The concept of the broadband epsilon-near-zero meta-atom consisting of layered stacks with specified metallic filling ratio and thickness is proposed based on the Bergman spectral representation of the effective permittivity. The steplike metal-dielectric multilayer structures are designed to achieve realistic broadband epsilon-near-zero meta-atoms in optical frequency range. These meta-atoms can be integrated as building blocks for unconventional optical components with exotic electromagnetic properties over a wide frequency range, such as the demonstrated broadband directional emission and phase front shaping.

We study spatial correlations in the ground state of a one-dimensional electron gas coupled to a dynamic quantum impurity. The system displays a nontrivial many-body effect known as the Fermi edge singularity: transitions between discrete internal states of the impurity have a power-law dependence on the internal energies of the impurity states. We present compact formulas for the static current-current correlator and the pair-correlation function. These reveal that spatial correlations induced by the impurity decay slowly (as the third inverse power of distance) and have a power-law energy dependence, characteristic of the Fermi edge singularity.

We study relaxation of an excited electron in the conduction band of intrinsic graphene at zero temperature due to production of interband electron-hole pairs by Coulomb interaction. The electronic band curvature, being anisotropic because of trigonal warping, is shown to suppress relaxation for a range of directions of the initial electron momentum. For other directions, relaxation is allowed only if the curvature exceeds a finite critical value; otherwise, a nondecaying quasiparticle state is found to exist.

We have simulated Edwards-Anderson (EA) as well as Sherrington-Kirkpatrick systems of $L^3$ spins. After averaging over large sets of EA system samples of $3\le L\le 10$, we obtain accurate numbers for distributions $p\left(q\right)$ of the overlap parameter $q$ at very low-temperature $T$. We find $p\left(0\right)/T\to 0.233\left(4\right)$ as $T\to 0$. This is in contrast with the droplet scenario of spin glasses. We also study the number of mismatched links—between replica pairs—that come with large scale excitations. Contributions from small scale excitations are discarded. We thus obtain for the fractal dimension of outer surfaces of $q\sim 0$ excitations in the EA model $d_s\to 2.59\left(3\right)$ as $T\to 0$. This is in contrast with $d_s\to 3$ as $T\to 0$ that is predicted by mean-field theory for the macroscopic limit.

Combining density functional calculations and solutions to the steady-state Boltzmann transport equation, the transport properties of osmium and osmium silicide are studied. The calculations of electrical and thermal resistivity take full account of the ab initio electron-phonon coupling, while the Seebeck coefficient is calculated in the relaxation-time approximation. Our predicted resistivities of Os are in good agreement with experiment at both low and high temperatures, which implies electron-electron scattering is less important than electron-phonon scattering, even for temperatures below 20 K. Intrinsic OsSi is a narrow-gap semiconductor, and our prediction of the band gap confirms the indirect estimation from the measured resistivity. This agreement results from a common cancellation of error in density functional theory with spin-orbit coupling. For the degenerate semiconductor OsSi, we determine the range of carrier concentrations present in experiments, through comparison with transport data. We further predict the power factor and figure of merit for both $n$ and $p$ types, with the dependence on doping concentration and temperature.

We examine the character of the itinerant magnetic transition of DyCo${}_2$ by different calorimetric methods, thereby separating the heat capacity and latent heat contributions to the entropy—allowing direct comparison to other itinerant electron metamagnetic systems. The heat capacity exhibits a large λ-like peak at the ferrimagnetic ordering phase transition, a signature that is remarkably similar to La(Fe,Si)${}_{13}$, where it is attributed to giant spin fluctuations. Using calorimetric measurements, we also determine the point at which the phase transition ceases to be first order: the critical magnetic field, ${\mu }_0$$H_{\mathrm{crit}}$ $=$ 0.4 ± 0.1 T and temperature $T_{\mathrm{crit}}$ $=$ 138.5 ± 0.5 K, and we compare these values to those obtained from analysis of magnetization by application of the Shimizu inequality for itinerant electron metamagnetism. Good agreement is found between these independent measurements, thus establishing the phase diagram and critical point with some confidence. In addition, we find that the often-used Banerjee criterion may not be suitable for determination of first order behavior in itinerant magnet systems.

We report on the study of the phase dynamics of high-critical-temperature superconductor Josephson junctions. We realized YBa${}_2$Cu${}_3$O${}_{7-x}$ grain boundary biepitaxial junctions in the submicron scale using low-loss substrates and analyzed their dissipation by comparing the transport measurements with Monte Carlo simulations. The behavior of the junctions can be fitted using a model based on two quality factors, which results in a frequency-dependent damping. Moreover, our devices can be designed to have Josephson energy of the order of the Coulomb energy. In this unusual energy range, phase delocalization strongly influences the device's dynamics, promoting the transition to a quantum phase diffusion regime. We study the signatures of such a transition by combining the outcomes of Monte Carlo simulations with the analysis of the device's parameters, the critical current, and the temperature behavior of the low-voltage resistance $R_0$.

The appearance of static magnetism, nanoscopically coexisting with superconductivity, is shown to be a general feature of optimally electron-doped $L$Fe${}_{1-x}$Ru${}_x$AsO${}_{1-y}$F${}_y$ superconductors ($L$ represents a lanthanide ion) upon isovalent substitution of Fe by Ru. The magnetic ordering temperature $T_N$ and the magnitude of the internal field display a domelike dependence on $x$, peaked around $x=1/4$, with higher $T_N$ values for those materials characterized by a larger $z$ cell coordinate of As. Remarkably, the latter are also those with the highest superconducting transition temperatures ($T_c$) for $x=0$. The reduction of $T_c\left(x\right)$ is found to be significant in the $x$ region of the phase diagram where the static magnetism develops. Upon increasing the Ru content superconductivity eventually disappears, but only at $x\simeq 0.6$.

Topological superconductors are gapped superconductors with gapless and topologically robust quasiparticles propagating on the boundary. In this paper, we present a topological field theory description of three-dimensional time-reversal invariant topological superconductors. In our theory the topological superconductor is characterized by a topological coupling between the electromagnetic field and the superconducting phase fluctuation, which has the same form as the coupling of “axions” with an Abelian gauge field. As a physical consequence of our theory, we predict the level crossing induced by the crossing of special “chiral” vortex lines, which can be realized by considering $s$-wave superconductors in proximity with the topological superconductor. Our theory can also be generalized to the coupling with a gravitational field.

Multiferroic materials in which magnetism and ferroelectricity coexist are a time-honored subject of study, opening the route for the next generation of electronic devices. Understanding the pronounced interplay between these properties is a current challenge, especially for the realization of multiferroic devices. One of the largest reversible magnetoelectric couplings has been evidenced in TbMn${}_2$O${}_5$, the reference compound of the so-called $R$Mn${}_2$O${}_5$ multiferroics. The origin of this strong coupling has been intensively studied from the static properties. Here we report an inelastic neutron scattering investigation of the dynamic properties in this famous compound. Our work sheds light on the electromagnon, a hybrid mode theoretically predicted in multiferroics, which couples magnetic excitations with polar lattice degrees of freedom. We were able to determine in TbMn${}_2$O${}_5$ the exchange constants of a model Heisenberg Hamiltonian and to identify the spin excitation that transforms in the electromagnon on passing from the ferroelectric and magnetically collinear phase to the low temperature incommensurate and weakly ferroelectric one. Our results point to the unusual feature of the dynamic magnetoelectric coupling in this series of multiferroics.

An exact analytical solution of the ground-state problem of the isotropic classical Heisenberg model on the Shastry-Sutherland lattice in external magnetic field $H$ is found for an arbitrary ratio of diagonal and edge exchange constants $J_2/J_1$. The phase diagram of this model in the ($J_2/J_1,H/J_1$) plane is presented. It includes spin-flop, spin-flip, and umbrella phases. The magnetization curves are found to be linear until saturation. It is shown numerically that the inclusion of the easy-axis anisotropy into the model leads to the appearance of the $1/3$ magnetization plateau, corresponding to the collinear up-up-down spin structure. This explains the appearance of the $1/3$ magnetization plateau in rare-earth tetraborides $R$B${}_4$. In particular, the magnetization curve of the compound HoB${}_4$ is explained.

A model of quantum depinning of magnetic vortex cores from line defects in a disk geometry and under the application of an in-plane magnetic field has been developed within the framework of the Caldeira-Leggett theory. The corresponding instanton solutions are computed for several values of the magnetic field. Expressions for the crossover temperature $T_c$ and for the depinning rate $\Gamma \left(T\right)$ are obtained. Fitting of the theory parameters to experimental data is presented.

It is well known that the Haldane phase of a one-dimensional spin-1 chain is a symmetry-protected topological (SPT) phase, which is described by a nonlinear sigma model (NLSM) with a $\Theta$ term at $\Theta =2\pi$. In this work we study a three-dimensional (3d) SPT phase of a SU(2$N$) antiferromagnetic spin system with a self-conjugate representation on every site. The spin-ordered Néel phase of this system has a ground state manifold $\mathcal{M}=\frac{\mathrm{U}\left(2N\right)}{\mathrm{U}\left(N\right)\times \mathrm{U}\left(N\right)}$, and this system is described by a NLSM defined with manifold $\mathcal{M}$. Since the homotopy group ${\pi }_4\left[\mathcal{M}\right]=\mathbb{Z}$ for $N>1$, this NLSM can naturally have a $\Theta$ term. We will argue that when $\Theta =2\pi$ this NLSM describes a SPT phase. This SPT phase is protected by the SU(2$N$) spin symmetry, or its subgroup SU($N$)$\times$SU$\left(N\right)⋊Z_2$, without assuming any other discrete symmetry. We will also construct a trial SU(2$N$) spin state on a 3d lattice; we argue that the long-wavelength physics of this state is precisely described by the aforementioned NLSM with $\Theta =2\pi$.

The time-averaged emission spectrum of single nitrogen-vacancy defects in diamond gives zero-phonon lines of both the negative charge state at 637 nm (1.945 eV) and the neutral charge state at 575 nm (2.156 eV). This occurs through photoconversion cycling between the two charge states. Due to strain in the diamond the zero-phonon lines of both charge states are split and it is found that the splitting and polarization of the two zero-phonon lines are the same. From this observation and consideration of the electronic structure of the nitrogen-vacancy center it is concluded that the excited state of the neutral center has $A_2$ orbital symmetry. The assignment of the 575-nm transition to a ${}^{2}E$-${}^2{A}_2$ transition has not been established previously.

We investigate the effect of an applied uniaxial strain on the ferromagnetic instability due to long-range Coulomb interaction between Dirac fermions in graphene. In the case of undeformed graphene the ferromagnetic exchange instability occurs at sufficiently strong interaction within the Hartree-Fock approximation. In this work we show that using the same theoretical framework but with an additional applied uniaxial strain, the transition can occur for much weaker interaction, within the range in suspended graphene. We also study the consequence of strain on the formation of localized magnetic states on adatoms in graphene. We systematically analyze the interplay between the anisotropic (strain-induced) nature of the Dirac fermions in graphene, onsite Hubbard interaction at the impurity, and the hybridization between the graphene and impurity electrons. The polarization of the electrons in the localized orbital is numerically calculated within the mean-field self-consistent scheme. We obtain a complete phase diagram containing nonmagnetic as well as magnetic regions, and our results can find prospective application in the field of carbon-based spintronics.

The quantum admittance of an interacting/coupled mesoscopic system and its series expansion are obtained by using the refermionization method. With the help of these nonperturbative results, it is possible to study the dependencies of the admittance according to the applied dc voltage, temperature, and frequency without any restriction on the relative values of these variables. Explicit expressions of the admittance are derived both in the limits of weak and strong interactions/coupling strength, giving clear indication of the inductive or capacitive nature of the mesoscopic system. They help to determine the conditions under which the phase of the current with respect to the ac voltage is positive.

Boron and nitrogen substitutional impurities in graphene are analyzed using a self-consistent tight-binding approach. An analytical result for the impurity Green's function is derived taking broken electron-hole symmetry into account and validated by comparison to numerical diagonalization. The impurity potential depends sensitively on the impurity occupancy, leading to a self-consistency requirement. We solve this problem using the impurity Green's function and determine the self-consistent local density of states at the impurity site and, thereby, identify acceptor and donor energy resonances.

Well-known conductive molecular wires, such as cumulene or polyyne, provide a model for interconnecting molecular electronics circuits. In recent experiments, the appearance of carbon wire bridging between two-dimensional electrodes, i.e., graphene sheets, was observed [C. Jin et al., Phys. Rev. Lett. 102, 205501 (2009)], thus demonstrating a mechanical way of producing cumulene. In this work, we studied the structure and conductance of carbon wire suspended between carbon nanotubes (CNTs) of different chiralities (zigzag and armchair), and corresponding conductance variation upon stretching. We found that the geometric structure of the carbon wire bridging CNTs was similar to the experimentally observed structures in carbon wire obtained between graphene electrodes. We show a way to modulate conductance by changing bridging sites between carbon wire and CNTs without breaking the wire. Observed current modulation via cumulene wire stretching or elongation together with CNT junction stability makes this a promising candidate for use in mechano-switching devices for molecular nanoelectronics.

We propose a microscopic model for a nanoelectromechanical system made by a radio-frequency driven suspended carbon nanotube (CNT) in the presence of an external magnetic field perpendicular to the current. As a main result, we show that, when the device is driven far from equilibrium, one can tune the CNT mechanical properties by varying the external magnetic field. Indeed, the magnetic field affects the CNT bending mode dynamics inducing an enhanced damping as well as a noise term due to the electronic phase fluctuations. The quality factor, as observed experimentally, exhibits a quadratic dependence on external magnetic field strength. Finally, CNT resonance frequencies as a function of gate voltage acquire, increasing the magnetic field strength, a peculiar dip-peak structure that should be experimentally observed.

Electronic nematic phases have been proposed to occur in various correlated electron systems and were recently claimed to have been detected in scanning tunneling microscopy (STM) conductance maps of the pseudogap states of the cuprate high-temperature superconductor ${\text{Bi}}_2{\text{Sr}}_2{\text{CaCu}}_2{\text{O}}_{8+\delta }$ (Bi-2212). We investigate the influence of anisotropic STM tip structures on such measurements and establish, with a model calculation, the presence of a tunneling interference effect within an STM junction that induces energy-dependent symmetry-breaking features in the conductance maps. We experimentally confirm this phenomenon on different correlated electron systems, including measurements in the pseudogap state of Bi-2212, showing that the apparent nematic behavior of the imaged crystal lattice is likely not due to nematic order but is related to how a realistic STM tip probes the band structure of a material. We further establish that this interference effect can be used as a sensitive probe of changes in the momentum structure of the sample's quasiparticles as a function of energy.