We propose a multiplexed-acoustic-metasurface hologram that can project multiple predesigned images at different frequencies. We demonstrate the generation of fourfold acoustic holographic images by one metasurface at 3500, 4500, 5500, and 6500 Hz, respectively, and discuss their fundamental physics. We uncover the design of this acoustic hologram which simultaneously realizes multiplexing and modulations of amplitude and phase. The proposed metasurface unit cell is composed of multiple acoustic resonant cavities leading to the multiplexing effect. We consider here both leaky and intrinsic losses, which provide simultaneous controllable manipulations of amplitude and phase. These proposed strategies significantly increase the degree of freedom of the classical metasurfaces, which may open up additional avenues for multifunctional compact acoustic devices.

Donor-doped transition metal oxides such as donor-doped strontium titanate ($n-{\mathrm{SrTiO}}_3$) are of fundamental importance for oxide electronic devices as well as for electronic surface and interface engineering. Here we quantitatively analyze the variable band alignment and the resulting space charge layer at the surface of $n-{\mathrm{SrTiO}}_3$, determined by its surface redox chemistry. Synchrotron-based ambient-pressure x-ray photoelectron spectroscopy conducted under applied thermodynamic bias is used to access electronic structure and chemistry of the surface. We find an electron depletion layer driven by cationic surface point defects that are controlled by adjusting the ambient atmosphere ($p{\mathrm{O}}_2$). We correlate the $p{\mathrm{O}}_2$ dependence to a response of the strontium sublattice, namely the precipitation of strontium oxide and the formation of charged strontium vacancies at the surface. We suggest the reversible conversion of surface-terminating strontium oxide into extended strontium oxide clusters as the responsible process by resolving chemical dynamics in situ. As we show, atomic control of these subtle changes in the surface redox chemistry allows us to tailor electrical transport properties along the $n-{\mathrm{SrTiO}}_3$ surface. Our study thereby gives access to engineering electronic band bending in transition metal oxides by the control of the surface chemistry.

Superconductivity (SC) in the Ba-122 family of iron-based compounds can be controlled by aliovalent or isovalent substitutions, applied external pressure, and strain, the combined effects of which are sometimes studied within the same sample. Most often, the result is limited to a shift of the SC dome to different doping values. In a few cases, the maximum SC transition at optimal doping can also be enhanced. In this work, we study the combination of charge doping together with isovalent P substitution and strain by performing ionic gating experiments on ${\mathrm{BaFe}}_2$(${\mathrm{As}}_{0.8}{\mathrm{P}}_{0.2}$)${}_2$ ultrathin films. We show that the polarization of the ionic gate induces modulations to the normal-state transport properties that can be mainly ascribed to surface charge doping. We demonstrate that ionic gating can only shift the system away from the optimal conditions, as the SC transition temperature is suppressed by both electron and hole doping. We also observe a broadening of the resistive transition, which suggests that the SC order parameter is modulated nonhomogeneously across the film thickness, in contrast with earlier reports on charge-doped standard BCS superconductors and cuprates.

Selected-area electron diffraction and convergent-beam electron diffraction (CBED) techniques are used to reveal the Ba doping effect on the crystal structure of (${\mathrm{Sr}}_{1-x}{\mathrm{Ba}}_x{)}_2{\mathrm{Nb}}_2{\mathrm{O}}_7$ [SBN(x)] in the temperature range from 293 to 693 K. Ba doping of ${\mathrm{Sr}}_2{\mathrm{Nb}}_2{\mathrm{O}}_7$ (SN) causes weakening of the incommensurate modulation and deviation from the C-centered lattice. On the other hand, symmetries of fundamental reflections in CBED patterns of SBN(x) are seen to be the same as those of SN, showing a small structural deviation from Cmc$2_1$. Crystal symmetries above and below the temperature of a relaxorlike dielectric anomaly in SBN ($x=0.32$) at 465 K remain unchanged. CBED experiments with a nanometer-sized electron probe do not show any indication of the presence of a nanodomain around this temperature. However, the presence of a few-micron-sized polar-inversion domains is found at around 573 K in SBN ($x=0.32$). The origin of the reported relaxorlike dielectric anomaly in SBN ($x=0.32$) is discussed on the basis of the experimental results of CBED.

Avalanche events occur during mechanical loading of many material systems and are characterized by stress drops and acoustic emission (AE). Stress drops are directly related to the macroscopic response of the investigated material, but their detection capability is restricted to relatively large and slow events. AE measurements can detect events with smaller amplitude and shorter duration, but their energy and duration are not directly related to the change of the system. In this paper, we present simultaneous measurements of stress drops and AE during mechanically induced twin boundary motion in Ni-Mn-Ga. We found that the probability of finding an AE event during a stress drop is $\sim 100$ times higher than between stress drops. Analysis of the relations between mechanical energy drops $\mathrm{\Delta }{U}_m$ and acoustic emitted energy $E_{\mathrm{AE}}$, on the level of individual events, reveals the existence of a lower bound for $E_{\mathrm{AE}}$, which is approximately proportional to $\mathrm{\Delta }{U}_m$. These results imply that the macroscopic stress changes generate acoustic waves, which contribute a well-defined amount of energy that is equal to the lower bound function. Furthermore, smaller scale events that are related to microscopic subprocesses by which the twin boundary moves generate additional AE energy. The latter contribution displays a power-law distribution, which implies that these processes are close to criticality.

In this work, we report an ab initio investigation based on density functional theory of the structural, energetic, and electronic properties of 2D layered chalcogenides compounds based in the combination of the transition-metals (Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W) and chalcogenides (S, Se, Te) in three polymorphic phases: trigonal prismatic (2H), octahedral (1T), and distorted octahedral ($1{\mathrm{T}}_{\text{d}}$). We determined the most stable phases for each compound, verifying the existence of the $1{\mathrm{T}}_{\text{d}}$ phase for a small number of the compounds and we have also identified the magnetic compounds. In addition, with the determination of the exfoliation energies, we indicated the potential candidates to form one layer material and we have also found a relation between the exfoliation energy and the effective Bader charge in the metal, suggesting that when the materials present small exfoliation energies, it is due to the Coulomb repulsion between the chalcogen planes. Finally, we analyzed the electronic properties, identifying the semiconductor, semimetal, and metal materials and predicting the band gap of the semiconductors. In our results, the dependence of the band gap on the $d$-orbital is explicit. In conclusion, we have investigated the properties of stable and metastable phases for a large set of TMD materials, and our findings may be auxiliary in the synthesis of metastable phases and in the development of new TMDs applications.

We investigated the pressure evolution of the electrical transport in the almost compensated Weyl semimetal TaP. In addition, we obtained information on the modifications of the Fermi-surface topology with pressure from the analysis of pronounced Shubnikov–de Haas (SdH) quantum oscillations present in the Hall-effect and magnetoresistance data. The simultaneous analysis of the Hall and longitudinal conductivity data in a two-band model revealed an only weak decrease in the electron and hole charge-carrier densities up to 1.2 GPa, while the mobilities are essentially pressure independent along the $a$ direction of the tetragonal crystal structure. Only weak changes in the SdH frequencies for $B\parallel a$ and $B\parallel c$ point at a robust Fermi-surface topology. In contrast to the stability of the Fermi-surface topology and of the density of charge carriers, our results evidence a strong pressure variation of the magnitude of transverse magnetoresistance for $B\parallel a$ contrary to the results for $B\parallel c$. We can relate the former to an increase in the charge-carrier mobilities along the crystallographic $c$ direction.

We present a theoretical study of the lattice thermal conductivity of ${\mathrm{SrTiO}}_3$ in its antiferrodistortive ferroelastic phase and of its dependence on an applied external electric field, via electrophononic couplings. The calculations are done by using second-principles density-functional theory and the full solution of the Boltzmann transport equation. Our results allow, on one hand, to identify and explain deviations from the usual temperature dependence of the thermal conductivity, revealing Poiseuille flow and a rare umklapp transport regime, in agreement with recent experimental results [Martelli et al., Phys. Rev. Lett. 120, 125901 (2018)]; on the other hand, they show that an external electric field, by reducing the symmetry of the lattice, activates different phonon-phonon scattering processes and thus yields a reduction of the thermal conductivity, supporting the generality of a heat control strategy previously reported by some of us [Seijas-Bellido et al., Phys. Rev. B 97, 184306 (2018)].

Production rates in long-term predictive radiation damage accumulation models are generally considered independent of the material's microstructure for reactor components. In this study, the effect of preexisting microstructural elements on primary damage production in α-Zr—and vice versa—is assessed by molecular dynamics (MD) simulations. $a$-type dislocation loops, $c$-component dislocation loops, and a tilt grain boundary (GB) were considered. Primary damage production is reduced in the presence of all these microstructural elements, and clustering behavior is dependent on the microstructure. Collision cascades do not cause $a$-type loop growth or shrinkage, but they cause $c$-component loop shrinkage. Cascades in the presence of the GBs produce more vacancies than interstitials. This result, as well as other theoretical, MD, and experimental evidence, confirms that vacancy loops will grow in the vacancy-supersaturated environment near GBs. Distinct temperature-dependent growth regimes are identified. Also, MD reveals cascade-induced events where $a$-type vacancy loops are absorbed by GBs. Fe segregation at the loops inhibits this cascade-induced absorption.

The interactions between electric and magnetic subsystems in a ferroelectric-ferromagnetic composite occur through mechanical forces. Here we discuss results of a systematic investigation on the strength of the magnetic response of the composite to an applied electric field, known as the converse magnetoelectric (CME) effect, and its dependence on the ferroic order parameters and volume fraction for the two phases. Studies were carried out on composites of lead zirconate titanate and 2–30-μm-thick nickel zinc ferrite (NZFO) films grown by liquid phase epitaxy on lattice matched (100) and (111) MgO substrates. Ferromagnetic resonance was utilized to determine the strength of CME from data on electric field E induced shift in the resonance frequency and its dependence on ferrite film orientation and thickness as well as MgO substrate thickness. The CME coupling coefficient A was found to be a factor of 2 to 4 higher in samples with NZFO films with (100) orientation than for (111) films. A decrease in A was measured with increasing ferrite film thickness and a very significant enhancement in the strength of CME was measured for decreasing MgO thickness. A model for CME that takes into consideration the influence of nonferroic MgO substrate was developed, and estimated A values are in very good agreement with the data. The results presented here are also of importance for a new class of electric field tunable ferrite microwave devices.

We investigate the stability of self-trapped holes (STHs) and the acceptor levels of substitutional Mg and N impurities in $\alpha$-, $\beta$-, $\delta$-, and $\varepsilon \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ using first-principles calculations based on the hybrid functional approach to assess their $p$-type dopability. When Fock-exchange and screening parameter values in the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional are optimized to satisfy the generalized Koopmans' theorem for a STH level in $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$, the band gap is slightly overestimated, while functionals that well reproduce the band gap show slight convex behavior against the fractional electron number. However, the absolute position of the STH level at a fixed geometry is nearly independent of the parameter value, showing that the results are robust as long as the STH geometry and localized electronic nature are appropriately described and the band edges are well reproduced. In all of the polymorphs, holes localize with high self-trapping energies rather than being delocalized. Furthermore, both Mg and N impurities introduce polaronic acceptor states, and their acceptor levels lie far above the valence band maximum in all the polymorphs. Thus, the $p$-type doping of the four ${\mathrm{Ga}}_2{\mathrm{O}}_3$ polymorphs seems unfeasible in terms of the STH formation and the related deep, polaronic acceptor nature of the Mg and N impurities.

Magnetic multilayer thin films with perpendicular magnetic anisotropy (PMA) and interfacial Dzyaloshinskii-Moriya interaction (iDMI) are of intense interest for realizing magnetic skyrmions and modifying topological spin textures. We systematically investigate interlayer exchange coupling (IEC) in Pt/Co/Ru(Ir) superlattices that have PMA and large iDMI. The IEC is greatly tunable by varying Ru(Ir) or Pt thickness and the antiferromagnetic IEC is as large as $1.3\mathrm{mJ}/{\mathrm{m}}^2$ that is on the same order of magnitude as the iDMI. We find unusual magnetic hysteresis loop crossing between field-ascending and -descending magnetization curves. Furthermore, we identify magnetic phase diagrams for antiferromagnetic IEC and hysteresis loop crossing with respect to Ru(Ir) and Pt thickness. Our experimental findings may open a way in the development of synthetic antiferromagnetic spintronics and/or the realization of antiferromagnetic skyrmions.

Based on first-principles calculations, the evolution of the electronic and magnetic properties of transition metal dihalides $M{X}_2(M=\text{V}$, Mn, Fe, Co, Ni; $X=\text{Cl}$, Br, I) is analyzed from the bulk to the monolayer limit. A variety of magnetic ground states is obtained as a result of the competition between direct exchange and superexchange. The results predict that $\mathrm{Fe}{X}_2,\mathrm{Ni}{X}_2,{\mathrm{CoCl}}_2$, and ${\mathrm{CoBr}}_2$ monolayers are ferromagnetic insulators with sizable magnetocrystalline anisotropies. This makes them ideal candidates for robust ferromagnetism at the single-layer level. Our results highlight the importance of spin-orbit coupling to obtain the correct ground state in these materials.

We provide an avenue to achieve optimized magnetic properties without relying on rare-earth elements. In particular, we focus on transition-metal nanoparticles with magnetic ordering that is rarely seen in bulk. By using first-principles real-space pseudopotential calculations, we show that Mn can exhibit ferromagnetic behavior similar to Fe when Mn is placed on the surface of a FeCo cluster. This ferromagnetic behavior of surface Mn is in sharp contrast with Mn bulk and Mn-based alloys, which are either antiferromagnetic or paramagnetic. The local magnetic moment carried by a surface Mn atom is larger than those of Fe and Co, leading to an increase of the total magnetic moment. We find that surface-Mn-induced magnetic enhancement is robust in ternary MnFeCo clusters with different sizes and various chemical compositions. Our theoretical findings provide a useful guide for designing new nanoparticles with a large magnetic moment.

We present an in-depth investigation of structural and electronic properties of GaAsBi epilayers. High (001) crystalline order is achieved using careful molecular beam epitaxy and surface preparation procedures. High surface order allows us to use x-ray, ultraviolet, and angle-resolved photoemission spectroscopy at variable photon energies and to disentangle electronic effects of an atomically thin Bi-rich surface layer with $(2\times 3)$ symmetry from those of Bi atoms incorporated in the GaAs bulk matrix. The influence of bulk-integrated Bi concentrations on the GaAs band structure becomes visible in angle-resolved photoemission after removing Bi-rich surface layers by a brief and mild ion bombardment and subsequent annealing treatment. Experimental observations are supported by density functional theory simulations of the valence band structure of bulk and surface-reconstructed GaAs with and without Bi. Bi-induced energy shifts in the dispersion of GaAs heavy and light hole bulk bands are evident both in experiment and theory, which are relevant for modulations in the optical band gap and thus optoelectronic applications.

Materials combining both a high refractive index and a wide band gap are of great interest for optoelectronic and sensor applications. However, these two properties are typically described by an inverse correlation with high refractive index appearing in small gap materials and vice versa. Here, we conduct a first-principles high-throughput study on more than 4000 semiconductors (with a special focus on oxides). Our data confirm the general inverse trend between refractive index and band gap but interesting outliers are also identified. The data are then analyzed through a simple model involving two main descriptors: the average optical gap and the effective frequency. The former can be determined directly from the electronic structure of the compounds, but the latter cannot. This calls for further analysis in order to obtain a predictive model. Nonetheless, it turns out that the negative effect of a large band gap on the refractive index can be counterbalanced in two ways: (i) by limiting the difference between the direct band gap and the average optical gap which can be realized by a narrow distribution in energy of the optical transitions and (ii) by increasing the effective frequency which can be achieved through either a high number of transitions from the top of the valence band to the bottom of the conduction band or a high average probability for these transitions. Focusing on oxides, we use our data to investigate how the chemistry influences this inverse relationship and rationalize why certain classes of materials would perform better. Our findings can be used to search for new compounds in many optical applications both in the linear and nonlinear regime (waveguides, optical modulators, laser, frequency converter, etc.).

We present a muon spin relaxation study of the Mott transition in ${\mathrm{BaCoS}}_2$ using two independent control parameters: (i) pressure $p$ to tune the electronic bandwidth and (ii) Ni substitution $x$ on the Co site to tune the band filling. For both tuning parameters, the antiferromagnetic insulating state first transitions to an antiferromagnetic metal and finally to a paramagnetic metal without undergoing any structural phase transition. ${\mathrm{BaCoS}}_2$ under pressure displays minimal change in the ordered magnetic moment $S_{ord}$ until it collapses abruptly upon entering the antiferromagnetic metallic state at $p_{cr}\sim 1.3\mathrm{GPa}$. In contrast, $S_{ord}$ in the Ni-doped system $\mathrm{Ba}\left({\mathrm{Co}}_{1-x}{\mathrm{Ni}}_x\right){\mathrm{S}}_2$ steadily decreases with increasing $x$ until the antiferromagnetic metallic region is reached at $x_{cr}\sim 0.22$. In both cases, significant phase separation between regions with static magnetic order and paramagnetic/nonmagnetic regions develops when approaching $p_{cr}$ or $x_{cr}$, and the antiferromagnetic metallic state is characterized by weak, random, static magnetism in a small volume fraction. No dynamical critical behavior is observed near the transition for either tuning parameter. These results demonstrate that the quantum evolution of both the bandwidth- and filling-controlled metal-insulator transition at zero temperature proceeds as a first-order transition. This behavior is common to magnetic Mott transitions in $R{\mathrm{NiO}}_3$ and ${\mathrm{V}}_2{\mathrm{O}}_3$, which are accompanied by structural transitions without the formation of an antiferromagnetic metal phase.

The interaction between electronic and nuclear spins in the presence of external magnetic fields can be described by a spin Hamiltonian, with parameters obtained from first principles, electronic structure calculations. We describe an approach to compute these parameters, applicable to both molecules and solids, which is based on density functional theory (DFT) and real-space, all-electron calculations using finite elements (FE). We report results for hyperfine tensors, zero field splitting tensors (spin-spin component), and nuclear quadrupole tensors of a series of molecules and of the nitrogen-vacancy center in diamond. We compare our results with calculations using Gaussian orbitals and plane-wave basis sets, and we discuss their numerical accuracy. We show that calculations based on FE can be systematically converged with respect to the basis set, thus allowing one to establish reference values for the spin Hamiltonian parameters, at a given level of DFT.