Single-atom catalysts (SACs) hold great promise for CO-selective NO reduction (CO-SCR) due to their maximal metal atom utilization and the tunable adsorption of reactants and intermediates. However, optimizing SACs remains a major challenge because it requires a delicate balance of adsorption strengths, which are governed by complex atomic-scale interactions. Here, we combine first-principles calculations with interpretable AI to systematically investigate over 200 transition metal (TM)-doped metal oxides for CO-SCR using a hierarchical learning workflow, which delivers both higher training accuracy and better predictive performance than the conventional nonhierarchical approach. Through thermodynamic screening, we significantly reduce the candidate pool to 20 promising SACs and ultimately identify Mn-doped $\mathrm{Zr}{\mathrm{O}}_2\left(111\right)$ as the most effective catalyst based on both thermodynamic and kinetic criteria. While no single catalyst feature shows a direct correlation with activity, our hierarchical learning approach uncovers nonlinear combinations of features, such as $d$-band center, atomic radius, and charge state, that reliably predict performance. In addition, subgroup discovery data mining reveals physically meaningful strategies for optimizing catalytic properties. This work demonstrates the value of integrating first-principles theory with hierarchical learning to accelerate the discovery of efficient catalysts for environmentally important reactions.

Rhombohedral-stacked transition metal dichalcogenides (TMDs) break inversion symmetry between adjacent layers, giving rise to an intrinsic out-of-plane ferroelectric polarization. Controlling the formation of this stacking polytype is therefore essential for harnessing ferroelectric effects in two-dimensional materials. In this work, we demonstrate the epitaxial growth of rhombohedral bilayer tungsten diselenide (3R-${\mathrm{WSe}}_2$) on a cubic W(110) single crystal by molecular beam epitaxy. We show that selenium passivation of the substrate is key to enable a quasi-van der Waals epitaxy, effectively suppressing strong interfacial bonding and promoting the growth of quasi free-standing bilayer films. The 3R stacking order is confirmed through a combination of Raman spectroscopy and high-resolution angle-resolved photoemission spectroscopy (ARPES), supported by density functional theory (DFT) calculations. ARPES and DFT reveals an indirect-gap electronic structure with the valence-band maximum at the Γ point, as well as a pronounced spin-orbit-driven splitting of $520\pm 20$ meV at the K point. Analysis of the measured dispersions yields hole effective masses of $0.46\pm 0.04m_e$ and $0.75\pm 0.06m_e$ for the upper and lower valence bands at K point, respectively. These results establish a robust route for synthesizing quasi-free-standing 3R-${\mathrm{WSe}}_2$ and provide a platform for exploring the electronic, optical, and ferroelectric functionalities that emerge from inversion-symmetry breaking in layered TMDs. Our findings further highlight the potential of cubic substrates for deterministic fabrication of rhombohedral TMD heterostructures and ferroelectric devices at the nanoscale.

We explore the molecular beam epitaxy synthesis of superconducting aluminum thin films grown on c-plane sapphire substrates at cryogenic temperatures of 6 K and compare their behavior with films synthesized at room temperature. We demonstrate that cryogenic growth increases structural disorder, producing crystalline grains that modify the optical, electrical, and superconducting properties of aluminum. We observe that cryogenic deposition changes the color of aluminum from fully reflective to yellow and correlate the pseudodielectric function and reflectance with structural changes in the film. We find that smaller grain sizes enhance the superconductivity of aluminum, increasing its critical temperature and critical field. We then estimate the superconducting gap and coherence length of Cooper pairs in aluminum in the presence of disorder. Finally, we fabricate superconducting microwave resonators on these films and find that, independently of the growth temperature, the system is dominated by two-level system loss with similar quality factors in the high- and low-power regimes. We further measure a higher kinetic inductance in the cryogenically grown films.

Atomic volume differences determine the structure and phase stability of alloys. To assess the size mismatch, atomic volumes of pure elements, derived from their elemental crystalline states, are normally used as a reference. Using ab initio calculations, we show that substitutional transition metal solute atoms in bcc iron or tungsten exhibit sizes substantially different from their elemental volumes. Analysis of electronic structure suggests that the significant oversizing of substitutional solutes in bcc $\alpha$-iron is caused by the asymmetric filling of electronic states of solute atoms. This produces a strong local magnetovolume effect where, for example, the apparent sizes of platinum and palladium solute atoms are from 35% to 40% larger than their elemental volumes. The analysis of nonlocal variation of electronic structure in the neighborhood of a solute atom shows that in bcc tungsten, the atoms near a substitutional $3d$ transition metal solute atom shrink, whereas in bcc iron, atoms next to a substitutional site swell, further increasing the apparent relaxation volume of the solute atom. Constrained magnetic DFT calculations show that manganese, iron, and cobalt solute impurity atoms in tungsten retain their magnetic moments, with collinear spin-flip barriers of 0.20, 0.52, and 0.17 eV, respectively. In other instances, elements like Cr in $\alpha$-Fe are nominally undersized if the elemental volumes are compared, but in a solute form the relaxation volume of Cr is positive ($+20$% of its atomic volume), stemming from the electron transfer effect and variation of the effective volume of neighboring host Fe atoms. Simulations suggest that application of strong magnetic field might also affect the relaxation volume of transition metal solutes and their solubility.

When two-dimensional atomic layers of different materials are brought into close proximity to form van der Waals (vdW) heterostructures, interlayer interactions can strongly influence their physicochemical properties. These effects are particularly pronounced when the interface exhibits local order and near-perfect structural alignment, giving rise to moiré patterns. Using density-functional theory calculations, we investigate a bilayer heterostructure composed of hexagonal boron nitride (hBN) and silicon carbide (SiC). We predict that introducing a boron vacancy, $V_{\mathrm{B}}$, at specific lattice sites alters the interlayer interaction from weak vdW coupling to localized silicon-nitrogen covalent bonding. Motivated by this mechanism, we examine the binding of transition-metal adatoms and identify principles for enhancing surface reactivity and stabilizing isolated single-metal atoms. Machine-learning molecular dynamics at finite temperature further demonstrate rapid and effectively irreversible trapping of Cu at $V_{\mathrm{B}}$ sites, and show how the $V_{\mathrm{B}}$:Cu ratio governs the transition from single-atom isolation to vacancy-directed aggregation. These results suggest the hBN/SiC heterostructure as a versatile platform for atomically precise transition-metal functionalization, with implications for catalytic energy-conversion materials.

We present a comprehensive optical characterization of 200-nm-thick CrN(111) films grown simultaneously on ${\mathrm{Al}}_2{\mathrm{O}}_3\left(0001\right)$ and $\mathrm{AlN}\text{/}{\mathrm{Al}}_2{\mathrm{O}}_3\left(0001\right)$ using plasma-assisted molecular beam epitaxy. Spectroscopic ellipsometry, spanning the far-infrared to ultraviolet range ($0.04–5.5\mathrm{eV}$), is conducted at room temperature to determine the optical constants $n$ and $k$ of the films. Spectral fits reveal two interband transitions at approximately 0.35 and 0.60 eV. In the infrared range, the ellipsometry data also reveal a pronounced Reststrahlen band stemming from transversal and longitudinal optical phonons at approximately 403 and $629{\mathrm{cm}}^{-1}$, respectively. The relative static and high-frequency permittivities are estimated to be about 39 and 15, respectively. A Born effective charge of approximately 2.7, extracted from the far-infrared region, indicates that CrN is partially ionic.

The search for room-temperature ferromagnetic two-dimensional materials is crucial for the development of future functional spintronic devices. In this study, combining a global minimum search method of the free energy surfaces by the CALYPSO software, we have predicted a two-dimensional cobalt structure. First-principles calculations demonstrate that the structure is a stable ferromagnetic metal exhibiting in-plane magnetic anisotropy, with an estimated Curie temperature of 1001 K significantly exceeding room temperature. Furthermore, the structure shows remarkable resistance to external strain, maintaining its ferromagnetic ground state and excellent stability under biaxial strains ranging from −4% to 5%. These findings indicate that the structure holds promising application prospects in spintronic devices.

We compute the thermodynamic phase diagram of 17 elemental metals with hexagonal-close-packed (hcp), face-centered-cubic (fcc), and body-centered-cubic (bcc) crystal structures using finite-temperature density functional theory. Helmholtz free-energy differences between competing hcp, fcc, and bcc phases are evaluated as functions of electronic temperature up to 7 eV, allowing us to identify solid-solid phase transitions driven by electronic entropy. The systems studied include Zr, Ti, Cd, Zn, Co, and Mg (hcp); Ni, Cu, Ag, Al, Pt, and Pb (fcc); and Cr, W, V, Nb, and Mo (bcc) in their ground-state structures. From the free-energy crossings, we extract the transition electronic temperatures and analyze systematic trends across the metallic systems. We found that all the studied systems go through one or two solid-solid phase transitions caused purely by electronic entropy except Mg and Pb. Our results establish electronic entropy as a key factor governing structural stability in metals under strong electronic excitation.

While intrinsic two-dimensional (2D) ferromagnetic (FM) materials hold great promise for spintronic applications, external control over both electrical and magnetic properties is crucial. Here, we demonstrate effective modulation of electronic and magnetic characteristics of ${\mathrm{Cr}}_2{\mathrm{Si}}_2{\mathrm{Te}}_6$ (CST) through ionic liquid gating (ILG). Upon electron doping via ILG, CST undergoes a semiconductor-to-metal transition, accompanied by remarkable enhancements in Curie temperature ($T_{\mathrm{c}}$, from 33 K to 110 K) and coercive field ($H_{\mathrm{c}}$, from 1.4 mT to 50 mT). Moreover, under high doping levels, the magnetic easy axis can be electrically switched from the out-of-plane to the in-plane direction. Theoretical calculations suggest a possible scenario in which electron doping strengthens FM coupling through a double-exchange mechanism between mixed-valence Cr ions, thereby boosting $T_{\mathrm{c}}$. These findings highlight ILG as a versatile strategy for tailoring magnetism in 2D ferromagnets and offering pathways for next-generation spintronic devices.

Comprehensive study using DC transport, specific heat, magnetization, and two-coil mutual inductance measurements unveils an understanding of three temperature regimes in $\mathrm{Sm}{\mathrm{B}}_6$: (i) $T\ge T^{*}$ ($\sim 62\mathrm{K}$), (ii) $T_g (\sim 23\mathrm{K})\le T<T^{*}$, and (iii) $T<T_g$. Onset of Kondo breakdown below $T^{*}$ releases disorder-driven magnetic fluctuations, which splits the bulk ($\sim 116\mathrm{K}$) and surface Kondo temperature ($T_k^s\approx$ 7 K). Below $T_g$, as magnetic fluctuations subside, surface Kondo screening revives, stabilizing the topological surface state and generating an in-gap feature ($\sim 2.2\mathrm{meV}$) across which Dirac-like carriers are excited. Nyquist impedance analysis reveals a crossover from purely capacitive to capacitive-inductive behavior, signalling a disorder-driven two-fluid phase of heavy quasiparticles and light, high-mobility carriers below $T_g$. We identify a characteristic length scale, ${\mathcal{L}}_{{\nu }_0}\left(T\right)$, associated with the high-mobility phase, exhibiting an almost divergent trend below $T_k^s$. These findings underscore the complex nature of surface conducting state in $\mathrm{Sm}{\mathrm{B}}_6$.

We investigate the effects of W incorporation into Cu-Zr thin film metallic glasses using molecular dynamics (MD) simulations combined with magnetron sputtering. All studies are carried out in the whole range of W concentrations (0 to 100 at.%) and the MD studies also in a wide range of incident energies (1 to 500 eV) and deposition angles (0 to ${60}^{\circ }$). Calculated x-ray diffractograms, packing factor, short-range order (bonding fractions and coordination numbers), medium-range order (network ring and common neighbor statistics), and stress are correlated with measured x-ray diffractograms and technologically important properties (hardness, hardness/Young's modulus ratio, and elastic recovery). The simulations explain the experimental results at the atomic level and provide a lot of information that is not available experimentally. Special attention is paid to nonmonotonic dependencies on the elemental composition and incident energy. Collectively, the results explain the role of W in modifying the structure and improving the mechanical properties of Cu-Zr metallic glasses, predict optimum compositions which maximize some of these properties, and contribute to the development of advanced materials for various applications.

Using molecular dynamics simulations, we show that a widely accepted theoretical prediction for glassy-polymeric strain-hardening moduli ($G_R\propto {\rho }_e$, where ${\rho }_e$ is the entanglement density) fails badly for semiflexible polymers. By postulating that the length, energy, and strain scales controlling $G_R$ are the Kuhn length ${\ell }_K$ and statistical-segment length $b=\sqrt{{\ell }_0{\ell }_K}$ (where ${\ell }_0$ is the backbone bond length), the intermonomer binding energy $u_0$, and the incremental elastic strain ${\mathcal{S}}_{\mathrm{c}}$ required to activate Kuhn-segment-scale plastic rearrangements, we develop a scaling theory predicting that $G_R={\mathcal{S}}_{\mathrm{c}}(u_0/{\ell }_0^3){(b/{\ell }_0)}^3$ in the athermal limit. This prediction agrees quantitatively with the simulated $G_R$ values for athermal uniaxial-stress extension, and semiquantitatively with the values for athermal constant-volume simple shear, for bead-spring polymer glasses spanning a range of ${\ell }_K/{\ell }_0$ that is wider than the range spanned by real systems.

Carbon exhibits both a layered ground-state structure that produces two-dimensional (2D) nanosheets and a nonlayered diamond structure created under high pressure conditions. Motivated by this metastability relationship, we revisit the ground state structure of metal dichalcogenides that are known to have a nonlayered pyrite-type structure. Ultrathin films of pyrite-type ${\mathrm{ZnSe}}_2$ spontaneously transform into a layered phase. This phase is identified as a ground state, and the monolayer exhibits strong elastic anisotropy and a semiconducting bandgap larger than that of the pyrite phase by a factor of two. We demonstrate that a two-valued but directional potential energy surface exists along a Bain-like distortion path, hiding the layered ground state. This work implies that many 2D materials are hidden in nonlayered materials and connects 2D materials science with surface and high-pressure science.

Intermetallic nuclear fuel materials, in particular the uranium alloyed with Zr, Mo, Nb, Ti, offer superior swelling, fabrication, and fuel-cladding chemical reaction properties, among others. However, similar to pure $\alpha$-uranium, the alloyed uranium compositions exhibit susceptibility to a charge-density-wave (CDW) instability at low temperatures, which has rarely been investigated in detail. Here, we unequivocally experimentally identify CDW instability in ${\gamma }^{\prime }\text{−}{\mathrm{U}}_2\mathrm{Mo}$ using diffraction and heat capacity measurements on hitherto unreported single-phase ${\gamma }^{\prime }\text{−}{\mathrm{U}}_2\mathrm{Mo}$. We find that ${\gamma }^{\prime }\text{−}{\mathrm{U}}_2\mathrm{Mo}$ undergoes phase transformation from ordered intermetallic tetragonal ($I4/mmm$) phase to monoclinic symmetry ($C2/c$) below $T_{\mathrm{CDW}}\sim 200\mathrm{K}$, which remains stable at least up to 10 K. Ab initio molecular dynamics simulations confirm the CDW instability, and the CDW phase is determined. A detailed analysis of electronic structures and the real and imaginary parts of electronic susceptibility suggests the role of Fermi-surface nesting and hidden nesting leading to phonon instabilities that induce the CDW distortion. Our results emphasize the need for careful investigation of low-temperature CDW instability in other single-phase intermetallic nuclear fuels.

Cerium diantimonide (${\mathrm{CeSb}}_2$) is a layered heavy-fermion Kondo lattice material that hosts complex magnetism and pressure-induced superconductivity. The interpretation of its in-plane anisotropy has remained unsettled due to structural twinning, which superimposes orthogonal magnetic responses. Here we combine controlled crystal growth with magnetization and rotational magnetometry to disentangle the effects of twinning. Nearly untwinned high-quality single crystals reveal the intrinsic in-plane anisotropy: The in-plane easy axis saturates at $M_{\text{easy}}\left(4\text{T}\right)\approx 1.8{\mu }_{\text{B}}/\mathrm{Ce}$, while the in-plane hard axis magnetization is strongly suppressed, nearly linear, and comparable to the out-of-plane response. These results resolve long-standing discrepancies in reported magnetic measurements, in which in-plane metamagnetic transition fields and saturation magnetization varied significantly across previous studies. Growth experiments demonstrate that avoiding the proposed $\alpha \beta$ structural transition—through Sb-rich flux and slower cooling—systematically reduces twinning. However, powder x-ray diffraction and differential thermal analysis measurements show no clear evidence of a distinct $\beta$ phase. Our results establish a consistent magnetic phase diagram and provide essential constraints for crystal-electric field models, enabling a clearer understanding of the interplay between anisotropic magnetism and unconventional superconductivity in ${\mathrm{CeSb}}_2$.

In Dirac and Weyl semimetals with a flat band, the Seebeck coefficient ($S$) can be enhanced by the energy filtering effect in wave number space through electron-phonon interaction (EPI). Especially, topological B20-type semimetal thin films are promising materials because their Dirac or Weyl fermions are robust against defect scattering, making EPI dominant carrier scattering in the thin films. Here, we propose a strategy of raising this EPI-induced $S$ enhancement effect (ESE) by enhancing EPI at room temperature (RT) via phonon frequency control. By using two atoms with a similar mass in topological B20 materials, the projected phonon density of states (DOS) of two atoms are mainly tuned within the frequency range below $200\mathrm{c}{\mathrm{m}}^{-1}$ (RT thermal energy), leading to the increased total phonon DOS at RT related to EPI enhancement. In this study, focusing on topological B20-CoGe with Dirac-like and flat bands, where Co and Ge have similar masses, we demonstrate raising ESE in an epitaxial B20-CoGe thin film experimentally and theoretically. Although B20-CoGe is unstable under atmospheric pressure, the epitaxial growth of B20-CoGe thin films is achieved on Si substrates by the seed-assisted epitaxy method. The good agreement between experimental results and calculated $S$ values with EPI is observed. The epitaxial B20-CoGe thin film shows the thermoelectric power factor of $\sim 5.8\mu \mathrm{W}\mathrm{c}{\mathrm{m}}^{-1}{\mathrm{K}}^{-2}$, which is comparable to group IV element-based typical thermoelectric semiconductor thin films although B20-CoGe is a semimetal. Furthermore, the use of a heavier Ge atom resulted in lower thermal conductivity. The proposed strategy opens a new approach to realize a high thermoelectric performance (at RT) thin film on a Si platform.