We observed in superconducting (Tl,Rb)${}_2$Fe${}_4$Se${}_5$ spin-wave branches that span an energy range from 6.5 to 209 meV. Spin dynamics are successfully described by a Heisenberg localized spin model whose dominant in-plane interactions include only the nearest ($J_1$ and $J_1^{\prime }$) and next nearest neighbor ($J_2$ and $J_2^{\prime }$) exchange terms within and between the tetramer spin blocks, respectively. These experimentally determined exchange constants would crucially constrain the theoretical viewpoints on magnetism and superconductivity in the Fe-based materials.

Upon cooling, the aperiodic inclusion compound $n$-nonadecane/urea presents a hexagonal-to-orthorhombic group-subgroup phase transition at $T_{\mathrm{cl}}$ that increases the structure's superspace dimensionality from four to five. This paper reports on pretransitional phenomena in such a high-dimensional space, generalizing the critical results previously reported at a lower dimensionality. Very high-resolution diffraction data reveal anomalously large correlation lengths along the aperiodic direction, with all correlation lengths diverging at $T_{\mathrm{cl}}$. This could be explained by low-frequency phason excitations that soften at $T_{\mathrm{cl}}$ at the critical wave vector, in accordance with an increase in the critical diffuse scattering intensity.

Single crystals of Sc${}_2$O${}_3$ were grown by a high-temperature solution-growth method. They were studied by x-ray single-crystal diffractometry and micro-Raman spectroscopy. In the polarized Raman spectra collected at room temperature all $4A_g+4E_g$ and 11 out of 14 $F_g$ Raman allowed lines were observed. The vibrational assignment was made by a comparison of the experimentally observed frequencies with calculated ones determined by density functional theory lattice-dynamical calculations.

Our neutron diffraction investigation of PrMn${}_2$Ge${}_{2-x}$Si${}_x$ reveals a clear separation into two magnetic phases, canted ferromagnetic (Fmc) and antiferromagnetic (AFmc), between $x$ $=$ 1.0 and 1.2 and a commensurate phase gap in the lattice, due to magnetostrictive distortion. This remarkable magnetoelastic phenomenon is driven by a nonuniform atomic distribution on the $X$ site which in turn produces subtle variations in the local lattice and abrupt changes in the Mn-Mn magnetic exchange interaction. Our results show that coexistence of Fmc and AFmc phases depends on lattice parameter, chemical pressure from the rare-earth and metalloid sites, and local lattice strain distributions. We demonstrate that these magnetostructural correlations act across the entire family of $R_{1-y}{R}_y^{\prime }{\mathrm{Mn}}_2{X}_{2-x}{X}_x^{\prime }$ compounds.

Conventional superconductivity is robust against the addition of impurities unless the impurities are magnetic in which case superconductivity is quickly suppressed. Here we present a study of the cuprate superconductor Bi${}_2$Sr${}_2$Ca${}_1$Cu${}_2$O${}_{8+\delta }$ that is intentionally doped with the magnetic impurity, Fe. Through the use of our tomographic density of states technique, we find that while the superconducting gap magnitude is essentially unaffected by the inclusion of iron, the onset of superconductivity, $T_{\mathrm{C}}$, and the pair-breaking rate are strongly dependent and correlated. These findings suggest that, in the cuprates, the pair-breaking rate is critical to the determination of $T_{\mathrm{C}}$ and that magnetic impurities do not disrupt the strength of pairing but rather the lifetime of the pairs.

We study the superconducting proximity effect in a quantum wire with broken time-reversal (TR) symmetry connected to a conventional superconductor. We consider the situation of a strong TR-symmetry breaking, so that Cooper pairs entering the wire from the superconductor are immediately destroyed. Nevertheless, some traces of the proximity effect survive: for example, the local electronic density of states (LDOS) is influenced by the proximity to the superconductor, provided that localization effects are taken into account. With the help of the supersymmetric sigma model, we calculate the average LDOS in such a system. The LDOS in the wire is strongly modified close to the interface with the superconductor at energies near the Fermi level. The relevant distances from the interface are of the order of the localization length, and the size of the energy window around the Fermi level is of the order of the mean level spacing at the localization length. Remarkably, the sign of the effect is sensitive to the way the TR symmetry is broken: In the spin-symmetric case (orbital magnetic field), the LDOS is depleted near the Fermi energy, whereas for the broken spin symmetry (magnetic impurities), the LDOS at the Fermi energy is enhanced.

We study the formation, mobility, and stability of self-trapped excitons (STE) and self-trapped holes and electrons in NaI and NaI(Tl) using embedded cluster hybrid density functional theory calculations. This method employs an array of classical charges to provide an environment simulating the interior of an ionic solid in which the electronic structure of a modestly sized quantum-mechanical cluster is computed including nonlocal exchange effects which are necessary to describe localized excitations in NaI. In contrast with previous models, we find that both carriers in pure NaI have similar mobilities, with an activation energy of $\sim 0.2$ eV. We propose an alternate interpretation including a new migration mechanism for the STE. In Tl-doped material excitons preferentially trap at dopants, inducing off-center distortions that have a structure unlike an STE and provide a mechanism for light emission at multiple wavelengths.

A system of two interacting cobalt atoms, at varying distances, was studied in a recent scanning tunneling microscope experiment by Bork et al. [Nat. Phys. 7, 901 (2011)]. We propose a microscopic model that explains, for all experimentally analyzed interatomic distances, the physics observed in these experiments. Our proposal is based on the two-impurity Anderson model, with the inclusion of a two-path geometry for charge transport. This many-body system is treated in the finite-$U$ slave boson mean-field approximation and the logarithmic-discretization embedded-cluster approximation. We physically characterize the different charge transport regimes of this system at various interatomic distances and show that, as in the experiments, the features observed in the transport properties depend on the presence of two impurities but also on the existence of two conducting channels for electron transport. We interpret the splitting observed in the conductance as the result of the hybridization of the two Kondo resonances associated with each impurity.

We predict the enhancement of the spin-rotation coupling due to the interband mixing. The Bloch wave functions in the presence of mechanical rotation are constructed with the generalized crystal momentum which includes a gauge potential arising from the rotation. Using the eight-band Kane model, the renormalized spin-rotation coupling is explicitly obtained. As a result of the renormalization, the rotational Doppler shift in electron spin resonance and the mechanical torque on an electron spin will be strongly modulated.

We compute the thermal conductivity of superlattice (SL) thin films and nanowires for various SL periods and total specimen lengths using nonequilibrium molecular dynamics. Both types of materials exhibit similar behaviors with respect to SL period but the thermal conductivity of the thin films exhibits a significantly higher sensitivity to the specimen length. Notably, the thermal conductivity of SL thin films is smaller than those of the corresponding nanowires for specimen lengths below approximately 35 nm. These results arise from the complex dependence of the conductivities of the interfaces and the SL components on the specimen size and period. These trends and observations are explained using a simple phonon model that builds on the relationship between the cumulative thermal conductivity and the phonon wavelength.

We report on the dual nature (capacitive and inductive) of the surface impedance of periodic graphene patches at low-terahertz frequencies. The transmission spectra of a graphene-dielectric stack shows that patterned graphene exhibits both the low-frequency (capacitive) passband of metal patch arrays and the higher-frequency (inductive) passband of metal aperture arrays in a single tunable configuration. The analysis is carried out using a transfer-matrix approach with two-sided impedance boundary conditions, and the results are verified using full-wave numerical simulations. In addition, the Bloch-wave analysis of the corresponding infinite periodic structure is presented in order to explain the passband and stopband characteristics of the finite graphene-dielectric stack.

Large Chern number phases in a Haldane model become possible if there is a multiplication of Dirac points in the underlying graphene model. This is realized by considering long-distance hopping integrals. Through variation of these integrals, it is possible to arrive at supermerging band touchings, which up to N7 graphene are unique in parameter space. They result from the synchronized motion of all supplementary Dirac points into the regular $\pm \mathbf{K}$ points of graphene. The energy dispersion power law is usually larger than the topological charge associated with them. Moreover, adding distant-neighbor hoppings in the Haldane mass allows one to sweep large Chern number phases in the topological insulator.

In the near field, radiative heat transfer can exceed the prediction from Planck's law by several orders of magnitude, when the interacting materials support surface polaritons in the infrared range. However, if the emitter and absorber are made from two different materials, which support surface polariton resonances at different frequencies, the mismatch between surface polariton resonance frequencies will drastically reduce near-field radiative heat transfer. Here, we present a broadband near-field thermal emitter/absorber based on hyperbolic metamaterials, which can significantly enhance near-field radiative heat transfer with infrared surface-polariton-resonance materials and maintain the monochromatic characteristic of heat transfer. Instead of using an effective medium approximation, we perform a direct numerical simulation to accurately investigate the heat transfer mechanisms of metamaterials based on the Wiener chaos expansion method.

We report ${}^{31}$P- and ${}^{75}$As-NMR studies on (Ca${}_4$Al${}_2$O${}_6$)Fe${}_2$(As${}_{1-x}$P${}_x$)${}_2$ with an isovalent substitution of P for As. We present the novel evolution of emergent phases that the nodeless superconductivity (SC) in $0\le x\le 0.4$ and the nodal one around $x=1$ are intimately separated by the onset of a commensurate stripe-type antiferromagnetic (AFM) order in $0.5\le x\le 0.95$, as an isovalent substitution of P for As decreases a pnictogen height $h_{Pn}$ measured from the Fe plane. It is demonstrated that the AFM order takes place under a condition of $1.32\AA \le h_{Pn}\le 1.42\AA$, which is also the case for other Fe pnictides with the Fe${}^{2+}$ state in (Fe$Pn$)${}^{-}$ layers. This novel phase evolution with the variation in $h_{Pn}$ points to the importance of electron correlation for the emergence of SC as well as AFM order.

This Rapid Communication reports dynamical mean-field calculations for the periodic Anderson model in which the conduction band is coupled to phonons. Motivated in part by recent attention to the role of phonons in the $\gamma$-$\alpha$ transition in Ce, this model yields a rich and unexpected phase diagram which is of intrinsic interest. Specifically, above a critical value of the electron-phonon interaction, a first-order transition with two coexisting phases develops in the temperature-hybridization plane, which terminates at a second-order critical point. The coexisting phases display the familiar Kondo screened and local-moment character, yet they also exhibit pronounced polaronic and bipolaronic properties, respectively.

We investigate the doping of AA-stacked graphene bilayers. By applying a mean field theory at zero temperature we find that, at half-filling, the bilayer is an antiferromagnetic insulator. Upon doping, the homogeneous phase becomes unstable with respect to phase separation. The separated phases are undoped antiferromagnetic insulator and metal with a nonzero concentration of charge carriers. At sufficiently high doping, the insulating areas shrink and disappear, and the system becomes a homogeneous metal. The conductivity changes drastically upon doping, so the bilayer may be used as a switch in electronic devices. The effects of finite temperature are also discussed.

Generally, the addition or removal of a single particle in a many-body system does not correspond to an exact eigenstate of the system. Thus the resulting coherent excitation evolves in time. As discussed here, the evolution at short times upon the excitation with the energy $\epsilon$ exhibits a quadratic decay [with the rate constant ${\sigma }^2\left(\epsilon \right)$]. Later on, after some time $\tau \left(\epsilon \right)$, the exponential decay sets in. It is governed by another rate constant $\gamma \left(\epsilon \right)$. This behavior is generic for many realistic finite and extended systems. For a finite system it is possible to assess this behavior full numerically using an exact solution of the many-body problem. We present a simple model for the electron spectral function that links together all three aforementioned parameters and give a prescription for how the energy uncertainty ${\sigma }^2\left(\epsilon \right)$ can be computed within the many-body perturbation theory. Our numerical results demonstrate that the model approach accurately reproduces the exact spectral function in a large range of energies even in the case of fragmented many-body states. We show that the central quantity of this study ${\sigma }^2\left(\epsilon \right)$ can easily be computed exactly or from approximate theories and, hence, can be used for their validation. We also point out how the set in time can be tested by means of attosecond spectroscopy.

A vector bosonic field coupled to the electronic spin is treated by means of the continuous-time quantum Monte Carlo method. In the Bose-Kondo model with a sub-Ohmic density of states ${\rho }_{\mathrm{B}}\left(\omega \right)\propto {\omega }^s$ with $s=0.2$, two contributions to the spin susceptibility, the Curie term $T^{-1}$ and the term $T^{-s}$ due to bosonic fluctuations, are observed separately. This result indicates the existence of a residual moment and a hidden critical behavior. By including hybridization with itinerant electrons, a quantum critical point is identified between this local-moment state and the Kondo singlet state. It is demonstrated that the energy scale of the bosonic fluctuations is not affected by the quantum phase transition.

It is shown that a frequently used relation between the interband momentum and dipole matrix elements (shortened to the “p-r relation”) in semiconductors acquires an additional correction term if applied to finite-volume crystals treated with periodic boundary conditions. The correction term, which is a generalization of the one obtained by Yafet [Phys. Rev. 106, 679 (1957)] for infinite crystals, does not vanish in the limit of infinite volume. We illustrate this with numerical examples for bulk GaAs and GaAs superlattices. The persistence of the correction term is traced to the subtle nature of the dipole matrix element with spatially extended wave functions. In contrast, a straightforward application of the findings by Blount [Solid State Phys. 13, 305 (1962)] and Haug [Theoretical Solid State Physics (Pergamon, Oxford, 1972)] yields the usual p-r relation in the distribution sense, without any corrections, when Bloch wave functions normalized to delta functions in crystal momentum space are used. Our findings therefore show that, for the interband dipole matrix element, using Bloch wave functions under periodic boundary conditions is not the proper way to approach the infinite-volume limit. From our numerical evaluations, we find that the correction term is large in the case of interband transitions in bulk GaAs, and that it can be chosen to be small in the case of intersubband transitions in superlattices, which are important in the context of terahertz (THz) radiation. We also show that one can interpret the infinite-volume p-r relation in terms of a limiting procedure using progressively broadened wave packet states that approach delta-normalized Bloch wave functions. Finally, we discuss the p-r relation for nanostructures in the envelope function approximation and show that the cell-envelope factorization of the nanostructure dipole matrix element into a cell-matrix element and an envelope overlap integral involves the cell gradient-$k$ rather than the cell dipole matrix element.

We study the Ruderman-Kittle-Kasuya-Yosida (RKKY) interaction in a monolayer MoS${}_2$. We show that the rotation of the itinerant electron spin due to the spin-orbit coupling causes a twisted interaction between two magnetic adatoms, which consists of different RKKY coupling terms: the Heisenberg, Dzyaloshinsky-Moriya, and Ising interactions. We find that the interaction terms are very sensitive to the Fermi energy values and change dramatically from doped to undoped systems. A finite doping causes all parts of the interaction to oscillate with the distance of two magnetic impurities $R$, and the interaction behaves like $R^{-2}$ for a long distance between two localized spins. We explore a beating pattern of oscillations of the RKKY interaction that occurs for the doped system.