Physics


https://sites.google.com/a/gauhati.ac.in/research/#ds11092019 Powerline communication

https://sites.google.com/a/gauhati.ac.in/research/physics/#kb14092019 Re-awakening of an X-Ray binary

https://sites.google.com/a/gauhati.ac.in/research/physics/#ds13062019 CDs-porous silicon photo sensor

https://sites.google.com/a/gauhati.ac.in/research/physics/#mpb12042019 Driving the electrons unstable

https://sites.google.com/a/gauhati.ac.in/research/physics/#bb05032019 Probing protons at LHC

https://sites.google.com/a/gauhati.ac.in/research/physics/#bks23022019 AZO transparent conductors

https://sites.google.com/a/gauhati.ac.in/research/physics/#kk20022019 Targets for nuclear reaction experiments

https://sites.google.com/a/gauhati.ac.in/research/physics/#ds19022019 Properties of PANI-SA composites

https://sites.google.com/a/gauhati.ac.in/research/physics/#kb11022019 One more neutrino

https://sites.google.com/a/gauhati.ac.in/research/physics/#mpb18012019 Modeling magnetospheric oscillations

https://sites.google.com/a/gauhati.ac.in/research/physics/#sh30112018 Detection of arsenic in water

https://sites.google.com/a/gauhati.ac.in/research/physics#mpb27102018 Dust levitation in an inverse sheath

https://sites.google.com/a/gauhati.ac.in/research/physics#hkk12082018 Graphene based soil moisture sensor

https://sites.google.com/a/gauhati.ac.in/research/physics/#sd28062018 Annealing effect on silver nanocomposite

https://sites.google.com/a/gauhati.ac.in/research/physics/#ds26062018 Room temperature ethanol sensor

https://sites.google.com/a/gauhati.ac.in/research/physics/#sb14062018 Properties of monolayer graphene

https://sites.google.com/a/gauhati.ac.in/research/physics/#bks30052018 ZnO based transparent conductor

https://sites.google.com/a/gauhati.ac.in/research/physics/#kb29052018 Broadening of GRB spectra

https://sites.google.com/a/gauhati.ac.in/research/physics/#agb Efficient ammonia gas sensor

https://sites.google.com/a/gauhati.ac.in/research/physics/#agb Porous silicon MSM device

https://sites.google.com/a/gauhati.ac.in/research/physics/#agb Relativistic Heavy Ion Collisions

https://sites.google.com/a/gauhati.ac.in/research/physics/#agb Coherence in firefly emission

https://sites.google.com/a/gauhati.ac.in/research/physics/#kb Probing the neutrinos at DUNE

https://sites.google.com/a/gauhati.ac.in/research/physics/#bks Gravitational theories at galactic centre

https://sites.google.com/a/gauhati.ac.in/research/physics/#bks Fabrication of Ag/ZnO heterostructure

https://sites.google.com/a/gauhati.ac.in/research/physics/#rg Solitary waves in dusty plasmas
Powerline communication

Banty Tiru, her graduate student, and collaborator explores if the powerlines (supplying electricity to our homes) can be used as a medium of communication. This research work is published in the Journal of Circuits, Systems and Computers.


Authors
Abstract
This paper deals with the development of a realistic power line channel simulator wherein power line communication devices can be tested before implementation to meet the massive need of data transfer. The statistics of the noise follow the experimentally observed in different sites, namely the time-varying non-white power spectral densities (PSDs) of the background noise and a target non Gaussian amplitude distribution. The procedure based on the inverse cumulative distribution function method of generation of random numbers and iteratively updating a target spectrum necessitates knowledge of a maximum of 17 parameters for successful implementation and has been validated for three sites in the low-frequency (< 500kHz) and high-frequency (1–30MHz) bands. The average percentage errors in prediction of the mean of the channel capacity (CC) are 12.68% and 10.66% in the two bands, respectively. The minimum correlations of the distribution of BER of OFDM in a channel corrupted by the simulated and observed noises are is 0.883 and 0.801 in the two bands which are high compared to 0.422 and 0.355, respectively, when the requirement of a target amplitude distribution is neglected. With low-frequency noise emulated by a data acquisition card, an average percentage error of 11.82% in the CC and a correlation of 0.867 (against 0.498) in BER are obtained. The noise thus generated can be used as a testbed for system testing, instead of the conventional static models (additive white Gaussian noise or with time-invariant coloured PSD), leading to better optimisation of the implemented devices.

Journal References
https://sites.google.com/a/gauhati.ac.in/research/physics/#top 
 



Re-awakening of an X-Ray binary

Kalyanee Boruah and collaborators report about a surprise outburst from the low-mass X-Ray binary GRS 1716-249. The last time an outburst was observed was 23 years ago! This work and another related works are published in the journal Monthly Notices of the Royal Astronomical Society (MNRAS).


Authors
Priya Bharali, Sunil Chandra1, Jaiverdhan Chauhan1, Javier A García1, Jayashree Roy1, Markus Boettcher1, and Kalyanee Boruah
from other institutions

Abstract
In this work, the author present a spectral and temporal analysis of Swift/XRT and NuSTAR observations of GRS 1716–249 during its recent 2016–2017 outburst. This low-mass X-ray binary underwent an extraordinary outburst after a long quiescence of 23 years, since its last major outburst in 1993. The source was observed over two different epochs during 2017 April 7 and 10. The best-fitting joint spectral fitting in the energy range 0.5–79.0 keV indicates that the spectrum is best described by relatively cold, weak disc blackbody emission, dominant thermal Comptonisation emission, and a relativistically broadened fluorescent iron Kα emission line. The authors observed a clear indication of a Compton hump around 30 keV. They also detected an excess feature of 1.3 keV. Assuming a lamp-post geometry of the corona, we constrained the inner disc radius for both observations to 11.92+8.62−11.92 RISCO (i.e. an upper limit) and 10.39+9.51−3.02 RISCO (where RISCO is the radius of the innermost stable circular orbit) for the first epoch (E1) and second epoch (E2), respectively. A significant (5σ) type C quasi-periodic oscillation (QPO) at 1.20 ± 0.04 Hz is detected for the first time for GRS 1716–249, which drifts to 1.55 ± 0.04 Hz (6σ) at the end of the second observation. The derived spectral and temporal properties show a positive correlation between the QPO frequency and the photon index.

In a subsequent report, the authors also report about the observation of a newly discovered X-Ray transient.

CDs-porous silicon nanostructured photo sensor

Deepali Sarkar and her coworkers are studying the photo sensing properties of CDs-proous silicon based metal-semiconductor-metal (MSM) hetero-structure. This work is published in Journal of Materials Science: Materials in Electronics.


Authors
Abstract
Present work reports light sensing property of metal–semiconductor–metal device based on nanostructured CdS-PS:p-Si hetero-structure. PS with thickness of ~ 1500 nm is prepared on p-Si (100) oriented wafer using electrochemical anodization method. Thin nanostructured n-CdS layer of ~ 170 nm is deposited on PS:p-Si substrate by vacuum evaporation. FESEM observation confirms growth of hierarchical flower like CdS nanostructure on PS:p-Si substrate. UV–Vis absorption spectrum gives bandgap of CdS nanostructure to be 2.6 eV. Reflectance measurement of CdS-PS:p-Si hetero-structure shows multiple interference pattern within the spectral range of 200–800 nm, with remarkable blue shift of the pattern compared to that of PS:p-Si structure. Photoluminescence study of the hetero-structure reveals presence of various luminescence bands peaked at ~ 450, 460, 468, 482, 560, and 590 nm. Dark current (Id) analysis shows low leakage current ~ 32 nA at − 2 V with ideality factor (n) and reverse saturation current (Is) values of 1.57 and 0.2 nA respectively. Spectral response of the hetero-structure at a bias voltage of − 2 V and irradiation wavelength of 400 nm shows maximum responsivity (Rλ) value of ~ 0.6 AW−1 and external quantum efficiency of ~ 180%. Response and recovery times of the device are ~ 160 and ~ 350 ms respectively. The prepared hetero-structure has been compared with other silicon based optoelectronic switching devices to find suitability of an alternate choice.

Driving the electrons unstable - a numerical simulation

Madhurjya P Bora and graduate student Suniti Changmai show how the photoemission in a plasma can drive the electron two-stream instability (ETSI) and can modify a plasma sheath. This research work is published in the journal Physics of Plasmas.



Authors

Abstract
Analysis related to the onset and evolution of the electron two-stream instability (ETSI) near a surface with a continuous photoemission flux is presented. With the help of a particle-in-cell simulation code, it is shown that when a surface emits photoelectrons due to exposure to ultraviolet photons (or due to similar mechanisms), it may lead to the onset of the ETSI due to the relative bulk velocity difference between the photoelectrons and the plasma electrons. It is shown that the ETSI thus developed is sustained through the lifetime of the plasma and prevents thermalisation of the electrons, which leads to a distortion in the electron velocity distribution function (EVDF) resulting a high energy tail, at least near the surface. We have shown that the resultant EVDF can be highly Lorentzian with a spectral index of ~1.5. The simulation results are supported with the corresponding theory, which are found to be in good agreement.

https://sites.google.com/a/gauhati.ac.in/research/physics/#top 
 



Probing the protons at Large Hadron Collider (LHC)

Buddhadeb Bhattacharjee and graduate student Pranjal Sarma explore a possible mechanism of intermittency during p-p collisions at Large Hadron Collider (LHC) in CERN. This work is published in Physical Review C.


Authors

Abstract
Nonstatistical fluctuation in pseudorapidity (η), azimuthal (φ), and pseudorapidity-azimuthal (η – φ) distribution spectra of primary particles of PYTHIA Monash (default) generated pp events at √s = 2.76, 7, and 13 TeV have been studied using the scaled factorial moment technique. A weak intermittent type of emission could be realised for minimum-bias (MB) pp events in χ(η – φ) space and a much stronger intermittency could be observed in high-multiplicity (HM) pp events in all χ(η), χ(φ), and χ(η – φ) spaces at all the studied energies. For HM pp events, at a particular energy, the intermittency index αq is found to be largest in two-dimensional χ(η – φ) space and least in χ(η) space, and no centre of mass energy dependence of αq could be observed. The anomalous dimensions dq are observed to be increased with the order of the moment q, suggesting a multifractal nature of the emission spectra of various studied events. While, the coefficient λq is found to decrease monotonically with the order of the moment q for two-dimensional analysis of MB pp events as well as for one-dimensional analysis of HM pp events, a clear minimum in λq values could be observed from the two-dimensional HM pp data analysis. For PYTHIA Monash generated sets of data, the strength of the intermittency is found to vary significantly with the variation of the strength of the colour reconnection (CR) parameter, i.e. reconnection range RR, for RR = 0.0, 1.8 and 3.0, thereby, establishing a strong connection between the CR mechanism and the observed intermittent type of emission of primary charged particles of the studied high-multiplicity pp events.

Journal Reference
https://sites.google.com/a/gauhati.ac.in/research/physics/#top 
 



The physics of transparent conductors

Bimal Kumar Sarma and his graduate students come up with an AZO transparent conductor with high figure of merit for possible optoelectronic applications. This research is published in the journal Applied Surface Science.


Authors
Bikash Sarma, Dipak Barman, and Bimal K Sarma

Abstract
This work presents a broad study of transparent and conducting Al-doped ZnO (AZO) thin films that could replace indium tin oxide (ITO) as transparent electrode in optoelectronic devices. AZO thin films are deposited by pulsed DC magnetron sputtering at a sputtering power of 80 W with different pulse frequencies in the range 50-100 kHz. Diffraction data confirm formation of doped ZnO and AZO thin films exhibit preferred orientation in the c-direction. The crystallite sizes of Al:ZnO are in the range 16–21 nm. The field emission scanning electron microscopy and atomic force microscopy of AZO thin films reveal nucleation and growth of uniform and dense films with better quality films deposited at a pulse frequency 75 kHz. A simple and non-destructive optical method is adopted to determine thickness and dispersion parameters such as dispersion energy, carrier concentration to effective mass ratio, and plasma frequency. AZO thin films offer excellent visible light transparency and limited transparency in the near-infrared region due to free carrier absorption. The sheet resistance of AZO thin films is recorded in the range 9-45 Ω/sq making these transparent conducting oxides (TCOs) suitable for optoelectronic applications. The figure of merit as high as 1.99 × 10−2 Ω−1 is achieved for AZO thin film deposited at a pulse frequency of 75 kHz. AZO thin film sputtered with a pulse frequency of 75 kHz is quite stable in ambient oxidising environment and surface adsorption sites might govern the initial oxidation of films when exposed to atmosphere. Excellent figure of merit and good stability of sputtered AZO thin films as TCO fulfil the requirements of a transparent electrode in photovoltaics.

Journal Reference
https://sites.google.com/a/gauhati.ac.in/research/physics/#top 
 



Fabricating targets for nuclear reaction experiments

Kushal Kalita, graduate student Nabendu Kumar Deb, and their collaborators report about fabrication of thin nickel targets for present and future nuclear reaction experiments. This research work is published in the journal Vacuum.


Authors
Nabendu Kumar Deb, Kushal Kalita, S R Abhilash1, Pankaj K Giri1, Rohan Biswas1, G R Umapathy1, D Kabiraj1, and S Chopra1
1from other institutions

Abstract
To perform nuclear reaction experiments at HIRA, IUAC, New Delhi, thin (100–150 ) and pure 61,62Ni targets of uniform thickness are required. Self supporting targets are preferable for such case but, instead, carbon-backed isotopic targets are fabricated by adapting physical vapour deposition technique as it was comparatively more stable and consistent. Around 25 thin targets of both 61,62Ni isotopes are prepared using the limited amount of available enriched target material (less than 100 mg). The carbon-backed slides along with the parting agents are prepared using a diffusion pump based coating unit and the target material is deposited over the carbon-backed slides in the turbo pump based coating unit. To obtain consistent and intact targets, some trials were done with deposited slides and was found that the material degrades in any of the slides when kept idle for few days. The thicknesses of the targets are verified using profilometer, α energy loss technique, and RBS technique. They were found to be in good agreement with each other using the three techniques. The purity and the uniformity of the fabricated targets are further confirmed after verification using the RBS, the EDS and the XRD techniques.

Journal Reference
https://sites.google.com/a/gauhati.ac.in/research/physics/#top 
 



Studying dielectric properties of PANI-SA composites

Deepali Sarkar and her graduate students study the dielectric properties of polyaniline (PANI) sulphonic acid (SA) composites. This work is published in the journal Synthetic Metals.


Authors
Mausumi Das, Ali Akbar, and D Sarkar

Abstract
Present work is primarily emphasised on the study of dielectric properties of sulphonic acids (SA) incorporated polyaniline (PANI). SA's function as dopants as well as surfactants for PANI. The PANI and PANI-SA samples are prepared by interfacial technique. The three composites prepared with three SA's [camphor sulphonic acid (CSA), dodecylbenzene sulphonic acid (DBSA) and polystyrene sulfonic acid (PSSA)] are characterised by X-ray diffraction (XRD), fourier transform infrared spectroscopy (FTIR), field effect scanning electron microscopy (FESEM) and thermogravimetric analysis (TGA) and differential thermogram (DTG) prior to dielectric studies. The results of PANI not incorporated with SA are also shown for comparison. XRD shows enhancement of crystallinity on SA doping. FTIR confirms the molecular structure with enhanced oxidation states on SA doping. FESEM shows fibrillar morphology with reduced fibre diameter from 86 nm in pure PANI to minimum of 50 nm in one of the composites. TGA and DTG results show better thermal stability on SA doping. The dielectric and AC conductivity measurements of the undoped and dopedPANI are studied in the frequency range of 42 kHz - 2 MHz and in the temperature range of 300 K–330 K. The dielectric constant for all the PANI composites increases with increase in temperature which indicates increase in mobility of the electric dipoles in the polymers. The dielectric constant for DBSA doped PANI is nearly 5000 which is much higher in context to interfacially polymerised nanofibres compared to that of undoped PANI where this value is about 800. Electric modulus studies show relaxation peaks in imaginary modulus spectra which ascertains about the type of relaxation process. The AC conductivity of the composites is found to be much higher than that of undoped PANI. The electrical conductivity increases with increase of temperature. This is suggested to be caused due to the activated trapped charge carriers.

Journal Reference
https://sites.google.com/a/gauhati.ac.in/research/physics/#top 
 



One more neutrino!

Kalpana Bora and her collaborators ask whether the sterile neutrino can fit into the existing neutrino hierarchy. Sterile neutrino is a possible dark matter candidate. This research is published in The European Physical Journal C.


Authors
Neelakshi Sarma, Kalpana Bora, and Debasish Borah1
1from other institution

Abstract
Motivated by the recent resurrection of the evidence for an eV scale sterile neutrino from the MiniBooNE experiment, the authors revisit one of the most minimal seesaw model known as the minimal extended seesaw that gives rise to a 3 + 1 light neutrino mass matrix. The authors consider the presence of A4 flavour symmetry which plays a nontrivial role in generating the structure of the neutrino mass matrix. Considering a diagonal charged lepton mass matrix and generic vacuum alignments of A4 triplet flavons, we classify the resulting mass matrices based on their textures. Keeping aside the disallowed texture zeros based on earlier studies of 3 + 1 neutrino textures, we categorise the remaining ones based on texture zeros, μ-τ symmetry in the 3 × 3 block and hybrid textures. After pointing out the origin of such 3 + 1 neutrino textures to A4 vacuum alignments, the authors use the latest 3 + 1 neutrino oscillation data and numerically analyse the texture zeros and μ–τ symmetric cases. The authors find that a few of them are allowed from each category predicting interesting cor- relations between neutrino parameters. They also find that all of these allowed cases prefer normal hierarchical pattern of light neutrino masses over inverted hierarchy.

Journal Reference
https://sites.google.com/a/gauhati.ac.in/research/physics/#top 
 



Modeling aperiodic magnetospheric oscillations

Graduate Student Murchana Khusroo and Madhurjya P Bora explain why the magnetic oscillations in earth's magnetosphere are always almost aperiodic. This research work is published in Physical Review E


(Featured in the Kaleidoscope of Phys Rev E for the Month of Jan 2019)

Authors

Abstract
The authors present an analysis of a Hall-magnetohydrodynamics model of the magnetospheric plasma with finite Larmor radius effect. Through a bifurcation analysis of the resultant nonlinear system, they show that this nonlinear model does not possess a limit cycle, which rules out regular periodic oscillations with constant amplitude. However, it does result in a train of magnetosonic solitons, localised in space, with amplitudes increasing in time, which are largely in agreement with what is usually observed in the magnetopause region. We call these oscillations aperiodic magnetospheric oscillations. The authors emphasise that most of the train of solitary oscillations observed by the Cluster fleet and other spacecrafts do not have constant amplitudes: they either continuously increase or decrease. These train of solitons with non-constant amplitudes is a primary solution of their mathematical model.

Journal Reference
https://sites.google.com/a/gauhati.ac.in/research/physics/#top 
 



Detection of arsenic in water

Simanta Hazarika and graduate student Chiranjib Rajkonwar describe a method which has the potential for being an in-situ method for detection of groundwater arsenic. This research is published in the journal Laser Physics.





Authors

Abstract
Ingestion of arsenic by the human body through drinking water induces numerous diseases. Therefore, detection and estimation of arsenic concentration in water, especially in groundwater, is important to protect the human world from its toxicity. In this work we demonstrate a laser induced fluorescence resonance energy transfer (LIFRET) method for the determination of the arsenic concentration in water. The fluorescence resonance energy transfer (FRET) is observed in an aqueous solution of 1, 8-naphthalimide and rhodamine-B by exciting it with a laser radiation of wavelength 405 nm. It is observed that if arsenic contaminated water is added to the pure solution of 1, 8-naphthalimide and rhodamine-B the wavelength of the peak in its FRET spectrum shifts from its pure value at 632.45 nm (peak position of the FRET spectrum of a pure solution of 1, 8-naphthalimide and rhodamine-B). These shifts of wavelengths are towards the lower wavelength side, corresponding to an increase in the concentration of arsenic added to the solution. In the present work, based on the shifting of the peak wavelength value of the FRET spectra corresponding to different concentrations of arsenic, we have described a method for the estimation of arsenic in water up to a lowest limit of 0.00001 gm l−1 (the maximum permissible limit of arsenic in water according to the World Health Organisation). The LIFRET method described in this work is portable, relatively cost effective, more convenient and has the potential for any in situ measurement of arsenic in the ground water of arsenic contaminated areas.

Journal Reference
https://sites.google.com/a/gauhati.ac.in/research/physics/#top 
 



Dust levitation in an inverse sheath

Rinku Deka and Madhurjya P Bora put forward a theory which could probably explain appearances of dust streaks above lunar surface. This research is published in the journal Physics of Plasmas.



Authors

Abstract
The results of an analysis of the physics of levitation of negatively charged dust particles over a surface (wall) in an inverse sheath are reported in this research work. It is shown that in a suitable parameter regime, the ion-drag force may balance the combined electrostatic and gravitational forces on the dust particles owing to its hollow profile as one moves away from the surface. The authors' analysis shows that the parameter regimes in which such a situation may result are realisable in laboratory and space plasma environments, particularly the near-surface dayside lunar plasma. The lunar surface and dust grains are electrostatically charged due to the interaction with the solar wind plasma environment and the photoemission of electrons due to solar UV radiation. This results in a process that charges the surface positively and generates a near-surface photoelectron inverse plasma sheath. The potential structure changes from a monotonic classical sheath to an inverse sheath as the emitted electron density becomes larger than the plasma electron density. In a relatively newer, recently developed charging model, called the Patched Charge Model, it was shown both theoretically and experimentally that even in a photoelectron-rich environment, dust particles lying on a regolith surface can attain large negative potential due to the formation of micro-cavities. This negative potential may reach such values so that dust mobilisation and lofting may become possible. In their work, the authors have assumed the existence of such negatively charged dust particles in a photoelectron-rich environment and argue that once the dust lofting is effected, the levitation can be sustained through the ion-drag force. The conditions of levitation are investigated for these dust particles, and the levitation distances from the lunar surface are calculated.

Journal Reference
https://sites.google.com/a/gauhati.ac.in/research/physics/#top 
 



Garphene based soil moisture microsensor

Dr Hemen Kumar Kalita and his collaborators have developed and an in-situ graphene-based microsensor for agricutural use, which opens up a range of novel future applications. This work is published in the journal Sensors and Actuators B: Chemical.




Authors
Vinay S Palaparthy1, Hemen Kalita, Sandeep G Surya1, Maryam Shojaei Baghini1, and M Aslam1
1 from other institutions

Abstract
In this work, the authors have designed a robust graphene oxide (GO) based capacitive sensor which is highly sensitive to soil moisture. The sensor consists of inter-digitated electrode (IDE) with chemically synthesised GO as the sensing platform. They have used the MEMS fabrication technique to fabricate the IDE and have performed the characterisation such as AFM, RAMAN and XPS on the as-prepared GO to understand the surface properties of the GO. The sensor response changes by 340% and 370% over soil moisture changes from 1% to 55% for red and black soil, respectively. GO sensor array shows a fast response time of 100–120 seconds for the soil moisture measurements. For in-situ soil moisture measurements, the diurnal temperature and salt concentration (soil conductivity) are the variable parameters, which might affect the sensor response. They observe that the sensor output changes by 6% when the ambient temperature varies from 25°C to 65°C, which leads to just 3% discrepancies for the soil moisture measurements. For the salt concentration (soil conductivity) measurements the sensor output changes by 4% when salt concentration in the soil sample varies from 0 mol to 0.35 mol (standard for the field measurement), and discrepancies in the soil moisture measurements is around 2%.

Effect of annealing temperature on  silver-PVA nanocomposite

Dr Sulochana Deb and her collaborator are reporting the effect of annealing temperature on the optical properties of silver nanoparticles embedded in polyvinyl alcohol (PVA) matrix. This work is published in the journal Optik.



Authors

Abstract
The present article reports the effect of annealing temperature on the morphology and optical properties of silver nanoparticles embedded in polyvinyl alcohol (PVA) matrix. The nanocomposite films prior to annealing shows spherical grains. These films are annealed at five different temperatures (373 K, 423 K, 473 K, 523 K and 573 K) in vacuum and the effects on surface morphology and optical properties are investigated. The unannealed and annealed films are characterised by FESEM, XRD, FTIR spectra, TGA, UV–vis absorption spectra and photoluminescence (PL) spectra. Scanning electron microscopy images show the change in shape and size with increase of annealing temperature. Spherical shapes of the grains in pre annealed samples get changed to nanocubes and nanorods at higher annealing temperatures of 523 K and 573 K respectively. UV–vis spectra show a remarkable change of surface plasmon resonance peak with the increase in annealing temperature. XRD shows a characteristic intense peak of silver nano at 2θ = 38ο for (111) crystalline plane with no significant change with annealing temperature. Photoluminescence spectra show a tendency of red shift of emission peak.

Journal Reference
https://sites.google.com/a/gauhati.ac.in/research/physics/#top 
 



Room temperature ethanol sensor

Dr Deepali Sarkar and her collaborators have developed an ethanol sensor from polypyrrole embedded in polyvinyl alcohol matrix. This work is published in Polymer Bulletin.


Authors

Abstract
Polypyrrole (PPy) is embedded in Polyvinyl Alcohol (PVA) matrix to give a homogeneous composite solution by dispersing PPy in PVA solution. Scanning Electron Microscopic (SEM) study on the film prepared from the composite solution shows uniform distribution of nearly spherical shaped PPy grains of average diameter of 20 nm. The X-ray diffraction (XRD), UV-Visible and Fourier Transform Infra-Red (FTIR) spectroscopic characterisations of the composite film are carried out for structural and optical properties. The enhanced thermal stability of the PPy-PVA composite film is confirmed by thermogravimetric analysis (TGA). XRD shows signature of amorphous nature for PVA along with some crystallinity arising for PPy. UV-Visible absorption spectrum shows two absorption peaks for π-π* and polaronic transition; FTIR spectrum suggests cross linking between PPy and PVA. Studies on sensing of the PPy-PVA composite film for Ethanol, Ammonia, Toluene, Chloroform and Acetone vapours at room temperature (300 K) are done for comparison. The composite film shows best response among all these vapours towards Ethanol requiring smallest response time.

Journal Reference
https://sites.google.com/a/gauhati.ac.in/research/physics/#top 
 



Structural, electronic and optical properties of heteroatom doped monolayer graphene

Dr Sankar Moni Borah and his coworkers present a theoretical study of the structural, electronic, and optical properties of doped monolayer graphene with the help of density functional theory (DFT). This research article is published in Optik.


Authors

Abstract
The authors present a theoretical work on the influence of doped and co-doped Al, Al-S, Al-N and Al-P heteroatoms in the mono layer graphene surface. The Density functional study reveals that, Al, P and S co-doping significantly modifies the neighbourhood bonding arrangement of the graphene sheet. The Natural population analysis revels that Al, P and S co-doping makes the graphene surface as electron rich system. From the molecular orbital analysis it is found that HOMO-LUMO energy gap decreases by starting from the pristine graphene in following manner Al doping > Al-S co-doping> Al-N co-doping >Al-P co-doping. On the other hand, the time dependent density functional theory (TD-DFT) calculation shows that the maximum absorbing wavelength of Al-P and Al-N co-doped graphene systems shifted towards the lower wavelength range with respect to Al doped graphene.

Journal Reference
https://sites.google.com/a/gauhati.ac.in/research/physics/#top 
 



ZnO based multilayer transparent conductor

Dr Bimal Kumar Sarma and his collaborator Bikash Sarma present electro-optical studies of ZnO (Zinc Oxide) based multilayer transparent conductor. This research work is published in Journal of Alloys and Compounds.



Authors

Abstract
A broad study on transparent and conducting ZnO/Ag/ZnO multilayer thin films is presented in this report. Bottom ZnO layer, mid Ag layer, and top ZnO layer are deposited on flexible and glass substrates by Pulsed DC and DC magnetron sputtering technique. X-ray diffraction (XRD) studies of multilayer structures reveal formation of nanocrystalline wurtzite ZnO which exhibit (002) preferable orientation along with the feeble appearance of (111) peak of Ag from the intermediate layer. High transmittance of 81.8% with low sheet resistance 17.0 U/sq are achieved for ZnO/Ag/ZnO thin films deposited on glass and transmittance of 77.3% with sheet resistance 14.7 U/sq are observed for ZnO/Ag/ZnO thin films deposited on PET. XRD studies facilitate to understand the role of texture and residual stress of ZnO on the electro- optical properties of ZnO/Ag/ZnO multilayer thin films. XRD analyses and electro-optical measurements confirm that the tensile residual stress of ZnO above certain level is favourable for lower sheet resistance primarily due to the lattice extension in the transverse direction to the film surface. Due to high Hall mobility, low sheet resistance and reasonably good figure of merit, sputtered ZnO/Ag/ZnO multilayer thin films meet the requirements of transparent conductors in photovoltaics.

Journal Reference
https://sites.google.com/a/gauhati.ac.in/research/physics/#top 
 



Broadening of spectra from gamma ray bursts (GRB)

Dr Kalyanee Boruah and her fellow researchers construct a model to explain the broadening of GRB spectra. GRBs are some of the most powerful emissions of electromagnetic radiation in the Universe. This research work has been published in New Astronomy.



Authors
Priya Bharali, Sunder Sahayanathan, Ranjeev Misra, and Kalyanee Boruah
Abstract
The observations of the prompt emission of gamma ray bursts (GRB) by GLAST Burst Monitor (GBM), on board Fermi Gamma-ray Space Telescope, suggest the presence of a significant thermal spectral component, whose origin is not well understood. Recently, it has been shown that for long duration GRBs, the spectral width as defined as the logarithm of the ratio of the energies at which the spectrum falls to half its peak value, lie in the range of 0.84-1.3 with a median value of 1.07. Thus, while most of the GRB spectra are found to be too narrow to be explained by synchrotron emission from an electron distribution, they are also significantly broader than a blackbody spectrum whose width should be 0.54. Here, the authors consider the possibility that an intrinsic thermal spectrum from a fire-ball like model, may be observed to be broadened if the system undergoes a rapid temperature evolution. A toy-model is being constructed here to show that for bursts with durations in the range 5-70 s, the widths of their 1 second time-averaged spectra can be at the most ≲ 0.557. Thus, while rapid temperature variation can broaden the detected spectral shape, the observed median value of  ~1.07 requires that there must be significant sub-photospheric emission and/or an anisotropic explosion to explain the broadening for most GRB spectra.

Journal Reference
https://sites.google.com/a/gauhati.ac.in/research/physics/#top 
 



Efficient ammonia gas sensor

Dr Deepali Sarkar and Mausumi Das report about the development of an efficient ammonia (NH3) gas sensor at room temperature from one-pot synthesised zinc oxide (ZnO). This research work is published in Ceramics International.


Authors
Mausumi Das and Deepali Sarkar

Abstract
Development of efficient room temperature ammonia (NH3) gas sensor from one pot synthesised zinc oxide (ZnO) – polyaniline (PANI) nanocomposite is reported in the present article. Prior to gas sensing study, the material is characterised to understand the structural, morphological, compositional, optical and thermal properties. Structural and morphological studies indicate good incorporation of ZnO particles in PANI matrix. The gas sensing efficiency of ZnO-PANI nanocomposite is examined at room temperature for ethanol (C2H5OH), methanol (CH3OH) and NH3 gas. The results confirm that ZnO-PANI nanocomposite to be highly selective for NH3 with fast response time and better stability. The response and recovery times are observed to be significantly dependent on NH3 concentration and the lowest detection limit of the sensor for NH3 is found 10 ppm. ZnO-PANI nanocomposite shows better gas sensing efficiency as compared to the sensors developed from single phase PANI film.
https://sites.google.com/a/gauhati.ac.in/research/physics/#top 
 



UV-Visible optical photodetection from porous silicon MSM device

Dr Deepali Sarkar and her collaborators describe the fabrication process of Metal-Semiconductor-Metal structure of porous silicon, which will be used for photodetection. This research article is published in Superlattices and Microstructures.



Authors
M Das, S Sarmah, and D Sarkar

Abstract
Silicon photodiodes have been in use as UV detectors and some compound semiconductors as visible detectors for a long time. However their implementation to the optoelectronic field is limited due to high fabrication cost as well as sophisticated prerequisites. The present research aims at fabricating porous silicon Metal-Semiconductor-Metal structure and its photodetection property for the UV wavelength range from 250 to 390 nm along with a portion of visible spectrum. Porous silicon thickness attained is ~ 2 mm with uniform distribution of pores. It shows characteristic visible yellow/green luminescence under UV-Visible irradiation. The responsivities, obtained through photoconductivity measurement of the device, are obtained as 1.42 and 2.00 AW-1 for UV and visible ranges respectively, whereas the response times in corresponding ranges as 0.70 and 1.00 s. These results suggest superiority of the device as a UV-Visible detector compared to silicon or other semiconductor detectors. However, the device shows ageing effect due to slow oxidation of the PS layer.
https://sites.google.com/a/gauhati.ac.in/research/physics/#top 




Rapidity and Relativistic Heavy Ion collisions

Dr Buddhadeb Bhattacharjee and his collaborators explore more about rapidity (a parameter which is a measure of relativistic velocity in collisions of sub-atomic particles) in Relativistic Heavy Ion Collisions in two research works published in Nuclear Physics A and Physical Review C.



Authors
Kalyan Dey and Buddhadeb Bhattacharjee &
Nur Hussain and Buddhadeb Bhattacharjee

Abstract #1
Strangeness enhancement factor (E_s) is considered to be one of the traditional signatures of formation of Quark Gluon Plasma (QGP) in relativistic heavy ion collisions (HIC). Generally, strangeness enhancement factor is believed to be maximum at mid-rapidity. However, in this work, it is shown that strangeness enhancement factor could be minimum as well at mid-rapidity depending upon the nature of the quark content of the produced particles of HIC. Rapidity dependent strangeness enhancement factors for the identified particles have been studied with the help of a string based hadronic transport model UrQMD-3.3 (Ultra-relativistic Quantum Molecular Dynamics) at FAIR energies. A strong rapidity dependent strangeness enhancement could be observed with our generated data for Au+Au collisions at the beam energy of 30A GeV. The strangeness enhancement is found to be maximum at mid-rapidity for the particles containing leading quarks while for particles consisting of produced quarks only, the situation is seen to be otherwise. Such rapidity dependent strangeness enhancement could be traced back to the dependence of rapidity width on centrality or otherwise on the distribution of net-baryon density.

Abstract #2
Width of the rapidity distributions of produced particles of heavy ion collisions carry a number of information about the collisions dynamics. NA49, a CERN collaboration, proposed by analysing CERN Super Proton Synchrotron (SPS) data, a universal mass ordering of the rapidity width of the produced charged particles. Such universal mass ordering was contradicted by GU group. In one of our recent works at AGS and low SPS energies [1], from a study on UrQMD-generated Monte Carlo events, as well as from existing experimental results, it has been shown that the universal mass ordering is violated with a jump in the rapidity width of lambda. A separate mass ordering of various mesons and baryons have been proposed. Width of the rapidity distribution of leading particles is shown to have non-trivial net baryon density distribution affect. In this work it has been shown with UrQMD-generated data that the observed jump in the rapidity width of lambda is a universal characteristics of heavy ion collisions data from AGS and SPS to RHIC and LHC energies.

Journal References
https://sites.google.com/a/gauhati.ac.in/research/physics/#top 




Coherence in firefly emission

There may be more than what we see in the light of the firefly in the summer sky! Dr Anurup Gohain Borua and his fellow researchers investigates. Find more about it in the research communication published in Current Science. The findings of this research have also been featured in India Science Wire - Vigyan Prasar on February 16, 2018.



Authors
Upamanyu Sharma, Angana Goswami, and Anurup Gohain Barua

Abstract
The emission spectrum of the Indian species of firefly Luciola Praeusta has been shown to consist of two broad green and red-coloured sectors, with a narrow yellow one in between. In this work, interference patterns of the light of this firefly-species have been shown to contain a laser-like emission. The value of the coherence length (implying the degree of monochromaticity) for the yellow-coloured light has been shown to be an astonishing 23.5 cm, while reasonably good spatial coherence for this radiation has been obtained.

Journal Reference
https://sites.google.com/a/gauhati.ac.in/research/physics/#top 




Probing the neutrinos

Dr Kalpana Bora and her collaborators explore the physics of Majorana neutrinos with reference to the Deep Underground Neutrino Experiment near Fermilab (DUNE, formerly LBNE). This work is published in Physical Review D.


Authors
Kalpana Bora, Debasish Borah, and Debajyoti Dutta

Abstract
This study probes different texture zero neutrino mass matrices at long baseline neutrino experiment DUNE particularly focusing on its sensitivity to the octant of atmospheric mixing angle θ23 and leptonic Dirac CP phase δcp. Assuming a diagonal charged lepton basis and Majorana nature of light neutrinos, the authors first classify the possible light neutrino mass matrices with one and two texture zeros and then numerically evaluate the parameter space which satisfies the texture zero conditions. Apart from using the latest global fit values of neutrino oscillation parameters, they also use the latest bound on the sum of absolute neutrino masses (Σi|mi|) from the Planck mission data and the updated bound on effective neutrino mass Mee from neutrinoless double beta decay (0νββ) experiment to find the allowed Majorana texture zero mass matrices. For the allowed texture zero mass matrices from all these constraints, they then feed the corresponding light neutrino parameter values satisfying the texture zero conditions into the numerical analysis in order to study the capability of DUNE to allow/exclude them once it starts taking data. The authors also find that the DUNE will be able to exclude some of these texture zero mass matrices which restrict the (θ23 − δcp) to a very specific range of values, depending on the values of the parameters that Nature has chosen.

Journal Reference
https://sites.google.com/a/gauhati.ac.in/research/physics/#top 




Gravitational theories near the galactic centre

Dr Sanjeev Kalita explores the properties of gravity near the galactic centre, the telltale sign of which may be detected by the Extremely Large Telescope (ELT). This work is published in Astrophysical Journal.

https://en.wikipedia.org/wiki/Extremely_Large_Telescope


Author
Sanjeev Kalita

Abstract
The author has used the Parameterised Post Newtonian (PPN) framework of General Relativity to examine the modification of the law of gravity near the Galactic Centre Black Hole, Sgr A*. Quantum gravity near black hole singularities largely expect such modifications. This is the first astrophysical test of new gravitational physics near the Galactic Centre. Periastron anomaly of the S-stars having compact orbits (within the central milli parsec, r = 50-100 au) near the black hole has been used to see the difference between General Relativity and Brans-Dicke scalar-tensor gravity and f(R) theories which are serious rivals to General Relativity and are capable of addressing several cosmological issues. Mild to strong values of spin and quadrupole of the black hole have been used. Possibility of detection of such deviation through upcoming astrometric observations of the Extremely Large Telescopes (ELTs) are discussed. New observational strategies for measuring the black hole spin and extracting modified gravity signals are also suggested. The work also reports possibility of black hole hair and super Kerr spin (naked singularity) which are otherwise unanticipated in existing black hole physics.

Journal Reference
https://sites.google.com/a/gauhati.ac.in/research/physics/#top 




Fabrication of Ag/ZnO heterostructure

Dr Bimal Kumar Sarma and his collaborator describe fabrication of Ag/ZnO heterostructure and the role of surface coverage of ZnO microrods by Ag nanoparticles on its photophysical and photocatalytic properties. This work is published in Applied Surface Sceince.



Authors
Bikash Sarma and Bimal K Sarma

Abstract
This report presents findings on microstructural, photophysical, and photocatalytic properties of Ag/ZnO heterostructure grown on flexible and silicon substrates. ZnO microrods are prepared by thermal decomposition method for different solute concentrations and Ag/ZnO heterostructure are fabricated by photo-deposition of Ag nanoparticles on ZnO microrods. X-ray diffraction and electron microscopy studies confirm that ZnO microrods belong to the hexagonal wurtzite structure and grown along [001] direction with random alignment showing that majority microrods are aligned with (100) face parallel to the sample surface. Plasmonic Ag nanoparticles are attached to different faces of ZnO. In the optical reflection spectra of Ag/ZnO heterostructure, the surface plasmon resonance peak due to Ag nanoparticles appears at 445 nm. Due to the oxygen vacancies the band gaps of ZnO microrods turn out to be narrower compared to that of bulk ZnO. The presence of Ag nanoparticles decreases the photoluminescence intensity which might be attributed to the non-radiative energy and direct electron transfer in the plasmon–exciton system. The quenching of photoluminescence in Ag/ZnO heterostructure at different growth conditions depend on the extent of surface coverage of ZnO by plasmonic Ag nanoparticles. Photocatalytic degradation efficiency of Ag/ZnO heterostructure is higher than that of ZnO microrods. The extent of surface coverage of ZnO microrods by Ag nanoparticles is crucial for the observed changes in photophysical and photochemical properties.

Journal Reference
https://sites.google.com/a/gauhati.ac.in/research/physics/#top 





Solitary waves in dense dusty plasmas

Rajita Goswami analyses a phenomena which is expected to be seen in dense dusty plasmas. This communication is published in Physics of Plasmas.



Authors
Rajita Goswami and Madhurjya P Bora

Abstract
In this report the authors present an analysis of the effect of polarization force on dust acoustic solitons with self-consistent inclusion of the polarization effect into the dust-charging model. This has been carried out considering the dependence of dust-charge number on dust potential which is affected by the polarization force. Their analysis shows that inclusion of polarization effect into the dust-charging model results solitons of much higher amplitudes.

Journal Reference
https://sites.google.com/a/gauhati.ac.in/research/physics/#top