arXiv subjects

Swastik Kar

Publications and source records attributed to Swastik Kar.

At least 19 recordsLinked to original sources

arXiv2026

Nanoscale imaging of ferromagnetic vortex dynamics with scanning NV magnetometry

The generation and manipulation of spin waves at the nanoscale via magnetic vortices are of considerable importance because of their broad applications across magnonic and quantum technologies. Previously, fixed nitrogen-vacancy (NV) centers in diamond have been used to locally characterize vortex dynamics, and scanning NV magnetometry (SNVM) has been used to image vortices' static stray fields. Here, we demonstrate SNVM imaging of both the static and microwave fields generated by vortices in mesoscopic permalloy structures with $\sim$50 nm spatial resolution, achieving excellent agreement with micromagnetic simulations, while revealing the effects of disorder. We further demonstrate a 40$\times$ microwave field enhancement near a vortex core and image the disorder-dependent, spatially varying, evanescent decay of these microwaves. Our ambient, tabletop technique surpasses diffraction-limited techniques' resolutions by at least 5$\times$, with far greater accessibility and throughput than synchrotron radiation-based techniques, offering new opportunities in the study and development of magnonic devices.

cond-mat.mes-hall↗

arXiv2026

Conductivity scaling of the anomalous Hall effect in the altermagnetic semiconductor {\alpha}-MnTe

{\alpha}-MnTe is a prototypical altermagnet exhibiting a strong anomalous Hall effect (AHE), despite having a nearly vanishing magnetization. Lately, sample-to-sample variations of the amplitude of the AHE have raised concerns of a possible defect related origin, especially in thin films. Here, we study the AHE in {\alpha}-MnTe films grown on SrF2 that have the crystal structure and m'm'm magnetic point group symmetry expected for bulk. By studying the scaling of the AHE with conductivity for those films and previously reported measurements in the literature, we find that sample-to-sample variations are well explained by a scaling law consistent with a hopping origin. Importantly, a comparison with other magnetic semiconductors reveals the colossal amplitude of the AHE of {\alpha}-MnTe compared to its measured spontaneous magnetization from magnetometry and polarized neutron reflectivity. Our findings address the important fundamental question of the origin of the AHE of {\alpha}-MnTe and further demonstrate the potential of altermagnets as promising spintronic materials.

cond-mat.str-el↗

arXiv2025

Flux channeling induced nano-confinement and enhancement of microwaves imaged by Rabi oscillation mapping

With rapid advances in qubit technologies, techniques for localizing, modulating, and measuring RF fields and their impact on qubit performance are of the utmost importance. Here, we demonstrate that flux-channeling from a permalloy nanowire can be used to achieve localized spatial modulation of an RF field and that the modulated field can be mapped with high resolution using the Rabi oscillations of an NV center. Rabi maps reveal ~100 mm wavelength microwaves concentrated in sub-300 nm-scale regions with up to ~16$\times$ power enhancement. This modulation is robust over a 20 dBm power range and has no adverse impact on NV $T_2$ coherence time. Micromagnetic simulations confirm that the modulated field results from the nanowire's stray field through its constructive/destructive interference with the incident RF field. Our findings provide a new pathway for controlling qubits, amplifying RF signals, and mapping local fields in various on-chip RF technologies.

cond-mat.mes-hall↗

arXiv2024

Enhanced heat dissipation and lowered power consumption in electronics using two-dimensional hexagonal boron nitride coatings

Miniaturization of electronic components has led to overheating, increasing power consumption and causing early circuit failures. Conventional heat dissipation methods are becoming inadequate due to limited surface area and higher short-circuit risks. This study presents a fast, low-cost, and scalable technique using 2D hexagonal boron nitride (hBN) coatings to enhance heat dissipation in commercial electronics. Inexpensive hBN layers, applied by drop casting or spray coating, boost thermal conductivity at IC surfaces from below 0.3 W/m-K to 260 W/m-K, resulting in over double the heat flux and convective heat transfer. This significantly reduces operating temperatures and power consumption, as demonstrated by a 17.4% reduction in a coated audio amplifier circuit board. Density functional theory indicates enhanced interaction between 2D hBN and packaging materials as a key factor. This approach promises substantial energy and cost savings for large-scale electronics without altering existing manufacturing processes.

cond-mat.mtrl-sci↗

arXiv2022

2022 Roadmap for Materials for Quantum Technologies

Quantum technologies are poised to move the foundational principles of quantum physics to the forefront of applications. This roadmap identifies some of the key challenges and provides insights on materials innovations underlying a range of exciting quantum technology frontiers. Over the past decades, hardware platforms enabling different quantum technologies have reached varying levels of maturity. This has allowed for first proof-of-principle demonstrations of quantum supremacy, for example quantum computers surpassing their classical counterparts, quantum communication with reliable security guaranteed by laws of quantum mechanics, and quantum sensors uniting the advantages of high sensitivity, high spatial resolution, and small footprints. In all cases, however, advancing these technologies to the next level of applications in relevant environments requires further development and innovations in the underlying materials. From a wealth of hardware platforms, we select representative and promising material systems in currently investigated quantum technologies. These include both the inherent quantum bit systems as well as materials playing supportive or enabling roles, and cover trapped ions, neutral atom arrays, rare earth ion systems, donors in silicon, color centers and defects in wide-band gap materials, two-dimensional materials and superconducting materials for single-photon detectors. Advancing these materials frontiers will require innovations from a diverse community of scientific expertise, and hence this roadmap will be of interest to a broad spectrum of disciplines.

quant-ph↗

arXiv2021

Second- and third-order optical susceptibilities in bidimensional semiconductors near excitons states

Semiconducting Transition Metal Dichalcogenides (TMDs) have significant nonlinear optical effects. In this work we have used second-harmonic generation (SHG) and the four-wave mixing (FWM) spectroscopy in resonance with the excitons in MoS2, MoSe2, and WS2 monolayers to characterize the nonlinear optical properties of these materials. We show that trions and excitons are responsible for enhancing the nonlinear optical response, and determine the exciton and trion energies by comparing with the photoluminescence spectra. Moreover, we extract the second and third order optical sheet susceptibility near exciton energies and compare with values found in the literature. We also demonstrate the ability to generate different nonlinear effects in a wide spectral range in the visible region for monolayer MoS2, opening the possibility of using two-dimensional materials for nonlinear optoelectronic and photonic applications.

cond-mat.mes-hall↗

arXiv2021

Twistronics: A turning point in 2D quantum materials

Moiré superlattices - periodic orbital overlaps and lattice-reconstruction between sites of high atomic registry in vertically-stacked 2D layered materials - are quantum-active interfaces where non-trivial quantum phases on novel phenomena can emerge from geometric arrangements of 2D materials, which are not intrinsic to the parent materials. Unexpected distortions in band-structure and topology lead to long-range correlations, charge-ordering, and several other fascinating quantum phenomena hidden within the physical space between the (similar or dissimilar) parent materials. Stacking, twisting, gate-modulating, and optically-exciting these superlattices open up a new field for seamlessly exploring physics from the weak to strong correlations limit within a many-body and topological framework. It is impossible to capture it all, and the aim of this review is to highlight some of the important recent developments in synthesis, experiments, and potential applications of these materials.

cond-mat.mes-hall↗

arXiv2020

2D-MoS2 with Narrowest Excitonic Linewidths Grown by Flow-Less Direct Heating of Bulk Powders

Developing techniques for high-quality synthesis of mono and few-layered 2D materials with lowered complexity and cost continues to remain an important goal, both for accelerating fundamental research and for applications development. We present the simplest conceivable technique to synthesize micrometer-scale single-crystal triangular monolayers of MoS2, i.e. by direct heating of bulk MoS2 powder onto proximally-placed substrates. Room-temperature excitonic linewidth values of our samples are narrower and more uniform than those of 2D-MoS2 obtained by most other techniques reported in literature, and comparable to those of ultraflat h-BN-capped mechanically exfoliated samples, indicative of their high quality. Feature-rich Raman spectra absent in samples grown or obtained by most other techniques, also stand out as a testament of the high quality of our samples. A contact-growth mode facilitates direct growth of crystallographically-strained circular samples, which allows us to directly compare the optoelectronic properties of flat vs. strained growth from the same growth runs. Our method allows, for the first time, to quantitatively compare the impact of strain on excitonic and Raman peak positions on identically-synthesized 2D-MoS2. Strain leads to average Red-shifts of ~ 30 meV in the A-exciton position, and ~ 2 cm-1 in the E12g Raman peak in these samples. Our findings open-up several new possibilities that expand 2D material research. By eliminating the need for carrier gas flow, mechanical motion or chemical reactions, our method can be either miniaturized for substantially low-cost, high-quality scientific research or potentially scaled-up for mass-production of 2D crystals for commercial purposes. Moreover, we believe this technique can also be extended to other transition metal dichalcogenides and other layered materials.

cond-mat.mtrl-sci↗

arXiv2020

Wafer-Scale Lateral Self-assembly of Mosaic Ti3C2Tx (MXene) Monolayer Films

Bottom-up assembly of two-dimensional (2D) materials into macroscale morphologies with emergent properties requires control of the material surroundings, so that energetically favorable conditions direct the assembly process. MXenes, a class of recently developed 2D materials, have found new applications in areas such as electrochemical energy storage, nanoscale electronics, sensors, and biosensors. In this report, we present a lateral self-assembly method for wafer-scale deposition of a mosaic-type 2D MXene flake monolayers that spontaneously order at the interface between two immiscible solvents. Facile transfer of this monolayer onto a flat substrate (Si, glass) results in high-coverage (>90%) monolayer films with uniform thickness, homogeneous optical properties, and good electrical conductivity. Multiscale characterization of the resulting films reveals the mosaic structure and sheds light on the electronic properties of the films, which exhibit good conductivity over cm-scale areas.

physics.app-ph↗

arXiv2020

Development of Use-specific High Performance Cyber-Nanomaterial Optical Detectors by Effective Choice of Machine Learning Algorithms

Due to their inherent variabilities,nanomaterial-based sensors are challenging to translate into real-world applications,where reliability/reproducibility is key.Recently we showed Bayesian inference can be employed on engineered variability in layered nanomaterial-based optical transmission filters to determine optical wavelengths with high accuracy/precision.In many practical applications the sensing cost/speed and long-term reliability can be equal or more important considerations.Though various machine learning tools are frequently used on sensor/detector networks to address these,nonetheless their effectiveness on nanomaterial-based sensors has not been explored.Here we show the best choice of ML algorithm in a cyber-nanomaterial detector is mainly determined by specific use considerations,e.g.,accuracy, computational cost,speed, and resilience against drifts/ageing effects.When sufficient data/computing resources are provided,highest sensing accuracy can be achieved by the kNN and Bayesian inference algorithms,but but can be computationally expensive for real-time applications.In contrast,artificial neural networks are computationally expensive to train,but provide the fastest result under testing conditions and remain reasonably accurate.When data is limited,SVMs perform well even with small training sets,while other algorithms show considerable reduction in accuracy if data is scarce,hence,setting a lower limit on the size of required training data.We show by tracking/modeling the long-term drifts of the detector performance over large (1year) period,it is possible to improve the predictive accuracy with no need for recalibration.Our research shows for the first time if the ML algorithm is chosen specific to use-case,low-cost solution-processed cyber-nanomaterial detectors can be practically implemented under diverse operational requirements,despite their inherent variabilities.

physics.app-ph↗

arXiv2019

Absorption and emission modulation in MoS2-GaN (0001) heterostructure by interface phonon-exciton coupling

Semiconductor heterostructures based on layered two-dimensional transition metal dichalcogenides (TMD) interfaced to gallium nitride (GaN) are excellent material systems to realize broadband light emitters and absorbers. The surface properties of the polar semiconductor, such as GaN are dominated by interface phonons, thus the optical properties of the vertical heterostructure depend strongly on the interface exciton-phonon coupling. The origin and activation of different Raman modes in the heterostructure due to coupling between interfacial phonons and optically generated carriers in a monolayer MoS2-GaN (0001) heterostructure was observed. This coupling strongly influences the non-equilibrium absorption properties of MoS2 and the emission properties of both semiconductors. Density functional theory (DFT) calculations were performed to study the band alignment of the interface, which revealed a type-I heterostructure. The optical excitation with interband transition in MoS2 at K-point strongly modulates the C excitonic band in MoS2. The overlap of absorption and emission bands of GaN with the absorption bands of MoS2 induces the energy and charge transfer across the interface with an optical excitation at Γ-point. A strong modulation of the excitonic absorption states is observed in MoS2 on GaN substrate with transient optical pump-probe spectroscopy. The interaction of carriers with phonons and defect states leads to the enhanced and blue shifted emission in MoS2 on GaN substrate. Our results demonstrate the relevance of interface coupling between phonons and carriers for the development of optical and electronic applications.

cond-mat.mtrl-sci↗

arXiv2019

Active control of coherent dynamics in hybrid plasmonic MoS2 monolayers with dressed phonons

The near-field interaction due to a strong electromagnetic field induced by resonant localized plasmons can result in a strong coupling of excitonic states or formation of hybrid exciton-plasmon modes in quantum confined structures. This coupling can be strengthed by designing a system with its vibronic states resonant to the energy of the driving field induced by the localized plasmon excitation. Silver nanoislands nucleated on molybdenum disulfide (MoS2) is an ideal platform for such interaction. The influence of localized plasmons (LSP) on the formation and dissociation of excitons due to resonant and off-resonant optical excitation of carriers to excitonic states is studied using ultrafast optical spectroscopy. The local field due to the Ag nanoparticles (Ag-NP) enhances the magnitude of the Raman modes in MoS2 in the presence of resonant excitation. An ultrashort pulsed optical excitation at 2.3 eV resonantly excites the LSP modes and the optical near-field resonantly drive the phonon modes, which leads to a coherent coupling of the A and B excitons in MoS2 with the plasmon modes. The resonant excitation of the LSP modes modulate the optical absorption of the probe field. The resonant excitation of C exciton due to an excitation source at 3.0 eV, which is off-resonant to the LSP mode increases the electrostatic screening in the presence of excess carriers from Ag NPs. It results in a faster dissociation of optically generated C excitons into free carriers that eventually increase the A and B exciton population. A 3-level density matrix theory in the presence of dressed vibronic states induced by a localized near-field optical driving source is applied to describe the coherent interaction process in the hybrid nano-plasmonic system.

cond-mat.mes-hall↗

arXiv2019

Nonlinear dark-field imaging of 1D defects in monolayer dichalcogenides

Extended defects with one dimensionality smaller than that of the host, such as 2D grain boundaries in 3D materials or 1D grain boundaries in 2D materials, can be particularly damaging since they directly impede the transport of charge, spin or heat, and can introduce a metallic character into otherwise semiconducting systems. Unfortunately, a technique to rapidly and non-destructively image 1D defects in 2D materials is lacking. Scanning transmission electron microscopy (STEM), Raman, photoluminescence and nonlinear optical spectroscopies, are all extremely valuable, but current implementations suffer from low throughput and a destructive nature (STEM) or limitations in their unambiguous sensitivity at the nanoscale. Here we demonstrate that dark-field second harmonic generation (SHG) microscopy can rapidly, efficiently, and non-destructively probe grain boundaries and edges in monolayer dichalcogenides (i.e. MoSe2, MoS2 and WS2). Dark-field SHG efficiently separates the spatial components of the emitted light and exploits interference effects from crystal domains of different orientations to localize grain boundaries and edges as very bright 1D patterns through a Cerenkov-type SHG emission. The frequency dependence of this emission in MoSe2 monolayers is explained in terms of plasmon-enhanced SHG related to the defects metallic character. This new technique for nanometer-scale imaging of the grain structure, domain orientation and localized 1D plasmons in 2D different semiconductors, thus enables more rapid progress towards both applications and fundamental materials discoveries.

cond-mat.mes-hall↗

arXiv2019

Bayesian Inference-enabled Precise Optical Wavelength Estimation using Transition Metal Dichalcogenide Thin Films

Despite its ability to draw precise inferences from large and complex datasets, the use of data analytics in the field of condensed matter and materials sciences -- where vast quantities of complex metrology data are regularly generated -- has remained surprisingly limited. Specifically, such approaches could dramatically reduce the engineering complexities of devices that directly exploit the physical properties of materials. Here, we present a cyber-physical system for accurately estimating the wavelength of any monochromatic light in the range of 325-1100nm, by applying Bayesian inference on the optical transmittance data from a few low-cost, easy-to-fabricate thin film "filters" of layered transition metal dichalcogenides (TMDs) such as MoS2 and WS2. Wavelengths of tested monochromatic light could be estimated with only 1% estimation error over 99% of the stated spectral range, with lowest error values reaching as low as a few ten parts per million (ppm) in a system with only eleven filters. By step-wise elimination of filters with the least contribution toward accuracy, mean estimation accuracy of 99% could be obtained even in a two-filter system. Furthermore, we provide a statistical approach for selecting the best "filter" material for any intended spectral range based on the spectral variation of transmittance within the desired range of wavelengths. And finally, we demonstrate that calibrating the data-driven models for the filters from time to time overcomes the minor drifts in their transmittance values, which allows using the same filters indefinitely. This work not only enables the development of simple cyber-physical photodetectors with high accuracy color-estimation, but also provides a framework for developing similar cyber-physical systems with drastically reduced complexity.

physics.app-ph↗

arXiv2019

Oxygen-induced in-situ manipulation of the interlayer coupling and exciton recombination in Bi2Se3/MoS2 2D heterostructures

2D heterostructures are more than a sum of the parent 2D materials, but are also a product of the interlayer coupling, which can induce new properties. In this paper we present a method to tune the interlayer coupling in Bi2Se3/MoS2 2D heterostructures by regulating the oxygen presence in the atmosphere, while applying laser or thermal energy. Our data suggests the interlayer coupling is tuned through the diffusive intercalation and de-intercalation of oxygen molecules. When one layer of Bi2Se3 is grown on monolayer MoS2, an influential interlayer coupling is formed that quenches the signature photoluminescence (PL) peaks. However, thermally annealing in the presence of oxygen disrupts the interlayer coupling, facilitating the emergence of the MoS2 PL peak. DFT calculations predict intercalated oxygen increases the interlayer separation ~17%, disrupting the interlayer coupling and inducing the layers to behave more electronically independent. The interlayer coupling can then be restored by thermally annealing in N2 or Ar, where the peaks will re-quench. Hence, this is an interesting oxygen-induced switching between "non-radiative" and "radiative" exciton recombination. This switching can also be accomplished locally, controllably, and reversibly using a low-power focused laser, while changing the environment from pure N2 to air. This allows for the interlayer coupling to be precisely manipulated with submicron spatial resolution, facilitating site-programmable 2D light-emitting pixels whose emission intensity could be precisely varied by a factor exceeding 200x. Our results show that these atomically-thin 2D heterostructures may be excellent candidates for oxygen sensing.

cond-mat.mtrl-sci↗

arXiv2018

Dynamics of liquid crystal on hexagonal lattice

Nematic liquid crystal (LC) molecules adsorbed on two dimensional materials are aligned along the crystal directions of the hexagonal lattice. It was demonstrated that short electric pulses can reorient the aligned LC molecules in the preferred armchair direction of hexagonal boron nitride (h-BN). Several states with a variety of colors were obtained by changing the direction and strength of the electric pulses. The ab initio calculations based on density functional theory was carried out to determine the favorable adsorption configurations of the LC molecules on the h-BN surface. A non-volatile display, in which pixel resolution can be determined by grains of hexagonal surface, is proposed, which can offer a pathway towards dynamic high-quality pixels with low power consumption, and could define a new paradigm for all non-volatile display applications.

cond-mat.soft↗

arXiv2015

Chemical Vapor Deposition Synthesized Atomically Thin Molybdenum Disulfide with Optoelectronic-Grade Crystalline Quality

The ability to synthesize high-quality samples over large areas and at low cost is one of the biggest challenges during the developmental stage of any novel material. While chemical vapor deposition (CVD) methods provide a promising low-cost route for CMOS compatible, large-scale growth of materials, it often falls short of the high-quality demands in nanoelectronics and optoelectronics. We present large-scale CVD synthesis of single- and few- layered MoS2 using direct vapor-phase sulfurization of MoO2, which enables us to obtain extremely high-quality single-crystal monolayer MoS2 samples with field-effect mobility exceeding 30 cm2/Vs in monolayers. These samples can be readily synthesized on a variety of substrates, and demonstrate a high-degree of optoelectronic uniformity in Raman and photoluminescence mapping over entire crystals with areas exceeding hundreds of square micrometers. Owing to their high crystalline quality, Raman spectroscopy on these samples reveal a range of multi-phonon processes through peaks with equal or better clarity compared to past reports on mechanically exfoliated samples. This enables us to investigate the layer thickness- and substrate-dependence of the extremely weak phonon processes at 285 cm-1 and 487 cm-1 in 2D MoS2. The ultra-high, optoelectronic-grade crystalline quality of these samples could be further established through photocurrent spectroscopy, which clearly reveal excitonic states at room temperature, a feat that has been previously demonstrated only on device fabricated with mechanically exfoliated that were artificially suspended across trenches. Our method reflects a big step in the development of atomically thin, 2D MoS2 for scalable, high-quality optoelectronics.

cond-mat.mtrl-sci↗

arXiv2015

Direct and Scalable Chemical Vapor Deposition of Ultrathin Low-Noise MoS2 Membranes on Apertures

We show that atomically thin molybdenum disulfide (MoS2) crystals can grow without any underlying substrates into free-standing atomically-thin layers, maintaining their planar 2D form. Using this property, we present a new mechanism for 2D crystal synthesis, i.e. reagent-limited nucleation near an aperture edge followed by reactions that allow crystal growth into the free-space of the aperture. Such an approach enables us, for the first time, the direct and selective growth of freestanding membranes of atomically thin MoS2 layers across micrometer-scale pre-fabricated solid-state apertures in SiNx membranes. Under optimal conditions, MoS2 grows preferentially across apertures, resulting in sealed membranes that are one to a few atomic layers thick. Since our method involves free-space growth and is devoid of either substrates or transfer, it is conceivably the most contamination-free method for obtaining 2D crystals reported so far. The membrane quality was investigated using atomic-resolution transmission electron microscopy, Raman spectroscopy, photoluminescence spectroscopy, and low-noise ion-current recordings through nanopores fabricated in such membranes.

cond-mat.mtrl-sci↗