Single crystals of ${\mathrm{Pr}}_3\mathrm{Mg}{\mathrm{Bi}}_5$ were successfully grown and their structural and physical properties were investigated systematically. ${\mathrm{Pr}}_3\mathrm{Mg}{\mathrm{Bi}}_5$ crystallizes in a hexagonal structure with the space group of $P{6}_3/mcm$ (No. 193), in which the ${Pr}^{3+}$ ions form a distorted kagome lattice in the $ab$ plane. Magnetic measurements and specific heat analysis reveal distinct anomalies at around 10.5 K, indicating an antiferromagnetic phase transition. Meanwhile, an obvious magnetic anisotropy and complex magnetic behavior are observed in this compound. The transport properties of ${\mathrm{Pr}}_3\mathrm{Mg}{\mathrm{Bi}}_5$ are strongly coupled to its magnetism. Specifically, for $H//ab$, a large and positive quasilinear magnetoresistance is observed at $T=2$ K, accompanied by anomalies at the metamagnetic transition in the magnetization-field curve. Additionally, a pronounced anomalous Hall effect is observed at low temperatures. Our study not only expands the $R{E}_3\mathrm{M}{\mathrm{Bi}}_5$ ($RE=\mathrm{rare}$ earth) family but also provides an excellent opportunity to explore unconventional spin structures arising from the interplay between geometrical frustration and long-range Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions.

The distinction and correlation between intrinsic and boundary-induced properties represent a core problem in material physics. While resonant modes do not exist within band gaps in infinite periodic materials, they may appear as in-gap localized edge modes once the material is truncated to form a finite periodic structure. Here we reveal the topological origins of truncation resonances in continuous periodic structures and introduce new avenues for their precise manipulation considering material-property modulation, nature of boundary truncation, number of unit cells, and the possibility of interaction with nontopological modes. Using theory and experiments, we consider elastic beams with sinusoidal and stepwise property modulations as representative examples. The topology of the band gaps is characterized based on the Chern number by variation of the modulation wavelength. This, in turn, allows the behavior of the truncation resonances to be elucidated as topological edge states that appear by varying the phason—a parameter that produces spatial shifts of the property modulation while continuously varying how the boundaries are truncated. Within this framework, we introduce the notion of a boundary phason for the modification of truncation and associated edge states at only one boundary independent of the other. The influence of the boundary conditions on mode couplings and implications on the convergence with the number of unit cells are then analyzed. Finally, nontopological in-gap resonances induced by a defect are investigated, showing that these can be coupled with topological modes when they spatially coincide. These novel features of truncation resonances may be exploited both in applications where in-gap resonances are not desired, such as vibration attenuation and thermal conductivity reduction, or where in-gap resonances provide a functional role, such as filtering, sensing, waveguiding, energy harvesting, and flow control by phononic subsurfaces.

We report our study on the growth of HoN thin films on MgO (100) and ${\mathrm{LaAlO}}_3$ (100) substrates. By using molecular beam epitaxy, we thermally evaporate holmium in an atmosphere of molecular nitrogen, forming HoN at slow rates, moderate temperatures and pressures. We are able to carefully and systematically vary the growth conditions, thereby tuning the nitrogen content of our samples. We explore the differences in the growth window by looking at the crystalline structure and composition of the films deposited on the different substrates. We find that HoN has an epitaxial, well-ordered growth on ${\mathrm{LaAlO}}_3$, in contrast to the three-dimensional growth that occurs on MgO. Using a combination of in situ electron diffraction and x-ray spectroscopies, as well as ex situ x-ray diffraction and SQUID magnetometry, we investigate the structural, electronic, and magnetic properties of the epitaxial HoN films.

Two-dimensional (2D) transition metal dichalcogenide (TMD) materials have versatile electronic and optical properties. TMD nanoribbons exhibit interesting properties due to reduced dimensionality, quantum confinement, and edge states, which make them suitable for various electronic and optoelectronic applications. In a previous work conducted by our group, we demonstrated that the edge bands evolved with bending can tune the optical properties for various widths of TMD nanoribbons. Defects are commonly present in 2D TMD materials, and can dramatically change the material properties. In this following work, we investigate the interaction between the edge and the defect states in tungsten disulfide $\left({\mathrm{WS}}_2\right)$ nanoribbons with lines of W- or S-atom vacancies defects under different bending conditions, using density functional theory (DFT). To gain understanding about the limits of density functional approximations, we compare results on band gaps and energies of defect states with quasiparticle GW. We reveal interesting semiconductor-metal phase transitions, suggesting potential applications in nanoelectronics or molecular electronics. We also calculate the optical absorption of the bent and defective nanoribbons with the many-body GW-BSE (Bethe-Salpeter equation) approach, revealing a tunable optical spectrum and diverse exciton states in the defective ${\mathrm{WS}}_2$ nanoribbons.

Refractory metals exhibit high strength at high temperature, but often lack ductility. Multiprinciple element alloys such as high entropy alloys offer the potential to improve ductility while maintaining strength, but we do not know a priori what compositions will be suitable. A number of measures have been proposed to predict the ductility of metals, notably the Pugh ratio and the Rice-Thomson D-parameter, among others. Here we examine direct ab initio simulation of deformation under tensile strain, and we apply this to a variety of Nb- and Mo-based binary alloys and to several quaternary alloy systems. Our results exhibit peak stresses for elastic deformation, beyond which defects such as lattice slip, stacking faults, transformation, and twinning relieve the stress. The peak stress grows strongly with increasing valence electron count. Correlations are examined among several physical properties, including the above-mentioned ductility parameters.

Magnetic damping within ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ (Permalloy, Py) thin films is studied via temperature- and frequency-dependent ferromagnetic resonance (FMR) experiments. While the Py thickness is kept constant at 20 nm, the environment at the film interfaces was systematically varied by fabricating a set of Py thin films grown on widely used substrates and capped with common layers, which are assumed to be suitable to prevent oxidation. The resulting frequency and temperature dependence of the FMR linewidth significantly deviates from the expected Gilbert-like behavior and especially for oxidic interfaces unwanted non-Gilbert-like contributions to the magnetic damping appear, in particular, at low temperatures. In contrast, metallic capping layers avoid non-Gilbert-like contributions. In particular, Py sandwiched in between Al metallic capping and buffer layers exhibits negligible inhomogeneous FMR linewidth broadening and a very small, purely Gilbert-like contribution of $\alpha =0.0066\left(2\right)$ down to the lowest temperature.

We studied barium niobates with a tetragonal tungsten bronze structure in which Ba is substituted by various rare earths including divalent Eu and trivalent La, Ce, Pr, Nd, and Sm. The magnitude of the negative magnetoresistance observed for ${\mathrm{Ba}}_{3-x}{\mathrm{Eu}}_x{\mathrm{Nb}}_5{\mathrm{O}}_{15}$ increases near the metal-insulator phase boundary of a series of compounds at $x\sim 2.2$, where the number of carriers estimated using the Hall coefficient decreases by two orders of magnitude from that of the parent compound. Various transport properties, specific heat, and the optical reflectivity spectra of the series of compounds were measured and it was found that all the results can be explained by the changes in the number of carriers $n$ and the effective mass $m^{*}$, except for the anomalous decrease in the Seebeck coefficient from $x=0$ to 1.

The Li-based layered perovskites ${\mathrm{Li}}_{2}A{B}_2{\mathrm{O}}_7$ $(A=\mathrm{Ca},\mathrm{Sr};B=\mathrm{Nb},\mathrm{Ta})$ host competing ferroelectric and antiferroelectric states which arise from coupled octahedral rotation distortions and (anti)polar instabilities. We combine density functional theory (DFT) calculations with group theoretic analysis to unravel the mechanism that controls the relative energies of these competing states. We identify transition paths between the competing polar and antipolar states with very low-energy barriers (<3 meV/f.u.), and show that stacking domain walls can facilitate an antipolar-polar transition. We furthermore show that epitaxial strain tunes the relative energy between the polar and antipolar phases. We compare the ${\mathrm{Li}}_{2}A{B}_2{\mathrm{O}}_7$ materials to other families of layered perovskite oxide (anti)ferroelectrics.

The 12442-type hybrid structure compound $\mathrm{Ba}{\mathrm{Th}}_2{\mathrm{Fe}}_4{\mathrm{As}}_4{\left({\mathrm{N}}_{0.7}{\mathrm{O}}_{0.3}\right)}_2$ is a recently synthesized inter-block-layer electron-transfer superconductor with separated double ${\mathrm{Fe}}_2{\mathrm{As}}_2$ layer and shows the superconducting transition at $T_{\mathrm{c}}^{\mathrm{onset}}\sim 30$ K and $T_{\mathrm{c}}^{\mathrm{zero}}\sim 20$ K. Here, we perform the resistivity measurements under various hydrostatic pressures up to $\sim 12$ GPa to track the evolution of its superconducting $T_{\mathrm{c}}$ and normal-state properties as a function of pressure. We found that $T_{\mathrm{c}}$($P$) exhibits a nonmonotonic variation with pressure forming a dome-shaped superconducting phase, i.e., $T_{\mathrm{c}}^{\mathrm{onset}}$ first increases quickly from $\sim 30$ K at ambient pressure to the optimal values of $\sim 46$ K at 2.6 GPa and then decreases gradually to $\sim 23$ K upon further increasing pressure to $\sim 12$ GPa. However, $T_{\mathrm{c}}^{\mathrm{zero}}$ merely increases from $\sim 21$ K at ambient pressure to $\sim 24$ K at 2.6 GPa and then to $\sim 12$ K at $\sim 12$ GPa, leading to an obvious broadening of the superconducting transition at $2.6$ GPa. Concomitant with the domed-shaped superconducting phase, the linear-temperature resistivity coefficient $A_1$ described by the empirical formula $\rho ={\rho }_0+A_{1}T+A_2{T}^2$ also exhibits a similar dome-shaped structure, which may establish the positive correlation between $T_{\mathrm{c}}$ and $A_1$ as found in the unconventional high-$T_{\mathrm{c}}$ superconductors. Our high-pressure x-ray diffraction measurements rule out the occurrence of structural transition or a half-collapsed phase transition in the measured pressure range. Thus, our results indicate that the application of pressure may optimize the superconducting $T_{\mathrm{c}}$ by enhancing the inter-block-layer electron-transfer and/or the spin fluctuations in $\mathrm{Ba}{\mathrm{Th}}_2{\mathrm{Fe}}_4{\mathrm{As}}_4{\left({\mathrm{N}}_{0.7}{\mathrm{O}}_{0.3}\right)}_2$.

Charged carriers in solids diffuse from hot to cold sides under a temperature gradient to induce the thermoelectric voltage. The carrier filtering effect, which only passes either electrons or holes for the conduction process, is an efficient method to enhance such voltage, although it is challenging to experimentally realize it especially in conventional metals with weak energy dependence of the density of states near the Fermi level. Here we measure the in-plane and out-of-plane thermopower of the layered perovskite ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ single crystals above room temperature. We find that the out-of-plane thermopower is largely enhanced with increasing temperature, while the in-plane one seems to remain a temperature-independent constant value expected from the Heikes formula. The observed large out-of-plane thermopower may originate from the recently proposed intriguing hole filtering effect in the body-centered tetragonal system, in which the carrier hopping through the centered atom is essential. Thus the present carrier filtering effect may be a universal property that is applicable in various materials belonging to such a crystal system.

We present a novel heterostructured approach to disentangle the mechanism of electrical transport of the strongly correlated $\mathrm{PrNi}{\mathrm{O}}_3$, by placing the nickelate under the photoconductor CdS. This enables the injection of carriers into $\mathrm{PrNi}{\mathrm{O}}_3$ in a controlled way, which can be used to interrogate its intrinsic transport mechanism. We find a nonvolatile resistance decrease when illuminating the system at temperatures below the $\mathrm{PrNi}{\mathrm{O}}_3$ metal-insulator transition. The photoinduced change becomes more volatile as the temperature increases. These data help understand the intrinsic transport properties of the nickelate-CdS bilayer. Together with data from a bare $\mathrm{PrNi}{\mathrm{O}}_3$ film, we find that the transport mechanism includes a combination of mechanisms, including both thermal activation and variable range hopping. At low temperatures without photoinduced carriers, the transport is governed by hopping, while at higher temperatures and intense illumination the activation mechanism becomes relevant. This work shows a new way to control optically control the low-temperature resistance of $\mathrm{PrNi}{\mathrm{O}}_3$.

Using both time-dependent density functional theory (TDDFT) and the “single-shot” GW plus Bethe-Salpeter equation (GW-BSE) approach, we compute optical band gaps and optical absorption spectra from first principles for eight common binary and ternary closed-shell metal oxides (MgO, ${\mathrm{Al}}_2{\mathrm{O}}_3$, CaO, ${\mathrm{TiO}}_2, {\mathrm{Cu}}_2\mathrm{O}$, ZnO, ${\mathrm{BaSnO}}_3$, and ${\mathrm{BiVO}}_4$), based on the nonempirical Wannier-localization-based, optimally tuned, screened range-separated hybrid functional. Overall, we find excellent agreement between our TDDFT and GW-BSE results and experiment, with a mean absolute error smaller than 0.4 eV, including for ${\mathrm{Cu}}_2\mathrm{O}$ and ZnO that are traditionally considered to be challenging for both methods.

Chiral crystals show promise for spintronic technologies on account of their high spin selectivity, which has led to significant recent interest in quantitative characterization and first-principles prediction of their spin-optoelectronics properties. Here, we outline a computational framework for efficient ab initio calculations of circular dichroism (CD) in crystalline materials. We leverage direct calculations of orbital angular momentum and quadrupole matrix element calculations in density-functional theory (DFT) and Wannier interpolation to calculate CD in complex materials, removing the need for band convergence and accelerating Brillouin-zone convergence compared to prior approaches. We find strong agreement with measured CD signals in molecules and crystals ranging in complexity from small bulk unit cells to 2D hybrid perovskites, and show the importance of the quadrupole contribution to the anisotropic CD in crystals. Spin-orbit coupling affects the CD of crystals with heavier atoms, as expected, but this is primarily due to changes in the electronic energies, rather than due to direct contributions from the spin matrix elements. We showcase the capability to predict CD for complex structures on a 2D hybrid perovskite, finding strong orientation dependence and identifying the eigen-directions of the unit cell with the strongest CD. We additionally decompose CD into separate contributions from inorganic, organic, and mixed organic-inorganic transitions, finding the chiral molecules to dominate the CD, with the inorganic lattice contributing at higher frequencies in specific directions. This unprecedented level of detail in CD predictions in crystals will facilitate experimental development of complex chiral crystals for spin selectivity.

It was shown earlier that applying a magnetic field during the growth of ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$, also known as “field alteration,” induces significant changes to its structural, magnetic, and transport properties. However, the microscopic nature of these changes is enigmatic. In this study, we employed resonant elastic and inelastic x-ray scattering, as well as Raman scattering, to investigate samples from two batches of ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ grown in magnetic fields of different strengths. Our findings reveal that samples grown in a weaker magnetic field have similar magnetic order to nonaltered samples, whereas those grown in a stronger field show a different stacking of weak in-plane ferromagnetic moments. Additionally, we observed significant softening and broadening of select Raman-active phonons in the field altered samples, with a stronger effect in the samples grown in the stronger field. We discuss insights that our results provide into the microscopic nature of field alteration in ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$.

Using density functional theory, we find that tailoring the surface composition provides a route to stabilize the polar phases of the promising ferroelectric material, ${\mathrm{Hf}}_x{\mathrm{Zr}}_{1\text{−}x}{\mathrm{O}}_2$. First, we show that for pure $\mathrm{Hf}{\mathrm{O}}_2$, controlling the positively polarized surface to be relatively oxygen rich adequately screens the ferroelectric surface charges and stabilizes the polar orthorhombic phase. We then demonstrate that the ferroelectric polarization, as measured by the structural polar displacements, increases with decreasing thickness, leading to the emergence of a polar rhombohedral-like phase at the ultrathin limit (1.5 unit cells). Our findings extend to the cases of ${\mathrm{Hf}}_{0.5}{\mathrm{Zr}}_{0.5}{\mathrm{O}}_2$ and $\mathrm{Zr}{\mathrm{O}}_2$, both of which have surface energy landscapes similar to that of $\mathrm{Hf}{\mathrm{O}}_2$. These findings are consistent with and offer insights into the observed absence of a ferroelectric thickness limit in ${\mathrm{Hf}}_x{\mathrm{Zr}}_{1\text{−}x}{\mathrm{O}}_2$-based thin films.

To establish first-principles calculation accuracy and methodology for a metal/water interface, we investigated the ${\mathrm{H}}_2\mathrm{O}$ adsorption monolayer on a Pt(111) surface and developed a density functional theory (DFT)-based method for analyzing the second-order nonlinear susceptibility ${\chi }_{zzz}^{\left(2\right)}$. The ${\mathrm{H}}_2\mathrm{O}$ adsorption structures of $\surd 3\times \surd 3$ with H-up/H-down models, $\surd 39\times \surd 39$ with a six-member ring model, $\surd 39\times \surd 39$, and $\surd 37\times \surd 37$ with five- and seven-member ring models were considered by applying DFT calculations. The last two structures were found to be the most stable, and their heats of adsorption, including zero-point vibration correction, were in good agreement with the activation energy of desorption in the experiment with the vdW-DF2-B86R functional. Furthermore, by employing the density functional perturbation formulation combined with the effective-screening medium (ESM) $+$ a finite electric field approach, we obtained the second-order nonlinear susceptibility ${\chi }_{zzz}^{\left(2\right)}$ of the $\surd 39$ and $\surd 37 5-7$-ring structures. The negative and positive signs of $\mathrm{Im}{\chi }_{zzz}^{\left(2\right)}$ in the high ($3000-3500\mathrm{c}{\mathrm{m}}^{-1}$) and low ($600-2400\mathrm{c}{\mathrm{m}}^{-1}$) frequency regions, respectively, were obtained. The spectra show negative double peaks in the high-frequency region, consistent with those obtained by heterodyne-detected sum frequency generation spectroscopy. This study provides a fundamental basis for improving our understanding of ${\mathrm{H}}_2\mathrm{O}$ adsorption on metal surfaces.

We present a crystal plasticity model tailored for high-pressure, high-strain-rate conditions that uses a multiscale treatment of dislocation-based slip kinetics. We use this model to analyze the pronounced plasticity-induced lattice rotations observed in shock-compressed polycrystalline tantalum via in situ x-ray diffraction. By making direct comparisons between experimentally measured and simulated texture evolution, we can explain how the details of the underlying slip kinetics control the degree of lattice rotation that ensues. Specifically, we show that only the highly nonlinear kinetics caused by dislocation nucleation can explain the magnitude of the rotation observed under shock compression. We demonstrate a good fit between our crystal plasticity model and x-ray diffraction data and exploit the data to quantify the dislocation nucleation rates that are otherwise poorly constrained by experiment in the dynamic compression regime.