Showing 1–25 of 25 results for author: Dash, A

Dispersive Readout of a SiMOS Quantum Dot Using a Flip-Chip Integrated Microwave Resonator

Authors: Vo Kim Hieu Van, Santiago Serrano, Cédric Bohémier, Ajit Dash, Fay E. Hudson, Tuomo Tanttu, Chih Hwan Yang, MengKe Feng, Ensar Vahapoglu, Florian K. Unseld, Wee Han Lim, Andrea Morello, Andrew S. Dzurak, Kok Wai Chan

Abstract: Heterogeneous integration provides a promising route to combine semiconductor quantum dot devices and superconducting microwave circuits, while allowing each component to be fabricated using an optimized process flow. Here, we demonstrate a flip-chip integrated platform for dispersive readout of silicon metal-oxide semiconductor (SiMOS) quantum dot devices. A SiMOS double quantum dot chip is bonde… ▽ More Heterogeneous integration provides a promising route to combine semiconductor quantum dot devices and superconducting microwave circuits, while allowing each component to be fabricated using an optimized process flow. Here, we demonstrate a flip-chip integrated platform for dispersive readout of silicon metal-oxide semiconductor (SiMOS) quantum dot devices. A SiMOS double quantum dot chip is bonded to a superconducting aluminum resonator chip using indium bump interconnects to enable microwave coupling to the quantum dot gate. We show that the developed flip-chip process is compatible with cryogenic operation of both the SiMOS device and the superconducting resonator, and demonstrate resonator-based detection of charge transitions in the quantum dot system. The readout signal-to-noise ratio follows a dependence of $\sqrt{t}$ with the integration time, reaching SNR = 1 at an integration time of approximately 0.3 ms. These results establish flip-chip bonding as a viable integration approach for SiMOS quantum dot devices operating at both dc and microwave frequencies, with potential applications for resonator-based techniques such as spin-photon coupling. △ Less

Submitted 25 August, 2026; v1 submitted 16 July, 2026; originally announced July 2026.

Comments: 6 pages, 3 figures

SiMOS quantum-dot spin qubits enabled by extreme-ultraviolet lithography

Authors: Thomas Van Caekenberghe, Paul Steinacker, Bart Raes, Sofie Beyne, Clement Godfrin, Jacques Van Damme, Sylvain Baudot, Arne Loenders, Gulzat Jaliel, Stefan Kubicek, Johan De Backer, Yannick Hermans, Sugandha Sharma, Shuchi Kaushik, Yuchao Jiang, Yosuke Shimura, Roger Loo, Vukan Levajac, Kristof Moors, George Simion, Florian K. Unseld, Ensar Vahapoglu, Ajit Dash, Tuomo Tanttu, Chris C. Escott , et al. (9 additional authors not shown)

Abstract: The realization of large-scale silicon quantum processors requires spin qubits compatible with advanced semiconductor manufacturing technologies, demanding lithographic processes that combine nanometer-scale precision with exceptional uniformity. Although the highest-performing silicon spin qubits demonstrated to date have relied on electron-beam (e-beam) lithography, its serial exposure process l… ▽ More The realization of large-scale silicon quantum processors requires spin qubits compatible with advanced semiconductor manufacturing technologies, demanding lithographic processes that combine nanometer-scale precision with exceptional uniformity. Although the highest-performing silicon spin qubits demonstrated to date have relied on electron-beam (e-beam) lithography, its serial exposure process limits reproducibility studies and wafer-scale fabrication. Here, we demonstrate high-performance silicon metal-oxide-semiconductor (SiMOS) spin qubits fabricated using extreme-ultraviolet (EUV) lithography in a 300 mm semiconductor pilot line. We report wafer-scale quantum-dot uniformity metrics, including 100 % room-temperature gate-to-gate leakage yield and sub-nanometer control of critical gate dimensions. We characterize four double-dot systems realized in two triple-quantum-dot devices. Gate set tomography (GST) reveals consistently high fidelities across all four systems, with values up to 99.8 % for SPAM, 99.9 % for single-qubit gates, and 99.1 % for two-qubit gates. The devices exhibit highly reproducible exchange turn-on characteristics of 10-13 dec/V, indicating high fabrication uniformity enabled by EUV patterning. These results establish EUV lithography as a viable manufacturing technology for quantum processors based on high-fidelity SiMOS spin qubits. △ Less

Submitted 14 July, 2026; originally announced July 2026.

Comments: 11 pages, 4 figures, 3 extended data figures

Generative modelling powered by room-temperature polariton condensates

Authors: Yuan Wang, Marcin Muszynski, Avinash Dash, Rishabh Kaurav, Vinod M. Menon, Oleksandr Kyriienko

Abstract: Generative modelling requires efficient stochastic nonlinear transformations and physical platforms that can naturally realise them. We experimentally demonstrate that nonlinear optical systems operating in the strong light-matter coupling regime can serve as physical transformation layers for conditional generative modelling. Specifically, we develop a workflow in which room-temperature exciton-p… ▽ More Generative modelling requires efficient stochastic nonlinear transformations and physical platforms that can naturally realise them. We experimentally demonstrate that nonlinear optical systems operating in the strong light-matter coupling regime can serve as physical transformation layers for conditional generative modelling. Specifically, we develop a workflow in which room-temperature exciton-polariton condensates formed in organic dye microcavities act as a physical stochastic transform within a generative adversarial network and enable conditional digit-to-image translation. By using the nonlinear many-body dynamics and intrinsic stochasticity of polariton condensates, the workflow outperforms baseline approaches based on digitally injected perturbations. We find that polariton-enabled sampling via generative adversarial network (Polariton GAN) yields improved inception score, digit preservation accuracy and structural similarity compared with both digital sampling and laser-based systems. We further show that spatially correlated output variations can naturally regularise adversarial training and enhance output diversity. Our results establish polariton condensation as a new computational resource for generative modelling, opening a pathway towards physics-enhanced machine learning systems. △ Less

Submitted 13 June, 2026; originally announced June 2026.

Comments: 9 pages and 4 figures in the main text; 17 pages SM; codes to be released

Gate Stack Engineering for High-Mobility and Low-Noise SiMOS Quantum Devices

Authors: Md. Mamunur Rahman, Ensar Vahapoglu, Kok Wai Chan, Tuomo Tanttu, Ajit Dash, Jonathan Yue Huang, Steve Yianni, Venkatesh Chenniappan, Jesús D. Cifuentes, Fay Hudson, Christopher C. Escott, Yik Kheng Lee, Nard Dumoulin Stuyck, Arne Laucht, Andrea Morello, Andre Saraiva, Jared H. Cole, Andrew S. Dzurak, Wee Han Lim

Abstract: We systematically investigate the interplay between materials engineering, quantum transport, and low-frequency charge noise in silicon metal--oxide--semiconductor (SiMOS) quantum devices. By combining Hall-bar transport measurements with charge-noise spectroscopy of gate-defined quantum dots, we identify correlations between gate-stack design, carrier mobility, and electrostatic noise, providing… ▽ More We systematically investigate the interplay between materials engineering, quantum transport, and low-frequency charge noise in silicon metal--oxide--semiconductor (SiMOS) quantum devices. By combining Hall-bar transport measurements with charge-noise spectroscopy of gate-defined quantum dots, we identify correlations between gate-stack design, carrier mobility, and electrostatic noise, providing an experimental case study of material and process dependencies relevant to low-noise, high-mobility operation. Hall-bar studies reveal that increasing the atomic-layer-deposition temperature of Al$_2$O$_3$ markedly enhances mobility, whereas the choice of oxidant has little impact. Devices incorporating HfO$_2$ exhibit improved carrier mobility, an interesting observation that can plausibly be attributed to defect passivation associated with aluminum diffusion from the gate metal into the HfO$_2$ layer. Charge-noise measurements show a strong correlation between higher mobility and reduced noise, with TiPd-gated devices displaying both degraded transport and elevated charge noise. In contrast, the poly-Si-gated CMOS-foundry device achieves the lowest noise levels. Finally, dual-feedback dot--sensor stability mapping demonstrates enhanced charge stability in devices with the gate stacks studied here, underscoring their promise for scalable, high-fidelity silicon spin-qubit platforms. △ Less

Submitted 5 April, 2026; v1 submitted 3 March, 2026; originally announced March 2026.

Comments: 26 pages, 7 figures

Gate-tuneable single-photon emitters in WSe2 monolayer created via AFM nanoindentation on rigid SiO2/Si substrates

Authors: Ajit Kumar Dash, Sanket Jugade, Manavendra Pratap Singh, Hardeep, Tilly Guyot, Cora Crunteanu-Stanescu, Indrajeet Dhananjay Prasad, Yunus Waheed, Sumitra Shit, Sébastien Roux, Santosh Kumar, Cedric Robert, Xavier Marie, Akshay Naik, Akshay Singh

Abstract: Single-photon emitters (SPEs) hosted by two-dimensional (2D) semiconducting materials are envisioned for next-generation quantum applications. However, SPE creation in 2D semiconductors on rigid substrates like SiO2/Si via nanoindentation is a technological gap, critical for interfacing SPEs with photonic circuits and cavities. Here, we report a protocol for deterministically creating SPEs in mono… ▽ More Single-photon emitters (SPEs) hosted by two-dimensional (2D) semiconducting materials are envisioned for next-generation quantum applications. However, SPE creation in 2D semiconductors on rigid substrates like SiO2/Si via nanoindentation is a technological gap, critical for interfacing SPEs with photonic circuits and cavities. Here, we report a protocol for deterministically creating SPEs in monolayer WSe2 on SiO2/Si substrates using a sharp diamond AFM (atomic force microscope) tip. A displacement-controlled indentation process is developed, allowing indent depths > 150 nm necessary for creating SPEs. Sharp defect peaks (~200 μeV) are observed in cryogenic (4K) photoluminescence (PL) spectrum at nanoindented sites and are stable upto ~ 120K. 76% of sites exhibit sharp defect-bound peaks confirmed by power-dependent, temperature-dependent, and time-resolved PL (TRPL). AFM and PL mapping link these peaks to indent periphery. The peaks show sub-linewidth spectral jitter, no blinking, and single-photon nature in second-order autocorrelation measurements. SPEs can be switched on/off, and background emissions suppressed using electrical gating. Gate-voltage dependent TRPL indicate that SPE dynamics can be tuned, depending on nature of SPE, pointing the way to higher-purity SPEs. Our work is directly applicable to other 2D materials and photonic circuit/cavity compatible rigid substrates and is a significant step for scalable SPE technologies. △ Less

Submitted 30 January, 2026; originally announced January 2026.

Comments: Main text and supplementary files included, 41 pages, 15 figures, 4 tables

In-Substrate Imaging of Diamond hBN FET Current via Widefield Quantum Diamond Microscopy

Authors: Anuj Bathla, Subrat Kumar Pradhan, Ajit Kumar Dash, Prabhat Anand, M. Girish Chandra, Kenji Watanabe, Takashi Taniguchi, Akshay Singh, Veeresh Deshpande, Kasturi Saha

Abstract: We demonstrate widefield magnetic imaging of current flow in hydrogen terminated diamond field effect transistors (FETs) through in-substrate nitrogen vacancy (NV) centers. Hydrogen termination of the diamond surface induces a two dimensional hole gas (2DHG), while an ensemble of near surface NV centers located $ \sim 1~μm$ below the surface enables noninvasive magnetic imaging of current flow wit… ▽ More We demonstrate widefield magnetic imaging of current flow in hydrogen terminated diamond field effect transistors (FETs) through in-substrate nitrogen vacancy (NV) centers. Hydrogen termination of the diamond surface induces a two dimensional hole gas (2DHG), while an ensemble of near surface NV centers located $ \sim 1~μm$ below the surface enables noninvasive magnetic imaging of current flow with micrometer scale spatial resolution. The FETs were electrically characterized over a range of drain source biases $V_{ds}= 0$ to $-15V$ and gate voltages,$V_{gs}= +3$ to $-9V$ followed by in situ widefield NV magnetometry during device operation. Magnetic field maps and reconstructed current density distributions directly visualize current injection at the source drain contacts and transport beneath the hBN gated channel. Magnetic field maps reveal current density variations in the channel region owing to non-uniformities or defects in the gate dielectric. In addition, we observe a pronounced enhancement of the drain current ($\sim 600-900 μA$) and a shift in the apparent threshold voltage during laser illumination, reflecting photo induced changes in channel electrostatics. By correlating gate dependent magnetic images with simultaneous electrical measurements, we directly link spatial current distributions to FET transfer characteristics, providing new insight into buried interface transport and non-uniform gating effects in the transistor channel. As the methodology is compatible with top gated FETs, it can be used to map channel current distributions with micrometer resolution in emerging channel materials, such as 2D materials and wide bandgap channels, and establish widefield NV magnetometry as a powerful platform for probing charge transport in transistors and Van der Waals dielectric heterostructures. △ Less

Submitted 21 January, 2026; originally announced January 2026.

Interplay of Zeeman Splitting and Tunnel Coupling in Coherent Spin Qubit Shuttling

Authors: Ssu-Chih Lin, Paul Steinacker, MengKe Feng, Ajit Dash, Santiago Serrano, Wee Han Lim, Kohei M. Itoh, Fay E. Hudson, Tuomo Tanttu, Andre Saraiva, Arne Laucht, Andrew S. Dzurak, Hsi-Sheng Goan, Chih Hwan Yang

Abstract: Spin shuttling offers a promising approach for developing scalable silicon-based quantum processors by addressing the connectivity limitations of quantum dots. In this work, we demonstrate high-fidelity bucket-brigade spin shuttling in a silicon MOS device, utilizing Pauli-spin-blockade readout. We achieve an average shuttling fidelity of \SI{99.8}{\percent}. The residual shuttling error is highly… ▽ More Spin shuttling offers a promising approach for developing scalable silicon-based quantum processors by addressing the connectivity limitations of quantum dots. In this work, we demonstrate high-fidelity bucket-brigade spin shuttling in a silicon MOS device, utilizing Pauli-spin-blockade readout. We achieve an average shuttling fidelity of \SI{99.8}{\percent}. The residual shuttling error is highly sensitive to the ratio between interdot tunnel coupling and Zeeman splitting, with tuning of these parameters enabling up to a 20-fold variation in error rate. An appropriate four-level Hamiltonian model supports our findings. These results provide valuable insights for optimizing high-performance spin-shuttling systems in future quantum architectures. △ Less

Submitted 6 March, 2026; v1 submitted 21 July, 2025; originally announced July 2025.

Journal ref: Phys. Rev. Applied 25, 034016 (2026)

Scalable quantum current source on commercial CMOS process technology

Authors: Ajit Dash, Suyash Pati Tripathi, Dimitrios Georgakopoulos, MengKe Feng, Steve Yianni, Ensar Vahapoglu, Md Mamunur Rahman, Shai Bonen, Owen Brace, Jonathan Y. Huang, Wee Han Lim, Kok Wai Chan, Will Gilbert, Arne Laucht, Andrea Morello, Andre Saraiva, Christopher C. Escott, Sorin P. Voinigescu, Andrew S. Dzurak, Tuomo Tanttu

Abstract: Many quantum technologies require a precise electrical current standard that can only be achieved with expensive cryogenics, or through the secondary standards, such as resistance or voltage. Silicon-based charge pumps could provide such a standard in an inherently scalable way, through their compatibility with complementary metal-oxide-semiconductor (CMOS) fabrication methods. However, coherent q… ▽ More Many quantum technologies require a precise electrical current standard that can only be achieved with expensive cryogenics, or through the secondary standards, such as resistance or voltage. Silicon-based charge pumps could provide such a standard in an inherently scalable way, through their compatibility with complementary metal-oxide-semiconductor (CMOS) fabrication methods. However, coherent quantized charge transfer has so far been demonstrated only in nanoscale devices that are custom-fabricated in academic cleanrooms or research technology foundries. Here, we show that a CMOS device manufactured with commercial 22-nm process node can be used to define a quantum current standard in the International System of Units (SI). We measure an accuracy of (1.2 +/- 0.1)E-3 A/A at 50 MHz with reference to SI voltage and resistance standards in a pumped helium system. We then propose a practical monolithic CMOS chip that incorporates one million parallel connected charge pumps along with on-chip control electronics. This chip could be operated as a table-top primary standard that can be easily integrated with CMOS electronics, generating quantum currents of up to microampere levels. △ Less

Submitted 7 July, 2025; v1 submitted 18 June, 2025; originally announced June 2025.

Comments: 16 pages, 4 figures, 3 extended data figures, 7 extended data tables

Ion-Exchange Doping of Semiconducting Single-Walled Carbon Nanotubes

Authors: Angus Hawkey, Aditya Dash, Xabier Rodríguez-Martínez, Zhiyong Zhao, Anna Champ, Sebastian Lindenthal, Michael Zharnikov, Martijn Kemerink, Jana Zaumseil

Abstract: Semiconducting single-walled carbon nanotubes (SWCNTs) are a promising thermoelectric material with high power factors after chemical p- or n-doping. Understanding the impact of dopant counterions on charge transport and thermoelectric properties of nanotube networks is essential to further optimize doping methods and to develop better dopants. Here, we utilize ion-exchange doping to systematicall… ▽ More Semiconducting single-walled carbon nanotubes (SWCNTs) are a promising thermoelectric material with high power factors after chemical p- or n-doping. Understanding the impact of dopant counterions on charge transport and thermoelectric properties of nanotube networks is essential to further optimize doping methods and to develop better dopants. Here, we utilize ion-exchange doping to systematically vary the size of counterions in thin films of small and large diameter, polymer-sorted semiconducting SWCNTs with AuCl3 as the initial p-dopant and investigate the impact of ion size on conductivity, Seebeck coefficients and power factors. Larger anions are found to correlate with higher electrical conductivities and improved doping stability, while no significant effect on the power factors is found. Importantly, the effect of counterion size on the thermoelectric properties of dense SWCNT networks is not obscured by morphological changes upon doping. The observed trends of carrier mobilities and Seebeck coefficients can be explained by a random resistor model for the nanotube network that accounts for overlapping Coulomb potentials leading to the formation of an impurity band whose depth depends on the carrier density and counterion size. These insights can be applied more broadly to understand the thermoelectric properties of doped percolating disordered systems, including semiconducting polymers. △ Less

Submitted 20 March, 2025; originally announced March 2025.

Journal ref: Adv. Mater. 2024, 36, 2404554

Quantum light generation with ultra-high spatial resolution in 2D semiconductors via ultra-low energy electron irradiation

Authors: Ajit Kumar Dash, Sharad Kumar Yadav, Sebastien Roux, Manavendra Pratap Singh, Kenji Watanabe, Takashi Taniguchi, Akshay Naik, Cedric Robert, Xavier Marie, Akshay Singh

Abstract: Single photon emitters (SPEs) are building blocks of quantum technologies. Defect engineering of 2D materials is ideal to fabricate SPEs, wherein spatially deterministic and quality-preserving fabrication methods are critical for integration into quantum devices and cavities. Existing methods use combination of strain and electron irradiation, or ion irradiation, which make fabrication complex, an… ▽ More Single photon emitters (SPEs) are building blocks of quantum technologies. Defect engineering of 2D materials is ideal to fabricate SPEs, wherein spatially deterministic and quality-preserving fabrication methods are critical for integration into quantum devices and cavities. Existing methods use combination of strain and electron irradiation, or ion irradiation, which make fabrication complex, and limited by surrounding lattice damage. Here, we utilise only ultra-low energy electron beam irradiation (5 keV) to create dilute defect density in hBN-encapsulated monolayer MoS2, with ultra-high spatial resolution (< 50 nm, extendable to 10 nm). Cryogenic photoluminescence spectra exhibit sharp defect peaks, following power-law for finite density of single defects, and characteristic Zeeman splitting for MoS2 defect complexes. The sharp peaks have low spectral jitter (< 200 μeV), and are tuneable with gate-voltage and electron beam energy. Use of low-momentum electron irradiation, ease of processing, and high spatial resolution, will disrupt deterministic creation of high-quality SPEs. △ Less

Submitted 16 September, 2024; originally announced September 2024.

Comments: 31 pages, 16 figures, 1 table. Supplementary Material included

Journal ref: Adv. Funct. Mater. 2025, 2421684

Towards a comprehensive understanding of the low energy luminescence peak in 2D materials

Authors: Keerthana S Kumar, Ajit Kumar Dash, Hasna Sabreen H, Manvi Verma, Vivek Kumar, Kenji Watanabe, Takashi Taniguchi, Gopalakrishnan Sai Gautam, Akshay Singh

Abstract: An intense low-energy broad luminescence peak (L-peak) is usually observed in 2D transition metal dichalcogenides (TMDs) at low temperatures. L-peak has earlier been attributed to bound excitons, but its origins are widely debated with direct consequences on optoelectronic properties. To decouple the contributions of physisorbed and chemisorbed oxygen, organic adsorbates, and strain on L-peak, we… ▽ More An intense low-energy broad luminescence peak (L-peak) is usually observed in 2D transition metal dichalcogenides (TMDs) at low temperatures. L-peak has earlier been attributed to bound excitons, but its origins are widely debated with direct consequences on optoelectronic properties. To decouple the contributions of physisorbed and chemisorbed oxygen, organic adsorbates, and strain on L-peak, we measured a series of monolayer (ML) MoS2 samples (mechanically exfoliated (ME), synthesized by oxygen-assisted chemical vapour deposition (O-CVD), hexagonal boron nitride (hBN) covered and hBN encapsulated). Emergence of L-peak below 150 K and saturation of photoluminescence (PL) intensity with laser power confirm bound nature of L-peak. Anomalously at room temperature, O-CVD samples show high A-exciton PL (c.f. ME), but reduced PL at low temperatures, which is attributed to strain-induced direct-to-indirect bandgap change in low defect O-CVD MoS2. Further, L-peak redshifts dramatically ~ 130 meV for O-CVD samples (c.f. ME). These observations are fully consistent with our predictions from density functional theory (DFT) calculations, considering effects of both strain and defects, and supported by Raman spectroscopy. In ME samples, charged oxygen adatoms are identified as thermodynamically favourable defects which can create in-gap states, and contribute to the L-peak. The useful effect of hBN is found to originate from reduction of charged oxygen adatoms and hydrocarbon complexes. This combined experimental-theoretical study allows an enriched understanding of L-peak and beneficial impact of hBN, and motivates collective studies of strain and defects with direct impact on optoelectronics and quantum technologies. △ Less

Submitted 22 December, 2023; originally announced December 2023.

Comments: 49 pages,21 figures

Silicon charge pump operation limit above and below liquid helium temperature

Authors: Ajit Dash, Steve Yianni, MengKe Feng, Fay Hudson, Andre Saraiva, Andrew S. Dzurak, Tuomo Tanttu

Abstract: Semiconductor tunable barrier single-electron pumps can produce output current of hundreds of picoamperes at sub ppm precision, approaching the metrological requirement for the direct implementation of the current standard. Here, we operate a silicon metal-oxide-semiconductor electron pump up to a temperature of 14 K to understand the temperature effect on charge pumping accuracy. The uncertainty… ▽ More Semiconductor tunable barrier single-electron pumps can produce output current of hundreds of picoamperes at sub ppm precision, approaching the metrological requirement for the direct implementation of the current standard. Here, we operate a silicon metal-oxide-semiconductor electron pump up to a temperature of 14 K to understand the temperature effect on charge pumping accuracy. The uncertainty of the charge pump is tunnel limited below liquid helium temperature, implying lowering the temperature further does not greatly suppress errors. Hence, highly accurate charge pumps could be confidently achieved in a $^4$He cryogenic system, further promoting utilization of the revised quantum current standard across the national measurement institutes and industries worldwide. △ Less

Submitted 11 September, 2023; originally announced September 2023.

Switching global correlations on and off in a many-body quantum state by tuning local entanglement

Authors: Colin Benjamin, Aditya Dash

Abstract: A quantum many-body state built on a classical 1D Ising model with locally entangled qubits is considered. This setup can model an infinite-player quantum Prisoner's dilemma game with each site representing two entangled players (or qubits). The local entanglement $γ$ between two qubits placed on a site in the 1D Ising model and classical coupling between adjacent sites of the Ising model has an a… ▽ More A quantum many-body state built on a classical 1D Ising model with locally entangled qubits is considered. This setup can model an infinite-player quantum Prisoner's dilemma game with each site representing two entangled players (or qubits). The local entanglement $γ$ between two qubits placed on a site in the 1D Ising model and classical coupling between adjacent sites of the Ising model has an apposite influence on qubits. It points to a counter-intuitive situation wherein local entanglement at a site can exactly cancel global correlations, signaling an artificial quantum many-body state wherein, by locally tuning the entanglement at a particular site, one can transition from a strongly correlated quantum state to an uncorrelated quantum state and then to a correlated classical state. In other words, we can simulate a state similar to a Type II superconducting state via local tuning of entanglement in a 1D Ising chain with entangled qubits. △ Less

Submitted 11 September, 2023; originally announced September 2023.

Comments: 8 pages, 4 figures

Journal ref: Chaos 33, 091104 (2023)

Controlled defect production in monolayer MoS2 via electron irradiation at ultralow accelerating voltages

Authors: Ajit Kumar Dash, Hariharan Swaminathan, Ethan Berger, Mainak Mondal, Touko Lehenkari, Pushp Raj Prasad, Kenji Watanabe, Takashi Taniguchi, Hannu-Pekka Komsa, Akshay Singh

Abstract: Control on spatial location and density of defects in 2D materials can be achieved using electron beam irradiation. Conversely, ultralow accelerating voltages (less than or equal to 5kV) are used to measure surface morphology, with no expected defect creation. We find clear signatures of defect creation in monolayer (ML) MoS2 at these voltages. Evolution of E' and A1' Raman modes with electron dos… ▽ More Control on spatial location and density of defects in 2D materials can be achieved using electron beam irradiation. Conversely, ultralow accelerating voltages (less than or equal to 5kV) are used to measure surface morphology, with no expected defect creation. We find clear signatures of defect creation in monolayer (ML) MoS2 at these voltages. Evolution of E' and A1' Raman modes with electron dose, and appearance of defect activated peaks indicate defect formation. To simulate Raman spectra of MoS2 at realistic defect distributions, while retaining density-functional theory accuracy, we combine machine-learning force fields for phonons and eigenmode projection approach for Raman tensors. Simulated spectra agree with experiments, with sulphur vacancies as suggested defects. We decouple defects, doping and carbonaceous contamination using control (hBN covered and encapsulated MoS2) samples. We observe cryogenic PL quenching and defect peaks, and find that carbonaceous contamination does not affect defect creation. These studies have applications in photonics and quantum emitters. △ Less

Submitted 28 March, 2023; v1 submitted 10 October, 2022; originally announced October 2022.

Comments: 49 pages, 24 figures, 4 tables

Nanomechanical Resonators: Toward Atomic Scale

Authors: Bo Xu, Pengcheng Zhang, Jiankai Zhu, Zuheng Liu, Alexander Eichler, Xu-Qian Zheng, Jaesung Lee, Aneesh Dash, Swapnil More, Song Wu, Yanan Wang, Hao Jia, Akshay Naik, Adrian Bachtold, Rui Yang, Philip X. -L. Feng, Zenghui Wang

Abstract: The quest for realizing and manipulating ever smaller man-made movable structures and dynamical machines has spurred tremendous endeavors, led to important discoveries, and inspired researchers to venture to new grounds. Scientific feats and technological milestones of miniaturization of mechanical structures have been widely accomplished by advances in machining and sculpturing ever shrinking fea… ▽ More The quest for realizing and manipulating ever smaller man-made movable structures and dynamical machines has spurred tremendous endeavors, led to important discoveries, and inspired researchers to venture to new grounds. Scientific feats and technological milestones of miniaturization of mechanical structures have been widely accomplished by advances in machining and sculpturing ever shrinking features out of bulk materials such as silicon. With the flourishing multidisciplinary field of low-dimensional nanomaterials, including one-dimensional (1D) nanowires/nanotubes, and two-dimensional (2D) atomic layers such as graphene/phosphorene, growing interests and sustained efforts have been devoted to creating mechanical devices toward the ultimate limit of miniaturization--genuinely down to the molecular or even atomic scale. These ultrasmall movable structures, particularly nanomechanical resonators that exploit the vibratory motion in these 1D and 2D nano-to-atomic-scale structures, offer exceptional device-level attributes, such as ultralow mass, ultrawide frequency tuning range, broad dynamic range, and ultralow power consumption, thus holding strong promises for both fundamental studies and engineering applications. In this Review, we offer a comprehensive overview and summary of this vibrant field, present the state-of-the-art devices and evaluate their specifications and performance, outline important achievements, and postulate future directions for studying these miniscule yet intriguing molecular-scale machines. △ Less

Submitted 20 November, 2022; v1 submitted 15 August, 2022; originally announced August 2022.

Comments: 41 pages, 30 figures

Journal ref: ACS Nano 2022

Body Diagonal Diffusion Couple Method for Estimation of Tracer Diffusion Coefficients in a Multi-Principal Element Alloy

Authors: Anuj Dash, Aloke Paul

Abstract: The estimation of (n-1)2 interdiffusion coefficients in an n component system requires (n-1) diffusion paths to intersect or pass closely in the (n-1) dimensional space according to the body diagonal diffusion couple method. These interdiffusion coefficients are related to n(n-1) intrinsic (or n tracer diffusion coefficients), which cannot be estimated easily following the Kirkendall marker experi… ▽ More The estimation of (n-1)2 interdiffusion coefficients in an n component system requires (n-1) diffusion paths to intersect or pass closely in the (n-1) dimensional space according to the body diagonal diffusion couple method. These interdiffusion coefficients are related to n(n-1) intrinsic (or n tracer diffusion coefficients), which cannot be estimated easily following the Kirkendall marker experiment in a multicomponent system despite their importance for understanding the atomic mechanism of diffusion and the physico-mechanical properties of materials. In this study, the estimation of tracer diffusion coefficients from only two diffusion profiles following the concept of the body diagonal diffusion couple method in a multicomponent system is demonstrated. Subsequently, one can estimate the intrinsic and interdiffusion coefficients. This reduces the overall effort up to a great extent since it needs only two instead of (n-1) diffusion profiles irrespective of the number of components, with an additional benefit of enabling the estimation of all types of diffusion coefficients. The available tracer diffusion coefficients estimated following the radiotracer method are compared to the data estimated in this study following this method. This method can also be extended to the systems in which the radiotracer method is not feasible. △ Less

Submitted 13 May, 2022; originally announced May 2022.

Comments: 3 figures and 9 tables

Optical microscope based universal parameter for identifying layer number in two-dimensional materials

Authors: Mainak Mondal, Ajit Kumar Dash, Akshay Singh

Abstract: Optical contrast is the most common preliminary method to identify layer number of two-dimensional (2D) materials, but is seldom used as a confirmatory technique. We explain the reason for variation of optical contrast between imaging systems. We introduce a universal method to quantify the layer number using the RGB (red-green-blue) and RAW optical images. For RGB images, the slope of 2D flake (M… ▽ More Optical contrast is the most common preliminary method to identify layer number of two-dimensional (2D) materials, but is seldom used as a confirmatory technique. We explain the reason for variation of optical contrast between imaging systems. We introduce a universal method to quantify the layer number using the RGB (red-green-blue) and RAW optical images. For RGB images, the slope of 2D flake (MoS2, WSe2, graphene) intensity vs. substrate intensity is extracted from optical images with varying lamp power. The intensity slope identifies layer number and is system independent. For RAW images, intensity slopes and intensity ratios are completely system and intensity independent. Intensity slope (for RGB) and intensity ratio (for RAW) are thus universal parameters for identifying layer number. A Fresnel-reflectance-based optical model provides an excellent match with experiments. Further, we have created a MATLAB-based graphical user interface that can identify layer number rapidly. This technique is expected to accelerate the preparation of heterostructures, and fulfil a prolonged need for universal optical contrast method. △ Less

Submitted 27 April, 2022; originally announced April 2022.

Device geometry dependent deterministic skyrmion generation from a skyrmionium

Authors: Adyashakti Dash, Brindaban Ojha, Shaktiranjan Mohanty, Ashish Kumar Moharana, Subhankar Bedanta

Abstract: A magnetic skyrmionium can be perceived as an association of two magnetic skyrmions with opposite topological charges. In this work, we have investigated the transformation of skyrmionium into multi-skyrmionic states via domain wall (DW) pairs in three different devices with variable geometric configurations. The same device geometries were considered for single ferromagnetic layer as well as synt… ▽ More A magnetic skyrmionium can be perceived as an association of two magnetic skyrmions with opposite topological charges. In this work, we have investigated the transformation of skyrmionium into multi-skyrmionic states via domain wall (DW) pairs in three different devices with variable geometric configurations. The same device geometries were considered for single ferromagnetic layer as well as synthetic antiferromagnetic (SAF) system. It is observed that by tuning the current density, deterministic generation of skyrmions is possible via the spin transfer torque (STT). The proposed device is efficiently adjustable to change the number of skyrmions . The results may lead to development of skyrmion-based devices for neuromorphic and unconventional computing. △ Less

Submitted 10 September, 2021; originally announced September 2021.

Effect of electron-irradiation on layered quantum materials

Authors: Ajit Kumar Dash, Mainak Mondal, Manvi Verma, Keerthana S Kumar, Akshay Singh

Abstract: Technological advancement towards the quantum era requires secure communication, quantum computation, and ultra-sensitive sensing capabilities. Layered quantum materials (LQMs) have remarkable optoelectronic and quantum properties that can usher us into the quantum era. Electron microscopy is the tool of choice for measuring these LQMs at an atomic and nanometer scale. On the other hand, electron-… ▽ More Technological advancement towards the quantum era requires secure communication, quantum computation, and ultra-sensitive sensing capabilities. Layered quantum materials (LQMs) have remarkable optoelectronic and quantum properties that can usher us into the quantum era. Electron microscopy is the tool of choice for measuring these LQMs at an atomic and nanometer scale. On the other hand, electron-irradiation of LQMs can modify various material properties, including the creation of structural defects. We review different types of structural defects, as well as electron elastic- and inelastic-scattering induced processes. Controlled modification of optoelectronic and quantum properties of LQMs using electron-irradiation, including creation of single photon emitters is discussed. Protection of electron-irradiation induced damage of LQMs via encapsulation by other layered materials is encouraged. We finally give insights into challenges and opportunities, including creating novel structures using an electron beam. △ Less

Submitted 9 March, 2021; originally announced March 2021.

Comments: 16 pages, 4 figures

Fabrication Of Economical Signal Amplified Vibrating Sample Magnetometer With Spiral Designed Detector

Authors: Aditya Dash, Rabindra Sarkar, Abel Mathew, Prakash Nath Vishwakarma

Abstract: The design and fabrication of a cost-effective vibrating sample magnetometer are explained. Improvement in the detected signal can be obtained using an operational amplifier circuit. The fabricated spiral detection coil design enhances the induced voltage obtained as shielding flux is reduced. A homemade setup is obtained by taking a woofer as an actuator and plexiglass as the vibrating medium. Lo… ▽ More The design and fabrication of a cost-effective vibrating sample magnetometer are explained. Improvement in the detected signal can be obtained using an operational amplifier circuit. The fabricated spiral detection coil design enhances the induced voltage obtained as shielding flux is reduced. A homemade setup is obtained by taking a woofer as an actuator and plexiglass as the vibrating medium. Lock-in amplifier analyzed the booster enhanced induced voltage signal by the principle of phase detection. Study of amplified and non amplified signals from the Nickel (99% purity) sample was done. Conversion of induced voltage to magnetization was done by calibration incorporating the coercivity and retentivity measurement. Magnetic oxide can be analyzed using this cost-effective designed Vibrating sample magnetometer. The data obtained from the VSM can conclude the successful operation of the designed magnetometer. △ Less

Submitted 14 November, 2020; originally announced November 2020.

A novel experimental method of estimating tracer and intrinsic diffusion coefficients from multicomponent diffusion profiles

Authors: Neelamegan Esakkiraja, Anuj Dash, Avik Mondal, K. C. Hari Kumar, Aloke Paul

Abstract: A few decades earlier, Kirkaldy and Lane proposed an indirect method of estimating the tracer and intrinsic diffusion coefficients in a ternary system (without showing experimental verification), which is otherwise impossible following the Kirkendall marker experiments. Subsequently, Manning proposed the relations between the tracer and intrinsic diffusion coefficients in the multicomponent system… ▽ More A few decades earlier, Kirkaldy and Lane proposed an indirect method of estimating the tracer and intrinsic diffusion coefficients in a ternary system (without showing experimental verification), which is otherwise impossible following the Kirkendall marker experiments. Subsequently, Manning proposed the relations between the tracer and intrinsic diffusion coefficients in the multicomponent system by extending the Onsager formalism (although could not be estimated by intersecting the diffusion couples). By solving these issues in this article, we have now proposed the equations and method for estimating these parameters in pseudo-ternary diffusion couples in which diffusion paths can be intersected in multicomponent space. We have chosen NiCoFeCr system for verification of this method because of the availability of good quality diffusion couple experiments and estimated tracer diffusion coefficients of all the components measured by the radiotracer method. An excellent match is found when the tracer diffusion coefficients estimated following the newly proposed method are compared with the data estimated following the radiotracer method. Following, the intrinsic diffusion coefficients are estimated experimentally in a multicomponent system for the first time highlighting diffusional interactions between the components. We have further shown that the intrinsic diffusion coefficients are the same (if the vacancy wind effect is negligible/neglected) when estimated from other types of diffusion couples (pseudo-binary and body diagonal) in the same multi-component system. This method can be now extended to the Al, Ga, Si containing systems in which the estimation of tracer diffusion coefficients following the radiotracer method is difficult/impossible because of various reasons. △ Less

Submitted 10 September, 2020; originally announced September 2020.

In-situ formation of 2D-TiCx in Cu-Ti2AlC composites: an interface reaction study

Authors: K. Dash, A. Dash

Abstract: In this paper, we present a concept to fabricate copper based Ti2AlC MAX phase composite focusing on the processing method and the reaction which takes place at the matrix-reinforcement interface to yield 2D TiCx. Copper was reinforced with Ti2AlC (agglomerate size ~40 micron) phase and sintered in vacuum by pressure-less sintering. The interface of consolidated samples was investigated using tran… ▽ More In this paper, we present a concept to fabricate copper based Ti2AlC MAX phase composite focusing on the processing method and the reaction which takes place at the matrix-reinforcement interface to yield 2D TiCx. Copper was reinforced with Ti2AlC (agglomerate size ~40 micron) phase and sintered in vacuum by pressure-less sintering. The interface of consolidated samples was investigated using transmission electron microscopy (TEM) to reveal the microstructural details. In the due course of consolidation of Cu-Ti2AlC; the formation of 2D TiCx from the reaction between Cu and Ti2AlC by forming solid solution between Cu and Al was facilitated. The reaction between Cu and Ti2AlC has been elaborated and analyzed in the light of corroborated results. Wavelength dispersive spectroscopy (WDS) throws light on the elemental distribution at the site of interfacial reaction. This investigation elaborates a proof of concept to process an in-situ 2D TiCx reinforced Cu metal matrix composite (MMC). △ Less

Submitted 14 July, 2020; originally announced July 2020.

Solving the issues of multicomponent diffusion in an equiatomic NiCoFeCr medium entropy alloy

Authors: Anuj Dash, Neelamegan Esakkiraja, Aloke Paul

Abstract: Estimating the diffusion coefficients experimentally in a four-component inhomogeneous alloy following the conventional diffusion couple method by intersecting three couples at the same composition is difficult unless a small composition range of constant diffusivity is identified. Additionally, the intrinsic diffusion coefficients of the components cannot be estimated in a system with more than t… ▽ More Estimating the diffusion coefficients experimentally in a four-component inhomogeneous alloy following the conventional diffusion couple method by intersecting three couples at the same composition is difficult unless a small composition range of constant diffusivity is identified. Additionally, the intrinsic diffusion coefficients of the components cannot be estimated in a system with more than two components. To solve these issues, we have followed the pseudo-binary and pseudo-ternary diffusion couple methods for estimating the diffusion coefficients at the equiatomic composition of NiCoFeCr medium entropy alloy. Along with the pseudo-binary interdiffusion coefficients, we have estimated the intrinsic diffusion coefficients of all the components by designing the pseudo-binary couples such that Ni and Co develop the diffusion profiles keeping Fe and Cr constant in one couple and Fe and Cr develop the diffusion profiles keeping Ni and Co constant in another couple. Subsequently, we have proposed the relations for calculating the tracer diffusion coefficients utilizing the thermodynamic details. We have found a good match with the data estimated directly following the radiotracer method at the equiatomic composition. Following, we have produced three pseudo-ternary diffusion couples intersecting at the compositions close to the equiatomic composition. The main pseudo-ternary interdiffusion coefficients of Fe are found to be higher than Ni and Co. Therefore, we have estimated different types of diffusion coefficients highlighting the complex diffusion process in the four-component NiCoFeCr medium entropy alloy. △ Less

Submitted 17 May, 2020; originally announced May 2020.

Journal ref: Acta Materialia 193 (2020) 163-171

Thermodynamic susceptibility as a measure of cooperative behavior in social dilemmas

Authors: Colin Benjamin, Aditya Dash

Abstract: The emergence of cooperation in the thermodynamic limit of social dilemmas is an emerging field of research. While numerical approaches (using replicator dynamics) are dime a dozen, analytical approaches are rare. A particularly useful analytical approach is to utilize a mapping between the spin-1/2 Ising model in 1-D and the social dilemma game and calculate the magnetization, which is the net di… ▽ More The emergence of cooperation in the thermodynamic limit of social dilemmas is an emerging field of research. While numerical approaches (using replicator dynamics) are dime a dozen, analytical approaches are rare. A particularly useful analytical approach is to utilize a mapping between the spin-1/2 Ising model in 1-D and the social dilemma game and calculate the magnetization, which is the net difference between the fraction of cooperators and defectors in a social dilemma. In this paper, we look at the susceptibility, which probes the net change in the fraction of players adopting a certain strategy, for both classical and quantum social dilemmas. The reason being, in statistical mechanics problems, the thermodynamic susceptibility as compared to magnetization is a more sensitive probe for microscopic behavior, e.g., observing small changes in a population adopting a certain strategy. In this paper, we find the thermodynamic susceptibility for reward, sucker's payoff and temptation in classical Prisoner's Dilemma to be positive, implying that the turnover from defect to cooperate is greater than vice-versa, although the Nash Equilibrium for the two-player game is to defect. In classical Hawk-Dove game, the thermodynamic susceptibility for resource suggests that the number of players switching to Hawk from Dove strategy is dominant. Entanglement in Quantum Prisoner's Dilemma (QPD) has a non-trivial role in determining the behavior of thermodynamic susceptibility. At maximal entanglement, we find that sucker's payoff and temptation increase the number of players switching to defect. In the zero-temperature limit, we find that there are two second-order phase transitions in the game, marked by a divergence in the susceptibility. This behavior is similar to that seen in Type-II superconductors wherein also two second-order phase transitions are seen. △ Less

Submitted 11 August, 2020; v1 submitted 9 October, 2019; originally announced October 2019.

Comments: 16 pages, 14 figures, accepted for publication in Chaos

Journal ref: Chaos 30, 093117 (2020)

alpha - HgS Nanocrystals: Synthesis, Structure and Optical Properties

Authors: A. K. Mahapatra, A. K. Dash

Abstract: Well-separated mercury sulfide (HgS) nanocrystals are synthesized by a wet chemical route. Transmission electron microscopy studies show that nanocrystals are nearly spherical in shape with average size of 9 nm. Grazing angle X-ray diffraction confirms that HgS nanocrystals are in cinnabar phase. Particle induced X-ray emission and Rutherford back scattering spectrometry analysis reveal HgS nanocr… ▽ More Well-separated mercury sulfide (HgS) nanocrystals are synthesized by a wet chemical route. Transmission electron microscopy studies show that nanocrystals are nearly spherical in shape with average size of 9 nm. Grazing angle X-ray diffraction confirms that HgS nanocrystals are in cinnabar phase. Particle induced X-ray emission and Rutherford back scattering spectrometry analysis reveal HgS nanocrystals are stoichiometric and free from foreign impurities. The optical absorption measurements show two excitonic peaks corresponding to electron-heavy hole and electron-light hole transitions, which are blue shifted by 0.1 and 0.2 eV, respectively, from its bulk value, due to quantum size effect. The experimental data obtained by optical absorption measurement is simulated with a theoretical model considering the particle size distribution as Gaussian. △ Less

Submitted 14 November, 2011; v1 submitted 13 May, 2006; originally announced May 2006.

Journal ref: Physica E 35 (2006) 9 - 15