In the binary alloy with composition $A_x{B}_{1-x}$ of two atoms with ionic energy scales $\pm \mathrm{\Delta }$, an apparent Anderson insulator (AI) is obtained as a result of randomness in the position of atoms. Using our recently developed technique that combines the local self-energy from strong-coupling perturbation theory with the transfer matrix method, we are able to address the problem of adding a Hubbard $U$ to the binary-alloy problem for millions of lattice sites on the honeycomb lattice. By adding the Hubbard interaction $U$, the resulting AI phase will become metallic, which in our formulation can be manifestly attributed to the screening of disorder by Hubbard $U$. Upon further increase in $U$, again the AI phase emerges, which can be understood in terms of the suppressed charge fluctuations due to residual Hubbard interaction. The randomness takes advantage of such suppression and localizes the quasiparticles of the metallic phase. The ultimate destiny of the system at very large $U$ is to become a Mott insulator. We construct the phase diagram of this model in the plane of $(U,\mathrm{\Delta })$ for various compositions $x$.

We propose a mechanism for destabilizing or stabilizing Bose-Einstein condensation (BEC) of interacting magnons in an antiferromagnet. We study how the interaction between magnons affects the magnon BEC in a two-sublattice antiferromagnet without and with an external magnetic field. We show that in the absence of the magnetic field the magnon BEC is destabilized by the attractive interband interaction, which is larger than the repulsive intraband interaction. We also show that in the presence of the magnetic field the magnon BEC is stabilized only if the magnetic field is large enough to make the intraband interaction larger than the interband interaction. Our results provide the first step for understanding the interaction effects on the magnon BEC in antiferromagnets and may open interaction physics of multicomponent BEC of magnons in magnets.

Spin correlations of the frustrated pyrochlore oxide ${\mathrm{Tb}}_{2+x}{\mathrm{Ti}}_{2-x}{\mathrm{O}}_{7+y}$ have been investigated by using inelastic neutron scattering on single-crystalline samples ($x=-0.007,0.000,$ and 0.003), which have the putative quantum-spin-liquid (QSL) or electric-quadrupolar ground states. Spin correlations, which are notably observed in nominally elastic scattering, show short-range correlations around $L$ points $[\mathit{q}=(\frac{1}{2},\frac{1}{2},\frac{1}{2})]$, tiny antiferromagnetic Bragg scattering at $L$ and $\mathrm{\Gamma }$ points, and pinch-point-type structures around $\mathrm{\Gamma }$ points. The short-range spin correlations were analyzed using a random-phase approximation (RPA) assuming the paramagnetic state and two-spin interactions among Ising spins. These analyses have shown that the RPA scattering intensity well reproduces the experimental data using temperature- and $x$-dependent coupling constants of up to tenth-neighbor site pairs. This suggests that no symmetry breaking occurs in the QSL sample and that a quantum treatment beyond the semiclassical RPA approach is required. Implications of the experimental data and the RPA analyses are discussed.

We study a continuum model of the weakly interacting Bose gas in the presence of an external field with minima forming a triangular lattice. The second lowest band of the single-particle spectrum ($p$ band) has three minima at nonzero momenta. We consider a metastable Bose condensate at these momenta and find that, in the presence of interactions that vary slowly over the lattice spacing, the order parameter space is isomorphic to $S^5$. We show that the enlarged symmetry leads to the loss of topologically stable vortices, as well as two extra gapless modes with quadratic dispersion. The former feature implies that this non-Abelian condensate is a “failed superfluid” that does not undergo a Berezinskii-Kosterlitz-Thouless (BKT) transition. Order-by-disorder splitting appears suppressed, implying that signatures of the $S^5$ manifold ought to be observable at low temperatures.

The success of many emerging molecular electronics concepts hinges on an atomistic understanding of the underlying electronic dynamics. We employ picosecond time-resolved x-ray photoemission spectroscopy (tr-XPS) to elucidate the roles of singlet and triplet excitons for photoinduced charge generation at a copper-phthalocyanine–${\mathrm{C}}_{60}$ heterojunction. Contrary to common belief, fast intersystem crossing to triplet excitons after photoexcitation is not a loss channel but contributes to a significantly larger extent to the time-integrated interfacial charge generation than the initially excited singlet excitons. The tr-XPS data provide direct access to the diffusivity of the triplet excitons $D_{\mathrm{CuPc}}=\left(1.8\pm 1.2\right)\times {10}^{-5}\mathrm{c}{\mathrm{m}}^2/\mathrm{s}$ (where CuPc is copper-phthalocyanine) and their diffusion length $L_{\mathrm{diff}}=\left(8\pm 3\right)\mathrm{nm}$.

We study the contribution of triplet exciton diffusion to the efficiency loss resulting from Förster-type triplet-triplet annihilation (TTA) in organic phosphorescent semiconductor host-guest systems, using kinetic Monte Carlo (KMC) simulations. Our study focusses on diffusion due to Förster-type guest-guest transfer, but includes also a comparison with simulation results for the case of Dexter-type guest-guest transfer. The simulations are carried out for a wide range of Förster radii, and for guest concentrations up to 100 mol%, with the purpose to support analyses of time-resolved photoluminescence experiments probing TTA. We find that the relative contribution of diffusion to the TTA-induced efficiency loss may be deduced quite accurately from a quantitative experimental measure for the shape of the time-dependent photoluminescence intensity, the so-called $r$ ratio. For small guest concentrations and Förster radii that are most relevant to organic light-emitting diodes (OLEDs), the diffusion contribution is in general quite small. Under these weak-diffusion conditions, the absolute diffusion contribution to the TTA-induced efficiency loss can be understood quantitatively using a capture radius formalism. The effective guest-guest diffusion coefficient that follows from the TTA simulations, using the capture radius formalism, agrees well with the diffusion coefficient that follows from direct KMC diffusion simulations. The simulations reveal that the diffusion coefficient is strongly affected by the randomness of the distribution of guest molecule locations.

The quadruple perovskite calcium manganite (${\mathrm{CaMn}}_7{\mathrm{O}}_{12}$) is a multiferroic material in which a giant magnetically induced ferroelectric polarization has been reported, making it potentially very interesting for magnetoelectric applications. Here we report the Raman spectroscopy study on this compound of both the phonon modes and the low energy excitations from 4 K to room temperature. A detailed study of the Raman active phonon excitations shows that three phonon modes evidence a spin-phonon coupling at $T_{N2}\simeq 50\mathrm{K}$. In particular, we show that the mode at 432 ${\mathrm{cm}}^{-1}$ associated with $\mathrm{Mn}\left(\mathrm{B}\right){\mathrm{O}}_6$ (B position of the perovskite) rotations around the [111] cubic diagonal is impacted by the magnetic transition at 50 K and its coupling to the new modulation of the Mn spin in the ($a,b$) plane. At low energies, two large low energy excitations are observed at 25 and 47 ${\mathrm{cm}}^{-1}$. The first one disappears at 50 K and the second one at 90 K. We have associated these excitations to electromagnetoactive modes.

The realization of terahertz (THz) sources is a fundamental aspect for a wide range of applications. Over different approaches, compact THz oscillators can be realized, taking advantage of dynamics in antiferromagnetic thin films driven by the spin Hall effect. Here we perform a systematic study of these THz oscillators within a full micromagnetic solver based on the numerical solution of two coupled Landau-Lifshitz-Gilbert-Slonczewski equations, considering ultrathin films. We find two different dynamical modes depending on the strength of the Dzyaloshinskii-Moriya interaction (DMI). At low DMI, a large-amplitude precession is excited, where both the magnetizations of the sublattices are in a uniform state and rotate in the same direction. At large enough DMI, the ground state of the antiferromagnet becomes nonuniform and the antiferromagnetic dynamics is characterized by ultrafast domain-wall motion.

We report evidence of spontaneous formation of a heterogeneous network of superdomains in two-dimensional square artificial spin ice nanostructures in externally applied magnetic fields. Coherent resonant soft-x-ray scattering from such textures gives rise to unique internal structure in Bragg peaks. The magnetic heterogeneity is locally disordered but has a zigzag texture at longer length scales. Our result suggests that the macroscopic magnetic texture is derived from the microscopic structure of the Dirac strings.

Neutron spectroscopy is used to probe transverse acoustic phonons near the $(2,2,0)$ Bragg position in colossal magnetoresistive ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3$ and ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_3$. Upon warming to temperatures near $T_c=257$ K the phonon peaks in ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3$ soften and damp significantly with the phonon half width at half maximum approaching 2.5 meV for phonons at a reduced wave vector of $\vec{q}=(0.2,0.2,0)$. Concurrently a quasielastic component develops that dominates the spectrum near the polaron position at high temperatures. This quasielastic scattering is $\approx 5$ times more intense near $T_c$ than in ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_3$ despite comparable structural distortions in the two. The damping becomes more significant near the polaron position with a temperature dependence similar to that of polaron structural distortions. An applied magnetic field of 9.5 T only partially reverses the damping and quasielastic component, despite smaller fields being sufficient to drive the colossal magnetoresistive effect. The phonon energy, on the other hand, is unaffected by field. The damping in ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_3$ near $T_c$ at a reduced wave vector of $\vec{q}=(0.25,0.25,0)$ is significantly smaller but displays a similar trend with an applied magnetic field.

Motivated by previous efforts in detecting topological orders from the ground state(s) wave function, we introduce a new quantum information tool, coined the information convex, to capture the bulk and boundary topological excitations of a 2D topological order. Defined as a set of reduced density matrices that minimizes the energy in a subsystem, the information convex encodes not only the bulk anyons but also the gapped boundaries of 2D topological orders. Using untwisted gapped boundaries of non-Abelian quantum doubles as an example, we show how the information convex reveals and characterizes deconfined bulk and boundary topological excitations, and the condensation rule relating them. Interference experiments in cold atoms provide potential measurements for the invariant structure of information convex.

We have developed and employed a quantum transport simulation method to study the transport properties in the presence of a spatially indirect exciton condensate between energy-gapped two-dimensional semiconductors, with contact to each end of each layer. This system differs from previous works on bilayer exciton transport properties in the consideration of the energy gap as well as the associated massive (parabolic band structure) electrons and holes. We observe transport properties in the presence of the condensate including the elimination of current counterflow—otherwise phenomenologically analogous to near-perfect Coulomb drag—absent overlap of the conduction and valence bands of opposite layers, as well as substantially greater critical interlayer current for a given assumed bare interlayer coupling.

Electron standing wave patterns around single hexa-peri-hexabenzocoronene (HBC) molecules adsorbed on the surface of thin KBr(001) films grown on Ag(111) are observed by scanning tunneling microscopy. We suggest that they result from the scattering, by the molecules, of the electrons populating the surface resonance associated with the lowest image potential state of the insulating film on a metal system. These patterns are nearly independent on the thickness of the film from two to five KBr monolayers, implying that they are localized near the surface of the insulating film. The electrons exhibit a parabolic dispersion relation which yields an effective mass $m^{*}=0.62\pm 0.2m_e$.

A Kekulé bond texture in graphene modifies the electronic band structure by folding the Brillouin zone and bringing the two inequivalent Dirac points to the center. This can result in the opening of a gap (Kek-O) or the locking of the valley degree of freedom with the direction of motion (Kek-Y). We analyze the effects of uniaxial strain on the band structure of Kekulé-distorted graphene for both textures. Using a tight-binding approach, we introduce strain by considering the hopping renormalization and corresponding geometrical modifications of the Brillouin zone. We numerically evaluate the dispersion relation and present analytical expressions for the low-energy limit. Our results indicate the emergence of a Zeeman-like term due to the coupling of the pseudospin with the pseudomagnetic strain potential which separates the valleys by moving them in opposite directions away from the center of the Brillouin zone. For the Kek-O phase, this results in a competition between the Kekulé parameter that opens a gap and the magnitude of strain which closes it, while for the Kek-Y phase, it results in a superposition of two shifted Dirac cones. As the Dirac cones are much closer in the superlattice reciprocal space than in pristine graphene, we propose strain as a control parameter for intervalley scattering.

Adsorbates can drastically alter physical properties of graphene. Particularly important are adatoms and admolecules that induce resonances at the Dirac point. Such resonances limit electron mobilities and spin relaxation times. We present a systematic tight-binding as well as analytical modeling to investigate the properties of resonant states in the presence of a quantizing magnetic field. Landau levels are strongly influenced by the resonances, especially close to the Dirac point. Here the cyclotron motion of electrons around a defect leads to the formation of circulating local currents which are manifested by the appearance of side peaks around the zero-energy Landau level. Our study is based on realistic parameters for H, F, and Cu adatoms, each exhibiting distinct spectral features in the magnetic field. We also show that by observing a local density of states around an adatom in the presence of Landau levels useful microscopic model parameters can be extracted.

We report on measurements of the anisotropic magnetoresistance (AMR) of doped $\mathrm{EuTi}{\mathrm{O}}_3$. It is shown that the primary contribution to the AMR is the crystalline component, which depends on the relative orientation between the magnetic moments and the crystal axes. With increasing magnetic field, a fourfold crystalline AMR undergoes a change in its alignment with respect to the crystal axes. The results are discussed in the context of the coupling between spin canting, electronic structure, and transport. We discuss the potential role of Weyl points in the band structure. At high fields, the AMR transitions to uniaxial symmetry, which is lower than that of the lattice, along with a crossover from positive to negative magnetoresistance.

Using density-functional dynamical mean-field theory (DFDMFT) we show the importance of multiorbital electronic correlations in determining the insulating state of ${\mathrm{BiFeO}}_3$, a multiferroic material with an electron band gap larger than its bare bandwidth. Within the ${\mathrm{Fe}}^{3+}$ oxidation state and using realistic values for the on-site Coulomb interaction, we unveil strongly correlated key features probed in x-ray photoelectron and absorption spectra, showing good qualitative theory-experiment agreement. We explore the electronic reconstruction hidden in ferromagnetic ${\mathrm{BiFeO}}_3$, predicting broad orbital- and spin-polarized features at low temperatures. Our proposal for ferromagnetic ${\mathrm{BiFeO}}_3$ is expected to be an important step to understanding the emergent correlated electronic structure of magnetoelectronic and spintronic materials with persisting ordered localized moments coexisting with Coulomb reconstructed electronic states.