The influence of dimensionality on the electronic properties of layered perovskite materials remains an outstanding issue. We address it here for ${\text{Sr}}_3{\text{Ir}}_2{\text{O}}_7$, the bilayer compound of the iridate ${\text{Sr}}_{n+1}{\text{Ir}}_n{\text{O}}_{3n+1}$ series. By angle-resolved photoemission spectroscopy we show that in this material the interlayer coupling is large and that the intercell coupling is, conversely, negligible. From a detailed mapping of the bilayer splitting, and from the intensity modulation of the bonding and antibonding bands with photon energy, we establish differences and similarities with the prominent case of the bilayer superconducting cuprates.

The Casimir force between two surfaces is attractive in most cases. Although stable suspension of nano-objects has been achieved, the sophisticated geometries make them difficult to be merged with well-established thin film processes. We find that by introducing thin film surface coating on porous substrates, a repulsive to attractive force transition is achieved when the separations are increased in planar geometries, resulting in a stable suspension of two surfaces near the force transition separation. Both the magnitude of the force and the transition distance can be flexibly tailored though modifying the properties of the considered materials, that is, thin film thickness, doping concentration, and porosity. This stable suspension can be used to design new nanodevices with ultralow friction. Moreover, it might be convenient to merge this thin film coating approach with micro- and nanofabrication processes in the future.

The reststrahlen bands of rare-earth garnets were investigated in the terahertz regime. Through this, it was found that the crystal orientation and light polarization directly influence the reststrahlen-related dielectric absorptions of ${\text{Gd}}_3$${\text{Ga}}_5$${\text{O}}_{12}$ (GGG), which is attributed to restriction in the vibrational motion of Gd ions. A theoretical fit to the dielectric absorption revealed that the phonon-polariton frequency in GGG crystals at around 85.5 cm${}^{-1}$ exhibits a narrow spectral width of 100 GHz at room temperature, as estimated using a Lorentz oscillator model. In addition, it was found that the reststrahlen bands can be readily manipulated within a range of 0.7–3.7 THz by changing the chemical composition of the garnet.

The insertion of a magnetic $\pi$ flux into a quantum spin Hall insulator creates four localized, spin-charge separated states: the charge and spin fluxons with either charge $Q=\pm 1$ or spin $S_z=\pm 1/2$, respectively. In the presence of repulsive Coulomb interactions, the charged states are gapped out and a local moment is formed. We consider the Kane-Mele-Hubbard model on a ribbon with zigzag edges to construct an impurity model where the spin fluxon is screened by the helical edge liquid. In the noninteracting model, the hybridization between fluxon and edge states is dominated by the extent of the latter. It becomes larger with increasing spin-orbit coupling $\lambda$ but only has nonzero values for even distances between the $\pi$ flux and the edge. For the interacting system, we use the continuous-time quantum Monte Carlo method, which we have extended by global susceptibility measurements to reproduce the characteristic Curie law of the spin fluxon. However, due to the finite extent of the fluxons, the local moment is formed at rather low energies. The screening of the spin fluxon leads to deviations from the Curie law that follow the universal behavior obtained from a data collapse. Additionally, the Kondo resonance arises in the local spectral function between the two low-lying Hubbard peaks.

We study point-contact tunneling in the integer quantum Hall state of bosons. This symmetry-protected topological state has electrical Hall conductivity equal to $2e^2/h$ and vanishing thermal Hall conductivity. In contrast to the integer quantum Hall state of fermions, a point contact can have a dramatic effect on the low-energy physics. In the absence of disorder, a point contact generically leads to a partially split Hall bar geometry. We describe the resulting intermediate fixed point via the two-terminal electrical (Hall) conductance of the edge modes. Disorder along the edge, however, both restores the universality of the two-terminal conductance and helps preserve the integrity of the Hall bar within the relevant parameter regime.

Plasmarons formed by the compound of photoelectrons and acoustic surface-plasmon excitations is investigated in the system p($2\times 2$)-K/graphite. The physics behind this type of plasmarons ($e$ plasmarons) differs from the physics of plasmarons recently found in graphene, where the loss feature is argued to result from the photohole-plasmon interaction ($h$ plasmarons). Based on first principles methods we calculate the dispersion of the $e$-plasmaron excitation rate, which yields a broad feature below the parabolic quantum-well band with a peak about 0.4 eV below the quantum-well band at the $\overline{\Gamma }$ point.

A study of previously overlooked structural and optical properties of InGaAs heterostructures grown on (111)$B$ oriented GaAs substrates patterned with inverted 7.5-μm pitch pyramidal recesses is presented. First, the composition of the confinement barrier material (GaAs in this work) and its growth temperature are shown as some of the key parameters that determine the main quantum dot properties, including nontrivial emission energy dependence, excitonic pattern, and unusual photoluminescence energetic ordering of the InGaAs ensemble nanostructures. Second, the formation of a formerly unidentified type of InGaAs nanostructures—three corner quantum dots—is demonstrated in our structures next to the well-known ones (a quantum dot and three lateral quantum wires and quantum wells). The findings show the complexity of the pyramidal quantum dot system which strongly depends on the sample design and which should be considered when selecting highly symmetric (central) quantum dots in newly designed experimental projects.

Motivated by the InSb nanowire superconductor system, we investigate a system where one-dimensional topological superconductors are placed in parallel. It would be simulated well by a ladder of the Kitaev chains. The system undergoes multiple topological phase transitions, where the number of Majorana fermions changes as a function of the interchain superconducting pairings. We analytically determine the topological phase diagram by explicitly calculating the topological number and the band structure. They show even-odd effects with respect to the number of legs of the ladder. When the relative phase between the inter- and intrachain superconducting pairings is 0 or $\pi$, the system belongs to the class BDI characterized by the $\mathbb{Z}$ index, and otherwise it belongs to the class D characterized by the ${\mathbb{Z}}_2$ index. This topological class change would be caused by applying the Josephson current or an external magnetic field, and could be observed by measuring the zero-bias differential conductance.

Superconducting properties under high magnetic fields for the ${\mathrm{BiS}}_2$-based layered superconductor ${\mathrm{LaO}}_{0.5}$${\mathrm{F}}_{0.5}$${\mathrm{BiS}}_2$ (polycrystalline sample prepared using the high-pressure synthesis method) are investigated. The anisotropy of the upper critical field is discussed by analyzing the temperature dependence of the temperature derivative of resistivity. The temperature dependence of the upper critical field shows an anomalous behavior with a characteristic magnetic field of 8 T. We reveal that the anomalous behavior is caused by the existence of three kinds of upper critical field ${\mu }_0{H}_{C2}^{max}$, ${\mu }_0{H}_{C2}^{\mathrm{mid}}$, and ${\mu }_0{H}_{C2}^{\min }$ in ${\mathrm{LaO}}_{0.5}$${\mathrm{F}}_{0.5}$${\mathrm{BiS}}_2$. The lowest upper critical field ${\mu }_0{H}_{C2}^{\min }$ is regarded as ${\mu }_0{H}_{c2}$, where the superconducting states of the grains with an orientation of $H\left|\right|c$ are suppressed. The ${\mu }_0{H}_{C2}^{max}$ and ${\mu }_0{H}_{C2}^{\mathrm{mid}}$ are regarded as ${\mu }_0{H}_{c2}$, where the superconducting states of the grains with an orientation of $H\left|\right|ab$ are suppressed. The difference between ${\mu }_0{H}_{C2}^{max}$ and ${\mu }_0{H}_{C2}^{\mathrm{mid}}$ could be explained by the anisotropy of superconducting states within the Bi-S plane. The estimated anisotropy parameter of the upper critical fields is about 7.4 for the high-pressure-synthesized ${\mathrm{LaO}}_{0.5}$${\mathrm{F}}_{0.5}$${\mathrm{BiS}}_2$ polycrystalline sample. Since this value is clearly lower than the anisotropy parameter of over 30 observed in previously reported ${\mathrm{LaO}}_{0.5}$${\mathrm{F}}_{0.5}$${\mathrm{BiS}}_2$ single crystals grown under ambient pressure, we conclude that a larger anisotropy of superconductivity is not essential for the enhancement of $T_c$ and lower anisotropy might be important for a higher $T_c$ in ${\mathrm{LaO}}_{1-x}$${\mathrm{F}}_x$${\mathrm{BiS}}_2$.

The penetration depth $l_{\mathrm{j}}$ in superconducting junctions is identified within the Ginzburg-Landau theory as a function of the interfacial pair breaking, of the magnetic field, and of the Josephson coupling strength. When the interfacial pair breaking goes up, $l_{\mathrm{j}}$ increases and an applicability of the local Josephson electrodynamics to junctions with a strong Josephson coupling is extended. In the junctions with strongly anharmonic current-phase relations, the magnetic field dependence of $l_{\mathrm{j}}$ is shown to lead to a significant difference between the weak-field penetration depth and the characteristic size of the Josephson vortex. For such junctions a nonmonotonic dependence of $l_{\mathrm{j}}$ and of the lower critical field on the Josephson coupling constant is found, and the specific features of spatial profiles of the supercurrent and the magnetic field in the Josephson vortex are established.

We performed synchrotron x-ray diffraction and infrared (IR) experiments combined with evolutionary structure predictions and band structure calculations on ${\text{Ag}}_2\text{Se}$ to ∼20 GPa. We present evidence for phase I (β-${\text{Ag}}_2\text{Se}$) as a potential three-dimensional topological insulator by its increase in optical band gap and the topologically nontrivial nature of its band structure. Higher pressures induce a triple-layer stacking pattern and significantly increase the crystallographic inequivalence of the two Ag sites, where $P$2${}_1$2${}_1$2${}_1$ phase I first reconstructs into a Pnma phase II, and then transforms into a topologically different Cmcm phase III. The radical changes in IR spectra and electronic band structures indicate the metallic nature of the high-pressure phases. Our results highlight the effects of pressure in tuning the crystal structure and electronic states of ${\text{Ag}}_2\text{Se}$.

Using hybrid density functional theory, we investigate the impact of native vacancies and hydrogen impurities on the electrical and optical properties of cubic ${\mathrm{SrZrO}}_3$. Oxygen vacancies ($V_{\mathrm{O}}$) form localized states and introduce deep donor levels in the gap. The formation energy of $V_{\mathrm{O}}$ is low when the Fermi level is near the valence band; $V_{\mathrm{O}}$ will thus be a compensating center in acceptor-doped ${\mathrm{SrZrO}}_3$. Sr and Zr vacancies ($V_{\mathrm{Sr}}$/$V_{\mathrm{Zr}}$) are acceptor-type defects, and have low formations energies when the Fermi level is near the conduction band. Hole localization on oxygen dangling bonds is an important feature of these vacancies. $V_{\mathrm{Sr}}$ is most prevalent and can account for the luminescence peak at 3.4 eV observed in Sr-deficient ${\mathrm{SrZrO}}_3$. Hydrogen impurities are found to preferentially incorporate at an interstitial site (${\mathrm{H}}_i$), forming an O-H bond and acting as a donor. ${\mathrm{H}}_i$ can also be stabilized as an acceptor, sitting midway between adjacent Sr atoms; the $\varepsilon (+/-)$ transition level is found at 0.44 eV below the conduction band. Hydrogen can also substitute on an oxygen site (${\mathrm{H}}_{\mathrm{O}}$), acting as a shallow donor. The formation energy of ${\mathrm{H}}_{\mathrm{O}}$ is high compared to ${\mathrm{H}}_i$, yet it is stable with respect to dissociation into ${\mathrm{H}}_i$ and $V_{\mathrm{O}}$.

The effect of the Dzyaloshinsky-Moriya interaction on the magnetic structure of an ordered FePt monolayer deposited on the $\mathrm{Pt}(\mathrm{111})$ surface has been investigated. In the ground state, the pronounced anisotropic geometry of the FePt layer with alternating Fe and Pt chains gives rise to a helimagnetic structure with a strong difference in the helicity period along the chains and perpendicular to them. In the presence of an external magnetic field, the region of stable Skyrmion magnetic structures in the B-T phase diagram has been demonstrated via Monte Carlo simulations using the parameters obtained within first-principles electronic structure calculations. The present study demonstrates clearly that the ratio of the exchange coupling parameters $J/D$ for a deposited magnetic film—being of central importance for the formation of Skyrmions—can be manipulated by growing an overlayer of 2-dimensional compounds with the atoms carrying spontaneous magnetic moments separated by the atoms of nonmagnetic elements.

We revisit the description of the magnetic field around antiferromagnetic magnetoelectrics in the context of recent developments regarding magnetoelectric monopoles. Using Maxwell's equations, we calculate the magnetic and electric fields associated with a free charge in a bulk uniaxial magnetoelectric, as well as in a finite sphere of magnetoelectric material. We show that a charge in the prototypical magnetoelectric ${\text{Cr}}_2$${\text{O}}_3$, which is uniaxial with a diagonal magnetoelectric response, induces an internal magnetic field with both monopolar and quadrupolar components, but that only the quadrupolar contribution extends beyond the sample surface. We discuss the behavior of the external quadrupolar field and compare its magnitude to those of magnetic fields from other sources.

We show here the existence of the indirect coupling of electron and magnetic or nuclear ion spins in self-assembled quantum dots mediated by electron-electron interactions. With a single localized spin placed in the center of the dot, only the spins of electrons occupying the zero angular momentum states couple directly to the localized spin. We show that when the electron-electron interactions are included, the electrons occupying finite angular momentum orbitals interact with the localized spin. This effective interaction is obtained using exact diagonalization of the microscopic Hamiltonian as a function of the number of electronic shells, shell spacing, and anisotropy of the electron-Mn exchange interaction. The effective interaction can be engineered to be either ferromagnetic or antiferromagnetic by tuning the parameters of the quantum dot.

We discuss the BCS theory for electrons in graphene with a superimposed electrical unidirectional superlattice (SL) potential. New Dirac points emerge together with van Hove singularities (VHSs) linking them. We obtain a superconducting transition temperature $T_c$ for chemical potentials close to the VHSs assuming that acoustic phonon coupling should be the dominant mechanism. Pairing of two onsite electrons with one electron close to the $\mathbf{K}$ and the other close to the $-\mathbf{K}$ point is the most stable pair formation. The resulting order parameter is almost constant over the entire SL.

We investigate electromagnetic coupling between plasmonic and molecular electronic resonances using single-molecular surface-enhanced resonance Raman scattering (SERRS) from single silver nanoparticle dimers. When dimers exhibit SERRS activity, their elastic light scattering spectra show two lines, which are temporally closing toward each other. The higher energy line eventually disappears at the time of SERRS quenching. A coupled-oscillator model composed of plasmonic and molecular electronic resonances consistently reproduces the above interesting results by decreasing coupling energy, indicating that SERRS can be a quantitative probe for strong coupling between the two resonances.

Using a weak-coupling renormalization group formalism, we study competing-ordered phases for repulsively interacting fermions on the bilayer honeycomb lattice away from half-filling, which can be realized experimentally as doped bilayer graphene. As electrons are added to the system, excitonic order is suppressed, and unconventional superconductivity appears generically in its place. In general, it is found that the maximum critical temperature for superconductivity appears directly adjacent to the dome of particle-hole order, illustrating the importance of fluctuations in these channels for the formation of unconventional superconductivity. We obtain the phase diagram showing characteristic ordering temperatures for both short- and long-ranged interactions, and show that the most likely superconducting instabilities occur in $d$-wave, $f$-wave, and pair density wave channels. The nature of and competition between these phases are further analyzed using both free-energy expansion and self-consistent mean-field theory. The effects of finite temperature and trigonal warping due to further-neighbor hopping are studied, and implications for experiments on bilayer graphene are discussed.

We present a theory for the cloaking of arbitrarily shaped objects and demonstrate electromagnetic scattering cancellation through designed homogeneous coatings. First, in the small-particle limit, we expand the dipole moment of a coated object in terms of its resonant modes. By zeroing the numerator of the resulting rational function, we accurately predict the permittivity values of the coating layer that abates the total scattered power. Then, we extend the applicability of the method beyond the small-particle limit, deriving the radiation corrections of the scattering-cancellation permittivity within a perturbation approach. Our method permits the design of invisibility cloaks for irregularly shaped devices such as complex sensors and detectors.

${\mathrm{BaFe}}_2$${\mathrm{As}}_2$ with transition-metal doping exhibits a variety of rich phenomena from the coupling of structure, magnetism, and superconductivity. Using density functional theory, we systematically compare the Fermi surfaces (FSs), formation energies ($\Delta E_f$), and densities of states (DOSs) of electron-doped Ba(${\mathrm{Fe}}_{1-x}$$M$${}_x$)${}_2$${\mathrm{As}}_2$ with $M=\{\mathrm{Co},\mathrm{Ni},\mathrm{Cu},\mathrm{Zn}\}$ in tetragonal ($I$$4/mmm$) and orthorhombic ($F$$mmm$) structures in nonmagnetic, antiferromagnetic, and paramagnetic (disordered local moment) states. We explain changes to the phase stability ($\Delta E_f$) and Fermi surfaces (and nesting) due to chemical and magnetic disorder. We compare our results to observed/assessed properties and contrast alloy theory with the results expected from the rigid-band model. With alloying, the DOS changes from common band (Co,Ni) to split band (Cu,Zn), which dictates $\Delta E_f$ and can overwhelm FS-nesting instabilities, as for the Cu and Zn cases.