arXiv subjects

Sudipta Kundu

Publications and source records attributed to Sudipta Kundu.

18 recordsLinked to original sources

arXiv2026

Non-Commutative Wiener-Wintner theorem for amenable group actions

Let $G$ be a locally compact, second countable, amenable group acting on a finite von Neumann algebra $(\mathcal{M},τ)$ by trace-preserving automorphisms. In this article, we establish a Jacobs-de Leeuw-Glicksberg decomposition for this action, yielding a decomposition of $\mathcal{M}$ into its almost periodic and weakly mixing components. We also prove a noncommutative version of the van der Corput lemma. As an application, we establish a noncommutative Wiener-Wintner theorem for amenable group actions on finite von Neumann algebras.

math.OA↗

arXiv2026

A geometric basis for materials families in inorganic solids

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.

cond-mat.mtrl-sci↗

arXiv2026

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

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.

cond-mat.mes-hall↗

arXiv2026

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

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.

cond-mat.mes-hall↗

arXiv2026

Moiré enabled spin pumping and preservation in MoSe2/WS2 heterobilayers

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.

cond-mat.mes-hall↗

arXiv2026

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

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.

cond-mat.str-el↗

arXiv2025

Non-Commutative Maximal Inequalities for State-Preserving Actions of amenable groups

In this article, we establish maximal inequalities and deduce ergodic theorems for state-preserving actions of amenable, locally compact, second-countable groups on tracial non-commutative $L^1$-spaces. As a further consequence, in combination with the Neveu decomposition, we obtain a stochastic ergodic theorem for amenable group actions.

math.OA↗

arXiv2025

Atomic relaxation and flat bands in strain-engineered transition metal dichalcogenide bilayer moiré systems

Strain-induced lattice mismatch leads to moiré patterns in homobilayer transition metal dichalcogenides (TMDs). We investigate the structural and electronic properties of such strained moiré patterns in TMD homobilayers. The moiré patterns in strained TMDs consist of several stacking domains which are separated by tensile solitons. Relaxation of these systems distributes the strain unevenly in the moiré superlattice, with the maximum strain energy concentrating at the highest energy stackings. The order parameter distribution shows the formation of aster topological defects at the same sites. In contrast, twisted TMDs host shear solitons at the domain walls, and the order parameter distribution in these systems shows the formation of vortex defects. The strained moiré systems also show the emergence of several well-separated flat bands at both the valence and conduction band edges, and we observe a significant reduction in the band gap. The flat bands in these strained moiré superlattices provide platforms for studying the Hubbard model on a triangular lattice as well as the ionic Hubbard model on a honeycomb lattice. Furthermore, we study the localization of the wave functions corresponding to these flat bands. The wave functions localize at different stackings compared to twisted TMDs, and our results are in excellent agreement with spectroscopic experiments.

cond-mat.mtrl-sci↗

arXiv2023

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

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.

cond-mat.mtrl-sci↗

arXiv2022

Exciton fine structure in twisted transition metal dichalcogenide heterostructures

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.

cond-mat.mtrl-sci↗

arXiv2022

Tuning exciton complexes in twisted bilayer WSe2 at intermediate misorientation

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.

physics.optics↗

arXiv2022

Moiré induced topology and flat bands in twisted bilayer WSe$_2$: A first-principles study

We study the influence of strong spin-orbit interaction on the formation of flat bands in relaxed twisted bilayer WSe$_2$. Flat bands, well separated in energy, emerge at the band edges for twist angles ($θ$) near 0$^{\circ}$ and 60$^{\circ}$. For $θ$ near 0$^{\circ}$, the interlayer hybridization together with a moiré potential determines the electronic structure. The bands near the valence band edge have nontrivial topology, with Chern numbers equal to +1 or $-$1. We propose that the nontrivial topology of the first band can be probed experimentally for twist angles less than a critical angle of 3.5$^{\circ}$. For $θ$ near 60$^{\circ}$, the flattening of the bands arising from the K point of the unit cell Brillouin zone is a result of atomic rearrangements in the individual layers. Our findings on the flat bands and the localization of their wavefunctions for both ranges of $θ$ match well with recent experimental observations.

cond-mat.mtrl-sci↗

arXiv2020

Population Analysis with Wannier Orbitals

We formulate Wannier orbital overlap population and Wannier orbital Hamilton population to describe the contribution of different orbitals to electron distribution and their interactions. These methods, which are analogous to the well known crystal orbital overlap population and crystal orbital Hamilton population, provide insight into the distribution of electrons at various atom centres and their bonding nature. We apply this formalism in the context of a plane-wave density functional theory calculation. This method provides a means to connect the non-local plane-wave basis to a localised basis by projecting the wave functions from a plane-wave density functional theory calculation on to localized Wannier orbital basis. The main advantage of this formulation is that the spilling factor is strictly zero for insulators and can systematically be made small for metals. We use our proposed method to study and obtain bonding and electron localization insights in five different materials.

physics.comp-ph↗

arXiv2020

Origin and Evolution of Ultraflatbands in Twisted Bilayer Transition Metal Dichalcogenides: Realization of Triangular Quantum Dot Array

Using a multiscale computational approach, we probe the origin and evolution of ultraflatbands in moiré superlattices of twisted bilayer MoS$_2$, a prototypical transition metal dichalcogenide. Unlike twisted bilayer graphene, we find no unique magic angles in twisted bilayer MoS$_2$ for flatband formation. Ultraflatbands form at the valence band edge for twist angles ($θ$) close to 0$^\circ$ and at both the valence and conduction band edges for $θ$ close to 60$^\circ$, and have distinct origins. For$ θ$ close to 0$^\circ$, inhomogeneous hybridization in the reconstructed moiré superlattice is sufficient to explain the formation of flatbands. For $θ$ close to 60$^\circ$, additionally, local strains cause the formation of modulating triangular potential wells such that electrons and holes are spatially separated. This leads to multiple energy-separated ultraflatbands at the band edges closely resembling eigenfunctions of a quantum particle in an equilateral triangle well. Twisted bilayer transition metal dichalcogenides are thus suitable candidates for the realisation of ordered quantum dot array.

cond-mat.mes-hall↗

arXiv2020

Native Point Defects in Mono-- and Bi--layer Phosphorene

We study the stability and electronic properties of intrinsic point defects, vacancy and self-interstitial, in mono- and bi-layer phosphorene. We calculate the formation energies, quasiparticle defect states and charge transition levels (CTLs) of these defects using \textit{ab initio} density functional theory (DFT) and GW approximation to the electron self-energy. Using the DFT + GW two paths formalism for studying interstitial in monolayer phosphorene, we show that with the inclusion of electrostatic corrections CTLs can be calculated reliably. Our calculations show that all the native point defects have low formation energies 0.9-1.6 eV in neutral state. Furthermore, we find that vacancy in phosphorene behaves as an acceptor-like defect which can explain the p-type conductivity in phosphorene. On the other hand, interstitial can show both acceptor- and donor-like behaviour.

cond-mat.mtrl-sci↗

arXiv2018

Opening of large band gap in metallic carbon nanotubes by mannose functionalized dendrimers: Experiments and theory

Despite many theoretical schemes, the direct experimental observation of supramolecular control on band gap opening in single-walled carbon nanotubes (SWNT) is still lacking. We report experimental and theoretical demonstration of metal to semiconductor transition with precisely measured large band gap in SWNTs due to the wrapping of mannose functionalized poly (propyl ether imine) dendrimer (DM) molecules. Semiconductor behaviour of SWNT-DM complex is comprehensively established with a band gap value of ~ 0.45 eV measured using scanning tunnelling spectroscopy (STS), ionic liquid top gated field effct transistor (FET) characteristics and Raman spectroscopy. Further, a validated molecular picture of SWNT-DM complex obtained from fully atomistic molecular dynamic (MD) simulations was used to carry out ab-initio density functional theory (DFT) and GW calculations of the electronic structure, evaluating experimentally estimated band gap value. We attribute this large band gap opening in SWNTs to the complexation induced asymmetric strain developed in the carbon-carbon bond length.

cond-mat.mtrl-sci↗

arXiv2018

PASTA: Python Algorithms for Searching Transition stAtes

Chemical reactions are often associated with an energy barrier along the reaction pathway which hinders the spontaneity of the reaction. Changing the energy barrier along the reaction pathway allows one to modulate the performance of a reaction. We present a module, Python Algorithms for Searching Transition stAtes (PASTA), to calculate the energy barrier and locate the transition state of a reaction efficiently. The module is written in python and can perform nudged elastic band, climbing image nudged elastic band and automated nudged elastic band calculations. These methods require the knowledge of the potential energy surface (and its gradient along some direction). This module is written such that it works in conjunction with density functional theory (DFT) codes to obtain this information. Presently it is interfaced with three well known DFT packages: Vienna Ab initio Simulation Package (VASP), Quantum Espresso and Spanish Initiative for Electronic Simulations with Thousands of Atoms (SIESTA). This module is easily extendable and can be interfaced with other DFT, force-field or empirical potential based codes. The uniqueness of the module lies in its user-friendliness. For users with limited computing resources, this module will be an effective tool as it allows to perform the calculations image by image. On the other hand, users with plentiful computing resources (such as users in a high performance computing environment) can perform the calculations for large number of images simultaneously. This module gives users complete flexibility, thereby enabling them to perform calculations on large systems making the best use of the available resources.

physics.comp-ph↗

arXiv2012

An Adaptive Modulation Scheme for Two-user Fading MAC with Quantized Fade State Feedback

With no CSI at the users, transmission over the two-user Gaussian Multiple Access Channel with fading and finite constellation at the input, is not efficient because error rates will be high when the channel conditions are poor. However, perfect CSI at the users is an unrealistic assumption in the wireless scenario, as it would involve massive feedback overheads. In this paper we propose a scheme which uses only quantized knowledge of CSI at the transmitters with the overhead being nominal. The users rotate their constellation without varying their transmit power to adapt to the existing channel conditions, in order to meet certain pre-determined minimum Euclidean distance requirement in the equivalent constellation at the destination. The optimal modulation scheme has been described for the case when both the users use symmetric M-PSK constellations at the input, where $ M=2^λ$, $ λ$ being a positive integer. The strategy has been illustrated by considering examples where both users use QPSK or 8-PSK signal sets at the input. It is shown that the proposed scheme has better throughput and error performance compared to the conventional non-adaptive scheme, at the cost of a feedback overhead of just $\lceil \log_2(\frac{M^2}{8}-\frac{M}{4}+2)\rceil + 1 $ bits, for the M-PSK case.

cs.IT↗