Using the experimental capability of the x-ray diffraction instrument available at the 25-T Florida split coil magnet at the NHMFL, we investigated the magnetostriction of polycrystalline $\mathrm{Al}{\mathrm{Fe}}_2{\mathrm{B}}_2$. The magnetostriction was measured in the vicinity of the ferromagnetic transition with $T_C=280\mathrm{K}$, at 250, 290, and 300 K. $\mathrm{Al}{\mathrm{Fe}}_2{\mathrm{B}}_2$ exhibits an anisotropic change in lattice parameters as a function of magnetic field near the Curie temperature, and a monotonic variation as a function of applied field has been observed, i.e., the $c$ axis increases significantly while the $a$ and $b$ axes decrease with increasing field in the vicinity of $T_C$, irrespective of the measurement temperature. The volume magnetostriction decreases with decreasing temperature and changes its sign across $T_C$. Density functional theory calculations for the nonpolarized and spin-polarized (ferromagnetic) models confirm that the observed changes in lattice parameters due to spin polarization are consistent with the experiment. The relationships for magnetostriction are estimated based on a simplified Landau model that agrees well with the experimental results.

We report a comprehensive investigation of $L{n}_2{\mathrm{NiIrO}}_6$ ($Ln=\text{La}$, Pr, Nd) using thermodynamic and transport properties, neutron powder diffraction, resonant inelastic x-ray scattering, and density-functional theory (DFT) calculations to investigate the role of A-site cations on the magnetic interactions in this family of hybrid $3d\text{−}5d\text{−}4f$ compositions. Magnetic structure determination using neutron diffraction reveals antiferromagnetism for ${\mathrm{La}}_2{\mathrm{NiIrO}}_6$, a collinear ferrimagnetic Ni and Ir state that is driven to long-range antiferromagnetism upon the onset of Nd ordering in ${\mathrm{Nd}}_2{\mathrm{NiIrO}}_6$, and a noncollinear ferrimagnetic Ni and Ir sublattice interpenetrated by a ferromagnetic Pr lattice for ${\mathrm{Pr}}_2{\mathrm{NiIrO}}_6$. For ${\mathrm{Pr}}_2{\mathrm{NiIrO}}_6$, heat-capacity results reveal the presence of two independent magnetic sublattices, and transport resistivity indicates insulating behavior and a conduction pathway that is thermally mediated. A first principles DFT calculation elucidates the existence of the two independent magnetic sublattices within ${\mathrm{Pr}}_2{\mathrm{NiIrO}}_6$ and offers insight into the behavior in ${\mathrm{La}}_2{\mathrm{NiIrO}}_6$ and ${\mathrm{Nd}}_2{\mathrm{NiIrO}}_6$. Resonant inelastic x-ray scattering is consistent with spin-orbit coupling splitting the $t_{2g}$ manifold of octahedral ${\mathrm{Ir}}^{4+}$ into a $J_{\mathrm{eff}}=\frac{1}{2}$ and $J_{\mathrm{eff}}=\frac{3}{2}$ state for all members of the series considered.

We report a study of polymorphic boron nitride (BN) samples. We interpret the photoluminescence (PL) line at $6.032\pm 0.005\mathrm{eV}$ that can be recorded at 8 K in $s{p}^2$-bonded BN as being the signature of the excitonic fundamental bandgap of the Bernal BN (bBN) [or graphitic BN (gBN)] polymorph. This is determined by advanced PL measurements combined with x-ray characterizations on pure hexagonal BN (hBN) and on polymorphic crystal samples, later compared with the theoretical predictions of Sponza et al., [Phys. Rev. B 98, 125206 (2018)]. The overall picture is consistent with a direct excitonic fundamental bandgap of the bBN (or gBN) polymorph. This value $d{X}_b=6.032\pm 0.005\mathrm{eV}$ is higher than the indirect bandgap of hBN $(i{X}_h=5.955\pm 0.005\mathrm{eV})$.

Nonstoichiometric ${\mathrm{ZrN}}_x$ (ZrN) thin films, ${\mathrm{ZrO}}_x{\mathrm{N}}_y$ (ZrON) thin films, and bilayer ZrN/ZrON structures were prepared, and the effects of stoichiometry and interface on their plasmonic properties were investigated. We find that the samples are all B1 structured with similar lattice constants. Higher nitrogen and oxygen content can reduce the screened plasma frequency ${\omega }_c$. Interestingly, the bilayer ZrN/ZrON structures with some ZrON thickness are more metallic than ZrN films, which should be reasonable since the mutual diffusion through the ZrN/ZrON interface may cause a ZrON buffer zone with substitute oxygen atoms generating more free carriers. The postulation is further confirmed by the calculations about the band structure, which indicates that substitute oxygen atoms can depress the interband transition level and cause more carriers in conduction band. This work implies that oxygen substitution is an effective method to enhance the performances of nitride-based plasmonic materials and devices.

We report unexpectedly fast phase transitions inside the track as pressure and temperature develop after a swift heavy ion impact. By means of molecular dynamics simulations we reveal the origin of the core-shell fine structure observed in a-${\mathrm{Si}}_{}{\mathrm{O}}_2$ and a-${\mathrm{Si}}_3{\mathrm{N}}_4$, as well as the dense core in a-Si. The fine structure of the track is composed of two distinct amorphous phases that result from the different pressure levels in the core and in the shell during solidification. We link this behavior to the tetrahedral network present in these materials.

Two-dimensional (2D) atomically thin boron layers designated as borophene or boraphene, attract great interest as promising post-graphene 2D materials. The borophene layer phonon structure has not yet been clarified by experimentation in spite of its importance for application for possible superconduction and thermal technologies. For this work we measured, by experimentation, the phonon dispersion of an incommensurate three-domain ${\chi }_3$-borophene formed on Ag(111) using high-resolution electron energy loss spectroscopy. Measured phonons were analyzed using first-principles theoretical calculations. Because the phonon band of the substrate Ag(111) lies in a low-energy region, the borophene phonon is well separated from the substrate. For our calculations, a commensurate slab supercell model was adopted, the periodicity of which was Ag(111) $3\times 3$. The lattice parameter of the model was determined using experimentally obtained phonon data. The highest frequency phonon of an isolated borophene layer was fitted to the data as a function of the electronic charge in it. The best fitted lattice parameter was used in the supercell model. The model well reproduced the experimentally observed phonon dispersion. No marked anomaly was found in the phonon structure.

${\mathrm{SrTiO}}_3$-based two-dimensional electron gases (2DEGs) can be formed through the deposition of epitaxial oxides like ${\mathrm{LaAlO}}_3$ or of reactive metals such as Al. Such 2DEGs possess a finite Rashba spin-orbit coupling that has recently been harnessed to interconvert charge and spin currents through the direct and inverse Edelstein and spin Hall effects. Here we compare the formation and properties of 2DEGs generated in ${\mathrm{SrTiO}}_3$ by the growth of Al, Ta, and Y ultrathin films by magnetron sputtering. By combining in situ and ex situ x-ray photoelectron spectroscopy (XPS) we gain insight into the reduction of the ${\mathrm{SrTiO}}_3$ and the appearance of ${\mathrm{Ti}}^{3+}$ states associated with 2DEG formation, its reoxidation by exposure to the air, and the transformation of the metal into its binary oxides. We extract the carrier densities through magnetotransport and compare them with the XPS data. Finally, working with samples covered by an extra layer of NiFe, we perform spin-pumping ferromagnetic resonance experiments and investigate spin-charge conversion as a function of gate voltage. We identify trends in the data across the different sample systems and discuss them as a function of the carrier density and the transparency of the metal oxide tunnel barrier.

Depositing disordered Al on top of $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ is a cheap and easy way to create a two-dimensional electron system in the $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ surface layers. To facilitate future device applications, we passivate the heterostructure by a disordered $\mathrm{La}\mathrm{Al}{\mathrm{O}}_3$ capping layer to study the electronic properties by complementary x-ray photoemission spectroscopy and transport measurements on the very same samples. We also tune the electronic interface properties by adjusting the oxygen pressure during film growth.

Flexoelectricity has garnered much attention owing to its ability to bring electromechanical functionality to nonpiezoelectric materials and its nanoscale significance. In order to move towards a more complete understanding of this phenomenon and improve the efficacy of flexoelectric-based devices, it is necessary to quantify microstructural contributions to flexoelectricity. Here we directly measure the flexoelectric response of bulk centrosymmetric ${\mathrm{LaAlO}}_3$ crystals with different twin-boundary microstructures. We show that twin-boundary flexoelectric contributions are comparable to intrinsic contributions at room temperature and enhance the flexoelectric response by $\sim 4\times$ at elevated temperatures. Additionally, we observe time-dependent and nonlinear flexoelectric responses associated with strain-gradient-induced twin-boundary polarization. These results are explained by considering the interplay between twin-boundary orientation, beam-bending strain fields, and pinning site interactions, and directly demonstrate that macroscopic flexoelectric responses are very sensitive to structural defects.

While potassium titanyl phosphate (KTP) is widely used for various optics applications, essentially nothing is known about its surfaces and electronic properties. Here the ground-state atomic structures of KTP [001] and [010] surfaces, frequently termed Z and Y cuts, respectively, have been determined using ground-state density-functional theory total-energy calculations. The calculated surface phase diagrams in dependence on the chemical potentials of the materials constituents show several stable nonstoichiometric terminations. A tendency to form oxygen-rich surfaces is observed. The ${\mathrm{Z}}^{+}$ and ${\mathrm{Z}}^{-}$ surfaces, discriminated by oppositely orientated internal electric fields, are found to differ with respect to their stoichiometry and structure. Occupied O-derived and in some cases unoccupied Ti-derived surface states appear in the lower and upper part of the bulk band gap, respectively.

Perovskite $\mathrm{SrTi}{\mathrm{O}}_3$ (STO) is an attractive photocatalyst for solar water splitting but suffers from a limited photoresponse in the visible spectral range due to its wide band gap. By means of hybrid density functional theory calculations, we systematically study engineering its band gap via doping $4d$ and $5d$ transition-metals $M$ ($M=\mathrm{Zr}$, Nb, Mo, Tc, Ru, Rh, Pd, Hf, Ta, W, Re, Os, Ir, and Pt) and chalcogen elements $Y$ ($Y=\mathrm{S}$ and Se). We find that transition-metal dopant $M$ either has no effect on the STO band gap or introduces detrimental midgap states except for Pd and Pt that are able to reduce the STO band gap. In contrast, doping S and Se significantly reduces STO's direct band gap, thus, leading to appreciable optical absorption transitions in the visible spectral range. Our findings provide that Pd-, S-, and Se-doped STO are potential promising photocatalysts for water splitting under visible light irradiation thereby providing insightful theoretical guides for experiments to improve the photocatalytic activity of STO.

A magnetic field parallel to an electrical current does not produce a Lorentz force on the charge carriers. Therefore, orbital longitudinal magnetoresistance is unexpected. Here we report on the observation of a large and nonsaturating magnetoresistance in lightly doped $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3-x}$ independent of the relative orientation of current and magnetic field. We show that this quasi-isotropic magnetoresistance can be explained if the carrier mobility along all orientations smoothly decreases with magnetic field. This anomalous regime is restricted to low concentrations when the dipolar correlation length is shorter than the distance between carriers. We identify cyclotron motion of electrons in a potential landscape tailored by polar domains as the cradle of quasi-isotropic orbital magnetoresistance. The result emerges as a challenge to theory and may be a generic feature of lightly doped quantum paralectric materials.

Negative magnetoresistances (NMRs) have been widely observed in two-dimensional electron gases (2DEGs). However, their origins are under debate. Here, we report on NMRs in the 2DEG in AlGaN/GaN heterostructures, aiming to uncover their origins by utilizing electric field gating. We systematically measured the magnetoresistances in magnetic fields up to 12 T and at temperatures between 3 and 260 K and observed NMRs over a wide range of temperatures from 3 to 170 K, which become more pronounced with decreasing temperature. We conducted electric field gating experiments to correlate the occurrence of NMRs with the relationship between electron mobility and density. The latter is governed by defect scattering sources in the sample and can be theoretically modeled. Comparison of the measured electron mobility and electron density relationship with theory reveals that interface roughness scattering plays a crucial role in obtaining large NMRs. Our work demonstrates that electric field gating provides a means not only to tune the values of NMRs but also to uncover their mechanisms.

We report the synthesis and structure of the second allotrope of technetium, $\beta \text{−}\mathrm{Tc}$. Transformative pathways are accessed at extreme conditions using the laser-heated diamond anvil cell and confirmed with in situ synchrotron x-ray diffraction and Raman spectroscopy. $\beta \text{−}\mathrm{Tc}$ is fully recoverable to ambient conditions, although counter to our DFT calculations predicting a face-centered-cubic lattice, we observe a tetragonal structure $(I4/mmm)$ that exhibits further tetragonal distortion with pressure. $\beta \text{−}\mathrm{Tc}$ has an expanded volume relative to the hcp ground state phase, that when doped with nitrogen has an unexpected volume lowering. Such anomalous behavior is possibly indicative of a rare electronic phase transition in a $4d$ element.

Ferroelectricity in hafnia is often regarded as a breakthrough discovery in ferroelectrics, potentially able to revolutionize the whole field. Despite increasing interests, a deep and comprehensive understanding of the many factors driving ferroelectric stabilization is still lacking. We here address the phase transition in terms of a Landau-theory-based approach, by analyzing symmetry-allowed distortions connecting the high-symmetry paraelectric tetragonal phase to the low-symmetry polar orthorhombic phase. By means of first-principles simulations, we find that the ${\mathrm{\Gamma }}_{3-}$ polar mode is only weakly unstable, whereas the other two symmetry-allowed distortions, ${\mathrm{Y}}_{2+}$ and ${\mathrm{Y}}_{4-}$ (showing a nonpolar and antipolar behavior, respectively), are hard modes. While none of the modes, taken alone or combined with one other mode, is able to drive the transition, the key factor in stabilizing the ferroelectric phase is identified as the strong trilinear coupling among the three modes. Furthermore, the experimentally acknowledged importance of substrate-induced effects in the growth of ${\mathrm{HfO}}_2$ ferroelectric thin films, along with the lack of a clear order parameter in the transition, suggested the extension of our analysis to strain effects. Our findings suggest a complex behavior of the ${\mathrm{Y}}_{2+}$ mode, which can become unstable under certain conditions (i.e., a tensile strain applied along the $a$ direction), and an overall weakly unstable behavior for the ${\mathrm{\Gamma }}_{3-}$ polar mode for all the strain conditions. In any case, a robust result emerges from our analysis: independently of the different applied strain (be it compressive or tensile, applied along the $a, b$, or $c$ orthorhombic axis), the need for a simultaneous excitation of the three coupled modes remains unaltered. Finally, when applied to mimic experimental growth conditions under strain, our analysis shows a further stabilization of the ferroelectric phase with respect to the unstrained case, in agreement with experimental findings.

${\mathrm{AgF}}_2$ is a correlated charge-transfer insulator with properties remarkably similar to insulating cuprates, which have raised hope that it may lead to a new family of unconventional superconductors upon doping. We use ab initio computations to study doping strategies leading to metallization. Because the upper Hubbard band is very narrow electron doping leads to undesired strongly self-trapped states (polarons). For the hole-doped case, the polaron tendency is stronger than for cuprates but still moderate enough to expect that heavily doped compounds may become metallic. Since the strong electron lattice coupling originates in the strong buckling we study also an hypothetically flat allotrope and show that it has excellent prospect to become metallic. We compare the ${\mathrm{AgF}}_2$ behavior with that for the hole-doped conventional cuprate ${\mathrm{La}}_2{\mathrm{CuO}}_4$ and electron-doped ${\mathrm{Nd}}_2{\mathrm{CuO}}_4$. Our results show a clear path to achieve high-temperature superconductivity in silver fluorides.

The discovery of novel materials with superior tailored properties is highly attractive for nano- and optoelectronics. Inspired by penta graphene, we introduce a novel family of two-dimensional group-IV materials with ${\mathrm{C}}_2$ dimers, namely penta carbides. The first-principles calculations reveal that the unitary, binary, and ternary penta carbides have excellent energetic, dynamical, thermal, and mechanical stability with remarkable properties. The unitary penta carbide displays auxetic behavior whereas the binary and ternary penta carbides posses near zero and positive Poisson's ratio. The phonon dispersion curves of penta carbides indicate a remarkable phononic gap, which can be tuned by alloy engineering and hydrogenation. They are semiconductor in nature with band-gap energy ranging from 1.35 and 2.39 eV. Alloying and strain engineering enable the direct modification of atomic bonding and thereby tuning of electronic band gap of penta carbides. Such novel tunable electronic and phononic properties of penta carbides can find applications in the field of nanoelectronics, sensors, and frequency filter applications. The band edge positions of penta carbides (except for Sn-based ones) straddle the redox potentials of water. Remarkably, penta carbides exhibit very high optical absorption in the visible and ultraviolet regions (up to ${10}^{-6} {\mathrm{cm}}^{-1}$). They have small and anisotropic carrier effective masses, indicating fast carrier transport characteristics and promoting the photo-generated electron-hole separation efficiency. The high specific surface areas, suitable and sizable band gaps, appropriate band edges, small effective carrier masses, and excellent optical absorption capability, all these exotic properties taken together, make penta carbides promising candidates for photocatalytic water splitting.

The heat-carrying acoustic excitations of amorphous silicon are of interest because their mean free paths may approach micron scales at room temperature. Despite extensive investigation, the origin of the weak acoustic scattering in the heat-carrying frequencies remains a topic of debate. Here, we report measurements of the thermal conductivity mean free path accumulation function in amorphous silicon thin films from 60 to 315 K using transient grating spectroscopy. With additional picosecond acoustics measurements and considering the power-law frequency dependence of scattering mechanisms in glasses, we reconstruct the mean free paths from $\sim 0.1–3$ THz. The mean free paths are independent of temperature and exhibit a Rayleigh scattering trend over most of this frequency range. The observed trend is inconsistent with the predictions of numerical studies based on normal mode analysis but agrees with diverse measurements on other glasses. The micron-scale MFPs in amorphous Si arise from the absence of anharmonic or thermally activated relaxation damping in the sub-THz frequencies, leading to heat-carrying acoustic excitations with room-temperature MFPs comparable to those of other glasses at cryogenic temperatures.

The discovery of superconductivity in the infinite-layer nickelates has opened new perspectives in the context of quantum materials. We analyze, via first-principles calculations, the electronic properties of ${\mathrm{La}}_2{\mathrm{NiO}}_3\mathrm{F}$—the first single-layer T'-type nickelate—and compare these properties with those of related nickelates and isostructural cuprates. We find that ${\mathrm{La}}_2{\mathrm{NiO}}_3\mathrm{F}$ is essentially a single-band system with a Fermi surface dominated by the Ni-$3d_{x^2-y^2}$ states with an exceptional 2D character. In addition, the hopping ratio is similar to that of the highest $T_c$ cuprates and there is a remarkable $e_g$ splitting together with a charge transfer energy of 3.6 eV. According to these descriptors, along with a comparison to ${\mathrm{Nd}}_2{\mathrm{CuO}}_4$, we thus indicate single-layer T'-type nickelates of this class as very promising analogs of cupratelike physics while keeping distinct ${\mathrm{Ni}}^{1+}$ features.