On the Ag(110) surface copper phthalocyanine (CuPc) orders in two structurally similar superstructures, as revealed by low-energy electron diffraction. Scanning tunneling microscopy (STM) shows that in both superstructures the molecular planes are oriented parallel to the surface and the long molecular axes, defined as diagonals of the square molecule, are rotated by $\simeq \pm {32}^{\circ }$ away from the high-symmetry directions $\left[1\overline{1}0\right]$ and [001] of the silver surface. Similarly to many other adsorbed metal phthalocyanines, the CuPc molecules on Ag(110) appear in STM as crosslike features with twofold symmetry. Photoemission tomography based on angle-resolved photoemission spectroscopy reveals a charge transfer from the substrate into the molecule. A symmetry analysis of experimental and theoretical constant binding energy maps of the photoemission intensity in the $k_x,k_y$-plane points to a preferential occupation of one of the two initially degenerate lowest unoccupied molecular orbitals (LUMOs) of $e_g$ symmetry. The occupied $e_g$ orbital is rotated by ${32}^{\circ }$ against the [001] direction of the substrate. The lifting of the degeneracy of the LUMOs and the related reduction of the symmetry of the adsorbed CuPc molecule are attributed to an anisotropy in the chemical reactivity of the Ag(110) surface.

We study the effects of charge degrees of freedom on the spin excitation dynamics in quasi-one-dimensional magnetic materials. Using the density matrix renormalization group method, we calculate the dynamical spin structure factor of the Hubbard model at half electronic filling on a chain and on a ladder geometry, and compare the results with those obtained using the Heisenberg model, where charge degrees of freedom are considered frozen. For both chains and two-leg ladders, we find that the Hubbard model spectrum qualitatively resembles the Heisenberg spectrum—with low-energy peaks resembling spinonic excitations—already at intermediate on-site repulsion as small as $U/t\sim 2–3$, although ratios of peak intensities at different momenta continue evolving with increasing $U/t$ converging only slowly to the Heisenberg limit. We discuss the implications of these results for neutron scattering experiments and we propose criteria to establish the values of $U/t$ of quasi-one-dimensional systems described by one-orbital Hubbard models from experimental information.

We introduce a variational implementation of cluster perturbation theory (CPT) to address the dynamics of spin systems driven out of equilibrium. We benchmark the method with the quantum Ising model subject to a sudden quench of the transverse magnetic field across the transition or within a phase. We treat both the one-dimensional case, for which an exact solution is available, as well the two-dimensional case, for which we have to resort to numerical results. Comparison with exact results shows that the approach provides a quite accurate description of the real-time dynamics up to a characteristic timescale $\tau$ that increases with the size of the cluster used for CPT. In addition, and not surprisingly, $\tau$ is small for quenches across the equilibrium phase transition point, but can be quite larger for quenches within the ordered or disordered phases.

Hydrothermally grown $n$-type ZnO samples, implanted with helium $\left({\text{He}}^{+}\right)$ at a sample temperature of $\sim 40$ K and fluences of $5\times {10}^9$ and $5\times {10}^{10}{\text{cm}}^{-2}$, have been studied in situ by capacitance voltage (CV) and junction spectroscopy measurements. The results are complemented by data from secondary ion mass spectrometry and Fourier transform infrared absorption measurements and first-principles calculations. Removal/passivation of an implantation-induced shallow donor center or alternatively growth of a deep acceptor defect are observed after annealing, monitored via charge carrier concentration $\left(N_d\right)$ versus depth profiles extracted from CV data. Isothermal anneals in the temperature range of 290–325 K were performed to study the evolution in $N_d$, revealing a first-order kinetics with an activation energy, $E_a\approx 0.7\text{eV}$ and frequency factor, $c_0\sim {10}^6{\text{s}}^{-1}$. Two models are discussed in order to explain these annealing results. One relies on transition of oxygen interstitials $\left({\text{O}}_i\right)$ from a split configuration (neutral state) to an octahedral configuration (deep double acceptor state) as a key feature. The other one is based on the migration of Zn interstitials (double donor) and trapping by neutral Zn-vacancy-hydrogen complexes as the core ingredient. In particular, the latter model exhibits good quantitative agreement with the experimental data and gives an activation energy of $\sim 0.75$ eV for the migration of Zn interstitials.

We propose an explicitly solvable model for collinear scattering of photoexcited carriers in intrinsic graphene irradiated by monochromatic light. We find that the collinear scattering rate is directly proportional to the photocarrier energy and derive an analytic expression for the corresponding relaxation time. The result agrees with the recent numerical prediction [Nat. Commun. 7, 11617 (2016)] and is able to describe the photocarrier evolution at low energies, where scattering on optical phonons is strongly suppressed.

Second harmonic generation (SHG) from three-dimensional topological insulators originates from both surface and bulk, which does not allow probing of surface states unless the measurement can separate the two contributions. In this paper, we used combined measurements of transmitted and reflected SHG from epitaxially grown $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ thin films of different thickness on $\mathrm{Ba}{\mathrm{F}}_2$, and a bulk $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ crystal, to deduce surface and bulk nonlinear susceptibilities of $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ separately. We found that the surface contribution to SHG was comparable to that from the bulk of the crystal, but becomes dominant in ultrathin films. In the latter case, contributions from both air/$\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ and $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3/\mathrm{Ba}{\mathrm{F}}_2$ interfaces were significant and exhibited a strong out-of-plane polar ordering. The bulk contribution came mainly from the space charge region (SCR), which was formed by Se vacancies aggregated at the air/$\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ interface; its magnitude can provide an estimate on the field strength in the SCR. Clarification of surface and bulk contributions to SHG can help nonlinear optical techniques be used as a versatile in situ probe for topological insulators.

After deposition of rare-earth elements (Dy, Tb) on Si(111) at elevated temperatures, a formerly unknown $\left(2\sqrt{3}\times \sqrt{3}\right)R{30}^{\circ }$ reconstruction is observed by low-energy electron diffraction, while scanning tunneling microscopy measurements exhibit a $\left(\sqrt{3}\times \sqrt{3}\right)R{30}^{\circ }$ reconstruction. On the basis of density-functional theory calculations, the structure of the larger unit cell is explained by periodically arranged subsurface Si vacancies. The vacancy network in the first subsurface layer has a $\left(\sqrt{3}\times \sqrt{3}\right)R{30}^{\circ }$ periodicity, while strain is released by a $\left(2\sqrt{3}\times \sqrt{3}\right)R{30}^{\circ }$ Si vacancy network in the second subsurface layer. In addition, this vacancy network forms quasi-one-dimensional structures (striped domains) separated by periodically arranged antiphase domain boundaries. The diffraction spot profiles are explained in detail by kinematic diffraction theory calculations, and average domain widths are deduced.

We investigate the collective plasmonic modes in a chain of metallic nanoparticles that are coupled by near-field interactions. The size- and momentum-dependent nonradiative Landau damping and radiative decay rates are calculated analytically within an open quantum system approach. These decay rates determine the excitation propagation along the chain. In particular, the behavior of the radiative decay rate as a function of the plasmon wavelength leads to a transition from an exponential decay of the collective excitation for short distances to an algebraic decay for large distances. Importantly, we show that the exponential decay is of a purely nonradiative origin. Our transparent model enables us to provide analytical expressions for the polarization-dependent plasmon excitation profile along the chain and for the associated propagation length. Our theoretical analysis constitutes an important step in the quest for the optimal conditions for plasmonic propagation in nanoparticle chains.

We present a thorough investigation of the electromagnetic resonant modes supported by systems of polaritonic rods placed at the vertices of canonical polygons. The study is conducted with rigorous finite-element eigenvalue simulations. To provide physical insight, the simulations are complemented with coupled mode theory (the analog of LCAO in molecular and solid state physics) and a lumped wire model capturing the coupling-caused reorganizations of the currents in each rod. The systems of rods, which form all-dielectric cyclic metamolecules, are found to support the unconventional toroidal dipole mode, consisting of the magnetic dipole mode in each rod. Besides the toroidal modes, the spectrally adjacent collective modes are identified. The evolution of all resonant frequencies with rod separation is examined. They are found to oscillate about the single-rod magnetic dipole resonance, a feature attributed to the leaky nature of the constituent modes. Importantly, we observe that ensembles of an odd number of rods produce larger frequency separation between the toroidal mode and its neighbor than the ones with an even number of rods. This increased spectral isolation, along with the low quality factor exhibited by the toroidal mode, favors the coupling of the commonly silent toroidal dipole to the outside world, rendering the proposed structure a prime candidate for controlling the observation of toroidal excitations and their interaction with the usually present electric dipole.

The application of Cartesian multipoles in irreducible representations provides the possibility to get explicit contributions of the toroidal multipole terms in the extinction and scattering power without the introduction of special form factors. In the framework of the Cartesian multipoles, we obtained multipole decomposition (up to the third order) of the induced polarization (current) inside an arbitrarily shaped scatterer (nanoparticle). The third-order decomposition includes the toroidal dipole, magnetic quadrupole, electric octupole terms, and also nonradiating terms. The corresponding multipole decomposition of the scattering cross section, taking into account the electric octupole term, is derived and compared with the multipole decomposition of the extinction cross section obtained using the optical theorem. We show that the role of multipoles in the optical theorem (light extinction) and scattering by arbitrarily shaped nanoparticles can be different. This can result in seemingly paradoxical conclusions with respect to the appearance of multipole contributions in the scattering and extinction cross sections. This fact is especially important for absorptionless nanoparticles, for which the scattering cross section can be calculated using the optical theorem, because in this case extinction is solely determined by scattering. Demonstrative results concerning the role of third-order multipoles in the resonant optical response of high-refractive-index dielectric nanodisks, with and without a through hole at the center, are presented. It is shown that the optical theorem results in a negligible role of the third-order multipoles in the extinction cross sections, whereas these multipoles provide the main contribution in the scattering cross sections.

We study the effects of mass asymmetry on the ground state properties of an electron-hole bilayer (EHBL) system at $T=0$ by using the quantum Monte Carlo method. Particularly, we use the variational Monte Carlo method to calculate the pair-correlation function $g\left(r\right)$ and the condensation fraction $c$ at a fixed density for different values of interplaner distance $d$ and extract the phases of the EHBL system. We use a single trial wave function that can describe fluid, excitonic, and biexcitonic phases. We find that the excitonic fluid phase is stable in the region of $d\ge 0.25\mathrm{a}.\mathrm{u}.$ and a transition from the excitonic fluid phase to the biexcitonic fluid phase at $d=0.24\mathrm{a}.\mathrm{u}.$

The Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between impurity spins is calculated for bilayer graphene in the presence of a layer symmetry-breaking external electric field. We find that for intercalated impurities (i.e., impurity atoms between the two constituent layers of the bilayer) the interaction is extraordinarily sensitive to such a field. In particular, (i) the form of the RKKY interaction may be tuned between oscillatory, ferromagnetic, and antiferromagnetic simply by varying the external field, and (ii) the strength of the RKKY interaction may be increased by an order of magnitude by application of an external field. This sensitivity arises directly from the “Mexican hat” form that the low-energy spectrum takes in an applied field. These finding suggest that heterostructures of intercalated magnetic atoms in bilayer graphene may represent a possible system for electrical control over magnetic structure.

The energy spectrum of a tight-binding Hamiltonian is studied for the two-dimensional quasiperiodic Rauzy tiling in a perpendicular magnetic field. This spectrum known as a Hofstadter butterfly displays a very rich pattern of bulk gaps that are labeled by four integers, instead of two for periodic systems. The role of phason-flip disorder is also investigated in order to extract genuinely quasiperiodic properties. This geometric disorder is found to only preserve main quantum Hall gaps.

We derive exact analytical formulas for the radiative heat transfer between parallel slabs separated by vacuum and subject to arbitrary temperature profiles. We show that, depending on the derivatives of the temperature at points close to the slab-vacuum interfaces, the flux can exhibit one of several different asymptotic low-distance $\left(d\right)$ behaviors, obeying either $1/d^2,1/d$, or logarithmic power laws, or approaching a constant. Tailoring the temperature profile within the slabs could enable unprecedented tunability over heat exchange, leading for instance to sign-flip transitions (where the flux reverses sign) at tunable distances. Our results are relevant to the theoretical description of on-going experiments measuring near-field heat transfer at nanometric distances, where the coupling between radiative and conductive transfer could result in temperature gradients.

An extension of the layer-multiple-scattering method to phononic crystals of poroelastic spheres immersed in a fluid medium is developed. The applicability of the method is demonstrated on specific examples of close-packed fcc crystals of submerged water-saturated meso- and macroporous silica microspheres. It is shown that, by varying the pore size and/or the porosity, the transmission, reflection, and absorption spectra of finite slabs of these crystals are significantly altered. Strong absorption, driven by the slow waves in the poroelastic material and enhanced by multiple scattering, leads to negligible transmittance over an extended frequency range, which might be useful for practical applications in broadband acoustic shielding. The results are analyzed by reference to relevant phononic dispersion diagrams in the viscous and inertial coupling limits, and a consistent interpretation of the underlying physics is provided.

We studied the nonlocal transport behavior of both electrically and thermally excited magnons in yttrium iron garnet (YIG) as a function of its thickness. For electrically injected magnons, the nonlocal signals decrease monotonically as the YIG thickness increases. For the nonlocal behavior of the thermally generated magnons, or the nonlocal spin Seebeck effect (SSE), we observed a sign reversal which occurs at a certain heater-detector distance, and it is influenced by both the opacity of the YIG/heater interface and the YIG thickness. Our nonlocal SSE results can be qualitatively explained by the bulk-driven SSE mechanism together with the magnon diffusion model. Using a two-dimensional finite element model (2D-FEM), we estimated the bulk spin Seebeck coefficient of YIG at room temperature. The quantitative disagreement between the experimental and modeled results indicates more complex processes going on in addition to magnon diffusion and relaxation, especially close to the contacts.

Domain wall dynamics in a perpendicularly magnetized system is studied by means of micromagnetic simulations in which disorder is introduced as a dispersion of both the easy-axis orientation and the anisotropy constant over regions reproducing a granular structure of the material. High field dynamics show a linear velocity-field relationship and an additional grain size dependent velocity shift, weakly dependent on both applied field and intrinsic Gilbert's damping parameter. We find the origin of this velocity shift in the nonhomogeneous in-plane effective field generated by the tilting of anisotropy easy axis introduced by disorder. We show that a one-dimensional analytical approach cannot predict the observed velocities and we augment it with the additional dissipation of energy arising from internal domain wall dynamics triggered by disorder. This way we prove that the main cause of higher velocity is the ability of the domain wall to irradiate energy into the domains, acquired with a precise feature of disorder.

Ferromagnets are believed to exhibit strongly anisotropic spin relaxation, with relaxation lengths for spin longitudinal to the magnetization significantly longer than those for spin transverse to the magnetization. Here, we characterize the anisotropy of spin relaxation in Co using the spin pumping contribution to Gilbert damping in noncollinearly magnetized ${\mathrm{Py}}_{1-x}{\mathrm{Cu}}_x$/Cu/Co trilayer structures. The static magnetization angle between ${\mathrm{Py}}_{1-x}{\mathrm{Cu}}_x$ and Co, adjusted under field bias perpendicular to film planes, controls the projections of longitudinal and transverse spin current pumped from ${\mathrm{Py}}_{1-x}{\mathrm{Cu}}_x$ into Co. We find nearly isotropic absorption of pure spin current in Co using this technique; fits to a diffusive transport model yield the longitudinal spin relaxation length $<2$ nm in Co. The longitudinal spin relaxation lengths found are an order of magnitude smaller than those determined by current-perpendicular-to-planes giant magnetoresistance measurements, but comparable with transverse spin relaxation lengths in Co determined by spin pumping.

A theoretical formulation of magnetoresistance effect in a metallic ferromagnetic/nonmagnetic bilayer originated from the charge-spin conversion by the anomalous Hall effect is presented. Analytical expressions of the longitudinal and transverse resistivities in both nonmagnet and ferromagnet are obtained by solving the spin diffusion equation. The magnetoresistance generated from charge-spin conversion purely caused by the anomalous Hall effect in the ferromagnet is found to be proportional to the square of the spin polarizations in the ferromagnet and has fixed sign. We also find additional magnetoresistances in both nonmagnet and ferromagnet arising from the mixing of the spin Hall and anomalous Hall effects. The sign of this mixing resistance depends on those of the spin Hall angle in the nonmagnet and the spin polarizations of the ferromagnet.