It is known that $\epsilon$-near-zero (ENZ) structures may provide exciting possibilities for light-matter interaction, including the phenomenon of supercoupling, i.e., channeling of wave energy through ultranarrow conduits connecting two waveguides. Here, we extend this concept to the surface waves using structures with effective ENZ properties, exploring how surface waves can tunnel through such structures with essentially no reflection, even in the presence of sharp bends. Such ENZ metastructures support tunneling of waves with longitudinal wave numbers near zero. The surface waves may include examples of transverse electric (TE) surface waves along the grounded dielectric slabs (e.g., in the microwave regime) and TE surface-plasmon polariton waves along the negative-permeability interfaces. The ENZ metastructures can transfer the surface wave energy from one side to another by an anomalous squeezing effect, which is effectively independent of the length and the shape of the ENZ metastructures. The interaction of electromagnetic waves with such systems offers exciting possibilities for the design of flat photonic devices and components.

We report measurements of Shubnikov–de Haas oscillations in single crystals of BiTeCl at magnetic fields up to 31 T and at temperatures as low as 0.4 K. Two oscillation frequencies were resolved at the lowest temperatures, $F_1=65\pm 4$ T and $F_2=156\pm 5$ T. We also measured the infrared optical reflectance $\left[\mathcal{R}\left(\omega \right)\right]$ and Hall effect; we propose that the two frequencies correspond respectively to the inner and outer Fermi sheets of the Rashba spin-split bulk conduction band. The bulk carrier concentration was $n_e\approx 1\times {10}^{19}{\mathrm{cm}}^{-3}$ and the effective masses $m_1^{*}=0.20m_0$ for the inner and $m_2^{*}=0.27m_0$ for the outer sheet. Surprisingly, despite its low effective mass, we found that the amplitude of $F_2$ is very rapidly suppressed with increasing temperature, being almost undetectable above $T\approx 4$ K.

Motivated by recently searching for topological states in organic materials as well as successful experimental synthesis of a graphitelike metal-organic framework ${\mathrm{Ni}}_3\left({\mathrm{C}}_{18}{\mathrm{H}}_{12}{\mathrm{N}}_6\right){}_2$ [Sheberla et al., J. Am. Chem. Soc. 136, 8859 (2014)], we systematically investigated the electronic and topological properties of the ${\mathrm{Ni}}_3\left({\mathrm{C}}_{18}{\mathrm{H}}_{12}{\mathrm{N}}_6\right){}_2$ monolayer using an ab initio method combined with a tight-binding model. Our calculations demonstrate that the material can be in a quantum spin Hall or $Z_2$ metallic state in different electron-doped concentrations, which are experimentally accessible with currently electrostatic gating technologies. The tight-binding model also shows that the real next-nearest-neighbor interaction is essential to drive the $Z_2$ metallic phase in ${\mathrm{Ni}}_3\left({\mathrm{C}}_{18}{\mathrm{H}}_{12}{\mathrm{N}}_6\right){}_2$-type lattices.

We investigate the electronic and magnetic structures and the character and direction of spin and orbital moments of the recently synthesized quadruple perovskite compound ${\mathrm{CaCo}}_3{\mathrm{V}}_4{\mathrm{O}}_{12}$ using a selection of methods from density functional theory. Implementing the generalized gradient approximation and the Hubbard $U$ correction $\left(\text{GGA}+U\right)$, ferromagnetic spin alignment leads to half-metallicity rather than the observed narrow gap insulating behavior. Including spin-orbit coupling (SOC) leaves a Mott insulating spectrum but with a negligible gap. SOC is crucial for the Mott insulating character of the V $d^1$ ion, breaking the $d_{m=\pm 1}$ degeneracy and also giving a substantial orbital moment. Evidence is obtained of the large orbital moments on Co that have been inferred from the measured susceptibility. Switching to the orbital polarization (OP) functional, GGA+OP+SOC also displays clear tendencies toward very large orbital moments but in its own distinctive manner. In both approaches, application of SOC, which requires specification of the direction of the spin, introduces large differences in the orbital moments of the three Co ions in the primitive cell. We study a fictitious but simpler cousin compound ${\mathrm{Ca}}_3{\mathrm{CoV}}_4{\mathrm{O}}_{12}$ (Ca replacing two of the Co atoms) to probe in a more transparent fashion the interplay of spin and orbital degrees of freedom with the local environment of the planar ${\mathrm{CoO}}_4$ units. The observation is made that the underlying mechanisms seem to be local to a ${\mathrm{CoO}}_4$ plaquette, and that there is very strong coupling of the size of the orbital moment to the spin direction. These facts strongly suggest noncollinear spins, not only on Co but on the V sublattice as well.

We report a magnetization study down to 1.7 K for the trilayered ruthenate ${\mathrm{Sr}}_4{\mathrm{Ru}}_3{\mathrm{O}}_{10}$ for magnetic fields applied within the $ab$ plane. Our measurements reveal that the metamagnetic jump appearing in the isothermal magnetization curves below $T_M=68$ K is actually composed of two consecutive metamagnetic transitions, where the increase in the magnetization due to the second transition is roughly one third of the overall metamagnetic jump. We show that the metamagnetic critical fields for these two transitions increase as the temperature decreases, and that their difference has a weak temperature dependence that opens up in the proximity of the critical region near $T_M$. We discuss the origin of this double metamagnetic transition being due to either the ordering of the Ru $4d$ magnetic moments in the two inequivalent crystallographic Ru sites or to the presence of two van Hove singularities in the near-Fermi-level density of states.

We observe unusually narrow donor-bound exciton transitions (400 µeV) in the photoluminescence spectra of GaN nanowire ensembles grown on Si(111) substrates at very high ($>850$ ${}^{\circ }\mathrm{C}$) temperatures. The spectra of these samples reveal a prominent transition of excitons bound to neutral Si impurities which is not observed for samples grown under standard conditions. Motivated by these experimental results, we investigate theoretically the impact of surface-induced internal electric fields on the binding energy of donors by a combined Monte Carlo and envelope function approach. We obtain the ranges of doping and diameter for which the potential is well described using the Poisson equation, where one assumes a spatially homogeneous distribution of dopants. Our calculations also show that surface donors in nanowires with a diameter smaller than 100 nm are ionized when the surface electric field is larger than about 10 kV/cm, corresponding to a doping level higher than $2\times {10}^{16}{\mathrm{cm}}^{-3}$. This result explains the experimental observation: since the (${\mathrm{D}}^{+}$,X) complex is not stable in GaN, surface-donor-bound excitons do not contribute to the photoluminescence spectra of GaN nanowires above a certain doping level, and the linewidth reflects the actual structural perfection of the nanowire ensemble.

Recently, Lee et al. [Nature (London) 502, 532 (2013)] used ∼1% tensile strain to induce a ferroelectric instability in thin films of $\mathrm{S}{{\mathrm{r}}_n}_{+1}\mathrm{T}{\mathrm{i}}_n{\mathrm{O}}_{3n+1}(n=1-6)$ phases. They showed that the Curie temperature $T_C$ gradually increased with $n$, reaching $180\mathrm{K}$ for $\mathrm{S}{\mathrm{r}}_7\mathrm{T}{\mathrm{i}}_6{\mathrm{O}}_{19}\left(n=6\right)$. The permittivity of this $\left(n=6\right)$ sample could also be tuned significantly by the application of an electric field with exceptionally low dielectric loss at 300 K, rivaling all known tunable microwave dielectrics. Here, we present microwave (MW), terahertz, and infrared spectra of strained $\mathrm{S}{{\mathrm{r}}_n}_{+1}\mathrm{T}{\mathrm{i}}_n{\mathrm{O}}_{3n+1}$ thin films deposited on (110) $\mathrm{DySc}{\mathrm{O}}_3$. Near the ferroelectric phase transitions, we observe the splitting and shifting of phonon and central mode frequencies, demonstrating the change of crystal symmetry below $T_C$. Moreover, our spectra reveal that the central mode contribution dominates MW loss. In the $\mathrm{S}{\mathrm{r}}_7\mathrm{T}{\mathrm{i}}_6{\mathrm{O}}_{19}$ thin film, the central mode vanishes at 300 K, explaining its low MW loss. Finally, we discuss the origin and general conditions for the appearance of central modes near ferroelectric phase transitions.

Two magnetic ordering transitions are found in $\mathrm{InMn}{\mathrm{O}}_3$, the paramagnetic to antiferromagnetic transition at ∼118 K and a lower possible spin rotation transition near 42 K. Multiple length scale structural measurements reveal enhanced local distortion found to be connected with tilting of the $\mathrm{Mn}{\mathrm{O}}_5$ polyhedra as temperature is reduced. Coupling is observed between the lattice and the spin manifested as changes in the structure near both of the magnetic ordering temperatures (at ∼42 and ∼118 K). External parameters, such as pressure, are expected to modify the coupling.

The quasiharmonic Debye approximation has been implemented within the aflow and Materials Project frameworks for high-throughput computational materials science (Automatic Gibbs Library, agl), in order to calculate thermal properties such as the Debye temperature and the thermal conductivity of materials. We demonstrate that the agl method, which is significantly cheaper computationally compared to the fully ab initio approach, can reliably predict the ordinal ranking of the thermal conductivity for several different classes of semiconductor materials. In particular, a high Pearson (i.e., linear) correlation is obtained between the experimental and agl computed values of the lattice thermal conductivity for a set of 75 compounds including materials with cubic, hexagonal, rhombohedral, and tetragonal symmetry.

The pressure dependence of the superconducting transition temperature $T_{\mathrm{c}}$, the local structure, and the electronic properties of the iron chalcogenide superconductor ${\mathrm{FeSe}}_{0.5}{\mathrm{Te}}_{0.5}$ was investigated by means of SQUID magnetometry and site-selective polarized x-ray absorption spectroscopy at the iron and selenium $K$-edges. The measurements reveal a discontinuous decrease of the local bond distances by crossing a tetragonal to monoclinic structural phase transition at $p_{\mathrm{S}}\sim 2.7$ GPa. The electronic structure, however, evolves evenly, but the superconducting properties are highly affected by changes in the local structure. These results underline the close connection of the lattice and the superconductivity in the iron-based superconductors.

We describe an improved method for extracting semiconductor band gap zero-point corrections from experimental data. We propose two extrapolation schemes and use first-principles calculations to show that they are up to an order of magnitude more accurate than existing schemes. We apply our schemes to diamond, extracting a value of the zero-point correction of $-0.41$ eV, in better agreement with first-principles results than previous estimates. Finally, we consider the low-temperature limit of the class of observables that includes the electronic band gap, obtaining a $T^4$ dependence in three dimensions, $T^2$ in two dimensions, and $T^{3/2}$ in one dimension.

A theoretical description of the low-temperature phase of short-range spin glasses has remained elusive for decades. In particular, it is unclear if theories that assert a single pair of pure states, or theories that are based on infinitely many pure states—such as replica symmetry breaking—best describe realistic short-range systems. To resolve this controversy, the three-dimensional Edwards-Anderson Ising spin glass in thermal boundary conditions is studied numerically using population annealing Monte Carlo. In thermal boundary conditions all eight combinations of periodic vs antiperiodic boundary conditions in the three spatial directions appear in the ensemble with their respective Boltzmann weights, thus minimizing finite-size corrections due to domain walls. From the relative weighting of the eight boundary conditions for each disorder instance a sample stiffness is defined, and its typical value is shown to grow with system size according to a stiffness exponent. An extrapolation to the large-system-size limit is in agreement with a description that supports the droplet picture and other theories that assert a single pair of pure states. The results are, however, incompatible with the mean-field replica symmetry breaking picture, thus highlighting the need to go beyond mean-field descriptions to accurately describe short-range spin-glass systems.

Using Monte Carlo simulations with a tunable uniaxial strain, the nematic susceptibility of the spin fermion model for the pnictides is calculated. The results are in excellent agreement with the experiments by Chu et al. [Science 337, 710 (2012)]. Via a Ginzburg-Landau analysis, our study suggests a nematicity in the spin fermion model primarily originating in magnetism, but with the lattice/orbital also playing a key role by boosting up critical temperatures and separating the structural $T_S$ and Néel $T_N$ transitions. At $T>T_S$, Curie-Weiss behavior is observed with a characteristic temperature $T^{*}$ being the $T_N$ of the purely electronic system. In this temperature regime, short-range magnetic order with wave vectors $(\pi ,0)\text{−}(0,\pi )$ induce local nematic fluctuations and a density-of-states pseudogap, compatible with several experiments. The present analysis relies on the study of a particular model for the iron superconductors; thus further studies are needed to conclusively establish the driver of nematicity in real materials.

We argue that the three key phenomena recently observed in solid ${}^4\mathrm{He}$—mass supertransport, anomalous isochoric compressibility (syringe effect), and giant plasticity—are closely linked to each other through the physics of an interconnected network of tilted quantum-rough dislocations. As immediate implications of this connection several predictions follow: In the absence of ${}^3\mathrm{He}$ impurities, the syringe effect and giant plasticity persist down to $T=0$; the dynamical low-frequency syringe and giant-plasticity responses are dispersionless; and similarly to giant plasticity but without direct relationship to the supertransport along the dislocation cores, ${}^3\mathrm{He}$ impurities should suppress the syringe effect partially or completely at appropriately low temperatures.

We present a comprehensive study on the low-temperature orthorhombic phase of $\mathrm{Ba}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right)}_2{\mathrm{As}}_2$ based on the Korringa-Kohn-Rostoker-Green function approach. Using this band-structure method in combination with the coherent potential approximation alloy theory we are able to investigate the evolution of the magnetic and electronic properties of this prototype iron pnictide for arbitrary concentrations $x$, while dealing with the chemical disorder without uncontrolled simplifications by using solely a rigid-band shift or the virtual crystal approximation. We discuss the development of the site-resolved magnetic moments for the experimentally observed stripe antiferromagnetic order together with the strong electronic anisotropy of the Fermi surface and compare it with angle-resolved photoemission spectroscopy measurements of detwinned crystals. We furthermore calculate magnetic exchange coupling parameters $J_{ij}$ and use them for Monte Carlo simulations on the basis of the classical Heisenberg model to get an insight on the temperature dependence of the magnetic ordering on the cobalt concentration.

We develop an analytical approach based on a combined perturbative and self-consistent mean-field treatment of interactions that is capable of capturing topological phase transitions beyond either method when used independently. As an illustration of the method, we study the effects of short-range interactions on the $Z_2$ topological insulator phase, also known as the quantum spin Hall phase, in two generalized versions of the Kane-Mele model at half-filling on the honeycomb lattice. The results are in excellent agreement with quantum Monte Carlo calculations on the same model and cannot be reproduced by either a perturbative treatment or a self-consistent mean-field treatment of the interactions. Our analytical approach helps to clarify how the symmetries of the one-body terms of the Hamiltonian determine whether interactions tend to stabilize or destabilize a topological phase. Moreover, our method should be applicable to a wide class of models where topological transitions due to interactions are in principle possible but are not correctly predicted by either perturbative or self-consistent treatments.