arXiv subjects

Kostya S. Novoselov

Publications and source records attributed to Kostya S. Novoselov.

At least 37 records · Page 2Linked to original sources

arXiv2026

Deep-Subwavelength and Broadband Quarter-Wave Retardation in Ultrathin Hyperbolic MoOCl2

The miniaturization of polarization-controlling optical components is one of the central pursuits in nanophotonics. While traditional anisotropic materials require large propagation lengths to achieve the desired phase shifts, metasurfaces mitigate this size constraint but often introduce narrow operational bandwidths and high fabrication complexities. To bridge this gap, we introduce MoOCl2 as a promising material for ultracompact and broadband phase retardation. Building on its giant optical anisotropy, we experimentally demonstrate MoOCl2 quarter-wave plates with thicknesses of 77 nm and 98 nm. These flakes exhibit achromatic quarter-wave retardation across broad visible (445 - 525 nm) and near-infrared (730 - 945 nm) spectral windows, surpassing the fundamental thickness and bandwidth limitations of both conventional optical materials and artificial nanostructures. Moreover, MoOCl2 waveplates demonstrate up to lambda/4500 retardance tolerance at central wavelengths. As a result, this study establishes MoOCl2 as a building block for ultracompact polarization optics.

physics.optics↗

arXiv2026

Scalable learning of macroscopic stochastic dynamics

Macroscopic dynamical descriptions of complex physical systems are crucial for understanding and controlling material behavior. With the growing availability of data and compute, machine learning has become a promising alternative to first-principles methods to build accurate macroscopic models from microscopic trajectory simulations. However, for spatially extended systems, direct simulations of sufficiently large microscopic systems that inform macroscopic behavior is prohibitive. In this work, we propose a framework that learns the macroscopic dynamics of large stochastic microscopic systems using only small-system simulations. Our framework employs a partial evolution scheme to generate training data pairs by evolving large-system snapshots within local patches. We subsequently identify the closure variables associated with the macroscopic observables and learn the macroscopic dynamics using a custom loss. Furthermore, we introduce a hierarchical upsampling scheme that enables efficient generation of large-system snapshots from small-system trajectory distributions. We empirically demonstrate the accuracy and robustness of our framework through a variety of stochastic spatially extended systems, including those described by stochastic partial differential equations, idealised lattice spin systems, and a more realistic NbMoTa alloy system.

physics.comp-ph↗

arXiv2026

Surface defects in carbon-doped hexagonal boron nitride for negative-contrast direct laser writing

Radiative defects in hexagonal boron nitride (hBN) are active in a broad spectral range from deep ultraviolet to near-infrared wavelengths. Representatives of these defects act as bright single photon sources, spin-1 systems, and multiproperty atomic-scale sensors. They are predominantly investigated in bulk hBN films, where defects are decoupled from surface and interfacial effects. Here, we demonstrate a novel class of surface defects optically active in the green/yellow visible spectral range, which exhibit photophysical properties distinct from their bulk counterparts. High-power resonant laser illumination quenched the emission from the ensemble of such defects, which was attributed to a light-driven structural reconfiguration. The quenched defects were found to recover their emissive capabilities via a thermal cycling process, revealing an activation energy of 24.5 meV for the structural transition. Alternatively, permanent quenching of the defects was triggered by surface chemistry, involving lithiation-enabled attachment of functional groups. These mechanisms were utilized to realize negative-contrast direct laser writing, designing arbitrary geometric emissive patterns on demand in a microscopic configuration. The surface-active radiative centers in hBN appear particularly attractive for exploring environmental sensitivity, surface science, and coupling to photonic structures or electronic devices by taking unique advantage of the two-dimensional characteristics of the host lattice.

physics.optics↗

arXiv2026

Quantum Hall Effect at 0.002T

Graphene enables precise carrier-density control via gating, making it an ideal platform for studying electronic interactions. However, sample inhomogeneities often limit access to the low-density regimes where these interactions dominate. Enhancing carrier mobility is therefore crucial for exploring fundamental properties and developing device applications. Here, we demonstrate a significant reduction in external inhomogeneity using a double-layer graphene architecture separated by an ultra-thin hexagonal boron nitride layer. Mutual screening between the layers reduces scattering from random Coulomb potentials, resulting in a quantum mobility exceeding. Shubnikov de-Haas oscillations emerge at magnetic fields below 1 mT, while integer quantum Hall features are observed at 0.002T. Furthermore, we identify a fractional quantum Hall plateau at a filling factor of at 2T. These results demonstrate the platform's suitability for investigating strongly correlated electronic phases in graphene-based heterostructures.

cond-mat.mes-hall↗

arXiv2025

Giant exciton binding energy in bulk CrCl3

Van der Waals (vdW) materials, with their unique combination of electronic, optical, and magnetic properties, are emerging as promising platforms for exploring excitonic phenomena. Thus far, the choice of materials with exceptional excitonic response has been limited to two-dimensional (2D) configurations of vdW materials. At the same time, large interlayer distance and the possibility to create a variety of heterostructures offers an opportunity to control the dielectric screening in van der Waals heterostructures and van der Waals 3D materials, thus engineering the excitonic properties. Here, we reveal that bulk vdW crystal CrCl3 answers this quest with a record exciton binding energy of 1.64 eV owing to a delicate interplay of quasi-2D electronic confinement and local magnetic correlations. We also suggest that the non-local magnetic correlations play an important role in the temperature dependence of photoluminescence intensity. Furthermore, we observe colossal binding energies in vdW crystals NbOCl2 (0.66 eV) and MoCl3 (0.35 eV) and formulate a universal exciton binding energy dependence on bandgap for 2D and 3D vdW materials. Hence, our findings establish a fundamental link between the layered structure of vdW materials and their excitonic properties.

cond-mat.mtrl-sci↗

arXiv2025

Dielectric permittivity tensor dynamics of in-plane hyperbolic van der Waals MoOCl2 and emergent chiral photonic applications

Van der Waals (vdW) crystals offer unique opportunities for modern nanophotonic applications owing to their intrinsic anisotropic nature. While most of them exhibit uniaxial anisotropy arising from weak out-of-plane vdW interaction, some of their representative families also exhibit an in-plane biaxial anisotropy. Among the latter, outstand vdW oxochlorides with in-plane axes of a different physical character (metallic or dielectric). Here, we present an accurate dynamics of dielectric permittivity tensor components of vdW MoOCl2 in the ultraviolet (UV) to visible (Vis) spectral region partly covering near-infrared (NIR). Addressing its enormously anisotropic optical constants, we focus on another hyperbolicity window of vdW MoOCl2 emerging in the UV spectral region that may potentially unlock rich light-matter interaction effects. Furthermore, we propose an approach towards designing nanoscale handedness preserved Vis light circular polarizers based on twisted helical vdW MoOCl2 heterostructures. Our findings display that vdW MoOCl2 provides a highly promising platform not only for hyperbolic, but also for chiral nanophotonic applications.

physics.optics↗

arXiv2025

Giant optical anisotropy and visible-frequency epsilon-near-zero in hyperbolic van der Waals MoOCl2

The realization of extreme optical anisotropy is foundational to nanoscale light manipulation. Van der Waals (vdW) crystal MoOCl2 has emerged as a promising candidate for this quest, hosting hyperbolic plasmon polaritons in the visible and near-infrared wavelengths. However, the fundamental anisotropic dielectric tensor governing this behavior has remained elusive. Here, we resolve this problem by providing the first experimental determination of the full dielectric tensor of hyperbolic vdW MoOCl2. Via spectroscopic ellipsometry, Mueller matrix, and reflectance measurements, we quantify the material's optical duality: a metallic optical response (ε_1 < 0) along the crystallographic a-axis and a dielectric response (ε_1 > 0) along the orthogonal directions. This dichotomy drives an epsilon-near-zero (ENZ) condition at \approx 512 nm and results in giant in-plane birefringence of δn \approx 2.2 for MoOCl2. As a result, our work provides the critical missing experimental parameters for MoOCl2, establishing it as a benchmark hyperbolic and ENZ material.

physics.optics↗

arXiv2025

Chemical vapor deposition growth of continuous monolayer antiferromagnetic CrOCl films

The discovery of two-dimensional magnetic materials has provided an ideal platform for exploring physical phenomena in the two-dimensional limit. However, intrinsic two-dimensional antiferromagnetic materials have been rarely reported, limiting systematic studies of their electronic properties. The discovery of novel intrinsic two-dimensional antiferromagnets and the development of robust synthesis strategies, therefore, remain significant challenges. Here, we report the chemical vapor deposition synthesis of CrOCl monolayer films and nanosheets that exhibit excellent air stability. The CrOCl morphology is tunable, ranging from two-dimensional nanosheets to three-dimensional flower-like structures, with lateral sizes ranging from several microns to continuous monolayer films. Structural characterization confirms the materials composition and high crystalline quality. Furthermore, magnetic measurements, supported by theoretical calculations, reveal a Néel temperature for CrOCl of ~14 K. This work provides a reliable route for preparing two-dimensional antiferromagnetic materials.

cond-mat.mes-hall↗

arXiv2025

Probing the features of electron dispersion by tunneling between slightly twisted bilayer graphene sheets

Tunneling conductance between two bilayer graphene (BLG) sheets separated by 2 nm-thick insulating barrier was measured in two devices with the twist angles between BLGs less than 1°. At small bias voltages, the tunneling occurs with conservation of energy and momentum at the points of intersection between two relatively shifted Fermi circles. Here, we experimentally found and theoretically described signatures of electron-hole asymmetric band structure of BLG: since holes are heavier, the tunneling conductance is enhanced at the hole doping due to the higher density of states. Another key feature of BLG that we explore is gap opening in a vertical electric field with a strong polarization of electron wave function at van Hove singularities near the gap edges. This polarization, by shifting electron wave function in one BLG closer to or father from the other BLG, gives rise to asymmetric tunneling resonances in the conductance around charge neutrality points, which result in strong sensitivity of the tunneling current to minor changes of the gate voltages. The observed phenomena are reproduced by our theoretical model taking into account electrostatics of the dual-gated structure, quantum capacitance effects, and self-consistent gap openings in both BLGs.

cond-mat.mes-hall↗

arXiv2025

Structure and flow of low-dimensional water

Water, a subject of human fascination for millennia, is likely the most studied substance on Earth, with an entire scientific field -- hydrodynamics -- dedicated to understanding water in motion. However, when water flows through one-dimensional or two-dimensional channels, its behavior deviates substantially from the principles of hydrodynamics. This is because reducing the dimensionality of any interacting physical system amplifies interaction effects that are beyond the reach of traditional hydrodynamic equations. In low-dimensional water, hydrogen bonds can become stable enough to arrange water molecules into an ordered state, causing water to behave not only like a liquid but also like a solid in certain respects. In this review, we explore the relationship between water's ordering and its ability to flow in low-dimensional channels, using viscosities of bulk water, vapor, and ice as benchmarks. We also provide a brief overview of the key theoretical approaches available for such analyses and discuss ionic transport, which is heavily influenced by the molecular structure of water.

cond-mat.soft↗

arXiv2025

Modeling crystal defects using defect-informed neural networks

Most AI-for-Materials research to date has focused on ideal crystals, whereas real-world materials inevitably contain defects that play a critical role in modern functional technologies. The defects break geometric symmetry and increase interaction complexity, posing particular challenges for traditional ML models. Here, we introduce Defect-Informed Equivariant Graph Neural Network (DefiNet), a model specifically designed to accurately capture defect-related interactions and geometric configurations in point-defect structures. DefiNet achieves near-DFT-level structural predictions in milliseconds using a single GPU. To validate its accuracy, we perform DFT relaxations using DefiNet-predicted structures as initial configurations and measure the residual ionic steps. For most defect structures, regardless of defect complexity or system size, only 3 ionic steps are required to reach the DFT-level ground state. Finally, comparisons with scanning transmission electron microscopy (STEM) images confirm DefiNet's scalability and extrapolation beyond point defects, positioning it as a valuable tool for defect-focused materials research.

cond-mat.mtrl-sci↗

arXiv2025

Stacking-induced ferroelectricity in tetralayer graphene

Recent studies have reported emergent ferroelectric behavior in twisted or moiré-engineered graphene-based van der Waals heterostructures, yet the microscopic origin of this effect remains under debate. Pristine mono- or few-layer graphene lacks a permanent dipole due to its centrosymmetric lattice, making the emergence of ferroelectricity unlikely. However, mixed-stacked graphene, such as the ABCB tetralayer configuration, breaks both inversion and mirror symmetry and has been theoretically predicted to support electrically switchable dipoles. ABCB graphene represents the simplest natural graphene polytype exhibiting intrinsic out-of-plane polarization, arising from asymmetric charge carrier distribution across its layers. Here, we report robust ferroelectric behavior in dual-gated, non-aligned ABCB tetralayer graphene encapsulated in hexagonal boron nitride. The device exhibits pronounced hysteresis in resistance under both top and bottom gate modulation, with the effect persisting up to room temperature. This hysteresis originates from reversible layer-polarized charge reordering, driven by gate-induced transitions between ABCB and BCBA stacking configurations -- without requiring moiré superlattices. Our findings establish stacking-order-induced symmetry breaking as a fundamental route to electronic ferroelectricity in graphene and open pathways for non-volatile memory applications based on naturally occurring mixed-stacked multilayer graphene.

cond-mat.mes-hall↗

arXiv2025

Multi-property directed generative design of inorganic materials through Wyckoff-augmented transfer learning

Accelerated materials discovery is an urgent demand to drive advancements in fields such as energy conversion, storage, and catalysis. Property-directed generative design has emerged as a transformative approach for rapidly discovering new functional inorganic materials with multiple desired properties within vast and complex search spaces. However, this approach faces two primary challenges: data scarcity for functional properties and the multi-objective optimization required to balance competing tasks. Here, we present a multi-property-directed generative framework designed to overcome these limitations and enhance site symmetry-compliant crystal generation beyond P1 (translational) symmetry. By incorporating Wyckoff-position-based data augmentation and transfer learning, our framework effectively handles sparse and small functional datasets, enabling the generation of new stable materials simultaneously conditioned on targeted space group, band gap, and formation energy. Using this approach, we identified previously unknown thermodynamically and lattice-dynamically stable semiconductors in tetragonal, trigonal, and cubic systems, with bandgaps ranging from 0.13 to 2.20 eV, as validated by density functional theory (DFT) calculations. Additionally, we assessed their thermoelectric descriptors using DFT, indicating their potential suitability for thermoelectric applications. We believe our integrated framework represents a significant step forward in generative design of inorganic materials.

cond-mat.mtrl-sci↗

arXiv2025

Dynamics of single Au nanoparticles on graphene simultaneously in real- and diffraction space by time-series convergent beam electron diffraction

Convergent beam electron diffraction (CBED) on two-dimensional materials allows simultaneous recording of the real-space image (tens of nanometers in size) and diffraction pattern of the same sample in one single-shot intensity measurement. In this study, we employ time-series CBED to visualize single Au nanoparticles deposited on graphene. The real-space image of the probed region, with the amount, size, and positions of single Au nanoparticles, is directly observed in the zero-order CBED disk, while the atomic arrangement of the Au nanoparticles is available from the intensity distributions in the higher-order CBED disks. From the time-series CBED patterns, the movement of a single Au nanoparticle with rotation up to 4° was recorded. We also observed facet diffraction lines - intense bright lines formed between the CBED disks of the Au nanoparticle, which we explain by diffraction at the Au nanoparticle's facets. This work showcases CBED as a useful technique for studying adsorbates on graphene using Au nanoparticles as a model platform, and paves the way for future studies of different objects deposited on graphene.

cond-mat.other↗

arXiv2025

Quantifying hydrogen bonding using electrically tunable nanoconfined water

Hydrogen bonding plays a crucial role in biology and technology, yet it remains poorly understood and quantified despite its fundamental importance. Traditional models, which describe hydrogen bonds as electrostatic interactions between electropositive hydrogen and electronegative acceptors, fail to quantitatively capture bond strength, directionality, or cooperativity, and cannot predict the properties of complex hydrogen-bonded materials. Here, we introduce a novel approach that conceptualizes the effect of hydrogen bonds as elastic dipoles in an electric field, which captures a wide range of hydrogen bonding phenomena in various water systems. Using gypsum, a hydrogen bond heterostructure with two-dimensional structural crystalline water, we calibrate the hydrogen bond strength through an externally applied electric field. We show that our approach quantifies the strength of hydrogen bonds directly from spectroscopic measurements and reproduces a wide range of key properties of confined water reported in the literature. Using only the stretching vibration frequency of confined water, we can predict hydrogen bond strength, local electric field, O-H bond length, and dipole moment. Our work also introduces hydrogen bond heterostructures - a new class of electrically and chemically tunable materials that offer stronger, more directional bonding compared to van der Waals heterostructures, with potential applications in areas such as catalysis, separation, and energy storage.

cond-mat.mes-hall↗

arXiv2025

MoSi$_2$N$_4$-like crystals -- the new family of two-dimensional materials

Recently-synthesised MoSi$_2$N$_4$ is the first septuple layer two-dimensional material, which doesn't naturally occurs as a layered crystal, and has been obtained with CVD growth. It can be considered as MoN$_2$ crystal (with a crystal structure of MoS2) intercalating Si2N2 two-dimensional layer (with the structure similar to InSe). Such classification gave rise to the understanding of the electronic properties of the material, but also to the prediction of other members of the family (many dozens of them) as well as to the way to classify those. Whereas the originally-synthesised MoSi$_2$N$_4$ is a semiconductor, some of the members of the family are also metallic and some even demonstrate magnetic properties. Interestingly, the room-temperature mobility predicted for such crystals can be as high few thousands cm$^2$/V*s (hole mobility typically higher than electron) with some record cases as high as $5\times 10^4$ cm$^2$/V*s, making these materials strong contenders for future electronic applications. The major interest towards these materials is coming from the septuple layer structure, which allows multiple crystal phases, but also complex compositions, in particular those with broken mirror-reflection symmetry against the layer of metal atoms.

cond-mat.mes-hall↗

arXiv2024

Limiting performance of graphene bilayer sub-terahertz detectors at large induced band gap

Electrically induced $p-n$ junctions in graphene bilayer (GBL) have shown superior performance for detection of sub-THz radiation at cryogenic temperatures, especially upon electrical induction of the band gap $E_g$. Still, the upper limits of responsivity and noise equivalent power (NEP) at very large $E_g$ remained unknown. Here, we study the cryogenic performance of GBL detectors at $f=0.13$ THz by inducing gaps up to $E_g \approx 90$ meV, a value close to the limits observed in recent transport experiments. High value of the gap is achieved by using high-$κ$ bottom hafnium dioxide gate dielectric. The voltage responsivity, current responsivity and NEP optimized with respect to doping do not demonstrate saturation with gap induction up to its maximum values. The NEP demonstrates an order-of-magnitude drop from $\sim450$ fW/Hz$^{1/2}$ in the gapless state to $\sim30$ fW/Hz$^{1/2}$ at the largest gap. At largest induced band gaps, plasmonic oscillations of responsivity become visible and important for optimization of sub-THz response.

cond-mat.mes-hall↗

arXiv2024

Hysteresis-controlled Van der Waals tunneling infrared detector enabled by selective layer heating

Mid-infrared (mid-IR) photodetectors play a crucial role in various applications, including the development of biomimetic vision systems that emulate neuronal function. However, current mid-IR photodetector technologies are limited by their cost and efficiency. In this work, we demonstrate a new type of photodetector based on a tunnel structure made of two-dimensional materials. The effect manifests when the upper and lower layers of the tunnel structure are heated differently. The photoswitching is threshold-based and represents a ``jump'' in voltage to another branch of the current-voltage characteristic when illuminated at a given current. This mechanism provides enormous photovoltage (0.05$-$1~V) even under weak illumination. Our photodetector has built-in nonlinearity and is therefore an ideal candidate for use in infrared vision neurons. Additionally, using this structure, we demonstrated the possibility of selective heating of layers in a van der Waals stack using mid-IR illumination. This method will allow the study of heat transfer processes between layers of van der Waals structures, opening new avenues in the physics of phonon interactions.

cond-mat.mes-hall↗