Experimental evidence is reported on the control of the directionality of guided electromagnetic microwaves by the polarization of the exciting wave. Experiments are conducted using a two-dimensional waveguide made of two parallel aluminum plates. The upper plate, which has a linear array of holes, is externally illuminated by a polarized wave whose incident wavevector is contained within the mirror-symmetry plane defined by the linear array. Surprisingly, the measurements show that the propagation inside the waveguide is highly asymmetrical, and it is controlled by the polarization of the incoming wave. This extraordinary phenomenon is explained in terms of a simple model involving a set of dipoles that are excited at the hole positions. Our finding provides a powerful method to sort different polarizations of a free-space beam to different propagation directions of guided electromagnetic waves.

The effect of magnetic field on an ultrathin magnetic topological insulator film with structural inversion asymmetry is investigated. We introduce the phase diagram, calculate the Landau level spectrum analytically, and simulate the transport behavior in Landauer-Büttiker formalism. The quantum anomalous Hall phase will survive increasing magnetic field. Due to the two spin-polarized zero modes of Landau levels, a nontrivial phase similar with the quantum spin Hall effect can be induced by a magnetic field, which is protected by structural inversion symmetry. Some exotic longitudinal and Hall resistance plateaus with fractional values are also found in a six-terminal Hall bar, arising from the coupling between edge states due to an inverted energy band and Landau levels.

We investigate the conductivity of charge carriers confined to a two-dimensional system with the nonparabolic dispersion $k^N$ with $N$ being an arbitrary natural number. A $\delta$-shaped scattering potential is assumed as the major source of disorder. We employ the exact solution of the Lippmann-Schwinger equation to derive an analytical Boltzmann conductivity formula valid for an arbitrary scattering potential strength. The range of applicability of our analytical results is assessed by a numerical study based on the finite size Kubo formula. We find that for any $N>1$, the conductivity demonstrates a linear dependence on the carrier concentration in the limit of a strong scattering potential strength. This finding agrees with the conductivity measurements performed recently on chirally stacked multilayer graphene where the lowest two bands are nonparabolic and the adsorbed hydrocarbons might act as strong short-range scatterers.

We study the electronic properties of the confined honeycomb lattice in the presence of the intrinsic spin-orbit (ISO) interaction and perpendicular magnetic field, and report on uncommon aspects of the quantum spin Hall conductance corroborated by peculiar properties of the edge states. The ISO interaction induces two specific gaps in the Hofstadter spectrum, namely the “weak” topological gap defined by Beugeling et al. [Phys. Rev. B 86, 075118 (2012)], and spin-imbalanced gaps in the relativistic range of the energy spectrum. We analyze the evolution of the helical states with the magnetic field and with increasing Anderson disorder. The “edge” localization of the spin-dependent states and its dependence on the disorder strength is shown. The quantum transport, treated in the Landauer-Büttiker formalism, reveals interesting new plateaus of the quantum spin Hall effect (QSHE), and also of the integer quantum Hall effect (IQHE), in the energy ranges corresponding to the spin-imbalanced gaps. The properties of the spin-dependent transmittance matrix that determine the symmetries with respect to the spin, energy, and magnetic field of the longitudinal and transverse resistance are shown.

We screen a large chemical space of perovskite alloys for systems with optimal properties to accommodate a morphotropic phase boundary (MPB) in their composition-temperature phase diagram, a crucial feature for high piezoelectric performance. We start from alloy end points previously identified in a high-throughput computational search. An interpolation scheme is used to estimate the relative energies between different perovskite distortions for alloy compositions with a minimum of computational effort. Suggested alloys are further screened for thermodynamic stability. The screening identifies alloy systems already known to host an MPB and suggests a few others that may be promising candidates for future experiments. Our method of investigation may be extended to other perovskite systems, e.g., (oxy-)nitrides, and provides a useful methodology for any application of high-throughput screening of isovalent alloy systems.

We investigate the low-temperature properties of the one-dimensional spin-1 Heisenberg model with geometric fluctuations induced by aperiodic but deterministic coupling distributions, involving two parameters. We focus on two aperiodic sequences, the Fibonacci sequence and the 6-3 sequence. Our goal is to understand how these geometric fluctuations modify the physics of the (gapped) Haldane phase, which corresponds to the ground state of the uniform spin-1 chain. We make use of different adaptations of the strong-disorder renormalization-group (SDRG) scheme of Ma, Dasgupta, and Hu, widely employed in the study of random spin chains, supplemented by quantum Monte Carlo and density-matrix renormalization-group numerical calculations, to study the nature of the ground state as the coupling modulation is increased. We find no phase transition for the Fibonacci chain, while we show that the 6-3 chain exhibits a phase transition to a gapless, aperiodicity-dominated phase similar to the one found for the aperiodic spin-1/2 XXZ chain. Contrary to what is verified for random spin-1 chains, we show that different adaptations of the SDRG scheme may lead to different qualitative conclusions about the nature of the ground state in the presence of aperiodic coupling modulations.

Ferromagnetic insulators with Dzyaloshinskii-Moriya interaction show the magnon Hall effect, i.e., a transverse heat current upon application of a temperature gradient. In this theoretical investigation we establish a close connection of the magnon Hall effect in two-dimensional kagome lattices with the topology of their magnon dispersion relation. From the topological phase diagram we predict systems which show a change of sign in the heat current in dependence on temperature. Furthermore, we derive the high-temperature limit of the thermal Hall conductivity; this quantity provides a figure of merit for the maximum strength of the magnon Hall effect. Eventually, we compare the temperature and field dependence of the magnon Hall conductivity of the three-dimensional pyrochlore ${\mathrm{Lu}}_2{\mathrm{V}}_2{\mathrm{O}}_7$ with experimental results.

We present time-of-flight inelastic neutron scattering measurements at low temperature on powder samples of the magnetic pyrochlore oxides ${\mathrm{Tb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ and ${\mathrm{Tb}}_2{\mathrm{Sn}}_2{\mathrm{O}}_7$. These two materials possess related, but different ground states, with ${\mathrm{Tb}}_2{\mathrm{Sn}}_2{\mathrm{O}}_7$ displaying “soft” spin ice order below $T_{\mathrm{N}}\sim 0.87$ K, while ${\mathrm{Tb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ enters a hybrid, glassy spin ice state below $T_{\mathrm{g}}\sim 0.2$ K. Our neutron measurements, performed at $T=1.5$ and 30 K, probe the crystal field states associated with the $J$ = 6 states of Tb${}^{3+}$ within the appropriate $Fd\overline{3}m$ pyrochlore environment. These crystal field states determine the size and anisotropy of the Tb${}^{3+}$ magnetic moment in each material's ground state, information that is an essential starting point for any description of the low-temperature phase behavior and spin dynamics in ${\mathrm{Tb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ and ${\mathrm{Tb}}_2{\mathrm{Sn}}_2{\mathrm{O}}_7$. While these two materials have much in common, the cubic stanate lattice is expanded compared to the cubic titanate lattice. As our measurements show, this translates into a factor of $\sim$2 increase in the crystal field bandwidth of the $2J+1=13$ states in ${\mathrm{Tb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ compared with ${\mathrm{Tb}}_2{\mathrm{Sn}}_2{\mathrm{O}}_7$. Our results are consistent with previous measurements on crystal field states in ${\mathrm{Tb}}_2{\mathrm{Sn}}_2{\mathrm{O}}_7$, wherein the ground-state doublet corresponds primarily to $m_J=|\pm 5\rangle$ and the first excited state doublet to $m_J=|\pm 4\rangle$. In contrast, our results on ${\mathrm{Tb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ differ markedly from earlier studies, showing that the ground-state doublet corresponds to a significant mixture of $m_J=|\pm 5\rangle$, $|\mp 4\rangle$, and $|\pm 2\rangle$, while the first excited state doublet corresponds to a mixture of $m_J=|\pm 4\rangle$, $|\mp 5\rangle$, and $|\pm 1\rangle$. We discuss these results in the context of proposed mechanisms for the failure of ${\mathrm{Tb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ to develop conventional long-range order down to 50 mK.

We report a low-temperature specific heat study of high-quality single crystals of the heavily hole-doped superconductor ${\mathrm{Ca}}_{0.32}{\mathrm{Na}}_{0.68}{\mathrm{Fe}}_2{\mathrm{As}}_2$. This compound exhibits bulk superconductivity with a transition temperature $T_{\mathrm{c}}\approx 34$ K, which is evident from the magnetization, transport, and specific heat measurements. The zero-field data manifest a significant electronic specific heat in the normal state with a Sommerfeld coefficient $\gamma \approx 53$ mJ/mol K${}^2$. Using a multiband Eliashberg analysis, we demonstrate that the dependence of the zero-field specific heat in the superconducting state is well described by a three-band model with an unconventional s${}_{\pm }$ pairing symmetry and gap magnitudes ${\Delta }_i$ of approximately 2.35, 7.48, and $-$7.50 meV. Our analysis indicates a non-negligible attractive intraband coupling, which contributes significantly to the relatively high value of $T_c$. The Fermi surface averaged repulsive and attractive coupling strengths are of comparable size and outside the strong coupling limit frequently adopted for describing high-$T_c$ iron pnictide superconductors. We further infer a total mass renormalization of the order of five, including the effects of correlations and electron-boson interactions.

The morphology of abrupt bursts of magnetic flux into superconducting films with engineered periodic pinning centers (antidots) has been investigated. Guided flux avalanches of thermomagnetic origin develop a treelike structure, with the main trunk perpendicular to the borders of the sample, while secondary branches follow well-defined directions determined by the geometrical details of the underlying periodic pinning landscape. Strikingly, we demonstrate that in a superconductor with relatively weak random pinning the morphology of such flux avalanches can be fully controlled by proper combinations of lattice symmetry and antidot geometry. Moreover, the resulting flux patterns can be reproduced, to the finest details, by simulations based on a phenomenological thermomagnetic model. In turn, this model can be used to predict such complex structures and to estimate physical variables of more difficult experimental access, such as the local values of temperature and electric field.

X-ray magnetic linear dichroism spectra measured in reflection (XMLD-R) on crystalline bcc Fe thin films across the 3$p$ absorption edges are reported. A series of measurements with varying orientation of the electric field vector of the linear polarized synchrotron radiation with respect to the crystal axes reveals a strong magnetocrystalline anisotropy in the XMLD-R spectra. The spectra agree well with theoretical spectra calculated within the framework of the density-functional theory accounting for the spin-orbital and exchange splitting of the $3p$ semicore states on an equal footing.

The pressure-dependent diffraction response of the superconducting phase-separated ${\mathrm{Cs}}_{0.72}$${\mathrm{Fe}}_{1.57}$${\mathrm{Se}}_2$ ($T_{\mathrm{c}}$ = 28.5 K) has been studied at room temperature using synchrotron radiation up to the pressure of 19 GPa. The main and secondary phases of ${\mathrm{Cs}}_{0.72}$${\mathrm{Fe}}_{1.57}$${\mathrm{Se}}_2$ have been observed in the whole pressure range. The main ordered phase has been found to undergo an order-disorder transition in the Fe sublattice at $P$ = 11 GPa with the corresponding kinetics on the order of hours. Contrary to the analogous temperature-induced transition, the secondary phase has not been suppressed suggesting that its stability pressure range is higher than 19 GPa or the corresponding transformation kinetics is too slow at room temperature. Together with the previously reported pressure-dependent resistivity and magnetic susceptibility measurements, this work indicates that superconductivity in the $A_x$${\mathrm{Fe}}_{2-y}$${\mathrm{Se}}_2$ ($A$: alkali metals) phases could be related to the Fe-vacancy ordering in the main phase.

We predict the properties of the propagating and nonpropagating vibrational modes in amorphous silica ($a$-SiO${}_2$) and amorphous silicon ($a$-Si) and, from them, thermal conductivity accumulation functions. The calculations are performed using molecular dynamics simulations, lattice dynamics calculations, and theoretical models. For $a$-SiO${}_2$, the propagating modes contribute negligibly to thermal conductivity (6$\%$), in agreement with the thermal conductivity accumulation measured by Regner et al. [Nat. Commun. 4, 1640 (2013)]. For $a$-Si, propagating modes with mean-free paths up to 1 $\mu$m contribute 40$\%$ of the total thermal conductivity. The predicted contribution to thermal conductivity from nonpropagating modes and the total thermal conductivity for $a$-Si are in agreement with the measurements of Regner et al. The accumulation in the measurements, however, takes place over a narrower band of mean-free paths (100 nm–1 $\mu$m) than that predicted (10 nm–1 $\mu$m).

Different magnetic coupling mechanisms have been identified for a few monolayers of Co-porphyrin molecules deposited on a graphene-covered Ni(111) single crystal. A relatively strong antiferromagnetic coupling of the first molecular layer via graphene to the Ni crystal in comparison to a weaker intermolecular coupling gives rise to a complex field-dependent response of this hybrid system. By continuously increasing the magnetic field strength, the net magnetization of the molecular system switches from antiparallel to parallel to the field direction at 2.5 T. Utilizing x-ray absorption spectroscopy and x-ray magnetic circular dichroism, the element-specific magnetization and field dependence was probed. The nature of the magnetic couplings is identified by means of density functional theory and orbital-dependent susceptibilities.

It has been reported that in a CoFeB$/$MgO$/$CoFeB based tunnel junction, B diffuses into the MgO layer. In this work we consider three possible locations of B in the MgO lattice upon diffusing into the MgO crystal: B substitutes for an Mg site, it substitutes for an O site, or it occupies an interstitial position. Apart from the possibility of introducing midgap states, various charge states of the defect could sustain a local magnetic moment. This could additionally modify the tunnel magneto-resistance ratio. We investigate the electronic structure, the local moment formation, and the formation energies of various defect states. While midgap states are introduced in all three cases, the low formation energy defects are found to be nonmagnetic.

We demonstrate that, contrary to common belief, the critical temperature $T_c$ of clean ${\mathrm{F}}_1/\mathrm{S}/{\mathrm{F}}_2$ spin valves can depend nonmonotonically on the angle between the magnetic moments of the ferromagnetic ${\mathrm{F}}_1$ and ${\mathrm{F}}_2$ layers. Depending on the system parameters the minimum of $T_c$ may correspond to a parallel, antiparallel, or noncollinear mutual orientation of magnetic moments. Such anomalous behavior can reveal itself only if the ferromagnetic layers differ from each other and it completely disappears in the dirty limit.

We focus on inducing a topological state from regular or irregular scattering in (i) $p$-wave superconducting wires and (ii) Rashba wires proximity coupled to an $s$-wave superconductor. We find that, contrary to common expectations, the topological properties of both systems are fundamentally different: In $p$-wave wires, disorder generally has a detrimental effect on the topological order, and the topological state is destroyed beyond a critical disorder strength. In contrast, in Rashba wires, which are relevant for recent experiments, disorder can induce topological order, reducing the need for quasiballistic samples to obtain Majorana fermions. Moreover, we find that the total phase space area of the topological state is conserved for long disordered Rashba wires and can even be increased in an appropriately engineered superlattice potential.

We consider the gapless modes along the vortex line of the fully gapped, momentum independent paired states of three-dimensional Dirac fermions. For this, we require the solution of fermion zero modes of the corresponding two-dimensional problem in the presence of a point vortex, in the plane perpendicular to the vortex line. Based on the spectral symmetry requirement for the existence of the zero mode, we identify the appropriate generalized Jackiw-Rossi Hamiltonians for different paired states. A four-dimensional generalized Jackiw-Rossi Hamiltonian possesses spectral symmetry with respect to an antiunitary operator, and gives rise to a single zero mode only for the odd vorticity, which is formally described by a $Z_2$ index. In the presence of generic perturbations such as chemical potential, Dirac mass, and Zeeman couplings, the associated two-dimensional problem for the odd parity topological superconducting state maps onto two copies of generalized Jackiw-Rossi Hamiltonian, and consequently an odd vortex binds two Majorana fermions. In contrast, there are no zero-energy states for the topologically trivial $s$-wave superconductor in the presence of any chiral symmetry breaking perturbation in the particle-hole channel, such as regular Dirac mass. We show that the number of one-dimensional dispersive modes along the vortex line is also determined by the index of the associated two-dimensional problem. For an axial superfluid state in the presence of various perturbations, we discuss the consequences of the $Z_2$ index on the anomaly equations.