arXiv subjects

Gaurav Shukla

Publications and source records attributed to Gaurav Shukla.

At least 19 recordsLinked to original sources

arXiv2026

ASTER -- Agentic Science Toolkit for Exoplanet Research

The expansion of exoplanet observations has created a need for flexible, accessible, and user-friendly workflows. Transmission spectroscopy has become a key technique for probing atmospheric composition of transiting exoplanets. The analyses of these data require the combination of archival queries, literature search, the use of radiative transfer models, and Bayesian retrieval frameworks, each demanding specialized expertise. Modern large language models enable the coordinated execution of complex, multi-step tasks by AI agents with tool integration, structured prompts, and iterative reasoning. In this study we present ASTER, an Agentic Science Toolkit for Exoplanet Research. ASTER is an orchestration framework that brings LLM capability to the exoplanetary community by enabling LLM-driven interaction with integrated domain-specific tools, workflow planning and management, and support for common data analysis tasks. Currently ASTER incorporates tools for downloading planetary parameters and observational datasets from the NASA Exoplanet Archive, as well as the generation of transit spectra from the TauREx radiative transfer model, and the completion of Bayesian retrieval of planetary parameters with TauREx. Beyond tool integration, the agent assists users by proposing alternative modeling approaches, reporting potential issues and suggesting solutions, and interpretations. We demonstrate ASTER's workflow through a complete case study of WASP-39b, performing multiple retrievals using observational data available on the archive. The agent efficiently transitions between datasets, generates appropriate forward model spectra and performs retrievals. ASTER provides a unified platform for the characterization of exoplanet atmospheres. Ongoing development and community contributions will continue expanding ASTER's capabilities toward broader applications in exoplanet research.

astro-ph.EP↗

arXiv2026

Growth and Transport Properties of InAsSb Nanoflags

The present work reports, for the first time, the growth of high-quality free-standing InAsSb nanoflags and their electronic properties. Different growth conditions have been explored, and zinc-blende InAsSb nanoflags of various composition have been obtained. In particular, InAs0.77Sb0.23 nanoflags are on average (2000+-180) nm long, (640+-50) nm wide, and (130+-30) nm thick. We show that these nanoflags have a Land\'e g-factor larger than InAs and InSb and a mobility comparable to those of the best performing InAs and InSb nanoflags. Besides, we show evidence for a surface Fermi level pinning in the conductance band of these InAs0.77Sb0.23 nanoflags, similar to the well-known behavior of InAs. This promises to make InAsSb easy to couple to superconductors, while keeping or improving many of the features that make InSb an interesting material for quantum applications.

cond-mat.mtrl-sci↗

arXiv2025

A Detailed Investigation of HD 209458 b HST & JWST Transmission Spectra with SANSAR

HD 209458 b is the first exoplanet on which an atmosphere was detected. Since then, its atmosphere has been investigated using multiple telescopes and instruments. However, many of its atmospheric constraints remain debatable. While HST observations suggested a highly sub-solar metallicity, recent JWST NIRCam observations by Xue et al. 2024 constrained a super-solar metallicity with highly sub-solar C/O. In this work, we show a detailed investigation of HD 209458 b transmission spectra observations from JWST and HST using SANSAR, a newly developed planetary atmosphere modeling framework, with free, equilibrium chemistry and self-consistent grid retrievals. The overall best-fitting model with free retrievals ($χ^2_{\rm{red}}$=1.21) constrains its metallicity and C/O to be highly sub-solar, while equilibrium chemistry and grid retrievals ($χ^2_{\rm{red}}$=1.27 and 1.30, respectively) are consistent with solar values using STIS+WFC3+NIRCam observations. The retrieved abundances of H$_2$O and CO$_2$ are almost three orders of magnitude lower (highly sub-solar) with STIS+WFC3+NIRCam compared to just NIRCam, using free retrievals. NIRCam observations alone also result in misleading constraints on metallicity and C/O, with equilibrium chemistry and grid retrieval. We find that the model choice of varying C/H or O/H to vary the C/O in equilibrium chemistry retrievals leads to different metallicity constraints with NIRCam, but similar constraints with STIS+WFC3+NIRCam. We conclude that NIRCam observations alone can lead to overestimation of abundances for exoplanet atmospheres and, therefore, should be used in combination with UV/Optical and near-infrared observations to obtain robust constraints on abundances, C/O, and metallicity. In particular, even though we can detect the CO$_2$ feature with just NIRCam, we cannot constrain its abundances robustly without the optical baseline.

astro-ph.EP↗

arXiv2025

Revisiting the Atmosphere of HD 149026b: The Role of Stellar Abundances and Choice of Opacities in Exoplanet Atmosphere Modeling

Planet formation occurs within the same molecular cloud as the host star, suggesting a link between the elemental abundances of star and the planet. Exoplanet atmosphere studies often assume solar abundances for host stars, however, specific host star abundances might lead to more accurate constraints. In this work, we perform sensitivity studies for a metal rich stellar host HD 149026 and its exoplanet HD 149026b, to understand the effect of solar versus stellar abundance choice on the $P$-$T$ profiles, equilibrium chemical abundances and emission spectra, using self-consistent atmosphere models. We find that the differences are dependent on the model parameters, particularly C/O ratio, and for HD 149026b the difference in the eclipse depth is maximum $\sim$80 ppm, for C/O between 0.75-0.85. Recent JWST NIRCam observations of HD 149026b have yielded widely varying metallicity ranges, highly super-solar (59-275$\times$) using chemical equilibrium retrievals and 12-31$\times$ solar using self-consistent models, both using solar abundances. In this work, we constrain the metallicity of HD 149026b to be 53-113$\times$ solar, with solar abundances and 39-78$\times$ stellar, with stellar abundances. We constrain the self-consistent $P$-$T$ profile of HD 149026b to be substantially cooler (upto 500 K) than the self-consistent best-fit model in the previous work, in the emission spectra probed region, thus requiring higher CO$_2$ abundance to explain the observations, leading to comparatively higher metallicity constraint. We find that the inclusion of Fe opacity in computing self-consistent $P$-$T$ profiles for HD 149026b in our models is the major reason for these differences. We constrain the C/O ratio to 0.47-0.68 and the heat redistribution factor to 0.70-0.76, indicating higher heat redistribution than previously estimated.

astro-ph.EP↗

arXiv2025

Supercurrent modulation in InSb nanoflag-based Josephson junctions by scanning gate microscopy

InSb nanoflags represent an interesting platform for quantum transport and have recently been exploited in the study of hybrid planar Josephson junctions. Due to the uncovered semiconductor surface, they are also good candidates for surface probe techniques. Here, we report the first Scanning Gate Microscopy (SGM) experiments on Nb-contacted InSb nanoflag-based Josephson junctions. In the normal state, sizable conductance modulation via the charged tip of the SGM is recorded. In the superconducting state, we report the first application of Scanning Gate Microscopy to superconducting weak links, demonstrating the possibility of manipulating the supercurrent flow across a semiconductor-superconductor heterostructure at a local level. The experimental findings are consistent with theoretical predictions and establish a new way of investigating the behavior of superconducting weak links, towards the local imaging of supercurrent flow.

cond-mat.supr-con↗

arXiv2025

Enhancing Measurement Precision of Non-Degenerate Two-Photon Absorption

Recent theoretical and experimental studies have shown that squeezed states of light can be engineered to enhance the resolution of nonlinear optical measurements. Here, we analyze non-degenerate two-photon absorption signals obtained from transmission measurements using two-mode squeezed light and compare different measurement strategies. In particular, we investigate how correlations between the light modes may be used to improve the achievable precision. We find that intensity correlation measurements offer the best performance compared to normalized intensity correlation and noise reduction factor approaches. Under experimental imperfections modeled as linear photon losses, the enhancements from intensity and noise reduction measurements are reduced. In contrast, the normalized intensity correlation remains robust to loss, though this comes at the cost of losing the enhancement from non-classical light fields. This establishes a trade-off between robustness to loss and the achievable quantum advantage.

quant-ph↗

arXiv2025

Superconducting Quantum Interference Devices based on InSb nanoflag Josephson junctions

Planar Josephson junctions (JJs) based on InSb nanoflags have recently emerged as an intriguing platform in superconducting electronics. This letter presents the fabrication and investigation of superconducting quantum interference devices (SQUIDs) employing InSb nanoflag JJs. We provide measurements of interference patterns in both symmetric and asymmetric geometries. The interference patterns in both configurations can be modulated by a back-gate voltage, a feature well reproduced through numerical simulations. The observed behavior aligns with the skewed current-phase relations of the JJs, demonstrating significant contributions from higher harmonics. We explore the magnetic field response of the devices across a wide range of fields ($\pm 30$ mT), up to the single-junction interference regime, where a Fraunhofer-like pattern is detected. Finally, we assess the flux-to-voltage sensitivity of the SQUIDs to evaluate their performance as magnetometers. A magnetic flux noise of $S^{1/2}_Φ= 4.4 \times 10^{-6} Φ_0 / \sqrt{Hz}$ is identified, indicating potential applications in nanoscale magnetometry.

cond-mat.mes-hall↗

arXiv2025

Structural and Transport Properties of Thin InAs Layers Grown on InxAl1-xAs Metamorphic Buffers

Indium Arsenide is a III-V semiconductor with low electron effective mass, a small band gap, strong spin-orbit coupling, and a large g-factor. These properties and its surface Fermi level pinned in the conduction band make InAs a good candidate for developing superconducting solid-state quantum devices. Here, we report the epitaxial growth of very thin InAs layers with thicknesses ranging from 12.5 nm to 500 nm grown by Molecular Beam Epitaxy on InxAl1-xAs metamorphic buffers. Differently than InAs substrates, these buffers have the advantage of being insulating at cryogenic temperatures, which allows for multiple device operations on the same wafer and thus making the approach scalable. The structural properties of the InAs layers were investigated by high-resolution X-ray diffraction, demonstrating the high crystal quality of the InAs layers. Furthermore, their transport properties, such as total and sheet carrier concentration, sheet resistance, and carrier mobility, were measured in the van der Pauw configuration at room temperature. A simple conduction model was employed to quantify the surface, bulk, and interface contributions to the overall carrier concentration and mobility.

cond-mat.mes-hall↗

arXiv2024

Enhancement in phase sensitivity of SU(1,1) interferometer with Kerr state seeding

A coherent seeded SU(1,1) interferometer provides a prominent technique in the field of precision measurement. We theoretically study the phase sensitivity of SU(1,1) interferometer with Kerr state seeding under single intensity and homodyne detection schemes. To find the lower bound in this case we calculate the quantum Cramér-Rao bound using the quantum Fisher information technique. We found that, under some conditions, the Kerr seeding performs better in phase sensitivity compared to the well-known vacuum and coherent seeded case. We expect that the Kerr state might act as an alternative non-classical state in the field of quantum information and sensing technologies.

quant-ph↗

arXiv2024

Quantum-enhanced super-sensitivity of Mach-Zehnder interferometer using squeezed Kerr state

We study the phase super-sensitivity of a Mach-Zehnder interferometer (MZI) with the squeezed Kerr and coherent states as the inputs. We discuss the lower bound in phase sensitivity by considering the quantum Fisher information (QFI) and corresponding quantum Cramer-Rao bound (QCRB). With the help of single intensity detection (SID), intensity difference detection (IDD) and homodyne detection (HD) schemes, we find that our scheme gives better sensitivity in both the lossless as well as in lossy conditions as compared to the combination of well-known results of inputs as coherent plus vacuum, coherent plus squeezed vacuum and double coherent state as the inputs. Because of the possibility of generation of squeezed Kerr state (SKS) with the present available quantum optical techniques, we expect that SKS may be an alternative nonclassical resource for the improvement in the phase super-sensitivity of the MZI under realistic scenario.

quant-ph↗

arXiv2022

The postperovskite transition in Fe- and Al-bearing bridgmanite: effects on seismic observables

The primary phase of the Earth's lower mantle, (Al, Fe)-bearing bridgmanite, transitions to the postperovskite (PPv) phase at Earth's deep mantle conditions. Despite extensive experimental and ab initio investigations, there are still important aspects of this transformation that need clarification. Here, we address this transition in (Al3+, Fe3+)-, (Al3+)-, (Fe2+)-, and (Fe3+)-bearing bridgmanite using ab initio calculations and validate our results against experiments on similar compositions. Consistent with experiments, our results show that the onset transition pressure and the width of the two-phase region depend distinctly on the chemical composition: a) Fe3+-, Al3+-, or (Al3+, Fe3+)-alloying increases the transition pressure, while Fe2+-alloying has the opposite effect; b) in the absence of coexisting phases, the pressure-depth range of the Pv-PPv transition seems quite broad to cause a sharp D" discontinuity (< 30 km); c) the average Clapeyron slope of the two-phase regions are consistent with previous measurements, calculations in MgSiO3, and inferences from seismic data. In addition, d) we observe a softening of the bulk modulus in the two-phase region. The consistency between our results and experiments gives us the confidence to proceed and examine this transition in aggregates with different compositions computationally, which will be fundamental for resolving the most likely chemical composition of the D" region by analyses of tomographic images.

cond-mat.mtrl-sci↗

arXiv2022

Federated Learning Enables Big Data for Rare Cancer Boundary Detection

Although machine learning (ML) has shown promise in numerous domains, there are concerns about generalizability to out-of-sample data. This is currently addressed by centrally sharing ample, and importantly diverse, data from multiple sites. However, such centralization is challenging to scale (or even not feasible) due to various limitations. Federated ML (FL) provides an alternative to train accurate and generalizable ML models, by only sharing numerical model updates. Here we present findings from the largest FL study to-date, involving data from 71 healthcare institutions across 6 continents, to generate an automatic tumor boundary detector for the rare disease of glioblastoma, utilizing the largest dataset of such patients ever used in the literature (25,256 MRI scans from 6,314 patients). We demonstrate a 33% improvement over a publicly trained model to delineate the surgically targetable tumor, and 23% improvement over the tumor's entire extent. We anticipate our study to: 1) enable more studies in healthcare informed by large and diverse data, ensuring meaningful results for rare diseases and underrepresented populations, 2) facilitate further quantitative analyses for glioblastoma via performance optimization of our consensus model for eventual public release, and 3) demonstrate the effectiveness of FL at such scale and task complexity as a paradigm shift for multi-site collaborations, alleviating the need for data sharing.

cs.LG↗

arXiv2021

Pressure induced emission enhancement and bandgap narrowing: experimental investigations and first principles theoretical simulations on a model halide perovskite

We report high-pressure photoluminescence, Raman scattering, and x-ray diffraction measurements on a lead-free halide perovskite $Cs_3Sb_2Br_9$. At about 3 GPa, an electronic transition manifests itself through a broad minimum in linewidth, a maximum in the intensity of $E_g$, $A_{1g}$ Raman modes, and the unusual change in the $c/a$ ratio of the trigonal lattice. The large compressibility and observed Raman anomalies indicate to a soft material with strong electron-phonon coupling. The observed below bandgap broadband emission in the photoluminescence measurement indicates the recombination of self-trapped excitons. The initial blueshift of the photoluminescence peak reinforces itself to the redshift at around 3 GPa due to the change in the electronic landscape. A first order trigonal to a monoclinic structural transition is also seen at 8 GPa. The first-principles density functional theory (DFT) calculations reveal that the electronic transition is associated with direct-to-indirect bandgap transition due to changes in the hybridization of $Sb-5s$ and $Br-4p$ orbitals near the Fermi level in the valence band. The experimentally observed Raman modes are assigned to their symmetry using the density functional perturbation theory. In addition, the DFT calculations predict a 27.5\% reduction of the bandgap in the pressure range 0-8 GPa.

cond-mat.mtrl-sci↗

arXiv2021

cij: A Python code for quasiharmonic thermoelasticity

The Wu-Wentzcovitch semi-analytical method (SAM) is a concise and predictive formalism to calculate the high-pressure and high-temperature (high-PT) thermoelastic tensor (Cij) of crystalline materials. This method has been successfully applied to materials across different crystal systems in conjunction with ab initio calculations of static elastic coefficients and phonon frequencies. Such results have offered first-hand insights into the composition and structure of the Earth's mantle. Here we introduce the cij package, a Python implementation of the SAM-Cij formalism. It enables a thermoelasticity calculation to be initiated from a single command and fully configurable from a calculation settings file to work with solids within any crystalline system. These features allow SAM-Cij calculations to work on a personal computer and to be easily integrated as a part of high-throughput workflows. Here we show the performance of this code for three minerals from different crystal systems at their relevant PTs: diopside (monoclinic), akimotoite (trigonal), and bridgmanite (orthorhombic).

physics.comp-ph↗

arXiv2020

Broadening the high sensitivity range of squeezing-assisted interferometers by means of two-channel detection

For a squeezing-enhanced SU(2) interferometer, we theoretically investigate the possibility to broaden the phase range of sub-shot-noise sensitivity. We show that this goal can be achieved by implementing detection in both output ports, with the optimal combination of the detectors outputs, leading to a phase sensitivity independent of the interferometer operation point. Provided that each detector is preceded by a phase-sensitive amplifier, this sensitivity could be also tolerant to the detection loss.

quant-ph↗

arXiv2017

Bullen's parameter as a seismic observable for spin crossovers in the lower mantle

Elastic anomalies produced by the spin crossover in ferropericlase have been documented by both first principles calculations and high pressure-temperature experiments. The predicted signature of this spin crossover in the lower mantle is, however, subtle and difficult to geophysically observe within the mantle. Indeed, global seismic anomalies associated with spin transitions have not yet been recognized in seismologic studies of the deep mantle. A sensitive seismic parameter is needed to determine the presence and amplitude of such a spin crossover signature. The effects of spin crossovers on Bullen's parameter, $η$, are assessed here for a range of compositions, thermal profiles, and lateral variations in temperature within the lower mantle. Velocity anomalies associated with the spin crossover in ferropericlase span a depth range near 1,000 km for typical mantle temperatures. Positive excursions of Bullen's parameter with a maximum amplitude of $\sim$ 0.03 are calculated to be present over a broad depth range within the mid-to-deep lower mantle: these are largest for peridotitic and harzburgitic compositions. These excursions are highest in amplitude for model lower mantles with large lateral thermal variations, and with cold downwellings having longer lateral length-scales relative to hot upwellings. We conclude that predicted deviations in Bullen's parameter due to the spin crossover in ferropericlase for geophysically relevant compositions may be sufficiently large to resolve in accurate seismic inversions of this parameter, and could shed light on both the lateral variations in temperature at depth within the lower mantle, and the amount of ferropericlase at depth.

physics.geo-ph↗

arXiv2016

Influence of the iron spin crossover in ferropericlase on the lower mantle geotherm

The iron spin crossover in ferropericlase introduces anomalies in its thermodynamics and thermoelastic properties. Here we investigate how these anomalies can affect the lower mantle geotherm. The effect is examined in mantle aggregates consisting of mixtures of bridgmanite, ferropericlase, and CaSiO$_3$ perovskite, with different Mg/Si ratios varying from harzburgitic to perovskitic (Mg/Si$\sim$1.5 to 0.8). We find that the anomalies introduced by the spin crossover increase the isentropic gradient and thus the geotherm proportionally to the amount of ferropericlase. The geotherms can be as much as $\sim$ 200K hotter than the conventional adiabatic geotherm at deep lower mantle conditions. Aggregate elastic moduli and seismic velocities are also sensitive to the spin crossover and the geotherm, which impacts analyses of lower mantle velocities and composition.

physics.geo-ph↗

arXiv2016

Thermoelasticity of Fe$^{3+}$- and Al-bearing bridgmanite

We report \textit{ab initio} (LDA + U$_{sc}$) calculations of thermoelastic properties of ferric iron (Fe$^{3+}$)- and aluminum (Al)-bearing bridgmanite (MgSiO$_3$ perovskite), the main Earth forming phase, at relevant pressure and temperature conditions and compositions. Three coupled substitutions, namely, [Al]$_{Mg}$-[Al]$_{Si}$, [Fe$^{3+}$]$_{Mg}$-[Fe$^{3+}$]$_{Si}$, and [Fe$^{3+}$]$_{Mg}$-[Al]$_{Si}$ have been investigated. Aggregate elastic moduli and sound velocities are successfully compared with limited experimental data available. In the case of the [Fe$^{3+}$]$_{Mg}$-[Fe$^{3+}$]$_{Si}$ substitution, the high-spin (S=5/2) to low-spin (S=1/2) crossover in [Fe$^{3+}$]$_{Si}$ induces a volume collapse and elastic anomalies across the transition region. However, the associated anomalies should disappear in the presence of aluminum in the most favorable substitution, i.e., [Fe$^{3+}$]$_{Mg}$-[Al]$_{Si}$. Calculated elastic properties along a lower mantle model geotherm suggest that the elastic behavior of bridgmanite with simultaneous substitution of Fe$_{2}$O$_3$ and Al$_{2}$O$_3$ in equal proportions or with Al$_{2}$O$_3$ in excess should be similar to that of (Mg,Fe$^{2+}$)SiO$_3$ bridgmanite. Excess Fe$_{2}$O$_3$ should produce elastic anomalies though.

cond-mat.mtrl-sci↗