Articles written in Pramana – Journal of Physics

    • Acoustic wave propagation in $Ni_3 R$ (𝑅 = Mo, Nb, Ta) compounds

      Pramod Kumar Yadawa

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      The ultrasonic properties of the hexagonal closed packed structured $Ni_3$Mo, $Ni_3$Nb and $Ni_3$Ta compounds were studied at room temperature for their characterization. For the investigations of ultrasonic properties, the second-order elastic constants using Lennard–Jones potential were computed. The velocities $V_1$ and $V_2$ have minima and maxima respectively at 45° with the unique axis of the crystal, while $V_3$ increases with respect to angle with the unique axis of the crystal. The inconsistent behaviour of angle-dependent velocities is associated with the action of second-order elastic constants. Debye average sound velocities of these compounds increase with the angle and has maximum at $55^{\circ}$ with the unique axis at room temperature. Hence, when a sound wave travels at $55^{\circ}$ with the unique axis of these materials, the average sound velocity is found to be maximum. The results achieved are discussed and compared with the available experimental and theoretical results.

    • A comparative study of graphene and graphite-based field effect transistor on flexible substrate


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      In the present era, there has been a great demand of cost-effective, biodegradable, flexible and wearable electronics which may open the gate to many applications like flexible displays, RFID tags, health monitoring devices, etc. Due to the versatile nature of plastic substrates, they have been extensively used in packaging, printing, etc. However, the fabrication of electronic devices requires specially prepared substrates with high quality surfaces, chemical compositions and solutions to the related fabrication issues along with its non-biodegradable nature. Therefore, in this report, a cost-effective, biodegradable cellulose paper as an alternative dielectric substrate material for the fabrication of flexible field effect transistor (FET) is presented. The graphite and liquid phase exfoliated graphene have been used as the material for the realisation of source, drain and channel on cellulose paper substrate for its comparative analysis.The mobility of fabricated FETs was calculated to be $\rm{83 cm^{2}/V s}$ (holes) and $\rm{33 cm^{2}/V s}$ (electrons) for graphite FET and $\rm{100 cm^{2}/V s$ (holes) and $\rm{52 cm^{2}/V s}$ (electrons) for graphene FET, respectively. The output characteristic of the device demonstrates the linear behaviour and a comprehensive increase in conductance as a function of gate voltages. The fabricated FETs may be used for strain sensing, health care monitoring devices, human motion detection, etc.

    • Characteristics of solar microflares as seen in soft X-ray emission


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      In this paper, we present the thermal and non-thermal characteristics of solar plasma producing microflares in 4–12 keV energy range. The X-ray spectra of 10 B-class solar microflares observed by the silicon(Si) detector (4–25 keV) on-board solar X-ray spectrometer (SOXS) mission were analysed in 4–12 keV energy range. We employed forward fitting for the spectral modelling of thermal and non-thermal components of X-ray spectra with isothermal, multithermal and single power-law functions in order to determine flare parameters. The fit results obtained from the combination of isothermal and single power-law functions yield a weighted mean value of emission measure $\rm{(EM) \approx 0.0203×10^{49}cm^{−3}}$, plasma temperature [Case (1)] $T (1) \approx 10.24$MK and non-thermal spectral index $\gamma (1) \approx 3.90$. The fit results obtained from the combination of multithermal and single power-law functions yield a weighted mean value of differential emission measure, $\rm{(DEM) \approx 0.00116 × 10^{49} cm^{−3} keV^{−1}}$, plasma temperature [Case (2)], $T (2) \approx 12.90$MK, thermal spectral index, $\delta \approx 4.06$ and non-thermal spectral index, $\gamma (2) \approx 3.81$. Further, we obtained the mean value of conduction cooling time, $\tau_{c}(T) \approx 283$ s at 11.6 MK, thermal energy, $E_{th} \approx 0.50×10^{29}$erg and thermal–non-thermal cross-over energy, $\epsilon_{th} \approx 9.23$ keV. In this analysis, the obtained results were found to be compatible with the earlier analysis carried out for the microflares through Reuven Ramety High Energy Solar Spectroscopic Imager (RHESSI), Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA) and NuSTAR observations. Here, we observed that EM decreases with increasing plasma temperature $(T)$. We find that $\tau_{c}(T)$ scale with plasma temperature $(T)$ with an inverse gradient exhibits time delay characteristic of the cooling process of plasma. The correlation of $E_{th}$ and temperature $(T)$ shows moderate anticorrelation. The present analysis demonstrates the multithermal plasma model and conduction cooling process during high temperature of microflares (similar to large flares) followed by radiative cooling in post-flare.

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