Showing 1–47 of 47 results for author: Pal, N

Magnetic anisotropy in the near-stoichiometric van der Waals ferromagnet Fe$_3$GeTe$_2$

Authors: Riju Pal, Joyal John Abraham, Satyabrata Bera, Mintu Mondal, Atindra Nath Pal, Bernd Buchner, Vladislav Kataev, Alexey Alfonsov

Abstract: Quasi-two-dimensional (2D) van der Waals (vdW) ferromagnets such as the series Fe$_{3-x}$GeTe$_2$, with a relatively high Curie temperature and robust metallicity, offer an ideal platform for investigating itinerant magnetism in reduced dimensions. Here, we present a comprehensive electron spin resonance (ESR) investigation of single-crystalline almost-stoichiometric Fe$_{3.03 \pm 0.03}$GeTe… ▽ More Quasi-two-dimensional (2D) van der Waals (vdW) ferromagnets such as the series Fe$_{3-x}$GeTe$_2$, with a relatively high Curie temperature and robust metallicity, offer an ideal platform for investigating itinerant magnetism in reduced dimensions. Here, we present a comprehensive electron spin resonance (ESR) investigation of single-crystalline almost-stoichiometric Fe$_{3.03 \pm 0.03}$GeTe$_{2}$ across wide ranges of frequencies, temperatures, and magnetic fields to gain quantitative insights into its magnetic anisotropy and spin dynamics. Frequency-dependent ESR measurements establish Fe$_{3}$GeTe$_{2}$ as an easy-axis ferromagnet. Temperature-dependent high-field ESR reveals a large internal field that gradually decreases at higher temperatures. Remarkably, this internal field persists even above $T_\mathrm{C}$, evidencing short-range spin correlations in Fe$_{3}$GeTe$_{2}$. Analysis of spin-wave modes yields a strong uniaxial magnetocrystalline anisotropy $K_{\text{int}} \approx - 5 \times 10^6$ erg cm$^{-3}$ at 3 K and a large magnon gap $Δ(3 \mathrm{K})$ $\approx$ 87.8 $\pm$ 13.7 GHz ($\approx$ 0.363 $\pm$ 0.057 meV). Our study highlights that small variations in Fe content in Fe$_{3}$GeTe$_{2}$ lead to substantial changes in the magnon gap at low temperatures, indicating the extreme sensitivity of spin dynamics to the chemical composition. These results establish Fe$_{3}$GeTe$_{2}$ as a model vdW ferromagnet for exploring tunable anisotropies and magnon excitations in metallic 2D magnets. △ Less

Submitted 27 July, 2026; originally announced July 2026.

Comments: 17 pages, 16 figures

Observation of correlation-driven topological transport and robust ferromagnetism in 2D CrS$_2$

Authors: Sk Md Obaidulla, M. Nur Hasan, Dayal Das, Rafiqul Alam, Antonio Supina, Muhammad Awais Aslam, Sherif Kamal, Iva Šarić Jankovic, Aleksandar Matkovic, Christian Teichert, Atindra Nath Pal, Heike C. Herper, Marko Kralj

Abstract: The realization of correlated layered magnets hosting robust ferromagnetism with emergent topological transport remains a key challenge in quantum materials. Here we report the first catalyst-free chemical vapour deposition growth of layered 1T-CrS$_2$, establishing a highly stable vdWs ferromagnet with an out-of-plane easy-axis anisotropy and a Curie temperature above room temperature. Transport… ▽ More The realization of correlated layered magnets hosting robust ferromagnetism with emergent topological transport remains a key challenge in quantum materials. Here we report the first catalyst-free chemical vapour deposition growth of layered 1T-CrS$_2$, establishing a highly stable vdWs ferromagnet with an out-of-plane easy-axis anisotropy and a Curie temperature above room temperature. Transport measurements reveal a semimetal--insulator crossover near 80 K and pronounced negative magnetoresistance up to 350 K. A topological Hall effect emerges below 30 K, a rare signature of correlated transport in layered transition-metal dichalcogenide ferromagnets. First-principles calculations show that spin--orbit coupling gaps Dirac-like crossings, while electronic correlations reconstruct the Fermi surface by suppressing electron pockets and reducing the carrier density, enhancing momentum-dependent out-of-plane spin polarization. Magnetic measurements, supported by Heisenberg exchange calculations, reveal strong nearest-neighbour ferromagnetic exchange that stabilizes long-range ferromagnetism. Our results establish 1T-CrS$_2$ as a rare correlated 3$d$ layered ferromagnet in which electronic correlations and spin--orbit coupling cooperatively drive emergent topological transport. △ Less

Submitted 26 July, 2026; originally announced July 2026.

Comments: 10 Pages, 4 figures and additional supplementary information

Spin-phonon interaction in a symmetry-enforced spin-polarized state

Authors: Suman Kalyan Pradhan, Dayal Das, Shubham Patel, Subhajit Mahapatra, Sachin Majee, Dibyendu Majee, Arnab Bera, Achintya Singha, Snehasish Nandy, Samik DuttaGupta, Atindra Nath Pal

Abstract: Symmetry-governed magnetic materials have emerged as a promising platform for spintronic functionalities without net magnetization or stray magnetic fields, motivating the exploration of how lattice dynamics couple to symmetry-derived spin-polarized electronic states. Understanding spin-phonon coupling in these systems is therefore essential for uncovering the microscopic origin of spin-lattice in… ▽ More Symmetry-governed magnetic materials have emerged as a promising platform for spintronic functionalities without net magnetization or stray magnetic fields, motivating the exploration of how lattice dynamics couple to symmetry-derived spin-polarized electronic states. Understanding spin-phonon coupling in these systems is therefore essential for uncovering the microscopic origin of spin-lattice interactions and for enabling their control in quantum materials. However, this mechanism remains poorly understood because spin polarization originates from crystal symmetry rather than conventional magnetic order. Here, we address this issue in the g-type altermagnet CoNb4Se8 using temperature- and polarization-resolved Raman spectroscopy, complemented by measurements on a structurally analogous Co-deficient compound lacking well-defined long-range magnetic order. We observe pronounced symmetry-selective phonon renormalization across the magnetic transition in CoNb4Se8, while related phonon anomalies persist in the Co-deficient system, demonstrating that the lattice response cannot be explained solely by conventional exchange-striction associated with coherent magnetic ordering. First-principles calculations reveal that spin-orbit coupling establishes a symmetry-dependent interaction channel between lattice vibrations and symmetry-governed electronic states. Our results identify an alternative mechanism for spin-phonon coupling in symmetry-governed magnetic materials and demonstrate that phonons provide a sensitive probe of symmetry-driven spin polarization even without robust magnetic order. More broadly, this work provides a framework for understanding and engineering spin-lattice functionality in symmetry-driven quantum materials, offering design principles for coupling lattice dynamics to spin-polarized electronic states. △ Less

Submitted 20 July, 2026; originally announced July 2026.

Impact of Disorder Dynamics and Multi-Domain Kinetics on the Sliding Ferroelectricity of CVD-Grown 3R-WSe2 Bilayers

Authors: Sourav Paul, Prasenjit Ghosh, Krishna Prasad Maity, Vineet Pandey, Abhijith M. B., Premananda Chatterjee, Kenji Watanabe, Takashi Taniguchi, Nicholas R. Glavin, Ajit K. Roy, Atindra Nath Pal, Vidya Kochat

Abstract: Sliding ferroelectricity in van der Waals (vdW) layered systems has emerged as a promising route toward non-volatile nanoscale devices, where interlayer displacement in non-centrosymmetric bilayers generates an out-of-plane polarization. In particular, 3R-stacked bilayer transition metal dichalcogenides (TMDs) grown via chemical vapor deposition (CVD) have been shown to host such polarization due… ▽ More Sliding ferroelectricity in van der Waals (vdW) layered systems has emerged as a promising route toward non-volatile nanoscale devices, where interlayer displacement in non-centrosymmetric bilayers generates an out-of-plane polarization. In particular, 3R-stacked bilayer transition metal dichalcogenides (TMDs) grown via chemical vapor deposition (CVD) have been shown to host such polarization due to broken inversion symmetry. However, a detailed investigation of the 2D ferroelectric (FE) properties of CVD-grown 2D films, particularly the role of intrinsic disorder, such as structural defects and domain structure, remains poorly understood. Here, we investigate the FE switching characteristics of CVD-grown 3R-stacked WSe2 using a graphene-based ferroelectric field-effect transistor (graphene-FE-FET) architecture, where graphene serves as a highly sensitive probe of induced charge modulation due to polarization switching of FEs. We show that the growth-induced structural disorder significantly impacts polarization switching, while multi-domain kinetics governs the evolution of the FE response. These findings provide important insights into the design and optimization of FE devices based on vdW materials. △ Less

Submitted 30 May, 2026; originally announced June 2026.

Journal ref: Nanoscale 2026

Intrinsic Electric Field Driven High Sensitive Photodetection in Alloy TMDC MoSSe

Authors: Chumki Nayak, Suvadip Masanta, Shubhadip Moulick, Manotosh Pramanik, Atanu Kabiraj, Satchidananda Rath, Sukanya Ghosh, Atindra Nath Pal, Bipul Pal, Achintya Singha

Abstract: Alloying offers an effective way to improve the functionality of transition metal dichalcogenides (TMDCs) in both fundamental research and optoelectronic applications, as it allows for engineering their electronic and optical properties. This study investigates the optoelectronic properties of CVD-synthesized alloy MoSSe, which exhibits an inherent out-of-plane dipole moment, arising from asymmetr… ▽ More Alloying offers an effective way to improve the functionality of transition metal dichalcogenides (TMDCs) in both fundamental research and optoelectronic applications, as it allows for engineering their electronic and optical properties. This study investigates the optoelectronic properties of CVD-synthesized alloy MoSSe, which exhibits an inherent out-of-plane dipole moment, arising from asymmetry in S and Se atoms on either side of the Mo layer, as confirmed by piezoelectric force microscopy, polarization-resolved second harmonic generation studies and theoretical first-principles calculations. Time-resolved photoluminescence measurements reveal an extended exciton radiative recombination lifetime in MoSSe, attributed to electron-hole wavefunction separation by the dipole moment, which improves photodetection by facilitating enhanced electron-hole separation before recombination. The device demonstrates significant responsivity over broad spectral range. By employing the photogating effect, the device response can be switched from slow to fast modes. These findings are further supported by illumination intensity-dependent photoluminescence and Raman measurements, underscoring the potential of polar TMDCs in future optoelectronic devices. △ Less

Submitted 3 March, 2026; originally announced March 2026.

Spin Reorientation Driven Renormalization of Spin-Phonon Coupling in Fe$_4$GeTe$_2$

Authors: Riju Pal, Md. Nur Hasan, Chumki Nayak, Mrinal Deka, Nastaran Salehi, Manuel Pereiro, Suchanda Mondal, Abhishek Misra, Achintya Singha, Prabhat Mandal, Debjani Karmakar, Atindra Nath Pal

Abstract: Quasi-2D van der Waals ferromagnet Fe$_4$GeTe$_2$, featuring the simultaneous presence of high Curie temperature ($T_\mathrm{C}$ $\sim 270$ K) and a spin-reorientation transition at $T_\mathrm{SR}$ $\sim 110$ K, is a rare system where strong interplay of spin dynamics, lattice vibrations, and electronic structure leads to a wide range of interesting phenomena. Here, we investigate the lattice resp… ▽ More Quasi-2D van der Waals ferromagnet Fe$_4$GeTe$_2$, featuring the simultaneous presence of high Curie temperature ($T_\mathrm{C}$ $\sim 270$ K) and a spin-reorientation transition at $T_\mathrm{SR}$ $\sim 110$ K, is a rare system where strong interplay of spin dynamics, lattice vibrations, and electronic structure leads to a wide range of interesting phenomena. Here, we investigate the lattice response of exfoliated Fe$_4$GeTe$_2$ nanoflakes using temperature-dependent Raman spectroscopy. Polarization-resolved measurements reveal that, while one Raman mode exhibits a purely out-of-plane character, the rest display mixed symmetry, reflecting interlayer vibrational nonuniformity and symmetry-driven mode degeneracies. Below $T_\mathrm{C}$, phonons harden, and the linewidth narrows, consistent with reduced anharmonicity, while across the spin reorientation transition at $T_\mathrm{SR}$ they display anomalous softening, linewidth broadening, and a peak in lifetime, which are signatures of strengthened spin-phonon coupling. Complementary DFT+DMFT calculations and atomistic spin dynamical simulations reveal temperature-dependent spin excitations whose energies overlap with the Raman-active phonons, providing a natural route for the observed magnon-phonon interaction. Together, these insights establish Fe$_4$GeTe$_2$ as a versatile platform for exploring intertwined spin, lattice, and electronic degrees of freedom, with relevance for dynamic spintronic and magneto-optic functionalities near technologically meaningful temperatures. △ Less

Submitted 20 December, 2025; originally announced December 2025.

Comments: Main Text: 10 pages, 4 figures; Supplementary Information: 31 pages, 27 figures

Disentangling the unusual magnetic anisotropy of the near-room-temperature ferromagnet Fe$_{4}$GeTe$_{2}$

Authors: Riju Pal, Joyal J. Abraham, Alexander Mistonov, Swarnamayee Mishra, Nina Stilkerich, Suchanda Mondal, Prabhat Mandal, Atindra Nath Pal, Jochen Geck, Bernd Büchner, Vladislav Kataev, Alexey Alfonsov

Abstract: In the quest for two-dimensional conducting materials with high ferromagnetic ordering temperature the new family of the layered Fe$_{n}$GeTe$_{2}$ compounds, especially the near-room-temperature ferromagnet Fe$_{4}$GeTe$_{2}$, receives a significant attention. Fe$_{4}$GeTe$_{2}$ features a peculiar spin reorientation transition at $T_\mathrm{SR} \sim 110$ K suggesting a non-trivial temperature ev… ▽ More In the quest for two-dimensional conducting materials with high ferromagnetic ordering temperature the new family of the layered Fe$_{n}$GeTe$_{2}$ compounds, especially the near-room-temperature ferromagnet Fe$_{4}$GeTe$_{2}$, receives a significant attention. Fe$_{4}$GeTe$_{2}$ features a peculiar spin reorientation transition at $T_\mathrm{SR} \sim 110$ K suggesting a non-trivial temperature evolution of the magnetic anisotropy (MA) - one of the main contributors to the stabilization of the magnetic order in the low-D systems. An electron spin resonance (ESR) spectroscopic study reported here provides quantitative insights into the unusual magnetic anisotropy of Fe$_{4}$GeTe$_{2}$. At high temperatures the total MA is mostly given by the demagnetization effect with a small contribution of the counteracting intrinsic magnetic anisotropy of an easy-axis type, whose growth below a characteristic temperature $T_{\rm shape} \sim 150$ K renders the sample seemingly isotropic at $T_\mathrm{SR}$. Below one further temperature $T_{\rm d} \sim 50$ K the intrinsic MA becomes even more complex. Importantly, all the characteristic temperatures found in the ESR experiment match those observed in transport measurements, suggesting an inherent coupling between magnetic and electronic degrees of freedom in Fe$_{4}$GeTe$_{2}$. This finding together with the observed signatures of the intrinsic two-dimensionality should facilitate optimization routes for the use of Fe$_{4}$GeTe$_{2}$ in the magneto-electronic devices, potentially even in the monolayer limit. △ Less

Submitted 9 December, 2025; originally announced December 2025.

Journal ref: Adv. Funct. Mater. 34, 2402551 (2024)

Single-Defect Spectroscopy via Random Telegraph Noise in Graphene-Contacted ReS$_2$-hBN Heterostructures

Authors: Shubhrasish Mukherjee, Gaurab Samanta, Shubhadip Moulick, Ruta Kulkarni, Kenji Watanabe, Takashi Taniguchi, Arumugum Thamizhavel, Atindra Nath Pal

Abstract: Defect spectroscopy in two-dimensional (2D) field-effect transistors (FETs) requires device architectures that suppress contact and disorder artifacts while preserving intrinsic carrier dynamics. Here, we demonstrate ReS$_2$-hBN FETs with few-layer graphene (FLG) van der Waals contacts that form nearly barrier-free interfaces, enabling intrinsic transport in ReS$_2$, an anisotropic, low-symmetry T… ▽ More Defect spectroscopy in two-dimensional (2D) field-effect transistors (FETs) requires device architectures that suppress contact and disorder artifacts while preserving intrinsic carrier dynamics. Here, we demonstrate ReS$_2$-hBN FETs with few-layer graphene (FLG) van der Waals contacts that form nearly barrier-free interfaces, enabling intrinsic transport in ReS$_2$, an anisotropic, low-symmetry TMDC rarely exhibiting disorder-free behavior. The clean ReS$_2$-FLG platform allows direct observation of random telegraph noise (RTN) even in micron-scale channels, manifested as discrete two-level current fluctuations between 90-150 K arising from stochastic trapping at localized hBN defect sites. With increasing temperature, the RTN evolves into a 1/f spectrum as multiple traps activate. Statistical analysis of RTN amplitudes and capture-emission kinetics identifies substitutional carbon-related centers in hBN as dominant defects. These findings establish a generalizable approach for probing dielectric-origin defect dynamics in intrinsically conducting, low-symmetry 2D semiconductors. △ Less

Submitted 21 November, 2025; originally announced November 2025.

Comments: 31 pages, 4 figures, including Supporting Information

Unconventional Electromechanical Response in Ferrocene Assisted Gold Atomic Chain

Authors: Biswajit Pabi, Štěpán Marek, Tal Klein, Arunabha Thakur, Richard Korytár, Atindra Nath Pal

Abstract: Atomically thin metallic chains serve as pivotal systems for studying quantum transport, with their conductance strongly linked to the orbital picture. Here, we report a non-monotonic electro-mechanical response in a gold-ferrocene junction, characterized by an unexpected conductance increase over a factor of ten upon stretching. This response is detected in the formation of ferrocene-assisted ato… ▽ More Atomically thin metallic chains serve as pivotal systems for studying quantum transport, with their conductance strongly linked to the orbital picture. Here, we report a non-monotonic electro-mechanical response in a gold-ferrocene junction, characterized by an unexpected conductance increase over a factor of ten upon stretching. This response is detected in the formation of ferrocene-assisted atomic gold chain in a mechanically controllable break junction at a cryogenic temperature. DFT based calculations show that tilting of molecules inside the chain modifies the orbital overlap and the transmission spectra, leading to such non-monotonic conductance evolution with stretching. This behavior, unlike typical flat conductance plateaus observed in metal atomic chains, pinpoints the unique role of conformational rearrangements during chain elongation. Our findings provide a deeper understanding of the role of orbital hybridization in transport properties and offer new opportunities for designing nanoscale devices with tailored electro-mechanical characteristics. △ Less

Submitted 2 September, 2025; originally announced September 2025.

Alloying as a new route to generating interlayer excitons

Authors: Suvadip Masanta, Chumki Nayak, Premananda Chatterjee, Atindra Nath Pal, Indrani Bose, Achintya Singha

Abstract: Heterobilayers formed by stacking two-dimensional atomic crystals are particularly promising for low-dimensional semiconductor optics, as they host interlayer excitons, bound states of electrons and holes residing in different layers. They inherit the valley-contrasting physics of the individual monolayers, leading to a range of unique properties that distinguish them from other solid-state nanost… ▽ More Heterobilayers formed by stacking two-dimensional atomic crystals are particularly promising for low-dimensional semiconductor optics, as they host interlayer excitons, bound states of electrons and holes residing in different layers. They inherit the valley-contrasting physics of the individual monolayers, leading to a range of unique properties that distinguish them from other solid-state nanostructures. Here, we propose a novel route for the generation of interlayer excitons based on the synthesis of a transition metal dichalcogenide bilayer alloy material, WS$_{2x}$Se$_{2(1-x)}$. Using piezoelectric force microscopy, we demonstrate the existence of an internal electric field oriented in the out-of-plane direction. Interlayer excitons have so far been mostly observed in heterostructures with a type-II band alignment. In the presence of an internal electric field, a similar alignment occurs in the alloy bilayer resulting in an efficient generation of interlayer excitons. Photoluminescence spectroscopy measurements involving circularly polarised light come up with key observations like a negative degree of circular polarization of the interlayer excitons which increases as a function of temperature. A simple theoretical model provides a physical understanding of the major experimentally observed features. With experimentally fitted parameter values, the dominant contribution to the degree of circular polarization is shown to arise from spin polarization and not from valley polarization, a consequence of the spin-valley-layer coupling characteristic of a TMDC bilayer. The room-temperature interlayer excitonic transition in bilayer TMDCs has key implications for fundamental physics, including Bose-Einstein condensation and high-temperature superfluidity, while enabling advanced valleytronic and quantum information functionalities. △ Less

Submitted 30 July, 2025; v1 submitted 27 March, 2025; originally announced March 2025.

Skyrmionic Transport and First Order Phase Transitions in Twisted Bilayer Graphene Quantum Hall Ferromagnet

Authors: Vineet Pandey, Prasenjit Ghosh, Riju Pal, Sourav Paul, Abhijith M B, Kenji Watanabe, Takashi Taniguchi, Atindra Nath Pal, Vidya Kochat

Abstract: Large-angle twisted bilayer graphene (TBLG) realizes a multicomponent quantum Hall (QH) platform of spin, valley and layer pseudospins with strong Coulomb interaction-driven symmetry broken phases. Here, we investigate the low energy Landau-level spectrum of layer-decoupled TBLG and identify skyrmion-textured charged excitations and a field-induced insulating transition to an intervalley coherent… ▽ More Large-angle twisted bilayer graphene (TBLG) realizes a multicomponent quantum Hall (QH) platform of spin, valley and layer pseudospins with strong Coulomb interaction-driven symmetry broken phases. Here, we investigate the low energy Landau-level spectrum of layer-decoupled TBLG and identify skyrmion-textured charged excitations and a field-induced insulating transition to an intervalley coherent state at zero-filling factor. Symmetric potential difference perpendicular to TBLG demonstrated layer coherent population of ground states with uniform energy barriers, while the charge imbalance in the layers at finite displacement field led to multidomain nucleation and a pronounced hysteresis in the exchange-dominated transport regime suggesting first order phase transitions between different QH ferromagnetic ground states. △ Less

Submitted 9 October, 2025; v1 submitted 26 March, 2025; originally announced March 2025.

High Rectification Ratio at Room Temperature in Rhenium(I) Compound

Authors: Subas Rajbangshi, Nila Pal, Robinur Rahman, Vladimir N. Nesterov, Lisa Roy, Shishir Ghosh, Prakash Chandra Mondal

Abstract: Electrical current rectification is an interesting electronic feature, popularly known as a diode. Achieving a high rectification ratio in a molecular junction has been a long-standing goal in molecular electronics. The present work describes mimicking electrical current rectification with pi-stacked rhenium(I) compound sandwiched between two electrical contacts. Among the two mononuclear rhenium… ▽ More Electrical current rectification is an interesting electronic feature, popularly known as a diode. Achieving a high rectification ratio in a molecular junction has been a long-standing goal in molecular electronics. The present work describes mimicking electrical current rectification with pi-stacked rhenium(I) compound sandwiched between two electrical contacts. Among the two mononuclear rhenium compounds studied here, [Re(CO)4(PPh3){(N)-saccharinate}] (1) and [Re(CO)3(phen){(N)-saccharinate}] (2), the latter show strong pi-pi interactions-induced high rectification ratio of ~ 4000 at 2.0 V at room temperature. Alternating current (AC)-based electrical measurements ensuring AC to DC electrical signal conversion at a frequency f of 1 KHz showing 2 can act as an excellent half-wave rectifier. Asymmetric charge injection barrier height at the electrode/Re(I) interfaces of the devices with a stacking configuration of p++-Si/Re compound31nm(2)/ITO originates the flow of electrical current unidirectionally. The charge transport mechanism governed by thermally activated hopping phenomena, and charge carrier propagation is explained through an energy profile considering the Fermi levels of two electrodes, and the energy of frontier molecular orbitals, HOMO, and LUMO, confirming rectification is of a molecular origin. The present work paves the way to combine different organometallic compounds as circuit elements in nanoelectronic devices to achieve numerous exciting electronic features. △ Less

Submitted 13 November, 2023; originally announced November 2023.

Comments: 16 pages, 5 figures

Valley polarization and photocurrent generation in transition metal dichalcogenide alloy MoS$_{2x}$Se$_{2(1-x)}$

Authors: Chumki Nayak, Suvadip Masanta, Sukanya Ghosh, Shubhadip Moulick, Atindra Nath Pal, Indrani Bose, Achintya Singha

Abstract: Monolayer transition metal dichalcogenides (TMDCs) constitute the core group of materials in the emerging semiconductor technology of valleytronics. While the coupled spin-valley physics of pristine TMDC materials and their heterstructures has been extensively investigated, less attention was given to TMDC alloys, which could be useful in optoelectronic applications due to the tunability of their… ▽ More Monolayer transition metal dichalcogenides (TMDCs) constitute the core group of materials in the emerging semiconductor technology of valleytronics. While the coupled spin-valley physics of pristine TMDC materials and their heterstructures has been extensively investigated, less attention was given to TMDC alloys, which could be useful in optoelectronic applications due to the tunability of their band gaps. We report here our experimental investigations of the spin-valley physics of the monolayer and bilayer TMDC alloy, MoS$_{2x}$Se$_{2(1-x)}$, in terms of valley polarization and the generation as well as electrical control of a photocurrent utilising the circular photogalvanic effect. Piezoelectric force microscopy provides evidence for an internal electric field perpendicular to the alloy layer, thus breaking the out-of-plane mirror symmetry. The experimental observation is supported by first principles calculations based on the density functional theory. A comparison of the photocurrent device, based on the alloy material, is made with similar devices involving other TMDC materials. △ Less

Submitted 11 November, 2023; v1 submitted 21 October, 2023; originally announced October 2023.

Resonant transport in a highly conducting single molecular junction via metal-metal covalent bond

Authors: Biswajit Pabi, Štepán Marek, Adwitiya Pal, Puja Kumari, Soumya Jyoti Ray, Arunabha Thakur, Richard Korytár, Atindra Nath Pal

Abstract: Achieving highly transmitting molecular junctions through resonant transport at low bias is key to the next-generation low-power molecular devices. Although, resonant transport in molecular junctions was observed by connecting a molecule between the metal electrodes via chemical anchors by applying a high source-drain bias (> 1V), the conductance was limited to < 0.1 G$_0$, G$_0$ being the quantum… ▽ More Achieving highly transmitting molecular junctions through resonant transport at low bias is key to the next-generation low-power molecular devices. Although, resonant transport in molecular junctions was observed by connecting a molecule between the metal electrodes via chemical anchors by applying a high source-drain bias (> 1V), the conductance was limited to < 0.1 G$_0$, G$_0$ being the quantum of conductance. Here, we report electronic transport measurements by directly connecting a Ferrocene molecule between Au electrodes at the ambient condition in a mechanically controllable break junction setup (MCBJ), revealing a conductance peak at ~ 0.2 G$_0$ in the conductance histogram. A similar experiment was repeated for Ferrocene terminated with amine (-NH2) and cyano (-CN) anchors, where conductance histograms exhibit an extended low conductance feature including the sharp high conductance peak, similar to pristine ferrocene. Statistical analysis of the data along with density functional theory-based transport calculation suggests the possible molecular conformation with a strong hybridization between the Au electrodes and Fe atom of Ferrocene molecule is responsible for a near-perfect transmission in the vicinity of the Fermi energy, leading to the resonant transport at a small applied bias (< 0.5V). Moreover, calculations including Van der Waals/dispersion corrections reveal a covalent like organometallic bonding between Au and the central Fe atom of Ferrocene, having bond energies of ~ 660 meV. Overall, our study not only demonstrates the realization of an air-stable highly transmitting molecular junction, but also provides an important insight about the nature of chemical bonding at the metal/organo-metallic interface. △ Less

Submitted 8 August, 2023; originally announced August 2023.

Comments: 23 pages, 6 figures, supplementary included

Journal ref: Nanoscale (2023)

Detection of nontrivial topology driven by charge density wave in a semi-Dirac metal

Authors: Rafiqul Alam, Prasun Boyal, Shubhankar Roy, Ratnadwip Singha, Buddhadeb Pal, Riju Pal, Prabhat Mandal, Priya Mahadevan, Atindra Nath Pal

Abstract: The presence of electron correlations in a system with topological order can lead to exotic ground states. Considering single crystals of LaAgSb2 which has a square net crystal structure, one finds multiple charge density wave transitions (CDW) as the temperature is lowered. We find large planar Hall (PHE) signals in the CDW phase, which are still finite in the high temperature phase though they c… ▽ More The presence of electron correlations in a system with topological order can lead to exotic ground states. Considering single crystals of LaAgSb2 which has a square net crystal structure, one finds multiple charge density wave transitions (CDW) as the temperature is lowered. We find large planar Hall (PHE) signals in the CDW phase, which are still finite in the high temperature phase though they change sign. Optimising the structure within first-principles calculations, one finds an unusual chiral metallic phase. This is because as the temperature is lowered, the electrons on the Ag atoms get more localized, leading to stronger repulsions between electrons associated with atoms on different layers. This leads to successive layers sliding with respect to each other, thereby stabilising a chiral structure in which inversion symmetry is also broken. The large Berry curvature associated with the low temperature structure explains the low temperature PHE. At high temperature the PHE arises from the changes induced in the tilted Dirac cone in a magnetic field. Our work represents a route towards detecting and understanding the mechanism in a correlation driven topological transition through electron transport measurements, complemented by ab-initio electronic structure calculations. △ Less

Submitted 8 August, 2023; v1 submitted 7 August, 2023; originally announced August 2023.

Comments: 11 pages, 4 figures, Includes supplementary information, Accepted in Advanced Functional Materials

Establishing Magnetic Coupling in Spin-crossover-2D Hybrid Nanostructures via Interfacial Charge-transfer Interaction

Authors: Shatabda Bhattacharya, Shubhadip Moulick, Chinmoy Das, Shiladitya Karmakar, Hirokazu Tada, Tanusri Saha-Dasgupta, Pradip Chakraborty, Atindra Nath Pal

Abstract: Despite a clear demonstration of bistability in spin-crossover (SCO) materials, the absence of long-range magnetic order and poor electrical conductivity limit their prospect in spintronic and nanoelectronic applications. Intending to create hybrid devices made of spin-crossover (SCO)-2D architecture, here, we report an easily processable Fe-based SCO nanostructures grown on 2D reduced graphene ox… ▽ More Despite a clear demonstration of bistability in spin-crossover (SCO) materials, the absence of long-range magnetic order and poor electrical conductivity limit their prospect in spintronic and nanoelectronic applications. Intending to create hybrid devices made of spin-crossover (SCO)-2D architecture, here, we report an easily processable Fe-based SCO nanostructures grown on 2D reduced graphene oxide (rGO). The heterostructure shows enhanced cooperativity due to formation of interfacial charge transfer induced inter-molecular interaction. The spin transition temperature is controlled by tuning the coverage area of SCO nanostructured networks over the 2D surfaces, thus manipulating hysteresis (aka memory) of the heterostructure. The enhanced magnetic coupling of the heterostructure leads to the spontaneous magnetization states with a large coercive field of $\sim$ 3000 Oe. Additionally, the low conductivity of the pristine SCO nanostructures is addressed by encapsulating them on suitable 2D rGO template, enabling detection of magnetic bistable spin states during high-spin/low-spin conductance change. This adds spin functionality in conductance switching for realizing hybrid 2D spintronic devices. Ab-inito calculations, on the experimentally proposed nanostructures, corroborate the enhanced magnetic interaction in the proposed architecture facilitated by interfacial charge transfer and provide insights on the microscopic mechanism. △ Less

Submitted 26 July, 2023; originally announced July 2023.

Comments: 23 pages, 6 figures and supplementary information included

Structural regulation of mechanical gating in molecular junctions

Authors: Biswajit Pabi, Jakub Šebesta, Richard Korytár, Oren Tal, Atindra Nath Pal

Abstract: In contrast to silicon-based transistors, single molecule junctions can be gated by simple mechanical means. Specifically, charge can be transferred between the junction's electrodes and its molecular bridge when the interelectrode distance is modified, leading to variations in the electronic transport properties of the junction. While this effect has been studied extensively, the influence of the… ▽ More In contrast to silicon-based transistors, single molecule junctions can be gated by simple mechanical means. Specifically, charge can be transferred between the junction's electrodes and its molecular bridge when the interelectrode distance is modified, leading to variations in the electronic transport properties of the junction. While this effect has been studied extensively, the influence of the molecule orientation on mechanical gating has not been addressed, despite its potential influence on the gating effectiveness. Here, we show that the same molecular junction can experience either clear mechanical gating or none, depending on the molecule orientation in the junctions. The effect is found in silver-ferrocene-silver break junctions, and analyzed in view of ab-initio and transport calculations, where the influence of molecular orbitals geometry on charge transfer to or from the molecule is revealed. The molecule orientation is thus a new degree of freedom that can be used to optimize mechanically-gated molecular junctions. △ Less

Submitted 6 April, 2023; originally announced April 2023.

Comments: 10 pages, 3 figures

Unusual magnetotransport and anomalous Hall effect in quasi-two-dimensional van der Waals ferromagnet Fe$_4$GeTe$_2$

Authors: Riju Pal, Buddhadeb Pal, Suchanda Mondal, Prabhat Mandal, Atindra Nath Pal

Abstract: Fe$_4$GeTe$_2$, an itinerant vdW ferromagnet (FM) having Curie temperature (T$_C$) close to room temperature ($\sim 270$ K), exhibits another transition (T$_{SR}$ $\sim$ 120 K) where the easy axis of magnetization changes from in-plane to the out-of-plane direction in addition to T$_C$. Here, we have studied the magnetotransport in a multilayer Hall bar device fabricated on 300 nm Si/SiO$_2$ subst… ▽ More Fe$_4$GeTe$_2$, an itinerant vdW ferromagnet (FM) having Curie temperature (T$_C$) close to room temperature ($\sim 270$ K), exhibits another transition (T$_{SR}$ $\sim$ 120 K) where the easy axis of magnetization changes from in-plane to the out-of-plane direction in addition to T$_C$. Here, we have studied the magnetotransport in a multilayer Hall bar device fabricated on 300 nm Si/SiO$_2$ substrate. Interestingly, the zero field resistivity shows a negligible change in resistivity near T$_C$ unlike the typical metallic FM, whereas, it exhibits a dramatic fall below T$_{SR}$. Also, the resistivity shows a weak anomaly at T $ \sim $ 38 K (T$_Q$), below which the resistivity shows a quadratic temperature dependence according to the Fermi liquid behavior. Temperature-dependent Hall data exhibits important consequences. The ordinary Hall coefficient changes sign near T$_{SR}$ indicating the change in majority carriers. In a similar manner, the magnetoresistance (MR) data shows significantly large negative MR near T$_{SR}$ and becomes positive below T$_Q$. The observations of anomaly in the resistivity, sign-change of the ordinary Hall coefficient and maximum negative MR near T$_{SR}$, together suggest a possible Fermi surface reconstruction associated with the spin reorientation transition. Furthermore, analysis of the Hall data reveals a significant anomalous Hall conductivity (AHC) from $\sim 123 Ω^{-1}$ cm$^{-1}$ (at T $\approx$ 5 K) to the maximum value of $\sim 366 Ω^{-1}$ cm$^{-1}$ near T$_{SR}$. While the low-temperature part may originate due to the intrinsic KL mechanism, our analysis indicates that the temperature-dependent AHC is primarily appearing due to the side-jump mechanism as a result of the spin-flip electron-magnon scattering. Our study demonstrates an interplay between magnetism and band topology and its consequence on electron transport in Fe$_4$GeTe$_2$. △ Less

Submitted 13 March, 2023; originally announced March 2023.

Comments: Main Text: 10 pages, 5 figures ; Supplementary Material: 9 pages, 8 figures

Journal ref: npj 2D Materials and Applications, 8 (1), 30 (2024)

Exciton-Plasmon Coupling Mediated Superior Photoresponse in 2D Hybrid Phototransistors

Authors: Shubhrasish Mukherjee, Didhiti Bhattacharya, Samit Kumar Ray, Atindra Nath Pal

Abstract: The possibility of creating heterostructure of two-dimensional (2D) materials has emerged as a viable route towards realizing novel optoelectronic devices. However, the low light absorption due to their small absorption cross section, limits their realistic application. While light-matter interaction mediated by strong exciton-plasmon coupling has been demonstrated to improve absorbance and sponta… ▽ More The possibility of creating heterostructure of two-dimensional (2D) materials has emerged as a viable route towards realizing novel optoelectronic devices. However, the low light absorption due to their small absorption cross section, limits their realistic application. While light-matter interaction mediated by strong exciton-plasmon coupling has been demonstrated to improve absorbance and spontaneous emission in a coupled TMDC and metallic nanostructures, the fabrication of tunable broadband phototransistor with high quantum yield is still a challenging task. By synthesizing Ag nanoparticles (Ag NPs) capped with a thin layer of polyvinylpyrrolidone (PVP) through chemical route, we report a lithography-free fabrication of a large area broadband superior gate-tunable hybrid phototransistor based on monolayer graphene decorated by WS$_2$-Ag NPs in a three-terminal device configuration. The fabricated device exhibits extremely high photoresponsivity (up to $3.2\times 10^4$ A/W) which is more than 5 times higher than the bare graphene/WS$_2$ hybrid device, along with a low noise equivalent power (NEP) (~10$^{-13}$ W/Hz$^{0.5}$, considering 1/f noise) and high specific detectivity ~1010 Jones in the wide (325-730 nm) wavelength region. The additional PVP capping of Ag NPs helps to suppress the direct charge and heat transfer and most importantly, increases the device stability by preventing the degradation of WS$_2$-Ag hybrid system. The enhanced optical properties of the hybrid device are explained via dipole mediated strong exciton-plasmon coupling, corroborated by COMSOL Multiphysics simulation. Our work demonstrates a strategy towards obtaining an environment-friendly, scalable, high-performance broadband phototransistor by tuning the exciton-plasmon coupling for new generation opto-electronic devices. △ Less

Submitted 12 March, 2023; originally announced March 2023.

Comments: 23 pages, 5 figures, Supplementary Information included

Observation of near room temperature thin film superconductivity of atmospherically stable Ag-Au mesoscopic thin film

Authors: Sobhan Hazra, Sandip Chatterjee, Bhola Nath Pal

Abstract: An environmentally stable mesoscopic thin film of Au of certain thickness has been deposited thermally on top of a Ag+ implanted oxide substrate to develop a close to room temperature superconductor. This thin film has been deposited in two different stages. Initially, a sol-gel derived ion conducting metal oxide (ICMO) thin film has been deposited by spin coating. Afterward, Ag+ has been introduc… ▽ More An environmentally stable mesoscopic thin film of Au of certain thickness has been deposited thermally on top of a Ag+ implanted oxide substrate to develop a close to room temperature superconductor. This thin film has been deposited in two different stages. Initially, a sol-gel derived ion conducting metal oxide (ICMO) thin film has been deposited by spin coating. Afterward, Ag+ has been introduced inside ICMO thin film by a chemical method. Following this, a thin layer of Au has been deposited on top of that Ag ion-implanted oxide via thermal evaporation. The temperature dependent resistivity (R-T) has been studied by four probe method. During high-to-low temperature sweep, around 240 K this thin film sample shows a sudden drop of resistance from 0.7 Ohm to 0.1 micro-Ohm. This 6-7 orders drop of resistance has been observed instantly within <0.1 K temperature variation of the sample. This transition temperature (TC) has been shifted toward the higher temperature by 5-6 degrees when temperature has been increased from low to the higher side. During 2nd and 3rd temperature cycling, both these transitions have been shifted by ~10 K towards room temperature w.r.t the earlier. However, after three successive temperature cycles, TC becomes stable and transitions occur close to 0 oC repeatedly. At the low resistance phase, current level has been varied from +100 mA to -100 mA which shows a random fluctuation of voltage drop within 10 nV range, indicating resistance under such circumstance is too low to measure by Delta mode electrical measurement (0.1 micro-Ohm). Besides, transition temperature reduces to lower temperature by 4 K, after applying 1 tesla magnetic field perpendicular to the thin film. Few YouTube video links of temperature dependent electrical characterization of such a thin film is given next to the acknowledgement section. △ Less

Submitted 27 February, 2023; v1 submitted 20 February, 2023; originally announced February 2023.

Comments: 9 pages, 7 figures

Enhanced coercivity and emergence of spin cluster glass state in 2D ferromagnetic material Fe3GeTe2

Authors: Satyabrata Bera, Suman Kalyan Pradhan, Riju Pal, Buddhadeb Pal, Arnab Bera, Sk Kalimuddin, Manjil Das, Deep Singha Roy, Hasan Afzal, Atindra Nath Pal, Mintu Mondal

Abstract: Two-dimensional (2D) van der Waals (vdW) magnetic materials with high coercivity and high $T_\text{C}$ are desired for spintronics and memory storage applications. Fe$_3$GeTe$_2$ (F3GT) is one such 2D vdW ferromagnet with a reasonably high $T_\text{C}$, but with a very low coercive field, $H_\text{c}$ ($\lesssim$100~Oe). Some of the common techniques of enhancing $H_\text{c}$ are by introducing pi… ▽ More Two-dimensional (2D) van der Waals (vdW) magnetic materials with high coercivity and high $T_\text{C}$ are desired for spintronics and memory storage applications. Fe$_3$GeTe$_2$ (F3GT) is one such 2D vdW ferromagnet with a reasonably high $T_\text{C}$, but with a very low coercive field, $H_\text{c}$ ($\lesssim$100~Oe). Some of the common techniques of enhancing $H_\text{c}$ are by introducing pinning centers, defects, stress, doping, etc. They involve the risk of undesirable alteration of other important magnetic properties. Here we propose a very easy, robust, and highly effective method of phase engineering by altering the sample growth conditions to greatly enhance the intrinsic coercivity (7-10 times) of the sample, without compromising its fundamental magnetic properties ($T_\text{C}\simeq$210K). The phase-engineered sample (F3GT-2) comprises of parent F3GT phase with a small percentage of randomly embedded clusters of a coplanar FeTe (FT) phase. The FT phase serves as both mosaic pinning centers between grains of F3GT above its antiferromagnetic transition temperature ($T_\text{C1}\sim$70~K) and also as anti-phase domains below $T_\text{C1}$. As a result, the grain boundary disorder and metastable nature are greatly augmented, leading to highly enhanced coercivity, cluster spin glass, and meta-magnetic behavior. The enhanced coercivity ($\simeq$1~kOe) makes F3GT-2 much more useful for memory storage applications and is likely to elucidate a new route to tune useful magnetic properties. Moreover, this method is much more convenient than hetero-structure and other cumbersome techniques. △ Less

Submitted 29 December, 2022; originally announced December 2022.

Comments: 12 pages, 11 figures

Sensing Remote Bulk Defects Through Resistance Noise in a Large Area Graphene Field Effect Transistor

Authors: Shubhadip Moulick, Rafiqul Alam, Atindra Nath Pal

Abstract: Substrate plays a crucial role in determining transport and low frequency noise behavior of graphene field effect devices. Typically, heavily dope Si/SiO$_2$ substrate is used to fabricate these devices for efficient gating. Trapping-detrapping processes closed to the graphene/substrate interface are the dominant sources of resistance fluctuations in the graphene channel, while Coulomb fluctuation… ▽ More Substrate plays a crucial role in determining transport and low frequency noise behavior of graphene field effect devices. Typically, heavily dope Si/SiO$_2$ substrate is used to fabricate these devices for efficient gating. Trapping-detrapping processes closed to the graphene/substrate interface are the dominant sources of resistance fluctuations in the graphene channel, while Coulomb fluctuations arising due to any remote charge fluctuations inside the bulk of the substrate are effectively screened by the heavily doped substrate. Here, we present electronic transport and low frequency noise characteristics of large area CVD graphene field effect transistor (FET) prepared on a lightly doped Si/SiO$_2$ substrate (N$_A$ $\sim$ 10$^{15}$cm$^{-3}$). Through a systematic characterization of transport, noise and capacitance at various temperature, we reveal that remote Si/SiO$_2$ interface can affect the charge transport in graphene severely and any charge fluctuations inside bulk of the silicon substrate can be sensed by the graphene channel. The resistance (R) vs. back gate voltage (V$_{bg}$) characteristics of the device shows a hump around the depletion region formed at the SiO$_2$/Si interface, confirmed by the capacitance (C) - Voltage (V) measurement. Low frequency noise measurement on these fabricated devices shows a peak in the noise amplitude close to the depletion region. This indicates that due to the absence of any charge layer at Si/SiO$_2$ interface, screening ability decreases and as a consequence, any fluctuations in the deep level coulomb impurities inside the silicon substrate can be observed as a noise in resistance in graphene channel via mobility fluctuations. Noise behavior on ionic liquid gated graphene on the same substrate exhibits no such peak in noise and can be explained by the interfacial trapping - detrapping processes closed to the graphene channel. △ Less

Submitted 18 October, 2022; originally announced October 2022.

Comments: 16 pages, 6 figures

Surface-phase superconductivity in Mg-deficient V-doped MgTi$_2$O$_4$ spinel

Authors: A. Rahaman, T. Paramanik, B. Pal, R. Pal, P. Maji, K. Bera, S. Mallik, D. K. Goswami, A. N. Pal, D. Choudhury

Abstract: Around fifty years ago, LiTi$_2$O$_4$ was reported to be first spinel oxide to exhibit a superconducting transition with highest T$_c$ $\approx$ 13.7 K. Recently, MgTi$_2$O$_4$ has been found to be the only other spinel oxide to reveal a superconducting transition with a T$_c$ $\approx$ 3 K, however, its superconducting state is realized only in thin film superlattices involving SrTiO$_3$. We find… ▽ More Around fifty years ago, LiTi$_2$O$_4$ was reported to be first spinel oxide to exhibit a superconducting transition with highest T$_c$ $\approx$ 13.7 K. Recently, MgTi$_2$O$_4$ has been found to be the only other spinel oxide to reveal a superconducting transition with a T$_c$ $\approx$ 3 K, however, its superconducting state is realized only in thin film superlattices involving SrTiO$_3$. We find that a V-doped Mg$_{1-x}$Ti$_2$O$_4$ phase, which gets stabilized as a thin surface layer on top of stoichiometric and insulating V-doped MgTi$_2$O$_4$ bulk sample, exhibits high-temperature superconductivity with T$_c$ $\approx$ 16 K. The superconducting transition is also confirmed through a concomitant sharp diamagnetic transition immediately below T$_c$. The spinel phase of the superconducting surface layer is elucidated through grazing-incidence X-ray diffraction and Micro-Raman spectroscopy. A small shift of the sharp superconducting transition temperature ($\sim$ 4 K) with application of a high magnetic field (upto 9 Tesla) suggests a very high critical field for the system, $\sim$ 25 Tesla. Thus, V-doped Mg$_{1-x}$Ti$_2$O$_4$ exhibits the highest T$_c$ among spinel superconductors and also possesses a very high critical field. △ Less

Submitted 25 June, 2023; v1 submitted 5 September, 2022; originally announced September 2022.

Comments: 5 pages, 4 figures

Journal ref: Phys. Rev. B 107, 245124 (2023)

An experimental set up to probe the quantum transport through single atomic/molecular junction at room temperature

Authors: Biswajit Pabi, Atindra Nath Pal

Abstract: Understanding the transport characteristics at the atomic limit is the prerequisite for futuristic nano-electronic applications. Among various experimental procedures, mechanically controllable break junction (MCBJ) is one of the well adopted experimental technique to study and control the atomic or molecular scale devices. Here, we present the details of the development of a piezo controlled tabl… ▽ More Understanding the transport characteristics at the atomic limit is the prerequisite for futuristic nano-electronic applications. Among various experimental procedures, mechanically controllable break junction (MCBJ) is one of the well adopted experimental technique to study and control the atomic or molecular scale devices. Here, we present the details of the development of a piezo controlled table top MCBJ set up, working at ambient condition, along with necessary data acquisition technique and analysis of the data. We performed conductance experiment on a macroscopic gold wire, which exhibits quantized conductance plateau upon pulling the wire with the piezo. Conductance peak up to $\sim 20 G_0$ ($G_0 = 2e^2/h$, $e$ being the electronic charge and $h$ being the plank's constant) could be resolved at room temperature. A well-known test bed molecule,$4, 4^\prime$ bipyridine, was introduced between the gold electrodes and conductance histogram exhibits two distinctive conductance peaks, confirming the formation of single molecular junction, in line with the previous reports. This demonstrate that our custom-designed MCBJ set up is capable of measuring quantum transport of a single molecular junction at ambient condition. △ Less

Submitted 14 July, 2022; originally announced July 2022.

Comments: 15 pages, 8 figures

High performance Broadband Photodetection Based on Graphene -- MoS$_{2x}$Se$_{2(1-x)}$ Alloy Engineered Phototransistors

Authors: Shubhrasish Mukherjee, Didhiti Bhattacharya, Samit Kumar Ray, Atindra Nath Pal

Abstract: The concept of alloy engineering has emerged as a viable technique towards tuning the bandgap as well as engineering the defect levels in two-dimensional transition metal dichalcognides (TMDC). Possibility to synthesize these ultrathin TMDC materials through chemical route has opened realistic possibilities to fabricate hybrid multi-functional devices. By synthesizing nanosheets with different com… ▽ More The concept of alloy engineering has emerged as a viable technique towards tuning the bandgap as well as engineering the defect levels in two-dimensional transition metal dichalcognides (TMDC). Possibility to synthesize these ultrathin TMDC materials through chemical route has opened realistic possibilities to fabricate hybrid multi-functional devices. By synthesizing nanosheets with different composites of MoS$_{2x}$Se$_{2(1-x)}$ (x = 0 to 1) using simple chemical methods, we systematically investigate the photo response properties of three terminal hybrid devices by decorating large area graphene with these nanosheets (x = 0, 0.5, 1) in 2D-2D configurations. Among them, graphene-MoSSe hybrid phototransistor exhibits superior optoelectronic properties than its binary counterparts. The device exhibits extremely high photoresponsivity (>10$^4$ A/W), low noise equivalent power (~10$^{-14}$ W/Hz$^{0.5}$), higher specific detectivity (~ 10$^{11}$ Jones) in the wide UV-NIR (365-810 nm) range with excellent gate tunability. The broadband light absorption of MoSSe, ultrafast charge transport in graphene, along with controllable defect engineering in MoSSe makes this device extremely attractive. Our work demonstrates the large area scalability with wafer-scale production of MoS$_{2x}$Se$_{2(1-x)}$ alloys, having important implication towards facile and scalable fabrication of high-performance optoelectronic devices and providing important insights into the fundamental interactions between van-der-Waals materials. △ Less

Submitted 30 June, 2022; originally announced June 2022.

Comments: 17 pages, 6 figures

Revisiting the magnetic ordering through anisotropic magnetic entropy change in quasi-two-dimensional metallic ferromagnet, Fe$_4$GeTe$_2$

Authors: Satyabrata Bera, Suman Kalyan Pradhan, Md Salman Khan, Riju Pal, Buddhadeb Pal, Sk Kalimuddin, Arnab Bera, Biswajit Das, Atindra Nath Pal, Mintu Mondal

Abstract: We have investigated the nature of ferromagnetic order and phase transitions in two dimensional (2D) van der Waals (vdW) layered material, Fe$_4$GeTe$_2$ through measurements of magnetization, magneto-caloric Effect (MCE), and heat capacity. Fe$_4$GeTe$_2$ hosts a complex magnetic phase with two distinct transitions: paramagnetic to ferromagnetic at around $T_\text{C}$ $\sim$ 266 K and another spi… ▽ More We have investigated the nature of ferromagnetic order and phase transitions in two dimensional (2D) van der Waals (vdW) layered material, Fe$_4$GeTe$_2$ through measurements of magnetization, magneto-caloric Effect (MCE), and heat capacity. Fe$_4$GeTe$_2$ hosts a complex magnetic phase with two distinct transitions: paramagnetic to ferromagnetic at around $T_\text{C}$ $\sim$ 266 K and another spin reorientation transition (SRT) at around $T_\text{SRT}$ $\sim $ 100 K. The magnetization measurements shows a prominent thermal hysteresis in proximity to $T_\text{SRT}$ at $H\parallel c$, which implies the first-order nature of SRT. Reasonable MCE has been observed around both transition temperatures ( at around $T_\text{C}$, -$Δ$S$_M^\text{max}$ = 1.95 and 1.99 J.Kg$^{-1}$K$^{-1}$ and at around $T_\text{SRT}$, -$Δ$S$_M^\text{max}$= 3.9 and 2.4 J.Kg$^{-1}$K$^{-1}$ along $H\parallel ab$ and $H\parallel c$ respectively) at 50 kOe magnetic field change. The above results reveal higher MCE value at $T_\text{SRT}$ compared to the values of MCE at $T_\text{C}$. The scaling analysis of MCE at $T_\text{C}$, shows that the rescaled $Δ$S$_M (T, H)$ follow a universal curve confirming the second-order character of the ferromagnetic transition. The same scaling analysis of MCE breaks down at $T_\text{SRT}$ suggesting that SRT is not a second order phase transition. The exponent $n$ from field dependence of magnetic entropy change presents a maximum of $|n|>2$ confirming the first-order nature of SRT. △ Less

Submitted 25 June, 2022; originally announced June 2022.

Comments: 12 pages, 10 figures + appendix

Superconductivity coexisting with ferromagnetism in a quasi-one dimensional non-centrosymmetric (TaSe$_4$)$_3$I

Authors: Arnab Bera, Sirshendu Gayen, Suchanda Mondal, Riju Pal, Buddhadeb Pal, Aastha Vasdev, Sandeep Howlader, Manish Jana, Tanmay Maiti, Rafikul Ali Saha, Biswajit Das, Biswarup Satpati, Atindra Nath Pal, Prabhat Mandal, Goutam Sheet, Mintu Mondal

Abstract: Low-dimensional materials with broken inversion symmetry and strong spin-orbit coupling can give rise to fascinating quantum phases and phase transitions. Here we report coexistence of superconductivity and ferromagnetism below 2.5\,K in the quasi-one dimensional crystals of non-centrosymmetric (TaSe$_4$)$_3$I (space group: $P\bar{4}2_1c$). The unique phase is a direct consequence of inversion sym… ▽ More Low-dimensional materials with broken inversion symmetry and strong spin-orbit coupling can give rise to fascinating quantum phases and phase transitions. Here we report coexistence of superconductivity and ferromagnetism below 2.5\,K in the quasi-one dimensional crystals of non-centrosymmetric (TaSe$_4$)$_3$I (space group: $P\bar{4}2_1c$). The unique phase is a direct consequence of inversion symmetry breaking as the same material also stabilizes in a centro-symmetric structure (space group: $P4/mnc$) where it behaves like a non-magnetic insulator. The coexistence here upfront contradicts the popular belief that superconductivity and ferromagnetism are two apparently antagonistic phenomena. Notably, here, for the first time, we have clearly detected Meissner effect in the superconducting state despite the coexisting ferromagnetic order. The coexistence of superconductivity and ferromagnetism projects non-centrosymmetric (TaSe$_4$)$_3$I as a host for complex ground states of quantum matter including possible unconventional superconductivity with elusive spin-triplet pairing. △ Less

Submitted 30 November, 2021; v1 submitted 29 November, 2021; originally announced November 2021.

Comments: 16 pages, 4 figures, Supplementary Information included in the anc directory

High Responsivity Gate Tunable UV-Visible Broadband Phototransistor Based on Graphene-WS2 Mixed Dimensional (2D-0D) Heterostructure

Authors: Shubhrasish Mukherjee, Didhiti Bhattacharya, Sumanti Patra, Sanjukta Paul, Rajib Kumar Mitra, Priya Mahadevan, Atindra Nath Pal, Samit Kumar Ray

Abstract: Recent progress in the synthesis of highly stable, eco-friendly, cost-effective transition metal-dichalcogenides (TMDC) quantum dots (QDs) with their broadband absorption spectrum and wavelength selectivity features have led to their increasing use in broadband photodetectors. With the solution based processing, we demonstrate a super large (~ 0.75 mm^2), UV-Vis broadband (365-633 nm), phototransi… ▽ More Recent progress in the synthesis of highly stable, eco-friendly, cost-effective transition metal-dichalcogenides (TMDC) quantum dots (QDs) with their broadband absorption spectrum and wavelength selectivity features have led to their increasing use in broadband photodetectors. With the solution based processing, we demonstrate a super large (~ 0.75 mm^2), UV-Vis broadband (365-633 nm), phototransistor made of WS_2 QDs decorated CVD graphene as active channel with extraordinary stability and durability in ambient condition (without any degradation of photocurrent till 4 months after fabrication). Here, colloidal 0D WS_2-QDs are used as the photo absorbing material and graphene acts as the conducting channel. A high photoresponsivity (3.1 x 10^2 A/W), higher detectivity (2.2 x 10^12 Jones) and low noise equivalent power (4 x 10^{-14} W/Hz^0.5) are obtained at a low bias voltage (V_{ds} = 1V) at an illumination of 365 nm with an optical power as low as 0.8 μW/cm^2, which can further be tuned by modulating the gate bias. While comparing the photocurrent between two different morphologies of WS_2 (QDs and 2D nanosheets), a significant enhancement of photocurrent is observed in case of QDs based device. Ab initio density functional theory based calculations further support our observation, revealing the role of quantum confinement for the enhanced photo response. Our work reveals a strategy towards making a scalable, cost-effective, highly performing hybrid two-dimensional (2D/0D) photo detector with graphene-WS_2 QDs, paving the way towards the next generation optoelectronic applications. △ Less

Submitted 9 November, 2021; originally announced November 2021.

Comments: 16 pages, 5 figures

Probing Metal-Molecule Contact at the Atomic Scale via Conductance Jump

Authors: Biswajit Pabi, Debayan Mondal, Priya Mahadevan, Atindra Nath Pal

Abstract: Understanding the formation of metal-molecule contact at the microscopic level is the key towards controlling and manipulating atomic scale devices. Employing two isomers of bipyridine, $4, 4^\prime$ bipyridine and $2, 2^\prime$ bipyridine between gold electrodes, here, we investigate the formation of metal-molecule bond by studying charge transport through single molecular junctions using a mecha… ▽ More Understanding the formation of metal-molecule contact at the microscopic level is the key towards controlling and manipulating atomic scale devices. Employing two isomers of bipyridine, $4, 4^\prime$ bipyridine and $2, 2^\prime$ bipyridine between gold electrodes, here, we investigate the formation of metal-molecule bond by studying charge transport through single molecular junctions using a mechanically controlled break junction technique at room temperature. While both molecules form molecular junctions during the breaking process, closing traces show the formation of molecular junctions unambiguously for $4, 4^\prime$ bipyridine via a conductance jump from the tunneling regime, referred as `jump to molecular contact', being absent for $2, 2^\prime$ bipyridine. Through statistical analysis of the data, along with, molecular dynamics and first-principles calculations, we establish that contact formation is strongly connected with the molecular structure of the electrodes as well as how the junction is broken during breaking process, providing important insights for using a single-molecule in an electronic device. △ Less

Submitted 9 September, 2021; originally announced September 2021.

Comments: 17 pages, 5 figures, , accepted as a letter

Journal ref: Phys. Rev. B (Letter) (2021)

Ephemeral Antibubbles: Spatiotemporal Evolution from Direct Numerical Simulations

Authors: Nairita Pal, Rashmi Ramadugu, Prasad Perlekar, Rahul Pandit

Abstract: Antibubbles, which consist of a shell of a low-density fluid inside a high-density fluid, have several promising applications. We show, via extensive direct numerical simulations (DNSs), in both two and three dimensions (2D and 3D), that the spatiotemporal evolution of antibubbles can be described naturally by the coupled Cahn-Hilliard-Navier-Stokes (CHNS) equations for a binary fluid. Our DNSs ca… ▽ More Antibubbles, which consist of a shell of a low-density fluid inside a high-density fluid, have several promising applications. We show, via extensive direct numerical simulations (DNSs), in both two and three dimensions (2D and 3D), that the spatiotemporal evolution of antibubbles can be described naturally by the coupled Cahn-Hilliard-Navier-Stokes (CHNS) equations for a binary fluid. Our DNSs capture elegantly the gravity-induced thinning and breakup of an antibubble via the time evolution of the Cahn-Hilliard scalar order parameter field $φ$, which varies continuously across interfaces, so we do not have to enforce complicated boundary conditions at the moving antibubble interfaces. To ensure that our results are robust, we supplement our CHNS simulations with sharp-interface Volume-of-Fluid (VoF) DNSs. We track the thickness of the antibubble and calculate the dependence of the lifetime of an antibubble on several parameters; we show that our DNS results agree with various experimental results; in particular, the velocity with which the arms of the antibubble retract after breakup scales as $σ^{1/2}$, where $σ$ is the surface tension. △ Less

Submitted 30 March, 2021; originally announced March 2021.

Comments: Ancillary folder contains movies and supplementary material -->

Dielectric/semiconductor interfacial doping to develop solution processed high performance 1 V ambipolar oxide-transistor and its application as CMOS inverter

Authors: Nitesh K. Chourasia, Anand Sharma, Nila Pal, Sajal Biring, Bhola N. Pal

Abstract: p-type doping from the dielectric/semiconductor interface of a SnO2 thin film transistor (TFT) has been utilized to develop high carrier mobility balanced ambipolar oxide-transistor. To introduce this interfacial-doping, bottom-gate top-contact TFTs have been fabricated by using two different ion-conducting oxide dielectrics which contain trivalent atoms. These ion-conducting dielectrics are LilnO… ▽ More p-type doping from the dielectric/semiconductor interface of a SnO2 thin film transistor (TFT) has been utilized to develop high carrier mobility balanced ambipolar oxide-transistor. To introduce this interfacial-doping, bottom-gate top-contact TFTs have been fabricated by using two different ion-conducting oxide dielectrics which contain trivalent atoms. These ion-conducting dielectrics are LilnO2 and LiGaO2 respectively, containing mobile Li+ ion. During SnO2 thin film fabrication on top of the ionic dielectric, those trivalent atoms allow p- doping to the interfacial SnO2 layer to introduce the hole conduction in channel of TFT. To realize this interfacial doping phenomena, a reference TFT has been fabricated with Li2ZnO2 dielectric under the same condition that contains divalent zinc (Zn) atom. Our comparative electrical data indicates that TFTs with LilnO2 and LiGaO2 dielectric are ambipolar in nature whereas, TFT with Li2ZnO2 dielectric is a unipolar n-channel transistor which reveals the interfacial doping of SnO2. Most interestingly, by using LilnO2 dielectric, we are capable to fabricated 1.0 V balanced ambipolar TFT with a high electron and hole mobility values of 7 cm2 V-1 s-1and 8 cm2 V-1 s-1 respectively with an on/off ratio >102 for both operations which has been utilized for low-voltage CMOS inverter fabrication. △ Less

Submitted 4 January, 2020; originally announced January 2020.

Exotic Multifractal Conductance Fluctuations in Graphene

Authors: Kazi Rafsanjani Amin, Samriddhi Sankar Ray, Nairita Pal, Rahul Pandit, Aveek Bid

Abstract: In quantum systems, signatures of multifractality are rare. They have been found only in the multiscaling of eigenfunctions at critical points. Here we demonstrate multifractality in the magnetic-field-induced universal conductance fluctuations of the conductance in a quantum condensed-matter system, namely, high-mobility single-layer graphene field-effect transistors. This multifractality decreas… ▽ More In quantum systems, signatures of multifractality are rare. They have been found only in the multiscaling of eigenfunctions at critical points. Here we demonstrate multifractality in the magnetic-field-induced universal conductance fluctuations of the conductance in a quantum condensed-matter system, namely, high-mobility single-layer graphene field-effect transistors. This multifractality decreases as the temperature increases or as doping moves the system away from the Dirac point. Our measurements and analysis present evidence for an incipient Anderson-localization near the Dirac point as the most plausible cause for this multifractality. Our experiments suggest that multifractality in the scaling behaviour of local eigenfunctions are reflected in macroscopic transport coefficients. We conjecture that an incipient Anderson-localization transition may be the origin of this multifractality. It is possible that multifractality is ubiquitous in transport properties of low-dimensional systems. Indeed, our work suggests that we should look for multifractality in transport in other low-dimensional quantum condensed-matter systems. △ Less

Submitted 12 April, 2018; originally announced April 2018.

Journal ref: Communications Physics, volume 1, Article number: 1 (2018)

Binary-Fluid Turbulence: Signatures of Multifractal Droplet Dynamics and Dissipation Reduction

Authors: Nairita Pal, Prasad Perlekar, Anupam Gupta, Rahul Pandit

Abstract: We present an extensive direct numerical simulation of statistically steady, homogeneous, isotropic turbulence in two-dimensional, binary-fluid mixtures with air-drag-induced friction by using the Cahn-Hilliard-Navier-Stokes equations. We choose parameters, e.g., the surface tension, such that we have a droplet of the minority phase moving inside a turbulent background of the majority phase. We ch… ▽ More We present an extensive direct numerical simulation of statistically steady, homogeneous, isotropic turbulence in two-dimensional, binary-fluid mixtures with air-drag-induced friction by using the Cahn-Hilliard-Navier-Stokes equations. We choose parameters, e.g., the surface tension, such that we have a droplet of the minority phase moving inside a turbulent background of the majority phase. We characterize the deformation of the droplet and show that it displays multifractal dynamics. The probability distribution functions of the components of the acceleration of the center of mass of the droplet exhibit wide, non-Gaussian tails. Our study reveals that the droplet enhances the energy spectrum $E(k)$ when the wavenumber $k$ is large; this enhancement leads to dissipation reduction. △ Less

Submitted 14 November, 2016; v1 submitted 31 December, 2015; originally announced December 2015.

Comments: 11 pages, 5 figures

Journal ref: Phys. Rev. E 93, 063115 (2016)

Two-dimensional Turbulence in Symmetric Binary-Fluid Mixtures: Coarsening Arrest by the Inverse Cascade

Authors: Prasad Perlekar, Nairita Pal, Rahul Pandit

Abstract: We study two-dimensional (2D) binary-fluid turbulence by carrying out an extensive direct numerical simulation (DNS) of the forced, statistically steady turbulence in the coupled Cahn-Hilliard and Navier-Stokes equations. In the absence of any coupling, we choose parameters that lead (a) to spinodal decomposition and domain growth, which is characterized by the spatiotemporal evolution of the Cahn… ▽ More We study two-dimensional (2D) binary-fluid turbulence by carrying out an extensive direct numerical simulation (DNS) of the forced, statistically steady turbulence in the coupled Cahn-Hilliard and Navier-Stokes equations. In the absence of any coupling, we choose parameters that lead (a) to spinodal decomposition and domain growth, which is characterized by the spatiotemporal evolution of the Cahn-Hilliard order parameter $φ$, and (b) the formation of an inverse-energy-cascade regime in the energy spectrum $E(k)$, in which energy cascades towards wave numbers $k$ that are smaller than the energy-injection scale $k_{inj}$ in the turbulent fluid. We show that the Cahn-Hilliard-Navier-Stokes coupling leads to an arrest of phase separation at a length scale $L_c$, which we evaluate from $S(k)$, the spectrum of the fluctuations of $φ$. We demonstrate that (a) $L_c \sim L_H$, the Hinze scale that follows from balancing inertial and interfacial-tension forces, and (b) $L_c$ is independent, within error bars, of the diffusivity $D$. We elucidate how this coupling modifies $E(k)$ by blocking the inverse energy cascade at a wavenumber $k_c$, which we show is $\simeq 2π/L_c$. We compare our work with earlier studies of this problem. △ Less

Submitted 29 June, 2015; originally announced June 2015.

Journal ref: Scientific Reports, 7, 44589 (2017)

Non-local transport via edge-states in InAs/GaSb coupled quantum wells

Authors: Susanne Mueller, Atindra Nath Pal, Matija Karalic, Thomas Tschirky, Christophe Charpentier, Werner Wegscheider, Klaus Ensslin, Thomas Ihn

Abstract: We have investigated low-temperature electronic transport on InAs/GaSb double quantum wells, a system which promises to be electrically tunable from a normal to a topological insulator. Hall bars of $50\,μ$m in length down to a few $μ$m gradually develop a pronounced resistance plateau near charge-neutrality, which comes along with distinct non-local transport along the edges. Plateau resistances… ▽ More We have investigated low-temperature electronic transport on InAs/GaSb double quantum wells, a system which promises to be electrically tunable from a normal to a topological insulator. Hall bars of $50\,μ$m in length down to a few $μ$m gradually develop a pronounced resistance plateau near charge-neutrality, which comes along with distinct non-local transport along the edges. Plateau resistances are found to be above or below the quantized value expected for helical edge channels. We discuss these results based on the interplay between imperfect edges and residual local bulk conductivity. △ Less

Submitted 27 April, 2015; originally announced April 2015.

Comments: 5 pages, 3 figures

Journal ref: Phys. Rev. B 92, 081303 (2015)

Influence of etching processes on electronic transport in mesoscopic InAs/GaSb quantum well devices

Authors: Atindra Nath Pal, Susanne Mueller, Thomas Ihn, Klaus Ensslin, Thomas Tschirky, Christophe Charpentier, Werner Wegscheider

Abstract: We report the electronic characterization of mesoscopic Hall bar devices fabricated from coupled InAs/GaSb quantum wells sandwiched between AlSb barriers, an emerging candidate for two-dimensional topological insulators. The electronic width of the etched structures was determined from the low field magneto-resistance peak, a characteristic signature of partially diffusive boundary scattering in t… ▽ More We report the electronic characterization of mesoscopic Hall bar devices fabricated from coupled InAs/GaSb quantum wells sandwiched between AlSb barriers, an emerging candidate for two-dimensional topological insulators. The electronic width of the etched structures was determined from the low field magneto-resistance peak, a characteristic signature of partially diffusive boundary scattering in the ballistic limit. In case of dry-etching the electronic width was found to decrease with electron density. In contrast, for wet etched devices it stayed constant with density. Moreover, the boundary scattering was found to be more specular for wet-etched devices, which may be relevant for studying topological edge states. △ Less

Submitted 3 July, 2015; v1 submitted 24 February, 2015; originally announced February 2015.

Comments: 5 pages, 2 figures

Journal ref: AIP Advances 5, 077106 (2015)

Fermi-edge transmission resonance in graphene driven by a single Coulomb impurity

Authors: Paritosh Karnatak, Srijit Goswami, Vidya Kochat, Atindra Nath Pal, Arindam Ghosh

Abstract: The interaction between the Fermi sea of conduction electrons and a non-adiabatic attractive impurity potential can lead to a power-law divergence in the tunneling probability of charge through the impurity. The resulting effect, known as the Fermi edge singularity (FES), constitutes one of the most fundamental many-body phenomena in quantum solid state physics. Here we report the first observatio… ▽ More The interaction between the Fermi sea of conduction electrons and a non-adiabatic attractive impurity potential can lead to a power-law divergence in the tunneling probability of charge through the impurity. The resulting effect, known as the Fermi edge singularity (FES), constitutes one of the most fundamental many-body phenomena in quantum solid state physics. Here we report the first observation of FES for Dirac Fermions in graphene driven by isolated Coulomb impurities in the conduction channel. In high-mobility graphene devices on hexagonal boron nitride substrates, the FES manifests in abrupt changes in conductance with a large magnitude $\approx e^{2}/h$ at resonance, indicating total many-body screening of a local Coulomb impurity with fluctuating charge occupancy. Furthermore, we exploit the extreme sensitivity of graphene to individual Coulomb impurities, and demonstrate a new defect-spectroscopy tool to investigate strongly correlated phases in graphene in the quantum Hall regime. △ Less

Submitted 15 June, 2014; originally announced June 2014.

Comments: 6 pages, 4 figures. To appear in Physical Review Letters

Spin-orbit splitting and effective masses in p-type GaAs two-dimensional hole gases

Authors: Fabrizio Nichele, Atindra Nath Pal, Roland Winkler, Christian Gerl, Werner Wegscheider, Thomas Ihn, Klaus Ensslin

Abstract: We present magnetotransport measurements performed on two-dimensional hole gases embedded in carbon doped p-type GaAs/AlGaAs heterostructures grown on [001] oriented substrates. A pronounced beating pattern in the Shubnikov-de Haas oscillations proves the presence of strong spin-orbit interaction in the device under study. We estimate the effective masses of spin-orbit split subbands by measuring… ▽ More We present magnetotransport measurements performed on two-dimensional hole gases embedded in carbon doped p-type GaAs/AlGaAs heterostructures grown on [001] oriented substrates. A pronounced beating pattern in the Shubnikov-de Haas oscillations proves the presence of strong spin-orbit interaction in the device under study. We estimate the effective masses of spin-orbit split subbands by measuring the temperature dependence of the Shubnikov-de Haas oscillations at different hole densities. While the lighter heavy-hole effective mass is not energy dependent, the heavier heavy-hole effective mass has a prominent energy dependence, indicating a strong spin-orbit induced non parabolicity of the valence band. The measured effective masses show qualitative agreement with self-consistent numerical calculations. △ Less

Submitted 17 February, 2014; v1 submitted 28 October, 2013; originally announced October 2013.

Journal ref: Physical Review B 89, 081306(R) (2014)

Suppression of bulk conductivity in InAs/GaSb broken gap composite quantum wells

Authors: Christophe Charpentier, Stefan Fält, Christian Reichl, Fabrizio Nichele, Atindra Nath Pal, Patrick Pietsch, Thomas Ihn, Klaus Ensslin, Werner Wegscheider

Abstract: The two-dimensional topological insulator state in InAs/GaSb quantum wells manifests itself by topologically protected helical edge channel transport relying on an insulating bulk. This work investigates a way of suppressing bulk conductivity by using gallium source materials of different degrees of impurity concentrations. While highest-purity gallium is accompanied by clear conduction through th… ▽ More The two-dimensional topological insulator state in InAs/GaSb quantum wells manifests itself by topologically protected helical edge channel transport relying on an insulating bulk. This work investigates a way of suppressing bulk conductivity by using gallium source materials of different degrees of impurity concentrations. While highest-purity gallium is accompanied by clear conduction through the sample bulk, intentional impurity incorporation lead to a bulk resistance over 1 MΩ. This resistance was found to be independent of applied magnetic fields. Ultra high electron mobilities for GaAs/AlGaAs structures fabricated in a molecular beam epitaxy system used for the growth of Sb-based samples are reported. △ Less

Submitted 16 August, 2013; v1 submitted 15 August, 2013; originally announced August 2013.

Journal ref: Appl. Phys. Lett. 103, 112102 (2013)

Insulating state and giant non-local response in an InAs/GaSb quantum well in the quantum Hall regime

Authors: Fabrizio Nichele, Atindra Nath Pal, Patrick Pietsch, Thomas Ihn, Klaus Ensslin, Christophe Charpentier, Werner Wegscheider

Abstract: We present transport measurements performed in InAs/GaSb double quantum wells. At the electron-hole crossover tuned by a gate voltage, a strong increase in the longitudinal resistivity is observed with increasing perpendicular magnetic field. Concomitantly with a local resistance exceeding the resistance quantum by an order of magnitude, we find a pronounced non-local resistance signal of almost s… ▽ More We present transport measurements performed in InAs/GaSb double quantum wells. At the electron-hole crossover tuned by a gate voltage, a strong increase in the longitudinal resistivity is observed with increasing perpendicular magnetic field. Concomitantly with a local resistance exceeding the resistance quantum by an order of magnitude, we find a pronounced non-local resistance signal of almost similar magnitude. The co-existence of these two effects is reconciled in a model of counter-propagating and dissipative quantum Hall edge channels providing backscattering, shorted by a residual bulk conductivity. △ Less

Submitted 19 December, 2013; v1 submitted 14 August, 2013; originally announced August 2013.

Journal ref: Phys. Rev. Lett 112, 036802 (2014)

Direct observation of valley-hybridization and universal symmetry of graphene with mesoscopic conductance fluctuations

Authors: Atindra Nath Pal, Vidya Kochat, Arindam Ghosh

Abstract: In graphene, the valleys represent spin-like quantities and can act as a physical resource in valley-based electronics to novel quantum computation schemes. Here we demonstrate a direct route to tune and read the valley quantum states of disordered graphene by measuring the mesoscopic conductance fluctuations. We show that the conductance fluctuations in graphene at low temperatures are reduced by… ▽ More In graphene, the valleys represent spin-like quantities and can act as a physical resource in valley-based electronics to novel quantum computation schemes. Here we demonstrate a direct route to tune and read the valley quantum states of disordered graphene by measuring the mesoscopic conductance fluctuations. We show that the conductance fluctuations in graphene at low temperatures are reduced by a factor of four when valley triplet states are gapped in the presence of short range potential scatterers at high carrier densities. We also show that this implies a gate tunable universal symmetry class which outlines a fundamental feature arising from graphene's unique crystal structure. △ Less

Submitted 18 June, 2012; originally announced June 2012.

Comments: 5 pages, 5 figures

High contrast imaging and thickness determination of graphene with in-column secondary electron microscopy

Authors: Vidya Kochat, Atindra Nath Pal, Sneha E. S., Arjun B. S., Anshita Gairola, S. A. Shivashankar, Srinivasan Raghavan, Arindam Ghosh

Abstract: We report a new method for quantitative estimation of graphene layer thicknesses using high contrast imaging of graphene films on insulating substrates with a scanning electron microscope. By detecting the attenuation of secondary electrons emitted from the substrate with an in-column low-energy electron detector, we have achieved very high thickness-dependent contrast that allows quantitative est… ▽ More We report a new method for quantitative estimation of graphene layer thicknesses using high contrast imaging of graphene films on insulating substrates with a scanning electron microscope. By detecting the attenuation of secondary electrons emitted from the substrate with an in-column low-energy electron detector, we have achieved very high thickness-dependent contrast that allows quantitative estimation of thickness up to several graphene layers. The nanometer scale spatial resolution of the electron micrographs also allows a simple structural characterization scheme for graphene, which has been applied to identify faults, wrinkles, voids, and patches of multilayer growth in large-area chemical vapor deposited graphene. We have discussed the factors, such as differential surface charging and electron beam induced current, that affect the contrast of graphene images in detail. △ Less

Submitted 27 March, 2012; originally announced March 2012.

Comments: 5 pages, 4 figures

Journal ref: Journal of Applied Physics 110, 014315 (2011)

The Nature of Electronic States in Atomically Thin MoS2 Field-Effect Transistors

Authors: Subhamoy Ghatak, Atindra Nath Pal, Arindam Ghosh

Abstract: We present low temperature electrical transport experiments in five field effect transistor devices consisting of monolayer, bilayer and trilayer MoS2 films, mechanically exfoliated onto Si/SiO2 substrate. Our experiments reveal that the electronic states in all films are localized well up to the room temperature over the experimentally accessible range of gate voltage. This manifests in two dimen… ▽ More We present low temperature electrical transport experiments in five field effect transistor devices consisting of monolayer, bilayer and trilayer MoS2 films, mechanically exfoliated onto Si/SiO2 substrate. Our experiments reveal that the electronic states in all films are localized well up to the room temperature over the experimentally accessible range of gate voltage. This manifests in two dimensional (2D) variable range hopping (VRH) at high temperatures, while below \sim 30 K the conductivity displays oscillatory structures in gate voltage arising from resonant tunneling at the localized sites. From the correlation energy (T0) of VRH and gate voltage dependence of conductivity, we suggest that Coulomb potential from trapped charges in the substrate are the dominant source of disorder in MoS2 field effect devices, which leads to carrier localization as well. △ Less

Submitted 26 March, 2012; originally announced March 2012.

Comments: 10 pages, 5 figures; ACS Nano (2011)

1/f noise as a probe for investigating band structure in graphene

Authors: Atindra Nath Pal, Subhamoy Ghatak, Vidya Kochat, Sneha E. S., Arjun B. S., Srinivasan Raghavan, Arindam Ghosh

Abstract: A distinctive feature of single layer graphene is the linearly dispersive energy bands, which in case of multilayer graphene become parabolic. Other than the quantum Hall effect, this distinction has been hard to capture in electron transport. Carrier mobility of graphene has been scrutinized, but many parallel scattering mechanisms often obscure its sensitivity to band structure. The flicker nois… ▽ More A distinctive feature of single layer graphene is the linearly dispersive energy bands, which in case of multilayer graphene become parabolic. Other than the quantum Hall effect, this distinction has been hard to capture in electron transport. Carrier mobility of graphene has been scrutinized, but many parallel scattering mechanisms often obscure its sensitivity to band structure. The flicker noise in graphene depends explicitly on its ability to screen local potential fluctuations. Here we show that the flicker noise is a sensitive probe to the band structure of graphene that vary differently with the carrier density for the linear and parabolic bands. Using devices of different genre, we find this difference to be robust against disorder in the presence or absence of substrate. Our results reveal the microscopic mechanism of noise in Graphene Field Effect Transistors (GraFET), and outline a simple portable method to separate the single from multi layered devices. △ Less

Submitted 30 September, 2010; v1 submitted 29 September, 2010; originally announced September 2010.

Comments: 5 pages, 4 figures

Large low-frequency resistance noise in chemical vapor deposited graphene

Authors: Atindra Nath Pal, Ageeth A. Bol, Arindam Ghosh

Abstract: We report a detailed investigation of resistance noise in single layer graphene films on Si/SiO$_2$ substrates obtained by chemical vapor deposition (CVD) on copper foils. We find that noise in these systems to be rather large, and when expressed in the form of phenomenological Hooge equation, it corresponds to Hooge parameter as large as $0.1 - 0.5$. We also find the variation in the noise magnit… ▽ More We report a detailed investigation of resistance noise in single layer graphene films on Si/SiO$_2$ substrates obtained by chemical vapor deposition (CVD) on copper foils. We find that noise in these systems to be rather large, and when expressed in the form of phenomenological Hooge equation, it corresponds to Hooge parameter as large as $0.1 - 0.5$. We also find the variation in the noise magnitude with the gate voltage (or carrier density) and temperature to be surprisingly weak, which is also unlike the behavior of noise in other forms of graphene, in particular those from exfoliation. △ Less

Submitted 5 October, 2010; v1 submitted 8 June, 2010; originally announced June 2010.

Comments: 4 pages, 4 figures

Journal ref: Applied Physics Letter, 97, 133504, 2010

Ultra-low noise field-effect transistor from multilayer graphene

Authors: Atindra Nath Pal, Arindam Ghosh

Abstract: We present low-frequency electrical resistance fluctuations, or noise, in graphene-based field-effect devices with varying number of layers. In single-layer devices the noise magnitude decreases with increasing carrier density, which behaved oppositely in the devices with two or larger number of layers accompanied by a suppression in noise magnitude by more than two orders in the latter case. Th… ▽ More We present low-frequency electrical resistance fluctuations, or noise, in graphene-based field-effect devices with varying number of layers. In single-layer devices the noise magnitude decreases with increasing carrier density, which behaved oppositely in the devices with two or larger number of layers accompanied by a suppression in noise magnitude by more than two orders in the latter case. This behavior can be explained from the influence of external electric field on graphene band structure, and provides a simple transport-based route to isolate single-layer graphene devices from those with multiple layers. △ Less

Submitted 27 May, 2009; originally announced May 2009.

Comments: 4 pages, 3 figures

Journal ref: Appl. Phys. Lett., 95, 082105 (2009)

Resistance noise in electrically biased bilayer graphene

Authors: Atindra Nath Pal, Arindam Ghosh

Abstract: We demonstrate that the low-frequency resistance fluctuations, or noise, in bilayer graphene is strongly connected to its band structure, and displays a minimum when the gap between the conduction and valence band is zero. Using double-gated bilayer graphene devices we have tuned the zero gap and charge neutrality points independently, which offers a versatile mechanism to investigate the low-en… ▽ More We demonstrate that the low-frequency resistance fluctuations, or noise, in bilayer graphene is strongly connected to its band structure, and displays a minimum when the gap between the conduction and valence band is zero. Using double-gated bilayer graphene devices we have tuned the zero gap and charge neutrality points independently, which offers a versatile mechanism to investigate the low-energy band structure, charge localization and screening properties of bilayer graphene. △ Less

Submitted 16 December, 2008; originally announced December 2008.

Comments: 5 pages, 4 figures

Journal ref: PRL 102, 126805 (2009)