Organic and inorganic lead halogen perovskites, and in particular, $\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\mathrm{Pb}{\mathrm{I}}_3$, have during the last years emerged as a class of highly efficient solar cell materials. Herein we introduce metalorganic halogen perovskite materials for energy-relevant applications based on alkaline-earth metals. Based on the classical notion of Goldschmidt's rules and quantum mechanical considerations, the three alkaline-earth metals, Ca, Sr, and Ba, are shown to be able to exchange lead in the perovskite structure. The three alkaline-earth perovskites, $\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\mathrm{Ca}{\mathrm{I}}_3,\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\mathrm{Sr}{\mathrm{I}}_3$, and $\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\mathrm{Ba}{\mathrm{I}}_3$, as well as the reference compound, $\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\mathrm{Pb}{\mathrm{I}}_3$, are in this paper investigated with density functional theory (DFT) calculations, which predict these compounds to exist as stable perovskite materials, and their electronic properties are explored. A detailed analysis of the projected molecular orbital density of states and electronic band structure from DFT calculations were used for interpretation of the band-gap variations in these materials and for estimation of the effective masses of the electrons and holes. Neglecting spin-orbit effects, the band gap of $\mathrm{MACa}{\mathrm{I}}_3,\mathrm{MASr}{\mathrm{I}}_3$, and $\mathrm{MABa}{\mathrm{I}}_3$ were estimated to be 2.95, 3.6, and 3.3 eV, respectively, showing the relative change expected for metal cation exchange. The shifts in the conduction band (CB) edges for the alkaline-earth perovskites were quantified using scalar relativistic DFT calculations and tight-binding analysis, and were compared to the situation in the more extensively studied lead halide perovskite, $\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\mathrm{Pb}{\mathrm{I}}_3$, where the change in the work function of the metal is the single most important factor in tuning the CB edge and band gap. The results show that alkaline-earth-based organometallic perovskites will not work as an efficient light absorber in photovoltaic applications but instead could be applicable as charge-selective contact materials. The rather high CB edge and the wide band gap together with the large difference of the electron and hole effective masses make them good candidates for $n$-type selective layers in hot carrier injection solar cell devices together with some light absorber candidates. The fact that they have similar lattice constants as the lead perovskite and suitable positions of the valence band edges open up the possibility to use them also as thin epitaxial $p$-type hole selective contacts in combination with the lead halogen perovskite materials. This can lead to both charge selectivity as well as to superior crystal growth of lead perovskite with less contact stress, which is interesting for further investigations.

We perform density functional theory based first-principles calculations to identify promising alloying elements $\left(X\right)$ capable of enhancing the compressive uniaxial theoretical (ideal) strength of the fcc Ni-matrix along the $〈001〉$ direction. The alloying element belongs to a wide range of $3d,4d$, and $5d$ series with nominal composition of 6.25 at. %. Additionally, a full elastic study is carried to investigate the ideal strength of fcc Ni and fcc Co. Our results indicate that the most desirable alloying elements are those with half $d\text{-band}$ filling, namely, Os, Ir, Re, and Ru.

The magnetic dissipative droplet is a strongly nonlinear wave structure that can be stabilized in a thin film ferromagnet exhibiting perpendicular magnetic anisotropy by use of spin transfer torque. These structures have been observed experimentally at room temperature, showcasing their robustness against noise. Here, we quantify the effects of thermal noise by deriving stochastic equations of motion for a droplet based on soliton perturbation theory. First, it is found that deterministic droplets are linearly unstable at large bias currents, subject to a drift instability. When the droplet is linearly stable, our framework allows us to analytically compute the droplet's generation linewidth and center variance. Additionally, we study the influence of nonlocal and Oersted fields with micromagnetic simulations, providing insight into their effect on the generation linewidth. These results motivate detailed experiments on the current and temperature-dependent linewidth as well as drift instability statistics of droplets, which are important figures-of-merit in the prospect of droplet-based applications.

We investigate the effect of isotope substitution on the electron-phonon interaction in the multiband superconductor ${\mathrm{MgB}}_2$ using tunable laser-based angle-resolved photoemission spectroscopy. The kink structure around 70 meV in the $\sigma$ band, which is caused by electron coupling to the $E_{2g}$ phonon mode, is shifted to higher binding energy by $\sim 3.5$ meV in $\mathrm{Mg}{}^{10}\mathrm{B}{}_2$ and the shift is not affected by superconducting transition. These results serve as the benchmark for investigations of isotope effects in known, unconventional superconductors and newly discovered superconductors where the origin of pairing is unknown.

By using a hydrothermal method, we have successfully grown crystals of the newly discovered superconductor FeS, which has an isostructure of the iron-based superconductor FeSe. The superconductivity appears at about 4.5 K, as revealed by both resistive and magnetization measurements. It is found that the upper critical field is relatively low, with, however, a rather large anisotropy, $\mathrm{\Gamma }={[(d{H}_{c2}^{ab}/dT)/(d{H}_{c2}^c/dT)]}_{T_c}\approx 5.8$. A huge magnetoresistivity ($290\%$ at 9 T and 10 K, $H\parallel c$ axis) together with a nonlinear behavior of Hall resistivity vs external field are observed. A two-band model is applied to fit the magnetoresistance and nonlinear transverse resistivity, yielding the basic parameters of the electron and hole bands.

The canonical Su-Schrieffer-Heeger (SSH) array is one of the basic geometries that have spurred significant interest in topological band-gap modes. Here, we show that the judicious inclusion of third-order Kerr nonlinearities in SSH arrays opens rich physics in topological insulators, including the possibility of supporting self-induced topological transitions, as a function of the applied intensity. We highlight the emergence of a class of topological solutions in nonlinear SSH arrays localized at the array edges and with unusual properties. As opposed to their linear counterparts, these nonlinear states decay to a plateau of nonzero amplitude inside the array, highlighting the local nature of topologically nontrivial band gaps in nonlinear systems. We study the conditions under which these states can be excited and their temporal dynamics as a function of the applied excitation, paving the way to interesting directions in the physics of topological edge states with robust propagation properties based on nonlinear interactions in suitably designed periodic arrays.

Disordered noninteracting systems in sufficiently high dimensions have been predicted to display a non-Anderson disorder-driven transition that manifests itself in the critical behavior of the density of states and other physical observables. Recently, the critical properties of this transition have been extensively studied for the specific case of Weyl semimetals by means of numerical and renormalisation-group approaches. Despite this, the values of the critical exponents at such a transition in a Weyl semimetal are currently under debate. We present an independent calculation of the critical exponents using a two-loop renormalization-group approach for Weyl fermions in $2-\varepsilon$ dimensions and resolve controversies currently existing in the literature.

Metal-semiconductor contacts are a pillar of modern semiconductor technology. Historically, their microscopic understanding has been hampered by the inability of traditional analytical and numerical methods to fully capture the complex physics governing their operating principles. Here we introduce an atomistic approach based on density functional theory and nonequilibrium Green's function, which includes all the relevant ingredients required to model realistic metal-semiconductor interfaces and allows for a direct comparison between theory and experiments via $I-V_{\mathrm{bias}}$ curve simulations. We apply this method to characterize an Ag/Si interface relevant for photovoltaic applications and study the rectifying-to-Ohmic transition as a function of the semiconductor doping. We also demonstrate that the standard “activation energy” method for the analysis of $I-V_{\mathrm{bias}}$ data might be inaccurate for nonideal interfaces as it neglects electron tunneling, and that finite-size atomistic models have problems in describing these interfaces in the presence of doping due to a poor representation of space-charge effects. Conversely, the present method deals effectively with both issues, thus representing a valid alternative to conventional procedures for the accurate characterization of metal-semiconductor interfaces.

The authors present a calculation model of surface roughness scattering (SRS) in nanowires (NWs) based on atomistic description of electronic states by an $s{p}^3{d}^5{s}^{*}$ tight-binding scheme, and then this model is applied to hole transport in rectangular cross-sectional germanium (Ge) NWs. In this SRS model, the change of electronic band structures due to width or height reduction is first computed, and then it is expressed using an equivalent potential near the surface. The perturbation corresponding to a surface roughness is calculated from this equivalent potential. Using the aforementioned SRS model, hole mobility in Ge NWs was computed taking into account phonon scattering and SRS. The impacts of SRS on hole mobility in Ge NWs were analyzed, focusing on the valence band structure and hole states of NWs. The main results are as follows. At low hole density, the impacts of SRS are strongly dependent on NW geometry, and Ge NWs with high phonon-limited hole mobility, such as rectangular cross-sectional [110]-oriented NWs with large height along the [001] direction and square cross-sectional [111]-oriented NWs, tend to be less affected by SRS. At high hole density, however, the geometry dependence of hole mobility becomes weaker. These are understood from the nature of hole states and the valence band structure.

We propose a theory of interference contributions to the two-dimensional exciton diffusion coefficient. The theory takes into account four spin states of the heavy-hole exciton. The interplay of the single-particle (electron and hole) spin splittings with the electron-hole exchange interaction gives rise to either localization or antilocalization behavior of excitons, depending on the system parameters. Possible experimental manifestations of exciton interference are discussed.

We study the quantum Andreev oscillation induced by interference of the edge chiral Majorana fermions in junctions made of quantum anomalous Hall (QAH) insulators and superconductors (SCs). We show two chiral Majorana fermions on a QAH edge with SC proximity generically have a momentum difference $\mathrm{\Delta }k$, which depends on the chemical potentials of both the QAH insulator and the SC. Due to the spatial interference induced by $\mathrm{\Delta }k$, the longitudinal conductance of QAH-SC junctions oscillates with respect to the edge lengths and the chemical potentials, which can be probed via charge transport. Furthermore, we show the dynamical SC phase fluctuation will give rise to a geometrical correction to the longitudinal conductance of the junctions.

The surface segregation profile of an intermetallic compound becomes vertically and laterally modulated upon epitaxial growth of a single-layer hexagonal boron nitride $(h$-BN) nanomesh. $h$-BN on PtRh(111) forms an 11-on-10 superhoneycomb, such as that on Rh(111) [Corso et al., Science 303, 217 (2004)], though with a smaller lattice constant of 2.73 nm. X-ray photoelectron diffraction shows that the $h$-BN layer reduces the Pt enrichment of the first layer by promoting site swapping of about 10 Pt-Rh pairs within the $10\times 10$ unit cell between the first and second layers. This segregation profile is confirmed by density-functional-theory-based cluster-expansion calculations. Generally, a strong modulation of the $h$-BN bonding strength and a higher affinity to one of the constituents leads to self-assembly of top layer patches underneath the nanomesh pores.

A necessary condition for superconductivity (SC) driven by electron correlations is that electron-electron (e-e) interactions enhance superconducting pair-pair correlations, relative to the noninteracting limit. We report high-precision numerical calculations of the ground state on four different finite lattices of up to 100 sites within the frustrated two-dimensional (2D) Hubbard Hamiltonian for a wide range of carrier concentration $\rho$ ($0<\rho <1$). The average long-range pair-pair correlation for each cluster is enhanced by Hubbard $U$ only for $\rho \approx 0.5$. At all other fillings e-e interactions mostly suppress pair correlations. Our work provides a key ingredient to the mechanism of SC in the 2D organic charge-transfer solids and many other unconventional superconductors with frustrated crystal lattices and $\rho \simeq 0.5$.

We discuss the paramagnetic and Néel-ordered phases of the Kondo lattice Hamiltonian on the two-dimensional square lattice by means of bond fermions. In the doped case we find two antiferromagnetic solutions, the first one with small ordered moment, heavy bands, and an antiferromagnetically folded large Fermi surface—i.e., including the localized spins—the second one with large ordered moment, light bands, and an antiferromagnetically folded conduction electron-only Fermi surface. The zero temperature phase diagram as a function of Kondo coupling and conduction electron density shows first- and second-order transition lines between the three different phases and agrees qualitatively with previous numerical studies. We compare to experiments on ${\mathrm{CeRh}}_{1-x}{\mathrm{Co}}_x{\mathrm{In}}_5$ and find qualitative agreement.

The atomic-scale structure of the mercury-electrolyte (0.01 M NaF) interface was studied as a function of temperature and potential by x-ray reflectivity and x-ray diffuse scattering measurements. The capillary wave contribution is determined and removed from the data, giving access to the intrinsic surface-normal electron density profile at the interface, especially to the surface layering in the Hg phase. A temperature dependent roughness anomaly known from the Hg-air interface is found to persist also at the Hg-electrolyte interface. Additionally, a temperature dependence of the layering period was discovered. The increase in the layer spacing with increasing temperature is approximately four times lager than the increase expected from thermal expansion. Finally, the interface is found to broaden towards the electrolyte side as the potential becomes more negative, in agreement with the Schmickler-Henderson theory. Our results favor a model for the interface structure, which is different to the model formerly used in comparable studies.

${\mathrm{Ni}}_2\mathrm{MnGa}$ exhibits ideal ferromagnetic shape memory properties, however, brittleness and a low-temperature martensite transition hinder its technological applications motivating the search for novel materials showing better mechanical properties as well as higher transition temperatures. In this work, the crystal structure, phase transitions, and the magnetic properties of quaternary ${\mathrm{Ni}}_{2-x}{\mathrm{Pt}}_x\mathrm{MnGa}\left(0\le x\le 1\right)$ shape memory alloys were studied experimentally by x-ray diffraction, magnetization measurements, and neutron diffraction and compared to ab initio calculations. Compositions within $0\le x\le 0.25$ exhibit the cubic austenite phase at room temperature. The $x\approx 0.3$ composition exhibits a seven-layer modulated monoclinic martensite structure. Within $0.4\le x\le 1$, the system stabilizes in the nonmodulated tetragonal structure. The martensite transition has very narrow thermal hysteresis $0\le x\le 0.3$, which is a typical characteristic of a shape memory alloy. By increasing $x$, the temperature of the martensite transition increases, while that of the magnetic transition decreases. The $x=1$ composition ($\text{NiPtMnGa}$) in the martensite phase undergoes a para-to-ferrimagnetic transition. The saturation magnetization exhibits a nontrivial behavior with increasing up to $x\approx 0.25$, above which, it suddenly decreases. Powder neutron diffraction reveals the presence of antisite disorder, with about 17% of the original Ga sites being occupied by Mn. Computations suggest that the antisite disorder triggers an antiferromagnetic coupling between two Mn atoms in different crystallographic positions, resulting into a sudden drop of the saturation magnetization for higher $x$.

Local entanglement between pairs of spins, as measured by concurrence, is investigated in a disordered spin model that displays a transition from an ergodic to a many-body localized phase in excited states. It is shown that the concurrence vanishes in the ergodic phase and becomes nonzero and increases in the many-body localized phase. This happen to be correlated with the transition in the spectral statistics from Wigner to Poissonian distribution. A scaling form is found to exist in the second derivative of the concurrence with the disorder strength. It also displays a critical value for the localization transition that is close to what is known in the literature from other measures. An exponential decay of concurrence with distance between spins is observed in the localized phase. Nearest neighbor spin concurrence in this phase is also found to be strongly correlated with the disorder configuration of on-site fields: nearly similar fields implying larger entanglement.

We study nonequilibrium dynamics of the quantum Ising chain at zero temperature when the transverse field is varied stochastically. In the equivalent fermion representation, the equation of motion of Majorana operators is derived in the form of a one-dimensional, continuous-time quantum random walk with stochastic, time-dependent transition amplitudes. This type of external noise gives rise to decoherence in the associated quantum walk and the semiclassical wave packet generally has a diffusive behavior. As a consequence, in the quantum Ising chain, the average entanglement entropy grows in time as $t^{1/2}$ and the logarithmic average magnetization decays in the same form. In the case of a dichotomous noise, when the transverse field is changed in discrete time steps, $\tau$, there can be excitation modes, for which coherence is maintained, provided their energy satisfies ${\epsilon }_k\tau \approx n\pi$ with a positive integer $n$. If the dispersion of ${\epsilon }_k$ is quadratic, the long-time behavior of the entanglement entropy and the logarithmic magnetization is dominated by these ballistically traveling coherent modes and both will have a $t^{3/4}$ time dependence.

Here we present a detailed study of the terahertz and far-infrared response of the mixed perovskite manganite system $\mathrm{E}{\mathrm{u}}_{1–x}\mathrm{H}{\mathrm{o}}_x\mathrm{Mn}{\mathrm{O}}_3$ for holmium concentrations $x=0.1$ and 0.3. We compare the magnetic excitations of the four different magnetically ordered phases ($A$-type antiferromagnetic, sinusoidally modulated collinear, helical phases with spin planes perpendicular to the crystallographic a and c axes). The transition between the two latter phases goes hand in hand with a switching of the ferroelectric polarization from $\mathbf{P}\parallel \mathbf{a}$ to $\mathbf{P}\parallel \mathbf{c}$. Special emphasis is paid to the temperature dependence of the excitations at this transition. We find a significant change of intensity indicating that the exchange-striction mechanism may not be the only mechanism to induce dipolar weight to spin-wave excitations. We also focus on excitations within the incommensurate collinear antiferromagnetic phase and find an excitation close to $40\mathrm{c}{\mathrm{m}}^{-1}$. A detailed analysis of optical weight gives a further unexpected result: In the multiferroic phase with $\mathbf{P}\parallel \mathbf{c}$ all the spectral weight of the electromagnons comes from the lowest-phonon mode. However, for the phase with the polarization $\mathbf{P}\parallel \mathbf{a}$ additional spectral weight must be transferred from higher frequencies.

Computationally characterizing magnetic properies of novel two-dimensional (2D) materials serves as an important first step of exploring possible applications. Using density-functional theory, we show that single-layer ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$ is a potential 2D material with sufficiently low formation energy to be synthesized by mechanical exfoliation from the bulk phase with a van der Waals layered structure. In addition, we calculated the phonon dispersion demonstrating that single-layer ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$ is dynamically stable. Furthermore, we find that similar to the bulk phase, 2D ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$ exhibits a magnetic moment that originates from a Stoner instability. In contrast to other 2D materials, we find that single-layer ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$ exhibits a significant uniaxial magnetocrystalline anisotropy energy of $920\mu \mathrm{eV}$ per Fe atom originating from spin-orbit coupling. Finally, we show that applying biaxial tensile strains enhances the anisotropy energy, which reveals strong magnetostriction in single-layer ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$ with a sizable magneostrictive coefficient. Our results indicate that single-layer ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$ is potentially useful for magnetic storage applications.