In this paper, we theoretically demonstrate that electromagnetic transparency can be induced by optical magnetism in a metamaterial, which is composed of metamolecules. Each metamolecule consists of a metallic split-ring resonator, as one bright meta-atom (which is optically magnetic), and also a cut-wire pair, as one dark meta-atom (which is optically nonmagnetic). It is found that magnetic resonances occur at optical frequencies due to the local magnetic interaction between “bright” meta-atoms and “dark” meta-atoms; thereafter, a transparency window emerges upon the original absorption background. The phenomenon is similar to the electromagnetically induced transparency (EIT) in atomic three-level systems, and a microscopic picture is given to compare it with the EIT. Furthermore, low loss and slow light in this metamaterial have also been verified. The investigations may achieve potential applications on integrated optical circuits.

By suitably patterning a metasurface, the phase velocity of surface waves may be manipulated. Here, a low-loss, thin (1/14th of the free-space wavelength), omnidirectional Luneburg lens, based upon a Sievenpiper “mushroom” array [Sievenpiper et al., IEEE Trans. Microwave Theory Tech. 47, 2059 (1999)], is fabricated and characterized at microwave frequencies. Surface waves excited using a near-field point source on the perimeter of the lens, exit the opposite side of the lens as planar wave fronts. The electric field of the surface wave is mapped out experimentally and compared to numerical simulations.

We calculate molecular hyperfine fields in organic magnetoresistance (OMAR) devices using ab initio calculations. To do so, we establish a protocol for the accurate determination of the average hyperfine field $B_{\text{hf}}$ and apply it to selected molecular ions: NPB, TPD, and Alq${}_3$. Then, we make devices with precisely the same molecules and perform measurements of the OMAR effect, in order to address the role of hole-transport layer in the characteristic magnetic field $B_0$ of OMAR. Contrary to common belief, we find that molecular hyperfine fields are not only caused by hydrogen nuclei. We also find that dipolar contributions to the hyperfine fields can be comparable to the Fermi contact contributions. However, such contributions are restricted to nuclei located in the same molecular ion as the charge carrier (intramolecular), as extramolecular contributions are negligible.

Computer image simulations provide a crucial aid to high-resolution transmission electron microscopy (HRTEM) in gaining fundamental understanding of the structure of materials. Interpretation of HRTEM images is, however, complicated due to continuous structure deformation caused by the imaging electron beam. A computational methodology has been implemented that takes into account the effects of the electron beam on deformation of sample structure during observation and imaging in HRTEM. The evolution of the sample structure is described as a sequence of externally initiated discrete damage events with a frequency determined by the cross section, which depends on the energy of the electron beam. A series of images showing structure evolution with time is obtained by coupling molecular dynamics simulations with the image simulation. These simulation parts are linked by two experimental parameters: the energy of the electron beam and the electron dose rate. As the energy of the electron beam also determines resolution and contrast of the obtained HRTEM image, a careful selection of its value is required to achieve a fine balance between reduction of the sample damage caused by the electron beam and the quality of the acquired image. The proposed computational approach is used to simulate the recently observed process of structural transformation of a small graphene flake into a fullerene cage. The simulated series of images showing the evolution of a graphene flake under the 80 keV electron beam closely reproduces experimental HRTEM images with regard to the structure evolution route, evolution rate, and signal-to-noise ratio. We show that under the increased electron beam energy of 200 keV a similar observation will be obscured by high damage rate or low signal-to-noise ratio.

We use driven Monte Carlo dynamics to study the resistive behavior of superconducting Josephson-junction arrays on a honeycomb lattice in a magnetic field corresponding to $f$ flux quantum per plaquette. While for $f=1/3$ the onset of zero resistance is found at nonzero temperature; for $f=1/2$ the results are consistent with a transition scenario where the critical temperature vanishes and the linear resistivity shows thermally activated behavior. We determine the thermal critical exponent of the zero-temperature transition for $f=1/2$ from a dynamic scaling analysis of the nonlinear resistivity. The resistive behavior agrees with recent results obtained for the phase-coherence transition from correlation-length calculations and with experimental observations on ultrathin superconducting films with a triangular pattern of nanoholes.

A combination of x-ray diffraction at high pressures and first-principles calculations reveals the sequence of crystal-structural phase transitions in AuGa${}_2$ from cubic ($Fm\overline{3}m$) to orthorhombic (Pnma) at 10 (±4) GPa and then to monoclinic ($P$2${}_1$/$n$) at 33 (±6) GPa. Neither theory nor experiment would have been adequate, on their own, in documenting this sequence of phases, but together they confirm a sequence differing from the $Fm\overline{3}m\to Pnma\to P{6}_3/mmc$ transitions predicted for CaF${}_2$ and Pnma → $P$112${}_1$/$a$ transition reported for PbCl${}_2$ and SnCl${}_2$. The combined results from theory and experiment also allow us to constrain the equations of state of the three phases of AuGa${}_2$. Calculations on the analog PbCl${}_2$ predict a transition to the $P$2${}_1$/$n$ phase seen in AuGa${}_2$ that could, therefore, be a common high-pressure phase for PbCl${}_2$-structured compounds.

Multiband superconductors support the excitation of vortices with fractional quantum flux, known as fractional vortices. In the ground state, the fractional vortices in different bands bond together to form a composite vortex with the standard flux quantum ${\Phi }_0$; thus it is difficult to stabilize the fractional vortices. Here we show that fractional vortex lattices can be stabilized in multiband superconductors with a periodic pinning array at the half-matching field of the composite system, when full matching of the fractionalized vortices occurs. In the presence of a high current, the fractional vortices in different bands decouple and move at different velocities. When the current is turned off suddenly, the fractional vortices in different bands may be trapped at different pinning sites. This system also exhibits rich dynamic behavior, and for fractional vortices in two-band superconductors we find phases where the vortices in one band are pinned and the vortices in the other band are moving. These different phases can be observed in transport measurements and produce a new type of self-induced Shapiro steps that can arise under the application of only a dc drive.

Recent experiments show oscillations of dominant period $h/2e$ in conductance vs magnetic flux of charge density wave (CDW) rings above 77 K, revealing macroscopically observable quantum behavior. The time-correlated soliton tunneling model discussed here is based on coherent, Josephson-like tunneling of microscopic quantum solitons of charge $2e$. The model interprets the CDW threshold electric field as a Coulomb blockade threshold for soliton pair creation, often much smaller than the classical depinning field but with the same impurity dependence (e.g., $\sim n_i^2$ for weak pinning). This picture draws upon the theory of time-correlated single-electron tunneling to interpret CDW dynamics above threshold. Similar to Feynman's derivation of the Josephson current-phase relation for a superconducting tunnel junction, the picture treats the Schrödinger equation as an emergent classical equation to describe the time-evolution of Josephson-coupled order parameters related to soliton dislocation droplets. Vector or time-varying scalar potentials can affect the order parameter phases to enable magnetic quantum interference in CDW rings or lead to interesting behavior in response to oscillatory electric fields. The ability to vary both magnitudes and phases is an aspect important to future applications in quantum computing.

The grand canonical numerical analysis recently developed for quantum many-body systems on a finite cluster [C. Hotta and N. Shibata, Phys. Rev. B 86, 041108(R) (2012)] is the technique to efficiently obtain the physical quantities in an applied field. There, the observables are the continuous and real functions of fields, mimicking their thermodynamic limit, even when a small cluster is adopted. We develop a theory to explain the mechanism of this analysis based on the deformation of the Hamiltonian. The deformation spatially scales down the energy unit from the system center toward zero at the open edge sites, which introduces the renormalization of the energy levels in a way reminiscent of Wilson's numerical renormalization group. However, compared to Wilson's case, our deformation generates a number of far well-localized edge states near the chemical potential level, which are connected via a very small quantum fluctuation in $k$ space with the “bulk” states which spread at the center of the system. As a response to the applied field, the particles on the cluster are self-organized to tune the particle number of the bulk states to their thermodynamic limit by using the “edges” as a buffer. We demonstrate the present analysis in two-dimensional quantum spin systems on square and triangular lattices, and determine the smooth magnetization curve with a clear $\frac{1}{3}$ plateau structure in the latter.

Kondo insulators and in particular their noncubic representatives have remained poorly understood. Here we report on the development of an anisotropic energy pseudogap in the tetragonal compound CeRu${}_4$Sn${}_6$ employing optical reflectivity measurements in broad frequency and temperature ranges. The low-energy optical conductivity shows semiconductor-like features within the $a$-$a$ plane but a Drude-like response along the $c$ axis, signaling weak metallicity. Local density approximation plus dynamical mean-field theory calculations reproduce these observations qualitatively and help to identify their origin. For large parts of the Brillouin zone, the strongly correlated band structure presents a narrow direct gap within the Kondo resonance. Only for the $c$ direction, due to anisotropy in the hybridization, there is a pronounced band crossing at the Fermi level.

We employ state-of-the-art ab initio simulations within the dynamical mean-field theory to study three likely phases of iron (hcp, fcc, and bcc) at the Earth's core conditions. We demonstrate that the correction to the electronic free energy due to correlations can be significant for the relative stability of the phases. The strongest effect is observed in bcc Fe, which shows a non-Fermi-liquid behavior, and where a Curie-Weiss behavior of the uniform susceptibility hints at a local magnetic moment still existing at 5800 K and 300 GPa. We predict that all three structures have sufficiently high magnetic susceptibility to stabilize the geodynamo.

Analytical expressions are presented for the intraband conductivity tensor of graphene that includes spatial dispersion for arbitrarily wave-vector values and the presence of a nonzero Fermi energy. The conductivity tensor elements are derived from the semiclassical Boltzmann transport equation under both the relaxation-time approximation and the Bhatnagar-Gross-Krook model (which allows for an extra degree of freedom to enforce number conservation). The derived expressions are based on linear electron dispersion near the Dirac points, and extend previous results that assumed small wave-vector values; these are shown to be inadequate for the very slow waves expected on graphene nanoribbons. The new expressions are also compared to results obtained by numerical integration over the first Brillouin zone using the exact (tight-binding) electron dispersion relation. Very good agreement is found between the new analytical expressions and the exact numerical results. Furthermore, a comparison with the longitudinal random-phase conductivity is also made. It is shown analytically that these new expressions lead to the correct value of the quantum capacitance of a graphene sheet and that ignoring spatial dispersion leads to serious errors in the propagation properties of fundamental modes on graphene nanoribbons.

The optically induced spin dynamics of a single Mn atom embedded into a single semiconductor quantum dot can be strongly influenced by using the optical Stark effect. The exchange interaction gives rise to simultaneous spin flips between the quantum dot electron and Mn. In the time domain, these flips correspond to exchange induced Rabi oscillations, which are typically off-resonant. By applying a detuned laser pulse, the states involved in the flipping can be brought into resonance by means of the optical Stark effect increasing the amplitude of the Rabi oscillations to one. In this paper, we study theoretically how this spin dynamics can be monitored in time-resolved spectroscopy. In the spectrum, the exchange interaction leads to a splitting of the exciton line into six lines, each corresponding to one of the six Mn spin states. The dynamical behavior of the Mn spin is reflected by the strength of the individual lines as a function of time. When an off-resonant optical pulse is applied, the spectral positions of the lines shift, but still the flipping dynamics is visible.

Spin-orbit (SO) fields in a spin-polarized electron gas are studied by angle-resolved inelastic light scattering on a CdMnTe quantum well. We demonstrate a striking organization and enhancement of SO fields acting on the collective spin excitation (spin-flip wave). While individual electronic SO fields have a broadly distributed momentum dependence, giving rise to D'yakonov-Perel' dephasing, the collective spin dynamics is governed by a single collective SO field which is drastically enhanced due to many-body effects. The enhancement factor is experimentally determined. These results provide a powerful indication that these constructive phenomena are universal to collective spin excitations of conducting systems.

We present a method of producing a densely ordered array of epitaxial graphene nanoribbons (GNRs) using vicinal SiC surfaces as a template, which consists of ordered pairs of (0001) terraces and nanofacets. Controlled selective growth of graphene on approximately 10 nm wide (0001) terraces with 10 nm spatial intervals allows GNR formation. By selecting the vicinal direction of SiC substrate, [1$\overline{1}$00], well-ordered GNRs with predominantly armchair edges are obtained. These structures, the high-density GNRs, enable us to observe the electronic structure at $K$ points by angle-resolved photoemission spectroscopy, showing a clear band-gap opening of at least 0.14 eV.

The electronic structure of the kagome staircase compounds, Ni${}_3$V${}_2$O${}_8$ and Co${}_3$V${}_2$O${}_8$, has been investigated using soft x-ray absorption, soft x-ray emission, and resonant inelastic x-ray scattering. Comparison between the two compounds, and with first-principles band structure calculations and crystal-field multiplet models, provides unique insight into the electronic structure of the two materials. Whereas the location of the narrow (Ni,Co) $d$ bands is predicted to be close to $E_{\mathrm{F}}$, we experimentally find they lie deeper in the occupied O $2p$ and unoccupied V $3d$ manifolds, and determine their energy via measured charge-transfer excitations. Additionally, we find evidence for a $dd$ excitation at 1.5 eV in Ni${}_3$V${}_2$O${}_8$, suggesting the V $d$ states may be weakly occupied in this compound, contrary to Co${}_3$V${}_2$O${}_8$. Good agreement is found between the crystal-field $dd$ excitations observed in the experiment and those predicted by atomic multiplet theory.

A metallic perovskite-type hydride LiNiH${}_3$ was synthesized based on first-principles prediction. We theoretically examined its electronic structure and found that half of the Ni-H derived antibonding states are occupied and that the modest thermodynamic stability depends on a delicate balance between (i) destabilization and (ii) alleviation of compression frustration in corner-sharing octahedra, both of which arise from occupation of antibonding states. Through density-functional analyses of the electronic structure and lattice instability extending over Li$T$H${}_3$ series ($T$ $=$ Fe, Co, Ni, and Cu), we showed that the balance is in fact reflected in their thermodynamic stability.

We present a state-of-the-art x-ray diffraction study of the charge density wave order in 1$T$–${\mathrm{TaS}}_2$ as a function of temperature and pressure. Our results prove that the charge density wave, which we characterize in terms of wave vector, amplitude, and the coherence length, indeed exists in the superconducting region of the phase diagram. The data further imply that the ordered charge density wave structure as a whole becomes superconducting at low temperatures, i.e., superconductivity and charge density waves coexist on a macroscopic scale in real space. This result is fundamentally different from a previously proposed separation of superconducting and insulating regions in real space and, instead, provides evidence that the superconducting and the charge density wave gap exist in separate regions of reciprocal space.

Amorphous semiconductors are known to give rise to greatly reduced conductivity relative to their crystalline counterparts, which makes the recent development of amorphous oxide semiconductors with high electron mobility unexpected. Using first-principles molecular dynamics and electronic structure simulations, we have analyzed the electronic and optical properties of covalent and ionic oxide amorphous semiconductors. We observe that in covalent systems, amorphization introduces deep defect states inside the gap, resulting in a substantial deterioration of electrical conductivity. In contrast, in ionic systems, such as the transparent conducting oxide ZnO, amorphization does not create deep carrier-recombination centers, so the oxides still exhibit good conductivity and visible transparency relative to the crystalline phases. The origin of the conductivity imbalance between covalent and ionic amorphous semiconductors can be explained using a band coupling mechanism.

We study the interactions between two semiconductor quantum dots (SQDs) coupled to a metal nanoparticle using different approximations. In particular, we identify and address issues in modeling the system using a semiclassical approach. We find that a semiclassical approach to model the coupling between the SQDs can lead to unstable, oscillatory, and chaotic behavior in a strong SQD–SQD coupling regime. This nonlinear behavior is shown to be due to a breaking of the identical particle symmetry. Additionally, we see that this chaotic behavior is closely related to the type of decoherence present in the system, specifically, whether the decoherence is collective or noncollective between the two SQDs. This provides insight into proper accounting of these important, but often neglected, interactions. When the system is modeled using a more quantum mechanical approach, this chaotic regime is absent. Finally, we compare the two models on a system with a strong plasmon-mediated interaction between the SQDs and a weak direct interaction between them. In this case, we find that while the results of the two models are similar, dipole blockade and the level splitting of the single-exciton states in the quantum model give rise to nontrivial differences between the two models.