We experimentally investigate the near field of a surface-plasmon wave that is interacting with a three-dimensional wedge-shaped resist structure using phase resolved near-field scanning optical microscopy. Upon entering the resist structure the surface-plasmon wave is exposed to a continuously increasing effective refractive index. As a consequence the dispersion relation of the surface plasmon varies accordingly. Similar to the shoaling of tsunami waves that are entering shallow water, the wavelength and the group velocity of the surface plasmon are reduced while the in-plane electric field component simultaneously increases. We provide measurements, simulations, and a theoretical analysis of the described behavior.

We consider the combined effect of a gap and the Zeeman interaction on the helical Dirac fermions that exist on the surface of a topological insulator. Magneto-optical properties, the magnetization, Hall effect, and the density of states are considered with emphasis on the particle-hole asymmetry, which arises when a subdominant Schrödinger piece is included along with the dominant Dirac part of the Hamiltonian. When appropriate, we compare our results with those of a single-valley gapped graphene system for which Zeeman splitting behaves differently. We provide a derivation of the phase offset in the magnetic oscillations brought about by the combined effect of the gap and Schrödinger term without requiring the semiclassical Onsager quantization condition. Our results agree with previous discussions based on semiclassical arguments.

The quasibound electromagnetic modes for the arrays of nanoholes perforated in thin gold film are analyzed both numerically by the rigorous coupled wave analysis (RCWA) method and semianalytically by the coupled mode method. It is shown that when the size of the nanohole occupies a large portion of the unit cell, the surface plasmon polaritons (SPPs) at both sides of the film are combined by the higher order waveguide modes of the holes to produce multipole surface plasmons: coupled surface plasmon modes with multipole texture on the elec-tric field distributions. Further, it is revealed that the multipole texture either enhances or suppresses the couplings between SPPs depending on their diffraction orders and also causes band inversion and reconstruction in the coupled SPP band structure. Due to the multipole nature of the quasibound modes, multiple dark modes coexist to produce a variety of Fano resonance structures on the transmission and reflection spectra.

We measured the optical conductivity $\tilde{\sigma }\left(\omega \right)$ spectra of photodoped silicon by optical-pump terahertz-probe spectroscopy and analyzed them with a two-carrier Drude model. Taking into account the values of electron (hole)-phonon scattering rates previously reported in chemically doped silicon, we evaluated the electron-hole scattering rates ${\gamma }_{\mathrm{e}-\mathrm{h}}$. From 293 to $90\mathrm{K}$, the magnitudes and temperature dependence of ${\gamma }_{\mathrm{e}-\mathrm{h}}$ were successfully reproduced by a theoretical model including the effects of Rutherford scattering, Coulomb screening, and Pauli exclusion. This suggests that these three factors dominate electron-hole scattering processes in silicon. Below $90\mathrm{K}, {\gamma }_{\mathrm{e}-\mathrm{h}}$ becomes larger than that of the theoretical curve, which is attributable to a prolongation of the relaxation time of hot carriers.

The pumping conductance of a disordered two-dimensional Chern insulator scales with increasing size and fixed disorder strength to sharp plateau transitions at well-defined energies between ordinary and quantum Hall insulators. When the disorder strength is scaled to zero as system size increases, the “metallic” regime of fluctuating Chern numbers can extend over the whole band. A simple argument leads to a sort of weighted equipartition of Chern number over minibands in a finite system with periodic boundary conditions: even though there must be strong fluctuations between disorder realizations, the mean Chern number at a given energy is determined by the clean Berry curvature distribution, as in the intrinsic anomalous Hall effect formula for metals. This estimate is compared to numerical results using recently developed operator algebra methods, and indeed the dominant variation of average Chern number is explained by the intrinsic anomalous Hall formula. A mathematical appendix provides more precise definitions and a model for the full distribution of Chern numbers.

We investigate the effect of parity-time (PT) symmetric optical potentials on the radiation of achiral and chiral dipole sources. Two properties unique to PT-symmetric potentials are observed. First, the dipole can be tuned to behave as a strong optical emitter or absorber based on the non-Hermiticity parameter and the dipole location. Second, exceptional points give rise to new system resonances that lead to orders-of-magnitude enhancements in the dipolar emitted or absorbed power. Utilizing these properties, we show that enantiomers of chiral molecules near PT-symmetric metamaterials exhibit a 4.5-fold difference in their emitted power and decay rate. The results of this work could enable new atom-cavity interactions for quantum optics, as well as all-optical enantioselective separation.

The stability of the topological order phase induced by the ${\mathbb{Z}}_3$ Kitaev model, which is a candidate for fault-tolerant quantum computation, against the local order phase induced by the three-state Potts model is studied. We show that the low-energy sector of the Kitaev-Potts model is mapped to the Potts model in the presence of transverse magnetic field. Our study relies on two high-order series expansions based on continuous unitary transformations in the limits of small and large Potts couplings as well as mean-field approximation. Our analysis reveals that the topological phase of the ${\mathbb{Z}}_3$ Kitaev model breaks down to the Potts model through a first-order phase transition. We capture the phase transition by analysis of the ground-state energy, one-quasiparticle gap, and geometric measure of entanglement.

We numerically study disorder effects in the Bernevig-Hughes-Zhang (BHZ) model, and we find that the Anderson transition of a quantum spin-Hall insulator (QSHI) is determined by model parameters. The BHZ Hamiltonian is equivalent to two decoupled spin blocks that belong to the unitary class. In contrast to the common belief that a two-dimensional unitary system scales to an insulator except at certain critical points, we find, through calculations scaling properties of the localization length, level statistics, and participation ratio, that a possible exotic metallic phase emerges between the QSHI and normal insulator phases in the InAs/GaSb-type BHZ model. On the other hand, direct transition from a QSHI to a normal insulator is found in the HgTe/CdTe-type BHZ model. Furthermore, we show that the metallic phase originates from the Berry phase and can survive both inside and outside the gap.

The surface of expanded face-centered tetragonal antiferromagnetic Mn films of a few atomic monolayers thickness grown epitaxially on $\mathrm{Co}/\mathrm{Cu}\left(001\right)$ was investigated at room temperature by scanning tunneling microscopy and scanning tunneling spectroscopy using a ferromagnetic ring-shaped bulk iron probe. We show that the main contribution to the contrast modulation observed as a function of Mn thickness in differential conductance maps is not due to spin-polarized tunneling from a layerwise antiferromagnetic spin alignment. Instead, it is mainly of electronic origin resulting from layer-dependent electronic properties of the Mn film, probably related to different levels of intermixing with Co atoms. On the atomic scale, the Mn surface demonstrates a geometrical reconstruction with a $(12\times 2)$ periodicity in two orthogonal domains on the fourfold symmetric substrate with an apparent surface corrugation of up to 0.3 Å. Simultaneously recorded differential conductance maps show different textures in the two orthogonal domains, providing evidence for noncollinearity in the Mn surface spin structure.

We report the temperature evolution of the detailed electronic band structure in FeSe single crystals measured by angle-resolved photoemission spectroscopy (ARPES), including the degeneracy removal of the $d_{xz}$ and $d_{yz}$ orbitals at the $\mathrm{\Gamma }$/Z and M points, and the orbital-selective hybridization between the $d_{xy}$ and $d_{xz/yz}$ orbitals. The temperature dependences of the splittings at the $\mathrm{\Gamma }$/Z and M points are different, indicating that they are controlled by different order parameters. The splitting at the M point is closely related to the structural transition and is attributed to orbital ordering defined on Fe-Fe bonds with a $d$-wave form in the reciprocal space that breaks the rotational symmetry. In contrast, the band splitting at the $\mathrm{\Gamma }$/Z points remains at temperature far above the structural transition. Although the origin of this latter splitting remains unclear, our experimental results exclude the previously proposed ferro-orbital ordering scenario.

The spectral phonon properties in defected graphene have been unclear due to the lack of advanced techniques for predicting the phonon-defect scattering rate without fitting parameters. Taking advantage of the extended phonon normal mode analysis, we obtained the spectral phonon relaxation time and mean free path (MFP) in defected graphene and studied the impacts of three common types of defects: Stone-Thrower-Wales (STW) defect, double vacancy (DV), and monovacancy (MV). The phonon-STW defect scattering rate is found to have no significant frequency dependence, and as a result, the relative contribution of long-wavelength phonons sharply decreases. In contrast, the phonon scattering by DVs or MVs exhibits a frequency dependence of ${\tau }_{p-d}^{-1}\sim {\omega }^{1.1-1.3}$ except for a few long-wavelength phonons, revisiting the traditionally used $\sim {\omega }^4$ dependence. We note that although MV-defected graphene has the lowest thermal conductivity as compared to the other two defected graphene samples at the same defect concentration, it has a portion of phonons with the longest MFP. The contribution from the long-MFP and long-wavelength phonons does not decrease much as the vacancy concentration increases. STW defect and MV block more out-of-plane modes than in-plane modes, while DV has less bias for which mode to block. As the MV concentration increases from 0 to 1.1%, the relative contribution from out-of-plane modes decreases from 30% to 18%, while that of the transverse acoustic mode remains at around 30%. These findings of spectral phonon properties can provide more insight than the effective properties and benefit the prospective phononic engineering.

An approach to compute exchange parameters of the Heisenberg model in plane-wave-based methods is presented. This calculation scheme is based on the Green's function method and Wannier function projection technique. It was implemented in the framework of the pseudopotential method and tested on such materials as NiO, FeO, ${\mathrm{Li}}_2{\mathrm{MnO}}_3$, and ${\mathrm{KCuF}}_3$. The obtained exchange constants are in a good agreement with both the total energy calculations and experimental estimations for NiO and ${\mathrm{KCuF}}_3$. In the case of FeO our calculations explain the pressure dependence of the Néel temperature. ${\mathrm{Li}}_2{\mathrm{MnO}}_3$ turns out to be a Slater insulator with antiferromagnetic nearest-neighbor exchange defined by the spin splitting. The proposed approach provides a unique way to analyze magnetic interactions, since it allows one to calculate orbital contributions to the total exchange coupling and study the mechanism of the exchange coupling.

We report ${\mathrm{FePd}}_3$ as a material for studying thermally active artificial spin ice (ASI) systems and use it to investigate both the square and kagome ice geometries. We readily achieve perfect ground state ordering in the square lattice and demonstrate the highest yet degree of monopole charge ordering in the kagome lattice. We find that smaller lattice constants in the kagome system generally produce larger domains of charge order. Monte Carlo simulations show excellent agreement with our data when a small amount of disorder is included in the simulation.

We study the effects of hybridization between the two electron pockets in Fe-based superconductors with $s$-wave gap with accidental nodes. We argue that hybridization reconstructs the Fermi surfaces and also induces an additional interpocket pairing component. We analyze how these two effects modify the gap structure by tracing the position of the nodal points of the energy dispersions in the superconducting state. We find three possible outcomes. In the first, the nodes simply shift their positions in the Brillouin zone; in the second, the nodes merge and disappear, in which case the gap function has either equal or opposite signs on the electron pockets; in the third, a new set of nodal points emerges, doubling the original number of nodes.

Motivated by the midinfrared scenario for high-temperature superconductivity proposed by Leggett, the effects of Umklapp processes on the density-density correlation function in the presence of long-range Coulomb interaction have been investigated on a microscopic model. We show that because Umklapp processes enable scatterings that conserve total momentum only up to $n\vec{K}$, where $n$ is an integer and $\vec{K}$ is the reciprocal wave vector, significant amounts of spectral weight in the plasmonic excitations at long wavelength are transferred into lower frequency around the midinfrared regime. We further find that regardless of the gap symmetry, superconductivity generally suppresses the Umklapp scatterings due to the formation of the electron pairing. This suppression is unique for the superconductivity due to the interplay between electron pairing and the odd parity of the matrix elements associated with Umklapp channels, which usually does not occur in other known competing orders. Specific predictions for the experimental signatures in optical conductivity and electron energy loss spectroscopy will be discussed.

A quantitative description of photoexcited scanning tunneling spectra is developed and applied to photoexcited spectra measured on $p$-doped nonpolar GaAs(110) surfaces. Under illumination, the experimental spectra exhibit an increase of the tunnel current at negative sample voltages only. In order to analyze the experimental data quantitatively, the potential and charge-carrier distributions of the photoexcited tip-vacuum-semiconductor system are calculated by solving the Poisson as well as the hole and electron continuity equations by a finite-difference algorithm. On this basis, the different contributions to the tunnel current are calculated using an extension of the model of Feenstra and Stroscio to include the light-excited carrier concentrations. The best fit of the calculated tunnel currents to the experimental data is obtained for a tip-induced band bending, which is limited by the partial occupation of the ${\mathrm{C}}_3$ surface state by light-excited electrons. The tunnel current at negative voltages is then composed of a valence band contribution and a photoinduced tunnel current of excited electrons in the conduction band. The quantitative description of the tunnel current developed here is generally applicable and provides a solid foundation for the quantitative interpretation of photoexcited scanning tunneling spectroscopy.

We study the thermal stability of a magnetic domain wall pinned in nanowires with various widths and thicknesses made of Co/Ni multilayers and analyze the effective volume that governs the thermal stability. We find that, above a critical wire width, the domain wall depinning is initiated by a subvolume excitation and that the critical width is dependent on the wire thickness. The obtained findings are supported by the distribution of critical current density for domain wall depinning and are qualitatively described by an analytical model in which the balance between the Zeeman energy and domain wall elastic energy is considered. We also show a different behavior between the device size dependence of the thermal stability and that of critical current, leading to an enhancement of domain wall motion efficiency with decreasing the device size.