arXiv subjects

Aleksandr Rodin

Publications and source records attributed to Aleksandr Rodin.

At least 19 recordsLinked to original sources

arXiv2026

Orbitals of Artificial Atoms in a Gapped Two-Dimensional Vacuum

Advances in nanotechnology now allow the creation of artificial atoms - engineered structures whose electronic states closely mimic those of real atoms. Understanding how these artificial atoms interact and bond is key to designing new materials with tailored electronic properties. Here, we use scanning tunnelling microscopy to visualise the bound states of nanostructures patterned in a two-dimensional molecular film featuring a parabolic band with multiple partial energy gaps. The lowest-energy states split off from the bottom of the band and resemble the familiar $s$ and $p$ orbitals of natural atoms, even bonding in the same way. Yet, artificial atoms go beyond this analogy: the gapped two-dimensional vacuum in which they reside gives rise to entirely new orbitals with no counterparts in real atoms. These quasi-one-dimensional localised states enrich the orbital vocabulary of chemistry, adding a new class of orbitals that are predominantly shaped by the surrounding electronic vacuum.

cond-mat.mes-hall↗

arXiv2026

Low-Field Metal-Insulator Transition in AB-Stacked Bilayer Graphene

We investigate the interplay of in-plane magnetic and transverse electric fields in AB-stacked bilayer graphene. In prior work neglecting trigonal warping, we demonstrated that this configuration induces an insulator-metal transition purely via orbital effects, albeit requiring impractically large magnetic fields ($>100$ T). Here, we extend the analysis to the ultra-low-energy regime by incorporating interlayer skew couplings. In a restricted region of momentum space, trigonal warping produces a fine splitting of Dirac cones leading to a compensated semimetallic state at zero external field. Application of a transverse electric field above a small threshold ($V_c\sim0.6$ meV) reinstates an insulating gap. When an in-plane magnetic field is applied, the orbital gauge vector immediately breaks the $C_3$ spatial symmetry of the lattice, and we uncover two sequential, field-driven transitions separated by an order of magnitude in scale. First, at fields ($B \approx 1$ T), the Zeeman effect drives an indirect insulator-to-semimetal transition at the ungated charge neutrality point; the cross-spin gap closes, generating distinct electron and hole pockets separated in momentum space. Second, as the field increases to $B \approx 10$--$25$ T, the orbital coupling closes the same-spin gap at an energy well away from the charge-neutral Fermi level. By electrostatically tuning the Fermi level to this specific gap-closing energy, we reveal the emergence of $C_3$-broken same-spin Fermi pockets, accompanied by a distinct step-like onset in the density of states. This dual-transition regime provides a highly sensitive platform for tunable, spin-selective transport.

cond-mat.mes-hall↗

arXiv2025

Time-local stochastic equation of motion for solid ionic electrolytes

Numerical studies of ionic motion through solid electrolytes commonly involve static nudged-elastic band (NEB) methods or costly \emph{ab initio} molecular dynamics (AIMD). Building on a time-local model of current carrier-electrolyte interaction and incorporating thermal motion, we introduce an approach that is intermediate between the two well-established methodologies by treating the electrolyte as an effective medium that interacts with the mobile particle. Through this coupling, the thermally vibrating electrolyte imparts energy to the charge carriers while also absorbing energy from them due to its own finite elasticity. Using a simple model system, we validate our approach through a series of numerical simulations. Our methodology reproduces both dissipative and diffusive behavior, and helps link microscopic system parameters to measurable macroscopic properties.

cond-mat.mtrl-sci↗

arXiv2024

Predicting ionic conductivity in solids from the machine-learned potential energy landscape

Discovering new superionic materials is essential for advancing solid-state batteries, which offer improved energy density and safety compared to the traditional lithium-ion batteries with liquid electrolytes. Conventional computational methods for identifying such materials are resource-intensive and not easily scalable. Recently, universal interatomic potential models have been developed using equivariant graph neural networks. These models are trained on extensive datasets of first-principles force and energy calculations. One can achieve significant computational advantages by leveraging them as the foundation for traditional methods of assessing the ionic conductivity, such as molecular dynamics or nudged elastic band techniques. However, the generalization error from model inference on diverse atomic structures arising in such calculations can compromise the reliability of the results. In this work, we propose an approach for the quick and reliable screening of ionic conductors through the analysis of a universal interatomic potential. Our method incorporates a set of heuristic structure descriptors that effectively employ the rich knowledge of the underlying model while requiring minimal generalization capabilities. Using our descriptors, we rank lithium-containing materials in the Materials Project database according to their expected ionic conductivity. Eight out of the ten highest-ranked materials are confirmed to be superionic at room temperature in first-principles calculations. Notably, our method achieves a speed-up factor of approximately 50 compared to molecular dynamics driven by a machine-learning potential, and is at least 3,000 times faster compared to first-principles molecular dynamics.

cond-mat.mtrl-sci↗

arXiv2024

Insulator-Metal Transition and Magnetic Crossover in Bilayer Graphene

In-plane magnetic fields offer a relatively unexplored opportunity to alter the band structure of stacks of 2D materials so that they exhibit desired physical properties. Here we show that an in-plane magnetic field combined with a transverse electric field can induce an insulator-metal (IM) transition in bilayer graphene. Our study of the magnetic response reveals that the orbital magnetic susceptibility changes from diamagnetic to paramagnetic around the transition point. We discuss several strategies to observe the IM transition, switch the diamagnetism, and more generally control the band structure of stacked 2D materials at experimentally accessible magnetic fields.

cond-mat.mes-hall↗

arXiv2024

On activation in solid ionic electrolytes

Ionic conductivity in solid electrolytes is commonly expected to exhibit Arrhenius dependence on temperature, determined by a well-defined activation energy. Consequently, a standard approach involves calculating this energy using quasi-static methods and using the Arrhenius form to extrapolate the numerical results from one temperature range to another. Despite the ubiquity of this Arrhenius-based modeling, disagreements frequently arise between theory and experiment, and even between different theoretical studies. By considering a tractable minimal model, we elucidate the reason behind the breakdown of the Arrhenius conductivity form. This breakdown is driven by non-trivial phase-space boundaries between conducting and non-conducting regimes, and depends on the kinetic properties of the system.

cond-mat.mes-hall↗

arXiv2024

Single-piston quantum engine

A single-piston quantum engine based on a harmonic oscillator acting as the working fluid is proposed. Using the fact that the interaction between the piston and the oscillator depends on the extent of the oscillator wavefunction, one can control this interaction by modifying the oscillator temperature. By retracting the piston when the interaction is weak (hot oscillator) and returning it to the original position when the coupling is strong (cold oscillator), useful work can be performed assuming the interaction is attractive. The cycle of the engine is simulated numerically using two different powering protocols: bath and measurement. Using the collision model for the baths, the engine is shown to reach a steady state with positive work output.

quant-ph↗

arXiv2023

Three-dimensional harmonic oscillator as a quantum Otto engine

A quantum Otto engine based on a three-dimensional harmonic oscillator is proposed. One of the modes of this oscillator functions as the working fluid, while the other two play the role of baths. The coupling between the working fluid and the baths is controlled using an external central potential. All four strokes of the engine are simulated numerically, exploring the nonadiabatic effects in the compression and expansion phases, as well as the energy transfer during the working fluid's contact with the baths. The efficiency and power of several realizations of the proposed engine are also computed with the former agreeing well with the theoretical predictions for the quantum Otto cycle.

quant-ph↗

arXiv2023

Advantages of non-Hookean coupling in a measurement-fueled two-oscillator engine

A quantum engine composed of two oscillators with a non-Hookean coupling is proposed. Unlike the more common quantum heat engines, the setup introduced here does not require heat baths as the energy for the operation originates from measurements. The engine follows the coupling $\rightarrow$ measurement $\rightarrow$ decoupling $\rightarrow$ extraction cycle. Using a Gaussian term as a prototypical non-harmonic interaction, it is shown that the fueling process facilitates the decoupling step. Numerical simulations are used to demonstrate the measurement-driven fueling, as well as the reduced decoupling energy.

quant-ph↗

arXiv2023

Dissipation and diffusion in one-dimensional solids

Using a nonperturbative classical model for ionic motion through one-dimensional (1D) solids, we explore how thermal lattice vibrations affect ionic transport properties. Based on analytic and numerical calculations, we find that the mean dissipation experienced by the mobile ion is similar to that of the non-thermal case, with thermal motion only contributing stochastic noise. A nonmonotonic dependence of drag on speed, predicted in earlier work, persists in the presence of thermal motion. The inverse relation between drag and speed at high speeds results in non-Fickian diffusion dominated by L\'{e}vy flights. This suppression of drag at high speeds, combined with enhanced activation frequency, improves the particle mobility at high temperatures, where typical particles move faster.

cond-mat.mes-hall↗

arXiv2023

Atomically-precise Vacancy-assembled Quantum Antidots

Patterning antidots ("voids") into well-defined antidot lattices creates an intriguing class of artificial structures for the periodic modulation of 2D electron systems, leading to anomalous transport properties and exotic quantum phenomena as well as enabling the precise bandgap engineering of 2D materials to address technological bottleneck issues. However, realizing such atomic-scale quantum antidots (QADs) is infeasible by current nanolithographic techniques. Here, we report an atomically-precise bottom-up fabrication of a series of atomic-scale QADs with elegantly engineered quantum states through a controllable assembly of a chalcogenide single vacancy (SV) in 2D PtTe2, a type-II Dirac semimetal. Te SVs as atomic-scale "antidots" undergo thermal migration and assembly into highly-ordered SV lattices spaced by a single Te atom, reaching the ultimate downscaling limit of antidot lattices. Increasing the number of SVs in QADs strengthens the cumulative repulsive potential and consequently enhances collective interference of multiple-pocket scattered quasiparticles inside QADs, creating multi-level quantum hole states with tunable gap from telecom to far-infrared regime. Moreover, precisely engineered quantum hole states of QADs are symmetry-protected and thus survive upon atom-by-atom oxygen substitutional doping. Therefore, SV-assembled QADs exhibit unprecedented robustness and property tunability, which not only holds the key to their future applications but also embody a wide variety of material technologies.

cond-mat.mes-hall↗

arXiv2022

Minimal model of drag in one-dimensional crystals

Using a non-perturbative classical approach, we study the dynamics of a mobile particle interacting with an infinite one-dimensional (1D) chain of harmonic oscillators. This minimal system is an effective model for many 1D transport phenomena, such as molecular motion in nanotubes and ionic conduction through solid-state materials. As expected, coupling between the mobile particle and the chain induces dissipation of the mobile particle's energy. However, both numerical and analytic results demonstrate an unconventional non-monotonic dependence of the drag on particle speed. In addition, when this system is subjected to a constant bias, it supports multiple steady-state drift velocities.

cond-mat.mes-hall↗

arXiv2022

Numerical package for QFT calculations of defect-induced phenomena in graphene

We introduce a computationally efficient method based on the path integral formalism to describe defect-modified graphene. By taking into account the entire Brillouin zone, our approach respects the lattice symmetry and can be used to investigate both short-range and long-range effects. The proposed method's key advantage is that the computational complexity does not increase with the system size, scaling, instead, with the number of defects. As a demonstration of our method, we explore the graphene-mediated RKKY interaction between multiple magnetic impurities. Our results concur with earlier findings by showing that the interaction strength and sign depend on various factors like impurity separation, sublattice arrangement, and system doping. We demonstrate that frustration can be introduced between the impurity spins by controlling their relative positions and that this frustration can be switched on and off by tuning the chemical potential of the system.

cond-mat.mes-hall↗

arXiv2021

Gate-tunable artificial nucleus in graphene

We report an atomically-precise integration of individual nitrogen (N) dopant as an in-plane artificial nucleus in a graphene device by atomic implantation to probe its gate-tunable quantum states and correlation effects. The N dopant creates the characteristic resonance state in the conduction band, revealing a giant carrier-dependent energetic renormalization up to 350 meV with respect to the Dirac point, accompanied by the observation of long-range screening effects. Joint density functional theory and tight-binding calculations with modified perturbation potential corroborate experimental findings and highlight the short-range character of N-induced perturbation.

cond-mat.mtrl-sci↗

arXiv2021

Microscopic theory of ionic motion in solid electrolytes

We propose a microscopic, first-principles description of the ionic conduction in crystals. This formalism allows us to gain new insights into the ideal characteristics of general ionic conducting materials and, in particular, solid electrolytes. Using \textit{ab initio} calculations, we show that our formalism results in ionic mobilities consistent with experiments for several materials. Our work opens the possibility of developing solid electrolytes based on fundamental physical principles rather than empirical descriptions of the underlying processes.

cond-mat.mtrl-sci↗

arXiv2021

Microscopic theory of ionic motion in solids

Drag and diffusion of mobile ions in solids are of interest for both purely theoretical and applied scientific communities. This article proposes a theoretical description of ion drag in solids that can be used to estimate ionic conductivities in crystals, and forms a basis for the rational design of solid electrolyte materials. Starting with a general solid-state Hamiltonian, we employ the non-equilibrium path integral formalism to develop a microscopic theory of ionic transport in solids in the presence of thermal fluctuations. As required by the fluctuation-dissipation theorem, we obtain a relation between the variance of the random force and friction. Because of the crystalline nature of the system, however, the two quantities are tensorial. We use the drag tensor to write down the formula for ionic mobility, determined by the potential profile generated by the crystal's ions.

cond-mat.mes-hall↗

arXiv2021

Electronic Self-passivation of Single Vacancy in Black Phosphorus via a Controlled Ionization

We report that mono-elemental black phosphorus presents a new electronic self-passivation scheme of single vacancy (SV). By means of low-temperature scanning tunneling microscopy and bond-resolved non-contact atomic force microscopy, we demonstrate that the local reconstruction and ionization of SV into negatively charged $\mathrm{SV}^-$ leads to the passivation of dangling bonds and thus the quenching of in-gap states, which can be achieved by mild thermal annealing or STM tip manipulation. SV exhibits a strong and symmetric Friedel oscillation (FO) pattern, while $\mathrm{SV}^-$ shows an asymmetric FO pattern with local perturbation amplitude reduced by one order of magnitude and a faster decay rate. The enhanced passivation by forming $\mathrm{SV}^-$ can be attributed to its weak dipole-like perturbation, consistent with density-functional theory and numerical calculations. Therefore, self-passivated $\mathrm{SV}^-$ is electronically benign and acts as a much weaker scattering center, which may hold the key to further enhance the charge mobility of BP and its analogs.

cond-mat.mtrl-sci↗

arXiv2021

Collective Excitations in 2D Materials

Research on 2D materials has been one of the fastest-growing fields in condensed matter physics and materials science in the past 10 years. The low dimensionality and strong correlations of 2D systems give rise to electronic and structural properties, in the form of collective excitations, that do not have counterparts in ordinary 3D materials used in modern technology. These 2D materials present extraordinary opportunities for new technologies, such as in flexible electronics. In this Review, we focus on plasmons, excitons, phonons and magnons in 2D materials. We discuss the theoretical formalism of these collective excitations and elucidate how they differ from their 3D counterparts.

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