arXiv subjects

Indrajit Maity

Publications and source records attributed to Indrajit Maity.

At least 19 recordsLinked to original sources

arXiv2026

Exciton multipolarity controls coherent and squeezed phonons in van der Waals heterostructures

Photoexcitation-driven changes in the electronic distribution displace atoms, generating coherent phonons on ultrafast timescales. Two-dimensional (2D) materials and their heterostructures offer a powerful platform for engineering these phonons. Yet, despite the widespread observation of photoexcited coherent phonons, a design principle for controlling their character remains elusive. Here, using detailed atomistic simulations of multilayers of alternating MoSe$_2$ and WSe$_2$, we reveal exciton multipolarity as a design principle for tuning photoinduced phonons from coherent to squeezed. These phonons are interlayer breathing modes, with dipolar excitons coupling linearly to generate coherent states and quadrupolar excitons coupling quadratically to produce squeezed states. Moreover, an out-of-plane electric field enables switch-like control, converting quadrupolar excitons into dipolar excitons and switching the phonon state from squeezed to coherent. For example, in trilayer WSe$_2$/MoSe$_2$/WSe$_2$, the photoexcited 1.04-THz breathing mode switches from a squeezed state at zero field to a coherent state under an applied vertical field. Experimentally, these phonon states can be directly probed by ultrafast X-ray or electron diffraction and indirectly through transient reflectivity. Our results open new avenues for ultrafast control of lattice and electronic dynamics on picosecond timescales, with implications for THz quantum phononics, nanophotonic technologies, and quantum-noise-limited sensing.

cond-mat.mtrl-sci↗

arXiv2026

Moir\'e Phonons and Emergent Exciton-Phonon Coupling in a Moir\'e Heterobilayer

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

cond-mat.mtrl-sci↗

arXiv2026

Moir\'e trapping of quadrupolar excitons in van der Waals trilayers

Quadrupolar excitons in van der Waals heterostructures - quantum superpositions of anti-aligned dipolar excitons - offer a novel platform to explore exotic many-body physics, with applications to quantum sensing and photonic devices. Yet their internal structure, symmetry, and real-space localisation remain largely unknown. Here, we reveal the atomic-scale structure of quadrupolar excitons in twisted WSe2/WS2/WSe2 trilayers by solving the Bethe-Salpeter equation within a large-scale atomistic framework. We discover that large atomic relaxations at small twist angles give rise to two distinct quadrupolar excitons trapped at moir\'e lattice sites, differing in the in-plane symmetry of the electron density about the hole: one azimuthally symmetric, with the density maximal at the hole, and one threefold symmetric, with a node at the hole. Moir\'e trapping, neglected in commonly used models of quadrupolar exciton formation, is critical to their many-exciton phases. Without moir\'e trapping, quadrupolar excitons transition into anti-parallel dipolar excitons on a bipartite square lattice, while with trapping, the same dipoles are confined to a triangular lattice and experience geometric frustration. Our study uncovers the highly non-trivial nature of quadrupolar excitons, with direct implications for simulating frustrated quantum magnetism in a fully tunable excitonic platform.

cond-mat.mtrl-sci↗

arXiv2025

Origin of trapped intralayer Wannier and charge-transfer excitons in moir\'e materials

Moir\'e materials offer a versatile platform for engineering excitons with unprecedented control, promising next-generation optoelectronic applications. While continuum models are widely used to study moir\'e excitons due to their computational efficiency, they often disagree with ab initio many-body approaches, as seen for intralayer excitons in WS$_2$/WSe$_2$ heterobilayers. Here, we resolve these discrepancies using an atomistic, quantum-mechanical framework based on the Bethe-Salpeter equation with localized Wannier functions as the basis for the electronic structure. We show that inclusion of dielectric screening due to hexagonal boron nitride (hBN) encapsulation is essential to reproduce the full set of experimentally observed features of moir\'e intralayer excitons. Our analysis reveals a competition between Wannier and charge transfer characters, driven by variations between direct and indirect band gaps at high symmetry stacking regions due to atomic relaxations and environmentally tunable electron-hole interactions. Building on this insight, we demonstrate that the lowest-energy bright excitons are Wannier-like in WS2/WSe2 heterobilayers but charge-transfer-like in twisted WSe2 homobilayers, despite having comparable moir\'e lengths when encapsulated in hBN. In the absence of hBN encapsulation, the lowest-energy bright exciton in twisted WSe$_2$ becomes Wannier-like. These results establish atomistic modeling as a powerful and efficient approach for designing and controlling excitonic phenomena in moir\'e materials.

cond-mat.mtrl-sci↗

arXiv2025

Phase engineering of 1T$'$ and 1T CrS2 and Cr2S3 by MOCVD

Layered Cr-chalcogenides compounds offer a rich range of crystal phases with different magnetic properties some of which have been predicted only but not experimentally verified. Here we demonstrate a previously unreported crystal phase of CrS2, namely the distorted octahedral (1T$'$) structure, synthesized via metal-organic chemical vapour deposition (MOCVD). We achieved the tuneable synthesis of either 1T$'$ or 1T phase CrS2. The structure were identified using polarized Raman spectroscopy, density functional perturbation theory (DFPT), as well as scanning electron diffraction (4D-STEM). 4D-STEM reveals that 1T$'$ crystals grow from an originally nucleated kinetically favoured 1T phase which then transform into the thermodynamically stable 1T$'$ phase. Magneto-optic Kerr imaging reveals that the 1T$'$ CrS2 crystals have soft ferromagnetic nature at low temperature. Our MOCVD growth of complex phases of 2D CrS2 with long-range magnetic order paves the way for the scalable synthesis of 2D magnets for ultrathin magnetic memories for logic-in-memory applications and spintronics.

cond-mat.mtrl-sci↗

arXiv2024

PARPHOM: PARallel PHOnon calculator for Moir\'e systems

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

cond-mat.mes-hall↗

arXiv2024

Atomistic theory of twist-angle dependent intralayer and interlayer exciton properties in twisted bilayer materials

Twisted bilayers of two-dimensional (2D) materials have emerged as a highly tunable platform to study and engineer properties of excitons. However, the atomistic description of these properties has remained a significant challenge as a consequence of the large unit cells of the emergent moir\'e superlattices. To address this problem, we introduce an efficient approach to solve the Bethe-Salpeter equation that exploits the localization of atomic Wannier functions. We then use this approach to study intra- and interlayer excitons in twisted WS$_{2}$/WSe$_{2}$ at a range of twist angles. In agreement with experiment, we find that the optical spectrum exhibits three low-energy peaks for twist angles small than $2^\circ$. The energy splitting between the peaks is described accurately. We also find two low-energy interlayer excitons with weak oscillator strengths. Our approach opens up new opportunities for the design of light-matter interactions in ultrathin materials.

cond-mat.mtrl-sci↗

arXiv2024

Phonon Linewidths in Twisted Bilayer Graphene near Magic Angle

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

cond-mat.mes-hall↗

arXiv2023

Valley-polarized Exitonic Mott Insulator in WS2/WSe2 Moir\'e Superlattice

Strongly enhanced electron-electron interaction in semiconducting moir\'e superlattices formed by transition metal dichalcogenides (TMDCs) heterobilayers has led to a plethora of intriguing fermionic correlated states. Meanwhile, interlayer excitons in a type-II aligned TMDC heterobilayer moir\'e superlattice, with electrons and holes separated in different layers, inherit this enhanced interaction and strongly interact with each other, promising for realizing tunable correlated bosonic quasiparticles with valley degree of freedom. We employ photoluminescence spectroscopy to investigate the strong repulsion between interlayer excitons and correlated electrons in a WS2/WSe2 moir\'e superlattice and combine with theoretical calculations to reveal the spatial extent of interlayer excitons and the band hierarchy of correlated states. We further find that an excitonic Mott insulator state emerges when one interlayer exciton occupies one moir\'e cell, evidenced by emerging photoluminescence peaks under increased optical excitation power. Double occupancy of excitons in one unit cell requires overcoming the energy cost of exciton-exciton repulsion of about 30-40 meV, depending on the stacking configuration of the WS2/WSe2 heterobilayer. Further, the valley polarization of the excitonic Mott insulator state is enhanced by nearly one order of magnitude. Our study demonstrates the WS2/WSe2 moir\'e superlattice as a promising platform for engineering and exploring new correlated states of fermion, bosons, and a mixture of both.

cond-mat.mes-hall↗

arXiv2023

Influence of atomic relaxations on the moir\'{e} flat band wavefunctions in antiparallel twisted bilayer WS$_{\text{2}}$

Twisting bilayers of transition metal dichalcogenides (TMDs) gives rise to a periodic moir\'{e} potential resulting in flat electronic bands with localized wavefunctions and enhanced correlation effects. In this work, scanning tunneling microscopy is used to image a WS$_{2}$ bilayer twisted approximately $3^{\circ}$ off the antiparallel alignment. Scanning tunneling spectroscopy reveals the presence of localized electronic states in the vicinity of the valence band onset. In particular, the onset of the valence band is observed to occur first in regions with a Bernal stacking in which S atoms are located on top of each other. In contrast, density-functional theory calculations on twisted bilayers which have been relaxed in vacuum predict the highest lying flat valence band to be localized in regions of AA' stacking. However, agreement with the experiment is recovered when the calculations are carried out on bilayers in which the atomic displacements from the unrelaxed positions have been reduced reflecting the influence of the substrate and finite temperature. This demonstrates the delicate interplay of atomic relaxations and the electronic structure of twisted bilayer materials.

cond-mat.mes-hall↗

arXiv2023

Electrons surf phason waves in moir\'e bilayers

We investigate the effect of thermal fluctuations on the atomic and electronic structure of a twisted MoSe$_{2}$/WSe$_{2}$ heterobilayer using a combination of classical molecular dynamics and \textit{ab-initio} density functional theory calculations. Our calculations reveal that thermally excited phason modes give rise to an almost rigid motion of the moir\'e lattice. Electrons and holes in low-energy states are localized in specific stacking regions of the moir\'e unit cell and follow the thermal motion of these regions. In other words, charge carriers surf phason waves that are excited at finite temperatures. Small displacements at the atomic scale are amplified at the moir\'e scale, which gives rise to significant surfing speeds. We also show that such surfing survives in the presence of a substrate and disorder. This effect has potential implications for the design of charge and exciton transport devices based on moir\'e materials.

cond-mat.mtrl-sci↗

arXiv2023

Phason-mediated interlayer exciton diffusion in WS2/WSe2 moir\'e heterostructure

Moir\'e potentials in two-dimensional materials have been proven to be of fundamental importance to fully understand the electronic structure of van der Waals heterostructures, from superconductivity to correlated excitonic states. However, understanding how the moir\'e phonons, so-called phasons, affect the properties of the system still remains an uncharted territory. In this work, we demonstrate how phasons are integral to properly describing and understanding low-temperature interlayer exciton diffusion in WS2/WSe2 heterostructure. We perform photoluminescence (PL) spectroscopy to understand how the coupling between the layers, affected by their relative orientation, impacts the excitonic properties of the system. Samples fabricated with stacking angles of 0{\deg} and 60{\deg} are investigated taking into account the stacking angle dependence of the two common moir\'e potential profiles. Additionally, we present spatially and time-resolved exciton diffusion measurements, looking at the photoluminescence emission in a temperature range from 30 K to 250 K. An accurate potential for the two configurations are computed via density functional theory (DFT) calculations. Finally, we perform molecular dynamics simulation in order to visualize the phasons motion, estimating the phason speed at different temperatures, providing novel insights into the mechanics of exciton propagation at low temperatures that cannot be explained within the frame of classical exciton diffusion alone.

cond-mat.mtrl-sci↗

arXiv2022

Chiral valley phonons and flat phonon bands in moiré materials

We investigate the chirality of phonon modes in twisted bilayer WSe2 and demonstrate distinct chiral behavior of the $K/K^\prime$ valley phonons for twist angles close to $0^{\circ}$ and close to $60^{\circ}$. In particular, multiple chiral non-degenerate $K/K^\prime$ valley phonons are found for twist angles near $60^{\circ}$ whereas no non-degenerate chiral modes are found for twist angles close to $0^\circ$. Moreover, we discover two sets of emergent chiral valley modes that originate from an inversion symmetry breaking at the moiré scale and find similar modes in moiré patterns of strain-engineered bilayers WSe2 and MoSe2/WSe2 heterostructures. At the energy gap between acoustic and optical modes, the formation of flat phonon bands for a broad range of twist angles is observed in the twisted bilayer WSe2. Our findings are relevant for understanding electron-phonon and exciton-phonon scattering in moiré materials and also for the design of phononic analogues of flat band electrons.

cond-mat.mtrl-sci↗

arXiv2021

Atomic relaxation and flat bands in strain-engineered transition metal dichalcogenide bilayer moir\'{e} systems

Strain-induced lattice mismatch leads to moir\'{e} patterns in homobilayer transition metal dichalcogenides (TMDs). We investigate the structural and electronic properties of such strained moir\'{e} patterns in TMD homobilayers. The moir\'{e} 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 moir\'{e} 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 moir\'{e} 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 moir\'{e} 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↗

arXiv2021

Reconstruction of moiré lattices in twisted transition metal dichalcogenide bilayers

An important step in understanding the exotic electronic, vibrational, and optical properties of the moiré lattices is the inclusion of the effects of structural relaxation of the un-relaxed moiré lattices. Here, we propose novel structures for twisted bilayer of transition metal dichalcogenides (TMDs). For $θ\gtrsim 58.4^{\circ}$, we show a dramatic reconstruction of the moiré lattices, leading to a trimerization of the unfavorable stackings. We show that the development of curved domain walls due to the three-fold symmetry of the stacking energy landscape is responsible for such lattice reconstruction. Furthermore, we show that the lattice reconstruction notably changes the electronic band-structure. This includes the occurrence of flat bands near the edges of the conduction as well as valence bands, with the valence band maximum, in particular, corresponding to localized states enclosed by the trimer. We also find possibilities for other complicated, entropy stabilized, lattice reconstructed structures.

cond-mat.mtrl-sci↗

arXiv2021

Twister: Construction and structural relaxation of commensurate moiré superlattices

Introduction of a twist between layers of two-dimensional materials which leads to the formation of a moiré pattern is an emerging pathway to tune the electronic, vibrational and optical properties. The fascinating properties of these systems is often linked to large-scale structural reconstruction of the moiré pattern. Hence, an essential first step in the theoretical study of these systems is the construction and structural relaxation of the atoms in the moiré superlattice. We present the Twister package, a collection of tools that constructs commensurate superlattices for any combination of 2D materials and also helps perform structural relaxations of the moiré superlattice. Twister constructs commensurate moiré superlattices using the coincidence lattice method and provides an interface to perform structural relaxations using classical forcefields.

cond-mat.mtrl-sci↗

arXiv2020

Anharmonicity in Raman-active phonon modes in atomically thin MoS$_2$

Phonon-phonon anharmonic effects have a strong influence on the phonon spectrum; most prominent manifestation of these effects are the softening (shift in frequency) and broadening (change in FWHM) of the phonon modes at finite temperature. Using Raman spectroscopy, we studied the temperature dependence of the FWHM and Raman shift of $\mathrm{E_{2g}^1}$ and $\mathrm{A_{1g}}$ modes for single-layer and natural bilayer MoS$_2$ over a broad range of temperatures ($8 < $T$ < 300$ K). Both the Raman shift and FWHM of these modes show linear temperature dependence for $T>100$ K, whereas they become independent of temperature for $T<100$ K. Using first-principles calculations, we show that three-phonon anharmonic effects intrinsic to the material can account for the observed temperature-dependence of the line-width of both the modes. It also plays an important role in determining the temperature-dependence of the frequency of the Raman modes. The observed evolution of the line-width of the A$_{1g}$ mode suggests that electron-phonon processes are additionally involved. From the analysis of the temperature-dependent Raman spectra of MoS$_2$ on two different substrates -- SiO$_2$ and hexagonal boron nitride, we disentangle the contributions of external stress and internal impurities to these phonon-related processes. We find that the renormalization of the phonon mode frequencies on different substrates is governed by strain and intrinsic doping. Our work establishes the role of intrinsic phonon anharmonic effects in deciding the Raman shift in MoS$_2$ irrespective of substrate and layer number.

cond-mat.mes-hall↗

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↗