Magnetic skyrmions belong to the most interesting spin structures for the development of future information technology as they have been predicted to be topologically protected. To quantify their stability, we use an innovative multiscale approach to simulating spin dynamics based on the Landau-Lifshitz-Gilbert equation. The multiscale approach overcomes the micromagnetic limitations that have hindered realistic studies using conventional techniques. We first demonstrate how the stability of a skyrmion is influenced by the refinement of the computational mesh and reveal that conventionally employed traditional micromagnetic simulations are inadequate for this task. Furthermore, we determine the stability quantitatively using our multiscale approach. As a key operation for devices, the process of annihilating a skyrmion by exciting it with a spin polarized current pulse is analyzed, showing that skyrmions can be reliably deleted by designing the pulse shape.

We propose a route towards engineering nonthermal states of matter, which show largely unexplored physics. The main idea relies on the adiabatic passage of a thermal ensemble under slow variations of the system Hamiltonian. If the temperature of the initial thermal ensemble is either zero or infinite, the ensemble after the passage is a simple thermal one with the same vanishing or infinite temperature. However, for any finite nonzero temperature, intriguing nonthermal ensembles can be achieved. We exemplify this in (a) a single oscillator, (b) a dimerized interacting one-dimensional chain of spinless fermions, (c) a BCS-type superconductor, and (d) the topological Kitaev chain. We solve these models with a combination of methods: either exactly, numerically using the density matrix renormalization group, or within an approximate functional renormalization group scheme. The designed states show strongly nonthermal behavior in each of the considered models. For example, for the chain of spinless fermions we exemplify how long-ranged nonthermal power-law correlations can be stabilized, and for the Kitaev chain we elucidate how the nonthermal ensemble can largely alter the transition temperature separating topological and trivial phases.

The asymptotical behavior of physical quantities, like the order parameter, the replicon, and longitudinal masses, is studied around the zero-field spin-glass transition point when a small external magnetic field is applied. An effective field theory to model this asymptotics contains a small perturbation in its Lagrangian which breaks the zero-field symmetry. A first-order renormalization group supplemented by perturbational results provides the scaling functions. The perturbative zero of the scaling function for the replicon mass defines a generic Almeida-Thouless surface stemming from the zero-field fixed point.

We use a multiscale approach linking ab initio calculations for the parametrization of an atomistic spin model with spin dynamics simulations based on the stochastic Landau-Lifshitz-Gilbert equation to investigate the thermal magnetic properties of the ferrimagnetic rare-earth transition-metal intermetallic ${\mathrm{DyCo}}_5$. Our theoretical findings are compared to elemental resolved measurements on ${\mathrm{DyCo}}_5$ thin films using the x-ray magnetic circular dichroism technique. With our model, we are able to accurately compute the complex temperature dependence of the magnetization. The simulations yield a Curie temperature of $T_{\mathrm{C}}=1030\text{K}$ and a compensation point of $T_{\mathrm{comp}}=164\text{K}$, which is in a good agreement with our experimental result of $T_{\mathrm{comp}}=120\text{K}$. The spin reorientation transition is a consequence of competing elemental magnetocrystalline anisotropies in connection with different degrees of thermal demagnetization in the Dy and Co sublattices. Experimentally, we find this spin reorientation in a region from $T_{\mathrm{SR}1,2}=320$ to $360\text{K}$, whereas in our simulations the Co anisotropy appears to be underestimated, shifting the spin reorientation to higher temperatures.

A repulsive Hubbard model with both spin-asymmetric hopping $\left(t_{\uparrow }\ne t_{\downarrow }\right)$ and a staggered potential (of strength $\mathrm{\Delta })$ is studied in one dimension. The model is a compound of the mass-imbalanced $\left(t_{\uparrow }\ne t_{\downarrow },\mathrm{\Delta }=0\right)$ and ionic $\left(t_{\uparrow }=t_{\downarrow },\mathrm{\Delta }>0\right)$ Hubbard models, and may be realized by cold atoms in engineered optical lattices. We use mostly mean-field theory to determine the phases and phase transitions in the ground state for a half-filled band (one particle per site). We find that a period-two modulation of the particle (or charge) density and an alternating spin density coexist for arbitrary Hubbard interaction strength, $U\ge 0$. The amplitude of the charge modulation is largest at $U=0$, decreases with increasing $U$ and tends to zero for $U\to \infty$. The amplitude for spin alternation increases with $U$ and tends to saturation for $U\to \infty$. Charge order dominates below a value $U_c$, whereas magnetic order dominates above. The mean-field Hamiltonian has two gap parameters, ${\mathrm{\Delta }}_{\uparrow }$ and ${\mathrm{\Delta }}_{\downarrow }$, which have to be determined self-consistently. For $U<U_c$ both parameters are positive, for $U>U_c$ they have different signs, and for $U=U_c$ one gap parameter jumps from a positive to a negative value. The weakly first-order phase transition at $U_c$ can be interpreted in terms of an avoided criticality (or metallicity). The system is reluctant to restore a symmetry that has been broken explicitly.

The indium coverage of the Si(111)-$\sqrt{7}\times \sqrt{3}$-In surface is investigated by means of x-ray photoelectron spectroscopy and first-principles density functional theory calculations. Both experimental and theoretical results indicate that the In coverage is a double layer rather than a single layer. Moreover, the atomic structure of the Si(111)-$\sqrt{7}\times \sqrt{3}$-In surface is discussed by comparing experimental with simulated scanning tunneling microscopy (STM) images and scanning tunneling spectra with the calculated density of states. Our structural assignment agrees with previous studies, except for the interpretation of experimental STM images.

Interference effects form a fundamental pillar of quantum mechanics. In this paper, we examine the interference in spin-orbit-coupled transverse magnetic focusing, where a weak magnetic field is used to focus charge carries over mesoscopic scales. We determine a semiclassical form for the Green's function in a weak magnetic field, for the case of both spinless and spin-orbit-coupled charge carriers. The obtained forms for the Greens' function are independent of particle dispersion and are thus applicable to a wide variety of systems.

The ground state of the diatomic molecules in nature is inevitably bonding, and its first excited state is antibonding. We demonstrate theoretically that, for a pair of distant adatoms placed buried in three-dimensional-Dirac semimetals, this natural order of the states can be reversed and an antibonding ground state occurs at the lowest energy of the so-called bound states in the continuum. We propose an experimental protocol with the use of a scanning tunneling microscope tip to visualize the topographic map of the local density of states on the surface of the system to reveal the emerging physics.

Degenerate perturbation theory from quantum mechanics is inadequate in density functional theory (DFT) because of nonlinearity in the Kohn-Sham potential. Herein, we develop the fully general perturbation theory for open-shell, degenerate systems in Kohn-Sham DFT, without assuming the presence of symmetry or equal occupation of degenerate orbitals. To demonstrate the resulting methodology, we apply it to the iron atom in the central field approximation, perturbed by an electric quadrupole. This system was chosen because it displays both symmetry required degeneracy, between the five $3d$ orbitals, as well as accidental degeneracy, between the $3d$ and $4s$ orbitals. The quadrupole potential couples the degenerate $3d$ and $4s$ states, serving as an example of the most general perturbation.

Ultrablack metamaterials are artificial materials able to harvest all the incident light regardless of wavelength, angle, or polarization. Here, we show the ultrablack properties of randomly aligned hierarchical carbon nanotube arrays with thicknesses below 200 nm. The thin coatings are realized by solution processing and dry-transfer deposition on different substrates. The hierarchical surface morphology of the coatings is biomimetic and provides a large effective area that improves the film optical absorption. Also, such a morphology is responsible for the moth-eye effect, which leads to the omnidirectional and polarization-independent suppression of optical reflection. The films exhibit an emissivity up to 99.36% typical of an ideal black body, resulting in the thinnest ultrablack metamaterial ever reported. Such a material may be exploited for thermal, optical, and optoelectronic devices such as heat sinks, optical shields, solar cells, light and thermal sensors, and light-emitting diodes.

The effect of steps on the herringbone pattern appearing at the $\mathrm{Au}\left(111\right)$ surface is explored. Scanning tunneling microscopy investigations show that the number of alternating fcc and hcp regions decreases with the decreasing width of the terrace, in fair agreement with atomistic simulations. It is demonstrated that the steps locally release the tensile surface stresses, leading to a reorganization of the herringbone pattern.

The existence of shear surface acoustic waves (SAWs) has been regarded as impossible in nonpiezoelectrics with homogeneous flat surfaces. We show that transverse shear SAWs can propagate near the flat surfaces of all crystalline dielectrics due to the omnipresent flexoelectric coupling. It appears that the penetration depth of the previously unexplored SAW is defined by the flexocoupling strength. Since the SAW occurs due to the flexoelectric coupling, we name it the flexoelectric SAW (flexo-SAW). We predict that the phonon spectra corresponding to the flexo-SAWs and bulk phonon modes can be separated in thin nonpiezoelectric films, such as strontium titanate.

We study localization and charge dynamics in a monochromatically driven one-dimensional Anderson insulator focusing on the low-frequency, strong-driving regime. We study this problem using a mapping of the Floquet Hamiltonian to a hopping problem with correlated disorder in one higher harmonic-space dimension. We show that (i) resonances in this model correspond to adiabatic Landau-Zener (LZ) transitions that occur due to level crossings between lattice sites over the course of dynamics; (ii) the proliferation of these resonances leads to dynamics that appear diffusive over a single drive cycle, but the system always remains localized; (iii) actual charge transport occurs over many drive cycles due to slow dephasing between these LZ orbits and is logarithmic in time, with a crucial role being played by far-off Mott-like resonances; and (iv) applying a spatially varying random phase to the drive tends to decrease localization, suggestive of weak-localization physics. We derive the conditions for the strong-driving regime, determining the parametric dependencies of the size of Floquet eigenstates, and time scales associated with the dynamics, and corroborate the findings using both numerical scaling collapses and analytical arguments.

Nanoscale single crystals possess modified phonon dispersions due to the truncation of the crystal. The introduction of surfaces alters the population of phonons relative to the bulk and introduces anisotropy arising from the breaking of translational symmetry. Such modifications exist throughout the Brillouin zone, even in structures with dimensions of several nanometers, posing a challenge to the characterization of vibrational properties and leading to uncertainty in predicting the thermal, optical, and electronic properties of nanomaterials. Synchrotron x-ray thermal diffuse scattering studies find that freestanding Si nanomembranes with thicknesses as large as 21 nm exhibit a higher scattering intensity per unit thickness than bulk silicon. In addition, the anisotropy arising from the finite thickness of these membranes produces particularly intense scattering along reciprocal-space directions normal to the membrane surface compared to corresponding in-plane directions. These results reveal the dimensions at which calculations of materials properties and device characteristics based on bulk phonon dispersions require consideration of the nanoscale size of the crystal.

Magnetoresistive and magnetoresonance measurements carried out on patterned perpendicular magnetic tunnel junction pillars and full-sheet films reveal magnetic inhomogeneities of FeCoB free layer grown on MgO and coated with Ta. At low FeCoB thicknesses, the layer behaves as an ensemble of weakly coupled grains resulting in a decrease of the free-layer thermal stability. In contrast, for thicker layers, the grains become more strongly coupled but strong magnetic inhomogeneities remain, yielding the emergence and further increase of a second-order magnetic anisotropy term ($\sim K_{2\mathrm{eff}}\mathrm{co}{\mathrm{s}}^4\theta$), eventually resulting in an easy-cone anisotropy. We show that the static and dynamic magnetic properties of such a free layer can be successfully described by a granular model with three thickness-dependent parameters: mean perpendicular anisotropy of the grains, grain-to-grain anisotropy distribution, and intergrain exchangelike coupling strength. Easy-cone anisotropy may help reduce the stochasticity of the spin transfer torque switching. However, it arises at intermediate values of the intergrain exchange coupling where the spin transfer torque (STT) switching efficiency is degraded, as shown by multimacrospin modeling. This is due to the excitation of exchange modes contributing weakly to the STT switching process while dissipating part of the STT energy.

Raman scattering experiments on LaFeAsO with distinct antiferromagnetic ($T_{\text{AFM}}=140$ K) and tetragonal-orthorhombic ($T_S=155$ K) transitions show a quasielastic peak (QEP) in $B_{2g}$ symmetry (2 Fe tetragonal cell) that fades away below $\sim T_{\text{AFM}}$ and is ascribed to electronic nematic fluctuations. A scaling of the reported shear modulus with the $T$ dependence of the QEP height rather than the QEP area indicates that magnetic degrees of freedom drive the structural transition. The large separation between $T_S$ and $T_{\text{AFM}}$ in LaFeAsO compared to ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ manifests itself in slower dynamics of nematic fluctuations in the former.

The recently discovered noncentrosymmetric superconductor ${\mathrm{ThCoC}}_2$ was observed to show unusual superconducting behavior with a critical temperature of $T_c=2.65\mathrm{K}$. Here we investigate the effect of nickel substitution on the superconducting state in $\mathrm{ThC}{\mathrm{o}}_{1-x}\mathrm{N}{\mathrm{i}}_x{\mathrm{C}}_2$. Magnetization, resistivity, and heat capacity measurements demonstrate Ni substitution has a dramatic effect with critical temperature increased up to $T_c=12.1$ K for $x=0.4$ Ni concentration, which is a rather high transition temperature for a noncentrosymmetric superconductor. In addition, the unusual superconducting characteristics observed in pure ${\mathrm{ThCoC}}_2$ appear to be suppressed or tuned with Ni substitution towards a more conventional fully gapped superconductor.

The pseudospin of Dirac electrons in graphene manifests itself in a peculiar momentum anisotropy for photoexcited electron-hole pairs. These interband excitations are in fact forbidden along the direction of the light polarization and are maximum perpendicular to it. Here, we use time- and angle-resolved photoemission spectroscopy to investigate the resulting unconventional hot carrier dynamics, sampling carrier distributions as a function of energy, and in-plane momentum. We first show that the rapidly-established quasithermal electron distribution initially exhibits an azimuth-dependent temperature, consistent with relaxation through collinear electron-electron scattering. Azimuthal thermalization is found to occur only at longer time delays, at a rate that depends on the substrate and the static doping level. Further, we observe pronounced differences in the electron and hole dynamics in $n$-doped samples. By simulating the Coulomb- and phonon-mediated carrier dynamics we are able to disentangle the influence of excitation fluence, screening, and doping, and develop a microscopic picture of the carrier dynamics in photoexcited graphene. Our results clarify new aspects of hot carrier dynamics that are unique to Dirac materials, with relevance for photocontrol experiments and optoelectronic device applications.

The ground state of $\mathrm{BaF}{\mathrm{e}}_{12}{\mathrm{O}}_{19}$ (BFO) is controversial as three different quantum states, namely, quantum paraelectric, frustrated antiferroelectric, and quantum electric dipole liquid (QEDL), have been proposed. We have investigated the quantum critical behavior of BFO as a function of chemical pressure (a nonthermal variable) generated by smaller isovalent ion $\mathrm{C}{\mathrm{a}}^{2+}$ at the $\mathrm{B}{\mathrm{a}}^{2+}$ site. Analysis of synchrotron x-ray diffraction data confirms that $\mathrm{C}{\mathrm{a}}^{2+}$ substitution generates positive chemical pressure. Our dielectric measurements reveal that $\mathrm{C}{\mathrm{a}}^{2+}$ substitution drives BFO away from its quantum critical point (QCP) and stabilizes a quantum electric dipolar glass state whose dielectric peak temperature $\left(T_c\right)$ increases with increasing $\mathrm{C}{\mathrm{a}}^{2+}$ content as $T_c\sim {(x-x_c)}^{1/2}$, a canonical signature of quantum phase transitions. Our dielectric measurements reveal that pure BFO is slightly away from its QCP with a $T_c$ of 2.91 K. Specific-heat measurements reveal excess specific heat of non-Debye and nonmagnetic origin with linear temperature dependence below $T_c$ which could be due to the QEDL state of BFO.