Many crystals, such as lanthanum zirconate $\left({\mathrm{La}}_2{\mathrm{Zr}}_2{\mathrm{O}}_7\right)$, exhibit a flat temperature dependence of thermal conductivity at elevated temperatures. This phenomenon has recently been attributed to the interband phonon tunneling (or diffuson) contribution using different formalisms. However, the contributions of finite-temperature corrections (e.g., higher-order phonon scattering, phonon renormalization, and phonon-scattering cross-section softening effects) and radiation at high temperatures remain unclear. In this work, we predict and compare the thermal conductivity of ${\mathrm{La}}_2{\mathrm{Zr}}_2{\mathrm{O}}_7$ using three distinct first-principles methods. The first method is Green-Kubo molecular dynamics (MD) based on temperature-dependent machine-learning interatomic potentials trained from ab initio MD simulations, which successfully predict the flat trend at ultrahigh temperatures. The second method is the Peierls Boltzmann transport equation (BTE), within the phonon particle framework, using phonon lifetime that includes all the finite-temperature corrections. Four-phonon scattering is found large but is canceled by the phonon-scattering cross-section softening effect. As a result, BTE with temperature corrections does not reproduce the flat thermal conductivity. The third method is Wigner formalism, which includes both phonon particle and wave contributions, which successfully reproduce the flat thermal conductivity. Diffuson and phonon contribute about 67 and 27% of thermal conductivity at 1800 K, respectively. The radiation contribution to thermal conductivity is calculated by the Rosseland model and found to be around 6%. The scaling laws of the phonon, diffuson, radiation, and total thermal conductivity are found to be $\sim T^{-0.97}$, $\sim T^{0.43}$, $\sim T^{2.01}$, and $\sim T^{-0.40}$, respectively. This work clarifies the thermal transport mechanisms in ${\mathrm{La}}_2{\mathrm{Zr}}_2{\mathrm{O}}_7$ at ultrahigh temperatures from different aspects.

We report the electronic structure of the ternary chalcopyrite ${\mathrm{CdSnAs}}_2$ using angle-resolved photoemission spectroscopy (ARPES) combined with the band-structure calculation. The tiny Fermi surface (FS) with the Fermi wave number $k_{\mathrm{F}}=0.012 {\AA }^{-1}$ was observed, and the carrier density $n=1.2\times {10}^{17}{\mathrm{cm}}^{-3}$ was estimated. The deduced carrier density $n$ indicates the electron density parameter $r_{\mathrm{s}}=240$, which corresponds to the extremely low density limit of the three-dimensional (3D) electron gas. On the other hand, the calculated band structure of ${\mathrm{CdSnAs}}_2$ well reproduced the band gap and the effective mass reported by the Hall measurement, the Shubnikov–de Haas (SdH) oscillation, and the optical measurement, quantitatively. Therefore, the ARPES results indicate that the carrier density $n$ decreases and there is a large deviation between the band calculation and the ARPES band structure. These results reveal that the extremely low electron density can be realized near the surface due to the bulk band-bending effect. Moreover, we found the high Fermi velocity of $v_{\mathrm{F}}=2.55\times {10}^6$ m/s and the extremely light mass of $m^{*}/m_0\sim 0.005$ comparable to the Dirac materials. This suggests that the effective mass $m^{*}/m_0$ is reduced due to the effect of the long-range Coulomb interaction in the extremely low-density limit. Our findings provide a venue to investigate the physics of the electron correlation in the extremely low-density electron gas as well as the Wigner crystallization or Anderson localization.

In underdoped cuprate high-$T_{\mathrm{c}}$ superconductors, various local orders and symmetry-breaking states, in addition to superconductivity, reside in the ${\mathrm{CuO}}_2$ planes. The confinement of the ${\mathrm{CuO}}_2$ planes can therefore play a fundamental role in modifying the hierarchy between the various orders and their intertwining with superconductivity. Here we present the growth of $a$-axis oriented ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ films, spanning the whole underdoped side of the phase diagram. In these samples, the ${\mathrm{CuO}}_2$ planes are confined by the film thickness, effectively forming unit-cell-thick nanoribbons. The unidirectional confinement at the nanoscale enhances the in-plane anisotropy of the films. By x-ray diffraction and resistance vs temperature measurements, we have discovered the suppression of the orthorhombic-to-tetragonal transition at low dopings, and a very high anisotropy of the normal state resistance in the $b\text{−}c$ plane, the latter being connected to a weak coupling between adjacent ${\mathrm{CuO}}_2$ nanoribbons. These findings show that the samples we have grown represent a novel system, different from the bulk, where future experiments can possibly shed light on the rich and mysterious physics occurring within the ${\mathrm{CuO}}_2$ planes.

Excursion sets of Gaussian random fields are used to model the three-dimensional (3D) morphology of differently manufactured porous glasses (PGs), which vary with respect to their mean pore widths measured by mercury intrusion porosimetry. The stochastic 3D model is calibrated by means of volume fractions and two-point coverage probability functions estimated from tomographic image data. Model validation is performed by comparing model realizations and image data in terms of morphological descriptors which are not used for model fitting. For this purpose, we consider mean geodesic tortuosity and constrictivity of the pore space, quantifying the length of the shortest transportation paths and the strength of bottleneck effects, respectively. Additionally, a stereological approach for parameter estimation is presented, i.e., the 3D model is calibrated using merely two-dimensional (2D) cross-sections of the 3D image data. Doing so, on average, a comparable goodness of fit is achieved as well. The variance of the calibrated model parameters is discussed, which is estimated on the basis of randomly chosen, individual 2D cross-sections. Moreover, interpolating between the model parameters calibrated to differently manufactured glasses enables the predictive simulation of virtual but realistic PGs with mean pore widths that have not yet been manufactured. The predictive power is demonstrated by means of cross-validation. Using the presented approach, relationships between parameters of the manufacturing process and descriptors of the resulting morphology of PGs are quantified, which opens possibilities for an efficient optimization of the underlying manufacturing process.

Here, we present a study of the influence of microstructure on the magnetic properties of polycrystalline samples of the ${\mathrm{La}}_{1.5}{\mathrm{Sr}}_{0.5}{\mathrm{CoMnO}}_6$ double perovskite, with primary attention to the spontaneous exchange bias effect, a fascinating recently discovered phenomena for which some materials exhibit unidirectional magnetic anisotropy after being cooled in zero magnetic fields. By sintering ${\mathrm{La}}_{1.5}{\mathrm{Sr}}_{0.5}{\mathrm{CoMnO}}_6$ at different temperatures, we obtained samples with distinct average grain sizes, ranging from 1.54–6.65 $\mu \mathrm{m}$. A detailed investigation of the material's structural, morphologic, electronic, and magnetic properties using x-ray powder diffraction, powder neutron diffraction, x-ray absorption near edge structure spectroscopy, scanning electron microscopy, and ac and dc magnetometry has revealed a systematic enhancement of the exchange bias effect with increasing the average grain size. This evolution is discussed in terms of changes in the material's porosity and grain morphology and its influence on the exchange couplings at the magnetic interfaces.

Engineering of magnetic heterostructures for spintronic applications has entered a new phase, driven by the recent discoveries of topological materials and exfoliated van der Waals materials. Their low-dimensional properties can be dramatically modulated in designer heterostructures via proximity effects from adjacent materials, thus enabling the realization of diverse quantum states and functionalities. Here we investigate spin-orbit-coupling proximity effects of Pt on the recently discovered quasi-two-dimensional ferromagnetic state at the FeSi surface. Skyrmionic bubbles (SkBs) are formed as a result of the enhanced interfacial Dzyloshinsky-Moriya interaction. The strong pinning effects on the SkBs are evidenced from the significant dispersion in size and shape of the SkBs and are further identified as a greatly enhanced threshold current density required for depinning of the SkBs. The robust integrity of the SkB assembly leads to the emergence of higher-order nonlinear Hall effects in the high current density regime, which originate from nontrivial Hall effects due to the noncollinearity of the spin texture, as well as from the current-induced magnetization dynamics via the augmented spin-orbit torque.

The equilibrium vacancy concentration and atomic diffusion coefficients in dilute and complex refractory alloys have been calculated using various computational methods. The most productive technique has been large-scale atomistic simulation in the form of a numerical experiment in which a crystal with free surfaces was simulated for a relatively long time. This method is based on the concept that the free surface acts as a source of point defects and provides a natural way to achieve an equilibrium concentration of the defects within the bulk after an initial annealing stage. For complex concentrated alloys (CCAs), this numerical experiment offers the possibility to study diffusion processes where standard analytical approaches are difficult to apply due to the large variety of microscopic states. As the simulation results, we found that the transition from dilute alloys to CCA is accompanied by a significant increase in atomic diffusion rate due to both a substantial increase in the vacancy concentration and their mobility.

Two-dimensional (2D) second-order topological insulators (SOTIs) have been extensively studied due to their unique feature of fractional charge at corners. In order to realize such kind of SOTI in natural materials, we reveal a class of experimentally synthesized 1T-phase transition metal dichalcogenides (TMDs) monolayers as candidates of SOTI. Taking the monolayer of 1T-${\mathrm{PtSe}}_2$ as an example, we identify its second-order topology by determining the nonzero fractional corner charge using first-principles calculations and symmetry analysis. Furthermore, we emphasize the role of crystalline symmetry in the emergence of corner states based on an effective edge theory. Due to the same symmetry and similar band structure, our analysis can be directly applied to other 1T-TMD monolayers. Our findings uncover the previously overlooked higher-order topology in 2D 1T-TMD materials, which may draw immediate experimental attention.

Recent studies have shown that transparent conducting oxides (TCOs) such as $\mathrm{I}{\mathrm{n}}_2{\mathrm{O}}_3$ or $\mathrm{Sn}{\mathrm{O}}_2$ exhibited enhanced mobility through resonant transition metal (TM) doping. However, our previous investigations of CdO, another TCO, doped with TM elements with partially filled 3d- or $4d$ shells, such as Sc, Ti, V, Mo, and W, could exhibit high electron concentration $N$ but experienced a notable reduction in electron mobility μ when the TM concentration $x_{\mathrm{TM}}$ exceeded $\sim 4\%$. Here, we conducted computational and experimental analyses to explore the electronic structure and optoelectronic properties of CdO doped with two distinct representative TM dopants, Ti and Ta, featuring partially filled 3d- and $5d$ shells, respectively, with varying $x_{\mathrm{TM}}$. Density-functional theory calculations unveiled that the localized $d$-donor states of TM interact with the extended CdO conduction-band states, resulting in a lower occupied $E^{-}$ subband and an upper unoccupied $E^{+}$ subband, consistent with predictions from the band anticrossing model. The reduced μ in the TM-doped CdO with relatively high $x_{\mathrm{TM}}$ can thus be attributed to both the relatively small dispersion of this $E^{-}$ conduction band and increased defect scattering. This is also supported by the increased electron effective mass $m^{*}$ and reduced mean scattering time τ derived from spectroscopic ellipsometry analysis using the Drude model. Furthermore, the decrease in $N$ with high $x_{\mathrm{TM}}$ might be due to compensation by oxygen interstitials ${\mathrm{O}}_i$ acceptors, enhanced electron correlation effect, and/or TM aggregation. Moreover, we find that CdO:TM films with $x_{\mathrm{TM}}>\sim 2\%$ demonstrated enhanced environmental stability in ambient air (with $\sim 45\%$ relative humidity), possibly attributed to a lower concentration of $V_{\mathrm{O}}-\mathrm{related}$ defects. Comparatively, Ta doping in CdO yielded electrical properties superior than Ti, owing to the relatively weaker electron correlation in Ta $5d$ orbitals. These findings offer valuable insights into TM doping of CdO and other TCOs.

The contribution of heavy-atom substituents to the overall spin-orbit interaction in two classes of organic radical molecular magnets is discussed. In “single-orbital” radicals, spin-orbit coupling (SOC) effects are well described with reference to pairwise anisotropic exchange interactions between singly occupied spin-bearing orbitals on neighboring molecules; anisotropy requires the presence of spin density on heavy-atom sites with principal quantum number $n>3$. In “multiorbital” radicals, SOC involving virtual orbitals also contributes to anisotropic exchange and, as a result, the presence of heavy ($n>3$) atoms in formally non-spin-bearing sites can enhance pseudodipolar ferromagnetic interaction terms. To demonstrate these effects, ferromagnetic and antiferromagnetic resonance spectroscopies have been used to probe the exchange anisotropy in two organic magnets, one a “single-orbital” ferromagnet, the other a “multiorbital” spin-canted antiferromagnet, both of which contain a heavy-atom iodine ($n=5$) substituent. While the symmetry of the singly occupied molecular orbital in both radicals precludes spin-orbit contributions from iodine to the overall exchange anisotropy, the symmetry and energetically low-lying nature of the lowest unoccupied molecular orbital in the latter allows for appreciable spin density at the site of iodine substitution and, hence, a large exchange anisotropy.

Within the realm of two-dimensional materials, monolayer transition metal dichalcogenide semiconductors boasting intrinsic band gaps of 1–2 eV are regarded as promising candidates for channel materials in next-generation transistors. The judicious choice of electrodes is paramount to achieving low-resistance contacts, thereby enhancing the performance of nanoelectronic devices. Therefore, the exploration of novel metal-semiconductor combinations and a comprehensive grasp of the atomistic nature of interfaces are indispensable. In this work, we present a systematic examination of vertical Moiré pattern contacts between WC and $\mathrm{W}{\mathrm{S}}_2$ or $\mathrm{WS}{\mathrm{e}}_2$, with the termination atoms (tungsten or carbon termination) being investigated through density-functional theory calculations. The Moiré pattern heterostructure is found to exhibit greater energetic favorability when compared to coherent epitaxial strain heterostructures. Our analyses encompass an in-depth exploration of the interface structure, effective potential, electron localization function, Bader charge, energy bands, and density of states within these heterostructures. These investigations reveal the formation of Schottky barriers within these systems, with the dominant carrier type and height of the Schottky barriers being under the control of the termination atoms. Metal-induced gap states formed in the interfaces give rise to a strong Fermi-level pinning. We conclude that the WC/$\mathrm{WS}{\mathrm{e}}_2$ heterostructure with carbon terminations in WC have the smallest $p$-type Schottky-barrier height of 0.08 eV among all other heterostructures considered. Transport properties are assessed using the Simmons tunneling injection model. These findings yield valuable insights that can be leveraged in the design of high-performance nanoelectronic d built upon two-dimensional materials.

In materials without inversion symmetry, the Berry curvature dipole (BCD) arises from the uneven distribution of Berry curvature in momentum space. This leads to nonlinear anomalous Hall effects even in systems with preserved time-reversal symmetry. A key goal is to engineer systems with prominent BCD near the Fermi level. Notably, TaAs, a type-I Weyl semimetal, exhibits substantial Berry curvature but a small BCD around the Fermi level. In this study, we employ first-principles methods to comprehensively investigate the BCD in TaAs. Our findings reveal significant cancellation effects not only within individual Weyl points, but crucially, among distinct Weyl point pairs in bulk TaAs. We propose a strategic approach to enhance the BCD in TaAs by employing a layer-stacking technique. This greatly amplifies the BCD compared to the bulk material. By tuning the number of slab layers, we can selectively target specific Weyl point pairs near the Fermi level, while quantum confinement effects suppress contributions from other pairs, mitigating cancellation effects. Specifically, the BCD of an eight-layer TaAs slab surpasses the bulk value near the Fermi level by orders of magnitude.

A general purpose machine-learning interatomic potential (MLIP) for the Cu-Zr system is presented based on the atomic cluster expansion formalism [R. Drautz, Phys. Rev. B 99, 014104 (2019)]. By using an extensive set of Cu-Zr training data generated withdensity functional theory, this potential describes a wide range of properties of crystalline as well as amorphous phases within the whole compositional range. Therefore, the machine learning interatomic potential (MLIP) can reproduce the experimental phase diagram and amorphous structure with considerably improved accuracy. A massively different short-range order compared to classica interatomic potentials is found in glassy Cu-Zr samples, shedding light on the role of the full icosahedral motif in the material. Tensile tests of B2-CuZr inclusions in an ${\mathrm{Cu}}_{50}{\mathrm{Zr}}_{50}$ amorphous matrix reveal the occurrence of martensitic phase transformations in this crystal-glass nanocomposite.

Energy-efficient operation and stable data retention are the key features of magnetic information devices. Simultaneous achievement of writing-energy reduction and data-stability enhancement has yet faced a dilemma, since both are subjected to the same governing mechanism of magnetization switching, and thus, it is not easy to reduce writing energy while keeping high thermal stability. Here, we propose a solution that bypasses the dilemma by introducing chiral spin alignment in magnetic structures, which assists the current-induced switching at pattern edges, with the energy barrier enhancement. Experiments on asymmetric Pt/Co/Cu/Pt films reveals that both the Dzyaloshinskii-Moriya interaction (DMI) and perpendicular magnetic anisotropy (PMA) are increased by inserting an ultrathin Cu layer. A large PMA enhances the thermal stability, whereas a large DMI reduces the switching current density by tilting the angle of chiral spin alignment at pattern edges. The present observation shows that an effective DMI engineering provides energy-efficient and highly stable magnetic structures suitable for spintronic applications.

Cycling stability of the photochromic effect in rare-earth oxyhydride thin films is of great importance for long-term applications such as smart windows. However, an increasingly slower bleaching rate upon photochromic cycling was found in yttrium oxyhydride thin films; the origin of this memory effect is yet unclear. In this work, the microstructural changes under six photodarkening-bleaching cycles in $\mathrm{Y}{\mathrm{H}}_x{\mathrm{O}}_y$ and $\mathrm{Gd}{\mathrm{H}}_x{\mathrm{O}}_y$ thin films are investigated by in situ illumination Doppler broadening positron annihilation spectroscopy, complemented by positron annihilation lifetime spectroscopy (PALS) investigations on $\mathrm{Y}{\mathrm{H}}_x{\mathrm{O}}_y$ films before and after one cycle. For the first three cycles, the Doppler broadening $S$ parameter after bleaching increases systematically with photodarkening-bleaching cycle, and correlates with the bleaching time constant extracted from optical transmittance measurements. This suggests that the microstructural evolution that leads to progressively slower bleaching involves vacancy creation and agglomeration. PALS suggests that during a photodarkening-bleaching cycle, divacancies are formed that are possibly composed of illumination-induced hydrogen vacancies and preexisting yttrium monovacancies, and vacancy clusters grow, which might be due to local removal of hydrogen. If bleaching is a diffusion-related process, the formed vacancy defects induced by illumination might affect the diffusion time by reducing the diffusion coefficient. Hydrogen loss could also be a key factor in the reduced bleaching kinetics. Other microstructural origins including domain growth, or formation of $\mathrm{O}{\mathrm{H}}^{-}$ hydroxide groups, are also discussed with respect to the slower bleaching kinetics. During the fourth to sixth photodarkening-bleaching cycle, reversible shifts in the Doppler $S$ and $W$ parameters are seen that are consistent with the reversible formation of metallic-like domains, previously proposed as a key factor in the mechanism for the photochromic effect.