We study the effects of quenched charge disorder on the phase reconstruction near itinerant ferromagnetic quantum critical points in three spatial dimensions. Combining a Replica-disorder average with a fermionic version of the quantum order-by-disorder mechanism, we show that weak disorder destabilizes the ferromagnetic state and enhances the susceptibility towards incommensurate, spiral magnetic ordering. The Goldstone modes of the spiral phase are governed by a $3d$-$XY$ model. The induced disorder in the pitch of the spiral generates a random anisotropy for the Goldstone modes, inducing vortex lines in the phase of the helical order and rendering the magnetic correlations short ranged with a strongly anisotropic correlation length.

We investigate magnetic ordering in metallic Ba(Fe${}_{1-x}$Mn${}_x$)${}_2$As${}_2$ and discuss the unusual magnetic phase, which was recently discovered for Mn concentrations $x>10$%. We argue that it can be understood as a Griffiths-type phase that forms above the quantum critical point associated with the suppression of the stripe-antiferromagnetic spin-density-wave (SDW) order in BaFe${}_2$As${}_2$ by the randomly introduced localized Mn moments acting as strong magnetic impurities. While the SDW transition at $x=0$, 2.5$\%$, and 5$\%$ remains equally sharp, in the $x=12$$\%$ sample we observe an abrupt smearing of the antiferromagnetic transition in temperature and a considerable suppression of the spin gap in the magnetic excitation spectrum. According to our muon-spin-relaxation, nuclear magnetic resonance and neutron-scattering data, antiferromagnetically ordered rare regions start forming in the $x=12$$\%$ sample significantly above the Néel temperature of the parent compound. Upon cooling, their volume grows continuously, leading to an increase in the magnetic Bragg intensity and to the gradual opening of a partial spin gap in the magnetic excitation spectrum. Using neutron Larmor diffraction, we also demonstrate that the magnetically ordered volume is characterized by a finite orthorhombic distortion, which could not be resolved in previous diffraction studies most probably due to its coexistence with the tetragonal phase and a microstrain-induced broadening of the Bragg reflections. We argue that Ba(Fe${}_{1-x}$Mn${}_x$)${}_2$As${}_2$ could represent an interesting model spin-glass system, in which localized magnetic moments are randomly embedded into a SDW metal with Fermi surface nesting.

Based on the recently introduced homogenization theory developed by Alù [Phys. Rev. B 84, 075153 (2011)], we propose a generalized retrieval method that allows extracting physically meaningful bulk effective parameters from conventional scattering measurements of periodic metamaterial samples composed of subwavelength inclusions. We show that, compared to conventional approaches, our method is able to capture the anomalous physics in the wave interaction with resonant metamaterials and return physically meaningful homogenized parameters that retain local properties in the long-wavelength limit. As a byproduct, we are also able to retrieve the polarizabilities of the constituent inclusions, which are shown to satisfy expected dispersion properties for passive inclusions, in contrast with conventional retrieval approaches.

We study Landau-Zener dynamics in a double quantum dot filled with two electrons, where the spin states can become correlated with charge states and the level velocity can be tuned in a time-dependent fashion. We show that a correct interpretation of experimental data is only possible when finite-time effects are taken into account. In addition, our formalism allows the study of partial adiabatic dynamics in the presence of phonon-mediated hyperfine relaxation and charge-noise-induced dephasing. Our findings demonstrate that charge noise severely impacts the visibility of Landau-Zener-Stückelberg-Majorana interference fringes. This indicates that charge coherence must be treated on an equal footing with spin coherence.

We propose the use of nanostructured photonic nanocavities made of ${\chi }^{\left(2\right)}$ nonlinear materials as prospective passive devices to generate strongly sub-Poissonian light via single-photon blockade of an input coherent field. The simplest scheme is based on the requirement that the nanocavity be doubly resonant, i.e., possess cavity modes with good spatial overlap at both the fundamental and second-harmonic frequencies. We discuss the feasibility of this scheme with state-of-the art nanofabrication technology and the possibility to use it as a passive single-photon source on demand.

The scaling of island and monomer density, capture zone distributions (CZDs), and island size distributions (ISDs) in reversible submonolayer growth was studied using the Clarke-Vvedensky model. An approach based on rate-equation results for irreversible aggregation (IA) models is extended to predict several scaling regimes in square and triangular lattices, in agreement with simulation results. Consistently with previous works, a regime I with fractal islands is observed at low temperatures, corresponding to IA with critical island size $i=1$, and a crossover to a second regime appears as the temperature is increased to $\epsilon R^{2/3}\sim 1$, where $\epsilon$ is the single bond detachment probability and $R$ is the diffusion-to-deposition ratio. In the square (triangular) lattice, a regime with scaling similar to IA with $i=3$ ($i=2$) is observed after that crossover. In the triangular lattice, a subsequent crossover to an IA regime with $i=3$ is observed, which is explained by the recurrence properties of random walks in two-dimensional lattices, which is beyond the mean-field approaches. At high temperatures, a crossover to a fully reversible regime is observed, characterized by a large density of small islands, a small density of very large islands, and total island and monomer densities increasing with temperature, in contrast to IA models. CZDs and ISDs with Gaussian right tails appear in all regimes for $R\sim {10}^7$ or larger, including the fully reversible regime, where the CZDs are bimodal. This shows that the Pimpinelli-Einstein approach for IA explains the main mechanisms for the large islands to compete for free adatom aggregation in the reversible model, and may be the reason for its successful application to a variety of materials and growth conditions.

The electronic and magnetic properties of bulk Cr tips for scanning tunneling microscopy have been investigated by means of density functional theory (DFT) calculations and scanning tunneling spectroscopy (STS) measurements. Spin-resolved densities of states (DOS) were calculated for model tips, i.e., Cr adatoms and clusters on ideal Cr surfaces. STS measurements on Au(111) and Si(111)-7$\times$7 have been interpreted by modeling the tunneling process in a Wentzel-Kramers-Brillouin approximation in order to ascertain the role of tip DOS features. Calculated spin-resolved electronic properties of Cr tips have been used to revisit spin-polarized (SP) STS measurements from the literature. The agreement between experimental findings and DFT calculations confirms the relevance of the knowledge of tip electronic properties and of an appropriate model for the tunneling process in the interpretation of STS and SP-STS data.

In previous publications, qualitative agreement between studies of surface plasmon excitations in nanoparticles by near-field light scattering and electron energy-loss spectroscopy (EELS) has been found for experiments and simulations. Here, we present a quantitative method for the comparison of light scattering and EELS for surface plasmons in metal spheres. Defining the Fourier transform of the modal component of the scattered electric field along the equivalent electron trajectory enables a direct evaluation of the relative weighting factor for light- and electron-excited surface plasmon modes. This common quantity for light scattering and EELS is examined for size, composition, and trajectory dependencies, facilitating the analysis of key differences between light and electron excitation. A single functional dependence on Drude model plasmon energies is identified to explain the relative modal weighting factors for light scattering and EELS. This method represents an important step toward the complete spectral and spatial reconstruction of EELS maps from near-field light scattering calculations.

We present full quantum-mechanical calculations of the hybridized plasmon modes of two nanowires at small separation, providing real-space visualization of the modes in the transition from the classical to the quantum tunneling regime. The plasmon modes are obtained as certain eigenfunctions of the dynamical dielectric function, which is computed using time-dependent density functional theory (TDDFT). For freestanding wires, the energy of both surface and bulk plasmon modes deviate from the classical result for low wire radii and high momentum transfer due to effects of electron spill-out, nonlocal response, and coupling to single-particle transitions. For the wire dimer, the shape of the hybridized plasmon modes are continuously altered with decreasing separation, and below 6 Å, the energy dispersion of the modes deviate from classical results due to the onset of weak tunneling. Below 2–3 Å separation, this mode is replaced by a charge-transfer plasmon, which blue shifts with decreasing separation in agreement with experiment and marks the onset of the strong tunneling regime.

Quantum conductance calculations on the mechanically deformed monolayers of MoS${}_2$ and WS${}_2$ were performed using the nonequlibrium Green's functions method combined with the Landauer-Büttiker approach for ballistic transport together with the density-functional-based tight binding method. Tensile strain causes significant changes in the electronic structure of transition-metal dichalcogenide single layers and eventually the semiconductor-metal transition occurs for elongations as large as 11$\%$ for the 2D-isotropic deformations in the hexagonal structure. This transition enhances the electron transport in otherwise semiconducting materials.

We demonstrate that flat bands with local Berry curvature arise naturally in chiral ($\mathit{ABC}$) multilayer graphene placed on a boron nitride (BN) substrate. The degree of flatness can be tuned by varying the number of graphene layers $N$. For $N=7$ the bands become nearly flat, with a small bandwidth $\sim 3.6$ meV. The two nearly flat bands coming from the $K$ and $K^{\prime }$ valleys cross along lines in the reduced zone. Weak intervalley tunneling turns the band crossing into an avoided crossing, producing two nearly flat bands with global Chern number zero, but with local Berry curvature. The flatness of the bands suggests that many body effects will dominate the physics, while the local Berry curvature of the bands endows the system with a nontrivial quantum geometry. The quantum geometry effects manifest themselves through the quantum distance (Fubini-Study) metric, rather than the more conventional Chern number. Multilayer graphene on BN thus provides a platform for investigating the effect of interactions in a system with a nontrivial quantum distance metric, without the complication of nonzero Chern numbers. We note in passing that flat bands with nonzero Chern number can also be realized by making use of magnetic adatoms, and explicitly breaking time reversal symmetry.

We investigate charge, valley, and spin transports in a normal/ferromagnetic/normal silicene junction. We show that the charge, valley, and spin conductances in this junction oscillate with the length of the ferromagnetic silicene. It is also found that the current through this junction is valley and spin polarized due to the coupling between valley and spin degrees of freedom, and the valley and spin polarizations can be tuned by local application of a gate voltage. In particular, we find a fully valley and spin polarized current by applying the electric field. We also obtain the condition for observing the fully valley and spin polarized current.

Graphene, a two-dimensional material with a high mobility and a tunable conductivity, is uniquely suited for plasmonics. The frequency dispersion of plasmons in bulk graphene has been studied both theoretically and experimentally, but no theoretical models have been reported and tested against experiments for confined plasmon modes in graphene microstructures. In this Rapid Communication, we present measurements as well as analytical and computational models for such confined modes. We show that plasmon modes can be described by an eigenvalue equation. We compare the experiments with the theory for plasmon modes in arrays of graphene strips and demonstrate a good agreement. This comparison also reveals the important role played by interaction among the plasmon modes of neighboring graphene structures.

We revisit the problem of the spin Drude weight $D$ of the integrable spin-$1/2$ XXZ chain using two complementary approaches, exact diagonalization (ED) and the time-dependent density-matrix renormalization group (tDMRG). We pursue two main goals. First, we present extensive results for the temperature dependence of $D$. By exploiting time translation invariance within tDMRG, one can extract $D$ for significantly lower temperatures than in previous tDMRG studies. Second, we discuss the numerical quality of the tDMRG data and elaborate on details of the finite-size scaling of the ED results, comparing calculations carried out in the canonical and grand-canonical ensembles. Furthermore, we analyze the behavior of the Drude weight as the point with SU(2)-symmetric exchange is approached and discuss the relative contribution of the Drude weight to the sum rule as a function of temperature.

We present a combined experimental and theoretical study of two-photon emission from the biexciton cascade in single GaN quantum dots. By changing the biexciton binding energy, pump power, and temperature, the balance between the one- and two-photon decay processes is controlled in this four-level system, which drastically affects the photon statistics of the resulting emission. As the most pronounced feature of this interplay we observe a bunching phenomenon and a transition from sub- to super-Poissonian photon statistics, originating from the complex nature of the biexciton cascade. This work highlights how photon statistics can be steered between one- and two-photon processes towards an increased, bunched two-photon emission probability up to 50 K with the perspective for efficient photon pair generation in the UV spectral range.

Quantum oscillations of work function and film stability as a function of the film thickness in Pb thin films on Si(111) are measured directly using scanning tunneling microscopy and spectroscopy in order to determine their phase relationship. The comparison of the phase relationship in quantum oscillations (surface energy vs work function) reveals a complete surprise: In contrast to a theoretically predicted quarter wavelength phase shift in the phase relationship, we found that their quantum oscillations have identical phase for this particular system. A conjecture to resolve this contradiction is also provided.

Helium-atom scattering angular distributions from Bi(111) show a number of selective-adsorption resonance features corresponding to three bound states of the He atom in the surface-averaged Bi(111) potential. They are well represented by a 3-9 potential with a potential depth of $8.3\text{meV}$. The bound-state resonance enhancement of inelastic scattering is shown to provide the mechanism for the observation of subsurface optical phonons and for their comparatively large intensity.

Using molecular dynamics simulations, we study the effects induced by the impact of Bi${}_n$ ($n\le 5$) clusters with energies in the range of a $3–20$ keV/atom into a Ge target. The target consists of Ge including a 10$\%$ contribution of randomly distributed Bi atoms. The impacting polyatomic clusters create a long-lived melt pool at the surface. After resolidification, the surface shows a characteristic meniscuslike depression; it is caused both by the missing sputtered atoms and by the volume change of Ge upon melting and amorphization. During cooling, Bi is driven towards the center of the melt pool. A large precipitate forms at the surface (for polyatomic impact) or in the center of the molten pocket (for monatomic impact). The remainder of the resolidified amorphous zone is purified from Bi. We argue that the reason for the demixing of Bi and Ge is the thermophoretic or Soret effect which is caused by the different diffusivities of Bi and Ge in the melt. Bi is sputtered preferentially from the sample. The extent of surface modification, the amount of Bi collected on the surface, the concentration of Bi, and structure of Bi precipitates in the former melt pool are analyzed in their dependence on projectile size, impact energy, and direction.

We propose a tensor network algorithm for the efficient sampling of quantum pure states belonging to a generalized microcanonical ensemble. The algorithm consists in an adaptation of the power method to a recently introduced ensemble of random matrix product states. The microcanonical ensemble that we consider is characterized by the fact that the participating energy eigenstates are not required to have identical statistical weight. To test the method we apply it to the Heisenberg model with an external magnetic field, and we find that the magnetization curves, due to the microcanonical constraint, are qualitatively different from those obtained in the canonical ensemble. Possible future applications include the study of isolated quantum systems evolving after a quantum quench.

A modeling approach for standing spin-wave resonances based on a finite-difference formulation of the Landau-Lifshitz-Gilbert equation is presented. In contrast to a previous study [C. Bihler et al., Phys. Rev. B 79, 045205 (2009)], this formalism accounts for elliptical magnetization precession and magnetic properties arbitrarily varying across the layer thickness, including the magnetic anisotropy parameters, the exchange stiffness, the Gilbert damping, and the saturation magnetization. To demonstrate the usefulness of our modeling approach, we experimentally study a set of (Ga,Mn)As samples grown by low-temperature molecular-beam epitaxy by means of angle-dependent standing spin-wave resonance spectroscopy and electrochemical capacitance-voltage measurements. By applying our modeling approach, the angle dependence of the spin-wave resonance data can be reproduced in a simulation with one set of simulation parameters for all external field orientations. We find that the approximately linear gradient in the out-of-plane magnetic anisotropy is related to a linear gradient in the hole concentrations of the samples.