Surface Plasmon Mediated Controllable Spin‐Resolved Transmission in Meta‐Hole Structures

Chiral responses are optical responses involving circular polarizations. Controlling the chiral response in a flexible way is very important in optical manipulations. Chiral metamaterials have thus drawn enormous interest due to their flexible designing feature. However, most of the previous studies are mainly realized by designing the structure of the individual meta‐atom. Meanwhile, to enhance the response, complex design and fabrication processes are typically required. Here, by introducing spin‐dependent propagating surface plasmons and spin‐selective interference, giant spin‐resolved transmission is achieved in a simple meta‐hole structure. In this interaction process, spin‐orbital angular momentum conversion plays an essential role. By controlling the phase difference between the interference components, controllable spin‐resolved transmission is achieved. Furthermore, such method can also be applied to realize spin‐resolved excitation of surface plasmons. The proposed controlling strategy offers a versatile platform for a variety of promising applications, such as polarization control, asymmetric transmission, surface plasmon excitation, and on‐chip chiral manipulation.     

The emergence of bound states in a superconducting gap at the topological insulator edge

We consider the edge of a superconducting topological insulator with the impurity in the presence of the Zeeman field. We analytically prove that in the trivial phase two Andreev bound states (ABSs) arise with energies moving from the superconducting gap edges to zero forming two Majorana-like bound states, as the impurity strength varies from 0 to ±2. When the Zeeman field is locally perturbed, ABSs arise both in the trivial and topological phases, but in the topological phase ABSs with energy near the gap edges cannot transform into Majorana bound states and vice versa.     

Spin‐Polarized Symmetric Electron‐Hole Quantum Bilayers: Finite width Effect

In this paper, the effect of finite width on ground‐state properties of a spin‐polarized symmetric electron‐hole quantum bilayers (EHBL) system is investigated at zero temperature. The quantum self‐consistent mean‐field approximation of Singwi, Tosi, Land and Sjölander (qSTLS) is adopted to explore intra‐ and interlayer properties such as the pair‐correlation function, the static density susceptibility, the local‐field corrections and the ground‐state energy. Interestingly, we noticed that due to the inclusion of finite width, the critical density for the onset of Wigner crystal (WC) phase is now lowered as compared to the recent spin‐polarized EHBL system without finite width and unpolarized EHBL system with finite width. Further, spin‐polarization effect is seem to introduce a marked change in the ground‐state energy of EHBL system as compared to that of unpolarized system. Results of ground‐state energy are also compared with the recent EHBL system without finite width (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Helical Luttinger liquid in topological insulator nanowires

We derive and analyze the effective low-energy theory for interacting electrons in a cylindrical nanowire made of a strong topological insulator. Three different approaches provide a consistent picture for the band structure, where surface states forming inside the bulk gap correspond to one-dimensional bands indexed by total angular momentum. When a half-integer magnetic flux pierces the nanowire, we find a strongly correlated helical Luttinger liquid topologically protected against weak disorder. We describe how transport experiments can detect this state.     

An EPR study of the temperature dependence of the energy gap in ytterbium dodecaboride

An EPR study of ytterbium dodecaboride (YbB₁₂) showed the presence of an energy gap with a width of 2Δ=12 meV in the energy spectrum of this Kondo insulator. The temperature dependence of the energy gap was determined by interpreting the experimental data within the framework of the exciton dielectric model: Δ(T)=72 K at an absolute zero and Δ(T)=0 at ～115 K. The temperature dependence of the EPR line-width exhibits a feature at 13–15 K, which is indicative of a finite density of states inside the gap. This can be related to the presence of impurity states or bound polaron excitations in the electron spectrum of YbB₁₂.     

Non‐magnetic defects in the bulk of two‐dimensional topological insulators

We found that non‐magnetic defects in two‐dimensional topological insulators induce bound states of two kinds for each spin orientation: electron‐ and hole‐like states. Depending on the sign of the defect potential these states can be also of two kinds with different distribution of the electron density. The density has a maximum or minimum in the center. A surprising effect caused by the topological order is a singular dependence of the bound‐state energy on the defect potential.

     

Interface-induced states at the boundary between a 3D topological insulator and a normal insulator

We show that, when a three-dimensional (3D) narrow-gap semiconductor with inverted band gap (“topological insulator,” TI) is attached to a 3D wide-gap semiconductor with non-inverted band gap (“normal insulator,” NI), two types of bound electron states having different spatial distributions and spin textures arise at the TI/NI interface. Namely, the gapless (“topological”) bound state can be accompanied by the emergence of the gapped (“ordinary”) bound state. We describe these states in the framework of the envelope function method using a variational approach for the energy functional; their existence hinges on the ambivalent character of the constraint for the envelope functions that correspond to the “open” or “natural” boundary conditions at the interface. The properties of the ordinary state strongly depend on the effective interface potential, while the topological state is insensitive to the interface potential variation.     

Astrophysical violations of the Kerr bound as a possible signature of string theory

In 4D general relativity, the angular momentum of a black hole is limited by the Kerr bound. We suggest that in string theory, this bound can be breached and compact black-hole-like objects can spin faster. Near such “superspinars”, the efficiency of energy transfer from the accreting matter to radiation can reach 100%, compared to the maximum efficiency of 42% of the extremal Kerr (or 6% of the Schwarzschild) black hole. Finding such superspinning objects as active galactic nuclei, GBHCs, or sources of gamma ray bursts, could be viewed as experimental support for string theory.     

Sizable band gap in organometallic topological insulator

Based on first principle calculation when Ceperley–Alder and Perdew–Burke–Ernzerh type exchange-correlation energy functional were adopted to LSDA and GGA calculation, electronic properties of organometallic honeycomb lattice as a two-dimensional topological insulator was calculated. In the presence of spin–orbit interaction bulk band gap of organometallic lattice with heavy metals such as Au, Hg, Pt and Tl atoms were investigated. Our results show that the organometallic topological insulator which is made of Mercury atom shows the wide bulk band gap of about ∼120 meV. Moreover, by fitting the conduction and valence bands to the band-structure which are produced by Density Functional Theory, spin–orbit interaction parameters were extracted. Based on calculated parameters, gapless edge states within bulk insulating gap are indeed found for finite width strip of two-dimensional organometallic topological insulators.     

Angular momentum population in fragmentation reactions

Using a relativistic transport model followed by a statistical sequential binary emission model, the population of metastable high-spin isomeric states are studied in relativistic projectile fragmentation reactions. The initial angular momentum distribution are generated from hole excitations. We find that the angular momentum distribution of the excited prefragments are considerably broadened due to light particle evaporation. The model reproduces the experimentally measured population of relatively low-lying states and underpredicts states with high angular momentum I?17?$I?17?$. We propose that coupling the spin of the excited and hole states in the prefragment will give a better understanding of the data.     

Hybrid density-functional theory and the insulating gap of UO2

We report the first calculations carried out with a periodic boundary condition code capable of examining hybrid density-functional theory (DFT) for f-element solids. We apply it to the electronic structure of the traditional Mott insulator UO2, and find that it correctly yields an antiferromagnetic insulator as opposed to the ferromagnetic metal predicted by the local spin density and generalized gradient approximations. The gap, density of states, and optimum lattice constant are all in good agreement with experiment. We stress that this results from the functional and the variational principle alone. We compare our results with the more traditional approximations.     

Magnetic quantum ring in two-dimensional topological insulators

The quantum properties of topological insulator magnetic quantum rings formed by inhomogeneous magnetic fields are investigated using a series expansion method for the modified Dirac equation. Cycloid-like and snake-like magnetic edge states are respectively found in the bulk gap for the normal and inverted magnetic field profiles. The energy spectra, current densities and classical trajectories of the magnetic edge states are discussed in detail. The bulk band inversion is found to manifest itself through the angular momentum transition in the ground state for the cycloid-like states and the resonance tunneling effect for the snake-like states.     

Fission barriers at high angular momentum and the ground state rotational band of the nucleus 254No

We study the superheavy nucleus 254No in the framework of the Hartree-Fock-Bogoliubov approximation with the finite-range density-dependent Gogny force, at zero and high angular momentum. The properties of the ground state rotational band and the fission barriers are discussed as a function of angular momentum. We found a two-humped barrier up to spin values of (30-40)Planck's over 2pi and a one-humped barrier for higher spins. We reproduce fairly well with the binding energy, the ground state deformation, the gamma-ray energies, and the bound on the fission barrier height measured at high spin.     

Optically driven Mott‐Hubbard systems out of thermodynamic equilibrium

We consider the Hubbard model at half filling, driven by an external, stationary laser field. This stationary, but periodic in time, electromagnetic field couples to the charge current, i.e. it induces an extra contribution to the hopping amplitude in the Hubbard Hamiltonian (photo‐induced hopping). We generalize the dynamical mean‐field theory (DMFT) for nonequilibrium with periodic‐in‐time external fields, using a Floquet mode representation and the Keldysh formalism. We calculate the non‐equilibrium electron distribution function, the density of states and the optical DC conductivity in the presence of the external laser field for laser frequencies above and below the Mott‐Hubbard gap. The results demonstrate that the system exhibits an insulator‐metal transition as the frequency of the external field is increased and exceeds the Mott‐Hubbard gap. This corresponds to photo‐induced excitations into the upper Hubbard band.     

Quantum behavior of orbitals in ferromagnetic titanates: novel orderings and excitations

We investigate the collective behavior of orbital angular momentum in the spin ferromagnetic state of a Mott insulator with t(2g) orbital degeneracy. The frustrated nature of the interactions leads to an infinite degeneracy of classical states. Quantum effects select four distinct orbital orderings. Two of them have a quadrupolar order, while the other states show in addition weak orbital magnetism. Specific predictions are made for neutron scattering experiments which might help to identify the orbital order in YTiO3 and to detect the elementary orbital excitations.     

Binary-black-hole encounters, gravitational bursts, and maximum final spin

The spin of the final black hole in the coalescence of nonspinning black holes is determined by the "residual" orbital angular momentum of the binary. This residual momentum consists of the orbital angular momentum that the binary is not able to shed in the process of merging. We study the angular momentum radiated, the spin of the final black hole, and the gravitational bursts in a sequence of equal mass encounters. The initial orbital configurations range from those producing an almost direct infall to others leading to numerous orbits before infall, with multiple bursts of radiation. Our sequence consists of orbits with fixed impact parameter. What varies is the initial linear momentum of the black holes. For this sequence, the final black hole of mass M_{h} gets a maximum spin parameter a/M_{h} approximately 0.823, with this maximum occurring for initial orbital angular momentum L/M_{h};{2} approximately 1.176.     

Geometric metasurface fork gratings for vortex‐beam generation and manipulation

In recent years, optical vortex beams possessing orbital angular momentum have received much attention due to their potential for high‐capacity optical communications. This capability arises from the unbounded topological charges of orbital angular momentum (OAM) that provide infinite freedoms for encoding information. The two most common approaches for generating vortex beams are through fork diffraction gratings and spiral phase plates. While realization of conventional spiral phase plate requires complicated 3D fabrication, the emerging field of metasurfaces has provided a planar and facile solution for generating vortex beams of arbitrary orbit angular momentum. Among various types of metasurfaces, the geometric phase metasurface has shown great potential for robust control of light‐ and spin‐controlled wave propagation. Here, we realize a novel type of geometric metasurface fork grating that seamlessly combine the functionality of a metasurface phase plate for vortex‐beam generation, and that of a linear phase gradient metasurface for controlling the wave‐propagation direction. The metasurface fork grating is therefore capable of simultaneously controlling both the spin and the orbital angular momentum of light.

     

Asymptotic level density in heterotic string theory and rotating black holes

We calculate the density of states with given mass and spin in string theory and obtain asymptotic formulas. We also compute the tree-level magnetic dipole moments of arbitrary physical states in the heterotic string theory. These results are then applied to study whether fundamental strings can consistently describe the microphysics of the black hole horizon in the case of a general classical solution characterized by mass, charge and angular momentum.     

Negatively Charged Excitons in a Quantum Disk

The method of few-body physics is applied to treating negatively charged excitons in a quantum disk. The energies of low-lying states of a negatively charged exciton are calculated for a few values of the electron-to-hole mass ratio. A new bound state of a negatively charged exciton in a quantum disk with orbital angular momentum L = 1 and the triplet state of the two bound electrons are predicted. The binding energy of a negatively charged exciton asfunction of disk radius for the heavy hole and the light hole is investigated.