Showing 1–50 of 119 results for author: Paul, D

Quasiparticle properties below coherence onset in YbAl3 nanostructures

Authors: Dale T. Lowder, Gage Eichman, Yuxin Wan, Karthik Rao, Ruiwen Xie, Hongbin Zhang, Debjoty Paul, Shouvik Chatterjee, Darrell G. Schlom, Kyle Shen, Emilia Morosan, Douglas Natelson

Abstract: Mesoscopic transport measurements are underexplored as probes of quasiparticles and their properties in correlated metals. The mixed valence compound YbAl$_3$ exhibits a single-ion Kondo temperature of 670 K, while thermodynamic and transport properties (probed with specific heat, magnetic susceptibility, Hall effect, and resistivity) imply the onset of coherence of heavy fermion quasiparticles at… ▽ More Mesoscopic transport measurements are underexplored as probes of quasiparticles and their properties in correlated metals. The mixed valence compound YbAl$_3$ exhibits a single-ion Kondo temperature of 670 K, while thermodynamic and transport properties (probed with specific heat, magnetic susceptibility, Hall effect, and resistivity) imply the onset of coherence of heavy fermion quasiparticles at T$* \approx$ 37 K. To characterize these quasiparticles, we utilize mesoscopic techniques familiar from weakly correlated conductors. In lithographically-defined nanowires etched from epitaxial films, we observe weak antilocalization magnetoresistance and universal conductance fluctuations, consistent with electronic coherence lengths of tens of nanometers. Additionally, analysis of Johnson-Nyquist noise measurements as a function of bias current reveal, within the context of a range of accepted models, a significant electron-phonon energy loss that increases with decreasing temperature, a finding that we contextualize within the broader properties of YbAl$_3$. △ Less

Submitted 17 March, 2026; originally announced March 2026.

Comments: 14 pages, 4 figures, + 11 pages supporting material, 7 supplemental figures

First-Principles Investigation of the Pressure Dependent Physical Properties of Intermetallic Kagome ZrRe2

Authors: Mst. Irin Naher, A. F. M. Yusuf Haider, Dholon Kumar Paul, Md Lutfor Rahman, Firoze H. Haque, Saleh Hasan Naqib

Abstract: We present a density functional theory investigation of the pressure dependent structural, electronic, mechanical, thermophysical, vibrational, and optical properties of the intermetallic Kagome compound ZrRe2. The calculated ground-state structural parameters are in excellent agreement with available experimental results. The estimated structural parameters, elastic constants, and phonon dispersi… ▽ More We present a density functional theory investigation of the pressure dependent structural, electronic, mechanical, thermophysical, vibrational, and optical properties of the intermetallic Kagome compound ZrRe2. The calculated ground-state structural parameters are in excellent agreement with available experimental results. The estimated structural parameters, elastic constants, and phonon dispersion confirm the structural, chemical, mechanical, and dynamical stability of ZrRe2 up to 25 GPa. The Kagome feature in the material has been identified from the electronic band structure for the first time. ZrRe2 exhibits topological feature at 0 GPa, which vanishes under 25 GPa. Fermi surface (FS) analysis predicts that ZrRe2 could potentially host a charge density wave (CDW) phase. The electronic and optical studies confirmed its metallic nature. The Debye temperature and phonon thermal conductivity are moderate, while the melting point is relatively high. Furthermore, ZrRe2 possesses moderate electron-phonon coupling, which weakens under pressure as the phonon modes harden. Consequently, the superconducting transition temperature decreases with increasing pressure. Most of the properties studied and analyses performed in this paper are novel in nature. △ Less

Submitted 17 March, 2026; originally announced March 2026.

Ab initio study of Coulomb drag driven electron-hole bifluidity in doped graphene

Authors: Dwaipayan Paul, Elena Trukhan, Nakib H. Protik

Abstract: Motivated by the notion that a preponderance of Coulomb interactions might lead to hydrodynamics, we carry out an ab initio calculation of the charge carrier transport properties of the electron-hole plasma of doped graphene. We include both the phonon and Coulomb interactions within a momentum and band resolved Boltzmann transport formalism. We find that, under suitable conditions, the strong Cou… ▽ More Motivated by the notion that a preponderance of Coulomb interactions might lead to hydrodynamics, we carry out an ab initio calculation of the charge carrier transport properties of the electron-hole plasma of doped graphene. We include both the phonon and Coulomb interactions within a momentum and band resolved Boltzmann transport formalism. We find that, under suitable conditions, the strong Coulomb drag effect induces effects like negative conductivity and joint electron-hole hydrodynamics (bifluidity) in the plasma. We also identify the exclusive electron or hole hydrodynamics. We find that there is a strong violation of the Wiedemann-Franz law in the low doped regimes. Our work elucidates the roles of the microscopic scattering mechanisms that drive these hydrodynamic phenomena. △ Less

Submitted 15 June, 2026; v1 submitted 4 December, 2025; originally announced December 2025.

Robust NbN on Si-SiGe hybrid superconducting-semiconducting microwave quantum circuit

Authors: Paniz Foshat, Samane Kalhor, Shima Poorgholam-khanjari, Douglas Paul, Martin Weides, Kaveh Delfanazari

Abstract: Advancing large-scale quantum computing requires superconducting circuits that combine long coherence times with compatibility with semiconductor technology. We investigate niobium nitride (NbN) coplanar waveguide resonators integrated with Si/SiGe quantum wells, creating a hybrid platform designed for CMOS-compatible quantum hardware. Using temperature-dependent microwave spectroscopy in the sing… ▽ More Advancing large-scale quantum computing requires superconducting circuits that combine long coherence times with compatibility with semiconductor technology. We investigate niobium nitride (NbN) coplanar waveguide resonators integrated with Si/SiGe quantum wells, creating a hybrid platform designed for CMOS-compatible quantum hardware. Using temperature-dependent microwave spectroscopy in the single-photon regime, we examine resonance frequency and quality factor variations to probe the underlying loss mechanisms. Our analysis identifies the roles of two-level systems, quasiparticles, and scattering processes, and connects these losses to wafer properties and fabrication methods. The devices demonstrate reproducible performance and stable operation maintained for over two years, highlighting their robustness. These results provide design guidelines for developing low-loss, CMOS-compatible superconducting circuits and support progress toward resilient, scalable architectures for quantum information processing. △ Less

Submitted 30 September, 2025; originally announced September 2025.

Physical Embodiment Enables Information Processing Beyond Explicit Sensing in Active Matter

Authors: Diptabrata Paul, Nikola Milosevic, Nico Scherf, Frank Cichos

Abstract: Living microorganisms have evolved dedicated sensory machinery to detect environmental perturbations, processing these signals through biochemical networks to guide behavior. Replicating such capabilities in synthetic active matter remains a fundamental challenge. Here, we demonstrate that synthetic active particles can adapt to hidden hydrodynamic perturbations through physical embodiment alone,… ▽ More Living microorganisms have evolved dedicated sensory machinery to detect environmental perturbations, processing these signals through biochemical networks to guide behavior. Replicating such capabilities in synthetic active matter remains a fundamental challenge. Here, we demonstrate that synthetic active particles can adapt to hidden hydrodynamic perturbations through physical embodiment alone, without explicit sensing mechanisms. Using reinforcement learning to control self-thermophoretic particles, we show that they learn navigation strategies to counteract unobserved flow fields by exploiting information encoded in their physical dynamics. Remarkably, particles successfully navigate perturbations that are not included in their state inputs, revealing that embodied dynamics can serve as an implicit sensing mechanism. This discovery establishes physical embodiment as a computational resource for information processing in active matter, with implications for autonomous microrobotic systems and bio-inspired computation. △ Less

Submitted 25 August, 2025; originally announced August 2025.

Dark Matter Haloscope with a Disordered Dielectric Absorber

Authors: Stewart Koppell, Otavio D. A. R. Bittencourt, Dip Joti Paul, Junwu Huang, Masha Baryakhtar, Karl K. Berggren

Abstract: Light dark matter candidates such as axions and dark photons generically couple to electromagnetism, yielding dark-matter-to-photon conversion as a key search strategy. In addition to resonant conversion in cavities and circuits, light dark matter bosons efficiently convert to photons on material interfaces, with a broadband power proportional to the total area of these interfaces. In this work, w… ▽ More Light dark matter candidates such as axions and dark photons generically couple to electromagnetism, yielding dark-matter-to-photon conversion as a key search strategy. In addition to resonant conversion in cavities and circuits, light dark matter bosons efficiently convert to photons on material interfaces, with a broadband power proportional to the total area of these interfaces. In this work, we make use of interface conversion to develop a new experimental dark matter detector design: the disordered dielectric detector. We show that a volume filled with dielectric powder is an efficient, robust, and broadband target for axion-to-photon or dark-photon-to-photon conversion. We perform semi-analytical and numerical studies in small-volume 2D and 3D disordered systems to compute the conversion power as a function of dark matter mass. We also discuss the power gathered onto a sensitive photodetector in terms of the bulk properties of the disordered material, making it possible to characterize the predicted dark-matter-to-photon conversion rate across a wide range of wavelengths. Finally, we propose DPHaSE: the Dielectric Powder Haloscope SNSPD Experiment which is composed of a disordered dielectric target, a veto system, and a photon collection chamber to maximize the coupling between the powder target and a low noise superconducting nanowire single photon detector (SNSPD). With ambitious but realistic improvements to sensor area and detection efficiency at low energy, the projected reach in the 10 meV-eV mass range is sensitive to QCD axion-photon couplings and exceeds current constraints on dark photon dark matter by up to 5 orders of magnitude. △ Less

Submitted 20 August, 2026; v1 submitted 30 May, 2025; originally announced June 2025.

Comments: 35 pages, 8 figures and appendices, comments welcomed

Journal ref: Phys. Rev. D 113, 083045 (2026)

Phonon Mean Free Path Spectroscopy By Raman Thermometry

Authors: Katharina Dudde, Mahmoud Elhajhasan, Guillaume Würsch, Julian Themann, Jana Lierath, Dwaipayan Paul, Nakib H. Protik, Giuseppe Romano, Gordon Callsen

Abstract: In this work, we exemplify on a bulk silicon sample that Raman thermometry is capable of phonon mean free path (PMFP) spectroscopy. Our experimental approach is similar to the variation of different characteristic length scales $l_{c}$ during thermal reflectance measurements in the time or frequency domain and transient thermal grating spectroscopy. In place of $l_{c}$, we vary the laser focus spo… ▽ More In this work, we exemplify on a bulk silicon sample that Raman thermometry is capable of phonon mean free path (PMFP) spectroscopy. Our experimental approach is similar to the variation of different characteristic length scales $l_{c}$ during thermal reflectance measurements in the time or frequency domain and transient thermal grating spectroscopy. In place of $l_{c}$, we vary the laser focus spot size ($w_{e}$) and the light penetration depth ($h_α$) during one-laser Raman thermometry (1LRT) measurements. For our largest $w_{e}$ values, the derived effective thermal conductivities $κ_{eff}$ converge towards the bulk thermal conductivity $κ_{bulk}$ for silicon. However, towards smaller $w_{e}$ values, we observe a pronounced increase for the $κ_{eff}$ values, which amounts up to a factor of 5.3 at 293K and even 8.3 at 200K. We mainly assign this phenomenon to quasi-ballistic phonon transport. As a result, we can compare our measured $κ_{eff}(w_{e})$ trends with the thermal accumulation function $κ_{cum}$ and its dependence on the phonon mean free path $l_{ph}$, which we derive from ab initio solutions of the linearized phonon Boltzmann transport equation (BTE). Since the variation of $w_{e}$ can be experimentally cumbersome, we also suggest varying $h_α(λ)$ via the applied Raman laser wavelength $λ$ during 1LRT. In this regard, we present proof-of-principle 1LRT measurements, yielding a step-like $κ_{eff}(λ)$ trend for four different $λ$ values, which we also interpret in terms of quasi-ballistic phonon transport. Our results shall seed future PMFP spectroscopy based on 1LRT, which can directly be benchmarked against state-of-art theory by comparison of $κ_{\text{cum}}$ trends and not only $κ$ values, aiming to test our understanding of the intricate phonon transport physics. △ Less

Submitted 20 May, 2025; originally announced May 2025.

Determination of Mid-Infrared Refractive Indices of Superconducting Thin Films Using Fourier Transform Infrared Spectroscopy

Authors: Dip Joti Paul, Tony X. Zhou, Karl K. Berggren

Abstract: In this work, we present a technique to determine the mid-infrared refractive indices of thin superconducting films using Fourier transform infrared spectroscopy (FTIR). In particular, we performed FTIR transmission and reflection measurements on 10-nm-thick NbN and 15-nm-thick MoSi films in the wavelength range of 2.5 to 25 $μ$m, corresponding to frequencies of 12-120 THz or photon energies of 50… ▽ More In this work, we present a technique to determine the mid-infrared refractive indices of thin superconducting films using Fourier transform infrared spectroscopy (FTIR). In particular, we performed FTIR transmission and reflection measurements on 10-nm-thick NbN and 15-nm-thick MoSi films in the wavelength range of 2.5 to 25 $μ$m, corresponding to frequencies of 12-120 THz or photon energies of 50-500 meV. To extract the mid-infrared refractive indices of these thin films, we used the Drude-Lorentz oscillator model to represent their dielectric functions and implemented an optimization algorithm to fit the oscillator parameters by minimizing the error between the measured and simulated FTIR spectra. We performed Monte Carlo simulations in the optimization routine to estimate error ranges in the extracted refractive indices resulting from multiple sources of measurement uncertainty. To evaluate the consistency of the extracted dielectric functions, we compared the refractive indices extrapolated from these dielectric functions in the UV to near-infrared wavelengths with the values separately measured using spectroscopic ellipsometry. We validated the applicability of the extracted mid-infrared refractive indices of NbN and MoSi at temperatures below their critical temperatures by comparing them with the Mattis-Bardeen model. This FTIR-based refractive index measurement approach can be extended to measure the refractive indices of thin films at wavelengths beyond 25 $μ$m, which will be useful for designing highly efficient photon detectors and photonic devices with enhanced optical absorption in the mid- and far-infrared wavelengths. △ Less

Submitted 16 May, 2025; v1 submitted 28 February, 2025; originally announced March 2025.

Comments: 9 figures

Journal ref: Appl. Phys. Lett. 126, 252601 (2025)

Thermofluidic non-equilibrium assembly of reconfigurable functional structures

Authors: Desmond J. Quinn, Diptabrata Paul, Frank Cichos

Abstract: Non-equilibrium assembly, driven by fluxes controllable by continuous external energy inputs, enables dynamic and reconfigurable structures. Such controlled 3D assembly is desired for the design of adaptive materials that exploit structure-function relationships, but has remained challenging. We present a non-equilibrium assembly of colloidal particles mediated by laser-induced local heating and c… ▽ More Non-equilibrium assembly, driven by fluxes controllable by continuous external energy inputs, enables dynamic and reconfigurable structures. Such controlled 3D assembly is desired for the design of adaptive materials that exploit structure-function relationships, but has remained challenging. We present a non-equilibrium assembly of colloidal particles mediated by laser-induced local heating and continuous heat dissipation. These 3D out-of-equilibrium structures, assembled in a matter of a few minutes, were highly ordered and exhibited tunable photonic stopbands. We quantify the particle fluxes from the underlying assembly processes and report the growth dynamics of the assembled structures. Furthermore, we demonstrate the modulation of the photonic stopband achieved by modulating the particle fluxes, highlighting the prospects of such thermofluidic assembly for creating reconfigurable functional structures. △ Less

Submitted 14 December, 2024; originally announced December 2024.

Regulated polarization of active particles in local osmotic flow fields

Authors: Lisa Rohde, Desmond J. Quinn, Diptabrata Paul, Frank Cichos

Abstract: Regulation to a well-defined target state is a fundamental requirement for achieving reliable functionality in living systems and maintaining specific non-equilibrium states. The control of certain properties and functionalities of systems on the microscale presents a particular challenge since thermal fluctuations and environmental perturbations dominate. While synthetic active matter has demonst… ▽ More Regulation to a well-defined target state is a fundamental requirement for achieving reliable functionality in living systems and maintaining specific non-equilibrium states. The control of certain properties and functionalities of systems on the microscale presents a particular challenge since thermal fluctuations and environmental perturbations dominate. While synthetic active matter has demonstrated remarkable self-organization capabilities, examples of autonomous regulation processes at the single-particle level remain scarce. Here, we show that the interplay of two non-equilibrium processes leads to a regulated polarization state of active particles in local osmotic flow fields. The balance between thermophoretic repulsion and attraction by thermo-osmotic boundary flows, both generated by a single heat source, yields a steady state at which active particles encircle the heat source at a distance that depends on the temperature of the heat source. The balance of both temperature-induced processes causes a polarization of the active particles that is independent of the heat source temperature. The individual control of heat source and active particles in the experiment allows a detailed investigation of the self-regulated polarization effect in which we find that hydrodynamic interactions dominate. As the effects rely on osmotic flows and phoretic interactions, we expect that the observed phenomena can be generalized to other active systems and flow fields. △ Less

Submitted 14 December, 2024; originally announced December 2024.

Four-fold Anisotropic Magnetoresistance in Antiferromagnetic Epitaxial Thin Films of MnPt$_{x}$Pd$_{1-x}$

Authors: Shivesh Yadav, Shikhar Kumar Gupta, Mohit Verma, Debjoty Paul, Abira Rashid, Bhagyashree Chalke, Rudheer Bapat, Nilesh Kulkarni, Abhay Gautam, Arti Kashyap, Shouvik Chatterjee

Abstract: Antiferromagnets are emerging as promising alternatives to ferromagnets in spintronics applications. A key feature of antiferromagnets is their anisotropic magnetoresistance (AMR), which has the potential to serve as a sensitive marker for the antiferromagnetic order parameter. However, the underlying origins of this behavior remains poorly understood, particularly, in thin film geometries. In thi… ▽ More Antiferromagnets are emerging as promising alternatives to ferromagnets in spintronics applications. A key feature of antiferromagnets is their anisotropic magnetoresistance (AMR), which has the potential to serve as a sensitive marker for the antiferromagnetic order parameter. However, the underlying origins of this behavior remains poorly understood, particularly, in thin film geometries. In this study, we report the observation of AMR in epitaxial thin films of the collinear L1$_{0}$ antiferromagnet MnPt$_{x}$Pd$_{1-x}$. In the thicker films, AMR is dominated by a non-crystalline two-fold component, which emerges from domain reconfiguration and spin canting under applied magnetic field. As the film thickness is reduced, however, a crystalline four-fold component emerges, accompanied by the appearance of uncompensated magnetic moment, which strongly modifies the magnetotransport properties in the thinner films. We demonstrate that interfacial interactions lead to a large density of states (DOS) at the Fermi level. This enhanced DOS, combined with disorder in the thinner films, stabilizes the uncompensated moment and results in a four-fold modulation of the DOS as the Neel vector rotates, explaining the observed AMR behavior. △ Less

Submitted 8 June, 2025; v1 submitted 5 December, 2024; originally announced December 2024.

Comments: 10 pages, 5 figures

Journal ref: Physical Review B, 112, 014410, 2025

Imprinting electrically switchable scalar spin chirality by anisotropic strain in a Kagome antiferromagnet

Authors: Debjoty Paul, Shivesh Yadav, Shikhar Gupta, Bikash Patra, Nilesh Kulkarni, Debashis Mondal, Kaushal Gavankar, Sourav K. Sahu, Biswarup Satpati, Bahadur Singh, Owen Benton, Shouvik Chatterjee

Abstract: Topological chiral antiferromagnets, such as Mn$_{3}$Sn, are emerging as promising materials for next-generation spintronic devices due to their intrinsic transport properties linked to exotic magnetic configurations. Here, we demonstrate that anisotropic strain in Mn$_{3}$Sn thin films offers a novel approach to manipulate the magnetic ground state, unlocking new functionalities in this material.… ▽ More Topological chiral antiferromagnets, such as Mn$_{3}$Sn, are emerging as promising materials for next-generation spintronic devices due to their intrinsic transport properties linked to exotic magnetic configurations. Here, we demonstrate that anisotropic strain in Mn$_{3}$Sn thin films offers a novel approach to manipulate the magnetic ground state, unlocking new functionalities in this material. Anisotropic strain reduces the point group symmetry of the manganese (Mn) Kagome triangles from $C_{3v}$ to $C_{1}$, significantly altering the energy landscape of the magnetic states in Mn$_{3}$Sn. This symmetry reduction enables even a tiny in-plane Dzyaloshinskii-Moriya (DM) interaction to induce canting of the Mn spins out of the Kagome plane. The modified magnetic ground state introduces a finite scalar spin chirality and results in a significant Berry phase in momentum space. Consequently, a large anomalous Hall effect emerges in the Kagome plane at room temperature - an effect that is absent in the bulk material. Moreover, this two-fold degenerate magnetic state enables the creation of multiple-stable, non-volatile anomalous Hall resistance (AHR) memory states. These states are field-stable and can be controlled by thermal assisted current-induced magnetization switching requiring modest current densities and small bias fields, thereby offering a compelling new functionality in Mn$_{3}$Sn for spintronic applications. △ Less

Submitted 8 June, 2025; v1 submitted 4 November, 2024; originally announced November 2024.

Comments: 14 pages, 5 figures

Journal ref: Advanced Science, 12, e02569 (2025)

Propagation of Enzyme-driven Active Fluctuations in Crowded Milieu

Authors: Rik Chakraborty, Arnab Maiti, Diptangshu Paul, Rajnandan Borthakur, K. R. Jayaprakash, Uddipta Ghosh, Krishna Kanti Dey

Abstract: We investigated the energy transfer from active enzymes to their surroundings in crowded environments by measuring the diffusion of passive microscopic tracers in active solutions of ficoll and glycerol. Despite observing lower rates of substrate turnover and relatively smaller enhancement of passive tracer diffusion in artificial crowded media compared to those in aqueous solutions, we found a si… ▽ More We investigated the energy transfer from active enzymes to their surroundings in crowded environments by measuring the diffusion of passive microscopic tracers in active solutions of ficoll and glycerol. Despite observing lower rates of substrate turnover and relatively smaller enhancement of passive tracer diffusion in artificial crowded media compared to those in aqueous solutions, we found a significantly higher relative diffusion enhancement in crowded environments in the presence of enzymatic activity. Our experimental observations, coupled with supporting analytical estimations, underscored the critical role of the intervening media in facilitating mechanical energy distribution around active enzymes. △ Less

Submitted 1 August, 2024; originally announced August 2024.

Emergence Of Directional Rotation In Optothermally Activated Colloidal System

Authors: Rahul Chand, Chaudhary Eksha Rani, Diptabrata Paul, G V Pavan Kumar

Abstract: We experimentally demonstrate the emergence of directional rotation in thermally active-passive colloidal structures under optical confinement. The observed handedness of rotation of the structure can be controlled by changing the relative position of the constituent colloids. We show that the angular velocity of rotation is sensitive to the intensity of the incident optical fields and the size of… ▽ More We experimentally demonstrate the emergence of directional rotation in thermally active-passive colloidal structures under optical confinement. The observed handedness of rotation of the structure can be controlled by changing the relative position of the constituent colloids. We show that the angular velocity of rotation is sensitive to the intensity of the incident optical fields and the size of the constituent colloidal entities. The emergence of rotational dynamics can be understood in the context of asymmetric temperature distribution in the system and the relative location of the active colloid, which creates a local imbalance of optothermal torques to the confined system. Our work demonstrates how localized optothermal fields lead to directional rotational dynamics without explicitly utilizing spin or orbital angular momentum of light. We envisage that our results will have implications in realizing Brownian engines, and can directly relate to rotational dynamics in biological and ecological systems. △ Less

Submitted 22 September, 2023; originally announced September 2023.

The 4D Camera: an 87 kHz direct electron detector for scanning/transmission electron microscopy

Authors: Peter Ercius, Ian J. Johnson, Philipp Pelz, Benjamin H. Savitzky, Lauren Hughes, Hamish G. Brown, Steven E. Zeltmann, Shang-Lin Hsu, Cassio C. S. Pedroso, Bruce E. Cohen, Ramamoorthy Ramesh, David Paul, John M. Joseph, Thorsten Stezelberger, Cory Czarnik, Matthew Lent, Erin Fong, Jim Ciston, Mary C. Scott, Colin Ophus, Andrew M. Minor, and Peter Denes

Abstract: We describe the development, operation, and application of the 4D Camera -- a 576 by 576 pixel active pixel sensor for scanning/transmission electron microscopy which operates at 87,000 Hz. The detector generates data at approximately 480 Gbit/s which is captured by dedicated receiver computers with a parallelized software infrastructure that has been implemented to process the resulting 10 - 700… ▽ More We describe the development, operation, and application of the 4D Camera -- a 576 by 576 pixel active pixel sensor for scanning/transmission electron microscopy which operates at 87,000 Hz. The detector generates data at approximately 480 Gbit/s which is captured by dedicated receiver computers with a parallelized software infrastructure that has been implemented to process the resulting 10 - 700 Gigabyte-sized raw datasets. The back illuminated detector provides the ability to detect single electron events at accelerating voltages from 30 - 300 keV. Through electron counting, the resulting sparse data sets are reduced in size by 10 - 300x compared to the raw data, and open-source sparsity-based processing algorithms offer rapid data analysis. The high frame rate allows for large and complex 4D-STEM experiments to be accomplished with typical STEM scanning parameters. △ Less

Submitted 19 May, 2023; originally announced May 2023.

Adherence of the rotating vortex lattice in the noncentrosymmetric superconductor Ru$_{7}$B$_{3}$ to the London model

Authors: A. S. Cameron, Y. V. Tymoshenko, P. Y. Portnichenko, A. S. Sukhanov, M. Ciomaga Hatnean, D. McK. Paul, G. Balakrishnan, R. Cubitt, D. S. Inosov

Abstract: The noncentrosymmetric superconductor Ru$_7$B$_3$ has in previous studies demonstrated remarkably unusual behaviour in its vortex lattice, where the nearest neighbour directions of the vortices dissociate from the crystal lattice and instead show a complex field-history dependence, and the vortex lattice rotates as the field is changed. In this study, we look at the vortex lattice form factor of R… ▽ More The noncentrosymmetric superconductor Ru$_7$B$_3$ has in previous studies demonstrated remarkably unusual behaviour in its vortex lattice, where the nearest neighbour directions of the vortices dissociate from the crystal lattice and instead show a complex field-history dependence, and the vortex lattice rotates as the field is changed. In this study, we look at the vortex lattice form factor of Ru$_7$B$_3$ during this field-history dependence, to check for deviations from established models, such as the London model. We find that the data is well described by the anisotropic London model, which is in accordance with theoretical predictions that the alterations to the structure of the vortices due to broken inversion symmetry should be small. From this, we also extract values for the penetration depth and coherence length. △ Less

Submitted 29 September, 2022; originally announced September 2022.

Journal ref: J. Phys.: Condens. Matter 35, 425602 (2023)

Modulation of trion and exciton formation in monolayer WS2 by dielectric and substrate engineering

Authors: Tamaghna Chowdhury, Diptabrata Paul, Divya Nechiyil, Gokul M. A, Kenji Watanabe, Takashi Taniguchi, G. V. Pavan Kumar, Atikur Rahman

Abstract: Photoluminescence (PL) of transition metal dichalcogenide (TMD) monolayers is strongly influenced by the dielectric environment. The defect states present in the substrate induces uncontrollable doping in the TMD monolayer and thereby modifies the PL spectra. There have been enormous efforts to tune and overcome the effect of inevitable subtract defects in PL spectra, but a proper understanding an… ▽ More Photoluminescence (PL) of transition metal dichalcogenide (TMD) monolayers is strongly influenced by the dielectric environment. The defect states present in the substrate induces uncontrollable doping in the TMD monolayer and thereby modifies the PL spectra. There have been enormous efforts to tune and overcome the effect of inevitable subtract defects in PL spectra, but a proper understanding and a convenient way are still lacking. Here, we systematically studied the effect of surface defects by gradually increasing the separation between WS2 monolayer and substrate. Hence, we could precisely modulate the exciton and trion contribution in the PL spectra of WS2. The excitation power dependant measurements on dielectric engineered and patterned substrates helped us to shed light on the mechanism of PL modulation in monolayer WS2. We have also studied the influence of the nature of the charge carried by substrate defects on the PL spectra. These results open a new pathway to modulate and obtain the desired PL spectra of TMDs by engineering the substrates. Our findings will be useful for fabricating excitonic interconnects, valleytronic, and single-photon devices. △ Less

Submitted 23 September, 2022; originally announced September 2022.

Comments: 16 pages, 5 figures

Journal ref: 2D Materials, Volume 8, Number 4, 2021

Optothermal evolution of active colloidal matter in defocused laser trap

Authors: Diptabrata Paul, Rahul Chand, G V Pavan Kumar

Abstract: Optothermal interaction of active colloidal matter can facilitate environmental cues which can influence the dynamics of active soft matter systems. The optically induced thermal effect can be harnessed to study non-equilibrium thermodynamics as well as applied to self-propel colloids and form assemblies. In this work, we employ a defocused laser trap to form self-evolving colloidal active matter.… ▽ More Optothermal interaction of active colloidal matter can facilitate environmental cues which can influence the dynamics of active soft matter systems. The optically induced thermal effect can be harnessed to study non-equilibrium thermodynamics as well as applied to self-propel colloids and form assemblies. In this work, we employ a defocused laser trap to form self-evolving colloidal active matter. The optothermal interaction of the active colloids in both focused and defocused optical trap has been investigated to ascertain their thermophoretic behavior, which shows a long-range attraction and a short-range repulsion between the colloids. The optical gradient field enabled attraction and the short-range repulsion between the active colloids have been harnessed to form re-configurable dynamic assembly. Additionally, the assembly undergoes self-evolution as a new colloid joins the structure. Further, we show that the incident polarization state of the optical field can be employed as a parameter to modulate the structural orientation of the active colloids. The simple defocused optical field-enabled assembly can serve as a model to understand the collective dynamics of active matter systems, and can be harnessed as re-configurable microscopic engine. △ Less

Submitted 3 September, 2022; v1 submitted 16 March, 2022; originally announced March 2022.

Singlet-triplet mixing in the order parameter of the noncentrosymmetric superconductor Ru$_{7}$B$_{3}$

Authors: A. S. Cameron, Y. S. Yerin, Y. V. Tymoshenko, P. Y. Portnichenko, A. S. Sukhanov, M. Ciomaga Hatnean, D. McK. Paul, G. Balakrishnan, R. Cubitt, A. Heinemann, D. S. Inosov

Abstract: One of the key effects which is predicted to arise in superconductors without a centre of inversion is the mixing of singlet and triplet order parameters, which are no longer good quantum numbers on their own due to parity. We have probed the gap structure in the noncentrosymmetric superconductor Ru$_7$B$_3$, through small-angle neutron diffraction from the vortex lattice, in order to search for t… ▽ More One of the key effects which is predicted to arise in superconductors without a centre of inversion is the mixing of singlet and triplet order parameters, which are no longer good quantum numbers on their own due to parity. We have probed the gap structure in the noncentrosymmetric superconductor Ru$_7$B$_3$, through small-angle neutron diffraction from the vortex lattice, in order to search for the proposed mixed order parameter. We find that the measured temperature dependence of the vortex-lattice form factor is well characterised by a model constructed to describe the effects of broken inversion symmetry on the superconducting state, indicating the presence of a mixed singlet-triplet gap and confirming the theoretical predictions. △ Less

Submitted 8 February, 2022; originally announced February 2022.

Journal ref: Phys. Rev. B 105, 094519 (2022)

Simultaneous detection of spin and orbital angular momentum of light through scattering from a single silver nanowire

Authors: Diptabrata Paul, Deepak K Sharma, G V Pavan Kumar

Abstract: In recent times the spin angular momentum (SAM) and orbital angular momentum (OAM) of light have gained prominence because of their significance in optical communication systems, micromanipulation, sub-wavelength position sensing. To this end, simultaneous detection of SAM and OAM of light beam is one of the important topics of research from both application and fundamental spin-orbit interaction… ▽ More In recent times the spin angular momentum (SAM) and orbital angular momentum (OAM) of light have gained prominence because of their significance in optical communication systems, micromanipulation, sub-wavelength position sensing. To this end, simultaneous detection of SAM and OAM of light beam is one of the important topics of research from both application and fundamental spin-orbit interaction (SOI) point of view. While interferometry and metasurface based approaches have been able to detect the states, our approach involves elastic scattering from a monocrystalline silver nanowire for the simultaneous detection of SAM and OAM state of a circularly polarized Laguerre-Gaussian (LG) beam. By employing Fourier plane (FP) microscopy, the transmitted scattered light intensity distribution in the FP is analyzed to reconstruct the SAM and OAM state unambiguously. The SAM and OAM induced transverse energy flow as well as the polarization dependent scattering characteristics of the nanowire is investigated to understand the detection mechanism. Our method is devoid of complex nanofabrication techniques required for metasurface based approaches and to our knowledge, is a first example of single nano-object based simultaneous SAM and OAM detection. The study will further the understanding of SOI effects and can be useful for on-chip optical detection and manipulation. △ Less

Submitted 23 March, 2022; v1 submitted 29 November, 2021; originally announced November 2021.

Comments: 28 pages, 14 figures including supplementary information

Mirror-Coupled Microsphere can narrow the Angular distribution of Photoluminescence from WS2 Monolayers

Authors: Shailendra K. Chaubey, Sunny Tiwari, Gokul M. A., Diptabrata Paul, Atikur Rahman, G. V. Pavan Kumar

Abstract: Engineering optical emission from two dimensional, transition metal dichalcogenides (TMDs) materials such as Tungsten disulphide (WS2) has implications in creating and understanding nanophotonic sources. One of the challenges in controlling the optical emission from 2D materials is to achieve narrow angular spread using a simple photonic geometry. In this paper, we study how the photoluminescence… ▽ More Engineering optical emission from two dimensional, transition metal dichalcogenides (TMDs) materials such as Tungsten disulphide (WS2) has implications in creating and understanding nanophotonic sources. One of the challenges in controlling the optical emission from 2D materials is to achieve narrow angular spread using a simple photonic geometry. In this paper, we study how the photoluminescence of a monolayer WS2 can be controlled when coupled to film coupled microsphere dielectric antenna. Specifically, by employing Fourier plane microscopy and spectroscopic techniques, we quantify the wavevector distribution in the momentum space. As a result, we show beaming of the WS2 photoluminescence with angular divergence of θ1/2 = 4.6°. Furthermore, the experimental measurements have been supported by three-dimensional numerical simulations. We envisage that the discussed results can be generalized to a variety of nanophotonic 2D materials, and can be harnessed in nonlinear and quantum technology. △ Less

Submitted 9 June, 2022; v1 submitted 20 October, 2021; originally announced October 2021.

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.

Beaming Elastic and SERS Emission from Bent-Plasmonic Nanowire on a Mirror Cavity

Authors: Sunny Tiwari, Adarsh B. Vasista, Diptabrata Paul, Shailendra K. Chaubey, G. V. Pavan Kumar

Abstract: We report on the experimental observation of beaming elastic and surface enhanced Raman scattering (SERS) emission from a bent-nanowire on a mirror (B-NWoM) cavity. The system was probed with polarization resolved Fourier plane and energy-momentum imaging to study the spectral and angular signature of the emission wavevectors. The out-coupled elastically scattered light from the kink occupies a na… ▽ More We report on the experimental observation of beaming elastic and surface enhanced Raman scattering (SERS) emission from a bent-nanowire on a mirror (B-NWoM) cavity. The system was probed with polarization resolved Fourier plane and energy-momentum imaging to study the spectral and angular signature of the emission wavevectors. The out-coupled elastically scattered light from the kink occupies a narrow angular spread. We used a self-assembled monolayer of molecules with a well-defined molecular orientation to utilize the out-of-plane electric field in the cavity for enhancing Raman emission from the molecules and in achieving beaming SERS emission. Calculated directionality for elastic scattering and SERS emission were found to be 16.2 and 12.5 dB respectively. The experimental data were corroborated with three-dimensional numerical finite element and finite difference time domain based numerical simulations. The results presented here may find relevance in understanding coupling of emitters with elongated plasmonic cavities and in designing on-chip optical antennas. △ Less

Submitted 17 June, 2021; originally announced June 2021.

Mirror enhanced directional out-coupling of SERS by remote excitation of a nanowire-nanoparticle cavity

Authors: Sunny Tiwari, Adarsh B Vasista, Diptabrata Paul, G. V. Pavan Kumar

Abstract: We report on the experimental observation of mirror enhanced directional surface enhanced Raman scattering (SERS) from a self-assembled monolayer of molecules coupled to a nanowire-nanoparticle (NW-NP) junction on a mirror in remote excitation configuration. Placing NW-NP junction on a metallic mirror generates multiple gap plasmon modes which have unique momentum space scattering signatures. We p… ▽ More We report on the experimental observation of mirror enhanced directional surface enhanced Raman scattering (SERS) from a self-assembled monolayer of molecules coupled to a nanowire-nanoparticle (NW-NP) junction on a mirror in remote excitation configuration. Placing NW-NP junction on a metallic mirror generates multiple gap plasmon modes which have unique momentum space scattering signatures. We perform Fourier plane imaging of SERS from NW-NP on a mirror to understand the effect of multiple hotspots on molecular emission. We systematically study the effect of ground plane on the directionality of emission from NW-NP junction and show that the presence of a mirror drastically reduces angular spread of emission. The effect of multiple hotspots in the geometry on directionality of molecular emission is studied using 3D numerical simulations. The results presented here will have implications in understanding plasmon hybridization in the momentum space and its effects on molecular emission. △ Less

Submitted 27 April, 2021; originally announced April 2021.

Comments: 19 pages, 10 figures

Experimental observation of transverse spin of plasmon polaritons in a single-crystalline silver nanowire

Authors: Chetna Taneja, Diptabrata Paul, G V Pavan Kumar

Abstract: We report the experimental observation of the transverse spin and associated spin-momentum locking of surface plasmon polaritons (SPPs) excited in a plasmonic single crystalline silver nanowire (AgNW). In contrast to the SPPs excited in metal films, the electromagnetic field components of the evanescent SPP mode propagating along the long axis ($x$ axis) of the NW can decay along two longitudinal… ▽ More We report the experimental observation of the transverse spin and associated spin-momentum locking of surface plasmon polaritons (SPPs) excited in a plasmonic single crystalline silver nanowire (AgNW). In contrast to the SPPs excited in metal films, the electromagnetic field components of the evanescent SPP mode propagating along the long axis ($x$ axis) of the NW can decay along two longitudinal planes ($x$-$y$ and $x$-$z$ planes), resulting in two orthogonal transverse spin components ($s_z$ and $s_y$). Analysis of the opposite circular polarization components of the decaying SPP mode signal in the longitudinal plane ($x$-$y$) reveals spin dependent biasing of the signal and hence the existence of transverse spin component ($s_z$). The corresponding transverse spin density ($s_3$) in the Fourier plane reveals spin-momentum locking, where the helicity of the spin is dictated by the wave-vector components of the SPP evanescent wave. Further, the results are corroborated with three-dimensional numerical calculations. The presented results showcase how a chemically prepared plasmonic AgNW can be harnessed to study optical spins in evanescent waves, and can be extrapolated to explore sub-wavelength effects including directional spin coupling and optical nano-manipulation. △ Less

Submitted 3 October, 2021; v1 submitted 19 April, 2021; originally announced April 2021.

Comments: 25 pages, 8 figures

Directional emission from WS2 monolayer coupled to plasmonic Nanowire-on-Mirror Cavity

Authors: Shailendra K. Chaubey, Gokul M. A., Diptabrata Paul, Sunny Tiwari, Atikur Rahman, G. V. Pavan Kumar

Abstract: Influencing spectral and directional features of exciton emission characteristics from 2D transition metal dichalcogenides by coupling it to plasmonic nano-cavities has emerged as an important prospect in nanophotonics of 2D materials. In this paper we experimentally study the directional photoluminescence emission from Tungsten disulfide (WS2) monolayer sandwiched between a single-crystalline pla… ▽ More Influencing spectral and directional features of exciton emission characteristics from 2D transition metal dichalcogenides by coupling it to plasmonic nano-cavities has emerged as an important prospect in nanophotonics of 2D materials. In this paper we experimentally study the directional photoluminescence emission from Tungsten disulfide (WS2) monolayer sandwiched between a single-crystalline plasmonic silver nanowire (AgNW) waveguide and a gold (Au) mirror, thus forming an AgNW-WS2-Au cavity. By employing polarization-resolved Fourier plane optical microscopy, we quantify the directional emission characteristics from the distal end of the AgNW-WS2-Au cavity. Given that our geometry simultaneously facilitates local field enhancement and waveguiding capability, we envisage its utility in 2D material-based, on-chip nanophotonic signal processing, including nonlinear and quantum optical regimes. △ Less

Submitted 1 March, 2021; originally announced March 2021.

Comments: To appear in Advanced Photonics Research (2021)

Focused linearly-polarized light scattering from a silver nanowire: Experimental characterization of optical spin-Hall effect

Authors: Diptabrata Paul, Deepak K. Sharma, G. V. Pavan Kumar

Abstract: Spin-orbit interactions (SOI) are a set of sub-wavelength optical phenomenon in which spin and spatial degrees of freedom of light are intrinsically coupled. One of the unique example of SOI, spin-Hall effect of light (SHEL) has been an area of extensive research with potential applications in spin controlled photonic devices as well as emerging fields of spinoptics and spintronics. Here, we repor… ▽ More Spin-orbit interactions (SOI) are a set of sub-wavelength optical phenomenon in which spin and spatial degrees of freedom of light are intrinsically coupled. One of the unique example of SOI, spin-Hall effect of light (SHEL) has been an area of extensive research with potential applications in spin controlled photonic devices as well as emerging fields of spinoptics and spintronics. Here, we report our experimental study on SHEL due to forward scattering of focused linearly polarized Gaussian and Hermite-Gaussian ($\textrm{HG}_{10}$) beams from a silver nanowire (AgNW). Spin dependent anti-symmetric intensity patterns are obtained when the polarization of the scattered light is analysed. The corresponding spin-Hall signal is obtained by computing the far-field longitudinal spin density ($s_3$). Furthermore, by comparing the $s_3$ distributions, significant enhancement of the spin-Hall signal is found for $\textrm{HG}_{10}$ beam compared to Gaussian beam. The investigation of the optical fields at the focal plane of the objective lens reveals the generation of longitudinally spinning fields as the primary reason for the effects. The experimental results are corroborated by 3-dimensional numerical simulations. The results lead to better understanding of SOI and can have direct implications on chip-scale spin assisted photonic devices. △ Less

Submitted 14 January, 2021; v1 submitted 27 August, 2020; originally announced August 2020.

Comments: 9 pages, 9 figues

Journal ref: Phys. Rev. A 103, 013520 (2021)

Large-scale optothermal assembly of colloids mediated by a gold microplate

Authors: Vandana Sharma, Diptabrata Paul, Shailendra K Chaubey, Sunny Tiwari, G. V. Pavan Kumar

Abstract: Light-activated colloidal assembly and swarming can act as model systems to explore non-equilibrium state of matter. In this context, creating new experimental platforms to facilitate and control two-dimensional assembly of colloidal crystals are of contemporary interest. In this paper, we present an experimental study of assembly of colloidal silica microparticles in the vicinity of a single-crys… ▽ More Light-activated colloidal assembly and swarming can act as model systems to explore non-equilibrium state of matter. In this context, creating new experimental platforms to facilitate and control two-dimensional assembly of colloidal crystals are of contemporary interest. In this paper, we present an experimental study of assembly of colloidal silica microparticles in the vicinity of a single-crystalline gold microplate evanescently excited by a 532 nm laser beam. The gold microplate acts as a source of heat and establishes a thermal gradient in the system. The created optothermal potential assembles colloids to form a two-dimensional poly-crystal, and we quantify the coordination number and hexagonal packing order of the assembly in such a driven system. Interestingly, we observe variation in assembly-size as a function of excitation-polarization. Furthermore, we observe that the assembly is colloidal-material dependent. Specifically, silica colloids assemble but polystyrene colloids do not, indicating an intricate behaviour of the forces under play. Our work highlights a promising direction in utilizing metallic, single crystalline microstructures that can be harnessed for optothermal colloidal crystal assembly and swarming studies. Our experimental system can be utilized to explore optically driven matter and photophoretic interactions in soft-matter including biological systems such as cells and micro organisms. △ Less

Submitted 18 August, 2020; v1 submitted 2 November, 2019; originally announced November 2019.

Comments: 14 pages, 7 figures in main manuscript, 6 figures in supplementary information, Supplementary movies: https://www.youtube.com/playlist?list=PLVIRTkGrtbrtdsiVWjngfYO6e_srUGolf

Coexistence of type-I and type-II superconductivity signatures in ZrB12 probed by muon spin rotation measurements

Authors: P. K. Biswas, F. N. Rybakov, R. P. Singh, Saumya Mukherjee, N. Parzyk, G. Balakrishnan, M. R. Lees, C. D. Dewhurst, E. Babaev, A. D. Hillier, D. Mc K. Paul

Abstract: Superconductors usually display either type-I or type-II superconductivity and the coexistence of these two types in the same material, for example at different temperatures is rare in nature. We the employed muon spin rotation (muSR) technique to unveil the superconducting phase diagram of the dodecaboride ZrB12 and obtained clear evidence of both type-I and type-II characteristics. Most importan… ▽ More Superconductors usually display either type-I or type-II superconductivity and the coexistence of these two types in the same material, for example at different temperatures is rare in nature. We the employed muon spin rotation (muSR) technique to unveil the superconducting phase diagram of the dodecaboride ZrB12 and obtained clear evidence of both type-I and type-II characteristics. Most importantly, we found a region showing unusual behavior where the usually mutually exclusive muSR signatures of type-I and type-II superconductivity coexist. We reproduced that behavior in theoretical modeling that required taking into account multiple bands and multiple coherence lengths, which suggests that material has one coherence length larger and another smaller than the magnetic field penetration length (the type-1.5 regime). At stronger fields, a footprint of the type-II mixed state showing square flux-line lattice was also obtained using neutron diffraction. △ Less

Submitted 27 October, 2020; v1 submitted 20 October, 2019; originally announced October 2019.

Journal ref: Phys. Rev. B 102, 144523 (2020)

Room temperature operation of n-type Ge/SiGe terahertz quantum cascade lasers predicted by non-equilibrium Green's functions

Authors: T. Grange, D. Stark, G. Scalari, J. Faist, L. Persichetti, L. Di Gaspare, M. De Seta, M. Ortolani, D. J. Paul, G. Capellini, S. Birner, M. Virgilio

Abstract: n-type Ge/SiGe terahertz quantum cascade laser are investigated using non-equilibrium Green's functions calculations. We compare the temperature dependence of the terahertz gain properties with an equivalent GaAs/AlGaAs QCL design. In the Ge/SiGe case, the gain is found to be much more robust to temperature increase, enabling operation up to room temperature. The better temperature robustness with… ▽ More n-type Ge/SiGe terahertz quantum cascade laser are investigated using non-equilibrium Green's functions calculations. We compare the temperature dependence of the terahertz gain properties with an equivalent GaAs/AlGaAs QCL design. In the Ge/SiGe case, the gain is found to be much more robust to temperature increase, enabling operation up to room temperature. The better temperature robustness with respect to III-V is attributed to the much weaker interaction with optical phonons. The effect of lower interface quality is investigated and can be partly overcome by engineering smoother quantum confinement via multiple barrier heights. △ Less

Submitted 30 November, 2018; originally announced November 2018.

Comments: 5 pages, 5 figures

Time-reversal symmetry breaking in Re-based superconductors

Authors: T. Shang, M. Smidman, S. K. Ghosh, C. Baines, L. J. Chang, D. J. Gawryluk, J. A. T. Barker, R. P. Singh, D. Mck. Paul, G . Balakrishnan, E. Pomjakushina, M. Shi, M. Medarde, A. D. Hillier, H. Q. Yuan, J. Quintanilla, J. Mesot, T. Shiroka

Abstract: To trace the origin of time-reversal symmetry breaking (TRSB) in Re-based superconductors, we performed comparative muon-spin rotation/relaxation ($μ$SR) studies of superconducting noncentrosymmetric Re$_{0.82}$Nb$_{0.18}$ ($T_c = 8.8$ K) and centrosymmetric Re ($T_c = 2.7$ K). In Re$_{0.82}$Nb$_{0.18}$, the low temperature superfluid density and the electronic specific heat evidence a fully-gappe… ▽ More To trace the origin of time-reversal symmetry breaking (TRSB) in Re-based superconductors, we performed comparative muon-spin rotation/relaxation ($μ$SR) studies of superconducting noncentrosymmetric Re$_{0.82}$Nb$_{0.18}$ ($T_c = 8.8$ K) and centrosymmetric Re ($T_c = 2.7$ K). In Re$_{0.82}$Nb$_{0.18}$, the low temperature superfluid density and the electronic specific heat evidence a fully-gapped superconducting state, whose enhanced gap magnitude and specific-heat discontinuity suggest a moderately strong electron-phonon coupling. In both Re$_{0.82}$Nb$_{0.18}$ and pure Re, the spontaneous magnetic fields revealed by zero-field $μ$SR below $T_c$ indicate time-reversal symmetry breaking and thus unconventional superconductivity. The concomitant occurrence of TRSB in centrosymmetric Re and noncentrosymmetric Re$T$ ($T$ = transition metal), yet its preservation in the isostructural noncentrosymmetric superconductors Mg$_{10}$Ir$_{19}$B$_{16}$ and Nb$_{0.5}$Os$_{0.5}$, strongly suggests that the local electronic structure of Re is crucial for understanding the TRSB superconducting state in Re and Re$T$. We discuss the superconducting order parameter symmetries that are compatible with the observations. △ Less

Submitted 5 December, 2018; v1 submitted 28 November, 2018; originally announced November 2018.

Comments: 3 figures, 5 pages, accepted by Physical Review Letters

Journal ref: Phys. Rev. Lett. 121, 257002 (2018)

Rotation of the magnetic vortex lattice in Ru7B3 driven by the effects of broken time-reversal and inversion symmetry

Authors: A. S. Cameron, Y. S. Yerin, Y. V. Tymoshenko, P. Y. Portnichenko, A. S. Sukhanov, M. Ciomaga Hatnean, D. McK. Paul, G. Balakrishnan, R. Cubitt, D. S. Inosov

Abstract: We observe a hysteretic reorientation of the magnetic vortex lattice in the noncentrosymmetric superconductor Ru7B3, with the change in orientation driven by altering magnetic field below Tc. Normally a vortex lattice chooses either a single or degenerate set of orientations with respect to a crystal lattice at any given field or temperature, a behavior well described by prevailing phenomenologica… ▽ More We observe a hysteretic reorientation of the magnetic vortex lattice in the noncentrosymmetric superconductor Ru7B3, with the change in orientation driven by altering magnetic field below Tc. Normally a vortex lattice chooses either a single or degenerate set of orientations with respect to a crystal lattice at any given field or temperature, a behavior well described by prevailing phenomenological and microscopic theories. Here, in the absence of any typical VL structural transition, we observe a continuous rotation of the vortex lattice which exhibits a pronounced hysteresis and is driven by a change in magnetic field. We propose that this rotation is related to the spontaneous magnetic fields present in the superconducting phase, which are evidenced by the observation of time-reversal symmetry breaking, and the physics of broken inversion symmetry. Finally, we develop a model from the Ginzburg-Landau approach which shows that the coupling of these to the vortex lattice orientation can result in the rotation we observe. △ Less

Submitted 9 October, 2018; originally announced October 2018.

Journal ref: Phys. Rev. B 100, 024518 (2019)

Multigap Superconductivity in Chiral Noncentrosymmetric TaRh$_{2}$B$_{2}$

Authors: D. A. Mayoh, A. D. Hillier, K. Götze, D. McK. Paul, G. Balakrishnan, M. R. Lees

Abstract: We report the first observation of multigap superconductivity in TaRh$_{2}$B$_{2}$. We show TaRh$_{2}$B$_{2}$ is a bulk type-II superconductor with a transition temperature, $T_{\mathrm{c}} = 6.00(5)$ K. We present transverse-field muon spin relaxation data where the superconducting gap can be fit using a two-gap $\left(s+s\right)$-wave model. We also report the zero-field electronic specific heat… ▽ More We report the first observation of multigap superconductivity in TaRh$_{2}$B$_{2}$. We show TaRh$_{2}$B$_{2}$ is a bulk type-II superconductor with a transition temperature, $T_{\mathrm{c}} = 6.00(5)$ K. We present transverse-field muon spin relaxation data where the superconducting gap can be fit using a two-gap $\left(s+s\right)$-wave model. We also report the zero-field electronic specific heat in the superconducting state that is best described by the same $\left(s+s\right)$ model providing further evidence of multiband behavior in this superconductor. Zero-field muon spin relaxation measurements show time-reversal symmetry is preserved in the superconducting state. We demonstrate that TaRh$_{2}$B$_{2}$ has an upper critical field of $15.2(1)$ T, which is significantly higher than previously reported and exceeds the Pauli limit. △ Less

Submitted 20 June, 2018; originally announced June 2018.

Comments: 5 pages, 3 figures. Accepted for publication in Physical Review B

Journal ref: Phys. Rev. B 98, 014502 (2018)

Quantum Interference in Silicon 1D Quasi-Ballistic Junctionless Nanowire Field Effect Transistors

Authors: Felix J. Schupp, Muhammad M. Mirza, Donald A. MacLaren, G. Andrew D. Briggs, Douglas J. Paul, Jan A. Mol

Abstract: We investigate the low temperature transport in 8 nm diameter Si junctionless nanowire field effect transistors fabricated by top down techniques with a wrap-around gate and two different activated doping densities. First we extract the intrinsic gate capacitance of the device geometry from a device that shows Coulomb blockade at 13 mK with over 500 Coulomb peaks across a gate voltage range of 6 V… ▽ More We investigate the low temperature transport in 8 nm diameter Si junctionless nanowire field effect transistors fabricated by top down techniques with a wrap-around gate and two different activated doping densities. First we extract the intrinsic gate capacitance of the device geometry from a device that shows Coulomb blockade at 13 mK with over 500 Coulomb peaks across a gate voltage range of 6 V indicating the formation of a single island in the entire nanowire channel. In two other devices, doped Si:P $4\times10^{19}\,\text{cm}^{-3}$ and $2\times10^{20}\,\text{cm}^{-3}$, we observe quantum interference and use the extracted gate coupling to determine the dominant energy scale and the corresponding mean-free paths. For the higher doped device the analysis yields a mean free path of $4\pm2\,\text{nm}$, which is on the order of the average dopant spacing and suggests scattering on unactivated or activated dopants. For the device with an activated dopant density of $4\times10^{19}\,\text{cm}^{-3}$ the quantum interference effects suggest a mean free path of $10\pm2\,\text{nm}$, which is comparable to the nanowire width, and thus quasi-ballistic transport. A temperature dependent analysis of Universal Conductance Fluctuations suggests a coherence length above the nanowire length for temperatures below 1.9 K and decoherence from 1D electron-electron interactions for higher temperatures. The mobility is limited by scattering on impurities rather than the expected surface roughness scattering for nanowires with diameters larger or comparable to the Fermi wavelength. Our measurements therefore provide insight into the performance limitations from dominant scattering and dephasing mechanisms in technologically relevant silicon device geometries. △ Less

Submitted 20 February, 2018; originally announced February 2018.

Journal ref: Phys. Rev. B 98, 235428 (2018)

Mid Infrared Nonlinear Plasmonics using Germanium Nanoantennas on Silicon Substrates

Authors: Marco P. Fischer, Aaron Riede, Kevin Gallacher, Jacopo Frigerio, Giovanni Pellegrini, Michele Ortolani, Douglas J. Paul, Giovanni Isella, Alfred Leitenstorfer, Paolo Biagioni, Daniele Brida

Abstract: We demonstrate third harmonic generation in plasmonic antennas made of highly doped germanium and designed to be resonant in the mid infrared. Owing to the near-field enhancement, the result is an ultrafast, sub-diffraction, coherent light source tunable between 3 and 5 micrometer wavelength on a silicon substrate. To observe nonlinearity in this challenging spectral region, a high-power femtoseco… ▽ More We demonstrate third harmonic generation in plasmonic antennas made of highly doped germanium and designed to be resonant in the mid infrared. Owing to the near-field enhancement, the result is an ultrafast, sub-diffraction, coherent light source tunable between 3 and 5 micrometer wavelength on a silicon substrate. To observe nonlinearity in this challenging spectral region, a high-power femtosecond laser system equipped with parametric frequency conversion in combination with an all-reflective confocal microscope setup is employed. We show spatially resolved maps of the linear scattering cross section and the nonlinear emission of single isolated antenna structures. A clear third order power dependence as well as the mid-infrared emission spectra prove the nonlinear nature of the light emission. Simulations support the observed resonance length of the double rod antenna and demonstrate that the field enhancement inside the antenna material is responsible for the nonlinear frequency mixing. △ Less

Submitted 12 February, 2018; originally announced February 2018.

Time Reversal Symmetry Breaking in noncentrosymmetric superconductor Re6Ti

Authors: D. Singh, K. P. Sajilesh, J. A. T. Barker, D. McK. Paul, A. D. Hillier, R. P. Singh

Abstract: We have investigated the superconducting state of the noncentrosymmetric superconductor Re6Ti (Tc = 6.0 K) using muon-spin rotation/relaxation (muSR) technique. The zero-field muon experiment shows the presence of spontaneous magnetic fields in the superconducting state, indicating time-reversal symmetry breaking (TRSB). However, the low-temperature transverse field muon measurements suggest nodel… ▽ More We have investigated the superconducting state of the noncentrosymmetric superconductor Re6Ti (Tc = 6.0 K) using muon-spin rotation/relaxation (muSR) technique. The zero-field muon experiment shows the presence of spontaneous magnetic fields in the superconducting state, indicating time-reversal symmetry breaking (TRSB). However, the low-temperature transverse field muon measurements suggest nodeless s-wave superconductivity. The time reversal symmetry breaking further confirmed in the stoichiometric composition Re24Ti5. These results indicate that the pairing symmetry is not affected by spin-orbital coupling Re6X family of compounds. Altogether these studies suggest unconventional nature (TRSB) of superconductivity is intrinsic to Re6X family of compounds and paves the way for further studies of this family of materials. △ Less

Submitted 9 January, 2018; originally announced January 2018.

Comments: 6 Page, 4 Figure

Journal ref: Phys. Rev. B 97, 100505 (2018)

Time-reversal symmetry breaking in noncentrosymmetric superconductor Re6Hf:further evidence for unconventional behaviour in the alpha-Mn family of materials

Authors: D. Singh, J. A. T. Barker, A. Thamizhavel, D. McK. Paul, A. D. Hillier, R. P. Singh

Abstract: The discovery of new families of unconventional superconductors is important both experimentally and theoretically, especially if it challenges current models and thinking. By using muon spin relaxation in zero-field, time-reversal symmetry breaking has been observed in Re6Hf. Moreover, the temperature dependence of the superfluid density exhibits s-wave superconductivity with an enhanced electron… ▽ More The discovery of new families of unconventional superconductors is important both experimentally and theoretically, especially if it challenges current models and thinking. By using muon spin relaxation in zero-field, time-reversal symmetry breaking has been observed in Re6Hf. Moreover, the temperature dependence of the superfluid density exhibits s-wave superconductivity with an enhanced electron-phonon coupling. This, coupled with the results from isostructural Re6Zr, shows that the Re6X family are indeed a new and important group of unconventional superconductors. △ Less

Submitted 24 October, 2017; originally announced October 2017.

Comments: 5 pages, 2 figures Accepted Physical Review B, Rapid Communication

Superconducting and normal-state properties of the noncentrosymmetric superconductor Re6Zr

Authors: D. A. Mayoh, J. A. T. Barker, R. P. Singh, G. Balakrishnan, D. McK. Paul, M. R. Lees

Abstract: We systematically investigate the normal and superconducting properties of non-centrosymmetric Re$_{6}$Zr using magnetization, heat capacity, and electrical resistivity measurements. Resistivity measurements indicate Re$_{6}$Zr has poor metallic behavior and is dominated by disorder. Re$_6$Zr undergoes a superconducting transition at $T_{\mathrm{c}} = \left(6.75\pm0.05\right)$ K. Magnetization mea… ▽ More We systematically investigate the normal and superconducting properties of non-centrosymmetric Re$_{6}$Zr using magnetization, heat capacity, and electrical resistivity measurements. Resistivity measurements indicate Re$_{6}$Zr has poor metallic behavior and is dominated by disorder. Re$_6$Zr undergoes a superconducting transition at $T_{\mathrm{c}} = \left(6.75\pm0.05\right)$ K. Magnetization measurements give a lower critical field, $μ_{0}H_{\mathrm{c1}} = \left(10.3 \pm 0.1\right)$ mT. The Werthamer-Helfand-Hohenberg model is used to approximate the upper critical field $μ_{0}H_{\mathrm{c2}} = \left(11.2 \pm 0.2\right)$ T which is close to the Pauli limiting field of 12.35 T and which could indicate singlet-triplet mixing. However, low-temperature specific-heat data suggest that Re$_{6}$Zr is an isotropic, fully gapped s-wave superconductor with enhanced electron-phonon coupling. Unusual flux pinning resulting in a peak effect is observed in the magnetization data, indicating an unconventional vortex state. △ Less

Submitted 5 September, 2017; originally announced September 2017.

Comments: 11 pages, 7 figures, 2 tables

Journal ref: Physical Review B 96, 064521 (2017)

Superconducting Properties and $μ$SR Study of the Noncentrosymmetric Superconductor Nb$_{0.5}$Os$_{0.5}$

Authors: D. Singh, J. A. T. Barker, A. Thamizhavel, A. D. Hillier, D. McK. Paul, R. P. Singh

Abstract: The properties of the noncentrosymmetric superconductor ($α$-$\textit{Mn}$ structure) Nb$_{0.5}$Os$_{0.5}$ is investigated using resistivity, magnetization, specific heat, and muon spin relaxation and rotation ($μ$SR) measurements. These measurements suggest that Nb$_{0.5}$Os$_{0.5}$ is a weakly coupled ($λ_{e-ph}$ $\sim$ 0.53) type-II superconductor ($κ_{GL}$ $\approx$ 61) having a bulk supercond… ▽ More The properties of the noncentrosymmetric superconductor ($α$-$\textit{Mn}$ structure) Nb$_{0.5}$Os$_{0.5}$ is investigated using resistivity, magnetization, specific heat, and muon spin relaxation and rotation ($μ$SR) measurements. These measurements suggest that Nb$_{0.5}$Os$_{0.5}$ is a weakly coupled ($λ_{e-ph}$ $\sim$ 0.53) type-II superconductor ($κ_{GL}$ $\approx$ 61) having a bulk superconducting transition temperature $T_c$ = 3.07 K. The specific heat data in the superconductive regime fits well with the single-gap BCS model indicating nodeless s-wave superconductivity in Nb$_{0.5}$Os$_{0.5}$. The $μ$SR measurements also confirm $\textit{s}$-wave superconductivity with the preserved time-reversal symmetry. △ Less

Submitted 29 April, 2017; originally announced May 2017.

Comments: 7 Page, 8 Figures

Electron-quasiparticle interaction in $\rm DyNi_2B_2C$ measured by point-contact spectroscopy

Authors: I. K. Yanson, N. L. Bobrov, C. V. Tomy, D. McK. Paul

Abstract: The electron-quasiparticle interaction (EQI) spectral function has been measured for $\rm DyNi_2B_2C$ in the normal state at low temperatures by means of point-contact spectroscopy (PCS). A low-frequency peak is found around $eV\sim 5\ meV$. It becomes measurable at $T_m^*\simeq 15\ K$ and grows in intensity with constant width as the temperature is lowered. We argue that this peak arises from the… ▽ More The electron-quasiparticle interaction (EQI) spectral function has been measured for $\rm DyNi_2B_2C$ in the normal state at low temperatures by means of point-contact spectroscopy (PCS). A low-frequency peak is found around $eV\sim 5\ meV$. It becomes measurable at $T_m^*\simeq 15\ K$ and grows in intensity with constant width as the temperature is lowered. We argue that this peak arises from the strong interaction of conduction electron with coupled crystal-electric-field-phonon excitations whose branches cross at low energy. The comparison with PC spectra for $\rm HoNi_2B_2C$ suggests that a similar peak also exists for this compound. The magnitude of the point-contact EQI parameter $λ_{PC}$ for $\rm DyNi_2B_2C$ is estimated. $©$2000 Published by Elsevier Science B.V. All rights reserved. △ Less

Submitted 12 March, 2017; originally announced March 2017.

Comments: 10 pages, 8 figures

Journal ref: Physica C 334 152-162 (2000)

Point-contact spectroscopy of superconducting energy gap in $\rm DyNi_2B_2C$

Authors: I. K. Yanson, N. L. Bobrov, C. V. Tomy, D. McK. Paul

Abstract: The superconducting energy gap in $\rm DyNi_2B_2C$ has been investigated using a point-contact technique based on the Andreev reflection from a normal (N)-superconductor (S) boundary, where N is Ag. The observed differential resistance $dV/dI$ is well described by the Blonder-Tinkham-Klapwijk (BTK) theory based on the BSC density of states with zero broadening parameter. Typically, the intensity o… ▽ More The superconducting energy gap in $\rm DyNi_2B_2C$ has been investigated using a point-contact technique based on the Andreev reflection from a normal (N)-superconductor (S) boundary, where N is Ag. The observed differential resistance $dV/dI$ is well described by the Blonder-Tinkham-Klapwijk (BTK) theory based on the BSC density of states with zero broadening parameter. Typically, the intensity of the gap structure amounts to several percentage of the normal state resistance, which is an order of magnitude less than predicted by the theory. For $\rm DyNi_2B_2C$ with $T_c<T_N$ (the Neel temperature), we found gap values satisfying the ratio of $2Δ_0/k_BT_c=3.63\pm 0.05$ similar to other superconducting nickel-borocarbides, both nonmagnetic and magnetic with $T_c\geq T_N$. The superconducting gap nonlinearity is superimposed on the antiferromagnetic structure in $dV/dI(V)$ which is suppressed at the magnetic field of the order of 3T applied nominally in the $ab$-plane and temperature $\geq 11~K$. The observed superconducting properties depend on the exact composition and structure at the surface of the crystal. △ Less

Submitted 19 February, 2017; originally announced February 2017.

Comments: 8 pages, 13 figures

Journal ref: Physica C 334, 33-43, (2000)

Suppression of magnetic excitations near the surface of the topological Kondo insulator SmB6

Authors: P. K. Biswas, M. Legner, G. Balakrishnan, M. Ciomaga Hatnean, M. R. Lees, D. McK. Paul, E. Pomjakushina, T. Prokscha, A. Suter, T. Neupert, Z. Salman

Abstract: We present a detailed investigation of the temperature and depth dependence of the magnetic properties of 3D topological Kondo insulator SmB6 , in particular near its surface. We find that local magnetic field fluctuations detected in the bulk are suppressed rapidly with decreasing depths, disappearing almost completely at the surface. We attribute the magnetic excitations to spin excitons in bulk… ▽ More We present a detailed investigation of the temperature and depth dependence of the magnetic properties of 3D topological Kondo insulator SmB6 , in particular near its surface. We find that local magnetic field fluctuations detected in the bulk are suppressed rapidly with decreasing depths, disappearing almost completely at the surface. We attribute the magnetic excitations to spin excitons in bulk SmB6 , which produce local magnetic fields of about ~1.8 mT fluctuating on a time scale of ~60 ns. We find that the excitonic fluctuations are suppressed when approaching the surface on a length scale of 40-90 nm, accompanied by a small enhancement in static magnetic fields. We associate this length scale to the size of the excitonic state. △ Less

Submitted 4 January, 2017; originally announced January 2017.

Comments: 5 pages, 5 figures, accepted for publication as a Rapid Communication in Phys. Rev. B

Journal ref: Phys. Rev. B 95, 020410 (2017)

Disentangling nonradiative recombination processes in Ge micro-crystals on Si substrates

Authors: F. Pezzoli, A. Giorgioni, K. Gallacher, F. Isa, P. Biagioni, R. W. Millar, E. Gatti, E. Grilli, E. Bonera, G. Isella, D. J. Paul, Leo Miglio

Abstract: We address nonradiative recombination pathways by leveraging surface passivation and dislocation management in micron-scale arrays of Ge crystals grown on deeply patterned Si substrates. The time decay photoluminescence (PL) at cryogenic temperatures discloses carrier lifetimes approaching 45 ns in band-gap engineered Ge micro-crystals. This investigation provides compelling information about the… ▽ More We address nonradiative recombination pathways by leveraging surface passivation and dislocation management in micron-scale arrays of Ge crystals grown on deeply patterned Si substrates. The time decay photoluminescence (PL) at cryogenic temperatures discloses carrier lifetimes approaching 45 ns in band-gap engineered Ge micro-crystals. This investigation provides compelling information about the competitive interplay between the radiative band-edge transitions and the trapping of carriers by dislocations and free surfaces. Furthermore, an in-depth analysis of the temperature dependence of the PL, combined with capacitance data and finite difference time domain modeling, demonstrates the effectiveness of GeO2 in passivating the surface of Ge and thus in enhancing the room temperature PL emission. △ Less

Submitted 29 March, 2016; originally announced March 2016.

Journal ref: Appl. Phys. Lett. 108, 262103 (2016)

Optical Activation of Germanium Plasmonic Antennas in the Mid Infrared

Authors: Marco P. Fischer, Christian Schmidt, Emilie Sakat, Johannes Stock, Antonio Samarelli, Jacopo Frigerio, Michele Ortolani, Douglas J. Paul, Giovanni Isella, Alfred Leitenstorfer, Paolo Biagioni, Daniele Brida

Abstract: Impulsive interband excitation with femtosecond near-infrared pulses establishes a plasma response in intrinsic germanium structures fabricated on a silicon substrate. This direct approach activates the plasmonic resonance of the Ge structures and enables their use as optical antennas up to the mid-infrared spectral range. The optical switching lasts for hundreds of picoseconds until charge recomb… ▽ More Impulsive interband excitation with femtosecond near-infrared pulses establishes a plasma response in intrinsic germanium structures fabricated on a silicon substrate. This direct approach activates the plasmonic resonance of the Ge structures and enables their use as optical antennas up to the mid-infrared spectral range. The optical switching lasts for hundreds of picoseconds until charge recombination red-shifts the plasma frequency. The full behavior of the structures is modeled by the electrodynamic response established by an electron-hole plasma in a regular array of antennas. △ Less

Submitted 10 June, 2016; v1 submitted 21 March, 2016; originally announced March 2016.

Journal ref: Phys. Rev. Lett. 117, 047401 (2016)

Tunability and Losses of Mid-infrared Plasmonics in Heavily Doped Germanium Thin Films

Authors: Jacopo Frigerio, Andrea Ballabio, Giovanni Isella, Emilie Sakat, Paolo Biagioni, Monica Bollani, Enrico Napolitani, Costanza Manganelli, Michele Virgilio, Alexander Grupp, Marco P. Fischer, Daniele Brida, Kevin Gallacher, Douglas J. Paul, Leonetta Baldassarre, Paolo Calvani, Valeria Giliberti, Alessandro Nucara, Michele Ortolani

Abstract: Heavily-doped semiconductor films are very promising for application in mid-infrared plasmonic devices because the real part of their dielectric function is negative and broadly tunable in this wavelength range. In this work we investigate heavily n-type doped germanium epilayers grown on different substrates, in-situ doped in the $10^{17}$ to $10^{19}$ cm$^{-3}$ range, by infrared spectroscopy, f… ▽ More Heavily-doped semiconductor films are very promising for application in mid-infrared plasmonic devices because the real part of their dielectric function is negative and broadly tunable in this wavelength range. In this work we investigate heavily n-type doped germanium epilayers grown on different substrates, in-situ doped in the $10^{17}$ to $10^{19}$ cm$^{-3}$ range, by infrared spectroscopy, first principle calculations, pump-probe spectroscopy and dc transport measurements to determine the relation between plasma edge and carrier density and to quantify mid-infrared plasmon losses. We demonstrate that the unscreened plasma frequency can be tuned in the 400 - 4800 cm$^{-1}$ range and that the average electron scattering rate, dominated by scattering with optical phonons and charged impurities, increases almost linearly with frequency. We also found weak dependence of losses and tunability on the crystal defect density, on the inactivated dopant density and on the temperature down to 10 K. In films where the plasma was optically activated by pumping in the near-infrared, we found weak but significant dependence of relaxation times on the static doping level of the film. Our results suggest that plasmon decay times in the several-picosecond range can be obtained in n-type germanium thin films grown on silicon substrates hence allowing for underdamped mid-infrared plasma oscillations at room temperature. △ Less

Submitted 20 January, 2016; originally announced January 2016.

Comments: 18 pages, 10 figures

Journal ref: Phys. Rev. B 94, 085202 (2016)

Graphene stabilization of two-dimensional gallium nitride

Authors: Zakaria Y. Al Balushi, Ke Wang, Ram Krishna Ghosh, Rafael A. Vilá, Sarah M. Eichfeld, Paul A. DeSario, Dennis F. Paul, Joshua D. Caldwell, Suman Datta, Joan M. Redwing, Joshua A. Robinson

Abstract: The spectrum of two-dimensional (2D) materials beyond graphene offers a remarkable platform to study new phenomena in condensed matter physics. Among these materials, layered hexagonal boron nitride (hBN), with its wide bandgap energy (~5.0-6.0 eV), has clearly established that 2D nitrides are key to advancing novel devices1. A gap, however, remains between the theoretical prediction of 2D nitride… ▽ More The spectrum of two-dimensional (2D) materials beyond graphene offers a remarkable platform to study new phenomena in condensed matter physics. Among these materials, layered hexagonal boron nitride (hBN), with its wide bandgap energy (~5.0-6.0 eV), has clearly established that 2D nitrides are key to advancing novel devices1. A gap, however, remains between the theoretical prediction of 2D nitrides beyond hBN and experimental realization of such structures. Here we demonstrate the synthesis of 2D gallium nitride (GaN) via a novel migration-enhanced encapsulated growth (MEEG) technique utilizing epitaxial graphene. We theoretically predict and experimentally validate that the atomic structure of 2D GaN grown via MEEG is notably different from reported theory. Moreover, we establish that graphene plays a critical role in stabilizing the direct-bandgap (nearly 5.0 eV), 2D buckled structure. Our results provide a foundation for discovery and stabilization of novel 2D nitrides that are difficult to prepare via traditional synthesis. △ Less

Submitted 12 May, 2016; v1 submitted 5 November, 2015; originally announced November 2015.

Electric Field Controlled Magnetization and Charge-Ordering in Pr0.6Ca0.4MnO3

Authors: Himanshu Sharma, Geetha Balakrishnan, Don McKenzie Paul, A. Tulapurkar, C. V. Tomy

Abstract: In this paper, we present the observation of the electric field control on the charge-ordering and metamagnetic transitions during the magnetization measurements in a single crystal of Pr0.6Ca0.4MnO3 (PCMO). We have demonstrated that the complete melting of charge ordering can be realized in a single crystal of PCMO by applying a voltage as small as 2.5 V, which otherwise needs magnetic fields in… ▽ More In this paper, we present the observation of the electric field control on the charge-ordering and metamagnetic transitions during the magnetization measurements in a single crystal of Pr0.6Ca0.4MnO3 (PCMO). We have demonstrated that the complete melting of charge ordering can be realized in a single crystal of PCMO by applying a voltage as small as 2.5 V, which otherwise needs magnetic fields in excess of 11 T. The maximum change in magnetization with applied voltage occurs across the charge-ordering transition temperature (TCO = 235 K). Even though the electric field does not seem to affect the magnetic ordering, we see a clear evidence at low temperatures for the occurrence of the metamagnetic transitions at higher fields with the application of electric field. △ Less

Submitted 28 October, 2015; originally announced October 2015.

Surface and Mechanical studies of Bismaleimide coatings

Authors: A. S. Bhattacharyya, D. Paul, P. P. Dutta, G. Bhattacharjee

Abstract: Bismaleimide (BMI) resins are a new breed of thermosetting resins used mainly for high temperature applications and have major usage in aerospace. BMI polymer coatings were deposited on aluminum and mild steel substrates. The effect of corrosion on mild steel and aluminum by Ringers Solution and there protection using BMI coatings were observed. X-ray diffraction studies showed crystalline nature… ▽ More Bismaleimide (BMI) resins are a new breed of thermosetting resins used mainly for high temperature applications and have major usage in aerospace. BMI polymer coatings were deposited on aluminum and mild steel substrates. The effect of corrosion on mild steel and aluminum by Ringers Solution and there protection using BMI coatings were observed. X-ray diffraction studies showed crystalline nature of the BMI coatings. Surface contact angle measurements were carried out using goniometer. △ Less

Submitted 3 September, 2015; originally announced September 2015.

Comments: 8 pages, 19 figures

Journal ref: Adv. Eng. Tec. Appl. 5, No. 1, 1 1 - 17 (2016)

Metallization and APPJ treatment of Bismaleimide

Authors: A. S. Bhattacharyya, S. Kumar, A. Sharma, D. Kumar, S. B. Patel, D. Paul, P. P. Dutta, G. Bhattacharjee

Abstract: Bismaleimide (BMI) resins are a new breed of thermosetting resins used mainly for high temperature applications and have major usage in aerospace. FTIR studies have shown the signatures of imide, CNC stretching, malemide and N-H stretching. These BMI polymer coatings were deposited on aluminum and mild steel substrates by sprinkling powers followed by baking. Thermo gravimetric analysis and Differ… ▽ More Bismaleimide (BMI) resins are a new breed of thermosetting resins used mainly for high temperature applications and have major usage in aerospace. FTIR studies have shown the signatures of imide, CNC stretching, malemide and N-H stretching. These BMI polymer coatings were deposited on aluminum and mild steel substrates by sprinkling powers followed by baking. Thermo gravimetric analysis and Differential scanning calorimetric studies showed the degradation temperature of these polymers around 370oC. Aluminum coatings were deposited on BMI previously deposited on Al and mild steel to make a metal-BMI-metal trilayer. These trilayers can solve the problem charging of the aircraft bodies at high altitudes. Atomic force microscopy was done to determine the morphology of the surface. Roughness and thickness measurements of the BMI coatings were carried out by surface profilometer. Vickers microhardness tests showed an increase in hardness of the metal-BMI-metal trilayer. FTIR spectrum showed signature of imides, CNC stretching, maleimide and N-H stretching in BMI. We observed that peak broadens at which shows the release of the stress during thermal treatment of the coating. The coating is subject to variable APPJ conditions which improve the properties at high temperature. △ Less

Submitted 29 June, 2016; v1 submitted 20 August, 2015; originally announced August 2015.

Comments: High Performance Polymers (per-referring, version 1)

Probing the superconducting ground state of the rare-earth ternary boride superconductors $R$RuB$_2$ ($R$ = Lu,Y) using muon-spin rotation and relaxation

Authors: Joel A. T. Barker, Ravi P. Singh, Adrian D. Hillier, Don McK. Paul

Abstract: The superconductivity in the rare-earth transition metal ternary borides $R$RuB$_2$ (where $R$ = Lu and Y) has been investigated using muon-spin rotation and relaxation. Measurements made in zero-field suggest that time-reversal symmetry is preserved upon entering the superconducting state in both materials; a small difference in depolarization is observed above and below the superconducting trans… ▽ More The superconductivity in the rare-earth transition metal ternary borides $R$RuB$_2$ (where $R$ = Lu and Y) has been investigated using muon-spin rotation and relaxation. Measurements made in zero-field suggest that time-reversal symmetry is preserved upon entering the superconducting state in both materials; a small difference in depolarization is observed above and below the superconducting transition in both compounds, however this has been attributed to quasistatic magnetic fluctuations. Transverse-field measurements of the flux-line lattice indicate that the superconductivity in both materials is fully gapped, with a conventional s-wave pairing symmetry and BCS-like magnitudes for the zero-temperature gap energies. The electronic properties of the charge carriers in the superconducting state have been calculated, with effective masses $m^*/ m_\mathrm{e} = $ $9.8\pm0.1$ and $15.0\pm0.1$ in the Lu and Y compounds, respectively, with superconducting carrier densities $n_\mathrm{s} = $ ($2.73\pm0.04$) $\times 10^{28}$ m$^{-3}$ and ($2.17\pm0.02$) $\times 10^{28}$ m$^{-3}$. The materials have been classified according to the Uemura scheme for superconductivity, with values for $T_\mathrm{c}/T_\mathrm{F}$ of $1/(414\pm6)$ and $1/(304\pm3)$, implying that the superconductivity may not be entirely conventional in nature. △ Less

Submitted 2 March, 2018; v1 submitted 3 August, 2015; originally announced August 2015.

Comments: 8 pages, 8 figures

ACM Class: J.2

Journal ref: Phys. Rev. B 97, 094506 (2018)