Several topological orders have been proposed to explain the quantum Hall plateau at $\nu =5/2$. The observation of an upstream neutral mode on the sample edge supports the non-Abelian anti-Pfaffian state. On the other hand, tunneling experiments favor the Halperin 331 state which exhibits no upstream modes. No proposed ground states agree with both types of experiments. We find a topological order, compatible with the results of both experiments. That order allows both finite and zero spin polarizations. It is Abelian but its signatures in Aharonov-Bohm interferometry can be similar to those of the Pfaffian and anti-Pfaffian states.

Recent experimental studies on graphene on hexagonal boron nitride (hBN) have demonstrated that hBN is not only a passive substrate that ensures superb electronic properties of graphene's carriers, but that it actively modifies their massless Dirac fermion character through a periodic moiré potential. Here we present a theory of the plasmon excitation spectrum of massless Dirac fermions in a moiré superlattice. We demonstrate that graphene-hBN stacks offer a rich platform for plasmonics in which control of plasmon modes can occur not only via electrostatic gating but also by adjusting, e.g., the relative crystallographic alignment.

In a previous paper, Baguet et al. [Phys. Rev. Lett. 111, 166402 (2013)], we presented the ground-state phase diagram of the homogeneous electron gas in three dimensions within the Hartree-Fock approximation yielding incommensurate crystal states at high density. Here, we analyze the properties of these solutions. In particular, at high density we find universal behavior of the incommensurate crystal, which strongly supports the existence of a spin-density-wave ground state.

The influence of the exciton spin on the formation and stability properties of periodic cavity polariton patterns is studied in a semiconductor microcavity operating in the strong-coupling regime. A linearly polarized optical beam excites polaritons formed by excitons with different spin orientations and left- and right-circularly polarized photons. The perturbation analysis of homogeneous solutions reveals a competition between these two spin states. The outcome of this competition is determined by the sign of the cross-phase modulation parameter. In particular, it is shown that linearly polarized patterns are preferred, if this parameter is positive. Otherwise, a spontaneous symmetry-breaking instability leads to the formation of transverse patterns with a spatial polarization asymmetry. In the regime of bistable homogeneous solutions we observe the spontaneous formation of domains framed by one-dimensional dark half solitons.

We suggest a novel approach for generating the second-harmonic radiation in subwavelength graphene waveguides. We demonstrate that the quadratic phase matching between plasmonic guided modes of different symmetries can be achieved in a planar double-layer graphene structure when conductivity of one of the layers becomes spatially modulated. We predict theoretically that, owing to graphene nonlocal conductivity, the second-order nonlinear processes can be actualized for interacting plasmonic modes with an effective grating coupler to allow external pumping of the structure with the generated radiation at the double frequency.

The structural stability of paramagnetic (PM) body-centered-cubic (bcc) Fe under pressure is investigated based on first-principles phonon calculations. Spin configurations of the PM phase are approximated using a binary special quasirandom structure with a supercell approach. The behavior of phonon modes can be associated with pressure-induced phase transitions to the face-centered-cubic (fcc) and hexagonal-close-packed (hcp) structures as follows: For the PM phase, it is found that the low-frequency transverse mode at the $N$ point ($N_4^{-}$ mode), which corresponds to a bcc-hcp phase transition pathway, exhibits strong softening under isotropic volume compression. The frequency of this mode becomes zero by a 2% volume decrease within the harmonic approximation. This result is not consistent with the experimental fact that the phase transition from the PM bcc to hcp phases does not occur under volume compression. The seeming contradiction can be explained only when the anharmonic behavior of the $N_4^{-}$ mode is taken into consideration; A potential energy curve along the $N_4^{-}$ mode becomes closer to a double-well shape for the PM phase under volume compression. On the other hand, softening of the longitudinal mode at the 2/3[111] point under volume compression is also found for the PM phase, which indicates the pressure-induced bcc-fcc phase transition along this mode. Such behaviors are not seen in ferromagnetic bcc Fe, implying that the magnetic structure plays an essential role on the phase transition mechanism.

A theoretical method recently developed is used to find all possible equilibrium magnetic states of a finite-size classical one-dimensional planar spin chain with competing nearest-neighbor (nn) and next-nearest-neighbor (nnn) exchange interactions. The energy of a classical planar model with $N$ spins is a function of $N$ absolute orientational angles or equivalently, due to the absence of in-plane anisotropy, of $(N-1)$ relative orientational angles. The lowest energy stable state (ground state) corresponds to a global minimum of the energy in the $(N-1)$-dimensional space, while metastable states correspond to local minima. For a given value of the ratio, $\gamma$, between nnn and nn exchange couplings, all the equilibrium configurations of the model were calculated with great accuracy for $N\le 16$, and a stability analysis was subsequently performed. For any value of $N$, the ground state was found to be “symmetric” with respect to the middle of the chain in the relative angles representation. For the chosen value of $\gamma$, the ground state consists of a helix whose chirality is constant in sign along the chain (i.e., all the spins turn clockwise, or all anticlockwise), but whose pitch varies owing to finite-size effects; e.g., for positive chirality we found that the chiral order parameter $\chi \left(N\right)>0$ increases monotonically with increasing $N$, approaching the value $\left(\chi =1\right)$ pertinent to the ground state in the limit $N\to \infty$. For finite but not too small values of $N$, we found metastable states characterized by one reversal of chirality, either localized just in the middle of the chain [“antisymmetric” state, with chiral order parameter $\chi \left(N\right)=0$], or shifted away from the middle of the chain, to the right or to the left [pairs of “ugly” states, with equal and opposite values of $\chi \left(N\right)\ne 0$; the attribute “ugly” refers to the absence of a definite symmetry in the relative angles representation]. Concerning the stability of these states with one reversal of chirality, two main results were found. First, the “antisymmetric” state is metastable for even $N$ and unstable for odd $N$. Second, an additional pair of “ugly” states is found whenever the number of spins in the chain is increased by 1; the states in each additional pair are unstable for even $N$ and metastable for odd $N$. Analysis of stable and metastable configurations in the framework of a discrete nonlinear mapping approach provides further support for the above results.

The atomic theory of elasticity of amorphous solids, based on the nonaffine response formalism, is extended into the nonlinear stress-strain regime by coupling with the underlying irreversible many-body dynamics. The latter is implemented in compact analytical form using a qualitative method for the many-body dynamics. The resulting nonlinear stress-strain (constitutive) relation is very simple, with few fitting parameters, yet contains all the microscopic physics. The theory is successfully tested against experimental data on metallic glasses, and it is able to reproduce the ubiquitous feature of stress-strain overshoot upon varying temperature and shear rate. A clear atomic-level interpretation is provided for the stress overshoot, in terms of the competition between the elastic instability caused by nonaffine deformation of the glassy cage and the stress buildup due to viscous dissipation.

The compound ${\mathrm{Fe}}_2\mathrm{Ti}{\mathrm{O}}_5$ (FTO) is a well-known uniaxial anisotropic spin-glass insulator with two successive glassy freezing temperatures, i.e., transverse $(T_{TF}=9\mathrm{K})$ and longitudinal $(T_{LF}=55\mathrm{K})$. In this article, we present the results of measurements of complex dielectric behavior, electric polarization as a function of temperature $\left(T\right)$, in addition to characterization by magnetic susceptibility and heat capacity, primarily to explore magnetoelectric (ME) coupling and multiglass properties in uniaxial anisotropic spin-cluster-glass FTO. The existence of two magnetic transitions is reflected in the isothermal magnetodielectric (MD) behavior in the sense that the sign of MD is different in the $T$ regime $T<T_{TF}$ and $T>T_{TF}$. The data in addition provide evidence for the glassy dynamics of electric dipoles; interestingly, this occurs at much higher temperature (∼100–150 K) than $T_{LF}$, with high remnant polarization at 10 K $(\sim 4000\mu \mathrm{C}/{\mathrm{m}}^2)$ attributable to short-range magnetic correlations, thereby offering a route to attain ME coupling above 77 K.

We report on the magnetization reversal mechanism of Co islands on Pt(111) as a function of their size and shape. We measure the zero-field susceptibility $\chi \left(T\right)$ and low-temperature magnetization curves $M\left(H\right)$ with in situ magneto-optical Kerr effect. Together with the island morphology deduced from scanning tunneling microscopy, this creates sufficient information to determine both the magnetization reversal mechanism and the distribution of anisotropy energies between perimeter and surface atoms. We find a transition from quasicoherent rotation to domain wall nucleation and propagation with a critical size of 350 atoms for ramified, and of 600 atoms for compact islands.

A theory for frequency-dependent magnetic susceptibility $\chi \left(\omega \right)$ is developed for thermally activated magnetic monopoles in a two-dimensional (2D) spin ice. By modeling the system in the vicinity of the ground-state manifold as a 2D Coulomb gas with an entropic interaction, and then as a 2D sine-Gordon model, we have shown that the susceptibility has a scaling form $\chi \left(\omega \right)/\chi \left(0\right)=\mathcal{F}(\omega /{\omega }_1)$, where the characteristic frequency ${\omega }_1$ is related to a charge correlation length between diffusively moving monopoles, and to the principal-breather excitation. The dynamical scaling is universal and applicable not only for kagome ice, but also for superfluid and superconducting films and generic 2D ices possibly including the artificial spin ice.

In this work, we investigate the microscopic nature of the magnetism in honeycomb iridium-based systems by performing a systematic study of how the effective magnetic interactions in these compounds depend on various electronic microscopic parameters. We show that the minimal model describing the magnetism in ${\mathrm{A}}_2{\mathrm{IrO}}_3$ includes both isotropic and anisotropic Kitaev-type spin-exchange interactions between nearest and next-nearest neighbor Ir ions, and that the magnitude of the Kitaev interaction between next-nearest neighbor Ir magnetic moments is comparable with nearest neighbor interactions. We also find that, while the Heisenberg and the Kitaev interactions between nearest neighbors are correspondingly antiferro- and ferromagnetic, they both change sign for the next-nearest neighbors. Using classical Monte Carlo simulations we examine the magnetic phase diagram of the derived super-exchange model. We find that the zigzag-type antiferromagnetic order occupies a large part of the phase diagram of this model and, for the ferromagnetic next-nearest neighbor Heisenberg interaction relevant for ${\mathrm{Na}}_2{\mathrm{IrO}}_3$, it can be stabilized at small and even at zero third nearest neighbor coupling. Our results suggest that a natural physical origin of the zigzag phase experimentally observed in ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ is due to the interplay of the Kitaev anisotropic interactions between nearest and next-nearest neighbors.

Time-dependent perturbations can drive a trivial two-dimensional band insulator into a quantum-Hall-like phase, with protected nonequilibrium states bound to its edges. We propose an experiment to probe the existence of these topological edge states that consists of passing a tunneling current through a small two-dimensional sample out of equilibrium. The signature is a nonquantized metallic conductance near the edges of the sample and, in contrast, an excitation gap in the bulk. This proposal is demonstrated for the case of a two-dimensional lattice model of Dirac electrons with tunable mass in a strong electromagnetic field. In addition, we also study the tunneling conductance of the driven resonant level model and find a phenomenon similar to dynamic localization in which certain transport channels are suppressed.

We present a detailed study of thermal transport in the disordered Fermi liquid with short-range interactions. At temperatures smaller than the impurity scattering rate, i.e., in the diffusive regime, thermal conductivity acquires nonanalytic quantum corrections. When these quantum corrections become large at low temperatures, the calculation of thermal conductivity demands a theoretical approach that treats disorder and interactions on an equal footing. In this paper, we develop such an approach by merging Luttinger's idea of using gravitational potentials for the analysis of thermal phenomena with a renormalization group calculation based on the Keldysh nonlinear sigma model. The gravitational potentials are introduced in the action as auxiliary sources that couple to the heat density. These sources are a convenient tool for generating expressions for the heat density and its correlation function from the partition function. Already in the absence of the gravitational potentials, the nonlinear sigma model contains several temperature-dependent renormalization group charges. When the gravitational potentials are introduced into the model, they acquire an independent renormalization group flow. We show that this flow preserves the phenomenological form of the correlation function, reflecting its relation to the specific heat and the constraints imposed by energy conservation. The main result of our analysis is that the Wiedemann-Franz law holds down to the lowest temperatures even in the presence of disorder and interactions and despite the quantum corrections that arise for both the electric and thermal conductivities.

Most of the functionality of modern electronic circuits relies on the possibility to modify the path followed by the electrons using, e.g., field effect transistors. Here we discuss the interplay between the modification of this path and the quantum dynamics of the electronic flow. Specifically, we study the propagation of charge pulses through the edge states of a two-dimensional electron gas in the quantum Hall regime. By sending radio-frequency (rf) excitations on a top gate capacitively coupled to the electron gas, we manipulate these edge states dynamically. We find that a fast rf change of the gate voltage can stop the propagation of the charge pulse inside the sample. This effect is intimately linked to the vanishing velocity of bulk states in the quantum Hall regime and the peculiar connection between momentum and transverse confinement of Landau levels. Our findings suggest possibilities for stopping, releasing, and switching the trajectory of charge pulses in quantum Hall systems.

We propose a simple mechanism through which elementary excitations in spin ice, namely, magnetic monopoles, lead to spontaneous macroscopic electric polarization by breaking the space inversion symmetry in the spin configuration. We show that while the magnetic monopole charge is conserved in hopping, the corresponding electric dipole moment cannot be a constant of motion. The possibilities as well as the limitations involved in electrical manipulation of the monopole dynamics are also discussed.

We have performed an extensive test of the ability of density functional theory within several approximations for the exchange-correlation functional, local density approximation + Hubbard $U$, and local density approximation + dynamic mean field theory to describe magnetic and electronic properties of ${\mathrm{SrRuO}}_3$. We focus on the ferromagnetic phase, illustrating differences between the orthorhombic low-temperature structure versus the cubic high-temperature structure. We assess how magnetism, spectral function, and cohesive properties are affected by methodology, onsite Hubbard $U$, and double-counting corrections. Further, we compare the impact of the impurity solver on the quasiparticle weight $Z$, which is in turn compared to experimental results. The spectral functions resulting from the different treatments are also compared to experimental data. Finally, the impact of spin-orbit coupling is studied, allowing us to determine the orbital moments. In the orthorhombic phase, the orbital moments are found to be tilted with respect to the spin moments, emphasizing the importance of taking into account the distortion of the oxygen octahedra.

Circulating orbital currents, originating from the spin-orbit interaction, are calculated for semiconductor nanostructures in the shape of spheres, disks, spherical shells, and rings for the electron ground state with spin oriented along a symmetry axis. The currents and resulting orbital and spin magnetic moments, which combine to yield the effective electron $g$ factor, are calculated using a recently introduced formalism that allows the relative contributions of different regions of the nanostructure to be identified at zero magnetic field. For all these spherically or cylindrically symmetric hollow or solid nanostructures, independent of material composition and whether the boundary conditions are hard or soft, the dominant orbital current originates from intermixing of valence-band states in the electron ground state, circulates within the nanostructure, and peaks approximately halfway between the center and edge of the nanostructure in the plane perpendicular to the spin orientation. For a specific material composition and confinement character, the confinement energy and orbital moment are determined by a single size-dependent parameter for spherically symmetrical nanostructures, whereas they can be independently tuned for cylindrically symmetric nanostructures.

We report numerical simulations of biexciton generation in coupled quantum dots (CQDs) placed in a static electric field and excited by a chirped laser pulse. Our simulations explicitly account for exciton-phonon interactions at finite temperature using a non-Markovian quantum jump approach to solve the excitonic dynamics. In the case of noninteracting quantum dots, the biexciton generation is severely limited by the biexciton binding energy. We demonstrate that the application of an axial electric field along the CQDs can yield a favorable excitonic level alignment that compensates for the biexciton binding energy and yields an optimum biexciton generation. On the contrary, well-defined values of the electric field lead to destructive quantum interference that completely inhibits the biexciton generation. We therefore demonstrate here the potential of chirped pulse excitations of CQDs for high-efficiency biexciton generation but also for the control of unique optoelectronic properties of complex quantum systems.

We investigate the edge states of real group IV honeycomb crystals by means of ab initio methods. We fully take spin-orbit coupling into account. Thereby germanene, the germanium-based counterpart to graphene, is used as the model system. We focus on the topological properties depending on the non-, antiferro-, and ferromagnetic edges of the ribbon. We show that, in contrast to model studies, the dispersion of the edge states is strongly nonlinear, destroying gapless Dirac bands. Applying an electric field we further study the topological phase transition to a trivial insulator above a critical field strength. The results are critically discussed and compared with tight-binding predictions.