Showing 1–50 of 62 results for author: Nayak, S

Particle-Wall Alignment Interaction and Active Brownian Diffusion Through Narrow Channels

Authors: Poulami Bag, Shubhadip Nayak, Pulak Kumar Ghosh

Abstract: We numerically examine the impacts of particle-wall alignment interactions on active species diffusion through a structureless narrow two-dimensional channel. We consider particle-wall interaction to depend on the self-propulsion velocity direction whereby some specific particle's alignments with respect to the boundary walls are stabilized most. Further, the alignment interaction is meaningful as… ▽ More We numerically examine the impacts of particle-wall alignment interactions on active species diffusion through a structureless narrow two-dimensional channel. We consider particle-wall interaction to depend on the self-propulsion velocity direction whereby some specific particle's alignments with respect to the boundary walls are stabilized most. Further, the alignment interaction is meaningful as long as particles are close to the confining boundaries. Unbiased diffusion of active particles for various possible stable velocity alignments against the walls has been examined. We show that for the most stable configuration leading to self-propulsion velocity direction perpendicular to the wall, diffusivity becomes inversely proportional to the square of alignment interaction torque. On the other hand, when the self-propulsion velocity direction making an acute angle to the channel walls is the most stable configuration, diffusion exponentially grows with strengthening alignment interaction. Hence, particle-wall interaction plays a pivotal role in the transport control of active particles through narrow channels. Moreover, the impacts of the alignment interactions on diffusion largely depend on the particle's self-propulsion properties and its chirality. Our simulation results can potentially be used to understand unbiased diffusion of artificial or living micro/nano-objects (such as virus, bacteria, Janus particles, etc.) though narrow confined structures. △ Less

Submitted 19 August, 2026; originally announced August 2026.

Journal ref: Soft Matter 20 (2024) 8267-8277

Orientation Dynamics of Gyrotactic Microswimmers in Turbulent Flows

Authors: Suraj Kumar Nayak, Vishwanath Shukla, Akshay Bhatnagar

Abstract: We study the dynamics of gyrotactic microswimmers suspended in homogeneous and isotropic turbulence by using direct numerical simulations (DNS). The swimmers are characterized by three non-dimensional parameters: their aspect ratio ($γ$), a dimensionless swimming speed ($φ$), and a dimensionless reorientation time ($ψ$). Strong gyrotaxis (smaller $ψ$) promotes vertical alignment of the swimmers, w… ▽ More We study the dynamics of gyrotactic microswimmers suspended in homogeneous and isotropic turbulence by using direct numerical simulations (DNS). The swimmers are characterized by three non-dimensional parameters: their aspect ratio ($γ$), a dimensionless swimming speed ($φ$), and a dimensionless reorientation time ($ψ$). Strong gyrotaxis (smaller $ψ$) promotes vertical alignment of the swimmers, while weak gyrotaxis leads to nearly isotropic orientations. At low swimming numbers, the orientation distribution is largely shape-independent with spheres and spheroids showing marginally greater vertical alignment than rods, whereas at higher activity the peaks of the distributions exhibit largely shape-independent behavior and the tails show a clear dependence on particle shape. However, at large $ψ$ rods exhibit a stronger alignment along the vertical. We observe that at small $ψ$ the rod-shaped swimmers respond to shear by aligning with the stretching direction of the strain-rate tensor, while at large $ψ$ the alignment with the vorticity vector is preferred. The orientation autocorrelation is found to decay exponentially, with a decay rate that scales as $1/(2ψ)$. Analysis of the mean-squared displacement (MSD) reveals a transition from a ballistic motion at short times to a diffusive regime at longer times. To assess the efficiency of vertical migration, we compute the probability distributions of vertical displacement over a fixed time interval and the time taken to migrate a specific vertical distance. Furthermore, we use a simplified two-dimensional model for spherical swimmers that qualitatively reproduces the key trends observed in the full three-dimensional (3D) simulations. △ Less

Submitted 5 August, 2026; v1 submitted 22 April, 2026; originally announced April 2026.

Comments: 11 pages, 8 figures, 37 references

Wafer-Scale Integration of Piezo- and Ferroelectric Al0.64Sc0.36N Thin Films by Reactive Sputtering

Authors: Sanjay Nayak, Venkata Raveendra Nallagatla, Ravindra Singh Bisht, Dmytro Solonenko, Demian Henzen, Washim Reza Ali, Carla Maria Lazzari, Robert JW Frost, Andrea Serafini, Davide Codegoni, Amalia Balsamo, Rossana Scaldaferri, Elaheh Allahyari, Anirban Ghosh, Martin Kratzer, Andrea Picco, Sonia Costantini, Andrea Rusconi, Mohssen Moridi, Humberto Campanella, Marco Deluca, Annalisa De Pastina

Abstract: Large-area deposition of Aluminium-Scandium-Nitride (Al1-xScxN) thin films with higher Sc content (x) remains challenging due to issues such as abnormal orientation growth, stress control, and the undesired crystal phase. These anomalies across the wafer hinder the development of high scandium-content AlScN films, which are critical for microelectromechanical systems applications. In this study, w… ▽ More Large-area deposition of Aluminium-Scandium-Nitride (Al1-xScxN) thin films with higher Sc content (x) remains challenging due to issues such as abnormal orientation growth, stress control, and the undesired crystal phase. These anomalies across the wafer hinder the development of high scandium-content AlScN films, which are critical for microelectromechanical systems applications. In this study, we report the sputter deposition of Al0.64Sc0.36N thin films from a 300 mm Al0.64Sc0.36 alloy target on 200 mm Si(100) wafers, achieving an exceptionally high deposition rate of 8.7 μm/h with less than 1% AOGs and controllable stress tuning. Comprehensive microstructural and electrical characterizations confirm the superior growth of high-quality Al0.64Sc0.36N films with exceptional wafer-average piezoelectric coefficients (d33,f =15.62 pm/V and e31,f = -2.9 C/m2) owing to low point defects density and grain mosaicity. This was accomplished through the implementation of an optimized seed layer and a refined electrode integration strategy, along with optimal process conditions. The wafer yield and device failure rates are analysed and correlated with the average stress of the films and their stress profiles along the diameter. The resulting films show excellent uniformity in structural, compositional, and piezoelectric properties across the entire 200 mm wafer, underscoring their strong potential for next-generation MEMS applications. △ Less

Submitted 30 December, 2025; originally announced December 2025.

Shear viscosity at finite magnetic field for graphene, non-relativistic and ultra-relativistic cases

Authors: Cho Win Aung, Thandar Zaw Win, Subhalaxmi Nayak, Sabyasachi Ghosh

Abstract: The present article has addressed the finite magnetic field extension of the previous work by Cho et al. (Phys. Rev. B 108, 235172, 2023) on microscopic calculation of shear viscosity for electron fluid in graphene system. Our calculation is based on the kinetic theory approach in the relaxation time approximation. In the absence of a magnetic field, transport is governed by a single shear viscosi… ▽ More The present article has addressed the finite magnetic field extension of the previous work by Cho et al. (Phys. Rev. B 108, 235172, 2023) on microscopic calculation of shear viscosity for electron fluid in graphene system. Our calculation is based on the kinetic theory approach in the relaxation time approximation. In the absence of a magnetic field, transport is governed by a single shear viscosity coefficient, whereas the application of a finite magnetic field induces anisotropy, giving rise to five independent shear viscosity coefficients associated with distinct velocity gradient tensors. These coefficients can be physically categorized into perpendicular, parallel, and Hall components relative to the magnetic field direction. When the scattering time equals the cyclotron time, the perpendicular component is suppressed by 80% and the parallel component by 50% and the Hall effect can reach maximum. Corresponding magnetic field strength for electron fluid in graphene is around 0.01-0.1 Tesla, and the same for non-relativistic electron fluid and ultra-relativistic quark fluid are around 10 Tesla and 10^{14} Tesla respectively. They may be considered as the required magnetic field strength in three different fluid systems to observe noticeable magnetic field response in their shear viscosity coefficients. △ Less

Submitted 11 March, 2026; v1 submitted 23 December, 2025; originally announced December 2025.

Comments: 13 pages, 4 Figures

Directed autonomous motion of active Janus particles induced by wall-particle alignment interactions

Authors: Poulami Bag, Tanwi Debnath, Shubhadip Nayak, Pulak K. Ghosh

Abstract: We propose a highly efficient mechanism to rectify the motion of active particles by exploiting particle-wall alignment interactions. Through numerical simulations of active particles' dynamics in a narrow channel, we demonstrate that a slight difference in alignment strength between the top and bottom walls or a small gravitational drag suffices to break upside-down symmetry, leading to rectifyin… ▽ More We propose a highly efficient mechanism to rectify the motion of active particles by exploiting particle-wall alignment interactions. Through numerical simulations of active particles' dynamics in a narrow channel, we demonstrate that a slight difference in alignment strength between the top and bottom walls or a small gravitational drag suffices to break upside-down symmetry, leading to rectifying the motion of chiral active particles with over 60% efficiency. In contrast, for achiral swimmers to achieve rectified motion using this protocol, an unbiased fluid flow is necessary that can induce orbiting motion in the particle's dynamics. Thus, an achiral particle subject to Couette flow exhibits spontaneous directed motion due to an upside-down asymmetry in particle-wall alignment interaction. The rectification effects caused by alignment we report are robust against variations in self-propulsion properties, particle's chirality, and the most stable orientation of self-propulsion velocities relative to the walls. Our findings offer insights into controlled active matter transport and could be useful to sort artificial as well as natural microswimmers (such as bacteria and sperm cells) based on their chirality and self-propulsion velocities. △ Less

Submitted 6 November, 2025; originally announced November 2025.

Journal ref: Physics of Fluids, 2025

Electron Hydrodynamics in Graphene : Experimental and Theoretical Status

Authors: Subhalaxmi Nayak, Cho Win Aung, Thandar Zaw Win, Ashutosh Dwibedi, Sabyasachi Ghosh, Sesha Vempati

Abstract: The present work comprehensively reviews electron hydrodynamics in graphene, highlighting both experimental observations and theoretical developments. Key experimental signatures such as negative vicinity resistance, Poiseuille flow, and significant violation of the Wiedemann-Franz (WF) law have been discussed, with special emphasis on Lorenz ratio measurements. In the theoretical direction, recen… ▽ More The present work comprehensively reviews electron hydrodynamics in graphene, highlighting both experimental observations and theoretical developments. Key experimental signatures such as negative vicinity resistance, Poiseuille flow, and significant violation of the Wiedemann-Franz (WF) law have been discussed, with special emphasis on Lorenz ratio measurements. In the theoretical direction, recent efforts have focused on developing hydrodynamic frameworks for calculating the thermodynamic and transport coefficients of electrons in graphene. The present work has briefly addressed the theoretical framework adopted by our group. △ Less

Submitted 14 September, 2025; originally announced September 2025.

Atomic Layer Etching of Aluminum Nitride: Mechanistic Insights from First-Principles Studies of Chlorine Chemistry

Authors: Sanjay Nayak, Nikolai Andrianov, Thomas Gruhn, Joaquin Miranda

Abstract: Using first-principles density functional theory calculations in combination with the climbing-image nudged elastic band method, we investigated the adsorption, desorption, and diffusion of atomic chlorine and molecular chlorine on the Al terminated 0001 surface of aluminum nitride. Our results reveal that both atomic Cl and Cl2 exhibit a chemisorption character with high binding energies. Calcula… ▽ More Using first-principles density functional theory calculations in combination with the climbing-image nudged elastic band method, we investigated the adsorption, desorption, and diffusion of atomic chlorine and molecular chlorine on the Al terminated 0001 surface of aluminum nitride. Our results reveal that both atomic Cl and Cl2 exhibit a chemisorption character with high binding energies. Calculations revealed that the splitting pathway of Cl2 on the Al terminated AlN 0001 surface is a barrierless and exothermic process. These findings provide new microscopic-scale insights into halogen-insulator surface interactions and opportunities for new strategies in optimizing AlN atomic layer etching process in semiconductor fabrication. △ Less

Submitted 9 September, 2025; originally announced September 2025.

Large-scale dynamics in visual quorum sensing chiral suspensions

Authors: Yuxin Zhou, Qingqing Yin, Shubhadip Nayak, Poulami Bag, Pulak K. Ghosh, Yunyun Li, Fabio Marchesoni

Abstract: Motility induced phase separation is an efficient aggregation mechanism of active matter, yet biological systems exhibit richer organization through communication among constituents. We investigate suspensions of active particles that change chirality when neighbor density within their visual cone exceeds a threshold, a communication based non-reciprocal interaction akin to quorum sensing. Tuning… ▽ More Motility induced phase separation is an efficient aggregation mechanism of active matter, yet biological systems exhibit richer organization through communication among constituents. We investigate suspensions of active particles that change chirality when neighbor density within their visual cone exceeds a threshold, a communication based non-reciprocal interaction akin to quorum sensing. Tuning the visual cone triggers programmable transitions: from disorder to phase separation to hyper-uniformity. Notably, phase separation triggers large-scale circulation, with robust edge currents persistently flowing around dense clusters, while particle distributions inside become effectively hyper-uniform. These are genuine non-reciprocal effects which occur even in the absence of steric interactions. Remarkably, in active-passive mixtures, only 5% quorum-sensing chiral particles suffice to induce collective circulation. Thus, simple perception-based rules can generate life-like order, offering design principles for programmable active materials and micro-robotic swarms. △ Less

Submitted 15 August, 2025; originally announced August 2025.

Interaction-Driven Giant Electrostatic Modulation of Ion Permeation in Atomically Small Capillaries

Authors: Dhal Biswabhusan, Yechan Noh, Sanat Nalini Paltasingh, Chandrakar Naman, Siva Sankar Nemala, Rathi Aparna, Kaushik Suvigya, Andrea Capasso, Saroj Kumar Nayak, Li-Hsien Yeh, Kalon Gopinadhan

Abstract: Manipulating the electrostatic double layer and tuning the conductance in nanofluidic systems at salt concentrations of 100 mM or higher has been a persistent challenge. The primary reasons are (i) the short electrostatic proximity length, ~3-10 Å, and (ii) difficulties in fabricating atomically small capillaries. Here, we successfully fabricate in-plane vermiculite laminates with transport height… ▽ More Manipulating the electrostatic double layer and tuning the conductance in nanofluidic systems at salt concentrations of 100 mM or higher has been a persistent challenge. The primary reasons are (i) the short electrostatic proximity length, ~3-10 Å, and (ii) difficulties in fabricating atomically small capillaries. Here, we successfully fabricate in-plane vermiculite laminates with transport heights of ~3-5 Å, which exhibit a cation selectivity close to 1 even at a 1000 mM concentration, suggesting an overlapping electrostatic double layer. For gate voltages from -2 V to +1 V, the K+-intercalated vermiculite shows a remarkable conductivity modulation exceeding 1400% at a 1000 mM KCl concentration. The gated ON/OFF ratio is mostly unaffected by the ion concentration (10-1000 mM), which confirms that the electrostatic double layer overlaps with the collective ion movement within the channel with reduced activation energy. In contrast, vermiculite laminates intercalated with Ca2+ and Al3+ ions display reduced conductance with increasing negative gate voltage, highlighting the importance of ion-specific gating effects under Å-scale confinement. Our findings contribute to a deeper understanding of electrostatic phenomena occurring in highly confined fluidic channels, opening the way to the exploration of the vast library of two-dimensional materials. △ Less

Submitted 1 July, 2025; originally announced July 2025.

Comments: 37 pages, 5 figures

Journal ref: Nat Commun 16, 8542 (2025)

Fast Interlayer Energy Transfer from the Lower Bandgap MoS2 to the Higher Bandgap WS2

Authors: Gayatri, Mehdi Arfaoui, Debashish Das, Tomasz Kazimierczuk, Sabrine Ayari, Natalia Zawadzka, Takashi Taniguchi, Kenji Watanabe, Adam Babinski, Saroj K. Nayak, Maciej R. Molas, Arka Karmakar

Abstract: Energy transfer (ET) is a dipole-dipole interaction, mediated by the virtual photon. Traditionally, ET happens from the higher (donor) to lower bandgap (acceptor) material. However, in some rare instances, ET can happen from the lower-to-higher bandgap material, depending on the strong overlap between the acceptor photoluminescence (PL) and the donor absorption spectra. In this work, we report an… ▽ More Energy transfer (ET) is a dipole-dipole interaction, mediated by the virtual photon. Traditionally, ET happens from the higher (donor) to lower bandgap (acceptor) material. However, in some rare instances, ET can happen from the lower-to-higher bandgap material, depending on the strong overlap between the acceptor photoluminescence (PL) and the donor absorption spectra. In this work, we report an ET process from the lower bandgap MoS2 to the higher bandgap WS2, due to a near 'resonant' overlap between the MoS2 B and WS2 A excitonic levels. Changing the MoS2 bandgap from direct-to-indirect by increasing the layer number results in a reduced ET rate, evidenced by the quenching of the WS2 PL emission. Our work shows at 300 K, the ET timescale of ~33 fs is faster than the reported thermalization of the MoS2 excitonic intervalley scattering (K to K') time and competing with the ultrafast charge transfer timescale. Thus, allowing us to open a new direction in understanding the competing inter/intralayer processes. △ Less

Submitted 16 December, 2025; v1 submitted 17 April, 2025; originally announced April 2025.

Report number: npj 2D Mater Appl 10, 25 (2026)

Diffusion Transients in Motility-Induced Phase Separation

Authors: Shubhadip Nayak, Poulami Bag, Pulak K. Ghosh, Yuxin Zhou, Qingqing Yin, Fabio Marchesoni, Franco Nori

Abstract: We numerically investigate normal diffusion in a two-dimensional athermal suspension of active particles undergoing motility-induced phase separation. The particles are modeled as achiral Janus disks with fixed self-propulsion speed and weakly fluctuating orientation. When plotted versus the overall suspension packing fraction, the relevant diffusion constant traces a hysteresis loop with sharp ju… ▽ More We numerically investigate normal diffusion in a two-dimensional athermal suspension of active particles undergoing motility-induced phase separation. The particles are modeled as achiral Janus disks with fixed self-propulsion speed and weakly fluctuating orientation. When plotted versus the overall suspension packing fraction, the relevant diffusion constant traces a hysteresis loop with sharp jumps in correspondence with the binodal and spinodal of the gaseous phase. No hysteresis loop is observed between the spinodal and binodal of the dense phase, as they appear to overlap. Moreover, even under steady-state phase separation, the particle displacement distributions exhibit non-Gaussian normal diffusion with transient fat (thin) tails in the presence (absence) of phase separation. △ Less

Submitted 19 February, 2025; originally announced February 2025.

Comments: 6 figures and 10 pages

Journal ref: Phys. Rev. Research 7, 013153 (2025)

On the Wiedemann-Franz law violation in Graphene and quark-gluon plasma systems

Authors: Ashutosh Dwibedi, Subhalaxmi Nayak, Sathe Subodh Kiran, Sabyasachi Ghosh, Sesha Vempati

Abstract: A comparative study of the thermodynamic and transport properties of the ultra-relativistic quark-gluon plasma produced in heavy ion collisions with the "quasi-relativistic" massless electron-hole plasma in graphene sample has been performed. We observe that the enthalpy per net charge carriers emerges as a useful physical quantity determining the transport variables in hydrodynamic domain. Lorenz… ▽ More A comparative study of the thermodynamic and transport properties of the ultra-relativistic quark-gluon plasma produced in heavy ion collisions with the "quasi-relativistic" massless electron-hole plasma in graphene sample has been performed. We observe that the enthalpy per net charge carriers emerges as a useful physical quantity determining the transport variables in hydrodynamic domain. Lorenz ratio is defined as thermal to electrical conductivity ratio, normalized by temperature and Lorenz number $L_{0}=\frac{π^{2}}{3}\left(\frac{k_{B}}{e}\right)^{2}$. The validity of the Wiedemann-Franz law can be checked by evaluating the Lorenz ratio, which is expected to be unity. We investigate the validity of the Wiedemann-Franz law by examining whether the Lorenz ratio equals unity or deviates from it. Our findings indicate that, within the fluid-based framework, the Lorenz ratio consistently leads to a violation of the Wiedemann-Franz law. This is attributed to the proportional relation between Lorenz ratio and enthalpy per net charge carriers in the fluid. Based on the experimental observation, graphene and quark-gluon plasma, both systems at a low net carrier density, violate the Wiedemann-Franz law due to their fluidic nature. However, graphene at a relatively high net carrier density obeys the Wiedemann-Franz law, followed by metals with high Fermi energy or electron density. It indicates a fluid to the non-fluid transition of the graphene system from low to high carrier density. In this regard, the fluid or non-fluid aspect of quark-gluon plasma at high density is yet to be explored by future facilities like Compressed Baryonic Matter and Nuclotron-based Ion Collider fAcility experiments. △ Less

Submitted 10 August, 2025; v1 submitted 31 December, 2024; originally announced January 2025.

Journal ref: Eur. Phys. J. B 98, 167 (2025)

Dependence of Exchange Bias on Interparticle Interactions in Co/CoO Core/shell Nanostructures

Authors: S. Goswami, Pushpendra Gupta, S. Nayak, Subhankar Bedanta, Òscar Iglesias, M. Chakraborty, Debajyoti De

Abstract: This article reports dependence of exchange bias (EB) effect on interparticle interactions in nanocrystalline Co/CoO core/shell structures, synthesized using conventional sol-gel technique. Analysis via powder X-Ray diffraction (PXRD) studies and transmission electron microscope (TEM) images confirm absence of crystalline phases other than core-shell Co-CoO with average particle size $\approx$18 n… ▽ More This article reports dependence of exchange bias (EB) effect on interparticle interactions in nanocrystalline Co/CoO core/shell structures, synthesized using conventional sol-gel technique. Analysis via powder X-Ray diffraction (PXRD) studies and transmission electron microscope (TEM) images confirm absence of crystalline phases other than core-shell Co-CoO with average particle size $\approx$18 nm. Volume fraction ($\varphi$) is varied (from 20\% to 1\%) by introduction of stoichiometric amount of non-magnetic amorphous silica matrix (SiO$_2$) which leads to a change in interparticle separation/interaction. The influence of exchange and dipolar interactions on the EB effect, caused by the variation in interparticle interaction/separation is studied for a series of Co/CoO core/shell nanoparticle systems. Studies of thermal variation of magnetization ($M- T$) and magnetic hysteresis loops ($M- H$) for the series point towards strong dependence of magnetic properties on dipolar interaction in concentrated assemblies whereas individual nanoparticle response is dominant in isolated nanoparticle systems. The analysis of the EB effect reveals a monotonic increase of coercivity ($H_C$) and EB field ($H_E$) with increasing volume fraction. When the nanoparticles are close enough and the interparticle interaction is significant, collective behavior leads to an increase in the effective antiferromagnetic (AFM) CoO shell thickness which results in high $H_C$, $H_E$. Moreover, in concentrated assemblies, the dipolar field superposes to the local exchange field and enhances the EB effect contributing as an additional source of unidirectional anisotropy. △ Less

Submitted 2 June, 2024; originally announced June 2024.

Comments: 14 pages, 7 figures

Journal ref: Nanomaterials 12, 3159 (2022)

Spinterface Mediated Magnetic Properties of Co20Fe60B20/Alq3 Heterostructures

Authors: Swayang Priya Mahanta, Antarjami Sahoo, Sagarika Nayak, T. P. A. Hase, Del Atkinson, Subhankar Bedanta

Abstract: Organic semiconductors (OSCs) are suitable materials for spintronics applications as they form a spinterface when placed next to a ferromagnet, which in turn leads to novel functionalities. The evolution of spinterface can tune the global magnetic anisotropy, magnetization reversal, magnetization dynamics, etc. Planar tris-(8-hydroxyquinoline)aluminum (Alq3) OSC has shown tremendous potential for… ▽ More Organic semiconductors (OSCs) are suitable materials for spintronics applications as they form a spinterface when placed next to a ferromagnet, which in turn leads to novel functionalities. The evolution of spinterface can tune the global magnetic anisotropy, magnetization reversal, magnetization dynamics, etc. Planar tris-(8-hydroxyquinoline)aluminum (Alq3) OSC has shown tremendous potential for spintronics applications, thanks to its efficient spin-polarized current transport ability. Here, we establish the spinterface when the Alq3 molecules are deposited on amorphous ferromagnet Co20Fe60B20(CFB). The $π$-d hybridization in CFB/Alq3 enhances the coercive field and significantly modifies the shape and size of the magnetic domains. A $\sim$100% increase in uniaxial anisotropic energies and a reduction in magnetic damping are also evident owing to the strong interfacial hybridization. △ Less

Submitted 31 December, 2023; originally announced January 2024.

Dynamic evolution of internal stress, grain growth, and crystallographic texture in arc-evaporated AlTiN thin films using in-situ synchrotron x-ray diffraction

Authors: Sanjay Nayak, Tun-Wei Hsu, Robert Boyd, Jens Gibmeier, Norbert Schell, Jens Birch, Lina Rogström, Magnus Odén

Abstract: Understanding the nucleation and growth of polycrystalline thin films is a long-standing goal. Polycrystalline films have many grains with different orientations that affect thin-film properties. Numerous studies have been done to determine these grain size and their preferred crystallographic orientation as well as stress in films. However most past studies have either employed an ex-situ methodo… ▽ More Understanding the nucleation and growth of polycrystalline thin films is a long-standing goal. Polycrystalline films have many grains with different orientations that affect thin-film properties. Numerous studies have been done to determine these grain size and their preferred crystallographic orientation as well as stress in films. However most past studies have either employed an ex-situ methodology or only monitor the development of macroscopic stress in real-time. There has never been any research done on the simultaneous determination of crystallographic texture, grain size, and microscopic stress in polycrystalline thin films. In this study, we simultaneously monitored the generation and temporal evolution of texture, grain size, and internal stress in cathodic arc evaporated Al0.50Ti0.50N thin films using a bespoke deposition apparatus designed for use with 2-dimensional synchrotron x-ray diffraction technique. The influence of the substrate temperature is investigated in terms of the emergence and development of texture, grain size and stress evolution. A dynamic evolution of the crystallographic texture is observed as the overall film thickness varies. We clearly resolved two regime of films growth based on stress evolution. Beyond a threshold grain size (~ 14 nm), the stress scales inversely to the average grain sizes, and as the film thickness increases, immediate compressive stress relaxation was seen. An extensive ex-situ evaluation of thin films using electron microscopies and electron diffraction was performed to support the in-situ x-ray diffraction results. △ Less

Submitted 20 December, 2023; originally announced December 2023.

Assembling PNIPAM-Capped Gold Nanoparticles in Aqueous Solutions

Authors: Binay P. Nayak, Hyeong Jin Kim, Srikanth Nayak, Wenjie Wang, Wei Bu, Surya K. Mallapragada, David Vaknin

Abstract: Employing small angle X-ray scattering (SAXS), we explore the conditions under which the assembly of gold nanoparticles (AuNPs) grafted with the thermo-sensitive polymer Poly(N-isopropylacrylamide) (PNIPAM) emerges. We find that short-range order assembly emerges by combining the addition of electrolytes or poly-electrolytes with raising the temperature of the suspensions above the lower-critical… ▽ More Employing small angle X-ray scattering (SAXS), we explore the conditions under which the assembly of gold nanoparticles (AuNPs) grafted with the thermo-sensitive polymer Poly(N-isopropylacrylamide) (PNIPAM) emerges. We find that short-range order assembly emerges by combining the addition of electrolytes or poly-electrolytes with raising the temperature of the suspensions above the lower-critical solution temperature (LCST) of PNIPAM. Our results show that the longer the PNIPAM chain is, the better organization in the assembled clusters. Interestingly, without added electrolytes, there is no evidence of AuNP assembly as a function of temperature, although untethered PNIPAM is known to undergo a coil-to-globule transition above its LCST. This study demonstrates another approach to assembling potential thermo-sensitive nanostructures for devices by leveraging the unique properties of PNIPAM. △ Less

Submitted 7 December, 2023; originally announced December 2023.

Comments: Published at ACS Macro Letters, DOI - https://doi.org/10.1021/acsmacrolett.3c00617

Journal ref: ACS Macro Lett. 2023, 12, XXX, 1659 to 1664

Driven transport of active particles through arrays of symmetric obstacles

Authors: Shubhadip Nayak, Sohom Das, Poulami Bag, Tanwi Debnath, Pulak K. Ghosh

Abstract: We numerically examine the driven transport of an overdamped self-propelled particle through a two-dimensional array of circular obstacles. A detailed analysis of transport quantifiers (mobility and diffusivity) has been performed for two types of channels, {\it channel I} and {\it channel II}, that respectively correspond to the parallel and diagonal drives with respect to the array axis. Our sim… ▽ More We numerically examine the driven transport of an overdamped self-propelled particle through a two-dimensional array of circular obstacles. A detailed analysis of transport quantifiers (mobility and diffusivity) has been performed for two types of channels, {\it channel I} and {\it channel II}, that respectively correspond to the parallel and diagonal drives with respect to the array axis. Our simulation results show that the signatures of pinning actions and depinning processes in the array of obstacles are manifested through excess diffusion peaks or sudden drops in diffusivity, and abrupt jumps in mobility with varying amplitude of the drive. The underlying depinning mechanisms and the associated threshold driving strength largely depend on the persistent length of self-propulsion. For low driving strength, both diffusivity and mobility are noticeably suppressed by the array of obstacles, irrespective of the self-propulsion parameters and direction of the drive. When self-propulsion length is larger than a channel compartment size, transport quantifiers are insensitive to the rotational relaxation time. Transport with diagonal drives features self-propulsion-dependent negative differential mobility. The amplitude of the negative differential mobility of an active particle is much larger than that of a passive one. The present analysis aims at understanding the driven transport of active species like, bacteria, virus, Janus Particle etc. in porous medium. △ Less

Submitted 13 October, 2023; originally announced October 2023.

Comments: Accepted for publication in JCP (October 2023)

Autonomous Ratcheting by Stochastic Resetting

Authors: Pulak K. Ghosh, Shubhadip Nayak, Jianli Liu, Yunyun Li, Fabio Marchesoni

Abstract: We propose a generalization of the stochastic resetting mechanism for a Brownian particle diffusing in a one-dimensional periodic potential: randomly in time, the particle gets reset at the bottom of the potential well it was in. Numerical simulations show that in mirror asymmetric potentials, stochastic resetting rectifies the particle's dynamics, with maximum drift speed for an optimal average r… ▽ More We propose a generalization of the stochastic resetting mechanism for a Brownian particle diffusing in a one-dimensional periodic potential: randomly in time, the particle gets reset at the bottom of the potential well it was in. Numerical simulations show that in mirror asymmetric potentials, stochastic resetting rectifies the particle's dynamics, with maximum drift speed for an optimal average resetting time. Accordingly, an unbiased Brownian tracer diffusing on an asymmetric substrate can rectify its motion by adopting an adaptive stop-and-go strategy. Our proposed ratchet mechanism can model directed autonomous motion of molecular motors and micro-organisms △ Less

Submitted 12 July, 2023; originally announced July 2023.

Comments: Journal of Chemical Physics 2023 (in press)

Journal ref: J. Chem. Phys. 159, 031101 (2023)

Inducing Quantum Phase Transitions in Non-Topological Insulators Via Atomic Control of Sub-Structural Elements

Authors: Thomas K. Reid, S. Pamir Alpay, Alexander V. Balatsky, Sanjeev K. Nayak

Abstract: Topological insulators (TIs) are an important family of quantum materials that exhibit a Dirac point (DP) in the surface band structure but have a finite band gap in bulk. A large degree of spin-orbit interaction and low bandgap is a prerequisite for stabilizing DPs on selective atomically flat cleavage planes. Tuning of the DP in these materials has been suggested via modifications to the atomic… ▽ More Topological insulators (TIs) are an important family of quantum materials that exhibit a Dirac point (DP) in the surface band structure but have a finite band gap in bulk. A large degree of spin-orbit interaction and low bandgap is a prerequisite for stabilizing DPs on selective atomically flat cleavage planes. Tuning of the DP in these materials has been suggested via modifications to the atomic structure of the entire system. Using the example of As$_2$Te$_3$ and ZnTe$_5$, which are not TIs, we show that a quantum phase transition can be induced in atomically flat and stepped surfaces, for As$_2$Te$_3$ and ZrTe$_5$, respectively. This is achieved by establishing a framework for controlling electronic properties that is focused on local perturbations at key locations that we call sub-structural elements (SSEs). We exemplify this framework through a novel method of isovalent sublayer anion doping and biaxial strain. △ Less

Submitted 11 April, 2023; originally announced April 2023.

Comments: 13 pages, 3 figures, 1 table

Journal ref: Phys. Rev. B 108, 235402 (2023)

Escape Kinetics of an Underdamped Colloidal Particle from a Cavity through Narrow Pores

Authors: Shubhadip Nayak, Tanwi Debnath, Shovan Das, Debajyoti Debnath, Pulak K. Ghosh

Abstract: It is often desirable to know the controlling mechanism of survival probability of nano - or microscale particles in small cavities such as, e.g., confined submicron particles in fiber beds of high-efficiency filter media or ions/small molecules in confined cellular structures. Here we address this issue based on numerical study of the escape kinetics of inertial Brownian colloidal particles from… ▽ More It is often desirable to know the controlling mechanism of survival probability of nano - or microscale particles in small cavities such as, e.g., confined submicron particles in fiber beds of high-efficiency filter media or ions/small molecules in confined cellular structures. Here we address this issue based on numerical study of the escape kinetics of inertial Brownian colloidal particles from various types of cavities with single and multiple pores. We consider both the situations of strong and weak viscous damping. Our simulation results show that as long as the thermal length is larger than the cavity size the mean exit time remains insensitive to the medium viscous damping. On further increasing damping strength, a linear relation between escape rate and damping strength emerges gradually. This result is in sharp contrast to the energy barrier crossing dynamics where the escape rate exhibits a turnover behavior as a function of the damping strength. Moreover, in the ballistic regime, the exit rate is directly proportional to the pore width and the thermal velocity. All these attributes are insensitive to the cavity as well as the pore structures. Further, we show that the effects of pore structure variation on the escape kinetics are conspicuously different in the low damping regimes compared to the overdamped situation. Apart from direct applications in biology and nanotechnology, our simulation results can potentially be used to understand diffusion of living or artificial micro/nano objects, such as bacteria, virus, Janus Particle etc. where memory effects play dictating roles. △ Less

Submitted 28 March, 2023; originally announced March 2023.

Journal ref: J. Phys. Chem. C 2020, 124, 34, 18747

Directed Autonomous Motion and Chiral Separation of Self-Propelled Janus Particles in Convection Roll Arrays

Authors: Poulami Bag, Shubhadip Nayak, Tanwi Debnath, Pulak K. Ghosh

Abstract: Self-propelled Janus particles exhibit autonomous motion thanks to engines of their own. However, due to randomly changing direction of such motion they are of little use for emerging nano-technological and bio-medical applications. Here, we numerically show that the motion of chiral active Janus can be directed subjecting them to a linear array of convection rolls. Rectification power of self-pro… ▽ More Self-propelled Janus particles exhibit autonomous motion thanks to engines of their own. However, due to randomly changing direction of such motion they are of little use for emerging nano-technological and bio-medical applications. Here, we numerically show that the motion of chiral active Janus can be directed subjecting them to a linear array of convection rolls. Rectification power of self-propulsion motion here can be made more than 60% which is much larger than earlier reports. We show that rectification of chiral Janus particle's motion leads to conspicuous segregation of dextrogyre and levogyre active particles from a racemic binary mixture. Further, we demonstrate how efficiently the rectification effect can be exploited to separate dextrogyre and levogyre particles when their intrinsic torques are distributed with Gaussian statistics. △ Less

Submitted 26 March, 2023; originally announced March 2023.

Journal ref: J. Phys. Chem. Lett. 2022, 13, 11413

Structure and diffusion of active-passive binary mixtures in a single-file

Authors: Tanwi Debnath, Shubhadip Nayak, Poulami Bag, Debajyoti Debnath, Pulak Kumar Ghosh

Abstract: We numerically study structure and dynamics of single files composed of active particles, as well as, active-passive binary mixtures. Our simulation results show that when the persistent length of self-propelled particles is much larger than the average inter-particle separation and the self-propulsion velocity is larger than the thermal velocity, particles in the file exist as clusters of various… ▽ More We numerically study structure and dynamics of single files composed of active particles, as well as, active-passive binary mixtures. Our simulation results show that when the persistent length of self-propelled particles is much larger than the average inter-particle separation and the self-propulsion velocity is larger than the thermal velocity, particles in the file exist as clusters of various sizes. Average cluster size and structures of the file are very sensitive to self-propulsion properties, thermal fluctuations and composition of the mixture. In addition to the variation of file composition, our study considers two sorts of mixture configurations. One corresponds to the uniform distribution of active passive throughout the mixture in the single file. In the other configuration, active particles are on one side of the file. For the both configurations, even a little fraction of active particles produces a large impact on the structure and dynamics of the file. △ Less

Submitted 14 March, 2023; originally announced March 2023.

Journal ref: J. Chem. Sci., 2023

In-situ real-time evolution of intrinsic stresses and microstructure during growth of cathodic arc deposited (Al,Ti)N coatings

Authors: Sanjay Nayak, Tun-Wei Hsu, Lina Rogström, Maiara Moreno, Jon M. Andersson, Mats P. Johansson-Jöesaar, Robert Boyd, Norbert Schell, Jens Gibmeier, Jens Birch, Magnus Odén

Abstract: The residual stress plays a vital role in determination of the device performance that uses thin films coating and thus the accurate determination of stress and its optimization with process parameters is an ongoing research work for many decades. In line with this, the microscopic origin of the stress at the atomic scale and its development during the thin film deposition is a matter of major sci… ▽ More The residual stress plays a vital role in determination of the device performance that uses thin films coating and thus the accurate determination of stress and its optimization with process parameters is an ongoing research work for many decades. In line with this, the microscopic origin of the stress at the atomic scale and its development during the thin film deposition is a matter of major scientific interests. The development of stress is a complex phenomenon and has a complex dependence to process parameters, film microstructure and its morphology. In this work, by utilizing a custom-designed cathodic arc deposition system and synchrotron radiation based 2D x-ray diffraction (XRD) technique, we determine the real-time evolution of stress, crystallite sizes and their preferential orientations of Aluminum-Titanium-Nitride (AlxTi1-xN) films with varied Al-content (x=0.0, 0.25, 0.50, and 0.67) on Si-100 substrate. The energies of incoming ions and hence stress in the films is tuned by applying different direct current substrate bias (Vs = floating potential, -20, -40, -60, -80, and -100 V). The instantaneous stress is evaluated by the well-known d vs. sin2ψ technique, while crystallite sizes are determined by analyzing line profiles of x-ray diffractograms. The evolution of stress and crystallite sizes are modelled with multiple numerical models from which kinetic parameters associated with the thin film depositions are extracted. The ex-situ microstructure characterizations of AlxTi1-xN coatings are carried out by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The formation of ex-situ microstructure of the films is discussed considering the results obtained from in-situ XRD data. Finally, we demonstrate that the method utilized here is a powerful approach towards estimation of the fracture toughness of thin film coatings. △ Less

Submitted 10 January, 2023; originally announced January 2023.

Electric Field Control of Magnetism of Mn dimer supported on Carbon-doped-h-BN surface

Authors: Mihir Ranjan Sahoo, Saroj Kumar Nayak, Kalpataru Pradhan

Abstract: Using density functional theory we show that the interaction between two Mn atoms can be tuned from anti-ferromagnetic (AFM) to ferromagnetic (FM) state by creating charge disproportion between the two on a 2D surface. The non-metallic planar heterostructures, the 2D surface, in our work is designed by doping carbon hexagon rings in a hexagonal boron nitride (h-BN) sheet. In addition, we show that… ▽ More Using density functional theory we show that the interaction between two Mn atoms can be tuned from anti-ferromagnetic (AFM) to ferromagnetic (FM) state by creating charge disproportion between the two on a 2D surface. The non-metallic planar heterostructures, the 2D surface, in our work is designed by doping carbon hexagon rings in a hexagonal boron nitride (h-BN) sheet. In addition, we show that an external electric field can be used to control the charge disproportion and hence the magnetism. In fact, our calculations demonstrate that the magnetic states of the dimer can be switched from AFM to FM or vice versa in an external electric field. The origin of this magnetic switching is explained using the charge transfer from (or to) the Mn dimer to (or from) the 2D material. The switching between anti-ferromagnetic to ferromagnetic states can be useful for future spintronic applications. △ Less

Submitted 1 December, 2021; originally announced December 2021.

Comments: 17 pages, 6 figures

Pairing symmetries in the Zeeman-coupled extended attractive Hubbard model

Authors: Swagatam Nayak, Navketan Batra, Sanjeev Kumar

Abstract: By introducing the possibility of equal- and opposite-spin pairings concurrently, we show that the extended attractive Hubbard model (EAHM) exhibits rich ground state phase diagrams with a variety of singlet, triplet, and mixed parity superconducting orders. We study the competition between these superconducting pairing symmetries invoking an unrestricted Hartree-Fock- Bogoliubov-de Gennes (HFBdG)… ▽ More By introducing the possibility of equal- and opposite-spin pairings concurrently, we show that the extended attractive Hubbard model (EAHM) exhibits rich ground state phase diagrams with a variety of singlet, triplet, and mixed parity superconducting orders. We study the competition between these superconducting pairing symmetries invoking an unrestricted Hartree-Fock- Bogoliubov-de Gennes (HFBdG) mean-field approach, and we use the d-vector formalism to characterize the nature of the stabilized superconducting orders. We discover that, while all other types of orders are suppressed, a non-unitary triplet order dominates the phase space in the presence of an in-plane external magnetic field. We also find a transition between a non-unitary to unitary superconducting phase driven by the change in average electron density. Our results serve as a reference for identifying and understanding the nature of superconductivity based on the symmetries of the pairing correlations. The results further highlight that EAHM is a suitable effective model for describing most of the pairing symmetries discovered in different materials. △ Less

Submitted 13 July, 2021; originally announced July 2021.

Journal ref: Scientific Reports 11, 22724 (2021)

Spin pumping and inverse spin Hall effect in CoFeB/C$_{60}$ bilayers

Authors: Purbasha Sharangi, Braj Bhusan Singh, Sagarika Nayak, Subhankar Bedanta

Abstract: Pure spin current based research is mostly focused on ferromagnet (FM)/heavy metal (HM) system. Because of the high spin orbit coupling (SOC) these HMs exhibit short spin diffusion length and therefore possess challenges for device application. Low SOC (elements of light weight) and large spin diffusion length make the organic semiconductors (OSCs) suitable for future spintronic applications. From… ▽ More Pure spin current based research is mostly focused on ferromagnet (FM)/heavy metal (HM) system. Because of the high spin orbit coupling (SOC) these HMs exhibit short spin diffusion length and therefore possess challenges for device application. Low SOC (elements of light weight) and large spin diffusion length make the organic semiconductors (OSCs) suitable for future spintronic applications. From theoretical model it is explained that, due to $π$ - $σ$ hybridization the curvature of the C$_{60}$ molecules may increase the SOC strength. Here, we have investigated spin pumping and inverse spin hall effect (ISHE) in CoFeB/C$_{60}$ bilayer system using coplanar wave guide based ferromagnetic resonance (CPW-FMR) set-up. We have performed angle dependent ISHE measurement to disentangle the spin rectification effects for example anisotropic magnetoresistance, anomalous Hall effect etc. Further, effective spin mixing conductance (g$_{eff}^{\uparrow\downarrow}$) and spin Hall angle ($θ_{SH}$) for C$_{60}$ have been reported here. The evaluated value for $θ_{SH}$ is 0.055. △ Less

Submitted 7 November, 2021; v1 submitted 12 June, 2021; originally announced June 2021.

Spinterface Induced Modification in Magnetic Properties in Co40Fe40B20/Fullerene Bilayers

Authors: Purbasha Sharangi, Esita Pandey, Shaktiranjan Mohanty, Sagarika Nayak, Subhankar Bedanta

Abstract: Organic semiconductor/ferromagnetic bilayer thin films can exhibit novel properties due to the formation of the spinterface at the interface. Buckminsterfullerene (C60) has been shown to exhibit ferromagnetism at the interface when it is placed next to a ferromagnet (FM) such as Fe or Co. Formation of spinterface occurs due to the orbital hybridization and spin polarized charge transfer at the int… ▽ More Organic semiconductor/ferromagnetic bilayer thin films can exhibit novel properties due to the formation of the spinterface at the interface. Buckminsterfullerene (C60) has been shown to exhibit ferromagnetism at the interface when it is placed next to a ferromagnet (FM) such as Fe or Co. Formation of spinterface occurs due to the orbital hybridization and spin polarized charge transfer at the interface. In this work, we have demonstrated that one can enhance the magnetic anisotropy of the low Gilbert damping alloy CoFeB by introducing a C60 layer. We have shown that anisotropy increases by increasing the thickness of C60 which might be a result of the formation of spinterface. However, the magnetic domain structure remains same in the bilayer samples as compared to the reference CoFeB film. △ Less

Submitted 7 November, 2021; v1 submitted 7 February, 2021; originally announced February 2021.

Comments: 20 pages, 8 figures

Non-Gaussian Normal Diffusion in Low Dimensional Systems

Authors: Qingqing Yin, Yunyun Li, Fabio Marchesoni, Shubhadip Nayak, Pulak Ghosh

Abstract: Brownian particles suspended in disordered crowded environments often exhibit non-Gaussian normal diffusion (NGND), whereby their displacements grow with mean square proportional to the observation time and non-Gaussian statistics. Their distributions appear to decay almost exponentially according to "universal" laws largely insensitive to the observation time. This effect is generically attribute… ▽ More Brownian particles suspended in disordered crowded environments often exhibit non-Gaussian normal diffusion (NGND), whereby their displacements grow with mean square proportional to the observation time and non-Gaussian statistics. Their distributions appear to decay almost exponentially according to "universal" laws largely insensitive to the observation time. This effect is generically attributed to slow environmental fluctuations, which perturb the local configuration of the suspension medium. To investigate the microscopic mechanisms responsible for the NGND phenomenon, we study Brownian diffusion in low dimensional systems, like the free diffusion of ellipsoidal and active particles, the diffusion of colloidal particles in fluctuating corrugated channels and Brownian motion in arrays of planar convective rolls. NGND appears to be a transient effect related to the time modulation of the instantaneous particle's diffusivity, which can occur even under equilibrium conditions. Consequently, we propose to generalize the definition of NGND to include transient displacement distributions which vary continuously with the observation time. To this purpose, we provide a heuristic one-parameter function, which fits all time-dependent transient displacement distributions corresponding to the same diffusion constant. Moreover, we reveal the existence of low dimensional systems where the NGND distributions are not leptokurtic (fat exponential tails), as often reported in the literature, but platykurtic (thin sub-Gaussian tails), i.e., with negative excess kurtosis. The actual nature of the NGND transients is related to the specific microscopic dynamics of the diffusing particle. △ Less

Submitted 18 January, 2021; originally announced January 2021.

Comments: accepted by frontiers of Physics

Effect of spin glass frustration on exchange bias in NiMn/CoFeB bilayers

Authors: Sagarika Nayak, Palash Kumar Manna, Braj Bhusan Singh, Subhankar Bedanta

Abstract: Exchange bias in ferromagnetic/antiferromagnetic systems can be explained in terms of various interfacial phenomena. Among these spin glass frustration can affect the magnetic properties in exchange bias systems. Here we have studied a NiMn/CoFeB exchange bias system in which spin glass frustration seems to play a crucial role. In order to account the effect of spin glass frustration on magnetic p… ▽ More Exchange bias in ferromagnetic/antiferromagnetic systems can be explained in terms of various interfacial phenomena. Among these spin glass frustration can affect the magnetic properties in exchange bias systems. Here we have studied a NiMn/CoFeB exchange bias system in which spin glass frustration seems to play a crucial role. In order to account the effect of spin glass frustration on magnetic properties, we have performed the temperature and cooling field dependence of exchange bias. We have observed the decrease of exchange bias field (HEB) with cooling field (HFC) whereas there is not significant effect on coercive field (HC). Exponential decay of HEB and HC is found in these exchange bias systems. Further, training effect measurements have been performed to study the spin relaxation mechanism. We have fitted the training effect data with frozen and rotatable spin relaxation model. We have investigated the ratio of relaxation rate of interfacial rotatable and frozen spins in this study. The training effect data are also fitted with various other models. Further, we observed the shifting of peak temperature towards higher temperature with frequency from the ac susceptibility data. △ Less

Submitted 31 October, 2020; originally announced November 2020.

Comments: 8 pages, 9 figures

Microscopic Origin of Structural Disorder in $δ$-NbN: Correlation of Superconductivity and Electronic Structure

Authors: Shailesh Kalal, Sanjay Nayak, Akhil Tayal, Jens Birch, Rajeev Rawat, Mukul Gupta

Abstract: Rock-salt type niobium nitride ($δ$-NbN) is a well-known superconductor having superconducting transition temperature (Tc) $\approx$ 18\,K and a large superconducting gap $\approx$3\,meV. The Tc of $δ$-NbN thin film exhibits a large scattering irrespective of the growth conditions and lattice parameter. In this work, we investigate the atomic origin of suppression of Tc in $δ$-NbN thin film by emp… ▽ More Rock-salt type niobium nitride ($δ$-NbN) is a well-known superconductor having superconducting transition temperature (Tc) $\approx$ 18\,K and a large superconducting gap $\approx$3\,meV. The Tc of $δ$-NbN thin film exhibits a large scattering irrespective of the growth conditions and lattice parameter. In this work, we investigate the atomic origin of suppression of Tc in $δ$-NbN thin film by employing combined methods of experiments and ab-initio simulations. Sputtered $δ$-NbN thin films with different disorder were analyzed through electrical resistivity and x-ray absorption spectroscopy. A strong correlation between the superconductivity and the atomic distortion induced electronic reconstruction was observed. The theoretical analysis revealed that under N-rich growth conditions, atomic and molecular N-interstitial defects assisted by cation vacancies form spontaneously and are responsible for the suppression of Tc in $δ$-NbN by smearing its electronic densities of states around Fermi level. △ Less

Submitted 9 October, 2020; originally announced October 2020.

Comments: 5 Pages, 4 figures

Study of magnetic interface and its effect in Fe/NiFe bilayers of alternating order

Authors: Sagarika Nayak, Sudhansu Sekhar Das, Braj Bhusan Singh, Timothy R. Charlton, Christy J. Kinane, Subhankar Bedanta

Abstract: We present a comprehensive study on the magnetization reversal in Fe/NiFe bilayer system by alternating the order of the magnetic layers. All the samples show growth-induced uniaxial magnetic anisotropy due to oblique angle deposition technique. Strong interfacial exchange coupling between the Fe and NiFe layers leads to the single-phase hysteresis loops in the bilayer system. The strength of coup… ▽ More We present a comprehensive study on the magnetization reversal in Fe/NiFe bilayer system by alternating the order of the magnetic layers. All the samples show growth-induced uniaxial magnetic anisotropy due to oblique angle deposition technique. Strong interfacial exchange coupling between the Fe and NiFe layers leads to the single-phase hysteresis loops in the bilayer system. The strength of coupling being dependent on the interface changes upon alternating the order of magnetic layers. The magnetic parameters such as coercivity HC, and anisotropy field HK become almost doubled when NiFe layer is grown over the Fe layers. This enhancement in the magnetic parameters is primarily dependent on the increase of the thickness and magnetic moment of Fe-NiFe interfacial layer as revealed from the polarized neutron reectivity (PNR) data of the bilayer samples. The difference in the thickness and magnetization of the Fe-NiFe interfacial layer indicates the modification of the microstructure by alternating the order of the magnetic layers of the bilayers. The interfacial magnetic moment increased by almost 18 % when NiFe layer is grown over the Fe layer. In spite of the different values of anisotropy fields and modified interfacial exchange coupling, the Gilbert damping constant values of the ferromagnetic bilayers remain similar to single NiFe layer. △ Less

Submitted 10 June, 2020; originally announced June 2020.

Comments: 9 pages, 9 figures

Gallium-Boron-Phosphide (GaBP$_2$): A New III-V Semiconductor for photovoltaics

Authors: Upendra Kumar, Sanjay Nayak, Soubhik Chakrabarty, Satadeep Bhattacharjee, Seung-Cheol Lee

Abstract: Using machine learning (ML) approach, we unearthed a new III-V semiconducting material having an optimal bandgap for high efficient photovoltaics with the chemical composition of Gallium-Boron-Phosphide(GaBP$_2$, space group: Pna2$_1$). ML predictions are further validated by state of the art ab-initio density functional theory (DFT) simulations. The stoichiometric Heyd-Scuseria-Ernzerhof (HSE) ba… ▽ More Using machine learning (ML) approach, we unearthed a new III-V semiconducting material having an optimal bandgap for high efficient photovoltaics with the chemical composition of Gallium-Boron-Phosphide(GaBP$_2$, space group: Pna2$_1$). ML predictions are further validated by state of the art ab-initio density functional theory (DFT) simulations. The stoichiometric Heyd-Scuseria-Ernzerhof (HSE) bandgap of GaBP$_2$ is noted to 1.65 eV, a close ideal value (1.4-1.5 eV) to reach the theoretical Queisser-Shockley limit. The calculated electron mobility is similar to that of silicon. Unlike perovskites, the newly discovered material is thermally, dynamically and mechanically stable. Above all the chemical composition of GaBP$_2$ are non-toxic and relatively earth-abundant, making it a new generation of PV material. Using ML, we show that with a minimal set of features the bandgap of III-III-V and II-IV-V semiconductor can be predicted up to an RMSE of less than 0.4 eV. We presented a set of scaling laws, which can be used to estimate the bandgap of new III-III-V and II-IV-V semiconductor, with three different crystal phases, within an RMSE of approx. 0.5 eV. △ Less

Submitted 27 September, 2019; originally announced September 2019.

The Role of Specific Ion Effects in Ion Transport: The Case of Nitrate and Thiocyanate

Authors: Kaitlin Lovering, Srikanth Nayak, Wei Bu, Ahmet Uysal

Abstract: The selective transport of trivalent rare earth metals from aqueous to organic environments with the help of amphiphilic "extractants" is an industrially important process. When the amphiphilic extractant is positively charged or neutral, the coextracted background anions are not only necessary for charge balance but also have a large impact on extraction efficiency and selectivity. In particular,… ▽ More The selective transport of trivalent rare earth metals from aqueous to organic environments with the help of amphiphilic "extractants" is an industrially important process. When the amphiphilic extractant is positively charged or neutral, the coextracted background anions are not only necessary for charge balance but also have a large impact on extraction efficiency and selectivity. In particular, the opposite selectivity trends observed throughout the lanthanide series in the presence of nitrate and thiocyanate ions have not been explained. To understand the role of background anions in the phase transfer of lanthanide cations, we use a positively charged long-chain aliphatic molecule, modeling a common extractant, and gain molecular level insight into interfacial headgroup-anion interactions. By combining surface sensitive sum frequency generation spectroscopy with X-ray reflectivity and grazing incidence X-ray diffraction, we observed qualitative differences in the orientational and overall interfacial structure of nitrate and thiocyanate solutions at a positively charged Langmuir monolayer. Though nitrate adsorbs without dramatic changes to the solvation structure at the interface or the monolayer ordering, thiocyanate significantly alters the water structure and reduces monolayer ordering. We suggest that these qualitatively different adsorption trends help explain a reversal in system selectivity toward lighter or heavier lanthanides in solvent extraction systems in the presence of nitrate or thiocyanate anions. △ Less

Submitted 10 January, 2020; v1 submitted 25 September, 2019; originally announced September 2019.

Journal ref: J. Phys. Chem. C 2020, 124, 1, 573-581

First Principles Modeling of Topological Insulators: Structural Optimization and Exchange Correlation Functionals

Authors: Thomas K. Reid, S. Pamir Alpay, Alexander V. Balatsky, Sanjeev K. Nayak

Abstract: Topological insulators (TIs) are materials that are insulating in the bulk but have zero band gap surface states with linear dispersion and are protected by time reversal symmetry. These unique characteristics could pave the way for many promising applications that include spintronic devices and quantum computations. It is important to understand and theoretically describe TIs as accurately as pos… ▽ More Topological insulators (TIs) are materials that are insulating in the bulk but have zero band gap surface states with linear dispersion and are protected by time reversal symmetry. These unique characteristics could pave the way for many promising applications that include spintronic devices and quantum computations. It is important to understand and theoretically describe TIs as accurately as possible in order to predict properties. Quantum mechanical approaches, specifically first principles density functional theory (DFT) based methods, have been used extensively to model electronic properties of TIs. Here, we provide a comprehensive assessment of a variety of DFT formalisms and how these capture the electronic structure of TIs. We concentrate on Bi$_2$Se$_3$ and Bi$_2$Te$_3$ as examples of prototypical TI materials. We find that the generalized gradient (GGA) and kinetic density functional (metaGGA) produce displacements increasing the thickness of the TI slab, whereas we see an opposite behavior in DFT computations using LDA. Accounting for van der Waals (vdW) interactions overcomes the apparent over-relaxations and retraces the atomic positions towards the bulk. Based on an intensive computational study, we show that GGA with vdW treatment is the most appropriate method for structural optimization. Electronic structures derived from GGA or metaGGA employing experimental lattice parameters are also acceptable. In this regard, we express a slight preference for metaGGA in terms of accuracy, but an overall preference for GGA due to compensatory improvements in computability in capturing TI behavior. △ Less

Submitted 24 July, 2019; originally announced July 2019.

Journal ref: Phys. Rev. B 101, 085140 (2020)

Electronic and Optical Properties of Zinc based Hybrid Organic-Inorganic Compounds for Ultraviolet Applications

Authors: Trilochan Sahoo, Uchenna A. Anene, S. Pamir Alpay, Sanjeev K. Nayak

Abstract: Hybrid organic-inorganic (HOI) compounds are excellent candidates for a wide spectrum of applications in diverse fields such as optics, electronics, energy and biotechnology. Their broad range of versatility is achieved by combining the functionalities of organic and inorganic materials to generate unique properties. Current research has mostly focused on perovskite HOIs due to their wide range of… ▽ More Hybrid organic-inorganic (HOI) compounds are excellent candidates for a wide spectrum of applications in diverse fields such as optics, electronics, energy and biotechnology. Their broad range of versatility is achieved by combining the functionalities of organic and inorganic materials to generate unique properties. Current research has mostly focused on perovskite HOIs due to their wide range of uses in solar cells, photo detectors and memory devices. However, drawbacks such as instability and lead toxicity limit further implementation into other new areas. Thus, there is a need to develop stable and non-toxic HOI perovskite materials. Zinc is an attractive substitute for Pb in HOIs. Here, we apply a functionality based materials selection approach to screen for Zn-based HOI compounds from two crystallographic repositories; Inorganic Crystal Structure Database and American Mineralogist Crystal Structure Database. We successfully identify thirteen Zn-based HOI compounds. The electronic structure and optical properties of these compounds are investigated using density functional theory. The calculated optical absorbance fall within the far ultra-violet (FUV) region of 200-112 nm wavelength. We selected four of these compounds and calculated their band gaps; they were found to range between 4.9-5.7 eV. Considering that the UV absorbance is three times larger than average tissue absorbance and the refractive index (> 1.49) is greater than typical tissue materials, one could consider these Zn-based HOI compounds for selective photothermolysis treatment and UV protectant coating for electronic devices. △ Less

Submitted 12 July, 2019; originally announced July 2019.

Exchange bias in Fe/Ir20Mn80 bilayers: Role of spin-glass like interface and bulk antiferromagnet spins

Authors: Sagarika Nayak, Palash Kumar Manna, Thiruvengadam Vijayabaskaran, Braj Bhusan Singh, Subhankar Bedanta

Abstract: We have performed magnetic measurements like temperature (T), cooling field (HFC) dependence of exchange bias (EB) and training effect to investigate the magnetic nature of the interface of the Fe/Ir20Mn80 systems. Thin film bilayer samples of different thicknesses of Ir20Mn80 have been prepared by dc magnetron sputtering at room temperature. The variation of exchange bias field (HEB) with the inc… ▽ More We have performed magnetic measurements like temperature (T), cooling field (HFC) dependence of exchange bias (EB) and training effect to investigate the magnetic nature of the interface of the Fe/Ir20Mn80 systems. Thin film bilayer samples of different thicknesses of Ir20Mn80 have been prepared by dc magnetron sputtering at room temperature. The variation of exchange bias field (HEB) with the increase in thickness of Ir20Mn80 predicts the antiferromagnet (AFM) bulk spins contribution to EB. Exponential decay of HEB and coercive field (HC) with temperature reveals the presence of spin glass (SG) like interface. Also, the decrease of HEB with increasing HFC confirms the SG like frustration at the interface. Further, the fitting of training effect experimental data envisages the presence of frozen and rotatable spins at the magnetically frustrated interface of these EB systems. △ Less

Submitted 4 June, 2019; originally announced June 2019.

Error Minimization in Predicting Accurate Adsorption Energies Using Machine Learning

Authors: Sanjay Nayak, Satadeep Bhattacharjee, Jung-Hae Choi, Seung Cheol Lee

Abstract: Finding the "ideal" catalyst is a matter of great interest in the communities of chemists and material scientists, partly because of its wide spectrum of industrial applications. Information regarding a physical parameter termed "adsorption energy", which dictates the degrees of adhesion of an adsorbate on a substrate is a primary requirement in selecting the catalyst for catalytic reactions. Both… ▽ More Finding the "ideal" catalyst is a matter of great interest in the communities of chemists and material scientists, partly because of its wide spectrum of industrial applications. Information regarding a physical parameter termed "adsorption energy", which dictates the degrees of adhesion of an adsorbate on a substrate is a primary requirement in selecting the catalyst for catalytic reactions. Both experiments and \textit{in-silico} modelling are extensively being used in estimating the adsorption energies, both of which are \textit{Edisonian} approach and demands plenty of resources and are time consuming. In this report, by employing a data centric approach almost instantly we predict the adsorption energies of atomic and molecular gases on the surfaces of many transition metals (TMs). With less than 10 sets of simple atomic features, our predictions of the adsorption energies are within a root-mean-squared-error (RMSE) of less than 0.4 eV with the quantum many-body perturbation theory estimates, a computationally expensive with good experimental agreement. Further, we minimized the RMSE up to 0.11 eV by using the precomputed adsorption energies obtained with conventional exchange and correlation (XC) functional as one component of the feature vector. Based on our results, we developed a set of scaling laws between the adsorption energies computed with many-body perturbation theory and conventional DFT XC-functionals. △ Less

Submitted 7 May, 2019; originally announced May 2019.

Comments: 6 pages, 4 Figure

Topological Transitions in a Model for Proximity Induced Superconductivity

Authors: Navketan Batra, Swagatam Nayak, Sanjeev Kumar

Abstract: Using a prototype model for proximity induced superconductivity on a bilayer square lattice, we show that interlayer tunneling can drive change in topology of the Bogoliubov quasiparticle bands. Starting with topologically trivial superconductors, transitions to a non-trivial $p_x + {\rm i} p_y$ state and back to another trivial state are discovered. We characterize these phases in terms of edge-s… ▽ More Using a prototype model for proximity induced superconductivity on a bilayer square lattice, we show that interlayer tunneling can drive change in topology of the Bogoliubov quasiparticle bands. Starting with topologically trivial superconductors, transitions to a non-trivial $p_x + {\rm i} p_y$ state and back to another trivial state are discovered. We characterize these phases in terms of edge-state spectra and Chern indices. We show that these transitions can also be controlled by experimentally viable control parameters, the bandwidth of the metallic layer and the gate potential. Insights from our results on a simple model for proximity induced superconductivity may open up a new route to discover topological superconductors. △ Less

Submitted 7 February, 2019; originally announced February 2019.

Comments: 8 pages, 6 figures

Journal ref: Phys. Rev. B 100, 214517 (2019)

Experimental and Numerical Investigation of the Fracture Behavior of Particle Reinforced Alkali Activated Slag Mortars

Authors: Sumeru Nayak, Ahmet Kizilkanat, Narayanan Neithalath, Sumanta Das

Abstract: This paper presents fracture response of alkali-activated slag (AAS) mortars with up to 30% (by volume) of slag being replaced by waste iron powder which contains a significant fraction of elongated particles. The elongated iron particles act as micro-reinforcement and improve the crack resistance of AAS mortars by increasing the area of fracture process zone (FPZ). Increased area of FPZ signifies… ▽ More This paper presents fracture response of alkali-activated slag (AAS) mortars with up to 30% (by volume) of slag being replaced by waste iron powder which contains a significant fraction of elongated particles. The elongated iron particles act as micro-reinforcement and improve the crack resistance of AAS mortars by increasing the area of fracture process zone (FPZ). Increased area of FPZ signifies increased energy-dissipation which is reflected in the form of significant increase in the crack growth resistance as determined from R-curves. Fracture response of notched AAS mortar beams under three-point bending is simulated using extended finite element method (XFEM) to develop a tool for direct determination of fracture characteristics such as crack extension and fracture toughness in particulate-reinforced AAS mortars. Fracture response simulated using the XFEM based framework correlates well with experimental observations. The comprehensive fracture studies reported here provide an economical and sustainable means towards improving the ductility of AAS systems which are generally more brittle than their conventional ordinary portland cement counterparts. △ Less

Submitted 4 January, 2019; originally announced January 2019.

Comments: This article has been accepted for publication in ASCE's Journal of Materials in Civil Engineering

Journal ref: Journal of Materials in Civil Engineering, 2019

Polarization rotation in Bi$_{\mathbf{4}}$Ti$_{\mathbf{3}}$O$_{\mathbf{12}}$ by isovalent doping at the fluorite sublattice

Authors: Kevin Co, Fu-Chang Sun S. Pamir Alpay, Sanjeev K. Nayak

Abstract: Bismuth titanate, Bi$_4$Ti$_3$O$_{12}$ (BiT), is a complex layered ferroelectric material that is composed of three perovskite-like units and one fluorite-like unit stacked alternatively along the $c$-direction. The ground state crystal structure is monoclinic with the spontaneous polarization (~50 $μ$C/cm$^{2}$) along the in-plane $b$-direction. BiT typically grows along the $c$-direction in thin… ▽ More Bismuth titanate, Bi$_4$Ti$_3$O$_{12}$ (BiT), is a complex layered ferroelectric material that is composed of three perovskite-like units and one fluorite-like unit stacked alternatively along the $c$-direction. The ground state crystal structure is monoclinic with the spontaneous polarization (~50 $μ$C/cm$^{2}$) along the in-plane $b$-direction. BiT typically grows along the $c$-direction in thin film form and having the polarization vector aligned with the growth orientation can be beneficial for several potential device applications. It is well known that judicious doping of ferroelectrics is an effective method in adjusting the magnitude and the orientation of the spontaneous polarization. Here, we show using first-principles density functional theory and a detailed phonon analysis that Bi atoms in the fluorite-like layers have significantly more impact on the magnitude and orientation of the spontaneous polarization vector as compared to the perovskite-like layer. The low energy hard phonon modes are characterized by fluorite-like layers experiencing transverse displacements and large changes in Born effective charges on Bi atoms. Thus, the breaking of symmetry caused by doping of Bi sites within the fluorite-like layer leads to the formation of uncancelled permanent dipole moments along the $c$-direction. This provides an opportunity for doping the Bi site in the fluorite-like layer. Isovalent dopants P, As, and Sb were studied. P is found to be most effective in the reorientation of the spontaneous polarization. It leads to a three-fold enhancement of the $c$-component of polarization and to a commensurate rotation of the spontaneous polarization vector by 36.2$^{\circ}$ towards the $c$-direction. △ Less

Submitted 21 August, 2018; originally announced August 2018.

Comments: 22 pages, 6 figures, 4 tables

Journal ref: Phys. Rev. B 99, 014101 (2019)

Spectroscopic signatures of native charge compensation in Mg doped GaN Nanorods

Authors: Rajendra Kumar, Sanjay Nayak, S. M. Shivaprasad

Abstract: We study the native charge compensation effect in Mg doped GaN nanorods (NRs), grown by Plasma Assisted Molecular Beam Epitaxy (PAMBE), using Raman, photoluminescence (PL) and X-ray photoelectron spectroscopies (XPS). The XPS valence band analysis shows that upon Mg incorporation the E$_F$-E$_{VBM}$ reduces, suggesting the compensation of the native n-type character of GaN NRs. Raman spectroscopic… ▽ More We study the native charge compensation effect in Mg doped GaN nanorods (NRs), grown by Plasma Assisted Molecular Beam Epitaxy (PAMBE), using Raman, photoluminescence (PL) and X-ray photoelectron spectroscopies (XPS). The XPS valence band analysis shows that upon Mg incorporation the E$_F$-E$_{VBM}$ reduces, suggesting the compensation of the native n-type character of GaN NRs. Raman spectroscopic studies on these samples reveal that the line shape of longitudinal phonon plasmon (LPP) coupled mode is sensitive to Mg concentration and hence to background n-type carrier density. We estimate a two order of native charge compensation in GaN NRs upon Mg-doping with a concentration of 10$^{19}$-10$^{20}$ atoms cm$^{-3}$. Room temperature (RT) PL measurements and our previous electronic structure calculations are used to identify the atomistic origin of this compensation effect. △ Less

Submitted 10 August, 2018; v1 submitted 3 July, 2018; originally announced July 2018.

Comments: 5 pages, 5 Figure

Vacancy defect in bulk and at (10$\overline{1}$0) surface of GaN: A combined first-principles theoretical and experimental analysis

Authors: Sanjay Nayak, Mit H. Naik, Manish Jain, U. V. Waghmare, S. M. Shivaprasad

Abstract: We determine atomic and electronic structure, formation energy, stability and magnetic properties of native point defects, such as Gallium (Ga) and Nitrogen (N) vacancies in bulk and at non-polar (10$\overline{1}$0) surface of wurtzite Gallium Nitride (\textit w-GaN) using, first-principles calculations based on Density Functional Theory (DFT). Under both Ga-rich and N-rich conditions, formation e… ▽ More We determine atomic and electronic structure, formation energy, stability and magnetic properties of native point defects, such as Gallium (Ga) and Nitrogen (N) vacancies in bulk and at non-polar (10$\overline{1}$0) surface of wurtzite Gallium Nitride (\textit w-GaN) using, first-principles calculations based on Density Functional Theory (DFT). Under both Ga-rich and N-rich conditions, formation energy of N-vacancies is significantly lower than that of Ga-vacancies in bulk and at (10$\overline{1}$0) surface. Experimental evidence of the presence of N-vacancies was noted from electron energy loss spectroscopy measurements which further correlated with the high electrical conductivity observed in GaN nanowall network. We find that the Fermi level pins at 0.35 $\pm$0.02 eV below Ga derived surface state. Presence of atomic steps in the nanostructure due to formation of N-vacancies at the (10$\overline{1}$0) surface makes its electronic structure metallic. Clustering of N-vacancies and Ga-Ga metallic bond formation near these vacancies, is seen to be another source of electrical conductivity of faceted GaN nanostructure that is observed experimentally. △ Less

Submitted 14 December, 2017; v1 submitted 16 October, 2017; originally announced October 2017.

Enhanced radial growth of Mg doped GaN nanorods: A combined experiment and first-principles based analysis

Authors: Sanjay Nayak, Rajendra Kumar, Nagaraja KK, S. M. Shivaprasad

Abstract: We discuss the microstructural origin of enhanced radial growth in magnesium (Mg) doped gallium nitride (GaN) nanorods (NRs) using electron microscopy and \textit{first-principles} Density Functional Theory calculations. Experimentally, we find the Mg incorporation increases surface coverage of the grown samples and the height of NRs decreases as a consequence of an increase radial growth rate. We… ▽ More We discuss the microstructural origin of enhanced radial growth in magnesium (Mg) doped gallium nitride (GaN) nanorods (NRs) using electron microscopy and \textit{first-principles} Density Functional Theory calculations. Experimentally, we find the Mg incorporation increases surface coverage of the grown samples and the height of NRs decreases as a consequence of an increase radial growth rate. We also observed the coalescence of NRs becomes prominent and the critical height of coalescence decreases with the increase in Mg concentration. From \textit{first-principles} calculations, we find the surface free energy of Mg doped surface reduces with increasing Mg concentration in the samples. The calculations further suggests a reduction in the diffusion barrier of Ga adatoms along [11$\overline{2}$0] on the side wall surface of the NRs, possibly the primary reason for the observed enhancement in the radial growth. △ Less

Submitted 22 September, 2017; v1 submitted 13 September, 2017; originally announced September 2017.

Journal ref: Journal of Applied Physics 123 (13), 135303 (2018)

On the origin of Blue Luminescence in Mg doped GaN

Authors: Sanjay Nayak, Mukul Gupta, Umesh V. Waghmare, S. M. Shivaprasad

Abstract: We uncover the origin of blue luminescence (BL) peak in Mg doped GaN thin film using a combination of experimental X-ray absorption near edge spectroscopy (XANES), first-principles calculations based on density functional theory and full multiple scattering theoretical analysis of various possible defect complexes and their XANES signatures. We demonstrate that a defect complex composed of Mg subs… ▽ More We uncover the origin of blue luminescence (BL) peak in Mg doped GaN thin film using a combination of experimental X-ray absorption near edge spectroscopy (XANES), first-principles calculations based on density functional theory and full multiple scattering theoretical analysis of various possible defect complexes and their XANES signatures. We demonstrate that a defect complex composed of Mg substituted at Ga site (MgGa) and Mg at interstitial site (Mgi) is primarily responsible for the observed BL by Donor-Acceptor Pair transition (DAP) associated with a deep donor state in the gap. It correlates with a higher (lower) oxidation state of N (Ga) in heavily Mg doped GaN than in its pristine structure, evident in our experiments as well as calculations. Physical and chemical mechanisms identified here point out a route to achieving efficient p-type GaN. △ Less

Submitted 18 August, 2017; v1 submitted 14 August, 2017; originally announced August 2017.

Journal ref: Phys. Rev. Applied 11, 014027 (2019)

Edge enhanced growth induced shape transition in the formation of GaN Nanowall Network

Authors: Sanjay Nayak, Rajendra Kumar, S. M. Shivaprasad

Abstract: We address the mechanism of early stages of growth and shape transition of the unique nanowall network (NwN) nanostructure of GaN by experimentally monitoring its controlled growth using PA-MBE and complementing it by \textit{first-principles} calculations. Using electron microscopy, we observe the formation of tetrahedron shaped (3 faced pyramid) islands at early stages of growth, which later gro… ▽ More We address the mechanism of early stages of growth and shape transition of the unique nanowall network (NwN) nanostructure of GaN by experimentally monitoring its controlled growth using PA-MBE and complementing it by \textit{first-principles} calculations. Using electron microscopy, we observe the formation of tetrahedron shaped (3 faced pyramid) islands at early stages of growth, which later grows anisotropically along their edges of the (20$\overline{2}$1) facets, to form the wall like structure. The mechanism of this crystal growth is discussed in light of surface free energies of the different surfaces, adsorption energy and diffusion barrier of Ga ad-atoms on the (20$\overline{2}$1) facets. By \textit{first-principles} calculations, we find that the diffusion barrier of ad-atoms decreases with decreasing width of facets, and is responsible for the anisotropic growth and formation of the nanowall network. This study suggest that formation of NwN is a archetype example of structure dependent attachment kinetic (SDAK) instability induced shape transition in thin film growth. △ Less

Submitted 10 August, 2017; originally announced August 2017.

Journal ref: Journal of Applied Physics 123, 014302 (2018)

Exotic Superconductivity in the Extended Attractive Hubbard Model

Authors: Swagatam Nayak, Sanjeev Kumar

Abstract: We show that the extended attractive Hubbard model on a square lattice hosts a variety of superconducting phases, including exotic mixed-symmetry phases with $d_{x^2-y^2} + {\rm i} [s + d_{x^2+y^2}]$ and $ d_{x^2-y^2} + p_{x}$ symmetries, and a novel $p_x + {\rm i} p_y$ state. The calculations are performed within the mean-field Bogoliubov-deGennes (BdG) framework. The ground states of the BdG Ham… ▽ More We show that the extended attractive Hubbard model on a square lattice hosts a variety of superconducting phases, including exotic mixed-symmetry phases with $d_{x^2-y^2} + {\rm i} [s + d_{x^2+y^2}]$ and $ d_{x^2-y^2} + p_{x}$ symmetries, and a novel $p_x + {\rm i} p_y$ state. The calculations are performed within the mean-field Bogoliubov-deGennes (BdG) framework. The ground states of the BdG Hamiltonian are obtained via a minimization scheme that does not impose symmetry constraints on the superconducting solutions, hence allowing a mixing of $s$-, $p$- and $d$-wave order parameters. Our results show that extended attractive Hubbard model can serve as an effective model for investigating properties of exotic superconducting states. △ Less

Submitted 12 July, 2017; originally announced July 2017.

Journal ref: Journal of Physics: Condensed Matter 30, 135601 (2018)

GaN Nanowall Network: A new possible route to obtain efficient p-GaN and enhanced light extraction

Authors: Sanjay Kumar Nayak, Mukul Gupta, S. M. Shivaprasad

Abstract: We demonstrate that GaN formed in a Nanowall Network (NwN) morphology can overcome fundamental limitations in optoelectronic devices, and enable high light extraction and effective Mg incorporation for efficient p-GaN. We report the growth of Mg doped GaN Nanowall network (NwN) by plasma assisted molecular beam epitaxy (PA-MBE) that is characterized by Photoluminescence (PL) spectroscopy, Raman sp… ▽ More We demonstrate that GaN formed in a Nanowall Network (NwN) morphology can overcome fundamental limitations in optoelectronic devices, and enable high light extraction and effective Mg incorporation for efficient p-GaN. We report the growth of Mg doped GaN Nanowall network (NwN) by plasma assisted molecular beam epitaxy (PA-MBE) that is characterized by Photoluminescence (PL) spectroscopy, Raman spectroscopy, high-resolution X-ray diffraction (HR-XRD), X-ray photoelectron spectroscopy (XPS) and Secondary ion mass spectroscopy (SIMS). We record a photo-luminescence enhancement ($ \approx $3.2 times) in lightly doped GaN as compared to that of undoped NwN. Two distinct (and broad) blue luminescence peaks appears at 2.95 and 2.7 eV for the heavily doped GaN (Mg $>10^{20}$ atoms $cm^{-3}$), of which the 2.95 eV peak is sensitive to annealing is observed. XPS and SIMS measurements estimate the incorporated Mg concentration to be $10^{20}$ atoms $cm^{-3}$ in GaN NwN morphology, while retaining its band edge emission at $\approx$ 3.4 eV. A higher Mg accumulation towards the GaN/Al$_2$O$_3$ interface as compared to the surface was observed from SIMS measurements. △ Less

Submitted 30 November, 2016; originally announced November 2016.

Soft Phonon Mode Dynamics in Aurivillius Type Structures

Authors: Deepam Maurya, Ali Charkhesht, Sanjeev K. Nayak, Fu-Chang Sun, Deepu George, Abhijit Pramanick, Min-Gyu Kang, Hyun-Cheol Song, Marshall M. Alexander, Djamila Lou, Giti A. Khodaparast, S. Pamir Alpay, N. Q. Vinh, Shashank Priya

Abstract: We report the dynamics of soft phonon modes and their role towards the various structural transformations in Aurivillius materials by employing terahertz frequency-domain spectroscopy, atomic pair distribution function analysis, and first-principles calculations. We have chosen Bi4Ti3O12 as a model system and identified soft phonon modes associated with the paraelectric tetragonal to the ferroelec… ▽ More We report the dynamics of soft phonon modes and their role towards the various structural transformations in Aurivillius materials by employing terahertz frequency-domain spectroscopy, atomic pair distribution function analysis, and first-principles calculations. We have chosen Bi4Ti3O12 as a model system and identified soft phonon modes associated with the paraelectric tetragonal to the ferroelectric monoclinic transition. Three soft phonon modes have been discovered which exhibit a strong temperature dependence. We have determined that the anharmonicity in Bi-O bonds plays a significant role in phonon softening and that Bi cations play an important role in the emergence of ferroelectricity. △ Less

Submitted 12 June, 2017; v1 submitted 3 September, 2016; originally announced September 2016.

Comments: 4 Figures, supplemental information

Journal ref: Phys. Rev. B 96, 134114 (2017)

The effect of hydrostatic pressure and uniaxial strain on the electronic structure of Pb$_{\text{1-x}}$Sn$_{\text{x}}$Te

Authors: Matthias Geilhufe, Sanjeev K. Nayak, Stefan Thomas, Markus Däne, Gouri S. Tripathi, Peter Entel, Wolfram Hergert, Arthur Ernst

Abstract: The electronic structure of Pb$_{1-x}$Sn$_{x}$Te is studied by using the relativistic Korringa-Kohn-Rostoker Green function method in the framework of density functional theory. For all concentrations $x$, Pb$_{1-x}$Sn$_{x}$Te is a direct semiconductor with a narrow band gap. In contrast to pure lead telluride, tin telluride shows an inverted band characteristic close to the Fermi energy. It will… ▽ More The electronic structure of Pb$_{1-x}$Sn$_{x}$Te is studied by using the relativistic Korringa-Kohn-Rostoker Green function method in the framework of density functional theory. For all concentrations $x$, Pb$_{1-x}$Sn$_{x}$Te is a direct semiconductor with a narrow band gap. In contrast to pure lead telluride, tin telluride shows an inverted band characteristic close to the Fermi energy. It will be shown that this particular property can be tuned, first, by alloying PbTe and SnTe and, second, by applying hydrostatic pressure or uniaxial strain. Furthermore, the magnitude of strain needed to switch between the regular and inverted band gap can be tuned by the alloy composition. Thus, there is range of potential usage of Pb$_{1-x}$Sn$_{x}$Te for spintronic applications. △ Less

Submitted 17 August, 2015; originally announced August 2015.

Journal ref: Phys. Rev. B 92, 235203 (2015)

Study of the Negative Magneto-Resistance of Single Proton-Implanted Lithium-Doped ZnO Microwires

Authors: I. Lorite, C. Zandalazini, P. Esquinazi, D. Spemann, S. Friedländer, A. Pöppl, T. Michalsky, M. Grundmann, J. Vogt, J. Meijer, S. P. Heluani, H. Ohldag, W. A. Adeagbo, S. K. Nayak, W. Hergert, A. Ernst, M. Hoffmann

Abstract: The magneto-transport properties of single proton-implanted ZnO and of Li(7\%)-doped ZnO microwires have been studied. The as-grown microwires were highly insulating and not magnetic. After proton implantation the Li(7\%) doped ZnO microwires showed a non monotonous behavior of the negative magneto-resistance (MR) at temperature above 150 K. This is in contrast to the monotonous NMR observed below… ▽ More The magneto-transport properties of single proton-implanted ZnO and of Li(7\%)-doped ZnO microwires have been studied. The as-grown microwires were highly insulating and not magnetic. After proton implantation the Li(7\%) doped ZnO microwires showed a non monotonous behavior of the negative magneto-resistance (MR) at temperature above 150 K. This is in contrast to the monotonous NMR observed below 50 K for proton-implanted ZnO. The observed difference in the transport properties of the wires is related to the amount of stable Zn vacancies created at the near surface region by the proton implantation and Li doping. The magnetic field dependence of the resistance might be explained by the formation of a magnetic/non magnetic heterostructure in the wire after proton implantation. △ Less

Submitted 30 April, 2015; originally announced April 2015.

Comments: 6 pages with 5 figures