We have studied the prototype hybrid organic-inorganic perovskite ${\text{CH}}_3{\text{NH}}_3{\text{PbI}}_3$ and its three close relatives, ${\text{CH}}_3{\text{NH}}_3{\text{SnI}}_3,{\text{CH}}_3{\text{NH}}_3{\text{PbCl}}_3$, and ${\text{CsPbI}}_3$, using relativistic density function theory. The long-range van der Waals (vdW) interactions were incorporated into the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional using the Tkatchenko-Scheffler pairwise scheme. Our results reveal that hydrogen bonding, which is well described by the PBE functional, plays a decisive role for the structural parameters of these systems, including the position and orientation of the organic cation as well as the deformation of the inorganic framework. The magnitude of the inorganic-framework deformation depends sensitively on the orientation of the organic cation, and directly influences the stability of the hybrid perovskites. Our results suggest that the organic and the inorganic components complement each other; the low symmetry of the organic cation is the origin of the inorganic-framework deformation, which then aids the overall stabilization of the hybrid perovskite structure. This stabilization is indirectly affected by vdW interactions, which lead to smaller unit-cell volumes than in PBE and therefore modulate the interaction between the organic cation and the inorganic framework. The vdW-induced lattice-constant corrections are system dependent and lead to PBE+vdW lattice constants in good agreement with experiment. Further insight is gained by analyzing the vdW contributions. In all iodide-based hybrid perovskites, the interaction between the organic cation and the iodide anions provides the largest lattice-constant change, followed by iodine-iodine and the organic cation—heavy-metal cation interaction. These corrections follow an almost linear dependence on the lattice constant within the range considered in our study and are therefore approximately additive.

We simulated the carrier dynamics in InGaAs after ultrafast photoexcitation. By using a finite-difference time-domain approach we were able to analyze the near terahertz field emission caused by the motion of such carriers. We found that both the current parallel and normal to the interface take a relevant role in the terahertz emission. We also found that the ballistic motion of the carriers after photoexcitation dominates the emission rather than diffusion.

Heusler alloys have been intensively studied due to the wide variety of properties that they exhibit. One of these properties is of particular interest for technological applications, i.e., the fact that some Heusler alloys are half-metallic. In the following, a systematic study of the magnetic properties of three different Heusler families ${\mathrm{Co}}_2\mathrm{Mn}\mathrm{Z},{\text{Co}}_2\text{Fe}\text{Z}$, and ${\mathrm{Mn}}_2\mathrm{V}\mathrm{Z}$ with $\text{Z}=\left(\text{Al,}\text{Si,}\text{Ga,}\text{Ge}\right)$ is performed. A key aspect is the determination of the Gilbert damping from first-principles calculations, with special focus on the role played by different approximations, the effect that substitutional disorder and temperature effects. Heisenberg exchange interactions and critical temperature for the alloys are also calculated as well as magnon dispersion relations for representative systems, the ferromagnetic ${\mathrm{Co}}_2\mathrm{Fe}\mathrm{Si}$ and the ferrimagnetic ${\mathrm{Mn}}_2\mathrm{V}\mathrm{Al}$. Correlation effects beyond standard density-functional theory are treated using both the local spin density approximation including the Hubbard $U$ and the local spin density approximation plus dynamical mean field theory approximation, which allows one to determine if dynamical self-energy corrections can remedy some of the inconsistencies which were previously reported for these alloys.

We present x-ray resonant magnetic reflectivity (XRMR) as a very sensitive tool to detect proximity induced interface spin polarization in Pt/FM heterostructures. Different XRMR experiments are carried out and the results are evaluated for their dependence on the magneto-optical depth profile, the photon energy, the optical parameters, and the ferromagnetic material. We demonstrate that a detailed analysis of the reflected x-ray intensity gives insight into the spatial distribution of the spin polarization of a nonmagnetic metal across the interface to a ferromagnetic layer. The evaluation of the experimental results with simulations based on optical data from ab initio calculations provides the induced magnetic moment per Pt atom in the spin-polarized volume adjacent to the ferromagnet. For a series with different ferromagnetic materials consisting of Pt/Fe, $\text{Pt}/{\mathrm{Ni}}_{33}{\mathrm{Fe}}_{67}, \text{Pt}/{\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$ (permalloy), and Pt/Ni bilayers we find the largest spin polarization in Pt/Fe and a much smaller magnetic proximity effect in Pt/Ni. Additional XRMR experiments with varying photon energy are in good agreement with the theoretical predictions for the energy dependence of the magneto-optical parameters and allow identifying the optical dispersion $\delta$ and absorption $\beta$ across the $\mathrm{Pt}{\mathrm{L}}_3$-absorption edge.

A general theory of collective spin-wave edge modes in semi-infinite and finite periodic arrays of magnetic nanodots having uniform dynamic magnetization (macrospin approximation) is developed. The theory is formulated using a formalism of multivectors of magnetization dynamics, which allows one to study edge modes in arrays having arbitrarily complex primitive cells and lattice structure. The developed formalism can describe spin-wave edge modes localized both at the physical edges of the array and at the internal “domain walls” separating the array regions existing in different static magnetization states. Using a perturbation theory, in the framework of the developed formalism, it is possible to calculate damping of the edge modes and to describe their excitation by external variable magnetic fields. The theory is illustrated on the following practically important examples: (i) calculation of the FMR absorption in a finite nanodot array having the shape of a right triangle; (ii) calculation of the spectra of nonreciprocal spin-wave edge modes, including the modes at the physical edges of an array and modes at the domain walls inside the array; and (iii) study of the influence of the domain wall modes on the FMR spectrum of an array existing in a nonideal chessboard antiferromagnetic ground state.

We consider excited vortex states, which are vortex states left inside a superconductor once the external applied magnetic field is switched off and whose energy is lower than of the normal state. We show that this state is paramagnetic and develop here a general method to obtain its Gibbs free energy through conformal mapping. The solution for any number of vortices in any cross-section geometry can be read off from the Schwarz-Christoffel mapping. The method is based on the first-order equations used by Abrikosov to discover vortices.

The thermal conductivity $\kappa$ of the iron-arsenide superconductor ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ was measured for heat currents parallel and perpendicular to the tetragonal $c$ axis at temperatures down to 50 mK and in magnetic fields up to 15 T. Measurements were performed on samples with compositions ranging from optimal doping ($x=0.34, T_{\mathrm{c}}=39$ K) down to dopings deep into the region where antiferromagnetic order coexists with superconductivity ($x=0.16, T_{\mathrm{c}}=7$ K). In zero field, there is no residual linear term in $\kappa \left(T\right)$ as $T\to 0$ at any doping, whether for in-plane or interplane transport. This shows that there are no nodes in the superconducting gap. However, as $x$ decreases into the range of coexistence with antiferromagnetism, the residual linear term grows more and more rapidly with applied magnetic field. This shows that the superconducting energy gap develops minima at certain locations on the Fermi surface and these minima deepen with decreasing $x$. We propose that the minima in the gap structure arise when the Fermi surface of ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ is reconstructed by the antiferromagnetic order.

We report a joint experimental and theoretical study of the optical properties of tetragonal bismuth oxide ($\beta -\mathrm{B}{\mathrm{i}}_2{\mathrm{O}}_3$) at high pressure by means of optical absorption measurements combined with ab initio electronic band structure calculations. Our results are consistent with previous results that show the presence of a second-order isostructural phase transition in $\mathrm{B}{\mathrm{i}}_2{\mathrm{O}}_3$ (from $\beta$ to ${\beta }^{\prime }$) around 2 GPa and a phase transition above 15 GPa combined with a pressure-induced amorphization above 17–20 GPa. In order to further understand the pressure-induced phase transitions and amorphization occurring in $\beta -\mathrm{B}{\mathrm{i}}_2{\mathrm{O}}_3$, we theoretically studied the mechanical and dynamical stability of the tetragonal structures of $\beta$- and ${\beta }^{\prime }-\mathrm{B}{\mathrm{i}}_2{\mathrm{O}}_3$ at high pressure through calculations of their elastic constants, elastic stiffness coefficients, and phonon dispersion curves. The pressure dependence of the elastic stiffness coefficients and phonon dispersion curves confirms that the isostructural phase transition near 2 GPa is of ferroelastic nature. Furthermore, a topological study of the electron density shows that the ferroelastic transition is not caused by a change in number of critical points (cusp catastrophe), but by the equalization of the electron densities of both independent O atoms in the unit cell due to a local rise in symmetry. Finally, from theoretical simulations, ${\beta }^{\prime }-\mathrm{B}{\mathrm{i}}_2{\mathrm{O}}_3$ is found to be mechanically and dynamically stable at least up to 26.7 GPa under hydrostatic conditions; thus, the pressure-induced amorphization reported above 17–20 GPa in powder ${\beta }^{\prime }-\mathrm{B}{\mathrm{i}}_2{\mathrm{O}}_3$ using methanol-ethanol-water as pressure-transmitting medium could be related to the frustration of a reconstructive phase transition at room temperature and the presence of mechanical or dynamical instabilities under nonhydrostatic conditions.

We present a comparison of the tunnel magneto-Seebeck effect for laser-induced and intrinsic heating. Therefore, ${\mathrm{Co}}_{40}{\mathrm{Fe}}_{40}{\mathrm{B}}_{20}/{\mathrm{MgAl}}_2{\mathrm{O}}_4$ and ${\mathrm{Co}}_{25}{\mathrm{Fe}}_{55}{\mathrm{B}}_{20}$/MgO magnetic tunnel junctions have been prepared. The TMS ratio of 3% in case of the MAO MTJ agrees well with ratios found for other barrier materials, while the TMS ratio of 23% of the MgO MTJ emphasizes the influence of the CoFe composition. We find results using the intrinsic method that differ in sign and magnitude in comparison to the results of the laser heating. The intrinsic contributions can alternatively be explained by the Brinkman model and the given junction properties. Especially, we are able to demonstrate that the symmetric contribution is solely influenced by the barrier asymmetry. Thus, we conclude that the symmetry analysis used for the intrinsic method is not suitable to unambiguously identify an intrinsic tunnel magneto-Seebeck effect.

Systems which rapidly evolve through symmetry-breaking transitions on timescales comparable to the fluctuation timescale of the single-particle excitations may behave very differently than under controlled near-ergodic conditions. A real-time investigation with high temporal resolution may reveal insights into the ordering through the transition that are not available in static experiments. We present an investigation of the system trajectory through a normal-to-superconductor transition in a prototype high-temperature superconducting cuprate in which such a situation occurs. Using a multiple pulse femtosecond spectroscopy technique we measure the system trajectory and time evolution of the single-particle excitations through the transition in ${\mathrm{La}}_{1.9}{\mathrm{Sr}}_{0.1}{\mathrm{CuO}}_4$ and compare the data to a simulation based on the time-dependent Ginzburg-Landau theory, using the laser excitation fluence as an adjustable parameter controlling the quench conditions in both experiment and theory. The comparison reveals the presence of significant superconducting fluctuations which precede the transition on short timescales. By including superconducting fluctuations as a seed for the growth of the superconducting order we can obtain a satisfactory agreement of the theory with the experiment. Remarkably, the pseudogap excitations apparently play no role in this process.

We present a microwave quantum refrigeration principle based on the Josephson effect. When a superconducting quantum interference device (SQUID) is pierced by a time-dependent magnetic flux, it induces changes in the macroscopic quantum phase and an effective finite bias voltage appears across the SQUID. This voltage can be used to actively cool, well below the lattice temperature, one of the superconducting electrodes forming the interferometer. The achievable cooling performance combined with the simplicity and scalability intrinsic to the structure pave the way to a number of applications in quantum technology.

We show that disruption of charge-density-wave (stripe) order by charge transfer excitation, enhances the superconducting phase rigidity in ${\mathrm{La}}_{1.885}{\mathrm{Ba}}_{0.115}{\mathrm{CuO}}_4$. Time-resolved resonant soft x-ray diffraction demonstrates that charge order melting is prompt following near-infrared photoexcitation whereas the crystal structure remains intact for moderate fluences. THz time-domain spectroscopy reveals that, for the first 2 ps following photoexcitation, a new Josephson plasma resonance edge, at higher frequency with respect to the equilibrium edge, is induced indicating enhanced superconducting interlayer coupling. The fluence dependence of the charge-order melting and the enhanced superconducting interlayer coupling are correlated with a saturation limit of $\sim 0.5\mathrm{mJ}/{\mathrm{cm}}^2$. Using a combination of x-ray and optical spectroscopies we establish a hierarchy of timescales between enhanced superconductivity, melting of charge order, and rearrangement of the crystal structure.

The boundary of a fractionalized topological phase can be gapped by condensing a proper set of bosonic quasiparticles. Interestingly, in the presence of a global symmetry, such a boundary can have different symmetry transformation properties. Here we present an explicit example of this kind, in the double semion state with time reversal symmetry. We find two distinct cases where the semionic excitations on the boundary can transform either as time reversal singlets or as time reversal (Kramers) doublets, depending on the coherent phase factor of the Bose condensate. The existence of these two possibilities are demonstrated using both field-theory argument and exactly solvable lattice models. Furthermore, we study the domain walls between these two types of gapped boundaries and find that the application of time reversal symmetry tunnels a semion between them.

We propose a general-purpose semilocal/nonlocal exchange-correlation functional approximation, named mBEEF-vdW. The exchange is a meta generalized gradient approximation, and the correlation is a semilocal and nonlocal mixture, with the Rutgers-Chalmers approximation for van der Waals (vdW) forces. The functional is fitted within the Bayesian error estimation functional (BEEF) framework [J. Wellendorff et al., Phys. Rev. B 85, 235149 (2012); J. Wellendorff et al., J. Chem. Phys. 140, 144107 (2014)]. We improve the previously used fitting procedures by introducing a robust MM-estimator based loss function, reducing the sensitivity to outliers in the datasets. To more reliably determine the optimal model complexity, we furthermore introduce a generalization of the bootstrap 0.632 estimator with hierarchical bootstrap sampling and geometric mean estimator over the training datasets. Using this estimator, we show that the robust loss function leads to a $10\%$ improvement in the estimated prediction error over the previously used least-squares loss function. The mBEEF-vdW functional is benchmarked against popular density functional approximations over a wide range of datasets relevant for heterogeneous catalysis, including datasets that were not used for its training. Overall, we find that mBEEF-vdW has a higher general accuracy than competing popular functionals, and it is one of the best performing functionals on chemisorption systems, surface energies, lattice constants, and dispersion. We also show the potential-energy curve of graphene on the nickel(111) surface, where mBEEF-vdW matches the experimental binding length. mBEEF-vdW is currently available in gpaw and other density functional theory codes through Libxc, version 3.0.0.

Semiconductor nanowires based on non-nitride III-V compounds can be synthesized under certain growth conditions to favor the appearance of the wurtzite crystal phase. Despite reports in the literature of ab initio band structures for these wurtzite compounds, we still lack effective multiband models and parameter sets that can be simply used to investigate physical properties of such systems, for instance, under quantum confinement effects. In order to address this deficiency, in this study we calculate the ab initio band structure of bulk InAs and InP in the wurtzite phase and develop an $8\times 8$ $k·p$ Hamiltonian to describe the energy bands around the $\mathrm{\Gamma }$ point. We show that our $k·p$ model is robust and can be fitted to describe the important features of the ab initio band structure. The correct description of the spin-splitting effects that arise due to the lack of inversion symmetry in wurtzite crystals is obtained with the $k$-dependent spin-orbit term in the Hamiltonian, often neglected in the literature. All the energy bands display a Rashba-like spin texture for the in-plane spin expectation value. We also provide the density of states and the carrier density as functions of the Fermi energy. Alternatively, we show an analytical description of the conduction band, valid close to the $\mathrm{\Gamma }$ point. The same fitting procedure is applied to the $6\times 6$ valence band Hamiltonian. However, we find that the most reliable approach is the $8\times 8$ $k·p$ Hamiltonian for both compounds. The $k·p$ Hamiltonians and parameter sets that we develop in this paper provide a reliable theoretical framework that can be easily applied to investigate electronic, transport, optical, and spin properties of InAs- and InP-based nanostructures.

We investigate the nonequilibrium transport properties of a double quantum-dot system connected in parallel to two leads, including intradot electron-electron interaction. In the absence of interactions, the system supports a bound state in the continuum. This state is revealed as a Fano antiresonance in the transmission when the energy levels of the dots are detuned. Using the Keldysh nonequilibrium Green's-function formalism, we find that the occurrence of the Fano antiresonance survives in the presence of Coulomb repulsion. We give precise predictions for the experimental detection of bound states in the continuum. First, we calculate the differential conductance as a function of the applied voltage and the dot level detuning and find that crossing points in the diamond structure are revealed as minima due to the transmission antiresonances. Second, we determine the thermoelectric current in response to an applied temperature bias. In the linear regime, quantum interference gives rise to sharp peaks in the thermoelectric conductance. Remarkably, we find interaction-induced strong current nonlinearities for large thermal gradients that may lead to several nontrivial zeros in the thermocurrent. The latter property is especially attractive for thermoelectric applications.

It has been recently suggested that the coexistence of ferroelectricity and Rashba-like spin-splitting effects due to spin-orbit coupling in a single material may allow for a nonvolatile electric control of spin degrees of freedom. In the present work, we compared the structural and ferroelectric properties of tetragonal and rhombohedral phases of ferroelectric ${\mathrm{BiAlO}}_3$ by means of density-functional calculations. In both phases, we carefully investigated Rashba and Dresselhaus effects, giving rise to spin-splitting in their bulk electronic structure, particularly near the conduction band minimum, supplementing our first-principles results with an effective $\mathit{k}·\mathit{p}$ model analysis. The full reversal of the spin texture with ferroelectric polarization switching was also predicted. ${\mathrm{BiAlO}}_3$ can therefore be considered as the first known oxide to exhibit a coexistence of ferroelectricity and Rashba-Dresselhaus effects.

Intercalation of Li in graphite and other layered structures is of interest for highly efficient energy storage devices. In this paper, we determine the extent to which Li intercalates at the different interfaces formed between graphene (G) and hexagonal boron nitride (hBN) heterostructures. We use ab initio calculations to explore in detail the position of the dispersed Li atoms, changes in the structure at the interfaces, energetic stability of the configurations, and the corresponding electronic structure with varying concentrations of the intercalant. We trace the origin of the energetic stability and maximum concentration of Li that intercalates into various layered structures to the ability of the interface to accept electrons. Our calculations indicate that Li intercalates easiest at G/G interfaces, followed by interfaces between G/hBN, whereas Li cannot intercalate in hBN/hBN interfaces. Our results provide a framework for the design of experimental setups with optimal Li intercalation and reveal the implications of intercalation on the dielectric properties of these materials and their possible application in plasmonics.

We study radiative energy transfer between a donor-acceptor pair across a hyperbolic metamaterial slab. We show that similar to a perfect lens a hyperbolic lens allows for giant energy transfer rates. For a realistic realization of a hyperbolic multilayer metamaterial we find an enhancement of up to three orders of magnitude with respect to the transfer rates across a plasmonic silver film of the same size especially for frequencies which coincide with the epsilon-near zero and the epsilon-near pole frequencies. Furthermore, we compare exact results based on the $S$-matrix method with results obtained from effective medium theory. Our finding of very large dipole-dipole interaction at distances of the order of a wavelength has important consequences for producing radiative heat transfer, quantum entanglement, etc.

The adsorption of benzene on the Si(100) surface is studied theoretically using the self-consistent van der Waals density functional (vdW-DF) method. The adsorption energies of two competing adsorption structures, butterfly (BF) and tight-bridge (TB) structures, are calculated with several vdW-DFs at saturation coverage. Our results show that recently proposed vdW-DFs with high accuracy all prefer TB to BF, in accord with more accurate calculations based on exact exchange and correlation within the random-phase approximation. Detailed analyses reveal the important roles played by the molecule-surface interaction and molecular deformation upon adsorption, and we suggest that their precise description is a prerequisite for accurate prediction of the most stable adsorption structure of organic molecules on semiconductor surfaces.