Showing 1–41 of 41 results for author: Sahu, R

Phase locking nuclear spins in silicon with spin-orbit coupling

Authors: Habitamu Y. Walelign, Manas Ranjan Sahu, John M. Nichol

Abstract: Because they have such long coherence times, nuclear spins have extraordinary potential for use in quantum information processing devices. However, coherent nuclear spin control generally requires external phase references, such as microwave control fields. Here, we phase-lock a $^{29}$Si nuclear spin ensemble in a silicon quantum dot using only the internal electronic spin-orbit coupling as a pha… ▽ More Because they have such long coherence times, nuclear spins have extraordinary potential for use in quantum information processing devices. However, coherent nuclear spin control generally requires external phase references, such as microwave control fields. Here, we phase-lock a $^{29}$Si nuclear spin ensemble in a silicon quantum dot using only the internal electronic spin-orbit coupling as a phase reference. When driven with the quantum-dot electrons, the nuclear spins align themselves to a phase determined by the electronic spin-orbit coupling and the timing of the drive protocol. This enables us to measure the coherent precession and inhomogeneous dephasing of the nuclear spins. We corroborate our results with detailed numerical simulations of the many-body electron nuclear system. Our work opens new routes for coherently controlling solid-state nuclear spin ensembles. △ Less

Submitted 18 June, 2026; originally announced June 2026.

Comments: 7+11 pages and 4+6 figures

Diamond-to-graphite transformation under hypersonic impact

Authors: Abhijit Biswas, Aniket Mote, Rajib Sahu, Marcelo Lopes Pereira Junior, Shuo Yang, Sudaice Kazibwe, Jishnu Murukeshan, Raphael Benjamim de Oliveira, Guilherme da Silva Lopes Fabris, Shreyasi Chattopadhyay, Gelu Costin, Jianhua Li, Robert Vajtai, Ching-Wu Chu, Lizhong Lang, Yu Zou, Liangzi Deng, Tobin Filleter, Douglas Soares Galvão, Christian Kübel, Thomas E Lacy Jr, Pulickel M. Ajayan

Abstract: Diamond to graphite transformation is a complex kinetically driven process which has been studied under various conditions for its fundamental importance. We report the transformation of diamond embedded ceramic matrix composites during hypersonic impact. Diamond particles embedded in cubic boron nitride matrix provide a superhard composite that was subjected to high impact collisions of metal pro… ▽ More Diamond to graphite transformation is a complex kinetically driven process which has been studied under various conditions for its fundamental importance. We report the transformation of diamond embedded ceramic matrix composites during hypersonic impact. Diamond particles embedded in cubic boron nitride matrix provide a superhard composite that was subjected to high impact collisions of metal projectiles travelling at speeds reaching Mach 8.45. Our observations suggest that the energy absorption and fracture of the composite is primarily enabled via the phase change of diamond into graphite. Characterization of the impact-fractured composite shows transformed diamond particles and provides details of the shock-induced phase transformation and the nature of diamond-graphite interfaces formed during rapid phase change. The study provides new understanding of phase transformation of diamond under extreme conditions. △ Less

Submitted 10 August, 2026; v1 submitted 13 February, 2026; originally announced February 2026.

Comments: 4 main figures, 31 supporting figures, 7 SI videos, published in Materials Today

Journal ref: Materials Today, 99, 103477 (2026)

Kerr-enhanced amplification of three-wave mixing and emergent masing regimes

Authors: Ragheed Alhyder, Rishabh Sahu, Johannes M. Fink, Mikhail Lemeshko, Georgios M. Koutentakis

Abstract: Integrated optical microresonators exploiting either second-order ($χ^{(2)}$) or third-order ($χ^{(3)}$) nonlinearities have become key platforms for frequency conversion, low-noise microwave photonics, and quantum entanglement generation. Here, we present an analytic theory of Kerr-enhanced three-wave mixing amplification in an electro-optic microresonator with both $χ^{(2)}$ and $χ^{(3)}$ nonlin… ▽ More Integrated optical microresonators exploiting either second-order ($χ^{(2)}$) or third-order ($χ^{(3)}$) nonlinearities have become key platforms for frequency conversion, low-noise microwave photonics, and quantum entanglement generation. Here, we present an analytic theory of Kerr-enhanced three-wave mixing amplification in an electro-optic microresonator with both $χ^{(2)}$ and $χ^{(3)}$ nonlinearities. We demonstrate that Kerr dressing hybridizes the optical sidebands, renormalizing the $χ^{(2)}$ couplings and detunings. As a result the system exhibits gain in regions where analogous bare $χ^{(2)}$ or $χ^{(3)}$ amplifiers are subthreshold. Time-domain Langevin simulations confirm this threshold reduction, mapping a practical design window for experiments. △ Less

Submitted 21 January, 2026; originally announced January 2026.

Comments: 15 pages, 4 figures, containing supplemental materials,

Pyroelectric effects in hybrid semiconductor-lithium niobate quantum devices

Authors: Manas Ranjan Sahu, Suraj Thapa Magar, Yadav Prasad Kandel, John M. Nichol

Abstract: Hybrid quantum devices using surface acoustic waves show promise as key elements of quantum information processors. We report measurements of integrated flip-chip devices consisting of semiconductor quantum dots and surface acoustic wave resonators in lithium niobate. We observed that the pyroelectric effect in lithium niobate inhibited the operation of quantum dots in the integrated devices. GaAs… ▽ More Hybrid quantum devices using surface acoustic waves show promise as key elements of quantum information processors. We report measurements of integrated flip-chip devices consisting of semiconductor quantum dots and surface acoustic wave resonators in lithium niobate. We observed that the pyroelectric effect in lithium niobate inhibited the operation of quantum dots in the integrated devices. GaAs/AlGaAs devices suffered from unintentional carrier depletion, and Si/SiGe devices suffered from electrostatic discharge. Our results highlight the importance of mitigating pyroelectric effects in semiconductor-lithium niobate hybrid devices for continued progress in quantum interconnects and transducers. △ Less

Submitted 22 December, 2025; originally announced December 2025.

Shear induced topological changes of local structure in dense colloidal suspensions

Authors: Ratimanasee Sahu, Abhishek Kumar Gupta, Peter Schall, Sarika Maitra Bhattacharyya, Vijayakumar Chikkadi

Abstract: Understanding the structural origins of glass formation and mechanical response remains a central challenge in condensed matter physics. Recent studies have identified the local caging potential experienced by a particle due to its nearest neighbors as a robust structural metric that links microscopic structure to dynamics under thermal fluctuations and applied shear. However, its connection to lo… ▽ More Understanding the structural origins of glass formation and mechanical response remains a central challenge in condensed matter physics. Recent studies have identified the local caging potential experienced by a particle due to its nearest neighbors as a robust structural metric that links microscopic structure to dynamics under thermal fluctuations and applied shear. However, its connection to locally favored structural motifs has remained unclear. Here, we analyze structural motifs in colloidal crystals and glasses and correlate them with the local caging potential. We find that icosahedral motifs in glasses are associated with deeper caging potentials than crystalline motifs such as face-centered cubic (FCC) and hexagonal close-packed (HCP) structures. Both crystalline and amorphous systems also contain large number of particles belonging to stable defective motifs, which are distortions of the regular motifs. Under shear, large clusters of defective motifs fragment into smaller ones, driving plastic deformation and the transition from a solid-like to a liquid-like state in amorphous suspensions. Particles that leave clusters of stable motifs are associated with shallower caging potentials and are more prone to plastic rearrangements, ultimately leading to motif disintegration during shear. Our results thus reveal that the loss of mechanical stability in amorphous suspensions is governed by the topological evolution of polytetrahedral motifs, uncovering a structural mechanism underlying plastic deformation and fluidization. △ Less

Submitted 21 August, 2025; originally announced August 2025.

Electron doping in single crystalline BaBiO$_3$: BaBiO$_{3-x}$F$_{x}$

Authors: Sathishkumar M, Asha Ann Abrahama, Rajesh Kumar Sahu, Soma Banik, Soham Manni

Abstract: Topological insulators are a new class of insulators with conducting surface state. Most of the topological insulators are chalcogenides, where a tiny amount of chalcogen vacancy destroys the predicted bulk insulating state and results in a metallic or semimetallic bulk electrical transport. BaBiO$_3$ (BBO) is an interesting large bandgap (0.7 eV) insulator that upon hole doping becomes a supercon… ▽ More Topological insulators are a new class of insulators with conducting surface state. Most of the topological insulators are chalcogenides, where a tiny amount of chalcogen vacancy destroys the predicted bulk insulating state and results in a metallic or semimetallic bulk electrical transport. BaBiO$_3$ (BBO) is an interesting large bandgap (0.7 eV) insulator that upon hole doping becomes a superconductor and is theoretically predicted to show a topological insulating state under electron doping. We have explored electron doping through the chemical substitution of fluorine atoms at the oxygen site. The single crystals of BBO and fluorine doped BBO were synthesized via a one-step solid-state technique. The single crystals of pure BBO and 10 % F -doped BBO (BaBiO$_{2.7}$F$_{0.3}$) are chemically single-phase samples and crystallize in monoclinic I2/m crystal structure. The core level and valence band X-ray photoelectron spectra confirm electron doping in the 10% fluorine-doped BBO. 20 % F-doped BBO appears to be a multiphase sample, confirmed by back-scattered electron (BSE) imaging and X-ray diffraction. This article reports on the successful growth of pure and F-doped BBO using a one-step solid-state technique and discusses the effect of F-doping on structural and electronic properties. △ Less

Submitted 22 July, 2025; originally announced July 2025.

Comments: 5 pages, 5 figures, 2 tables

Localized Radiofrequency Heating for Enhanced Thermoelectric Energy Generation Using Natural Galena Ore

Authors: Karthik R, Yiwen Zheng, Rajesh Kumar Sahu, Punathil Raman Sreeram, Soma Banik, Aniruddh Vashisth, Chandra Sekhar Tiwary

Abstract: The efficiency of thermoelectric devices can be significantly enhanced by maintaining a stable temperature gradient, which can be achieved through localized heating. Radio waves serve as an ideal heat source for this purpose. In this study, we demonstrate the enhancement of thermoelectric performance in earth-abundant natural ore Galena (PbS) through localized radio frequency (RF) heating. RF heat… ▽ More The efficiency of thermoelectric devices can be significantly enhanced by maintaining a stable temperature gradient, which can be achieved through localized heating. Radio waves serve as an ideal heat source for this purpose. In this study, we demonstrate the enhancement of thermoelectric performance in earth-abundant natural ore Galena (PbS) through localized radio frequency (RF) heating. RF heating experiments conducted at frequencies between \SI{35}{MHz} and \SI{45}{MHz} induced substantial localized heating in PbS, generating a temperature gradient of \SI{32}{K}. This resulted in a Seebeck voltage of \SI{-5.8}{mV/K}, approximately 13 times greater than the conventional Seebeck coefficient of PbS (\SI{440}{\micro V/K}). Additionally, a power factor of \SI{151}{mWm^{-1}K^{-2}} and an overall RF to thermoelectric conversion efficiency of 15\% were achieved. Molecular dynamics simulations corroborate the experimental findings, providing insights into the mechanism of thermal transport and RF-induced heating in PbS. These results highlight the potential of localized RF heating as an effective strategy for enhancing thermoelectric performance, with promising implications for ambient thermoelectric energy harvesting applications. △ Less

Submitted 23 February, 2025; originally announced February 2025.

Comments: 26 pages, 6 figures

Structural origin of relaxation in dense colloidal suspensions

Authors: Ratimanasee Sahu, Mohit Sharma, Peter Schall, Sarika Maitra Bhattacharyya, Vijayakumar Chikkadi

Abstract: Amorphous solids relax via slow molecular rearrangement induced by thermal fluctuations or applied stress. Although microscopic structural signatures predicting these structural relaxations have long been sought, a physically motivated structural measure relevant to diverse systems remains elusive. Here, we introduce a structural order parameter derived from the mean-field caging potential experie… ▽ More Amorphous solids relax via slow molecular rearrangement induced by thermal fluctuations or applied stress. Although microscopic structural signatures predicting these structural relaxations have long been sought, a physically motivated structural measure relevant to diverse systems remains elusive. Here, we introduce a structural order parameter derived from the mean-field caging potential experienced by the particles due to their neighbors, which reliably predicts the occurrence of structural relaxations. The parameter, derived from density functional theory, is a measure of susceptibility to particle rearrangements that can effectively identify weak or defect-like regions in disordered systems. Using experiments on dense colloidal suspensions, we demonstrate a causal relationship between this order parameter and the structural relaxations of the amorphous solid. In quiescent suspensions, increasing the density leads to stronger correlations between the structure and dynamics. Under applied shear, the mean structural order parameter increases with increasing strain, signaling shear-induced softening, which is accompanied by the proliferation of plastic events. In both cases, the order parameter reliably identifies weak regions where the plastic rearrangements due to thermal fluctuation or applied shear preferentially occur. Our study paves the way to a structural understanding of the relaxation of a wide range of amorphous solids, from suspensions to metallic glasses. △ Less

Submitted 9 August, 2024; v1 submitted 4 March, 2024; originally announced March 2024.

Ground state phase diagram and "parity flipping'' microwave transitions in a gate-tunable Josephson Junction

Authors: M. R. Sahu, F. J. Matute-Cañadas, M. Benito, P. Krogstrup, J. Nygård, M. F. Goffman, C. Urbina, A. Levy Yeyati, H. Pothier

Abstract: We probed a gate-tunable InAs nanowire Josephson weak link by coupling it to a microwave resonator. Tracking the resonator frequency shift when the weak link is close to pinch-off, we observe that the ground state of the latter alternates between a singlet and a doublet when varying either the gate voltage or the superconducting phase difference across it. The corresponding microwave absorption sp… ▽ More We probed a gate-tunable InAs nanowire Josephson weak link by coupling it to a microwave resonator. Tracking the resonator frequency shift when the weak link is close to pinch-off, we observe that the ground state of the latter alternates between a singlet and a doublet when varying either the gate voltage or the superconducting phase difference across it. The corresponding microwave absorption spectra display lines that approach zero energy close to the singlet-doublet boundaries, suggesting parity flipping transitions, which are in principle forbidden in microwave spectroscopy and expected to arise only in tunnel spectroscopy. We tentatively interpret them by means of an ancillary state isolated in the junction acting as a reservoir for individual electrons. △ Less

Submitted 20 December, 2023; originally announced December 2023.

Journal ref: Phys. Rev. B 109, 134506 (2024)

Multiple exciton generation in VO2

Authors: S. R. Sahu, S. Khan, A. Tripathy, K. Dey, N. Bano, S. Raj Mohan, M. P. Joshi, S. Verma, B. T. Rao, V. G. Sathe, D. K. Shukla

Abstract: Multiple exciton generation (MEG) is a widely studied phenomenon in semiconductor nanocrystals and quantum dots, aimed at improving the energy conversion efficiency of solar cells. MEG is the process wherein incident photon energy is significantly larger than the band gap, and the resulting photoexcited carriers relax by generating additional electron-hole pairs, rather than decaying by heat dissi… ▽ More Multiple exciton generation (MEG) is a widely studied phenomenon in semiconductor nanocrystals and quantum dots, aimed at improving the energy conversion efficiency of solar cells. MEG is the process wherein incident photon energy is significantly larger than the band gap, and the resulting photoexcited carriers relax by generating additional electron-hole pairs, rather than decaying by heat dissipation. Here, we present an experimental demonstration of MEG in a prototype strongly correlated material, VO2, through photocurrent spectroscopy and ultrafast transient reflectivity measurements, both of which are considered the most prominent ways for detecting MEG in working devices. The key result of this paper is the observation of MEG at room temperature (in a correlated insulating phase of VO2), and the estimated threshold for MEG is 3Eg. We demonstrate an escalated photocurrent due to MEG in VO2, and quantum efficiency is found to exceed 100%. Our studies suggest that this phenomenon is a manifestation of expeditious impact ionization due to stronger electron correlations and could be exploited in a large number of strongly correlated materials. △ Less

Submitted 23 October, 2023; originally announced October 2023.

Comments: 6 pages, 5 figures, Physical Review B

Journal ref: Physical Review B 108, 125133 (2023)

Operando Insights on the Degradation Mechanisms of Rhenium-doped and Undoped Molybdenum Disulfide Nanocatalysts for Electrolyzer Applications

Authors: Raquel Aymerich-Armengol, Miquel Vega-Paredes, Zhenbin Wang, Andrea M. Mingers, Luca Camuti, Jeeung Kim, Jeongwook Bae, Ilias Efthimiopoulos, Rajib Sahu, Filip Podjaski, Martin Rabe, Christina Scheu, Joohyun Lim, Siyuan Zhang

Abstract: MoS2 nanostructures are promising catalysts for proton-exchange-membrane (PEM) electrolyzers to replace expensive noble metals. Their broadscale application demands high activity for the hydrogen evolution reaction (HER) as well as robust durability. Doping is commonly applied to enhance the HER activity of MoS2-based nanocatalysts, but the effect of dopants in the electrochemical and structural s… ▽ More MoS2 nanostructures are promising catalysts for proton-exchange-membrane (PEM) electrolyzers to replace expensive noble metals. Their broadscale application demands high activity for the hydrogen evolution reaction (HER) as well as robust durability. Doping is commonly applied to enhance the HER activity of MoS2-based nanocatalysts, but the effect of dopants in the electrochemical and structural stability is yet to be discussed. Herein, we correlate operando electrochemical measurements to the structural evolution of the materials down to the nanometric scale by identical location electron microscopy and spectroscopy. The range of stable operation for MoS2 nanocatalysts with and without rhenium doping is experimentally defined. The responsible degradation mechanisms at first electrolyte contact, open circuit stabilization and HER conditions are experimentally identified and confirmed with the calculated Pourbaix diagram of Re-doped MoS2. Doping MoS2-based nanocatalysts is validated as a promising strategy for the continuous improvement of high performance and durable PEM electrolyzers. △ Less

Submitted 21 April, 2024; v1 submitted 16 September, 2023; originally announced September 2023.

Scalable fabrication of gap-plasmon-based dynamic and chromogenic nanostructures by capillary-interaction driven self-assembly of liquid-metal

Authors: Renu Raman Sahu, Alwar Samy Ramasamy, Santosh Bhonsle S, Mark Vailshery D C, Tapajyoti Das Gupta

Abstract: Dynamically tunable nanoengineered structures for coloration show promising applications in sensing, displays, and communication. However, their potential challenge remains in having a scalable manufacturing process over large scales in tens of cm of area. For the first time, we report a novel approach for fabricating chromogenic nanostructures that respond to mechanical stimuli by utilizing the f… ▽ More Dynamically tunable nanoengineered structures for coloration show promising applications in sensing, displays, and communication. However, their potential challenge remains in having a scalable manufacturing process over large scales in tens of cm of area. For the first time, we report a novel approach for fabricating chromogenic nanostructures that respond to mechanical stimuli by utilizing the fluidic properties of polydimethylsiloxane (PDMS) as a substrate and the interfacial tension of liquid metal-based plasmonic nanoparticles. Relying on the PDMS tunable property and a physical deposition method, our approach is single-step, scalable, and does not rely on high carbon footprint lithographic processes. By tuning the oligomer content in PDMS, we show that varieties of structural colors covering a significant gamut in CIE coordinates are achieved. We develop a model which depicts the formation of Ga nanodroplets from the capillary interaction of oligomers in PDMS with Ga. We showcase the capabilities of our processing technique by presenting prototypes of reflective displays and sensors for monitoring body parts, smart bandages, and the capacity of the nanostructured film to map force in real time. These examples illustrate this technology's broad range of applications, such as large-area displays, devices for human-computer interactions, healthcare, and visual communication. △ Less

Submitted 13 April, 2023; originally announced April 2023.

Kinetically Decoupled Electrical and Structural Phase Transitions in VO2

Authors: S. R. Sahu, S. S. Majid, A. Ahad, A. Tripathy, K. Dey, S. Pal, B. K. De, Wen-Pin Hsieh, R. Rawat, V. G. Sathe, D. K. Shukla

Abstract: Vanadium dioxide (VO2) has drawn significant attention for its near room temperature insulator to metal transition and associated structural phase transition. The underlying Physics behind the temperature induced insulator to metal and concomitant structural phase transition in VO2 is yet to be fully understood. We have investigated the kinetics of the above phase transition behaviors of VO2 with… ▽ More Vanadium dioxide (VO2) has drawn significant attention for its near room temperature insulator to metal transition and associated structural phase transition. The underlying Physics behind the temperature induced insulator to metal and concomitant structural phase transition in VO2 is yet to be fully understood. We have investigated the kinetics of the above phase transition behaviors of VO2 with the help of resistivity measurements and Raman spectroscopy. Resistance thermal hysteresis scaling and relaxation measurements across the temperature induced insulator to metal transition reveal the unusual behaviour of this first-order phase transition, whereas Raman relaxation measurements show that the temperature induced structural phase transition in VO2 follows usual behaviour and is consistent with mean field prediction. At higher temperature sweeping rates decoupling of insulator to metal transition and structural phase transition have been confirmed. The observed anomalous first order phase transition behavior in VO2 is attributed to the unconventional quasi particle dynamics, i.e. significantly lowered electronic thermal conductivity across insulator to metal transition, which is confirmed by ultrafast optical pump-probe time domain thermoreflectance measurements. △ Less

Submitted 6 October, 2022; originally announced October 2022.

Comments: 25 pages, 8 figures

Monoclinic symmetry at the nanoscale in lead-free ferroelectric BaZr$_{x}$Ti$_{1-x}$O$_{3}$ ceramics

Authors: Koushik Dey, Abinash Tripathy, Shikha Rani Sahu, Himanshu Srivastava, Archna Sagdeo, Joerg Strempfer, Dinesh Kumar Shukla

Abstract: Local structural symmetries play a key role in the functionalities of ferroelectric materials and are often found different from average symmetry. Here, we study the real space nanoscale structure in Pb-free BaZr$_{x}$Ti$_{1-x}$O$_{3}$ (x $\leq$ 0.10) by pair distribution function measurements, complemented by transmission electron microscopy and x-ray diffraction. Our observations show existence… ▽ More Local structural symmetries play a key role in the functionalities of ferroelectric materials and are often found different from average symmetry. Here, we study the real space nanoscale structure in Pb-free BaZr$_{x}$Ti$_{1-x}$O$_{3}$ (x $\leq$ 0.10) by pair distribution function measurements, complemented by transmission electron microscopy and x-ray diffraction. Our observations show existence of the rhombohedrally distorted unit cells; however, at intermediate length scales, at least up to 5 nm, there exist nano-scale correlated regions of monoclinic symmetry. This is complemented by the observation of curved frustrated nanodomains. Further, the average structure is found to have coexisting monoclinic and rhombohedral symmetries. Our observation of a two-phase ferroelectric state is in contrast to interferroelectric instabilities of conventional polymorphic phase boundaries reported for doped BaTiO$_{3}$. △ Less

Submitted 12 May, 2022; originally announced May 2022.

Comments: 7 pages, 6 figures

Electrically switchable tunneling across a graphene pn junction: evidence for canted antiferromagnetic phase in $ν=0$ state

Authors: Arup Kumar Paul, Manas Ranjan Sahu, Kenji Watanabe, Takashi Taniguchi, J. K. Jain, Ganpathy Murthy, Anindya Das

Abstract: The ground state of a graphene sheet at charge neutrality in a perpendicular magnetic field remains enigmatic, with various experiments supporting canted antiferromagnetic, bond ordered, and even charge density wave phases. A promising avenue to elucidating the nature of this state is to sandwich it between regions of different filling factors, and study spin-dependent tunneling across the edge mo… ▽ More The ground state of a graphene sheet at charge neutrality in a perpendicular magnetic field remains enigmatic, with various experiments supporting canted antiferromagnetic, bond ordered, and even charge density wave phases. A promising avenue to elucidating the nature of this state is to sandwich it between regions of different filling factors, and study spin-dependent tunneling across the edge modes at the interfaces. Here we report on tunnel transport through a $ν=0$ region in a graphite-gated, hexagonal boron nitride ($hBN$) encapsulated monolayer graphene device, with the $ν=0$ strip sandwiched by spin-polarized $ν=\pm1$ quantum Hall states. We observe finite tunneling ($t \sim 0.3-0.6$) between the $ν=\pm1$ edges at not too small magnetic fields ($B>3T$) and low tunnel bias voltage ($<30-60μV$), which is surprising because electrons at the edge states nominally have opposite spins. Hartree-Fock calculations elucidate these phenomena as being driven by the formation of a CAF order parameter in the $ν=0$ region at zero bias (for wide enough junctions) leading to non-orthogonal spins at the edges. Remarkably, this tunneling can be controllably switched off by increasing bias; bias voltage leads to a pileup of charge at the junction, leading to a collapse of the CAF order and a suppression of the tunneling. △ Less

Submitted 2 May, 2022; originally announced May 2022.

Novel Increase of Superconducting Critical Temperature of an Iron-Superconductor due to Ion Implantation

Authors: Kriti R Sahu, Thomas Wolf, A K Mishra, A Banerjee, V Ganesan, Udayan De

Abstract: Energetic ion irradiation usually decreases superconducting critical temperature(Tc), with the few exceptions involving increases up to a few K only. However, our recent 2.5X10^15 Ar/cm2 irradiations by 1.5 MeV Ar6+ enhanced Tc of the single crystal Fe-superconductor Ba(Fe0.943Co0.057)2As2 by 8.2 K from its initial onset Tc of ~16.9 K as measured from the real part of the magnetic susceptibility,… ▽ More Energetic ion irradiation usually decreases superconducting critical temperature(Tc), with the few exceptions involving increases up to a few K only. However, our recent 2.5X10^15 Ar/cm2 irradiations by 1.5 MeV Ar6+ enhanced Tc of the single crystal Fe-superconductor Ba(Fe0.943Co0.057)2As2 by 8.2 K from its initial onset Tc of ~16.9 K as measured from the real part of the magnetic susceptibility, matching measurements from the imaginary part, electrical resistivity and magnetization. Ozaki et al. (2016) explained their Tc increase of 0.5 K in FeSe0.5Te0.5 films with the thickness (t) < the irradiating proton range (R), as due to a nanoscale compressive strain developed from radiation damage of the lattice. Here, Ar irradiation with t > R results in an Ar implanted layer in our crystal. Implanted inert gas atoms often agglomerate into high-pressure bubbles to exert a large compressive strain on the lattice. We suggest that this additional compressive strain could be the reason for such a large (~49%) Tc increase. △ Less

Submitted 7 April, 2022; originally announced April 2022.

Multiple exciton generation and giant external quantum efficiency in VO$_2$

Authors: S. R. Sahu, A. Tripathy, K. Dey, N. Mansuri, V. G. Sathe, D. K. Shukla

Abstract: Multiple exciton generation (MEG) is a widely studied phenomenon in semiconductor nanocrystals and quantum dots wherein photo-excited carriers relax by generating additional electron-hole pairs. Here, we present the first experimental observation of MEG and the same leading to giant external quantum efficiency (EQE) in VO$_2$, a prototype strongly correlated material. By employing a photoexcitatio… ▽ More Multiple exciton generation (MEG) is a widely studied phenomenon in semiconductor nanocrystals and quantum dots wherein photo-excited carriers relax by generating additional electron-hole pairs. Here, we present the first experimental observation of MEG and the same leading to giant external quantum efficiency (EQE) in VO$_2$, a prototype strongly correlated material. By employing a photoexcitation (lamda ~ 488 nm) of ~ 4.2 times the bandgap, EQE in VO$_2$ is enhanced up to ~ 170 % at room temperature. Temperature dependent experiments exhibit the direct relation between MEG and strength of electron correlation and suggest that such a phenomenon could be exploited in large number of strongly correlated materials for high performance solar cell research in near future. △ Less

Submitted 26 October, 2023; v1 submitted 10 February, 2022; originally announced February 2022.

Comments: New version is available at arXiv:2310.14835

Enhancement of optical properties and dielectric nature of Sm$_3$+doped Na$_2$O-ZnO-TeO$_2$ Glass materials

Authors: Jyotindra Nath Mirdda, Subhadipta Mukhopadhyay, Kriti Ranjan Sahu, Makhanlal Nanda Goswami

Abstract: Samarium doped Na$_2$O-ZnO-TeO$_2$ (NZT) glasses were prepared by the melt quenching method. The glass-forming ability and glass stability of prepared glass was estimated by Hruby parameter using Differential Thermal Analysis (DTA) and Thermo-gravimetric Analysis (TGA). The study of FTIR spectra and X-ray diffraction described the ionic nature and the amorphous pattern of glass respectively. The a… ▽ More Samarium doped Na$_2$O-ZnO-TeO$_2$ (NZT) glasses were prepared by the melt quenching method. The glass-forming ability and glass stability of prepared glass was estimated by Hruby parameter using Differential Thermal Analysis (DTA) and Thermo-gravimetric Analysis (TGA). The study of FTIR spectra and X-ray diffraction described the ionic nature and the amorphous pattern of glass respectively. The absorption peaks were observed for the transitions 6H5/2-4P3/2 at 402 nm,6H5/2-4M19/2 at 418 nm, 6H5/2-4I15/2 at 462 nm and 6H5/2-4I11/2 at 478 nm in the absorption spectra. The optical band gap energy (Eg) was calculated and observed to be decreased from 2.95 eV to 1.50 eV with doping concentration. The visible emission band was observed in the Sm3+ doped glass samples. The variation of dielectric constant with frequency was found to be independent for the frequency range 3 kHz - 2 MHz. The measurement of temperature-dependent dc conductivity showed Arrhenius type mechanism of conduction. △ Less

Submitted 22 January, 2022; originally announced January 2022.

Optothermal pulling, trapping, and assembly of colloids using nanowire plasmons

Authors: Vandana Sharma, Sunny Tiwari, Diptabrata Paul, Ratimanasee Sahu, Vijayakumar Chikkadi, G. V. Pavan Kumar

Abstract: Optical excitation of colloids can be harnessed to realize soft matter systems that are out of equilibrium. In this paper, we present our experimental studies on the dynamics of silica colloids in the vicinity of a silver nanowire propagating surface plasmon polaritons (SPPs). Due to the optothermal interaction, the colloids are directionally pulled towards the excitation point of the nanowire. Ha… ▽ More Optical excitation of colloids can be harnessed to realize soft matter systems that are out of equilibrium. In this paper, we present our experimental studies on the dynamics of silica colloids in the vicinity of a silver nanowire propagating surface plasmon polaritons (SPPs). Due to the optothermal interaction, the colloids are directionally pulled towards the excitation point of the nanowire. Having reached this point, they are spatio-temporally trapped around the excitation location. By increasing the concentration of colloids in the system, we observe multi-particle assembly around the nanowire. This process is thermophoretically driven and assisted by SPPs. Furthermore, we find such an assembly to be sensitive to the excitation polarization at input of the nanowire. Numerically-simulated temperature distribution around an illuminated nanowire corroborates sensitivity to the excitation polarization. Our study will find relevance in exploration of SPPs-assisted optothermal pulling, trapping and assembly of colloids, and can serve as test-beds of plasmon-driven active matter. △ Less

Submitted 25 November, 2021; v1 submitted 20 September, 2021; originally announced September 2021.

Optical and electrical properties of Nd3+doped Na2O-ZnO-TeO2 Material

Authors: J. N. Mirdda, S. Mukhopadhyay, K. R. Sahu, M. N. Goswami

Abstract: Neodymium doped Na2O-ZnO-TeO2 (NZT) glasses were prepared by the conventional melt quenching technique. DTA and TG were used to confirmation of glass preparation through the glass transition temperature at 447°C for the glass system. The analysis of FTIR spectra and X-ray diffraction described the nature of the samples were ionic and amorphous respectively. The optical bandgap energy was estimated… ▽ More Neodymium doped Na2O-ZnO-TeO2 (NZT) glasses were prepared by the conventional melt quenching technique. DTA and TG were used to confirmation of glass preparation through the glass transition temperature at 447°C for the glass system. The analysis of FTIR spectra and X-ray diffraction described the nature of the samples were ionic and amorphous respectively. The optical bandgap energy was estimated using absorption spectra and found to be decreased from 2.63eV to 1.32 eV due to the increase of doping concentration. The intensity of the emission spectra was enhanced for the higher concentration of Nd3+ ions. The dielectric constant of the glass samples was found to be constant for the large range of frequency (3 kHz to 1 MHz). The variation of conductivity with the temperature of the samples had shown the Arrhenius mechanism of conduction. △ Less

Submitted 8 June, 2021; originally announced June 2021.

Quantized conductance with non-zero shot noise as a signature of Andreev edge state

Authors: Manas Ranjan Sahu, Arup Kumar Paul, Jagannath Sutradhar, K. Watanabe, T. Taniguchi, Vibhor Singh, Subroto Mukerjee, Sumilan Banerjee, Anindya Das

Abstract: Electrical conductance measurements have limited scope in identifying Andreev edge states (AESs), which form the basis for realizing various topological excitations in quantum Hall (QH) - superconductor (SC) junctions. To unambiguously detect AESs, we measure shot noise along with electrical conductance in a graphene based QH-SC junction at integer filling nu=2. Remarkably, we find that the Fano f… ▽ More Electrical conductance measurements have limited scope in identifying Andreev edge states (AESs), which form the basis for realizing various topological excitations in quantum Hall (QH) - superconductor (SC) junctions. To unambiguously detect AESs, we measure shot noise along with electrical conductance in a graphene based QH-SC junction at integer filling nu=2. Remarkably, we find that the Fano factor of shot noise approaches half when the bias energy is less than the superconducting gap, whereas it is close to zero above the superconducting gap. This is striking, given that, at the same time, the electrical conductance remains quantized at 2e^2/h within and above the superconducting gap. A quantized conductance is expected to produce zero shot noise due to its dissipationless flow. However, at a QH-SC interface, AESs carry the current in the zero-bias limit and an equal mixing of electron and hole like states produces half of the Poissonian shot noise with quantized conductance. The observed results are in accord with our detailed theoretical calculations of electrical conductance and shot noise based on non-equilibrium Green's function method in the presence of disorder. Our results pave the way in using shot noise as a detection tool in the search of exotic topological excitations in QH-SC hybrids. △ Less

Submitted 27 May, 2021; originally announced May 2021.

Journal ref: Phys. Rev. B 104, 081404 (2021)

Direct imaging of dopant and impurity distributions in 2D MoS$_2$

Authors: Se-Ho Kim, Joohyun Lim, Rajib Sahu, Olga Kasian, Leigh T. Stephenson, Christina Scheu, Baptiste Gault

Abstract: Molybdenum disulfide (MoS$_2$) nanosheet is a two-dimensional material with high electron mobility and with high potential for applications in catalysis and electronics. We synthesized MoS$_2$ nanosheets using a one-pot wet-chemical synthesis route with and without Re-doping. Atom probe tomography revealed that 3.8 at.% Re is homogeneously distributed within the Re-doped sheets. Other impurities a… ▽ More Molybdenum disulfide (MoS$_2$) nanosheet is a two-dimensional material with high electron mobility and with high potential for applications in catalysis and electronics. We synthesized MoS$_2$ nanosheets using a one-pot wet-chemical synthesis route with and without Re-doping. Atom probe tomography revealed that 3.8 at.% Re is homogeneously distributed within the Re-doped sheets. Other impurities are found also integrated within the material: light elements including C, N, O, and Na, locally enriched up to 0.1 at.%, as well as heavy elements such as V and W. Analysis of the non-doped sample reveals that the W and V likely originate from the Mo precursor. △ Less

Submitted 10 February, 2021; originally announced February 2021.

Interplay of filling fraction and coherence in symmetry broken graphene p-n junction

Authors: Arup Kumar Paul, Manas Ranjan Sahu, Chandan Kumar, Kenji Watanabe, Takashi Taniguchi, Anindya Das

Abstract: The coherence of quantum Hall (QH) edges play the deciding factor in demonstrating an electron interferometer, which has potential to realize a topological qubit. A Graphene p-n junction (PNJ) with co-propagating spin and valley polarized QH edges is a promising platform for studying an electron interferometer. However, though a few experiments have been attempted for such PNJ via conductance meas… ▽ More The coherence of quantum Hall (QH) edges play the deciding factor in demonstrating an electron interferometer, which has potential to realize a topological qubit. A Graphene p-n junction (PNJ) with co-propagating spin and valley polarized QH edges is a promising platform for studying an electron interferometer. However, though a few experiments have been attempted for such PNJ via conductance measurements, the edge dynamics (coherent or incoherent) of QH edges at a PNJ, where either spin or valley symmetry or both are broken, remain unexplored. In this work, we have carried out the measurements of conductance together with shot noise, an ideal tool to unravel the dynamics, at low temperature (~ 10mK) in a dual graphite gated hexagonal boron nitride (hBN) encapsulated high mobility graphene device. The conductance data show that the symmetry broken QH edges at the PNJ follow spin selective equilibration. The shot noise results as a function of both p and n side filling factors reveal the unique dependence of the scattering mechanism with filling factors. Remarkably, the scattering is found to be fully tunable from incoherent to coherent regime with the increasing number of QH edges at the PNJ, shedding crucial insights into graphene based electron interferometer. △ Less

Submitted 1 October, 2020; originally announced October 2020.

Journal ref: Communications Physics volume 3, Article number: 171 (2020)

Anomalous thermopower oscillations in graphene-InAs nanowire vertical heterostructures

Authors: Richa Mitra, Manas Ranjan Sahu, Aditya Sood, Takashi Taniguchi, Kenji Watanabe, Hadas Shtrikman, Subroto Mukerjee, A. K. Sood, Anindya Das

Abstract: Thermoelectric measurements have the potential to uncover the density of states of low-dimensional materials. Here, we present the anomalous thermoelectric behaviour of mono-layer graphene-nanowire (NW) heterostructures, showing large oscillations as a function of doping concentration. Our devices consist of InAs NW and graphene vertical heterostructures, which are electrically isolated by thin (… ▽ More Thermoelectric measurements have the potential to uncover the density of states of low-dimensional materials. Here, we present the anomalous thermoelectric behaviour of mono-layer graphene-nanowire (NW) heterostructures, showing large oscillations as a function of doping concentration. Our devices consist of InAs NW and graphene vertical heterostructures, which are electrically isolated by thin ($\sim$ 10nm) hexagonal boron nitride (hBN) layers. In contrast to conventional thermoelectric measurements, where a heater is placed on one side of a sample, we use the InAs NW (diameter $\sim 50$ nm) as a local heater placed in the middle of the graphene channel. We measure the thermoelectric voltage induced in graphene due to Joule heating in the NW as a function of temperature (1.5K - 50K) and carrier concentration. The thermoelectric voltage in bilayer graphene (BLG)- NW heterostructures shows sign change around the Dirac point, as predicted by Mott's formula. In contrast, the thermoelectric voltage measured across monolayer graphene (MLG)-NW heterostructures shows anomalous large-amplitude oscillations around the Dirac point, not seen in the Mott response derived from the electrical conductivity measured on the same device. The anomalous oscillations are a signature of the modified density of states in MLG by the electrostatic potential of the NW, which is much weaker in the NW-BLG devices. Thermal calculations of the heterostructure stack show that the temperature gradient is dominant in the graphene region underneath the NW, and thus sensitive to the modified density of states resulting in anomalous oscillations in the thermoelectric voltage. Furthermore, with the application of a magnetic field, we detect modifications in the density of states due to the formation of Landau levels in both MLG and BLG. △ Less

Submitted 22 December, 2020; v1 submitted 18 September, 2020; originally announced September 2020.

Comments: 27 pages, 16 figures

Anomalous Coulomb Drag between InAs Nanowire and Graphene Heterostructures

Authors: Richa Mitra, Manas Ranjan Sahu, Kenji Watanabe, Takashi Taniguchi, Hadas Shtrikman, A. K Sood, Anindya Das

Abstract: Correlated charge inhomogeneity breaks the electron-hole symmetry in two-dimensional (2D) bilayer heterostructures which is responsible for non-zero drag appearing at the charge neutrality point. Here we report Coulomb drag in novel drag systems consisting of a two-dimensional graphene and a one dimensional (1D) InAs nanowire (NW) heterostructure exhibiting distinct results from 2D-2D heterostruct… ▽ More Correlated charge inhomogeneity breaks the electron-hole symmetry in two-dimensional (2D) bilayer heterostructures which is responsible for non-zero drag appearing at the charge neutrality point. Here we report Coulomb drag in novel drag systems consisting of a two-dimensional graphene and a one dimensional (1D) InAs nanowire (NW) heterostructure exhibiting distinct results from 2D-2D heterostructures. For monolayer graphene (MLG)-NW heterostructures, we observe an unconventional drag resistance peak near the Dirac point due to the correlated inter-layer charge puddles. The drag signal decreases monotonically with temperature ($\sim T^{-2}$) and with the carrier density of NW ($\sim n_{N}^{-4}$), but increases rapidly with magnetic field ($\sim B^{2}$). These anomalous responses, together with the mismatched thermal conductivities of graphene and NWs, establish the energy drag as the responsible mechanism of Coulomb drag in MLG-NW devices. In contrast, for bilayer graphene (BLG)-NW devices the drag resistance reverses sign across the Dirac point and the magnitude of the drag signal decreases with the carrier density of the NW ($\sim n_{N}^{-1.5}$), consistent with the momentum drag but remains almost constant with magnetic field and temperature. This deviation from the expected $T^2$ arises due to the shift of the drag maximum on graphene carrier density. We also show that the Onsager reciprocity relation is observed for the BLG-NW devices but not for the MLG-NW devices. These Coulomb drag measurements in dimensionally mismatched (2D-1D) systems, hitherto not reported, will pave the future realization of correlated condensate states in novel systems. △ Less

Submitted 23 February, 2020; originally announced February 2020.

Comments: Accepted for publication in Physical Review Letters

Enhanced shot noise at bilayer graphene -- superconductor junction

Authors: Manas Ranjan Sahu, Arup Kumar Paul, Abhiram Soori, K. Watanabe, T. Taniguchi, Subroto Mukerjee, Anindya Das

Abstract: Transport properties of graphene - superconductor junction has been studied extensively to understand the interplay of the relativistic Dirac quasiparticles and superconductivity. Though shot noise measurements in graphene has been performed to realize many theoretical predictions, both at zero magnetic field as well as quantum Hall (QH) regime, its junction with superconductor remain unexplored.… ▽ More Transport properties of graphene - superconductor junction has been studied extensively to understand the interplay of the relativistic Dirac quasiparticles and superconductivity. Though shot noise measurements in graphene has been performed to realize many theoretical predictions, both at zero magnetic field as well as quantum Hall (QH) regime, its junction with superconductor remain unexplored. Here, we have carried out the shot noise measurements in an edge contacted bilayer graphene - Niobium superconductor junction at zero magnetic field as well as QH regime. At the Dirac point we have observed a Fano factor ~ 1/3 above the superconducting gap and a transition to an enhanced Fano factor ~ 0.5 below the superconducting gap. By changing the carrier density we have found a continuous reduction of Fano factor for both types of carriers, however the enhancement of Fano factor within the superconducting gap by a factor of ~ 1.5 is always preserved. The enhancement of shot noise is also observed in the QH regime, where the current is carried by the edge state, below the critical magnetic field and within the superconducting gap. These observations clearly demonstrate the enhanced charge transport at the graphene-superconductor interface. △ Less

Submitted 14 November, 2019; originally announced November 2019.

Comments: accepted in Physical Review B

Journal ref: Phys. Rev. B 100, 235414 (2019)

Role of the V-V dimerization in insulator-metal transition and optical transmittance of pure and doped VO2 thin films

Authors: S. S. Majid, S. R. Sahu, A. Ahad, K. Dey, K. Gautam, F. Rahman, P. Behera, U. Deshpande, V. G. Sathe, D. K. Shukla

Abstract: Insulator to metal (IMT) transition (T$_t$ $\sim$ 341 K) in the VO2 accompanies transition from an infrared (IR) transparent to IR opaque phase. Tailoring of the IMT and associated IR switching behavior can offer potential thermochromic applications. Here we report on effects of the W and the Tb doping on the IMT and associated structural, electronic structure and optical properties of the VO2 thi… ▽ More Insulator to metal (IMT) transition (T$_t$ $\sim$ 341 K) in the VO2 accompanies transition from an infrared (IR) transparent to IR opaque phase. Tailoring of the IMT and associated IR switching behavior can offer potential thermochromic applications. Here we report on effects of the W and the Tb doping on the IMT and associated structural, electronic structure and optical properties of the VO2 thin film. Our results show that the W doping significantly lowers IMT temperature ($\sim$ 292 K to $\sim$ 247 K for 1.3\% W to 3.7\% W) by stabilizing the metallic rutile, $\it{R}$, phase while Tb doping does not alter the IMT temperature much and retains the insulating monoclinic, $\it{M1}$, phase at room temperature. It is observed that the W doping albeit significantly reduces the IR switching temperature but is detrimental to the solar modulation ability, contrary to the Tb doping effects where higher IR switching temperature and solar modulation ability is observed. The IMT behavior, electrical conductivity and IR switching behavior in the W and the Tb doped thin films are found to be directly associated with the spectral changes in the V 3$\it{d_{\|}}$ states. △ Less

Submitted 30 July, 2019; originally announced July 2019.

Comments: 11 pages and 11 figures

Journal ref: Phys. Rev. B 101, 014108 (2020)

Universal quantized thermal conductance in graphene

Authors: Saurabh Kumar Srivastav, Manas Ranjan Sahu, K. Watanabe, T. Taniguchi, Sumilan Banerjee, Anindya Das

Abstract: The universal quantization of thermal conductance provides information on the topological order of a state beyond electrical conductance. Such measurements have become possible only recently, and have discovered, in particular, that the value of the observed thermal conductance of the 5/2 state is not consistent with either the Pfaffian or the anti-Pfaffian model, motivating several theoretical ar… ▽ More The universal quantization of thermal conductance provides information on the topological order of a state beyond electrical conductance. Such measurements have become possible only recently, and have discovered, in particular, that the value of the observed thermal conductance of the 5/2 state is not consistent with either the Pfaffian or the anti-Pfaffian model, motivating several theoretical articles. The analysis of the experiments has been made complicated by the presence of counter-propagating edge channels arising from edge reconstruction, an inevitable consequence of separating the dopant layer from the GaAs quantum well. In particular, it has been found that the universal quantization requires thermalization of downstream and upstream edge channels. Here we measure the thermal conductance in hexagonal boron nitride encapsulated graphene devices of sizes much smaller than the thermal relaxation length of the edge states. We find the quantization of thermal conductance within 5% accuracy for ν = 1, 4/3, 2 and 6 plateaus and our results strongly suggest the absence of edge reconstruction for fractional quantum Hall in graphene, making it uniquely suitable for interference phenomena exploiting paths of exotic quasiparticles along the edge. △ Less

Submitted 18 February, 2019; originally announced February 2019.

Inter-Landau-level Andreev Reflection at the Dirac Point in a Graphene Quantum Hall State Coupled to a NbSe2 Superconductor

Authors: Manas Ranjan Sahu, Xin Liu, Arup Kumar Paul, Sourin Das, Pratap Raychaudhuri, J. K. Jain, Anindya Das

Abstract: Superconductivity and quantum Hall effect are distinct states of matter occurring in apparently incompatible physical conditions. Recent theoretical developments suggest that the coupling of quantum Hall effect with a superconductor can provide a fertile ground for realizing exotic topological excitations such as non-abelian Majorana fermions or Fibonacci particles. As a step toward that goal, we… ▽ More Superconductivity and quantum Hall effect are distinct states of matter occurring in apparently incompatible physical conditions. Recent theoretical developments suggest that the coupling of quantum Hall effect with a superconductor can provide a fertile ground for realizing exotic topological excitations such as non-abelian Majorana fermions or Fibonacci particles. As a step toward that goal, we report observation of Andreev reflection at the junction of a quantum Hall edge state in a single layer graphene and a quasi-two dimensional niobium diselenide (NbSe2) superconductor. Our principal finding is the observation of an anomalous finite-temperature conductance peak located precisely at the Dirac point, providing a definitive evidence for inter-Landau level Andreev reflection in a quantum Hall system. Our observations are well supported by detailed numerical simulations, which offer additional insight into the role of the edge states in Andreev physics. This study paves the way for investigating analogous Andreev reflection in a fractional quantum Hall system coupled to a superconductor to realize exotic quasiparticles. △ Less

Submitted 31 August, 2018; v1 submitted 19 July, 2018; originally announced July 2018.

Comments: published verion

Journal ref: Phys. Rev. Lett. 121, 086809 (2018)

Chemically stabilized epitaxial wurtzite-BN thin film

Authors: Badri Vishal, Rajendra Singh, Abhishek Chaturvedi, Ankit Sharma, M. B. Sreedhara, Rajib Sahu, Usha Bhat, Upadrasta Ramamurty, Ranjan Datta

Abstract: We report on the chemically stabilized epitaxial w-BN thin film grown on c-plane sapphire by pulsed laser deposition under slow kinetic condition. Traces of no other allotropes such as cubic (c) or hexagonal (h) BN phases are present. Sapphire substrate plays a significant role in stabilizing the metastable w-BN from h-BN target under unusual PLD growth condition involving low temperature and pres… ▽ More We report on the chemically stabilized epitaxial w-BN thin film grown on c-plane sapphire by pulsed laser deposition under slow kinetic condition. Traces of no other allotropes such as cubic (c) or hexagonal (h) BN phases are present. Sapphire substrate plays a significant role in stabilizing the metastable w-BN from h-BN target under unusual PLD growth condition involving low temperature and pressure and is explained based on density functional theory calculation. The hardness and the elastic modulus of the w-BN film are 37 & 339 GPa, respectively measured by indentation along <0001> direction. The results are extremely promising in advancing the microelectronic and mechanical tooling industry. △ Less

Submitted 21 January, 2018; originally announced January 2018.

Enhanced Specular Andreev reflection in bilayer graphene

Authors: Abhiram Soori, Manas Ranjan Sahu, Anindya Das, Subroto Mukerjee

Abstract: Andreev reflection in graphene is special since it can be of two types- retro or specular. Specular Andreev reflection (SAR) dominates when the position of the Fermi energy in graphene is comparable to or smaller than the superconducting gap. Bilayer graphene (BLG) is an ideal candidate to observe the crossover from retro to specular since the Fermi energy broadening near the Dirac point is much w… ▽ More Andreev reflection in graphene is special since it can be of two types- retro or specular. Specular Andreev reflection (SAR) dominates when the position of the Fermi energy in graphene is comparable to or smaller than the superconducting gap. Bilayer graphene (BLG) is an ideal candidate to observe the crossover from retro to specular since the Fermi energy broadening near the Dirac point is much weaker compared to monolayer graphene. Recently, the observation of signatures of SAR in BLG have been reported experimentally by looking at the enhancement of conductance at finite bias near the Dirac point. However, the signatures were not very pronounced possibly due to the participation of normal quasi-particles at bias energies close to the superconducting gap. Here, we propose a scheme to observe the features of enhanced SAR even at zero bias at a normal metal (NM)-superconductor (SC) junction on BLG. Our scheme involves applying a Zeeman field to the NM side of the NM-SC junction on BLG (making the NM ferromagnetic), which energetically separates the Dirac points for up-spin and down-spin. We calculate the conductance as a function of chemical potential and bias within the superconducting gap and show that well-defined regions of specular- and retro-type Andreev reflection exist. We compare the results with and without superconductivity. We also investigate the possibility of the formation of a p-n junction at the interface between the NM and SC due to a work function mismatch. △ Less

Submitted 4 August, 2018; v1 submitted 2 January, 2018; originally announced January 2018.

Comments: 9 pages, 10 captioned figures. Published version

Journal ref: Phys. Rev. B 98, 075301 (2018)

Distinct photoluminescence in multilayered van der Waals heterostructures of MoS2/WS2/ReS2 and BN

Authors: U. Bhat, R. Singh, B. Vishal, A. Sharma, H. Sharona, R. Sahu, R. Datta

Abstract: Van der Waals heterostructures of (TMDL=1/BNL=1-4/TMDL=1/BNL=1-4), [TMD = MoS2, WS2, and ReS2] are grown on c-plane sapphire substrate by pulsed laser deposition under slow kinetic condition. The heterostructure systems show strong emission around 2.3 eV and subsidiary peaks around 2.8, 1.9, 1.7 and 1.5 eV. BN and TMDs forms type-I heterojunction and the emission peaks observed are explained in te… ▽ More Van der Waals heterostructures of (TMDL=1/BNL=1-4/TMDL=1/BNL=1-4), [TMD = MoS2, WS2, and ReS2] are grown on c-plane sapphire substrate by pulsed laser deposition under slow kinetic condition. The heterostructure systems show strong emission around 2.3 eV and subsidiary peaks around 2.8, 1.9, 1.7 and 1.5 eV. BN and TMDs forms type-I heterojunction and the emission peaks observed are explained in terms of various band to band recombination processes and considering relative orientation of Brillouin Zones. The emission peak around 2.3eV is promising for solar and photovoltaic application. The observation is almost similar for three different heterostructure systems. △ Less

Submitted 11 October, 2017; originally announced October 2017.

Comments: 24 pages, 10 figures

Nature of low dimensional structural modulations and relative phase stability in MoS2/WS2-ReS2 transition metal dichalcogenide alloys

Authors: R. Sahu, U. Bhat, N. M. Batra, H. Sharona, B. Vishal, S. Sarkar, S. Assa Aravindh, S. C. Peter, I. S. Roqan, P. M. F. J. D. Costa, R. Datta

Abstract: We report on the various types of Peierls like two dimensional structural modulations and relative phase stability of 2H and 1T poly-types in MoS2-ReS2 and WS2-ReS2 alloy system. Theoretical calculation predicts a polytype phase transition cross over at ~50 at.% of Mo and W in ReS2 in both monolayer and bulk form, respectively. Experimentally, two different types of structural modulations at 50% a… ▽ More We report on the various types of Peierls like two dimensional structural modulations and relative phase stability of 2H and 1T poly-types in MoS2-ReS2 and WS2-ReS2 alloy system. Theoretical calculation predicts a polytype phase transition cross over at ~50 at.% of Mo and W in ReS2 in both monolayer and bulk form, respectively. Experimentally, two different types of structural modulations at 50% and a modulation corresponding to trimerization at 75% alloy composition is observed for MoS2-ReS2 and only one type of modulation is observed at 50% WS2-ReS2 alloy system. The 50% alloy system is found to be a suitable monolithic candidate for metal semiconductor transition with minute external perturbation. ReS2 is known to be in 2D Peierls distorted 1Td structure and forms a chain like superstructure. Incorporation of Mo and W atoms in the ReS2 lattice modifies the metal-metal hybridization between the cations and influences the structural modulation and electronic property of the system. The results offer yet another effective way to tune the electronic structure and poly-type phases of this class of materials other than intercalation, strain, and vertical stacking arrangement. △ Less

Submitted 2 November, 2016; originally announced November 2016.

Substrate induced tuning of compressive strain and phonon modes in large area MoS2 and WS2 van der Waals epitaxial thin films

Authors: Rajib Sahu, Dhanya Radhakrishnan, Badri Vishal, Devendra Singh Negi, Anomitra Sil, Chandrabhas Narayana, Ranjan Datta

Abstract: Large area MoS2 and WS2 van der Waals epitaxial thin films with complete control over number of layers including monolayer is grown by pulsed laser deposition utilizing slower growth kinetics. The films grown on c-plane sapphire show stiffening of A1g and E12g phonon modes with decreasing number of layers for both MoS2 and WS2. The observed stiffening translate into the compressive strain of 0.52… ▽ More Large area MoS2 and WS2 van der Waals epitaxial thin films with complete control over number of layers including monolayer is grown by pulsed laser deposition utilizing slower growth kinetics. The films grown on c-plane sapphire show stiffening of A1g and E12g phonon modes with decreasing number of layers for both MoS2 and WS2. The observed stiffening translate into the compressive strain of 0.52 % & 0.53 % with accompanying increase in fundamental direct band gap to 1.74 and 1.68 eV for monolayer MoS2 and WS2, respectively. The strain decays with the number of layers. HRTEM imaging directly reveals the nature of atomic registry of van der Waals layers with the substrate and the associated compressive strain. The results demonstrate a practical route to stabilize and engineer strain for this class of material over large area device fabrication. △ Less

Submitted 18 October, 2016; originally announced October 2016.

Andreev reflection near the Dirac point at Graphene - NbSe2 junction

Authors: Manas Ranjan Sahu, Pratap Raychaudhuri, Anindya Das

Abstract: Despite extensive search for about a decade, specular Andreev reflection is only recently realized in bilayer graphene-superconductor interface. However, the evolution from the typical retro type Andreev reflection to the unique specular Andreev reflection in single layer graphene has not yet been observed. We investigate this transition by measuring the differential conductance at the van der Wal… ▽ More Despite extensive search for about a decade, specular Andreev reflection is only recently realized in bilayer graphene-superconductor interface. However, the evolution from the typical retro type Andreev reflection to the unique specular Andreev reflection in single layer graphene has not yet been observed. We investigate this transition by measuring the differential conductance at the van der Walls interface of single layer graphene and NbSe2 superconductor. We find that the normalized conductance becomes suppressed as we pass through the Dirac cone via tuning the Fermi level and bias energy, which manifests the transition from retro to non-retro type Andreev reflection. The suppression indicates the blockage of Andreev reflection beyond a critical angle of the incident electron with respect to the normal between the single layer graphene and the superconductor junction. The results are compared with a theoretical model of the corresponding setup. △ Less

Submitted 8 June, 2016; originally announced June 2016.

Journal ref: Phys. Rev. B 94, 235451 (2016)

Gravitational Drainage of Thin Films of Trisiloxane-(Poly)ethoxylate Superspreaders

Authors: Soumyadip Sett, Rakesh P. Sahu, Suman Sinha-Ray, Alexander Yarin

Abstract: Gravitational drainage of vertical films supported on a wire frame of superspreader SILWET L-77 and its cousin non-superspreader SILWET L-7607 revealed drastic differences. The superspreader films showed complicated dynamic turbulent-like interferometric patterns in distinction from the ordered color bands of the cousin non-superspreader which reminded those of the ordinary surfactants. Neverthele… ▽ More Gravitational drainage of vertical films supported on a wire frame of superspreader SILWET L-77 and its cousin non-superspreader SILWET L-7607 revealed drastic differences. The superspreader films showed complicated dynamic turbulent-like interferometric patterns in distinction from the ordered color bands of the cousin non-superspreader which reminded those of the ordinary surfactants. Nevertheless the superspreader films stabilized themselves at the thickness below 50 nm and revealed an order of magnitude longer life time before bursting compared to the cousin non-superspreader. Notably, the superspreader revealed drastic differences from the non-superspreader in aqueous solutions with no contact with any solid Teflon surface. The theoretical part of the work attributed the self-stabilization of the superspreader films to significant disjoining pressure associated with the van der Waals repulsion of the fluffy surfaces of the film formed by long superspreader bilayers hanging from the free surfaces. The non-superspreaders do not possess any significant disjoining pressure in the film with thicknesses even in the range 30-50 nm. The results show that gravitational drainage of vertical films is a useful simple tool for measuring disjoining pressure. △ Less

Submitted 17 July, 2013; originally announced July 2013.

Comments: There are videos included

Density functional theory studies of interactions of graphene with its environment: substrate, gate dielectric and edge effects

Authors: Priyamvada Jadaun, Bhagawan R. Sahu, Leonard F. Register, Sanjay K. Banerjee

Abstract: This paper reviews the theoretical work undertaken using density functional theory (DFT) to explore graphene's interactions with its surroundings. We look at the impact of substrates, gate dielectrics and edge effects on the properties of graphene. In particular, we focus on graphene-on-quartz and graphene-on-alumina systems, exploring their energy spectrum and charge distribution. Silicon-termina… ▽ More This paper reviews the theoretical work undertaken using density functional theory (DFT) to explore graphene's interactions with its surroundings. We look at the impact of substrates, gate dielectrics and edge effects on the properties of graphene. In particular, we focus on graphene-on-quartz and graphene-on-alumina systems, exploring their energy spectrum and charge distribution. Silicon-terminated quartz is found to not perturb the linear graphene spectrum. On the other hand, oxygen-terminated quartz and both terminations of alumina bond with graphene, leading to the opening of a band gap. Significant charge transfer is seen between the graphene layer and the oxide in the latter cases. Additionally, we review the work of others regarding the effect of various substrates on the electronic properties of graphene. Confining graphene to form nanoribbons also results in the opening of a band gap. The value of the gap is dependent on the edge properties as well as width of the nanoribbon. △ Less

Submitted 31 January, 2012; originally announced February 2012.

Journal ref: Solid State Communications, 152, 1497 (2012)

Raman evidence for Orbiton-Mediated Multiphonon Scattering in Multiferroic TbMnO$_3$

Authors: Pradeep Kumar, Surajit Saha, D. V. S. Muthu, J. R. Sahu, A. K. Sood, C. N. R. Rao

Abstract: Temperature-dependent Raman spectra of TbMnO$_3$ from 5 K to 300 K in the spectral range of 200 to 1525 cm$^{-1}$ show five first-order Raman allowed modes and two high frequency modes. The intensity ratio of the high frequency Raman band to the corresponding first order Raman mode is nearly constant and high ($\sim$ 0.6) at all temperatures, suggesting a orbiton-phonon mixed nature of the high fr… ▽ More Temperature-dependent Raman spectra of TbMnO$_3$ from 5 K to 300 K in the spectral range of 200 to 1525 cm$^{-1}$ show five first-order Raman allowed modes and two high frequency modes. The intensity ratio of the high frequency Raman band to the corresponding first order Raman mode is nearly constant and high ($\sim$ 0.6) at all temperatures, suggesting a orbiton-phonon mixed nature of the high frequency mode. One of the first order phonon modes shows anomalous softening below T$_N$ ($\sim$ 46 K), suggesting a strong spin-phonon coupling. △ Less

Submitted 9 April, 2010; originally announced April 2010.

Comments: 14 pages, 3 figures, 1 table

Journal ref: J. Phys.: Condens. Matter vol. 22, pp. 115403 (2010)

Ab Initio Theory of Gate Induced Gaps in Graphene Bilayers

Authors: Hongki Min, B. R. Sahu, Sanjay K. Banerjee, A. H. MacDonald

Abstract: We study the gate voltage induced gap that occurs in graphene bilayers using \textit{ab initio} density functional theory. Our calculations confirm the qualitative picture suggested by phenomenological tight-binding and continuum models. We discuss enhanced screening of the external interlayer potential at small gate voltages, which is more pronounced in the \textit{ab initio} calculations, and… ▽ More We study the gate voltage induced gap that occurs in graphene bilayers using \textit{ab initio} density functional theory. Our calculations confirm the qualitative picture suggested by phenomenological tight-binding and continuum models. We discuss enhanced screening of the external interlayer potential at small gate voltages, which is more pronounced in the \textit{ab initio} calculations, and quantify the role of crystalline inhomogeneity using a tight-binding model self-consistent Hartree calculation. △ Less

Submitted 26 April, 2007; v1 submitted 10 December, 2006; originally announced December 2006.

Comments: 7 pages, 7 figures; the effect of r3 coupling included; typo corrected

Journal ref: Phys. Rev. B 75, 155115 (2007)

Intrinsic and Rashba Spin-orbit Interactions in Graphene Sheets

Authors: Hongki Min, J. E. Hill, N. A. Sinitsyn, B. R. Sahu, Leonard Kleinman, A. H. MacDonald

Abstract: Starting from a microscopic tight-binding model and using second order perturbation theory, we derive explicit expressions for the intrinsic and Rashba spin-orbit interaction induced gaps in the Dirac-like low-energy band structure of an isolated graphene sheet. The Rashba interaction parameter is first order in the atomic carbon spin-orbit coupling strength $ξ$ and first order in the external e… ▽ More Starting from a microscopic tight-binding model and using second order perturbation theory, we derive explicit expressions for the intrinsic and Rashba spin-orbit interaction induced gaps in the Dirac-like low-energy band structure of an isolated graphene sheet. The Rashba interaction parameter is first order in the atomic carbon spin-orbit coupling strength $ξ$ and first order in the external electric field $E$ perpendicular to the graphene plane, whereas the intrinsic spin-orbit interaction which survives at E=0 is second order in $ξ$. The spin-orbit terms in the low-energy effective Hamiltonian have the form proposed recently by Kane and Mele. \textit{Ab initio} electronic structure calculations were performed as a partial check on the validity of the tight-binding model. △ Less

Submitted 20 October, 2006; v1 submitted 20 June, 2006; originally announced June 2006.

Comments: 5 pages, 2 figures; typos corrected, references updated

Journal ref: Phys. Rev. B 74, 165310 (2006)

Large Orbital Magnetic Moment and Coulomb Correlation effects in FeBr2

Authors: S. J. Youn, B. R. Sahu, Kwang S. Kim

Abstract: We have performed an all-electron fully relativistic density functional calculation to study the magnetic properties of FeBr2. We show for the first time that the correlation effect enhances the contribution from orbital degrees of freedom of $d$ electrons to the total magnetic moment on Fe$^{2+}$ as opposed to common notion of nearly total quenching of the orbital moment on Fe$^{2+}$ site. The… ▽ More We have performed an all-electron fully relativistic density functional calculation to study the magnetic properties of FeBr2. We show for the first time that the correlation effect enhances the contribution from orbital degrees of freedom of $d$ electrons to the total magnetic moment on Fe$^{2+}$ as opposed to common notion of nearly total quenching of the orbital moment on Fe$^{2+}$ site. The insulating nature of the system is correctly predicted when the Hubbard parameter U is included. Energy bands around the gap are very narrow in width and originate from the localized Fe-3$d$ orbitals, which indicates that FeBr2 is a typical example of the Mott insulator. △ Less

Submitted 20 December, 2001; originally announced December 2001.

Comments: 4 pages, 3 figures, revtex4, PRB accepted