Showing 1–50 of 102 results for author: Jain, M

Comparative Assessment of Thermal Transport Theories: Dual-Channel Mechanism Dictates Heat Transport in Ultralow-$κ$ Materials

Authors: Soham Mandal, Ashutosh Srivastava, Tanmoy Das, Manish Jain, Abhishek Kumar Singh, Prabal K. Maiti

Abstract: Anomalous heat transport in strongly anharmonic crystalline solids poses both a fundamental challenge to the theoretical understanding and an opportunity for thermoelectric and thermal barrier coating applications. Although Green-Kubo theory reproduces experimental thermal conductivity ($κ$) at high temperatures, it lacks microscopic insight and neglects the Bose-Einstein statistics of lattice vib… ▽ More Anomalous heat transport in strongly anharmonic crystalline solids poses both a fundamental challenge to the theoretical understanding and an opportunity for thermoelectric and thermal barrier coating applications. Although Green-Kubo theory reproduces experimental thermal conductivity ($κ$) at high temperatures, it lacks microscopic insight and neglects the Bose-Einstein statistics of lattice vibrations. On the other hand, the conventional Boltzmann transport equation (BTE) framework, based on a phonon-gas picture, fails due to strong anharmonicity-induced overdamped phonons. Herein, the thermal transport properties in TlAgSe, a metal chalcogenide, and Cs$_2$PbI$_2$C$_2$, an all-inorganic layered Ruddlesden-Popper perovskite, are investigated by explicitly accounting for temperature-dependent lattice dynamics through machine learning interatomic potentials and employing the Wigner transport equation (WTE) framework. Crucially, heat conduction is governed not only by higher-order phonon scattering-dominated populations' transport channel described within the BTE, but also by a coherences' channel in the WTE framework arising from wave-like interbranch coherence between eigenstates. Incorporating four-phonon scattering, WTE predicts average room-temperature $κ$ values of 0.31 Wm$^{-1}$K$^{-1}$ (TlAgSe) and 0.38 Wm$^{-1}$K$^{-1}$ (Cs$_2$PbI$_2$C$_2$), in excellent agreement with experiments. Phonon scattering-rate analysis reveals strong coherences' contributions and prevalent overdamped phonon modes, demonstrating the breakdown of the conventional BTE framework based on the phonon quasiparticle picture with only first-order anharmonic perturbation. This computational approach provides a unified description of heat transport in ultralow-$κ$ materials, offering a basis for the rational design of phononic and thermoelectric devices. △ Less

Submitted 21 August, 2026; originally announced August 2026.

Barium Hexaferrite Thin Films as a Scalable Magnetic-Insulator Platform for Proximity-Engineered Spintronics

Authors: Shyam Sundar Poriah, Sanjana D. S., Agrim Sharma, Sreelakshmi M. Nair, Pankaj Bhardwaj, Laxmipriya Nanda, Aryaman Das, Jagadish Rajendran, R. S. Patel, Manish Jain, Dhavala Suri

Abstract: Rare-earth iron garnets, such as yttrium iron garnet (YIG) and thulium iron garnet (TmIG), are the benchmark magnetic insulators for spintronic and magnonic devices, but achieving usable perpendicular magnetic anisotropy (PMA) in these materials typically relies on substrate strain- engineering, requiring careful lattice-matching and specific growth conditions that constrain ma- terial accessibili… ▽ More Rare-earth iron garnets, such as yttrium iron garnet (YIG) and thulium iron garnet (TmIG), are the benchmark magnetic insulators for spintronic and magnonic devices, but achieving usable perpendicular magnetic anisotropy (PMA) in these materials typically relies on substrate strain- engineering, requiring careful lattice-matching and specific growth conditions that constrain ma- terial accessibility. Here we establish sputter grown barium hexaferrite (BaFe12O19, BaM) as a magnetic-insulator alternative with strong intrinsic perpendicular anisotropy, requiring no strain engineering. X-ray diffraction, transmission electron microscopy and Raman spectroscopy confirm stoichiometric films with atomically smooth surfaces, while first-principles calculations corroborate a robust ferrimagnetic ground state. The films exhibit square out-of-plane hysteresis with a coercive field of nearly 0.1 T. Unlike rare-earth garnets, the perpendicular anisotropy in BaM is intrinsic to its magnetoplumbite crystal structure, arising independent of highly ordered strain. Interfaced with Pt and with exfoliated BiSbTeSe2 (BSTS), BaM induces proximity induced anomalous Hall trans- port, confirming efficient interfacial exchange coupling, while the BSTS/BaM heterostructure shows an additional Hall contribution suggestive of non-collinear interfacial spin textures. These results position BaM thin films as a scalable magnetic-insulator platform for spintronic and topological heterostructure devices beyond the constraints of garnet chemistry. △ Less

Submitted 15 August, 2026; originally announced August 2026.

Moiré Phonons and Emergent Exciton-Phonon Coupling in a Moiré Heterobilayer

Authors: Can B. Uzundal, Woochang Kim, Zhiyuan Cui, Yuxuan Wei, Zheyu Lu, Qixin Feng, Francis L. Hong, Indrajit Maity, Takashi Taniguchi, Kenji Watanabe, Manish Jain, Mit H. Naik, Yoseob Yoon, Michael F. Crommie, Steven G. Louie, Feng Wang

Abstract: Moiré superlattices have emerged as a new platform for engineering electronic and optical properties in van der Waals heterostructures, enabling control over correlated and excitonic phenomena. Yet the impact of moiré superlattices on exciton-phonon coupling remains largely unexplored. Here we demonstrate emergent, layer-selective coupling between moiré phonons and moiré excitons in angle-aligned… ▽ More Moiré superlattices have emerged as a new platform for engineering electronic and optical properties in van der Waals heterostructures, enabling control over correlated and excitonic phenomena. Yet the impact of moiré superlattices on exciton-phonon coupling remains largely unexplored. Here we demonstrate emergent, layer-selective coupling between moiré phonons and moiré excitons in angle-aligned WS2/WSe2 heterobilayers. Using a broadband terahertz phonon transducer, we coherently launch moiré phonons that resonantly perturb the excitonic states. We show that the exciton-phonon coupling is intrinsically modified by the moiré superlattice in a layer-selective manner. A driven oscillator model captures the dynamics, revealing three moiré phonon resonances with distinct coupling to the moiré excitons. First principles calculations show that many moiré phonon modes can arise with distinct strongly hybridized in-plane and out-of-plane vibrations in the moiré unit cells. The calculations further identify the three experimentally observed moiré phonons and their emergent characteristic coupling to the moiré excitons. △ Less

Submitted 26 June, 2026; originally announced June 2026.

Layer-Polarization-Driven Metal-Insulator Transition in multi-band Graphene Moire' Superlattices

Authors: Harsimran Kaur Mann, Simrandeep Kaur, Harsimran Singh, Yashashwani Garg, Amogh Waghmare, Mohit Kumar Jat, Kenji Watanabe, Takashi Taniguchi, Manish Jain, Aveek Bid

Abstract: Graphene/hBN moiré superlattices provide a highly tunable platform for exploring emergent quantum phases in low-dimensional systems. Here, we investigate the moiré superlattice formed between hBN and ABA-stacked trilayer graphene (TLG), an inherently multi-band system. We demonstrate that the moiré potential is not merely a perturbation but a tool to hybridize the distinct massless and massive ele… ▽ More Graphene/hBN moiré superlattices provide a highly tunable platform for exploring emergent quantum phases in low-dimensional systems. Here, we investigate the moiré superlattice formed between hBN and ABA-stacked trilayer graphene (TLG), an inherently multi-band system. We demonstrate that the moiré potential is not merely a perturbation but a tool to hybridize the distinct massless and massive electronic sectors of TLG. By applying a perpendicular displacement field to tune layer polarization, we drive a fundamental reconstruction of the electronic band structure. Specifically, increasing the displacement field evolves the system from a multi-band regime to an effectively single-band regime at low energies, accompanied by a metal--insulator transition at the hole-doped secondary Dirac point. This transition originates from a redistribution of carriers across graphene layers that selectively enhances their coupling to the extrinsic moiré potential. Quantum capacitance measurements provide direct evidence for the suppression of the density of states at the hole-side secondary Dirac point, consistent with gap opening and the emergence of a displacement-field-tuned band gap. Theoretical calculations reproduce these observations and identify layer-selective coupling to the moiré potential as the underlying mechanism. These results demonstrate electrical control of an emergent insulating phase in a low-dimensional moiré system, and highlight that layer polarization and layer-selective coupling in multi-band moiré heterostructures provide a powerful route for engineering topological and correlated phases through band structure reconstruction and electron interactions. △ Less

Submitted 5 June, 2026; originally announced June 2026.

Electronic Coherence Evolution at the Nearly Commensurate Incommensurate CDW Boundary of 1T-TaS2

Authors: Turgut Yilmaz, Yi Sheng Ng, Menka Jain, Xiao Tong, Thipusa Wongpinij, Pat Photongkam, Anil Rajapitamahuni, Asish K. Kundu, Jin-Cheng Zheng, Elio Vescovo

Abstract: Transition metal dichalcogenides host a variety of charge density wave phases that couple lattice, charge, and correlation effects. In 1T-TaS2, the commensurate and nearly commensurate states are well characterized, yet the transition near 350 K into the incommensurate phase has lacked direct momentum resolved insight. Here we use temperature dependent angle resolved photoemission spectroscopy to… ▽ More Transition metal dichalcogenides host a variety of charge density wave phases that couple lattice, charge, and correlation effects. In 1T-TaS2, the commensurate and nearly commensurate states are well characterized, yet the transition near 350 K into the incommensurate phase has lacked direct momentum resolved insight. Here we use temperature dependent angle resolved photoemission spectroscopy to track the electronic structure across this transition. We observe a suppression of quasiparticle spectral weight at the Brillouin zone center, coincident with the transport anomaly, but without clear evidence of a full band gap opening. The transition appears to involve momentum dependent redistribution of spectral weight, consistent with a loss of coherence that reshapes the Fermi surface while leaving conduction dispersions largely intact. These results suggest that the nearly commensurate incommensurate transition may not align with a conventional metal insulator transition picture, but rather as an electronic reconstruction driven by loss of coherence. Our work provides new microscopic insight into the resistivity anomaly near room temperature and may guide design principles for collective electronic switching in Transition metal dichalcogenides. △ Less

Submitted 11 March, 2026; originally announced March 2026.

Signatures of moiré intralayer biexcitons and exciton-phason coupling in WSe2/WS2 heterostructures

Authors: Ranju Dalal, Harsimran Singh, Rwik Dutta, Hariharan Swaminathan, Kenji Watanabe, Takashi Taniguchi, Mit H Naik, Manish Jain, Akshay Singh

Abstract: Interactions among electronic and lattice degrees-of-freedom are foundational to various phases in condensed-matter physics, yet the dynamic interplay between excitonic and phononic quasiparticles represents an equivalent, underexplored frontier. Moiré superlattices provide an ideal platform for realizing these interactions by offering localized intralayer excitons (IALX) and ultralow-energy colle… ▽ More Interactions among electronic and lattice degrees-of-freedom are foundational to various phases in condensed-matter physics, yet the dynamic interplay between excitonic and phononic quasiparticles represents an equivalent, underexplored frontier. Moiré superlattices provide an ideal platform for realizing these interactions by offering localized intralayer excitons (IALX) and ultralow-energy collective lattice modes, such as phasons. Here, by optically suppressing ultrafast charge-transfer (CT) to interlayer excitons in WSe2/WS2 heterostructures, we uncover dynamics of moiré IALX revealing long lifetimes (τ > 1000 ps) arising from localized Wannier and in-plane CT nature. We then observe moiré intralayer intervalley biexcitons with binding energy ~ 16 meV, with long lifetimes due to moiré confinement. Furthermore, we find time-domain signatures of strong coupling between moiré-IALX and ~ 10 micro-eV phasons, evidenced as twist-angle-dependent GHz oscillations in IALX dynamics. Our findings establish moiré superlattices as interacting hybrid quantum systems and for engineering non-equilibrium phenomena, as well as for GHz-scale optoelectronics. △ Less

Submitted 6 January, 2026; originally announced January 2026.

Comments: Main text and Supplementary figures included, having total 40 pages. Main text contains 4 figures

Lanthanide Ion Electronic Structure Controls Magnetic Excitations in Topological Quantum Ferrimagnets $LnMn_{6}Sn_{6}$ (Ln = Tb, Dy, Ho)

Authors: Kelsey A. Collins, Jacob Pfund, Michael R. Page, Menka Jain, Michael A. Susner, Michael J. Newburger

Abstract: The $LnMn_{6}Sn_{6}$ family of topological magnets is a promising platform for next-generation spintronic and magnonic technologies. However, the influence of the lanthanide ion ($Ln^{3+}$) on the excited-state spin dynamics, or magnons, remains a critical knowledge gap. Here, we present the first comparative study of the magnetic dynamics in $LnMn_{6}Sn_{6}$ materials (Ln = Tb, Dy, Ho) using Bril… ▽ More The $LnMn_{6}Sn_{6}$ family of topological magnets is a promising platform for next-generation spintronic and magnonic technologies. However, the influence of the lanthanide ion ($Ln^{3+}$) on the excited-state spin dynamics, or magnons, remains a critical knowledge gap. Here, we present the first comparative study of the magnetic dynamics in $LnMn_{6}Sn_{6}$ materials (Ln = Tb, Dy, Ho) using Brillouin light scattering. Our findings reveal a direct correlation between the lanthanide ion's intrinsic properties and the magnon behavior. We demonstrate that the magnon frequency in the absence of an applied magnetic field is primarily dictated by the strength of the lanthanide exchange coupling, as modeled by its relationship with the de Gennes factor. The response of the magnon to an applied field is influenced by material's gyromagnetic ratio and the overall anisotropy of the material, which are dictated by total angular momentum and the anisotropy of the lanthanide sublattice, respectively. These results establish that simple lanthanide substitution provides a powerful and predictable method for tuning magnon properties, enabling the rational design of materials for advanced technological applications. △ Less

Submitted 6 July, 2026; v1 submitted 19 December, 2025; originally announced December 2025.

Comments: 12 pages main text, 41 total including supplementary info, 4 main figures, 40 supplementary figures

Report number: AFRL-2025-5776

Selective trapping of bacteria in porous media by cell length

Authors: David Gao, Zeyuan Wang, Mihika Jain, Arnold J. T. M. Mathijssen, Ran Tao

Abstract: Bacteria commonly inhabit porous environments such as host tissues, soil, and marine sediments, where complex geometries constrain and redirect their motion. Although bacterial motility has been studied in porous media, the roles of cell length and pore shape in navigating these environments remain poorly understood. Here, we investigate how cell morphology and pore architecture jointly determine… ▽ More Bacteria commonly inhabit porous environments such as host tissues, soil, and marine sediments, where complex geometries constrain and redirect their motion. Although bacterial motility has been studied in porous media, the roles of cell length and pore shape in navigating these environments remain poorly understood. Here, we investigate how cell morphology and pore architecture jointly determine bacterial spreading behavior. Using genetically engineered E. coli with tunable cell length, we performed single-cell tracking in microfluidic devices that mimic ordered and disordered porous structures. We find that elongated bacteria traverse ordered pore networks more effectively than short cells, exhibiting straighter paths, greater directional persistence, and enhanced exploration efficiency. In contrast, in disordered porous media, elongated bacteria become trapped in dead-end regions for extended periods, resulting in markedly reduced navigational efficiency. Together, these results reveal how cell shape and environmental geometry interact to govern bacterial transport. Moreover, we suggest a new mechanism for separating antimicrobial-resistant (AMR) bacteria from elongated susceptible cells in designer porous media. △ Less

Submitted 18 December, 2025; originally announced December 2025.

Comments: 13 pages, 5 figures

Journal ref: Integrative and Comparative Biology, Volume 66, 2026, icag018

Quantum Emission in Monolayer WSe2 Transferred onto InP Nanowires

Authors: Palwinder Singh, Jasleen Kaur Jagde, Megha Jain, Edith Yeung, David B. Northeast, Simona Moisa, Seid J. Mohammed, Jean Lapointe, Una Rajnis, Annika Kienast, Philip J. Poole, Dan Dalacu, Kimberley C. Hall

Abstract: Localized quantum emitters in transition-metal dichalcogenides (TMDs) have recently emerged as solid-state candidates for on-demand sources of single photons. Due to the role of strain in the site-selective creation of TMD emitters, their hybrid integration into photonic structures such as cavities and waveguides is possible using pick-and-place methods. Here we investigate quantum emission from a… ▽ More Localized quantum emitters in transition-metal dichalcogenides (TMDs) have recently emerged as solid-state candidates for on-demand sources of single photons. Due to the role of strain in the site-selective creation of TMD emitters, their hybrid integration into photonic structures such as cavities and waveguides is possible using pick-and-place methods. Here we investigate quantum emission from a hybrid structure consisting of a monolayer of WSe2 interfaced with horizontally aligned InP nanowires (NWs). Our experiments reveal multiple narrow and bright emission peaks in the 715-785 nm spectral range and g(2)(0) as low as 0.049, indicating strong antibunching and good single photon purity. The faceted nature of III-V NWs provides unique opportunities for strain engineering, including the potential for placement of emitters on the top surface for optimal coupling. Our findings pave the way for realizing hybrid quantum light sources for integrated quantum photonics that could combine III-V quantum dots with TMD emitters into a single platform. △ Less

Submitted 5 October, 2025; originally announced October 2025.

Emergent Rashba spin-orbit coupling in bulk gold with buried network of nanoscale interfaces

Authors: Shreya Kumbhakar, Banashree Debnath, Tuhin Kumar Maji, Binita Tongbram, Shinjan Mandal, T. Phanindra Sai, T. V. Ramakrishnan, Manish Jain, H. R. Krishnamurthy, Anshu Pandey, Arindam Ghosh

Abstract: The Rashba effect, which plays a crucial role in fundamental materials physics and potential spintronics applications, has been engineered in diverse systems, including semiconductor quantum wells, oxide heterostructures, metallic surfaces, topological insulators, ferroelectrics, etc. However, generating it in systems that preserve bulk inversion symmetry (BIS), for example, in bulk metals, has no… ▽ More The Rashba effect, which plays a crucial role in fundamental materials physics and potential spintronics applications, has been engineered in diverse systems, including semiconductor quantum wells, oxide heterostructures, metallic surfaces, topological insulators, ferroelectrics, etc. However, generating it in systems that preserve bulk inversion symmetry (BIS), for example, in bulk metals, has not been possible so far. We demonstrate a unique strategy to introduce and tune Rashba spin-orbit interaction (SOI) to unprecedented magnitudes in inversion-symmetric solids, by incorporating ultra-small silver nanoparticles in bulk gold. The near-identical lattice constants of Ag and Au allowed dense packing of the Ag/Au hetero-interfaces without compromising the global BIS. By varying the density of embedded nanoparticles, we generate Rashba SOI in a bulk metal with a coupling strength of ~15 meV.Angstrom, higher than any known system preserving BIS globally, and up to ~20 times increase in the spin-orbit scattering rate. We argue that the combined effect of charge-transfer at the interfaces and polaronic localization enhances the SOI. △ Less

Submitted 3 September, 2025; originally announced September 2025.

Comments: 10 pages,5 figures

Symmetries in zero and finite center-of-mass momenta excitons

Authors: Robin Bajaj, Namana Venkatareddy, H. R. Krishnamurthy, Manish Jain

Abstract: We present a symmetry-based framework for the analysis of excitonic states, incorporating both time-reversal and space-group symmetries. We demonstrate the use of time-reversal and space-group symmetries to obtain exciton eigenstates at symmetry-related center-of-mass momenta in the entire Brillouin zone from eigenstates calculated for center-of-mass momenta in the irreducible Brillouin zone. Furt… ▽ More We present a symmetry-based framework for the analysis of excitonic states, incorporating both time-reversal and space-group symmetries. We demonstrate the use of time-reversal and space-group symmetries to obtain exciton eigenstates at symmetry-related center-of-mass momenta in the entire Brillouin zone from eigenstates calculated for center-of-mass momenta in the irreducible Brillouin zone. Furthermore, by explicitly calculating the irreducible representations of the little groups, we classify excitons according to their symmetry properties across the Brillouin zone. Using projection operators, we construct symmetry-adapted linear combinations of electron-hole product states, which block diagonalize the Bethe-Salpeter equation (BSE) Hamiltonian at both zero and finite exciton center-of-mass momenta. This enables a transparent organization of excitonic states and provides direct access to their degeneracies, selection rules, and symmetry-protected features. As a demonstration, we apply this formalism to monolayer MoS$_2$, where the classification of excitonic irreducible representations and the block structure of the BSE Hamiltonian show excellent agreement with compatibility relations derived from group theory. Beyond this material-specific example, the framework offers a general and conceptually rigorous approach to the symmetry classification of excitons, enabling significant reductions in computational cost for optical spectra, exciton-phonon interactions, and excitonic band structure calculations across a wide range of materials. △ Less

Submitted 11 December, 2025; v1 submitted 22 August, 2025; originally announced August 2025.

Comments: 18 pages, 2 figures, 2 Tables

Journal ref: Phys. Rev. B 112, 245127 (2025)

Double excitations in molecules

Authors: Namana Venkatareddy, Victor Ghosh, H. R. Krishnamurthy, Manish Jain

Abstract: Double excitations in organic molecules have garnered significant interest as a result of their importance in singlet fission and photophysics. These excitations play a crucial role in understanding the photoexcitation processes in polyenes. To describe photoexcited states with both single and double excitation character, we use a first-principles many-body theory that combines the GW / Bethe-Salp… ▽ More Double excitations in organic molecules have garnered significant interest as a result of their importance in singlet fission and photophysics. These excitations play a crucial role in understanding the photoexcitation processes in polyenes. To describe photoexcited states with both single and double excitation character, we use a first-principles many-body theory that combines the GW / Bethe-Salpeter equation and the configuration interaction (CI) methods. Specifically, we develop and employ two CI-based methods: screened configuration interaction singles and doubles (scrCISD) and screened configuration interaction singles with perturbative doubles (scrCIS(D)), applied to an effective many-body Hamiltonian that incorporates screening. We apply these methods to Thiel's set of molecules, which exhibit excited states predominantly characterized by single excitations with a partial double excitation character. Our results indicate that the scrCISD method systematically underestimates the excitation energies compared to the best theoretical estimates, while the scrCIS(D) method shows good agreement with these estimates. Furthermore, we used the scrCISD method to calculate the binding energies of the dominantly doubly excited correlated triplet pair states, $\mathrm{TT^1}$, in pentacene dimers, finding that the $\mathrm{TT^1}$ binding energies agree well with empirical calculations. △ Less

Submitted 1 September, 2025; v1 submitted 22 August, 2025; originally announced August 2025.

Enhanced Phonon-Assisted Tunneling in Metal -- Twisted Bilayer Graphene Junctions

Authors: Radhika Soni, Suvronil Datta, Robin Bajaj, Saisab Bhowmik, Shinjan Mandal, Baladitya Suri, Kenji Watanabe, Takashi Taniguchi, Manish Jain, U. Chandni

Abstract: We report planar tunneling spectroscopy measurements on metal-WSe$_2$-twisted bilayer graphene heterostructures across a broad range of gate and bias voltages. The observed experimental features are attributed to phonon-assisted tunneling and the significantly high density of states within the moiré bands. A notable finding is the enhanced phonon-assisted tunneling in twisted bilayer graphene comp… ▽ More We report planar tunneling spectroscopy measurements on metal-WSe$_2$-twisted bilayer graphene heterostructures across a broad range of gate and bias voltages. The observed experimental features are attributed to phonon-assisted tunneling and the significantly high density of states within the moiré bands. A notable finding is the enhanced phonon-assisted tunneling in twisted bilayer graphene compared to Bernal bilayer graphene, which arises from a more relaxed in-plane momentum matching criterion. Theoretical calculations of phonon dispersions enable us to identify low-energy phonon modes in both Bernal and twisted bilayers of graphene, thereby elucidating the underlying mechanism of tunneling. Our results establish planar tunneling as a versatile tool to further understand electron-phonon coupling in twisted van der Waals materials. △ Less

Submitted 17 July, 2025; originally announced July 2025.

Comments: 25+12 pages, 4+5 figures

Journal ref: ACS Nano 2025

Probing Phonon Modes in Reconstructed twisted Homo and Hetero Bilayer System

Authors: Sushil Kumar Sahu, Robin Bajaj, Syed Ummair Ali, Ajay Bhut, Roshan Jesus Mathew, Shinjan Mandal, Kenji Watanabe, Takashi Taniguchi, Manish Jain, Chandan Kumar

Abstract: Twist angle engineering in van der Waals homo and hetero-bilayers introduces profound modifications in their electronic, optical and mechanical properties due to lattice reconstruction. In these systems, the interlayer coupling and atomic rearrangement strongly depend on the twist angle, leading to the formation of periodic Moire superlattices. At small twist angles, significant lattice relaxation… ▽ More Twist angle engineering in van der Waals homo and hetero-bilayers introduces profound modifications in their electronic, optical and mechanical properties due to lattice reconstruction. In these systems, the interlayer coupling and atomic rearrangement strongly depend on the twist angle, leading to the formation of periodic Moire superlattices. At small twist angles, significant lattice relaxation results in the emergence of domain structures separated by one dimensional soliton networks, influencing electronic band structures and phonon modes. Here we systematically investigate the impact of lattice reconstruction on phonon renormalization in twisted bilayer graphene (TBLG,homo) and graphene-hBN Moire superlattices(hetero). Using Raman spectroscopy, we identify distinct phonon behaviours across different twist angle regimes. In TBLG, we observe the evolution of the G peak, including broadening, splitting, and the emergence of additional peaks in the small angle range 0.3 to 1 degree, attributed to Moire modified phonon interactions. At large twist angles, the peaks gradually merge back into a single feature, reflecting the reduced impact of lattice reconstruction. Similarly, in hBN graphene Moire superlattices, we detect Moire induced Raman peaks above and below the G peak, while the central G peak remains largely invariant to twist angle variation. The theoretical calculations uncover Moire phonon modes originating from different stacking regions providing insights into phonon renormalization. Our results establish a direct link between twist angle, lattice reconstruction, Moire phonons, and interlayer coupling, offering a fundamental framework for understanding phonon engineering in twisted bilayer systems. These findings pave the way for controlling phononic, optoelectronic and heat flow properties in next generation van der Waals heterostructures. △ Less

Submitted 24 June, 2025; originally announced June 2025.

Accelerating point defect photo-emission calculations with machine learning interatomic potentials

Authors: Kartikeya Sharma, Antoine Loew, Haiyuan Wang, Fredrik A. Nilsson, Manjari Jain, Miguel A. L. Marques, Kristian S. Thygesen

Abstract: We introduce a computational framework leveraging universal machine learning interatomic potentials (MLIPs) to dramatically accelerate the calculation of photoluminescence (PL) spectra of atomic or molecular emitters with ab initio accuracy. By replacing the costly density functional theory (DFT) computation of phonon modes with much faster MLIP phonon mode calculations, our approach achieves spee… ▽ More We introduce a computational framework leveraging universal machine learning interatomic potentials (MLIPs) to dramatically accelerate the calculation of photoluminescence (PL) spectra of atomic or molecular emitters with ab initio accuracy. By replacing the costly density functional theory (DFT) computation of phonon modes with much faster MLIP phonon mode calculations, our approach achieves speed improvements exceeding an order of magnitude with minimal precision loss. We benchmark the approach using a dataset comprising ab initio emission spectra of 791 color centers spanning various types of crystal point defects in different charge and magnetic states. The method is also applied to a molecular emitter adsorbed on a hexagonal boron nitride surface. Across all the systems, we find excellent agreement for both the Huang-Rhys factor and the PL lineshapes. This application of universal MLIPs bridges the gap between computational efficiency and spectroscopic fidelity, opening pathways to high-throughput screening of defect-engineered materials. Our work not only demonstrates accelerated calculation of PL spectra with DFT accuracy, but also makes such calculations tractable for more complex materials. △ Less

Submitted 8 September, 2025; v1 submitted 2 May, 2025; originally announced May 2025.

Comments: 9 pages, 5 figures, 1 table

Exploring the role of four-phonon scattering in the lattice thermal transport of LaMoN$_3$

Authors: Manjari Jain, Sanchi Monga, Saswata Bhattacharya

Abstract: In this work, we systematically investigate the lattice thermal conductivity ($κ_L$) of LaMoN$_3$ in the $C$2/$c$ and $R$3$c$ phases using first-principles calculations combined with the Boltzmann transport equation. In the $C$2/$c$ phase, $κ_L$ exhibits strong anisotropy, with values of 0.75 W/mK, 1.89 W/mK, and 0.82 W/mK along the a, b, and c axes, respectively, at 300 K. In contrast, the $R$3… ▽ More In this work, we systematically investigate the lattice thermal conductivity ($κ_L$) of LaMoN$_3$ in the $C$2/$c$ and $R$3$c$ phases using first-principles calculations combined with the Boltzmann transport equation. In the $C$2/$c$ phase, $κ_L$ exhibits strong anisotropy, with values of 0.75 W/mK, 1.89 W/mK, and 0.82 W/mK along the a, b, and c axes, respectively, at 300 K. In contrast, the $R$3$c$ phase shows nearly isotropic thermal conductivity, with values of 6.28 W/mK, 7.05 W/mK, and 7.31 W/mK along the a, b, and c directions. In both phases, acoustic phonons dominate thermal transport. However, in the $C$2/$c$ phase, the absence of an acoustic-optical gap results in increased three-phonon scattering leading to smaller values of $κ_L$. Additionally, four-phonon scattering plays a dominant role in the C2/c phase, reducing $κ_L$ by approximately 96\%, whereas in the $R3c$ phase, it leads to a smaller but still significant reduction of ~50\%. These results highlight the critical role of four-phonon interactions in determining the thermal transport properties of LaMoN$_3$ and reveal the stark contrast in thermal conductivity between its two structural phases. △ Less

Submitted 1 April, 2025; originally announced April 2025.

Even-denominator fractional quantum Hall states in the zeroth Landau level of ABA trilayer graphene

Authors: Tanima Chanda, Simrandeep Kaur, Harsimran Singh, Kenji Watanabe, Takashi Taniguchi, Manish Jain, Udit Khanna, Ajit C. Balram, Aveek Bid

Abstract: Even-denominator fractional quantum Hall states (FQHSs) at half filling are of particular interest because they can host non-Abelian quasiparticles. Here we report the emergence of such states in the zeroth Landau level ($N=0$) of ABA trilayer graphene (TLG), challenging the conventional expectation that they are confined to the first excited Landau level. We observe robust incompressible states a… ▽ More Even-denominator fractional quantum Hall states (FQHSs) at half filling are of particular interest because they can host non-Abelian quasiparticles. Here we report the emergence of such states in the zeroth Landau level ($N=0$) of ABA trilayer graphene (TLG), challenging the conventional expectation that they are confined to the first excited Landau level. We observe robust incompressible states at $ν=7/2$, $9/2$, and $5/2$ with their associated Levin--Halperin daughter states: $ν=59/17$ and $46/13$ near $7/2$; $ν=58/13$ and $77/17$ near $9/2$; and $ν=43/17$ near $5/2$. These states appear exclusively within a finite displacement-field window coincident with crossings between symmetry-broken $N=0$ Landau levels carrying distinct isospin indices. The quantitative correspondence between the calculated crossing loci and the experimentally determined stability regions identifies Landau-level mixing as the microscopic origin. We attribute the stabilization of these even-denominator states to inversion-symmetry breaking in TLG, which enhances valley-resolved Landau-level hybridization and renormalizes short-range Coulomb interactions. Our results expand the landscape of even-denominator FQHSs to multilayer graphene and establish TLG as a tunable platform for realizing non-Abelian anyons. △ Less

Submitted 16 July, 2026; v1 submitted 10 February, 2025; originally announced February 2025.

Comments: 34 pages

Journal ref: Phys. Rev. Lett. 137, 036601 (2026)

Controlling particle-hole symmetry of fractional quantum hall states in trilayer graphene

Authors: Simrandeep Kaur, Harsimran Singh, Kenji Watanabe, Takashi Taniguchi, Unmesh Ghorai, Manish Jain, Rajdeep Sensarma, Aveek Bid

Abstract: We present a detailed experimental study of the particle-hole symmetry (PHS) of the fractional quantum Hall (FQH) states about half filling in a multiband system. Specifically, we focus on the lowest Landau level of the monolayer-like band of Bernal stacked trilayer graphene (TLG). In pristine TLG, the excitation energy gaps, Landé g-factor, effective mass, and disorder broadening of the odd-denom… ▽ More We present a detailed experimental study of the particle-hole symmetry (PHS) of the fractional quantum Hall (FQH) states about half filling in a multiband system. Specifically, we focus on the lowest Landau level of the monolayer-like band of Bernal stacked trilayer graphene (TLG). In pristine TLG, the excitation energy gaps, Landé g-factor, effective mass, and disorder broadening of the odd-denominator FQH states are identical to their hole-conjugate counterpart. This precise PH symmetry stems from the lattice mirror symmetry that precludes Landau-level mixing. Introducing a non-zero displacement field \(D\) disrupts this mirror symmetry, facilitating the hybridization between the monolayer-like and bilayer-like Landau levels. This inter-band coupling enhances the Landau level mixing factor $η$ and activates three-body interactions -- both of which explicitly break the PHS of FQHs. As a result, conventional FQHs are completely destabilized, offering a route to engineer symmetry breaking of FQHs in a controlled way. We establish that the PHS breaking in TLG is of extrinsic origin and is fundamentally distinct from the intrinsic, interaction-driven symmetry breaking observed in the lowest Landau levels of single-layer and bilayer graphene. △ Less

Submitted 13 May, 2025; v1 submitted 27 November, 2024; originally announced November 2024.

Comments: 32 pages

PARPHOM: PARallel PHOnon calculator for Moiré systems

Authors: Shinjan Mandal, Indrajit Maity, H R Krishnamurthy, Manish Jain

Abstract: The introduction of a twist between two layers of two-dimensional materials has opened up a new and exciting field of research known as twistronics. In these systems, the phonon dispersions show significant renormalization and enhanced electron-phonon interactions as a function of the twist angle. However, the large system size of the resulting moiré patterns in these systems makes phonon calculat… ▽ More The introduction of a twist between two layers of two-dimensional materials has opened up a new and exciting field of research known as twistronics. In these systems, the phonon dispersions show significant renormalization and enhanced electron-phonon interactions as a function of the twist angle. However, the large system size of the resulting moiré patterns in these systems makes phonon calculations computationally challenging. In this paper, we present PARPHOM, a powerful code package designed to address these challenges. PARPHOM enables the generation of force constants, computation of phononic band structures, and determination of density of states in twisted 2D material systems. Moreover, PARPHOM provides essential routines to investigate the finite temperature dynamics in these systems and analyze the chirality of the phonon bands. This paper serves as an introduction to PARPHOM, highlighting its capabilities and demonstrating its utility in unraveling the intricate phononic properties of twisted 2D materials. △ Less

Submitted 28 October, 2024; originally announced October 2024.

Comments: 10 pages, 7 figures

Possibilities for enhanced electron-phonon interactions and high-$T_c$ superconductivity in engineered bimetallic nano-structured superlattices

Authors: Shinjan Mandal, Shrihari Soundararajan, Manish Jain, H. R. Krishnamurthy

Abstract: We explore theoretically the properties of engineered bimetallic nano-structured superlattices where an array of nano-clusters of a simple (single band) metal are embedded periodically inside another simple metal with a different work function. The exploration is done using a simplified tight-binding model with Coulomb interactions included, as well as density functional theory. Taking arrays of "… ▽ More We explore theoretically the properties of engineered bimetallic nano-structured superlattices where an array of nano-clusters of a simple (single band) metal are embedded periodically inside another simple metal with a different work function. The exploration is done using a simplified tight-binding model with Coulomb interactions included, as well as density functional theory. Taking arrays of "Ag" clusters of fixed sizes and configurations (when unrelaxed) embedded periodically in an "Au" matrix as an example, we show that a significant enhancement of electron-phonon interactions ensues, implying possibilities for high-$T_c$ superconductivity. The enhancement stems from a strong coupling, via Coulomb interactions, between the dipolar charge distribution that forms at the Au-Ag interfaces and the breathing and other modes of vibration of the light Ag atoms caged inside the heavier Au matrix. The interface dipoles form because of the interplay between the mismatch of the local potential seen by the conduction electrons localised in Wannier orbitals at the Ag and Au sites (the Ag sites being slightly repulsive relative to the Au sites) and the (long-range) Coulomb repulsion between electrons occupying these Wannier orbitals. We also discuss the DC transport in such systems. △ Less

Submitted 28 August, 2024; originally announced August 2024.

Comments: 24 pages, 17 figures

Journal ref: Phys. Rev. B 111, 184507 (2025)

Emergent inhomogeneity and non-locality in a graphene field-effect transistor on a near-parallel moire superlattice of transition metal dichalcogenides

Authors: Shaili Sett, Rahul Debnath, Arup Singha, Shinjan Mandal, Jyothsna K, Monika Bhakar, Kenji Watanabe, Takashi Taniguchi, Varun Raghunathan, Goutam Sheet, Manish Jain, Arindam Ghosh

Abstract: At near-parallel orientation, twisted bilayer of transition metal dichalcogenides exhibit inter-layer charge transfer-driven out-of-plane ferroelectricity that may lead to unique electronic device architectures. Here we report detailed electrical transport in a dual-gated graphene field-effect transistor placed on 3R stacked twisted bilayer of WSe2 at a twist angle of 2.1 degree. We observe hyster… ▽ More At near-parallel orientation, twisted bilayer of transition metal dichalcogenides exhibit inter-layer charge transfer-driven out-of-plane ferroelectricity that may lead to unique electronic device architectures. Here we report detailed electrical transport in a dual-gated graphene field-effect transistor placed on 3R stacked twisted bilayer of WSe2 at a twist angle of 2.1 degree. We observe hysteretic transfer characteristics and an emergent charge inhomogeneity with multiple local Dirac points as the electric displacement field (D) is increased. Concomitantly, we also observe a strong non-local voltage signal at D = 0 V/nm that decreases rapidly with increasing D. A linear scaling of the non-local signal with longitudinal resistance suggests edge mode transport, which we attribute to the breaking of valley symmetry of the graphene channel due to the spatially fluctuating electric field from the moire domains of the underlying twisted WSe2. A quantitative analysis connecting the non-locality and channel inhomogeneity suggests emergence of finite-size domains in the graphene channel that modulate the charge and the valley currents simultaneously. This work underlines efficient control and impact of interfacial ferroelectricity that can trigger a new genre of devices for twistronic applications. △ Less

Submitted 28 May, 2024; originally announced May 2024.

Comments: 16 pages, 15 figures

Journal ref: Nano Lett. 2024, 24, 30, 9245-9252

Engineering ultra-strong electron-phonon coupling and nonclassical electron transport in crystalline gold with nanoscale interfaces

Authors: Shreya Kumbhakar, Tuhin Kumar Maji, Binita Tongbram, Shinjan Mandal, Shri Hari Soundararaj, Banashree Debnath, T. Phanindra Sai, Manish Jain, H. R. Krishnamurthy, Anshu Pandey, Arindam Ghosh

Abstract: Electrical resistivity in good metals, particularly noble metals such as gold (Au), silver (Ag), or copper, increases linearly with temperature ($T$) for $T > Θ_{\mathrm{D}}$, where $Θ_{\mathrm{D}}$ is the Debye temperature. This is because the coupling ($λ$) between the electrons and the lattice vibrations, or phonons, in these metals is rather weak with $λ\sim 0.1-0.2$, and a perturbative analys… ▽ More Electrical resistivity in good metals, particularly noble metals such as gold (Au), silver (Ag), or copper, increases linearly with temperature ($T$) for $T > Θ_{\mathrm{D}}$, where $Θ_{\mathrm{D}}$ is the Debye temperature. This is because the coupling ($λ$) between the electrons and the lattice vibrations, or phonons, in these metals is rather weak with $λ\sim 0.1-0.2$, and a perturbative analysis suffices to explain the $T$-linear electron-phonon scattering rate. In this work, we outline a new nanostructuring strategy of crystalline Au where this foundational concept of metallic transport breaks down. We show that by embedding a distributed network of ultra-small Ag nanoparticles (AgNPs) of radius $\sim1-2$ nm inside a crystalline Au shell, an unprecedented enhancement in the electron-phonon interaction, with $λ$ as high as $\approx 20$, can be achieved. This is over hundred times that of bare Au or Ag, and ten times larger than any known metal. With increasing AgNP density, the electrical resistivity deviates from $T$-linearity, and approaches a saturation to the Mott-Ioffe-Regel scale $ρ_{\mathrm{MIR}}\sim h a /e^2$ for both disorder ($T\to 0$) and phonon ($T \gg Θ_{\mathrm{D}}$)-dependent components of resistivity (here, $a=0.3$~nm, is the lattice constant of Au). This giant electron-phonon interaction, which we suggest arises from the coulomb interaction-induced coupling of conduction electrons to the localized phonon modes at the buried Au-Ag hetero-interfaces, allows experimental access to a regime of nonclassical metallic transport that has never been probed before. △ Less

Submitted 23 May, 2024; originally announced May 2024.

Comments: 7+12 pages, total 4+8 figures

Theoretical Insights into Inorganic Antiperovskite Nitrides (X$_3$NA; X = Mg, Sr, Ca, Ba; A = Sb, As): An Emerging Class of Materials for Photovoltaics

Authors: Sanchi Monga, Manjari Jain, Claudia Draxl, Saswata Bhattacharya

Abstract: Antiperovskite nitrides are potential candidates for applications harvesting solar light. With a comprehensive state-of-the-art approach combining hybrid density-functional theory, many-body perturbation theory, the Wannier-Mott model, density-functional perturbation theory, and the Feynman polaron model, we explore excitonic and polaronic effects in X$_3$NA (X: Mg, Ca, Sr, Ba, A = Sb, As). For al… ▽ More Antiperovskite nitrides are potential candidates for applications harvesting solar light. With a comprehensive state-of-the-art approach combining hybrid density-functional theory, many-body perturbation theory, the Wannier-Mott model, density-functional perturbation theory, and the Feynman polaron model, we explore excitonic and polaronic effects in X$_3$NA (X: Mg, Ca, Sr, Ba, A = Sb, As). For all of them, we uncover a significant influence of the ionic dielectric screening on the static dielectric constant. Small exciton binding energies, weak electron-phonon coupling, and high charge-carrier mobilities facilitate enhanced charge transport in Mg$_3$NSb, Sr$_3$NSb, and Ba$_3$NSb. Our results highlight the potential of these nitrides as optimal candidates for efficient photovoltaic absorbers. △ Less

Submitted 25 April, 2024; originally announced April 2024.

Phonon Linewidths in Twisted Bilayer Graphene near Magic Angle

Authors: Shinjan Mandal, Indrajit Maity, H. R. Krishnamurthy, Manish Jain

Abstract: We present a computational study of the phonon linewidths in twisted bilayer graphene arising from electron-phonon interactions and anharmonic effects. The electronic structure is calculated using distance-dependent transfer integrals based on the atomistic Slater-Koster tight-binding formalism, including electron-electron interactions treated at the Hartree level, and the phonons are calculated u… ▽ More We present a computational study of the phonon linewidths in twisted bilayer graphene arising from electron-phonon interactions and anharmonic effects. The electronic structure is calculated using distance-dependent transfer integrals based on the atomistic Slater-Koster tight-binding formalism, including electron-electron interactions treated at the Hartree level, and the phonons are calculated using classical force fields. These ingredients are used to calculate the phonon linewidths arising from electron-phonon interactions. Furthermore, anharmonic effects on the linewidths are computed using the mode-projected velocity autocorrelation function obtained from classical molecular dynamics. We predict a moiré potential induced splitting of this mode, which arises due to contributions from high symmetry stacking regions. Our findings show that both electron-phonon and anharmonic effects have a significant impact on the linewidth of the Raman active G mode near the magic angle. △ Less

Submitted 2 July, 2024; v1 submitted 15 February, 2024; originally announced March 2024.

Comments: 6 pages, 4 figures (Supplementary 13 pages, 11 figures)

Engineering and Revealing Dirac Strings in Spinor Condensates

Authors: Gui-Sheng Xu, Mudit Jain, Xiang-Fa Zhou, Guang-Can Guo, Mustafa A. Amin, Han Pu, Zheng-Wei Zhou

Abstract: Artificial monopoles have been engineered in various systems, yet there has been no systematic study of the singular vector potentials associated with the monopole field. We show that the Dirac string, the line singularity of the vector potential, can be engineered, manipulated, and made manifest in a spinor atomic condensate. We elucidate the connection among spin, orbital degrees of freedom, and… ▽ More Artificial monopoles have been engineered in various systems, yet there has been no systematic study of the singular vector potentials associated with the monopole field. We show that the Dirac string, the line singularity of the vector potential, can be engineered, manipulated, and made manifest in a spinor atomic condensate. We elucidate the connection among spin, orbital degrees of freedom, and the artificial gauge, and show that there exists a mapping between the vortex filament and the Dirac string. We also devise a proposal where preparing initial spin states with relevant symmetries can result in different vortex patterns, revealing an underlying correspondence between the internal spin states and the spherical vortex structures. Such a mapping also leads to a new way of constructing spherical Landau levels, and monopole harmonics. Our observation provides insights into the behavior of quantum matter possessing internal symmetries in curved spaces. △ Less

Submitted 9 April, 2024; v1 submitted 22 February, 2024; originally announced February 2024.

Comments: 5 pages + 5 figures + 5 appendices. In comparison with the previous version, we have (1) added supplementary material as appendices, and (2) made some minor revisions in the main text. Also, a simulation video for creating two artificial Dirac Strings with a mixed spin-1 state, is available at https://youtu.be/PCQGMqd-DfQ

Electric field tunable superconductivity with competing orders in twisted bilayer graphene near magic-angle

Authors: Ranit Dutta, Ayan Ghosh, Shinjan Mandal, K. Watanabe, T. Taniguchi, H. R. Krishnamurthy, Sumilan Banerjee, Manish Jain, Anindya Das

Abstract: Superconductivity (SC) in twisted bilayer graphene (tBLG) has been explored by varying carrier concentrations, twist angles, and screening strength, with the aim of uncovering its origin and possible connections to strong electronic correlations in narrow bands and various resulting broken symmetries. However, the link between the tBLG band structure and the onset of SC and other orders largely re… ▽ More Superconductivity (SC) in twisted bilayer graphene (tBLG) has been explored by varying carrier concentrations, twist angles, and screening strength, with the aim of uncovering its origin and possible connections to strong electronic correlations in narrow bands and various resulting broken symmetries. However, the link between the tBLG band structure and the onset of SC and other orders largely remains unclear. In this study, we address this crucial gap by examining in-situ band structure tuning of a near magic-angle ($θ\approx0.95^\circ$) tBLG device with displacement field ($D$) and reveal remarkable competition between SC and other broken symmetries. At zero $D$, the device exhibits superconducting signatures without the resistance peak at half-filling, a characteristic signature with a strong electronic correlation. As $D$ increases, the SC is suppressed, accompanied by the appearance of a resistance peak at half-filling. Hall density measurements reveal that at zero $D$, SC arises around the van Hove singularity (vHs) from an isospin or spin-valley unpolarized band. At higher $D$, the suppression of SC coincides with broken isospin symmetry near half-filling with lifted degeneracy ($g_d \sim 2$). Additionally, as the SC phase becomes weaker with $D$, vHs shifts to higher fillings, highlighting the modification of the underlying band structure with the applied electric field. These findings, with recent theoretical study on SC in tBLG, highlight the competition, rather being connected concomitantly, between SC and other orders promoted by broken symmetries. △ Less

Submitted 12 September, 2024; v1 submitted 18 February, 2024; originally announced February 2024.

Ab initio Investigation of Thermal Transport in Insulators: Unveiling the Roles of Phonon Renormalization and Higher-Order Anharmonicity

Authors: Soham Mandal, Manish Jain, Prabal K. Maiti

Abstract: The occurrence of thermal transport phenomena is widespread, exerting a pivotal influence on the functionality of diverse electronic and thermo-electric energy-conversion devices. The traditional first-principles theory governing the thermal and thermodynamic characteristics of insulators relies on the perturbative treatment of interatomic potential and ad-hoc displacement of atoms within supercel… ▽ More The occurrence of thermal transport phenomena is widespread, exerting a pivotal influence on the functionality of diverse electronic and thermo-electric energy-conversion devices. The traditional first-principles theory governing the thermal and thermodynamic characteristics of insulators relies on the perturbative treatment of interatomic potential and ad-hoc displacement of atoms within supercells. However, the limitations of these approaches for highly anharmonic and weakly bonded materials, along with discrepancies arising from not considering explicit finite temperature effects, highlight the necessity for a well-defined quasiparticle approach to the lattice vibrations. To address these limitations, we present a comprehensive numerical framework in this study, designed to compute the thermal and thermodynamic characteristics of crystalline semiconductors and insulators. The self-consistent phonon renormalization method we have devised reveals phonons as quasiparticles, diverging from their conventional characterization as bare normal modes of lattice vibration. The extension of the renormalization impact to interatomic force constants (IFCs) of third and fourth orders is also integrated and demonstrated. For the comprehensive physical insights, we employed an iterative solution of the Peierls-Boltzmann transport equation (PBTE) to determine thermal conductivity and carry out Helmholtz free energy calculations, encompassing anharmonicity effects up to the fourth order. In this study, we utilize our numerical framework to showcase its applicability through an examination of phonon dispersion, phonon linewidth, anharmonic phonon scattering, and temperature-dependent lattice thermal conductivity in both highly anharmonic materials (NaCl and AgI) and weakly anharmonic materials (cBN and 3C-SiC). △ Less

Submitted 5 February, 2024; originally announced February 2024.

Journal ref: Computer Physics Communications (2026)

Calculation of Gilbert damping and magnetic moment of inertia using torque-torque correlation model within ab initio Wannier framework

Authors: Robin Bajaj, Seung-Cheol Lee, H. R. Krishnamurthy, Satadeep Bhattacharjee, Manish Jain

Abstract: Magnetization dynamics in magnetic materials are well described by the modified semiclassical Landau-Lifshitz-Gilbert (LLG) equation, which includes the magnetic damping $α$ and the magnetic moment of inertia $\mathrm{I}$ tensors as key parameters. Both parameters are material-specific and physically represent the time scales of damping of precession and nutation in magnetization dynamics. $α$ and… ▽ More Magnetization dynamics in magnetic materials are well described by the modified semiclassical Landau-Lifshitz-Gilbert (LLG) equation, which includes the magnetic damping $α$ and the magnetic moment of inertia $\mathrm{I}$ tensors as key parameters. Both parameters are material-specific and physically represent the time scales of damping of precession and nutation in magnetization dynamics. $α$ and $\mathrm{I}$ can be calculated quantum mechanically within the framework of the torque-torque correlation model. The quantities required for the calculation are torque matrix elements, the real and imaginary parts of the Green's function and its derivatives. Here, we calculate these parameters for the elemental magnets such as Fe, Co and Ni in an ab initio framework using density functional theory and Wannier functions. We also propose a method to calculate the torque matrix elements within the Wannier framework. We demonstrate the effectiveness of the method by comparing it with the experiments and the previous ab initio and empirical studies and show its potential to improve our understanding of spin dynamics and to facilitate the design of spintronic devices. △ Less

Submitted 1 January, 2024; originally announced January 2024.

Fabrication and extreme micromechanics of additive metal microarchitectures

Authors: Sung-Gyu Kang, Barbara Bellon, Lalithkumar Bhaskar, Siyuan Zhang, Alexander Gotz, Janis Wirth, Benjamin Apeleo Zubiri, Szilvia Kalacska, Manish Jain, Amit Sharma, Wabe Koelmans, Giorgio Ercolano, Erdmann Spiecker, Johann Michler, Jakob Schwiedrzik, Gerhard Dehm, Rajaprakash Ramachandramoorthy

Abstract: The mechanical performance of metallic metamaterials with 3-dimensional solid frames is typically a combination of the geometrical effect ("architecture") and the characteristic size effects of the base material ("microstructure"). In this study, for the first time, the temperature- and rate-dependent mechanical response of copper microlattices has been investigated. The microlattices were fabrica… ▽ More The mechanical performance of metallic metamaterials with 3-dimensional solid frames is typically a combination of the geometrical effect ("architecture") and the characteristic size effects of the base material ("microstructure"). In this study, for the first time, the temperature- and rate-dependent mechanical response of copper microlattices has been investigated. The microlattices were fabricated via a localized electrodeposition in liquid (LEL) process which enables high-precision additive manufacturing of metal at the micro-scale. The metal microlattices possess a unique microstructure with micron sized grains that are rich with randomly oriented growth twins and near-ideal nodal connectivity. Importantly, copper microlattices exhibited unique temperature (-150 and 25 degree C) and strain rate (0.001~100 s-1) dependent deformation behavior during in situ micromechanical testing. Systematic compression tests of fully dense copper micropillars, equivalent in diameter and length to the struts of the microlattice at comparable extreme loading conditions, allow us to investigate the intrinsic deformation mechanism of copper. Combined with the post-mortem microstructural analysis, substantial shifts in deformation mechanisms depending on the temperature and strain rate were revealed. On the one hand, at room temperature (25 degree C), dislocation slip based plastic deformation occurs and leads to a localized deformation of the micropillars. On the other hand, at cryogenic temperature (-150 degree C), mechanical twinning occurs and leads to relatively homogeneous deformation of the micropillars. Based on the intrinsic deformation mechanisms of copper, the temperature and strain rate dependent deformation behavior of microlattices could be explained. △ Less

Submitted 3 April, 2024; v1 submitted 23 November, 2023; originally announced November 2023.

Mott insulating negative thermal expansion perovskite TiF3

Authors: Donal Sheets, Kaitlin Lyszak, Menka Jain, Gayanath W. Fernando, Ilya Sochnikov, Jacob Franklin, R. Mattias Geilhufe, Jason N. Hancock

Abstract: We characterize perovskite TiF_3, a material which displays significant negative thermal expansion at elevated temperatures above its cubic-to-rhombohedral structural phase transition at 330 K. We find the optical response favors an insulating state in both structural phases, which we show can be produced in density functional theory calculations only through the introduction of an on-site Coulomb… ▽ More We characterize perovskite TiF_3, a material which displays significant negative thermal expansion at elevated temperatures above its cubic-to-rhombohedral structural phase transition at 330 K. We find the optical response favors an insulating state in both structural phases, which we show can be produced in density functional theory calculations only through the introduction of an on-site Coulomb repulsion. Analysis of the magnetic susceptibility data gives a S=1/2 local moment per Ti+3 ion and an antiferromagnetic exchange coupling. Together, these results show that TiF_3 is a strongly correlated electron system, a fact which constrains possible mechanisms of strong negative thermal expansion in the Sc_1-xTi_xF3 system. We consider the relative strength of the Jahn-Teller and electric dipole interactions in driving the structural transition. △ Less

Submitted 14 November, 2023; originally announced November 2023.

Comments: 8 pages, 4 figures, in review Physical Review B

Controlling Umklapp scattering in bilayer graphene moir'e superlattice

Authors: Mohit Kumar Jat, Shubhankar Mishra, Harsimran Kaur Mann, Robin Bajaj, Kenji Watanabe, Takashi Taniguchi, H. R. Krishnamurthy, Manish Jain, Aveek Bid

Abstract: In this Letter, we present experimental findings on electron-electron scattering in a two-dimensional moir'e heterostructure with tunable Fermi wave vector, reciprocal lattice vector, and band gap. We achieve this in high-mobility aligned heterostructures of bilayer graphene (BLG) and hBN. Around half-filling, the primary contribution to the resistance of BLG/hBN aligned superlattices arises from… ▽ More In this Letter, we present experimental findings on electron-electron scattering in a two-dimensional moir'e heterostructure with tunable Fermi wave vector, reciprocal lattice vector, and band gap. We achieve this in high-mobility aligned heterostructures of bilayer graphene (BLG) and hBN. Around half-filling, the primary contribution to the resistance of BLG/hBN aligned superlattices arises from electron-electron Umklapp (Uee) scattering, making the resistance of graphene/hBN moir'e devices significantly larger than that of non-aligned devices (where Uee is forbidden). We quantify the strength of the Uee scattering and find that it follows a universal scaling with Fermi energy and has a non-monotonic dependence on the charge carrier density. The Uee scattering is strongly electric field tunable and affected by layer-polarization of BLG. It has a strong particle-hole asymmetry - the resistance when the chemical potential is in the conduction band is significantly lesser than when it is in the valence band, making the electron-doped regime more practical for potential applications. △ Less

Submitted 15 February, 2024; v1 submitted 13 October, 2023; originally announced October 2023.

Comments: Comments and suggestion are welcome

Journal ref: Nano Letters (2024)

Controlling the interactions in a cold atom quantum impurity system

Authors: Thomas Hewitt, Tom Bertheas, Manan Jain, Yusuke Nishida, Giovanni Barontini

Abstract: We implement an experimental architecture in which a single atom of K is trapped in an optical tweezer, and is immersed in a bath of Rb atoms at ultralow temperatures. In this regime, the motion of the single trapped atom is confined to the lowest quantum vibrational levels. This realizes an elementary and fully controllable quantum impurity system. For the trapping of the K atom, we use a species… ▽ More We implement an experimental architecture in which a single atom of K is trapped in an optical tweezer, and is immersed in a bath of Rb atoms at ultralow temperatures. In this regime, the motion of the single trapped atom is confined to the lowest quantum vibrational levels. This realizes an elementary and fully controllable quantum impurity system. For the trapping of the K atom, we use a species-selective dipole potential, that allows us to independently manipulate the quantum impurity and the bath. We concentrate on the characterization and control of the interactions between the two subsystems. To this end, we perform Feshbach spectroscopy, detecting several inter-dimensional confinement-induced Feshbach resonances for the KRb interspecies scattering length, that parametrizes the strength of the interactions. We compare our data to a theory for inter-dimensional scattering, finding good agreement. Notably, we also detect a series of p-wave resonances stemming from the underlying free-space s-wave interactions. We further determine how the resonances behave as the temperature of the bath and the dimensionality of the interactions change. Additionally, we are able to screen the quantum impurity from the bath by finely tuning the wavelength of the light that produces the optical tweezer, providing us with a new effective tool to control and minimize the interactions. Our results open a range of new possibilities in quantum simulations of quantum impurity models, quantum information, and quantum thermodynamics, where the interactions between a quantized system and the bath is a powerful yet largely underutilized resource. △ Less

Submitted 28 May, 2024; v1 submitted 4 October, 2023; originally announced October 2023.

Journal ref: Quantum Sci. Technol. 9 035039 (2024)

Quantum MASALA: Quantum MAterialS Ab initio eLectronic-structure pAckage

Authors: Shri Hari Soundararaj, Agrim Sharma, Manish Jain

Abstract: We present Quantum MASALA, a compact package that implements different electronic structure methods in Python using the plane-wave basis. Within just 8100 lines of pure Python code, we have implemented Density Functional Theory (DFT), Time-dependent Density Functional Theory (TD-DFT) and the GW Method. The program can run across multiple processors and in Graphical Processing Units (GPU) with the… ▽ More We present Quantum MASALA, a compact package that implements different electronic structure methods in Python using the plane-wave basis. Within just 8100 lines of pure Python code, we have implemented Density Functional Theory (DFT), Time-dependent Density Functional Theory (TD-DFT) and the GW Method. The program can run across multiple processors and in Graphical Processing Units (GPU) with the help of easily accessible Python libraries. With Quantum ESPRESSO and BerkeleyGW input interfaces implemented, it can also be used as a substitute for small and medium scale calculations, making it a perfect learning tool for ab initio methods. The package is aimed to provide a framework with its modular and simple code design to rapidly build and test new methods for first-principles calculation. △ Less

Submitted 22 October, 2024; v1 submitted 14 August, 2023; originally announced August 2023.

Comments: 50 pages, 6 figures, 7 tables

Quantum fluctuations lead to glassy electron dynamics in the good metal regime of electron doped KTaO3

Authors: Shashank Kumar Ojha, Sankalpa Hazra, Surajit Bera, Sanat Kumar Gogoi, Prithwijit Mandal, Jyotirmay Maity, A. Gloskovskii, C. Schlueter, Smarajit Karmakar, Manish Jain, Sumilan Banerjee, Venkatraman Gopalan, Srimanta Middey

Abstract: One of the central challenges in condensed matter physics is to comprehend systems that have strong disorder and strong interactions. In the strongly localized regime, their subtle competition leads to glassy electron dynamics which ceases to exist well before the insulator-to-metal transition is approached as a function of doping. Here, we report on the discovery of glassy electron dynamics deep… ▽ More One of the central challenges in condensed matter physics is to comprehend systems that have strong disorder and strong interactions. In the strongly localized regime, their subtle competition leads to glassy electron dynamics which ceases to exist well before the insulator-to-metal transition is approached as a function of doping. Here, we report on the discovery of glassy electron dynamics deep inside the good metal regime of an electron-doped quantum paraelectric system: KTaO$_3$. We reveal that upon excitation of electrons from defect states to the conduction band, the excess injected carriers in the conduction band relax in a stretched exponential manner with a large relaxation time, and the system evinces simple aging phenomena - a telltale sign of glassy dynamics. Most significantly, we observe a critical slowing down of carrier dynamics below 35 K, concomitant with the onset of quantum paraelectricity in the undoped KTaO$_3$. Our combined investigation using second harmonic generation technique, density functional theory and phenomenological modeling demonstrates quantum fluctuation-stabilized soft polar modes as the impetus for the glassy behavior. This study addresses one of the most fundamental questions regarding the potential promotion of glassiness by quantum fluctuations and opens a route for exploring glassy dynamics of electrons in a well-delocalized regime. △ Less

Submitted 5 June, 2024; v1 submitted 26 June, 2023; originally announced June 2023.

Comments: 51 pages and 18 figures including supplemental

Journal ref: Nature Communications 15, 3830 (2024)

Observation of c-axis Magnetization at Low Temperatures in Weak Ferromagnet FeBO$_3$ Reveals a Spin-Reorientation Transition

Authors: Jacob Franklin, Jacob Pfund, Joshua Bedard, Weiguo Zhang, P. Shiv Halasyamani, Menka Jain, Ilya Sochnikov

Abstract: The weak ferromagnet FeBO$_3$ is well known for being a unique system for modelling and testing magnetic dynamics primarily due to relatively simple and localized magnetic structure and its interesting spin wave dynamics. At room temperature, it has slightly canted iron moments lying in the a-b plane that result in a strong antiferromagnetic moment and a weak ferromagnetic moment, which results in… ▽ More The weak ferromagnet FeBO$_3$ is well known for being a unique system for modelling and testing magnetic dynamics primarily due to relatively simple and localized magnetic structure and its interesting spin wave dynamics. At room temperature, it has slightly canted iron moments lying in the a-b plane that result in a strong antiferromagnetic moment and a weak ferromagnetic moment, which results in pronounced ferromagnetic and antiferromagnetic spin modes. However, some previous studies have shown unusual low-temperature behavior that suggests a phase transition. By performing low-temperature magnetization measurements, both in bulk and on the mesoscale, we have observed a low temperature magnetic texture in this material in which a large c-axis magnetization occurs. Magnetic fields along the c-axis as high as 1300 Oe were observed close to the sample surface. This presents evidence for the onset of a Morin transition or another type of spin-reorientation phase transition wherein the Fe3+ moments would acquire a c-axis component to their canting below a critical temperature. The observation of this c-axis magnetization suggests that there is a different ground state in this material than has been previously expected and could be due to as yet unexplored intricacies of the Dzyaloshinskii-Moriya interaction. △ Less

Submitted 25 May, 2023; originally announced May 2023.

i-SPin 2: An integrator for general spin-s Gross-Pitaevskii systems

Authors: Mudit Jain, Mustafa A. Amin, Han Pu

Abstract: We provide an algorithm for evolving general spin-$s$ Gross-Pitaevskii / non-linear Schrödinger systems carrying a variety of interactions, where the $2s+1$ components of the `spinor' field represent the different spin-multiplicity states. We consider many nonrelativistic interactions up to quartic order in the Schrödinger field (both short and long-range, and spin-dependent and spin-independent i… ▽ More We provide an algorithm for evolving general spin-$s$ Gross-Pitaevskii / non-linear Schrödinger systems carrying a variety of interactions, where the $2s+1$ components of the `spinor' field represent the different spin-multiplicity states. We consider many nonrelativistic interactions up to quartic order in the Schrödinger field (both short and long-range, and spin-dependent and spin-independent interactions), including explicit spin-orbit couplings. The algorithm allows for spatially varying external and/or self-generated vector potentials that couple to the spin density of the field. Our work can be used for scenarios ranging from laboratory systems such as spinor Bose-Einstein condensates (BECs), to cosmological/astrophysical systems such as self-interacting bosonic dark matter. As examples, we provide results for two different setups of spin-$1$ BECs that employ a varying magnetic field and spin-orbit coupling, respectively, and also collisions of spin-$1$ solitons in dark matter. Our symplectic algorithm is second-order accurate in time, and is extensible to the known higher-order accurate methods. △ Less

Submitted 2 May, 2023; originally announced May 2023.

Comments: 13 pages, 3 figures, 2 appendices

Journal ref: Phys. Rev. E 108, 055305, 15 November 2023

Higher-order Bragg gaps in the electronic band structure of bilayer graphene renormalized by recursive supermoiré potential

Authors: Mohit Kumar Jat, Priya Tiwari, Robin Bajaj, Ishita Shitut, Shinjan Mandal, Kenji Watanabe, Takashi Taniguchi, H. R. Krishnamurthy, Manish Jain, Aveek Bid

Abstract: This letter presents our findings on the recursive band gap engineering of chiral fermions in bilayer graphene doubly aligned with hBN. By utilizing two interfering moiré potentials, we generate a supermoiré pattern which renormalizes the electronic bands of the pristine bilayer graphene, resulting in higher-order fractal gaps even at very low energies. These Bragg gaps can be mapped using a uniqu… ▽ More This letter presents our findings on the recursive band gap engineering of chiral fermions in bilayer graphene doubly aligned with hBN. By utilizing two interfering moiré potentials, we generate a supermoiré pattern which renormalizes the electronic bands of the pristine bilayer graphene, resulting in higher-order fractal gaps even at very low energies. These Bragg gaps can be mapped using a unique linear combination of periodic areas within the system. To validate our findings, we used electronic transport measurements to identify the position of these gaps as functions of the carrier density and establish their agreement with the predicted carrier densities and corresponding quantum numbers obtained using the continuum model. Our work provides direct experimental evidence of the quantization of the area of quasi-Brillouin zones in supermoiré systems. It fills essential gaps in understanding the band structure engineering of Dirac fermions by a recursive doubly periodic superlattice potential. △ Less

Submitted 4 April, 2023; originally announced April 2023.

Comments: 29 pages (including Supplementary Materials)

Giant electromechanical response from defective non-ferroelectric epitaxial BaTiO3 integrated on Si 100

Authors: Sandeep Vura, Shubham Kumar Parate, Subhajit Pal, Upanya Khandelwal, Rajeev Kumar Rai, Sri Harsha Molleti, Vishnu Kumar, Rama Satya Sandilya Ventrapragada, Girish Patil, Mudit Jain, Ambresh Mallya, Majid Ahmadi, Bart Kooi, Sushobhan Avasthi, Rajeev Ranjan, Srinivasan Raghavan, Saurabh Chandorkar, Pavan Nukala

Abstract: Lead free, silicon compatible materials showing large electromechanical responses comparable to, or better than conventional relaxor ferroelectrics, are desirable for various nanoelectromechanical devices and applications. Defect-engineered electrostriction has recently been gaining popularity to obtain enhanced electromechanical responses at sub 100 Hz frequencies. Here, we report record values o… ▽ More Lead free, silicon compatible materials showing large electromechanical responses comparable to, or better than conventional relaxor ferroelectrics, are desirable for various nanoelectromechanical devices and applications. Defect-engineered electrostriction has recently been gaining popularity to obtain enhanced electromechanical responses at sub 100 Hz frequencies. Here, we report record values of electrostrictive strain coefficients (M31) at frequencies as large as 5 kHz (1.04 x 10-14 m2 per V2 at 1 kHz, and 3.87 x 10-15 m2 per V2 at 5 kHz) using A-site and oxygen-deficient barium titanate thin-films, epitaxially integrated onto Si. The effect is robust and retained even after cycling the devices >5000 times. Our perovskite films are non-ferroelectric, exhibit a different symmetry compared to stoichiometric BaTiO3 and are characterized by twin boundaries and nano polar-like regions. We show that the dielectric relaxation arising from the defect-induced features correlates very well with the observed giant electrostrictive response. These films show large coefficient of thermal expansion (2.36 x 10-5/K), which along with the giant M31 implies a considerable increase in the lattice anharmonicity induced by the defects. Our work provides a crucial step forward towards formulating guidelines to engineer large electromechanical responses even at higher frequencies in lead-free thin films. △ Less

Submitted 6 March, 2023; originally announced March 2023.

Comments: 26 pages, 4 figures, 8 supplementary figures

Theoretical evaluation of oxynitride, oxyfluoride and nitrofluoride perovskites with promising photon absorption properties for solar water splitting

Authors: Manjari Jain, Deepika Gill, Sanchi Monga, Saswata Bhattacharya

Abstract: Photocatalytic water splitting represents a very promising but at the same time very challenging contribution to a clean and renewable route to produce hydrogen fuel. Developing efficient and cost-effective photocatalysts for water splitting is a growing need. For this purpose, semiconductor photocatalysts have attracted much more attention due to their stability and low manufacturing cost. Here,… ▽ More Photocatalytic water splitting represents a very promising but at the same time very challenging contribution to a clean and renewable route to produce hydrogen fuel. Developing efficient and cost-effective photocatalysts for water splitting is a growing need. For this purpose, semiconductor photocatalysts have attracted much more attention due to their stability and low manufacturing cost. Here, we have systematically applied several state-of-the-art advanced first-principles-based methodologies, viz., hybrid density functional theory, many-body perturbation theory (G$_0$W$_0$) and density functional perturbation theory (DFPT), to understand the electronic structure properties of ABX$_2$Y perovskites. We have chosen the vast composition space of ABX$_2$Y type perovskites where A and B are cations and X and Y can be nitrogen, oxygen, or fluorine. These perovskites exhibit direct band gaps ranging from 1.6 to 3.3 eV. Further, to evaluate the feasibility of the visible light catalytic performance, we calculate the structural, electronic, and optical properties of ABX$_2$Y perovskites. In addition, from hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) mechanism, BaInO$_2$F, InSnO$_2$N, CsPbO$_2$F and LaNbN$_2$O are found as probable photocatalysts. △ Less

Submitted 11 January, 2023; originally announced January 2023.

Does twist angle affect the properties of water confined inside twisted bilayer graphene?

Authors: Jeet Majumdar, Subhadeep Dasgupta, Soham Mandal, Mohd Moid, Manish Jain, Prabal K. Maiti

Abstract: Graphene nanoslit pore is used for nanofluidic devices like water desalination, ion-selective channels, ionic transistors, sensing, molecular sieving, blue energy harvesting, and protein sequencing. It is a strenuous task to prepare nanofluidic devices because a small misalignment leads to a significant alteration in various properties of the devices. Here we focus on the rotational misalignment b… ▽ More Graphene nanoslit pore is used for nanofluidic devices like water desalination, ion-selective channels, ionic transistors, sensing, molecular sieving, blue energy harvesting, and protein sequencing. It is a strenuous task to prepare nanofluidic devices because a small misalignment leads to a significant alteration in various properties of the devices. Here we focus on the rotational misalignment between two parallel graphene sheets. Using molecular dynamics simulation, we probe the structure and dynamics of monolayer water confined inside graphene nanochannels for a range of commensurate twist angles. With SPC/E and TIP4P/2005 water model, our simulations reveal the independence of equilibrium number density $(n \sim 13 nm^{-2})$ for SPC/E and $(n \sim 11.5 nm^{-2})$ for TIP4P/2005) across twists. Based on the respective densities of water models, the structure and dielectric constant are invariant of twist angles. The confined water structure at this shows square ice ordering for SPC/E water only. TIP4P/2005 shows ordering at the vicinity of a critical density $(n \sim 12.5 nm^{-2})$. The average perpendicular dielectric constant of the confined water remains anomalously low ($\sim 2$ for SPC/E and $\sim 6$ for TIP4P/2005) for studied twist angles. We find that the friction coefficient of confined water molecules varies for small twist angles while becoming independent for twists greater than $5.1^{o}$. Our results indicate that small angular misalignment will not impair the dielectric properties of monolayer water within graphene slit-pore but can significantly influence its dynamics. △ Less

Submitted 23 December, 2022; originally announced December 2022.

Comments: 29 pages, 12 figures

i-SPin: An integrator for multicomponent Schrödinger-Poisson systems with self-interactions

Authors: Mudit Jain, Mustafa A. Amin

Abstract: We provide an algorithm and a publicly available code to numerically evolve multicomponent Schrödinger-Poisson (SP) systems with a SO($n$) symmetry, including attractive or repulsive self-interactions in addition to gravity. Focusing on the case where the SP system represents the non-relativistic limit of a massive vector field, non-gravitational self-interactions (in particular spin-spin interact… ▽ More We provide an algorithm and a publicly available code to numerically evolve multicomponent Schrödinger-Poisson (SP) systems with a SO($n$) symmetry, including attractive or repulsive self-interactions in addition to gravity. Focusing on the case where the SP system represents the non-relativistic limit of a massive vector field, non-gravitational self-interactions (in particular spin-spin interactions) introduce complexities related to mass and spin conservation which are not present in purely gravitational systems. We address them with an analytical solution for the `kick' step in the algorithm, where we are able to decouple the multicomponent system completely. Equipped with this analytical solution, the full field evolution is second order accurate, preserves spin and mass to machine precision, and is reversible. Our algorithm allows for an expanding universe relevant for cosmology, and the inclusion of external potentials relevant for laboratory settings. △ Less

Submitted 19 November, 2022; v1 submitted 15 November, 2022; originally announced November 2022.

Comments: 18 pages, 3 figures, 4 appendices. A python code based on our algorithm in provided at, https://github.com/mudit-jain90/i-SPin . Animations of the numerical simulation results can be found at, https://www.youtube.com/watch?v=KUbVtULiBtE . (Added discussions of another algorithm, along with its generalization.)

Journal ref: JCAP 04 (2023) 053, 20 April 2023

Electron Holographic Mapping of Structural and Electronic Reconstruction at Mono- and Bilayer Steps of h-BN

Authors: Subakti Subakti, Mohammadreza Daqiqshirazi, Daniel Wolf, Martin Linck, Felix L. Kern, Mitisha Jain, Silvan Kretschmer, Arkady V. Krasheninnikov, Thomas Brumme, Axel Lubk

Abstract: Here, by making use of medium and high resolution autocorrected off-axis electron holography, we directly probe the electrostatic potential as well as in-plane and out-of-plane charge delocalization at edges and steps in multilayer hexagonal boron nitride. In combination with ab-initio calculations, the data allows to directly reveal the formation of out-of-plane covalent bonds at folded zig-zag e… ▽ More Here, by making use of medium and high resolution autocorrected off-axis electron holography, we directly probe the electrostatic potential as well as in-plane and out-of-plane charge delocalization at edges and steps in multilayer hexagonal boron nitride. In combination with ab-initio calculations, the data allows to directly reveal the formation of out-of-plane covalent bonds at folded zig-zag edges and steps comprising two monolayers and the absence of which at monolayer steps. The technique paves the way for studying other charge (de)localization phenomena in 2D materials, e.g., at polar edges, topological edge states and defects. △ Less

Submitted 8 October, 2022; originally announced October 2022.

Vacancy-Ordered Double Perovskites Cs$_2$BI$_6$ (B = Pt, Pd, Te, Sn): An Emerging Class of Thermoelectric Materials

Authors: Preeti Bhumla, Manjari Jain, Sajjan Sheoran, Saswata Bhattacharya

Abstract: Vacancy-ordered double perovskites (A$_2$BX$_6$), being one of the environmentally friendly and stable alternatives to lead halide perovskites, have garnered considerable research attention in the scientific community. However, their thermal transport has not been explored much despite their potential applications. Here, we explore Cs$_2$BI$_6$ (B = Pt, Pd, Te, Sn) as potential thermoelectric mate… ▽ More Vacancy-ordered double perovskites (A$_2$BX$_6$), being one of the environmentally friendly and stable alternatives to lead halide perovskites, have garnered considerable research attention in the scientific community. However, their thermal transport has not been explored much despite their potential applications. Here, we explore Cs$_2$BI$_6$ (B = Pt, Pd, Te, Sn) as potential thermoelectric materials using the state-of-the-art first-principles based methodologies, viz., density functional theory combined with many-body perturbation theory (G$_0$W$_0$) and spin-orbit coupling. %The phonon dispersion plots and Poisson's and Pugh's ratios show the dynamical and mechanical stability of this class of perovskites. The absence of polyhedral connectivity in vacancy-ordered perovskites gives rise to additional degrees of freedom leading to lattice anharmonicity. The presence of anharmonic lattice dynamics leads to strong electron-phonon coupling, which is well captured by Fröhlich mesoscopic model. % to investigate the interaction of longitudinal optical phonon modes with the carriers that strongly influence the carrier mobility. The lattice anharmonicity is further studied using {\it ab initio} molecular dynamics and electron localization function. The maximum anharmonicity is observed in Cs$_2$PtI$_6$, followed by Cs$_2$PdI$_6$, Cs$_2$TeI$_6$ and Cs$_2$SnI$_6$. Also, the computed average thermoelectric figure of merit ($zT$) for Cs$_2$PtI$_6$, Cs$_2$PdI$_6$, Cs$_2$TeI$_6$ and Cs$_2$SnI$_6$ are 0.88, 0.85, 0.95 and 0.78, respectively, which reveals their promising renewable energy applications. △ Less

Submitted 30 April, 2023; v1 submitted 18 September, 2022; originally announced September 2022.

Comments: 43 pages, 11 figures

Exciton fine structure in twisted transition metal dichalcogenide heterostructures

Authors: Sudipta Kundu, Tomer Amit, H. R. Krishnamurthy, Manish Jain, Sivan Refaely-Abramson

Abstract: Moiré superlattices of transition metal dichalcogenide (TMD) heterostructures give rise to rich excitonic phenomena associated with the interlayer twist angle and induced changes in the involved quantum states. Theoretical calculations of excitons in such systems are typically based on model moiré potentials to mitigate the computational cost. However, an ab initio understanding of the electron-ho… ▽ More Moiré superlattices of transition metal dichalcogenide (TMD) heterostructures give rise to rich excitonic phenomena associated with the interlayer twist angle and induced changes in the involved quantum states. Theoretical calculations of excitons in such systems are typically based on model moiré potentials to mitigate the computational cost. However, an ab initio understanding of the electron-hole coupling dominating the excitations is crucial to realize the twist-induced modifications of the optical selection rules. In this work we use many-body perturbation theory to compute and analyze the relation between twist angle and exciton properties in twisted TMD heterostructures. We present a general approach for unfolding excitonic states from the moiré Brillouin zone onto the Brillouin zones of the separate layers. Applying this method to a twisted MoS$_2$/MoSe$_2$ bilayer, we find that the optical excitation spectrum is dominated by mixed transitions between electrons and holes with different momenta in the separate monolayers, leading to unexpected and angle-dependent hybridization between interlayer and intralayer excitons. Our findings offer a design pathway for tuning exciton layer-localization in TMD heterostructures as a function of twist angle. △ Less

Submitted 12 September, 2022; originally announced September 2022.

Tuning exciton complexes in twisted bilayer WSe2 at intermediate misorientation

Authors: Rahul Debnath, Shaili Sett, Sudipta Kundu, Rabindra Biswas, Varun Raghunathan, Manish Jain, Arindam Ghosh, Akshay Singh

Abstract: Twist angle modifies the band alignment, screening, and interlayer (IL) coupling in twisted bilayers (tBLs) of transition metal dichalcogenides. Intermediate misorientation (twist angles > 15 degrees) bilayers (BLs) offer a unique opportunity to tune excitonic behavior within these concurrent physical mechanisms but are seldom studied. In this paper, we measure many-body excitonic complexes in mon… ▽ More Twist angle modifies the band alignment, screening, and interlayer (IL) coupling in twisted bilayers (tBLs) of transition metal dichalcogenides. Intermediate misorientation (twist angles > 15 degrees) bilayers (BLs) offer a unique opportunity to tune excitonic behavior within these concurrent physical mechanisms but are seldom studied. In this paper, we measure many-body excitonic complexes in monolayer (ML), natural BL, and tBL WSe2. Neutral biexciton (XX) is observed in tBL, while being undetected in nonencapsulated ML and BL, demonstrating unique effects of disorder screening in tBLs. The XX as well as charged biexciton are robust to thermal dissociation and are controllable by electrostatic doping. Vanishing of momentum-indirect IL excitons with increasing electron doping is demonstrated in tBL, resulting from the near alignment of Q-K and K-K valleys. Intermediate misorientation samples offer a high degree of control of excitonic complexes while offering possibilities for studying exciton-phonon coupling, band alignment, and screening. △ Less

Submitted 7 September, 2022; originally announced September 2022.

Report number: Vol. 106, No. 12

Journal ref: Physical Review B 2022

Electrocatalytic Study for Hydrogen Evolution Reaction on MoS$_2$/BP and MoSSe/BP in Acidic Media

Authors: Arunima Singh, Preeti Bhumla, Manjari Jain, Saswata Bhattacharya

Abstract: Molecular hydrogen (H$_2$) production by electrochemical hydrogen evolution reaction (HER) is being actively explored for non-precious-metal based electrocatalysts that are earth-abundant and low cost like MoS$_2$. Although it is acid-stable, its applicability is limited by catalytically inactive basal plane, poor electrical transport and inefficient charge transfer at the interface. Therefore, th… ▽ More Molecular hydrogen (H$_2$) production by electrochemical hydrogen evolution reaction (HER) is being actively explored for non-precious-metal based electrocatalysts that are earth-abundant and low cost like MoS$_2$. Although it is acid-stable, its applicability is limited by catalytically inactive basal plane, poor electrical transport and inefficient charge transfer at the interface. Therefore, the present work examines its bilayer van der Waals heterostructure (vdW HTS). The second constituent monolayer Boron Phosphide (BP) is advantageous as an electrode material owing to its chemical stability in both oxygen and water environments. Here, we have performed first-principles based calculations under the framework of density functional theory (DFT) for HER in an electrochemical double layer model with the BP monolayer, MoS$_2$/BP and MoSSe/BP vdW HTSs. The climbing image nudged elastic band method (CI-NEB) has been employed to determine the minimum energy pathways for Tafel and Heyrovsky reactions. The calculations yield that Tafel reaction shows no reaction barrier. Thereafter, for Heyrovsky reaction, we have obtained low reaction barrier in the vdW HTSs as compared to that in the BP monolayer. Subsequently, we have observed no significant difference in the reaction profile of MoS$_2$/BP and MoSSe/BP vdW HTSs in case of high coverage (25 %) and 1/3 H$^+$ concentration (conc.). However, in the case of small coverage (11 %) and 1/3 H$^+$ conc., MoSSe/BP shows feasible Heyrovsky reaction with no reaction barrier. Finally, on comparing the coverages with 1/4 H$^+$ conc., we deduce high coverage with low conc. and low coverage with high conc. to be apt for HER via Heyrovsky reaction path. △ Less

Submitted 16 August, 2022; originally announced August 2022.

Fully relativistic $GW$/Bethe-Salpeter calculations in BerkeleyGW: implementation, symmetries, benchmarking, and performance

Authors: Bradford A. Barker, Jack Deslippe, Johannes Lischner, Manish Jain, Oleg V. Yazyev, David A. Strubbe, Steven G. Louie, .

Abstract: Computing the $GW$ quasiparticle bandstructure and Bethe-Salpeter Equation (BSE) absorption spectra for materials with spin-orbit coupling has commonly been done by treating $GW$ corrections and spin-orbit coupling as separate perturbations to density-functional theory. However, accurate treatment of materials with strong spin-orbit coupling often requires a fully relativistic approach using spino… ▽ More Computing the $GW$ quasiparticle bandstructure and Bethe-Salpeter Equation (BSE) absorption spectra for materials with spin-orbit coupling has commonly been done by treating $GW$ corrections and spin-orbit coupling as separate perturbations to density-functional theory. However, accurate treatment of materials with strong spin-orbit coupling often requires a fully relativistic approach using spinor wavefunctions in the Kohn-Sham equation and $GW$/BSE. Such calculations have only recently become available, in particular for the BSE. We have implemented this approach in the plane-wave pseudopotential $GW$/BSE code BerkeleyGW, which is highly parallelized and widely used in the electronic-structure community. We present reference results for quasiparticle bandstructures and optical absorption spectra of solids with different strengths of spin-orbit coupling, including Si, Ge, GaAs, GaSb, CdSe, Au, and Bi$_2$Se$_3$. The calculated quasiparticle band gaps of these systems are found to agree with experiment to within a few tens of meV. The absorption spectrum of GaSb calculated with the fully-relativistic $GW$-BSE captures the large spin-orbit splitting of peaks in the spectrum. For Bi$_2$Se$_3$, we find a drastic change in the low-energy bandstructure compared to that of DFT, with the fully-relativistic treatment of the $GW$ approximation correctly capturing the parabolic nature of the valence and conduction bands after including off-diagonal self-energy matrix elements. We present the detailed methodology, approach to spatial symmetries for spinors, comparison against other codes, and performance compared to spinless $GW$/BSE calculations and perturbative approaches to SOC. This work aims to spur further development of spinor $GW$/BSE methodology in excited-state research software. △ Less

Submitted 1 June, 2022; originally announced June 2022.

Journal ref: Phys. Rev. B 106, 115127 (2022)

Exploring strong and weak topological states on isostructural substitutions in TlBiSe2

Authors: Ankita Phutela, Preeti Bhumla, Manjari Jain, Saswata Bhattacharya

Abstract: Topological Insulators (TIs) are unique materials where insulating bulk hosts linearly dispersing surface states protected by the Time-Reversal Symmetry (TRS). These states lead to dissipationless current flow, which makes this class of materials highly promising for spintronic applications. Here, we predict new TIs via high-throughput screening by employing state-of-the-art first-principles based… ▽ More Topological Insulators (TIs) are unique materials where insulating bulk hosts linearly dispersing surface states protected by the Time-Reversal Symmetry (TRS). These states lead to dissipationless current flow, which makes this class of materials highly promising for spintronic applications. Here, we predict new TIs via high-throughput screening by employing state-of-the-art first-principles based methodologies, viz., Density Functional Theory (DFT) and many-body perturbation theory (G0W0) combined with Spin-Orbit Coupling (SOC). For this, we take a well-known 3D TI, TlBiSe2 and perform complete substitution with suitable materials at different sites to check if the obtained isostructural materials exhibit topological properties. Subsequently, we scan these materials based on SOC-induced parity inversion at Time-Reversal Invariant Momenta (TRIM). Later, to confirm the topological nature of selected materials, we plot their surface states along with calculation of Z2 invariants. Our results show that GaBiSe2 is a Strong Topological Insulator (STI). Besides, we report six Weak Topological Insulators (WTIs) viz. PbBiSe2, SnBiSe2, SbBiSe2, Bi2Se2, TlSnSe2 and PbSbSe2. We have further verified that all the reported TIs are dynamically stable showing all real phonon modes of vibration. △ Less

Submitted 18 May, 2022; originally announced May 2022.

Structural and electronic properties of rare earth chromites: A computational and experimental study

Authors: Jianhang Shi, Gayanath W. Fernando, Yanliu Dang, Steven L. Suib, Menka Jain

Abstract: In this work, the structural, optical, and electronic properties of rare-earth perovskites of the general formula RCrO3, where R represents the rare-earth Gd, Tb, Dy, Ho, Er, and Tm, have been studied in detail. These compounds were synthesized through a facile citrate route. X-ray diffraction, Raman spectroscopy, and UV-Vis spectroscopy were used to reveal the structural evolutions in RCrO3. The… ▽ More In this work, the structural, optical, and electronic properties of rare-earth perovskites of the general formula RCrO3, where R represents the rare-earth Gd, Tb, Dy, Ho, Er, and Tm, have been studied in detail. These compounds were synthesized through a facile citrate route. X-ray diffraction, Raman spectroscopy, and UV-Vis spectroscopy were used to reveal the structural evolutions in RCrO3. The lattice parameter, Cr3+-O2--Cr3+ bond angle, and CrO6 octahedral distortions were found to strongly depend on the ionic radii of the rare-earth element. First-principles calculations based on density-functional theory within the generalized gradient approximation (GGA) of Perdew- Burke- Ernzerhof (PBE) and strongly constrained-and-appropriately normed (SCAN) meta-GGA were also employed to calculate the structural and electronic properties of RCrO3. The ground-state energy, lattice constants, electronic structure, and density of states (DOS) of RCrO3 were calculated. These provide some insights into the electronic characteristics of the series of RCrO3 compounds. The calculated values of lattice parameters and band gaps with Hubbard U correction (SCAN+U) agree well with values measured experimentally and show more accuracy in predicting the ground-state crystal structure and band structure compared to PBE+U approximation. The band gap of RCrO3 is found to be independent of the ionic radii of the element R from both experiments and calculations △ Less

Submitted 25 April, 2022; originally announced April 2022.

Comments: 17 pages, 10 figures, 8 tables

Metalated Porous-Organic-Polymer Renders Mustard-Gas Simulant Harmless: Core Planarity Matters

Authors: Ratul Paul, Chitra Sarkar, Manjari Jain, Shaojun Xu, Kashmiri Borah, Duy Quang Dao, Chih-Wen Pao, Saswata Bhattacharya, John Mondal

Abstract: The presence of open active metal sites in Metal-Organic Frameworks (MOFs) exhibit higher catalytic activity. However, rational accomplishment of MOFs in heterogeneous catalysis is limited due to coordination bonds. Recently balanced characteristic feature with combination of both the covalent bonds (structural stability) and open metal sites (single site catalysis) introduced an entirely organic… ▽ More The presence of open active metal sites in Metal-Organic Frameworks (MOFs) exhibit higher catalytic activity. However, rational accomplishment of MOFs in heterogeneous catalysis is limited due to coordination bonds. Recently balanced characteristic feature with combination of both the covalent bonds (structural stability) and open metal sites (single site catalysis) introduced an entirely organic alternative architecture named as Metalated Porous-Organic-Polymers (M-POPs). In this contribution, we demonstrate successful construction of two Fe-POPs (Fe-Tt-POP & Fe-Rb-POP) by ternary copolymerization approach for catalytic oxidative decontamination of different sulfur-based mustard gas simulants. Fe-Tt-POP exhibited superior catalytic performance for oxidation of the thioanisole (TA) studied in terms of conversion (99% after 13 h) in comparison with Fe-Rb-POP (43% after 13h). The remarkable difference in the mechanistic pathways towards catalytic performance for oxidation of TA was investigated by in situ operando Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) analysis, complemented by Density Functional Theory (DFT) computational study. △ Less

Submitted 23 February, 2022; originally announced February 2022.

Comments: 9 pages excluding SI, 49 pages in total