arXiv subjects

Viet-Hung Nguyen

Publications and source records attributed to Viet-Hung Nguyen.

At least 19 recordsLinked to original sources

arXiv2026

Fine-Tuned Machine-Learned Interatomic Potentials for Structural and Vibrational Properties of Twisted 2D Materials

Twisted van der Waals bilayers form moiré superlattices whose structural and vibrational properties are highly sensitive to variations in local stacking registry and the degree of atomic reconstruction, yet accurate atomistic modeling of these systems at the DFT level remains computationally prohibitive at small twist angles. We investigate machine-learned interatomic potentials for moiré systems, using twisted bilayer graphene, \textit{h}-BN, and MoS$_2$ as representative materials spanning a broad spectrum of mechanical compliance and atomic reconstruction behavior. We show that fine-tuning universal atomistic foundation models is essential to achieve DFT accuracy for layered materials, as broadly trained foundation models prove insufficient for resolving the subtle interlayer energetics that govern atomic reconstruction. Through local strain tensor analysis and the phonon band unfolding technique, our fine-tuned MACE model reveals a consistent reconstruction-induced strain landscape in all three materials, with extended low-energy stacking domains separated by narrow soliton lines where deformation concentrates. The system progressively optimizes the local stacking registry within each domain, giving rise to a spatially structured deformation field whose amplitude scales with the mechanical compliance of the material and can be further tuned by external perturbation. The obtained results of both atomic reconstructed structures and moiré phonon spectra present a good agreement with the reported experiments, thereby demonstrating the accuracy and efficiency of our methodology in modeling of these large scale nanomaterials.

cond-mat.mtrl-sci↗

arXiv2025

Mapping the twist angle dependence of quasi-Brillouin zones in doubly aligned graphene/BN heterostructures

When monolayer graphene is crystallographically aligned to hexagonal boron nitride (BN), a moiré superlattice is formed, producing characteristic satellite Dirac peaks in the electronic band structure. Aligning a second BN layer to graphene creates two coexisting moiré patterns, which can interfere to produce periodic, quasi-periodic or non-periodic superlattices, depending on their relative alignment. Here, we investigate one of the simplest realizations of such a double-moiré structure, graphene encapsulated between two BN layers, using dynamically rotatable van der Waals heterostructures. Our setup allows \textit{in situ} control of the top BN alignment while keeping the bottom BN fixed. By systematically mapping the charge transport as a function of BN angular alignment, we identify the simultaneous signatures of the original moirés, super-moirés, and a third set of features corresponding to quasi-Brillouin zones (qBZ) formed when the system's periodicity becomes ill-defined. Comparing our measurements with theoretical models, we provide the first experimental mapping of the qBZs as a function of angular alignment. Our results establish a direct experimental link between moiré interference and qBZ formation, opening new avenues for engineering electronic structures in multi-aligned 2D heterostructures.

cond-mat.mes-hall↗

arXiv2025

Proximity Effects Between the Graphene Quasicrystal and Magic-Angle Twisted Bilayer Graphene

We present a numerical study of three-layer graphene heterostructures in which the layers are twisted by the magic angle ($\sim$1.1$^\circ$) or by $\sim$$30^\circ$ to form a graphene quasicrystal. The heterostacks are described using realistic structural relaxations and tight-binding Hamiltonians, and their transport properties are computed for both pristine and disordered systems containing up to $\sim$8 million atoms. Owing to the weak interlayer coupling, we resolve the hybridization between magic-angle flat bands and quasicrystalline states, which are modified in distinct ways across low- and high-energy windows, revealing a new hybrid electronic regime to explore.

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arXiv2025

Disorder-Induced Delocalization in Magic-Angle Twisted Bilayer Graphene

Flat bands in moiré systems are exciting new playgrounds for the generation and study of exotic many-body physics phenomena in low-dimensional materials. Such physics is attributed to the vanishing kinetic energy and strong spatial localization of the flat-band states. Here we use numerical simulations to examine the electronic transport properties of such flat bands in magic-angle twisted bilayer graphene in the presence of disorder. We find that while a conventional downscaling of the mean free path with increasing disorder strength occurs at higher energies, in the flat bands the mean free path can actually increase with increasing disorder strength.This phenomenon is also captured by the disorder-dependent quantum metric, which is directly linked to the ground state localization.This disorder-induced delocalization suggests that weak disorder may have a strong impact on the exotic physics of magic-angle bilayer graphene and other related moiré systems.

cond-mat.mes-hall↗

arXiv2024

Recursive Green's functions optimized for atomistic modelling of large superlattice-based devices

The Green's function method is recognized to be a very powerful tool for modelling quantum transport in nanoscale electronic devices. As atomistic calculations are generally expensive, numerical methods and related algorithms have been developed accordingly to optimize their computation cost. In particular, recursive techniques have been efficiently applied within the Green's function calculation approach. Recently, with the discovery of Moiré materials, several attractive superlattices have been explored using these recursive Green's function techniques. However, numerical difficulty issues were reported as most of these superlattices have relatively large supercells, and consequently a huge number of atoms to be considered. In this article, improvements to solve these issues are proposed in order to keep optimizing the recursive Green's function calculations. These improvements make the electronic structure calculations feasible and efficient in modelling large superlattice-based devices. As an illustrative example, twisted bilayer graphene superlattices are computed and presented to demonstrate the efficiency of the method.

cond-mat.mes-hall↗

arXiv2024

Quantifying the local mechanical properties of twisted double bilayer graphene

Nanomechanical measurements of minimally twisted van der Waals materials remained elusive despite their fundamental importance for device realisation. Here, we use Ultrasonic Force Microscopy (UFM) to locally quantify the variation of out-of-plane Young's modulus in minimally twisted double bilayer graphene (TDBG). We reveal a softening of the Young's modulus by 7\% and 17\% along single and double domain walls, respectively. Our experimental results are confirmed by force-field relaxation models. This study highlights the strong tunability of nanomechanical properties in engineered twisted materials, and paves the way for future applications of designer 2D nanomechanical systems.

cond-mat.mes-hall↗

arXiv2024

Thermoelectric Properties of Graphene through BN-ring Doping: A Theoretical Investigation

Graphene has been widely studied for various applications due to its outstanding electrical and mechanical properties. However, its potential in thermoelectric applications has been limited by a low Seebeck coefficient and high thermal conductivity. Efforts to enhance its thermoelectric properties have involved the usage of carbon-based nanoribbons, strain engineering, and heteroatom co-doping, particularly with Nitrogen and/or Boron atoms. In this work, multiscale simulation approaches combining DFT calculations and semi-empirical models are used to explore the potential improvement of the thermoelectric properties via borazine (B$_3$N$_3$)-ring doping. As bandgap engineering can be obtained with this doping, the thermoelectric properties of graphene are significantly enlarged, albeit at the cost of reduced conductance. The effects observed are not only dependent on the concentration of BN within the graphene lattice but are also notably influenced by the relative rotational alignment of the BN rings. Furthermore, the effect of the distribution and rotational disorder are considered, showing reduced electronic conductance compared to the periodic case highlighting the importance of precise control over the doping parameters of BN-ring. Lastly, the thermal lattice conductance is estimated revealing a substantial reduction of up to 40$\%$ compared to pristine graphene opening up the possibility of enhancing the thermoelectric efficiency of BNC materials. The present theoretical approach highlights how BN-ring doping can refine the thermoelectric properties of 2D graphene, offering a pathway for enhancing its suitability in practical thermoelectric applications.

cond-mat.mtrl-sci↗

arXiv2024

Exploring Dielectric Properties in Models of Amorphous Boron Nitride

We report a theoretical study of dielectric properties of models of amorphous Boron Nitride, using interatomic potentials generated by machine learning. We first perform first-principles simulations on small (about $100$ atoms in the periodic cell) sample sizes to explore the emergence of mid-gap states and its correlation with structural features. Next, by using a simplified tight-binding electronic model, we analyse the dielectric functions for complex three dimensional models (containing about $10.000$ atoms) embedding varying concentrations of ${\rm sp^{1}, sp^{2}}$ and ${\rm sp^3}$ bonds between B and N atoms. Within the limits of these methodologies, the resulting value of the zero-frequency dielectric constant is shown to be influenced by the population density of such mid-gap states and their localization characteristics. We observe nontrivial correlations between the structure-induced electronic fluctuations and the resulting dielectric constant values. Our findings are however just a first step in the quest of accessing fully accurate dielectric properties of as-grown amorphous BN of relevance for interconnect technologies and beyond.

cond-mat.mtrl-sci↗

arXiv2023

Klein tunneling degradation and enhanced Fabry-Pérot interference in graphene/h-BN moiré-superlattice devices

Hexagonal boron-nitride (h-BN) provides an ideal substrate for supporting graphene devices to achieve fascinating transport properties, such as Klein tunneling, electron optics and other novel quantum transport phenomena. However, depositing graphene on h-BN creates moiré superlattices, whose electronic properties can be significantly manipulated by controlling the lattice alignment between layers. In this work, the effects of these moiré structures on the transport properties of graphene are investigated using atomistic simulations. At large misalignment angles (leading to small moiré cells), the transport properties (most remarkably, Klein tunneling) of pristine graphene devices are conserved. On the other hand, in the nearly aligned cases, the moiré interaction induces stronger effects, significantly affecting electron transport in graphene. In particular, Klein tunneling is significantly degraded. In contrast, strong Fabry-Pérot interference (accordingly, strong quantum confinement) effects and non-linear I-V characteristics are observed. P-N interface smoothness engineering is also considered, suggesting as a potential way to improve these transport features in graphene/h-BN devices.

cond-mat.mes-hall↗

arXiv2022

First-principles study of the structural and electronic properties of BN-ring doped graphene

Since advanced Silicon-based device components are moderately chemically tunable, doped graphene has emerged as a promising candidate to replace this semiconducting material in flexible miniaturized electronic devices. Indeed, heteroatom co-doping (i.e. with boron and/or nitrogen) is an appealing strategy to tune both its structural and electronic properties, possibly inducing a band gap in graphene. However, presently synthesized BN-doped carbon-based materials are randomly doped, leading to their electronic properties not being reproducible. Using first-principles techniques, the present study investigates the periodic doping of graphene with borazine-like rings in order to search for an entirely new class of BCN hybrid 2D materials exhibiting high stabilities and optimized band gaps for opto-electronic applications. Ab initio calculations show that BN-ring doped graphene displays cohesive energies comparable with benchmark ideal periodic BCN systems (such as BC$_3$, C$_3$N$_4$, BC$_2$N) with a decreasing linear trend toward high concentrations of BN-rings. Band gaps of BN-ring doped graphene systems are calculated using many-body perturbation techniques and are found to be sensitive to the doping pattern and to be considerably larger for high concentrations of BN rings exhibiting the same orientation. These predictions suggest that BN-ring doped graphene materials could be interesting candidates for the next generation of optoelectronic devices and open new opportunities for their synthesis using chemical bottom-up approaches.

cond-mat.mtrl-sci↗

arXiv2022

Towards Optimized Charge Transport in Multilayer Reduced Graphene Oxides

In the context of graphene-based composite applications, a complete understanding of charge conduction in multilayer reduced graphene oxides (rGO) is highly desirable. However, these rGO compounds are characterized by multiple and different sources of disorder depending on the chemical method used for their synthesis. Most importantly the precise role of interlayer interaction in promoting or jeopardizing electronic flow remains unclear. Here, thanks to the development of a multiscale computational approach combining first-principles calculations with large scale transport simulations, the transport scaling laws in multilayer rGO are unraveled, explaining why diffusion worsens with increasing film thickness. In contrast, contacted films are found to exhibit an opposite trend when the mean free path becomes shorter than the channel length, since conduction becomes predominantly driven by interlayer hopping. These predictions are favourably compared with experimental data and open a road towards the optimization of graphene-based composites with improved electrical conduction.

cond-mat.mes-hall↗

arXiv2021

Graphene whisperitronics: transducing whispering gallery modes into electronic transport

When confined in circular cavities, graphene relativistic charge carriers occupy whispering gallery modes (WGM) in analogy to classical acoustic and optical fields. The rich geometrical patterns of the WGM decorating the local density of states offer promising perspectives to devise new disruptive quantum devices. However, exploiting these highly sensitive resonances requires the transduction of the WGMs to the outside world through source and drain electrodes, a yet unreported configuration. Here we create a circular p-n island in a graphene device using a polarized scanning gate microscope tip, and probe the resulting WGMs signatures in in-plane electronic transport through the p-n island. Combining tight-binding simulations and exact solution of the Dirac equation, we assign the measured device conductance features to WGMs, and demonstrate mode selectivity by displacing the p-n island with respect to a constriction. This work therefore constitutes a proof of concept for graphene whisperitronics devices.

cond-mat.mes-hall↗

arXiv2021

Electron-phonon coupling in a magic-angle twisted-bilayer graphene device

The importance of phonons in the strong correlation phenomena observed in twisted bilayer graphene (TBG) at the so-called magic-angle is under debate. Here we apply gate-dependent micro-Raman spectroscopy to monitor the G band linewidth in TBG devices of twist angles $θ=0^{\circ}$, $\sim 1.1^{\circ}$ (magic-angle) and $\sim 7^{\circ}$ (large angle). The results show a broad and p/n-asymmetric doping behavior at the magic-angle, in clear contrast to the behavior observed in twist angles above and below. Atomistic modeling reproduces the experimental observations, revealing how the unique electronic structure of magic-angle TBGs influences the electron-phonon coupling and, consequently, the G band linewidth. Our findings indicate a connection between electron-phonon coupling and experimental observations of strongly correlated phenomena in magic-angle TBG.

cond-mat.mes-hall↗

arXiv2020

Lattice dynamics localization in low-angle twisted bilayer graphene

A low twist angle between the two stacked crystal networks in bilayer graphene enables self-organized lattice reconstruction with the formation of a periodic domain. This superlattice modulates the vibrational and electronic structures, imposing new rules for electron-phonon coupling and the eventual observation of strong correlation and superconductivity. Direct optical images of the crystal superlattice in reconstructed twisted bilayer graphene are reported here, generated by the inelastic scattering of light in a nano-Raman spectroscope. The observation of the crystallographic structure with visible light is made possible due to lattice dynamics localization, the images resembling spectral variations caused by the presence of strain solitons and topological points. The results are rationalized by a nearly-free-phonon model and electronic calculations that highlight the relevance of solitons and topological points, particularly pronounced for structures with small twist angles. We anticipate our discovery to play a role in understanding Jahn-Teller effects and electronic Cooper pairing, among many other important phonon-related effects, and it may be useful for characterizing devices in the most prominent platform for the field of twistronics.

cond-mat.mes-hall↗

arXiv2019

Optimizing Dirac fermions quasi-confinement by potential smoothness engineering

With the advent of high mobility encapsulated graphene devices, new electronic components ruled by Dirac fermions optics have been envisioned and realized. The main building blocks of electron-optics devices are gate-defined p-n junctions, which guide, transmit and refract graphene charge carriers, just like prisms and lenses in optics. The reflection and transmission are governed by the p-n junction smoothness, a parameter difficult to tune in conventional devices. Here we create p-n junctions in graphene, using the polarized tip of a scanning gate microscope, yielding Fabry-Pérot interference fringes in the device resistance. We control the p-n junctions smoothness using the tip-to-graphene distance, and show increased interference contrast using smoother potential barriers. Extensive tight-binding simulation reveal that smooth potential barriers induce a pronounced quasi-confinement of Dirac fermions below the tip, yielding enhanced interference contrast. On the opposite, sharp barriers are excellent Dirac fermions transmitters and lead to poorly contrasted interferences. Our work emphasizes the importance of junction smoothness for relativistic electron optics devices engineering.

cond-mat.mes-hall↗

arXiv2019

Imaging Dirac fermions flow through a circular Veselago lens

Graphene charge carriers behave as relativistic massless fermions, thereby exhibiting a variety of counter-intuitive behaviors. In particular, at p-n junctions, they behave as photons encountering a negative index media, therefore experiencing a peculiar refraction known as Veselago lensing. However, the way Dirac fermions flow through a Veselago lens remains largely unexplored experimentally. Here, a novel approach to create a movable and tunable circular p-n junction in graphene is proposed, using the polarized tip of a scanning gate microscope. Scanning the tip in the vicinity of a graphene constriction while recording the device conductance yields images related to the electron flow through a circular Veselago lens, revealing a high current density in the lens core, as well as two low current density zones along transport axis. Tight-binding simulations reveal the crucial role of the p-n junction smoothness on these phenomena. The present research adds new dimensions in the control and understanding of Dirac fermions optical elements, a prerequisite to engineer relativistic electron optics devices.

cond-mat.mes-hall↗

arXiv2013

Quantum calculations of the carrier mobility in thin films: Methodology, Matthiessen's rule and comparison with semi-classical approaches

We discuss the calculation of the carrier mobility in silicon films within the quantum Non-Equilibrium Green's Functions (NEGF) framework. We introduce a new method for the extraction of the carrier mobility that is free from contact resistance contamination, and provides accurate mobilities at a reasonable cost, with minimal needs for ensemble averages. We then introduce a new paradigm for the definition of the partial mobility $μ_{M}$ associated with a given elastic scattering mechanism "M", taking phonons (PH) as a reference ($μ_{M}^{-1}=μ_{PH+M}^{-1}-μ_{PH}^{-1}$). We argue that this definition makes better sense in a quantum transport framework as it is free from long range interference effects that can appear in purely ballistic calculations. As a matter of fact, these mobilities satisfy Matthiessen's rule for three mechanisms [surface roughness (SR), remote Coulomb scattering (RCS) and phonons] much better than the usual, single mechanism calculations. We also discuss the problems raised by the long range spatial correlations in the RCS disorder. Finally, we compare semi-classical Kubo-Greenwood (KG) and quantum NEGF calculations. We show that KG and NEGF are in reasonable agreement for phonon and RCS, yet not for SR. We point to possible deficiencies in the treatment of SR scattering in KG, opening the way for further improvements.

cond-mat.mes-hall↗

arXiv2013

Interplay between Aharonov-Bohm interference and parity selective tunneling in zigzag graphene nanoribbon rings

We report a numerical study on Aharonov-Bohm (AB) effect and parity selective tunneling in pn junctions based on zigzag graphene nanoribbon rings. We find that when applying a magnetic field to the ring, the AB interference can reverse the parity symmetry of incoming waves and hence can strongly modulate the parity selective transmission through the system. Therefore, the transmission between two states of different parity exhibits the AB oscillations with a π-phase shift, compared to the case of states of same parity. On this basis, it is shown that interesting effects such as giant (both positive and negative) magnetoresistance and strong negative differential conductance can be achieved in this structure. Our study thus presents a new property of the AB interference, which could be helpful to further understand the transport properties of graphene mesoscopic-systems.

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