Using our recently proposed Bethe-Salpeter $G_0{W}_0$ formulation, we explore the optical absorption spectra of fullerene (${\mathrm{C}}_{60}$) near coinage metal surfaces (Cu, Ag, and Au). We pay special attention to how the surface plasmon ${\omega }_S$ influences the optical activity of fullerene. We find that the lower-energy fullerene excitons at $3.77$ and $4.8$ eV only weakly interact with the surface plasmon. However, we find that the surface plasmon strongly interacts with the most intense fullerene $\pi$ exciton, i.e., the dipolar mode at $\hslash {\omega }_{+}\approx 6.5$ eV, and the quadrupolar mode at $\hslash {\omega }_{-}\approx 6.8$ eV. When fullerene is close to a copper surface ($z_0\approx 5.3$ Å), the dipolar mode ${\omega }_{+}$ and “localized” surface plasmons in the molecule/surface interface hybridize to form two coupled modes which both absorb light. As a result, the molecule gains an additional optically active mode. Moreover, in resonance, when ${\omega }_S\approx {\omega }_{\pm }$, the strong interaction with the surface plasmon destroys the ${\omega }_{-}$ quadrupolar character and it becomes an optically active mode. In this case, the molecule gains two additional very intense optically active modes. Further, we find that this resonance condition, ${\omega }_S\approx {\omega }_{\pm }$, is satisfied by silver and gold metal surfaces.

The fascinating optical properties of metamaterials and metasurfaces are intrinsically wave-vector (k) dependent and spatial dispersion effects induce a complex optical response. Here, Mueller matrix spectroscopic ellipsometry, providing both amplitude and phase information in the visible, is used in a large frequency and k-space range to characterize a plasmonic meander and assign the polarization effects to the microscopic plasmonic excitations of a metasurface. This leads to a fundamental physical insight into the optical properties of the plasmonic meanders: the effect of closed-film resonant coupling is used for large polarization rotation and high transmission, and multiple optical functions are created within one compact design, which cannot be obtained by any natural crystal.

We present a computational study on the topology, energetics, and structural deformations for a large number of experimentally observed defect configurations in graphene. We find that both the number of lost hexagonal carbon rings and introduced nonhexagonal rings increase linearly as a function of the vacancy order (number of missing atoms). The formation energies of the defects increase by about 2.2 eV per missing atom after an initial offset, establishing these defects as the lowest energy vacancy configurations studied in graphene to date. In addition, we find that even small point defects, which have been until now assumed flat, cause graphene to bend out of plane when not restricted into prohibitively confined geometries. This effect reaches to relative long distances even for some of the smallest defects, significantly reducing the stress otherwise imposed on the surrounding lattice.

High-throughput density functional calculations of solids are highly time-consuming. As an alternative, we propose a machine learning approach for the fast prediction of solid-state properties. To achieve this, local spin-density approximation calculations are used as a training set. We focus on predicting the value of the density of electronic states at the Fermi energy. We find that conventional representations of the input data, such as the Coulomb matrix, are not suitable for the training of learning machines in the case of periodic solids. We propose a novel crystal structure representation for which learning and competitive prediction accuracies become possible within an unrestricted class of $spd$ systems of arbitrary unit-cell size.

We investigate to which extent a many-body Bloch-Redfield master-equation description of quantum transport is consistent with the exact generalized equilibrium conditions known as exchange fluctuation theorems. Thereby, we identify a class of master equations for which this is the case. Beyond this class, we find deviations which exhibit characteristic scaling laws as functions of the dot-lead tunneling, the interdot tunneling, and the temperature. These deviations are accompanied by an increase of lead energy fluctuations inherent in the Bloch-Redfield equation beyond a rotating-wave approximation. We illustrate our results with numerical data for a double quantum dot attached to four leads.

We demonstrate the occurrence of compensated spin configurations in Fe clusters and monolayers on Ru(0001) and Rh(111) by a combination of x-ray magnetic circular dichroism experiments, first-principles calculations, and Monte Carlo simulations. Our results reveal complex intracluster exchange interactions which depend strongly on the substrate $4d$-band filling, the cluster geometry, as well as lateral and vertical structural relaxations. The importance of substrate $4d$-band filling manifests itself also in small nearest-neighbor exchange interactions in Fe dimers and in a nearly inverted trend of the Ruderman-Kittel-Kasuya-Yosida coupling constants for Fe adatoms on the Ru and Rh surface.

We use first-principles calculations based on density functional theory to investigate the magnetic exchange interaction of Fe clusters on Rh(111) and Ru(0001). We consider dimers, trimers, tetramers, and pentamers of different shape in fcc and hcp stacking as well as infinite atomic and biatomic chains. From the dimer calculations, we extract the exchange interaction as a function of adatom distance by mapping total energies to a Heisenberg model. The nearest-neighbor (NN) exchange constant is about one order of magnitude smaller than reported for other substrates due to the strong hybridization between the Fe atoms and the partly filled $4d$ band of the surface. We also find a transition from a ferromagnetic NN exchange interaction for Fe dimers on Rh(111) to an antiferromagnetic one on Ru(0001). The distance-dependent exchange coupling displays a RKKY-like oscillatory behavior, which is nearly inverted for Fe dimers on the Rh(111) surface compared to those on Ru(0001). Unexpectedly, for Fe clusters beyond dimers, a complex trend of the magnetic ground state is observed which alternates between ferro- and antiferromagnetic configurations depending on cluster size and shape. In view of the exchange constants obtained for dimers, it is surprising that on both surfaces small compact clusters are ferromagnetic while open structures such as linear trimers or tetramers become antiferromagnetic. We demonstrate that both vertical and lateral structural relaxations of the clusters are crucial in order to understand this unexpected trend of magnetic order and connected to the competition of direct ferromagnetic exchange among Fe atoms in the cluster and the hybridization with the substrate.

Skutterudites are considered as interesting material for thermoelectric applications. Filling foreign atoms into the cagelike structure of a ${\mathrm{CoSb}}_3$ skutterudite is beneficial to its thermoelectric properties by increasing phonon scattering while maintaining the electrical conductivity. In this paper we demonstrate the generation and detection of coherent acoustic phonons in thin films of ${\mathrm{CoSb}}_3$ and partially filled ${\mathrm{Yb}}_x{\mathrm{Co}}_4{\mathrm{Sb}}_{12}$ skutterudites using femtosecond pump-probe spectroscopy. By using a pulse-echo method, the longitudinal sound velocity of amorphous and polycrystalline ${\mathrm{CoSb}}_3$ thin films is obtained. For partially filled ${\mathrm{Yb}}_x{\mathrm{Co}}_4{\mathrm{Sb}}_{12}$ thin films, an obvious decrease of the longitudinal sound velocity is observed at high filling fraction. Concomitantly, the high frequency acoustic phonon modes are strongly damped as the $\mathrm{Yb}$ filling fraction increases, which gives direct evidence for acoustic phonon scattering processes. It is shown that the reduction of lattice thermal conductivity after Yb filling is mainly achieved by the strong scattering of acoustic phonons.

With pulsed laser deposition, we have grown $c$ axis oriented thin films of the nominal composition ${\text{LaMnO}}_3$ (LMO) on LSAT(001) substrates. We find that, depending on the oxygen background pressure during growth, the LMO films contain sizeable amounts of La and/or Mn vacancies that strongly influence their electronic and magnetic properties. Specifically, we show that the Mn/La ratio can be systematically varied from 0.92 at 0.11 mbar to 1.09 at 0.30 mbar of oxygen. The cationic vacancies have markedly different effects that become most pronounced once the samples are fully oxygenated and thus strongly hole doped. All as-grown and thus slightly oxygen-deficient LMO films are ferromagnetic insulators with saturation moments in excess of 2.5 ${\mu }_{\mathrm{B}}$ per Mn ion, their transport and optical properties can be understood in terms of trapped ferromagnetic polarons. Upon oxygen annealing, the most La-deficient films develop a metallic response with an even larger ferromagnetic saturation moment of 3.8 ${\mu }_{\mathrm{B}}$ per Mn ion. In contrast, in the oxygenated Mn-deficient films, the ferromagnetic order is strongly suppressed to less than 0.5 ${\mu }_{\mathrm{B}}$ per Mn ion, and the transport remains insulatorlike. We compare our results with the ones that were previously obtained on bulk samples and present an interpretation in terms of the much stronger disruption of the electronic and magnetic structure by the Mn vacancies as compared to the La vacancies. We also discuss the implications for the growth of LMO thin films with well-defined physical properties that are a prerequisite for the study of interface effects in multilayers.

The ${\text{YCo}}_3$${\text{B}}_2$ compound is studied by polarized neutron powder diffraction in search of magnetic order. It is found that ${\text{YCo}}_3$${\text{B}}_2$ remains paramagnetic down to 10 K. Neutron powder diffraction study of the induced transition in the Van Vleck paramagnet ${\text{TbCo}}_3$${\text{B}}_2$ revealed a significant contribution of magnetic diffuse scattering at small angles and near the (100) reflection. Analysis of these results leads to a revised magnetic structure of ${\text{TbCo}}_3$${\text{B}}_2$, in which the Tb sublattice orders ferromagnetically at 31 K, with no Co ordering. Also, short-range magnetic order of the Tb sublattice, extending into the paramagnetic region, was found. The temperature evolution of this short-range order's correlation length, in the paramagnetic temperatures region, shows a significant anomaly unique to induced transition magnetic ordering.

Experimental signatures of Majorana zero modes in a single superconducting quantum wire with spin-orbit coupling have been reported as zero-bias peaks in tunneling spectroscopy. We study whether these zero modes can persist in an array of coupled wires, and if not, what their remnant might be. The bulk exhibits topologically distinct gapped phases and an intervening gapless phase. Even though the bulk pairing structure is topological, the interaction between Majorana zero modes and superfluid phases leads to spontaneous time-reversal symmetry breaking. Consequently, edge supercurrent loops emerge and edge Majorana fermions are in general gapped out except when the number of chains is odd, in which case one Majorana fermion survives.

We study the phase diagram of a dipolar Fermi gas at half-filling in a cubic optical lattice with dipole moments aligned along the $z$ axis. The anisotropic dipole-dipole interaction leads to the competition between $p_z$-wave superfluid and nematic charge-density-wave (CDW) orders at low temperatures. We find that the superfluid phase survives with weak interactions and the CDW phase dominates with strong interactions. In-between, the supersolid phase appears as a balance between superfluid and CDW orders. The superfluid density is anisotropic in the supersolid and superfluid phases. In the CDW phase, there is a semimetal-to-insulator transition with increase of the interaction strength. Experimental implications are discussed.

The interaction behaviors of solitons are defining characteristics of these nonlinear, coherent structures. Due to recent experimental observations, thin ferromagnetic films offer a promising medium in which to study the scattering properties of two-dimensional magnetic droplet solitons, particle-like, precessing dipoles. Here, a rich set of two-droplet interaction behaviors are classified through micromagnetic simulations. Repulsive and attractive interaction dynamics are generically determined by the relative phase and speeds of the two droplets and can be classified into four types: (1) merger into a breather bound state, (2) counterpropagation trapped along the axis of symmetry, (3) reflection, and (4) violent droplet annihilation into spin wave radiation and a breather. Utilizing a nonlinear method of images, it is demonstrated that these dynamics describe repulsive/attractive scattering of a single droplet off of a magnetic boundary with pinned/free spin boundary conditions, respectively. These results explain the mechanism by which propagating and stationary droplets can be stabilized in a confined ferromagnet.

Chains of magnetic impurities placed on a superconducting substrate and forming helical spin order provide a promising venue for realizing a topological superconducting phase. An effective tight-binding description of such helical Shiba chains involves long-range (power-law) hopping and pairing amplitudes which induce an unconventional topological critical point. At the critical point, we find exponentially localized Majorana bound states with a short localization length unrelated to a topological gap. Away from the critical point, this exponential decay develops a power-law tail. Our analytical results have encouraging implications for experiment.

From strain-rate- and temperature-dependent deformation studies on nanocrystalline PdAu alloys with grain sizes $\le$10 nm, the shear activation volume (6 ${\text{b}}^3$), strain-rate sensitivity ($0.03$), as well as the Helmholtz (0.9 eV) and Gibbs free energy of activation ($\Delta G=0.2$ eV) have been extracted. The close similarity to values found for metallic glasses indicates that grain boundary mediated shear shuffling dominates plasticity at the low end of the nanoscale. More fundamentally, we find that the energy barrier height exhibits universal scaling behavior $\Delta G\propto \Delta {\tau }^{3/2}$, where $\Delta \tau$ is a residual load, giving rise to a generalization of the Johnson-Samwer $T^{2/3}$ scaling law of yielding in metallic glasses.

For spintronics application, which requires fast field switching, it is important to have a kind of soft magnetic material with large damping coefficient. Here, we present the studies of the Nd dopant-level-dependent damping coefficient of Nd${}_x$-Py${}_{(1-x)}$ thin films (30 nm) in a dilute region utilizing ferromagnetic resonance (FMR). With the Nd content increasing, the film structure was found to be changing from polycrystalline to amorphous when the Nd content is around 3.4%. Meanwhile, the magnetization decreases linearly. Interestingly, we find that along the easy axis, both low coercivity and high hysteresis squareness are simultaneously maintained in the system; i.e., the magnetic softness has been well kept. By theoretical fitting of the angular dependence of the FMR field, the first- and second-order magnetic anisotropy constants, $K_1$ and $K_2$, and the Lande $g$ factor are obtained and discussed quantitatively. The measurements of angular and frequency dependence of the ferromagnetic resonance linewidth, as well as the theoretical fitting by considering the contributions of Gilbert damping, two-magnon scattering, and inhomogeneous broadening, show that the damping coefficient $\alpha$ increases rapidly (about 25-fold) as the Nd content increases to 11.6%, which is mainly due to the enhanced spin-orbit coupling by the Nd additives, supported by x-ray magnetic circular dichroism measurements.

We calculate the effects of the spin-lattice coupling on the magnon spectrum of thin ferromagnetic films consisting of the magnetic insulator yttrium iron garnet. The magnon-phonon hybridization generates a characteristic minimum in the spin dynamic structure factor which quantitatively agrees with recent Brillouin light scattering experiments. We also show that at room temperature the phonon contribution to the magnon damping exhibits a rather complicated momentum dependence: In the exchange regime the magnon damping is dominated by Cherenkov type scattering processes, while in the long-wavelength dipolar regime these processes are subdominant and the magnon damping is two orders of magnitude smaller. We supplement our calculations by actual measurements of the magnon relaxation in the dipolar regime. Our theory provides a simple explanation of a recent experiment probing the different temperatures of the magnon and phonon gases in yttrium iron garnet.

In rare-earth ($\mathit{R}$) ferroborates, R${\mathrm{Fe}}_3$(${\mathrm{BO}}_3$)${}_4$ with R = Eu, Gd, and Tb, the magnetoelectric (ME) responses appear to stem from both the antiferromagnetic order of the iron (Fe) spins and the magnetic moments on the $\mathit{R}$ ions. We measured the electric polarization ($P$) along the $a$ axis while rotating a magnetic field ($H$) around the $a$ axis and found that the target compounds show mutually distinctive $H$-direction dependencies. ${\mathrm{EuFe}}_3$(${\mathrm{BO}}_3$)${}_4$ (R = Eu) shows an almost constant spontaneous $P$ with a slight modulation when $H$ is slanted from the $c$ axis. The $H$-angle (${\theta }_H$) dependence of the $P$ can be described by a formula $P=P_0-\Lambda {sin}^2{\theta }_H$. As for ${\mathrm{GdFe}}_3$(${\mathrm{BO}}_3$)${}_4$ and ${\mathrm{TbFe}}_3$(${\mathrm{BO}}_3$)${}_4$, they show highly anisotropic ${\theta }_H$ dependence of $P$, which characterizes the respective ME responses from their $\mathit{R}$ magnetic moments. In certain regions of ${\theta }_H$, the $P$ can be described by $P=P_0-Ksin2{\theta }_H$ and $P=P_0\mp \Gamma sin{\theta }_H$ for R = Gd and Tb, respectively. We devised a theory for the ME response of the individual magnetic ions in a R${\mathrm{Fe}}_3$(${\mathrm{BO}}_3$)${}_4$ crystal and applied it to these compounds focusing on their local symmetry and their ground-state multiplet structures. The above formulas successfully reproduce the observed results as the summation of $P$ from each magnetic subsystem, which in turn enables us to assign the first and second terms to the spontaneous $P$ due to a collinear antiferromagnetic ordering of the Fe spins and the ME response of the $\mathit{R}$ ion under $H$, respectively. The thermal and $H$-induced evolutions of the magnetic-ion resolved $P$ quantitatively agree with the theoretical predictions, ensuring the relevant microscopic ME mechanism for each magnetic ion. The measurement of angular dependence of $P$ is particularly useful to decompose the overlapped ME responses into the respective origins in the system with multiple magnetic subsystems.

The dynamics of free carriers in polycrystalline ${\mathrm{CuIn}}_{1-x}$${\mathrm{Ga}}_x$${\mathrm{Se}}_2$ (CIGS) thin films were studied using picosecond time-resolved photoluminescence (PL) and femtosecond transient-absorption (TA) measurements. The PL spectrum and the TA decay component due to the band-to-band recombination of free carriers were observed in the picosecond time region. From excitation-photon-energy-dependent TA measurements, we identified a slow intraband relaxation of free carriers in the CIGS thin films. Collectively, the combination of PL and TA experiments reveal a global feature of energy relaxation and recombination processes of free carriers in the femtosecond to nanosecond time regions.