arXiv subjects

Divya Rawat

Publications and source records attributed to Divya Rawat.

At least 19 recordsLinked to original sources

arXiv2025

Lattice Mismatch Driven In Plane Strain Engineering for Enhanced Upper Critical Fields in Mo2N Superconducting Thin Films

Transition metal nitrides are a fascinating class of hard coating material that provide an excellent platform for investigating superconductivity and fundamental electron phonon interactions. In this work the structural morphological and superconducting properties have been studied for Mo2N thin films deposited via direct current magnetron sputtering on cplane Al2O3 and MgO substrates to elucidate the effect of internal strain on superconducting properties. High resolution X Ray diffraction and time of flight elastic recoil detection analysis confirms the growth of single phase Mo2N thin films exhibiting epitaxial growth with twin domain structure. Low temperature electrical transport measurements reveal superconducting transitions at 5.2 K and 5.6 K with corresponding upper critical fields of 5 T and 7 T for the films deposited on Al2O3 and MgO, respectively. These results indicate strong type II superconductivity and the observed differences in superconducting properties are attributed to substrate induced strain which leads to higher e ph coupling for the film on MgO substrate. These findings highlight the tunability of superconducting properties in Mo2N films through strategic substrate selection.

cond-mat.supr-con↗

arXiv2025

Contemporaneous X-ray and Optical Polarization of EHSP Blazar H 1426+428

We present the first contemporaneous X-ray and optical polarimetric measurement of the extremely high synchrotron peaked (EHSP) blazar H 1426+428. The X-ray polarimetric observations were undertaken using the Imaging X-ray Polarimetry Explorer (\textit{IXPE}) on 2024 May 27, and 2024 July 5. The \textit{IXPE} pointings were accompanied by contemporaneous optical observations of the Observatorio de Sierra Nevada, Calar Alto Observatory and the Perkins Telescope Observatory. While we observed the X-ray degree of polarization to be $>20\%$, the polarization in the optical band was found to be only $1-3\%$. This trend has been observed in several HSP blazars with available optical and X-ray polarimetric data and is typically explained in terms of energy stratification downstream of a shock. However, we observed a significant difference between the optical and X-ray polarization angles, a feature that has been observed in certain HSP blazars, such as Mrk 421, but remains a relatively rare or underreported phenomenon. We discuss possible scenarios for these findings within the framework of a partially turbulent jet model.

astro-ph.HE↗

arXiv2025

Evolution of the Comptonizing medium of the black-hole candidate Swift J1727.8$-$1613 along the hard to hard-intermediate state transition using NICER

We analyse the properties of the Comptonizing medium in the black-hole X-ray binary Swift J1727.8$-$1613 using the time-dependent Comptonization model vkompth, using NICER observations of type-C QPOs in the hard and hard-intermediate states. During the 2023 outburst of the source, we measure the rms and phase lags of the QPO across 45 observations as the QPO frequency, $\nu_{\rm QPO}$, evolves from $\sim 0.3$ Hz to $\sim 7$ Hz. By simultaneously fitting the time-averaged spectrum of the source and the rms and lag spectra of the QPO, we derive the evolution of the disk and corona parameters. At $\nu_{\rm QPO} = 0.34$ Hz, the QPO phase lags are hard, with 10 keV photons lagging 0.5 keV photons by $\sim 0.5$ rad. As $\nu_{\rm QPO}$ increases, the lags for the same energy bands decrease, reaching near zero at $\nu_{\rm QPO} \sim 1.2$ Hz, and then reverse to soft lags of $\sim -1.1$ rad at $\nu_{\rm QPO} \sim 7$ Hz. Initially, the inner radius of the accretion disk is truncated at $\sim 30-40 R_g$ (assuming a 10 solar-mass black hole) and, as the QPO frequency increases, the truncation radius decreases down to $\sim 10 R_g$. Initially, two coronas of sizes of $\sim 6.5 \times 10^3$ km and $\sim 2 \times 10^3$ km, extend over the disk and are illuminated by different regions of the disk. As the QPO frequency increases, both the coronas shrink to $\sim 2 \times 10^3$ km at $\nu_{\rm QPO} = 2.5$ Hz. Following a data gap, one corona expands again, peaking at a size of $\sim 2 \times 10^4$ km. We interpret the evolution of the coronal size in the context of accompanying radio observations, discussing its implications for the interplay between the corona and the jet.

astro-ph.HE↗

arXiv2025

Electron-phonon interactions and tensor analysis in topological insulator bismuth telluride using angle resolved polarized Raman spectroscopy

We report on the angle-resolved polarized Raman spectroscopy and estimation of the Raman tensor elements using both classical and quantum treatments to analyse the polarized Raman spectra of single crystal Bismuth Telluride. The observed polar patterns and systematic variations in the relative intensities of four characteristic Raman active modes indicate a higher differential polarizability along the c-axis, accompanied by anisotropic photon-phonon interactions. This interplay of electron-photon-phonon interactions is crucial for understanding the lattice dynamics of Bismuth Telluride, which underpin its thermoelectric performance and topological properties.

cond-mat.mtrl-sci↗

arXiv2024

Emergence of spin-phonon coupling in a Gd-doped Y$_2$CoMnO$_6$ double perovskite oxide: a combined experimental and ab-initio study

We present Raman spectroscopy results backed by first-principles calculations and investigate the nature of possible spin-phonon coupling (SPC) in a Gd-doped Y$_2$CoMnO$_6$ (YGCMO) double perovskite oxide. The influence of Gd substitution, A-site ordering, and anti-site disorder is also studied. YGCMO exhibits anti-site disorder leading to both ferromagnetic (between Co and Mn) and antiferromagnetic interactions (Co-Co, Mn-Mn, Gd-Co/Mn), making the SPC quite intriguing. An analysis of the temperature-dependent phonon frequencies for the stretching modes of YGCMO indicates that SPC here possibly emerges from the simultaneous presence of competing ferromagnetic and antiferromagnetic interactions. The SPC strength comes out to be 0.29 cm$^{-1}$. Our density functional theory (DFT) calculations show that Phonon modes shifted towards lower frequency with Gd doping. Similarly, A-site ordring and anti-site disorder significantly alter the Raman spectra. Experimental findings are also corroborated by first-principles DFT calculations, which indicate that anti-site disorder and Gd doping enhances SPC in YGCMO. This implies a strong influence of A-site cationic radii, and B-site (Co/Mn) ordering on SPC in the bulk double perovskite systems. The phonon dynamics of YGCMO are, therefore, correlated with magnetic ordering, indicating potential applications in spintronics devices.

cond-mat.mtrl-sci↗

arXiv2023

Multiphonon interactions and collective excitations in charge density wave of transition metal dichalcogenides

Charge density wave (CDW) is a periodic modulation of electronic charge leading to a reconstruction of the lattice, an emergence of zone folded mode along with collective excitations.1 Transition metal chalcogenides have shown a great potential to study the underlying physics of multi body interactions like electron-phonon (e-ph) as well as plasmons-phonon coupling,2,3 excitonic complexes4 and CDW instabilities.5,6 We are elaborating on the collective excitation and multiphonon interactions like amplitude, zone-folded and two phonon (2_ph) mode associated with CDW in 2H-TaS2 and 2H-NbSe2. The 2_ph is a unique excitation appears due to anharmonicity mediated longitudinal acoustic modes associated with the Kohn anomaly and provides a substantial evidences of e-ph coupling.7 The experimental observations of anomalous response of 2_ph mode across CDW is clarified with phonon self-energy formalism. Transport measurements shows that both materials exhibit incommensurate CDW (TI-CDW) at ~ 76 K (2H-TaS2) and ~ 30 K (2H-NbSe2). As revealed by the estimated e-ph coupling (l ~ 0.007 for 2H-NbSe2 and ~ 0.013 for 2H-TaS2) and anharmonicity constant (d ~ 6.92 for 2H-NbSe2 and ~ 2.723 for 2H-TaS2), we emphasize on higher e-ph coupling in 2H-TaS2 while larger anharmonicity 2H-NbSe2.

cond-mat.mtrl-sci↗

arXiv2023

Unveiling the X-ray polarimetric properties of LMC X-3 with IXPE, NICER, and Swift/XRT

The incoming Imaging X-ray Polarimetry Explorer (IXPE) observations of X-ray binaries provide a new tool to investigate the underlying accretion geometry. Here we report the first measurements of X-ray polarization of the extra-galactic black-hole X-ray binary LMC X$-$3. We find a polarization fraction of $\sim 3%$ at a polarization angle of $\sim 135^\circ$ in the $2-8$ keV energy band with statistical significance at the 7$\sigma$ level. This polarization measurement significantly exceeds the minimum detectable polarization threshold of 1.2% for the source, ascertained at a 99% confidence level within the $2-8$ keV energy band. The simultaneous spectro-polarimetric fitting of NICER, Swift/XRT, and IXPE revealed the presence of a disc with a temperature of $\sim$1 keV and a Comptonized component with a power-law index of $\sim$ 2.4, confirming the soft nature of the source. The polarization degree increases with energy from $\sim$3% in the $2-5.7$ keV band to $\sim$9% in the $5.7-8$ keV band, while the polarization angle is energy independent. The observed energy dependence and the sudden jump of polarization fraction above 5 keV supports the idea of a static slab coronal geometry for the Comptonizing medium of LMC X$-$3.

astro-ph.HE↗

arXiv2023

Spectropolarimetric study of 4U 1630$-$47 in Steep Power law state with IXPE and NICER

We probe the spectropolarimetric properties of the black-hole binary source 4U 1630$-$47 in the steep power law state. We detect a significant polarization fraction of $\sim$7 % at a polarization angle of $\sim$21 $^\circ$. The $2-12$ keV NICER spectrum can be fitted with a combination of a thermal and a Comptonization component, the latter characterized by a spectral index, $\Gamma \sim$2.1, along with a reflection feature at $\sim$7.0 keV. In the $2-8$ keV band, the degree of polarization of 4U 1630$-$47 in the steep power law state is 4.4 $\sigma$ different from the value previously measured in the high soft state. In the steep power law state, the polarization fraction increases as a function of energy but exhibits an overall drop in each energy band compared to that of the high soft state. We propose that the decrease in the polarization fraction in the steep power law state could be attributed to the presence of a corona. The observed polarization properties in both states can be explained by the self-irradiation of the disk around a Kerr black hole, likely influenced by the frame-dragging effect.

astro-ph.HE↗

arXiv2023

Testing the dynamic origin of Quasi-periodic Oscillations in MAXI J1535-571 and H 1743-322

We investigate spectro-temporal properties for two black hole X-ray binary sources, MAXI J1535$-$571 and H 1743$-$322, during their hard and hard-intermediate states. For MAXI J1535$-$571, we analyze Swift/XRT, NuSTAR and NICER observations, specifically focusing on the occurrence of type-C Quasi-periodic Oscillations (QPOs). Regarding H 1743$-$322, we analyze multi-epoch observations of NICER and AstroSat, identifying a type-C QPO with centroid frequency ranging from 0.1--0.6 Hz. In both sources, we fit the spectra with a relativistic truncated disk and a power law component. In MAXI J1535$-$571, we also observe an additional relativistically smeared iron line. Through temporal and spectral analysis, we estimate the QPO centroid frequency and spectral parameters, such as the accretion rate and inner disc radii. We test the origin of type-C QPOs as relativistic precession frequency and dynamic frequency (i.e. the inverse of the sound crossing time $\frac{r}{c_s(r)}$). The dependence of QPO frequency on both the accretion rate and inner disc radii favours the QPO origin as dynamic frequency. We discuss the implications of these results.

astro-ph.HE↗

arXiv2023

Symmetry breaking and structure instability in ultra-thin 2H-TaS2 across charge density wave transition

Ultra-thin 2D materials have shown complete paradigm shift of understanding of physical and electronic properties because of confinement effects, symmetry breaking and novel phenomena at nanoscale. Bulk 2H-TaS2 undergoes an incommensurate charge density wave (I-CDW) transition temperature, TI-CDW - 76 K, however, onset of CDW in atomically thin layers is not clear. We explored the evidence of CDW instability in exfoliated atomically thin 2H-TaS2 using low temperature Raman spectroscopy. We have emphasized on CDW associated modes, M1 - 125 cm-1, M2 -158 cm-1, and M3 -334 cm-1, with thickness - 3 nm (one unit-cell). The asymmetric (Fano) line shape of M2 suggests evidence of strong electron-phonon coupling, which mainly drives the CDW instability. Our observations provide key evidence that the CDW can persists even in one-unit cell with a TI-CDW well above - 200 K, which is higher than bulk 2H-TaS2.

cond-mat.mes-hall↗

arXiv2023

Evolution of QPOs in GX 339-4 and EXO 1846-031 with Insight-HXMT and NICER

We conduct a spectral and timing analysis of GX 339-4 and EXO 1846-031 with the aim of studying the evolution of Type-C QPOs with spectral parameters. The high cadence data from Insight-HXMT and NICER allow us to track them. Type-C QPOs appear at the end of low-hard state and/or hard-intermediate state. The results reveal that the QPO frequency is closely related to the inner disk radius and mass accretion rate in the two sources. Such a correlation is nicely consistent with the dynamic frequency model.

astro-ph.HE↗

arXiv2023

Flux-resolved Spectropolarimetric Evolution of the X-Ray Pulsar Hercules X-1 Using IXPE

We conduct a spectro-polarimetric study of the accreting X-ray pulsar Hercules X-1 using observations with the Imaging X-ray Polarimetry Explorer (IXPE). IXPE monitored the source in three different Epochs, sampling two Main-on and one Short-on state of the well-known super-orbital period of the source. We find that the 2-7 keV polarization fraction increases significantly from ~ $7-9$ % in the Main-on state to ~ $15-19$ % in the Short-on state, while the polarization angle remains more or less constant or changes slightly, ~ $47-59$ degrees, in all three Epochs. The polarization degree and polarization angle are consistent with being energy-independent for all three Epochs. We propose that in the Short-on state, when the neutron star is partially blocked by the disk warp, the increase in the polarization fraction can be explained as a result of the preferential obstruction of one of the magnetic poles of the neutron star.

astro-ph.HE↗

arXiv2023

Detection of X-ray polarized emission and accretion-disk winds with IXPE and NICER in the black-hole X-ray binary 4U 1630-47

We detect a high level of polarization in the X-ray emission of the black-hole binary 4U 1630-47 in an observation with the Imaging X-ray Polarimetry Explorer. The 2-8 keV polarization degree is 8 % at a position angle of $18^\circ$, with the polarization degree increasing significantly with energy, from ~6 % at ~2 keV to ~ 11 % at ~8 keV. The continuum emission in the spectrum of simultaneous observations with the Neutron Star Interior Composition Explorer, NICER, is well described with only a thermal disc spectrum, with stringent upper limits to any Comptonized emission from the corona. Together with the lack of significant variability in the Fourier power spectrum, this suggests that the source was in the high-soft state at the time of these observations. The NICER spectrum reveals the presence of several absorption lines in the 6-9 keV band that we fit with two ionized absorbers, providing evidence of the presence of a strong disk wind, which supports the idea that the source was in the soft state. Previous measurements of X-ray polarization in other sources in harder states were associated with the corona or the jet in those systems. Given that the corona is significantly absent in this observation of 4U 1630-47, and that the jet in black-hole binaries is quenched in the high-soft state, we speculate that in this observation of 4U 1630-47, the polarization likely arises from the direct and reflected radiation of the accretion disk in this source.

astro-ph.HE↗

arXiv2023

A NICER look at the jet-like corona of MAXI J1535-571 through type-B quasi-periodic oscillations

MAXI J1535-571 is a black-hole X-ray binary that in 2017 exhibited a very bright outburst which reached a peak flux of up to 5 Crab in the 2-20 keV band. Given the high flux, several X-ray space observatories obtained unprecedented high signal-to-noise data of key parts of the outburst. In our previous paper we studied the corona of MAXI J1535-571 in the hard-intermediate state (HIMS) with Insight-HXMT. In this paper we focus on the study of the corona in the soft-intermediate state (SIMS) through the spectral-timing analysis of 26 NICER detections of the type-B quasi-periodic oscillations (QPOs). From simultaneous fits of the energy, rms and lag spectra of these QPOs with our time-dependent Comptonization model, we find that in the SIMS the corona size is ~ 6500 km and vertically extended. We detect a narrow iron line in the energy spectra, which we interpret to be due to the illumination of the outer part of the accretion disk by this large corona. We follow the evolution of the corona and the radio jet during the HIMS-SIMS transition, and find that the jet flux peaks after the time when the corona extends to its maximum vertical size. The jet flux starts to decay after the corona contracts vertically towards the black hole. This behavior points to a connection between the X-ray corona and the radio jet similar to that seen in other sources.

astro-ph.HE↗

arXiv2023

The comptonizing medium of the black-hole X-ray binary MAXI~J1535$-$571 through type-C quasi-periodic oscillations

We present a detailed spectral and temporal analysis of the black-hole candidate MAXI~J1535$-$571 using NICER observations in September and October 2017. We focus specifically on observations in the hard-intermediate state when the source shows type-C quasi-periodic oscillations (QPOs). We fitted the time-averaged spectrum of the source and the rms and phase-lag spectra of the QPO with a one-component time-dependent Comptonization model. We found that the corona contracts from $\sim 10^4$ to $\sim 3 \times 10^3$ km as the QPO frequency increases from $\sim 1.8$ Hz to $\sim 9.0$ Hz. The fits suggest that the system would consists of two coronas, a small one that dominates the time-averaged spectrum and a larger one, possibly the jet, that dominates the rms and lag spectra of the QPO. We found a significant break in the relation of the spectral parameters of the source and the properties of the QPO, including its lag spectra, with QPO frequency. The change in the relations happens when the QPO frequency crosses a critical frequency $\nu_c \approx 3.0$ Hz. Interestingly, the QPO reaches this critical frequency simultaneously as the radio emission from the jet in this source is quenched.

astro-ph.HE↗

arXiv2023

Anisotropic Light-Matter Interactions in Single Crystal Topological Insulator Bismuth Selenide

Anisotropy of light-matter interactions in materials give remarkable information about the phonons and their interactions with electrons. We report the angle-resolved polarized Raman spectroscopy of single-crystal of Bi2Se3 to obtain the elements of Raman tensor for understanding the strength of polarization along different crystallographic orientations. Intensity variation in the polar plots corresponding to E_g^1 ~ 37 cm-1, A_1g^1 ~71 cm-1, E_g^2 ~ 130 cm-1, and A_1g^2 ~ 173 cm-1 suggests the higher differential polarizability along cross-plane (bc-plane). The polar patterns and the differences in elements of the Raman tensor provides the evidence of the fundamental electron-phonon and anisotropic light matter interactions in Bi2Se3.

cond-mat.mtrl-sci↗

arXiv2022

The evolution of the corona in MAXI J1535-571 through type-C quasi-periodic oscillations with Insight-HXMT

Type-C quasi-periodic oscillations (QPOs) in black hole X-ray transients can appear when the source is in the low-hard and hard-intermediate states. The spectral-timing evolution of the type-C QPO in MAXI J1535-571 has been recently studied with Insight-HXMT. Here we fit simultaneously the time-averaged energy spectrum, using a relativistic reflection model, and the fractional rms and phase-lag spectra of the type-C QPOs, using a recently developed time-dependent Comptonization model when the source was in the intermediate state. We show, for the first time, that the time-dependent Comptonization model can successfully explain the X-ray data up to 100 keV. We find that in the hard-intermediate state the frequency of the type-C QPO decreases from 2.6 Hz to 2.1 Hz, then increases to 3.3 Hz, and finally increases to ~ 9 Hz. Simultaneously with this, the evolution of corona size and the feedback fraction (the fraction of photons up-scattered in the corona that return to the disc) indicates the change of the morphology of the corona. Comparing with contemporaneous radio observations, this evolution suggests a possible connection between the corona and the jet when the system is in the hard-intermediate state and about to transit into the soft-intermediate state.

astro-ph.HE↗

arXiv2022

Time-resolved spectroscopy on the heartbeat state of GRS 1915+105 using AstroSat

AstroSat spectra of the black hole system GRS 1915+105 during the heartbeat state (with a varying oscillation period from 150 to 100 secs) were analysed using a truncated relativistic disc model along with a Comptonization component. Spectra were fitted for segments of length ~ 24 secs. The oscillation can be described as coordinated variations of the accretion rate, Comptonised flux, and the inner disc radius, with the latter ranging from 1.235-5 gravitational radii. Comparison with results from the $\chi$ and Intermediate states shows that while the accretion rate and the high energy photon index were similar, the inner disc radius and the fraction of Comptonised photons were larger for these states than for the heartbeat one. The coronal efficiency $\eta \equiv L_{ac}/\dot M c^2$, where $L_{ac}$ is the radiative luminosity generated in the corona is found to be approximately $\propto \dot M^{-2/3}$ for all the observations. The efficiency decreases with inner radii for the heartbeat state but has similar values for the $\chi$ and Intermediate states where the inner radii is larger. The implications of these results are discussed.

astro-ph.HE↗