 monolayers in a chemical capacitor setup

Flat monolayers of silver II fluoride, which could be obtained by epitaxial deposition on an appropriate substrate, have been recently predicted to exhibit very strong antiferromagnetic superexchange and to have large potential for ambient pressure superconductivity if doped to an optimal level. It was shown that AgF2 could become a magnetic glue based superconductor with a critical superconducting temperature approaching 200 K at optimum doping. In the current work we calculate the optimum doping to correspond to 14% of holes per formula unit, i.e. quite similar to that for oxocuprates II. Furthermore, using DFT calculations we show that flat AgF2 single layers can indeed be doped to a controlled extent using a recently proposed chemical capacitor setup. Hole doping associated with formation of Ag III proves to be difficult to achieve in the setup explored in this work as it falls at verge of charge stability of fluoride anions and does not affect the d x2 minus y2 manifold . However, in the case of electron doping, manipulation of different factors, such as number of dopant layers and the thickness of the separator, permits fine tuning of the doping level - and concomitantly TC - all the way from underdoped to overdoped regime (in a similar manner as chemical doping for the Nd2CuO4 analogue).

cond-mat.supr-con↗

arXiv2021

Understanding the ferromagnetic insulating state in Cr doped VO$_2$

Experimentally Cr doping in the rutile phase of VO$_2$ is found to stabilize a charge ordered ferromagnetic insulating state in the doping range of 10\% to 20\%. In this work, we investigated its origin at 12.5\% Cr doping using a combination of ab-initio electronic structure calculations as well as microscopic modeling. Our calculations are found to reproduce the ferromagnetic insulating state as well as a charge ordering at the V and Cr sites. The mapping of the ab-initio band structure onto a tight-binding Hamiltonian allows one to calculate the energy gain from different exchange pathways. This gain is quantified in this work for the first time and the role of charge ordering in stabilizing a ferromagnetic insulating state is understood.

cond-mat.mtrl-sci↗

arXiv2020

Dynamical correlation enhanced orbital magnetization in VI$_{3}$

The effect of electronic correlations on the orbital magnetization in real materials has not been explored beyond a static mean-field level. Based on the dynamical mean-field theory, the effect of electronic correlations on the orbital magnetization in layered ferromagnet VI$_3$ has been studied. A comparison drawn with the results obtained from density functional theory calculations robustly establishes the crucial role of dynamical correlations in this case. In contrast to the density functional theory that leads to negligible orbital magnetization in VI$_3$, in dynamical mean-field approach the orbital magnetization is greatly enhanced. Further analysis show that this enhancement is mainly due to the enhanced local circulations of electrons, which can be attributed to a better description of the localization behavior of correlated electrons in VI$_3$. The conclusion drawn in our study could be applicable to a wide range of layered materials in this class.

cond-mat.str-el↗

arXiv2017

The driving force for charge ordering in rare earth nickelates

We show that charge ordering (more precisely, two-sublattice bond disproportionation) in the rare earth nickelate perovskites is intimately related to a negative charge transfer energy. By adding an additional potential on the Ni d states we are able to vary the charge tranfer energy and compute relaxed structures within an ab-initio framework. We show that the difference in Ni-O bond lengths and the value of the ordered state magnetic moment correlate with the charge transfer energy and that the transition to the bond-disproportionated state occurs when the effective charge transfer energy becomes negative.

cond-mat.str-el↗

arXiv2016

Driving force for martensitic transformation in Ni$_{2}$Mn$_{1+x}$Sn$_{1-x}$

The martensitic transformation in Ni$_{2}$Mn$_{1+x}$Sn$_{1-x}$ alloys has been investigated within ab-initio density functional theory. The experimental trend of a martensitic transition happening beyond $x$ = 0.36 is captured within these calculations. The microscopic considerations leading to this are traced to increased Ni-Mn hybridization which results from the Ni atom experiencing a resultant force along a lattice parameter and moving towards the Mn atoms above a critical concentration. The presence of the lone pair electrons on Sn forces the movement of Ni atoms away from Sn. While band Jahn Teller effects have been associated with this transition, we show quantitatively that atleast in this class of compounds they have a minor role.

cond-mat.mtrl-sci↗