Interlayer van der Waals (vdW) coupling is generic in two-dimensional materials such as graphene and transition-metal dichalcogenides, which can induce very low-energy phonon modes. Using high-resolution inelastic hard x-ray scattering, we uncover the ultralow energy phonon mode along the Cu-O bond direction in the high-$T_c$ cuprate (Bi,${\mathrm{Pb})}_2$(Sr,${\mathrm{La})}_2{\mathrm{CuO}}_{6+\delta }$ (Bi2201). The energy and full-width half-maximum (FWHM) of these modes are independent of temperature, while their intensity decreases with doping in accordance with an increasing $c$-axis lattice parameter. We compare the experimental results to first-principles density functional theory simulations and identify the observed mode as a van der Waals phonon, which arises from the shear motion of the adjacent Bi-O layers. This shows that Bi-based cuprate has vibrational properties similar to graphene and transition-metal dichalcogenides, which can be exploited to engineer novel heterostructures.

Leveraging the flexible material designability of organic-inorganic hybrid systems, spatial inversion symmetry breaking and spin-orbit coupling (SOC) were simultaneously controlled in a series of two-dimensional organic-inorganic hybrid perovskite (2D-OIHP) copper halides with ferromagnetic exchange interaction (FMI). In chiral molecule-incorporated 2D-OIHP copper halides, an increase in Br component over Cl component at halogen sites leads to systematic variations in magnetic phase diagrams owing to the competition between the FMI and Dzyaloshinskii-Moriya interaction induced by the SOC. The crucial role of noncentrosymmetry in the competition for the magnetic interactions was confirmed by comparing the magnetic phase diagrams of chiral and achiral 2D-OIHP copper bromides.

Strong magnetic anisotropy is crucial for spintronic devices, but the strength of in-plane anisotropy (IMA) has constrained its applications in magnetic storage devices in contrast with the perpendicular magnetic anisotropy (PMA). In our study, we observed a giant interfacial uniaxial IMA in Co/Pt(110) bilayers that is approximately five times larger than the PMA in Co/Pt(111) systems. By adding another interface to form Pt/Co/Pt trilayers, the total interfacial uniaxial IMA can be further enhanced to $13.1\pm 0.3\mathrm{erg}/{\mathrm{cm}}^2$, making it a promising platform for high-density storage devices. The interfacial IMA in Co/Pt(110) bilayers is related to the Co at the interface and can be controlled by inserting an ultrathin Cu layer. Our results demonstrated the strong in-plane anisotropy in Co/Pt(110) systems, making it a promising platform for studying the spintronic phenomenon in strongly anisotropic systems.

Infinite-layer nickelates $R_{1-x}{\mathrm{Sr}}_x{\mathrm{NiO}}_2$ ($R$ = La, Pr, Nd) are a class of superconductors with structural similarities to cuprates. Although long-range antiferromagnetic order has not been observed for these materials, magnetic effects such as antiferromagnetic spin fluctuations and spin-glass behavior have been reported. Different experiments have drawn different conclusions about whether the pairing symmetry is $s$ or $d$ wave. In this paper, we applied a scanning superconducting quantum interference device (SQUID) to probe the magnetic behavior of film samples of three infinite-layer nickelates (${\mathrm{La}}_{0.85}{\mathrm{Sr}}_{0.15}{\mathrm{NiO}}_2$, ${\mathrm{Pr}}_{0.8}{\mathrm{Sr}}_{0.2}{\mathrm{NiO}}_2$, and ${\mathrm{Nd}}_{0.775}{\mathrm{Sr}}_{0.225}{\mathrm{NiO}}_2$) grown on ${\mathrm{SrTiO}}_3$ (STO), each with a nominal thickness of 20 unit cells. In all three films, we observed a ferromagnetic background. We also measured the magnetic susceptibility above the superconducting critical temperature in ${\mathrm{Pr}}_{0.8}{\mathrm{Sr}}_{0.2}{\mathrm{NiO}}_2$ and ${\mathrm{La}}_{0.85}{\mathrm{Sr}}_{0.15}{\mathrm{NiO}}_2$ and identified a non-Curie-Weiss dynamic susceptibility. Both magnetic features are likely due to ${\mathrm{NiO}}_x$ nanoparticles. Additionally, we investigated superconductivity in ${\mathrm{Pr}}_{0.8}{\mathrm{Sr}}_{0.2}{\mathrm{NiO}}_2$ and ${\mathrm{Nd}}_{0.775}{\mathrm{Sr}}_{0.225}{\mathrm{NiO}}_2$, which exhibited inhomogeneous diamagnetic screening. The superfluid density inferred from the diamagnetic susceptibility in relatively homogeneous regions shows $T$-linear behavior in both samples. Finally, we observed superconducting vortices in ${\mathrm{Nd}}_{0.775}{\mathrm{Sr}}_{0.225}{\mathrm{NiO}}_2$. We determined a Pearl length of $330\mu \mathrm{m}$ for ${\mathrm{Nd}}_{0.775}{\mathrm{Sr}}_{0.225}{\mathrm{NiO}}_2$ at 300 mK, both from the strength of the diamagnetism and from the size and shape of the vortices. These results highlight the importance of considering ${\mathrm{NiO}}_x$ particles when interpreting experimental results for these films.

First-principles density functional theory (DFT) calculations of supercell structures based on N-doped $\mathrm{Fm}\overline{3}\mathrm{m}{\mathrm{LuH}}_3$ reveal configurations of $\mathrm{Fm}\overline{3}\mathrm{m}{\mathrm{Lu}}_8{\mathrm{H}}_{23-x}\mathrm{N}$ that exhibit novel electronic properties such as flat bands, sharply peaked densities of states (van Hove singularities, vHs), and intersecting Dirac cones near the Fermi energy (${\mathrm{E}}_F$). These electronic properties are present when N substitutes H in the octahedral interstices of $\mathrm{Fm}\overline{3}\mathrm{m}{\mathrm{LuH}}_3$. These structures also exhibit an interconnected metallic hydrogen network, a common feature of high-${\mathrm{T}}_c$ hydride superconductors. Electronic property systematics gives an estimate of ${\mathrm{T}}_c$ for one structure that is well above the critical temperatures predicted for structures considered previously. $\text{DFT}+\text{U}$ has an especially strong effect on one of the structures considered, enhancing the vHs and flat bands near ${\mathrm{E}}_F$. These results provide a basis for understanding the electronic properties observed for nitrogen-doped lutetium hydride.

We report quantum oscillation measurements on the kagome compounds ${\mathrm{ATi}}_3{\mathrm{Bi}}_5$ (A=Rb, Cs) in magnetic fields up to 41.5 T and temperatures down to 350 mK. In addition to the frequencies observed in previous studies, we have observed multiple unreported frequencies above 2000 T in ${\mathrm{CsTi}}_3{\mathrm{Bi}}_5$ using a tunnel diode oscillator technique. We compare these results against density functional theory calculations and find good agreement with the calculations in the number of peaks observed, frequency, and the dimensionality of the Fermi surface. For ${\mathrm{RbTi}}_3{\mathrm{Bi}}_5$ we have obtained a different quantum oscillation spectrum, although calculated quantum oscillation frequencies for the Rb compound are remarkably similar to the Cs compound, calling for further studies.

In chiral crystals crystalline symmetries can protect multifold fermions, pseudorelativistic masless quasiparticles that have no high-energy counterparts. Their realization in transition metal monosilicides has exemplified their intriguing physical properties, such as long Fermi arc surface states and unusual optical responses. Recent experimental studies on amorphous transition metal monosilicides suggest that topological properties may survive beyond crystals, even though theoretical evidence is lacking. Motivated by these findings, we theoretically study a tight-binding model of amorphous transition metal monosilicides. We find that topological properties of multifold fermions survive in the presence of structural disorder that converts the semimetal into a diffusive metal. We characterize this topological diffusive metal phase with the spectral localizer, a real-space topological indicator that we show can signal multifold fermions. Our findings showcase how topological properties can survive in disordered metals, and how they can be uncovered using the spectral localizer.

Hydrogen dynamics in the nanoscale region of ${\mathrm{VO}}_2$ was investigated by muon spin rotation/relaxation ($\mu \mathrm{SR}$) technique. Positively charged muon acts as a light radioisotope of protons and can be used as a probe to explore the inside of materials from an atomic perspective. By analyzing the muon hopping rate and the spatial distribution of the ${}^{51}\mathrm{V}$ nuclear magnetic moments, we have identified two types of diffusion paths in ${\mathrm{VO}}_2$ (via oxygen-muon bonds or defects) and the potential of a high diffusion coefficient in the ${10}^{-10}{\mathrm{cm}}^2/\mathrm{s}$ range at ambient temperature. Our results provide valuable information for developing hydrogen-driven ${\mathrm{VO}}_2$-related electronic devices.

The spin, charge, and lattice degrees of freedom and their interplay in high entropy oxides were intensively investigated in recent years. However, how the orbital degree of freedom is affected by the extreme disorder in high entropy oxides has not been studied. In this work, using perovskite structured $R{\mathrm{VO}}_3$ as a materials playground, we report how the disorder arising from mixing different rare earth ions at the rare earth site affects the orbital ordering of ${\mathrm{V}}^{3+}{\mathrm{t}}_{2g}$ electrons. Since each member of $R{\mathrm{VO}}_3$ ($R$ = rare earth and Y) crystallizes into the same orthorhombic Pbnm structure, the configurational entropy should not be critical for the stability of ($R{}_1$,…,$R{}_n){\mathrm{VO}}_3$. The spin and orbital ordering was studied by measuring magnetic properties and specific heat of single crystals. Rather than the number and type of rare earth ions, the average and variance of ionic radius are the key factors determining the spin and orbital order in ($R{}_1$,…,$R{}_n){\mathrm{VO}}_3$. When the size variance is small, the average ionic radius takes precedence in dictating spin and orbital order. Increasing size variance suppresses the G-type orbital order (G-OO) and C-type antiferromagnetic order (C-AF) but favors the C-OO/G-AF state and spin-orbital entanglement. These findings suggest that the extreme disorder introduced by mixing multiple rare earth ions in high entropy perovskites might be employed to preserve the orbital degree of freedom to near the magnetic order, which is necessary for the electronic driven orbital ordering in a Kugel-Khomskii compound.

The copper substituted lead oxyapatite, ${\mathrm{Pb}}_{10-x}{\mathrm{Cu}}_x{\left({\mathrm{PO}}_{3.92\left(4\right)}\right)}_6{\mathrm{O}}_{0.96\left(3\right)} [x$ = 0.94(6)] was studied using neutron and x-ray diffraction and neutron spectroscopy techniques. The crystal structure of the main phase of our sample, which has come to be colloquially known as LK-99, is verified to possess a hexagonal structure with space group $P{6}_3/m$, alongside the presence of impurity phases Cu and ${\mathrm{Cu}}_2\mathrm{S}$. We determine the primary substitution location of the Cu as the Pb1 $\left(6h\right)$ site, with a small substitution at the Pb2 $\left(4f\right)$ site. Consequently, no clear Cu-doping-induced structural distortion was observed in the investigated temperature region between 10 K and 300 K. Specially, we did not observe a reduction of coordinate number at the Pb2 site or a clear tilting of ${\mathrm{PO}}_4$ tetrahedron. Magnetic characterization reveals a diamagnetic signal in the specimen, accompanied by a very weak ferromagnetic component at 2 K. No long-range magnetic order down to 10 K was detected by the neutron diffraction. Inelastic neutron scattering measurements did not show magnetic excitations for energies up to 350 meV. There is no sign of a superconducting resonance in the excitation spectrum of this material. The measured phonon density of states compares well with density functional theory calculations performed for the main LK-99 phase and its impurity phases. Our study may shed some insight into the role of the favored substitution site of copper in the absence of structural distortion and superconductivity in LK-99.

${\mathrm{Mn}}_{3}X$ ($X=\text{Sn}$, Ge, Ga) kagome Weyl semimetals have attracted significant research interest due to their large anomalous Hall, thermal, and optical effects originating from their nontrivial band topology. These large topological effects together with the antichiral antiferromagnetic order that can be manipulated through various experimental means provide unique platforms for developing high-speed spintronics. ${\mathrm{Mn}}_3\mathrm{Ga}$ is known to have the largest Néel temperature ($T_{\mathrm{N}}\approx 480$ K), which is useful for developing antiferromagnetic spintronics. Here, we establish the epitaxial growth of antiferromagnetic ${\mathrm{Mn}}_3\mathrm{Ga}$ films by magnetron sputtering and present their structure, magnetotransport, terahertz properties, and exchange bias effect in ${\mathrm{Mn}}_3\mathrm{Ga}$/NiFe bilayers, establishing their remarkable properties essential for future investigations towards device applications.

Dislocation-mediated deformation mechanisms in body-centered-cubic solid solutions are expected to be influenced by spatial fluctuations in screw dislocation core energies. In refractory high-entropy alloys, the formation of chemical short-range order has been demonstrated to decrease the heterogeneity of this energy landscape, narrowing the distribution of dislocation core energies. It is, however, unclear if multicomponent compositionally complex systems display any unique effects or if these results are more generally applicable. To answer this question, this study computationally investigates how system chemistry, compositional complexity, and the presence of various degrees of chemical short-range order affect the distribution of screw dislocation energies in binary and ternary subsystems of the NbMoTaW alloy. We report the calculated averages and variances for the diffuse antiphase boundary energy and the dislocation core energy with various degrees of chemical short-range order. While short-range order negligibly affects the average core energies, their distributions are notably narrowed in some, but not all, systems, primarily depending on chemistry rather than the number of components.

A uniform 1-unit-cell-high step on the $\mathrm{SrTi}{\mathrm{O}}_3$ (STO) substrate is a prerequisite for growing high-quality epitaxial oxide heterostructures. However, it is inevitable that defects induced by mixed substrate-surface termination exist at the interface, significantly impacting the properties of ultrathin films. In this study, we microscopically identify the origin for the lateral inhomogeneity in the growth of ultrathin $\mathrm{SrRu}{\mathrm{O}}_3$ films due to the step effects of $\mathrm{SrTi}{\mathrm{O}}_3$ (001). By using atomic-resolved scanning transmission electron microscopy, we observe two distinct types of step propagation along the [011] and $\left[0\overline{1}1\right]$ crystallographic direction in $\mathrm{SrTi}{\mathrm{O}}_3-\mathrm{SrRu}{\mathrm{O}}_3$ heterostructures, respectively. In particular, the type-II $\left[0\overline{1}1\right]$ step results in lateral discontinuity of monolayer $\mathrm{SrRu}{\mathrm{O}}_3$ and originates from the SrO-terminated regions along the $\mathrm{Ti}{\mathrm{O}}_2-\mathrm{terminated}$ step edge. Such an induced lateral discontinuity should be responsible for the distinct electronic and magnetic properties of monolayer $\mathrm{SrRu}{\mathrm{O}}_3$. Our findings underscore the critical importance of using single-termination STO substrate to achieve high-quality termination-selective films and to unveil the intrinsic properties of epitaxial films in the atomic limit.

Crystalline sheets (e.g., graphene and transition metal dichalcogenides) liberated from a substrate are a paradigm for materials at criticality, because flexural phonons can fluctuate into the third dimension. Although studies of static critical behaviors (e.g., the scale-dependent elastic constants) are plentiful, investigations of dynamics remain limited. Here, we use molecular dynamics to study the time dependence of the midpoint (the height center of mass) of doubly clamped nanoribbons, as prototypical graphene resonators, under a wide range of temperature and strain conditions. By treating the ribbon midpoint as a Brownian particle confined to a nonlinear potential (which assumes a double-well shape beyond the buckling transition), we formulate an effective theory describing the ribbon's transition rate across the two wells and its oscillations inside a given well. We find that, for nanoribbbons compressed above the Euler buckling point and thermalized above a temperature at which the nonlinear effects due to thermal fluctuations become significant, the exponential term (the ratio between energy barrier and temperature) depends only on the geometry but not the temperature, unlike the usual Arrhenius behavior. Moreover, we find that the natural oscillation time for small strain shows a nontrivial scaling ${\tau }_{\mathrm{o}}\sim L_0^z{T}^{-\eta /4}$, with $L_0$ being the ribbon length, $z=2-\eta /2$ being the dynamic critical exponent, $\eta =0.8$ being the scaling exponent describing scale-dependent elastic constants, and $T$ being the temperature. These unusual scale- and temperature-dependent dynamics thus exhibit dynamic criticality and could be exploited in the development of graphene-based nanoactuators.

We study the optical generation and control of coherent phonons in elemental bismuth (Bi) and antimony (Sb) using a classical equation of motion informed by first-principles calculations of the potential energy surface and the frequency-dependent macroscopic dielectric function along the zone-centered optical phonon coordinates. Using this approach, we demonstrate that phonons with the largest optomechanical couplings also have the strongest degree of anharmonicity among the zone-centered modes, a result of the broken-symmetry structural ground state of Bi and Sb. We show how this anharmonicity, explaining the light-induced phonon softening observed in experiments, prevents the application of standard phonon-amplification and annihilation protocols. We introduce a simple linearization protocol that extends the use of such protocols to the case of anharmonic phonons in broken-symmetry materials, and demonstrate its efficiency at high displacement amplitudes. Our formalism and results provide a path for improving optical control in nonlinear phononics.

For the past few years, two-dimensional (2D) ferroelectric materials have attracted strong interest for their potential in future nanoelectronics devices. The recent discovery of 2D ferroelectricity in twisted layers of insulating hexagonal boron nitride, one of the most used 2D materials, has opened the route to its integration into complex van der Waals heterostructures combining hybrid properties. Here we show that opposite polarizations in ferroelectric domains of a folded $h\mathrm{BN}$ layer can imprint local $n$ and $p$ doping in a semiconducting transition metal dichalcogenide ${\mathrm{WSe}}_2$ monolayer. We demonstrate that ${\mathrm{WSe}}_2$ can be used as an optical probe of ferroelectricity in $h\mathrm{BN}$ and show that the doping density and type can be controlled with the position of the semiconductor with respect to the ferroelectric interface. Our results establish the ferroelectric $h\mathrm{BN}$/${\mathrm{WSe}}_2$ van der Waals stacking as a promising optoelectronic structure.

Nested sampling is a promising method for calculating phase diagrams of materials. However, if accuracy at the level of ab initio calculations is required, the computational cost limits its applicability. In the present work, we report on the efficient use of a neural-network force field in conjunction with the nested-sampling algorithm. We train our force fields on a recently reported database of silicon structures evaluated at the level of density functional theory and demonstrate our approach on the low-pressure region of the silicon pressure-temperature phase diagram between 0 and $16\mathrm{GPa}$. The simulated phase diagram shows good agreement with experimental results, closely reproducing the melting line. Furthermore, all of the experimentally stable structures within the investigated pressure range are also observed in our simulations. We point out the importance of the choice of exchange-correlation functional for the training data and show how the r2SCAN meta-generalized gradient approximation plays a pivotal role in achieving accurate thermodynamic behavior. We furthermore perform a detailed analysis of the potential energy surface exploration and highlight the critical role of a diverse and representative training data set.

The layered ${\mathit{AMP}}_2$ ($A$ = alkali-earth or rare-earth atom, $M$ = transition metal, $P$ = Sb, Bi) compounds are widely studied for their rich magnetism and electronic structure topology. We provide a detailed characterization of the magnetic and transport properties of ${\mathrm{LaMn}}_x{\mathrm{Sb}}_2$, an understudied member of the ${\mathit{AMP}}_2$ family. ${\mathrm{LaMn}}_x{\mathrm{Sb}}_2$ forms with intrinsic Mn vacancies, and we demonstrate that by varying the starting ratio of La, Mn, and Sb, we can synthetically control the Mn occupancy and produce single crystals with $x=0.74–0.97$. Magnetization and transport measurements indicate ${\mathrm{LaMn}}_x{\mathrm{Sb}}_2$ has a rich temperature-composition ($T\text{–}x$) magnetic phase diagram with physical properties strongly influenced by the Mn occupancy. ${\mathrm{LaMn}}_x{\mathrm{Sb}}_2$ orders antiferromagnetically at $T_1=130–180$ K, where $T_1$ increases with $x$. Below $T_1$, the $T\text{–}x$ phase diagram is complicated. At high $x$, there is a second transition $T_2$ that decreases in temperature as $x$ is lowered, vanishing below $x \le 0.85$. A third, first-order, transition $T_3$ is detected at $x \approx 0.92$, and the transition temperature increases as $x$ is lowered, crossing above $T_2$ near $x \approx 0.9$. On moving below $x <0.79$, we find the crystal structure changes from the $P$4/nmm arrangement to an $I\overline{4}2m$ structure with partially ordered Mn vacancies. The change in crystal structure results in the sudden appearance of two new low-temperature phases and a crossover between regimes of negative and positive magnetoresistance when $x \le 0.78$. Finally, we provide powder neutron diffraction for $x=0.93$, and find that the high-$x$ compositions first adopt a $G$-type antiferromagnetic structure with the Mn moments aligned within the $\mathit{ab}$ plane, which is followed upon further cooling by a second transition to a different, noncollinear structure where the moments are rotated within the basal plane. Our results demonstrate that ${\mathrm{LaMn}}_x{\mathrm{Sb}}_2$ is a highly tunable material with six unique magnetically ordered phases, depending on $T$ and $x$.

Dealloyed nanoporous metals with a hierarchical structure provide model systems for low-density structural and functional nanomaterials. It has been suggested that these materials are distinguished by particularly stringent design principles, with precisely defined characteristic length scales, and with geometrically similar structures on each hierarchy level, and that the length scales can be independently tuned on each level. Studying nanoporous gold made by two-step dealloying, we here demonstrate the tunability of the microstructure, independently for the upper and the lower hierarchy level. Small-angle (SAXS) and ultrasmall-angle x-ray scattering (USAXS) revealed sharp interference peaks corresponding to each of the two levels, confirming the stringent structural definition. Exploiting USAXS, we resolve and study upper-hierarchy-level ligament spacings of up to 600 nm. The length scales inferred from the peak positions correlate excellently with structure sizes determined by analysis of electron micrographs. This suggests a scaling factor that allows for size conversion between the two approaches. Furthermore, the analysis of the small-angle scattering enables a characterization of the volume-specific surface area, in good agreement with the estimate based on the ligament size and the leveled-wave model as an approximate description of the material's microstructure.

Heterostructures of two-dimensional (2D) van der Waals (vdW) magnets and topological insulators (TI) are of substantial interest as candidate materials for efficient spin-torque switching, quantum anomalous Hall effect, and chiral spin textures. However, since many of the vdW magnets have Curie temperatures below room temperature, we want to understand how materials can be modified to stabilize their magnetic ordering to higher temperatures. In this work, we utilize molecular beam epitaxy to systematically tune the Curie temperature ($T_C$) in thin film ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2/{\mathrm{Bi}}_2{\mathrm{Te}}_3$ from bulklike values ($\sim 220$ K) to above room temperature by increasing the growth temperature from 300 ${}^{\circ }\mathrm{C}$ to 375 ${}^{\circ }\mathrm{C}$. For samples grown at 375 ${}^{\circ }\mathrm{C}$, cross-sectional scanning transmission electron microscopy (STEM) reveals the spontaneous formation of different ${\mathrm{Fe}}_m{\mathrm{Ge}}_n{\mathrm{Te}}_2$ compositions (e.g., ${\mathrm{Fe}}_5{\mathrm{Ge}}_2{\mathrm{Te}}_2$ and ${\mathrm{Fe}}_7{\mathrm{Ge}}_6{\mathrm{Te}}_2$) as well as intercalation in the vdW gaps, which are possible origins of the enhanced Curie temperature. This observation paves the way for developing various ${\mathrm{Fe}}_m{\mathrm{Ge}}_n{\mathrm{Te}}_2$/TI heterostructures with novel properties.