Surface anisotropy energy density is a key quantity in the description of the dynamics of surface spins in ferromagnetic thin films and is known to result in a ferromagnetic resonance fine structure of a thin film: A multipeak spectrum is observed in an effect known as spin-wave resonance (SWR). The so-called surface pinning parameter, directly correlated with the surface anisotropy energy, plays a fundamental role in determining the character of an SWR spectrum in the theory of SWR based on the surface inhomogeneity model. Here we extend the theory of SWR by introducing a new formula representing the surface pinning parameter as a series of contributions from different anisotropies existing in (Ga,Mn)As thin films. We show how to determine the coefficients in this series by comparing the SWR theory with data obtained in a resonance experiment. The proposed procedure allows one to determine the proportion with which each type of the anisotropy contributes to the total surface anisotropy of (Ga,Mn)As, and, consequently, to assess to what extent the proportions on the surface differ from those in the bulk. The presented considerations can be used for the study of any other thin-film system, since we provide also a general rule that allows one to express the surface pinning parameter of a thin film, as well as its surface anisotropy energy, by the free energy of the considered thin-film system. This enables the interpretation of experimental SWR spectra in terms of the free-energy density of the thin film.

The complex hexagonal reconstructions of the (001) surfaces of platinum and gold have been under debate for decades. Here, the structural details of the Pt(001) reconstruction have been quantitatively reinvestigated by combining the high resolving power of scanning tunneling microscopy (STM) and spot profile analysis low energy electron diffraction (SPA-LEED). In addition, LEED simulations based on a Moiré approach have been applied. Annealing temperatures around 850 °C yield a superstructure that approaches a commensurable $c(26.6\times 118)$ substrate registry. It evolves from a Moiré-like buckling of a compressed hexagonal top layer (hex) where atomic rows of the hex run parallel to atomic rows of the square substrate. Annealing at 920 °C stimulates a continuous rotation of the hex where all angles between ±0.7° are simultaneously realized. At temperatures around 1080 °C, the nonrotated hex coexists with a hex that is rotated by about 0.75°. Annealing at temperatures around 1120 °C yield a locking of the hex in fixed rotation angles of 0.77°, 0.88°, and 0.94°. At temperatures around 1170 °C, the Pt(001)-hex-R 0.94° prevails as the energetically most favored form of the rotated hex.

We present a theoretical and experimental analysis of the cantilever motions detected in photoinduced force microscopy (PiFM) using the sideband coupling detection scheme. In sideband coupling, the cantilever dynamics are probed at a combination frequency of a fundamental mechanical eigenmode and the modulation frequency of the laser beam. Using this detection mode, we develop a method for reconstructing the modulated photoinduced force gradient from experimental parameters in a quantitative manner. We show evidence, both theoretically and experimentally, that the sideband coupling detection mode provides PiFM images with superior contrast compared to images obtained when detecting the cantilever motions directly at the laser modulation frequency.

Bismuth Selenide $(\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3)$ is a topological insulator with a two-dimensional layered structure that enables clean and well-ordered surfaces to be prepared by cleaving. Although some studies have demonstrated that the cleaved surface is terminated with Se, as expected from the bulk crystal structure, other reports have indicated either a Bi- or mixed-termination. Low-energy ion scattering (LEIS), low energy electron diffraction (LEED) and x-ray photoelectron spectroscopy (XPS) are used here to compare surfaces prepared by ex situ cleaving, in situ cleaving, and ion bombardment and annealing (IBA) in ultrahigh vacuum (UHV). Surfaces prepared by in situ cleaving and IBA are well ordered and Se-terminated. Ex situ cleaved samples could be either Se-terminated or Bi-rich, are less well ordered and have adsorbed contaminants. This suggests that a chemical reaction involving atmospheric contaminants, which may preferentially adsorb at surface defects, could contribute to the nonreproducibility of the termination.

We use neutron scattering to compare the magnetic excitations in the hidden order (HO) and antiferromagnetic (AFM) phases in ${\mathrm{URu}}_{2-x}{\mathrm{Fe}}_x{\mathrm{Si}}_2$ as a function of Fe concentration. The magnetic excitation spectra change significantly between $x=0.05$ and $x=0.10$, following the enhancement of the AFM ordered moment, in good analogy to the behavior of the parent compound under applied pressure. Prominent lattice-commensurate low-energy excitations characteristic of the HO phase vanish in the AFM phase. The magnetic scattering is dominated by strong excitations along the Brillouin zone edges, underscoring the important role of electron hybridization to both HO and AFM phases and the similarity of the underlying electronic structure. The stability of the AFM phase is correlated with enhanced local-itinerant electron hybridization.

Recent reports of spin-orbit coupling enhancement in chemically modified graphene have opened doors to studies of the spin Hall effect with massless chiral fermions. Here, we theoretically investigate the interaction and impurity density dependence of the extrinsic spin Hall effect in spin-orbit coupled graphene. We present a nonperturbative quantum diagrammatic calculation of the spin Hall response function in the strong-coupling regime that incorporates skew scattering and anomalous impurity density-independent contributions on equal footing. The spin Hall conductivity dependence on Fermi energy and electron-impurity interaction strength reveals the existence of experimentally accessible regions where anomalous quantum processes dominate. Our findings suggest that spin-orbit-coupled graphene is an ideal model system for probing the competition between semiclassical and bona fide quantum scattering mechanisms underlying the spin Hall effect.

Extended dynamical mean-field theory (EDMFT) is insufficient to describe nonlocal effects in strongly correlated systems, since corrections to the mean-field solution are generally large. We present an efficient scheme for the construction of diagrammatic extensions of EDMFT that avoids the usual double-counting problem by using an exact change of variables (the Dual Boson formalism) to distinguish the correlations included in the dynamical mean-field solution and those beyond. With a computational efficiency comparable to the $\mathrm{EDMFT}+\mathrm{GW}$ approach, our scheme significantly improves on the charge order transition phase boundary in the extended Hubbard model.

Macroscopic ferroelectric order in $\alpha$-GeTe with its noncentrosymmetric lattice structure leads to a giant Rashba spin splitting in the bulk bands due to strong spin-orbit interaction. Direct measurements of the bulk band structure using soft x-ray angle-resolved photoemission (ARPES) reveals the three-dimensional electronic structure with spindle torus shape. By combining high-resolution and spin-resolved ARPES as well as photoemission calculations, the bulk electronic structure is disentangled from the two-dimensional surface electronic structure by means of surface capping, which quenches the complex surface electronic structure. This unravels the bulk Rashba-split states in the ferroelectric Rashba $\alpha$-GeTe(111) semiconductor exhibiting a giant spin splitting with Rashba parameter ${\alpha }_R$ around 4.2 eV Å, the highest of so-far known materials.

The combined effect of a lateral square superlattice potential and the Coulomb interaction on the ground state of a two-dimensional electron gas in a perpendicular magnetic field is studied for different rational values of $\mathrm{\Gamma }$, the inverse of the number of flux quanta per unit cell of the external potential, at filling factor $\nu =1$ in Landau-level $N=0.$ When Landau-level mixing and disorder effects are neglected, increasing the strength $W_0$ of the potential induces a transition at a critical strength of $W_0^{\left(c\right)}$ from a uniform and fully spin-polarized state to a two-dimensional charge density wave (CDW) with a meronlike spin texture at each maximum and minimum of the CDW. The collective excitations of this “vortex CDW” are similar to those of the Skyrme crystal that is expected to be the ground-state near filling factor $\nu =1$. In particular, a broken $U\left(1\right)$ symmetry in the vortex CDW results in an extra gapless phase mode that could provide a fast channel for the relaxation of nuclear spins. The average spin-polarization $S_z$ changes in a continuous or discontinuous manner as $W_0$ is increased depending on whether $\mathrm{\Gamma }\in [1/2,1]$ or $\mathrm{\Gamma }\in [0,1/2]$. The phase mode and the meronlike spin texture disappear at a large value of $W_0$ leaving as the ground state a partially spin-polarized CDW if $\mathrm{\Gamma }\ne 1/2$ or a spin-unpolarized CDW if $\mathrm{\Gamma }=1/2.$

This paper investigates the effects of cesium (Cs) deposited on pentacene (5A) and sexiphenyl (6P) monolayers on the Ag(110) substrate. The process of doping and the energy level alignment are studied quantitatively and contrasted. While ultimately for both molecules lowest unoccupied molecular orbital (LUMO) filling on charge transfer upon Cs dosing is observed, the doping processes are tellingly different. In the case of 5A, hybrid molecule-substrate states and doping states coexist at lowest Cs exposures, while for 6P doping states appear only after Cs has completely decoupled the monolayer from the substrate. With the support of density functional theory calculations, this different behavior is rationalized by the local character of electrostatic potential changes induced by dopants in relation to the spatial extent of the molecules. This also has severe effects on the energy level alignment, which for most dopant/molecule systems cannot be considered continuous but discrete.

We present a method to calculate short-time nonequilibrium universal exponents within the functional-renormalization-group scheme. As an example, we consider the classical critical dynamics of the relaxational model A after a quench of the temperature of the system and calculate the initial-slip exponent which characterizes the nonequilibrium universal short-time behavior of both the order parameter and correlation functions. The value of this exponent is found to be consistent with the result of a perturbative dimensional expansion and of Monte Carlo simulations in three spatial dimensions.

Uranium nitride (UN) is a candidate fuel for current GEN III fission reactors, for which it is investigated as an accident-tolerant fuel, as well as for future GEN IV reactors. In this study, we investigate the kinetic properties of gas fission products (Xe and Kr) in UN. Binding and migration energies are obtained using density functional theory, with an added Hubbard correlation to model $f$ electrons, and the occupation matrix control scheme to avoid metastable states. These energies are then used as input for the self-consistent mean field method which enables to determine transport coefficients for vacancy-mediated diffusion of Xe and Kr on the $U$ sublattice. The magnetic ordering of the UN structure is explicitly taken into account, for both energetic and transport properties. Solute diffusivities are compared with experimental measurements and the effect of various parameters on the theoretical model is carefully investigated. We find that kinetic correlations are very strong in this system, and that despite atomic migration anisotropy, macroscopic solute diffusivities show limited anisotropy. Our model indicates that the discrepancy between experimental measurements probably results from different irradiation conditions, and hence different defect concentrations.

We report the Hanle magnetoresistance (HMR) due to the spin precession of edge spin accumulation and interfacial spin current. Because of spin-orbit coupling (SOC), an electric current is accompanied by a transverse spin current, which builds up the spin accumulation at surfaces of Pt and the spin current across the YIG/Pt interface. Once a magnetic field is applied, the precession of spins will decrease the edge spin accumulation and interfacial spin current, which leads to an increased resistance of Pt via ISHE. Spin relaxation governs the HMR from edge spin accumulation, while spin diffusion and spin conversion play important roles in the HMR from interfacial spin current. This work provides another method to investigate the spin-orbit coupling by electrical measurement.

We developed a general theory that allows us to predict the power factor $n$ in the asymptotics of the tunneling magnetoresistance (TMR), $\text{TMR}\propto N^n$, in the limit of large number of the tunnel barrier layers, $N$, for a magnetic tunnel junction (MTJ) system that has the so-called symmetry spin-filtering properties. Within this theory the only information required to determine $n$ is the knowledge of the symmetries of the wave functions of the bulk electrode and barrier materials at the $\mathrm{\Gamma }$ point in the in-plane surface Brillouin zone. In particular, we show that for a MTJ that has the in-plane square symmetry only three values for the power factor are allowed: $n=0,1,$ and 2 for the asymptotics of the TMR enhanced due to the symmetry spin-filtering mechanism. To verify our theory we performed the density functional theory calculations of transmission functions and TMR for a Fe/MgO/Fe MTJ which confirm predicted values of the power factor $n=0,1,$ or 2 in specific ranges of energies (in particular, $n=1$ at the Fermi energy).

There are two known ludwigites containing a single transition metal element, ${\text{Fe}}_3{\text{O}}_2{\text{BO}}_3$ and ${\text{Co}}_3{\text{O}}_2{\text{BO}}_3$. The structure of these materials has low-dimensional units in the form of three-legged ladders (3LL) that confer to each of them unique magnetic and electronic properties. ${\text{Fe}}_3{\text{O}}_2{\text{BO}}_3$ presents a staggered charge density wave (CDW) transition in the ladders near room temperature and two magnetic transitions. It has remained a mystery why the other compound ${\text{Co}}_3{\text{O}}_2{\text{BO}}_3$ behaves so conventionally, with a single magnetic transition and no CDW in spite of similar structural and electronic configurations. Neutron diffraction results presented here in this system finally unravel these differences. Far from a trivial explanation, we uncover a coexistence of low and high spin Co ions in well-defined octahedral sites. Our results allow one to solve the contrasting behavior of the Fe and Co ludwigites in terms of a subtle and unique charge ordering mechanism occurring at the microscopic level of the rungs of the 3LL.

We present a systematic study of the temperature dependence of diffusive magnon spin transport using nonlocal devices fabricated on a 210-nm yttrium iron garnet film on a gadolinium gallium garnet substrate. In our measurements, we detect spin signals arising from electrical and thermal magnon generation, and we directly extract the magnon spin diffusion length ${\lambda }_m$ for temperatures from 2 to 293 K. Values of ${\lambda }_m$ obtained from electrical and thermal generation agree within the experimental error with ${\lambda }_m=9.6\pm 0.9\mu \mathrm{m}$ at room temperature to a minimum of ${\lambda }_m=5.5\pm 0.7\mu \mathrm{m}$ at 30 K. Using a two-dimensional finite element model to fit the data obtained for electrical magnon generation we extract the magnon spin conductivity ${\sigma }_m$ as a function of temperature, which is reduced from ${\sigma }_m=3.7\pm 0.3\times {10}^5\mathrm{S}/\mathrm{m}$ at room temperature to ${\sigma }_m=0.9\pm 0.6\times {10}^4\mathrm{S}/\mathrm{m}$ at 5 K. Finally, we observe an enhancement of the signal originating from thermally generated magnons for low temperatures where a maximum is observed around $T=7\mathrm{K}$. An explanation for this low-temperature enhancement is however still missing and requires additional investigation.

The second-layer phase diagrams of ${}^4\mathrm{He}$ and ${}^3\mathrm{He}$ adsorbed on graphite are investigated. Intrinsically rounded specific-heat anomalies are observed at 1.4 and 0.9 K, respectively, over extended density regions in between the liquid and incommensurate solid phases. They are identified to anomalies associated with the Kosterlitz-Thouless-Halperin-Nelson-Young type two-dimensional melting. The prospected low temperature phase (C2 phase) is a commensurate phase or a quantum hexatic phase with quasi-bond-orientational order, both containing zero-point defectons. In either case, this would be the first atomic realization of the quantum liquid crystal, a new state of matter. From the large enhancement of the melting temperature over ${}^3\mathrm{He}$, we propose to assign the observed anomaly of ${}^4\mathrm{He}\text{−}\mathrm{C}2$ phase at 1.4 K to the hypothetical supersolid or superhexatic transition.