We have studied the electronic structure of Ba(Fe${}_{1-x}$Mn${}_x$)${}_2$As${}_2$ ($x=0.08$), which fails to become a superconductor in spite of the formal hole doping like Ba${}_{1-x}$K${}_x$Fe${}_2$As${}_2$, with photoemission spectroscopy and x-ray absorption spectroscopy (XAS). With decreasing temperature, a transition from the paramagnetic phase to the antiferromagnetic phase was clearly observed by angle-resolved photoemission spectroscopy. XAS results indicated that the substituted Mn atoms form a strongly hybridized ground state. Resonance-photoemission spectra at the Mn $L_3$ edge revealed that the Mn $3d$ partial density of states is distributed over a wide energy range of 2–13 eV below the Fermi level ($E_F$), with little contribution around $E_F$. This indicates that the dopant Mn 3$d$ states are localized in spite of the strong Mn $3d$–As $4p$ hybridization and split into the occupied and unoccupied parts due to the on-site Coulomb and exchange interaction. The absence of superconductivity in Ba(Fe${}_{1-x}$Mn${}_x$)${}_2$As${}_2$ can thus be ascribed both to the absence of carrier doping in the FeAs plane and to the stabilization of the competing G-type antiferromagnetic order by the Mn impurities.

The field-induced evolution of the magnetic ordering in (CuBr)Sr${}_2$Nb${}_3$O${}_{10}$ with a $⅓$ magnetization plateau has been investigated by neutron diffraction under magnetic fields up to 10 T. With an increasing magnetic field, the zero-field helical antiferromagnetic (AFM) phase, AF1, with κ $=$ [0 $⅜$ $½$] is replaced by a simple ferromagnetic phase with κ $=$ [0 0 0], the formation of which is, however, retarded by the appearance of a second AFM, AF2, with κ $=$ [0 $⅓\sim 0.46$]. Upon further increasing of the magnetic field, the AF2 phase disappears and only the ferromagnetic phase persists. The results clearly show that the magnetization plateau, induced by the competition between field-induced ferromagnetic, F, and AF2 phases, is coincidentally situated at $M$ $=$ $⅓$ $M_S$ of the dc magnetization curve. The AF1 and AF2 phases have strongly differing magnetic propagation vectors and are therefore not directly related.

To describe anomalous zone-boundary softening and damping of magnons in manganites we present a phenomenological two-fluid model containing ferromagnetic Fermi-liquid and non-Fermi-liquid components associated with the itinerant and core electrons of Mn. The observed discontinuous increase of magnon damping is explained by the intersection of magnon dispersions with the electron-hole Stoner continuum arising due to the breakdown of the half-metallic ground state of manganites supported by the experiments and analysis of zero-point effects. Coupling of the Fermi-liquid and non-Fermi-liquid fluids yields conventional long wavelength magnons damped due to their interaction with longitudinal spin fluctuations.

We studied current-voltage characteristics of long one-dimensional Josephson junction chains with Josephson energy much larger than charging energy, $E_J\gg E_C$. In this regime, typical I-V curves of the samples consist of a supercurrent-like branch at low-bias voltages followed by a voltage-independent chain current branch, $I_{\mathrm{chain}}$ at high bias. Our experiments showed that $I_{\mathrm{chain}}$ is not only voltage-independent but it is also practically temperature-independent up to $T=0.7T_C$. We have successfully model the transport properties in these chains using a capacitively shunted junction model with nonlinear damping.

Using a radio-frequency tunnel diode oscillator technique, we measured the temperature dependence of the in-plane London penetration depth $\Delta {\lambda }_{ab}\left(T\right)$ in Fe${}_{1+y}$(Te${}_{1-x}$Se${}_x)$ single crystals, down to temperatures as low as 50 mK. A significant number of samples, with nominal Se concentrations $x=0.36$, 0.40, 0.43, and 0.45, respectively, were studied and in many cases we found that $\Delta {\lambda }_{ab}\left(T\right)$ shows an upturn below 0.7 K, indicative of a paramagnetic-type contribution. After subtracting the magnetic background, the low-temperature behavior of penetration depth is best described by a power law with exponent $n\approx 2$ and with no systematic dependence on the Se concentration. Most importantly, in the limit of $T\to 0$, in some samples we observed a narrow region of linear temperature dependence of penetration depth, suggestive of nodes in the superconducting gap of Fe${}_{1+y}$(Te${}_{1-x}$Se${}_x)$.

The dynamical cluster approximation (DCA) is a systematic extension beyond the single-site approximation in dynamical mean field theory, to include spatially nonlocal correlations in quantum many-body simulations of strongly correlated systems. We extend the DCA with a continuous lattice self-energy in order to achieve better convergence with cluster size. This method, which we call DCA${}^{+}$, cures the cluster-shape dependence problems of the DCA, without suffering from causality violations of previous attempts to interpolate the cluster self-energy. A practical approach based on standard inference techniques is given to deduce the continuous lattice self-energy from an interpolated cluster self-energy. We study the pseudogap region of a hole-doped two-dimensional Hubbard model and find that, in the DCA${}^{+}$algorithm, the self-energy and pseudogap temperature $T^{*}$ converge monotonously with cluster size. Introduction of a continuous lattice self-energy eliminates artificial long-range correlations and thus significantly reduces the sign problem of the quantum Monte Carlo cluster solver in the DCA${}^{+}$algorithm compared to the normal DCA. Simulations with much larger cluster sizes thus become feasible, which, along with the improved convergence in cluster size, raises hope that precise extrapolations to the exact infinite cluster size limit can be reached for other physical quantities as well.

We have grown Ca${}_{2-x}$Sr${}_x$RuO${}_4$ ($x$ $=$ 0, 0.1, 0.5, and 2) epitaxial thin films using a pulsed laser deposition method and characterized their structures and magnetotransport properties. We find that the $x$ $=$ 0, 0.1, and 0.5 films grown on LaAlO${}_3$ substrates exhibit coherent strain with tetragonal structure. The nature of strain is dependent on Sr content: the Ca${}_2$RuO${}_4$ ($x$ $=$ 0) film features biaxial compressive strain, while the $x$ $=$ 0.5 film shows biaxial tensile strain. The strain in the $x$ $=$ 0.1 film is relatively weak and strongly anisotropic, with compressive strain along the $a$ axis and tensile strain along the $b$ axis. In contrast, the Sr${}_2$RuO${}_4$ films show strain relaxation. The epitaxial strain effect leads the properties of the $x=0$, 0.1, and 0.5 films to be distinct from those of bulk materials. The bulk material shows antiferromagnetic Mott-insulating properties for $x$ < 0.2 and a nearly ferromagnetic state for $x$ ∼ 0.5 [Nakatsuji and Maeno, Phys. Rev. Lett. 84, 2666 (2000)], whereas the film displays itinerant ferromagnetism for $x$ $=$ 0 and 0.1 and paramagnetic metal for $x$ $=$ 0.5. Furthermore, in the $x$ $=$ 0 and 0.1 films, we observed distinct fourfold ferromagnetic anisotropy, with the minimum magnetoresistivity along the diagonal directions for $x$ $=$ 0 and $a$ and $b$ directions for $x$ $=$ 0.1. Such evolution of magnetic anisotropy may be associated with the tuning of the spin-orbit coupling by the epitaxial strain.

We show that near a quantum-critical point generating quantum criticality of strongly correlated metals where the density of electron states diverges, the quasiclassical physics remains applicable to the description of the resistivity $\rho$ of strongly correlated metals due to the presence of a transverse zero-sound collective mode, reminiscent of the phonon mode in solids. We demonstrate that at $T$, being in excess of an extremely low Debye temperature $T_D$, the resistivity $\rho \left(T\right)$ changes linearly with $T$, since the mechanism, forming the $T$ dependence of $\rho \left(T\right)$, is the same as the electron-phonon mechanism that prevails at high temperatures in ordinary metals. Thus, in the region of the $T$-linear resistivity, electron-phonon scattering leads to near material independence of the lifetime $\tau$ of quasiparticles that is expressed as the ratio of the Planck constant $\hslash$ to the Boltzmann constant $k_B$, $T\tau \sim \hslash /k_B$. We find that at $T<T_D$ there exists a different mechanism, maintaining the $T$-linear dependence of $\rho \left(T\right)$, and making the constancy of $\tau$ fail in spite of the presence of $T$-linear dependence. Our results are in good agreement with exciting experimental observations.

We demonstrate the correspondence between charge transition levels of localized point defects in hybrid density functional theory and in $G_0{W}_0$ many-body perturbation theory. To achieve this correspondence, it is necessary to properly combine the treatments of the finite-size effect, the delocalization error, and the path dependency in the $G_0{W}_0$ scheme. In particular, we introduce a beyond-monopole finite-size electrostatic correction for the eigenvalue which is fully consistent with the analogous correction for the total energy. We illustrate how the comparison with experiment is affected by the calculated band-edge positions, which unlike the defect levels are sensitive to the adopted electronic-structure method.

Conventionally a flat, shiny metal surface can be used as a reflector. Here we show that a microstructured metal surface can be switched from a perfect reflector to a perfect absorber by merely changing the polarization of the incident light. The structure consists of arrays of three-dimensional standing U-shaped resonators (SUSRs), which are fabricated by two-photon polymerization followed by blanket coating of the metal. For the incidence with a specific polarization, light can be perfectly reflected. By changing the polarization of incident light for 90°, light neither transmits nor reflects. Since the electromagnetic resonance occurs in between the standing arms of a U-shaped resonator, the heat generated in the absorption process is expected to dissipate more easily. The thermal stability of the SUSR absorber has also been investigated.

Recent revisions of defect formation energy calculations based on bandgap corrected hybrid functionals have raised concerns about the validity of earlier results based on standard density functionals and about the reliability of the theoretical prediction of electrical properties in semiconductor materials in general. We show here that a close agreement between the two types of functionals can be achieved by determining appropriate values for the electronic and atomic reference energies, thereby mitigating uncertainties associated with the choice of the underlying functional.

The electronic properties of single Fe atoms in the surface of $p$-type GaAs(110) are investigated by scanning tunneling microscopy (STM) and spectroscopy. In STM images, Fe atoms show a characteristic structure with an orientation that matches the GaAs substrate lattice. Fe is assumed to occupy substitutional sites in the surface. Spectra of the differential conductance on Fe reveal several distinct peaks. They are discussed in the context of electronic states that originate from hybridization between $d$ orbitals of the Fe atom and $p$-like orbitals of neighboring As atoms.

We investigate the effects of nonlocal response on the surface-plasmon polariton guiding properties of the metal-insulator (MI), metal-insulator-metal (MIM), and insulator-metal-insulator (IMI) waveguides. The nonlocal effects are described by a linearized hydrodynamic model, which includes the Thomas-Fermi internal kinetic energy of the free electrons in the metal. We derive the nonlocal dispersion relations of the three waveguide structures taking into account also retardation and interband effects, and examine the delicate interplay between nonlocal response and absorption losses in the metal. We also show that nonlocality breaks the complementarity of the MIM and IMI waveguides found in the nonretarded limit.

We present here a detailed analysis of the shapes of two-dimensional Ag islands of various sizes observed during the early stages of coarsening on the Ag(111) surface, using kinetic Monte Carlo (KMC) simulations, and show that selectivity is due to the formation of kinetically stable island shapes that survive longer than nonselected sizes, which decay into nearby selected sizes. The stable shapes have a closed-shell structure—one in which every atom on the periphery has at least three nearest neighbors. These findings further explain our earlier study in which we found that in the early stages coarsening proceeds as a sequence of selected island sizes resulting in peaks and valleys in the island size distribution [G. Nandipati, A. Kara, S. I. Shah, and T. S. Rahman, J. Phys.: Condens. Matter 23, 262001 (2011)]. This selectivity is dictated by the relative energetics of edge-atom diffusion and detachment and attachment processes and by the large activation barrier for kink detachment. Our simulations were carried out using a very large database of processes identified by each atom's unique local environment using the self-learning KMC scheme. The activation barriers were calculated using semiempirical interaction potentials based on the embedded-atom method.

Planar thin-film solar cells can exploit Fabry-Perot resonances in the spectral domain of weak absorption to make them more efficient. However, their spectral width decreases and their number increases for a thicker solar cell, both being detrimental. Here, we show that an optically complementary material, i.e., a material with a negative but otherwise identical refractive index to that of the solar cell material and with the same impedance, allows a single, spectrally immensely broad, Fabry-Perot resonance to be obtained. This is useful to enhance the absorption across the entire spectral domain of interest. If a material with the required optical response were available, a solar cell could be implemented that performs close to the Lambertian limit. However, since the idealistic material properties assumed herein cannot be achieved, an actual implementation may exploit nanostructures such as, e.g., dispersion engineered photonic crystals. In a reasonable spectral range their optical response can be close to that required.

Transport and thermoelectric coefficients (including also spin thermopower) of silicene nanoribbons with zigzag edges are investigated by ab initio numerical methods. Local spin density of such nanoribbons reveals edge magnetism. As in graphene, one finds antiferromagnetic and ferromagnetic ordering, with spin polarization at one edge antiparallel or parallel to that at the other edge, respectively. Thermoelectric properties, especially the Seebeck coefficient, significantly depend on the electronic band structure and are enhanced when the Fermi level is in the energy gap. However, the thermoelectric efficiency is significantly reduced when the phonon contribution to the heat conductance is included. This phonon contribution has been calculated numerically by two different methods. Transition from antiferromagnetic to ferromagnetic states leads to a large magnetoresistance as well as to a considerable magnetothermopower. Thermoelectric parameters in the antiparallel configuration, when spin polarization in the left part of the nanoribbon is opposite to that in the right part, are also analyzed.

We propose a theoretical framework for reconstructing tip-surface interactions using the intermodulation technique when more than one eigenmode is required to describe the cantilever motion. Two particular cases of bimodal motion are studied numerically: one bending and one torsional mode, and two bending modes. We demonstrate the possibility of accurate reconstruction of a two-dimensional conservative force field for the former case, while dissipative forces are studied for the latter.

We present an analysis of the Rashba effects in a two-dimensional electron gas induced by the microwave electric and magnetic fields. We show that in the frame of the Drude model the Rashba interaction can be described by magnetic and electric corrections affecting the spin and the velocity of the electrons. We describe the rf currents making use of the conductivity tensor formalism. The electromagnetic power absorption is obtained as a function of the external electric and magnetic rf fields up to the second order of the Rashba parameter. The channels of energy transfer due to Rashba field corrections are analyzed.

Silicene is a graphene-like honeycomb structure made of silicon atoms. It is a two-dimensional quantum spin-Hall insulator due to the spin-orbit interaction. According to the bulk-edge correspondence we expect zero-energy edge channels to appear in silicene nanoribbons. The behaviors of the helical edge channels are completely different between the armchair and the zigzag edges. Zero-energy states disappear in armchair nanoribbons despite the bulk-edge correspondence, while they appear as zigzag nanoribbons even if the width is quite narrow. The difference originates in the penetration depth of the helical edge channel, which is antiproportional to the spin-orbit gap for the armchair edge, while it remains as short as the lattice constant for the zigzag edge. These properties make clear distinctions between silicene and graphene nanoribbons, especially for armchair edges: In silicene edge states emerge as required by its topology, though the zero-energy states disappear from the energy spectrum, whereas in graphene no edge states exist. The emergence of edge states in armchair nanoribbons must be experimentally detectable by scanning tunneling microscopy, and may well serve as an experimental signal that silicene is a topological insulator.

We report on the infrared photoresponse of high-mobility graphene in high magnetic fields. Two types of photoresponse signals were observed that were due to photovoltaic and bolometric effects. The photovoltaic signal was observed in the quantum Hall regime, whereas the bolometric signal was dominant in the quantum Hall transition regime. The photovoltaic effect, which was induced by cyclotron resonance, was observable up to 180 K. The polarity of the photovoltaic photoresponse was shown to be systematically reversed on reversal of the applied magnetic field direction and measurement geometry, suggesting that the photovoltage signals were generated along the quantum Hall edge channel.