Showing 1–50 of 197 results for author: Singh, N

A single platform with van der Pauw geometry for measurement of Seebeck coefficient, resistivity, and Hall effect of thin films

Authors: Niraj Kumar Singh, Martin Falk, Per-Anton Schön Wallander, Per Sandström, Per Eklund, Arnaud le Febvrier

Abstract: A modular thermoelectric properties measurement setup in van der Pauw configuration was developed for a straightforward and simultaneous measurement of electrical resistivity and Seebeck coefficient in an extensive temperature range of 25°C - 600°C and can also perform Hall measurements at room temperature. The setup is optimized for accurate measurement of voltages and temperatures gradients by m… ▽ More A modular thermoelectric properties measurement setup in van der Pauw configuration was developed for a straightforward and simultaneous measurement of electrical resistivity and Seebeck coefficient in an extensive temperature range of 25°C - 600°C and can also perform Hall measurements at room temperature. The setup is optimized for accurate measurement of voltages and temperatures gradients by minimizing possible errors from offset voltages, wire contributions and thermal contact resistances which helps getting reliable data. The setup is user friendly, and the measurements are fully automated and controlled using a LabVIEW program. The detachable modules make this setup quite versatile and provide an all-in-one (except thermal conductivity) solution for thermoelectric measurements. △ Less

Submitted 26 June, 2026; originally announced June 2026.

Communication Heterogeneity and Collective Consensus in Neural Cellular Automata

Authors: Nishit Singh

Abstract: Reaching global agreement from purely local interactions is a defining problem of collective intelligence, and most models of it assume that all agents share a single communication protocol. We ask what happens when they do not. Using a Neural Cellular Automaton in which a population of cells must solve the density classification task, agreeing on a global majority that no individual can observe,… ▽ More Reaching global agreement from purely local interactions is a defining problem of collective intelligence, and most models of it assume that all agents share a single communication protocol. We ask what happens when they do not. Using a Neural Cellular Automaton in which a population of cells must solve the density classification task, agreeing on a global majority that no individual can observe, we introduce ``languages'' as sub-populations that read one another's messages through a translation with a tunable ``linguistic distance''. We find that linguistic distance slows consensus, that it produces mild divergence between groups rather than full fragmentation, and that a collective whose shared rule was trained under diverse protocols is robust to mismatch; a homogeneously trained one is not. The findings hold on both a ring and a two-dimensional grid, and admit a natural reading as Ising relaxation, in which a foreign-language region acts as a boundary defect that leaves the system in a higher-energy, partially ordered state. These patterns are qualitatively consistent with effects reported in human group studies, suggesting that distance between communication protocols is a minimal mechanism sufficient to produce them, without anything language-specific. △ Less

Submitted 13 August, 2026; v1 submitted 19 June, 2026; originally announced June 2026.

Comments: 8 pages, 11 figures

Collective charge measurement in quantum dot chains: controlling barrier occupation and tunneling current

Authors: Alok Nath Singh, Rafael Sánchez, Andrew N. Jordan

Abstract: We investigate nonequilibrium transport in a triple-quantum-dot (TQD) system, where the central dot acts as a discrete tunnel barrier, subject to continuous monitoring by a quantum point contact (QPC) that is capacitively coupled to all three dots with independently tunable strengths. We show that this global measurement scheme affects transport in a qualitatively distinct manner from single-site… ▽ More We investigate nonequilibrium transport in a triple-quantum-dot (TQD) system, where the central dot acts as a discrete tunnel barrier, subject to continuous monitoring by a quantum point contact (QPC) that is capacitively coupled to all three dots with independently tunable strengths. We show that this global measurement scheme affects transport in a qualitatively distinct manner from single-site measurement. By engineering structured dephasing, measurement provides a significant improvement in the barrier occupation and tunneling current. In the strong-measurement limit, the steady state becomes independent of the underlying Hamiltonian parameters, and the barrier occupation can approach 1/2 for suitable measurement configurations. We identify an optimal measurement configuration that maximizes the steady-state current and show that near-optimal performance can be achieved with a simple central-dot readout scheme. △ Less

Submitted 18 May, 2026; originally announced May 2026.

Conformal Elastodynamics in 2D Dilational Metamaterials

Authors: Neel Singh, Audrey A. Watkins, Giovanni Bordiga, Vincent Tournat, Katia Bertoldi, Zeb Rocklin

Abstract: Flexible mechanical structures can undergo large deformations under small loads, enabling large, complex, and nonlinear wave responses under finite-frequency driving. Here, we study a dynamically driven canonical flexible mechanical metamaterial composed of rigid squares connected at their corners by flexible hinges. This metamaterial supports a uniform dilational mechanism and, in the limit of id… ▽ More Flexible mechanical structures can undergo large deformations under small loads, enabling large, complex, and nonlinear wave responses under finite-frequency driving. Here, we study a dynamically driven canonical flexible mechanical metamaterial composed of rigid squares connected at their corners by flexible hinges. This metamaterial supports a uniform dilational mechanism and, in the limit of ideal joints, exhibits a Poisson ratio of -1. The presence of this dilational mode of deformation gives rise to a conformal symmetry, in which the dynamics are approximately invariant under a wide class of physical transformations -- conformal maps. We find that the low-frequency response of the system is dominated by conformal deformations consisting of spatially varying rotations and dilations concentrated at the boundary. Even at high frequencies, each conformal map implies a conserved spatially complex momentum. We explore how experimental parameters such as material stiffnesses and the geometry and number of unit cells allow experimental conformal momenta to approach this conservation, varying slowly compared to the non-conformal momenta of same order. These results constitute a new framework opening fundamental avenues for the study of conformal wave phenomena in dilational metamaterials as well as potential strategies for controlling nonlinear waves and vibrations. △ Less

Submitted 17 April, 2026; originally announced April 2026.

Epitaxial MgSnN2 on 4H-SiC (0001): An Earth-Abundant Nitride for Green Optoelectronics and Photovoltaics

Authors: D. Gogova, D. Tran, V. Stanishev, D. Shafizadeh, C. -L. Hsiao, M. Kim, B. Pécz, A. Kovács, K. Frey, A. Sulyok, N. K. Singh, A. Le Febvrier, P. Eklund, V. Darakchieva

Abstract: Group II-IV nitrides have recently emerged as a novel class of semiconductors composed of earth-abundant elements. Owing to their tunable bandgaps, comparable to those of III-nitrides, these materials are attractive candidates for replacing expensive Ga-based alloys in photovoltaics and green-gap optoelectronics. In this work, epitaxial growth of MgSnN2 layers on 4H-SiC(0001) substrates by direct… ▽ More Group II-IV nitrides have recently emerged as a novel class of semiconductors composed of earth-abundant elements. Owing to their tunable bandgaps, comparable to those of III-nitrides, these materials are attractive candidates for replacing expensive Ga-based alloys in photovoltaics and green-gap optoelectronics. In this work, epitaxial growth of MgSnN2 layers on 4H-SiC(0001) substrates by direct current magnetron sputtering is demonstrated. Mg and Sn metal targets have been co-sputtered in nitrogen-containing atmosphere at growth temperatures up to 500 °C. X-ray diffraction and cross-sectional transmission electron microscopy confirm the MgSnN2 layers grow epitaxially in a wurtzite crystal structure, exhibiting the epitaxial relationships with the substrate: MgSnN2 [0001]//4H-SiC [0001] and MgSnN2 [10-10]//4H-SiC[10-10]. Improved crystalline quality is observed for higher deposition temperatures and near-stoichiometric composition, as evidenced by the narrowing of rocking curve linewidths. Optical characterization reveals high absorption coefficients (1e5 cm-1) in the visible spectrum, comparable to that of GaAs, highlighting the suitability of MgSnN2 for photovoltaic applications. A photoluminescence emission band at ~2.4 eV is detected, highly desirable for optoelectronic devices operating in the challenging green spectral region. These results establish MgSnN2 as an earth-abundant, environmentally friendly material, structurally compatible with III-nitrides, with potential for cost-efficient components in sustainable optoelectronics and photovoltaics. △ Less

Submitted 6 April, 2026; originally announced April 2026.

Long-Lived Mechanically-Detected Molecular Spins for Quantum Sensing

Authors: Sahand Tabatabaei, Pritam Priyadarsi, Daniel Tay, Namanish Singh, Pardis Sahafi, Andrew Jordan, Raffi Budakian

Abstract: Quantum sensors based on individual spins provide unprecedented access to local magnetic fields in condensed matter, chemistry, and biology, with solid-state defect spins emerging as the leading platform. However, their molecular-sensing capabilities are limited by confinement to a host lattice, which prevents placement in close proximity to a target molecule. Molecular spins offer an alternative,… ▽ More Quantum sensors based on individual spins provide unprecedented access to local magnetic fields in condensed matter, chemistry, and biology, with solid-state defect spins emerging as the leading platform. However, their molecular-sensing capabilities are limited by confinement to a host lattice, which prevents placement in close proximity to a target molecule. Molecular spins offer an alternative, enabling chemical tunability and flexible positioning relative to the target system. Here we present a nanoscale sensing platform that combines molecular electron spins, ultrasensitive mechanical readout, and Hamiltonian engineering. Using a modified XYXY dipolar decoupling sequence, we suppress electron-electron dipolar interactions across a broad distribution of control fields, extending coherence times to $\sim 400~μ$s in an attoliter-scale droplet containing $\sim$100 trityl-OX063 radicals. Leveraging this sequence, we demonstrate frequency-selective detection of nanotesla-scale AC fields and perform sensing and spectroscopy of small, local nuclear-spin ensembles. Collectively, these results establish SQUINT (Spin-based QUantum Integrated Nanomechanical Transduction) as a framework for quantum sensing that affords molecular-level control over sensor properties and enables direct integration into complex molecular targets. △ Less

Submitted 28 July, 2026; v1 submitted 4 March, 2026; originally announced March 2026.

Comments: Main text: 15 pages, 5 figures, 1 table; supplemental material: 14 pages, 8 figures, 3 tables

Journal ref: Phys. Rev. X 16, 031018 (2026)

Pressure induced electronic band evolution and observation of superconductivity in the Dirac semimetal ZrTe5

Authors: Sanskar Mishra, Nagendra Singh, Vinod K. Gangwar, Rajan Walia, Jianping Sun, Genfu Chen, Dilip Bhoi, Sandip Chatterjee, Yoshiya Uwatoko, Jinguang Cheng, Prashant Shahi

Abstract: We report a comprehensive investigation of the pressure effects on the magnetotransport properties of the topological material ZrTe5 within 1 to 8 GPa pressure range. With increasing pressure, the characteristic peak (Tp) in its electrical resistivity first shifts to higher temperature and then moves quickly towards the lower temperature before disappearing eventually at 6 GPa. Beyond 6 GPa, the s… ▽ More We report a comprehensive investigation of the pressure effects on the magnetotransport properties of the topological material ZrTe5 within 1 to 8 GPa pressure range. With increasing pressure, the characteristic peak (Tp) in its electrical resistivity first shifts to higher temperature and then moves quickly towards the lower temperature before disappearing eventually at 6 GPa. Beyond 6 GPa, the system exhibits metallic behavior across the entire temperature range, and superconductivity emerges below Tc = 1.8 K at 8 GPa. Based on the systematic magnetotransport measurement under pressure, we demonstrate that the superconductivity occurs following a significant electronic structure modulation possibly due to pressure induced structural changes near 6 GPa, which coincides with dramatic enhancement of the magnetoresistance (MR) reaching up to 1400 percent. Our experimental results are substantiated by density functional theory calculations as the application of pressure drastically alters the density of states near the Fermi level. Notably, multiple hole pockets emerge at the Fermi level from 4 GPa onward, and their contributions are further enhanced with increasing pressure. The combined experimental and theoretical investigation reveals a comprehensive evolution of electronic structure of Dirac semimetal ZrTe5 under pressure and suggest a possible link between the Fermi surface reconstruction in the pressure range of structural transition and emergence of superconductivity △ Less

Submitted 9 February, 2026; originally announced February 2026.

Comments: 14 Pages, 16 Figures

Report number: 195113

Journal ref: Physical Review B 113, 195113 (2026)

Atomic Alignment in PbS Nanocrystal Superlattices with Compact Inorganic Ligands via Reversible Oriented Attachment of Nanocrystals

Authors: Ahhyun Jeong, Aditya N. Singh, Josh Portner, Xiaoben Zhang, Saghar Rezaie, Justin C. Ondry, Zirui Zhou, Junhong Chen, Ye Ji Kim, Richard D. Schaller, Youssef Tazoui, Zehan Mi, Sadegh Yazdi, David T. Limmer, Dmitri V. Talapin

Abstract: Nanocrystals (NCs) serve as versatile building blocks for the creation of functional materials, with NC self-assembly offering opportunities to enable novel material properties. Here, we demonstrate that PbS NCs functionalized with strongly negatively charged metal chalcogenide complex (MCC) ligands, such as $Sn_2S_6^{4-}$ and $AsS_4^{3-}$, can self-assemble into all-inorganic superlattices with b… ▽ More Nanocrystals (NCs) serve as versatile building blocks for the creation of functional materials, with NC self-assembly offering opportunities to enable novel material properties. Here, we demonstrate that PbS NCs functionalized with strongly negatively charged metal chalcogenide complex (MCC) ligands, such as $Sn_2S_6^{4-}$ and $AsS_4^{3-}$, can self-assemble into all-inorganic superlattices with both long-range superlattice translational and atomic-lattice orientational order. Structural characterizations reveal that the NCs adopt unexpected edge-to-edge alignment, and numerical simulation clarifies that orientational order is thermodynamically stabilized by many-body ion correlations originating from the dense electrolyte. Furthermore, we show that the superlattices of $Sn_2S_6^{4-}$-functionalized PbS NCs can be fully disassembled back into the colloidal state, which is highly unusual for orientationally attached superlattices with atomic-lattice alignment. The reversible oriented attachment of NCs, enabling their dynamic assembly and disassembly into effectively single-crystalline superstructures, offers a pathway toward designing reconfigurable materials with adaptive and controllable electronic and optoelectronic properties. △ Less

Submitted 17 January, 2026; originally announced January 2026.

Morphology-engineered nanostructured silver- and antimony-telluride films for flexible thermoelectric generators

Authors: Ankit Kashyap, Conner Wallace, Geetu Sharma, Collin Rowe, Mahima Sasikumar, Niraj Kumar Singh, Per Eklund, Theodorian Borca-Tasciucc, Ganpati Ramanath, Ajay Soni

Abstract: Harvesting low-grade heat to electricity is attractive for powering wearable electronic devices. Here, we demonstrate nW-scale thermoelectric power generation in devices from thin film assemblies of microwave-synthesized p-Sb2Te3 nanoplates and n-Ag2Te nanowires on polyvinylidene fluoride membranes. While microwave cycling is crucial for Ag2Te nanocrystal shaping, Sb2Te3 formation is sensitive to… ▽ More Harvesting low-grade heat to electricity is attractive for powering wearable electronic devices. Here, we demonstrate nW-scale thermoelectric power generation in devices from thin film assemblies of microwave-synthesized p-Sb2Te3 nanoplates and n-Ag2Te nanowires on polyvinylidene fluoride membranes. While microwave cycling is crucial for Ag2Te nanocrystal shaping, Sb2Te3 formation is sensitive to precursors and surfactant concentrations. Introducing S doping in Sb2Te3 in the 1 - 1.5 atomic percent range via thioglycolic acid during synthesis yields an up to eightfold higher power-factor, due to a fivefold increase in electrical conductivity and 25% increase in Seebeck coefficient. Our microfilm devices generate up to 33.6 mV from 5 deg C to 50 deg C thermal gradients, with 120 nW maximum power output at Delta T 30 deg C, which is sixtyfold higher than Sb2Te3 paper devices. Mechanical bending can increase device resistance by up to 125% due to diminished inter-nanostructure electronic transport. These findings provide insights for integrating synthesis, morphology engineering and device design for next-generation wearable thermoelectric systems. △ Less

Submitted 7 December, 2025; originally announced December 2025.

Comments: 14 Pages and 6 figure

Making the Virtual Real: Measurement-Powered Tunneling Engines

Authors: Rafael Sánchez, Alok Nath Singh, Andrew N. Jordan, Bibek Bhandari

Abstract: Quantum tunneling allows electrons to be transferred between two regions separated by an energetically forbidden barrier. Performing a position measurement that finds a particle in the barrier forces the tunneling electrons to transition from having a classically forbidden energy to an energy above the barrier height. We exploit this effect to define quantum tunneling engines that can use the unco… ▽ More Quantum tunneling allows electrons to be transferred between two regions separated by an energetically forbidden barrier. Performing a position measurement that finds a particle in the barrier forces the tunneling electrons to transition from having a classically forbidden energy to an energy above the barrier height. We exploit this effect to define quantum tunneling engines that can use the unconditioned detection of virtually occupied states as a resource for power generation and cooling. Leveraging energy exchange with the detector, we show that the device can operate in a hybrid regime, enabling simultaneous cooling and power generation. Furthermore, we demonstrate measurement-assisted autonomous refrigeration and "checkpoint" cooling driven purely by a thermal bias, without the need for an applied potential. We also find a "purification-by-noise" effect when the measurement drives the system into a stationary dark state. These results underscore the intriguing dual role of measurement as a thermodynamic resource and a dark state generator. △ Less

Submitted 25 October, 2025; originally announced October 2025.

Comments: 10 pages, 7 figures

Journal ref: Phys. Rev. B 113, 125414 (2026)

Universal Thickness-Dependent Absorption in Solids at the Nanoscale: Anomalous Enhancement in the Ultrathin Limit

Authors: Bhumika Chauhan, Nikhil Singh, Subhrajit Dalai, Abhisek Saidarsan, Sayantan Patra, Sourabh Jain, Aparna Deshpande, Ashish Arora

Abstract: Through systematic experimental and theoretical studies of layer-thickness-dependent absorption in semiconducting MoSe$_2$ and WS$_2$ across the visible to near-infrared spectral range, we demonstrate a universal absorption behavior in solids at nanoscale thicknesses. With increasing thickness, a non-monotonic evolution of absorption integrated over the measured spectral region is revealed which i… ▽ More Through systematic experimental and theoretical studies of layer-thickness-dependent absorption in semiconducting MoSe$_2$ and WS$_2$ across the visible to near-infrared spectral range, we demonstrate a universal absorption behavior in solids at nanoscale thicknesses. With increasing thickness, a non-monotonic evolution of absorption integrated over the measured spectral region is revealed which is accompanied by pronounced oscillatory features. This shows a strong deviation from the expected Beer-Lambert law. Below 10 nm, we observe a sharp anomalous increase in absorption, with deviations from Beer's law exceeding 50% in layered semiconductors. Our conclusions hold irrespective of the presence of any optical resonances such as excitons or plasmons within the spectral window. The observed behavior has origins in the electromagnetic interference effects taking place between the two surfaces of the thin crystals. The present work on 2D semiconductors is extendable to all kinds of solids such as conventional semiconductors (e.g. Si, GaAs, GaN, InP), (semi) metals (e.g. Al, Ag, Au, c-HOPG) and 2D magnetic materials (e.g. CrSBr and NiPS$_3$). Our results provide fundamental insights into light-matter interactions in solids at the nanoscale and are vital for optimally designing the new generation of absorption-based flexible optoelectronic devices. △ Less

Submitted 26 July, 2026; v1 submitted 24 October, 2025; originally announced October 2025.

Comments: 13 pages, 4 figures in main text, 17 figures in supplementary materials

Universal scaling of shear thickening suspensions under acoustic perturbation

Authors: Anna R. Barth, Navneet Singh, Stephen J. Thornton, Pranav Kakhandiki, Edward Y. X. Ong, Meera Ramaswamy, Abhishek M. Shetty, Bulbul Chakraborty, James P. Sethna, Itai Cohen

Abstract: Tuning shear thickening behavior is a longstanding problem in the field of dense suspensions. Acoustic perturbations offer a convenient way to control shear thickening in real time, opening the door to a new class of smart materials. However, complete control over shear thickening requires a quantitative description for how suspension viscosity varies under acoustic perturbation. Here, we achieve… ▽ More Tuning shear thickening behavior is a longstanding problem in the field of dense suspensions. Acoustic perturbations offer a convenient way to control shear thickening in real time, opening the door to a new class of smart materials. However, complete control over shear thickening requires a quantitative description for how suspension viscosity varies under acoustic perturbation. Here, we achieve this goal by experimentally probing suspensions with acoustic perturbations and incorporating their effect on the suspension viscosity into a universal scaling framework where the viscosity is described by a scaling function, which captures a crossover from the frictionless jamming critical point to a frictional shear jamming critical point. Our analysis reveals that the effect of acoustic perturbations may be explained by the introduction of an effective interparticle repulsion whose magnitude is roughly equal to the acoustic energy density. Furthermore, we demonstrate how this scaling framework may be leveraged to produce explicit predictions for the viscosity of a dense suspension under acoustic perturbation. Our results demonstrate the utility of the scaling framework for experimentally manipulating shear thickening systems. △ Less

Submitted 13 October, 2025; originally announced October 2025.

Bipolar surface charging by evaporating water droplets

Authors: Nitish Singh, Aaron D. Ratschow, Nabeel Aslam, Dan Daniel

Abstract: Surface charging is a ubiquitous phenomenon with important consequences. On one hand, surface charging underpins emerging technologies such as triboelectric nanogenerators; on the other, uncontrolled charging can damage delicate nanostructures and devices. Despite its significance, surface charging by evaporating water droplets remains poorly understood. Here, using Kelvin Probe Force Microscopy,… ▽ More Surface charging is a ubiquitous phenomenon with important consequences. On one hand, surface charging underpins emerging technologies such as triboelectric nanogenerators; on the other, uncontrolled charging can damage delicate nanostructures and devices. Despite its significance, surface charging by evaporating water droplets remains poorly understood. Here, using Kelvin Probe Force Microscopy, we spatially resolve the surface-charge patterns from evaporating droplets and propose a physical model that quantitatively explains the origin of bipolar charging. △ Less

Submitted 12 August, 2025; originally announced August 2025.

Data-driven Discovery of Novel High-performance Quaternary Chalcogenide Photovoltaics

Authors: Nikhil Singh, Mohammad Ubaid, Pabitra Kumar Nayak, Jiangang He, Dibyajyoti Ghosh, Chris Wolverton, Koushik Pal

Abstract: Photovoltaic materials facilitate the conversion of sunlight into electricity by harnessing the interaction between light and matter, offering an eco-friendly and cost-efficient energy solution. Combining data-driven approaches with static and time-dependent density functional theories and nonadiabatic molecular dynamics simulations, we predict 14 high-performance photoabsorber materials from a fa… ▽ More Photovoltaic materials facilitate the conversion of sunlight into electricity by harnessing the interaction between light and matter, offering an eco-friendly and cost-efficient energy solution. Combining data-driven approaches with static and time-dependent density functional theories and nonadiabatic molecular dynamics simulations, we predict 14 high-performance photoabsorber materials from a family of known quaternary semiconductors. Among these, we investigate four compounds - SrCuGdSe3, SrCuDyTe3, BaCuLaSe3, and BaCuLaTe3 in greater detail. Hybrid density functional theory calculations including spin-orbit coupling reveal that SrCuGdSe3, SrCuDyTe3, BaCuLaSe3 and BaCuLaTe3 possess direct band gaps of 1.65, 1.79, 1.05, and 1.01 eV, respectively. These band gap values lie close to an optimal range ideal for visible-light absorption. Consequently, the calculated optical absorption coefficient and spectroscopic limited maximum efficiency for these compounds become comparable or larger than crystalline silicon, GaAs, and methylammonium lead iodide. Calculated exciton binding energies for these compounds are relatively small (30-32 meV), signifying easy separation of the electron-hole pairs, and hence enhanced power conversion efficiencies. Investigations of photoexcited carrier dynamics reveal a relatively long carrier lifetime (~ 30-40 ns), suggesting suppressed nonradiative recombination and enhanced photo-conversion efficiencies. We further determined the defect formation energies in these compounds, which showed that despite the likely formation of cation vacancies and interstitial defects, midgap states remain absent making these defects non-detrimental to carrier recombination. Our theoretical predictions invite experimental verification and encourage further investigations of these and similar compounds in this quaternary semiconductor family. △ Less

Submitted 21 July, 2025; originally announced July 2025.

V. J. Emery and P. W. Anderson's views and related issues regarding the basics of cuprates: a re-look

Authors: Navinder Singh

Abstract: In 1991, V. J. Emery in his important review article entitled "Some aspects of the theory of high temperature superconductors"\cite{emery1} argued against the Zhang-Rice reduction of three-band to an effective one-band model. In his words "...therefore it seems that the simple $t-J$ model does not account for the properties of high temperature superconductors". Over approximately 35 years after th… ▽ More In 1991, V. J. Emery in his important review article entitled "Some aspects of the theory of high temperature superconductors"\cite{emery1} argued against the Zhang-Rice reduction of three-band to an effective one-band model. In his words "...therefore it seems that the simple $t-J$ model does not account for the properties of high temperature superconductors". Over approximately 35 years after the initial debates\cite{debates} much has happened in the field pertaining to this topic. Even though it is one of the most discussed issue, a comprehensive account and the required resolution are lacking. Connected to the debate over one-band versus three-band models is another discussion: the one-component versus two-component model for cuprates. The two-component model is most strongly advocated by Barzykin and Pines\cite{bp}. In this article the author attempts a perspective and a re-look on some of these issues. After an analysis of a large body of literature, author finds that V. J. Emery's criticism of the Zhang-Rice reduction was correct. Many central experimental features of cuprates cannot be rationalized within the one-band model, and Johnston-Nakano scaling is one such example. Other examples are also discussed. Author introduces a simple-minded toy model to illustrate the core issues involved. △ Less

Submitted 3 June, 2025; v1 submitted 29 May, 2025; originally announced May 2025.

Comments: 12 pages, and 7 figures

Spectrum Selective Interfaces and Materials towards Non-photothermal Saltwater Evaporation: Demonstration with a White Ceramic Wick

Authors: Navindra D. Singh, James Leung, Ji Feng, Alma K. González-Alcalde, Arial Tolentino, David Tuft, Juchen Guo, Luat T. Vuong

Abstract: Most solar desalination efforts are photothermal: they evaporate water with ``black'' materials that absorb as much sunlight as possible. Such ``brine-boiling'' methods are limited by the high thermal mass of water, i.e., its capacity to store and release heat. Here, we study the light-enhanced evaporation by a hard, white, aluminum nitride wick, and propose a route to selectively target salt-wate… ▽ More Most solar desalination efforts are photothermal: they evaporate water with ``black'' materials that absorb as much sunlight as possible. Such ``brine-boiling'' methods are limited by the high thermal mass of water, i.e., its capacity to store and release heat. Here, we study the light-enhanced evaporation by a hard, white, aluminum nitride wick, and propose a route to selectively target salt-water bonds instead of bulk heating via deep-UV interactions. Through experiments and analyses that isolate the effects of light absorption and heating in aluminum nitride, we provide experimental evidence of a light-driven, spectrum-selective path to non-photothermal saltwater evaporation. Leverage of these light-matter interactions in white ceramic wicks may achieve low-cost, low-energy desalination, reduce the heat island effects of traditional solar technologies, and contribute to future cooling technologies where drought is also a concern. △ Less

Submitted 8 October, 2025; v1 submitted 28 May, 2025; originally announced May 2025.

Comments: Accepted for publication in ACS: Applied Materials & Interfaces, October 2025

DFT Investigations of Major Defects in Quartz Crystal: Implications for Luminescence and ESR Dosimetry and Dating

Authors: Jalaja Pandya, Malika Singhal, Navinder Singh, Naveen Chauhan

Abstract: Quartz is extensively used for luminescence and ESR dosimetry as well as dating. These techniques use inherent defects introduced in quartz crystal during its crystallization in nature. The defect comprises both intrinsic as well as extrinsic defects. These defects give important luminescence properties to quartz but are not yet well understood from a theoretical perspective. Specifically, in case… ▽ More Quartz is extensively used for luminescence and ESR dosimetry as well as dating. These techniques use inherent defects introduced in quartz crystal during its crystallization in nature. The defect comprises both intrinsic as well as extrinsic defects. These defects give important luminescence properties to quartz but are not yet well understood from a theoretical perspective. Specifically, in case of luminescence dosimetry the nature of traps and their involvement in luminescence production is not exactly known. Thus, present work attempts to understand the basic physics of defects and their implication for luminescence and ESR techniques via Density Functional Theory (DFT) modelling. The work uses DFT to model the presence of some possible major impurities in quartz. Several interesting novel results are obtained that will have implications for ongoing research in Luminescence and ESR methods. The DFT modelling suggested that Oxygen deficiency in quartz crystal results in the formation of both electron and hole trapping centres. However, it is observed that these centres can be passivated by the introduction of charge compensating OH or H ions. Further, it is found that peroxy defects can be formed in the presence of either excess Oxygen or due to the absence of Silicon (Si4+), however, the nature of the traps formed in both cases is different. Besides these intrinsic defects, Al and Fe are the major impurities which are observed as defects in quartz. The modelling of these impurities suggested that negligible change in DOS is observed for Al defect and Fe generally forms a recombination centre or hole trap. In addition to these, there are several interesting first-time observations that are not reported and will be helpful for progressing luminescence and ESR dosimetry research. △ Less

Submitted 25 April, 2025; originally announced April 2025.

Comments: 26 pages, 18 figures

Effect of pressure on the transport properties and thermoelectric performance of Dirac semimetal ZrTe5

Authors: Sanskar Mishra, Nagendra Singh, V. K. Gangwar, Rajan Walia, Manindra Kumar, Udai Bhan Singh, Deepash Sekhar Saini, Jianping Sun, Genfu Chen, Dilip Bhoi, Sandip Chatterjee, Yoshiya Uwatoko, Jinguang Cheng, Prashant Shahi

Abstract: In this study, we have investigated and compared the effect of hydrostatic pressure up to ~20 kbar on the transport properties of ZrTe5 single crystals grown by chemical vapor transport (CVT) and flux methods. With the application of pressure, the electrical resistivity Rho(T) and thermopower S(T) of both crystals were found to increase in the whole temperature range unlike the other known thermoe… ▽ More In this study, we have investigated and compared the effect of hydrostatic pressure up to ~20 kbar on the transport properties of ZrTe5 single crystals grown by chemical vapor transport (CVT) and flux methods. With the application of pressure, the electrical resistivity Rho(T) and thermopower S(T) of both crystals were found to increase in the whole temperature range unlike the other known thermoelectric materials, such as Bi2Te3, SnSe etc. This observation is supported by the complementary first-principles band structure calculation as the application of pressure widens the direct bandgap at Γ point. Moreover, the analysis of the pressure dependent magneto-transport and Shubnikov de-Hass oscillation results revealed an increase in carrier concentration and effective mass along with the reduction of mobility as pressure rises. Furthermore, with the application of pressure, the flux-grown ZrTe5 crystals display a transition from unipolar to bipolar charge transport as evidenced by the emergence of resistivity peak at T* under high pressure, unlike the CVT-grown ZrTe5 crystals where the bipolar charge transport near its characteristic resistivity peak (Tp) remains unaffected. △ Less

Submitted 23 April, 2025; originally announced April 2025.

Comments: 14 pages, 11 Figures, For submission in Physical Review B

Report number: Volume 112, 165140

Journal ref: Physical Review B 2025

Capturing an Electron in the Virtual State

Authors: Alok Nath Singh, Bibek Bhandari, Rafael Sánchez, Andrew N. Jordan

Abstract: We address a foundational question in quantum mechanics: Can a particle be directly found in a classically forbidden virtual state? We instantiate this conceptual question by investigating the traversal of electrons through a tunnel barrier, which we define in a triple quantum dot (TQD) system where the occupation of the central dot is energetically avoided. The motivation behind this setup is to… ▽ More We address a foundational question in quantum mechanics: Can a particle be directly found in a classically forbidden virtual state? We instantiate this conceptual question by investigating the traversal of electrons through a tunnel barrier, which we define in a triple quantum dot (TQD) system where the occupation of the central dot is energetically avoided. The motivation behind this setup is to answer whether the central dot is occupied or not during a virtual transition when it is being explicitly monitored. We investigate this problem in two different limits of continuous measurements: the stochastic quantum diffusion and the quantum jump. We find that, even though individual trajectories differ considerably across these limits, measuring leads to a higher occupation in the central dot on average. Our results demonstrate that the act of observation fundamentally reshapes tunneling dynamics, resolving the seeming paradox of detecting a particle in a classically forbidden region: weak measurements partially localize the particle, while strong measurements enforce a discontinuous either/or detection or no detection outcome. △ Less

Submitted 20 July, 2026; v1 submitted 13 March, 2025; originally announced March 2025.

Comments: v3: Updated to match the published version in New Journal of Physics

Journal ref: New J. Phys. 28 074503 (2026)

Anomalies in the electronic, magnetic and thermal behavior near the Invar compositions of Fe-Ni alloys

Authors: Ananya Sahoo, Ayusa Aparupa Biswal, S. K. Parida, V. R. R. Medicherla, Soumya Shephalika Behera, M. N. Singh, A. Sagdeo, Sawani Datta, Abhishek Singh, Kalobaran Maiti

Abstract: The structural and magnetic properties of Fe$_{1-x}$Ni$_x$~($x$ = 0.32, 0.36, 0.40, 0.50) alloys have been investigated using synchrotron based x-ray diffraction (XRD) technique with x-rays of wavelength 0.63658 Å down to 50 K temperature, magnetic measurement using superconducting quantum interference device (SQUID) magnetometer and high resolution x-ray photoelectron spectroscopy (XPS) with mono… ▽ More The structural and magnetic properties of Fe$_{1-x}$Ni$_x$~($x$ = 0.32, 0.36, 0.40, 0.50) alloys have been investigated using synchrotron based x-ray diffraction (XRD) technique with x-rays of wavelength 0.63658 Å down to 50 K temperature, magnetic measurement using superconducting quantum interference device (SQUID) magnetometer and high resolution x-ray photoelectron spectroscopy (XPS) with monochromatic AlK$_α$ radiation. The XRD studies suggest a single phase with fcc structure for $x$ = 0.36, 0.40, and 0.50 ~alloys and a mixed phase for $x$ = 0.32 alloy containing both bcc and fcc structures. The lattice parameter of the alloys exhibits a linear dependence on temperature giving rise to a temperature independent coefficient of thermal expansion (CTE). The lowest CTE is observed for $x$ = 0.36 Invar alloy as expected while $x$ = 0.50 alloy exhibits the highest CTE among the alloys studied. The CTE of the fcc component of mixed phase alloy is close to that of Invar alloy. The temperature dependence of magnetization of the alloys down to 2 K reveals an overall antiferromagnetic interactions within the ferromagnetic phase causing the magnetization decreasing with cooling. The field cooled and zero field cooled data show larger differences for the Invar compositions; this is also manifested in the magnetic hysteresis data at 2 K and 300 K. △ Less

Submitted 15 February, 2025; originally announced February 2025.

Comments: 7 pages, 6 figures, regular article

Variational path sampling of rare dynamical events

Authors: Aditya N. Singh, Avishek Das, David T. Limmer

Abstract: This article reviews the concepts and methods of variational path sampling. These methods allow computational studies of rare events in systems driven arbitrarily far from equilibrium. Based upon a statistical mechanics of trajectory space and leveraging the theory of large deviations, they provide a perspective with which dynamical phenomena can be studied with the same types of ensemble reweight… ▽ More This article reviews the concepts and methods of variational path sampling. These methods allow computational studies of rare events in systems driven arbitrarily far from equilibrium. Based upon a statistical mechanics of trajectory space and leveraging the theory of large deviations, they provide a perspective with which dynamical phenomena can be studied with the same types of ensemble reweighting ideas that have been used for static equilibrium properties. Applications to chemical, material, and biophysical systems are highlighted. △ Less

Submitted 3 February, 2025; originally announced February 2025.

Reactive path ensembles within nonequilibrium steady-states

Authors: Aditya N. Singh, David T. Limmer

Abstract: The modern theory of rare events is grounded in near equilibrium ideas, however many systems of modern interest are sufficiently far from equilibrium that traditional approaches do not apply. Using the recently developed variational path sampling methodology, we study systems evolving within nonequilibrium steady states to elucidate how reactive processes are altered away from equilibrium. Variati… ▽ More The modern theory of rare events is grounded in near equilibrium ideas, however many systems of modern interest are sufficiently far from equilibrium that traditional approaches do not apply. Using the recently developed variational path sampling methodology, we study systems evolving within nonequilibrium steady states to elucidate how reactive processes are altered away from equilibrium. Variational path sampling provides access to ensembles of reactive events, and a means of quantifying the relative importance of each dynamical degree of freedom in such processes. With it, we have studied the conformational change of a solute in an active bath. We illustrate how energy injection generically enhances the rates of rare events, even when energy is not directed into specific reactive modes. By studying the folding and unfolding transitions of a grafted polymer under shear, we illustrate how nonequilibrium reactive processes do not follow gradient paths due to the emergence of persistent currents. The breaking of detailed balance allows for the mechanisms of forward and backward reactions to be distinct, enabling novel pathways to be explored and designed, and states unstable in equilibrium to become stabilized kinetically away from it. The analysis presented in this work establishes some basic principles for nonequilibrium reactive events, and is made possible by the use of a numerical method that does not invoke proximity to equilibrium or requires strong prior assumptions about the mechanism of reaction. △ Less

Submitted 13 February, 2025; v1 submitted 31 January, 2025; originally announced January 2025.

Comments: 24 pages, 18 figures, comments welcome

A comprehensive study of electronic and piezoelectric properties of Li-based Tin-halide perovskites from GGA and Meta-GGA

Authors: Celestine Lalengmawia, Zosiamliana Renthlei, Shivraj Gurung, Lalhriat Zuala, Lalrinthara Pachuau, Ningthoujam Surajkumar Singh, Lalmuanpuia Vanchhawng, Karthik Gopi, A. Yvaz, D. P. Rai

Abstract: Wide bandgap semiconductors (WBGs) are predicted to be the potential materials for energy generation and storing. In this work, we used density functional theory (DFT) that incorporates generalized gradient approximation (GGA) and meta-generalized gradient approximation (mGGA) methods to explore the various properties of the LiSnCl3 and LiSnBr3 perovskites. The structural stabilities, charge trans… ▽ More Wide bandgap semiconductors (WBGs) are predicted to be the potential materials for energy generation and storing. In this work, we used density functional theory (DFT) that incorporates generalized gradient approximation (GGA) and meta-generalized gradient approximation (mGGA) methods to explore the various properties of the LiSnCl3 and LiSnBr3 perovskites. The structural stabilities, charge transfer, electronic, optical, mechanical, and piezoelectric properties are studied. Herein, we report that these rarely studied materials are eco-friendly and look promising for optoelectronics and piezoelectric applications. △ Less

Submitted 6 December, 2024; originally announced December 2024.

Hot (non-equilibrium) electron relaxation: A review of the ultra-fast phenomena in metals and superconductors (PART I)

Authors: Navinder Singh

Abstract: The famous Two-Temperature Model (TTM) used extensively in the investigations of energy relaxation in photo-excited systems originated in the seminal work of M. I. Kaganov, I. M. Lifshitz, and L. V. Tanatarov (KLT) in 1957. The idea that with an ultra short laser pulse a temporal (transient) state of electrons in a metal can be created in which electrons after absorbing energy from the laser pulse… ▽ More The famous Two-Temperature Model (TTM) used extensively in the investigations of energy relaxation in photo-excited systems originated in the seminal work of M. I. Kaganov, I. M. Lifshitz, and L. V. Tanatarov (KLT) in 1957. The idea that with an ultra short laser pulse a temporal (transient) state of electrons in a metal can be created in which electrons after absorbing energy from the laser pulse heat up and their temperature becomes substantially greater than that of lattice was originated in the work of S. I. Anisimov, B. L. Kapeliovich, and T. L. Perel'man in 1974. The heated electron sub-system ("hot" electrons) loses its energy to phonon sub-system via electron-phonon scattering (relaxation) and thermodynamic equilibrium re-establishes over a time scale of a few pico-seconds (psecs) in metals. This field saw great developments in the 1980s and 1990s with the advent of femto-second (fsec) pump-probe spectroscopy. And from 2000 onwards focus shifted from non-equilibrium phenomena in simple metals to that in more complex systems including strongly correlated systems such as high Tc cuprate superconductors. In 1987, P. B. Allen re-visits the calculations of KLT and re-writes the electron-phonon heat transfer coefficient $α$ in terms of a very important parameter in the theory of superconductivity. Year 2000 onwards, field saw the development of models that go beyond the original TTM. Very recently, field enters into atto-second domain. In this article, author attempts a rigorous and concise account of the entire development of this very fascinating area of research including a summary of the current research in this field. △ Less

Submitted 25 October, 2024; originally announced October 2024.

Comments: 10 pages and 4 figures

Rigidity transitions in anisotropic networks happen in multiple steps

Authors: William Y. Wang, Stephen J. Thornton, Bulbul Chakraborty, Anna Barth, Navneet Singh, Japheth Omonira, Jonathan A. Michel, Moumita Das, James P. Sethna, Itai Cohen

Abstract: We study how the rigidity transition in a triangular lattice changes as a function of anisotropy by preferentially filling bonds on the lattice in one direction. We discover that the onset of rigidity in anisotropic spring networks arises in at least two steps, reminiscent of the two-step melting transition in two dimensional crystals. In particular, our simulations demonstrate that the percolatio… ▽ More We study how the rigidity transition in a triangular lattice changes as a function of anisotropy by preferentially filling bonds on the lattice in one direction. We discover that the onset of rigidity in anisotropic spring networks arises in at least two steps, reminiscent of the two-step melting transition in two dimensional crystals. In particular, our simulations demonstrate that the percolation of stress-supporting bonds happens at different critical volume fractions along different directions. By examining each independent component of the elasticity tensor, we determine universal exponents and develop universal scaling functions to analyze isotropic rigidity percolation as a multicritical point. We expect that these results will be important for elucidating the underlying mechanical phase transitions governing the properties of biological materials ranging from the cytoskeletons of cells to the extracellular networks of tissues such as tendon where the networks are often preferentially aligned. △ Less

Submitted 13 September, 2024; originally announced September 2024.

Comments: 13 pages, 17 figures

Unifying principle for Hall coefficient in systems near magnetic instability

Authors: Jalaja Pandya, Navinder Singh

Abstract: Typically, Hall coefficient of materials near magnetic instabilities exhibits pronounced temperature dependence. To explore the reasons involved, we studied the temperature dependence of Hall coefficient in $Cr_{1-x}V_x$, $V_{2-y}O_3$ and some high-$T_c$ superconducting cuprates. We argue that it can be rationalized using the following unifying principle:\textit{ When a system is near a magnetic i… ▽ More Typically, Hall coefficient of materials near magnetic instabilities exhibits pronounced temperature dependence. To explore the reasons involved, we studied the temperature dependence of Hall coefficient in $Cr_{1-x}V_x$, $V_{2-y}O_3$ and some high-$T_c$ superconducting cuprates. We argue that it can be rationalized using the following unifying principle:\textit{ When a system is near a magnetic instability and temperature is reduced towards the instability, there is a progressive "loss" of carriers (progressive "tying down" of electrons) as they participate in long-lived and long-ranged magnetic correlations.} In other words, magnetic correlations grow in space and are longer-lived as temperature is reduced towards the magnetic instability. This is the mechanism behind reduced carrier density with reducing temperature and leads to an enhancement of the Hall coefficient. This unifying principle is implemented and quantitative analysis is done using the Gor'kov Teitel'baum Thermal Activation (GTTA) model. We also show that the Hall angle data can be understood using one relaxation time (in contrast to the "two-relaxation" times idea of Anderson) by taking into consideration of temperature dependence of carrier density. This unifying principle is shown to be working in above studied systems, but authors believe that it is of much more general validity. △ Less

Submitted 5 July, 2024; originally announced July 2024.

Comments: 15 pages, 17 figures, comments are welcomed

Giant Thermoelectric Response of Fluxons in Superconductors

Authors: Alok Nath Singh, Bibek Bhandari, Alessandro Braggio, Francesco Giazotto, Andrew N. Jordan

Abstract: Thermoelectric devices that operate on quantum principles have been under extensive investigation in the past decades. These devices are at the fundamental limits of miniaturized heat engines and refrigerators, advancing the field of quantum thermodynamics. Most research in this area concerns the use of conduction electrons and holes as charge and heat carriers, and only very recently have superco… ▽ More Thermoelectric devices that operate on quantum principles have been under extensive investigation in the past decades. These devices are at the fundamental limits of miniaturized heat engines and refrigerators, advancing the field of quantum thermodynamics. Most research in this area concerns the use of conduction electrons and holes as charge and heat carriers, and only very recently have superconductors been considered as thermal engines and thermoelectric devices. Here, we investigate the thermoelectric response of an Abrikosov vortex in type-II superconductors in the deep quantum limit. We consider two thermoelectric geometries, a type-II SIN junction and a local Scanning Tunneling Microscope (STM)-tip normal metal probe over the superconductor. We exploit the strong breaking of particle-hole symmetry in bound states at sub-gap energies within the superconducting vortex to realize a giant thermoelectric response in the presence of fluxons. We predict a thermovoltage of a few mV/K at sub-Kelvin temperatures using both semi-analytic and numerical self-consistent solutions of the Bogoliubov-de Gennes equations. Relevant thermoelectric coefficients and figures of merit are found within our model, both in linear and nonlinear regimes. The ZT of the SIN junction is around 1, rising to above 3 for the STM junction centered at the vortex core. We also discuss how this system can be used as a sensitive thermocouple, diode, or localized bolometer to detect low-energy single photons. △ Less

Submitted 8 May, 2024; originally announced May 2024.

Comments: 11 pages, 4 figures

Journal ref: Phys. Rev. Lett. 133, 256002 (2024)

Connecting physics to systems with modular spin-circuits

Authors: Kemal Selcuk, Saleh Bunaiyan, Nihal Sanjay Singh, Shehrin Sayed, Samiran Ganguly, Giovanni Finocchio, Supriyo Datta, Kerem Y. Camsari

Abstract: An emerging paradigm in modern electronics is that of CMOS + $\sf X$ requiring the integration of standard CMOS technology with novel materials and technologies denoted by $\sf X$. In this context, a crucial challenge is to develop accurate circuit models for $\sf X$ that are compatible with standard models for CMOS-based circuits and systems. In this perspective, we present physics-based, experim… ▽ More An emerging paradigm in modern electronics is that of CMOS + $\sf X$ requiring the integration of standard CMOS technology with novel materials and technologies denoted by $\sf X$. In this context, a crucial challenge is to develop accurate circuit models for $\sf X$ that are compatible with standard models for CMOS-based circuits and systems. In this perspective, we present physics-based, experimentally benchmarked modular circuit models that can be used to evaluate a class of CMOS + $\sf X$ systems, where $\sf X$ denotes magnetic and spintronic materials and phenomena. This class of materials is particularly challenging because they go beyond conventional charge-based phenomena and involve the spin degree of freedom which involves non-trivial quantum effects. Starting from density matrices $-$ the central quantity in quantum transport $-$ using well-defined approximations, it is possible to obtain spin-circuits that generalize ordinary circuit theory to 4-component currents and voltages (1 for charge and 3 for spin). With step-by-step examples that progressively become more complex, we illustrate how the spin-circuit approach can be used to start from the physics of magnetism and spintronics to enable accurate system-level evaluations. We believe the core approach can be extended to include other quantum degrees of freedom like valley and pseudospins starting from corresponding density matrices. △ Less

Submitted 10 September, 2024; v1 submitted 30 April, 2024; originally announced April 2024.

Journal ref: NPJ Spintronics (2024)

Jamming memory into acoustically trained dense suspensions under shear

Authors: Edward Y. X. Ong, Anna R. Barth, Navneet Singh, Meera Ramaswamy, Abhishek Shetty, Bulbul Chakraborty, James P. Sethna, Itai Cohen

Abstract: Systems driven far from equilibrium often retain structural memories of their processing history. This memory has, in some cases, been shown to dramatically alter the material response. For example, work hardening in crystalline metals can alter the hardness, yield strength, and tensile strength to prevent catastrophic failure. Whether memory of processing history can be similarly exploited in flo… ▽ More Systems driven far from equilibrium often retain structural memories of their processing history. This memory has, in some cases, been shown to dramatically alter the material response. For example, work hardening in crystalline metals can alter the hardness, yield strength, and tensile strength to prevent catastrophic failure. Whether memory of processing history can be similarly exploited in flowing systems, where significantly larger changes in structure should be possible, remains poorly understood. Here, we demonstrate a promising route to embedding such useful memories. We build on work showing that exposing a sheared dense suspension to acoustic perturbations of different power allows for dramatically tuning the sheared suspension viscosity and underlying structure. We find that, for sufficiently dense suspensions, upon removing the acoustic perturbations, the suspension shear jams with shear stress contributions from the maximum compressive and maximum extensive axes that reflect the acoustic training. Because the contributions from these two orthogonal axes to the total shear stress are antagonistic, it is possible to tune the resulting suspension response in surprising ways. For example, we show that differently trained sheared suspensions exhibit: 1) different susceptibility to the same acoustic perturbation; 2) orders of magnitude changes in their instantaneous viscosities upon shear reversal; and 3) even a shear stress that increases in magnitude upon shear cessation. To further illustrate the power of this approach for controlling suspension properties, we demonstrate that flowing states well below the shear jamming threshold can be shear jammed via acoustic training. Collectively, our work paves the way for using acoustically induced memory in dense suspensions to generate rapidly and widely tunable materials. △ Less

Submitted 24 April, 2024; originally announced April 2024.

Comments: To be published in Physical Review X

Efficient Electrochemical CO2 Reduction Reaction over Cu-decorated Biphenylene

Authors: Radha N Somaiya, Muhammad Sajjad, Nirpendra Singh, Aftab Alam

Abstract: Developing efficient electrocatalysts for CO$_2$ reduction into value-added products is crucial for the green economy. Inspired by the recent synthesis of Biphenylene (BPH), we have systematically investigated pristine, defective, and Cu-decorated BPH as an electrocatalyst for the CO$_2$ reduction reactions (CRR). Our first-principles calculations show the CO$_2$ molecules weakly interact with the… ▽ More Developing efficient electrocatalysts for CO$_2$ reduction into value-added products is crucial for the green economy. Inspired by the recent synthesis of Biphenylene (BPH), we have systematically investigated pristine, defective, and Cu-decorated BPH as an electrocatalyst for the CO$_2$ reduction reactions (CRR). Our first-principles calculations show the CO$_2$ molecules weakly interact with the pristine BPH surface while defective BPH facilitates the CO$_2$ adsorption with a binding energy ($E_b$) of -3.22 eV, indicating the detrimental process for the CRR on the surface of both systems. Furthermore, we have investigated the binding energy and kinetic stability of Cu-decorated BPH as a single-atom-catalyst (SAC). The molecular dynamics simulations confirm the kinetic stability, revealing that the Cu-atom avoids agglomeration under low metal dispersal conditions. The CO$_2$ molecule gets adsorbed horizontally on the Cu-BPH surface with $E_b$ of -0.52 eV. The CRR mechanism is investigated using two pathways beginning with two different initial intermediate states, formate ($\mathrm{^*OCOH}$) and the carboxylic ($\mathrm{^*COOH}$) pathways. The formate pathway confirms the conversion of $\mathrm{^*OCOH}$ to $\mathrm{^*HCOOH}$ with the rate-limiting potential ($U_L$) of 0.57 eV for the production of HCOOH, while for the carboxylic pathway, the conversion of $\mathrm{^*COH}$ to $\mathrm{^*CHOH}$ has $U_L$ of 0.49 eV for the production of CH$_3$OH. We have also investigated the effect of protons using charged hydrogen pseudopotential, which hints towards the possible formation of CH$_3$OH as fuel. Our findings propose Cu-BPH as an efficient single-atom catalyst for CO$_2$ conversion compared to the well-known Cu metal. △ Less

Submitted 17 April, 2024; v1 submitted 16 April, 2024; originally announced April 2024.

Comments: 9 pages, 6 figures, 8 figures in Supplementary Information

A review: The Gor'kov-Teitel'baum thermal activation model for cuprates

Authors: Navinder Singh

Abstract: While closing their famous paper entitled "Pseudogap: friend or foe of high-Tc?" Norman, Pines, and Kallin underlined that before we have a microscopic theory, we must have a consistent phenomenology. This was in 2005. As it turns out in 2006 a phenomenological theory of the pseudogap state was proposed by Gor'kov and Teitel'baum. This originated from their careful analysis of the Hall effect data… ▽ More While closing their famous paper entitled "Pseudogap: friend or foe of high-Tc?" Norman, Pines, and Kallin underlined that before we have a microscopic theory, we must have a consistent phenomenology. This was in 2005. As it turns out in 2006 a phenomenological theory of the pseudogap state was proposed by Gor'kov and Teitel'baum. This originated from their careful analysis of the Hall effect data, and it has been very successful model as numerous investigations over the years has shown. In this mini-review the essence of the idea of Gor'kov and Teitel'baum is presented. The pseudogap obtained by them from the Hall effect data agrees very well with that obtained from the ARPES data. This famous Gor'kov-Teitel'baum Thermal Activation model (in short GTTA model) not only presents a consistent phenomenology of the pseudogap state but also it rationalizes the Hall angle data and it presents a strong case against the famous "two-relaxation times" idea of Anderson and collaborators. △ Less

Submitted 26 March, 2024; originally announced April 2024.

Comments: 6 pages and 3 figures

Electronic structure and thermoelectric properties of epitaxial Sc1-xVxNy thin films grown on MgO(001)

Authors: Susmita Chowdhury, Niraj Kumar Singh, Sanath Kumar Honnali, Grzegorz Greczynski, Per Eklund, Arnaud le Febvrier, Martin Magnuson

Abstract: The electronic structure of Sc1-xVxNy epitaxial films with different alloying concentrations of V are investigated with respect to effects on thermoelectric properties. Band structure calculations on Sc0.75V0.25N indicate that V 3d states lie in the band gap of the parent ScN compound in the vicinity of the Fermi level. Thus, theoretically the presence of light (dispersive) bands at the Γ-point wi… ▽ More The electronic structure of Sc1-xVxNy epitaxial films with different alloying concentrations of V are investigated with respect to effects on thermoelectric properties. Band structure calculations on Sc0.75V0.25N indicate that V 3d states lie in the band gap of the parent ScN compound in the vicinity of the Fermi level. Thus, theoretically the presence of light (dispersive) bands at the Γ-point with band multiplicity is expected to lead to lower electrical resistivity while flat (heavy) bands at X-W-K symmetry points are associated with higher Seebeck coefficient than that of ScN. With this aim, epitaxial Sc1-xVxNy thin film samples were deposited on MgO(001) substrates. All the samples showed N substoichiometry and pseudocubic crystal structure. The N-vacancy-induced states were visible in the Sc 2p XAS spectra. The reference ScN and Sc1-xVxNy samples up to x = 0.12 were n type, exhibiting carrier concentration of 1021 cm-3, typical for degenerate semiconductors. For the highest V alloying of x = 0.15, holes became the majority charge carrier as indicated by the positive Seebeck coefficient. The underlying electronic structure and bonding mechanism in Sc1-xVxNy influence the electrical resistivity, Seebeck coefficient, and Hall effect. Thus, the work contributes to the fundamental understanding of the correlated defects and thermoelectric properties to the electronic structure in Sc-N system with V alloying. △ Less

Submitted 1 April, 2024; originally announced April 2024.

Comments: 29 pages, 8 figures, 4 tables

Effect of cathode porosity on the Lithium air cell oxygen reduction reaction

Authors: Jeongwook Seo, Shrihari Sankarasubramanian, Nikhilendra Singh, Fuminori Mizuno, Kensuke Takechi, Jai Prakash

Abstract: The kinetics of the oxygen reduction reaction (ORR) on the practical air cathode in a Lithium air cell, which is conventionally composed of porous carbon with or without catalysts supported on it, was investigated. The mechanism and kinetics of the oxygen reduction reaction (ORR) was studied on a porous carbon electrode in an oxygen saturated solution of 0.1M Lithium bis-trifluoromethanesulfonimid… ▽ More The kinetics of the oxygen reduction reaction (ORR) on the practical air cathode in a Lithium air cell, which is conventionally composed of porous carbon with or without catalysts supported on it, was investigated. The mechanism and kinetics of the oxygen reduction reaction (ORR) was studied on a porous carbon electrode in an oxygen saturated solution of 0.1M Lithium bis-trifluoromethanesulfonimidate (LiTFSI) in Dimethoxyethane (DME) using cyclic voltammetery (CV) and the rotating ring-disk electrode (RRDE) technique. The oxygen reduction and evolution reactions were found to occur at similar potentials to those observed on a smooth, planar glassy carbon (GC) electrode. The effect of the porosity and the resultant increase in surface area were readily observed in the increase in the transient time required for the intermediates to reach the ring and the much larger disk currents (compared to smooth, planar GC) recorded respectively. The RRDE data was analyzed using a kinetic model previously developed by us and the rate constant of the elementary reactions calculated. The rates constant for the electrochemical reactions were found to be similar in magnitude to the rate constants calculated for smooth GC disks. The porosity of the electrode was found to decrease the rate of desorption of the intermediate and the product and delay their diffusion by shifting it from a Fickian regime in the electrolyte bulk to the Knudsen regime in the film pores. Thus, it is shown that the effect of the electrode porosity on the kinetics of the ORR is physical rather than electrochemical. △ Less

Submitted 31 March, 2024; originally announced April 2024.

Journal ref: Electrochimica Acta 248, 570-577 (2017)

Large-Enhancement Nanoscale Dynamic Nuclear Polarization Near a Silicon Nanowire Surface

Authors: Sahand Tabatabaei, Pritam Priyadarsi, Namanish Singh, Pardis Sahafi, Daniel Tay, Andrew Jordan, Raffi Budakian

Abstract: Dynamic nuclear polarization (DNP) has revolutionized the field of NMR spectroscopy, expanding its reach and capabilities to investigate diverse materials, biomolecules, and complex dynamic processes. Bringing high-efficiency DNP to the nanometer scale would open new avenues for studying nanoscale nuclear spin ensembles, such as single biomolecules, virus particles, and condensed matter systems. C… ▽ More Dynamic nuclear polarization (DNP) has revolutionized the field of NMR spectroscopy, expanding its reach and capabilities to investigate diverse materials, biomolecules, and complex dynamic processes. Bringing high-efficiency DNP to the nanometer scale would open new avenues for studying nanoscale nuclear spin ensembles, such as single biomolecules, virus particles, and condensed matter systems. Combining pulsed DNP with nanoscale force-detected magnetic resonance measurements, we demonstrated a 100-fold enhancement in the Boltzmann polarization of proton spins in nanoscale sugar droplets at 6 K and 0.33 T. Crucially, this enhancement corresponds to a factor of 200 reduction in the averaging time compared to measurements that rely on the detection of statistical fluctuations in nanoscale nuclear spin ensembles. These results significantly advance the capabilities of force-detected magnetic resonance detection as a practical tool for nanoscale imaging. △ Less

Submitted 25 February, 2024; originally announced February 2024.

Comments: Main text: 26 pages, 5 figures, 3 tables; supplemental material: 36 pages, 15 figures, 4 tables

Splitting probabilities as optimal controllers of rare reactive events

Authors: Aditya N. Singh, David T. Limmer

Abstract: The committor constitutes the primary quantity of interest within chemical kinetics as it is understood to encode the ideal reaction coordinate for a rare reactive event. We show the generative utility of the committor, in that it can be used explicitly to produce a reactive trajectory ensemble that exhibits numerically exact statistics as that of the original transition path ensemble. This is don… ▽ More The committor constitutes the primary quantity of interest within chemical kinetics as it is understood to encode the ideal reaction coordinate for a rare reactive event. We show the generative utility of the committor, in that it can be used explicitly to produce a reactive trajectory ensemble that exhibits numerically exact statistics as that of the original transition path ensemble. This is done by relating a time-dependent analogue of the committor that solves a generalized bridge problem, to the splitting probability that solves a boundary value problem under a bistable assumption. By invoking stochastic optimal control and spectral theory, we derive a general form for the optimal controller of a bridge process that connects two metastable states expressed in terms of the splitting probability. This formalism offers an alternative perspective into the role of the committor and its gradients, in that they encode forcefields that guarantee reactivity, generating trajectories that are statistically identical to the way that a system would react autonomously. △ Less

Submitted 3 July, 2024; v1 submitted 8 February, 2024; originally announced February 2024.

Pseudogap in \(Sr_{2-x}La_{x}IrO_{4}\): Gor'kov-Teitel'baum thermal activation model

Authors: Jalaja Pandya, Devarshi Dave, Navinder Singh

Abstract: Recently, Hall effect measurements are done on Lanthanum doped Strontium Iridate \(Sr_{2-x}La_{x}IrO_{4}\) which is 5d analogue of cuprates \cite{hsu2023carrier}. Hall effect measurements show that the effective carrier density \(n_{H}\) exhibits a crossover from \(n_{H} \sim x\) to \(n_{H} \sim 1+x\) near \(x\simeq 0.16\) . This is very similar to that found in cuprates around $p\simeq0.19$. It i… ▽ More Recently, Hall effect measurements are done on Lanthanum doped Strontium Iridate \(Sr_{2-x}La_{x}IrO_{4}\) which is 5d analogue of cuprates \cite{hsu2023carrier}. Hall effect measurements show that the effective carrier density \(n_{H}\) exhibits a crossover from \(n_{H} \sim x\) to \(n_{H} \sim 1+x\) near \(x\simeq 0.16\) . This is very similar to that found in cuprates around $p\simeq0.19$. It is proposed that a pseudogap (PG) state in \(Sr_{2-x}La_{x}IrO_{4}\) exists and is ending at \(x\simeq 0.16\) \cite{hsu2023carrier}. However, PG boundary (in doping-temperature phase diagram) remains unknown. In this work, we apply a very successfull the Gor'kov-Teitel'baum Thermal Activation (GTTA) model to \(Sr_{2-x}La_{x}IrO_{4}\) and obtain its PG phase boundary and draw an updated phase diagram. Our results agree with previously known signatures of PG phase in this system \cite{seo2017infrared,de2015collapse}. Using results from GTTA model we also obtain the evolution of "Fermi arcs" in this system as a function %of both doping and temperature for doping concentration $x\simeq0.08$ which are in qualitative agreement with the reported results. △ Less

Submitted 25 January, 2024; originally announced January 2024.

Comments: 6 pages and 7 figures

Ternary Alkali Metal Copper Chalcogenides ACuX (A= Na, K and X= S, Se, Te): Promising Candidate for Solar Harvesting Applications

Authors: Gurudayal Behera, Surabhi Suresh Nair, Nirpendra Singh, K. R. Balasubramaniam, Aftab Alam

Abstract: We report a comprehensive first-principles study of the relative stability of the various possible crystal structures, and the electronic and optical properties of ternary alkali metal chalcogenides ACuX (A= Na/K and X= S/Se/Te) compounds through density functional theory (DFT) calculations. The energetics and phonon spectra of greater than 700 structures were compared, and seven possible stabiliz… ▽ More We report a comprehensive first-principles study of the relative stability of the various possible crystal structures, and the electronic and optical properties of ternary alkali metal chalcogenides ACuX (A= Na/K and X= S/Se/Te) compounds through density functional theory (DFT) calculations. The energetics and phonon spectra of greater than 700 structures were compared, and seven possible stabilized structures of six ACuX compounds were identified using the fixed composition evolutionary search method. Our electronic band structure simulation confirms that all the ternary ACuX compounds are direct band gap semiconductors, with the band gap lying between 0.83 eV to 2.88 eV. These compounds exhibit directly allowed electronic transitions from the valence band to the conduction band, which leads to a significant strength of optical transition probability. This yields a sharp rise in the optical absorption spectra (ranging between 10$^4$ to 10$^5$ cm$^{-1}$) near the energy gap. The estimated spectroscopic limited maximum efficiency (SLME) is about 18% for an 8 $μ$m thick NaCuTe film. For other ACuX compounds, the SLME ranges between 10% to 13%. In addition, we also explored the feasibility of these ternary ACuX compounds for photocatalytic water splitting applications and found that they can be promising candidates as photocathodes for hydrogen evolution reactions. With a large spread in the band gap and interesting band topology near Fermi level, these chalcogenides can be quite fertile for other energy applications such as thermoelectric, LED, etc. △ Less

Submitted 26 December, 2023; originally announced December 2023.

Pressure-Induced Topological Dirac Semimetallic Phase in KCdP

Authors: Shivendra Kumar Gupta, Nikhilesh Singh, Saurabh Kumar Sen, Poorva Singh

Abstract: Dirac semimetals (DSMs), characterized by linear dispersion relations in their electronic band structure, have gained prominence due to their unique topological features and potential applications in electronic devices. Through systematic calculation, we explore the electronic structure evolution of KCdP under varying negative pressure conditions. Our findings reveal a compelling transition from a… ▽ More Dirac semimetals (DSMs), characterized by linear dispersion relations in their electronic band structure, have gained prominence due to their unique topological features and potential applications in electronic devices. Through systematic calculation, we explore the electronic structure evolution of KCdP under varying negative pressure conditions. Our findings reveal a compelling transition from a normal semiconductor to a triple point semimetal when spin-orbit coupling (SOC) is not introduced, whereas in the SOC case, it converts into a Dirac semimetallic state in KCdP under negative triaxial pressure. The electronic band structure exhibits distinct Dirac cones at the Fermi level, indicating the presence of massless Dirac fermions. Moreover, the negative pressure-induced Dirac semimetallic phase in this compound is found to be robust and is protected by crystal symmetry. We provide a symmetry analysis of the bandgap, Fermi surface, Fermi velocity, and other relevant electronic properties, offering insights into the pressure-driven phase transition in KCdP. The tunability of this material under external pressure suggests the potential utility in next-generation electronic devices and quantum technologies. △ Less

Submitted 28 March, 2026; v1 submitted 9 December, 2023; originally announced December 2023.

Effects of W alloying on the electronic structure, phase stability and thermoelectric power factor in epitaxial CrN thin films

Authors: Niraj Kumar Singh, Victor Hjort, Sanath Kumar Honnali, Davide Gambino, Arnaud le Febvrier, Ganpati Ramanath, Björn Alling, Per Eklund

Abstract: CrN-based alloy thin films are of interest as thermoelectric materials for energy harvesting. Ab initio calculations show that dilute alloying of CrN with 3 at.% W substituting Cr, induce flat electronic bands and push the Fermi level EF into the conduction band, while retaining dispersive Cr 3d bands. These features are conducive for both high electrical conductivity σand high Seebeck coefficient… ▽ More CrN-based alloy thin films are of interest as thermoelectric materials for energy harvesting. Ab initio calculations show that dilute alloying of CrN with 3 at.% W substituting Cr, induce flat electronic bands and push the Fermi level EF into the conduction band, while retaining dispersive Cr 3d bands. These features are conducive for both high electrical conductivity σand high Seebeck coefficient α, and hence the thermoelectric power factor α^2σ. To investigate this possibility, epitaxial CrWxNz films were grown on c-plane sapphire by dc-magnetron sputtering. However, even films with the lowest W concentration (x = 0.03) in our study contained metallic h-Cr2N, which is not conducive for a high α. Nevertheless, the films exhibit a sizeable power factor of α^2σ~ 4.7 x 10-4 Wm-1K-2 due to high σ~ 700 Scm-1, and a moderate α~ -25 ~{^^^^00b5}V/K. Increasing h-Cr2N fractions in the 0.03 < x \le 0.19 range monotonically increases σ, but severely diminishes αleading to two orders of magnitude decrease in α^2σ. This trend continues with x > 0.19 due to W precipitation. These findings indicate that dilute W additions below its solubility limit in CrN is important for realizing high thermoelectric power factor in CrWxNz alloy films. △ Less

Submitted 25 June, 2024; v1 submitted 4 November, 2023; originally announced November 2023.

Review cuprates -- Anderson's unhappy electrons and their fate

Authors: Navinder Singh

Abstract: In cuprates, as doping $p$ is reduced from the overdoped side through the quantum critical point $p^*$, a transition in Hall number density of carriers is observed, in which this number of carriers reduces from $1+p$ holes per copper site to $p$ holes per copper site. The connection of this $1+p$ to $p$ transition with pseudogap and with superconductivity is discussed. A "panoramic" view or a "bro… ▽ More In cuprates, as doping $p$ is reduced from the overdoped side through the quantum critical point $p^*$, a transition in Hall number density of carriers is observed, in which this number of carriers reduces from $1+p$ holes per copper site to $p$ holes per copper site. The connection of this $1+p$ to $p$ transition with pseudogap and with superconductivity is discussed. A "panoramic" view or a "broad-brush" discussion is presented in which Anderson's "unhappy" electrons take on a central stage. △ Less

Submitted 9 August, 2023; originally announced August 2023.

Comments: 6 pages and 4 figures

Site-specific stable deterministic single photon emitters with low Huang-Rhys value in layered hexagonal boron nitride at room temperature

Authors: Amit Bhunia, Pragya Joshi, Nitesh Singh, Biswanath Chakraborty, Rajesh V Nair

Abstract: Development of stable room-temperature bright single-photon emitters using atomic defects in hexagonal-boron nitride flakes (h-BN) provides significant promises for quantum technologies. However, an outstanding challenge in h-BN is creating site-specific, stable, high emission rate single photon emitters with very low Huang-Rhys (HR) factor. Here, we discuss the photonic properties of site-specifi… ▽ More Development of stable room-temperature bright single-photon emitters using atomic defects in hexagonal-boron nitride flakes (h-BN) provides significant promises for quantum technologies. However, an outstanding challenge in h-BN is creating site-specific, stable, high emission rate single photon emitters with very low Huang-Rhys (HR) factor. Here, we discuss the photonic properties of site-specific, isolated, stable quantum emitter that emit single photons with a high emission rate and unprecedented low HR value of 0.6 at room temperature. Scanning confocal image confirms site-specific single photon emitter with a prominent zero-phonon line at ~578 nm with saturation photon counts of 105 counts/second. The second-order intensity-intensity correlation measurement shows an anti-bunching dip of ~0.25 with an emission lifetime of 2.46 ns. Low-energy electron beam irradiation and subsequent annealing are important to achieve stable single photon emitters. △ Less

Submitted 21 July, 2023; originally announced July 2023.

Ultrahigh electrostrain > 1% in lead-free piezoceramics: A critical review

Authors: Gobinda Das Adhikary, Digivijay Narayan Singh, Getaw Abebe Tina, Gudeta Jafo Muleta, Rajeev Ranjan

Abstract: Recently, a series of reports showing ultra-high electrostrain (> 1 %) have appeared in several Pb-free piezoceramics. The ultrahigh electrostrain has been attributed exclusively to the defect dipoles created in these systems. We examine these claims based on another report arXiv:2208.07134 which demonstrated that the measured electric field driven strain increased dramatically simply by reducing… ▽ More Recently, a series of reports showing ultra-high electrostrain (> 1 %) have appeared in several Pb-free piezoceramics. The ultrahigh electrostrain has been attributed exclusively to the defect dipoles created in these systems. We examine these claims based on another report arXiv:2208.07134 which demonstrated that the measured electric field driven strain increased dramatically simply by reducing the thickness of the ceramic discs. We prepared some representative Pb-free compositions reported to exhibit ultrahigh strain and performed electrostrain measurements. We found that these compositions do not show ultrahigh electrostrain if the thickness of the discs is above 0.3 mm (the disc diameters were in the range 10- 12 mm diameter). The ultrahigh strain values were obtained when the thickness was below 0.3 mm. We compare the electrostrain obtained from specimens designed to exhibit defect dipoles with specimens that were not designed to have defect dipoles in Na0.5Bi0.5TiO3 (NBT) and K0.5Na0.5NbO3 (KNN) -based lead-free systems and could obtain much higher strain levels (4- 5 %) in the defect dipole free piezoceramics in the small thickness regime. Our results do not favor the defect dipole theory as the exclusive factor for causing ultrahigh strain in piezoceramics. A new approach is called for to understand the phenomenon of ultrahigh electrostrain caused by the thickness reduction of piezoceramic discs. △ Less

Submitted 5 June, 2023; originally announced June 2023.

Comments: 9 pages, 5 figures, lab report

CMOS + stochastic nanomagnets: heterogeneous computers for probabilistic inference and learning

Authors: Nihal Sanjay Singh, Keito Kobayashi, Qixuan Cao, Kemal Selcuk, Tianrui Hu, Shaila Niazi, Navid Anjum Aadit, Shun Kanai, Hideo Ohno, Shunsuke Fukami, Kerem Y. Camsari

Abstract: Extending Moore's law by augmenting complementary-metal-oxide semiconductor (CMOS) transistors with emerging nanotechnologies (X) has become increasingly important. One important class of problems involve sampling-based Monte Carlo algorithms used in probabilistic machine learning, optimization, and quantum simulation. Here, we combine stochastic magnetic tunnel junction (sMTJ)-based probabilistic… ▽ More Extending Moore's law by augmenting complementary-metal-oxide semiconductor (CMOS) transistors with emerging nanotechnologies (X) has become increasingly important. One important class of problems involve sampling-based Monte Carlo algorithms used in probabilistic machine learning, optimization, and quantum simulation. Here, we combine stochastic magnetic tunnel junction (sMTJ)-based probabilistic bits (p-bits) with Field Programmable Gate Arrays (FPGA) to create an energy-efficient CMOS + X (X = sMTJ) prototype. This setup shows how asynchronously driven CMOS circuits controlled by sMTJs can perform probabilistic inference and learning by leveraging the algorithmic update-order-invariance of Gibbs sampling. We show how the stochasticity of sMTJs can augment low-quality random number generators (RNG). Detailed transistor-level comparisons reveal that sMTJ-based p-bits can replace up to 10,000 CMOS transistors while dissipating two orders of magnitude less energy. Integrated versions of our approach can advance probabilistic computing involving deep Boltzmann machines and other energy-based learning algorithms with extremely high throughput and energy efficiency. △ Less

Submitted 23 February, 2024; v1 submitted 12 April, 2023; originally announced April 2023.

Journal ref: Nature Communications volume 15, Article number: 2685 (2024)

Interaction of Acoustic and Optical Phonons in Soft Bonded Cu-Se Framework of Large Unit Cell Minerals with Anionic Disorders

Authors: Kewal Singh Rana, Raveena Gupta, Debattam Sarkar, Niraj Kumar Singh, Somnath Acharya, Satish Vitta, Chandan Bera, Kanishka Biswas, Ajay Soni

Abstract: Large unit cell copper-chalcogenide based minerals with high crystalline anharmonicity have a potential for thermoelectric applications owing to their inherent poor lattice thermal conductivity. Here, the softening of copper-selenium bonding and hence crystal framework plays an important role in superionic conduction and thermal conductivity. We have studied Cu26Nb2Sn6Se32, Cu26Nb2Sn6Se31.5 and Cu… ▽ More Large unit cell copper-chalcogenide based minerals with high crystalline anharmonicity have a potential for thermoelectric applications owing to their inherent poor lattice thermal conductivity. Here, the softening of copper-selenium bonding and hence crystal framework plays an important role in superionic conduction and thermal conductivity. We have studied Cu26Nb2Sn6Se32, Cu26Nb2Sn6Se31.5 and Cu26Nb2Sn6Se30Te2 minerals with a strategically tailored anionic disorders. These compounds have p-type degenerate behavior with carrier concentration ranging between 1020 cm-3 at 300 K, high power factor and low lattice thermal conductivity at 640 K. The existence of two low frequency Raman active optical modes associated with soft Cu and Se atoms, three localized Einstein modes in specific heat, suggest high scattering between acoustic and optical branches with very short phonon lifetime less than 1 ps. The excess vibrational density of states at low energies with compressed and flat optical branches strongly hinders the heat transport in these crystalline mineral. Comparatively, Cu26Nb2Sn6Se30Te2 is a promising thermoelectric material because of high crystalline anharmonicity and softening of Cu-Se framework due to heavier tellurium atom. △ Less

Submitted 16 March, 2023; originally announced March 2023.

Comments: 20 Pages, 14 figures in main text and supporting information

Report number: 045202

Journal ref: Phys. Rev. B 108, 045202 (2023)

Variational deep learning of equilibrium transition path ensembles

Authors: Aditya N. Singh, David T. Limmer

Abstract: We present a time dependent variational method to learn the mechanisms of equilibrium reactive processes and efficiently evaluate their rates within a transition path ensemble. This approach builds off variational path sampling methodology by approximating the time dependent commitment probability within a neural network ansatz. The reaction mechanisms inferred through this approach are elucidated… ▽ More We present a time dependent variational method to learn the mechanisms of equilibrium reactive processes and efficiently evaluate their rates within a transition path ensemble. This approach builds off variational path sampling methodology by approximating the time dependent commitment probability within a neural network ansatz. The reaction mechanisms inferred through this approach are elucidated by a novel decomposition of the rate in terms of the components of a stochastic path action conditioned on a transition. This decomposition affords an ability to resolve the typical contribution of each reactive mode and their couplings to the rare event. The associated rate evaluation is variational and systematically improvable through the development of a cumulant expansion. We demonstrate this method in both over- and under-damped stochastic equations of motion, in low-dimensional model systems and the isomerization of solvated alanine dipeptide. In all examples, we find that we can obtain quantitatively accurate estimates of the rates of the reactive events with minimal trajectory statistics, and gain unique insight into the transitions through the analysis of their commitment probability. △ Less

Submitted 7 July, 2023; v1 submitted 28 February, 2023; originally announced February 2023.

Enhancement of the Rate of Surface Reactions by Elasto-capillary Effect

Authors: Nitish Singh, Animangsu Ghatak

Abstract: Rate of a reaction is enhanced by increase in temperature, partial pressure or the concentration of reactants, and/or via use of a catalyst or an enzyme. Whereas the former increases the probability of collision rate between different molecules, the latter provides an alternate reaction path that diminishes the activation energy barrier for reaction, both effects increase the reaction rate. The de… ▽ More Rate of a reaction is enhanced by increase in temperature, partial pressure or the concentration of reactants, and/or via use of a catalyst or an enzyme. Whereas the former increases the probability of collision rate between different molecules, the latter provides an alternate reaction path that diminishes the activation energy barrier for reaction, both effects increase the reaction rate. The deformability of the substrate on which a reaction occurs is not known to affect this rate. In contrast, by carrying out reduction of Gold and Silver salt on soft, crosslinked layers of poly(dimethylsiloxane) (PDMS), we show here that surface tension driven deformation of the solid too can increase the reaction rate. The PDMS contains SiH groups as organosilanes which reduces the salt thereby producing the corresponding metallic nano-particles. When an aqueous solution of the salt is dispensed as a sessile drop on a sufficiently soft PDMS layer, nano-particles get generated at ~3 times the rate over which when the salt solution is dispensed as a pool on the same surface. Unlike the pool, the drop has a three-phase contact line, where it pulls up the solid forming a ridge. By balancing the resultant surface stresses at the vicinity of this ridge, we have estimated the excess surface free energy associated with it. We show that this excess energy can diminish the activation energy barrier and increase the reaction rate to the extent that would have been achieved by increasing the reaction temperature by over 30o C. △ Less

Submitted 22 February, 2023; originally announced February 2023.

Comments: 27 pages, 4 figures

Robust fabrication of ultra-soft tunable PDMS microcapsules as a biomimetic model for red blood cells

Authors: Qi Chen, Naval Singh, Kerstin Schirrmann, Qi Zhou, Igor Chernyavsky, Anne Juel

Abstract: Microcapsules with liquid cores encapsulated by thin membranes have many applications in science, medicine and industry. In this paper, we design a suspension of microcapsules which flow and deform like red blood cells (RBCs), as a valuable tool to investigate microhaemodynamics. A reconfigurable and easy-to-assemble 3D nested glass capillary device is used to robustly fabricate water-oil-water do… ▽ More Microcapsules with liquid cores encapsulated by thin membranes have many applications in science, medicine and industry. In this paper, we design a suspension of microcapsules which flow and deform like red blood cells (RBCs), as a valuable tool to investigate microhaemodynamics. A reconfigurable and easy-to-assemble 3D nested glass capillary device is used to robustly fabricate water-oil-water double emulsions which are then converted into spherical microcapsules with hyperelastic membranes by cross-linking the polydimethylsiloxane (PDMS) layer coating the droplets. The resulting capsules are monodisperse to within 1% and can be made in a wide range of size and membrane thickness. We use osmosis to deflate by 36% initially spherical capsules of diameter 350 μm and a membrane thickness of 4% of their radius, in order to match the reduced volume of biconcave RBCs. We compare the propagation of initially spherical and deflated capsules under constant volumetric flow in cylindrical capillaries of different confinements. We find that only deflated capsules deform broadly similarly to RBCs over a similar range of capillary numbers (Ca) -- the ratio of viscous to elastic forces. Similarly to the RBCs, the microcapsules transition from a symmetric 'parachute' to an asymmetric 'slipper'-like shape as Ca increases within the physiological range, demonstrating intriguing confinement-dependent dynamics. In addition to biomimetic RBC properties, high-throughput fabrication of tunable ultra-soft microcapsules could be further functionalized and find applications in other areas of science and engineering △ Less

Submitted 21 February, 2023; v1 submitted 19 February, 2023; originally announced February 2023.

Comments: 14 pages, 10 figures

Continuous manipulation and characterization of colloidal beads and liposomes via diffusiophoresis in single- and double-junction microchannels

Authors: Adnan Chakra, Naval Singh, Goran T. Vladisavljević, François Nadal, Cécile Cottin-Bizonne, Christophe Pirat, Guido Bolognesi

Abstract: We reveal an unreported physical mechanism that enables the pre-concentration, sorting and characterization of charged polystyrene nanobeads and liposomes dispersed in a continuous flow within a straight micron-sized channel. Initially, a single $Ψ$-junction microfluidic chip is used to generate a steady-state salt concentration gradient in the direction perpendicular to the flow. As a result, flu… ▽ More We reveal an unreported physical mechanism that enables the pre-concentration, sorting and characterization of charged polystyrene nanobeads and liposomes dispersed in a continuous flow within a straight micron-sized channel. Initially, a single $Ψ$-junction microfluidic chip is used to generate a steady-state salt concentration gradient in the direction perpendicular to the flow. As a result, fluorescent nanobeas dispersed in the electrolyte solutions accumulate into symmetric regions of the channel, appearing as two distinct symmetric stripes when the channel is observed from top via epi-fluorescence microscopy. Depending on the electrolyte flow configuration and, thus, the direction of the salt gradient field, the fluorescent stripes get closer to or apart from each other as the distance from the inlet increases. Our numerical and experimental analysis shows that, although diffusiophoresis and hydrodynamic effects are involved in the accumulation process, diffusioosmosis plays a crucial role in the observed particles dynamics. In addition, we developed a proof-of-concept double $Ψ$-junction microfluidic device which exploits this new accumulation mechanism for the size-based separation and size detection of nanobeads as well as for the measurement of zeta potential and charged lipid composition of liposomes under continuous flow settings. This device is also used to investigate the effect of fluid-like or gel-like states of the lipid membranes on the liposome diffusiophoretic response. The proposed strategies for solute-driven manipulation of colloids have great potential for microfluidic bio-analytical testing applications, including bioparticle pre-concentration, sorting, sensing and analysis. △ Less

Submitted 7 March, 2023; v1 submitted 11 February, 2023; originally announced February 2023.

Comments: Manuscript 38 pages, 6 figures + Supplementary Information 4 pages, 2 figures

Journal ref: July 17th, 2023 - ACS Nano

Intermediate-range order governs dynamics in dense colloidal liquids

Authors: Navneet Singh, Zhen Zhang, A. K. Sood, Walter Kob, Rajesh Ganapathy

Abstract: The conventional wisdom is that liquids are completely disordered and lack non-trivial structure beyond nearest-neighbor distances. Recent observations have upended this view and demonstrated that the microstructure in liquids is surprisingly rich and plays a critical role in numerous physical, biological, and industrial processes. However, approaches to uncover this structure are either system-sp… ▽ More The conventional wisdom is that liquids are completely disordered and lack non-trivial structure beyond nearest-neighbor distances. Recent observations have upended this view and demonstrated that the microstructure in liquids is surprisingly rich and plays a critical role in numerous physical, biological, and industrial processes. However, approaches to uncover this structure are either system-specific or yield results that are not physically intuitive. Here, through single-particle resolved three-dimensional confocal microscope imaging and the use of a recently introduced four-point correlation function, we show that bidisperse colloidal liquids have a highly non-trivial structure comprising alternating layers with icosahedral and dodecahedral order, which extends well-beyond nearest-neighbor distances and grows with supercooling. By quantifying the dynamics of the system on the particle level, we establish that it is this intermediate-range order, and not the short-range order, which has a one-to-one correlation with dynamical heterogeneities, a property directly related to the relaxation dynamics of glassy liquids. Our experimental findings provide a direct and much sought-after link between the structure and dynamics of liquids and pave the way for probing the consequences of this intermediate-range order in other liquid state processes. △ Less

Submitted 10 February, 2023; originally announced February 2023.

Comments: 18 pages, and 3 figures

Anisotropic Light-Matter Interactions in Single Crystal Topological Insulator Bismuth Selenide

Authors: Divya Rawat, Aditya Singh, Niraj Kumar Singh, Ajay Soni

Abstract: Anisotropy of light-matter interactions in materials give remarkable information about the phonons and their interactions with electrons. We report the angle-resolved polarized Raman spectroscopy of single-crystal of Bi2Se3 to obtain the elements of Raman tensor for understanding the strength of polarization along different crystallographic orientations. Intensity variation in the polar plots corr… ▽ More Anisotropy of light-matter interactions in materials give remarkable information about the phonons and their interactions with electrons. We report the angle-resolved polarized Raman spectroscopy of single-crystal of Bi2Se3 to obtain the elements of Raman tensor for understanding the strength of polarization along different crystallographic orientations. Intensity variation in the polar plots corresponding to E_g^1 ~ 37 cm-1, A_1g^1 ~71 cm-1, E_g^2 ~ 130 cm-1, and A_1g^2 ~ 173 cm-1 suggests the higher differential polarizability along cross-plane (bc-plane). The polar patterns and the differences in elements of the Raman tensor provides the evidence of the fundamental electron-phonon and anisotropic light matter interactions in Bi2Se3. △ Less

Submitted 1 January, 2023; originally announced January 2023.

Comments: 11 pages and 4 figures

Report number: 155203

Journal ref: Phys. Rev. B 107, 155203 (2023)