We study the Smith-Purcell light emission produced by electrons moving parallel to linear aperiodic particle arrays. This constitutes a generalization of this type of phenomenon from periodic to aperiodic structures. As in the periodic case, the emission is found to exhibit intense features in its angular and frequency distributions, associated with the condition of constructive interference between the contributions arising from different particles in the array. This condition can also be expressed in terms of momentum conservation involving reciprocal wave-vector transfers from the array. We consider two examples of quasiperiodic and hyperuniform aperiodic arrays that allow us to illustrate this idea. Our study provides insight into the interaction of fast electrons with aperiodic arrays characterized by strong features in reciprocal space, which dominate the electron-array coupling.

Laterally periodic nanostructures were investigated with grazing incidence small angle x-ray scattering. To support an improved reconstruction of nanostructured surface geometries, we investigated the origin of the contributions to the diffuse scattering pattern which is correlated to the surface roughness. Resonant diffuse scattering leads to a palmlike structure of intensity sheets. Dynamic scattering generates the so-called Yoneda band caused by a resonant scatter enhancement at the critical angle of total reflection and higher-order Yoneda bands originating from a subsequent diffraction of the Yoneda enhanced scattering at the grating. Our explanations are supported by modeling using a solver for the time-harmonic Maxwell's equations based on the finite-element method.

We present anisotropic transport and optical spectroscopy studies on ${\mathrm{EuCd}}_2{\mathrm{As}}_2$. The measurements reveal that ${\mathrm{EuCd}}_2{\mathrm{As}}_2$ is a low carrier density semimetal with moderate anisotropic resistivity ratio. The charge carriers experience very strong scattering from Eu magnetic moments, resulting in a Kondo-like increase of resistivity at low temperature. Below the antiferromagnetic transition temperature at $T_N=9.5$ K, the resistivity drops sharply due to the reduced scattering from the ordered Eu moments. Nevertheless, the anisotropic ratio of ${\rho }_c/{\rho }_{ab}$ keeps increasing, suggesting that the antiferromagnetic coupling is along the $c$ axis. The optical spectroscopy measurement further reveals, besides an overdamped reflectance plasma edge at low energy, a strong coupling between phonon and electronic continuum. Our study suggests that ${\mathrm{EuCd}}_2{\mathrm{As}}_2$ is a promising candidate displaying intriguing interplay among charge, magnetism, and the underlying crystal lattice.

We discuss (2+1)-dimensional gapless surface theories of bulk (3+1)-dimensional topological phases, such as the BF theory at level $\mathrm{K}$, and its generalization. In particular, we put these theories on a flat (2+1)-dimensional torus $T^3$ parameterized by its modular parameters, and compute the partition functions obeying various twisted boundary conditions. We show the partition functions are transformed into each other under $SL(3,\mathbb{Z})$ modular transformations, and furthermore establish the bulk-boundary correspondence in (3+1) dimensions by matching the modular $\mathcal{S}$ and $\mathcal{T}$ matrices computed from the boundary field theories with those computed in the bulk. We also propose the three-loop braiding statistics can be studied by constructing the modular $\mathcal{S}$ and $\mathcal{T}$ matrices from an appropriate boundary field theory.

We perform density functional calculations to investigate the structure of the intermetallic alloy FeRh under epitaxial strain. Bulk FeRh exhibits a metamagnetic transition from a low-temperature antiferromagnetic (AFM) phase to a ferromagnetic phase at 350 K, and its strain dependence is of interest for tuning the transition temperature to the room-temperature operating conditions of typical memory devices. We find an unusually strong dependence of the structural energetics on the choice of exchange-correlation functional, with the usual local density approximation yielding the wrong ground-state structure, and generalized gradient (GGA) extensions being in better agreement with the bulk experimental structure. Using the GGA we show the existence of a metastable face-centered-cubic-like AFM structure that is reached from the ground-state body-centered-cubic-like AFM structure by following the epitaxial Bain path. We show that the behavior is well described using nonlinear elasticity theory, which captures the softening and eventual sign change of the orthorhombic shear modulus under compressive strain, consistent with this structural instability. Finally, we predict the existence of an additional unit-cell-doubling lattice instability, which should be observable at low temperature.

In 1949, Forsbergh, Jr. reported spontaneous spatial ordering in the birefringence patterns seen in flux-grown $\mathrm{BaTi}{\mathrm{O}}_3$ crystals under the transmission polarized light microscope [Phys. Rev. 76, 1187 (1949)]. Stunningly regular square-net arrays were often only found within a finite temperature window and could be induced on both heating and cooling, suggesting genuine thermodynamic stability. At the time, Forsbergh rationalized the patterns to have resulted from the impingement of ferroelastic domains, creating a complex tessellation of variously shaped domain packets. However, no direct evidence for the intricate microstructural arrangement proposed by Forsbergh has subsequently been found. Moreover, there are no robust thermodynamic arguments to explain the finite region of thermal stability, its occurrence just below the Curie temperature, and the apparent increase in entropy associated with the loss of the Forsbergh pattern on cooling. Despite decades of research on ferroelectrics, this ordering phenomenon and its thermodynamic origin have hence remained a mystery. In this paper, we reexamine the microstructure of flux-grown $\mathrm{BaTi}{\mathrm{O}}_3$ crystals, which show Forsbergh birefringence patterns. Given an absence of any obvious arrays of domain polyhedra or even regular shapes of domain packets, we suggest an alternative origin for the Forsbergh pattern in which sheets of orthogonally oriented ferroelastic stripe domains simply overlay one another. We show explicitly that the Forsbergh birefringence pattern occurs if the periodicity of the stripe domains is above a critical value. Moreover, by considering well-established semiempirical models, we show that the significant domain coarsening needed to generate the Forsbergh birefringence is fully expected in a finite window below the Curie temperature. We hence present a much more straightforward rationalization of the Forsbergh pattern than that originally proposed in which exotic thermodynamic arguments are unnecessary.

The phenomenon of self-organization of domains into a “square-net pattern” in single-crystal, flux-grown $\mathrm{BaTi}{\mathrm{O}}_3$ several degrees below the ferroelectric to paraelectric phase transition was investigated using in situ synchrotron x-ray topography. The tetragonal distortion of the crystal was determined by measuring the angular separation between the diffraction images received from 90° $a$ and $c$ domains in the projection topographs, and shows a rapid decrease towards 110 °C, the onset temperature for self-organization. The onset of self-organization is accompanied by bending of the $\left\{100\right\}$ lattice planes parallel to the crystal surface, which produces a strain that persists up to and beyond the Curie temperature, where the crystal becomes cubic and the self-organized domains disappear. At the Curie point, the bending angle ${\alpha }_{100}=8.1(\pm 0.3)\mathrm{mrad}$ is at a maximum and corresponds to the radius of curvature of the surface being 16.3(±0.6) mm.

The structure and magnetic properties of orthorhombic $\mathrm{D}{\mathrm{y}}_2\mathrm{Ti}{\mathrm{O}}_5$ have been investigated using x-ray diffraction, neutron diffraction, and alternating current (ac)/direct current (dc) magnetic susceptibility measurements. We report a continuous structural distortion below 100 K characterized by negative thermal expansion in the [0 1 0] direction. Neutron diffraction and magnetic susceptibility measurements revealed that two-dimensional (2D) magnetic ordering begins at 3.1 K, which is followed by a three-dimensional magnetic transition at 1.7 K. The magnetic structure has been solved through a representational analysis approach and can be indexed with the propagation vector $k=\left[01/20\right]$. The spin structure corresponds to a coplanar model of interwoven 2D “sheets” extending in the [0 1 0] direction. The local crystal field is different for each $\mathrm{D}{\mathrm{y}}^{3+}$ ion (Dy1 and Dy2), one of which possesses strong uniaxial symmetry indicative of Ising-like magnetic ordering. Consequently, two succeeding transitions under magnetic field are observed in the ac susceptibility, which are associated with flipping each $\mathrm{D}{\mathrm{y}}^{3+}$ spin independently.

$AB{\mathrm{O}}_3$ perovskite is a kind of very important functional material with versatile physical properties. Although $B$-site chemical substitution with various magnetic ions has been widely investigated, the $A$-site doping with magnetic transition metal is little known. Here we report $A{A}_3^{\prime }{B}_2{B}_2^{\prime }{\mathrm{O}}_{12}$-type $A$- and $B$-site ordered ferrimagnet $\mathrm{CaC}{\mathrm{u}}_3\mathrm{F}{\mathrm{e}}_2\mathrm{O}{\mathrm{s}}_2{\mathrm{O}}_{12}$ with magnetic transition metals occupying three different atomic sites ($A^{\prime }, B$, and $B^{\prime }$ sites). This compound is synthesized by a special high-pressure annealing process. It possesses a much higher Curie temperature $T_{\mathrm{C}}$ of 580 K compared with that of the $B$-site-only ordered $\mathrm{C}{\mathrm{a}}_2\mathrm{FeOs}{\mathrm{O}}_6$ ($T_{\mathrm{C}}=320\mathrm{K}$) without magnetic ion at the $A$ site. First-principles numerical calculations reveal that this enhancement primarily originates from the additional spin interaction between the $A^{\prime }$-site $\mathrm{C}{\mathrm{u}}^{2+}$ and the $B^{\prime }$-site $\mathrm{O}{\mathrm{s}}^{5+}$, generating a strong $\mathrm{C}{\mathrm{u}}^{2+}(\uparrow )\mathrm{F}{\mathrm{e}}^{3+}(\uparrow )\mathrm{O}{\mathrm{s}}^{5+}(\downarrow )$ ferrimagnetic spin coupling. This work opens up an alternative way for enhancing the spin ordering temperature by introducing $A$-site magnetic ions.

Concentrating on zero temperature quantum Monte Carlo calculations of electronic systems, we give a general description of the theory of finite size extrapolations of energies to the thermodynamic limit based on one- and two-body correlation functions. We introduce effective procedures, such as using the potential and wave function split up into long and short range functions to simplify the method, and we discuss how to treat backflow wave functions. Then we explicitly test the accuracy of our method to correct finite size errors on example hydrogen and helium many-body systems and show that the finite size bias can be drastically reduced for even small systems.

We apply effective medium theory (EMT) to metamaterials consisting of a varying number of consecutive sheets of graphene and hexagonal boron nitride, and compare this with a full calculation of the permittivity and the reflection based on the tight binding method and the transfer matrix method in order to study the convergence to EMT. We find that convergence is reached for both in-plane and out-of-plane directions already for five sheets but that for $\approx 30$ sheets multiple reflection effects causes the reflection spectrum to differ from EMT. We show that modes that are evanescent in air are extremely sensitive to the electronic details of the sheets near the structure boundary and that EMT estimates poorly the reflection of these modes, causing an overestimation of the Purcell factor. Finally, we offer an improved EMT, which gives far better convergence in the low-energy regime.

The weak-field Hall conductivity of disordered two-dimensional systems with strong Rashba spin-orbit interaction is studied in a self-consistent Born approximation. Explicit numerical results are obtained for scatterers with a Gaussian potential and for charged impurities. The singular behavior associated with a conelike crossing band appears only in the case of scatterers with a long-range Gaussian potential, which do not cause mixing with the outer band. In the case of more realistic scatterers such as charged impurities, the singularity is completely removed except the presence of a weak steplike feature. The Hall conductivity associated with the spin-Zeeman energy is also strongly reduced by interband mixing and generally remains much smaller than the orbital contribution.

The dynamics at the interface between a close-packed porphyrin monolayer and Au(111) is investigated by time-dependent scanning tunneling microscopy, detecting the motion of single-interface adatoms in real space. Imaging sequences reveal predominant switching of the molecular appearance in adjacent molecules, pointing to a spatial correlation that is consistent with adatom diffusion from one molecule to the next. In some cases, the number of switching molecules is drastically increased, indicating collective switching events. In addition to the thermally induced motion of adatoms at the interface, also voltage pulses from the microscope tip can induce the process—revealing different yields in agreement with the model of adatom hopping.

We investigate numerically the possibility to detect the spatial profile of Majorana fermions (MFs) by using STM tips that are made of either normal or superconducting material. In both cases, we are able to resolve the localization length and the oscillation period of the MF wave function. We show that the tunneling between the substrate and the tip, necessary to get the information on the wave-function oscillations, has to be weaker in the case of a superconducting probe. In the strong tunneling regime, the differential conductance saturates making it more difficult to observe the exponential decay of MFs. The temperature broadening of the profile is strongly suppressed in the case of the superconducting tip resulting, generally, in better resolution.

We study wave propagation in interacting Bose liquid, where the short-range part of the interaction between atoms is of a hard-core type, and its long-range part scales with a distance as a power law. The cases of Coulomb, dipole-dipole and van der Waals interaction are considered. We employ a hydrodynamic approach, based on the exact solution of the Lieb–Liniger model, and study the evolution of a density pulse instantly released from a potential trap. We analyze semiclassical Euler and continuity equations and construct the corresponding Riemann invariants. We supplement our analysis with numerical calculations and discuss experimental applications for ultracold atom experiments.

Renowned for its sensitivity to detect the presence of numerous substances, graphene is an excellent chemical sensor. Unfortunately, which general features a dopant must have in order to enter the list of substances detectable by graphene are not exactly known. Here we demonstrate with a simple model calculation implemented in three different ways that one of such features is the symmetry properties of the impurity binding to graphene. In particular, we show that electronic scattering is suppressed when dopants are bound symmetrically to both graphene sublattices, giving rise to impurity invisibility. In contrast, dopants that affect the two sublattices asymmetrically are more strongly scattered and therefore the most likely candidates to being chemically sensed by graphene. Furthermore, we demonstrate that impurity invisibility is lifted with the introduction of a symmetry-breaking perturbation such as uniaxial strain. In this case, graphene with sublattice-symmetric dopants will function as efficient strain sensors. We argue that classifying dopants through their bonding symmetry leads to a more efficient way of identifying suitable components for graphene-based sensors.