We report switching of magnetic anisotropy (MA) from in-plane to perpendicular with increasing the thickness $d$ of a (001)-oriented ferromagnetic-semiconductor (FMS) $\left(\mathrm{G}{\mathrm{a}}_{0.7},\mathrm{F}{\mathrm{e}}_{0.3}\right)\mathrm{Sb}$ layer with a high Curie temperature ($T_{\mathrm{C}}>320$ K), using ferromagnetic resonance at room temperature. We show that the total MA energy ($E_{\perp }$) along the [001] direction changes its sign from positive (in-plane) to negative (perpendicular) with increasing $d$ above an effective critical value $d_{\mathrm{C}}^{*}\sim 42\mathrm{nm}$. We reveal that (Ga,Fe)Sb has twofold symmetry in the film plane. Meanwhile, in the plane perpendicular to the film including the in-plane [110] axis, the twofold symmetry with the easy magnetization axis along [110] changes to fourfold symmetry with the easy magnetization axis along $\langle 001\rangle$ with increasing $d$. This peculiar behavior is different from that of (Ga,Mn)As, in which only the in-plane MA depends on the film thickness and has fourfold symmetry due to its dominant cubic anisotropy along the $\langle 100\rangle$ axes. This work provides an important guide for controlling the easy magnetization axis of high-$T_{\mathrm{C}}$ FMS (Ga,Fe)Sb for room-temperature device applications.

The band alignments at nonpolar surfaces and heterointerfaces of ${\mathrm{Al}}_2{\mathrm{O}}_3$, ${\mathrm{Ga}}_2{\mathrm{O}}_3$, and ${\mathrm{In}}_2{\mathrm{O}}_3$ polymorphs, and three related group-III oxides, namely, ${\mathrm{Sc}}_2{\mathrm{O}}_3$, ${\mathrm{Y}}_2{\mathrm{O}}_3$, and ${\mathrm{La}}_2{\mathrm{O}}_3$, are investigated by using first-principles calculations. A non-self-consistent dielectric-dependent hybrid functional approach is adopted on top of semilocal density-functional calculations by using the Perdew-Burke-Ernzerhof functional tuned for solids (PBEsol) to accelerate the band alignment evaluation that involves surface and interface calculations. Among the five crystal structures considered, namely, corundum, β-gallia, κ-alumina, bixbyite (C-type rare earth), and A-type rare earth, the lowest energy phases are corundum, β-gallia, A-type rare earth, and bixbyite for ${\mathrm{Al}}_2{\mathrm{O}}_3$, ${\mathrm{Ga}}_2{\mathrm{O}}_3$, ${\mathrm{La}}_2{\mathrm{O}}_3$, and the others, respectively, within PBEsol calculations. The ionization potential typically decreases in the order ${\mathrm{Al}}_2{\mathrm{O}}_3$, ${\mathrm{Ga}}_2{\mathrm{O}}_3$, ${\mathrm{In}}_2{\mathrm{O}}_3$, ${\mathrm{Sc}}_2{\mathrm{O}}_3$, ${\mathrm{Y}}_2{\mathrm{O}}_3$, and ${\mathrm{La}}_2{\mathrm{O}}_3$ within the same crystal structure and surface termination. This tendency is enhanced by the atomic relaxation-induced surface dipoles, where smaller cations tend to relax toward the bulk side compared to O ions, while larger cations tend to relax toward the vacuum side. The ionization potential and electron affinity differences at unrelaxed surfaces are good indicators of the interfacial valence- and conduction-band offsets, respectively, for ${\mathrm{Al}}_2{\mathrm{O}}_3$/${\mathrm{Ga}}_2{\mathrm{O}}_3$ with a relatively small mismatch in the lattice parameters of the two phases.

The energy density of ruthenia (${\mathrm{RuO}}_2$) pseudocapacitor electrodes is critically dependent on their surface structure. To understand this dependence, we simulate the electrochemical response of ${\mathrm{RuO}}_2$(110), ${\mathrm{RuO}}_2$(100), and ${\mathrm{RuO}}_2$(101) in aqueous environments using a self-consistent continuum solvation (SCCS) model of the solid-liquid interface. The insertion of protons into the ${\mathrm{RuO}}_2$(110) sublayer is found to profoundly affect the voltage-dependent characteristics of the system, leading to a sharp transition from a battery-type to capacitor-type response. The calculated charge-voltage properties for ${\mathrm{RuO}}_2$(101) are in qualitative agreement with experiment, albeit with a pseudocapacitance that is significantly underestimated. In contrast, the ${\mathrm{RuO}}_2$(100) facet is correctly predicted to be pseudocapacitive over a wide voltage window, with a calculated pseudocapacitance in close agreement with experimental voltammetry. These results establish the SCCS model as a reliable approach to predict and optimize the facet-dependent pseudocapacitance of polycrystalline systems.

Plastic flow at small scales is generally observed to be intermittent, whereas the stress-strain behavior of bulk crystals is mostly smooth. Here we find that when the external deformation rate of small-scale crystals approaches the speed of the crystallographic slip velocity, an intermittent-to-smooth transition of plastic flow is observed. By defining a rate-dependent intermittency parameter, this phenomenon can be captured with a power law covering 5.5 orders of magnitude for Au and Nb micron-sized single crystals with experiments and via simulations for Nb crystals. Our results indicate that the transition to smooth flow is driven by a gradual truncation of the underlying truncated power law that describes the intermittently evolving system. This is caused by a competition of internal and external rates, which aligns with the well-known transitions from serrated to nonserrated flow in metallic glasses or materials with dynamic strain aging.

In the present work, we perform a systematic investigation on $p$-type codoping in (Ga,Mn)As. Through gradually increasing Zn doping concentration, the hole concentration increases, which should theoretically lead to an increase of the Curie temperature ($T_{\mathrm{C}}$) according to the p-d Zener model. Unexpectedly, although the film keeps its epitaxial structure, both $T_{\mathrm{C}}$ and the magnetization decrease. The samples present a phase transition from ferromagnetism to paramagnetism upon increasing hole concentration. In the intermediate regime, we observe a signature of antiferromagnetism. By using channeling Rutherford backscattering spectrometry and particle-induced x-ray emission, the substitutional Mn atoms are observed to shift to interstitial sites, while more Zn atoms occupy Ga sites, which explains the observed behavior. This is also consistent with first-principles calculations, showing that the complex of substitutional Zn and interstitial Mn has the lowest formation energy.

One approach for finding new transparent conductors involves taking advantage of electronic correlations in metallic transition metal oxides, such as ${\mathrm{SrVO}}_3$, to enhance the electronic effective mass and suppress the plasma frequency (${\omega }_P$) to infrared. Success of this approach relies on finding a compound with the right electron effective mass and quasiparticle weight $Z$. Biaxial strain can in principle be a fruitful way to manipulate the electronic properties of materials to tune both of these quantities. In this paper, we elucidate the behavior of the electronic properties of early transition metal oxides ${\mathrm{SrVO}}_3$, ${\mathrm{SrNbO}}_3$, and ${\mathrm{SrMoO}}_3$ under strain, using first-principles density-functional theory and dynamical mean-field theory. We show that strain is not an effective way to manipulate the plasma frequency, but dimensionality of the crystal structure and origin of electronic correlations strongly affect the trends in both ${\omega }_P$ and $Z$.

Cadmium arsenide $\left(\mathrm{C}{\mathrm{d}}_3\mathrm{A}{\mathrm{s}}_2\right)$ is one of the first materials to be discovered to belong to the class of three-dimensional topological semimetals. Reported room-temperature crystal structures of $\mathrm{C}{\mathrm{d}}_3\mathrm{A}{\mathrm{s}}_2$ differ subtly in the way the Cd vacancies are arranged within its antifluorite-derived structure, which determines if an inversion center is present and if $\mathrm{C}{\mathrm{d}}_3\mathrm{A}{\mathrm{s}}_2$ is a Dirac or Weyl semimetal. Here, we apply convergent beam electron diffraction (CBED) to determine the point group of $\mathrm{C}{\mathrm{d}}_3\mathrm{A}{\mathrm{s}}_2$ thin films grown by molecular-beam epitaxy. Using CBED patterns from multiple zone axes, high-angle annular dark-field images acquired in scanning transmission electron microscopy, and Bloch wave simulations, we show that $\mathrm{C}{\mathrm{d}}_3\mathrm{A}{\mathrm{s}}_2$ belongs to the tetragonal 4/mmm point group, which is centrosymmetric. The results show that CBED can distinguish very subtle differences in the crystal structure of a topological semimetal, a capability that will be useful for designing materials and thin film heterostructures with topological states that depend on the presence of certain crystal symmetries.

We report a detailed investigation of the low-temperature transport properties (5–300 K) on polycrystalline samples of $\mathrm{S}{\mathrm{n}}_{1+x}\mathrm{Te}$ ($x=0$ and 0.03) prepared by melt quenching in water and slow cooling. These two different synthetic routes result in variations in the hole concentration over more than one order of magnitude, allowing for a systematic investigation of the influence of Sn vacancies on the transport properties. The results evidence a strong correlation between the details of the synthetic process and the concentration of Sn vacancies. Transmission electron microscopy and Mössbauer spectroscopy show that the excess Sn, which helps to lower the hole concentration, segregates at grain boundaries. Interestingly, Hall-effect measurements reveal that charge transport is dominated near 300 K by alloy scattering regardless of the hole concentration. In addition to dictating the electronic properties, the concentration of Sn vacancies has also a significant impact on the thermal transport, with the magnitude of the low-temperature Umklapp peak observed in the lattice thermal conductivity near 30 K scaling with the concentration of Sn vacancies that act as efficient point-defect scatterers.

While experimental and computational studies abound demonstrating the diverse range of phenomena caused by grain interactions under quasistatic loading conditions, far less attention has been given to these interactions under the comparatively dramatic conditions of shock compression. The consideration of grain interactions is essential within the context of contemporary shock-compression experiments that exploit the distinctive x-ray diffraction patterns of highly textured (and therefore strongly anisotropic) targets in order to interrogate local structural evolution. We present here a study of grain interaction effects in shock-compressed, body-centered cubic tantalum nanocrystals characterized by a columnar geometry and a strong fiber texture using large-scale molecular dynamics simulations. Our study reveals that contiguous grains deform cooperatively in directions perpendicular to the shock, driven by the gigapascal-scale stress gradients induced over their boundaries by the uniaxial compression, and in so doing are able to reach a state of reduced transverse shear stress. We compare the extent of this relaxation for two different columnar geometries (distinguished by their square or hexagonal cross-sections), and quantify the attendant change in the transverse elastic strains. We further show that cooperative deformation is able to replace ordinary plastic deformation mechanisms at lower shock pressures, and, under certain conditions, activate new mechanisms at higher pressures.

To explore the formation of noncollinear magnetic configurations in materials with strongly correlated electrons, we derive a noncollinear $\mathrm{LSDA}+U$ model involving only one parameter $U$, as opposed to the difference between the Hubbard and Stoner parameters $U-J$. Computing $U$ in the constrained random phase approximation, we investigate noncollinear magnetism of uranium dioxide ${\mathrm{UO}}_2$ and find that the spin-orbit coupling (SOC) stabilizes the $3\mathbf{k}$ ordered magnetic ground state. The estimated SOC strength in ${\mathrm{UO}}_2$ is as large as 0.73 eV per uranium atom, making spin and orbital degrees of freedom virtually inseparable. Using a multipolar pseudospin Hamiltonian, we show how octupolar and dipole-dipole exchange coupling help establish the $3\mathbf{k}$ magnetic ground state with canted ordering of uranium $f$ orbitals. The cooperative Jahn-Teller effect does not appear to play a significant part in stabilizing the noncollinear $3\mathbf{k}$ state, which has the lowest energy even in an undistorted lattice. The choice of parameter $U$ in the $\mathrm{LSDA}+U$ model has a notable quantitative effect on the predicted properties of ${\mathrm{UO}}_2$, in particular on the magnetic exchange interaction and, perhaps trivially, on the band gap: The value of $U=3.46\mathrm{eV}$ computed fully ab initio delivers the band gap of 2.11 eV in good agreement with experiment, and a balanced account of other pertinent energy scales.

The structure of the SiC($000\overline{1}$) surface, the C-face of the {0001} SiC surfaces, is studied as a function of temperature and of pressure in a gaseous environment of disilane ($\mathrm{S}{\mathrm{i}}_2{\mathrm{H}}_6$). Various surface reconstructions are observed, both with and without the presence of an overlying graphene layer (which spontaneously forms at sufficiently high temperatures). Based on cross-sectional scanning transmission electron microscopy measurements, the interface structure that forms in the presence of the graphene is found to contain 1.4–1.7 monolayers (ML) of Si, a somewhat counter-intuitive result since, when the graphene forms, the system is actually under C-rich conditions. Using ab initio thermodynamics, it is demonstrated that there exists a class of Si-rich surfaces containing about 1.3 ML of Si that are stable on the surface (even under C-rich conditions) at temperatures above ∼400 K. The structures that thus form consist of Si adatoms atop a Si adlayer on the C-face of SiC, with or without the presence of overlying graphene.

Segregation in a series of ${\mathrm{Ni}}_2{\mathrm{Mn}}_{1+x}{(\mathrm{In},\mathrm{Sn},\mathrm{Ga},\mathrm{Al})}_{1-x}$ and ${\mathrm{Mn}}_2{\mathrm{Ni}}_{1+x}$(Ga, Al) as well as ${\mathrm{Ni}}_{2+x}{\mathrm{Mn}}_{1-x}\mathrm{Ga}$ Heusler alloys is studied by first-principles calculations. We show that Mn-rich ${\mathrm{Ni}}_2{\mathrm{Mn}}_{1+x}{(\mathrm{In},\mathrm{Sn},\mathrm{Al})}_{1-x}$ compounds are at low temperatures unstable in the whole concentration range against decomposition into a dual-phase system consisting of an $L{2}_1$-cubic ${\mathrm{Ni}}_2\mathrm{Mn}X$ phase with ferromagnetic order and an $L{1}_0$-tetragonal NiMn phase ordered antiferromagnetically. In contrast, ${\mathrm{Ni}}_2{\mathrm{Mn}}_{1+x}{\mathrm{Ga}}_{1-x}$ and ${\mathrm{Mn}}_2{\mathrm{Ni}}_{1+x}{(\mathrm{Ga},\mathrm{Al})}_{1-x}$ are stable in the narrow concentration range near the 2-1-1 stoichiometry. This concentration range depends on the presence of a martensitic transformation and becomes wider with increasing energy difference between austenite and martensite phases in a parent system. We find that ferromagnetic Ni-rich ${\mathrm{Ni}}_{2+x}{\mathrm{Mn}}_{1-x}\mathrm{Ga}$ is stable in the concentration range $0<x\lesssim 0.6$.

Ultrafast demagnetization of Co/Pt heterostructures induced by a femtosecond 800-nm laser pulse launches a spin current from Co to Pt and subsequent conversion of the spin current to a charge current in the Pt layer due to the inverse spin-Hall effect. At the same time, due to the spin-dependent photogalvanic effect, a circularly polarized femtosecond laser pulse also generates a photocurrent at the Co/Pt interface. Both ultrashort photocurrent pulses are effectively detected in a contactless way by measuring the THz radiation they emit. Here we aim to understand how the properties of the Co/Pt interface affect the photocurrents in the bilayers. By varying the interfacial roughness, crystal structure, and interfacial intermixing, as well as having an explicit focus on the cases when THz emissions from these two photocurrents reveal opposite trends, we identify which interface properties play a crucial role for the photocurrents. In particular, we show that by reducing the roughness, the THz emission due to the spin-dependent photogalvanic effect reduces to zero while the strength of the THz emission from the photocurrent associated with the inverse spin-Hall effect increases by a factor of 2. On the other hand, while intermixing strongly enhances the THz emission from the inverse spin-Hall effect by a factor of 4.2, THz emission related to the spin-dependent photogalvanic effect reveals the opposite trend. These findings indicate that microstructural properties of the Co-Pt interface play a decisive role in the generation of photocurrents.

Ferroelectric Rashba semiconductors (FERSC) are a novel class of multifunctional materials showing a giant Rashba spin splitting which can be reversed by switching the electric polarization. Although they are excellent candidates as channels in spin field effect transistors, the experimental research has been limited so far to semiconducting GeTe, in which ferroelectric switching is often prevented by heavy doping and/or large leakage currents. Here, we report that ${\mathrm{CsBiNb}}_2{\mathrm{O}}_7$, a layered perovskite of Dion-Jacobson type, is a robust ferroelectric with sufficiently strong spin-orbit coupling and spin texture reversible by electric field. Moreover, we reveal that its topmost valence band's spin texture is quasi-independent from the momentum, as a result of the low symmetry of its ferroelectric phase. The peculiar spin-polarization pattern in the momentum space may yield the so-called “persistent spin helix,” a specific spin-wave mode which protects the spin from decoherence in the diffusive transport regime, potentially ensuring a very long spin lifetime in this material.

We report the discovery of superconductivity in binary alloy ${\mathrm{Sn}}_{1-x}{\mathrm{Sb}}_x$ with $x$ in the range of 0.43 to 0.6, which possesses a modulated rhombohedral structure due to the incommensurate ordering of Sn and Sb layers along the $c$ axis. The specific heat measurements indicate a weakly coupled, fully gapped superconducting state in this homogeneity range with a maximum bulk $T_{\mathrm{c}}$ of 1.58 K at $x=0.46$, though the electronic specific heat and Hall coefficients remain nearly $x$ independent. The nonmonotonic dependence of the bulk $T_{\mathrm{c}}$ is discussed in relation to the effects of Sb-layer intercalation between the [${\mathrm{Sn}}_4{\mathrm{Sb}}_3$] seven-layer lamellae that are the essential building blocks for superconductivity. On the other hand, a zero-resistivity transition is found to take place well above the bulk superconducting transition, and the corresponding $T_{\mathrm{c}}$ increases monotonically with $x$ from 2.06 K to 3.29 K. This contrast, together with the uniform elements distribution revealed by energy dispersive x-ray mapping, implies that the resistive transition is due to the strain effect at the grain boundary rather than the compositional inhomogeneity. The first-principles calculations on the representative composition ${\mathrm{Sn}}_4{\mathrm{Sb}}_3$ ($x=0.43$) indicate that it is topologically nontrivial similar to Sb, but with different ${\mathrm{Z}}_2$ invariants (0;111). Our results not only identify a second superconducting region in the Sn-Sb phase diagram, but also provide a viable platform to study the interplay between structural modulation, nontrivial band topology, and superconductivity.

Be-B- and B-Ta-based compounds are of great interest for potential technological applications because of their high density and high hardness. However, to date, ternary Be-B-Ta compounds have neither been synthesized nor predicted. In this paper, variable-composition evolutionary crystal structure prediction calculations, based on first-principles calculations, have been performed to determine the stable crystal structures containing Be-B-Ta under ambient conditions. The predicted five compounds ${\mathrm{BeB}}_2\mathrm{Ta}$, ${\mathrm{BeB}}_3{\mathrm{Ta}}_2$ (high-pressure phase), BeBTa, ${\mathrm{BeBTa}}_2$, and ${\mathrm{Be}}_2{\mathrm{B}}_2\mathrm{Ta}$ are found to be highly dense and very hard materials. All these compounds are metallic and the effect of spin-orbit coupling (SOC) is significant in them. Two of them (${\mathrm{BeB}}_2\mathrm{Ta}$ and ${\mathrm{Be}}_2{\mathrm{B}}_2\mathrm{Ta}$) are found to be superconductors within Migdal-Eliashberg theory. The calculated critical temperatures, including SOC effects, are 6 and 7 K for ${\mathrm{BeB}}_2\mathrm{Ta}$ and ${\mathrm{Be}}_2{\mathrm{B}}_2\mathrm{Ta}$, respectively. Because of their energetic and dynamic stability, these compounds might be favorable to synthesize in the laboratory.

The ground state of the parent compounds of many high-temperature superconductors is an antiferromagnetically ordered phase, where superconductivity emerges when the antiferromagnetic phase transition is suppressed by doping or application of pressure. This behavior implies a close relation between the two orders. Examining the interplay between them promises a better understanding of how the superconducting condensate forms from the antiferromagnetically ordered background. Here we explore this relation in real space at the atomic scale using low-temperature spin-polarized scanning tunneling microscopy and spectroscopy. We investigate the transition from antiferromagnetically ordered ${\mathrm{Fe}}_{1+y}\mathrm{Te}$ via the spin-glass phase in ${\mathrm{Fe}}_{1+y}{\mathrm{Se}}_{0.1}{\mathrm{Te}}_{0.9}$ to superconducting ${\mathrm{Fe}}_{1+y}{\mathrm{Se}}_{0.15}{\mathrm{Te}}_{0.85}$. In ${\mathrm{Fe}}_{1+y}{\mathrm{Se}}_{0.1}{\mathrm{Te}}_{0.9}$ we observe an atomic-scale coexistence of superconductivity and short-ranged bicollinear antiferromagnetic order. However, a direct correlation between the two orders is not observed, supporting the scenario of $s_{\pm }$ superconducting symmetry in this material. Our work demonstrates a direct probe of the relation between the two orders, which is indispensable for our understanding of high-temperature superconductivity.

Dielectric metasurfaces allow us to realize many unique effects in optics, and they can serve as the building blocks of the modern photonic technologies. Here, we suggest theoretically and demonstrate experimentally the effect of high transparency of all-dielectric metasurfaces with meta-atoms supporting the so-called transverse Kerker effect. In contrast to the well-known Huygens' metasurfaces, in our case both phase and amplitude of the incoming wave remain unperturbed at the resonant frequency and, consequently, our novel metasurfaces totally operate in the high-transparency regime. We prove experimentally, in the microwave frequency range, that both phase and amplitude of the waves transmitted through these metasurfaces remain almost unaffected. Also, we demonstrate numerically and experimentally and explain theoretically a novel mechanism for achieving a perfect absorption of the incident light enabled by the resonant response of the dielectric metasurfaces placed on a conducting substrate. In the subdiffractive limit, we show that these effects are mainly determined by the optical response of the constituting meta-atoms rather than collective lattice contributions. With the spectrum scalability, our finding can be extended to the optical frequencies to be employed for energy harvesting, nonlinear phenomena, and filtering of light.

The physical impact of thermal treatment applied to an organic material hosting alkali-metal atoms can be rather complex. Therefore, especially for ultrathin films, only little is known about annealing-induced effects on the molecular scale. In this work, we study the annealing of a model system consisting of ultrathin layers of potassium-doped 3,4,9,10-perylene tetracarboxylic dianhydride (${\mathrm{K}}_x\mathrm{PTCDA}$) on Ag(111) [C. Zwick et al., ACS Nano 10, 2365 (2016)]. We evidence three fundamental categories of annealing-induced structural changes, ascertained for distinct doping ratios $x$ of the as-grown ${\mathrm{K}}_x\mathrm{PTCDA}$ phases: order-to-disorder ($x=4$), order-to-order ($x=2$), and disorder-to-order ($x=0.5$) transitions can be demonstrated. We provide an in-depth structural analysis of all well-ordered phases found by combining distortion-corrected low-energy electron diffraction and scanning tunneling microscopy with submolecular resolution. The changes of the epitaxial relations (including epitaxy type, lattice parameters, and unit-cell composition) are extracted with high precision, eventually allowing for an interpretation of the interactions yielding a particular adsorbate layer structure.

We investigated the stiffness of the interfaces between hard-sphere colloidal face-centered cubic crystals sedimented onto (100), (110), and (111) oriented templates and their equilibrium liquid with special attention to the in-plane anisotropy. The stiffness was determined from thermal fluctuations in the interface position imaged by confocal microscopy. The colloidal particles of diameter $\sigma$ are nearly density-matched with the suspending fluid so that the amount of liquid above the interface was much greater than in an earlier similar hard-sphere colloid experiment. We find stiffness values of $0.47k_{B}T/{\sigma }^2$ for the (100) interface $0.53k_{B}T/{\sigma }^2$ for the (110) interface and $0.41k_{B}T/{\sigma }^2$ for the (111) interface. These values are generally closer to those found in computer simulations than to the earlier colloid results. No evidence was found however for the expected in-plane anisotropy of the (110) interface. This surprising finding confirms that of the earlier colloid work.