The quality of kinetic Monte Carlo (KMC) simulations of microstructure evolution in alloys relies on the parametrization of point-defect migration rates, which are complex functions of the local chemical composition and can be calculated accurately with ab initio methods. However, constructing reliable models that ensure the best possible transfer of physical information from ab initio to KMC is a challenging task. This work presents an innovative approach, where the transition rates are predicted by artificial neural networks trained on a database of 2000 migration barriers, obtained with density functional theory (DFT) in place of interatomic potentials. The method is tested on copper precipitation in thermally aged iron alloys, by means of a hybrid atomistic-object KMC model. For the object part of the model, the stability and mobility properties of copper-vacancy clusters are analyzed by means of independent atomistic KMC simulations, driven by the same neural networks. The cluster diffusion coefficients and mean free paths are found to increase with size, confirming the dominant role of coarsening of medium- and large-sized clusters in the precipitation kinetics. The evolution under thermal aging is in better agreement with experiments with respect to a previous interatomic-potential model, especially concerning the experiment time scales. However, the model underestimates the solubility of copper in iron due to the excessively high solution energy predicted by the chosen DFT method. Nevertheless, this work proves the capability of neural networks to transfer complex ab initio physical properties to higher-scale models, and facilitates the extension to systems with increasing chemical complexity, setting the ground for reliable microstructure evolution simulations in a wide range of alloys and applications.

We present an analytic and numerical study of the creation of a unidirectional spin-wave emission in ultrathin ferromagnetic films sandwiched in an asymmetric layer stack. For this we extend the analytical description of spin waves in spin-wave waveguides by incorporating the influence of the interfacial Dzyaloshinskii-Moriya interaction on the spin-wave propagation. By exploring the model system ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}/\mathrm{Pt}$, we show that it is possible to achieve a unidirectional spin-wave emission by combining wave-vector selective excitation sources with the frequency splitting that arises from the interfacial Dzyaloshinskii-Moriya interaction. Hereby, we focus on device feature sizes and spin-wave wavelengths that are compatible with state-of-the art excitation and detection schemes. We demonstrate that the presented analytical formalism allows for an approximate prediction of the nonreciprocal emission.

We have studied the effect of Se substitution on Eu valence in a layered ${\mathrm{Eu}}_{0.5}{\mathrm{La}}_{0.5}{\mathrm{FBiS}}_{2-x}{\mathrm{Se}}_x$ superconductor using a combined analysis of x-ray absorption near-edge structure (XANES) and x-ray photoelectron spectroscopy (XPS) measurements. Eu $L_3$-edge XANES spectra reveal that Eu is in the mixed valence state with coexisting ${\mathrm{Eu}}^{2+}$ and ${\mathrm{Eu}}^{3+}$. The average Eu valence decreases sharply from $\sim 2.3$ for $x=0.0$ to $\sim 2.1$ for $x=0.4$. Consistently, Eu $3d$ XPS shows a clear decrease in the average valence by Se substitution. Bi $4f$ XPS indicates that effective charge carriers in the ${\mathrm{BiCh}}_2$ (Ch = S, Se) layers are slightly increased by Se substitution. On the basis of the present results it has been discussed that the metallic character induced by Se substitution in ${\mathrm{Eu}}_{0.5}{\mathrm{La}}_{0.5}{\mathrm{FBiS}}_{2-x}{\mathrm{Se}}_x$ is likely to be due to increased in-plane orbital overlap driven by reduced in-plane disorder that affects the carrier mobility.

We calculate the finite temperature linear dc conductance of a generic single-impurity Anderson model containing an arbitrary number of Fermi liquid leads, and apply the formalism to closed and open long Aharonov-Bohm-Kondo (ABK) rings. We show that, as with the short ABK ring, there is a contribution to the conductance from the connected four-point Green's function of the conduction electrons. At sufficiently low temperatures this contribution can be eliminated, and the conductance can be expressed as a linear function of the T matrix of the screening channel. For closed rings we show that at temperatures high compared to the Kondo temperature, the conductance behaves differently for temperatures above and below $v_F/L$, where $v_F$ is the Fermi velocity and $L$ is the circumference of the ring. For open rings, when the ring arms have both a small transmission and a small reflection, we show from the microscopic model that the ring behaves like a two-path interferometer, and that the Kondo temperature is unaffected by details of the ring. Our findings confirm that ABK rings are potentially useful in the detection of the size of the Kondo screening cloud, the $\pi /2$ scattering phase shift from the Kondo singlet, and the suppression of Aharonov-Bohm oscillations due to inelastic scattering.

The fundamental properties of a wave precludes it from being localized to subwavelength distances in all dimensions of the wave's existence. The inability to focus electromagnetic waves to an all-direction subwavelength spot limits the 3D resolution of a conventional imaging system to about half the imaging wavelength. A plethora of super-resolution imaging systems have been designed which obtain super-resolution in one or two (but not all) dimensions, but they suffer various restrictions in working distance and the classes of objects they can image. In this paper, we report a first investigation into a wave that is focused to subwavelength dimensions in all directions. After reviewing the physics of wave dispersion and diffraction which seemingly preclude this phenomenon, we sidestep these preclusions using a broadband superoscillation waveform and synthesize an all-direction subwavelength focus. We report the salient spatial and temporal features of this wave, and apply it to achieve 3D super-resolution imaging.

The plasmon dispersion in $2H$ transition metal dichalcogenides has been reported to be negative, which is a strong renormalization with respect to the expectations for a homogeneous electron gas. This has been interpreted to be either due to the interaction of the plasmon oscillations with charge density fluctuations or due to the particular band structure of these materials. We present a separation of the momentum dependence of the charge carrier plasmon from that of single particle excitations (band-band transitions). We demonstrate that the (momentum dependent) screening of the plasmon oscillations by the single particle excitations predominantly determines the observed negative plasmon dispersion. However, even without this screening, the plasmon dispersion deviates from that of a simple metal, which indicates the additional, but smaller, impact of the charge density wave fluctuations.

We have studied the electronic structure of ${\mathrm{BaV}}_{10}{\mathrm{O}}_{15}$ across the metal-insulator transition with V trimerization by means of hard-x-ray photoemission spectroscopy (HAXPES) and mean-field calculations. The V $2p$ HAXPES indicates ${\mathrm{V}}^{2.5+}-{\mathrm{V}}^{3+}$ charge fluctuation in the metallic phase, and ${\text{V}}^{2+}-{\text{V}}^{3+}$ charge order in the insulating phase. The ${\mathrm{V}}^{2.5+}-{\mathrm{V}}^{3+}$ charge fluctuation is consistent with the mean-field solution where a V $3d a_{1g}$ electron is shared by two V sites with face-sharing ${\mathrm{VO}}_6$ octahedra. The valence-band HAXPES of the metallic phase exhibits pseudogap opening at the Fermi level associated with the charge fluctuation, and a band gap $\sim 200$ meV is established in the insulating phase due to the switching of charge correlation.

In this work we study a two-dimensional topological superconductor on a square lattice. It contains hopping, spin-orbit coupling and a time reversal symmetry breaking Zeeman term. This realizes, in combination with a singlet pairing term, an effective triplet pairing that leads to a topological superconductor. As previously found numerically, the transitions are seen in the entanglement entropy as cusps as a function of model parameters. We complement this through studying the entanglement entropy analytically by keeping only its most important components. Our study is based on the intuition that the number of Fermi surfaces in the system controls the topological invariant. With our approximate expression for the entanglement entropy we are able to extract the divergent entanglement entropy derivative close to the phase transition.

We study the spreading of correlations after a local quench in a nonrelativistic quantum field theory. We focus on noninteracting nonrelativistic fermions and study the time evolution after two identical systems in their ground states are suddenly joined together with a localized impurity at the junction. We find that, even if the quasiparticles of the system have unbounded dispersion, the correlations show light cone effects. We carry out a detailed study of these effects by developing an accurate asymptotic expansion of the two-point function and determining exactly the density of particles at any time after the quench. In particular, we find that the width of the light cone region is $\propto t^{1/2}$. The structure of correlations, however, does not show a pure light cone form: “superluminal corrections” are much larger than in the bounded-dispersion case. These findings can be explained by inspecting the structure of excitations generated by the initial state. We show that a similar picture also emerges in the presence of a harmonic trapping potential and when more than two systems are suddenly joined at a single point.

We have investigated $\mathrm{DyMn}{}^{16}\mathrm{O}_3$ and its isotopically substituted counterpart $\mathrm{DyMn}{}^{18}\mathrm{O}_3$ by neutron powder diffraction, x-ray diffraction, and heat capacity measurements to investigate the mechanism leading to its magnetically induced electric polarization. ${}^{18}\mathrm{O}$ isotope substitution does not influence the magnetic ordering temperature of the Mn ions $T_{\mathrm{N},\mathrm{Mn}}$ or the multiferroic ordering temperature $T_{\mathrm{l}}$ coinciding with the onset of the spin spiral phase; however, it does reduce the ordering temperature of Dy into its incommensurate magnetic state $T_{\mathrm{N},\mathrm{Dy}}$ from 7.0(1) K to 5.9(1) K. The temperature dependence of the magnetic propagation vector, ${\mathbf{q}}_{\mathrm{IC}}$, changes with ${}^{18}\mathrm{O}$ substitution, while $T_{\mathrm{l}}$ remains almost constant, independent of ${\mathbf{q}}_{\mathrm{IC}}$. Pronounced changes in the lattice parameters occur at the various phase transitions. Furthermore, distinct subpicometer-scale distortions of the $\mathrm{Mn}{\mathrm{O}}_6$ octahedra and displacements of the Dy ions are observed below the ferroelectric phase transition at $T_{\mathrm{l}}$ in both samples, pointing toward the mechanism for electric polarization and its coupling to the orbital degrees of freedom.

Using the fixed-node diffusion Monte Carlo (FN-DMC) method, we evaluate the formation energies and charge transition levels of substitutional nitrogen defects in the wide-band-gap semiconductor zinc oxide. The use of a direct-solution, many-body approach inherently secures a good description of electron-electron interactions, achieving high accuracy without adjustable parameters. According to FN-DMC nitrogen is a deep acceptor with a charge transition level 1.0(3) eV above the valence-band maximum when 72-atom supercells are used. This result falls on the lower end of typically reported hybrid density functional results for the same size supercells, which range from 1.0 to 1.8 eV. Further, residual finite-size effects due to charged defect image interactions in the 72-atom supercells are estimated by supercell extrapolation within hybrid density functional theory. When the finite-size correction is included, we obtain a deep acceptor at 1.6(3) eV. This result is in good agreement with recent experimental measurements. We also analyze the local compressibility of charge according to FN-DMC and common density functionals and find that the use of hybrid functionals obtains compressibilities in better agreement with the many-body theory. Our work illustrates the application of the FN-DMC method to a challenging point defect problem, demonstrating that uncertainties and approximations can be well controlled.

An analytical $N$-layer model for charge transport close to a surface is derived from the solution of Poisson's equation and used to describe distance-dependent electrical four-point measurements on the microscale. As the $N$-layer model comprises a surface channel, multiple intermediate layers, and a semi-infinite bulk, it can be applied to semiconductors in combination with a calculation of the near-surface band bending to model very precisely the measured four-point resistance on the surface of a specific sample and to extract a value for the surface conductivity. For describing four-point measurements on sample geometries with mixed 2D-3D conduction channels, often a very simple parallel-circuit model has so far been used in the literature, but the application of this model is limited, as there are already significant deviations, when it is compared to the lowest possible case of the $N$-layer model, i.e., the three-layer model. Furthermore, the $N$-layer model is applied to published distance-dependent four-point resistance measurements obtained with a multitip scanning tunneling microscope (STM) on germanium(100) and silicon(100) with different bulk doping concentrations resulting in the determination of values for the surface conductivities of these materials.

It is theoretically demonstrated that the figure of merit ($ZT$) of quantum dot (QD) junctions can be significantly enhanced when the degree of degeneracy of the energy levels involved in electron transport is increased. The theory is based on the the Green-function approach in the Coulomb blockade regime by including all correlation functions resulting from electron-electron interactions associated with the degenerate levels ($L$). We found that electrical conductance ($G_e$) as well as electron thermal conductance (${\kappa }_e$) are highly dependent on the level degeneracy ($L$), whereas the Seebeck coefficient ($S$) is not. Therefore, the large enhancement of $ZT$ is mainly attributed to the increase of $G_e$ when the phonon thermal conductance (${\kappa }_{ph}$) dominates the heat transport of the QD junction system. In the serially coupled double-QD case, we also obtain a large enhancement of $ZT$ arising from higher $L$. Unlike $G_e$ and ${\kappa }_e, S$ is found almost independent on electron interdot hopping strength.

Since Morin discovered that vanadium dioxide $\left(\mathrm{V}{\mathrm{O}}_2\right)$ undergoes a reversible and dramatic structural phase transition coupled with an abrupt metal-insulator transition, extensive attention has been paid to $\mathrm{V}{\mathrm{O}}_2$ due to its importance in fundamental condensed state physics and its potential technological applications. Here, we observed that the precipitated phases of $\mathrm{V}{\mathrm{O}}_2$ (insulating and metallic polymorphs) could be controlled by relaxing the dimerization of the vanadium-vanadium (V-V) atomic chain. In particular, the monoclinic metallic phase can be stabilized even at room temperature with the assistance of the angle-distortion-induced $\left(\beta ={120}^{\circ }\right)$ metallization through symmetry matching between the $\mathrm{V}{\mathrm{O}}_2$ epitaxial thin films and the (0001)-oriented sapphire substrates. Concomitantly, the insulating phase $(\text{M}1,\beta =122.6^{\circ })$ that separates from the metallic matrix may supply another driving force for stabilizing the metallic phase, as indicated by scattering-type scanning near-field optical infrared microscopy and further confirmed by synchrotron radiation high-resolution x-ray diffraction characterizations. Soft x-ray absorption spectroscopy results showed that the orbital features of the monoclinic metallic phase are analogous to those of the high-temperature metallic rutile $\mathrm{V}{\mathrm{O}}_2$ (R) phase. First-principles calculations further demonstrate the angle-distortion-induced reduction of the V-V atomic dimerization, which enhances the electron delocalization and thus the conductivity. Therefore, the angle distortion results in the metallic monoclinic phase and stabilizes it with the assistance of the nanoscale insulating $\mathrm{V}{\mathrm{O}}_2$ (M1) domains at room temperature. These results are of great importance for understanding the contributions of various polymorphs to the metal-insulator transition and for the design of novel artificially heterointerfacial devices based on $\mathrm{V}{\mathrm{O}}_2$ nanoscale polymorphs.

We introduce a novel class of interaction-enabled topological crystalline insulators in two- and three-dimensional electronic systems, which we call “topological crystalline magnet.” It is protected by the product of the time-reversal symmetry $\mathcal{T}$ and a mirror symmetry or a rotation symmetry $\mathcal{R}$. A topological crystalline magnet exhibits two intriguing features: (i) it cannot be adiabatically connected to any Slater insulator and (ii) the edge state is robust against coupling electrons to the edge. These features are protected by the anomalous symmetry transformation property ${\left(\mathcal{R}\mathcal{T}\right)}^2=-1$ of the edge state. An anisotropic response to the external magnetic field can be an experimental signature.

Spin injection from epitaxial iron into InGaAs/InAs quantum wells is observed using an all-electric nonlocal setup. From the choice of material, a significant spin-orbit interaction (SOI) is expected. The contact separation of the spin-valve devices is in the order of the mean free path so that the transport is at the transition between diffusive and ballistic. With an established purely diffusive model a spin-injection efficiency of $77\%$ is determined from the data. This value is very large compared to previous observations on diffusive spin-valve devices on similar material systems. Motivated by similar results on ballistic spin-valve devices in a material system with small spin-orbit coupling, a recent model was suggested in which a ballistic spin-dephasing length was pointed out to be the crucial length scale. With this model and an experimentally determined spin-orbit coupling parameter of $\alpha =4\times {10}^{-12}$ eV m, very high spin-injection efficiencies are still determined in our quantum wells. We suggest that the spin-dephasing length to be used in the model must be larger due to the crystallographic anisotropy of the spin-orbit coupling, i.e., in our setup the SOI stabilizes the spin in the crystal direction of the spin-polarized current.

Two-dimensional (2D) transition metal dichalogenides exhibit a unique band structure: In contrast to many direct-gap classical semiconductors, their band-gap minimum is not at the center of the Brillouin zone, but at finite values of the $k$ vector. We report on clear indications that this momentum mismatch fundamentally influences the carrier transfer between a 2D ${\mathrm{WS}}_2$ crystal and a three-dimensional (3D) GaN layer: Populating different local band extrema of the ${\mathrm{WS}}_2$ in $k$ space by selective laser excitation leads to a pronounced difference in the ${\mathrm{WS}}_2$ photoluminescence signal. These findings may be of high importance for future 2D-3D semiconductor devices.

Considerable efforts have been made to trap massless Dirac fermions in a graphene monolayer, but only quasibound states have been realized in continuous graphene sheets up to now. Here, we demonstrate the realization of bound states in nanoscale graphene quantum dots (GQDs) in a continuous graphene sheet. The GQDs are electronically isolated from the surrounding continuous graphene sheet by circular boundaries, which are generated by strong coupling between graphene and the substrate. By using scanning tunneling microscopy (STM), we observe single-electron charging states of the GQDs, seen as Coulomb oscillations in the tunneling conductance. The evolution of single-electron tunneling of the GQDs between the Coulomb blockade regime and the Coulomb staircase regime is observed by tuning the STM tip-sample distances. Spatial maps of the local electronic densities reveal concentric rings inside the GQDs with each ring corresponding to a single Coulomb oscillation of the tunneling spectra. These results indicate explicitly that the electrons are completely trapped inside the nanoscale GQDs.

We study the long time dynamics in closed quantum systems periodically driven via time dependent parameters with two frequencies ${\omega }_1$ and ${\omega }_2=r{\omega }_1$. Tuning of the ratio $r$ there can unleash plenty of dynamical phenomena to occur. Our study includes integrable models like Ising and $XY$ models in $d=1$ and the Kitaev model in $d=1$ and 2 and can also be extended to Dirac fermions in graphene. We witness the wave-function overlap or dynamic freezing that occurs within some small/ intermediate frequency regimes in the $({\omega }_1,r)$ plane (with $r\ne 0$) when the ground state is evolved through a single cycle of driving. However, evolved states soon become steady with long driving, and the freezing scenario gets rarer. We extend the formalism of adiabatic-impulse approximation for many cycle driving within our two-rate protocol and show the near-exact comparisons at small frequencies. An extension of the rotating wave approximation is also developed to gather an analytical framework of the dynamics at high frequencies. Finally we compute the entanglement entropy in the stroboscopically evolved states within the gapped phases of the system and observe how it gets tuned with the ratio $r$ in our protocol. The minimally entangled states are found to fall within the regime of dynamical freezing. In general, the results indicate that the entanglement entropy in our driven short-ranged integrable systems follow a genuine nonarea law of scaling and show a convergence (with a $r$ dependent pace) towards volume scaling behavior as the driving is continued for a long time.