arXiv subjects

S. Roy

Publications and source records attributed to S. Roy.

At least 37 records · Page 2Linked to original sources

arXiv2021

Relaxation timescales and electron-phonon coupling in optically-pumped YBa$_2$Cu$_3$O$_{6+x}$ revealed by time-resolved Raman scattering

Time resolved measurements provide a new way to disentangle complex interactions in quantum materials due to their different timescales. We used pump-probe Raman scattering to investigate the apical oxygen vibration in YBa$_2$Cu$_3$O$_{6+x}$ under nonequilibrium conditions. Time-dependence of the phonon population demonstrated strong electron-phonon coupling. Most importantly, the phonon shifts to a higher energy due to transient smearing of the Fermi surface in a remarkable agreement with diagrammatic theory. We also discuss new insights into photoinduced superconductivity reported at lower doping that follow from these results.

cond-mat.supr-con↗

arXiv2021

Simultaneous ambient pressure X-ray photoelectron spectroscopy and grazing incidence X-ray scattering in gas environments

We have developed an experimental system to simultaneously observe surface structure, morphology, composition, chemical state, and chemical activity for samples in gas phase environments. This is accomplished by simultaneously measuring X-ray photoelectron spectroscopy (XPS) and grazing incidence X-ray scattering (GIXS) in gas pressures as high as the multi-Torr regime, while also recording mass spectrometry. Scattering patterns reflect near-surface sample structures from the nano- to the meso-scale. The grazing incidence geometry provides tunable depth sensitivity while scattered X-rays are detected across a broad range of angles using a newly designed pivoting-UHV-manipulator for detector positioning. At the same time, XPS and mass spectrometry can be measured, all from the same sample spot and in ambient conditions. To demonstrate the capabilities of this system, we measured the chemical state, composition, and structure of Ag-behenate on a Si(001) wafer in vacuum and in O$_2$ atmosphere at various temperatures. These simultaneous structural, chemical, and gas phase product probes enable detailed insights into the interplay between structure and chemical state for samples in gas phase environments. The compact size of our pivoting-UHV-manipulator makes it possible to retrofit this technique into existing spectroscopic instruments installed at synchrotron beamlines. Because many synchrotron facilities are planning or undergoing upgrades to diffraction limited storage rings with transversely coherent beams, a newly emerging set of coherent X-ray scattering experiments can greatly benefit from the concepts we present here.

physics.ins-det↗

arXiv2020

Attenuation of electromagnetic radiation in Nuclear Track Detectors

A systematic study of the attenuation of electromagnetic radiation in Nuclear Track Detectors (NTDs) is carried out. The attenuation of gamma-ray, X-ray, UV, visible, and infrared radiation in NTDs are investigated using NaI(Tl) detector, Gas Electron Multiplier (GEM) detector, UV-Vis spectrophotometer, and FTIR spectrophotometer respectively. The values of some important parameters (e.g., optical depth, attenuation coefficient, etc.) of three commercially available NTDs (PET, Makrofol r and CR-39 r ), at the relevant region of the electromagnetic spectrum, is determined. The details of the experimental techniques and the results are also presented in this paper.

hep-ex↗

arXiv2020

Cosmic ray flux and lockdown due to COVID19 in Kolkata -- any correlation?

Cosmic ray muon flux is measured by the coincidence technique using plastic scintillation detectors in the High Energy Physics detector laboratory at Bose Institute, Kolkata. Due to the COVID19 outbreak and nationwide complete lockdown, the laboratory was closed from the end of March 2020 till the end of May 2020. After lockdown, although the city is not in its normal state, we still were able to take data on some days. The lockdown imposed a strict restriction on the transport service other than the emergency ones and also most of the industries were shut down in and around the city. This lockdown has significant effect on the atmospheric conditions in terms of change in the concentration of air pollutants. We have measured the cosmic ray flux before and after the lockdown to observe the apparent change if any due to change in the atmospheric conditions. In this article, we report the measured cosmic ray flux at Kolkata (22.58$^{\circ}$N 88.42$^{\circ}$E and 11~m Above Sea Level) along with the major air pollutants present in the atmosphere before and after the lockdown.

hep-ex↗

arXiv2020

Stability study and time resolution measurement of Straw Tube detectors

Straw tube detectors are single wire proportional counters that are widely used as a tracking device. We have carried out R$\&$D with a straw tube detector prototype. The motivation of this work is to study the stability of the performance in terms of gain and energy resolution of the straw tube detectors under high rate radiation. Two different methods are incorporated to perform this study. The gain and energy resolution of the detector are studied along with its variation with ambient temperature and pressure. X-ray from a radioactive source is used to irradiate the detector and the same source is also used to monitor the energy spectra simultaneously for calculation of gain. Variation of the gain and energy resolution of the straw tube detector under X-ray irradiation in Ar/CO$_2$ gas mixture is discussed in this article. We have also estimated the time resolution of the straw tube detectors that can be best achieved with cosmic rays as trigger for the same gas mixture. The details of the measurement process and the experimental results are presented in this article.

physics.ins-det↗

arXiv2020

Study of charging up effect in a triple GEM detector

The advancement of Micro Pattern Gaseous Detector technology offers us different kinds of detectors with good spatial resolution and high rate capability and the Gas Electron Multiplier (GEM) detector is one of them. Typically GEM is made up of a thin polyimide foil having a thickness of 50 micrometers with 5 micrometers copper cladding on top and bottom sides. The presence of polyimide changes the gain of the detector under the influence of external radiation and the phenomenon is referred to as the charging up effect. The charging up effect is investigated with a double mask triple GEM detector prototype with Ar/CO$_2$ gas mixture in 70/30 ratio under continuous irradiation from a strong Fe$^{55}$ X-ray source. The detailed method of measurements and the test results are presented in this article.

physics.ins-det↗

arXiv2020

A study of the secondary cosmic $γ$ -ray flux in India during the Great American solar eclipse on 21$^{st}$ August 2017

We present the results from the measurement of secondary cosmic gamma-ray flux using a NaI(Tl) scintillator detector during a total solar eclipse. The unique feature of this experiment is that it was carried out at a place where the solar eclipse was not observable. The total solar eclipse of August 21, 2017, was visible in most of the regions of North America during the day, whereas India, falling on the other half of the globe missed this particular eclipse. Our aim was to measure and examine if there are any variations in the secondary cosmic ray (SCR) flux at Kolkata, India due to the occurrence of the eclipse in America. Detailed experimental techniques used for this experiment are mentioned in this article. Method of data analysis and results are presented. We observe unexpected dehancement and enhancement of SCR flux in certain energy regions.

astro-ph.HE↗

arXiv2020

Generation of wakefields and electromagnetic solitons in relativistic degenerate plasmas

The nonlinear interaction of intense linearly polarized electromagnetic waves (EMWs) with longitudinal electron density perturbations is revisited in relativistic degenerate plasmas. The nonlinear dynamics of the EMWs and the longitudinal field, driven by the EMW ponderomotive force, is governed by a coupled set of nonlinear partial differential equations. A numerical simulation of these coupled equations reveals that the generation of wakefields is possible in weakly relativistic degenerate plasmas with $R_0\equiv p_F/mc\ll1$ and $v_g/c\sim1$, where $p_F$ is the Fermi momentum, $m$ is the mass of electrons, $c$ is the speed of light in vacuum, and $v_g$ is the EMW group velocity. However, when the ratio $v_g/c$ is reduced to $\sim0.1$, the wakefield generation is suppressed, instead the longitudinal fields get localized to form soliton-like structures. On the other hand, in the regimes of moderate $(R_0\lesssim1)$ or strong relativistic degeneracy $(R_0>1)$ with $v_g/c\sim0.1$, only the EM solitons can be formed.

physics.plasm-ph↗

arXiv2019

Spontaneous Magnetic Superdomain Wall Fluctuations in an Artificial Antiferromagnet

Collective dynamics often play an important role in determining the stability of ground states for both naturally occurring materials and metamaterials. We studied the temperature dependent dynamics of antiferromagnetically ordered superdomains in a square artificial spin lattice using soft x-ray photon correlation spectroscopy. We observed an exponential slowing down of superdomain wall motion below the AF onset temperature, similar to the behavior of typical bulk antiferromagnets. Using a continuous time random walk model we show that these superdomain walls undergo low-temperature ballistic and high-temperature diffusive motions.

cond-mat.mes-hall↗

arXiv2019

Identification of point defects on Co-Ni co-doping in SnO$_{2}$ nanocrystals and their effect on the structural and optical properties

Sn$_{0.97-y}$Co$_{0.03}$Ni$_{y}$O$_{2}$ (0 $\leq y \leq$ 0.04) nanocrystals, with average crystallite size in the range of 7.3 nm ($y$=0.00) to 5.6 nm ($y$=0.04), have been synthesized using pH-controlled chemical co-precipitation technique. The non-stoichiometric Sn related defects and the O related stoichiometric Frenkel defects arising in the nanocrystals because of co-doping have been identified and their effect on the structural and optical properties of the nanocrystals have been extensively studied. It has been observed, using XPS that on increasing the Ni co-doping concentration ($y$), the non-stoichiometric Sn defect Sn$_{\text{Sn}}^{"}$ increases in compensation of existing defect Sn$_{i}^{....}$ for $y$ = 0.00 nanocrystals. High resolution transmission electron microscopy (HR-TEM) also confirms the existence of Sn$_{\text{Sn}}^{"}$. Regarding the Frenkel defect, XPS results indicate that the concentration of $V_{\text{O}}$ and O$_{i}$, manifested in the form of dangling bond related surface defect states,increases with increase in $y$. Temperature dependent magnetisation measurement of the nanocrystals confirm the charge state of $V_{\text{O}}$. The point defects have been found to affect the structural properties in a way that distortion in octahedral geometry of complete Sn-O octahderon effectively reduces whereas distortion in the trigonal planar coordination geometry of O increases. The investigation of Urbach edge indicates an enhancement in the disorder in the nanocrystals on co-doping. The optical band gap of the nanocrystals has been found to be red shifted upto $y$=0.02 and then a gradual blue shift has been observed. A direct effect of the O related defect has been observed on the blue luminescence of the nanocrystals such that the spectral contribution of blue luminescence in the total emission intensity increases by 72% for $y$=0.04 as compared to $y$=0.00.

cond-mat.mtrl-sci↗

arXiv2019

ChAKRA : The high resolution charged particle detector array at VECC

A large 4$π$ array of charged particle detectors has been developed at Variable Energy Cyclotron Centre to facilitate high resolution charged particle reaction and spectroscopy studies by detecting event-by-event the charged reaction products emitted in heavy ion reactions at energy $\sim$ 10-60 MeV/A. The forward part ($θ\sim \pm $ $7^{0}$ - $\pm 45^{0}$) of the array consists of 24 highly granular, high resolution charged particle telescopes, each of which is made by three layers [single sided silicon strip($Δ$E) + double sided silicon strip (E/$Δ$E) + CsI(Tl)(E)]of detectors. The backward part of the array consists of 112 CsI(Tl) detectors which are capable of detecting primarily the light charged particles (Z $\le$ 2) emitted in the angular range of $θ\sim \pm $ $45^{0}$ - $\pm 175^{0}$. The extreme forward part of the array ($θ\sim \pm $ $3^{0}$ - $\pm 7^{0}$) is made up of 32 slow-fast plastic phoswich detectors that are capable of detecting light (Z $\le$2) and heavy charged particles (3 $\le$ Z $\lesssim$ 20) as well as handling high count rates. The design, construction and characterization of the array has been described.

physics.ins-det↗

arXiv2019

Modeling meso-scale energy localization in shocked HMX, Part II: training machine-learned surrogate models for void shape and void-void interaction effects

Surrogate models for hotspot ignition and growth rates were presented in Part I, where the hotspots were formed by the collapse of single cylindrical voids. Such isolated cylindrical voids are idealizations of the void morphology in real meso-structures. This paper therefore investigates the effect of non-cylindrical void shapes and void-void interactions on hotspot ignition and growth. Surrogate models capturing these effects are constructed using a Bayesian Kriging approach. The training data for machine learning the surrogates are derived from reactive void collapse simulations spanning the parameter space of void aspect ratio (AR), void orientation ($θ$), and void fraction ($ϕ$). The resulting surrogate models portray strong dependence of the ignition and growth rates on void aspect ratio and orientation, particularly when they are oriented at acute angles with respect to the imposed shock. The surrogate models for void interaction effects show significant changes in hotspot ignition and growth rates as the void fraction increases. The paper elucidates the physics of hotspot evolution in void fields due to the creation and interaction of multiple hotspots. The results from this work will be useful not only for constructing meso-informed macro-scale models of HMX, but also for understanding the physics of void-void interactions and sensitivity due to void shape and orientation.

physics.comp-ph↗

arXiv2019

Dynamical properties of acoustic-gravity waves in the atmosphere

We study the dynamical behaviors of a system of five coupled nonlinear equations that describes the dynamics of acoustic-gravity waves in the atmosphere. A linear stability analysis together with the analysis of Lyapunov exponents spectra are performed to show that the system can develop from ordered structures to chaotic states. Numerical simulation of the system of equations reveals that an interplay between the order and chaos indeed exists depending on whether the control parameter $s_1$, associated with the density scale height of acoustic-gravity waves, is below or above its critical value.

nlin.CD↗

arXiv2019

A model of mesons in finite extra-dimension

Recently,problem of stability of H-atom has been reported in extra-finite dimension,and found out that it is stable in extra-finite dimension of size,$R\leq\frac{a_0}{4}$,where,$a_0$ is the Bohr radius.Assuming that,the heavy flavoured mesons have also such stability controlled by the scale of coupling constant,we obtain corresponding QCD Bohr radius and it is found to be well within the present theoretical and experimental limit of higher dimension.We then study its consequences in their masses using effective string inspired potential model in higher dimension pursued by us.Within the uncertainty of masses of known Heavy Flavoured mesons the allowed range of extra dimension is $L\leq10^{-16}m$,which is well below the present theoretical and experimental limit,and far above the Planck length $\simeq1.5\times10^{-35}$ m.

hep-ph↗

arXiv2018

A new type of RPC with very low resistive material

There are several working groups that are currently working on high rate RPC's using different materials such as Si-based Ceramics, Low-resistive Glass, low-resistive bakelite etc. A new type of single gap RPC has been fabricated using very low-resistive carbon-loaded PTFE material to compete with all these other groups and materials. In terms of bulk resistivity, this material is the lowest and should in principle be able to work at the highest rates, provided the material can withstand working bias and radiation. The efficiency and noise rate of the RPC are measured with cosmic rays. The detail method of fabrication and first experimental results are presented.

physics.ins-det↗

arXiv2018

Study of uniformity of characteristics over the surface for triple GEM detector

Study of the uniformity of gain, energy resolution and count rate over active area of a triple GEM detector has been performed using a strong Fe55 X-ray source with premixed gas of Argon and CO2 in 70/30 ratio and conventional NIM electronics. The detail method of measurement and experimental results are presented in this article.

physics.ins-det↗

arXiv2018

Plastic scintillator detector array for detection of cosmic ray air shower

An air shower array of seven plastic scintillation detectors has been built and commissioned at an altitude of 2200 meter above sea level in the Eastern Himalayas (Darjeeling). Continuous measurement of shower rate using this array is going on since the end of January, 2018. The method of measurement and experimental results are presented in this article.

physics.ins-det↗

arXiv2018

Giant magnetocaloric effect in exchange-frustrated GdCrTiO5 antiferromagnet

We report the effect of exchange frustration on the magnetocaloric properties of GdCrTiO$_5$ compound. Due to the highly exchange-frustrated nature of magnetic interaction, in GdCrTiO$_5$, the long-range antiferromagnetic ordering occurs at much lower temperature $T_N$=0.9 K and the magnetic cooling power enhances dramatically relative to that observed in several geometrically frustrated systems. Below 5 K, isothermal magnetic entropy change (-$ΔS_{\rm m}$) is found to be 36 J kg$^{-1}$ K$^{-1}$, for a field change ($ΔH$) of 7 T. Further, -$ΔS_{\rm m}$ does not decrease from its maximum value with decreasing in $T$ down to very low temperatures and is reversible in nature. The adiabatic temperature change, $ΔT_{\rm ad}$, is 15 K for $ΔH$=7 T. These magnetocaloric parameters are significantly larger than that reported for several potential magnetic refrigerants, even for small and moderate field changes. The present study not only suggests that GdCrTiO$_5$ could be considered as a potential magnetic refrigerant at cryogenic temperatures but also promotes further studies on the role of exchange frustration on magnetocaloric effect. In contrast, only the role of geometrical frustration on magnetocaloric effect has been previously reported theoretically and experimentally investigated on very few systems.

cond-mat.mtrl-sci↗