arXiv subjects

Rohit Sharma

Publications and source records attributed to Rohit Sharma.

At least 37 records · Page 2Linked to original sources

arXiv2026

Microscopic origin of an exceptionally large phonon thermal Hall effect from charge puddles in a topological insulator

We present the experimental observation of a drastically enhanced thermal Hall effect in the topological insulator material TlBi$_{0.15}$Sb$_{0.85}$Te$_2$. Although heat transport is dominated by phonons, moderate magnetic fields generate a thermal Hall ratio ($κ_{xy}/κ_{xx}$) above 2\%, an unprecedented value for a nonmagnetic material. The transverse thermal conductivity $κ_{xy}$ exhibits a pronounced maximum in fields of a few Tesla. This characteristic field dependence allows us to identify the microscopic origin of the thermal Hall effect in this system. Small densities of charged impurities induce locally conducting regions, so-called charge puddles, within the bulk insulating matrix. Via electron-phonon coupling, these charge puddles imprint a large thermal Hall effect onto the phonons accounting for both the magnitude and the magnetic-field dependence of the observed effect.

cond-mat.str-el↗

arXiv2025

A Physical Analogy between Molecular Ordering and SAT-to-Ising Annealing

As temperature drops, molecular systems may undergo spontaneous ordering, moving from random behavior to orderly structure. This research demonstrates a direct analogy between this type of thermodynamic ordering in molecular systems and the development of coherent logic in computationally complex problem sets. We have proposed a mapping of Boolean SAT problem instances to pairwise Ising Hamiltonian models. Using simulated annealing, we then applied phenomenal cooling to the system through thermal evolution from high entropy random assignment to lower entropy, ordered assignments (the energy minima) using molecular cooling analogs. This indicated that there was a rapid "first-order" or "logical crystallization" of satisfiable logical configurations. The degree of backbone rigidity did not strongly correlate with the level of physical ordering observed in the system; thus, it appears that there is primarily a local alignment of constraint satisfaction occurring in the system. Thus, we have provided empirical evidence that satisfiable logical configurations are analogous to the low energy crystalline states observed in molecular systems and provide evidence for a unified thermodynamic view of computational coherence and complexity.

cs.LO↗

arXiv2025

Deep learning approach for identification of HII regions during reionization in 21-cm observations -- III. image recovery

The low-frequency component of the upcoming Square Kilometre Array Observatory (SKA-Low) will be sensitive enough to construct 3D tomographic images of the 21-cm signal distribution during reionisation. However, foreground contamination poses challenges for detecting this signal, and image recovery will heavily rely on effective mitigation methods. We introduce \texttt{SERENEt}, a deep-learning framework designed to recover the 21-cm signal from SKA-Low's foreground-contaminated observations, enabling the detection of ionised (HII) and neutral (HI) regions during reionisation. \texttt{SERENEt} can recover the signal distribution with an average accuracy of 75 per cent at the early stages ($\overline{x}_\mathrm{HI}\simeq0.9$) and up to 90 per cent at the late stages of reionisation ($\overline{x}_\mathrm{HI}\simeq0.1$). Conversely, HI region detection starts at 92 per cent accuracy, decreasing to 73 per cent as reionisation progresses. Beyond improving image recovery, \texttt{SERENEt} provides cylindrical power spectra with an average accuracy exceeding 93 per cent throughout the reionisation period. We tested \texttt{SERENEt} on a 10-degree field-of-view simulation, consistently achieving better and more stable results when prior maps were provided. Notably, including prior information about HII region locations improved 21-cm signal recovery by approximately 10 per cent. This capability was demonstrated by supplying \texttt{SERENEt} with ionising source distribution measurements, showing that high-redshift galaxy surveys of similar observation fields can optimise foreground mitigation and enhance 21-cm image construction.

astro-ph.CO↗

arXiv2025

Karabo: A versatile SKA Observation Simulation Framework

Karabo is a versatile Python-based software framework simplifying research with radio astronomy data. It bundles existing software packages into a coherent whole to improve the ease of use of its components. Karabo includes useful abstractions, like strategies to scale and parallelize typical workloads or science-specific Python modules. The framework includes functionality to access datasets and mock observations to study the Square Kilometer Array (SKA) instruments and their expected accuracy. SKA will address problems in a wide range of fields of astronomy. We demonstrate the application of Karabo to some of the SKA science cases from HI intensity mapping, mock radio surveys, radio source detection, the epoch of re-ionisation and heliophysics. We discuss the capabilities and challenges of simulating large radio datasets in the context of SKA.

astro-ph.IM↗

arXiv2025

READ: Reinforcement-based Adversarial Learning for Text Classification with Limited Labeled Data

Pre-trained transformer models such as BERT have shown massive gains across many text classification tasks. However, these models usually need enormous labeled data to achieve impressive performances. Obtaining labeled data is often expensive and time-consuming, whereas collecting unlabeled data using some heuristics is relatively much cheaper for any task. Therefore, this paper proposes a method that encapsulates reinforcement learning-based text generation and semi-supervised adversarial learning approaches in a novel way to improve the model's performance. Our method READ, Reinforcement-based Adversarial learning, utilizes an unlabeled dataset to generate diverse synthetic text through reinforcement learning, improving the model's generalization capability using adversarial learning. Our experimental results show that READ outperforms the existing state-of-art methods on multiple datasets.

cs.CL↗

arXiv2024

Optimizing Gastrointestinal Diagnostics: A CNN-Based Model for VCE Image Classification

In recent years, the diagnosis of gastrointestinal (GI) diseases has advanced greatly with the advent of high-tech video capsule endoscopy (VCE) technology, which allows for non-invasive observation of the digestive system. The MisaHub Capsule Vision Challenge encourages the development of vendor-independent artificial intelligence models that can autonomously classify GI anomalies from VCE images. This paper presents CNN architecture designed specifically for multiclass classification of ten gut pathologies, including angioectasia, bleeding, erosion, erythema, foreign bodies, lymphangiectasia, polyps, ulcers, and worms as well as their normal state.

cs.CV↗

arXiv2024

Phonon thermal Hall effect in non-magnetic Y$_2$Ti$_2$O$_7$

We report an investigation of the phonon thermal Hall effect in single crystal samples of Y$_2$Ti$_2$O$_7$, Dy$_2$Ti$_2$O$_7$, and DyYTi$_2$O$_7$. We measured the field-linear thermal Hall effect in all three samples. The temperature dependence of thermal Hall conductivities shows a peak around 15 K, which coincides with the peak positions of the longitudinal thermal conductivities. The temperature-dependent longitudinal thermal conductivities indicates that phonons dominate thermal transport in all three samples. However, the presence of Dy$^{3+}$ magnetic ions introduces significant effects on the field dependence of longitudinal thermal conductivities. The thermal Hall ratio is sizeable in all three samples and consistent with the values reported for other insulating materials exhibiting a phononic thermal Hall effect, though their exact underlying mechanism remains yet to be identified. The thermal Hall ratio is nearly the same for $\rm Y_2Ti_2O_7$ and DyYTi$_2$O$_7$, and slightly larger for Dy$_2$Ti$_2$O$_7$, suggesting that magnetic impurities are less significant in generating the phononic thermal Hall effect. Our observations of the phononic thermal Hall effect support an intrinsic origin in Y$_2$Ti$_2$O$_7$, but suggest a combination of intrinsic and extrinsic effects in Dy$_2$Ti$_2$O$_7$ and DyYTi$_2$O$_7$.

cond-mat.mtrl-sci↗

arXiv2024

Study of Particle Acceleration using Fine Structures and Oscillations in Microwaves from Electron Cyclotron Maser

The accelerated electrons during solar flares produce radio bursts and nonthermal X-ray signatures. The quasi-periodic pulsations (QPPs) and fine structures in spatial-spectral-temporal space in radio bursts depend on the emission mechanism and the local conditions, such as magnetic fields, electron density, and pitch angle distribution. Radio burst observations with high frequency-time resolution imaging provide excellent diagnostics. In converging magnetic field geometries, the radio bursts can be produced via the electron-cyclotron maser (ECM). Recently, using observations made by the Karl G. Jansky Very Large Array (VLA) at 1--2 GHz, \cite{Yu2023} reported a discovery of long-lasting auroral-like radio bursts persistent over a sunspot and interpreted them as ECM-generated emission. Here, we investigate the detailed second and sub-second temporal variability of this continuous ECM source. We study the association of 5-second period QPPs with a concurrent GOES C1.5-class flare, utilizing VLA's imaging spectroscopy capability with an extremely high temporal resolution (50 ms). We use the density and magnetic field extrapolation model to constrain the ECM emission to the second harmonic o-mode. Using the delay of QPPs from X-ray emission times, combined with X-ray spectroscopy and magnetic extrapolation, we constrain the energies and pitch angles of the ECM-emitting electrons to $\approx$4-8 keV and $>26^{\circ}$. Our analysis shows that the loss-cone diffusion continuously fuels the ECM via Coulomb collisions and magnetic turbulence between a 5 Mm--100 Mm length scale. We conclude that the QPP occurs via the Lotka-Volterra system, where the electron from solar flares saturates the continuously operating ECM and causes temporary oscillations.

astro-ph.SR↗

arXiv2024

Phonon thermal Hall effect in charge-compensated topological insulators

From a systematic study of thermal and charge transport in various single crystals of compensated topological insulators we identify the evolution of a large low-temperature thermal Hall effect as a characteristic common feature. In order to separate phononic and electronic contributions in the measured longitudinal and transverse thermal conductivity, the electronic contributions are estimated from corresponding electrical resisivity and Hall effect measurements on the same samples by using the Wiedemann-Franz law. As may be expected for charge-compensated topological insulators the longitudinal thermal conductivity is phonon-dominated in all samples. However, we also find a pronounced field-linear thermal Hall effect that becomes most pronounced in the low-temperature range, where all samples are good electrical insulators. This indicates an underlying phononic mechanism of the thermal Hall effect and in this respect the topological insulators resemble other, mainly ionic, insulators, which have been reported to show a phonon-induced thermal Hall effect, but its underlying phononic mechanism remains to be identified. Our observation of a comparable thermal Hall ratio in topological insulators supports a theoretical scenario that explains a thermal Hall effect through skew scattering on charged impurities.

cond-mat.str-el↗

arXiv2023

Detection of long-lasting aurora-like radio emission above a sunspot

Auroral radio emissions in planetary magnetospheres typically feature highly polarized, intense radio bursts, usually attributed to electron cyclotron maser (ECM) emission from energetic electrons in the planetary polar region that features a converging magnetic field. Similar bursts have been observed in magnetically active low-mass stars and brown dwarfs, often prompting analogous interpretations. Here we report observations of long-lasting solar radio bursts with high brightness temperature, wide bandwidth, and high circular polarization fraction akin to these auroral/exo-auroral radio emissions, albeit two to three orders of magnitude weaker than those on certain low-mass stars. Spatially, spectrally, and temporally resolved analysis suggests that the source is located above a sunspot where a strong, converging magnetic field is present. The source morphology and frequency dispersion are consistent with ECM emission due to precipitating energetic electrons produced by recurring flares nearby. Our findings offer new insights into the origin of such intense solar radio bursts and may provide an alternative explanation for auroral-like radio emissions on other flare stars with large starspots.

astro-ph.SR↗

arXiv2023

Glass-like thermal conductivity and narrow insulating gap of EuTiO$_3$

Crystals and glasses differ by the amplitude and the temperature dependence of their thermal conductivity. However, there are crystals known to display glass-like thermal conductivity. Here, we show that EuTiO$_3$, a quantum paraelectric known to order antiferromagnetically at 5.5 K, is one such system. The temperature dependence of resistivity and Seebeck coefficient yield an insulating band gap of $\sim 0.22$ eV. Thermal conductivity is drastically reduced. Its amplitude and temperature dependence are akin to what is seen in amorphous silica. Comparison with non-magnetic perovskite solids, SrTiO$_3$, KTaO$_3$, and EuCoO$_3$, shows that what impedes heat transport are $4f$ spins at Eu$^{2+}$ sites, which couple to phonons well above the ordering temperature. Thus, in this case, superexchange and valence fluctuations, not magnetic frustration, are the drivers of the glass-like thermal conductivity.

cond-mat.mtrl-sci↗

arXiv2023

Model Explainability in Physiological and Healthcare-based Neural Networks

The estimation and monitoring of SpO2 are crucial for assessing lung function and treating chronic pulmonary diseases. The COVID-19 pandemic has highlighted the importance of early detection of changes in SpO2, particularly in asymptomatic patients with clinical deterioration. However, conventional SpO2 measurement methods rely on contact-based sensing, presenting the risk of cross-contamination and complications in patients with impaired limb perfusion. Additionally, pulse oximeters may not be available in marginalized communities and undeveloped countries. To address these limitations and provide a more comfortable and unobtrusive way to monitor SpO2, recent studies have investigated SpO2 measurement using videos. However, measuring SpO2 using cameras in a contactless way, particularly from smartphones, is challenging due to weaker physiological signals and lower optical selectivity of smartphone camera sensors. The system includes three main steps: 1) extraction of the region of interest (ROI), which includes the palm and back of the hand, from the smartphone-captured videos; 2) spatial averaging of the ROI to produce R, G, and B time series; and 3) feeding the time series into an optophysiology-inspired CNN for SpO2 estimation. Our proposed method can provide a more efficient and accurate way to monitor SpO2 using videos captured from consumer-grade smartphones, which can be especially useful in telehealth and health screening settings.

cs.CV↗

arXiv2023

Single-crystal study of the honeycomb XXZ magnet BaCo$_2$(PO$_4$)$_2$ in magnetic fields

We present a study of high-quality BaCo$_2$(PO$_4$)$_2$ single crystals via magnetization, heat-capacity, thermal-expansion and magnetostriction measurements. Sharp anomalies in the thermodynamic properties at $T_N=3.4\,$K reveal a long-range antiferromagnetic order in these single-crystalline samples, which is absent in polycrystalline BaCo$_2$(PO$_4$)$_2$. The temperature dependent magnetic susceptibilities for in-plane and out-of-plane magnetic fields are strongly anisotropic and reveal a pronounced easy-plane anisotropy. A Curie-Weiss analysis implies strong orbital magnetism, as it is known from the sister compound BaCo$_2$(AsO$_4$)$_2$ that is discussed as a potential Kitaev spin-liquid material. When applying in-plane magnetic fields at low temperature, BaCo$_2$(PO$_4$)$_2$ is driven to another ordered phase at a critical field $H_{C1}\approx 0.11\,$T and then undergoes a further field-induced transition to a highly polarized paramagnetic phase at $H_{C2}\approx 0.3\,$T, which is again similar to the case of BaCo$_2$(AsO$_4$)$_2$. In addition, our lowest-temperature data reveal that the field-induced transitions in BaCo$_2$(PO$_4$)$_2$ become dominated by thermally assisted domain-wall motion.

cond-mat.str-el↗

arXiv2022

Multiple Regions of Nonthermal Quasi-Periodic Pulsations during the Impulsive Phase of a Solar Flare

Flare-associated quasiperiodic pulsations (QPPs) in radio and X-ray wavelengths, particularly those related to nonthermal electrons, contain important information about the energy release and transport processes during flares. However, the paucity of spatially resolved observations of such QPPs with a fast time cadence has been an obstacle for us to further understand their physical nature. Here, we report observations of such a QPP event that occurred during the impulsive phase of a C1.8-class eruptive solar flare using radio imaging spectroscopy data from the Karl G. Jansky Very Large Array (VLA) and complementary X-ray imaging and spectroscopy data. The radio QPPs, observed by the VLA in the 1--2 GHz with a subsecond cadence, are shown as three spatially distinct sources with different physical characteristics. Two radio sources are located near the conjugate footpoints of the erupting magnetic flux rope with opposite senses of polarization. One of the sources displays a QPP behavior with a ~5 s period. The third radio source, located at the top of the postflare arcade, coincides with the location of an X-ray source and shares a similar period of ~25--45 s. We show that the two oppositely polarized radio sources are likely due to coherent electron cyclotron maser (ECM) emission. On the other hand, the looptop QPP source, observed in both radio and X-rays, is consistent with incoherent gyrosynchrotron and bremsstrahlung emission, respectively. We conclude that the concurrent, but spatially distinct QPP sources must involve multiple mechanisms which operate in different magnetic loop systems and at different periods.

astro-ph.SR↗

arXiv2022

ULTRA: A Data-driven Approach for Recommending Team Formation in Response to Proposal Calls

We introduce an emerging AI-based approach and prototype system for assisting team formation when researchers respond to calls for proposals from funding agencies. This is an instance of the general problem of building teams when demand opportunities come periodically and potential members may vary over time. The novelties of our approach are that we: (a) extract technical skills needed about researchers and calls from multiple data sources and normalize them using Natural Language Processing (NLP) techniques, (b) build a prototype solution based on matching and teaming based on constraints, (c) describe initial feedback about system from researchers at a University to deploy, and (d) create and publish a dataset that others can use.

cs.IR↗

arXiv2022

Preparing for Solar and Heliospheric Science with the SKAO: An Indian Perspective

The Square Kilometre Array Observatory (SKAO) is perhaps the most ambitious radio telescope envisaged yet. It will enable unprecedented studies of the Sun, the corona and the heliosphere and help to answer many of the outstanding questions in these areas. Its ability to make a vast previously unexplored phase space accessible, also promises a large discovery potential. The Indian solar and heliospheric physics community have been preparing for this science opportunity. A significant part of this effort has been towards playing a leading role in pursuing science with SKAO precursor instruments. This article briefly summarises the current status of the various aspects of work done as a part of this enterprise and our future goals.

astro-ph.IM↗

arXiv2022

Detection of weak ubiquitous impulsive nonthermal emissions from the solar corona

A ubiquitous presence of weak energy releases is one of the most promising hypotheses to explain coronal heating, referred to as the nanoflare hypothesis. The accelerated electrons associated with such weak heating events are also expected to give rise to coherent impulsive emission via plasma instabilities in the meterwave radio band, making this a promising spectral window to look for their presence. Recently \citet{Mondal2020b} reported the presence of weak impulsive emissions from quiet Sun regions which seem to meet the requirements of being radio counterparts of the hypothesized nanoflares. Detection of such low-contrast weak emission from the quiet Sun is challenging and, given their implications, it is important to confirm their presence. In this work, using data from the Murchison Widefield Array, we explore the use of an independent robust approach for their detection by separating the dominant slowly varying component of emission from the weak impulsive one in the visibility domain. We detect milli-SFU level bursts taking place all over the Sun and characterize their brightness temperatures, distributions, morphologies, durations and association with features seen in EUV images. We also attempt to constraint the energies of the nonthermal particles using inputs from the FORWARD coronal model along with some reasonable assumptions and find them to lie in the sub-pico flare ($\sim 10^{19}-10^{21}$ ergs) range. In the process, we also discover perhaps the weakest type III radio burst and another one that shows clear signatures of weakest quasi-periodic pulsations.

astro-ph.SR↗

arXiv2021

Multiple electron acceleration instances during a series of solar microflares observed simultaneously at X-rays and microwaves

Even small solar flares can display a surprising level of complexity regarding their morphology and temporal evolution. Many of their properties, such as energy release and electron acceleration can be studied using highly complementary observations at X-ray and radio wavelengths. We present X-ray observations from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and radio observations from the Karl G. Jansky Very Large Array (VLA) of a series of GOES A3.4 to B1.6 class flares observed on 2013 April 23. The flares, as seen in X-ray and extreme ultraviolet (EUV), originated from multiple locations within active region NOAA 11726. A veritable zoo of different radio emissions between 1 GHz and 2 GHz was observed co-temporally with the X-ray flares. In addition to broad-band continuum emission, broad-band short-lived bursts and narrow-band spikes, indicative of accelerated electrons, were observed. However, these sources were located up to 150 arcsec away from the flaring X-ray sources but only some of these emissions could be explained as signatures of electrons that were accelerated near the main flare site. For other sources, no obvious magnetic connection to the main flare site could be found. These emissions likely originate from secondary acceleration sites triggered by the flare, but may be due to reconnection and acceleration completely unrelated to the co-temporally observed flare. Thanks to the extremely high sensitivity of the VLA, not achieved with current X-ray instrumentation, it is shown that particle acceleration happens frequently and at multiple locations within a flaring active region.

astro-ph.SR↗