The structure of $\mathrm{LuF}{\mathrm{e}}_2{\mathrm{O}}_4$ below the charge ordering temperature of $T_{\mathrm{CO}}=320$ K has been studied immensely due to the question of whether charge ordering on the mixed valence Fe sublattice leads to a macroscopic electric polarization. In contrast, the local structure associated with polaron fluctuations above $T_{\mathrm{CO}}$, from which the charge ordered structure emerges, is largely unknown. In this work, we characterize this local structure using the x-ray three-dimensional difference pair distribution function ($3\mathrm{D}\text{−}\mathrm{\Delta }\mathrm{PDF}$). Atomic correlations extracted directly from the $3\mathrm{D}\text{−}\mathrm{\Delta }\mathrm{PDF}$ are used to propose models for the local structure, which are simulated using Monte Carlo methods. It is found that the primary Fourier components of the diffuse scattering can be described by the large, correlated Lu displacements. The local structure of Lu displacements draws conceptual parallels to the classical models of triangular Ising antiferromagnets with the added complexity of interplane interactions. The weaker features of the $3\mathrm{D}\text{−}\mathrm{\Delta }\mathrm{PDF}$ are modeled by displacement correlations involving Fe and O, the latter resulting in an increased coordination number of Lu compared with the average structure. Analysis of the Fe-coordination environments in the atomic configurations obtained from Monte Carlo simulations reveals a strong correlation between distortion of the coordination environment and the +II oxidation state of Fe, which suggests a Jahn-Teller polaronic nature of the charge carriers.

We combine (scanning) transmission electron microscopy, electron energy loss spectroscopy (EELS), and x-ray photoelectron spectroscopy (XPS) with density functional theory (DFT) to investigate the complex interplay of surface energetics and polar discontinuity compensation in the interface formation of (001) ${\mathrm{LaInO}}_3/{\mathrm{BaSnO}}_3$-based heterostructures. We present evidence from both experiment and theory that the $\mathrm{BaSn}{\mathrm{O}}_3$ surface with BaO termination is energetically favored over a wide range of chemical potentials. However, overgrowth of the nonpolar BaO-terminated surface with $\mathrm{LaIn}{\mathrm{O}}_3$ results in a $\mathrm{Sn}{\mathrm{O}}_2$-terminated interface. EELS and XPS show that this interfacial termination exchange is mediated by Ba segregation to the growth surface. Our DFT calculations show that the efficient compensation of the polar discontinuity, attributed to polar distortions within $\mathrm{BaSn}{\mathrm{O}}_3$ at the $\mathrm{Sn}{\mathrm{O}}_2$-terminated interface, serves as the driving force behind the observed Ba segregation. This intricate interplay underscores the importance of polar discontinuity compensation as a pivotal factor influencing interface formation in perovskite systems.

The fracture of polymer networks and gels has a significant impact on the performance of these versatile and widely used materials, and a molecular-level understanding of the fracture process is crucial for the design of new materials. Combining molecular dynamics simulations and network analysis techniques, we demonstrate that in the initial undeformed state of model end-linked polymer networks, polymer strands with fewer topological defects in their local surroundings, higher geodesic edge betweenness centrality values compared to the system average, and greater alignment to the loading direction are more prone to breaking under uniaxial tensile deformation.

The quasi-one-dimensional chiral compound ${\left({\mathrm{TaSe}}_4\right)}_2\mathrm{I}$ has been extensively studied as a prime example of a topological Weyl semimetal. Upon crossing its phase transition temperature $T_{\mathrm{CDW}}\approx 263\mathrm{K}, {\left({\mathrm{TaSe}}_4\right)}_2\mathrm{I}$ exhibits incommensurate charge density wave (CDW) modulations described by the well-defined propagation vector $\sim (0.05,0.05,0.11)$, oblique to the ${\mathrm{TaSe}}_4$ chains. Although optical and transport properties greatly depend on chirality, there is no systematic report about chiral domain size for ${\left({\mathrm{TaSe}}_4\right)}_2\mathrm{I}$. In this study, our single-crystal scattering refinements reveal a bulk iodine deficiency, and Flack parameter measurements on multiple crystals demonstrate that separate ${\left({\mathrm{TaSe}}_4\right)}_2\mathrm{I}$ crystals have uniform handedness, supported by direct imaging and helicity-dependent terahertz emission spectroscopy. Our single-crystal x-ray scattering and calculated diffraction patterns identify multiple diffuse features and create a real-space picture of the temperature-dependent ${\left({\mathrm{TaSe}}_4\right)}_2\mathrm{I}$ crystal structure. The short-range diffuse features are present at room temperature and decrease in intensity as the CDW modulation develops. These transverse displacements, along with electron pinning from the iodine deficiency, help explain why ${\left({\mathrm{TaSe}}_4\right)}_2\mathrm{I}$ behaves as an electronic semiconductor at temperatures above and below $T_{\mathrm{CDW}}$, despite a metallic band structure calculated from density functional theory of the ideal structure.

Quantum technologies based on quantum point defects in crystals require control over the defect charge state. Here we tune the charge state of shallow nitrogen-vacancy and silicon-vacancy centers by locally oxidizing a hydrogenated surface with moderate optical excitation and simultaneous spectral monitoring. The loss of conductivity and change in work function due to oxidation are measured in atmosphere using conductive atomic force microscopy and Kelvin probe force microscopy (KPFM). We correlate these scanning probe measurements with optical spectroscopy of the nitrogen-vacancy and silicon-vacancy centers created via implantation 15–25 nm beneath the diamond surface and annealing. The observed charge state of the defects as a function of optical exposure demonstrates that laser oxidation provides a way to precisely tune the Fermi level over a range of at least 2.00 eV. We also observe a significantly larger oxidation rate for implanted surfaces compared to unimplanted surfaces under ambient conditions. Combined with knowledge of the electron affinity of a surface, these results suggest KPFM is a powerful, high-spatial-resolution technique to advance surface Fermi level engineering for charge stabilization of quantum defects.

We investigated changes in the structural and electronic properties of single-crystalline $\mathrm{T}{\mathrm{i}}_3{\mathrm{O}}_5$ films across temperature-induced phase transitions. Based on resistivity and synchrotron x-ray diffraction measurements, we determined the phase diagram of $\mathrm{T}{\mathrm{i}}_3{\mathrm{O}}_5$. The characteristic properties of $\mathrm{T}{\mathrm{i}}_3{\mathrm{O}}_5$ were classified into three temperature regions: the insulating λ phase at $T$ ≤ 350 K, the conductive λ phase at 350 K ≤ $T$ ≤ 460 K via the metal-insulator transition, and the metallic α phase at $T$ ≥ 460 K. Detailed crystallographic analysis revealed that the monoclinic λ phase exhibited a second-order structural phase transition to the orthorhombic α phase, accompanied by a gradual deformation of the monoclinic angle. These results suggest that unusual temperature-induced changes in $\mathrm{T}{\mathrm{i}}_3{\mathrm{O}}_5$ originate from similar atomic conformations of the λ and α phases. Our findings provide guidelines for controlling multiple phase transitions in $\mathrm{T}{\mathrm{i}}_3{\mathrm{O}}_5$ polymorphs using external stimuli, which is crucial for optoelectronic and heat-storage applications of $\mathrm{T}{\mathrm{i}}_3{\mathrm{O}}_5$.

We describe a first-principles statistical mechanics method to calculate the free energies of crystalline alloys that depend on temperature, composition, and strain. The approach relies on an extension of the alloy cluster expansion to include an explicit dependence on homogeneous strain in addition to site occupation variables that track the degree of chemical ordering. The method is applied to the Si-Ge binary alloy and is used to calculate free energies that describe phase stability under arbitrary epitaxial constraints. We find that while the incoherent phase diagram (in which coexisting phases are not affected by coherency constraints) hosts a miscibility gap, coherent phase equilibrium predicts ordering and negative enthalpies of mixing. Instead of chemical instability, the chemomechanical free energy exhibits instabilities along directions that couple the composition of the alloy with a volumetric strain order parameter. This has fundamental implications for phase field models of spinodal decomposition as it indicates the importance of gradient energy coefficients that couple gradients in composition with gradients in strain.

We report a set of scanning tunneling microscopy (STM) and spectroscopy (STS) experiments studying native defects in CVT grown $1T\text{−}{\mathrm{TaS}}_2$. Six different sample surfaces from four bulk crystals were investigated. Wide area imaging reveals a prevalence of nanometer-scale electronic inhomogeneities due to native defects, with pristine regions interspersed. These inhomogeneities appear in typical as-grown crystals and coexist with a well-formed commensurate charge density wave of $1T\text{−}{\mathrm{TaS}}_2$ at low temperatures. Electronic inhomogeneities show up both as variations in the apparent height in STM and in the local density of states in STS; the bands can shift by 60 meV and the gap varies by more than 100 meV across inhomogeneities. These inhomogeneities are present in similar concentration across large-scale areas of all samples studied, but do not influence the charge density wave formation on local or global scales. The commensurate charge density wave exhibits long-range order and remains locally intact in the presence of these inhomogeneities.

Solid phase epitaxy is a crystallization technique used to produce high-quality thin films. Lateral solid phase epitaxy furthermore enables the realization of nonplanar structures, which are interesting, e.g., in the field of spintronics. Here, we demonstrate lateral solid phase epitaxy of yttrium iron garnet over an artificial edge, such that the crystallization direction is perpendicular to the initial seed. We use single-crystalline garnet seed substrates partially covered by a ${\mathrm{SiO}}_x$ film to study the lateral crystallization over the ${\mathrm{SiO}}_x$ mesa. The yttrium iron garnet layer retains the crystal orientation of the substrate not only when in direct contact with the substrate but also across the edge on top of the ${\mathrm{SiO}}_x$ mesa. By controlling the crystallization dynamics it is possible to almost completely suppress the formation of polycrystals and to enable epitaxial growth of single-crystalline yttrium iron garnet on top of mesas made from ceramic materials. From a series of annealing experiments, we extract an activation energy of $3.0\mathrm{eV}$ and a velocity prefactor of $6.5\times {10}^{14}\mathrm{nm}/\mathrm{s}$ for the lateral epitaxial crystallization along the $\langle 100\rangle$ direction. Our results pave the way to engineer single-crystalline nonplanar yttrium iron garnet structures with controlled crystal orientation.

The nitrogen-vacancy (NV) center in diamond, a well-studied, optically active spin defect, is the prototypical system in many state-of-the-art quantum sensing and communication applications. In addition to the enticing properties intrinsic to the NV center, its diamond host's nuclear and electronic spin baths can be leveraged as resources for quantum information rather than considered solely as sources of decoherence. However, current synthesis approaches result in stochastic defect spin positions, reducing the technology's potential for deterministic control and yield of NV spin bath systems, as well as scalability and integration with other technologies. Here, we demonstrate the use of theoretical calculations of electronic central spin decoherence as an integral part of an NV spin bath synthesis workflow, providing a path forward for the quantitative design of NV center-based quantum sensing systems. We use computationally generated coherence data to characterize the properties of single NV center qubits across relevant growth parameters to find general trends in coherence time distributions dependent on spin bath dimensionality and density. We then build a maximum likelihood estimator with our theoretical model, enabling the characterization of a test sample through NV $T_2^{*}$ measurements. Finally, we explore the impact of dimensionality on the yield of strongly coupled electron spin systems. The methods presented herein are general and applicable to other qubit platforms that can be appropriately simulated.

Rydberg states of excitons can reach microns in size and require extremely pure crystals. We introduce an experimental method for the rapid and spatially resolved characterization of Rydberg excitons in copper oxide (${\mathrm{Cu}}_2\mathrm{O}$) with submicron resolution over large zones. Our approach involves illuminating and imaging the entire sample on a camera to realize a spatially resolved version of resonant absorption spectroscopy, without any mobile part. This yields spatial maps of Rydberg exciton properties, including their energy, linewidth, and peak absorption, providing a comprehensive quality assessment of the entire sample in a single shot. Furthermore, by imaging the sample photoluminescence over the same zone, we establish a strong relationship between the spectral quality map and the photoluminescence map of charged oxygen vacancies. This results in an independent, luminescence-based quality map that closely matches the results obtained through resonant spectroscopy. Our findings reveal that Rydberg excitons in natural ${\mathrm{Cu}}_2\mathrm{O}$ crystals are predominantly influenced by optically active charged oxygen vacancies, which can be easily mapped. Together, these two complementary methods provide valuable insights into ${\mathrm{Cu}}_2\mathrm{O}$ crystal properties.

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.