This article investigates the equilibrium states of square-planar arrays of magnetic dipoles. It has been demonstrated that in the presence of an exchange interaction the main equilibrium states are the configurations of dipoles oriented along the system diagonal, along its side, as well as configurations with vortex structures, which may differ by location of the vortex center and, respectively, by magnitude and direction of the magnetic moment of the system. Also the conditions for transitions in the equilibrium configurations, when influenced by a plane field affecting the whole array, or by a normal local field affecting a part of the system dipoles, were considered. The possibility to control magnetic moment of the dipoles system through transitions between different vortex configurations, including a configuration with zero total magnetic moment, has been shown.

We used nuclear inelastic scattering (NIS) to investigate the lattice dynamics in [Fe(pyrazine)(Ni(CN)${}_4\left)\right]$ spin crossover nanoparticles. The vibrational density of states of iron was extracted from the NIS data, which allowed to determine characteristic thermodynamical and lattice dynamical parameters as well as their spin-state dependence. The optical part of the NIS spectra compares well with the Raman scattering data reflecting the expansion/contraction of the coordination octahedron during the spin transition. From the acoustic part, we extracted the sound velocity in the low-spin $(v_{\mathrm{LS}}=2073\pm 31$ m ${\mathrm{s}}^{-1})$ and high-spin $(v_{\mathrm{HS}}=1942\pm 23$ m ${\mathrm{s}}^{-1})$ states of the particles. The spin-state dependence of this parameter is of primary interest to rationalize the spin-transition behavior in solids as well as its dynamics and finite size effects.

We develop a general quantum theory of the coupled plasmonic modes resulting from the near-field interaction between localized surface plasmons in a heterogeneous metallic nanoparticle dimer. In particular, we provide analytical expressions for the frequencies and decay rates of the bright and dark plasmonic modes. We show that, for sufficiently small nanoparticles, the main decay channel for the dark plasmonic mode, which is weakly coupled to light and, hence, immune to radiation damping, is of nonradiative origin and corresponds to Landau damping, i.e., decay into electron-hole pairs.

The advances in phonon spectroscopy in homogeneous solids have unveiled extremely useful physics regarding the contribution of phonon energies and mean-free paths to the thermal transport in solids. However, as material systems decrease to length scales less than the phonon mean-free paths, thermal transport can become much more impacted by scattering and transmission across interfaces between two materials than the intrinsic relaxation in the homogeneous solid. To elucidate the fundamental interactions driving this thermally limiting interfacial phonon scattering process, we analytically derive and experimentally measure a thermal boundary conductance accumulation function. We develop a semiclassical theory to calculate the thermal boundary conductance accumulation function across interfaces using the diffuse mismatch model, and validate this derivation by measuring the interface conductance between eight different metals on native oxide/silicon substrates and four different metals on sapphire substrates. Measurements were performed at room temperature using time-domain thermoreflectance and represent the first-reported values for interface conductance across several metal/native oxide/silicon and metal/sapphire interfaces. The various metal films provide a variable bandwidth of phonons incident on the metal/substrate interface. This method of varying phonons' cutoff frequency in the film while keeping the same substrate allows us to mimic the accumulation of thermal boundary conductance and thus provides a direct method to experimentally validate our theory. We show that the accumulation function can be written as the product of a weighted average of the interfacial phonon transmission function and the accumulation of the temperature derivative of the phonon flux incident on the interface; this provides the framework to extract an average, spectrally dependent phonon transmissivity from a series of thermal boundary conductance measurements. Our approach provides a platform for analyzing the spectral phononic contribution to interfacial thermal transport in our experimentally measured data of metal/substrate thermal boundary conductance. Based on the assumptions made in this work and the measurement results on different metals on native oxide/silicon and sapphire substrates, we demonstrate that high-frequency phonons dictate the transport across metal/Si interfaces, especially in low Debye temperature metals with low-cutoff frequencies.

Tuning of band gaps of semiconductors is a way to optimize materials for applications within photovoltaics or as photocatalysts. One way to achieve this is through applying strain to the materials. We investigate the effect of strain on a range of Sn-containing semiconductors using density functional theory and many-body perturbation theory calculations. We find that the band gaps of bulk Sn oxides with ${\text{SnO}}_6$ octahedra are highly sensitive to volumetric strain. By applying a small isotropic strain of 2% $\left(-2\%\right)$, a decrease (increase) of band gaps as large as 0.8 to 1.0 eV are obtained. We attribute the ultrahigh strain sensitivity to the pure Sn $s$-state character of the conduction-band edges. Other Sn-containing compounds may show both increasing and decreasing gaps under tensile strain and we show that the behavior can be understood by analyzing the role of the Sn $s$ states in both the valence and the conduction bands.

Based on first-principles calculations, we reveal that the interactions between extended line defects (ELDs) of type “585” (formed by five and eight membered rings) ELDs embedded in graphene can induce ordered magnetism and self-doping of graphene. By reducing the distance between 585 ELDs, a distinct charge transfer is predicted from the center of 585 ELDs to their edges, which induces a Dirac point shift below the Fermi level, resulting in distance- or density-dependent $n$-type doping in the graphene. Relevant to the above finding, we found a distance-dependent spin polarization at the edges of 585 ELDs, attributable to the rigidity of the π electronic structure. Our finding suggests a promising approach for achieving $n$-type graphene for spintronic devices by creating 585 ELDs.

Sulfur-metal complexes, containing only a few atoms, can open new, highly efficient pathways for transport of metal atoms on surfaces. For example, they can accelerate changes in the shape and size of morphological features, such as two-dimensional nanoclusters, over time. In this study we perform STM under conditions that are designed to specifically isolate such complexes. We find a new, unexpected S-Cu complex on the $\mathrm{Cu}\left(111\right)$ surface, which we identify as $\mathrm{C}{\mathrm{u}}_2{\mathrm{S}}_3$. We propose that $\mathrm{C}{\mathrm{u}}_2{\mathrm{S}}_3$ enhances mass transport in this system, which contradicts a previous proposal based on $\mathrm{C}{\mathrm{u}}_3{\mathrm{S}}_3$. We analyze bonding within these Cu-S complexes, identifying a principle for stabilization of sulfur complexes on coinage metal surfaces.

The structural characterization of polycyclic aromatic hydrocarbon molecules adsorbed on graphene is of fundamental importance in view of the use of graphene or graphene nanoribbons for electronic applications. Before reaching this point, one has to determine the structure of the adsorbed molecules. Here, we study the case of benzene, coronene, and hexabenzocoronene on a graphene layer. First, the adsorption properties of single molecules are calculated using first-principles calculations at the level of density functional theory. We benefit from a recent scheme, particularly adapted for weakly adsorbed molecules, allowing us to precisely calculate the van der Waals contribution. Then, scanning tunneling microscopy (STM) is used to produce images of self-assembled molecules comparing different theoretical approaches to experimental observations. Finally, we consider the imaging of isolated molecules, and we show how the STM tip influences the molecule position by soft mechanical interaction during the scanning process.

Inelastic neutron scattering was performed on silicon powder to measure the phonon density of states (DOS) from 100 to 1500 K. The mean fractional energy shifts with temperature of the modes were $\langle \Delta {\varepsilon }_i/{\varepsilon }_i\Delta T\rangle =-0.07$, giving a mean isobaric Grüneisen parameter of $+6.95\pm 0.67$, which is significantly different from the isothermal parameter of +0.98. These large effects are beyond the predictions from quasiharmonic models using density functional theory or experimental data, demonstrating large effects from phonon anharmonicity. At 1500 K the anharmonicity contributes $0.15k_{\mathrm{B}}$/atom to the vibrational entropy, compared to $0.03k_{\mathrm{B}}$/atom from quasiharmonicity. Excellent agreement was found between the entropy from phonon DOS measurements and the reference NIST-JANAF thermodynamic entropy from calorimetric measurements.

We report on the spin densities in CeRuSn determined at elevated and at low temperatures using polarized neutron diffraction. At 285 K, where the CeRuSn crystal structure contains two different crystallographic Ce sites, we observe that a Ce site with larger nearest-neighbor distances is clearly more susceptible to the applied magnetic field, whereas the other is hardly polarizable. This finding clearly documents that different local environment of the two Ce sites causes the Ce ions to split into magnetic ${\text{Ce}}^{3+}$ and nonmagnetic ${\text{Ce}}^{(4-\delta )+}$ valence states. With lowering the temperature, the crystal structure transforms to a structure incommensurately modulated along the $c$ axis. This leads to new inequivalent crystallographic Ce sites resulting in a redistribution of spin densities. Our analysis using the simplest structural approximant shows that in this metallic system Ce ions coexist in different valence states. Localized $4f$ states that fulfill the third Hund's rule are found to be close to the ideal ${\text{Ce}}^{3+}$ state (at sites with the largest Ce-Ru interatomic distances), whereas ${\text{Ce}}^{(4-\delta )+}$ valence states are found to be itinerant and situated at Ce sites with much shorter Ce-Ru distances. The similarity to the famous $\gamma \text{−}\alpha$ transition in elemental cerium is discussed.

We compare the laser-driven demagnetization dynamics of the rare earths gadolinium and terbium by mapping their transient valance band structures with time- and angle-resolved photoelectron spectroscopy. In both metals, the minority and majority spin valence bands evolve independently with different time constants after optical excitation. The ultrafast shift of the partially unoccupied minority spin bulk band to higher binding energy and of the majority spin surface state to lower binding energy suggests spin transport between surface and bulk. The slower response of the fully occupied majority spin band follows the lattice temperature and is attributed to Elliott-Yafet type spin-flip scattering. Terbium shows a stronger and faster decay of the exchange splitting, pointing to ultrafast magnon emission via $4f$ spin-to-lattice coupling.

We present first-principles calculations of the coupling of quasiparticles to spin fluctuations in iron selenide and discuss which types of superconducting instabilities this coupling gives rise to. We find that strong antiferromagnetic stripe-phase spin fluctuations lead to large coupling constants for superconducting gaps with $s_{\pm }$ symmetry, but these coupling constants are significantly reduced by other spin fluctuations with small wave vectors. An accurate description of this competition and an inclusion of band-structure and Stoner parameter renormalization effects lead to a value of the coupling constant for an $s_{\pm }$-symmetric gap which can produce a superconducting transition temperature consistent with experimental measurements.

The ab initio calculation of charged supercells within density-functional theory is a necessary step to access several important properties of matter. The relaxation volume of charged point defects or the partial molar volume of ions in solution are two such examples. However, the total energy and therefore the pressure of charged systems is not uniquely defined when periodic boundary conditions are employed. This problem is tightly related to the origin of the electrostatic potential in periodic systems. This effect can be easily observed by modifying the electrostatic convention or modifying the local ionic potential details. We propose an approach to uniquely define the pressures in charged supercells with the use of the absolute deformation potentials. Only with such a definition could the ab initio calculations provide meaningful values for the relaxation volumes and for the elastic interactions for charged defects in semiconductors or ions in solution. The proposed scheme allows one to calculate sensible data even when charge neutrality is not enforced, thus going beyond the classical force-field-based approaches.

Kramers-Kronig analysis is commonly used to estimate the optical properties of new materials. The analysis typically uses data from far infrared through near ultraviolet (say 40–$40000{\mathrm{cm}}^{-1}$ or 5 meV–5 eV) and uses extrapolations outside the measured range. Most high-frequency extrapolations use a power law, 1/${\omega }^n$, transitioning to $1/{\omega }^4$ at a considerably higher frequency and continuing this free-carrier extension to infinity. The midrange power law is adjusted to match the slope of the data and to give pleasing curves, but the choice of power (usually between 0.5 and 3) is arbitrary. Instead of an arbitrary power law, it is better to use x-ray atomic scattering functions such as those presented by Henke and co-workers. These basically treat the solid as a linear combination of its atomic constituents and, knowing the chemical formula and the density, allow the computation of dielectric function, reflectivity, and other optical functions. The “Henke reflectivity” can be used over photon energies of 10 eV to 34 keV, after which a $1/{\omega }^4$ continuation is perfectly fine. The bridge between experimental data and the Henke reflectivity as well as two corrections made to the latter are discussed.

Powder x-ray diffraction (PXRD) and single-crystal neutron scattering were used to study in detail the structural properties of the ${\mathrm{Cs}}_2\mathrm{Cu}\left({\mathrm{Cl}}_{4-x}{\mathrm{Br}}_x\right)$ series, good realizations of layered triangular antiferromagnets. Detailed temperature-dependent PXRD reveal a pronounced anisotropy of the thermal expansion for the three different crystal directions of the orthorhombic structure without any structural phase transition down to 20 K. Remarkably, the anisotropy of the thermal expansion varies for different $x$, leading to distinct changes of the geometry of the local Cu environment as a function of temperature and composition. The refinement of the atomic positions confirms that for $x=1$ and 2, the Br atoms occupy distinct halogen sites in the $\left[{\text{Cu}X}_4\right]$ tetrahedra ($X$ = Cl, Br). The precise structure data are used to calculate the magnetic exchange couplings using density functional methods for $x=0$. We observe a pronounced temperature dependence of the calculated magnetic exchange couplings, reflected in the strong sensitivity of the magnetic exchange couplings on structural details. These calculations are in good agreement with the experimentally established values for ${\mathrm{Cs}}_2{\mathrm{CuCl}}_4$ if one takes the low-temperature structure data as a starting point.

As the isotopic concentration of ultrathin graphite is varied from natural abundance to nearly pure ${}^{13}\mathrm{C}$, the thermal conductivity displays a slight dependence on the isotope concentration at temperatures near its maximum $\sim 150\mathrm{K}$. The strength of phonon-isotope scattering in the high-isotope impurity-concentration regime is found to be well below that given by a commonly used incoherent and independent isotope impurity scattering model. This finding is in agreement with some recent theoretical predictions that coherent multiple scattering of phonons is important in the measured thermal conductivity of low-dimensional materials in the high-isotope impurity-concentration regime.

The three-electron configuration of gate-defined double quantum dots encodes a promising qubit for quantum information processing. I propose a two-qubit entangling gate using a pulse-gated manipulation procedure. The requirements for high-fidelity entangling operations are equivalent to the requirements for the pulse-gated single-qubit manipulations that have been successfully realized for Si QDs. This two-qubit gate completes the universal set of all-pulse-gated operations for the three-electron double-dot qubit and paves the way for a scalable setup to achieve quantum computation.

The spin polarization of organic-ferromagnetic interfaces in an organic spin valve critically affects the efficiency of spin injection or detection. We examined the chemical and electronic properties of ferromagnetic Co deposited on organic $\mathrm{Al}{\mathrm{q}}_3$ and the interfacial spin-polarized capability of the electronic states. Our x-ray photoemission spectra and calculations with density-functional theory indicate a sequential and unequal distribution of charge from Co clusters to N and then to O atoms in $\mathrm{Al}{\mathrm{q}}_3$. The preferential orbital hybridization at specific functional sites produces efficient spin polarization of organic molecules. Element-specific measurements of x-ray magnetic circular dichroism demonstrate the preferential spin polarization in the lowest unoccupied molecular orbital state of N atoms at the complex interface for Co atop $\mathrm{Al}{\mathrm{q}}_3$, which agrees satisfactorily with calculation. Our results indicate that an induced interfacial spin polarization on engineering the dominant reaction of Co with mainly N and O atoms in $\mathrm{Al}{\mathrm{q}}_3$ might pave a way for effective spin filtering in organic spintronics.

Electron interaction in nanoscale metal structures is of major interest in understanding and designing nanoscale electronic devices. In this study, electron interaction in epitaxially grown 20-nm-thick copper film on Si(100) was studied using femtosecond pump-probe technique with tunable probe photon energy near the $d\text{-band}$ to Fermi-level transition. It was found that probe-photon-energy dependence on the transient reflectivity signal can be divided into four regimes, and the borders of these regimes depend on the pump power. Based on a phenomenological model, the excited electrons were separated into nonthermalized and thermalized electrons, and it was seen that the probe-photon-energy dependence of the signal can be explained by the difference of the ratios of nonthermalized to thermalized electron components. Furthermore, it was found that each of these two components can be selectively excited by properly choosing the probe photon energy. Each electron-phonon (el-ph) coupling and electron internal thermalization time were investigated individually without interference from each other's processes. We show that el-ph coupling factor is larger in epitaxial copper film than the previously reported value in polycrystalline-copper film. In addition, the electron internal thermalization time is smaller than the reported value for the polycrystalline gold and silver films.