arXiv subjects

Subhajit Sarkar

Publications and source records attributed to Subhajit Sarkar.

At least 19 recordsLinked to original sources

arXiv2026

A Homogeneous Survey of JWST MIRI Transmission Spectra of 10 Exoplanets

The launch of JWST has enabled unprecedented atmospheric characterization of exoplanets. In particular, the JWST-MIRI instrument in the mid-infrared enables probing of molecular species that are harder to detect in the near-infrared. This has come to the fore with the first MIRI-LRS observation of a candidate hycean world, K2-18 b, with potential signs of dimethyl sulfide in its atmosphere. The statistical significance of this result has been debated in subsequent works, arguing instead for the possibility of random noise, instrumental systematics, or a different molecule. The same applies to similar spectra reported for several other exoplanets using MIRI-LRS and interpreted with similar retrieval architectures. In this work, we distinguish between these possibilities with a homogeneous survey of MIRI-LRS transmission spectra of 10 diverse exoplanets, from temperate sub-Neptunes to hot rocky planets and gas giants. We first perform a comparative assessment of the spectra, considering fits with featureless models and pairwise correlations, followed by extensive atmospheric retrievals. With the retrievals, we search for over 150 trace species in the atmosphere of each planet. We find hints of complex molecules in the temperate sub-Neptunes and candidate hycean worlds K2-18 b, TOI-732 c and TOI-270 d, similar to previous works, but not in the other seven planets. Our results indicate that the observed spectral features are likely due to molecular absorption rather than instrument systematics or noise, and underscore these temperate sub-Neptunes as a unique class of planets. More observations are required to robustly identify the specific molecules and constrain the underlying chemical processes, biotic or abiotic.

astro-ph.EP↗

arXiv2026

Origin, strength, and speed of the nonlinear optical response of transparent conducting oxides to single-cycle optical pulses

We present a first-principles study of the nonlinear optical response of transparent conducting oxides at the nanoscale due to excitation by intense, extremely short pulses based on a density matrix framework. We identify a strong ($O(1)$) thermal nonlinearity, which is complemented with stimulated emission and excited-state absorption; it yields a cumulative permittivity change decorated by quantum coherent oscillations. Further, rigorous calculations under far-from-equilibrium conditions show that electron-electron thermalization occurs within a few femtoseconds, supporting interpretations of high-harmonic generation measurements and in agreement with a generalization of Fermi liquid theory.

physics.optics↗

arXiv2026

The Mid-Infrared Transmission Spectrum of the Temperate Sub-Neptune TOI-270 d

Observations of temperate sub-Neptunes with JWST have ushered in a new era for atmospheric characterization of small exoplanets. In particular, the MIRI instrument provides a unique opportunity to search for molecules that are not easily accessible in the near-infrared, as demonstrated by recent mid-infrared observations of K2-18 b. In this work, we present the first mid-infrared transmission spectrum of TOI-270 d observed using the JWST MIRI LRS (5-12 $\mu$m) instrument. By leveraging archival MIRI LRS data, we establish a new empirical relation between the detector settling timescale and the flux, which helps accurately model the spectral light curves and improve the precision of the transmission spectrum. We find that there is notable evidence of molecular features in the MIRI transmission spectrum of TOI-270 d, favouring the presence of atmospheric absorption at $\ln B$ = 2.8-5.3 when comparing physically plausible atmospheric models with and without molecular line absorption. The data show excess absorption beyond what could be attributed to CH$_4$ and CO$_2$ detected previously, in line with recent near-infrared results. Through an agnostic search for 203 species, we identify several candidate trace molecules, most of which are complex molecules, evaluate their physical plausibility, and compare them against inferences from near-infrared observations. We also compare the MIRI spectrum of TOI-270 d to that of K2-18 b and find that random or systematic noise is unlikely to explain these observations. Future follow-up observations are necessary to definitively identify the additional absorbers beyond CH$_4$ and CO$_2$. These observations demonstrate the unique capability of JWST MIRI for atmospheric characterisation of temperate sub-Neptunes.

astro-ph.EP↗

arXiv2026

Transmission Spectrum of the Benchmark Temperate Exo-Neptune TOI-1231 b

The JWST is revolutionizing our understanding of the temperate sub-Neptune population through atmospheric spectroscopy. The nature of these planets remains debated, as their bulk properties are compatible with a range of interior scenarios, including mini-Neptunes, hycean worlds, and gas dwarfs, with different predicted atmospheric compositions. While theoretical studies have predicted compositional diagnostics for shallow- versus deep-atmosphere scenarios, there is a critical need for empirical constraints for a temperate planet that is a priori known to possess a deep H$_2$-rich atmosphere. The temperate exo-Neptune TOI-1231 b provides one such benchmark target. In this work, we present the JWST near-infrared (0.65--5.2 $\mu$m) transmission spectrum of TOI-1231 b, observed with NIRISS single-object slitless spectroscopy and NIRSpec G395H, representing the first for a temperate exo-Neptune. The density of TOI-1231 b requires a thick H$_2$-rich atmosphere, making the planet a keystone reference case for testing mini-Neptune scenarios for sub-Neptunes. We report a strong detection of CH$_4$ ($\ln B = 54.5$-$69.6$) and moderate to strong evidence for CO$_2$ ($\ln B = 2.9$-$6.6$). We do not find significant evidence for any other prominent molecule, although we find high 95\% upper limits on the mixing ratios of NH$_3$ and CO, both of which are expected in deep H$_2$-rich atmospheres. We also do not find any significant evidence for sulfur-bearing species that have been inferred for some temperate sub-Neptunes. This composition is consistent with expectations for a temperate Neptune possessing a deep H$_2$-rich atmosphere with no distinct surface. We discuss the implications of our results for the characterization of temperate sub-Neptunes.

astro-ph.EP↗

arXiv2026

Spinterface-like mechanism of the chirality-induced spin selectivity in donor chiral-bridge acceptor complexes

The chirality-induced spin selectivity (CISS) effect has been invoked to explain recent reports of differences in the time-resolved EPR signals between chiral and achiral molecules. However, the microscopic origin of these differences and their connection to CISS remains contested, particularly since these systems lack a metal interface. Here we introduce an intramolecular spinterface-like mechanism that naturally arises within donor-chiral bridge-acceptor (D--$\chi$B--A) complexes and quantitatively reproduces experimentally reported observed spin polarization in time-resolved EPR studies. In our two-electron Lindblad model, the photoexcited charge-transfer electron traversing the chiral bridge exchanges with the residual donor electron, which acts as a localized magnetic moment analogous to an induced magnetic moment on an electrode surface. The resulting through-bridge charge current produces an effective solenoidal field at the donor--bridge interface, breaking spin degeneracy and directional symmetry, thus enabling spin-selective transport without invoking intrinsic spin-orbit coupling on the bridge. We show that the interplay between this current-induced field, donor thermalization (which breaks time-reversal symmetry), and bridge spin mixing yields tens-of-percent polarization over realistic experimental conditions and charge-transfer time scales, matching reported CISS signatures in triads and DNA hairpins. By explicitly resolving the dependence on solenoidal coupling strength, temperature, and spin-mixing rates, the model identifies the regime in which internal spinterfaces can generate robust CISS-like spin filtering. These findings demonstrate that CISS-like signals in isolated D--$\chi$B--A complexes are fully compatible with a spinterface mechanism, providing a unified conceptual framework for interpreting both device-based and molecule-internal CISS platforms.

cond-mat.mes-hall↗

arXiv2026

Quantum-optical theory of the few femtosecond nonlinear optical response of Drude metals with a non-parabolic conduction band

We develop an energy-space density matrix framework to investigate the interaction of extremely short optical pulses (ESPs) with transparent conducting oxides (TCOs). This approach captures not only electron populations, material polarization, and the permittivity, but also the quantum coherences between states. Compared to traditional momentum-space models, the energy-space formulation offers substantial computational simplification while retaining accuracy. Building on but going beyond the scope of Ref.~\cite{single_cycle_nlty_Letter}, we focus on dynamical features previously unexplored. Our formulation reveals clear signatures of quantum coherence in the net absorption dynamics and highlights the emergence of strong excited-state absorption under intense excitation. It also clarifies that spontaneous emission can be neglected in this regime. Furthermore, we investigate the influence of pump pulse intensity on the local field's duration, spectral broadening and shift, and phase induced by carrier dynamics, highlighting the absorptive nature of the nonlinear response. Our results provide a unified framework for understanding nonlinear light-matter interaction in dispersive, low-density electron systems driven far from equilibrium by intense broadband excitation.

physics.optics↗

arXiv2025

Transformational astrophysics and exoplanet science with Habitable Worlds Observatory's High Resolution Imager

Habitable Worlds Observatory (HWO) will be NASA's flagship space telescope of the 2040s, designed to search for life on other planets and to transform broad areas of astrophysics. NASA are seeking international partners, and the UK is well-placed to lead the design and construction of its imaging camera - which is likely to produce the mission's most visible public impact. Early participation in the mission would return investment to UK industry, and bring generational leadership for the UK in space science, space technology, and astrophysics.

astro-ph.IM↗

arXiv2025

A JWST Transmission Spectrum of the Temperate Sub-Neptune TOI-732 c

In recent years, JWST has facilitated detections of carbon-bearing molecules in the atmospheres of temperate sub-Neptunes orbiting M dwarfs, ushering in a new era in the characterization of this intriguing planetary regime. We report the transmission spectrum of the temperate sub-Neptune TOI-732 c, observed with JWST NIRISS, NIRSpec G395H and MIRI LRS between 0.9-12 $\mu$m. The observations provide evidence for methane (CH$_4$) in a H$_2$-rich atmosphere, at a volume mixing ratio of $\sim$1\%, and non-detection of NH$_3$ and HCN, along with nominal constraints on other prominent molecules H$_2$O, CO and CO$_2$, which are typically expected in H$_2$-rich atmospheres. We conduct a comprehensive survey of 250 chemical species and find moderate to strong evidence (up to $\ln B\sim 5.9$, $3.9\sigma$) for additional absorption due to one or more complex molecules including higher-order hydrocarbons and/or sulfur-bearing molecules. The spectral features are strongly degenerate among these molecules and with methane, which we find at $\ln B=3.2-8.8$ (up to $3.0-4.6$$\sigma$) significance. Two complex molecules are preferred with at least moderate evidence ($\ln B \gtrsim 2.5$) in both the near- and mid-infrared, while several others show such evidence in at least one of the two wavelength ranges. The preferred molecules are found in trace quantities on Earth, with no significant sources identified in other planetary atmospheres, requiring future work to assess their physical plausibility in this planet. Future observations are required to resolve the degeneracies and place more robust constraints on these species. We highlight the need for further theoretical and experimental work to robustly characterize the atmospheric and internal composition of TOI-732 c and similar sub-Neptunes.

astro-ph.EP↗

arXiv2025

The spinterface mechanism for the chiral-induced spin selectivity effect: A Critical Perspective

The chiral-induced spin selectivity (CISS) effect, whereby chiral molecules preferentially transmit electrons of one spin orientation, remains one of the most intriguing and debated phenomena at the interface of spintronics, molecular electronics, and quantum materials. Despite extensive experimental observations across diverse platforms - including transport junctions, photoemission, and enantioselective chemistry - a comprehensive theoretical framework is still lacking. In this perspective, we critically examine the spinterface mechanism as a unifying explanation for the CISS effect. The spinterface model, which hypothesizes a feedback interaction between electron motion in chiral molecules and fluctuating surface magnetic moments, is shown to quantitatively reproduce experimental data across various systems and conditions. We contrast it with some existing theoretical models, highlighting key experimental features. Importantly, we also address open questions and criticisms of this model, including the nature of surface magnetism, the role of dissipation, and the applicability of the mechanism to non-helical or electrode-free systems. By offering falsifiable predictions and reconciling theory with experimental raw data, this work aims to sharpen the dialogue surrounding the microscopic origin of CISS and stimulate further experimental and theoretical progress.

cond-mat.mes-hall↗

arXiv2025

Approaching the scaling limit of transport through lattices with dephasing

We examine the stationary--state equations for lattices with generalized Markovian dephasing and relaxation. When the Hamiltonian is quadratic, the single--particle correlation matrix has a closed system of equations even in the presence of these two processes. The resulting equations have a vectorized form related to, but distinct from, Lyapunov's equation. We present an efficient solution that helps to achieve the scaling limit, e.g., of the current decay with lattice length. As an example, we study the super--diffusive--to--diffusive transition in a lattice with long--range hopping and dephasing. The approach enables calculations with up to $10^4$ sites, representing an increase of $10$ to $40$ times over prior studies. This enables a more precise extraction of the diffusion exponent, enhances agreement with theoretical results, and supports the presence of a phase transition. There is a wide range of problems that have Markovian relaxation, noise, and driving. They include quantum networks for machine--learning--based classification and extended reservoir approaches (ERAs) for transport. The results here will be useful for these classes of problems.

quant-ph↗

arXiv2025

New Constraints on DMS and DMDS in the Atmosphere of K2-18 b from JWST MIRI

The sub-Neptune frontier has opened a new window into the rich diversity of planetary environments beyond the solar system. The possibility of hycean worlds, with planet-wide oceans and H$_2$-rich atmospheres, significantly expands and accelerates the search for habitable environments elsewhere. Recent JWST transmission spectroscopy of the candidate hycean world K2-18 b in the near-infrared led to the first detections of carbon-bearing molecules CH$_4$ and CO$_2$ in its atmosphere, with a composition consistent with predictions for hycean conditions. The observations also provided a tentative hint of dimethyl sulfide (DMS), a possible biosignature gas, but the inference was of low statistical significance. We report a mid-infrared transmission spectrum of K2-18 b obtained using the JWST MIRI LRS instrument in the ~6-12 $\mu$m range. The spectrum shows distinct features and is inconsistent with a featureless spectrum at 3.4-$\sigma$ significance compared to our canonical model. We find that the spectrum cannot be explained by most molecules predicted for K2-18 b with the exception of DMS and dimethyl disulfide (DMDS), also a potential biosignature gas. We report new independent evidence for DMS and/or DMDS in the atmosphere at 3-$\sigma$ significance, with high abundance ($\gtrsim$10 ppmv) of at least one of the two molecules. More observations are needed to increase the robustness of the findings and resolve the degeneracy between DMS and DMDS. The results also highlight the need for additional experimental and theoretical work to determine accurate cross sections of important biosignature gases and identify potential abiotic sources. We discuss the implications of the present findings for the possibility of biological activity on K2-18 b.

astro-ph.EP↗

arXiv2025

Bipartite particle number fluctuations in dephased long-range lattice systems

We investigate the dynamics of subsystem particle number fluctuations in a long-range system with power-law decaying hopping strength characterized by exponent $\mu$ and subjected to a local dephasing at every site. We introduce an efficient {\it bond length} representation for the four-point correlator, enabling the large-scale simulation of the dynamics of particle number fluctuations from translationally invariant initial states. Our results show that the particle number fluctuation dynamics exhibit one-parameter Family-Vicsek scaling, with superdiffusive scaling exponents for $\mu < 1.5$ and diffusive scaling exponents for $\mu \geq 1.5$. Finally, exploiting the bond-length representation, we provide an exact analytical expression for the particle number fluctuations and their scaling exponents in the short-range limit ($\mu \to \infty)$.

cond-mat.stat-mech↗

arXiv2024

Time Crystals from single-molecule magnet arrays

Time crystals, a unique non-equilibrium quantum phenomenon with promising applications in current quantum technologies, mark a significant advance in quantum mechanics. Although traditionally studied in atom-cavity and optical lattice systems, pursuing alternative nanoscale platforms for time crystals is crucial. Here we theoretically predict discrete time-crystals in a periodically driven molecular magnet array, modeled by a spin-S Heisenberg Hamiltonian with significant quadratic anisotropy, taken with realistic and experimentally relevant physical parameters. Surprisingly, we find that the time-crystal response frequency correlates with the energy levels of the individual magnets and is essentially independent of the exchange coupling. The latter is unexpectedly manifested through a pulse-like oscillation in the magnetization envelope, signaling a many-body response. These results show that molecular magnets can be a rich platform for studying time-crystalline behavior and possibly other out-of-equilibrium quantum many-body dynamics.

quant-ph↗

arXiv2024

Protecting coherence from the environment via Stark many-body localization in a Quantum-Dot Simulator

Semiconductor platforms are emerging as a promising architecture for storing and processing quantum information, e.g., in quantum dot spin qubits. However, charge noise coming from interactions between the electrons is a major limiting factor, along with the scalability of many qubits, for a quantum computer. We show that a magnetic field gradient can be implemented in a semiconductor quantum dot array to induce a local quantum coherent dynamical $\ell-$bit exhibiting the potential to be used as logical qubits. These dynamical $\ell-$bits are responsible for the model being many-body localized. We show that these dynamical $\ell-$bits and the corresponding many-body localization are protected from all noises, including phonons, for sufficiently long times if electron-phonon interaction is not non-local. We further show the implementation of thermalization-based self-correcting logical gates. This thermalization-based error correction goes beyond the standard paradigm of decoherence-free and noiseless subsystems. Our work thus opens a new venue for passive quantum error correction in semiconductor-based quantum computers.

quant-ph↗

arXiv2024

Exoplanet Transit Spectroscopy with JWST NIRSpec: Diagnostics and Homogeneous Case Study of WASP-39 b

The JWST has ushered in a new era of exoplanet transit spectroscopy. Among the JWST instruments, the Near-Infrared Spectrograph (NIRSpec) has the most extensive set of configurations for exoplanet time series observations. The NIRSpec Prism and G395H grating represent two extremes in NIRSpec instrument modes, with the Prism spanning a wider spectral range (0.6-5.3 $μ$m at lower resolution (R$\sim$100) compared to G395H (2.87-5.14 $μ$m; R$\sim$2700). In this work, we develop a new data reduction framework, JexoPipe, to conduct a homogeneous assessment of the two NIRSpec modes for exoplanet spectroscopy. We use observations of the hot Saturn WASP-39 b obtained as part of the JWST Transiting Exoplanets ERS program to assess the spectral quality and stability between the two instrument modes at different epochs. We explore the noise sources, effect of saturation, and offsets in transmission spectra between the different instrument modes and also between the two G395H NRS detectors. We find an inter-detector offset in G395H of $\sim$ 40-50 ppm, consistent with recent studies. We find evidence for correlated noise in the Prism white light curve. We find the G395H spectrum to be of higher precision compared to the Prism at the same resolution. We also compare the JexoPipe spectra with those reported from other pipelines. Our work underscores the need for robust assessment of instrument performance and identification of optimal practices for JWST data reduction and analyses.

astro-ph.EP↗

arXiv2024

Ballistic vs. diffusive transport in metals

Using the Boltzmann transport model, we show that, somewhat unintuitively, ballistic transport of electrons in metals is weaker than diffusive transport. This happens because the femtosecond-scale collision rates of the non-thermal electrons makes their mean-free path negligible. Our predictions are correlated with various photoluminescence and nonlinear optics experimental examples both for Continuous Wave (CW) and pulsed illumination, and open the way to easy modelling of the non-thermal electron distributions in metal nanostructures of arbitrary complexity.

cond-mat.mes-hall↗

arXiv2024

The Elusive member of the Ti-Al-C MAX family- Ti4AlC3

We report here perhaps the first successful synthesis and structural characterization of the n=3 family member of Tin+1AlCn, i. e. Ti4AlC3. X-ray Powder diffraction (XRD) data shows characteristic reflections of from corresponding to reflections from the (002), (004), (006), (008), (100), (102), (104), (0010), (105), (106), (0012), (1011) and (1012) planes at 2θ =7.640, 15.170, 22.760, 30.50, 350, 37.40 38.30, 39.20, 41.30, 46.220, 55.240, 58.620 and 60.780 (double structure) respectively. Rietveld refinement of the XRD data reveals a phase purity of about 79 % for alpha-Ti4AlC3, 15 % for beta-Ti4AlC3 and the rest mostly that of cubic TiC (6 %). The primary crystal symmetry of the two dominant phases is the hexagonal P63/mmc. The precursors chosen were TiH2, Al metal powder and Carbon powder in a molar ratio of 3:1.2:2, which build the case for an Al-deficient condition. We adopted the pressureless sintering technique at 13500 C with a dwelling time of 4 hours under ultra-high vacuum of 10-7 mbar. The co-existence of trace amount of Ti2AlC at 1350 deg C is proven by the small structure at 2θ=13.130. No trace of oxides like Al2O3 or TiO2 was found in the end product. The line profile width of XRD data indicates average grain size of the order of micro meter. The Scanning Electron Microscopy images show highly lamellar stacked growth of almost a pure MAX (alpha or beta) phase and grain size of micron order, agreeing well with the XRD data.

cond-mat.mtrl-sci↗

arXiv2024

Cool Gaseous Exoplanets: surveying the new frontier with Twinkle

Cool gaseous exoplanets ($1.75\ R_\oplus < R_\text{p} < 3\ R_\text{J}$, $200$ K $ $5σ$ significance. We find that an injected mass-metallicity trend is well-recovered, demonstrating Twinkle's ability to elucidate this fundamental relationship into cool regime. We also find Twinkle will be able to detect cloud layers at 3$σ$ or greater in all cool gaseous planets for clouds at $\leq$ 10 Pa pressure level, but will be insensitive to clouds deeper than $10^4$ Pa in all cases. With these results we demonstrate the capability of the Twinkle mission to greatly expand the current knowledge of cool gaseous planets, enabling key insights and constraints to be obtained for this poorly-charted region of exoplanet parameter space.

astro-ph.EP↗