Showing 1–50 of 68 results for author: Chandan

Vertical Gallium Oxide Isolated Source Electrode Field Effect Transistors (ISEFET) Without Planarization or Mid-Gap Acceptor Blocking Layers

Authors: Akilesh Srikanth, Md Saklain Morshed, Chandan Joishi, Ahmad E. Islam, Siddharth Rajan

Abstract: We propose and demonstrate the first vertical Gallium oxide device architecture without the use of planarization etch back processes or mid-gap acceptor regions. The Isolated Source Electrode Field Effect Transistor (ISEFET) incorporates a dielectric blocking layer to access an isolated source pad extending from the top fin metal. Scaled multi-fin channels were formed by electron beam lithography… ▽ More We propose and demonstrate the first vertical Gallium oxide device architecture without the use of planarization etch back processes or mid-gap acceptor regions. The Isolated Source Electrode Field Effect Transistor (ISEFET) incorporates a dielectric blocking layer to access an isolated source pad extending from the top fin metal. Scaled multi-fin channels were formed by electron beam lithography with a width of 200 nm along with the source pads and then etched to a trench depth of ~1.2 um. The fabricated devices showed enhancement mode operation with threshold voltage of 2 V and on-off ratio > 1e7 with excellent gate modulation characteristics. The resulting device proved to be comparable to existing vertical transistors and suitable for high-throughput prototyping and large-scale manufacturing of future Gallium oxide and other wide bandgap semiconductor devices. △ Less

Submitted 13 August, 2026; originally announced August 2026.

Comments: 11 pages, 5 figures

Multi-objective Bayesian optimization of rigid and flexible nozzles for energy-efficient pulsed jet propulsion

Authors: Paras Singh, Yukesh Karki, Victor Hernandez, Daehyun Choi, Saad Bhamla, Chandan Bose

Abstract: The biomechanics of pulsed-jet propulsion in aquatic animals, including squids and jellyfish, provide valuable insights into energy-efficient locomotion. In these organisms, flexible funnel deformation enables rapid acceleration and maneuverability while minimizing energy use. Drawing inspiration from these biological systems, this study investigates performance trade-offs between rigid and flexib… ▽ More The biomechanics of pulsed-jet propulsion in aquatic animals, including squids and jellyfish, provide valuable insights into energy-efficient locomotion. In these organisms, flexible funnel deformation enables rapid acceleration and maneuverability while minimizing energy use. Drawing inspiration from these biological systems, this study investigates performance trade-offs between rigid and flexible nozzle geometries in pulsed-jet propulsion systems. A multi-objective Bayesian optimization framework integrated with three-dimensional fluid-structure interaction (FSI) simulations identifies nozzle designs that maximize hydrodynamic impulse and minimize jet energy input. The optimization reveals fundamentally distinct performance characteristics for rigid and flexible nozzles. Rigid nozzles achieve the highest impulse amplification, up to 5 times that of a baseline cylindrical nozzle, but at substantially increased energy expenditure. In contrast, flexible nozzles yield lower peak impulse enhancement of about 2.5 times while achieving significantly greater propulsion efficiency. The maximum normalized impulse-to-energy ratio for flexible nozzles is about 1.8 times higher than that of rigid configurations, indicating more effective conversion of input energy into useful propulsive output. Analysis of the flow physics shows that optimized rigid nozzles enhance performance through geometry-induced internal entrainment, secondary vortex formation, and contraction-driven jet acceleration. This results in stronger vortex circulation and downstream convection. Flexible nozzles use traveling expansion-contraction deformation waves that promote additional entrainment during expansion and accelerate the internally entrained fluid during contraction to improve pressure recovery, reduce pressure-energy expenditure, and mitigate negative pressure impulse contributions. △ Less

Submitted 17 June, 2026; originally announced June 2026.

A Promising Ohmic Contacts Approach for High-Al AlxGa1-xN (x>0.6) Channel HEMTs with AlN/GaN Digital Alloy Channel

Authors: Tariq Jamil, Abdullah Al Mamun Mazumder, S M Tazbiul Hasan, Mafruda Rahman, Muhammad Ali, Ankit Malik, Kamal Hussain, Chandan Joishi, Mansura Sadek, James G. Fiorenza, Grigory Simin, Asif Khan

Abstract: In this paper we report a novel ohmic contact formation scheme for Extreme Bandgap (EBG) AlxGa1-xN (x>0.6) channel HEMTs with undoped barrier layers. Our approach consists of using a new low temperature (LT) pulsed metal-organic chemical vapor deposition (PMOCVD) doping scheme for the n++-GaN regrown contacts and an AlxGa1-xN digital alloy (DA) channel layer comprising short period super lattices… ▽ More In this paper we report a novel ohmic contact formation scheme for Extreme Bandgap (EBG) AlxGa1-xN (x>0.6) channel HEMTs with undoped barrier layers. Our approach consists of using a new low temperature (LT) pulsed metal-organic chemical vapor deposition (PMOCVD) doping scheme for the n++-GaN regrown contacts and an AlxGa1-xN digital alloy (DA) channel layer comprising short period super lattices (SPSL) of AlN and GaN. Pulsed growth and doping yield a sheet resistivity which is a factor of 3-5 lower than that of conventional doped n++-GaN layers grown under identical conditions. Moreover, the regrown n++-GaN layer has no hetero-barrier with the GaN layers of the AlxGa1-xN DA channel. These innovations led to MOCVD regrown linear ohmic contacts and a record-low contact resistance Rc ~6.5 Ω-mm to the Al0.62Ga0.38N DA channel layer of a HEMT with AlN barrier layer. △ Less

Submitted 22 May, 2026; originally announced May 2026.

High Performance TiO2 Ferroelectric Field Effect Transistors with HfZrO2 for Neuromorphic Computing

Authors: Chandan Samanta, Elia Palmese, Ziyu Ouyang, Tuofu Zhama, Robinson Pino, Yuping Zeng

Abstract: TiO2 ferroelectric field effect transistors (FeFETs) with HfZrO2 (HZO) ferroelectric dielectric layers and bottom gate topology are fabricated for applications in neuromorphic systems. Two sets of devices are fabricated with different gate topologies by varying the thickness of the ferroelectric gate stack. Different device architectures are studied by varying the source drain length (LSD) and gat… ▽ More TiO2 ferroelectric field effect transistors (FeFETs) with HfZrO2 (HZO) ferroelectric dielectric layers and bottom gate topology are fabricated for applications in neuromorphic systems. Two sets of devices are fabricated with different gate topologies by varying the thickness of the ferroelectric gate stack. Different device architectures are studied by varying the source drain length (LSD) and gate length (LG). The devices have high on/off ratios up to 10^7 with low leakage off currents <10^-12 A. Repeated cycle testing shows high reliability and a stable memory window. The devices have large memory windows ranging from 3 to 8 V. △ Less

Submitted 19 May, 2026; originally announced May 2026.

Comments: This work has been submitted to the IEEE for possible publication

Computational and reduced-order modelling of elastic wave-driven impulse enhancement in pulsed jets through passively flexible nozzles

Authors: Daehyun Choi, Paras Singh, Saad Bhamla, Chandan Bose

Abstract: Elastic wave propagation and energy exchange in passively deforming cylindrical nozzles are investigated through three-dimensional, two-way fluid-structure interaction simulations. Flexible nozzles with varying stiffness (Eh = 75 to 500 N/m, E is Young's modulus, h is thickness) are subjected to pulsatile jet inflow at low Reynolds number (Re ~ 4400). Increased flexibility reduces deformation-wave… ▽ More Elastic wave propagation and energy exchange in passively deforming cylindrical nozzles are investigated through three-dimensional, two-way fluid-structure interaction simulations. Flexible nozzles with varying stiffness (Eh = 75 to 500 N/m, E is Young's modulus, h is thickness) are subjected to pulsatile jet inflow at low Reynolds number (Re ~ 4400). Increased flexibility reduces deformation-wave speed following MoensKorteweg scaling, prolonging the expansion phase. This delayed expansion enhances jet entrainment and elastic energy storage while suppressing early shear-layer roll-up and vortex formation. During contraction, released elastic energy increases jet acceleration and vortex formation. For the most flexible nozzle, primary vortex-ring circulation increases by 52%, vortex convection distance by 9%, and peak outlet kinetic energy flux 4.6-fold versus a rigid nozzle, resulting in a 62% increase in total hydrodynamic impulse. A reduced-order model represents the coupled response as a lumped store-and-release oscillator, derived as a single-mode projection of the inviscid one-dimensional wave equation and closed at the exit by two terms: (i) an inertial end correction that adds the external fluid column of length Le = R accelerating with the jet, and (ii) a vortex-radiation damping term, active only during ejection, determined by the discharged-jet momentum theorem. This damping reproduces the post-overshoot velocity decay undamped closures fail to capture. The model predicts the simulated resonance frequency within 6% and momentum impulse within 4% across Eh = 75 to 500 N/m, and recovers energy histories. Outlet kinetic energy flux is predicted within 6% for the three stiffer nozzles and 14% for the most compliant. △ Less

Submitted 10 August, 2026; v1 submitted 17 May, 2026; originally announced May 2026.

Squid-inspired soft superpropulsion

Authors: Daehyun Choi, Paras Singh, Ian Bergerson, Minho Kim, Jieun Park, Halley J. Wallace, Kenny Zhang, Sandy Y. Hsieh, Aqua T. Asberry, Theodore A. Uyeno, William F. Gilly, Hyungmin Park, Daeshik Kang, Chandan Bose, Saad Bhamla

Abstract: Squid span four orders of magnitude in size yet rely on pulsed jets. We show that the funnel (siphon) is a compliant nozzle whose dilation and recoil lag mantle contraction, storing and returning energy within each pulse, a mechanism we term superpropulsion. Histology reveals a collagen sheath, and chromatophore tracking in two squid species quantifies a repeatable phase lag. Engineered nozzles, 3… ▽ More Squid span four orders of magnitude in size yet rely on pulsed jets. We show that the funnel (siphon) is a compliant nozzle whose dilation and recoil lag mantle contraction, storing and returning energy within each pulse, a mechanism we term superpropulsion. Histology reveals a collagen sheath, and chromatophore tracking in two squid species quantifies a repeatable phase lag. Engineered nozzles, 3D fluid-structure simulations, and a reduced-order mathematical model predict > 300% impulse amplification when nozzle response time matches jet acceleration (tau/T = 0.2-0.4), overlapping in vivo timing. Tuned nozzles extend jet reach, enhance plume dispersion, and improve jet-driven boat transport, with gains persisting after 40x miniaturization. Superpropulsion recasts pulsed jets as impedance matching, with a soft nozzle acting as an elastic capacitor that passively shapes impulse delivery in soft robotic thrusters and fluidic actuators. △ Less

Submitted 4 May, 2026; originally announced May 2026.

Efficacy of the Weak Formulation of Sparse Nonlinear Identification in Predicting Vortex-Induced Vibrations

Authors: Haimi Jha, Hibah Saddal, Chandan Bose

Abstract: Vortex-induced vibrations (VIV) remain a canonical yet complex manifestation of fluid-structure interactions, where coupled nonlinear dynamics govern the motion of bluff bodies. For several years, we have relied on traditional reduced-order mathematical models derived from empirical and oscillator-based formulations; however, such models often fail to reproduce the quantitative dynamics observed i… ▽ More Vortex-induced vibrations (VIV) remain a canonical yet complex manifestation of fluid-structure interactions, where coupled nonlinear dynamics govern the motion of bluff bodies. For several years, we have relied on traditional reduced-order mathematical models derived from empirical and oscillator-based formulations; however, such models often fail to reproduce the quantitative dynamics observed in realistic flow environments. In this study, we explore a data-driven framework that leverages sparse identification of nonlinear dynamics (SINDy) and its weak formulation to uncover the governing equations of a single-degree-of-freedom cylinder undergoing VIV, using both data generated from previously developed reduced-order models and high-fidelity simulation results to assess the interpretation and efficacy of models discovered from a purely data-driven approach, particularly when the underlying dynamics are not fully known. The weak formulation (WSINDy), which replaces numerical differentiation with an integral-based representation, demonstrates marked robustness for aperiodic dynamics in particular. A complementary analysis using proper orthogonal decomposition (POD) is employed to extract the dominant spatio-temporal structures of the flow and to assess whether the temporal evolution of the wake can be represented on a reduced-dimensional manifold. The findings establish that data-driven identification can recover interpretable, quantitatively reliable models of VIV, providing a robust and computationally efficient pathway for modelling fluid-structure interactions directly from data. In particular, WSINDy is shown to be a more robust and interpretable alternative to standard SINDy for discovering VIV equations from aperiodic response dynamics, paving the way for predictive, data-informed design of fluid-structure interaction systems. △ Less

Submitted 29 March, 2026; originally announced March 2026.

Effects of Plunging Acceleration on the Passive Morphing of Avian-Inspired Flexible Foils

Authors: Hibah Saddal, Lucky Babu Jayswal, Chandan Bose

Abstract: This study investigates the dynamics of passively morphing foils under accelerated plunging, establishing mechanistic links between transient kinematics, structural compliance, and aerodynamic performance. Two-way coupled simulations are performed for three wing geometries: a symmetric NACA0012 foil and two bio-inspired geometries based on falcon and owl wing sections, across non-dimensional bendi… ▽ More This study investigates the dynamics of passively morphing foils under accelerated plunging, establishing mechanistic links between transient kinematics, structural compliance, and aerodynamic performance. Two-way coupled simulations are performed for three wing geometries: a symmetric NACA0012 foil and two bio-inspired geometries based on falcon and owl wing sections, across non-dimensional bending rigidity values, chordwise flexible segment extents from the trailing-edge (25%, 50%, and 75%), and transition speed parameters. The present findings reveal that flexible trailing-edge configurations exhibit improved aerodynamic performance relative to stiffer foils, and the aerodynamic benefit of trailing-edge compliance is strongly influenced by wing geometry. A geometry-specific optimal bending stiffness exists beyond which additional flexibility degrades performance. The extent of the chordwise flexible segment critically governs the aeroelastic response. Whilst a 25% flexible segment produces behaviour indistinguishable from a rigid wing, extending flexibility to 75% of the chord induces highly unsteady lift fluctuations, particularly for the NACA0012 foil, for which the root-mean-square lift coefficient increases sharply. The bio-inspired foils, in contrast, exhibit a moderate reduction in root-mean-square lift coefficient for the 50% and 75% cases, reflecting the stabilising influence of their cambered geometry. Increasing the transition speed parameter monotonically amplifies trailing-edge deflection, strengthens the leading- and trailing-edge vortices, and intensifies the coupling between structural deformation and instantaneous lift. These findings provide new physical insight into bio-inspired propulsion and manoeuvring strategies, with implications for the design of passively adaptive lifting surfaces in unsteady environments. △ Less

Submitted 29 March, 2026; originally announced March 2026.

Inverse Faraday Effect in Rashba two-dimensional electron systems: interplay of spin and orbital effects

Authors: Jaglul Hasan, Chandan Setty

Abstract: The inverse Faraday effect (IFE) refers to the generation of a DC magnetization by circularly polarized light through the transfer of optical angular momentum to electronic degrees of freedom. In conducting systems, this response can arise from two microscopic channels - spin polarization of itinerant electrons and orbital magnetization generated by circulating charge currents. However, the orbita… ▽ More The inverse Faraday effect (IFE) refers to the generation of a DC magnetization by circularly polarized light through the transfer of optical angular momentum to electronic degrees of freedom. In conducting systems, this response can arise from two microscopic channels - spin polarization of itinerant electrons and orbital magnetization generated by circulating charge currents. However, the orbital contribution to the inverse Faraday effect in spin-orbit-coupled conducting systems remains largely unexplored. We present a theoretical analysis of the IFE in disordered two-dimensional electron systems with Rashba spin-orbit coupling using both the quantum kinetic equation and Green's-function diagrammatics. We find that in a noninteracting Rashba metal the orbital magnetization is strongly modified by spin-orbit coupling and can become comparable to, or exceed, the spin magnetization for realistic parameter regimes. When the radiation frequency approaches the Rashba spin splitting, both spin and orbital magnetizations exhibit resonant enhancement. These results clarify the microscopic origin of light-induced magnetization and highlight the interplay of spin and orbital mechanisms in optically driven magnetization dynamics in low-dimensional electronic systems. △ Less

Submitted 13 March, 2026; originally announced March 2026.

Comments: 10 pages, 5 figures

Nonlinear Mode Coupling in Silicon Nitride Membrane Resonators

Authors: Soumya Kanti Das, Nishta Arora, Hridhay A S, Akshay Naik, Chandan Samanta

Abstract: Nonlinear interactions between vibrational modes play a crucial role in understanding the dynamical response of nanomechanical resonators. Here, we report the experimental observation and theoretical modeling of nonlinear mode coupling in a high-stress square silicon nitride membrane resonator. We quantify frequency shifts of the fundamental mode arising from tension-mediated geometric nonlinearit… ▽ More Nonlinear interactions between vibrational modes play a crucial role in understanding the dynamical response of nanomechanical resonators. Here, we report the experimental observation and theoretical modeling of nonlinear mode coupling in a high-stress square silicon nitride membrane resonator. We quantify frequency shifts of the fundamental mode arising from tension-mediated geometric nonlinearity by increasing the amplitude of the fundamental mode and higher-order flexural modes. A quantitative theoretical framework based on Kirchhoff-Love plate theory is developed, which incorporates both intrinsic Duffing nonlinearity and nonlinear intermodal coupling and shows good agreement with experimental measurements for the (1,1)-(2,1) and (1,1)-(2,2) mode pairs. We further compute the nonlinear coupling matrix across mode families, revealing the role of mode symmetry and spatial overlap in governing intermodal interactions. These results establish nonlinear mode coupling as a controllable resource for multimode frequency tuning and mechanical transduction. △ Less

Submitted 9 March, 2026; originally announced March 2026.

Comments: Total 18 pages, includes Supplementary Information

Emission of nitrogen-vacancy centers in diamond shaped by topological photonic waveguide modes

Authors: Raman Kumar, Chandan, Gabriel I. López Morales, Richard Monge, Anton Vakulenko, Svetlana Kiriushechkina, Alexander B. Khanikaev, Johannes Flick, Carlos A. Meriles

Abstract: As the ability to integrate single photon emitters into photonic architectures improves, so does the need to characterize and understand their interaction. Here, we use a scanning diamond nanocrystal to investigate the interplay between the emission of room-temperature nitrogen-vacancy (NV) centers and a proximal topological waveguide. In our experiments, NVs serve as local, spectrally broad light… ▽ More As the ability to integrate single photon emitters into photonic architectures improves, so does the need to characterize and understand their interaction. Here, we use a scanning diamond nanocrystal to investigate the interplay between the emission of room-temperature nitrogen-vacancy (NV) centers and a proximal topological waveguide. In our experiments, NVs serve as local, spectrally broad light sources which we exploit to characterize the waveguide bandwidth as well as the correspondence between light injection site and directionality of wave propagation. Further, we find that near-field coupling to the waveguide influences the spectral shape and ellipticity of the NV photoluminescence, hence allowing us to reveal nanostructured light fields with a spatial resolution defined by the nanoparticle size. Our results expand on the sensing modalities afforded by color centers, and portend novel opportunities in the development of on-chip, quantum optics devices leveraging topological photonics to best manipulate and readout single-photon emitters. △ Less

Submitted 24 January, 2026; originally announced January 2026.

Journal ref: Nat. Nanotech. 20, 1605 (2025)

Scaled Ultra-Wide Bandgap AlGaN Polarization-Graded FET with Ultra-thin Buffer Layer

Authors: Yinxuan Zhu, Ashley Wissel-Garcia, Kidus Guye, Chandan Joishi, Can Cao, Seungheon Shin, Kyle Liddy, Emils G. B. Jurcik, Agnes Maneesha Dominic Merwin Xavier, Andrew A. Allerman, Brianna A. Klein, Andrew Amrstrong, James S. Speck, Samuel Graham, Siddharth Rajan

Abstract: We report on the design and demonstration of ultra-wide bandgap AlGaN polarization-graded field effect transistors with ultra-thin channels to enable excellent current density and high-frequency performance while significantly reducing thermal resistance. We use polarization-graded AlGaN layers and ultra-thin pseudomorphic AlGaN buffer layers to enable low thermal resistance and excellent structur… ▽ More We report on the design and demonstration of ultra-wide bandgap AlGaN polarization-graded field effect transistors with ultra-thin channels to enable excellent current density and high-frequency performance while significantly reducing thermal resistance. We use polarization-graded AlGaN layers and ultra-thin pseudomorphic AlGaN buffer layers to enable low thermal resistance and excellent structural quality. The polarization-graded field effect transistors (PolFETs) demonstrated here show Imax over 800mA/mm and current/power gain cutoff frequency (fT/fmax) of 26/28 GHz. Small signal modeling and analysis were used to determine parasitic/transit delays, and gate-resistance thermometry was implemented to thermally characterize AlGaN PolFET and benchmark against state-of-the-art AlGaN HEMTs. The ultra-thin AlGaN PolFET showed thermal resistance of 12 K.mm/W, representing a significant reduction from typical AlGaN transistors. These results show state-of-art combination of high current density, excellent fT-LG product for ultra-wide bandgap AlGaN transistors, and superior thermal performance, and highlight the promise of AlGaN transistors for future RF and mm-wave applications. △ Less

Submitted 19 December, 2025; originally announced December 2025.

Tree Tensor Networks Methods for Efficient Calculation of Molecular Vibrational Spectra

Authors: Shuo Sun, Richard M. Milbradt, Stefan Knecht, Chandan Kumar, Christian B. Mendl

Abstract: We develop and employ general Tree Tensor Networks (TTNs) to compute the vibrational spectra for two model systems: a set of 64-dimensional coupled oscillators and acetonitrile. We explore various tree architectures, ranging from the simple linear structure of Matrix Product States (MPS), to trees where only the leaf nodes carry a physical leg -- as commonly seen in the underlying ansatz of the Mu… ▽ More We develop and employ general Tree Tensor Networks (TTNs) to compute the vibrational spectra for two model systems: a set of 64-dimensional coupled oscillators and acetonitrile. We explore various tree architectures, ranging from the simple linear structure of Matrix Product States (MPS), to trees where only the leaf nodes carry a physical leg -- as commonly seen in the underlying ansatz of the Multilayer Multiconfiguration Time-Dependent Hartree (ML-MCTDH) method -- and further to more general trees in which all nodes are allowed to possess a physical leg. In addition, we implement Locally Optimal Block Preconditioned Conjugate Gradient (LOBPCG) methods and Inverse Iteration methods as eigensolvers. Benchmarking runtime and accuracy shows that all tested topologies can reach high accuracy. For acetonitrile, inverse-iteration refinement brings all 84 computed states below 1~cm$^{-1}$ error, while the fork-4 tree, a comb-like tree with four backbone nodes, provides the best overall balance between accuracy and cost. MPS remains computationally attractive, whereas more connected trees generally improve accuracy at fixed bond dimension. All numerical simulations were performed using PyTreeNet, a Python package designed for flexible tensor network computations. △ Less

Submitted 16 July, 2026; v1 submitted 17 December, 2025; originally announced December 2025.

Journal ref: J. Chem. Phys. 165, 034111 (2026)

Evaluating Large Language Models in Scientific Discovery

Authors: Zhangde Song, Jieyu Lu, Yuanqi Du, Botao Yu, Thomas M. Pruyn, Yue Huang, Kehan Guo, Xiuzhe Luo, Yuanhao Qu, Yi Qu, Yinkai Wang, Haorui Wang, Jeff Guo, Jingru Gan, Parshin Shojaee, Di Luo, Andres M Bran, Gen Li, Qiyuan Zhao, Shao-Xiong Lennon Luo, Yuxuan Zhang, Xiang Zou, Wanru Zhao, Yifan F. Zhang, Wucheng Zhang , et al. (31 additional authors not shown)

Abstract: Large language models (LLMs) are increasingly applied to scientific research, yet prevailing science benchmarks probe decontextualized knowledge and overlook the iterative reasoning, hypothesis generation, and observation interpretation that drive scientific discovery. We introduce a scenario-grounded benchmark that evaluates LLMs across biology, chemistry, materials, and physics, where domain exp… ▽ More Large language models (LLMs) are increasingly applied to scientific research, yet prevailing science benchmarks probe decontextualized knowledge and overlook the iterative reasoning, hypothesis generation, and observation interpretation that drive scientific discovery. We introduce a scenario-grounded benchmark that evaluates LLMs across biology, chemistry, materials, and physics, where domain experts define research projects of genuine interest and decompose them into modular research scenarios from which vetted questions are sampled. The framework assesses models at two levels: (i) question-level accuracy on scenario-tied items and (ii) project-level performance, where models must propose testable hypotheses, design simulations or experiments, and interpret results. Applying this two-phase scientific discovery evaluation (SDE) framework to state-of-the-art LLMs reveals a consistent performance gap relative to general science benchmarks, diminishing return of scaling up model sizes and reasoning, and systematic weaknesses shared across top-tier models from different providers. Large performance variation in research scenarios leads to changing choices of the best performing model on scientific discovery projects evaluated, suggesting all current LLMs are distant to general scientific "superintelligence". Nevertheless, LLMs already demonstrate promise in a great variety of scientific discovery projects, including cases where constituent scenario scores are low, highlighting the role of guided exploration and serendipity in discovery. This SDE framework offers a reproducible benchmark for discovery-relevant evaluation of LLMs and charts practical paths to advance their development toward scientific discovery. △ Less

Submitted 7 May, 2026; v1 submitted 17 December, 2025; originally announced December 2025.

A DEM-driven machine learning framework for abrasive wear prediction

Authors: Prassana Chandan, Amiya Prakash Das, Shakti Swaroop Choudhury, Ratna Kumar Annabattula

Abstract: Particle-induced wear is a critical concern in bulk material handling systems, where abrasive interactions accelerate equipment degradation, increase maintenance needs, and raise operational costs. The Discrete Element Method (DEM) and Archard's wear model are widely adopted for predicting particle-surface wear processes. However, DEM is computationally prohibitive for real-time design and predict… ▽ More Particle-induced wear is a critical concern in bulk material handling systems, where abrasive interactions accelerate equipment degradation, increase maintenance needs, and raise operational costs. The Discrete Element Method (DEM) and Archard's wear model are widely adopted for predicting particle-surface wear processes. However, DEM is computationally prohibitive for real-time design and predictive maintenance, often requiring hours to days for a single parametric analysis. We propose a DEM-machine learning (ML) framework to address this limitation that combines physics-based simulations with data-driven efficiency. A dataset of 200 DEM simulations is generated by systematically varying particle size, material, and contacting plate geometric parameters. A few ML models -- linear regression, Lasso and Ridge regularization, decision trees, and a genetic algorithm-optimized artificial neural network (GA-ANN) -- were trained and evaluated. Feature selection revealed that Archard's wear constant, particle size, plate angle, and impingement velocity are the dominant predictors of wear. While linear models offered interpretability, their accuracy was limited. The GA-ANN achieved the highest performance $(R^2 = 0.91)$, effectively capturing nonlinear wear dynamics while reducing computational cost by orders of magnitude. This study demonstrates that physics-informed ML provides a scalable pathway for accurate, real-time wear prediction, enabling predictive maintenance and optimized design in bulk material handling industries. △ Less

Submitted 10 September, 2025; originally announced September 2025.

Exceptional point in a PT symmetric non-Hermitian terahertz plasmonic metasurface

Authors: Anshul Bhardwaj, Maidul Islam, Chandan Kumar, Anuraj Panwar, Gagan Kumar

Abstract: In this paper, we experimentally demonstrate a non-Hermitian open PT-symmetric terahertz metasurface comprising complementary plasmonic structures capable of exhibiting an exceptional point (EP). The metasurface consists of two resonators of different sizes, representing effective gain and loss elements, placed orthogonally in close proximity to realize a non-Hermitian configuration leading to a P… ▽ More In this paper, we experimentally demonstrate a non-Hermitian open PT-symmetric terahertz metasurface comprising complementary plasmonic structures capable of exhibiting an exceptional point (EP). The metasurface consists of two resonators of different sizes, representing effective gain and loss elements, placed orthogonally in close proximity to realize a non-Hermitian configuration leading to a PT symmetry state. A diagonal displacement of one resonator within this strongly coupled near-field configuration leads to the emergence of an exceptional point, where the system undergoes a sudden phase transition from a PT symmetric to a PT-asymmetric state. Terahertz time-domain spectroscopy (THz-TDS) is performed on the fabricated samples to experimentally validate the phase transition observed in numerical simulations. We employ coupled mode theory (CMT) to analyse and distinguish between the PT-symmetric, exceptional point, and PT-asymmetric states. This theoretical framework enables the calculation of eigenvalues, phase spectra, and eigenmodes associated with the metamaterial design, thereby corroborating the simulation results. Furthermore, we construct Poincare sphere to visualize the orientation of the polarization states of the eigenmodes, which further indicates the presence of the exceptional point. This comprehensive study of exceptional point in a plasmonic system holds potential for the development of practical, highly sensitive terahertz devices, addressing limitations of conventional PT-symmetric systems that rely on traditional gain and loss media. △ Less

Submitted 22 June, 2025; originally announced June 2025.

Comments: total 14 pages and 7 figures

MSC Class: 81Q80; 78M25

Quantitative agreement between experiment and theory for Vibrational Circular Dichroism enhanced by electronically excited states

Authors: Mariia Sapova, Chandan Kumar, Sahar Ashtari-Jafari, Wybren J. Buma, Lucas Visscher

Abstract: Intensity enhancement in vibrational circular dichroism (VCD) arises in open-shell transition metal complexes from coupling between ground-state vibrational transitions and magnetic dipole-allowed transitions to low-lying excited states (LLESs). In this work we apply Nafie's vibronic coupling theory to M(II)-(-)-sparteine-Cl$_2$ (M=Zn, Co, Ni) complexes to investigate these enhancement effects. We… ▽ More Intensity enhancement in vibrational circular dichroism (VCD) arises in open-shell transition metal complexes from coupling between ground-state vibrational transitions and magnetic dipole-allowed transitions to low-lying excited states (LLESs). In this work we apply Nafie's vibronic coupling theory to M(II)-(-)-sparteine-Cl$_2$ (M=Zn, Co, Ni) complexes to investigate these enhancement effects. We show that the VCD intensity is extremely sensitive to the excitation energies that neither time-dependent density functional theory (TDDFT) nor state-averaged complete active space self consistent field (SA-CASSCF) calculations can predict with sufficient accuracy. We argue that instead of using more accurate quantum chemistry methods these excitation energies can be treated as parameters and optimized against experimental spectra. With this approach we obtain simulated VCD similarity scores above 0.4, a threshold considered reliable for absolute configuration assignment. The ability to quantitatively reproduce enhanced experimental spectra with computations opens up new research areas, offering amongst else unique possibilities for the study of chiral structure of systems such as transition metal complexes and metalloproteins that so far remained intractable. △ Less

Submitted 7 July, 2025; v1 submitted 19 June, 2025; originally announced June 2025.

High Breakdown Electric Field (> 5 MV/cm) in UWBG AlGaN Transistors

Authors: Seungheon Shin, Hridibrata Pal, Jon Pratt, John Niroula, Yinxuan Zhu, Chandan Joishi, Brianna A. Klein, Andrew Armstrong, Andrew A. Allerman, Tomás Palacios, Siddharth Rajan

Abstract: We report on the design and demonstration of ultra-wide bandgap (UWBG) AlGaN-channel metal-insulator heterostructure field effect transistors (HEFTs) for high-power, high-frequency applications. We find that the integration of gate dielectrics and field plates greatly improves the breakdown field in these devices, with state-of-art average breakdown field of 5.3 MV/cm (breakdown voltage > 260 V) w… ▽ More We report on the design and demonstration of ultra-wide bandgap (UWBG) AlGaN-channel metal-insulator heterostructure field effect transistors (HEFTs) for high-power, high-frequency applications. We find that the integration of gate dielectrics and field plates greatly improves the breakdown field in these devices, with state-of-art average breakdown field of 5.3 MV/cm (breakdown voltage > 260 V) with an associated maximum current density of 342 mA/mm, and cut-off frequency of 9.1 GHz. Furthermore, low trap-related impact was observed from minimal gate and drain lag estimated from pulsed I-V characteristics. The reported results provide the potential of UWBG AlGaN HEFTs for the next generation high-power radio frequency applications. △ Less

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

Comments: 14 pages, 10 figures

Energy Bands and Breakdown Characteristics in Al2O3/UWBG AlGaN Heterostructures

Authors: Seungheon Shin, Kyle Liddy, Yinxuan Zhu, Chandan Joishi, Brianna A. Klein, Andrew Armstrong, Andrew A. Allerman, Siddharth Rajan

Abstract: We report on energy bands and breakdown characteristics of Al2O3 dielectrics on ultra-wide bandgap (UWBG) AlGaN heterostructures. Metal-dielectric-semiconductor structures are important to sustain high fields needed for future high-performance UWBG transistors. Using systematic experiments, we determined the fixed charge density (> 1013 cm-2), the dielectric/interface, and electric fields in the o… ▽ More We report on energy bands and breakdown characteristics of Al2O3 dielectrics on ultra-wide bandgap (UWBG) AlGaN heterostructures. Metal-dielectric-semiconductor structures are important to sustain high fields needed for future high-performance UWBG transistors. Using systematic experiments, we determined the fixed charge density (> 1013 cm-2), the dielectric/interface, and electric fields in the oxide of under flat-band conditions in the semiconductor. Low gate-to-drain leakage current of up to 5 x 10-7 A/cm2 were obtained in the metal-oxide-semiconductor structures. In lateral metal-semiconductor-insulator test structures, breakdown voltage exceeding 1 kV was obtained with a channel sheet charge density of 1.27 x 1013 cm-2. The effective peak electric field and average breakdown field were estimated to be > 4.27 MV/cm and 1.99 MV/cm, respectively. These findings demonstrate the potential of Al2O2 integration for enhancing the breakdown performance of UWBG AlGaN HEMTs. △ Less

Submitted 17 April, 2025; v1 submitted 1 April, 2025; originally announced April 2025.

Comments: 12 pages, 7 figures, and 3 tables

Trion Engineered Multimodal Transistors in Two dimensional Bilayer Semiconductor Lateral Heterostructures

Authors: Baisali Kundu, Poulomi Chakrabarty, Avijit Dhara, Roberto Rosati, Chandan Samanta, Suman K. Chakraborty, Srilagna Sahoo, Sajal Dhara, Saroj P. Dash, Ermin Malic, Saurabh Lodha, Prasana K. Sahoo

Abstract: Multimodal device operations are essential to advancing the integration of 2D semiconductors in electronics, photonics, information and quantum technology. Precise control over carrier dynamics, particularly exciton generation and transport, is crucial for finetuning the functionality of optoelectronic devices based on 2D semiconductor heterostructure. However, the traditional exciton engineering… ▽ More Multimodal device operations are essential to advancing the integration of 2D semiconductors in electronics, photonics, information and quantum technology. Precise control over carrier dynamics, particularly exciton generation and transport, is crucial for finetuning the functionality of optoelectronic devices based on 2D semiconductor heterostructure. However, the traditional exciton engineering methods in 2D semiconductors are mainly restricted to the artificially assembled vertical pn heterostructures with electrical or strain induced confinements. In this study, we utilized bilayer 2D lateral npn multijunction heterostructures with intrinsically spatially separated energy landscapes to achieve preferential exciton generation and manipulation without external confinement. In lateral npn FET geometry, we uncover unique and nontrivial properties, including dynamic tuning of channel photoresponsivity from positive to negative. The multimodal operation of these 2D FETs is achieved by carefully adjusting electrical bias and the impinging photon energy, enabling precise control over the trions generation and transport. Cryogenic photoluminescence measurement revealed the presence of trions in bilayer MoSe2 and intrinsic trap states in WSe2. Measurements in different FET device geometries show the multifunctionality of 2D lateral heterostructure phototransistors for efficient tuning and electrical manipulation of excitonic characteristics. Our findings pave the way for developing practical exciton-based transistors, sensors, multimodal optoelectronic and quantum technologies △ Less

Submitted 2 November, 2024; originally announced November 2024.

Report number: 2026

Journal ref: Adv. Funct. Mater. 36, no. 26 (2026): e17486

Growing length and time scales in activity-mediated glassy dynamics in confluent cell monolayers

Authors: Souvik Sadhukhan, Chandan Dasgupta, Saroj Kumar Nandi

Abstract: Activity-mediated unjamming of a confluent glassy system is crucial for several biological processes, such as embryogenesis and cancer metastasis. During these processes, the cells progressively change their junction properties, characterized by an interaction parameter $p_0$, and become motile. Here, we study the effect of nonequilibrium active fluctuations, in the form of self-propulsion, on the… ▽ More Activity-mediated unjamming of a confluent glassy system is crucial for several biological processes, such as embryogenesis and cancer metastasis. During these processes, the cells progressively change their junction properties, characterized by an interaction parameter $p_0$, and become motile. Here, we study the effect of nonequilibrium active fluctuations, in the form of self-propulsion, on the glassy dynamics in a confluent system. We simulate the active Vertex model and use the analytical mode-coupling theory (MCT) to show that the nature of the transition in the presence of activity remains similar to that in a thermal system where the fluctuations are temperature-like. The agreement of the simulation results with the MCT predictions demonstrates that the structure-dynamics feedback mechanism controls the relaxation dynamics. In addition, we present the first computation of a dynamic length scale, $ξ_d$, in confluent systems using finite-size scaling, and show that the growing relaxation time exhibita a power-law dependence on $ξ_d$. Furthermore, unlike particulate glasses, the static length that governs the finite-size scaling of the relaxation time is proportional to $ξ_d$, revealing the unique nature of the glassy dynamics in confluent systems. △ Less

Submitted 9 May, 2025; v1 submitted 26 September, 2024; originally announced September 2024.

Comments: 9 pages, 4 figures

Characterisation of Front-End Electronics of ChaSTE experiment onboard Chandayaan-3 lander

Authors: K. Durga Prasad, Chandan Kumar, Sanjeev K. Mishra, P. Kalyana S. Reddy, Janmejay Kumar, Tinkal Ladiya, Arpit Patel, Anil Bhardwaj

Abstract: Chandra Surface Thermophysical Experiment (ChaSTE) is one of the payloads flown onboard the Chandrayaan-3 lander. The objective of the experiment is in-situ investigation of thermal behaviour of outermost 100 mm layer of the lunar surface by deploying a thermal probe. The probe consists of 10 temperature sensors (Platinum RTDs) mounted at different locations along the length of the probe to measur… ▽ More Chandra Surface Thermophysical Experiment (ChaSTE) is one of the payloads flown onboard the Chandrayaan-3 lander. The objective of the experiment is in-situ investigation of thermal behaviour of outermost 100 mm layer of the lunar surface by deploying a thermal probe. The probe consists of 10 temperature sensors (Platinum RTDs) mounted at different locations along the length of the probe to measure lunar soil temperatures as a function of depth. A heater is also mounted on the probe for thermal conductivity measurements. The onboard electronics of ChaSTE has two parts, Front-End Electronics (FEE) and processing electronics (PE). The front-end electronics (FEE) card is responsible for carrying out necessary sensor signal conditioning,which includes exciting the RTD sensors,acquiring analog voltages and then converting the acquired analog signals to digital signals using an Analog to Digital Converter(ADC). The front-end card is further interfaced with the processing electronics card for digital processing and spacecraft interface.The calibration, characterisation and functional test activities of Front-End Electronics of ChaSTE were carried out with the objective of testing and ensuring proper functionality and performance.A two phase calibration process involving electronic offset correction and temperature calibration were carried out. All these activities were successfully completed and the results from them provided us with a really good understanding of the behaviour of the FEE under different thermal and electrical conditions as well as when subjected to the simulated conditions of the actual ChaSTE experiment. The performance of the ChaSTE front-end electronics was very much within the design margins and its behaviour in simulated lunar environment was as desired. The data from these activities is useful in the interpretation of the actual science data of ChaSTE. △ Less

Submitted 30 August, 2024; originally announced September 2024.

Comments: 15 pages, 14 figures

Journal ref: Journal of Spacecraft Technology, 34(2), July-Dec. 2023, Publisher: U.R.Rao.Satellite Centre, ISRO, Bangalore, ISSN: 0971-1600

Selective-injection GaN Heterojunction Bipolar Transistors with 275 kA/cm$^2$ Current Density

Authors: Zhanbo Xia, Chandan Joishi, Shahadat H. Sohel, Andy Xie, Edward Beam, Yu Cao, Siddharth Rajan

Abstract: We design and demonstrate selective injection GaN heterojunction bipolar transistors that utilize a patterned base for selective injection of electrons from the emitter. The design maneuvers minority carrier injection through a thin p-GaN base region, while the majority carrier holes for base current are injected from thick p-GaN regions adjacent to the thin p-GaN base. The design is realized usin… ▽ More We design and demonstrate selective injection GaN heterojunction bipolar transistors that utilize a patterned base for selective injection of electrons from the emitter. The design maneuvers minority carrier injection through a thin p-GaN base region, while the majority carrier holes for base current are injected from thick p-GaN regions adjacent to the thin p-GaN base. The design is realized using a regrowth emitter approach with SiO$_2$ as a spacer between the emitter layer and the thick p-GaN base contact regions. The fabricated device demonstrated state-of-art output current density (I$_{C, max}$) ~275 kA/cm$^2$ with a current gain ($β$) of 9, and 17 for the planar HBT design (I$_{C, max}$ =150 kA/cm$^2$). The reported results highlight the potential of the selective injection design to overcome the persistent GaN HBT design tradeoff between base resistance and current gain, paving the way for next-generation radio frequency and mm-Wave applications. △ Less

Submitted 26 August, 2024; originally announced August 2024.

Ultrahigh pyroelectricity in monoelemental 2D tellurium

Authors: Hari Krishna Mishra, Ayushi Jain, Dalip Saini, Bidya Mondal, Chandan Bera, Shanker Ram, Dipankar Mandal

Abstract: We report an ultrahigh pyroelectric response in van der Waals bonded layers of two-dimensional (2D) tellurium (Te) nanosheets (thickness, d = 4 to 5 nm) at periodic on-off temperature oscillations. For the first time a large pyroelectric coefficient, Pc ~ 3 mC.m-2.K-1, is observed which is eightfold higher than the traditional state-of-the-art pyroelectrics (lead zirconate titanate, PZT). The firs… ▽ More We report an ultrahigh pyroelectric response in van der Waals bonded layers of two-dimensional (2D) tellurium (Te) nanosheets (thickness, d = 4 to 5 nm) at periodic on-off temperature oscillations. For the first time a large pyroelectric coefficient, Pc ~ 3 mC.m-2.K-1, is observed which is eightfold higher than the traditional state-of-the-art pyroelectrics (lead zirconate titanate, PZT). The first-principles calculations point out that the breakdown of centro-symmetry in the 1-3 Te-layers (P-3m1 space group) of a non-centrosymmetry (higher-order symmetry of C2 space group) on an angular twist in the Te-Te bonds of an exotic electronic state in 2D Te. The angular Te-Te twisting elicits a surface-enhanced Raman band at 101 cm-1 (absent in bulk Te). The stimulation of the Born effective charge, in-plane piezoelectricity and thermal expansion coefficient are shown to tailor the large pyroelectricity. Thus, 2D Te nanosheets present a new paradigm for the wide application of pyroelectric materials for developing thermal energy-based flexible electronics. △ Less

Submitted 26 July, 2024; originally announced July 2024.

Comments: Research Article

Anisotropic Cage Evolution in Quasi-two-dimensional Colloidal Fluids

Authors: Noman Hanif Barbhuiya, Chandan K. Mishra

Abstract: We experimentally explore the morphological evolution of cages in quasi-two-dimensional suspensions of colloidal fluids, uncovering a complex dynamic restructuring in the fluid. Although cages display isotropic evolution in the laboratory frame, we observe a striking anisotropy when analyzed in the displacement frame of the caged particles. Moreover, our findings reveal that particles in specific… ▽ More We experimentally explore the morphological evolution of cages in quasi-two-dimensional suspensions of colloidal fluids, uncovering a complex dynamic restructuring in the fluid. Although cages display isotropic evolution in the laboratory frame, we observe a striking anisotropy when analyzed in the displacement frame of the caged particles. Moreover, our findings reveal that particles in specific but distinct regions of the cage predominantly contribute to either its persistence or relaxation. Thus, our study provides a coarse-grained microscopic picture of the structural relaxation of these fluids through cage evolution, which has broader implications for the flow and phase behavior of complex fluids in confined geometry. △ Less

Submitted 10 August, 2024; v1 submitted 25 July, 2024; originally announced July 2024.

Comments: 3 figures

Shear-Layer Perturbation Responses from Time-Resolved Schlieren Data

Authors: Spencer L. Stahl, Chandan Kumar, Datta V. Gaitonde

Abstract: A novel combination of physics-based and data-driven post-processing techniques is proposed to extract acoustic-related shear-layer perturbation responses directly from spatio-temporally resolved schlieren video. The physics-based component is derived from a momentum potential theory extension that extracts irrotational (acoustic and thermal) information from density gradients embedded in schliere… ▽ More A novel combination of physics-based and data-driven post-processing techniques is proposed to extract acoustic-related shear-layer perturbation responses directly from spatio-temporally resolved schlieren video. The physics-based component is derived from a momentum potential theory extension that extracts irrotational (acoustic and thermal) information from density gradients embedded in schlieren pixel intensities. For the unheated shear layer, the method spotlights acoustic structures and tones otherwise hidden. The filtered data is then subjected to a data-driven Dynamic Mode Decomposition Reduced Order Model (DMD-ROM), which provides the response to forced perturbations. This method applies a learned linear model to isolate and quantify growth rates of acoustic phenomena suited for efficient parametric studies. A shear-layer comprised of two streams at Mach 2.461 and 0.175, corresponding to a convective Mach number 0.88 and containing shocks, is adopted for illustration. The overall perturbation response is first obtained using an impulse forcing in the wall normal direction of the splitter plate, extending in both subsonic and supersonic streams. Subsequently, impulse and harmonic forcings are independently applied in a local pixel-by-pixel manner for a precise receptivity study. The acoustic response shows a convective wavepacket and an acoustic burst from the splitter plate. The interaction with the primary shock and associated wave dispersion emits a second, slower, acoustic wave. Harmonic forcing indicates higher frequency-dependent sensitivity in the supersonic stream, with the most sensitive location near the outer boundary layer region. Excitation here yields an order of magnitude larger acoustic response compared to disturbances in the subsonic stream. Some receptive forcing inputs do not generate significant acoustic waves, which may guide excitation with low noise impact. △ Less

Submitted 7 July, 2024; originally announced July 2024.

Comments: 27 pages

Journal ref: J. Fluid Mech. 1007 (2025) A77

Effect of sweep angle on three-dimensional vortex dynamics over plunging wings

Authors: Alex Cavanagh, Chandan Bose, Kiran Ramesh

Abstract: The effects of sweep angle and reduced frequency on the leading-edge vortex (LEV) structure over flapping swept wings in the Reynolds number ($Re$) range of $\mathbf{O}(10^4)$ are yet to be completely understood. With increasing interest in designing bio-inspired micro-air-vehicles (MAVs), understanding LEV dynamics in such scenarios is imperative. This study investigates the effects of three diff… ▽ More The effects of sweep angle and reduced frequency on the leading-edge vortex (LEV) structure over flapping swept wings in the Reynolds number ($Re$) range of $\mathbf{O}(10^4)$ are yet to be completely understood. With increasing interest in designing bio-inspired micro-air-vehicles (MAVs), understanding LEV dynamics in such scenarios is imperative. This study investigates the effects of three different sweep angles ($Λ= 0^\circ$, $30^\circ$ and $60^\circ$) on LEV dynamics through high-fidelity improved delayed detached eddy simulation (IDDES) to analyze the underlying flow physics. Plunge ramp kinematics at two different reduced frequencies ($k = 0.05$ and $0.4$) are studied to investigate the unsteady motion effects on LEV characteristics. The leading-edge suction parameter (LESP) concept is applied to determine LEV initiation, and the results are verified against flow field visualization for swept-wing geometries. The force partitioning method (FPM) is used to investigate the spanwise lift distribution resulting from the LEV. Distinct peaks in the lift coefficient occur for the high reduced frequency case due to the impulse-like plunging acceleration. This causes the LEV to detach from the leading edge more quickly and convect faster, significantly affecting the lift generated by the wing. As reduced frequency increases, the LEV breakdown mechanism switches from vortex bursting to LEV leg-induced instabilities. These results provide insights into the complex vortex structures surrounding swept wings at $Re = 20,000$, and the impact both sweep angle and reduced frequency have on the lift contribution of these flow features. △ Less

Submitted 6 July, 2024; originally announced July 2024.

Comments: 31 pages, 13 figures

Effect of a fixed downstream cylinder on the flow-induced vibration of an elastically-supported primary cylinder

Authors: Junlei Wang, Shenfang Li, Daniil Yurchenko, Hongjun Zhu, Chandan Bose

Abstract: This paper numerically investigates the influence of a fixed downstream control cylinder on the flow-induced vibration of an elastically-supported primary cylinder. These two cylinders are situated in a tandem arrangement with small dimensionless centre-to-centre spacing ($L/D$, $L$ is the intermediate spacing, and $D$ is the cylinder diameter). The present two-dimensional (2D) simulations are car… ▽ More This paper numerically investigates the influence of a fixed downstream control cylinder on the flow-induced vibration of an elastically-supported primary cylinder. These two cylinders are situated in a tandem arrangement with small dimensionless centre-to-centre spacing ($L/D$, $L$ is the intermediate spacing, and $D$ is the cylinder diameter). The present two-dimensional (2D) simulations are carried out in the low Reynolds number ($Re$) regime. The primary focus of this study is to reveal the underlying flow physics behind the transition from vortex-induced vibration to galloping in the response of the primary cylinder due to the presence of another fixed downstream cylinder. Two distinct flow field regimes, namely steady flow and alternate attachment regimes, are observed for different $L/D$ and Re values. Depending on the evolution of the near-field flow structures, four different wake patterns - `2S', `2P', `2C', and `aperiodic' - are observed. The corresponding vibration response of the upstream cylinder is characterized as interference galloping and extended vortex-induced vibration. As the $L/D$ ratio increases, the lift enhancement due to flow-induced vibration is seen to be weakened. The detailed correlation between the force generation and the near-wake interactions is investigated. The present findings will augment the understanding of vibration reduction or flow-induced energy harvesting of tandem cylindrical structures. △ Less

Submitted 14 May, 2024; originally announced May 2024.

Effect of structural parameters on the synchronization characteristics in a stall-induced aeroelastic system

Authors: Dheeraj Tripathi, Chandan Bose, Sirshendu Mondal, J Venkatramani

Abstract: This study focuses on discerning the role of structural parameters on the bifurcation characteristics and the underlying synchronization mechanism in an aeroelastic system undergoing nonlinear stall behaviour. To that end, wind tunnel experiments are performed on a NACA 0012 airfoil capable of undergoing bending (plunging) and torsional (pitching) oscillations under scenarios involving nonlinear a… ▽ More This study focuses on discerning the role of structural parameters on the bifurcation characteristics and the underlying synchronization mechanism in an aeroelastic system undergoing nonlinear stall behaviour. To that end, wind tunnel experiments are performed on a NACA 0012 airfoil capable of undergoing bending (plunging) and torsional (pitching) oscillations under scenarios involving nonlinear aerodynamic loads, i.e., dynamic stall conditions. Flow conditions under both deterministic/sterile flows and fluctuating/stochastic flows are fostered. The structure possesses continuous or polynomial-type stiffness nonlinearities, and therefore, is an aeroelastic experiment involving both structural and aerodynamic nonlinearities. We discern the bifurcation routes for a range of key structural parameters such as frequency ratio, static imbalance, and the extent of structural nonlinearity. In addition to interesting and atypical routes to stall-induced instabilities, we systematically demonstrate the role of modal interactions - via a synchronization analysis - over the manifestation of these instabilities. To the best of the authors' knowledge, this is perhaps the first study to document the role of multiple structural parameters on a stall-induced aeroelastic system, and in turn, cast the physical mechanism behind these dynamical transitions from the vantage of synchronization. △ Less

Submitted 22 April, 2024; originally announced April 2024.

Tunnel Junction-Enabled Monolithically Integrated GaN Micro-Light Emitting Transistor

Authors: Sheikh Ifatur Rahman, Mohammad Awwad, Chandan Joishi, Zane-Jamal Eddine, Brendan Gunning, Andrew Armstrong, Siddharth Rajan

Abstract: GaN/InGaN microLEDs are a very promising technology for next generation displays. Switching control transistors and their integration are key components in achieving high-performance, efficient displays. Monolithic integration of microLEDs with GaN switching devices provides an opportunity to control microLED output power with capacitive (voltage) control rather than current controlled schemes. Th… ▽ More GaN/InGaN microLEDs are a very promising technology for next generation displays. Switching control transistors and their integration are key components in achieving high-performance, efficient displays. Monolithic integration of microLEDs with GaN switching devices provides an opportunity to control microLED output power with capacitive (voltage) control rather than current controlled schemes. This approach can greatly reduce system complexity for the driver circuit arrays while maintaining device opto-electronic performance. In this work, we demonstrate a 3-terminal GaN micro-light emitting transistor that combines a GaN/InGaN blue tunneling-based microLED with a GaN n-channel FET. The integrated device exhibits excellent gate control, drain current control and optical emission control. This work provides a promising pathway for future monolithic integration of GaN FETs with microLED to enable fast switching high efficiency microLED display and communication systems. △ Less

Submitted 29 May, 2024; v1 submitted 7 April, 2024; originally announced April 2024.

Design and Simulation of a III-Nitride Light Emitting Transistor

Authors: Mohammad Awwad, Sheikh Ifatur Rahman, Chandan Joishi, Betty Lise Anderson, Siddharth Rajan

Abstract: This paper describes the design and characteristics of monolithically integrated three-terminal gated III-Nitride light emitting diodes (LEDs) devices. The impact of channel doping and thickness on the voltage penalty of the transistor-LED hybrid device is analyzed, and it is shown that with appropriate design, low voltage drop can be realized across integrated gated LED structures. The impact of… ▽ More This paper describes the design and characteristics of monolithically integrated three-terminal gated III-Nitride light emitting diodes (LEDs) devices. The impact of channel doping and thickness on the voltage penalty of the transistor-LED hybrid device is analyzed, and it is shown that with appropriate design, low voltage drop can be realized across integrated gated LED structures. The impact of device design on the switching charge is investigated, and it is shown that the adoption of an integrated LED/transistor structure can reduce the switching charge necessary for operation of a switched LED display device by an order of magnitude when compared with stand-alone light-emitting diodes. △ Less

Submitted 7 April, 2024; originally announced April 2024.

Controlling the chaotic wake of a flapping foil by tuning its chordwise flexibility

Authors: Chhote Lal Shah, Dipanjan Majumdar, Chandan Bose, Sunetra Sarkar

Abstract: Effects of chord-wise flexibility as an instrument to control chaotic transitions in the wake of a flexible flapping foil have been studied here using an immersed boundary method-based in-house fluid-structure-interaction solver. The ability of the flapping foil at an optimum level of flexibility to inhibit chaotic transition, otherwise encountered in a similar but rigid configuration, has been hi… ▽ More Effects of chord-wise flexibility as an instrument to control chaotic transitions in the wake of a flexible flapping foil have been studied here using an immersed boundary method-based in-house fluid-structure-interaction solver. The ability of the flapping foil at an optimum level of flexibility to inhibit chaotic transition, otherwise encountered in a similar but rigid configuration, has been highlighted. The rigid foil manifests chaotic transition through a quasi-periodic-intermittency route at high dynamic plunge velocities; whereas, increasing the level of flexibility gradually regularises the aperiodic behaviour through a variety of interesting wake patterns. If flexibility is increased beyond an optimum level, aperiodicity sets in again and robust chaos is restored at very high flexibility levels. The mechanisms of triggering the order-to-chaos transition are different between the rigid and the high flexibility cases. Along the route to order and back to chaos, the flexible foil exhibits different flow-field behaviours, including far-wake switching, primary \& secondary vortex streets, bifurcated wakes and interactive vortices between the bifurcated wakes. The underlying interaction mechanisms of the flow-field vortices responsible for the associated dynamical signatures of the wake have been closely tracked. This study further examines the optimum propulsive performance range of the flexible flapper and investigates its connection with the periodicity/regularity of the system. △ Less

Submitted 26 March, 2024; originally announced March 2024.

Motility driven glassy dynamics in confluent epithelial monolayers

Authors: Souvik Sadhukhan, Manoj Kumar Nandi, Satyam Pandey, Matteo Paoluzzi, Chandan Dasgupta, Nir Gov, Saroj Kumar Nandi

Abstract: As wounds heal, embryos develop, cancer spreads, or asthma progresses, the cellular monolayer undergoes glass transition between solid-like jammed and fluid-like flowing states. During some of these processes, the cells undergo an epithelial-to-mesenchymal transition (EMT): they acquire in-plane polarity and become motile. Thus, how motility drives the glassy dynamics in epithelial systems is crit… ▽ More As wounds heal, embryos develop, cancer spreads, or asthma progresses, the cellular monolayer undergoes glass transition between solid-like jammed and fluid-like flowing states. During some of these processes, the cells undergo an epithelial-to-mesenchymal transition (EMT): they acquire in-plane polarity and become motile. Thus, how motility drives the glassy dynamics in epithelial systems is critical for the EMT process. However, no analytical framework that is indispensable for deeper insights exists. Here, we develop such a theory inspired by a well-known glass theory. One crucial result of this work is that the confluency affects the effective persistence time-scale of active force, described by its rotational diffusivity, $D_r^{\text{eff}}$. $D_r^{\text{eff}}$ differs from the bare rotational diffusivity, $D_r$, of the motile force due to cell shape dynamics, which acts to rectify the force dynamics: $D_r^{\text{eff}}$ is equal to $D_r$ when $D_r$ is small and saturates when $D_r$ is large. We test the theoretical prediction of $D_r^{\text{eff}}$ and how it affects the relaxation dynamics in our simulations of active Vertex model. This novel effect of $D_r^{\text{eff}}$ is crucial to understanding the new and previously published simulation data of active glassy dynamics in epithelial monolayers. △ Less

Submitted 12 August, 2024; v1 submitted 13 March, 2024; originally announced March 2024.

Comments: 15 pages, 6 figures. It is published in Soft Matter

Journal ref: Soft Matter, 20 (2024), 6160-6175

Selective injection AlGaN/GaN heterojunction bipolar transistors with patterned regrown base contacts

Authors: Chandan Joishi, Sheikh Ifatur Rahman, Zhanbo Xia, Shahadat H. Sohel, Siddharth Rajan

Abstract: We demonstrate graded AlGaN/GaN heterojunction bipolar transistors (HBTs) with selective injection of minority carriers across a p-GaN base and patterned regrown base contacts. The selective injection design regulates minority carrier transport under emitter-base forward bias through a thin base region, while thick and highly doped p$^+$ GaN regrown layers patterned alongside the thin base regions… ▽ More We demonstrate graded AlGaN/GaN heterojunction bipolar transistors (HBTs) with selective injection of minority carriers across a p-GaN base and patterned regrown base contacts. The selective injection design regulates minority carrier transport under emitter-base forward bias through a thin base region, while thick and highly doped p$^+$ GaN regrown layers patterned alongside the thin base regions are utilized to lower the base contact resistance. With SiO$_2$ employed as a spacer between the emitter and the p$^+$ regrown layers, the device with an interdigitated emitter/base-contact stripe design displayed a maximum collector current density (I$_C$) of 101 kA/cm$^2$, a maximum current gain ($β$) of 70 at I$_C$ $\sim$ 1 kA/cm$^2$ and $\sim$ 11 for I$_C$ $>$ 50 kA/cm$^2$. The reported results demonstrate the potential of the selective injection approach to break the long-existing HBT design tradeoff between base resistance and current gain for next-generation radio frequency and mm-Wave applications. △ Less

Submitted 18 September, 2023; originally announced September 2023.

Platinum-based Catalysts for Oxygen Reduction Reaction simulated with a Quantum Computer

Authors: Cono Di Paola, Evgeny Plekhanov, Michal Krompiec, Chandan Kumar, Emanuele Marsili, Fengmin Du, Daniel Weber, Jasper Simon Krauser, Elvira Shishenina, David Muñoz Ramo

Abstract: Hydrogen has emerged as a promising energy source, holding the key to achieve low-carbon and sustainable mobility. However, its applications are still limited by modest conversion efficiency in the electrocatalytic oxygen reduction reaction (ORR) within fuel cells. Consequently, the development of novel catalysts and a profound understanding of the underlying reactions have become of paramount imp… ▽ More Hydrogen has emerged as a promising energy source, holding the key to achieve low-carbon and sustainable mobility. However, its applications are still limited by modest conversion efficiency in the electrocatalytic oxygen reduction reaction (ORR) within fuel cells. Consequently, the development of novel catalysts and a profound understanding of the underlying reactions have become of paramount importance. The complex nature of the ORR potential energy landscape and the presence of strong electronic correlations present challenges to atomistic modelling using classical computers. This scenario opens new avenues for the implementation of novel quantum computing workflows to address these molecular systems. Here, we present a pioneering study that combines classical and quantum computational approaches to investigate the ORR on pure platinum and platinum/cobalt surfaces. Our research demonstrates, for the first time, the feasibility of implementing this workflow on the H1-series trapped-ion quantum computer and identify the challenges of the quantum chemistry modelling of this reaction. The results highlight the involvement of strongly correlated species in the cobalt-containing catalyst, suggesting their potential as ideal candidates for showcasing quantum advantage in future applications. △ Less

Submitted 12 April, 2024; v1 submitted 28 July, 2023; originally announced July 2023.

Comments: Main paper: 29 pages, 4 figures, Supplementary Material: 10 pages, 5 figures, 5 tables

Effect of transverse gust on free-falling plates

Authors: Jawahar Sivabharathy Samuthira Pandi, Ahmet Gungor, Chandan Bose, Antonio Attili, Ignazio Maria Viola

Abstract: The effects of transverse gusts on free-falling plates are investigated using two-way coupled fluid-structure interaction simulations for a Galilei number (Ga) between 10 and 50 and a density ratio (rho) between 5 and 50. We consider gust ratios (GR) of up to 5, where GR is the ratio of the free-stream velocity change to an estimate of the terminal velocity. We demonstrate that the plate experienc… ▽ More The effects of transverse gusts on free-falling plates are investigated using two-way coupled fluid-structure interaction simulations for a Galilei number (Ga) between 10 and 50 and a density ratio (rho) between 5 and 50. We consider gust ratios (GR) of up to 5, where GR is the ratio of the free-stream velocity change to an estimate of the terminal velocity. We demonstrate that the plate experiences the gust as a transient horizontal force, which displaces it horizontally. This results in a transient reduction in the angle of attack, an increase in absolute velocity and the generation of circulation. The vertical component of the latter increases the upward aerodynamic force, slowing down the vertical descent of the plate. Furthermore, the plate's horizontal displacement with respect to its original wake results in a further transient increase in the upward aerodynamic force. The altitude gained by the plate in response to the gust is maximum for rho=15, and increases non-monotonically with Ga and GR. The non-monotonic trend is due to plate pitch: if the maximum pitch of the plate in response to the gust is close to vertical, the plate temporarily falls faster, losing some of the altitude it has gained. The present findings reveal an energy-harvesting mechanism that free-falling bodies can exploit to increase their time afloat. △ Less

Submitted 13 August, 2026; v1 submitted 9 April, 2023; originally announced April 2023.

Journal ref: Pandi JSS, Gungor A, Bose C, Attili A, Viola IM. Effect of transverse gust on free-falling plates. Journal of Fluid Mechanics. 2026;1040:A23

Porous plates at incidence

Authors: Chandan Bose, Callum Bruce, Ignazio Maria Viola

Abstract: This paper investigates the effect of permeability on two-dimensional rectangular plates at incidences. The flow topology is investigated for Reynolds number ($Re$) values between 30 and 90, and the forces on the plate are discussed for $Re=30$, where the wake is found to be steady for any value of the Darcy number ($Da$) and the flow incidence ($α$). At $Re=30$, for a plate normal to the stream a… ▽ More This paper investigates the effect of permeability on two-dimensional rectangular plates at incidences. The flow topology is investigated for Reynolds number ($Re$) values between 30 and 90, and the forces on the plate are discussed for $Re=30$, where the wake is found to be steady for any value of the Darcy number ($Da$) and the flow incidence ($α$). At $Re=30$, for a plate normal to the stream and vanishing $Da$, the wake shows a vortex dipole with a stagnation point on the plate surface. With increasing $Da$, the separation between the vortex dipole and the plate increases; the vortex dipole shortens and is eventually annihilated at a critical $Da$. For any value of $Da$ below the critical one, the vortex dipole disappears with decreasing $α$. However, at low $Da$, the two saddle-node pairs merge at the same $α$, annihilating the dipole; while at high $Da$, they merge at different $α$, resulting in a single recirculating region for intermediate incidences. The magnitudes of lift, drag, and torque decrease with $Da$. Nevertheless, there exists a range of $Da$ and $α$, where the magnitude of the plate-wise force component increases with $Da$, driven by the shear on the plate's pressure side. Finally, the analysis of the fluid impulse suggests that the lift and drag reduction with $Da$ are associated with the weakening of the leading and trailing edge shear layer, respectively. The present findings will be directly beneficial in understanding the role of permeability on small porous wings. △ Less

Submitted 13 August, 2026; v1 submitted 23 March, 2023; originally announced March 2023.

Comments: 13 pages, 9 figures, 4 tables

Journal ref: Bose, C., Bruce, C. & Viola, I.M. Porous plates at incidence. Theor. Comput. Fluid Dyn. 39, 19 (2025)

$β$-Ga$_2$O$_3$ Trench Schottky Diodes by Novel Low-Damage Ga-Flux Etching

Authors: Sushovan Dhara, Nidhin Kurian Kalarickal, Ashok Dheenan, Sheikh Ifatur Rahman, Chandan Joishi, Siddharth Rajan

Abstract: $β$-Ga$_2$O$_3… ▽ More $β$-Ga$_2$O$_3$ trench Schottky barrier diodes fabricated through a Gallium atomic beam etching technique, with excellent field strength and power device figure of merit, are demonstrated. Trench formation was accomplished by a low-damage Ga flux etch that enables near-ideal forward operating characteristics that are independent of fin orientation. The reverse breakdown field strength of greater than 5.10 MV/cm is demonstrated at breakdown voltage as of 1.45 kV. This result demonstrates the potential for Ga atomic beam etching and high-quality dielectric layers for improved performance in $β$-Ga$_2$O$_3$ vertical power devices. △ Less

Submitted 8 March, 2023; originally announced March 2023.

Comments: 10 pages, 5 figures

Room temperature energy-efficient spin-orbit torque switching in wafer-scale all-vdW heterostructure

Authors: Haiyu Wang, Hao Wu, Yingjie Liu, Dongdong Chen, Chandan Pandey, Jialiang Yin, Dahai Wei, Na Lei, Jie Zhang, Haichang Lu, Shuyuan Shi, Peng Li, Albert Fert, Kang L. Wang, Tianxiao Nie, Weisheng Zhao

Abstract: The emergent two-dimensional (2D) ferromagnetic materials with unique magnetic properties have endowed great potential for next-generation spintronic devices with extraordinary merits of high flexibility, easy controllability, and high heretointegrability, which is expected to promote the development of Moore's Law continuously. However, it is extremely challenging to realize magnetic switching wi… ▽ More The emergent two-dimensional (2D) ferromagnetic materials with unique magnetic properties have endowed great potential for next-generation spintronic devices with extraordinary merits of high flexibility, easy controllability, and high heretointegrability, which is expected to promote the development of Moore's Law continuously. However, it is extremely challenging to realize magnetic switching with ultra-low power consumption at room temperature. Here, we demonstrate the room-temperature spin-orbit torque (SOT) driven magnetization switching in a well-epitaxial all-van der Waals (vdW) heterostructure. The topological insulator Bi2Te3 not only helps to elevate the Curie temperature of Fe3GeTe2 (FGT) through interfacial exchange coupling but also works as a spin current source allowing to switch FGT at a low current density of 2.2 * 106 A cm2. A large SOT efficiency of 0.7 is measured at room temperature, and the thickness of FGT is further adjusted to reduce the influence of the thermal contribution on the second-harmonic signal. Furthermore, the temperature and thickness-dependent SOT efficiency prove that the large SOT in our system mainly originates from the nontrivial origin of topological materials. Our experiment has enabled an all-vdW SOT structure and lays a solid foundation for the implementation of room-temperature all-vdW spintronic devices in the future. △ Less

Submitted 28 November, 2021; originally announced November 2021.

Directional Message Passing on Molecular Graphs via Synthetic Coordinates

Authors: Johannes Gasteiger, Chandan Yeshwanth, Stephan Günnemann

Abstract: Graph neural networks that leverage coordinates via directional message passing have recently set the state of the art on multiple molecular property prediction tasks. However, they rely on atom position information that is often unavailable, and obtaining it is usually prohibitively expensive or even impossible. In this paper we propose synthetic coordinates that enable the use of advanced GNNs w… ▽ More Graph neural networks that leverage coordinates via directional message passing have recently set the state of the art on multiple molecular property prediction tasks. However, they rely on atom position information that is often unavailable, and obtaining it is usually prohibitively expensive or even impossible. In this paper we propose synthetic coordinates that enable the use of advanced GNNs without requiring the true molecular configuration. We propose two distances as synthetic coordinates: Distance bounds that specify the rough range of molecular configurations, and graph-based distances using a symmetric variant of personalized PageRank. To leverage both distance and angular information we propose a method of transforming normal graph neural networks into directional MPNNs. We show that with this transformation we can reduce the error of a normal graph neural network by 55% on the ZINC benchmark. We furthermore set the state of the art on ZINC and coordinate-free QM9 by incorporating synthetic coordinates in the SMP and DimeNet++ models. Our implementation is available online. △ Less

Submitted 5 April, 2022; v1 submitted 8 November, 2021; originally announced November 2021.

Comments: Published as a conference paper at NeurIPS 2021. Author name changed from Johannes Klicpera to Johannes Gasteiger

Nb$_{2}$O$_{5}$ high-k dielectric enabled electric field engineering of $β$-Ga$_{2}$O$_{3}$ metal-insulator-semiconductor (MIS) diode

Authors: Prabhans Tiwari, Jayeeta Biswas, Chandan Joishi, Saurabh Lodha

Abstract: We demonstrate an Nb$_{2}$O$_{5}$/$β$-Ga$_{2}$O$_{3}$ metal-insulator-semiconductor (MIS) hetero-junction diode with Nb$_{2}$O$_{5}$ as the high-k dielectric insulator for more efficient electric field management resulting in enhanced breakdown characteristics compared to a $β$-Ga$_{2}$O$_{3}$ Schottky barrier diode. The Nb$_{2}$O$_{5}$ dielectric films were grown using atomic layer deposition and… ▽ More We demonstrate an Nb$_{2}$O$_{5}$/$β$-Ga$_{2}$O$_{3}$ metal-insulator-semiconductor (MIS) hetero-junction diode with Nb$_{2}$O$_{5}$ as the high-k dielectric insulator for more efficient electric field management resulting in enhanced breakdown characteristics compared to a $β$-Ga$_{2}$O$_{3}$ Schottky barrier diode. The Nb$_{2}$O$_{5}$ dielectric films were grown using atomic layer deposition and exhibited a high dielectric constant of 50. The high dielectric constant resulted in a 5$\times$ lower electric field at the metal/dielectric interface in the MIS diode compared to the metal/$β$-Ga$_{2}$O$_{3}$ interface in the Schottky barrier diode. With good electron conduction in forward bias enabled by the negative conduction band offset of Nb$_{2}$O$_{5}$ w.r.t $β$-Ga$_{2}$O$_{3}$, the MIS design led to a 3$\times$ improvement in the reverse blocking voltage with a slight trade-off in the specific on-resistance. Overall, a 3.3$\times$ increase in the power figure of merit was observed (3.25 MW/cm$^2$ for the Schottky diode and 10.8 MW/cm$^2$ for the MIS diode). A detailed analysis of the energy band line-up, and the forward and reverse current transport mechanisms are also presented using analytical modeling and 2-D TCAD simulations. △ Less

Submitted 25 October, 2021; originally announced October 2021.

Comments: The following article has been submitted to 'Journal of Applied Physics' and the link will be updated if it gets published

Real-space nanoimaging of THz polaritons in the topological insulator Bi2Se3

Authors: Shu Chen, Andrei Bylinkin, Zhengtianye Wang, Martin Schnell, Greeshma Chandan, Peining Li, Alexey Y. Nikitin, Stephanie Law, Rainer Hillenbrand

Abstract: Plasmon polaritons in topological insulators attract attention from a fundamental perspective and for potential THz photonic applications. Although polaritons have been observed by THz far-field spectroscopy on topological insulator microstructures, real-space imaging of propagating THz polaritons has been elusive so far. Here, we show spectroscopic THz near-field images of thin Bi2Se3 layers (pro… ▽ More Plasmon polaritons in topological insulators attract attention from a fundamental perspective and for potential THz photonic applications. Although polaritons have been observed by THz far-field spectroscopy on topological insulator microstructures, real-space imaging of propagating THz polaritons has been elusive so far. Here, we show spectroscopic THz near-field images of thin Bi2Se3 layers (prototypical topological insulators) revealing polaritons with up to 12 times increased momenta as compared to photons of the same energy and decay times of about 0.48 ps, yet short propagation lengths. From the images we determine and analyze the polariton dispersion, showing that the polaritons can be explained by the coupling of THz radiation to various combinations of Dirac and massive carriers at the Bi2Se3 surfaces, massive bulk carriers and optical phonons. Our work provides critical insights into the nature of THz polaritons in topological insulators and establishes instrumentation and methodology for imaging of THz polaritons. △ Less

Submitted 24 March, 2022; v1 submitted 22 July, 2021; originally announced July 2021.

Journal ref: Nat Commun 13, 1374 (2022)

Sub-Rayleigh characterization of a binary source by spatially demultiplexed coherent detection

Authors: Chandan Datta, Yink Loong Len, Karol Łukanowski, Konrad Banaszek, Marcin Jarzyna

Abstract: We investigate theoretically coherent detection implemented simultaneously on a set of mutually orthogonal spatial modes in the image plane as a method to characterize properties of a composite thermal source below the Rayleigh limit. A general relation between the intensity distribution in the source plane and the covariance matrix for the complex field amplitudes measured in the image plane is d… ▽ More We investigate theoretically coherent detection implemented simultaneously on a set of mutually orthogonal spatial modes in the image plane as a method to characterize properties of a composite thermal source below the Rayleigh limit. A general relation between the intensity distribution in the source plane and the covariance matrix for the complex field amplitudes measured in the image plane is derived. An algorithm to estimate parameters of a two-dimensional symmetric binary source is devised and verified using Monte Carlo simulations to provide super-resolving capability for high ratio of signal to detection noise (SNR). Specifically, the separation between two point sources can be meaningfully determined down to $\textrm{SNR}^{-1/2}$ in the units determined by the spatial spread of the transfer function of the imaging system. The presented algorithm is shown to make a nearly optimal use of the measured data in the sub-Rayleigh region. △ Less

Submitted 6 October, 2021; v1 submitted 15 June, 2021; originally announced June 2021.

Comments: 10 pages, 3 figures. Accepted for publication in Optics Express

Journal ref: Optics Express 29, 35592-35601 (2021)

Studies on optical signal due to oxygen effect on hydrogenated amorphous/crystalline silicon thin-films

Authors: Meenakshi Rana, Chandan Banerjee, Papia Chowdhury

Abstract: We have studied the effects of oxygen on hydrogenated amorphous/crystalline silicon films in terms of their structural and optical properties. Different hydrogenated silicon oxide (SiO:H) and silicon (Si:H) films are fabricated between microcrystalline and amorphous transition region. X-ray diffraction, Raman, FTIR and UV-Vis emission spectrometry have been used to characterize different films. A… ▽ More We have studied the effects of oxygen on hydrogenated amorphous/crystalline silicon films in terms of their structural and optical properties. Different hydrogenated silicon oxide (SiO:H) and silicon (Si:H) films are fabricated between microcrystalline and amorphous transition region. X-ray diffraction, Raman, FTIR and UV-Vis emission spectrometry have been used to characterize different films. A comparison of the results with those of different types of films like hydrogenated amorphous silicon oxide (a-SiO:H), hydrogenated amorphous silicon (a-Si:H) and microcrystalline silicon ($μ$c-Si:H) films reveal their superiority as an excellent substance for solar cell. X-ray diffraction, FTIR and Raman spectral analysis show that difference of the H dilution effect has a major effect on the structure of the film and the optical properties. Photoluminescence analysis of amorphous silicon-oxygen and silicon-hydride alloy films has established their efficient application appropriate as Si based light emitting devices. A large optical band gap of 1.83 eV and appearance of strong photo luminescence at 2.0 eV validates the applicability of a-SiO:H film as a better alternative for the solar cells. △ Less

Submitted 8 November, 2020; originally announced November 2020.

Comments: Oxygen effect have been studied on hydrogenated amorphous/crystalline silicon thin-films for solar cell applications

Multiphoton Bell-type inequality: a tool to unearth nonlocality of continuous variable quantum optical systems

Authors: Chandan Kumar, Gaurav Saxena, Arvind

Abstract: We consider a multiphoton Bell-type inequality to study nonlocality in four-mode continuous variable systems, which goes beyond two-photon states and can be applied to mixed as well as states with fluctuating photon number. We apply the inequality to a wide variety of states such as pure and mixed Gaussian states (including squeezed thermal states) and non-Gaussian states. We consider beam splitte… ▽ More We consider a multiphoton Bell-type inequality to study nonlocality in four-mode continuous variable systems, which goes beyond two-photon states and can be applied to mixed as well as states with fluctuating photon number. We apply the inequality to a wide variety of states such as pure and mixed Gaussian states (including squeezed thermal states) and non-Gaussian states. We consider beam splitters as a model for leakage and show that the inequality is able to detect nonclassicality of noisy Gaussian states as well. Finally, we investigate nonlocality in pair-coherent states and entangled coherent states, which are prominent examples of nonclassical, non-Gaussian states. △ Less

Submitted 19 August, 2020; originally announced August 2020.

Comments: Revtex 11 pages 9 figures

Journal ref: Phys. Rev. A 103, 042224 (2021)

Sub-Rayleigh resolution of two incoherent sources by array homodyning

Authors: Chandan Datta, Marcin Jarzyna, Yink Loong Len, Karol Łukanowski, Jan Kołodyński, Konrad Banaszek

Abstract: Conventional incoherent imaging based on measuring the spatial intensity distribution in the image plane faces the resolution hurdle described by the Rayleigh diffraction criterion. Here, we demonstrate theoretically using the concept of the Fisher information that quadrature statistics measured by means of array homodyne detection enables estimation of the distance between two incoherent point so… ▽ More Conventional incoherent imaging based on measuring the spatial intensity distribution in the image plane faces the resolution hurdle described by the Rayleigh diffraction criterion. Here, we demonstrate theoretically using the concept of the Fisher information that quadrature statistics measured by means of array homodyne detection enables estimation of the distance between two incoherent point sources well below the Rayleigh limit for sufficiently high signal-to-noise ratio. This capability is attributed to the availability of spatial coherence information between individual detector pixels acquired using the coherent detection technique. A simple analytical approximation for the precision attainable in the sub-Rayleigh region is presented. Furthermore, an estimation algorithm is proposed and applied to Monte Carlo simulated data. △ Less

Submitted 27 November, 2020; v1 submitted 18 May, 2020; originally announced May 2020.

Comments: 10 pages, 6 figures, accepted for publication in Phys. Rev. A

Journal ref: Phys. Rev. A 102, 063526 (2020)

Charge trap layer enabled positive tunable V$_{fb}$ in $β$-Ga$_{2}$O$_{3}$ gate stacks for enhancement mode transistors

Authors: Dipankar Biswas, Chandan Joishi, Jayeeta Biswas, Prabhans Tiwari, Saurabh Lodha

Abstract: $β$-Ga$_{2}$O$_{3}$ based enhancement mode transistor designs are critical for the realization of low loss, high efficiency next generation power devices with rudimentary driving circuits. A novel approach towards attaining a high positive flat band voltage (V$_{fb}$) of 10.6 V in $β$-Ga$_{2}$O$_{3}… ▽ More $β$-Ga$_{2}$O$_{3}$ based enhancement mode transistor designs are critical for the realization of low loss, high efficiency next generation power devices with rudimentary driving circuits. A novel approach towards attaining a high positive flat band voltage (V$_{fb}$) of 10.6 V in $β$-Ga$_{2}$O$_{3}$ metal-oxide-semiconductor capacitors (MOSCAPs), with the ability to fine tune it between 3.5 V to 10.6 V, using a polycrystalline AlN charge trap layer has been demonstrated. This can enable enhancement mode operation over a wide doping range. Excellent V$_{fb}$ retention of ${\sim}$97% for 10$^{4}$ s at 55 $^{\circ}$C was exhibited by the gate stacks after charge trapping, hence reducing the requirement of frequent charge injection cycles. In addition, low gate leakage current density (J$_{g}$) for high negative gate voltages (V$_{g}$${\sim}$-60 V) indicates the potential of this gate stack to enable superior breakdown characteristics in enhancement mode transistors. △ Less

Submitted 11 May, 2020; originally announced May 2020.

Comments: Single file (Manuscript and Supplementary material combined) of 19 pages. Total 5 and 4 figures in manuscript and supplementary material, respectively

Deep-recessed $β$-Ga$_2$O$_3$ delta-doped field effect transistors with in situ epitaxial passivation

Authors: Chandan Joishi, Zhanbo Xia, John S. Jamison, Shahadat H. Sohel, Roberto C. Myers, Saurabh Lodha, Siddharth Rajan

Abstract: We introduce a deep-recessed gate architecture in $β$-Ga$_2$O$_3$ delta-doped field effect transistors for improvement in DC-RF dispersion and breakdown properties. The device design incorporates an unintentionally doped $β$-Ga$_2$O$_3$ layer as the passivation dielectric. To fabricate the device, the deep-recess geometry was developed using BCl$_3$ plasma based etching at ~5 W RIE to ensure minim… ▽ More We introduce a deep-recessed gate architecture in $β$-Ga$_2$O$_3$ delta-doped field effect transistors for improvement in DC-RF dispersion and breakdown properties. The device design incorporates an unintentionally doped $β$-Ga$_2$O$_3$ layer as the passivation dielectric. To fabricate the device, the deep-recess geometry was developed using BCl$_3$ plasma based etching at ~5 W RIE to ensure minimal plasma damage. Etch damage incurred with plasma etching was mitigated by annealing in vacuum at temperatures above 600 $°$C. A gate-connected field-plate edge termination was implemented for efficient field management. Negligible surface dispersion with lower knee-walkout at high V$_\mathrm{DS}$, and better breakdown characteristics compared to their unpassivated counterparts were achieved. A three terminal off-state breakdown voltage of 315 V, corresponding to an average breakdown field of 2.3 MV/cm was measured. The device breakdown was limited by the field-plate/passivation edge and presents scope for further improvement. This demonstration of epitaxially passivated field effect transistors is a significant step for $β$-Ga$_2$O$_3$ technology since the structure simultaneously provides control of surface-related dispersion and excellent field management. △ Less

Submitted 22 April, 2020; originally announced April 2020.

Optimization and effect of UV-ozone exposure of electron transport layer on the efficiency of the dye-sensitized solar cells

Authors: Chandan Dawo, Mohammad Adil Afroz, Parameswar Krishnan Iyer, Harsh Chaturvedi

Abstract: The surface states of the active TiO2 layer is crucial while fabricating an efficient solar cell. This work experimentally analyses the effect of exposing TiO2 based electron transport layer (ETL) to the ultraviolet-ozone (UV-O3) and optimizes the exposure time for improving power conversion efficiency (PCE) of fabricated dye-sensitized solar cells (DSSCs). These results demonstrate that the perfo… ▽ More The surface states of the active TiO2 layer is crucial while fabricating an efficient solar cell. This work experimentally analyses the effect of exposing TiO2 based electron transport layer (ETL) to the ultraviolet-ozone (UV-O3) and optimizes the exposure time for improving power conversion efficiency (PCE) of fabricated dye-sensitized solar cells (DSSCs). These results demonstrate that the performance of DSSCs can be improved significantly by UV-O3 exposure of sintered TiO2 photoanode surface, with the duration of exposure being a critical parameter. Fabricated devices show 33.01 % increase in PCE for the optimum exposure. Nevertheless, overexposure of the sample beyond the optimum time decreases the efficiency of the fabricated solar cells. The device with optimum exposure exhibits the highest PCE of 8.34% with short circuit current density (Jsc) of 15.15 mA/cm2, open circuit voltage (Voc) of 756 mV and Fill factor (FF) of 71.10%. This increase in efficiency is attributed to the enhanced crystallization and reduction in the organic contaminants C-C/C-H from 57.90 to 52.40% as shown by the X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), respectively. The XPS result further indicates an increase in oxygen vacancy from 12.40 to 23.40% for O 1s state and from 9.30 to 14.30% for Ti 2p state of Ti3+. Results from the Atomic Force Microscope (AFM) also confirms the minimized surface roughness of 16.36 nm for the optimally exposed TiO2 film, and increase in hydrophilicity leading to improved efficiency of the solar cells which were optimally exposed to UV-O3. △ Less

Submitted 27 February, 2020; originally announced February 2020.

Resolution limits of spatial mode demultiplexing with noisy detection

Authors: Yink Loong Len, Chandan Datta, Michał Parniak, Konrad Banaszek

Abstract: We consider the problem of estimating the spatial separation between two mutually incoherent point light sources using the super-resolution imaging technique based on spatial mode demultiplexing with noisy detectors. We show that in the presence of noise the resolution of the measurement is limited by the signal-to-noise ratio (SNR) and the minimum resolvable spatial separation has a characteristi… ▽ More We consider the problem of estimating the spatial separation between two mutually incoherent point light sources using the super-resolution imaging technique based on spatial mode demultiplexing with noisy detectors. We show that in the presence of noise the resolution of the measurement is limited by the signal-to-noise ratio (SNR) and the minimum resolvable spatial separation has a characteristic dependence of $\sim$SNR$^{-1/2}$. Several detection techniques, including direct photon counting, as well as homodyne and heterodyne detection are considered. △ Less

Submitted 12 November, 2019; originally announced November 2019.

Comments: 16 pages, 5 figures

Journal ref: Int. J. Quantum Inf. 18, 1941015 (2020)