Lithium niobate-on-insulator (LNOI) is an emerging photonic platform with high potential for scalable quantum information processing due to its strong second-order nonlinearity. However, little progress has been made in developing on-chip single-photon detectors on LNOI. Niobium titanium nitride (NbTiN) superconducting nanowire single-photon detectors (SNSPDs) are a promising candidate for this application. In this work, we use DC reactive magnetron sputtering to grow high-quality NbTiN thin films using an ultrahigh vacuum deposition system with a base pressure lower than $2\times {10}^{-10}$ mbar. Enabled by the low concentration of background impurities in this system, we investigate the impact of substrate temperature during NbTiN growth. We achieve four nm thick superconducting films with a critical temperature ($T_c$) of 12.3 K grown at a substrate temperature of 825 K. We find that the NbTiN films grow in the (111) orientation and evolve from a porous pillar structure when grown at low temperatures to densely packed fibrous grains at higher temperatures. Furthermore, we demonstrate that the increased substrate temperature reduces the oxygen concentration in our films and improves the overall stoichiometry. In addition, we integrate these films with the LNOI platform and investigate the obtained interface quality. Lastly, we fabricate SNSPDs from the NbTiN film on LNOI and characterize the detector performance.

GeTe-based degenerate semiconductors offer a promising platform for thermoelectric optimization owing to their responsive electronic structure and intrinsically low lattice thermal conductivity. However, neither the pure rhombohedral nor the pure cubic GeTe phase offers optimal band degeneracy for maximizing the thermoelectric power factor. Here, we demonstrate that defect-engineered co-doping enables simultaneous control of crystal symmetry and thereby electronic band structure in GeTe. Cr substitution suppresses the rhombohedral phase fraction and induces valence band convergence, leading to an enhanced $m_{\text{DOS}}^{*}$ and nearly 35% increase in the power factor (53 $\mu \mathrm{W}{\text{cm}}^{-1}{\text{K}}^{-2}$) at 750 K, while excessive Cr (3.5%) destabilizes the rhombohedral phase. Temperature-dependent synchrotron x-ray diffraction further elucidates the role of Cr in lowering the onset temperature of cubic symmetry ($\sim 663$ K). A simple two-band phenomenological model incorporating the L–$\mathrm{\Sigma }$ valence-band offset offers a transparent framework to rationalize the contrasting thermopower evolution in GeTe as compared to 2.5% Cr doped GeTe. In contrast, Se substitution stabilizes the rhombohedral lattice and suppresses the bipolar activation of minority carriers. Specifically, 2% Cr, 5% Se co-doped GeTe exhibits significant suppression of bipolar transport in the high-temperature thermoelectric transport. Ultraviolet photoelectron spectroscopy reveals a deeper Fermi level and increased work function upon Se incorporation, consistent with an increased minority carrier activation energy. These results establish defect-engineered co-doping as an effective strategy to couple phase evolution with valence band convergence, enabling enhanced thermoelectric performance in GeTe across a broad temperature range.

The introduction of high-valence actinides, such as thorium and uranium, into molten alkali halide induces complex structural correlations and intermediate-range ordering, presenting a daunting challenge for computational materials science. Because these dynamic topological features span length scales and timescales that are inaccessible to traditional ab initio molecular dynamics, capturing their influence on macroscopic transport properties requires highly scalable, high-fidelity sampling. Here, we employ an uncertainty-driven active-learning framework to construct a robust machine-learned interatomic potential (MLIP) for the multicomponent ${\text{LiF-NaF-ThF}}_4$ ionic liquid. With the resulting MLIP, we performed 400 ps simulations of $100000$-atom systems at six temperatures spanning 973–1473 K with near–density functional theory accuracy. These large-scale simulations allowed us to compute key thermophysical, structural, and transport properties over a broad temperature range while substantially reducing the finite-size effects that limit the convergence of Green-Kubo transport coefficients in highly correlated liquids. Our simulations reveal that the solvation of ${\mathrm{Th}}^{4+}$ drives the formation of transient, medium-range (${\mathrm{ThF}}_n$) clusters. We demonstrate that this emergent structural inhomogeneity fundamentally alters the transport mechanisms of the melt: the bulky clusters impede ionic mobility and disrupt collective vibrational modes. This interplay between dynamic local structure and energy dissipation leads to an increased shear viscosity and a significantly reduced thermal conductivity relative to the pure eutectic salt. Collectively, these results elucidate the profound impact of dynamic structural heterogeneities on dissipative transport, underscoring MLIP-driven simulations as a necessary tool to achieve the spatiotemporal resolution required to study correlated alkali halide melts.

Rare-earth perovskite nickelates are prototypical strongly correlated oxides. The physical properties of these materials can be efficiently tuned via doping with small cations such as ${\mathrm{H}}^{+}$ or ${\mathrm{Li}}^{+}$. Hydrogenation induces reversible phase transitions in perovskite nickelates accompanied by a dramatic increase in electrical resistivity, enhanced visible-to-mid-infrared optical transmittance, lattice expansion, and suppression of magnetic order. These multi-field coupled responses provide a unique platform for exploring the cooperative evolution of charge, spin, orbital, and lattice degrees of freedom in strongly correlated systems, while simultaneously enabling applications in dynamic optical modulation, high-sensitivity chemical sensing, and neuromorphic computing. Despite significant progress, the microscopic mechanisms underlying electronic structure reconstruction during hydrogenation of perovskite nickelates remain incompletely understood, and a systematic overview of proton conduction kinetics and its control strategies is still lacking. This review focuses on hydrogenated perovskite nickelate thin films, summarizing the evolution of electronic structure induced by electron doping, the mechanisms and control of proton conduction, and recent advances in functional device applications, while highlighting current challenges and future perspectives.

To maximize the efficiency of tandem solar cells and simultaneously decrease their environmental impact, absorbers with a band gap around 1.7 eV and synthesized with earth-abundant elements are needed. In this work single-crystalline ZnMgN thin films alloys with Mg concentrations up to 55% are epitaxially grown on MgO (100) substrates by plasma-assisted molecular beam epitaxy. While the band gap can be tuned from 1.1 eV for pure ${\mathrm{Zn}}_3{\mathrm{N}}_2$ to 2.1 eV, corresponding to (${\mathrm{Zn}}_{0.45}{\mathrm{Mg}}_{0.55}{)}_3{\mathrm{N}}_2$, across the optimum value of around 1.7 eV for silicon tandem solar cells, their long-term stability in air remains an open issue. Voluntary incorporation of oxygen during the molecular beam epitaxy growth and formation of ZnMgNO alloys is demonstrated to solve the stability issue while at the same time enabling control of charge carrier concentration without degrading the thin films' mobility ($50{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$). A thorough EDX-STEM-HAADF analysis, coupled with an x-ray photoelectron spectroscopy profiling study, reveals the homogeneous incorporation of oxygen throughout the whole layer thickness and the preservation of crystalline quality, opening the door for exploiting earth-abundant ZnMgNO alloys in tandem solar cells.

Understanding and tuning exchange coupling which enables the control of the Curie temperature are still one of the major challenges in the field of two-dimensional magnets. In this article, using density functional theory calculations, we investigate the interlayer exchange coupling of two-dimensional $\mathrm{Cr}{\mathrm{X}}_3$ bilayers that have been intercalated with $3d$ transition metals (TMs). Our results show that the interlayer exchange coupling is greatly enhanced in the intercalated bilayers compared to the pristine bilayers due to additional ferromagnetic or antiferromagnetic couplings between the intercalated atoms and the pristine Cr atoms. Electronic structure analysis reveals that the magnitude of the exchange coupling is mainly determined by the transition energy between the electron-doped $e_g$ band of Cr and the narrow $d$-orbital bands of the TM. In addition, ferromagnetic and antiferromagnetic couplings with host Cr atoms are favored for less than half filled and more than half filled $d$ orbitals of the TM, respectively, because of the Pauli exclusion principle and Hund's rule. Our work provides not only a detailed study but also a strategy to tune the interlayer exchange coupling and to raise the Curie temperature of two-dimensional magnets.

Ferromagnetic insulators are uncommon due to the prevalence of antiferromagnetic ordering in insulating materials. In this work, we investigate a family of centrosymmetric $A_2{BX}_3$ compounds whose building blocks lack inversion symmetry; each transition metal is fivefold coordinated and forms chains of edge-sharing square pyramids in one dimension, resembling wires. Using density functional theory, we explore the structural, electronic, and magnetic properties of four compounds: ${\mathrm{K}}_2{\mathrm{VO}}_3, {\mathrm{K}}_2{\mathrm{VS}}_3, {\mathrm{Rb}}_2{\mathrm{VO}}_3$, and ${\mathrm{Rb}}_2{\mathrm{VS}}_3$. All four compounds were found to be ferromagnetic insulators with magnetizations of 1 ${\mu }_B$ per formula unit. K and Rb oxides and the Rb sulfide form nonpolar Cmcm ground states, while ${\mathrm{K}}_2{\mathrm{VS}}_3$ relaxes to an antipolar Pnma phase. Total energy differences and dynamical stability preclude the formation of polar metastable states. Electronic band structure calculations predict indirect band gaps of 3.2 eV for the oxides, while the sulfides are predicted to have direct band gaps of 0.9 eV. Bader and Born effective charge analyses revealed stronger covalent bonding in the sulfides and that the ${\mathrm{K}}^{+}$ and ${\mathrm{Rb}}^{+}$ ions have no electronic or magnetic influence on the wirelike square pyramid chains. Collectively, the electronic-structure, charge-density, spin-density, and exchange-coupling results establish the microscopic mechanism by which ligand-mediated V–$X$–V interactions stabilize robust ferromagnetic ordering in these ferromagnetic wires. We also demonstrate the likelihood of synthesis through thermodynamic and dynamical stability analyses for this set of predicted compounds. These results establish $A_2{BX}_3$ square-pyramidal frameworks as viable platforms for designing ferromagnetic insulating materials with tunable electronic structure.

In recent years, counterparts of phenomena studied in spintronics have been actively explored in the orbital sector. The relationship between orbital degrees of freedom and crystal chirality has also been intensively investigated, although the distinction from gyrotropic properties has not been fully clarified. In this work, we investigate spin and orbital Edelstein effects as well as the nonlinear responses in the ternary transition-metal chalcogenide ${\mathrm{Cu}}_2{\mathrm{WSe}}_4$, which has a gyrotropic but achiral crystal structure. We find that in the Edelstein effect, magnetization is dominated by the orbital contribution rather than the spin contribution. On the other hand, both the nonlinear chiral thermoelectric (NCTE) Hall effect—a response to the cross product of the electric field and the temperature gradient—and the nonlinear Hall effect—a conventional second-order response to the electric field—are found to be dominated by the Berry-curvature dipole. We further find that spin-orbit coupling plays only a minor role in these effects, whereas the orbital degrees of freedom are essential. Finally, we demonstrate that the orbital magnetic-moment contributions to both the Edelstein effect and the NCTE Hall effect are closely linked to chirality, and we discuss the possibility of using them as a chirality indicator.

We demonstrate that thin metallic glasses of ${\mathrm{V}}_x{\mathrm{Zr}}_{100-x}$, deposited by direct current magnetron sputtering, closely approximate the structural characteristics of theoretically predicted maximally amorphous materials. Experimentally, x-ray reflectometry reveals atomically flat films with well-defined thickness and low surface roughness, while Rutherford backscattering spectrometry confirms V/Zr stochiometry within 1 at.% and a compact, systematically varying density with composition. Transmission electron microscopy shows no evidence of columnar growth, consistent with fully amorphous and homogeneous films. Using ab initio stochastic quenching to generate representative maximally amorphous configurations, we find excellent agreement with experiment in terms of mass density and pair distribution functions. The calculated structures are mechanically stable, elastically isotropic, and exhibit high Poisson ratios and Pugh ratios indicating a characteristically ductile response. The combination of compactness and ductility suggests potential for applications in mechanically resilient coatings and for hydrogen storage, where deformation tolerance and structural integrity are critical. The agreement between simulation and experiment across structural and compositional descriptors supports the conclusion that sputtered V–Zr metallic glasses can be viewed as experimental realizations of statistically maximally amorphous states within the framework of the random valley approximation. We propose such structures can serve as well defined benchmarks for the maximal degree of amorphousness.

Bismuth-doped yttrium iron garnet (BiYIG) has attracted significant attention for magnonics and spintronics due to its perpendicular magnetic anisotropy and exceptionally low Gilbert damping. However, realizing BiYIG thin films with precisely tailored magnetic properties–including effective magnetization $\left(M_{\mathrm{eff}}\right)$, inhomogeneous linewidth broadening $\left({\mu }_0\mathrm{\Delta }{H}_0\right)$, and damping constant $\left(\alpha \right)$–requires careful optimization of the growth conditions. In this work, we report the growth and detailed characterization of BiYIG thin films deposited on (111)-oriented substituted gadolinium gallium garnet (sGGG) substrates by pulsed-laser deposition. High-quality films were obtained by varying the substrate temperature $(420-510{}^{\circ }\mathrm{C})$ at fixed laser fluence (40 mJ), and by varying the fluence (30–55 mJ) at fixed temperature $\left(495{}^{\circ }\mathrm{C}\right)$. For $\sim 20\mathrm{nm}$-thick films, we achieved stoichiometric BiYIG with in situ x-ray photoelectron spectroscopy, confirming the chemical integrity. Broadband ferromagnetic resonance measurements reveal an ultra-low damping constant of $\alpha \sim 1.1\times {10}^{-4}$, an effective magnetization of ${\mu }_0{M}_{\mathrm{eff}}\sim -7.1\mathrm{mT}$, and inhomogeneous broadening as low as ${\mu }_0\mathrm{\Delta }{H}_0\sim 0.9\mathrm{mT}$. In selected samples, magneto-optical imaging further confirms the presence of stable magnetic bubble domains. Overall, this work provides a practical methodology for reproducibly obtaining targeted magnetic properties in BiYIG thin films, including low damping, narrow linewidth, and tunable anisotropy, suitable for future magnonic and spintronic applications.

Vacancy-screw dislocation interactions in bcc Nb are studied with near density functional theory accuracy using a machine-learning potential, specifically a moment tensor potential (MTP), in large-scale atomistic simulations. The MTP predicts attractive vacancy-dislocation interactions with threefold symmetry in the (111) plane. These attractive interactions lead to a decrease in the Peierls stress by up to 28%, depending on the vacancy position relative to the dislocation. The interactions also lead to the formation of new complex topological defects (CTDs), when the vacancy is close to the dislocation core. A CTD can be geometrically characterized as a screw dislocation with multiple kinks and with a (partially) dissociated vacancy along the dislocation line. CTDs form under shear when the vacancy-dislocation distance is less than ∼6 $\AA$, providing a new mechanism for pinning screw dislocations. The formation of CTDs confirms theoretical predictions made in the early 1990s based on differential geometry and underscores the importance of machine-learning potentials and large-scale simulations for accurately describing the properties of extended defects in metals.

Ni-based catalysts are promising alternatives to Pt for the hydrogen evolution reaction (HER), and topological Ni-based materials are particularly appealing because their robust nontrivial surface states can enhance charge transfer and intermediate adsorption. Motivated by these advantages, our high-throughput computational screening of the latest Materials Project database identifies 46 Ni-based potential topological HER catalysts, exhibiting a Gibbs free energy change $|\mathrm{\Delta }{G}_{{\mathrm{H}}^{*}}|<$ 0.2 eV. Furthermore, we have conducted a detailed study of two representative candidates, ${\mathrm{Ni}}_3\mathrm{Ge}$ and NiBi, which show $|\mathrm{\Delta }{G}_{{\mathrm{H}}^{*}}|$ of 0.117 and 0.08 eV, respectively. By considering different surfaces and tuning the corresponding bulk topology via strain or doping, we show that catalytic performance scales with the density of topological surface states, which strongly influences charge transfer during the HER process. These findings establish a strong correlation between topological electronic structure and HER activity, providing a predictive descriptor and a general strategy for the design of efficient topological HER catalysts.

Understanding and overcoming the bottlenecks that limit carrier mobility in wide-band-gap III-nitrides is essential for advancing high-performance electronic and optoelectronic technologies. Here we employ state-of-the-art ab initio Boltzmann transport calculations, incorporating fully resolved electron-phonon interactions, to evaluate the carrier mobilities of GaN and AlN in their wurtzite, hexagonal, and haeckelite (4|8) phases. We identify a robust mobility ordering of haeckelite > wurtzite > hexagonal for both electrons and holes, with mobility enhancements exceeding 50–250% in the haeckelite phases. By analyzing mode- and energy-resolved scattering spectra, we demonstrate that this improvement originates from a symmetry-driven suppression of piezoelectric acoustic scattering when the noncentrosymmetric wurtzite lattice transforms into the centrosymmetric haeckelite structure. In the absence of piezoelectric coupling, acoustic scattering reduces to deformation-potential behavior, while optical scattering becomes the dominant channel. The resulting decrease in low-energy scattering rates produces markedly enhanced mobilities, particularly for holes, where traditional III-nitride materials have long faced intrinsic limitations. Our findings reveal phase engineering as an effective route to mitigating phonon-limited transport in III-nitrides and suggest haeckelite structures as promising candidates for next-generation high-mobility electronic and quantum devices.

Density functional theory (DFT) provides a powerful tool for describing the electronic properties of materials, however, self-interaction errors in semilocal functionals complicate the accurate modeling of correlated materials. The $\mathrm{DFT}+U$ method is a popular and computationally cost-effective solution for mitigating self-interaction; though, this reduces the ab initio aspect of DFT calculations, as the outcome now becomes dependent on the user's choice of $U$ parameters. Atomistic modeling of plutonium dioxide (${\mathrm{PuO}}_2$) is employed to provide insight into its evolution in storage or into its properties as mixed-oxide fuel. There is no single $U$ parameter that can reproduce all the experimental properties of ${\mathrm{PuO}}_2$ accurately and, as such, it is important that the $U$ parameter is selected with careful consideration. In this work, we use noncollinear $\mathrm{DFT}+U$ simulations to thoroughly examine the defect chemistry of ${\mathrm{PuO}}_2$ using $U=4$ and $7\mathrm{eV}$, in order to understand the implication the choice of $U$ parameter can have on the predicted defect chemistry. We find that both $U$ parameters predict the same intrinsic defect chemistry, with the main discrepancy being on the preferred charge state of the oxygen vacancies: $+2$ for $U=4\mathrm{eV}$ and neutral for $U=7\mathrm{eV}$. Additionally, we show that the choice of $U$ can impact the defect formation energies and preferred charge states of a dopant. When uranium is placed onto a plutonium lattice site, it tends to favor the neutral and $+1$ charge state (indicative of ${\mathrm{U}}^{4+}$ and ${\mathrm{U}}^{5+}$) with $U=4\mathrm{eV}$, whereas with $U=7\mathrm{eV}$ the $-1$ and neutral charge states are more stable.

Structural, thermodynamic, and electrical transport properties of TbAuPb were investigated on single crystals. The compound was found to crystallize with the cubic MgAgAs-type structure characteristic of half-Heusler materials. It orders antiferromagnetically at $T_{\mathrm{N}}=5\mathrm{K}$ and undergoes a transition into a different antiferromagnetic phase emerging in high magnetic fields. Electrical transport in TbAuPb exhibits a multiband character, with a predominance of hole-like carriers. Angular magnetoresistance evolves systematically with applied magnetic field and changes its symmetry near the spin-reorientation transition, highlighting strong coupling between the charge transport and the magnetic order. The results of first-principles calculations indicate that TbAuPb is a band inverted semimetal in the nonmagnetic state, which becomes topologically trivial in the field-induced ferromagnetic state.

We employ atomistic machine-learning simulations to explore the atomic structures resulting from buckyball-graphene sandwich systems, where ${\mathrm{C}}_{60}$ molecules are confined between graphene layers under high temperature and pressure. We find that, depending on the thermodynamic conditions, ${\mathrm{C}}_{60}$ molecules can transform into dimers, trimers, and fullerene peanuts, as well as collapse into two- and three-dimensional amorphous condensed phases, including amorphous graphene and amorphous diamond. All of these forms have different electronic properties. Notably, the graphene layers maintain their structural integrity well with minimal changes, collapsing only under the most extreme conditions explored. Our study provides a unified framework for understanding the atomistic properties of fullerene-derived nanomaterials consistent with experiments and their dependence on the synthesis conditions. These findings suggest that graphene could function as an effective nanoscale “reactor” to synthesize novel carbon-based materials with diverse properties using ${\mathrm{C}}_{60}$ as a precursor.

The microstructural characteristics and morphology of interfaces in metals can be crucial in governing the initiation of plasticity and early deformation mechanisms under extreme conditions. During high-strain-rate deformation, these interfaces significantly affect dislocation nucleation, twinning, and other mechanisms that directly impact material strength and failure. Despite their importance, a substantial knowledge gap remains between the observed macroscopic material behavior and the underlying role of bimetal interfaces in plasticity initiation. To address this gap, large-scale molecular dynamics simulations are performed on Al/Ti bimetal structures to examine the effect of interface characteristics on the onset of plasticity under uniaxial compression. Specifically, this study investigates how interface roughness (flat vs waveform interfaces) modifies the initiation of plastic events in the microstructure. Atomistic simulations indicate that interface roughness (a microscopic behavior) reduces the stress required for dislocation nucleation, thereby reducing the peak stress relative to a flat interface. For the square interface, the step height strongly influences plasticity initiation by setting the separation of locally flat regions. Varying the interface rotation relative to the loading direction (macroscopic behavior) reveals that the peak stress for both flat and waveform interfaces initially decreases and then increases with rotation, accompanied by a shift in dominant mechanisms—from Al twinning (flat) to interface sliding and Ti-dominated phase transformation and twinning at lower angles. The change in the stress-strain slope during initial compression reflects how rotation alters the resolved shear stress and activates different slip systems. Overall, the study provides valuable insights into the role of bimetal interfaces in controlling plasticity initiation and early deformation pathways for Al/Ti under extreme loading conditions.

Theoretical modeling of bonding dynamics in metal oxides is required for predicting their thermal conductivity, catalytic activity, and mechanical properties. A primary challenge is the scarcity of experimental methods for validating theoretical predictions of these atomic-scale dynamics. This work presents a workflow that uses experimental extended x-ray absorption fine structure (EXAFS) data collected at high temperatures to validate an interatomic force field for uranium dioxide ($\mathrm{U}{\mathrm{O}}_2$), an important model material. The validated force field is then used to drive computationally intensive molecular dynamics (MD) simulations and as input for the much faster dynamical matrix Debye-Waller (DMDW) method. The predicted values of the Debye-Waller factors from the DMDW calculations are in good agreement with those obtained from the MD simulations, with residual pair-specific differences attributable to quantum zero-point motion at low temperatures and lattice anharmonicity at high temperatures. We further show that theoretical EXAFS spectra constructed directly from DMDW-derived Debye-Waller factors reproduce the experimental data (at relatively low temperatures) with accuracy comparable to full MD-EXAFS, providing an additional validation of the choice of the potential. This study establishes a validated, rapid computational pathway for modeling bond dynamics, naturally incorporating quantum nuclear statistics absent in classical simulations, which are essential for the mechanistic understanding of complex oxide materials.