CyberSec Research

Understanding the emergence of unconventional superconductivity, where the order parameter deviates from simple isotropic s-wave pairing, is a central puzzle in condensed matter physics. Transition-metal dichalcogenides (TMDCs), though generally regarded as conventional superconductors, display signatures of this unusual behavior and thus provide a particularly intriguing platform to explore how exotic states arise. Here we investigate the misfit compound (SnS)$_{1.15}$(TaS$_2$), a heterostructure composed of alternating SnS and 1H-TaS$_2$ layers. Using transport, photoemission, and scanning tunneling spectroscopy, we demonstrate that the SnS layers effectively decouple the TaS$_2$ into electronically isolated 1H sheets. In this limit, the tunneling density of states reveals a clear two-gap superconducting spectrum with T$_c \sim$ 3.1 K. A theoretical model based on lack of inversion symmetry and finite-range attraction reproduces the observed multi-gap structure as a mixed singlet-triplet state. These results establish misfit compounds as a powerful platform for studying unconventional superconductivity in isolated 1H layers and for realizing multiple uncoupled superconductors within a single crystal.

Strain provides a powerful knob to tailor the electronic properties of semiconductors. Simple yet accurate approximations that capture strain effects in demanding simulations of mesoscopic nanostructures are therefore highly desirable. However, for III-V compounds, key materials for quantum applications, such approaches remain comparatively underdeveloped. In this work, we derive a compact, effective Hamiltonian that describes the conduction band of zincblende III-V semiconductors incorporating strain effects. Starting from the eight-band k$\cdot$p model with Bir-Pikus corrections, we perform a folding-down procedure to obtain analytical expressions for conduction-band strain-renormalized parameters, including the effective mass, chemical potential, spin-orbit coupling, and $g$-factor. The model reproduces full multiband results under small to moderate strain, while retaining a form suitable for device-scale calculations. We benchmark the model for bulk deformations and apply it to representative nanostructures, such as core.shell nanowires and planar heterostructures. Our results provide a practical and versatile tool for incorporating strain into the design of III-V semiconductor devices, enabling reliable predictions of their properties with direct implications for spintronic, straintronic, optoelectronic, and topological quantum technologies.

Two-dimensional (2D) materials with large spin Hall effect (SHE) have attracted significant attention due to their potential applications in next-generation spintronic devices. In this work, we perform high-throughput (HTP) calculations to obtain the spin Hall conductivity (SHC) of 4486 non-magnetic compounds in the \texttt{2Dmatpedia} database and identify six materials with SHC exceeding $500\,(\hbar/e)\,(\mathrm{S/cm})$, surpassing those of previously known materials. Detailed analysis reveals that the significant SHC can be attributed to spin-orbit coupling (SOC)-induced gap openings at Dirac-like band crossings. Additionally, the presence of mirror symmetry further enhances the SHC. Beyond the high-SHC materials, 57 topological insulators with quantized SHCs have also been identified. Our work enables rapid screening and paves the way for experimental validation, potentially accelerating the discovery of novel 2D materials optimized for spintronics applications.

Using \textit{ab initio} methodology, we reveal a strain-mediated approach to precisely tune the magnetoelectric coupling and spin-driven emergent polarization of NiX$_2$ (X = I, Br) monolayers. In the absence of strain, these systems spontaneously stabilize non-collinear spin states that break the inversion symmetry, inducing a ferroelectric polarization in the plane of the material. We show that biaxial and uniaxial strains broadly modulate the magnetoelectric response in these materials through two distinct mechanisms: (i) direct modification of the magnetoelectric tensor components, and (ii) tuning of the characteristic propagation vectors of a spin texture. This dual mechanism enables precise control over the magnitude of the spin-induced electric polarization of these materials. With respect to the achievable magnitude of the electric polarization, we demonstrate the critical role of third-nearest-neighbor spin-pair contributions, which can increase under strain to levels that compete with or even exceed the polarization driven by first-nearest-neighbor effects. These findings offer important insights into low-dimensional piezo-magnetoelectricity and expand the possibilities for designing multifunctional two-dimensional straintronic devices.

Silver chromate ($\mathrm{Ag_{2}CrO_{4}}$) has attracted considerable attention in recent years due to its promising photocatalytic performance, which strongly depends on the crystallographic orientation of its exposed surfaces. A detailed understanding of the structural stability of these surfaces under realistic conditions is therefore essential for advancing its applications. In this work, we combine density functional theory (DFT) with first-principles atomistic thermodynamics to systematically investigate the stability of multiple surface orientations and terminations of $\mathrm{Ag_{2}CrO_{4}}$. The surface Gibbs free energy was evaluated as a function of oxygen and silver chemical potentials, enabling the construction of stability trends under non-vacuum environments. Our results reveal that the degree of coordination of surface chromium-oxygen clusters plays a decisive role in determining surface stability. Furthermore, Wulff constructions predict morphology evolution as a function of external conditions, allowing us to identify the atomic structures of the exposed facets in the equilibrium crystal shape. These insights provide a fundamental framework for understanding surface-dependent photocatalytic activity in $\mathrm{Ag_{2}CrO_{4}}$ and related silver-based oxides.

Electro-viscoelastic theory for polymer melts has been extensively studied experimentally for the past century, primarily for manufacturing purposes. However, the modeling and theory for this have been minimal, leaving many questions on the mechanisms and behavior of an arbitrary flow scheme. To remedy this, previously solved overdamped Langevin equations for the Doi-Rouse model are modified to include charge and electric field potential forces. The charge sequence on the chain is hypothesized to be a cosine sequence along the chain, resembling multiple electric dipoles that conveniently correspond to a Rouse mode of the chain. These are then solved for the shear stress under homogeneous shear rates and electric fields to find directional viscosity increases depending on the shearing and electric field orientation. Using the newly derived shear stress from the Doi-Rouse approach, a continuum model is proposed that resembles a modified upper-convected Maxwell model, including polarization stresses in terms of an electric field dyadic. This new continuum model, named the upper-convected electro-Maxwell model, is verified using Kremer-Grest polymer chains simulated with molecular dynamics for multiple flow schemes and a specified charge sequence along the chain. Furthermore, the MD results verified the difference in the overall and charge sequence relaxation times through the shear and normal stress polarizations, showing the necessity for the upper-convected derivative of the electric field dyadic to correct the viscosity scaling. Finally, the dynamic properties of the polarized polymer melt are examined analytically, finding that the phase shift is unaffected by the electric field contribution.

Lattice relaxation profoundly reshapes electronic structures in twisted materials. Prevailing treatments, however, typically rely on large-scale density functional theory (DFT), which is computationally costly and mechanistically opaque. Here, we develop a unified analytical framework to overcome these limitations. From continuum elastic theory, we derive closed-form solutions for both in-plane and out-of-plane relaxation fields. We further introduce an analytical phase factor expansion theory that maps relaxation into the electronic Hamiltonian. By applying this framework, the relaxation-mediated single-particle and many-body topological phase transitions in twisted MoTe$_{2}$ is accurately captured, and the evolution of flat bands in magic-angle graphene is quantitatively reproduced. Our work transforms the research of moir\'e relaxation from black-box numerical fitting to an analytical paradigm, offering fundamental insights, exceptional efficiency, and general applicability to a wide range of twisted materials.

Tip-enhanced Raman spectroscopy (TERS) is a powerful method for imaging vibrational motion and chemically characterizing surface-bound systems. Theoretical simulations of TERS images often consider systems in isolation, ignoring any substrate support, such as metallic surfaces. Here, we show that this omission leads to deviations from experimentally measured data through simulations with a new finite-field formulation of first-principles simulation of TERS spectra that can address extended, periodic systems. We show that TERS images of tetracyanoethylene on Ag(100) and defective MoS$_2$ monolayers calculated using isolated molecules or cluster models are qualitatively different from those calculated when accounting for the periodicity of the substrate. For Mg(II)-porphine on Ag(100), a system for which a direct experimental comparison is possible, these simulations prove to be crucial for explaining the spatial variation of TERS intensity patterns and allow us to uncover fundamental principles of TERS spectroscopy. We explain how and why surface interactions affect images of out-of-plane vibrational modes much more than those of in-plane modes, providing an important tool for the future interpretation of these images in more complex systems.

Boron-doped diamond crystals (BDD, C$_{1-x}$B$_{x}$) exhibit exceptional mechanical strength, electronic tunability, and resistance to radiation damage. This makes them promising materials for use in gamma-ray crystal-based light sources. To better understand and quantify the structural distortions introduced by doping, which are critical for maintaining channelling efficiency, we perform atomistic-level molecular dynamics simulations on periodic C$_{1-x}$B$_{x}$ systems of various sizes. These simulations allow the influence of boron concentration on the lattice constant and the (110) and (100) inter-planar distances to be evaluated over the concentration range from pure diamond (0%) to 5% boron at room temperature (300 K). Linear relationships between both lattice constant and inter-planar distance with increasing dopant concentration are observed, with a deviation from Vegard's Law. This deviation is larger than that reported by other theoretical and computational studies; however, this may be attributed to an enhanced crystal quality over these studies, a vital aspect when considering gamma-ray crystal light source design. The methodology presented here incorporates several refinements to closely reflect the conditions of microwave plasma chemical vapour deposition (MPCVD) crystal growth. Validation of the methodology is provided through a comprehensive statistical analysis of the structure of our generated crystals. These results enable reliable atomistic modelling of doped diamond crystals and support their use in the design and fabrication of periodically bent structures for next-generation gamma-ray light source technologies.

The persistence of ferroelectricity in ultrathin HfO$_2$ films challenges conventional theories, particularly given the paradoxical observation that the out-of-plane lattice spacing increases with decreasing thickness. We resolve this puzzle by revealing that this anomalous lattice expansion is counterintuitively coupled to suppressed out-of-plane polarization. First-principles calculations combined with analytical modeling identify two mechanisms behind this expansion: a negative longitudinal piezoelectric response to the residual depolarization field and a positive surface stress that becomes significant at reduced thickness. Their interplay quantitatively reproduces the experimentally observed lattice expansion. Furthermore, (111)-oriented HfO$_2$ films can support out-of-plane polarization even under open-circuit conditions, in contrast to (001) films that stabilize a nonpolar ground state. This behavior points to the emergence of orientation-induced hyperferroelectricity, an unrecognized mechanism that enables polarization persistence through orientation engineering without electrode screening. The principle also extends to perovskite ferroelectrics, offering a strategy to eliminate critical thickness limits by selecting appropriate film orientations.

La-Co-Ni oxides were fabricated in the form of thin-film materials libraries by combinatorial reactive co-sputtering and analyzed for structural and functional properties over large compositional ranges: normalized to the metals of the film they span about 0 - 70 at.-% for Co, 18 - 81 at.-% for La and 11 - 25 at.-% for Ni. Composition-dependent phase analysis shows formation of three areas with different phase constitutions in dependance of Co-content: In the La-rich region with low Co content, a mixture of the phases La2O3, perovskite, and La(OH)3 is observed. In the Co-rich region, perovskite and spinel phases form. Between the three-phase region and the Co-rich two-phase region, a single-phase perovskite region emerges. Surface microstructure analysis shows formation of additional crystallites on the surface in the two-phase area, which become more numerous with increasing Ni-content. Energy-dispersive X-ray analysis indicates that these crystallites mainly contain Co and Ni, so they could be spinels growing on the surface. The analysis of the oxygen evolution reaction (OER) electrocatalytic activity over all compositions and phase constitutions reveals that the perovskite/spinel two-phase region shows the highest catalytic activity, which increases with higher Ni-content. The highest OER current density was measured as 2.24 mA/cm2 at 1.8 V vs. RHE for the composition La11Co20Ni9O60.

This article explores the recent advancements in atomically thin two-dimensional transition metal dichalcogenides (2D TMDs) and their potential applications in various fields, including nanoelectronics, photonics, sensing, energy storage, and optoelectronics. Specifically, the focus is on TMDs such as MoS2, WS2, MoSe2, and WSe2, promising for next-generation electronics and optoelectronics devices based on ultra-thin atomic layers. One of the main challenges in utilising TMDs for practical applications is the scalable production of defect-free materials on desired substrates. However, innovative growth strategies have been developed to address this issue and meet the growing demand for high-quality and controllable TMD materials. These strategies are compatible with conventional and unconventional substrates, opening up new possibilities for practical implementation. Furthermore, the article highlights the development of novel 2D TMDs with unique functionalities and remarkable chemistry. These advancements contribute to expanding the range of applications and capabilities of TMD materials, pushing the boundaries of what can be achieved with these ultra-thin layers. In addition to electronics, the article delves into the significant efforts dedicated to exploring the potential of 2D TMDs in energy and sensor applications. These materials have shown promising characteristics for energy storage and have been extensively studied for their sensing capabilities, showcasing their versatility and potential impact in these fields. This article provides a comprehensive overview of the recent progress in 2D TMDs, emphasising their applications in electronics, optoelectronics, energy, and sensing. The continuous research and development in this area is promising for advancing these materials and their integration into practical devices and systems.

The search for high-performance spintronic materials motivates the exploration of Heusler alloys with unconventional electronic properties. Using density functional theory with Hubbard correction (DFT+$U$, $U = 4$~eV), we investigate X$_2$MnGa (X = Ti, Ir) alloys, which stabilize in the ferromagnetic L2$_1$-type structure with strong thermodynamic stability. Electronic structure calculations reveal contrasting behaviors: Ti$_2$MnGa transitions from a metallic L2$_1$-type phase to a spin gapless semiconductor (SGS) in the XA-type, while Ir$_2$MnGa exhibits gapless half-metallicity behavior in the L2$_1$-type but becomes half-metallic in the XA-type. The magnetic properties are governed by spd hybridization between Mn-3$d$ and X-$d$/Ga-$p$ states, which stabilizes ferromagnetism and tailors electronic states near the Fermi level. The Hubbard $U$ correction proves essential for accurately describing the correlated Mn-3$d$ electrons. These alloys combine structural stability with tunable electronic and magnetic properties, offering a promising platform for spin-polarized transport in next-generation spintronic devices.

Crystals and other condensed phases are defined primarily by their inherent symmetries, which play a crucial role in dictating their structural properties. In crystallization studies, local order parameters (OPs) that describe bond orientational order are widely employed to investigate crystal formation. Despite their utility, these traditional metrics do not directly quantify symmetry, an important aspect for understanding the development of order during crystallization. To address this gap, we introduce a new set of OPs, called Point Group Order Parameters (PGOPs), designed to continuously quantify point group symmetry order. We demonstrate the strength and utility of PGOP in detecting order across different crystalline systems and compare its performance to commonly used bond-orientational order metrics. PGOP calculations for all non-infinite point groups are implemented in the open-source package SPATULA (Symmetry Pattern Analysis Toolkit for Understanding Local Arrangements), written in parallelized C++ with a Python interface. The code is publicly available on GitHub.

Atomic-scale phase-field modeling formulates the probability densities of atomic vibrations as Gaussian distributions and derives a free energy functional using variational Gaussian theory and interatomic potentials. This framework permits per-Gaussian decomposition of the free energy, providing a description of local thermodynamic states with atomic resolution. However, existing formulations are limited to classical pairwise interatomic potentials, restricting their applicability to specific materials and compromising quantitative accuracy. In this work, we extend the atomic-scale phase-field methodology by incorporating universal machine learning interatomic potentials, thereby generalizing the free energy functional to many-body systems. This extension enhances both the accuracy and transferability of the approach. We demonstrate the method by applying it to bulk copper under NVT and NPT ensembles, where the predicted pressures and equilibrium lattice constants show excellent agreement with molecular dynamics simulations, validating the theoretical framework. Furthermore, we apply the method to {\Sigma}5(310)[001] grain boundaries in copper, enabling the visualization of local free energy distributions with atomic-scale resolution. The results reveal a pronounced free energy concentration at the grain boundary core, capturing the thermodynamic signature of the interface. This study establishes a versatile and accurate framework for atomic-scale thermodynamic modeling, significantly broadening the scope of phase-field approaches to include complex materials and defect structures.

Recent advancements have led to the development of bright and heavy metal-free blue-emitting quantum dot light-emitting diodes (QLEDs). However, consensus understanding of their distinct photophysical and electroluminescent dynamics remains elusive. This work correlates the chemical and electronic changes occurring in a QLED during operation using depth-resolved and operando techniques. The results indicate that oxygen vacancy forms in the ZnMgO layer during operation, with important implications on the charge injection and electrochemical dynamics. Taken together, the results suggest a causal relationship between oxygen vacancy formation and operational degradation of the blue-emitting ZnSeTe-based QLEDs.

Heavy metal-free quantum-dot light-emitting devices (QD-LEDs) have demonstrated remarkable brightness, saturated color, and high efficiencies across a broad spectral range. However, in contrast to organic LEDs (OLEDs), QD-LED operational lifetimes remain limited, with the underlying degradation mechanisms not fully understood. In the present study, we show that InP/ZnSe/ZnS (red-emitting) and ZnTeSe/ZnSe/ZnS (blue-emitting) cadmium-free colloidal QD-LEDs undergo nanoscale morphological changes during operation. Specifically,interparticle coarsening and layer thinning are observed in the electron transport layer (ETL) consisting of ZnMgO nanoparticles (NPs), in the QD emissive layer, and in the organic hole transport layer. This is accompanied by the generation and diffusion of compositional oxygen- and hydrogen-radicals throughout the device, with oxygen accumulating at the electrode/ETL interfance. Moreover, in situ transmission electron microscopy reveals the electron beam exposure, in the presence of hydrogen radicals, accelerates ZnMgO NPs coarsening. To mitigate these degradation pathway, we show that acrylate-based resin-encapsulation treatment stabilize the ETL/QD layers by suppressing the radical formation and halting morphology changes. This approach achieves dramatic stability enhancements, exhibits an 8-fold and 5000-fold lifetime improvement on InP/ZnSe/ZnS and ZnTeSe/ZnSe/ZnS QD-LEDs, respectively. Our findings establish the causal relationships between the morphological degradation, interlayer radical dynamics, and state-of-the-art QD-LEDs instability, providing new insights into a scalable encapsulation treatment that enables efficient and long-lived Cd-free QD-LEDs.

Two-dimensional (2D) semiconductors show great potential to sustain Moore's law in the era of ultra-scaled electronics. However, their scalable applications are severely constrained by low hole mobility. In this work, we take 2D-GaAs as a prototype of III-V semiconductors to investigate the effects of quantum anharmonicity (QA) on hole transport, employing the stochastic self-consistent harmonic approximation assisted by the machine learning potential. It is found that the room-temperature hole mobility of 2D-GaAs is reduced by $\sim$44% as the QA effects are incorporated, which is attributed to the enhanced electron-phonon scattering from the out-of-plane acoustic polarization. The valence band edge shift (VBES) strategy is proposed to increase the hole mobility by $\sim$1600% at room temperature, which can be realized by 1% biaxial compressive strain. The electron-phonon scattering rate is dramatically decreased due to the full filtering of the original interband electron-phonon scattering channels that existed in the flat hole pocket. The VBES strategy can be further extended to other 2D III-V semiconductors to promote their hole mobilities.