We present a comprehensive study on the silver bismuthate ${\mathrm{Ag}}_2{\mathrm{BiO}}_3$, synthesized under high-pressure, high-temperature conditions, which has been the subject of recent theoretical work on topologically complex electronic states. We present x-ray photoelectron spectroscopy results showing two different bismuth states and x-ray absorption spectroscopy results on the oxygen $K$ edge showing holes in the oxygen bands. These results support a bond disproportionated state with holes on the oxygen atoms for ${\mathrm{Ag}}_2{\mathrm{BiO}}_3$. We estimate a band gap of $\sim 1.25$ eV for ${\mathrm{Ag}}_2{\mathrm{BiO}}_3$ from optical conductivity measurements, which matches the band gap in density functional calculations of the electronic band structure in the nonsymmorphic space group $Pnn2$, which supports two inequivalent Bi sites. In our band structure calculations the disproportionated ${\mathrm{Ag}}_2{\mathrm{BiO}}_3$ is expected to host Weyl nodal chains, one of which is located $\sim 0.5$ eV below the Fermi level. Furthermore, we highlight similarities between ${\mathrm{Ag}}_2{\mathrm{BiO}}_3$ and the well-known disproportionated bismuthate ${\mathrm{BaBiO}}_3$, including breathing phonon modes with similar energy. In both compounds, hybridization of Bi $6s$ and O $2p$ atomic orbitals is important in shaping the band structure, but in contrast to the Ba $5p$ bands in ${\mathrm{BaBiO}}_3$, the Ag $4d$ bands in ${\mathrm{Ag}}_2{\mathrm{BiO}}_3$ extend up to the Fermi level.

Among the different piezoceramics, ${\mathrm{Na}}_{0.5}{\mathrm{Bi}}_{0.5}\mathrm{Ti}{\mathrm{O}}_3$ (NBT)-based systems have the unique distinction of exhibiting very large electrostrain $\left(>0.5\%\right)$ when pushed at the threshold of ergodic–nonergodic relaxor transition. However, this strategy severely compromises the system's weak-signal piezoelectric response ($d_{33}$). Here, we show that a similar level of electrostrain is feasible together with high $d_{33}$ by a different approach. In our approach, the system is kept well within the nonergodic relaxor regime close to an interferroelectric instability, and the inherent structural (tilt) disorder is exploited for large reverse switching of the ferroelastic domains in the field-stabilized ferroelectric phase. A synergy of this combination is demonstrated in $\left(1-x\right){\mathrm{Na}}_{0.5}{\mathrm{Bi}}_{0.5}\mathrm{Ti}{\mathrm{O}}_{3-}\left(x\right){\mathrm{K}}_{0.5}{\mathrm{Bi}}_{0.5}\mathrm{Ti}{\mathrm{O}}_3$; $x=0.25$ which exhibit electrostrain of ∼0.6% and $d_{33}$ (∼210 pC/N). With the added advantage of high depolarization temperature $(T_d\sim 185{}^{\circ }\mathrm{C})$ this piezoceramic stands unique for showing the best combination of the three most sought after properties.

Dielectric relaxation in ferroelectric perovskites can result from different inherent chemical and charge disorders within their crystal structures. Despite several theories on dielectric relaxations, the relationship between the concentration of oxygen vacancy ($V_{\mathrm{O}}$) and dielectric relaxation has not been studied in perovskite oxide thin films. In this work, we report a systematic investigation on the influence of the $V_{\mathrm{O}}^{\prime }\mathrm{s}$ on the dielectric relaxation of ${\mathrm{Ba}}_{0.85}{\mathrm{Ca}}_{0.15}{\mathrm{Ti}}_{0.9}{\mathrm{Zr}}_{0.1}{\mathrm{O}}_3$ (BCZT) epitaxial thin films grown with ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_3$ (LSMO) top and bottom electrodes on single-crystal ${\mathrm{SrTiO}}_3$ (100) substrates using pulsed laser deposition. The ambient oxygen pressures during film growth were systematically varied to control the concentration of $V_{\mathrm{O}}$ in the epitaxial BCZT thin films. Low ambient oxygen pressure was found to enhance the tetragonality of the BCZT films and a systematic decrease in the tetragonality was observed at high oxygen pressure conditions as evidenced from x-ray diffraction (XRD) studies. Temperature dependent XRD analyses indicated a low-temperature anomaly near $\sim 160\mathrm{K}$ in the BCZT/LSMO heterostructures. Low-temperature dielectric measurements revealed relaxor dielectric response with broad frequency dependence of the dielectric constant. It is proposed that the low-temperature dielectric relaxation is possibly caused by dipolar clusters that are formed by the aggregation of correlated $V_{\mathrm{O}}$-induced dipolar defects within the films deposited under different ambient oxygen pressures. High-resolution transmission electron microscopy suggested the presence of the nanometer-sized $V_{\mathrm{O}}$ clusters as represented by the structural defects of average diameters of $\sim 5\mathrm{nm}$ in the BCZT films grown at variable ambient oxygen pressure. Activation energies of the aforesaid dipolar clusters, obtained from the Vogel-Fulcher fitting of the dielectric dispersion, showed a gradual increase in magnitude, as well as decreasing freezing temperatures, with decreasing concentrations of $V_{\mathrm{O}}$ in the BCZT films. From the theoretical analysis of the experimental data, it is postulated that the presence of the $V_{\mathrm{O}}$-induced dipolar defects affects the low-temperature dielectric relaxation in the BCZT thin films. This work significantly extends the understanding of $V_{\mathrm{O}}$-mediated dielectric relaxor behavior in complex perovskite oxide films.

Materials composed of elements from the third and fifth columns of the periodic table display a very rich behavior, with the phase diagram usually containing a metallic liquid phase and a polar semiconducting solid. As a consequence, it is very hard to achieve transferable empirical models of interactions between the atoms that can reliably predict their behavior across the temperature and composition range that is relevant to the study of the synthesis and properties of III/V nanostructures and devices. We present a machine-learning potential trained on density functional theory reference data that provides a general-purpose model for the ${\mathrm{Ga}}_x{\text{As}}_{1-x}\text{system}$. We provide a series of stringent tests that showcase the accuracy of the potential, and its applicability across the whole binary phase space, computing with ab initio accuracy a large number of finite-temperature properties as well as the location of phase boundaries. We also show how a committee model can be used to reliably determine the uncertainty induced by the limitations of the machine-learning model on its predictions, to identify regions of phase space that are predicted with insufficient accuracy, and to iteratively refine the training set to achieve consistent, reliable modeling.

Recent developments in two-dimensional (2D) magnetism have motivated the search for van der Waals (vdW) magnetic materials to explore magnetic phenomena in the 2D limit. Metal thiophosphates, $M\mathrm{P}{X}_3$, are a class of magnetic vdW materials with antiferromagnetic ordering persisting down to the atomically thin limit. The magnetism in this material family has been found to be highly dependent on the choice of transition metal $M$. In this work, we have synthesized the intermediate compounds ${\mathrm{Ni}}_{1\text{−}x}{\mathrm{Mn}}_x\mathrm{P}{\mathrm{S}}_3$ ($0\le x\le 1$) and investigated their magnetic properties. Our study reveals that the variation of Ni and Mn content in ${\mathrm{Ni}}_{1\text{−}x}{\mathrm{Mn}}_x\mathrm{P}{\mathrm{S}}_3$ can efficiently tune the spin-flop transition, likely due to the modulation of the magnetic anisotropy. Such effective tuning offers a promising candidate to engineer 2D magnetism for future device applications.

Carbon vacancy ${\mathrm{V}}_{\mathrm{C}}$ is the dominant detrimental defect in SiC, and hydrogen passivation of ${\mathrm{V}}_{\mathrm{C}}$ is often used to facilitate its application in electronic devices. However, the exact nature of hydrogen passivation of ${\mathrm{V}}_{\mathrm{C}}$ in $4H$-SiC remains inconclusive in view of the available divergent experiment and theoretical findings. Here, using the Heyd-Scuseria-Ernzerhof screened hybrid density functional calculations, we demonstrate that the ${\mathrm{V}}_{\mathrm{C}}$ defect can capture up to four hydrogens, and the electrically active levels within the band gap can be entirely passivated, in line with recent reported experimental observations. This paper, thus, casts light on the role of hydrogen passivation in SiC.

High-fluence gallium $\left({\mathrm{Ga}}^{+}\right)$ implantation at medium energies is proven to be an effective tool in forming superconducting (SC) thin films in germanium (Ge). By changing the post-implantation annealing conditions nanocrystalline to single-crystalline Ge matrices have been produced. Irrespective of crystallinity, such processes have mostly led to supersaturated Ge:Ga films where superconductivity is controlled by the extent of coherent coupling between Ga precipitates. Here we use ${\mathrm{Ga}}^{+}$ implantation energy as a means to tailor the spatial distribution and the coupling energy of the Ga precipitates. By systematic structural and magneto-transport studies, we unravel the complex connection between the internal structure of Ge:Ga films and their global SC parameters. At the shallowest implantation depth, we observe the strongest coupling leading to a robust superconductivity that sustains parallel magnetic fields as high as 9.95 T, above the conventional Pauli paramagnetic limit and consistent with a quasi-2D geometry. Further measurements at mK temperatures revealed an anomalous upturn in perpendicular critical field ${\mathrm{B}}_{\perp }$ vs temperature whose curvature and thus origin may be tuned between weakly coupled SC arrays and vortex glass states with quenched disorder. This warrants future investigations into Ge:Ga films for applications where tunable disorder is favorable, including test-beds for quantum phase transitions and superinductors in quantum circuits.

The electrical conductivity of the polyimide R-BAPB polymer filled with single-wall carbon nanotubes (CNT) is modeled using a multiscale approach. The modeling starts with molecular dynamics simulations of time-dependent atomic configurations of polymer-filled CNTs junctions. Then the atomic positions obtained in the first step are used to perform fully first-principles microscopic calculations of the CNTs junctions contact resistances using the quantum transport technique based on Green's functions. Finally, those contact resistances are supplied as an input to a statistical calculation of a CNTs ensemble conductivity using a Monte Carlo percolation model. We discuss the effects of various geometrical peculiarities of CNTs mutual orientation, including an angle $\phi$ between nanotubes axes, a CNTs overlap, a separation between CNTs, as well as CNTs sizes, chiralities, CNTs functionalization on the contact resistance of CNTs junctions. The results of the first-principles calculations show that of all the considered geometrical peculiarities the angle dependence of CNTs intersections has the most significant influence on contact resistance of polymer-filled CNTs junctions. A simple fitting model, describing the dependence of a junction conductance of that angle, is proposed. Incorporating into the percolation model this strong dependence as well as CNTs agglomeration pushed the calculated values of electrical conductivity of the composite just above the percolation threshold below 0.01 S/m, which is within the experimental range for composites with various base polymers. Possible mechanisms for further reduction of composites conductivity are discussed.

We explore the possibility of the production of Si polytypes via the total energies and free energies obtained in the density-functional theory and the density-functional perturbation theory. We first calculate the total energies of three polytypes of the tetrahedrally coordinated $s{p}^3$ Si atoms having the two-layer, three-layer, and four-layer stacking periods in the hexagonal unit cell (“$2H$”, “$3H$”, and “$4H$”) as a function of the in-plane lattice constant $a$ using the local-density approximation (LDA) and generalized gradient approximation (GGA) comparatively. In the LDA calculation, the $4H$ phase is energetically the most favorable among three phases in the range of 3.55 Å $\le$ $a$ $\le$ 3.73 Å. While the $4H$ phase is found to be more stable than the $2H$ phase at a relatively wide range of the $a$ value we studied, the $2H$ phase is found to be more stable than the $3H$ ($3C$) phase in the range of 3.57 Å $\le a\le$ 3.61 Å. In the GGA calculation, on the other hand, the $4H$ phase becomes the most favorable phase for almost the same range as the LDA case, 3.57 Å $\le a\le$ 3.73 Å. The $2H$ phase also becomes as stable as the $3H$ phase as in the case of the LDA around the small $a$ value of 3.62 Å, although the $3H$ phase energy is always lower than that of the $2H$ phase in the GGA calculation. Considering the chemical vapor deposition (CVD) growth process, we calculate the biaxial stress values to reduce the lattice constant of the substrate Si crystal to realize the small $a$ values, which can lead to the homoepitaxial growth of the $4H$ phase. The values obtained are 4.69 GPa and 8.14 GPa for the LDA and GGA, respectively. Next, we study the thermal effect on the relative stabilities of the 3 phases. We calculate the phonon dispersion, bulk modulus and thermal expansion including the vibrational and thermal effect. From the comparison with experimental values of the $3C$ phase, it is found that the LDA results show much better agreement with experiment than the GGA, indicating that the LDA calculation is reliable to predict thermal properties of the real systems. Finally, we derive the free energy as a function of $a$ under several designated temperatures. The relative stabilities of the $2H$ and $4H$ phases with respect to the $3H$ phase are found to be enhanced. At around $a=3.65$ Å, stabilities of $2H$ and $4H$ phases relative to the $3H$ phase become most prominent. Although stabilities of the $2H$ and $4H$ phases relative to the $3H$ phase are reduced with increasing temperature at around $a$ = 3.60 Å, $2H$ phase is found to be as stable as $3H$ and $4H$ phases at higher temperatures, indicating the possibility of the production of the $2H$ phase via high-temperature CVD process with the biaxial stress.

The quasiparticle band structure can be properly described by Hedin's GW approximation (GW), at a high computational cost. For band gaps, semilocal density functionals up to the generalized gradient approximation (GGA) level cannot compete with the accuracy of hybrid-based approximations or GW. Meta-GGA density functionals with a strong dependence on the kinetic energy density ingredient can potentially give wider band gaps compared with GGAs. The recent TASK meta-GGA density functional from Aschebrock and Kümmel [Phys. Rev. Research 1, 033082 (2019)], is constructed with an enhanced nonlocality in the generalized Kohn-Sham scheme and therefore harbors great opportunities for band gap prediction. Although this approximation was found to yield excellent band gaps of bulk solids, this accuracy cannot be straightforwardly transferred to low-dimensional materials. The reduced screening of these materials results in larger band gaps compared with their bulk counterparts, as an additional barrier to overcome. In this paper we demonstrate how the alteration of this functional affects the band gaps of monolayers and nanoribbons and present accurate band gaps competing with the revised Heyd-Scuseria-Ernzerhof (HSE06) approximation. In order to achieve this goal, we have modified the TASK functional (a) by changing the tight upper bound for one- or two-electron systems ($h_X^0$) from 1.174 to 1.29 and (b) by changing the limit of the interpolation function $f_X(\alpha \to \infty$) of the TASK functional that interpolates the exchange enhancement factor $F_X(s,\alpha )$ from $\alpha =0$ to 1. The resulting modified TASK (mTASK) was tested for various materials from three dimensions to two dimensions to one dimension (nanoribbons) and was compared with the results of the higher-level hybrid functional HSE06 or with the ${\mathrm{G}}_0{\mathrm{W}}_0$ approximation within many-body perturbation theory. We find that mTASK systematically improves the band gaps and band structures of two-dimensional (2D) and 1D systems, without significantly affecting the accuracy of the original TASK for the bulk 3D materials, when compared with the Perdew-Burke-Ernzerhof (PBE) GGA and the strongly constrained and appropriately normed (SCAN) meta-GGA. We further demonstrate the applicability of mTASK by assessing the band structures of transition metal dichalcogenide nanoribbons with respect to various bending curvatures.

${\mathrm{CrI}}_3$ has recently been shown to exhibit low-dimensional, long-range magnetic ordering from few layers to single layers of ${\mathrm{CrI}}_3$. The properties of ${\mathrm{CrI}}_3$ bulk and few-layered systems are uniquely defined by a combination of short-range intralayer and long-range interlayer interactions, including strong correlations, exchange, and spin-orbit coupling. Unfortunately, both the long-range van der Waals interactions, which are driven by dynamic, many-body electronic correlations, and the competing strong intralayer correlations, present a formidable challenge for the local or semilocal mean-field approximations employed in workhorse electronic structure approaches like density-functional theory. In this paper we employ a sophisticated many-body approach that can simultaneously describe long- and short-range correlations. We establish that the fixed-node diffusion Monte Carlo (FNDMC) method reproduces the experimental interlayer separation distance of bulk ${\mathrm{CrI}}_3$ for the high-temperature monoclinic phase with a reliable prediction of the interlayer binding energy. We subsequently employed the FNDMC results to benchmark the accuracy of several density-functional theory exchange-correlation approximations.

Investigation of the spin Hall effect in gold has triggered increasing interest over the past decade, since gold combines the properties of a large bulk spin-diffusion length and strong interfacial spin-orbit coupling. However, discrepancies between the values of the spin Hall angle of gold reported in the literature have brought into question the microscopic origin of the spin Hall effect in Au. Here, we investigate the thickness dependence of the effective spin-charge conversion efficiency in single Au films and ultrathin Au/Si multilayers by nonlocal transport and spin-torque ferromagnetic resonance measurements. We show that the inferred effective spin-charge conversion efficiency is strongly enhanced in ultrathin Au/Si multilayers, reaching exceedingly large values of 0.99 ± 0.34 when the thickness of the individual Au layers is scaled down to 2 nm. These findings reveal the coexistence of a strong interfacial spin-to-charge conversion effect, which becomes dominant in ultrathin Au, and bulk spin Hall effect with a relatively low bulk spin Hall angle of 0.012 ± 0.005. Our experimental results suggest the key role of the intrinsic spin Hall effect enhancement along with a strong interfacial spin-orbit coupling-related effect in the large spin-to-charge conversion in ultrathin Au.

In spintronics, the propagation of spin-wave excitations in magnetically ordered materials can also be used to transport and process information. One of the most popular materials in this regard is the ferrimagnetic insulator yttrium iron garnet due its exceptionally small spin-wave damping parameter. While the small relaxation rate allows for large propagation length of magnetic excitations, it also leads to nonlocality of the magnetic properties. By imaging spin waves, their band structure is mapped with high-frequency resolution using a magneto-optic super-Nyquist sampling technique. In doing so, wave-vector selection is shown to suppress dispersion effects to a large extent, allowing for local measurements of spin relaxation. Moreover, we demonstrate even higher control of magnon propagation by employing the wave-vector selectivity near an avoided crossing of different spin-wave modes where the group velocity approaches zero. Here the local engineering of the dispersion allows us to construct magnonic waveguides, and at the same time it reveals the local relaxation properties.

Measurements of the magnetization in quasistatic and pulsed magnetic fields with different sweep rates, measurements of the specific heat in various magnetic fields, and direct measurements of the adiabatic temperature change have been employed to study the metamagnetic phase transition from an antiferromagnetic (AF) to the ferromagnetic (FM) state in an ${\left({\mathrm{Fe}}_{0.98}{\mathrm{Ni}}_{0.02}\right)}_{49}{\mathrm{Rh}}_{51}$ alloy with a critical AF-FM transition temperature, $T_{\mathrm{tr}}$, reduced to 266 K. Based on the obtained results, a magnetic phase diagram for this alloy has been constructed. The AF-FM transition induced by the magnetic field below 10 K is found to occur in a steplike fashion in contrast to smooth behavior at $10\mathrm{K}<T<T_{\mathrm{tr}}$. The adiabatic temperature change $\mathrm{\Delta }{T}_{ad}$ in the magnetic field of 2 T exceeds 6.5 K in pulsed fields (∼100 T/s) and in the Halbach setup (∼0.5 T/s), which is in agreement with the estimation from the S-T diagram constructed based on the specific heat measurements. The reversible $\mathrm{\Delta }{T}_{ad}$ reaches −4.6 K under cyclic conditions in the Halbach setup (2 T). A complete transformation to the FM state in the whole temperature range requires a magnetic field of 14 T. Direct measurements of $\mathrm{\Delta }{T}_{ad}$ in pulsed fields of 14 T revealed an irreversible part of the magnetocaloric effect associated with the presence of magnetic hysteresis and respective losses during the magnetization process.

In this paper, we introduce a quantum photonic device, which we term the photonic Bier-Glas cavity. We discuss its fabrication and functionality, which is based on the coupling of integrated In(Ga)As quantum dots to a broadband photonic cavity resonance. By design, the device architecture uniquely combines the Purcell enhancement of a photonic micropillar structure with a broadband photonic mode shaping of a vertical, tapered waveguide, making it an interesting candidate to enable the efficient extraction of entangled photon pairs. We detail the epitaxial growth of the heterostructure as well as the necessary lithography steps to approach a GaAs-based photonic device with a height exceeding 15 μm, supported on a pedestal that can be as thin as 20 nm. We further describe its key performance parameters using a Fourier-modal method. Finally, we present an optical characterization, which confirms the presence of broadband optical resonances, in conjunction with amplified spontaneous emission of single photons.

Despite their outstanding optoelectronic properties, metal halide perovskites have not yet seen widespread use in commercial applications. This is mostly due to their lack of stability with respect to several external factors, e.g., humidity, heat, light, and oxygen. Even though extensive studies have been carried out over the last decade, a lot of questions regarding their thermal stability still remain, and various publications have put forth different approaches for measuring and quantifying the conditions of their decomposition. However, differences in the experimental setups and in the reported values make comparisons of the reported results challenging. We show an approach, where ${\mathrm{MAPbI}}_3$ thin films are thermally decomposed in high vacuum within a temperature range from $220{}^{\circ }\mathrm{C}$ to $250{}^{\circ }\mathrm{C}$, while monitoring changes in the crystal structure using an in situ x-ray diffraction setup. This process reveals the complete phase evolution of the thin films from ${\mathrm{MAPbI}}_3$ into ${\mathrm{PbI}}_2$. The time resolved data was then evaluated in view of the reaction kinetics using three different approaches. First, an Arrhenius fit was applied to $lnk$ over $1/T$, where the rate constant $k$ was determined by fitting a first order exponential decay onto the decreasing peak area of the most prominent diffraction peaks. Secondly, a model fitting approach was used, where the data was tested against a set of different reaction models. Lastly, a model free isoconversional approach was applied. By doing this, we succeed in characterizing the decomposition and determine the kinetic triplet, consisting of the activation energy $E$, the frequency factor $A$, and the reaction model $f\left(\alpha \right)$. With the help of the kinetic triplet the decomposition reaction can be expressed in a physically meaningful way and allows us to predict the decomposition dynamics of ${\mathrm{MAPbI}}_3$ thin films for varying temperatures.

Recent works have shown that gels with a large amount of solvents can still exhibit a glass transition behavior with the thermomechanical properties exhibiting tremendous change over a narrow temperature region. Similar to dry polymers, the rheological behaviors of gels also show a broad distribution of relaxation times. However, so far the effects of solvents on the relaxation spectrum have rarely been studied. In this work, we first develop a viscoelastic model to relate the rheological properties of gels and the corresponding properties in the dry state. We then apply the theoretical model to analyze the complex modulus of gels measured in the small strain dynamic tests. The results show that the breadth of relaxation spectrum has been expanded by increasing the swelling ratio. The physical mechanism behind this observation needs a deep investigation in further study and also raises a significant challenge for developing constitutive models for gels.