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Dai-Nam Le

Publications and source records attributed to Dai-Nam Le.

At least 19 recordsLinked to original sources

Tuning of quantum nanoscaled friction within the Prandtl-Tomlinson model

Nanoscaled friction is a fundamental tribological phenomenon with complex behavior of its dynamical force. Here, we utilize the Prandtl-Tomlinson framework to investigate systematically the different means of control of the frictional force at the quantum and classical levels. It is found that the frictional dynamics can be controlled by the corrugation and characteristic length ratio parameters dependent upon properties of the nanoparticle-chain system. In addition to the stick-slip regime, other types of motion are uncovered, highlighting the richness of the frictional dynamics. The importance of Landau-Zener tunneling for the quantum motion is also analyzed. These findings provide valuable insights for interpreting experimental observations and controlling quantum frictional behavior in nanoscale systems.

quant-ph

Quantum stick-slip motion in nanoscaled friction

Friction in atomistic systems is usually described by the classical Prandtl-Tomlinson model suitable for capturing the dragging force of a nanoparticle in a periodic potential. Here we consider the quantum mechanical version of this model in which the dissipation is facilitated by releasing heat to an external bath reservoir. The time evolution of the system is captured with the Liouville-von Neumann equation through the density matrix of the system in the Markov approximation. We examine several kinetic and dissipative properties of the nanoparticle by delineating classical vs quantum mechanical effects. We find that the Landau-Zener tunneling is a key factor in the overall reduction of the frictional dissipation when compared to the classical motion in which such tunneling is absent. Other regimes of motion, controlled by the corrugation parameter and other properties, are also found. This in-depth study analyzes the interplay between velocity, strength of interaction, and temperature to control the frictional {force} and provide useful guidelines for experimental data interpretation.

quant-ph

Anisotropic two-dimensional magnetoexciton with exact center-of-mass separation

Excitons in anisotropic two-dimensional (2D) materials, defined by direction-dependent effective masses, are of pronounced interest for their roles in excitonic and magneto-optical phenomena. A perpendicular magnetic field complicates the separation of center-of-mass (c.m.) and relative motions, especially when electron and hole masses are comparable. Conventional theories often employ an approximate c.m. separation using factorized wave functions, modifying magnetic Hamiltonian terms and possibly introducing inaccuracies in magnetoexciton energy predictions. This work develops an exact analytical framework for c.m. and relative motion separation in anisotropic 2D magnetoexcitons, without resorting to the stationary-c.m. approximation. Starting from the full electron-hole Hamiltonian in a homogeneous magnetic field, the formalism uses the conserved pseudomomentum to derive a relative-motion Hamiltonian, revealing new anisotropy-dependent couplings and magnetic coefficients absent in approximate models. The resulting Schrödinger equation is treated via the Feranchuk-Komarov operator method and Levi-Civita transformation, allowing non-perturbative, systematically convergent solutions. Application to monolayer black phosphorus and titanium trisulfide, both freestanding and encapsulated in hexagonal boron nitride, yields magnetoexciton energies, diamagnetic coefficients, and probability densities for the ten lowest states across considerable magnetic-field ranges. The results demonstrate the significant influence of anisotropy-dependent coupling on magnetic response in systems with strong mass anisotropy. This formalism is generalizable to other anisotropic 2D semiconductors, establishing a foundation for advanced magneto-optical studies.

cond-mat.mes-hall

Casimir interactions between two parallel graphene sheets carrying steady-state drift currents

We investigate the fluctuation-induced Casimir interactions between two parallel graphene sheets carrying steady-state drift currents. The graphene properties are modeled based on the shifted Fermi disk model to capture the non-equilibrium optical response of the system. We find that the drift current introduces a repulsive correction to the perpendicular to the layers Casimir interaction, thereby reducing the overall attractive force. Although the correction is repulsive, it does not overcome the underlying attraction between the layers. It also generates a lateral force that opposes the carrier flow direction. Both contributions are studied in terms of distance and drift velocity functionalities showing pathways for Casimir force control.

quant-ph

Prediction of a measurable sign change in the Casimir force using a magnetic fluid

We demonstrate quantum levitation controlled by Casimir forces acting between a polystyrene surface and a Teflon-coated metallic substrate immersed in a mixture of Toluene and magnetite particles. This system experiences repulsion-attraction transitions in the Casimir interaction for distances where the effect is measurable. This Casimir trapping can be controlled by clever choices of metallic and ferrofluid materials, which are directly linked to the emergence of the trapping effect. Thermal and quantum contributions are investigated in detail, showing how the optical and magnetic properties of the ferrofluid and other materials affect the magnitude of the trapping and its distance range of observability.

quant-ph

Phononic Casimir Effect in Planar Materials

The Phononic Casimir effect between planar objects is investigated by deriving a formalism from the quantum partition function of the system following multiscattering approach. This fluctuation-induced coupling is mediated by phonons modeled as an effective elastic medium. We find that excitations with three types of polarizations arise from the resolved boundary conditions, however the coupling is dominated by only one of these degrees of freedom due to exponential suppression effects in the other two. The obtained scaling laws and dependence on materials properties and temperature suggest effective pathways of interaction control. Scenarios of materials combinations are envisioned where the Phononic Casimir effect is of similar order as the standard Casimir interaction mediated by electromagnetic fluctuations.

quant-ph

Retrieval of fundamental material parameters of monolayer transition metal dichalcogenides from experimental exciton energies: An analytical approach

We propose a straightforward and highly accurate method for extracting material parameters such as screening length, bandgap energy, exciton reduced mass, and the dielectric constant of the surrounding medium from experimental magnetoexciton energies available for monolayer transition metal dichalcogenides (TMDCs). Our approach relies on analytical formulations that allow us to calculate the screening length $r_0$ and bandgap energy $E_g$ directly from the experimental $s$-state exciton energies $E_{1s}$, $E_{2s}$, and $E_{3s}$. We also establish a relationship between the surrounding dielectric constant $κ$ and the exciton reduced mass $μ$. This relationship simplifies the Schr{ö}dinger equation for a magnetoexciton in a TMDC monolayer, transforming it into a one-parameter equation that depends solely on the single material parameter $μ$. Furthermore, we develop an analytical formula with high accuracy for magnetoexciton energies as a function of the exciton reduced mass: $E(B,μ)$. Then, the inverse of this formula allows us to calculate the exciton reduced mass from experimental data on magnetoexciton energies. By applying this method, we extract key material parameters, $E_g$, $r_0$, $μ$, and $κ$, from the magnetoexciton energies of monolayer TMDCs, including WSe$_2$, WS$_2$, MoSe$_2$, and MoS$_2$, encapsulated by hexagonal boron nitride (hBN) slabs in various current experiments. The material properties we retrieve complement and correct existing experimental and theoretical data. Additionally, we develop an analytical method for calculating diamagnetic coefficients and exciton radii with high accuracy compared to numerical calculations. Based on this method, we provide diamagnetic coefficients and exciton radii computed using the extracted material parameters.

cond-mat.mes-hall

Radiative Heat Transfer and 2D Transition Metal Dichalcogenide Materials

Radiative heat transfer is of great interest from a fundamental point of view and for energy harvesting applications. This is a material dependent phenomenon where confined plasmonic excitations, hyperbolicity and other properties can be effective channels for enhancement, especially at the near field regime. Materials with reduced dimensions may offer further benefits of enhancement compared to the bulk systems. Here we study the radiative thermal power in the family of transition metal dichalcogenide monolayers in their H- and T-symmetries. For this purpose, the computed from first principles electronic and optical properties are then used in effective models to understand the emerging scaling laws for metals and semiconductors as well as specific materials signatures as control knobs for radiative heat transfer. Our combined approach of analytical modeling with properties from ab initio simulations can be used for other materials families to build a materials database for radiative heat transfer.

cond-mat.mes-hall

Interface Magnetism in Vanadium-doped MoS$_2$/Graphene Heterostructures

Magnetism in two-dimensional materials is of great importance in discovering new physical phenomena and developing new devices at the nanoscale. In this paper, first-principles simulations are used to calculate the electronic and magnetic properties of heterostructures composed of Graphene and MoS$_2$ considering the influence of point defects and Vanadium doping. It is found that the concentration of the dopants and the types of defects can result in induced magnetic moments leading to ferromagnetically polarized systems with sharp interfaces. This provides a framework for interpreting the experimental observations of enhanced ferromagnetism in both MoS$_2$/Graphene and V-doped MoS$_2$/Graphene heterostructures. The computed electronic and spin polarizations give a microscopic understanding of the origin of ferromagnetism in these systems and illustrate how doping and defect engineering can lead to targeted property tunability. Our work has demonstrated that through defects engineering, ferromagnetism can be achieved in V-doped MoS$_2$/Graphene heterostructures, providing a potential way to induce magnetization in other TMDC/Graphene materials and opening new opportunities for their applications in nano-spintronics.

cond-mat.mtrl-sci

Twisting in h-BN bilayers and their angle-dependent properties

In this paper, we systematically investigate the structural and electronic properties of twisted h-BN bilayers to understand the role of the twisting angle. Using first-principles methods with relaxation taken into account, we simulate h-BN bilayers with commensurate supercells with the smallest angle being $2.88^{\circ}$ until $60^{\circ}$. We find that the interlayer separation is not constant throughout each bilayer because of the various stacking patterns of AA, AA', AB, AB', and A'B throughout the layers, which also play a significant role in their unique charge redistribution. The calculations for the 110 generated structures show the existence of flat bands in several twisted h-BN bilayers, as well as the emergence of different trends in their properties as a function of the twist angle. These results are useful for establishing a systematic base line of registry-dependent relations for the development of more advanced computational methods to access incommensurate h-BN bilayers.

cond-mat.mtrl-sci

Dissecting van der Waals interactions with Density Functional Theory -- Wannier-basis approach

A new scheme for the computation of dispersive interactions from first principles is presented. This cost-effective approach relies on a Wannier function representation compatible with density function theory descriptions. This is an electronic-based many-body method that captures the full electronic and optical response properties of the materials. It provides the foundation to discern van der Waals and induction energies as well as the role of anisotropy and different stacking patterns when computing dispersive interactions in systems. Calculated results for binding energies in benchmarked materials and layered materials, such as graphite, hBN, and MoS$_2$ give encouraging comparisons with available experimental data. Strategies for broadened computational descriptions of dispersive interactions are also discussed. Our investigation aims at stimulating new experimental studies to measure van der Waals energies in a wider range of materials, especially in layered systems.

cond-mat.mtrl-sci

Phonon-assisted Casimir interactions between piezoelectric materials

The strong coupling between electromagnetic field and lattice oscillation in piezoelectric materials gives rise to phonon polariton excitations. Such quasiparticles open up new directions in modulating the ubiquitous Casimir force. Here by utilizing the generalized Born-Huang hydrodynamics model, three types of phonons in piezoelectrics are studied: longitudinal optical phonon, transverse optical phonon and phonon polariton. The phonon-electromagnetic coupling results in a complex set of Fresnel reflection matrices which prevents the utilization of the standard Lifshitz approach for calculating Casimir forces in the imaginary frequency domain. Our calculations are based on an approach within real frequency and finite temperatures, through which various regimes of the Casimir interaction are examined. Our study shows that piezoelectrics emerge as a set of materials where this ubiquitous force can be controlled via phonon properties for the first time. The Casimir interaction appears as a suitable means to distinguish between different types of surface phonon polaritons associated with different structural piezoelectric polytypes.

cond-mat.mes-hall

Analytical exciton energies in monolayer transition-metal dichalcogenides

We derive an analytical expression for $s$-state exciton energies in monolayer transition-metal dichalcogenides (TMDCs): $E_{\text{ns}}=-{\text{Ry}}^*\times P_n/{(n-1/2+0.479\, r^*_0/κ)^2}$, $n=1,2,...$, where $r^*_0$ and $κ$ are the dimensionless screening length and dielectric constant of the surrounding medium; $\text{Ry}^*$ is an effective Rydberg energy scaled by the dielectric constant and exciton reduced mass; $P_n(r^*_0/κ)$ is a function of variables $n$ and $r^*_0/κ$. Its values are around 1.0 so we consider it a term that corrects the Rydberg energy. Despite the simple form, the suggested formula gives exciton energies with high precision compared to the exact numerical solutions that accurately describe recent experimental data for a large class of TMDC materials, including WSe$_2$, WS$_2$, MoSe$_2$, MoS$_2$, and MoTe$_2$. To achieve these results, we have developed a so-called regulated perturbation theory by combining the conventional perturbation method with several elements of the Feranchuk-Komarov operator method, including the Levi-Civita transformation, the algebraic calculation technique via the annihilation and creation operators, and the introduction of a free parameter to optimize the convergence rate of the perturbation series. This universal form of exciton energies could be helpful in various physical analyses, including retrieval of the material parameters such as reduced exciton mass and screening length from the available measured exciton energies.

cond-mat.mtrl-sci

Vanadium-Doped Molybdenum Disulfide Monolayers with Tunable Electronic and Magnetic Properties: Do Vanadium-Vacancy Pairs Matter?

Monolayers of molybdenum disulfide (MoS2) are the most studied two-dimensional (2D) transition-metal dichalcogenides (TMDs), due to its exceptional optical, electronic, and opto-electronic properties. Recent studies have shown the possibility of incorporating a small amount of magnetic transition metals (e.g., Fe, Co, Mn, V) into MoS2 to form a 2D dilute magnetic semiconductor (2D-DMS). However, the origin of the observed ferromagnetism has remained elusive, due to the presence of randomly generated sulfur vacancies during synthesis that can pair with magnetic dopants to form complex dopant-vacancy configurations altering the magnetic order induced by the dopants. By combining high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging with first-principles density functional theory (DFT) calculations and magnetometry data, we demonstrate the critical effects of sulfur vacancies and their pairings with vanadium atoms on the magnetic ordering in V-doped MoS2 (V-MoS2) monolayers. Additionally, we fabricated a series of field effect transistors on these V-MoS2 monolayers and observed the emergence of p-type behavior as the vanadium concentration increased. Our study sheds light on the origin of ferromagnetism in V-MoS2 monolayers and provides a foundation for future research on defect engineering to tune the electronic and magnetic properties of atomically thin TMD-based DMSs.

cond-mat.mtrl-sci

Giant anisotropy and Casimir phenomena: the case of carbon nanotube metasurfaces

The Casimir interaction and torque are related phenomena originating from the exchange of electromagnetic excitations between objects. While the Casimir force exists between any types of objects, the materials or geometrical anisotropy drives the emergence of the Casimir torque. Here both phenomena are studied theoretically between dielectric films with immersed parallel single wall carbon nanotubes in the dilute limit with their chirality and collective electronic and optical response properties taken into account. It is found that the Casimir interaction is dominated by thermal fluctuations at sub-micron separations, while the torque is primarily determined by quantum mechanical effects. This peculiar quantum vs. thermal separation is attributed to the strong influence of reduced dimensionality and inherent anisotropy of the materials. Our study suggests that nanostructured anisotropic materials can serve as novel platforms to uncover new functionalities in ubiquitous Casimir phenomena.

quant-ph

Nonlinear effects in many-body van der Waals interactions

Van der Waals interactions are ubiquitous and they play an important role for the stability of materials. Current understanding of this type of coupling is based on linear response theory, while optical nonlinearities are rarely considered in this context. Many materials, however, exhibit strong optical nonlinear response, which prompts further evaluation of dispersive forces beyond linear response. Here we present a $\textit{Discrete Coupled Nonlinear Dipole}$ approach that takes into account linear and nonlinear properties of all dipolar nanoparticles in a given system. This method is based on a Hamiltonian for nonlinear dipoles, which we apply in different systems uncovering a complex interplay of distance, anisotropy, polarizibilities, and hyperpolarizabilities in the vdW energy. This investigation broadens our basic understanding of dispersive interactions, especially in the context of nonlinear materials.

cond-mat.mtrl-sci

Retrieval of material properties of monolayer transition-metal dichalcogenides from magnetoexciton energy spectra

Reduced exciton mass, polarizability, and dielectric constant of the surrounding medium are essential properties for semiconducting materials, and they have been extracted recently from the magnetoexciton energies. However, the acceptable accuracy of the suggested method requires very high magnetic intensity. Therefore, in the present paper, we propose an alternative method of extracting these material properties from recently available experimental magnetoexciton s-state energies in monolayer transition-metal dichalcogenides (TMDCs). The method is based on the high sensitivity of exciton energies to the material parameters in the Rytova-Keldysh model. It allows us to vary the considered material parameters to get the best fit of the theoretical calculation to the experimental exciton energies for the $1s$, $2s$, and $3s$ states. This procedure gives values of the exciton reduced mass and $2D$ polarizability. Then, the experimental magnetoexciton spectra compared to the theoretical calculation also determine the average dielectric constant. Concrete applications are presented only for monolayers WSe$_2$ and WS$_2$ from the recently available experimental data; however, the presented approach is universal and can be applied to other monolayer TMDCs. The mentioned fitting procedure requires a fast and effective method of solving the Schrödinger equation of an exciton in monolayer TMDCs with a magnetic field. Therefore, we also develop such a method in this paper for highly accurate magnetoexciton energies.

cond-mat.mes-hall

Thermal effect on magnetoexciton energy spectra in monolayer transition metal dichalcogenides

It is widely comprehended that temperature may cause phonon-exciton scattering, enhancing the energy level's linewidth and leading to some spectrum shifts. However, in the present paper, we suggest a different mechanism that allows the thermal motion of the exciton's center of mass (c.m.) to affect the magnetoexciton energies in monolayer dichalcogenides (TMDCs). By the nontrivial but precise separation of the c.m. motion from an exciton in a monolayer TMDC with a magnetic field, we obtain an equation for the relative motion containing a motional Stark term proportional to the c.m. pseudomomentum, related to the temperature of the exciton gas but neglected in the previous studies. Solving the Schrödinger equation without omitting the motional Stark potential at room temperature shows approximately a few meV thermal-magnetic shifts in the exciton energies, significant enough for experimental detection. Moreover, this thermal effect causes a change in exciton radius and diamagnetic coefficient and enhances the exciton lifetime as a consequence. Surprisingly, the thermoinduced motional Stark potential breaks the system's SO(2) symmetry, conducting new peaks in the exciton absorption spectra at room temperature besides those of the $s$ states. This mechanism could be extended for other magnetoquasiparticles such as trions and biexcitons.

cond-mat.mes-hall