Showing 1–50 of 63 results for author: Tyagi, A

Spin Glass Mapping of the Parallel Minority Game

Authors: Aryan Tyagi, Soumyaditya Das, Soumyajyoti Biswas, Anirban Chakraborti

Abstract: The parallel minority game (PMG) extends the classical minority game to many choices, with each agent restricted to two predetermined alternatives. In this condition, minimizing the population variance across all choices is a complex combinatorial optimization problem. We show that this minimization is exactly equivalent to finding the ground state of an Ising spin glass in the mean-field limit, i… ▽ More The parallel minority game (PMG) extends the classical minority game to many choices, with each agent restricted to two predetermined alternatives. In this condition, minimizing the population variance across all choices is a complex combinatorial optimization problem. We show that this minimization is exactly equivalent to finding the ground state of an Ising spin glass in the mean-field limit, i.e., the Sherrington-Kirkpatrick model. By encoding the agent choices as spin variables, the variance becomes a quadratic Hamiltonian with quenched random couplings $J_{ij}$ and random fields $h_i$. This mapping reveals inherent frustration and connects the PMG to the well developed theory of spin glasses, providing a new perspective on the frozen, sub-optimal configurations observed in stochastic strategies. △ Less

Submitted 21 May, 2026; originally announced May 2026.

Comments: 5 pages, 2 figures

Active-Absorbing Phase Transitions in the Parallel Minority Game

Authors: Aryan Tyagi, Soumyajyoti Biswas, Anirban Chakraborti

Abstract: The Parallel Minority Game (PMG) is a synchronous adaptive multi-agent model that exhibits active-absorbing transitions characteristic of non-equilibrium statistical systems. We perform a comprehensive numerical study of the PMG under two families of microscopic decision rules: (i) agents update their choices based on instantaneous population in their alternative choices, and (ii) threshold-based… ▽ More The Parallel Minority Game (PMG) is a synchronous adaptive multi-agent model that exhibits active-absorbing transitions characteristic of non-equilibrium statistical systems. We perform a comprehensive numerical study of the PMG under two families of microscopic decision rules: (i) agents update their choices based on instantaneous population in their alternative choices, and (ii) threshold-based activation that activates agents movement only after overcrowding density crossing a threshold. We measure time-dependent and steady state limits of activity $A(t)$, overcrowding fraction $F(t)$ as functions of the control parameter $g=N/D$, where $N$ is the number of agents and $D$ is the total number of sites. Instantaneous rules display mean-field directed-percolation (MF-DP) scaling with $β\approx1.00$, $δ\approx0.5$, and $ν_{\parallel}\approx2.0$. Threshold rules, however, produce a distinct non-mean-field universality class with $β\approx0.75$ and a systematic failure of dynamical scaling. We show that thresholding acts as a relevant perturbation to the critical behavior of the model. The results highlight how minimal cognitive features at the agent level fundamentally alter large-scale critical behavior in socio-economic and active systems. △ Less

Submitted 28 January, 2026; v1 submitted 28 December, 2025; originally announced December 2025.

Comments: 6 pages, 3 figures

High-temperature and high-pressure study on columbite structured ZnNb2O6

Authors: A. Tyagi, P. Botella, A. B. Garg, J. Sanchez-Martin, D. Diaz-Anichtchenko, R. Turnbull, S. Anzellini, C. Popescu, D. Errandonea

Abstract: High-temperature and high-pressure experiments were conducted on columbite-type ZnNb2O6, reaching temperatures up to 873 K at ambient pressure and pressures up to 30 GPa at ambient temperature, respectively. Through systematic analysis employing synchrotron powder X-ray diffraction and Raman spectroscopy, we examined the crystal structure and phonon behavior. Within the specified temperature range… ▽ More High-temperature and high-pressure experiments were conducted on columbite-type ZnNb2O6, reaching temperatures up to 873 K at ambient pressure and pressures up to 30 GPa at ambient temperature, respectively. Through systematic analysis employing synchrotron powder X-ray diffraction and Raman spectroscopy, we examined the crystal structure and phonon behavior. Within the specified temperature range, the orthorhombic phase of ZnNb2O6 (space group: Pbcn) demonstrated notable phase stability, with a thermal expansion coefficient similar to that of isomorphic compounds. Notably, a reversible phase transition was observed under compression at 10 GPa, with diffraction experiments indicating a shift to a monoclinic structure (space group P2/a), which remained stable up to 30 GPa. Changes in Raman modes, lattice parameters, and the unit-cell volume were monitored. A significant 2.5% discontinuity in the unit-cell volume at the phase transition pressure from orthorhombic to monoclinic suggests a first-order phase transition. The bulk moduli of the orthorhombic and monoclinic phases were estimated as 165(7) GPa and 230(9) GPa, respectively- We also found that both phases exhibit an anisotropic response to pressure. Furthermore, first-principles calculations support consistently with experimental observations. △ Less

Submitted 5 August, 2025; originally announced August 2025.

Comments: 34 pages, 17 figures, 4 tables

Report number: Results in Physics 60, 2024, 107681

Orbital fluctuations and spin-orbital-lattice coupling in Bi2Fe4O9

Authors: Aditya Prasad Roy, M. K. Chattopadhyay, Ranjan Mittal, Srungarpu N. Achary, Avesh K. Tyagi, Manh Duc Le, Dipanshu Bansal

Abstract: Magnetic frustrations and degeneracies profoundly affect ground-state magnetic properties emerging from competing exchange interactions. Controlling such frustrations using orbital and phonon engineering via the Kugel-Khomskii-type (KK-type) interactions has recently enabled the orbital enhancement of magnetoelectric (ME) coupling. Using combined spectroscopic techniques and first-principle simula… ▽ More Magnetic frustrations and degeneracies profoundly affect ground-state magnetic properties emerging from competing exchange interactions. Controlling such frustrations using orbital and phonon engineering via the Kugel-Khomskii-type (KK-type) interactions has recently enabled the orbital enhancement of magnetoelectric (ME) coupling. Using combined spectroscopic techniques and first-principle simulations, here we demonstrate that the magnetically frustrated Cairo lattice, Bi2Fe4O9, exhibits a strong KK-type interaction resulting in a coupled spin-orbital phase below 1.8 times the Neel temperature (TN = 245 K). We observe an order of magnitude change in phonon linewidths that is not explainable considering spin-phonon coupling channels alone. Instead, the observed change is reminiscent of orbitally active materials, which we explicitly confirm by measuring the T-dependence of low-energy orbital excitations. We further find that Bi2Fe4O9 harbors an unstable polar mode, driving the lattice to a symmetry-lowered ferroelectric (FE) phase below TN, in line with the previously reported hysteresis in polarization. Nonetheless, the FE phase leads to extremely small calculated superlattice Bragg peak intensities that are yet to be experimentally confirmed. Moreover, thermal conductivity measurements do not show any measurable effect of KK-type interactions on thermal transport across TN. But, we observe a repeatable anomaly near 57 K appearing only in the heating cycle, which co-occurs with the 400 meV broad continuum observed in Raman measurements. The observed KK-type interaction in Bi2Fe4O9 provides an opportunity for orbital enhancement of ME coupling by phonon control of superexchange interactions. △ Less

Submitted 7 January, 2025; originally announced January 2025.

Comments: 7 figures

Physics Informed Deep Learning for Strain Gradient Continuum Plasticity

Authors: Ankit Tyagi, Uttam Suman, Mariya Mamajiwala, Debasish Roy

Abstract: We use a space-time discretization based on physics informed deep learning (PIDL) to approximate solutions of a class of rate-dependent strain gradient plasticity models. The differential equation governing the plastic flow, the so-called microforce balance for this class of yield-free plasticity models, is very stiff, often leading to numerical corruption and a consequent lack of accuracy or conv… ▽ More We use a space-time discretization based on physics informed deep learning (PIDL) to approximate solutions of a class of rate-dependent strain gradient plasticity models. The differential equation governing the plastic flow, the so-called microforce balance for this class of yield-free plasticity models, is very stiff, often leading to numerical corruption and a consequent lack of accuracy or convergence by finite element (FE) methods. Indeed, setting up the discretized framework, especially with an elaborate meshing around the propagating plastic bands whose locations are often unknown a-priori, also scales up the computational effort significantly. Taking inspiration from physics informed neural networks, we modify the loss function of a PIDL model in several novel ways to account for the balance laws, either through energetics or via the resulting PDEs once a variational scheme is applied, and the constitutive equations. The initial and the boundary conditions may either be imposed strictly by encoding them within the PIDL architecture, or enforced weakly as a part of the loss function. The flexibility in the implementation of a PIDL technique often makes for its ready interface with powerful optimization schemes, and this in turn provides for many possibilities in posing the problem. We have used freely available open-source libraries that perform fast, parallel computations on GPUs. Using numerical illustrations, we demonstrate how PIDL methods could address the computational challenges posed by strain gradient plasticity models. Also, PIDL methods offer abundant potentialities, vis-á-vis a somewhat straitjacketed and poorer approximant of FE methods, in customizing the formulation as per the problem objective. △ Less

Submitted 13 August, 2024; originally announced August 2024.

Soft Anharmonic Coupled Vibrations of Li and SiO4 Enable Li-ion Diffusion in Amorphous Li2Si2O5

Authors: Sajan Kumar, Mayanak K. Gupta, Prabhatasree Goel, Ranjan Mittal, Sanghamitra Mukhopadhyay, Manh Duc Le, Rakesh Shukla, Srungarpu N. Achary, Avesh K. Tyagi, Samrath L. Chaplot

Abstract: We present the investigations on atomic dynamics and Li+ diffusion in crystalline and amorphous Li2Si2O5 using quasielastic (QENS) and inelastic neutron scattering (INS) studies supplemented by ab-initio molecular dynamics simulations (AIMD). The QENS measurements in the amorphous phase of Li2Si2O5 show a narrow temperature window (700 < T < 775 K), exhibiting significant quasielastic broadening c… ▽ More We present the investigations on atomic dynamics and Li+ diffusion in crystalline and amorphous Li2Si2O5 using quasielastic (QENS) and inelastic neutron scattering (INS) studies supplemented by ab-initio molecular dynamics simulations (AIMD). The QENS measurements in the amorphous phase of Li2Si2O5 show a narrow temperature window (700 < T < 775 K), exhibiting significant quasielastic broadening corresponding to the fast Li+ diffusion and relaxation of SiO4 units to the crystalline phase. Our INS measurements clearly show the presence of large phonon density of states (PDOS) at low energy (low-E) in the superionic amorphous phase, which disappear in the non-superionic crystalline phase, corroborating the role of low-E modes in Li+ diffusion. The frustrated energy landscape and host flexibility (due to random orientation and vibrational motion of SiO4 polyhedral units) play an essential role in diffusing the Li+. We used AIMD simulations to identify that these low-E modes involve a large amplitude of Li vibrations coupled with SiO4 vibrations in the amorphous phase. At elevated temperatures, these vibrational dynamics accelerate the Li+ diffusion via a paddle-wheel like coupling mechanism. Above 775 K, these SiO4 vibrational dynamics drive the system into the crystalline phase by locking SiO4 and Li+ into deeper minima of the free energy landscape and disappear in the crystalline phase. Both experiments and simulations provide valuable information about the atomic level stochastic and vibrational dynamics in Li2Si2O5 and their role in Li+ diffusion and vitrification. △ Less

Submitted 17 October, 2022; originally announced October 2022.

Comments: 31 pages, 7 Figures, Supplementary Materials

Ultra-clean high-mobility graphene on technologically relevant substrates

Authors: Ayush Tyagi, Vaidotas Mišeikis, Leonardo Martini, Stiven Forti, Neeraj Mishra, Zewdu M. Gebeyehu, Marco A. Giambra, Jihene Zribi, Mathieu Frégnaux, Damien Aureau, Marco Romagnoli, Fabio Beltram, Camilla Coletti

Abstract: Graphene grown via chemical vapour deposition (CVD) on copper foil has emerged as a high-quality, scalable material, that can be easily integrated on technologically relevant platforms to develop promising applications in the fields of optoelectronics and photonics. Most of these applications require low-contaminated high-mobility graphene (i.e., approaching 10 000 $cm^2 V^{-1} s^{-1}$) at room te… ▽ More Graphene grown via chemical vapour deposition (CVD) on copper foil has emerged as a high-quality, scalable material, that can be easily integrated on technologically relevant platforms to develop promising applications in the fields of optoelectronics and photonics. Most of these applications require low-contaminated high-mobility graphene (i.e., approaching 10 000 $cm^2 V^{-1} s^{-1}$) at room temperature) to reduce device losses and implement compact device design. To date, these mobility values are only obtained when suspending or encapsulating graphene. Here, we demonstrate a rapid, facile, and scalable cleaning process, that yields high-mobility graphene directly on the most common technologically relevant substrate: silicon dioxide on silicon (SiO$_2$/Si). Atomic force microscopy (AFM) and spatially-resolved X-ray photoelectron spectroscopy (XPS) demonstrate that this approach is instrumental to rapidly eliminate most of the polymeric residues which remain on graphene after transfer and fabrication and that have adverse effects on its electrical properties. Raman measurements show a significant reduction of graphene doping and strain. Transport measurements of 50 Hall bars (HBs) yield hole mobility $μ_h$ up to 9000 $cm^2 V^{-1} s^{-1}$ and electron mobility $μ_e$ up to 8000 $cm^2 V^{-1} s^{-1}$, with average values $μ_h$ 7500 $cm^2 V^{-1} s^{-1}$ and $μ_e$ 6300 $cm^2 V^{-1} s^{-1}$. The carrier mobility of ultraclean graphene reach values nearly double of that measured in graphene HBs processed with acetone cleaning, which is the method widely adopted in the field. Notably, these mobility values are obtained over large-scale and without encapsulation, thus paving the way to the adoption of graphene in optoelectronics and photonics. △ Less

Submitted 1 September, 2021; originally announced September 2021.

Comments: 20 pages, 11 figures

Sodium Diffusion and Dynamics in Na2Ti3O7: Neutron Scattering and Ab-initio Simulations

Authors: Ranjan Mittal, Sajan Kumar, Mayanak K. Gupta, Sanjay K. Mishra, Sanghamitra Mukhopadhyay, Manh Duc Le, Rakesh Shukla, Srungarpu N. Achary, Avesh K. Tyagi, Samrath L. Chaplot

Abstract: We have performed quasielastic and inelastic neutron scattering (QENS and INS) measurements from 300 K to 1173 K to investigate the Na-diffusion and underlying host dynamics in Na2Ti3O7. The QENS data show that the Na atoms undergo localized jumps up to 1173 K. The ab-initio molecular dynamics (AIMD) simulations supplement the measurements and show 1-d long-ranged diffusion along the a-axis above… ▽ More We have performed quasielastic and inelastic neutron scattering (QENS and INS) measurements from 300 K to 1173 K to investigate the Na-diffusion and underlying host dynamics in Na2Ti3O7. The QENS data show that the Na atoms undergo localized jumps up to 1173 K. The ab-initio molecular dynamics (AIMD) simulations supplement the measurements and show 1-d long-ranged diffusion along the a-axis above 1500 K. The simulations indicate that the occupancy of the interstitial site is critical for long-range diffusion. The nudged-elastic-band (NEB) calculation confirmed that the activation energy barrier is lowest for diffusion along the a-axis. In the experimental phonon spectra the peaks at 10 and 14 meV are dominated by Na dynamics that disappear on warming, suggesting low-energy phonons significantly contribute to large Na vibrational amplitude at elevated temperatures that enhances the Na hopping probability. We have also calculated the mode Grüneisen parameters of the phonons and thereby calculated the volume thermal expansion coefficient, which is found to be in excellent agreement with available experimental data. △ Less

Submitted 21 July, 2021; originally announced July 2021.

Comments: 22 Pages, 10 Figures

Stoichiometric Tuning of Lattice Flexibility and Na Diffusion in NaAlSiO4: Quasielastic Neutron Scattering Experiment and Ab-initio Molecular Dynamics Simulations

Authors: Mayanak K. Gupta, Ranjan Mittal, Sajan Kumar, Baltej Singh, Niina H Jalarvo Olivier Delaire, Rakesh Shukla, Srungarpu N. Achary, Alexander I. Kolesnikov, Avesh K. Tyagi, Samrath L. Chaplot

Abstract: We have performed quasielastic neutron scattering (QENS) experiments up to 1243 K and ab-initio molecular dynamics (AIMD) simulations to investigate the Na diffusion in various phases of NaAlSiO4 (NASO), namely, low-carnegieite (L-NASO; trigonal), high-carnegieite (H-NASO; cubic) and nepheline (N-NASO; hexagonal) phases. The QENS measurements reveal Na ions localized diffusion behavior in L-NASO a… ▽ More We have performed quasielastic neutron scattering (QENS) experiments up to 1243 K and ab-initio molecular dynamics (AIMD) simulations to investigate the Na diffusion in various phases of NaAlSiO4 (NASO), namely, low-carnegieite (L-NASO; trigonal), high-carnegieite (H-NASO; cubic) and nepheline (N-NASO; hexagonal) phases. The QENS measurements reveal Na ions localized diffusion behavior in L-NASO and N-NASO, but long-range diffusion behavior in H-NASO. The AIMD simulation supplemented the QENS measurements and showed that excess Na ions in H-NASO enhance the host network flexibility and activate the AlO4/SiO4 tetrahedra rotational modes. These framework modes enable the long-range diffusion of Na across a pathway of interstitial sites. The simulations also show Na diffusion in Na-deficient N-NASO through vacant Na sites along the hexagonal c-axis. △ Less

Submitted 19 April, 2021; originally announced April 2021.

Comments: 14 Pages, 5 Figures

Phonons and Lithium diffusion in LiAlO$_2$

Authors: Mayanak K. Gupta, Ranjan Mittal, Baltej Singh, Olivier Delaire, Srungarpu N. Achary, Stephane Rols, Avesh K. Tyagi, Samrath L. Chaplot

Abstract: We report on investigations of phonons and lithium diffusion in LiAlO$_2$ based on inelastic neutron scattering (INS) measurements of the phonon density of states (DOS) in γ-LiAlO$_2$ from 473 K to 1073 K, complemented with ab-initio molecular dynamics (AIMD) simulations. We find that phonon modes related to Li vibrations broaden on warming as reflected in the measured phonon DOS and reproduced in… ▽ More We report on investigations of phonons and lithium diffusion in LiAlO$_2$ based on inelastic neutron scattering (INS) measurements of the phonon density of states (DOS) in γ-LiAlO$_2$ from 473 K to 1073 K, complemented with ab-initio molecular dynamics (AIMD) simulations. We find that phonon modes related to Li vibrations broaden on warming as reflected in the measured phonon DOS and reproduced in simulations. Further, the AIMD simulations probe the nature of lithium diffusion in the perfect crystalline phase (γ-LiAlO$_2$), as well as in a structure with lithium vacancies and a related amorphous phase. Almost liquid-like super-ionic diffusion is observed in AIMD simulations of the three structures at high temperatures; with predicted onset temperatures of 1800 K, 1200 K, and 600 K in the perfect structure, vacancy structure and the amorphous phase, respectively. In the ideal structure, the Li atoms show correlated jumps; while simple and correlated jumps are both seen in the vacancy structure, and a mix of jumps and continuous diffusion occur in the amorphous structure. Further, we find that the Li-diffusion is favored in all cases by a large librational amplitude of the neighbouring AlO4 tetrahedra, and that the amorphous structure opens additional diffusion pathways due to a broad distribution of AlO4 tetrahedra orientations. △ Less

Submitted 7 August, 2020; originally announced August 2020.

Comments: 31 Pages, 14 figures

Journal ref: Phys. Rev. B 103, 174109 (2021)

Dynamics of Na Ion in the Amorphous Na2Si2O5 Using Quasielastic Neutron Scattering and Molecular Dynamics Simulations

Authors: Mayanak K. Gupta, Sanjay K. Mishra, Ranjan Mittal, Baltej Singh, Prabhatasree Goel, Sanghamitra Mukhopadhyay, Rakesh Shukla, Srungarpu N. Achary, Avesh K. Tyagi, Samrath L. Chaplot

Abstract: We have investigated the dynamics of Na ions in amorphous Na2Si2O5, a potential solid electrolyte material for Na-battery. We have employed quasielastic neutron scattering (QENS) technique in the amorphous Na2Si2O5 from 300 to 748 K to understand the diffusion pathways and relaxation timescales of Na atom dynamics. The microscopic analysis of the QENS data has been performed using ab-initio and cl… ▽ More We have investigated the dynamics of Na ions in amorphous Na2Si2O5, a potential solid electrolyte material for Na-battery. We have employed quasielastic neutron scattering (QENS) technique in the amorphous Na2Si2O5 from 300 to 748 K to understand the diffusion pathways and relaxation timescales of Na atom dynamics. The microscopic analysis of the QENS data has been performed using ab-initio and classical molecular dynamics simulations (MD) to understand the Na-ion diffusion in the amorphous phase. Our experimental studies show that the traditional model, such as the Hall and Ross (H-R) model, fairly well describe the diffusion in the amorphous phase giving a mean jump length of ~3 Å and residence time about 9.1 picoseconds. Our MD simulations have indicated that the diffusion of Na+ ions occurs in the amorphous phase of Na2Si2O5 while that is not observed in the crystalline orthorhombic phase even up to 1100 K. The MD simulations have revealed that in the amorphous phase, due to different orientations of silicon polyhedral units, accessible pathways are opened up for Na+ diffusions. These pathways are not available in the crystalline phase of Na2Si2O5 due to rigid spatial arrangement of silicon polyhedral units. △ Less

Submitted 16 September, 2019; originally announced September 2019.

Comments: 24 Pages, 12 figures

Journal ref: Phys. Rev. Materials 4, 045802 (2020)

Electronic properties of A2Zr2O7 (A= Gd, Nd) ceramic

Authors: Algama Masud, A. K. Himanshu, Ratnesh Pandey, J. Lahiri, Nisith Das, Bijay K Singh, Kaustava Bhattacharyya, Ravi Kumar, A. K. Tyagi

Abstract: The density functional theory with generalized gradient approximation has been used to investigate the electronic structure of gadolinium pyrochlore A2Zr2O7 (A=Gd, Nd) ceramic synthesized in polycrystalline form by solid state reaction. Structural characterization of the compound was done through X-ray diffraction (XRD) followed by Rietveld analysis of the XRD pattern. The Zr-K edge X-ray absorpti… ▽ More The density functional theory with generalized gradient approximation has been used to investigate the electronic structure of gadolinium pyrochlore A2Zr2O7 (A=Gd, Nd) ceramic synthesized in polycrystalline form by solid state reaction. Structural characterization of the compound was done through X-ray diffraction (XRD) followed by Rietveld analysis of the XRD pattern. The Zr-K edge X-ray absorption (XAFS) spectra of A2Zr2O7 (A=Gd, Nd) were analysed together with those Zr-foil, which was used as reference compounds. X-ray photoemission spectroscopy (XPS), X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) for A2Zr2O7 (A=Gd, Nd) has been employed to obtain quantitative structural information on the Zr-local environment. The band gap is estimated using UV-Vis spectroscopy. The crystal structure is face centered cubic, space group being Fd-3m (No. 227). The total energies in this work were calculated using the generalized gradient approximation to DFT plus on-site repulsion (U) method. △ Less

Submitted 4 July, 2019; v1 submitted 30 June, 2019; originally announced July 2019.

Comments: 4 figures, 2 table

Selective Defect Formation in Hexagonal Boron Nitride

Authors: Irfan H. Abidi, Noah Mendelson, Toan Trong Tran, Abhishek Tyagi, Minghao Zhuang, Lu-Tao Weng, Barbaros Ozyilmaz, Igor Aharonovich, Milos Toth, Zhengtang Luo

Abstract: Luminescent defect-centers in hexagonal boron nitride (hBN) have emerged as a promising 2D-source of single photon emitters (SPEs) due to their high brightness and robust operation at room temperature. The ability to create such emitters with well-defined optical properties is a cornerstone towards their integration into on-chip photonic architectures. Here, we report an effective approach to fabr… ▽ More Luminescent defect-centers in hexagonal boron nitride (hBN) have emerged as a promising 2D-source of single photon emitters (SPEs) due to their high brightness and robust operation at room temperature. The ability to create such emitters with well-defined optical properties is a cornerstone towards their integration into on-chip photonic architectures. Here, we report an effective approach to fabricate hBN single photon emitters (SPEs) with desired emission properties in two isolated spectral regions via the manipulation of boron diffusion through copper during atmospheric pressure chemical vapor deposition (APCVD)--a process we term gettering. Using the gettering technique we deterministically place the resulting zero-phonon line (ZPL) between the regions 550-600 nm or from 600-650 nm, paving the way for hBN SPEs with tailored emission properties across a broad spectral range. Our ability to control defect formation during hBN growth provides a simple and cost-effective means to improve the crystallinity of CVD hBN films, and lower defect density making it applicable to hBN growth for a wide range of applications. Our results are important to understand defect formation of quantum emitters in hBN and deploy them for scalable photonic technologies. △ Less

Submitted 24 February, 2019; v1 submitted 21 February, 2019; originally announced February 2019.

Enhanced radiation tolerance of YSZ at high temperature against swift heavy ions: key role of interplay between material microstructure and irradiation temperature

Authors: Parswajit Kalita, Santanu Ghosh, Udai B. Singh, Pawan K. Kulriya, Vinita Grover, Rakesh Shukla, A. K. Tyagi, Gael Sattonnay, Devesh K. Avasthi

Abstract: Yttria stabilized Zirconia (YSZ) pellets with different crystallite sizes were irradiated with 80 MeV Ag$^{6+}$ ions at room temperature and 1000 K to understand the role of crystallite size/material microstructure and irradiation temperature on the radiation tolerance against high electronic energy loss (S$_e$). X-ray diffraction and Raman spectroscopy measurements reveal that, irrespective of th… ▽ More Yttria stabilized Zirconia (YSZ) pellets with different crystallite sizes were irradiated with 80 MeV Ag$^{6+}$ ions at room temperature and 1000 K to understand the role of crystallite size/material microstructure and irradiation temperature on the radiation tolerance against high electronic energy loss (S$_e$). X-ray diffraction and Raman spectroscopy measurements reveal that, irrespective of the irradiation temperature, the nano-crystalline samples suffered more damage as compared to the bulk-like sample. A reduction in the irradiation damage i.e. improvement in the radiation tolerance, was observed for all the samples irradiated at 1000 K. The reduction in the damage, however, was remarkably higher for the two nano-crystalline samples compared to the bulk-like sample, and hence the difference in the damage between the bulk-like and nano-crystalline samples was also significantly lower at 1000 K than that at room temperature. The irradiation damage, against S$_e$, was thus found to be critically dependent on the interplay between the irradiation temperature and crystallite size. These results are explained with the help of detailed theoretical calculations/simulations based on the 'in-elastic thermal spike' model by taking into consideration the combined effect of crystallite size and environmental (irradiation) temperature on the electron-phonon coupling factor and lattice thermal conductivity (and hence on the resulting thermal spike). Our results are crucial from the fundamental perspective of comprehending the size and temperature dependent radiation damage against S$_e$ ; and also for a number of applications, in various radiation environments, where nano-materials are being envisioned for use. △ Less

Submitted 27 July, 2018; v1 submitted 26 June, 2018; originally announced June 2018.

Comments: This version has been communicated (for publication)

Pressure impact on the stability and distortion of the crystal structure of CeScO3

Authors: Daniel Errandonea, David Santamaria-Perez, Domingo Martinez-Garcia, Oscar Gomis, Rakesh Shukla, S. Nagabhusan Achary, Avesh K. Tyagi, Catalin Popescu

Abstract: The effects of high pressure on the crystal structure of orthorhombic (Pnma) perovskite type cerium scandate have been studied in situ under high pressure by means of synchrotron x-ray powder diffraction, using a diamond anvil cell. We have found that the perovskite type crystal structure remains stable up to 40 GPa, the highest pressure reached in the experiments. The evolution of unit-cell param… ▽ More The effects of high pressure on the crystal structure of orthorhombic (Pnma) perovskite type cerium scandate have been studied in situ under high pressure by means of synchrotron x-ray powder diffraction, using a diamond anvil cell. We have found that the perovskite type crystal structure remains stable up to 40 GPa, the highest pressure reached in the experiments. The evolution of unit-cell parameters with pressure has indicated an anisotropic compression. The room-temperature pressure-volume equation of state is obtained from the experiments. From the evolution of microscopic structural parameters like bond distances and coordination polyhedra of cerium and scandium, the macroscopic behavior of CeScO3 under compression has been explained and reasoned for its large pressure stability. The reported results are discussed in comparison with high-pressure results from other perovskites. △ Less

Submitted 17 August, 2017; originally announced August 2017.

Comments: 39 pages, 1 table, 9 figures

Journal ref: Inorg. Chem., 56, pp 8363-8371 (2017)

Flipping growth orientation of nanographitic structures by plasma enhanced chemical vapor deposition

Authors: Subrata Ghosh, K. Ganesan, S. R. Polaki, S. Ilango, S. Amirthapandian, S. Dhara, M. Kamruddin, A. K. Tyagi

Abstract: Nanographitic structures (NGSs) with multitude of morphological features are grown on SiO2/Si substrates by electron cyclotron resonance - plasma enhanced chemical vapor deposition (ECR-PECVD). CH4 is used as source gas with Ar and H2 as dilutants. Field emission scanning electron microscopy, high resolution transmission electron microscopy (HRTEM) and Raman spectroscopy are used to study the stru… ▽ More Nanographitic structures (NGSs) with multitude of morphological features are grown on SiO2/Si substrates by electron cyclotron resonance - plasma enhanced chemical vapor deposition (ECR-PECVD). CH4 is used as source gas with Ar and H2 as dilutants. Field emission scanning electron microscopy, high resolution transmission electron microscopy (HRTEM) and Raman spectroscopy are used to study the structural and morphological features of the grown films. Herein, we demonstrate, how the morphology can be tuned from planar to vertical structure using single control parameter namely, dilution of CH4 with Ar and/or H2. Our results show that the competitive growth and etching processes dictate the morphology of the NGSs. While Ar-rich composition favors vertically oriented graphene nanosheets, H2-rich composition aids growth of planar films. Raman analysis reveals dilution of CH4 with either Ar or H2 or in combination helps to improve the structural quality of the films. Line shape analysis of Raman 2D band shows nearly symmetric Lorentzian profile which confirms the turbostratic nature of the grown NGSs. Further, this aspect is elucidated by HRTEM studies by observing elliptical diffraction pattern. Based on these experiments, a comprehensive understanding is obtained on the growth and structural properties of NGSs grown over a wide range of feedstock compositions. △ Less

Submitted 7 April, 2017; originally announced April 2017.

Comments: 19 pages, 8 figures, 1 table

Journal ref: RSC Adv., 5, 91922-91931 (2015)

The role of substrate bias and nitrogen doping on the structural evolution and local elastic modulus of diamond-like carbon films

Authors: S. R. Polaki, K. Ganesan, S. K. Srivastava, M. Kamruddin, A. K. Tyagi

Abstract: Diamond-like carbon (DLC) films are synthesized on Si using plasma enhanced chemical vapor deposition. The role of substrate bias and nitrogen doping on the structural evolution and local elastic modulus of DLC films are systematically investigated. Raman spectroscopic studies reveal that the amount of graphitic C=C sp2 bonding increases with substrate bias and nitrogen doping. The density and hyd… ▽ More Diamond-like carbon (DLC) films are synthesized on Si using plasma enhanced chemical vapor deposition. The role of substrate bias and nitrogen doping on the structural evolution and local elastic modulus of DLC films are systematically investigated. Raman spectroscopic studies reveal that the amount of graphitic C=C sp2 bonding increases with substrate bias and nitrogen doping. The density and hydrogen concentration in the films are found to vary from 0.7 to 2.2 g/cm3 and 16 to 38 atomic %, respectively, depending upon the substrate bias and nitrogen concentration in the DLC films. Atomic force acoustic microscopic (AFAM) analysis shows a direct correlation between local elastic modulus and structural properties estimated by Raman spectroscopy, Rutherford back scattering and elastic recoil detection analysis. AFAM analysis further confirms the evolution of soft second phases at high substrate biases (> -150V) in undoped DLC films. Further, N doping leads to formation of such soft second phases in DLC films even at lower substrate bias of -100 V. The AFAM studies provide a direct microscopic evidence for the "sub-implantation growth model" which predicts the formation of graphitic second phases in DLC matrix at high substrate biases. △ Less

Submitted 27 March, 2017; originally announced March 2017.

Comments: Accepted in J. Phys. D Appl. Phys

Journal ref: J. Phys. D: Appl. Phys. 50 175601 (2017)

Geometrically frustrated GdInO$_3$: An exotic system to study negative thermal expansion and spin-lattice coupling

Authors: Barnita Paul, Swastika Chatterjee, Anushree Roy, A. Midya, P. Mandal, Vinita Grover, A. K. Tyagi

Abstract: In this article, we report negative thermal expansion and spin frustration in hexagonal GdInO$_{3}$. Rietveld refinement of the XRD patterns reveal that the negative thermal expansion in the temperature range of 50-100K stems from the triangular lattice of Gd$^{3+}$ ions. At low temperature, the downward deviation of the inverse susceptibility ($χ^{-1}$) vs. $T$ plot from the Curie-Weiss law indic… ▽ More In this article, we report negative thermal expansion and spin frustration in hexagonal GdInO$_{3}$. Rietveld refinement of the XRD patterns reveal that the negative thermal expansion in the temperature range of 50-100K stems from the triangular lattice of Gd$^{3+}$ ions. At low temperature, the downward deviation of the inverse susceptibility ($χ^{-1}$) vs. $T$ plot from the Curie-Weiss law indicates spin frustration which inhibits long-range magnetic ordering down to 2K. Magnetostriction measurements clearly demonstrate a strong spin-lattice coupling. Low temperature anomalous phonon softening, as obtained from temperature dependent Raman measurements, also reveals the same. Our experimental observations are supported by first principles density functional theory calculations of the electronic and phonon dispersion of GdInO$_3$. The calculations suggest that the GdInO$_3$ lattice is highly frustrated at low temperature. Further, the calculated normal mode frequencies of the Gd related $Γ$ point phonons are found to depend on the magnetic structure of the lattice, suggesting significant magneto-elastic coupling. △ Less

Submitted 20 August, 2016; originally announced August 2016.

Comments: 16 pages, 10 figures

Journal ref: Phys. Rev. B 95, 054103 (2017)

A comparative study on defect estimation using XPS and Raman spectroscopy in few layer nanographitic structures

Authors: K. Ganesan, Subrata Ghosh, Nanda Gopala Krishna, S. Ilango, M. Kamruddin, A. K. Tyagi

Abstract: Defects in planar and vertically oriented nanographitic structures (NGSs) synthesized by plasma enhanced chemical vapor deposition (PECVD) has been investigated using Raman and X-ray photoelectron spectroscopy. While Raman spectra reveal the dominance of vacancy and boundary type defects respectively in vertical and planar NGSs, XPS provides additional information on vacancy related defect peaks a… ▽ More Defects in planar and vertically oriented nanographitic structures (NGSs) synthesized by plasma enhanced chemical vapor deposition (PECVD) has been investigated using Raman and X-ray photoelectron spectroscopy. While Raman spectra reveal the dominance of vacancy and boundary type defects respectively in vertical and planar NGSs, XPS provides additional information on vacancy related defect peaks at C 1s spectrum that originate from non-conjugated carbon atoms in hexagonal lattice. Although an excellent correlation prevails between these two techniques, our results show that estimation of surface defects by XPS is more accurate than Raman analysis. Nuances of these techniques are discussed in the context of assessing defects in nanographitic structures. △ Less

Submitted 15 July, 2016; originally announced July 2016.

Comments: 21 pages, 5 figures, 2 tables

Journal ref: Phys. Chem. Chem. Phys., 18, (2016) 22160-22167

Electrochemical supercapacitor performance of SnO2 quantum dots

Authors: V. Bonu, B. Gupta, S. Chandra, A Das, S. Dhara, A. K. Tyagi

Abstract: Metal oxide nanostructures are widely used in energy applications like super capacitors and Li-on battery. Smaller size nanocrystals show better stability, low ion diffusion time, higher-ion flux and low pulverization than bigger size nanocrystals during electrochemical operation. Studying the distinct properties of smaller size nanocrystals such as quantum dots (QDs) can improve the understanding… ▽ More Metal oxide nanostructures are widely used in energy applications like super capacitors and Li-on battery. Smaller size nanocrystals show better stability, low ion diffusion time, higher-ion flux and low pulverization than bigger size nanocrystals during electrochemical operation. Studying the distinct properties of smaller size nanocrystals such as quantum dots (QDs) can improve the understanding on reasons behind the better performance and it will also help in using QDs or smaller size nanoparticles (NPs) more efficiently in different applications. Aqua stable pure SnO2 QDs with compositional stability and high surface to volume ratio are studied as an electrochemical super capacitor material and compared with bigger size NPs of size 25 nm. Electron energy-loss spectroscopic study of the QDs revealed dominant role of surface over the bulk. Temperature dependent study of low frequency Raman mode and defect Raman mode of QDs indicated no apparent volume change in the SnO2 QDs within the temperature range of 80-300 K. The specific capacitance of these high surface area and stable SnO2 QDs has showed only 9% loss while increasing the scan rate from 20 mV/S to 500 mV/S. Capacitance loss for the QDs is less than 2% after 1000 cycles of charging discharging, whereas for the 25 nm SnO2 NPs, the capacitance loss is 8% after 1000 cycles. Availability of excess open volume in QDs leading to no change in volume during the electro-chemical operation and good aqua stability is attributed to the better performance of QDs over bigger sized NPs. △ Less

Submitted 11 May, 2016; originally announced May 2016.

Comments: 31 pages, 9 figures, Journal

Journal ref: Electrochemica Acta 203 (2016) 230-237

Localized tip enhanced Raman spectroscopic study of impurity incorporated single GaN nanowire in the sub-diffraction limit

Authors: A. Patsha, S. Dhara, A. K. Tyagi

Abstract: The localized effect of impurities in single GaN nanowires in the sub-diffraction limit is reported using the study of lattice vibrational modes in the evanescent field of Au nanoparticle assisted tip enhanced Raman spectroscopy (TERS). GaN nanowires with the O impurity and the Mg dopants were grown by the chemical vapor deposition technique in the catalyst assisted vapor-liquid-solid process. Sym… ▽ More The localized effect of impurities in single GaN nanowires in the sub-diffraction limit is reported using the study of lattice vibrational modes in the evanescent field of Au nanoparticle assisted tip enhanced Raman spectroscopy (TERS). GaN nanowires with the O impurity and the Mg dopants were grown by the chemical vapor deposition technique in the catalyst assisted vapor-liquid-solid process. Symmetry allowed Raman modes of wurtzite GaN are observed for undoped and doped nanowires. Unusually very strong intensity of the non-zone center zone boundary mode is observed for the TERS studies of both the undoped and the Mg doped GaN single nanowires. Surface optical mode of A1 symmetry is also observed for both the undoped and the Mg doped GaN samples. A strong coupling of longitudinal optical (LO) phonons with free electrons, however is reported only in the O rich single nanowires with the asymmetric A1(LO) mode. Study of the local vibration mode shows the presence of Mg as dopant in the single GaN nanowires. △ Less

Submitted 11 May, 2016; originally announced May 2016.

Comments: 12 pages, 3 figures, Journal

Journal ref: Appl. Phys. Lett. 107 (2015) 123108

Localized Charge Transfer Process and Surface Band Bending in Methane Sensing by GaN Nanowires

Authors: A. Patsha, P. Sahoo, S. Amirthapandian, A. K. Prasad, A. Das, A. K. Tyagi, M. A. Cotta, S. Dhara

Abstract: The physicochemical processes at the surfaces of semiconductor nanostructures involved in electrochemical and sensing devices are strongly influenced by the presence of intrinsic or extrinsic defects. To reveal the surface controlled sensing mechanism, intentional lattice oxygen defects are created on the surfaces of GaN nanowires for the elucidation of charge transfer process in methane (CH4) sen… ▽ More The physicochemical processes at the surfaces of semiconductor nanostructures involved in electrochemical and sensing devices are strongly influenced by the presence of intrinsic or extrinsic defects. To reveal the surface controlled sensing mechanism, intentional lattice oxygen defects are created on the surfaces of GaN nanowires for the elucidation of charge transfer process in methane (CH4) sensing. Experimental and simulation results of electron energy loss spectroscopy (EELS) studies on oxygen rich GaN nanowires confirmed the possible presence of 2(ON) and VGa-3ON defect complexes. A global resistive response for sensor devices of ensemble nanowires and a localized charge transfer process in single GaN nanowires are studied in situ scanning by Kelvin probe microscopy (SKPM). A localized charge transfer process, involving the VGa-3ON defect complex on nanowire surface is attributed in controlling the global gas sensing behavior of the oxygen rich ensemble GaN nanowires. △ Less

Submitted 11 May, 2016; originally announced May 2016.

Comments: 42 pages, 6 figures, Journal

Journal ref: J. Phys. Chem. C 119 (2015) 21251-21260

Nonpolar p-GaN/n-Si heterojunction diode characteristics: A comparison between ensemble and single nanowire devices

Authors: Avinash Patsha, Ramanathaswamy Pandian, Sandip Dhara, A. K. Tyagi

Abstract: The electrical and photodiode characteristics of ensemble and single p-GaN nanowire and n-Si heterojunction devices were studied. Ideality factor of the single nanowire p-GaN/n-Si device was found to be about three times lower compared to that of the ensemble nanowire device. Apart from the deep-level traps in p-GaN nanowires, defect states due to inhomogeneity in Mg dopants in the ensemble nanowi… ▽ More The electrical and photodiode characteristics of ensemble and single p-GaN nanowire and n-Si heterojunction devices were studied. Ideality factor of the single nanowire p-GaN/n-Si device was found to be about three times lower compared to that of the ensemble nanowire device. Apart from the deep-level traps in p-GaN nanowires, defect states due to inhomogeneity in Mg dopants in the ensemble nanowire device are attributed to the origin of high ideality factor. Photovoltaic mode of ensemble nanowire device showed an improvement in the fill-factors up to 60 percent over the single nanowire device with fill-factors up to 30 percent. Reponsivity of the single nanowire device in photoconducting mode was found to be enhanced by five orders, at 470 nm. The enhanced photoresponse of the single nanowire device also confirms the photoconduction due to defect states in p-GaN nanowires. △ Less

Submitted 11 May, 2016; originally announced May 2016.

Comments: 24 pages, 6 figures,Journal

Journal ref: J. Phys. D: Appl. Phys. 48 (2015) 395102

Carrier mediated reduction of stiffness in nanoindented crystalline Si(100)

Authors: S. Kataria, Sandip Dhara, S. Dash, A. K. Tyagi

Abstract: We report the observation of carrier mediated decrease in the stiffness of crystalline (c)-Si(100) under nanoindentation. The apparent elastic modulii of heavily dopes (1E21 cm-3) p- and n-type c-Si are observed to be lower by 5.-7.5 percent that the estimated value for intrinsic (1E14 cm-3) c-Si. The deviation observed with respect to elastic modulus remarkably matches with the estimated value wh… ▽ More We report the observation of carrier mediated decrease in the stiffness of crystalline (c)-Si(100) under nanoindentation. The apparent elastic modulii of heavily dopes (1E21 cm-3) p- and n-type c-Si are observed to be lower by 5.-7.5 percent that the estimated value for intrinsic (1E14 cm-3) c-Si. The deviation observed with respect to elastic modulus remarkably matches with the estimated value while considering the electronic elastic strain effect on carrier concentration as an influence of negative pressure coefficient of band gap for Si. The value is predominantly higher than the reported value of a decrease of 1-3 percent in stiffness as an effect of impurity in c-Si. △ Less

Submitted 11 May, 2016; originally announced May 2016.

Comments: 13 pages, 3 figures, Journal

Journal ref: J. Appl. Phys. 118 (2015) 035702

Role of Phonons in Li-Diffusion, Thermal Expansion and Phase Transitions in beta-eucryptite: Inelastic Neutron Scattering and Lattice Dynamics Studies

Authors: Baltej Singh, Mayanak Kumar Gupta, Ranjan Mittal, Mohamed Zbiri, Stephane Rols, Sadequa Jahedkhan Patwe, Srungarpu Nagabhusan Achary, Helmut Schober, Avesh Kumar Tyagi, Samrath Lal Chaplot

Abstract: Beta eucryptite (LiAlSiO4) shows one-dimensional super-ionic conductivity as well as anisotropic thermal expansion behavior. We have performed inelastic neutron scattering measurements in beta eucryptite over 300 to 900 K and calculated the phonon spectrum using ab initio density functional theory method. The calculated energy profile for cooperative lithium ion displacements indicates preferentia… ▽ More Beta eucryptite (LiAlSiO4) shows one-dimensional super-ionic conductivity as well as anisotropic thermal expansion behavior. We have performed inelastic neutron scattering measurements in beta eucryptite over 300 to 900 K and calculated the phonon spectrum using ab initio density functional theory method. The calculated energy profile for cooperative lithium ion displacements indicates preferential movement of Li ion along the hexagonal c-axis in the high temperature phase. However, the energy barrier for Li ion diffusion is significantly reduced when a Schottky defect is introduced in the crystal. Further, the anisotropic stress dependence of the phonon spectrum is calculated to obtain the thermal expansion behavior along various axes. The calculations show that the Gruneisen parameters of the low-energy phonon modes around 10 meV have large negative values and govern the negative thermal expansion behavior both along the a and c axes. On the other hand, anisotropic elasticity along with anisotropic positive values of the Gruneisen parameters of the high-energy modes in the range 30 to 70 meV are responsible for positive thermal expansion in the a-b plane and negative expansion along the c-axis. The analysis of the polarization vectors of the phonon modes sheds light on the mechanism of the anomalous thermal expansion behavior. We extend the study to discuss the relationship of the soft phonons in the Brillouin zone with the observed high-pressure and high-temperature phase transitions as reported in the literature. △ Less

Submitted 26 February, 2016; originally announced February 2016.

Comments: 31 pages, 15 Figures

Journal ref: Journal of Applied Physics 121, 085106 (2017)

Pressure-induced phase transformation in zircon-type orthovanadate SmVO4 from experiment and theory

Authors: C Popescu, Alka B Garg, D Errandonea, J A Sans, P Rodriguez-Hernandez, S Radescu, A Munoz, S N Achary, A K Tyagi

Abstract: The compression behavior of zircon-type samarium orthovanadate, SmVO4, has been investigated using synchrotron-based powder x-ray diffraction and ab-initio calculations up to 21 GPa. The results indicate the instability of ambient zircon phase at around 6 GPa, which transforms to a high-density scheelite-type phase. The high-pressure phase remains stable up to 21 GPa, the highest pressure reached… ▽ More The compression behavior of zircon-type samarium orthovanadate, SmVO4, has been investigated using synchrotron-based powder x-ray diffraction and ab-initio calculations up to 21 GPa. The results indicate the instability of ambient zircon phase at around 6 GPa, which transforms to a high-density scheelite-type phase. The high-pressure phase remains stable up to 21 GPa, the highest pressure reached in the present investigations. On pressure release, the scheelite phase is recovered. Crystal structure of high-pressure phase and equations of state (EOS) for the zircon- and scheelite-type phases have been determined. Various compressibilities such as bulk, axial and bond, estimated from the experimental data are found to be in good agreement with the results obtained from theoretical calculations. Calculated elastic constants show that the zircon structure becomes mechanically unstable beyond the transition pressure. Overall there is good agreement between experimental and theoretical findings. △ Less

Submitted 21 January, 2016; originally announced January 2016.

Comments: 13 pages, 12 figures, 3 tables

MSC Class: 86

Journal ref: J. Phys.: Condens. Matter 28 (2016) 035402

XPS studies on AlN thin films grown by ion beam sputtering in reactive assistance of N+/N2+ ions: Substrate temperature induced compositional variations

Authors: Neha Sharma, S. Ilango, S. Dash, A. K. Tyagi

Abstract: We report on an XPS study of AlN thin films grown on Si(100) substrates by ion beam sputter deposition (IBSD) in reactive assistance of N+/N2+ ions to unravel the compositional variation of their surface when deposited at different substrate temperatures. The temperature of the substrate was varied as room temperature (RT), 100oC and 500oC. The binding energy of Al-2p, N-1s and O-1s core electrons… ▽ More We report on an XPS study of AlN thin films grown on Si(100) substrates by ion beam sputter deposition (IBSD) in reactive assistance of N+/N2+ ions to unravel the compositional variation of their surface when deposited at different substrate temperatures. The temperature of the substrate was varied as room temperature (RT), 100oC and 500oC. The binding energy of Al-2p, N-1s and O-1s core electrons indicate the formation of 2H polytypoid of AlN. The increase in concentration of AlN with substrate temperature during deposition is elucidated through detailed analysis with calculated elemental atomic concentrations (at. %) of all possible phases at the film surface. Our results show that predominate formation of AlN as high as 74 at. % is achievable using substrate temperature as the only process parameter. This high fraction of AlN in thin film surface composition is remarkable when compared to other growth techniques. Also, the formation of other phases is established based on their elemental concentrations. △ Less

Submitted 2 October, 2015; originally announced October 2015.

Comments: 13 pages, 6 figures, 2 tables, submitted to the journal "Thin Solid Films"

Novel single phase vanadium dioxide nanostructured films for methane sensing near room temperature

Authors: A. K. Prasad, S. Amirthapandian, S. Dhara, S. Dash, N. Murali, A. K. Tyagi

Abstract: Methane (CH_4) gas sensing properties of novel vanadium dioxide (VO_2) nanostructured films is reported for the first time. The single phase nanostructures are synthesized by pulsed dc-magnetron sputtering of V target followed by oxidation in O_2 atmosphere at 550 ^oC. The partial pressure of O_2 is controlled to obtain stoichiometric VO_2 with the samples showing rutile monoclinic crystalline sym… ▽ More Methane (CH_4) gas sensing properties of novel vanadium dioxide (VO_2) nanostructured films is reported for the first time. The single phase nanostructures are synthesized by pulsed dc-magnetron sputtering of V target followed by oxidation in O_2 atmosphere at 550 ^oC. The partial pressure of O_2 is controlled to obtain stoichiometric VO_2 with the samples showing rutile monoclinic crystalline symmetry and regions of rod shaped nano-architectures. These nanostructured films exhibit a reversible semiconductor to metal transition in the temperature range of 60-70 ^oC. Gas sensing experiments are carried out in the temperature span from 25 ^oC to 200 ^oC in presence of CH_4. These experiments reveal that the films respond very well at temperatures as low as 50 ^oC, in the semiconducting state. △ Less

Submitted 1 September, 2015; originally announced September 2015.

Comments: 18 pages, 4 figures, journal

Journal ref: Sensors and Actuators B 2014, 191, 252

Optical Properties of Mono-Dispersed AlGaN Nanowires in the Single-Prong Growth Mechanism

Authors: A. K. Sivadasan, Avinash Patsha, S. Polaki, S. Amirthapandian, Sandip Dhara, Anirban Bhattacharya, B. K. Panigrahi, A. K. Tyagi

Abstract: Growth of mono-dispersed AlGaN nanowires of ternary wurtzite phase is reported using chemical vapour deposition technique in the vapour-liquid-solid process. The role of distribution of Au catalyst nanoparticles on the size and the shape of AlGaN nanowires are discussed. These variations in the morphology of the nanowires are understood invoking Ostwald ripening of Au catalyst nanoparticles at hig… ▽ More Growth of mono-dispersed AlGaN nanowires of ternary wurtzite phase is reported using chemical vapour deposition technique in the vapour-liquid-solid process. The role of distribution of Au catalyst nanoparticles on the size and the shape of AlGaN nanowires are discussed. These variations in the morphology of the nanowires are understood invoking Ostwald ripening of Au catalyst nanoparticles at high temperature followed by the effect of single and multi-prong growth mechanism. Energy-filtered transmission electron microscopy is used as an evidence for the presence of Al in the as-prepared samples. A significant blue shift of the band gap, in the absence of quantum confinement effect in the nanowires with diameter about 100 nm, is used as a supportive evidence for the AlGaN alloy formation. Polarized resonance Raman spectroscopy with strong electron-phonon coupling along with optical confinement due to the dielectric contrast of nanowire with respect to that of surrounding media are adopted to understand the crystalline orientation of a single nanowire in the sub-diffraction limit of about 100 nm using 325 nm wavelength, for the first time. The results are compared with the structural analysis using high resolution transmission microscopic study. △ Less

Submitted 1 September, 2015; originally announced September 2015.

Comments: 33 pages, 7 figures, journal

Journal ref: Cryst. Growth Des. 2015, 15, 1311

Raoult's Formalism in Understanding Low Temperature Growth of GaN Nanowires using Binary Precursor

Authors: Kishore K. Madapu, S. Dhara, S. Amirthapandian, S. Polaki, A. K. Tyagi

Abstract: Growth of GaN nanowires are carried out via metal initiated vapor-liquid-solid mechanism, with Au as the catalyst. In chemical vapour deposition technique, GaN nanowires are usually grown at high temperatures in the range of 900-1100 ^oC because of low vapor pressure of Ga below 900 ^oC. In the present study, we have grown the GaN nanowires at a temperature, as low as 700 ^oC. Role of indium in th… ▽ More Growth of GaN nanowires are carried out via metal initiated vapor-liquid-solid mechanism, with Au as the catalyst. In chemical vapour deposition technique, GaN nanowires are usually grown at high temperatures in the range of 900-1100 ^oC because of low vapor pressure of Ga below 900 ^oC. In the present study, we have grown the GaN nanowires at a temperature, as low as 700 ^oC. Role of indium in the reduction of growth temperature is discussed in the ambit of Raoult's law. Indium is used to increase the vapor pressure of the Ga sufficiently to evaporate even at low temperature initiating the growth of GaN nanowires. In addition to the studies related to structural and vibrational properties, optical properties of the grown nanowires are also reported for detailed structural analysis. △ Less

Submitted 31 August, 2015; originally announced August 2015.

Comments: 24 pages, 7 figures, journal

Journal ref: J. Phys. Chem. C 2013, 117, 21930

Growth of InN quantum dots to nanorods: A competition between nucleation and growth rates

Authors: Kishore K. Madapu, Sandip Dhara, S. Polaki, S. Amirthapandian, A. K. Tyagi

Abstract: Growth evolution of InN nanostructures via a chemical vapor deposition technique is reported using In_2O_3 as precursor material and NH_3 as reactive gas in the temperature range of 550-700 ^oC. Morphology of the nanostructures solely depends on the growth temperature, evolving from quantum dot sized nanoparticles to nanorods. It is found that 630 ^oC is the threshold temperature for nanorod growt… ▽ More Growth evolution of InN nanostructures via a chemical vapor deposition technique is reported using In_2O_3 as precursor material and NH_3 as reactive gas in the temperature range of 550-700 ^oC. Morphology of the nanostructures solely depends on the growth temperature, evolving from quantum dot sized nanoparticles to nanorods. It is found that 630 ^oC is the threshold temperature for nanorod growth. At 630 ^oC, nucleation starts with multifaceted particle having {10-12} surface planes. Subsequently, hexagonal polyhedral NRs are grown along the [0001] direction with non-polar surfaces of m-planes {10-10}. A comprehensive study is carried out to understand the evolution of nanorods as a function of growth parameters like temperature, time and gas flow rate. Change in the morphology of nanostructures is explained based on the nucleation rate and the growth rates during the phase formation. Raman studies of these nanostructures show that a biaxial strain is developed because of unintentional impurity doping with the increase in growth temperature. △ Less

Submitted 31 August, 2015; originally announced August 2015.

Comments: 31 pages, 10 figures, journal

Journal ref: CrystEngComm, 2015, 17, 3139

Invoking forbidden modes in SnO_2 nanoparticles using tip enhanced Raman spectroscopy

Authors: Venkataramana Bonu, A. Das, A. K. Sivadasan, A. K. Tyagi, Sandip Dhara

Abstract: Raman forbidden modes and surface defect related Raman features in SnO_2 nanostructures carry information about disorder and surface defects which strongly influence important technological applications like catalysis and sensing. Due to the weak intensities of these peaks, it is difficult to identify these features by using conventional Raman spectroscopy. Tip enhanced Raman spectroscopy (TERS) s… ▽ More Raman forbidden modes and surface defect related Raman features in SnO_2 nanostructures carry information about disorder and surface defects which strongly influence important technological applications like catalysis and sensing. Due to the weak intensities of these peaks, it is difficult to identify these features by using conventional Raman spectroscopy. Tip enhanced Raman spectroscopy (TERS) studies conducted on SnO_2 nanoparticles (NPs) of size 4 and 25 nm have offered significant insights of prevalent defects and disorders. Along with one order enhancement in symmetry allowed Raman modes, new peaks related to disorder and surface defects of SnO_2 NPs were found with significant intensity. Temperature dependent Raman studies were also carried out for these NPs and correlated with the TERS spectra. For quasi-quantum dot sized 4 nm NPs, the TERS study was found to be the best technique to probe the finite size related Raman forbidden modes. △ Less

Submitted 31 August, 2015; originally announced August 2015.

Comments: 11 pages, 6 figures, journal. J. Raman Spectrosc (Rapid Communication) 2015

Surface functionalization enhanced magnetism in SnO_2 nanoparticles and its correlation to photoluminescence properties

Authors: Venkataramana Bonu, Arindam Das, Manas Sardar, Sandip Dhara, Ashok Kumar Tyagi

Abstract: High value of magnetic moment 0.08 emu/g at room temperature for SnO_2 nanoparticles (NPs) was observed. Surface functionalization with octadecyltrichlorosilane (OTS) enhanced the saturation magnetic moment of NPs to an anomalously high value of 0.187 emu/g by altering the electronic configuration on NPs surface. Surface functionalization also suppressed photoluminescence (PL) peaks arising from o… ▽ More High value of magnetic moment 0.08 emu/g at room temperature for SnO_2 nanoparticles (NPs) was observed. Surface functionalization with octadecyltrichlorosilane (OTS) enhanced the saturation magnetic moment of NPs to an anomalously high value of 0.187 emu/g by altering the electronic configuration on NPs surface. Surface functionalization also suppressed photoluminescence (PL) peaks arising from oxygen defects around 2 eV and caused an increase in the intensities of two peaks near violet region (2.6 - 3 eV). PL studies under uniform external magnetic field enriched understanding of the role of OTS. Both OTS and external magnetic field significantly modulated the luminescence spectra, by altering the surface electronic structure of NPs. Extra spins on the surface of SnO2 NPs created by the surface functionalization process and their influence on resultant magnetic moment and luminescence properties are discussed in details. △ Less

Submitted 31 August, 2015; originally announced August 2015.

Comments: 8 pages, 7 figures, journal

Journal ref: J. Mater. Chem. C, 2015, 3, 1261

Photoluminescence of oxygen vacancies and hydroxyl group surface functionalized SnO_2 nanoparticles

Authors: Venkataramana Bonu, Arindam Das, S. Amirthapandian, Sandip Dhara, Ashok Kumar Tyagi

Abstract: We report, for the first time, the luminescence property of the hydroxyl group surface functionalized quantum dots (QDs) and nanoparticles (NPs) of SnO_2 using low energy excitations of 2.54 eV (488 nm) and 2.42 eV (514.5 nm). This luminescence is in addition to generally observed luminescence from 'O' defects. The as-prepared SnO_2 quantum dots (QDs) are annealed at different temperatures in ambi… ▽ More We report, for the first time, the luminescence property of the hydroxyl group surface functionalized quantum dots (QDs) and nanoparticles (NPs) of SnO_2 using low energy excitations of 2.54 eV (488 nm) and 2.42 eV (514.5 nm). This luminescence is in addition to generally observed luminescence from 'O' defects. The as-prepared SnO_2 quantum dots (QDs) are annealed at different temperatures in ambient conditions to create varied NPs in size. Subsequently, average size of the NPs is calculated from the acoustic vibrations observed at low frequencies in the Raman spectra and by the transmission electron microscopic measurements. Detailed photoluminescence studies with 3.815 eV (325 nm) excitation reveal the nature of in-plane and bridging 'O' vacancies as well as adsorption and desorption occurred at different annealing temperatures. X-ray photoelectron spectroscopy studies also support this observation. Defect level related to the surface -OH functional groups shows a broad luminescence peak around 1.96 eV in SnO_2 NPs which is elaborated with the help of temperature dependent studies. △ Less

Submitted 31 August, 2015; originally announced August 2015.

Comments: 10 pages, 11 figures, journal

Journal ref: Phys. Chem. Chem. Phys. 2015, 17, 9794

Influence of in-plane and bridging oxygen vacancies of SnO_2 nanostructures on CH_4 sensing at low operating temperatures

Authors: Venkataramana Bonu, A. Das, Arun K Prasad, Nanda Gopala Krishna, Sandip Dhara, A. K. Tyagi

Abstract: Role of 'O' defects in sensing pollutant with nanostructured SnO_2 is not well understood, especially at low temperatures. SnO_2 nanoparticles were grown by soft chemistry route followed by subsequent annealing treatment under specific conditions. Nanowires were grown by chemical vapor deposition technique. A systematic photoluminescence (PL) investigation of 'O' defects in SnO_2 nanostructures re… ▽ More Role of 'O' defects in sensing pollutant with nanostructured SnO_2 is not well understood, especially at low temperatures. SnO_2 nanoparticles were grown by soft chemistry route followed by subsequent annealing treatment under specific conditions. Nanowires were grown by chemical vapor deposition technique. A systematic photoluminescence (PL) investigation of 'O' defects in SnO_2 nanostructures revealed a strong correlation between shallow donors created by the in-plane and the bridging 'O' vacancies and gas sensing at low temperatures. These SnO_2 nanostructures detected methane (CH_4), a reducing and green house gas at a low temperature of 50 ^oC. Response of CH_4 was found to be strongly dependent on surface defect in comparison to surface to volume ratio. Control over 'O' vacancies during the synthesis of SnO2 nanomaterials, as supported by X-ray photoelectron spectroscopy and subsequent elucidation for low temperature sensing are demonstrated. △ Less

Submitted 29 August, 2015; originally announced August 2015.

Comments: 10 pages, 4 figures, journal

Journal ref: Applied Physics Letters 105, 243102 (2014)

Optical band gap and associated band-tails in nanocrystalline AlN thin films grown by reactive IBSD at different substrate temperatures

Authors: Neha Sharma, Shilpam Sharma, K. Prabakar, S. Amirthapandian, S. Ilango, S. Dash, A. K. Tyagi

Abstract: AlN thin films have been grown on Si (100) substrates by reactive ion beam sputter deposition (IBSD) at different substrate temperatures varying from room temperature (RT) to 500oC. Substrate temperature induced microstructural transition from amorphous at RT, nanocrystalline at 300oC to microcrystalline at 400oC has been observed by Transmission Electron Microscopy (TEM). Average surface roughnes… ▽ More AlN thin films have been grown on Si (100) substrates by reactive ion beam sputter deposition (IBSD) at different substrate temperatures varying from room temperature (RT) to 500oC. Substrate temperature induced microstructural transition from amorphous at RT, nanocrystalline at 300oC to microcrystalline at 400oC has been observed by Transmission Electron Microscopy (TEM). Average surface roughness (Ra) and morphology has been explored by using Atomic Force Microscopy (AFM). UV-VIS spectrophotometry has been employed to probe the substrate temperature induced changes in optical band-gap (Eg) of grown thin films in reflectance mode. It was found that Eg was increased from 5.08 to 5.21 eV as substrate temperature was increased from RT to 500oC. Urbach energy tail (Eu) along with weak absorption tail (WAT) energy (Et) have been estimated to account for the optical disorder which was found to decrease with associated increase in Eg. △ Less

Submitted 20 February, 2016; v1 submitted 17 July, 2015; originally announced July 2015.

Comments: 22 pages, 6 figures, 2tables

Exploring the high-pressure behavior of the three known polymorphs of BiPO4: Discovery of a new polymorph

Authors: D. Errandonea, O. Gomis, D. Santamaria-Perez, B. Garcia-Domene, A. Munoz, P. Rodriguez-Hernandez, S. N. Achary, A. K. Tyagi, C. Popescu

Abstract: We have studied the structural behavior of bismuth phosphate under compression. We performed x-ray powder diffraction measurements up to 31.5 GPa and ab initio calculations. Experiments were carried out on different polymorphs; trigonal (phase I) and monoclinic (phases II and III). Phases I and III, at low pressure (0.2-0.8 GPa), transform into phase II, which has a monazite-type structure. At roo… ▽ More We have studied the structural behavior of bismuth phosphate under compression. We performed x-ray powder diffraction measurements up to 31.5 GPa and ab initio calculations. Experiments were carried out on different polymorphs; trigonal (phase I) and monoclinic (phases II and III). Phases I and III, at low pressure (0.2-0.8 GPa), transform into phase II, which has a monazite-type structure. At room temperature, this polymorph is stable up to 31.5 GPa. Calculations support these findings and predict the occurrence of an additional transition from the monoclinic monazite-type to a tetragonal scheelite-type structure (phase IV). This transition was experimentally found after the simultaneous application of pressure (28 GPa) and temperature (1500 K), suggesting that at room temperature the transition might by hindered by kinetic barriers. Calculations also predict an additional phase transition at 52 GPa, which exceeds the maximum pressure achieved in the experiments. This transition is from phase IV to an orthorhombic barite-type structure (phase V). We also studied the axial and bulk compressibility of BiPO4. Room-temperature pressure-volume equations of state are reported. BiPO4 was found to be more compressible than isomorphic rare-earth phosphates. The discovered phase IV was determined to be the less compressible polymorph of BiPO4. On the other hand, the theoretically predicted phase V has a bulk modulus comparable with that of monazite-type BiPO4. Finally, the isothermal compressibility tensor for the monazite-type structure is reported at 2.4 GPa showing that the direction of maximum compressibility is in the (010) plane at approximately 15 (21) degrees to the a axis for the case of our experimental (theoretical) study. △ Less

Submitted 1 March, 2015; originally announced March 2015.

Comments: 30 pages, 6 figures, 3 tables, to appear in J. Appl. Phys

Journal ref: Journal of Applied Physics 117, 105902 (2015)

Mass spectral analysis and quantification of Secondary Ion Mass Spectrometry data

Authors: A. K. Balamurugan, S. Dash, A. K. Tyagi

Abstract: This work highlights the possibility of improving the quantification aspect of Cs-complex ions in SIMS (Secondary Ion Mass Spectrometry), by combining the intensities of all possible Cs-complexes. Identification of all possible Cs-complexes requires quantitative analysis of mass spectrum from the material of interest. The important steps of this mass spectral analysis include constructing fingerpr… ▽ More This work highlights the possibility of improving the quantification aspect of Cs-complex ions in SIMS (Secondary Ion Mass Spectrometry), by combining the intensities of all possible Cs-complexes. Identification of all possible Cs-complexes requires quantitative analysis of mass spectrum from the material of interest. The important steps of this mass spectral analysis include constructing fingerprint mass spectra of the constituent species from the table of isotopic abundances of elements, constructing the system(s) of linear equations to get the intensities of those species, solving them, evaluating the solutions and employing a regularization process when required. These steps are comprehensively described and the results of their application on a SIMS mass spectrum obtained from D9 steel are presented. It is demonstrated that results from the summation procedure, which covers entire range of sputtered clusters, is superior to results from single Cs-complex per element. The result of employing a regularization process in solving a mass spectrum from an SS316LN steel specimen is provided to demonstrate the necessity of regularization. △ Less

Submitted 29 October, 2014; v1 submitted 26 September, 2014; originally announced September 2014.

Comments: 10 pages, 3 figures; added reference for section "Theory", a few sentences modified for clarity

Growth Kinetics of Ion Beam Sputtered Al-thin films by Dynamic Scaling Theory

Authors: Neha Sharma, K. Prabakar, S. Dash, A. K. Tyagi

Abstract: This paper reports the study of growth kinetics of ion beam sputtered aluminum thin films. Dynamic scaling theory was used to derive the kinetics from AFM measurements. AFM imaging revealed that surface incorporates distinctly different morphologies. Variation in deposition times resulted in such distinctiveness. The growth governing static (alpha) as well as dynamic (beta) scaling exponents have… ▽ More This paper reports the study of growth kinetics of ion beam sputtered aluminum thin films. Dynamic scaling theory was used to derive the kinetics from AFM measurements. AFM imaging revealed that surface incorporates distinctly different morphologies. Variation in deposition times resulted in such distinctiveness. The growth governing static (alpha) as well as dynamic (beta) scaling exponents have been determined. The exponent (alpha) decreased as the deposition time increased from 3 to 15 minutes. Consequently, the interfacial width (xi) also decreased with critical length (Lc), accompanied with an increase in surface roughness. Surface diffusion becomes a major surface roughening phenomenon that occurs during deposition carried out over a short period of 3 minutes. Extension of deposition time to 15 minutes brought in bulk diffusion process to dominate which eventually led to smoothening of a continuous film. △ Less

Submitted 23 September, 2014; originally announced September 2014.

MSC Class: 00-02

Journal ref: Thin Solid Films 573 (2014) 84

Growth Kinetic studies of ion beam sputtered AlN-thin films: Effect of reactive assistance of nitrogen plasma

Authors: Neha Sharma, K. Prabhakar, S. Ilango, S. Dash, A. K. Tyagi

Abstract: Reactive dual ion beam sputter deposition of AlN thin films was carried out for the analysis of surface growth characteristics by Atomic Force Microscopy. The variation of roughness as a function of deposition time was analysed by Dynamic Scaling Theory (DST). Two distinct exponents, static and dynamic were used to unravel the film growth characteristics. As the deposition time increased, static s… ▽ More Reactive dual ion beam sputter deposition of AlN thin films was carried out for the analysis of surface growth characteristics by Atomic Force Microscopy. The variation of roughness as a function of deposition time was analysed by Dynamic Scaling Theory (DST). Two distinct exponents, static and dynamic were used to unravel the film growth characteristics. As the deposition time increased, static scaling exponent decreased gradually and substrate surface coverage was increased which is indicated by a decrease in critical length Lc. The rms roughness of the film was increased from 1.99 to 3.42 nm as the deposition time was increased from 3 minutes to 15 minutes. Dynamic scaling exponent was found to be 0.36. During the growth, surface diffusion (n = 4) becomes the major roughening phenomenon while Bulk diffusion (n = 3) turns into the dominating smoothening phenomenon. △ Less

Submitted 19 September, 2014; originally announced September 2014.

Comments: 10 pages, 8 figures

MSC Class: 00-02

Journal ref: Applied Surface Science 347 (2015) 875

Inelastic Neutron Scattering Studies of Phonon Spectra and Simulations in Tungstates, AWO4 (A = Ba, Sr, Ca and Pb)

Authors: Prabhatasree Goel, M. K. Gupta, R. Mittal, S. Rols, S. N. Achary, A. K. Tyagi, S. L. Chaplot

Abstract: Lattice dynamics and high pressure phase transitions in AWO4 (A = Ba, Sr, Ca and Pb) have been investigated using inelastic neutron scattering experiments, ab-initio density functional theory calculations and extensive molecular dynamics simulations. The vibrational modes that are internal to WO4 tetrahedra occur at the highest energies consistent with the relative stability of WO4 tetrahedra. The… ▽ More Lattice dynamics and high pressure phase transitions in AWO4 (A = Ba, Sr, Ca and Pb) have been investigated using inelastic neutron scattering experiments, ab-initio density functional theory calculations and extensive molecular dynamics simulations. The vibrational modes that are internal to WO4 tetrahedra occur at the highest energies consistent with the relative stability of WO4 tetrahedra. The neutron data and the ab-initio calculations are found to be in excellent agreement. The neutron and structural data are used to develop and validate an interatomic potential model. The model is used for classical molecular dynamics simulations to study their response to high pressure. We have calculated the enthalpies of the scheelite and fergusonite phases as a function of pressure, which confirms that the scheelite to fergusonite transition is second order in nature. With increase in pressure, there is a gradual change in the AO8 polyhedra, while there is no apparent change in the WO4 tetrahedra. We found that that all the four tungstates amorphize at high pressure. This is in good agreement with available experimental observations which show amorphization at around 45 GPa in BaWO4 and 40 GPa in CaWO4. On amorphization, there is an abrupt increase in the coordination of the W atom while the bisdisphenoids around A atom are considerably distorted. The pair correlation functions of the various atom pairs corroborate these observations. Our observations aid in predicting the pressure of amorphization in SrWO4 and PbWO4, which have not been experimentally reported. △ Less

Submitted 25 July, 2014; originally announced July 2014.

Comments: 15 pages, 8 figures

Journal ref: Phys. Rev. B 91, 094304 (2015)

Evolution and defect analysis of vertical graphene nanosheets

Authors: Subrata Ghosh, K. Ganesan, Shyamal R. Polaki, T. R. Ravindran, Nanda Gopala Krishna, M. Kamruddin, A. K. Tyagi

Abstract: We report catalyst-free direct synthesis of vertical graphene nanosheets (VGNs) on SiO2/Si and quartz substrates using microwave electron cyclotron resonance - plasma enhanced chemical vapor deposition. The evolution of VGNs is studied systematically at different growth stages. Raman analysis as a function of growth time reveals that two different disorder-induced competing mechanisms contributing… ▽ More We report catalyst-free direct synthesis of vertical graphene nanosheets (VGNs) on SiO2/Si and quartz substrates using microwave electron cyclotron resonance - plasma enhanced chemical vapor deposition. The evolution of VGNs is studied systematically at different growth stages. Raman analysis as a function of growth time reveals that two different disorder-induced competing mechanisms contributing to the defect band intensity. The VGNs grown on SiO2/Si substrates predominantly consists of both vacancy-like and hopping defects. On the other hand, the VGNs grown on quartz substrates contain mainly boundary-like defects. XPS studies also corroborate Raman analysis in terms of defect density and vacancy-like defects for the VGNs grown on SiO2/Si substrates. Moreover, the grown VGNs exhibit a high optical transmittance from 95 to 78 % at 550 nm and the sheet resistance varies from 30 to 2.17 kohms/square depending on growth time. △ Less

Submitted 31 May, 2014; v1 submitted 10 February, 2014; originally announced February 2014.

Comments: 19 pages, 8 figures, 1 table; Accepted in Journal of Raman Spectroscopy (2014)

Phonons and Thermodynamics of LiMPO4 (M=Mn, Fe)

Authors: Prabhatasree Goel, M. K. Gupta, R. Mittal, S. Rols, S. J. Patwe, S. N. Achary, A. K. Tyagi, S. L. Chaplot

Abstract: Lithium transition metal phospho-olivines are useful electrode materials, owing to their stability, high safety, low cost and cyclability. We report phonon studies using neutron inelastic scattering experiments, ab-initio density functional theory calculations and potential model calculations on LiMPO4 (M=Mn, Fe) at ambient and high temperature to understand the microscopic picture of Li sub-latti… ▽ More Lithium transition metal phospho-olivines are useful electrode materials, owing to their stability, high safety, low cost and cyclability. We report phonon studies using neutron inelastic scattering experiments, ab-initio density functional theory calculations and potential model calculations on LiMPO4 (M=Mn, Fe) at ambient and high temperature to understand the microscopic picture of Li sub-lattice. The experiments are in good agreement with calculations. The lattice dynamics calculations indicate instability of a zone-centre as well as zone-boundary modes along (100) at volume corresponding to high temperature. The unstable phonon modes show mainly large vibration of Li atoms in the x-z plane of the orthorhombic structure (space group Pbnm). Molecular dynamics simulations with increasing temperature indicate large mean square displacement of Li as compared to other constituent atoms. The computed pair-correlations between various atom pairs show that there is local disorder occurring in the lithium sub-lattice with increasing temperature, while other pairs show minimal changes. The results find the two compounds to be thermally stable up to high temperatures, which is a desirable trait for its battery applications. △ Less

Submitted 12 December, 2013; originally announced December 2013.

Comments: 19 Pages, 12 Figures

Journal ref: J. Mater. Chem. A 2, 14729 (2014)

Temperature dependence of dielectric constants in Titanium Nitride

Authors: S. Tripura Sundari, R. Ramaseshan, Feby Jose, S. Dash, A. K. Tyagi

Abstract: The temperature dependence of optical constants of titanium nitride thin film is investigated using spectroscopic ellipsometry between 1.4 to 5 eV in the temperature range 300 K to 650 K in steps of 50 K. The real and imaginary parts of the dielectric functions "1(E) and "2(E) increase marginally with increase in temperature. A Drude Lorentz dielectric analysis based on free electron and oscillato… ▽ More The temperature dependence of optical constants of titanium nitride thin film is investigated using spectroscopic ellipsometry between 1.4 to 5 eV in the temperature range 300 K to 650 K in steps of 50 K. The real and imaginary parts of the dielectric functions "1(E) and "2(E) increase marginally with increase in temperature. A Drude Lorentz dielectric analysis based on free electron and oscillator model are carried out to describe the temperature behavior. The parameters of the Lorentz oscillator model also showed that the relaxation time decreased with temperature while the oscillator energies increased. This study shows that owing to marginal change in the refractive index with temperature, titanium nitride can be employed for surface plasmon sensor applications even in environments where rise in temperature is imminent. △ Less

Submitted 2 August, 2013; originally announced August 2013.

Comments: 12 pages, 7 figures

Optical properties and hardness of highly a-axis oriented AlN films

Authors: Feby Jose, R. Ramaseshan, S. Tripura Sundari, S. Dash, M. S. R. N. Kiran, A. K. Tyagi, U. Ramamurty

Abstract: This paper reports optical and nanomechanical properties of seldom studied highly a-axis oriented AlN thin films for the first time. These films were deposited by reactive DC magnetron sputtering technique at an optimal target to substrate distance of 180 mm. Bragg-Brentano geometry X-ray and rocking curve (FWHM = 52 arcsec) studies confirmed the preferred orientation. Spectroscopic ellipsometry r… ▽ More This paper reports optical and nanomechanical properties of seldom studied highly a-axis oriented AlN thin films for the first time. These films were deposited by reactive DC magnetron sputtering technique at an optimal target to substrate distance of 180 mm. Bragg-Brentano geometry X-ray and rocking curve (FWHM = 52 arcsec) studies confirmed the preferred orientation. Spectroscopic ellipsometry revealed that these films exhibit a refractive index of 1.93 at a wavelength of 546 nm. The hardness and elastic modulus of these films were 17 GPa and 190 GPa, respectively. The mechanical properties obtained here are much higher than the earlier reported and therefore can be useful as protective coating in thermo printing devices, piezoelectric films in bulk acoustic wave resonators. △ Less

Submitted 26 November, 2012; v1 submitted 27 August, 2012; originally announced August 2012.

Comments: 7 pages, 4 figures

Journal ref: APL 2012

Phonons and Colossal Thermal Expansion Behavior of Ag3Co(CN)6 and Ag3Fe(CN)6

Authors: R Mittal, M. Zbiri, H. Schober, S. N. Achary, A. K. Tyagi, S. L. Chaplot

Abstract: Recently colossal positive volume thermal expansion has been found in the framework compounds Ag3Co(CN)6 and Ag3Fe(CN)6. Phonon spectra have been measured using the inelastic neutron scattering technique as a function of temperature and pressure. The data has been analyzed using ab-initio calculations. We find that the bonding is very similar in both compounds. At ambient pressure modes in the int… ▽ More Recently colossal positive volume thermal expansion has been found in the framework compounds Ag3Co(CN)6 and Ag3Fe(CN)6. Phonon spectra have been measured using the inelastic neutron scattering technique as a function of temperature and pressure. The data has been analyzed using ab-initio calculations. We find that the bonding is very similar in both compounds. At ambient pressure modes in the intermediate frequency part of the vibrational spectra in the Co compound are shifted to slightly higher energies as compared to the Fe compound. The temperature dependence of the phonon spectra gives evidence for large explicit anharmonic contribution to the total anharmonicity for low-energy modes below 5 meV. We found that modes are mainly affected by the change in the size of unit cell, which in turn changes the bond lengths and vibrational frequencies. Thermal expansion has been calculated via the volume dependence of phonon spectra. Our analysis indicates that Ag phonon modes in the energy range from 2 to 5 meV are strongly anharmonic and major contributors to thermal expansion in both compounds. The application of pressure hardens the low-energy part of the phonon spectra involving Ag vibrations and confirms the highly anharmonic nature of these modes. △ Less

Submitted 9 May, 2012; originally announced May 2012.

Comments: 19 pages, 14 figures and one table

Journal ref: J. Phys.: Condens. Matter 24, 505404 (2012)

New high-pressure phase and equation of state of Ce2Zr2O8

Authors: D. Errandonea, R. S. Kumar, S. N. Achary, O. Gomis, F. J. Manjon, R. Shukla, A. K. Tyagi

Abstract: In this paper we report a new high-pressure rhombohedral phase of Ce2Zr2O8 observed from high-pressure angle-dispersive x-ray diffraction and Raman spectroscopy studies up to nearly 12 GPa. The ambient-pressure cubic phase of Ce2Zr2O8 transforms to a rhombohedral structure beyond 5 GPa with a feeble distortion in the lattice. Pressure evolution of unit-cell volume showed a change in compressibilit… ▽ More In this paper we report a new high-pressure rhombohedral phase of Ce2Zr2O8 observed from high-pressure angle-dispersive x-ray diffraction and Raman spectroscopy studies up to nearly 12 GPa. The ambient-pressure cubic phase of Ce2Zr2O8 transforms to a rhombohedral structure beyond 5 GPa with a feeble distortion in the lattice. Pressure evolution of unit-cell volume showed a change in compressibility above 5 GPa. The unit-cell parameters of the high-pressure rhombohedral phase at 12.1 GPa are ah = 14.6791(3) Å, ch = 17.9421(5) Å, V = 3348.1(1) Å3. The structure relation between the parent cubic (P2_13) and rhombohedral (P3_2) phases were obtained by group-subgroup relations. All the Raman modes of the cubic phase showed linear evolution with pressure with the hardest one at 197 cm-1. Some Raman modes of the high-pressure phase have a non-linear evolution with pressure and softening of one low-frequency mode with pressure is found. The compressibility, equation of state, and pressure coefficients of Raman modes of Ce2Zr2O8 are also reported. △ Less

Submitted 9 March, 2012; originally announced March 2012.

Comments: 33 pages, 8 figures, 6 tables

Journal ref: JOURNAL OF APPLIED PHYSICS 111, 053519 (2012)

In situ high-pressure synchrotron x-ray diffraction study of CeVO4 and TbVO4 up to 50 GPa

Authors: D. Errandonea, R. S. Kumar, S. N. Achary, A. K. Tyagi

Abstract: Room temperature angle-dispersive x-ray diffraction measurements on zircon-type TbVO4 and CeVO4 were performed in a diamond-anvil cell up to 50 GPa using neon as pressure-transmitting medium. In TbVO4 we found at 6.4 GPa evidence of a non-reversible pressure-induced structural phase transition from zircon to a scheelite-type structure. A second transition to an M-fergusonite-type structure was fou… ▽ More Room temperature angle-dispersive x-ray diffraction measurements on zircon-type TbVO4 and CeVO4 were performed in a diamond-anvil cell up to 50 GPa using neon as pressure-transmitting medium. In TbVO4 we found at 6.4 GPa evidence of a non-reversible pressure-induced structural phase transition from zircon to a scheelite-type structure. A second transition to an M-fergusonite-type structure was found at 33.9 GPa, which is reversible. Zircon-type CeVO4 exhibits two pressure-induced transitions. First an irreversible transition to a monazite-type structure at 5.6 GPa and second at 14.7 GPa a reversible transition to an orthorhombic structure. No additional phase transitions or evidences of chemical decomposition are found in the experiments. The equations of state and axial compressibility for the different phases are also determined. Finally, the sequence of structural transitions and the compressibilities are discussed in comparison with other orhtovanadates and the influence of non-hydrostaticity commented. △ Less

Submitted 25 January, 2012; originally announced January 2012.

Comments: 28 pages, 6 figures, 38 references

Journal ref: Physical Review B 84, 024111 (2011)

High-Temperature Phonon Spectra of Multiferroic BiFeO3 from Inelastic Neutron Spectroscopy

Authors: M. Zbiri, H. Schober, N. Choudhury, R. Mittal, S. L. Chaplot, S. J. Patwe, S. N. Achary, A. K. Tyagi

Abstract: We report inelastic neutron scattering measurements of the phonon spectra in a pure powder sample of the multiferroic material BiFeO3. A high-temperature range was covered to unravel the changes in the phonon dynamics across the Neel (T_N ~ 650 K) and Curie (T_C ~ 1100 K) temperatures. Experimental results are accompanied by ab-initio lattice dynamical simulations of phonon density of states to en… ▽ More We report inelastic neutron scattering measurements of the phonon spectra in a pure powder sample of the multiferroic material BiFeO3. A high-temperature range was covered to unravel the changes in the phonon dynamics across the Neel (T_N ~ 650 K) and Curie (T_C ~ 1100 K) temperatures. Experimental results are accompanied by ab-initio lattice dynamical simulations of phonon density of states to enable microscopic interpretations of the observed data. The calculations reproduce well the observed vibrational features and provide the partial atomic vibrational components. Our results reveal clearly the signature of three different phase transitions both in the diffraction patterns and phonon spectra. The phonon modes are found to be most affected by the transition at the T_C. The spectroscopic evidence for the existence of a different structural modification just below the decomposition limit (T_D ~ 1240 K) is unambiguous indicating strong structural changes that may be related to oxygen vacancies and concomitant Fe3+ to Fe2+ reduction and spin transition. △ Less

Submitted 5 April, 2012; v1 submitted 7 January, 2012; originally announced January 2012.

Journal ref: Applied Physics Letters 100 (2012) 142901

Exchange Bias in BiFe_{0.8}Mn_{0.2}O_3 Nanoparticles with an Antiferromagnetic Core and a Diluted Antiferromagnetic Shell

Authors: P. K. Manna, S. M. Yusuf, R. Shukla, A. K. Tyagi

Abstract: We have observed conventional signature of exchange bias (EB), in form of shift in field-cooled (FC) hysteresis loop, and training effect, in BiFe0.8Mn0.2O3 nanoparticles. From neutron diffraction, thermoremanent magnetization and isothermoremanent magnetization measurements, the nanoparticles are found to be core-shell in nature, consisting of an antiferromagnetic (AFM) core, and a 2-dimensional… ▽ More We have observed conventional signature of exchange bias (EB), in form of shift in field-cooled (FC) hysteresis loop, and training effect, in BiFe0.8Mn0.2O3 nanoparticles. From neutron diffraction, thermoremanent magnetization and isothermoremanent magnetization measurements, the nanoparticles are found to be core-shell in nature, consisting of an antiferromagnetic (AFM) core, and a 2-dimensional diluted AFM (DAFF) shell with a net magnetization under a field. The analysis of the training effect data using the Binek's model shows that the observed loop shift arises entirely due to an interface exchange coupling between core and shell, and the intrinsic contribution of the DAFF shell to the total loop shift is zero. A significantly high value of EB field has been observed at room temperature. The present study is useful to understand the origin of EB in other DAFF-based systems as well. △ Less

Submitted 10 May, 2011; originally announced May 2011.

Comments: 20 pages, 7 figures, Phys. Rev. B 2011 (accepted)