arXiv subjects

Pritam Das

Publications and source records attributed to Pritam Das.

At least 19 recordsLinked to original sources

arXiv2026

Where Does the Anion-Redox Hole Go? Reliability of Open-Shell Projection-Based Embedding for Li-Rich Cathodes, from DFT Environments to Quantum Solvers

Anion redox in Li-rich layered oxides places holes on oxygen next to an antiferromagnetic Mn(IV) d3 lattice, a strongly correlated open-shell problem for which projection-based wavefunction-in-DFT embedding offers a route to correlated and quantum solvers, but whose errors are known mainly for closed-shell molecules. We develop a spin-unrestricted projection-based embedding, exact at the DFT-in-DFT level to the microhartree scale for closed-shell, broken-symmetry and high-spin references with point charges, and apply DMRG-in-DFT to point-charge clusters of delithiated Li(2-x)MnO3, varying the environment functional (0-100% exact exchange), geometry, active region, active-space spin, virtual space, cluster size and magnetic reference. In every setting the lowest SCF state places the hole on an oxygen pair, with no O-O bond. How the hole is shared depends on the functional together with the geometry: hybrids give a full, uncoupled hole at the DFT+U geometry but a partial, weakly covalent hole at their own geometry, a change of up to 0.5 e on one oxygen. Where spin spreads across the active/environment boundary, the integer spin projection of the active space becomes a choice worth up to 0.26 e, and at most 0.09 e once the two Mn neighbors are included. An extended state search found missed lower states for two functionals, changing a pair-oxygen spin by up to 0.45 e. Across all 82 runs the embedded mean field gives the same hole (median DMRG change 0.02-0.04 e); correlation mainly adds O-O covalency. State control and geometry consistency, not the solver, dominate the uncertainty, and total energies are not converged at 30-40 orbitals. For quantum solvers, the embedded Hamiltonian must be spin-unrestricted: a shared-orbital form reverses the hole, while sample-based quantum diagonalization reproduces exact diagonalization, as accurately as, but not better than, classical selected CI.

cond-mat.mtrl-sci↗

arXiv2026

Phase fluctuations dominated superconductivity in a thick Granular Boron-Doped Diamond film

We investigate the superconducting properties of a 1.2-μm-thick granular boron-doped diamond film, where tightly packed grains form a disordered network of Josephson junctions. The film exhibits a sharp superconducting transition at a critical temperature, T_c ~ 3 K. Its transport properties are well described by the Ambegaokar-Halperin theory of phase fluctuations. Direct measurements of the penetration depth using a two-coil mutual inductance technique reveal classic phase-fluctuation-dominated behavior: below T_c, the superfluid density (n_s) varies linearly over a wide temperature range - a hallmark of thermal phase fluctuations - before crossing over to a T^2 dependence at ultra-low temperatures. Both the large low-temperature penetration depth, λ(0) ~ 8.8 μm, and the slope of the linear ns variation in the thermal regime reveal a remarkably low superfluid stiffness in the range of 0.5-0.8 K. These results establish granular boron-doped diamond films as an excellent model system for investigating the role of long-wavelength phase fluctuations in three-dimensional superconductors.

cond-mat.supr-con↗

arXiv2026

From Dirac Neutrino Mass to Dark Radiation: An $A_4$-Symmetric FIMP Dark Matter Framework

We propose a unified $A_4\times Z_2\times U(1)_L$ framework for generating small Dirac neutrino masses and address dark matter through feeble interactions in an extended particle physics model. The model extends the Standard Model with a pair of heavy vector-like neutral $A_4$ fermions $N(N_{L,R})$, two additional flavon fields $η$ and $ϕ$, among which $η$ being a $Z_2$-odd inert scalar and a dark-sector vector-like fermion field $χ$. We numerically explore the Dirac-sector Yukawa couplings and identify the parameter region that is consistent with the {\tt NuFit 6.1} neutrino oscillation data at the $3σ$ level. The dark matter candidate $χ$, stabilized by the $Z_2$ symmetry, is produced non-thermally through the decay $N_L\rightarrowχ+η$. By solving the Boltzmann equation, we find that the observed relic abundance, $Ω_{\rm DM}h^2\simeq0.12$, can be reproduced for freeze-in couplings of order $λ\sim10^{-5.5}$--$10^{-7.5}$ over the considered dark matter mass range. For the chosen parameter space, we also verify the interaction rate remains below the Hubble expansion rate, confirming $χ$ never reaches thermal equilibrium. Thus, our results demonstrate that the model provides a consistent realization of Dirac neutrino masses and FIMP dark matter within a common symmetry-based setup. The presence of light Dirac RH neutrinos can contribute to the dark radiation and hence the effective degrees of freedom via thermal and non-thermal processes. We further estimate the contribution of the light right-handed neutrinos to $ΔN_{\rm eff}$, finding a maximum characteristic value of $ΔN_{\rm eff}\simeq0.14$ in the benchmark parameter region.

hep-ph↗

arXiv2026

Neutrino texture-zeros after JUNO's first results: Implications for long-baseline neutrino experiments

The recent results from the JUNO reactor neutrino experiment have significantly improved our knowledge of the solar mixing angle $θ_{12}$ and the solar mass splitting $Δm^2_{21}$. We study the impact of these improved estimates on the validity of texture-zeros in the light neutrino mass matrix by assuming neutrinos to be of Majorana nature. Considering a diagonal charged lepton basis, we revisit the previously allowed one-zero and two-zero textures and check their validity by using updated neutrino data from JUNO. While JUNO data rule out one previously allowed two-zero texture, they also make predictions for other neutrino parameters more precise. We finally study the prospects of probing the currently allowed texture-zeros and their predicted correlations among neutrino parameters at the Deep Underground Neutrino Experiment (DUNE). The inclusion of JUNO and reactor experiments strengthens DUNE's ability to constrain the allowed parameter space of both one-zero and two-zero textures. We also observe that DUNE benefits substantially from the complementarity with the T2HK experiment.

hep-ph↗

arXiv2026

Cosmological signature and light Dark Matter in Dirac $L_μ-L_τ$ model

We revisit an anomaly-free extension of the Standard Model (SM) $viz.$ gauged ${L_μ-L_τ}$ model in the Dirac framework, where the local $U(1)_{L_μ-L_τ}$ symmetry breaks and gives rise to a new gauge boson $Z'$ and corresponding gauge coupling $g_{μτ}$. Three additional heavy vector-like fermions, three light right-handed neutrinos and two heavy singlet scalars are added to complete the model framework for Dirac neutrinos. Another singlet vector-like fermion is added with a new gauge charge, which serves as a viable DM candidate, and the correct relic abundance is obtained via the resonance effect. The resonance effect triggers early kinetic decoupling of the DM candidate, which significantly influences the cosmological sector of this study. The parameter space is considered after satisfying the current bounds on $M_{Z'}$ and the gauge coupling $g_{μτ}$. The influence of dark radiations coming from the additional light degrees of freedoms are studied in connection with the dark matter. After imposing the relevant theoretical and experimental constraints, the viable parameter space is found to be significantly restricted while remaining accessible to ongoing and near-future experiments. The framework provides a consistent setting in which neutrino masses, dark matter, and cosmological observables can be studied simultaneously. In particular, we identify viable regions of the parameter space that satisfy the constraints from neutrino masses, dark matter relic abundance, and cosmological observables.

hep-ph↗

arXiv2026

CME Kinematics with Optical Flow: Multi-Coronagraph Insights into Internal Velocity Dispersion

Coronal mass ejections (CMEs) exhibit poorly understood internal velocity distributions. In this work, we introduce an optical flow-based method, DOFCAT (Dense Optical Flow CME Analysis Tool), applied to high-resolution data from ASPIICS/Proba-3 and METIS/Solar Orbiter to examine the internal velocity dispersion of different substructures within CMEs in the Middle Corona. We also introduce a Gaussian-tapered Fourier (GTF) filter that suppresses brightness artifacts in running-difference images from the ASPIICS wideband channel. We then apply DOFCAT to four CME events observed across ASPIICS and METIS, extracting their internal velocity information. Our results show that the velocity profiles of both the CME front and core change with position angle and are non-uniform. For impulsive CMEs, the height offset between the front and core begins to increase following the impulsive acceleration phase, while CMEs without a clear impulsive phase show a more gradual evolution of the height offset. We find that this increase in height difference occurs rapidly between 2.3--3.0\,R$_\odot$. Furthermore, the front shows a higher velocity spread than the core, with impulsiveness evident in the CME leading half. We further compute the radial self-similarity coefficient separately for the front and the core, finding that the front expands non-self-similarly initially and tends toward self-similarity at larger heights, while the core remains relatively self-similar. These findings confirm that CMEs contain complex internal flows and layered substructures within the traditional three-part morphology. Our results demonstrate the utility of DOFCAT on high-resolution coronagraphs for probing CME dynamics and provide new insights into CME initiation and evolution.

astro-ph.SR↗

arXiv2026

Inelastic Self-interacting Dark Matter and LUX-ZEPLIN 248 keV Event in a Dirac Modular Inverse Seesaw

We propose a novel framework that simultaneously addresses the origin of Dirac neutrino masses and the nature of self-interacting dark matter (SIDM). The model is based on an $A_{4}$ modular symmetry to ensure the Diracness of neutrinos as well as the stability of the DM. The neutrino sector realizes a Dirac Inverse Seesaw mechanism where the smallness of the neutrino mass is governed by the vacuum expectation value (VEV) of a singlet scalar $ϕ$. This same scalar couples to a vector-like fermion DM candidate, inducing a tiny Majorana mass splitting that renders the DM pseudo-Dirac and inelastic. Crucially, the scalar also acts as a light mediator for DM self-interactions, potentially solving the small-scale structure problems of Cold DM. In light of the recent 248 keV nuclear-recoil event, LZ230616, observed by LUX-ZEPLIN (LZ) DM direct detection experiment, we demonstrate that our inelastic SIDM parameter space naturally accommodates this signal via endothermic scattering kinematics. Furthermore, the spontaneous breaking of the dark parity required for this inelasticity produces a network of cosmological domain walls. We show that the explicit symmetry breaking needed to safely annihilate these walls generates a stochastic gravitational wave background. The non-holomorphic modular symmetry reduces the free parameters, correlating neutrino observables, addressing DM phenomenology, $ΔN_{\rm eff}$, and gravitational-wave signatures.

hep-ph↗

arXiv2026

A high-frequency type II radio burst associated with an X2.3 class flare

Radio observations provide a powerful diagnostic of the solar corona, enabling investigations of dynamic phenomena associated with solar flares, coronal mass ejections (CMEs), and shock waves. We present a multiwavelength analysis of a rare high-frequency type II radio burst (starting frequency of $\sim$750 MHz and frequency drift rate of $\sim$0.5 MHz s$^{-1}$) associated with the X2.3-class solar flare that occurred on 6 November 2024. A propagating EUV disturbance was observed shortly after the flare onset in the SDO/AIA field of view, while radio spectrographs recorded the type II burst between 13:46 and 13:56 UT over a frequency range of $\sim$750 to 45 MHz. Radio imaging observations from the Nançay Radioheliograph (NRH) show that the radio sources propagate southward during the event. X-ray spectroscopy from HEL1OS onboard Aditya-L1 and imaging observations from STIX onboard Solar Orbiter reveal signatures of efficient non-thermal electron acceleration associated with the flare. An NLFFF extrapolation identifies a pre-eruptive magnetic flux rope in the source region, while white-light coronagraph observations obtained during the event show no detectable large-scale CME. The speed of the erupting flux rope, derived from stereoscopic EUV observations, is consistent, within measurement uncertainties, with the shock speed inferred from the radio dynamic spectrum. Together, these observations suggest that compact flux rope eruptions, even in the absence of a detectable white-light CME, can generate low-coronal shocks capable of producing high-frequency type II radio emission.

astro-ph.SR↗

arXiv2026

Observation of Intrinsic Anderson Localization in Few-Layer ReS$_2$

Electron localization phenomena are expected to play an important role in the transport properties of two-dimensional materials. Rhenium disulfide (ReS$_2$), with its narrow conduction bandwidth, is uniquely susceptible to this effect. However, extrinsic disorder caused by fabrication methods obscures inherent localization behavior arising from reduced dimensionality and degrades transport properties. We report intrinsic Anderson localization in few-layer ReS$_2$ by eliminating extrinsic fabrication-induced disorder through all-dry van der Waals assembly and suppressing interface charge trapping through a hexagonal boron nitride (hBN) gate dielectric. Temperature-dependent transport reveals a crossover from nearest-neighbor hopping to two-dimensional (2D) Mott variable-range hopping (VRH). The non-monotonic gate-voltage dependence of activation energy provides direct access to the energy-resolved band-tail density of states of the ReS$_2$ conduction band. 2D Mott VRH yields a localization length of (3.5 $\pm$ 0.1) nm, an order of magnitude larger than disorder-dominated devices, providing a quantitative characterization of intrinsic Anderson localization in ReS$_2$.

cond-mat.mes-hall↗

arXiv2026

Phase diagram of the vortex state in an amorphous Re6Zr thin film exhibiting inverse melting

In Type II superconductors, the vortex lattice can exhibit "inverse melting," transitioning from a liquid to a crystalline solid as temperature increases. While recently observed via scanning tunneling microscopy in a 20 nm thick amorphous Re6Zr thin film, this work investigates the corresponding d.c. transport and low-frequency magnetic screening responses. By identifying distinct signatures of these transitions and integrating scanning tunneling spectroscopy imaging, we construct a comprehensive vortex-state phase diagram in the magnetic field-temperature parameter space. Furthermore, we demonstrate that inverse melting is thickness-dependent: a 5 nm film retains an inhomogeneous liquid state, while a 50 nm film maintains a crystalline solid structure except near the upper critical field.

cond-mat.supr-con↗

arXiv2026

Anisotropic exciton-polaritons reveal non-Hermitian topology in van der Waals materials

Topological band theory has expanded into various domains in applied physics, offering significant potential for future technologies. Recent developments indicate that unique bulk band topology perceived for electrons can be realized in a system of light-matter quasiparticles with reduced crystal symmetry by utilizing tunable light-matter interaction. In this work we realize topologically non-trivial energy band dispersion of exciton-polaritons confined in two-dimensional anisotropic materials inside an optical microcavity, and show the emergence of exceptional points (EPs) due to non-Hermitian topology arising from excitonic dipole oscillators with finite quasiparticle lifetime. Fourier-plane imaging reveals two pairs of EPs connected by bulk Fermi arcs for each of the transverse electric and magnetic polarized modes. An anisotropic Lorentz oscillator model captures the exact band dispersion observed in our experiment in two-dimensional momentum space. Our findings establish anisotropic two-dimensional materials as a platform for exploring non-Hermitian topological physics, with implications for polarization-controlled optical technologies.

physics.optics↗

arXiv2025

Local Symmetry Breaking in Skyrmion-Hosting Centrosymmetric Hexagonal Compounds

Dzyaloshinskii-Moriya interaction (DMI) plays a crucial role in stabilizing the exotic topologically stable skyrmion spin textures in the noncentrosymmetric crystals. The recent discovery of biskyrmions and skyrmions in the globally centrosymmetric crystals has raised debate about the role of the DMI in causing the spin textures, since DMI vanishes in such crystal structures. Theoretical studies, on the other hand, suggest non-vanishing DMI even if there is local inversion symmetry breaking in an otherwise globally centrosymmetric crystal structure. Motivated by such theoretical predictions, we present here the results of a systematic crystal structure study of two skyrmion-hosting Ni2In-type centrosymmetric hexagonal compounds, MnNiGa and MnPtGa, using the atomic pair distribution function (PDF) technique. Our result provides information about structural correlations in the short-range (SR), medium-range (MR) and long-range (LR) regimes simultaneously. The analysis of the experimental PDFs, obtained from high flux, high energy, and high Q synchrotron x-ray powder diffraction patterns, reveals that the local SR structure of both MnNiGa and MnPtGa compounds corresponds to the noncentrosymmetric trigonal space group P3m1, while the structure in the MR+LR regimes remains hexagonal in the centrosymmetric P63/mmc space group. These findings are also supported by theoretical DFT calculations. Our results, in conjunction with the previous theoretical predictions, provide a rationale for the genesis of skyrmions in centrosymmetric materials in terms of non-vanishing DMI due to local inversion symmetry breaking. We believe that our findings would encourage a systematic search of skyrmionic textures and other topological phenomena in a vast family of centrosymmetric materials.

cond-mat.mtrl-sci↗

arXiv2025

Enhancement of spin Hall angle by an order of magnitude via Cu intercalation in MoS$_2$/CoFeB heterostructures

Transition metal dichalcogenides (TMDs) are a novel class of quantum materials with significant potential in spintronics, optoelectronics, valleytronics, and opto-valleytronics. TMDs exhibit strong spin-orbit coupling, enabling efficient spin-charge interconversion, which makes them ideal candidates for spin-orbit torque-driven spintronic devices. In this study, we investigated the spin-to-charge conversion through ferromagnetic resonance in MoS$_2$/Cu/CoFeB heterostructures with varying Cu spacer thicknesses. The conversion efficiency, quantified by the spin Hall angle, was enhanced by an order of magnitude due to Cu intercalation. Magneto-optic Kerr effect microscopy confirmed that Cu did not significantly modify the magnetic domains, indicating its effectiveness in decoupling MoS$_2$ from CoFeB. This decoupling preserves the spin-orbit coupling (SOC) of MoS$_2$ by mitigating the exchange interaction with CoFeB, as proximity to localized magnetization can alter the electronic structure and SOC. First-principles calculations revealed that Cu intercalation notably enhances the spin Berry curvature and spin Hall conductivity, contributing to the increased spin Hall angle. This study demonstrates that interface engineering of ferromagnet/TMD-based heterostructures can achieve higher spin-to-charge conversion efficiencies, paving the way for advancements in spintronic applications.

cond-mat.mtrl-sci↗

arXiv2025

Origin of unexpected weak Gilbert damping in the LSMO/Pt bilayer system

This study presents a first-principles and semiclassical analysis of the puzzling observation that a La$_{0.7}$Sr$_{0.3}$MnO$_3$ (LSMO) thin film exhibits larger Gilbert damping than an LSMO/Pt bilayer, contrary to conventional spin-pumping expectations. Density functional theory with Wannier interpolation yields an intrinsic damping of $α_{\mathrm{int}}^{\mathrm{LSMO}}\!\approx\!1.4\times10^{-3}$, supporting an extrinsic origin of the high experimental value. Guided by the self-induced inverse spin Hall effect (ISHE) demonstrated in LSMO [Gupta et al., Phys.Rev. B 109, 014437 (2024)], we argue that the large spin Hall angle $|θ_{\mathrm{SH}}|\simeq 0.093$ and low longitudinal conductivity of LSMO enable an efficient conversion of spin current to charge current boosting the effective damping. In the LSMO/Pt heterostructure the Pt cap shunts the charge current, raising $σ_{xx}$ and reducing the interfacial $|θ_{\mathrm{SH}}|$ to~0.007. A Valet-Fert analysis for layer-resolved ab-initio spin accumulation gives the Pt spin-diffusion length and a non-negligible antidamping SOT coefficient, qualitatively accounting for the observed damping reduction under current bias. The seemingly anomalous damping hierarchy is thus reconciled without invoking additional interfacial mechanisms. The distinct length scales governing spin-pumping normalization, namely, the short absorption depth relevant to self-pumping in a single LSMO film versus the full magnetic thickness applicable to an LSMO/Pt bilayer, are crucial in this context. This observation suggests a practical design strategy: by simultaneously tuning the spin Hall-to-longitudinal conductivity ratio and the spin-diffusion length, one can engineer heterostructures with minimized magnetic losses for spin-orbitronics applications.

cond-mat.mtrl-sci↗

arXiv2025

Giant Resonance Raman Scattering via Anisotropic Excitons in ReS2

Anisotropic two-dimensional (2D) semiconductors recently have emerged as a promising platform for polarization-controlled Raman amplification. In this study, we probe energy-dependent resonant Raman scattering in few layer ReS2 under different polarization configurations. We identify two distinct excitation regimes, each characterized by a resonance condition where either the pump or the Stokes photon energy aligns with an excitonic transition. A two-order-of-magnitude enhancement in Raman intensity is observed when the pump energy is tuned near the exciton resonance. Under Stokes-resonant conditions, additional Raman lines accompanied by excitonic photoluminescence are observed, suggesting the participation of non-Bloch intermediate states in the scattering process. These findings shed light into the influence of excitons in modulating nonlinear optical phenomena in anisotropic 2D materials, offering valuable insights for the design of tunable photonic and optoelectronic devices based on anisotropic layered materials.

physics.optics↗

arXiv2025

Light thermal dark matter via type-I seesaw portal

We propose a minimal scenario for light thermal dark matter (DM) in sub-GeV to GeV range by incorporating a scalar singlet DM in a type-I seesaw scenario extended by an additional Higgs doublet $ϕ_2$. The latter permits efficient annihilation of light scalar DM into leptonic final states including right-handed neutrinos (RHN). We keep the charged fermion as well as neutral fermion final states in forbidden regime to avoid bounds from indirect search as well as cosmic microwave background (CMB) data. After studying the purely forbidden DM scenario with neutral and charged fermion final states separately, we discuss the interplay of forbidden and non-forbidden channels in generating light thermal DM relic by considering neutral fermions in non-forbidden mode due to relatively weaker constraints. The model can also explain the anomalous magnetic moment of muon, W-mass anomaly and saturate experimental bounds on charged lepton flavour violation and DM direct detection while offering tantalising detection prospects of RHN, the mass of which is kept approximately in the same range as DM.

hep-ph↗

arXiv2025

Neutrino mass genesis in Scoto-Inverse Seesaw with Modular $A_4$

We propose a hybrid scotogenic inverse seesaw framework in which the Majorana mass term is generated at the one-loop level through the inclusion of a singlet fermion. This singlet Majorana fermion also serves as a viable thermal relic dark matter candidate due to its limited interactions with other fields. To construct the model, we adopt an $A_4$ flavour symmetry in a modular framework, where the odd modular weight of the fields ensures their stability, and the specific modular weights of the couplings yield distinctive modular forms, leading to various phenomenological consequences. The explicit flavour structure of the mass matrices produces characteristic correlation patterns among the parameters. Furthermore, we examine several testable implications of the model, including neutrinoless double beta decay ($0νββ$), charged lepton flavour violation (cLFV), and direct detection prospects for the dark matter candidate. These features make our model highly testable in upcoming experiments.

hep-ph↗

arXiv2025

Exploring unconventional superconductivity in PdTe via Point Contact Spectroscopy

Palladium Telluride (PdTe), a non-layered intermetallic crystalline compound, has captured attention for its unique superconducting properties and strong spin-orbit coupling. In this work, we investigate the superconducting state of PdTe using point-contact Andreev reflection (PCAR) spectroscopy. The experimental data are analyzed using the Blonder-Tinkham-Klapwijk (BTK) model for s, p and d wave symmetries. Our results reveal clear evidence of unconventional superconductivity. The superconducting gap showing features consistent with either p-wave or d-wave pairing symmetries but cannot be fitted with s-wave symmetry. The observed anisotropic gap structure and deviations from conventional BCS behaviour highlight the complex nature of the pairing interactions in PdTe. These findings provide strong evidence of unconventional pairing symmetry in this material.

cond-mat.supr-con↗