Showing 1–50 of 121 results for author: Kundu, S

Jamming states in random sequential adsorption of diffusion-limited aggregates

Authors: Fahad Puthalath, Dipanjan Mandal, Sumanta Kundu

Abstract: Motivated by the ubiquity of ramified fractal deposits in nature and engineered systems, we investigate the irreversible adsorption of diffusion-limited aggregation (DLA) clusters on a square lattice. We study the role of cluster shape diversity on jamming properties of the system by systematically controlling the number of distinct shapes used across and within realizations, encompassing both mon… ▽ More Motivated by the ubiquity of ramified fractal deposits in nature and engineered systems, we investigate the irreversible adsorption of diffusion-limited aggregation (DLA) clusters on a square lattice. We study the role of cluster shape diversity on jamming properties of the system by systematically controlling the number of distinct shapes used across and within realizations, encompassing both monodisperse and polydisperse model variants. Our large-scale simulations over a broad range of cluster sizes $2\leqslant k\leqslant4096$ show that the jamming density decreases with cluster size as a power-law $p_j(k)-p_j^\infty\sim k^{-α}$. Both $α$ and $p_j^\infty$ are found to depend on the degree of shape diversity, with $α$ ranging from $0.374(2)$ to $0.417(1)$. It is observed that increasing shape polydispersity promotes denser packing. Importantly, the fluctuations of the jamming density exhibit distinct scaling behavior: $σ(L)\sim1/L$ for a fixed pool of cluster shape(s), but remain $L$-independent when the pool of shape(s) is refreshed across different realizations. Furthermore, our results demonstrate that the differences between the model variants systematically diminish with increasing $k$ and are expected to vanish as $k\to\infty$ due to the statistical self-similarity of the DLA clusters. △ Less

Submitted 24 August, 2026; originally announced August 2026.

Exchange-mediated exciton splitting and linear dichroism in monolayer transition metal dichalcogenide induced by ferroelectric substrates

Authors: Sudipta Kundu, Felipe H. da Jornada

Abstract: Valley-polarized excitons in two-dimensional transition metal dichalcogenides (TMDs) offer a promising platform for quantum applications, yet the addressability and decoherence of these states remain fundamental challenges. Here, by developing a first-principles electrostatic embedding approach and performing large-scale GW plus Bethe-Salpeter equation calculations, we reveal novel excitons that e… ▽ More Valley-polarized excitons in two-dimensional transition metal dichalcogenides (TMDs) offer a promising platform for quantum applications, yet the addressability and decoherence of these states remain fundamental challenges. Here, by developing a first-principles electrostatic embedding approach and performing large-scale GW plus Bethe-Salpeter equation calculations, we reveal novel excitons that emerge in TMD monolayers when supported by a ferroelectric twisted bilayer hBN substrate. We predict two competing low-energy excitons whose ordering depends on the dielectric environment: optically dark, charge-transfer excitons, and quasi-one-dimensional Wannier excitons with linear optical dichroism. The spatial localization of Wannier excitons, together with intervalley exchange interactions in monolayer TMDs, splits valley-degenerate excitons by about 3~meV without external magnetic fields. Our ab initio calculations clarify the role of the interfacial twist angle and the spatial localization of fringe fields, establishing design rules for engineering long-lived two-level systems in TMD monolayers supported by ferroelectric substrates. △ Less

Submitted 3 June, 2026; originally announced June 2026.

Comments: 5 figures

Moiré enabled spin pumping and preservation in MoSe2/WS2 heterobilayers

Authors: Leo Yu, Kateryna Pistunova, Sudipta Kundu, Jenny Hu, Kenji Watanabe, Takashi Taniguchi, Felipe H. da Jornada, Tony F. Heinz

Abstract: The spin degree of freedom is a fundamental quantum mechanical attribute with implications spanning from magnetism to quantum computing. Consequently, the relaxation of spin states for extended, Bloch electrons in solids has been studied for decades as it defines many of their properties and applications. We show that moiré patterns in layered materials can extend spin relaxation times by two orde… ▽ More The spin degree of freedom is a fundamental quantum mechanical attribute with implications spanning from magnetism to quantum computing. Consequently, the relaxation of spin states for extended, Bloch electrons in solids has been studied for decades as it defines many of their properties and applications. We show that moiré patterns in layered materials can extend spin relaxation times by two orders of magnitude to 1 millisecond and beyond. This is achieved by suppressing spin mixing for electrons in 2D semiconductor heterostructures, particularly in the MoSe2/WS2 system, as we elucidate both experimentally and theoretically. The extended longitudinal lifetime facilitates spin alignment over 50% using only nanowatt levels of optical power. Our findings highlight the potential of moiré engineering for future quantum sensing and information processing. △ Less

Submitted 2 June, 2026; originally announced June 2026.

Emergence of Dynamical Anisotropy induced by Demixing in a Binary System with Differential Diffusivity under an External Potential

Authors: Rashmi Trivedi, Subhajit Paul, Sumanta Kundu, Sunita Kumari

Abstract: Spontaneous demixing in active matter is a ubiquitous phenomenon that is crucial for numerous living processes ranging from bacterial swarming to sorting of cells in dense tissues. Here, we systematically investigate the effect of spatially varying potential acting along one direction and packing fraction on the binary mixture of particles with different diffusivities. Our results indicate that th… ▽ More Spontaneous demixing in active matter is a ubiquitous phenomenon that is crucial for numerous living processes ranging from bacterial swarming to sorting of cells in dense tissues. Here, we systematically investigate the effect of spatially varying potential acting along one direction and packing fraction on the binary mixture of particles with different diffusivities. Our results indicate that the presence of an external potential promotes demixing over a larger range of packing fractions, while also fostering a more pronounced 'hexatic order' within the bands of less diffusive "cold") particles formed near the minima of the potential. The mean-squared displacements (MSD) of "cold" and "hot" particles in different directions exhibit a distinct behavior. In contrast to the long-time sub-diffusive behavior of the "cold" particles, the "hot" ones display diffusive nature following an intermediate plateau. However, in the direction transverse to the applied potential, both types of particles undergo normal diffusion. Furthermore, interesting non-Gaussian characteristics are observed, corresponding to the spatial distribution of the displacement of "hot" and "cold" particles. Interestingly, our results reveal the formation of a 'percolating band', and the emergence of such dynamic anisotropy is not observed in the absence of an external potential. These aspects are highly relevant to the dynamics of various systems-including densely packed tissues, bacterial motility in confined spaces, and granular segregation in the pharmaceutical industry. △ Less

Submitted 28 May, 2026; originally announced May 2026.

Generalized Shift Vector as the Intrinsic Dipole of Many-Body Correlated Electronic States

Authors: Jiaming Hu, Sudipta Kundu, Zhichao Guo, Joshua J. P. Thompson, Wenbin Li, Hua Wang, Bartomeu Monserrat

Abstract: Shift vectors play a central role in nonlinear optics and transport phenomena, where they are usually understood as charge-center shifts associated with transitions between quantum states. Here we show that the same geometric structure can be more fundamentally understood as the intrinsic dipole moment of a single correlated state. Our derivation clarifies the local and global aspects of gauge inv… ▽ More Shift vectors play a central role in nonlinear optics and transport phenomena, where they are usually understood as charge-center shifts associated with transitions between quantum states. Here we show that the same geometric structure can be more fundamentally understood as the intrinsic dipole moment of a single correlated state. Our derivation clarifies the local and global aspects of gauge invariance, the origin of the phase-gradient term, and its connection to the internal coherence structure of many-body correlations. The single-state shift character appears both as a displacement of the real-space joint probability density and as a linear electric-field modification in energy space. Applying this framework to optically induced correlations, electron-phonon-mediated processes, and excitonic electron-hole states, we recover previously proposed shift vectors and the standard expression for the shift current as special cases. Our results establish a common physical foundation for shift vectors as intrinsic dipolar properties of correlated electronic states. △ Less

Submitted 7 June, 2026; v1 submitted 22 May, 2026; originally announced May 2026.

Comments: 17 pages, 2 figures

Transition Metal Dichalcogenide Excitons in Periodic Electrostatic Potentials: Center-of-Mass Models

Authors: Jose M. Torres-Lopez, Sudipta Kundu, Felipe H. da Jornada, Tony Heinz, Allan H. MacDonald

Abstract: Two-dimensional (2D) van-der-Waals materials are a promising platform for exciton state engineering. In this paper, we study the properties of excitons in 2D group VI transition-metal dichalcogenide (TMD) semiconductors that are modified by a periodic electrostatic potential through the quadratic Stark effect. Using a model that retains only center-of-mass and valley degrees-of-freedom, we find th… ▽ More Two-dimensional (2D) van-der-Waals materials are a promising platform for exciton state engineering. In this paper, we study the properties of excitons in 2D group VI transition-metal dichalcogenide (TMD) semiconductors that are modified by a periodic electrostatic potential through the quadratic Stark effect. Using a model that retains only center-of-mass and valley degrees-of-freedom, we find that electrostatic potentials can drive optical valley splitting up to 10meVs and induce valley selective exciton dispersion. We explain why both properties are sensitive to the rotational symmetry of the electrostatic trapping potential using a combination of numerical results and analytical approximations. An important consequence of valley-splitting is that the lowest exciton band is non-degenerate and has a linear dispersion around $γ$ that is expected to suppress thermal excitations, allowing true Bose condensation and superfluidity of excitons in two space dimensions. △ Less

Submitted 6 May, 2026; v1 submitted 4 May, 2026; originally announced May 2026.

Comments: 10 pages, 10 figures

Adsorption energies and decomposition barrier heights for ethylene carbonate on the surface of lithium from cluster-based quantum chemistry

Authors: Ethan A. Vo, Hung T. Vuong, Zachary K. Goldsmith, Hong-Zhou Ye, Yujing Wei, Sohang Kundu, Ardavan Farahvash, Garvit Agarwal, Richard A. Friesner, Timothy C. Berkelbach

Abstract: For ethylene carbonate on the (100) surface of lithium, we calculate the adsorption energy in two binding motifs as well as the barrier height for a ring-opening decomposition reaction. We validate a scheme for producing results in the thermodynamic limit by correcting results obtained on finite lithium clusters containing only 40-100 atoms, which enables the use of hybrid density functionals, the… ▽ More For ethylene carbonate on the (100) surface of lithium, we calculate the adsorption energy in two binding motifs as well as the barrier height for a ring-opening decomposition reaction. We validate a scheme for producing results in the thermodynamic limit by correcting results obtained on finite lithium clusters containing only 40-100 atoms, which enables the use of hybrid density functionals, the random-phase approximation, and correlated wavefunction theories such as coupled-cluster theory and auxiliary-field quantum Monte Carlo. We find that the high-level theories agree to within 2-5 kcal/mol and can therefore serve as benchmarks for more affordable methods. Using our reference data, we demonstrate that generalized gradient approximation functionals, such as PBE, are not sufficiently accurate for reaction barrier heights, and we identify $ω$B97X-V as an especially promising functional for the interfacial chemistry of electrolyte solvents at lithium metal anodes. △ Less

Submitted 23 March, 2026; originally announced March 2026.

Comments: 7 pages, 4 figures, plus Supplementary Material

Vacancy-induced local moments in quantum paramagnetic phases: An SU($N$) designer Hamiltonian study

Authors: Md Zahid Ansari, Souvik Kundu, Kedar Damle

Abstract: We explore the effects of non-magnetic impurities (vacancy disorder) on the quantum paramagnetic phases stabilized by SU($N$) designer Hamiltonians on bipartite lattices. Using the results of our quantum Monte Carlo simulations, we demonstrate that isolated vacancies seed emergent spin $S=1/2$ moments in their vicinity when the low-temperature state has valence bond solid order. Indeed, our quantu… ▽ More We explore the effects of non-magnetic impurities (vacancy disorder) on the quantum paramagnetic phases stabilized by SU($N$) designer Hamiltonians on bipartite lattices. Using the results of our quantum Monte Carlo simulations, we demonstrate that isolated vacancies seed emergent spin $S=1/2$ moments in their vicinity when the low-temperature state has valence bond solid order. Indeed, our quantum Monte Carlo results for the low-temperature susceptibility in such regimes shows clear evidence of the vacancy-induced Curie tails associated with these emergent moments, and our zero-temperature projector Monte Carlo results on the ground-state wavefunction in the valence bond basis provide additional evidence in support of this picture. Further, for such designer Hamiltonians on the Lieb lattice with two additional sites on each bond of a square lattice, we identify a low-temperature spin liquid-like regime with no sign of spin or valence bond order. This liquid-like regime serves as a test bed for validating a recently-developed argument concerning the effects of vacancy disorder in such low temperature regimes. Consistent with this argument, we find that isolated vacancies do not seed emergent local moments in such spin liquids. Instead, in the presence of vacancy disorder, emergent local moments are associated with the presence of monomers in maximum-density dimer packings of the corresponding diluted lattice. △ Less

Submitted 27 February, 2026; originally announced February 2026.

Comments: 11 pages, 9 figures

Arrested Relaxation in a Disorder-Free Coulomb Spin Liquid

Authors: Souvik Kundu, Arnab Seth, Sthitadhi Roy, Subhro Bhattacharjee, Roderich Moessner

Abstract: We investigate Coulomb spin liquids in classical spin-3/2 ice and show that the enlarged on-site Hilbert space gives rise to a qualitatively new class of such phases. Beyond the conventional magnetic monopoles of spin-1/2 ice, the system hosts additional low-energy crystal-field excitations, whose interplay with monopoles significantly modifies both equilibrium and non-equilibrium properties. Foll… ▽ More We investigate Coulomb spin liquids in classical spin-3/2 ice and show that the enlarged on-site Hilbert space gives rise to a qualitatively new class of such phases. Beyond the conventional magnetic monopoles of spin-1/2 ice, the system hosts additional low-energy crystal-field excitations, whose interplay with monopoles significantly modifies both equilibrium and non-equilibrium properties. Following a thermal quench, we find a pronounced dynamical arrest manifested in an exponentially long-lived {athermal} plateau in spin autocorrelations. This constitutes a rare example of dynamical arrest in a short-range interacting, disorder-free system. We demonstrate that the arrested dynamics originate from novel composite excitation structures unique to spin-3/2 ice and from kinetically constrained relaxation pathways that require activated processes. Our results establish higher-spin ice as a fertile platform for realising unconventional Coulomb spin liquids and dynamical arrest without quenched disorder. △ Less

Submitted 26 February, 2026; originally announced February 2026.

Comments: 8 pages, 9 figures + Supplementary Material (2 pages)

$Z_3$ confined and deconfined Coulomb liquids in $S_{\rm eff} = 3/2$ pyrochlore magnets

Authors: Jay Pandey, Souvik Kundu, Kedar Damle

Abstract: We identify an interesting regime in the physics of pyrochlore magnets in which spin-orbit and crystal field effects lead to {\em two} low-lying magnetic doublets that can be modeled as an effective spin $S=3/2$ degree of freedom that sees a dominant easy-axis antiferromagnetic exchange $J>0$ favoring the local $[111]$ axes, which competes with a comparably strong single-ion anisotropy $Δ= J+μ/2$… ▽ More We identify an interesting regime in the physics of pyrochlore magnets in which spin-orbit and crystal field effects lead to {\em two} low-lying magnetic doublets that can be modeled as an effective spin $S=3/2$ degree of freedom that sees a dominant easy-axis antiferromagnetic exchange $J>0$ favoring the local $[111]$ axes, which competes with a comparably strong single-ion anisotropy $Δ= J+μ/2$ (with $|μ| \ll J$) favoring the perpendicular planes. For a precise analysis, we study the $T/J \rightarrow 0$ limit in which $w \equiv \exp(-μ/T)$ is the control variable. In this limit, we find {\em two topologically distinct} zero-field Coulomb phases separated by a first-order $Z_3$ confinement transition at $w_c \approx 2.02$. Both Coulomb phases admit a description in terms of the fluctuations of a coarse-grained divergence-free polarization field. However, the flux of this polarization field is restricted to integer multiples of $3$, and only charges that are multiples of 3 are deconfined in one of these phases, while all integer fluxes are allowed and all integer charges are deconfined in the other phase. Experimental systems with small negative $μ$ ({\em i.e.}, $-J \ll μ< 0$) are therefore predicted to exhibit signatures of this topological transition when cooled below $T_c \approx 1.42|μ|$. △ Less

Submitted 26 February, 2026; originally announced February 2026.

Comments: 5 2-column pages and a 2-page appendix

A geometric basis for materials families in inorganic solids

Authors: Justin Tahmassebpur, Sarvesh Chaudhari, Cristóbal Méndez, Rushil Choudhary, Sudipta Kundu, Raymond E. Schaak, Héctor Abruña, Peter Frazier, Tomás Arias

Abstract: The thermodynamic stability of inorganic solids spans a vast compositional space, yet materials scientists have long organized their intuition around a manageable number of materials families. Here we show that this organization has a precise geometric basis. The formation-energy convex hull of all inorganic compounds from the Materials Project, spanning 92-dimensional elemental composition space,… ▽ More The thermodynamic stability of inorganic solids spans a vast compositional space, yet materials scientists have long organized their intuition around a manageable number of materials families. Here we show that this organization has a precise geometric basis. The formation-energy convex hull of all inorganic compounds from the Materials Project, spanning 92-dimensional elemental composition space, is captured to near DFT accuracy by a polyhedron with only seven facets. Each facet corresponds to a family of materials sharing similar chemical potentials. This low-dimensional structure is not merely an economical description of energies: without retraining or structural input, the same framework reproduces trends in DFT-calculated defect energies and elemental spatial correlations in high-entropy nanoparticles. These results reveal that a small number of material families, corresponding to geometric features of composition-energy space, govern bulk stability, defect energetics, and elemental mixing, and provide a unified, interpretable framework for rapid screening across diverse materials systems. △ Less

Submitted 13 June, 2026; v1 submitted 30 January, 2026; originally announced February 2026.

Multispectral UV Imaging on Capacitive CMOS Arrays Enabled by Solution-Processed Metal-Oxide Nanoparticles

Authors: Suman Kundu, Tao Shen, Kai Betlem, Murali K Ghatkesar, Peter G Steeneken, Frans P Widdershoven

Abstract: Ultraviolet (UV) imagers are important for a variety of applications, such as quality inspection in the semiconductor industry, forensics and food quality inspection, but are often costly because they require dedicated semiconductor process flows. Here, an imaging chip is introduced that has been fabricated using standard 40 nm complementary metal-oxidesemiconductor (CMOS) technology. Instead of u… ▽ More Ultraviolet (UV) imagers are important for a variety of applications, such as quality inspection in the semiconductor industry, forensics and food quality inspection, but are often costly because they require dedicated semiconductor process flows. Here, an imaging chip is introduced that has been fabricated using standard 40 nm complementary metal-oxidesemiconductor (CMOS) technology. Instead of using a conventional charge-based photodetection principle, the imager uses a capacitive operation principle where UV-light causes capacitance changes via the photodielectric effect in a functionalization layer, which are measured by the underlying CMOS circuitry. This spin-coated or inkjet-printed functionalization layer consists of solution-processed, wide-bandgap, semiconducting metaloxide nanoparticles, and facilitates multispectral imaging. The sensors exhibit low noiseequivalent powers (17-138 fW Hz^-1/2) across the UV bands. Unlike conventional silicon CMOS imagers, the present capacitive-CMOS platform is inherently visible-blind, providing selective UV detection. This work positions late-functionalized capacitive-CMOS arrays as a route toward reducing the cost of UV imagers, which can lead to their more widespread implementation in consumer and low-volume application-specific products. △ Less

Submitted 1 January, 2026; originally announced January 2026.

Stretching helical molecular springs: the peculiar evolution of electron transport in helicene junctions

Authors: Anil Kumar Singh, Yuta Ito, León Martin, Lukas Krieger, Matea Sršen, Stephan Korsager Pedersen, Axel Houssin, Satyaki Kundu, Carlos Sabater, Narcis Avarvari, Michael Pittelkow, Fabian Pauly, Oren Tal

Abstract: Single-molecule junctions represent electromechanical systems at the edge of device miniaturization. Despite extensive studies on the interplay between mechanical manipulation and electron transport in molecular junctions, a thorough understanding of conducting molecular springs remains elusive. Here, we investigate the impact of mechanical elongation and compression on the electron transport and… ▽ More Single-molecule junctions represent electromechanical systems at the edge of device miniaturization. Despite extensive studies on the interplay between mechanical manipulation and electron transport in molecular junctions, a thorough understanding of conducting molecular springs remains elusive. Here, we investigate the impact of mechanical elongation and compression on the electron transport and electronic structure of helicene-based spring-like single-molecule junctions, utilizing 2,2'-dithiol-[6]helicene and thioacetyl-[13]helicene molecules bridging two gold electrodes. We observe robust, reversible U-shaped conductance variations with interelectrode distance. Ab-initio electronic structure and quantum transport calculations reveal that this behavior stems from destructive quantum interference, induced mainly by modifications of the coupling at the metal-molecule interface as a peculiar outcome of the helical backbone deformation. These findings highlight the central role of the helical geometry in combination with contact properties in the electromechanical response of conducting molecular springs, offering insights for designing functional electromechanical devices that leverage similar mechanisms. △ Less

Submitted 25 November, 2025; originally announced November 2025.

Comments: 16 pages, 5 figures, 1 table

A light-induced charge order mode in a metastable cuprate ladder

Authors: Hari Padma, Prakash Sharma, Sophia F. R. TenHuisen, Filippo Glerean, Antoine Roll, Pan Zhou, Sarbajaya Kundu, Arnau Romaguera, Elizabeth Skoropata, Hiroki Ueda, Biaolong Liu, Eugenio Paris, Yu Wang, Seng Huat Lee, Zhiqiang Mao, Mark P. M. Dean, Edwin W. Huang, Elia Razzoli, Yao Wang, Matteo Mitrano

Abstract: We report the observation of an emergent charge order mode in the optically-excited cuprate ladder Sr$_{14}$Cu$_{24}$O$_{41}$. Near-infrared light in the ladder plane drives a symmetry-protected electronic metastable state together with a partial melting of the equilibrium charge order. Our time-resolved resonant inelastic x-ray scattering measurements at the upper Hubbard band reveal a gapless co… ▽ More We report the observation of an emergent charge order mode in the optically-excited cuprate ladder Sr$_{14}$Cu$_{24}$O$_{41}$. Near-infrared light in the ladder plane drives a symmetry-protected electronic metastable state together with a partial melting of the equilibrium charge order. Our time-resolved resonant inelastic x-ray scattering measurements at the upper Hubbard band reveal a gapless collective excitation dispersing from the charge-order wavevector up to 0.8 eV with a slope on the order of the quasiparticle velocity. These findings reveal a regime where correlated carriers acquire itinerant character at finite momentum, and charge order becomes dynamically fluctuating, offering a platform to explore light-induced pairing instabilities. △ Less

Submitted 28 October, 2025; originally announced October 2025.

Comments: Main Text with 8 pages, 3 figures, and Supplementary Material with 13 pages, 7 figures

Journal ref: Physical Review Letters 136, 196501 (2026)

Flux confinement-deconfinement transition of dimer-loop models on three-dimensional bipartite lattices

Authors: Souvik Kundu, Kedar Damle

Abstract: Motivated by recent work that mapped the low-temperature properties of a class of frustrated spin $S=1$ kagome antiferromagnets with competing exchange and single-ion anisotropies to the fully-packed limit (with each vertex touched by exactly one dimer or nontrivial loop) of a system of dimers and nontrivial (length $s > 2$) loops on the honeycomb lattice, we study this fully-packed dimer-loop mod… ▽ More Motivated by recent work that mapped the low-temperature properties of a class of frustrated spin $S=1$ kagome antiferromagnets with competing exchange and single-ion anisotropies to the fully-packed limit (with each vertex touched by exactly one dimer or nontrivial loop) of a system of dimers and nontrivial (length $s > 2$) loops on the honeycomb lattice, we study this fully-packed dimer-loop model on the three-dimensional bipartite cubic and diamond lattices as a function of $w$, the relative fugacity of dimers. We find that the $w \rightarrow 0$ O($1$) loop-model limit is separated from the $w \rightarrow \infty$ dimer limit by a geometric phase transition at a nonzero finite critical fugacity $w_c$: The $w>w_c$ phase has short loops with an exponentially decaying loop-size distribution, while the $w<w_c$ phase is dominated by large loops whose loop-size distribution is governed by universal properties of the critical O($1$) loop soup. This transition separates two {\em distinct} Coulomb liquid phases of the system: Both phases admit a description in terms of a fluctuating divergence-free polarization field $P_μ(\mathbf{r})$ on links of the lattice and are characterized by dipolar correlations at long distances. The transition at $w_c$ is a flux confinement-deconfinement transition. Equivalently, and independent of boundary conditions, half-integer test charges $q=\pm 1/2$ are confined for $w>w_c$, but become deconfined in the small-$w$ phase. Although both phases are unstable to a nonzero fugacity for the charge $\pm 1/2$ excitations, the destruction of the $w >w_c$ Coulomb liquid is characterized by an interesting slow crossover, since test charges with $q=\pm 1/2$ are confined in this phase. △ Less

Submitted 13 October, 2025; originally announced October 2025.

Emulating microbial run-and-tumble and tactic motion by stochastically reorienting synthetic active Brownian particles

Authors: Sandip Kundu, Dibyendu Mondal, Arup Biswas, Arnab Pal, Manas Khan

Abstract: Replicating efficient and adaptable microbial navigation strategies, such as run and tumble (RnT) and tactic motions to synthetic active agents has been an enduring quest. To this end, we introduce a stochastic orientational reset (SOR) protocol, in which the propulsion direction of an active Brownian particle (ABP) is reassigned to a random orientation within a defined reset-cone. When the reset-… ▽ More Replicating efficient and adaptable microbial navigation strategies, such as run and tumble (RnT) and tactic motions to synthetic active agents has been an enduring quest. To this end, we introduce a stochastic orientational reset (SOR) protocol, in which the propulsion direction of an active Brownian particle (ABP) is reassigned to a random orientation within a defined reset-cone. When the reset-cone is aligned with the instantaneous propulsion direction, ABPs reproduce the RnT dynamics of E. coli; when set along an attractant gradient, they exhibit taxis - with extensive adaptability in persistence through the angular width of the reset-cone and reset rate. We establish the robustness of this protocol across a broad range of swimming speeds using experiments, simulations, and analytical theory. △ Less

Submitted 26 September, 2025; originally announced September 2025.

Comments: 9 pages, 7 figures, and Supplementary materials

Reaction dynamics of lithium-mediated electrolyte decomposition using machine learning potentials

Authors: Sohang Kundu, Diana Chamaki, Hong-Zhou Ye, Garvit Agarwal, Timothy C. Berkelbach

Abstract: We study the ring-opening decomposition of ethylene carbonate in the presence of a single lithium atom and on the surface of lithium metal. Combining accurate electronic structure theory, enhanced sampling, and machine learning, we fine-tune the MACE-MP0 foundation model and apply the resulting machine learning potentials to obtain statistically converged free energy profiles and reaction rates. W… ▽ More We study the ring-opening decomposition of ethylene carbonate in the presence of a single lithium atom and on the surface of lithium metal. Combining accurate electronic structure theory, enhanced sampling, and machine learning, we fine-tune the MACE-MP0 foundation model and apply the resulting machine learning potentials to obtain statistically converged free energy profiles and reaction rates. We confirm that the level of electronic structure theory is important, and inaccurate density functionals can overestimate the reaction rate by up to nine orders of magnitude. We also find that harmonic transition state theory underestimates reaction rates by about one order of magnitude. For the surface reaction, we find and characterize a new, ultrafast decomposition pathway wherein the carbonyl is deeply inserted into the lithium surface and bent by about 70$^\circ$. This reaction, which occurs in a few tens of picoseconds, generates a ring-opened intermediate that is a precursor for CO or CO$_2$ formation; by contrast, an alternative pathway that yields CO$_3^{2-}$ and ethylene is found to be non-competitive, occurring on a timescale of tens of nanoseconds. △ Less

Submitted 17 September, 2025; originally announced September 2025.

Comments: 7 pages, 4 figures

Direct Observation of the Lindhard Continuum using Resonant Inelastic X-ray Scattering

Authors: Eder G. Lomeli, Sarbajaya Kundu, Yi-De Chuang, Zengqing Zhuo, Ke Chen, Xiaoxing Xi, Lingjia Shen, Georgi L. Dakovski, Stephan Geprägs, Brian Moritz, Thomas P. Devereaux, John Vinson, Matthias F. Kling, Edwin W. Huang, Daniel Jost

Abstract: Understanding the excitations of quantum materials is essential for unraveling how their microscopic constituents interact. Among these, particle-hole excitations form a particularly important class, as they govern fundamental processes such as screening, dissipation, and transport. In metals, the continuum of electron-hole excitations is described by the Lindhard function. Although central to the… ▽ More Understanding the excitations of quantum materials is essential for unraveling how their microscopic constituents interact. Among these, particle-hole excitations form a particularly important class, as they govern fundamental processes such as screening, dissipation, and transport. In metals, the continuum of electron-hole excitations is described by the Lindhard function. Although central to the theory of Fermi liquids, the corresponding Lindhard continuum has remained experimentally elusive. Here, we report its direct observation in the weakly correlated metal MgB$_{2}$ using ultra-soft resonant inelastic X-ray scattering (RIXS). We resolve a linearly dispersing excitation with velocity comparable to the Fermi velocity and find quantitative agreement with simulations of the non-interacting charge susceptibility. A detailed analysis and decomposition of the simulations reveal the intra-band origin of this low-energy excitation, confirming it as the Lindhard continuum. Our results establish ultra-soft RIXS as a momentum-resolved probe of the fermiology in metals and call for deeper investigations of continuum features in RIXS and related spectroscopy of other materials beyond MgB$_{2}$. △ Less

Submitted 12 September, 2025; originally announced September 2025.

Spin liquid state in a three-dimensional pyrochlore-like frustrated magnet

Authors: U. Jena, S. Kundu, Suheon Lee, Q. Faure, F. Damay, S. Rols, Adam Berlie, S. Petit, Kwang-Yong Choi, P. Khuntia

Abstract: The three-dimensional frustrated spin lattice in MgCrGaO4, where Cr3+ ions occupy a pyrochlore-like network, exemplifies a quantum magnet with competing interactions, macroscopic degeneracy, and exotic low-energy excitations. Using thermodynamic, electron spin resonance (ESR), muon spin relaxation (muSR), and inelastic neutron scattering (INS) techniques, we observe no magnetic order or spin freez… ▽ More The three-dimensional frustrated spin lattice in MgCrGaO4, where Cr3+ ions occupy a pyrochlore-like network, exemplifies a quantum magnet with competing interactions, macroscopic degeneracy, and exotic low-energy excitations. Using thermodynamic, electron spin resonance (ESR), muon spin relaxation (muSR), and inelastic neutron scattering (INS) techniques, we observe no magnetic order or spin freezing down to 57 mK, despite a sizable exchange interaction (J= 58 K) between Cr3+ (S=3/2) moments and inherent site disorder. Below the characteristic exchange energy scale, all experimental probes detect the emergence of antiferromagnetic short-range spin correlations, corroborated by magnetic diffuse scattering in the wave vector dependence of low-energy magnetic excitations centered on Q = 1.5 A^-1 in inelastic neutron scattering experiments. The low-temperature specific heat follows a near-quadratic dependence without a gap, consistent with algebraic spin correlations. These results establish MgCrGaO4 as a rare three-dimensional classical spin liquid featuring a highly degenerate ground-state manifold and gapless excitations, offering a strong impetus for the experimental realization of spin liquids in higher-dimensional frustrated quantum magnets. △ Less

Submitted 7 July, 2025; originally announced July 2025.

Journal ref: Phys. Rev. B (Letter), 113, L100404 (2026)

Variational autoencoders understand knot topology

Authors: Anna Braghetto, Sumanta Kundu, Marco Baiesi, Enzo Orlandini

Abstract: Supervised machine learning (ML) methods are emerging as valid alternatives to standard mathematical methods for identifying knots in long, collapsed polymers. Here, we introduce a hybrid supervised/unsupervised ML approach for knot classification based on a variational autoencoder enhanced with a knot type classifier (VAEC). The neat organization of knots in its latent representation suggests tha… ▽ More Supervised machine learning (ML) methods are emerging as valid alternatives to standard mathematical methods for identifying knots in long, collapsed polymers. Here, we introduce a hybrid supervised/unsupervised ML approach for knot classification based on a variational autoencoder enhanced with a knot type classifier (VAEC). The neat organization of knots in its latent representation suggests that the VAEC, only based on an arbitrary labeling of three-dimensional configurations, has grasped complex topological concepts such as chirality, unknotting number, braid index, and the grouping in families such as achiral, torus, and twist knots. The understanding of topological concepts is confirmed by the ability of the VAEC to distinguish the chirality of knots $9_{42}$ and $10_{71}$ not used for its training and with a notoriously undetected chirality to standard tools. The well-organized latent space is also key for generating configurations with the decoder that reliably preserves the topology of the input ones. Our findings demonstrate the ability of a hybrid supervised-generative ML algorithm to capture different topological features of entangled filaments and to exploit this knowledge to faithfully reconstruct or produce new knotted configurations without simulations. △ Less

Submitted 5 April, 2025; originally announced April 2025.

Comments: 10 pages

Journal ref: Phys. Rev. E 112, 025418 (2025)

How to Reliably Measure Carrier Mobility in Highly Resistive Lead Halide Perovskites with Photo-Hall Experiment

Authors: Soumen Kundu, Yeswanth Pattipati, Krishnamachari Lakshmi Narasimhan, Sushobhan Avasthi

Abstract: Mobility measurements in highly resistive methylammonium lead iodide (MAPI) are challenging due to high impedance, ion drift, and low mobility. We show that we can address the challenge using intensity-dependent photo-Hall measurements. The key is an improved photo-Hall setup, which enables reliable Hall measurements in the dark and under low-intensity illumination. By tuning the illumination over… ▽ More Mobility measurements in highly resistive methylammonium lead iodide (MAPI) are challenging due to high impedance, ion drift, and low mobility. We show that we can address the challenge using intensity-dependent photo-Hall measurements. The key is an improved photo-Hall setup, which enables reliable Hall measurements in the dark and under low-intensity illumination. By tuning the illumination over four orders of magnitude, we get the additional information to simultaneously extract hole mobility, electron mobility, and background doping. For the first time, we show that a MAPI single crystal, exhibiting n-type behaviour in the dark, can show p-type behaviour under light due to the difference in hole and electron mobility. The data partly explains the variability in mobility reported in the literature. We show that one can erroneously extract any mobility from 0 to 25 cm2/Vs if we restrict the experiment to a small range of illumination intensities. For our MAPI (310) crystal, the measured hole and electron mobility is 40 cm2/Vs and 25.5 cm2/Vs, respectively. △ Less

Submitted 2 April, 2025; originally announced April 2025.

Comments: 45 pages (including supplementary) 4 main figures, 16 supplementary figures

Evidence of Athermal Metastable Phase in a Halide Perovskite: Optically Tracked Thermal-Breach Memory

Authors: Kingshuk Mukhuti, Satyaki Kundu, Debasmita Pariari, Deepesh Kalauni, Ashutosh Mohanty, Aniket Bajaj, D. D. Sarma, Bhavtosh Bansal

Abstract: Halide perovskite materials have been extensively studied in the last decade because of their impressive optoelectronic properties. However, their one characteristic that is uncommon for semiconductors is that many undergo thermally induced structural phase transitions. The transition is hysteretic, with the hysteresis window marking the boundary of the metastable phase. We have discovered that in… ▽ More Halide perovskite materials have been extensively studied in the last decade because of their impressive optoelectronic properties. However, their one characteristic that is uncommon for semiconductors is that many undergo thermally induced structural phase transitions. The transition is hysteretic, with the hysteresis window marking the boundary of the metastable phase. We have discovered that in methylammonium lead iodide, this hysteretic metastable phase is athermal, meaning it shows almost no temporal phase evolution under isothermal conditions. We also show that a large number of distinguishable metastable states can be prepared following different thermal pathways. Furthermore, under a reversible thermal perturbation, the states in the metastable phase either show return-point memory or undergo a systematic nonrecoverable phase evolution, depending on the thermal history and the sign of the temperature perturbation. Since the phase fraction can be probed with extreme sensitivity via luminescence, we have an optically retrievable memory that reliably records any breach in temperature stability. Such thermal-breach memory in athermal martensites, of which there are numerous examples, may be useful for tagging packages requiring strict temperature control during transportation or preservation. △ Less

Submitted 6 February, 2025; originally announced February 2025.

Comments: 21 Pages, 21 figures

A novel Gapless Quantum Spin Liquid in the S = 1 4d4-honeycomb material Cu$_3$LiRu$_2$O$_6$

Authors: Sanjay Bachhar, Nashra Pistawala, S. Kundu, Maneesha Barik, M. Baenitz, Jorg Sichelschmidt, Koji Yokoyama, P. Khuntia, Surjeet Singh, A. V. Mahajan

Abstract: We report the discovery of a novel gapless quantum spin liquid in the S=1 honeycomb system Cu$_3$LiRu$_2$O$_6$ with Ru$^{4+}$ ($4d^4$) where moments remain dynamic down to 50 mK. Heat capacity measurements show no sign of magnetic ordering down to 60 mK in spite of a Curie-Weiss temperature = -222 K indicating a strong antiferromagnetic interaction. In zero field, magnetic heat capacity shows a li… ▽ More We report the discovery of a novel gapless quantum spin liquid in the S=1 honeycomb system Cu$_3$LiRu$_2$O$_6$ with Ru$^{4+}$ ($4d^4$) where moments remain dynamic down to 50 mK. Heat capacity measurements show no sign of magnetic ordering down to 60 mK in spite of a Curie-Weiss temperature = -222 K indicating a strong antiferromagnetic interaction. In zero field, magnetic heat capacity shows a linear T-dependence with Sommerfeld coefficient = 107 mJ/mol K$^2$ is much larger than that found in typical Fermi liquids. Our local probe $^7$Li nuclear magnetic resonance (NMR) measurements find a significant temperature-independent $^7$Li NMR shift (and hence a non-zero spin susceptibility) at low-T and a linear T-variation of the $^7$Li NMR spin-lattice relaxation rate 1/T$_1$ at low-T reminiscent of fermionic excitations. Muon spin relaxation measurements detect neither long-range ordering nor spin freezing down to 50 mK and the temperature variation of the muon depolarization rate shows a gradual increase with decreasing temperature and a leveling off below about 1 K evincing a persistent spin dynamics common to several spin liquid candidates. Our results provide strong signatures of a quantum spin liquid in the titled honeycomb material. △ Less

Submitted 28 January, 2025; originally announced January 2025.

Comments: 5 pages and 6 figures in main, 8 pages and 16 figures in the supplement. arXiv admin note: text overlap with arXiv:2405.06380

Phase behavior of hard sheared cube family

Authors: Kaustav Chakraborty, Sumitava Kundu, Avisek Das

Abstract: A sheared cube is made out of a cube by giving a shear to the body in one direction keeping one of the faces fixed. We investigate here the thermodynamic phase behavior of a family of such regular hard sheared cubes, each of the members of the family having a distinct angle made by the faces with the perpendicular on the fixed face. Hard particle Monte Carlo (HPMC) has been performed with these an… ▽ More A sheared cube is made out of a cube by giving a shear to the body in one direction keeping one of the faces fixed. We investigate here the thermodynamic phase behavior of a family of such regular hard sheared cubes, each of the members of the family having a distinct angle made by the faces with the perpendicular on the fixed face. Hard particle Monte Carlo (HPMC) has been performed with these anisotropic building blocks resulting entropy-driven self assembly. Thereby computational evidence of discrete plastic crystal phase has been found in crystal. The discrete plastic crystal phase is known to form through the spontaneous self-assembly of certain polyhedra. Throughout the entire solid regime particle orientations exhibit strong specific correlations before melting into a liquid, without any evidence of freely rotating plastic crystal at lower density solid. It has been thoroughly observed that geometrical attributes of the shapes don't determine any of the properties that designate this orientational disorder phase reported here. We also find that particle's rotational symmetric axes and one of the rotational symmetric axes of the unit cell of the crystal have a strong relationship in their alignment in space. These results, achieved with shapes having crystallographic point group symmetry, are investigated as being consistent with the phenomenology of discrete plastic crystal phase established in earlier works with hard particles having non-crystallographic point group symmetry. △ Less

Submitted 13 December, 2024; originally announced December 2024.

Comments: 37 pages, 65 references and 11 figures in the main text. 4 pages, 4 figures in the supplementary information

The effects of network architecture on the photomechanical performance of azo-acrylate liquid crystal elastomers

Authors: Anastasiia Svanidze, Sudarshan Kundu, Olena Iadlovska, Anil K. Thakur, Xiaoyu Zheng, Peter Palffy-Muhoray

Abstract: Azo-containing liquid crystal elatomers are photomechanical materials which can be actuated by illumination. The photomechanical response is a result of the photoisomerization of the azo moiety, which produces bulk stresses in the material. These stresses arise via two distinct and competing mechanisms: order parameter change induced stress and direct contractile stress. We describe thermomechanic… ▽ More Azo-containing liquid crystal elatomers are photomechanical materials which can be actuated by illumination. The photomechanical response is a result of the photoisomerization of the azo moiety, which produces bulk stresses in the material. These stresses arise via two distinct and competing mechanisms: order parameter change induced stress and direct contractile stress. We describe thermomechanical and photomechanical experiments aimed at assessing the relative contributions of these. we discuss our results and summarize our findings. △ Less

Submitted 7 December, 2024; originally announced December 2024.

Comments: 13 pages, 9 figures

Predictive orientational phase behavior in convex polyhedral entropic crystals

Authors: Sumitava Kundu, Kaustav Chakraborty, Avisek Das

Abstract: Hard convex polyhedra, idealized models for anisotropic colloids and nanoparticles, are known to form variety of orientational phases despite the regular arrangement of particles in the crystalline assemblies. Based on the orientational behavior of the constituents particles, such phases could be categorized into freely rotating plastic crystals (PC), discrete plastic crystals (DPC) and orientatio… ▽ More Hard convex polyhedra, idealized models for anisotropic colloids and nanoparticles, are known to form variety of orientational phases despite the regular arrangement of particles in the crystalline assemblies. Based on the orientational behavior of the constituents particles, such phases could be categorized into freely rotating plastic crystals (PC), discrete plastic crystals (DPC) and orientationally ordered crystals (OC). In this article, we report an extensive Monte Carlo computer simulation study of sixty hard convex polyhedral shape indicating a direct predictive relationship between the nature of orientational phases in the crystalline assemblies and single-particle shape attributes. The influence of three attributes namely; (i) Isoperimetric Quotient (IQ) i.e., the extent of asphericity; (ii) isotropy of the moment of inertia tensor in the principal frame and (iii) number of symmetry operations in the point group of the particle and self-assembled crystal structure, were observed to control the orientational phase behavior of the entire solid region in many-body system. The translational order in the crystal appeared to play significant role only in the DPC phase, where as, other two phases were completely governed by the combination of two attributes. In this study, the role of shape attributes were characterized by sequential appearance of one or two of the aforementioned rotational phases across the phase diagram in a pressure dependent manner which could be regarded as an important stepping stone towards fully predictive self-assembly behavior of hard particle systems. △ Less

Submitted 29 November, 2024; originally announced November 2024.

Comments: 27 pages article, 64 references, 5 figures, 5 pages supplementary information

Diabatic states of charge transfer with constrained charge equilibration

Authors: Sohang Kundu, Hong-Zhou Ye, Timothy C. Berkelbach

Abstract: Charge transfer (CT) processes that are electronically non-adiabatic are ubiquitous in chemistry, biology, and materials science, but their theoretical description requires diabatic states or adiabatic excited states. For complex systems, these latter states are more difficult to calculate than the adiabatic ground state. Here, we propose a simple method to obtain diabatic states, including energi… ▽ More Charge transfer (CT) processes that are electronically non-adiabatic are ubiquitous in chemistry, biology, and materials science, but their theoretical description requires diabatic states or adiabatic excited states. For complex systems, these latter states are more difficult to calculate than the adiabatic ground state. Here, we propose a simple method to obtain diabatic states, including energies and charges, by constraining the atomic charges within the charge equilibration framework. For two-state systems, the exact diabatic coupling can be determined, from which the adiabatic excited-state energy can also be calculated. The method can be viewed as an affordable alternative to constrained density functional theory (CDFT), and so we call it constrained charge equilibration (CQEq). We test the CQEq method on the anthracene-tetracyanoethylene CT complex and the reductive decomposition of ethylene carbonate on a lithium metal surface. We find that CQEq predicts diabatic energies, charges, and adiabatic excitation energies in good agreement with CDFT, and we propose that CQEq is promising for combination with machine learning force fields to study non-adiabatic CT in the condensed phase. △ Less

Submitted 7 November, 2024; originally announced November 2024.

Comments: 9 pages, 5 figures

Backbone Mediated Electrical Transport in a Double-Stranded DNA

Authors: Sourav Kundu, Siddhartha Lal

Abstract: In the field of DNA nanotechnology, it is common wisdom that charge transport occurs through the π stacked bases of double-stranded DNA. However, recent experimental findings by Zhuravel et. al. [Nat. Nanotech. 15, 836 (2020)] suggest that electronic transport happens through the backbone channels instead of π-π interaction of the nitrogen bases. These new experimental insights call for a detail i… ▽ More In the field of DNA nanotechnology, it is common wisdom that charge transport occurs through the π stacked bases of double-stranded DNA. However, recent experimental findings by Zhuravel et. al. [Nat. Nanotech. 15, 836 (2020)] suggest that electronic transport happens through the backbone channels instead of π-π interaction of the nitrogen bases. These new experimental insights call for a detail investigation. In keeping with this, we examine charge transport properties of three characteristic double-stranded DNA sequences (periodic GC, periodic AT and random ATGC sequences) within a tight-binding framework where backbones form the main conduction channels. Using techniques based on the Green function method, we inspect the single-particle density of states and localization properties of DNA in the presence of discontinuities (nicks) along the backbone channels. We also investigate the effect of these nicks on current-voltage response using the Landauer - Buttiker formalism for a two-terminal geometry where the source electrode is attached to one backbone strand and the drain to the other. We observe that the periodic DNA sequence of GC bases is metallic in nature, while the periodic AT sequence and the random ATGC sequence are insulating. Further, the effects of nicks on the transport properties of the periodic GC sequence is interesting: while a single nick on the upper backbone does not affect electronic transport, the addition of a second nick on the lower backbone causes the current to vanish altogether. This is found to be robust against changes in the positions of the nicks, as well as the alternation of the source and drain electrodes. △ Less

Submitted 30 December, 2024; v1 submitted 21 October, 2024; originally announced October 2024.

Optical Micromanipulation of Soft Materials: Applications in Devices and Technologies

Authors: Sanatan Halder, Debojit Chanda, Dibyendu Mondal, Sandip Kundu, Manas Khan

Abstract: Since its invention by Arthur Ashkin and colleagues at Bell Labs in the 1970s, optical micromanipulation, also known as optical tweezers or laser tweezers, has evolved remarkably to become one of the most convenient and versatile tools for studying soft materials, including biological systems. Arthur Ashkin received the Nobel Prize in Physics in 2018 for enabling these extraordinary scientific adv… ▽ More Since its invention by Arthur Ashkin and colleagues at Bell Labs in the 1970s, optical micromanipulation, also known as optical tweezers or laser tweezers, has evolved remarkably to become one of the most convenient and versatile tools for studying soft materials, including biological systems. Arthur Ashkin received the Nobel Prize in Physics in 2018 for enabling these extraordinary scientific advancements. Essentially, a focused laser beam is used to apply and measure minuscule forces from a few piconewtons to femtonewtons by utilizing light-matter interaction at mesoscopic length scales. Combined with advanced microscopy and position-sensing techniques, optical micromanipulations enable us to investigate diverse aspects of functional soft materials. These include studying mechanical responses through force-elongation measurements, examining the structural properties of complex fluids employing microrheology, analyzing chemical compositions using spectroscopy, and sorting cells through single-cell analysis. Furthermore, it is utilized in various soft-matter-based devices, such as laser scissors and optical motors in microfluidic channels. This chapter presents an overview of optical micromanipulation techniques by describing fundamental theories and explaining the design considerations of conventional single-trap and dual-trap setups as well as recent improvisations. We further discuss their capabilities and applications in probing exotic soft-matter systems and in developing widely utilized devices and technologies based on functional soft materials. △ Less

Submitted 22 July, 2024; originally announced July 2024.

Comments: 73 pages, 13 figures

Journal ref: Advanced Structured Materials, Springer Nature Singapore, 2024, pp 415-469

Role of symmetry in the orientationally disordered crystals of hard convex polyhedra

Authors: Sumitava Kundu, Kaustav Chakraborty, Avisek Das

Abstract: The crystalline solids with lack of orientational ordering of anisotropic particles serve the purpose of studying the disordered systems with many fundamental applications in contemporary research. Despite the orientational disorder, multiple unique orientations with fixed angular differences exist in the crystal structures giving rise of "discrete plastic crystal" phase where the particles jump d… ▽ More The crystalline solids with lack of orientational ordering of anisotropic particles serve the purpose of studying the disordered systems with many fundamental applications in contemporary research. Despite the orientational disorder, multiple unique orientations with fixed angular differences exist in the crystal structures giving rise of "discrete plastic crystal" phase where the particles jump discretely within the unique orientations. We report the computational evidence of the role of symmetries between polyhedral particles and respective crystalline structures in controlling the existence of such phase at comparatively higher range of packing fractions beyond the freely rotating plastic crystals. The point groups of the particle and crystal structure were found to be directly connected in terms of the parallel alignment between the highest order rotational symmetry axes of the particle point group and any rotational axes of crystallographic point group, as a characteristic feature of this phase giving rise of discrete orientations. Based on our previous research [Kundu \textit{et al.}, arXiv:2311.06799, 2023] and new findings reported here, this symmetry relationship appeared to occur at the unit cells of the crystal structures which acted as the source of correlation, where as, all previously reported conserved orientational attributes i.e., number of unique orientations with fixed angular differences, equal population densities within the unique orientations, could be thought as the signatures of correlation present in the entire system. This relationship appeared to control all the aspects of phase which might be useful to draw fundamental insights about the disordered phases with orientational correlation as well as designing the disorder in the crystals. △ Less

Submitted 31 October, 2024; v1 submitted 12 July, 2024; originally announced July 2024.

Comments: (33+8) pages, (7+6) figures, 69 references

Algorithmic detection of crystal structures from computer simulation data

Authors: Sumitava Kundu, Kaustav Chakraborty, Avisek Das

Abstract: Detection of crystal structures from particle positions of crystalline assemblies formed in computer simulations is an unsolved problem. The standard protocol, formulated in the reciprocal space, for structure determination from experimental diffraction data is not suitable for analysis of computer simulation data, after converting them to the Fourier space. There is a long history of attempts to… ▽ More Detection of crystal structures from particle positions of crystalline assemblies formed in computer simulations is an unsolved problem. The standard protocol, formulated in the reciprocal space, for structure determination from experimental diffraction data is not suitable for analysis of computer simulation data, after converting them to the Fourier space. There is a long history of attempts to tackle this problem by analyzing the system in the real space by using ideas of local neighbors and broken symmetries of the crystalline state. In this paper, we propose a heuristic solution to this problem by detecting all possible unit cells directly from particle coordinates obtained in a typical computer simulation. The method is based on well known facts about crystal structures, some of which are underutilized in the context of the current problem. These include, the symmetry of the coordination polyhedron and its empirical relationship with directions of lattice vectors for a simple Bravais lattice, and the fact that any complex crystal can be systematically decomposed into multiple Bravais lattices. By using these ideas, along with standard computational techniques like search, clustering and convex hull construction, we were able to handle complex basis and construct all crystallographically viable unit cells from the coordinates. The method is capable of handling statistical noise by employing certain cutoffs and deals with multicomponent systems in a transparent manner. We validated it on real Monte Carlo simulation data and variety of test systems, including crystals with tens of particles in the basis. Our heuristic algorithm, which requires minimal human intervention and computational resources, provides a solution to the long standing problem and would be beneficial to the wider communities of condensed matter physics and computational materials science. △ Less

Submitted 26 April, 2025; v1 submitted 11 July, 2024; originally announced July 2024.

Comments: 63 pages, 23 figures, 66 references

Determining the purity of single-helical proteins from electronic specific heat measurements

Authors: Sourav Kundu, Siddhartha Lal

Abstract: We present a theoretical investigation of the electronic specific heat (ESH) at constant volume (Cv) of single-helical proteins modeled within the tight-binding (TB) framework. We study the effects of helical symmetry, long-range hopping, environment and biological defects on thermal properties. We employ a general TB model to incorporate all parameters relevant to the helical structure of the pro… ▽ More We present a theoretical investigation of the electronic specific heat (ESH) at constant volume (Cv) of single-helical proteins modeled within the tight-binding (TB) framework. We study the effects of helical symmetry, long-range hopping, environment and biological defects on thermal properties. We employ a general TB model to incorporate all parameters relevant to the helical structure of the protein. In order to provide additional insights into our results for the ESH, we also study the electronic density of states for various disorder strengths. We observe that the variation of the specific heat with disorder is very different in low and high temperature regimes, though the variation of ESH with temperature possesses a universal pattern upon varying disorder strengths related to environmental effects. Lastly, we propose an interesting application of the ESH spectra of proteins. We show that by studying the ESH of single-helical proteins, one can distinguish a defective sample from a pure one. This observation can serve as the basis of a screening technique that can be applied prior to a whole genome testing, thereby saving valuable time & resources. △ Less

Submitted 21 May, 2024; originally announced May 2024.

Random Sequential Adsorption with Correlated Defects: A Series Expansion Approach

Authors: G Palacios, A M S Macêdo, Sumanta Kundu, M A F Gomes

Abstract: The Random Sequential Adsorption (RSA) problem holds crucial theoretical and practical significance, serving as a pivotal framework for understanding and optimizing particle packing in various scientific and technological applications. Here the problem of the one-dimensional RSA of k-mers onto a substrate with correlated defects controlled by uniform and power-law distributions is theoretically in… ▽ More The Random Sequential Adsorption (RSA) problem holds crucial theoretical and practical significance, serving as a pivotal framework for understanding and optimizing particle packing in various scientific and technological applications. Here the problem of the one-dimensional RSA of k-mers onto a substrate with correlated defects controlled by uniform and power-law distributions is theoretically investigated: the coverage fraction is obtained as a function of the density of defects and several scaling laws are examined. The results are compared with extensive Monte Carlo simulations and more traditional methods based on master equations. Emphasis is given in elucidating the scaling behavior of the fluctuations of the coverage fraction. The phenomenon of universality breaking and the issues of conventional gaussian fluctuations and the Lévy type fluctuations from a simple perspective, relying on the Central Limit Theorem, are also addressed. △ Less

Submitted 7 April, 2024; originally announced April 2024.

Field tuning Kitaev systems for spin fractionalization and topological order

Authors: Jagannath Das, Sarbajaya Kundu, Aman Kumar, Vikram Tripathi

Abstract: The honeycomb Kitaev model describes a $Z_2$ spin liquid with topological order and fractionalized excitations consisting of gapped $π$-fluxes and free Majorana fermions. Competing interactions, even when not very strong, are known to destabilize the Kitaev spin liquid. Magnetic fields are a convenient parameter for tuning between different phases of the Kitaev systems, and have even been investig… ▽ More The honeycomb Kitaev model describes a $Z_2$ spin liquid with topological order and fractionalized excitations consisting of gapped $π$-fluxes and free Majorana fermions. Competing interactions, even when not very strong, are known to destabilize the Kitaev spin liquid. Magnetic fields are a convenient parameter for tuning between different phases of the Kitaev systems, and have even been investigated for potentially counteracting the effects of other destabilizing interactions leading to a revival of the topological phase. Here we review the progress in understanding the effects of magnetic fields on some of the perturbed Kitaev systems, particularly on fractionalization and topological order. △ Less

Submitted 13 January, 2025; v1 submitted 29 March, 2024; originally announced March 2024.

Comments: 25 pages, 3 figures

Journal ref: J. Phys.: Condens. Matter 36 443001 (2024)

Mixed Stochastic-Deterministic Approach for Many-Body Perturbation Theory Calculations

Authors: Aaron R. Altman, Sudipta Kundu, Felipe H. da Jornada

Abstract: We present an approach for GW calculations of quasiparticle energies with quasi-quadratic scaling by approximating high-energy contributions to the Green's function in its Lehmann representation with effective stochastic vectors. The method is easy to implement without altering the GW code, converges rapidly with stochastic parameters, and treats systems of various dimensionality and screening res… ▽ More We present an approach for GW calculations of quasiparticle energies with quasi-quadratic scaling by approximating high-energy contributions to the Green's function in its Lehmann representation with effective stochastic vectors. The method is easy to implement without altering the GW code, converges rapidly with stochastic parameters, and treats systems of various dimensionality and screening response. Our calculations on a 5.75$^\circ$ twisted MoS$_2$ bilayer show how large-scale GW methods include geometry relaxations and electronic correlations on an equal basis in structurally nontrivial materials. △ Less

Submitted 5 December, 2023; originally announced December 2023.

Comments: 29 pages, 7 figures. Accepted in Physical Review Letters

Understanding orientational disorder in crystalline assemblies of hard convex polyhedra

Authors: Sumitava Kundu, Kaustav Chakraborty, Avisek Das

Abstract: Spontaneous self-assembly of hard convex polyhedra are known to form orientationally disordered crystalline phases, where particle orientations do not follow the same pattern as the positional arrangement of the crystal. A distinct type of orientational phase with discrete rotational mobility has been reported in hard particle systems. In this paper, we present a new analysis method for characteri… ▽ More Spontaneous self-assembly of hard convex polyhedra are known to form orientationally disordered crystalline phases, where particle orientations do not follow the same pattern as the positional arrangement of the crystal. A distinct type of orientational phase with discrete rotational mobility has been reported in hard particle systems. In this paper, we present a new analysis method for characterizing orientational phase of a crystal, which is based on algorithmic detection of unique orientations. Using this method we collected complete statistics of discrete orientations along the Monte Carlo simulation trajectories and observed that particles were equally partitioned among them, with specific values of pairwise orientational differences. These features remained constant across the pressure range and did not depend on rotational mobility. The discrete mobility was characteristic of a distinct equilibrium thermodynamic phase, qualitatively different from the freely rotating plastic phase with continuous orientations. The high pressure behavior with frozen particle orientations was part of that the same description and not a non-equilibrium arrested state. We introduced a precise notion of orientational order and demonstrated that the system was maximally disordered at the level of unit cell, even though individual particles could only take few discrete orientations. We report the existence of this phase in five polyhedral shapes and in systematically curated shape families constructed around two of them. The symmetry mismatch between the particle and the crystallographic point groups was found to be a predictive indicator for the occurrence of this phase. △ Less

Submitted 29 November, 2024; v1 submitted 12 November, 2023; originally announced November 2023.

Comments: 46 pages, 85 references and 8 figures and 1 table

Journal ref: J. Chem. Phys. 21 November 2024; 161 (20): 204907

CDMFT+HFD : an extension of dynamical mean field theory for nonlocal interactions applied to the single band extended Hubbard model

Authors: Sarbajaya Kundu, David Sénéchal

Abstract: We examine the phase diagram of the extended Hubbard model on a square lattice, for both attractive and repulsive nearest-neighbor interactions, using CDMFT+HFD, a combination of Cluster Dynamical Mean Field theory (CDMFT) and a Hartree-Fock mean-field decoupling of the inter-cluster extended interaction. For attractive non-local interactions, this model exhibits a region of phase separation near… ▽ More We examine the phase diagram of the extended Hubbard model on a square lattice, for both attractive and repulsive nearest-neighbor interactions, using CDMFT+HFD, a combination of Cluster Dynamical Mean Field theory (CDMFT) and a Hartree-Fock mean-field decoupling of the inter-cluster extended interaction. For attractive non-local interactions, this model exhibits a region of phase separation near half-filling, in the vicinity of which we find pockets of d-wave superconductivity, decaying rapidly as a function of doping, with disconnected patches of extended s-wave order at smaller (higher) electron densities. On the other hand, when the extended interaction is repulsive, a Mott insulating state at half-filling is destabilized by hole doping, in the strong-coupling limit, in favor of d-wave superconductivity. At the particle-hole invariant chemical potential, we find a first-order phase transition from antiferromagnetism (AF) to d-wave superconductivity as a function of the attractive nearest-neighbor interaction, along with a deviation of the density from the half-filled limit. A repulsive extended interaction instead favors charge-density wave (CDW) order at half-filling. △ Less

Submitted 24 October, 2023; originally announced October 2023.

Comments: 13 pages, 14 figures

Journal ref: SciPost Phys. Core 7, 033 (2024)

Competition between Neel, Haldane nematic, plaquette valence bond solid, and $(π,π)$ valence bond solid phases in SU(N) analogs of $S=1$ square-lattice antiferromagnets

Authors: Souvik Kundu, Nisheeta Desai, Kedar Damle

Abstract: We use stochastic series expansion (SSE) quantum Monte Carlo (QMC) methods to study the phases and transitions displayed by a class of sign-free designer Hamiltonians for SU($N$) analogs of spin $S=1$ quantum antiferromagnets on the square lattice. The SU($N$) spins are generators of the single-row two-column representation (complex conjugate of single-row two-column representation) on $A$ ($B$) s… ▽ More We use stochastic series expansion (SSE) quantum Monte Carlo (QMC) methods to study the phases and transitions displayed by a class of sign-free designer Hamiltonians for SU($N$) analogs of spin $S=1$ quantum antiferromagnets on the square lattice. The SU($N$) spins are generators of the single-row two-column representation (complex conjugate of single-row two-column representation) on $A$ ($B$) sublattices of the square lattice, and the Hamiltonian is designed to explore the competition between the nearest neighbour antiferromagnetic exchange couplings $J$ and four-spin interactions $Q$ that favor a plaquette-ordered valence bond solid (p-VBS) ground state. We find that this state is indeed established at large $Q/J$ for all $N > 3$. For $3< N \leq 9$, the ground state exhibits a direct first order quantum phase transition from a small-$Q/J$ Néel ordered antiferromagnetic state to this large-$Q/J$ p-VBS state. The ground state at $Q/J=0$ for $N \geq 10$ has been previously reported to be a valence bond nematic state, dubbed the Haldane nematic in recent literature. For small nonzero $Q/J$ and $N \geq 10$, we additionally find an unusual intermediate state in which the bond energy has a Bragg peak at wavevector $(π,π)$ with no accompanying Bragg peaks at wavevectors $(π,0)$ and $(0, π)$. This $(π, π)$ state also appears to be metastable at $Q/J =0$, as evidenced by low temperature histograms of the Haldane nematic and $(π, π)$ order parameters. Deep in the p-VBS phase of the ground state phase diagram, we find the temperature-driven melting of the p-VBS order is in the Ashkin-Teller universality class. In this regime, we identify an interesting signature of Ashkin-Teller criticality in bond correlations at wavevector $(π, π)$; this is in addition to the expected critical fluctuations of the conventional p-VBS order parameter at wavevectors $(π,0)$ and $(0,π)$. △ Less

Submitted 21 September, 2023; originally announced September 2023.

Journal ref: Phys. Rev. B 109 195146 (May, 2024)

Conformation and dynamics of partially active linear polymers

Authors: Marin Vatin, Sumanta Kundu, Emanuele Locatelli

Abstract: We perform numerical simulations of isolated, partially active polymers, driven out-of-equilibrium by a fraction of their monomers. We show that, if the active beads are all gathered in a contiguous block, the position of the section along the chain determines the conformational and dynamical properties of the system. Notably, one can modulate the diffusion coefficient of the polymer from {active-… ▽ More We perform numerical simulations of isolated, partially active polymers, driven out-of-equilibrium by a fraction of their monomers. We show that, if the active beads are all gathered in a contiguous block, the position of the section along the chain determines the conformational and dynamical properties of the system. Notably, one can modulate the diffusion coefficient of the polymer from {active-like to passive-like} just by changing the position of the active block. Further, in special cases, enhancement of diffusion can be achieved by decreasing the overall polymer activity. Our findings may help in the modelization of active biophysical systems, such as filamentous bacteria or worms. △ Less

Submitted 31 August, 2023; originally announced September 2023.

Comments: 12 pages, 10 figures + supplemental 4 pages, 7 figures

Dynamic hysteresis at a noisy saddle-node shows power-law scaling but nonuniversal exponent

Authors: Satyaki Kundu, Ranjan Kumar Patel, Srimanta Middey, Bhavtosh Bansal

Abstract: Dynamic hysteresis, viz., delay in switching of a bistable system on account of the finite sweep rate of the drive has been extensively studied in dynamical and thermodynamic systems. Dynamic hysteresis results from slowing of the response around a saddle-node bifurcation. As a consequence, the hysteresis area increases with the sweep rate. Mean-field theory, relevant for noise-free situations, pr… ▽ More Dynamic hysteresis, viz., delay in switching of a bistable system on account of the finite sweep rate of the drive has been extensively studied in dynamical and thermodynamic systems. Dynamic hysteresis results from slowing of the response around a saddle-node bifurcation. As a consequence, the hysteresis area increases with the sweep rate. Mean-field theory, relevant for noise-free situations, predicts power law scaling with the area scaling exponent of 2/3. We have experimentally investigated the dynamic hysteresis for a thermally-driven metal-insulator transition in a high quality NdNiO$_3$ thin film and found the scaling exponent to be about 1/3, far less than the mean field value. To understand this, we have numerically studied Langevin dynamics of the order parameter and found that noise, which can be thought to parallel finite temperature effects, influences the character of dynamic hysteresis by systematically lowering the dynamical exponent to as small as 0.2. The power law scaling character, on the other hand, is unaffected in the range of chosen parameters. This work rationalizes the ubiquitous power law scaling of the dynamic hysteresis as well as the wide variation in the scaling exponent between 0.66 and 0.2 observed in different systems over the last 30 years. △ Less

Submitted 18 August, 2023; originally announced August 2023.

Comments: 13 pages, 13 figures

Journal ref: Phys. Rev. E 108, 024101 (2023)

Flux fractionalization transition in anisotropic $S=1$ antiferromagnets and dimer-loop models

Authors: Souvik Kundu, Kedar Damle

Abstract: We demonstrate that the low temperature ($T$) properties of a class of anisotropic spin $S=1$ kagome (planar pyrochlore) antiferromagnets on a field-induced $\frac{1}{3}$-magnetization ($\frac{1}{2}$-magnetization) plateau are described by a model of fully-packed dimers and loops on the honeycomb (square) lattice, with a temperature-dependent relative fugacity $w(T)$ for the dimers. The fully-pack… ▽ More We demonstrate that the low temperature ($T$) properties of a class of anisotropic spin $S=1$ kagome (planar pyrochlore) antiferromagnets on a field-induced $\frac{1}{3}$-magnetization ($\frac{1}{2}$-magnetization) plateau are described by a model of fully-packed dimers and loops on the honeycomb (square) lattice, with a temperature-dependent relative fugacity $w(T)$ for the dimers. The fully-packed O(1) loop model ($w=0$) and the fully-packed dimer model ($w=\infty$) limits of this dimer-loop model are found to be separated by a phase transition at a finite and nonzero critical fugacity $w_c$, with interesting consequences for the spin correlations of the frustrated magnet. The $w>w_c$ phase has short loops and spin correlations dominated by power-law columnar order (with subdominant dipolar correlations), while the $w<w_c$ phase has dominant dipolar spin correlations and long loops governed by a power-law distribution of loop sizes. Away from $w_c$, both phases are described by a long-wavelength Gaussian effective action for a scalar height field that represents the coarse-grained electrostatic potential of fluctuating dipoles. The destruction of power-law columnar spin order below $w_c$ is driven by an unusual {\em flux fractionalization} mechanism, topological in character but quite distinct from the usual Kosterlitz-Thouless mechanism for such transitions: Fractional electric fluxes which are bound into integer values for $w>w_c$, proliferate in the $w<w_c$ phase and destroy power-law columnar order. △ Less

Submitted 18 February, 2024; v1 submitted 11 May, 2023; originally announced May 2023.

Comments: revised and expanded version resubmitted to Phys. Rev

Journal ref: Phys. Rev. X 15 011018 (January 2025)

Physical properties of a generalized model of multilayer adsorption of dimers

Authors: G Palacios, Sumanta Kundu, L A P Santos, M A F Gomes

Abstract: We investigate the transport properties of a complex porous structure with branched fractal architectures formed due to the gradual deposition of dimers in a model of multilayer adsorption. We thoroughly study the interplay between the orientational anisotropy parameter $p_0$ of deposited dimers and the formation of porous structures, as well as its impact on the conductivity of the system, throug… ▽ More We investigate the transport properties of a complex porous structure with branched fractal architectures formed due to the gradual deposition of dimers in a model of multilayer adsorption. We thoroughly study the interplay between the orientational anisotropy parameter $p_0$ of deposited dimers and the formation of porous structures, as well as its impact on the conductivity of the system, through extensive numerical simulations. By systematically varying the value of $p_0$, several critical and off-critical scaling relations characterizing the behavior of the system are examined. The results demonstrate that the degree of orientational anisotropy of dimers plays a significant role in determining the structural and physical characteristics of the system. We find that the Einstein relation relating to the size scaling of the electrical conductance holds true only in the limiting case of $p_0 \to 1$. Monitoring the fractal dimension of the interface of the multilayer formation for various $p_0$ values, we reveal that in a wide range of $p_0 > 0.2$ interface shows the characteristic of a self-avoiding random walk, compared to the limiting case of $p_0 \to 0$ where it is characterized by the fractal dimension of the backbone of ordinary percolation cluster at criticality. Our results thus can provide useful information about the fundamental mechanisms underlying the formation and behavior of wide varieties of amorphous and disordered systems that are of paramount importance both in science and technology as well as in environmental studies. △ Less

Submitted 12 July, 2023; v1 submitted 11 April, 2023; originally announced April 2023.

Inequality of avalanche sizes in models of fracture

Authors: Diksha, Sumanta Kundu, Bikas K. Chakrabarti, Soumyajyoti Biswas

Abstract: Prediction of an imminent catastrophic event in a driven disordered system is of paramount importance - from the laboratory scale controlled fracture experiment to the largest scale of mechanical failure i.e., earthquakes. It has been long conjectured that the statistical regularities in the energy emission time series mirrors the "health" of such driven systems and hence have the potential for fo… ▽ More Prediction of an imminent catastrophic event in a driven disordered system is of paramount importance - from the laboratory scale controlled fracture experiment to the largest scale of mechanical failure i.e., earthquakes. It has been long conjectured that the statistical regularities in the energy emission time series mirrors the "health" of such driven systems and hence have the potential for forecasting imminent catastrophe. Among other statistical regularities, a measure of how unequal the avalanche sizes are, is potentially a crucial indicator of imminent failure. The inequalities of avalanche sizes are quantified using inequality indices traditionally used in socio-economic systems: the Gini index (g), the Hirsch index (h) and the Kolkata index (k). It is then shown analytically (for mean field) and numerically (for non mean field) in models of quasi-brittle materials that the indices show universal behavior near the breaking points in such models and hence could serve as indicators of imminent breakdown of stressed disordered systems. △ Less

Submitted 14 June, 2023; v1 submitted 17 March, 2023; originally announced March 2023.

Comments: Accepted for publication in Phys. Rev. E

Critical Slowing Down at the Abrupt Mott Transition: When the First-Order Phase Transition Becomes Zeroth-Order and Looks Like Second-Order

Authors: Satyaki Kundu, Tapas Bar, Rajesh Kumble Nayak, Bhavtosh Bansal

Abstract: We report that the thermally-induced Mott transition in vanadium sesquioxide shows critical-slowing-down and enhanced variance ('critical opalescence') of the order parameter fluctuations measured through low-frequency resistance-noise spectroscopy. Coupled with the observed increase of also the phase-ordering time, these features suggest that the strong abrupt transition is controlled by a critic… ▽ More We report that the thermally-induced Mott transition in vanadium sesquioxide shows critical-slowing-down and enhanced variance ('critical opalescence') of the order parameter fluctuations measured through low-frequency resistance-noise spectroscopy. Coupled with the observed increase of also the phase-ordering time, these features suggest that the strong abrupt transition is controlled by a critical-like singularity in the hysteretic metastable phase. The singularity is identified with the spinodal point and is a likely consequence of the strain-induced long-range interaction. △ Less

Submitted 19 January, 2023; originally announced January 2023.

Comments: 14 pages, 16 figures

Journal ref: Phys. Rev. Lett. 124, 095703 (2020)

Machine learning understands knotted polymers

Authors: Anna Braghetto, Sumanta Kundu, Marco Baiesi, Enzo Orlandini

Abstract: Simulated configurations of flexible knotted rings confined inside a spherical cavity are fed into long-short term memory neural networks (LSTM NNs) designed to distinguish knot types. The results show that they perform well in knot recognition even if tested against flexible, strongly confined and therefore highly geometrically entangled rings. In agreement with the expectation that knots are del… ▽ More Simulated configurations of flexible knotted rings confined inside a spherical cavity are fed into long-short term memory neural networks (LSTM NNs) designed to distinguish knot types. The results show that they perform well in knot recognition even if tested against flexible, strongly confined and therefore highly geometrically entangled rings. In agreement with the expectation that knots are delocalized in dense polymers, a suitable coarse-graining procedure on configurations boosts the performance of the LSTMs when knot identification is applied to rings much longer than those used for training. Notably, when the NNs fail, usually the wrong prediction still belongs to the same topological family of the correct one. The fact that the LSTMs are able to grasp some basic properties of the ring's topology is corroborated by a test on knot types not used for training. We also show that the choice of the NN architecture is important: simpler convolutional NNs do not perform so well. Finally, all results depend on the features used for input: surprisingly, coordinates or bond directions of the configurations provide the best accuracy to the NNs, even if they are not invariant under rotations (while the knot type is invariant). Other rotational invariant features we tested are based on distances, angles, and dihedral angles. △ Less

Submitted 22 December, 2022; originally announced December 2022.

Journal ref: Macromolecules 56, 2899 (2023)

Monolithic TCAD Simulation of Phase-Change Memory (PCM/PRAM) + Ovonic Threshold Switch (OTS) Selector Device

Authors: M. Thesberg, Z. Stanojevic, O. Baumgartner, C. Kernstock, D. Leonelli, M. Barci, X. Wang, X. Zhou, H. Jiao, G. L. Donadio, D. Garbin, T. Witters, S. Kundu, H. Hody, R. Delhougne, G. Kar, M. Karner

Abstract: Owing to the increasing interest in the commercialization of phase-change memory (PCM) devices, a number of TCAD models have been developed for their simulation. These models formulate the melting, amorphization and crystallization of phase-change materials as well as their extreme conductivity dependence on both electric field and temperature into a set of self-consistently-solved thermoelectric… ▽ More Owing to the increasing interest in the commercialization of phase-change memory (PCM) devices, a number of TCAD models have been developed for their simulation. These models formulate the melting, amorphization and crystallization of phase-change materials as well as their extreme conductivity dependence on both electric field and temperature into a set of self-consistently-solved thermoelectric and phase-field partial-differential equations. However, demonstrations of the ability of such models to match actual experimental results are rare. In addition, such PCM devices also require a so-called selector device - such as an Ovonic Threshold Switching (OTS) device - in series for proper memory operation. However, monolithic simulation of both the PCM and OTS selector device in a single simulation is largely absent from the literature, despite its potential value for material- and design-space explorations. It is the goal of this work to first characterize a PCM device in isolation against experimental data, then to demonstrate the qualitative behavior of a simulated OTS device in isolation and finally to perform a single monolithic simulation of the PCM + OTS device within the confines of a commercially available TCAD solver: GTS Framework. △ Less

Submitted 11 November, 2022; originally announced November 2022.

Journal ref: Solid-State Electronics, Volume 199, January 2023, 108504

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

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

Authors: S. Kundu, A. Pal, Amit Chauhan, K. Patro, K. Anand, S. Rana, V. G. Sathe, Amish G. Joshi, P. Pal, K. Sethupathi, B. R. K. Nanda, P. Khuntia

Abstract: Double perovskite-based magnets wherein frustration and competition between emergent degrees of freedom are at play can lead to novel electronic and magnetic phenomena. Herein, we report the electronic structure and magnetic properties of an ordered double perovskite material Ho2CoMnO6. In the double perovskite with general class A2BB'O6, the octahedral B and B'-site has a distinct crystallographi… ▽ More Double perovskite-based magnets wherein frustration and competition between emergent degrees of freedom are at play can lead to novel electronic and magnetic phenomena. Herein, we report the electronic structure and magnetic properties of an ordered double perovskite material Ho2CoMnO6. In the double perovskite with general class A2BB'O6, the octahedral B and B'-site has a distinct crystallographic site. The Rietveld refinement of XRD data reveals that Ho2CoMnO6 crystallizes in the monoclinic P21/n space group. The X-ray photoelectron spectroscopy confirms the charge state of cations present in this material. The temperature dependence of magnetization and specific heat exhibit a long-range ferromagnetic ordering at Tc ~ 76 K owing to the presence of super exchange interaction between Co2+ and Mn4+ moments. Furthermore, the magnetization isotherm at 5 K shows a hysteresis curve that confirms ferromagnetic behavior of this double perovskite. We observed a re-entrant glassy state in the intermediate temperature regime, which is attributed to inherent anti-site disorder and competing interactions. A large magnetocaloric effect has been observed much below the ferromagnetic transition temperature. The temperature-dependent Raman spectroscopy studies support the presence of spin-phonon coupling and short-range order above Tc in this double perovskite. The stabilization of magnetic ordering and charge states is further analyzed through electronic structure calculations. The latter also infers the compound to be a narrow band gap insulator with the gap arising between the lower and upper Hubbard Co-d subbands. Our results demonstrate that anti-site disorder and complex 3d-4f exchange interactions in the spin-lattice account for the observed electronic and magnetic properties in this promising double perovskite material. △ Less

Submitted 19 September, 2022; v1 submitted 22 May, 2022; originally announced May 2022.

Journal ref: Physical Review Materials 6, 104401 (2022)

Energy effective mass dependence of electron tunneling through CdS/CdSe, AlxGa1-xAs/GaAs and AlSb/InAs Multiple Quantum Barriers

Authors: Jatindranath Gain, Madhumita Dassarkar, Sudakhina Kundu

Abstract: Tunneling of electrons through the barriers in heterostructures devices is investigated by using the unified Transfer Matrix Method. The effect of barrier width on electron transmission coefficients has also been examined for different pairs of semiconductor devices of significant research interest in current years. Such Pairs involve AlxGa1-xAs/GaAs, AlSb/InAs, and CdS/CdSe quantum barriers with… ▽ More Tunneling of electrons through the barriers in heterostructures devices is investigated by using the unified Transfer Matrix Method. The effect of barrier width on electron transmission coefficients has also been examined for different pairs of semiconductor devices of significant research interest in current years. Such Pairs involve AlxGa1-xAs/GaAs, AlSb/InAs, and CdS/CdSe quantum barriers with varying dimensions reduced from 20 nm to 5nm to observe how tunneling properties are affected by scaling. The effective electron masses in the well and barrier regions typically vary with constituent materials. It has been shown that the transmission coefficients are significantly changed due to the coupling. The effective mass-dependent transmission coefficients for electron energy have been evaluated in terms of the mass discontinuity metrics. The electron transmission coefficients for each pair of quantum structures are plotted with the variation of its electron energy, normalized to its potential energy. The resonant state obtained here will be beneficial for designing detectors, optical filters, photonic-switching devices and other optoelectronic and photonic devices. △ Less

Submitted 23 March, 2022; originally announced March 2022.

Comments: 6 pages,7 figures, Published in UGC care-I Journal, ICTACT Journal on Microelectronics, Volume: 07, Issue: 03, October 2021 ISSN: 2395-1680 (Online)

Report number: IJME0893

Journal ref: ICTACT Journal on Microelectronics (UGC care I Journal), VOLUME: 07, ISSUE: 03, OCTOBER 2021,ISSN: 2395-1680 (ONLINE)