We discuss the van der Waals (Casimir) free energies and pressures of thin metallic films, consisting of from one to fifteen atomic layers, with regard to the anisotropy in their dielectric properties. Both freestanding films and films deposited on a dielectric substrate are considered. The computations are performed for a Au film and a sapphire substrate. According to our results, for freestanding Au films consisting of one and three atomic layers the respective relative error arising from the use of an isotropic (bulk) dielectric permittivity is equal to 73% and 37% for the van der Waals energy, and 70% and 35% for the pressure. We tabulate the energy and pressure van der Waals coefficients of thin Au films computed with account of their anisotropy. It is shown that the bulk permittivity of Au can be used for the films consisting of more than 30 atomic layers, i.e., more than approximately 7 nm thickness. The role of relativistic effects is also investigated and shown to be important even for the films consisting of two or three layers. The obtained results can find applications in the investigation of the stability of thin films and the development of novel nanoscale devices.

We studied the far-field thermal emission properties of finite arrays of resonant gold dipole nanoantennas at equilibrium temperature. We numerically investigated the transition from the super-Planckian emission of the single resonant antenna to the sub-Planckian emission inherent to infinite periodic arrays. Increasing the number of unit cells of the array, the overall size of the system increases, and the relative emissivity quickly converges to values lower than the unity. Nevertheless, if the separation between nanoantennas in the array is small compared to the wavelength, the near-field interaction makes the emission of each unit cell multipolar. This opens the doors for additional tailoring of the emitted power and directionality of thermal radiation.

Previous studies have indicated that the figure of merit ($\mathit{ZT}$) of half-Heusler (HH) alloys with composition $M\mathrm{NiSn}$ ($M=\mathrm{Ti}$, Zr, or Hf) is greatly enhanced when the alloys contain a nano-scale full-Heusler (FH) $\mathrm{MN}{\mathrm{i}}_2\mathrm{Sn}$ second phase. However, the formation mechanism of the FHnanostructures in the HH matrix and their vibrational properties are still not well understood. We report on first-principles studies of thermodynamic phase equilibria in the MNiSn-$\mathrm{MN}{\mathrm{i}}_2\mathrm{Sn}$ pseudobinary system as well as HH and FH vibrational properties. Thermodynamic phase diagrams as functions of temperature and Ni concentration were developed using density functional theory (DFT) combined with a cluster expansion and Monte Carlo simulations. The phase diagrams show very low excess Ni solubility in HH alloys even at high temperatures, which indicates that any Ni excess will decompose into a two-phase mixture of HH and FH compounds. Vibrational properties of HH and FH alloys are compared. Imaginary vibrational modes in the calculated phonon dispersion diagram of $\mathrm{TiN}{\mathrm{i}}_2\mathrm{Sn}$ indicate a dynamical instability with respect to cubic [001] transverse acoustic modulations. Displacing atoms along unstable vibrational modes in cubic $\mathrm{TiN}{\mathrm{i}}_2\mathrm{Sn}$ reveals lower-energy structures with monoclinic symmetry. The energy of the monoclinic structures is found to depend strongly on the lattice parameter. The origin of the instability in cubic $\mathrm{TiN}{\mathrm{i}}_2\mathrm{Sn}$ and its absence in cubic $\mathrm{ZrN}{\mathrm{i}}_2\mathrm{Sn}$ and $\mathrm{HfN}{\mathrm{i}}_2\mathrm{Sn}$ is attributed to the small size of the Ti $3d$ shells compared to those of Zr and Hf atoms. Lattice constants and heat capacities calculated by DFT agree well with experiment.

Microscopic polar clusters can play an important role in the phase transition of ferroelectric perovskite oxides such as $\mathrm{BaTi}{\mathrm{O}}_3$, which have shown coexistence of both displacive and order-disorder dynamics, although their topological and dynamical characteristics are yet to be clarified. Here, we report sharp increases in the widths and intensities of Bragg peaks from a $\mathrm{BaTi}{\mathrm{O}}_3$ single crystal, which are measured in situ during heating and cooling within a few degrees of its phase transition temperature $T_C$, using the neutron time-of-flight Laue technique. Most significantly sharper and stronger increases in peak widths and peak intensities were found to occur during cooling compared to that during heating through $T_C$. A closer examination of the Bragg peaks revealed their elongated shapes in both the paraelectric and ferroelectric phases, the analysis of which indicated the presence of microdomains that have correlated $\langle 111\rangle$-type polarization vectors within the {110}-type crystallographic planes. No significant increase in the average size of the microdomains $(\sim 10\mathrm{nm})$ near $T_C$ could be observed from diffraction measurements, which is also consistent with small changes in the relaxation times for motion of Ti ions measured with quasielastic neutron scattering. The current observations do not indicate that the paraelectric-ferroelectric phase transition in $\mathrm{BaTi}{\mathrm{O}}_3$ is primarily caused by an increase in the size of the microscopic polar clusters or critical slowing down of Ti ionic motion. The sharp and strong increases in peak widths and peak intensities during cooling through $T_C$ are explained as a result of microstrains that are developed at microdomain interfaces during paraelectric-ferroelectric phase transition.

Subpicosecond double-pulse laser ablation of metals is simulated using a hybrid model that combines classical molecular dynamics and an energy equation for free electrons. The key advantage of our model is the usage of the Helmholtz wave equation for the description of the laser energy absorption. Applied together with the wide-range coefficients of optical and transport properties of the electron subsystem, the model gives the possibility to correctly describe the second pulse absorption on an arbitrary profile of the nascent plasma plume produced by the first pulse. We show that the integral absorption of the second pulse drastically increases with the delay between pulses, which varies in the simulation from 0 to 200 ps. As a result, the electron temperature in the plume increases up to three times with the delay variation from 0 to 200 ps. Thus the results of simulation resemble the previous experimental observations of the luminosity increase in the double-pulse irradiation for the delay interval from 100 to 200 ps. Besides, we bring to light two mechanisms of suppression of ablation responsible for the monotonic decrease of the ablation crater depth when the delay between pulses increases.

We study the dynamics of skyrmions under spin-transfer torque in Dzyaloshinskii-Moriya materials with easy-axis anisotropy. In particular, we study the motion of a topological skyrmion with skyrmion number $Q=1$ and a nontopological skyrmionium with $Q=0$ using their linear momentum, virial relations, and numerical simulations. The nontopological $Q=0$ skyrmionium is accelerated in the direction of the current flow and it either reaches a steady state with constant velocity, or it is elongated to infinity. The steady-state velocity is given by a balance between current and dissipation and has an upper limit. In contrast, the topological $Q=1$ skyrmion converges to a steady state with constant velocity at an angle to the current flow. When the spin current stops the $Q=1$ skyrmion is spontaneously pinned, whereas the $Q=0$ skyrmionium continues propagation. Exact solutions for the propagating skyrmionium are identified as solutions of equations given numerically in a previous work. Further exact results for propagating skyrmions are given in the case of the pure exchange model. The traveling solutions provide arguments that a spin-polarized current will cause rigid motion of a skyrmion or a skyrmionium.

We demonstrate the generation and detection of spin-torque ferromagnetic resonance in $\text{Pt/}{\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ (YIG) bilayers. A unique attribute of this system is that the spin Hall effect lies at the heart of both the generation and detection processes and no charge current is passing through the insulating magnetic layer. When the YIG undergoes resonance, a dc voltage is detected longitudinally along the Pt that can be described by two components. One is the mixing of the spin Hall magnetoresistance with the microwave current. The other results from spin pumping into the Pt being converted to a dc current through the inverse spin Hall effect. The voltage is measured with applied magnetic field directions that range from in plane to nearly perpendicular. When compared with theory, we find that the real and imaginary parts of the spin mixing conductance have out-of-plane angular dependences.

We use inelastic neutron scattering to study the temperature and energy dependence of the spin excitation anisotropy in uniaxial-strained electron-doped iron pnictide ${\mathrm{BaFe}}_{1.9}{\mathrm{Ni}}_{0.1}{\mathrm{As}}_2$ near optimal superconductivity ($T_c=20\mathrm{K}$). Our work has been motivated by the observation of in-plane resistivity anisotropy in the paramagnetic tetragonal phase of electron-underdoped iron pnictides under uniaxial pressure, which has been attributed to a spin-driven Ising-nematic state or orbital ordering. Here we show that the spin excitation anisotropy, a signature of the spin-driven Ising-nematic phase, exists for energies below $\sim 60$ meV in uniaxial-strained ${\mathrm{BaFe}}_{1.9}{\mathrm{Ni}}_{0.1}{\mathrm{As}}_2$. Since this energy scale is considerably larger than the energy splitting of the $d_{xz}$ and $d_{yz}$ bands of uniaxial-strained $\mathrm{Ba}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right)}_2{\mathrm{As}}_2$ near optimal superconductivity, spin Ising-nematic correlations are likely the driving force for the resistivity anisotropy and associated electronic nematic correlations.

A first-principles based approach to calculating self-diffusion rates in bcc Fe is discussed with particular focus on the magnetic free energy associated with diffusion activation. First, the enthalpies and entropies of vacancy formation and migration in ferromagnetic bcc Fe are calculated from standard density functional theory methods in combination with transition state theory. Next, the shift in diffusion activation energy when going from the ferromagnetic to the paramagnetic state is estimated by averaging over random spin states. Classical and quantum mechanical Monte Carlo simulations within the Heisenberg model are used to study the effect of spin disordering on the vacancy formation and migration free energy. Finally, a quasiempirical model of the magnetic contribution to the diffusion activation free energy is applied in order to connect the current first-principles results to experimental data. The importance of the zero-point magnon energy in modeling of diffusion in bcc Fe is stressed.

We study the direct calculation of total energy derivatives for lattice dynamics and electron-phonon coupling calculations using supercell matrices with nonzero off-diagonal elements. We show that it is possible to determine the response of a periodic system to a perturbation characterized by a wave vector with reduced fractional coordinates $(m_1/n_1,m_2/n_2,m_3/n_3)$ using a supercell containing a number of primitive cells equal to the least common multiple of $n_1,n_2$, and $n_3$. If only diagonal supercell matrices are used, a supercell containing $n_1{n}_2{n}_3$ primitive cells is required. We demonstrate that the use of nondiagonal supercells significantly reduces the computational cost of obtaining converged zero-point energies and phonon dispersions for diamond and graphite. We also perform electron-phonon coupling calculations using the direct method to sample the vibrational Brillouin zone with grids of unprecedented size, which enables us to investigate the convergence of the zero-point renormalization to the thermal and optical band gaps of diamond.

We report ${}^{75}\mathrm{As}$ nuclear magnetic resonance (NMR) measurements of single-crystalline $\mathrm{Ca}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right)}_2{\mathrm{As}}_2$ ($x=0.023$, 0.028, 0.033, and 0.059) annealed at $350{}^{\circ }\mathrm{C}$ for 7 days. From the observation of a characteristic shape of ${}^{75}\mathrm{As}$ NMR spectra in the stripe-type antiferromagnetic (AFM) state, as in the case of $x=0$ ($T_{\mathrm{N}}=170$ K), clear evidence for the commensurate AFM phase transition with the concomitant structural phase transition is observed in $x=0.023$ ($T_{\mathrm{N}}=106$ K) and $x=0.028$ ($T_{\mathrm{N}}=53$ K). Through the temperature dependence of the Knight shifts and the nuclear spin lattice relaxation rates ($1/T_1$), although stripe-type AFM spin fluctuations are realized in the paramagnetic state as in the case of other iron pnictide superconductors, we found a gradual decrease of the AFM spin fluctuations below a crossover temperature $T^{*}$ that was nearly independent of Co-substitution concentration, and it is attributed to a pseudogaplike behavior in the spin excitation spectra of these systems. The $T^{*}$ feature finds correlation with features in the temperature-dependent interplane resistivity, ${\rho }_c\left(T\right)$, but not with the in-plane resistivity ${\rho }_a\left(T\right)$. The temperature evolution of anisotropic stripe-type AFM spin fluctuations is tracked in the paramagnetic and pseudogap phases by the $1/T_1$ data measured under magnetic fields parallel and perpendicular to the $c$ axis. Based on our NMR data, we have added a pseudogaplike phase to the magnetic and electronic phase diagram of $\mathrm{Ca}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right)}_2{\mathrm{As}}_2$.

Strongly correlated systems with geometric frustrations can host the emergent phases of matter with unconventional properties. Here, we study the spin $S=1$ Heisenberg model on the honeycomb lattice with the antiferromagnetic first- ($J_1$) and second-neighbor ($J_2$) interactions ($0.0\le J_2/J_1\le 0.5$) by means of density-matrix renormalization group. In the parameter regime $J_2/J_1\lesssim 0.27$, the system sustains a Néel antiferromagnetic phase. At the large $J_2$ side $J_2/J_1\gtrsim 0.32$, a stripe antiferromagnetic phase is found. Between the two magnetic ordered phases $0.27\lesssim J_2/J_1\lesssim 0.32$, we find a nonmagnetic intermediate region with a plaquette valence-bond order. Although our calculations are limited within a six unit-cell width on cylinder, we present evidence that this plaquette state could be a strong candidate for this nonmagnetic region in the thermodynamic limit. We also briefly discuss the nature of the quantum phase transitions in the system. We gain further insight into the nonmagnetic phases in the spin-1 system by comparing its phase diagram with the spin-$1/2$ system.

We present data on the optical conductivity of ${\mathrm{URu}}_{2-x}{(\mathrm{Fe},\mathrm{Os})}_x{\mathrm{Si}}_2$. While the parent material ${\mathrm{URu}}_2{\mathrm{Si}}_2$ enters the enigmatic hidden order (HO) phase below 17.5 K, an antiferromagnetic (AFM) phase is induced by the substitution of Fe or Os onto the Ru sites. We find that both the HO and the AFM phases exhibit an identical gap structure that is characterized by a loss of conductivity below the gap energy with spectral weight transferred to a narrow frequency region just above the gap, the typical optical signature of a density wave. The AFM phase is marked by strong increases in both transition temperature and the energy of the gap associated with the transition. In the normal phase just above the transition the optical scattering rate varies as ${\omega }^2$. We find that in both the HO and the AFM phases, our data are consistent with elastic resonant scattering of a Fermi liquid. This indicates that the appearance of a coherent state is a necessary condition for either ordered phase to emerge. Our measurements favor models in which the HO and the AFM phases are driven by the common physics of a nesting-induced density wave gap.

We observed commensurability magnetoresistance arising from ballistic electron transport in a triangular antidot lattice of high-mobility graphene. For both the monolayer and bilayer, magnetoresistance peaks were observed at the commensurability magnetic fields of the cyclotron orbit with an antidot lattice. This condition was approximately unchanged for massless and massive Dirac fermions. The peaks appeared when the carrier mean free path was roughly larger than the lattice constant of the antidot, which indicates that the effect stems from the short-range characteristics of the carrier's scattering with antidots. We also found that the magnitude of the commensurability peak diminished with changing the gate voltages to the charge neutrality point. This arose from the screening of the charged impurity in graphene, which is dependent on the carrier density.

Our first-principles study demonstrates for the first time that by increasing the molecule-surface binding strength, the interlayer magnetic coupling of a ferromagnetic metal can be drastically reduced with respect to that of a clean substrate. Importantly, for a weakly chemisorbed molecule the rehybridization of metal atomic $d$ states within the molecule-induced surface geometry (geometrical effect) plays a crucial role in obtaining interlayer magnetic softening. On the contrary, for a strongly chemisorbed molecule the interlayer magnetic coupling is further reduced due to an interplay between the geometrical effect and the hybridization of atomic $d$ states with molecular ones.

Multiple frequency quantum beats of a system of the coherently excited quantum confined exciton states in a high-quality semiconductor structure containing a wide InGaAs/GaAs quantum well are experimentally detected by the spectrally resolved pump-probe method. The beat signal is observed both at positive and at negative delays between the pump and probe pulses. Several quantum beat (QB) frequencies are observed in the experiments, which coincide with the interlevel spacings in the exciton system. A theoretical model is developed, which allows one to attribute the QBs at negative delay to the four-wave mixing (FWM) signal detected at the nonstandard direction. The beat signal is strongly enhanced by the interference of the FWM signal with the secondary emission induced by the probe pulse. At positive delays, the QBs are due to the interference of the quantum confined exciton states. The decay time for QBs is of the order of several picoseconds both at positive and negative delays. This is close to the relaxation time of the exciton population that allows one to consider the exciton depopulation as the main mechanism of the coherence relaxation in the system under study.

Thin films of Au ranging from 7–24 nm were grown on ${\mathrm{MoS}}_2$ at room temperature using thermal evaporation and studied using scanning tunneling microscopy and ballistic electron emission spectroscopy. Topographic images show the surface morphology of Au transitions from terraced triangles to a mix of terraced hexagonal and irregular-shaped structures as film thickness exceeded 16 nm. Raman spectra reveal the presence of tensile strain in the ${\mathrm{MoS}}_2$ with thicker Au films and is likely the driving force behind this transition. All samples exhibit a Schottky barrier significantly lower than that predicted by the Schottky-Mott model due to Fermi-level pinning at the interface. The pinning mechanism is thought to be caused, in part, by the presence of gap states induced by a weakening of the interlayer Mo-S bonding in the presence of the Au film. Although relatively consistent in thinner films, the Schottky barrier increases concurrently with structural changes on the surface. At the same time, transmission through the interface begins to drop at an increased exponential rate with film thickness. These observations are consistent with a widening separation between the Au and ${\mathrm{MoS}}_2$ that would reduce the number of gap states and cause transmission through the interface to be more characteristic of quantum tunneling. An increased separation such as this could result from changes in equilibrium conditions at the interface with increasing strain.