We investigate the formation of ion-induced patterns on single-crystalline Ni(001) bombarded with a $20-\mathrm{keV}{\mathrm{Ga}}^{+}$ ion beam. For near normal ion incidence isotropic roughness forms on the surface. By moving to grazing incidence this pattern gradually transforms into a pronounced ripple pattern with wave vector perpendicular to the ion beam. By using an in situ scanning electron microscope we were able to analyze the real-time dynamics of the pattern formation process and evaluate the direction and velocity of the net in-plane morphology propagation for different angles of ion incidence. We compare the experimental results to the predictions of the classical theory for the pattern formation.

We show how the two-dimensional (2D) topological insulator evolves, by stacking, into a strong or weak topological insulator with different topological indices, proposing a new conjecture that goes beyond an intuitive picture of the crossover from quantum spin Hall to weak topological insulator. Studying the conductance under different boundary conditions, we demonstrate the existence of two conduction regimes in which conduction happens through either surface or edge conduction channels. We show that the two conduction regimes are complementary and exclusive. Conductance maps in the presence and absence of disorder are introduced, together with 2D ${\mathbb{Z}}_2$-index maps, describing the dimensional crossover of the conductance from the 2D to the 3D limit. Stacking layers is an effective way to invert the gap, an alternative to controlling the strength of spin-orbit coupling. The emerging quantum spin Hall insulator phase is not restricted to the case of odd numbers of layers.

The quasiparticle interferences (QPIs) of the featureless Mott insulators are investigated by a $T$-matrix formalism implemented with the dynamical mean field theory ($T$-DMFT). In the Mott insulating state, due to the singularity at zero frequency in the real part of the electron self-energy $\left[\mathrm{Re}\mathrm{\Sigma }\right(\omega )\sim \eta /\omega ]$ predicted by DMFT, where $\eta$ can be considered as the “order parameter” for the Mott insulating state, QPIs are completely washed out at small bias voltages. However, a further analysis shows that $\mathrm{Re}\mathrm{\Sigma }\left(\omega \right)$ serves as an energy-dependent chemical potential shift. As a result, the effective bias voltage seen by the system is $e{V}^{\prime }=eV-\mathrm{Re}\mathrm{\Sigma }\left(eV\right)$, which leads to a critical bias voltage $e{V}_c\sim \sqrt{\eta }$ satisfying $e{V}^{\prime }=0$ if and only if $\eta$ is nonzero. Consequently, the same QPI patterns produced by the noninteracting Fermi surfaces appear at this critical bias voltage $e{V}_c$ in the Mott insulating state. We propose that this reentry of noninteracting QPI patterns at $e{V}_c$ could serve as an experimental signature of the Mott insulating state, and the order parameter can be experimentally measured as $\eta \sim {\left(e{V}_c\right)}^2$.

We theoretically study the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between magnetic impurities in both Dirac and Weyl semimetals (SMs). We find that the internode process, as well as the unique three-dimensional spin-momentum locking, has significant influences on the RKKY interaction, resulting in both a Heisenberg and an Ising term, and an additional Dzyaloshinsky-Moriya term if the inversion symmetry is absent. These interactions can lead to rich spin textures and possible ferromagnetism in Dirac and time-reversal symmetry-invariant Weyl SMs. The effect of anisotropic Dirac and Weyl nodes on the RKKY interaction is also discussed. Our results provide an alternative scheme to engineer topological SMs and shed new light on the application of Dirac and Weyl SMs in spintronics.

Fabrication of devices made by isolated graphene layers has opened up possibility of examining highly correlated states of electron systems in parts of their phase diagram that is impossible to access in their counterpart devices such as semiconductor heterostructures. An example of such states are graphene double monolayer electron-hole systems under strong magnetic fields where the separation between layers can be adjusted to be as small as one magnetic length with interlayer tunneling still suppressed. In those separations, it is known that correlations between electrons and holes are of crucial importance and must be included in determination of observable quantities. Here we report the results of our full numerical Hartree-Fock study of coherent and crystalline ground states of the interacting balanced electron-hole graphene systems in small and intermediate separations with each layer occupying up to four lowest lying Landau levels. We show that in the Hartree-Fock approximation the electrons and holes pair to form a homogeneous Bose-condensed (excitonic) state, while crystalline states of such exciton systems remain incoherent at intermediate layer separations. Our results of calculation of capacitance of such states as a function of interlayer separation and filling factor provides quantitative and qualitative signatures that can be examined in real experiments. We show that the capacitance of some crystallized states as well as uniform coherent states are significantly enhanced compared to geometrical values solely due to Coulomb interactions and quantum corrections.

$\mathrm{S}{\mathrm{r}}_2\mathrm{Ir}{\mathrm{O}}_4$ is a magnetic insulator assisted by strong spin-orbit coupling (SOC) whereas $\mathrm{S}{\mathrm{r}}_2\mathrm{Ru}{\mathrm{O}}_4$ is a $p$-wave superconductor. The contrasting ground states have been shown to result from the critical role of the strong SOC in the iridate. Our investigation of structural, transport, and magnetic properties reveals that substituting $4d\mathrm{R}{\mathrm{u}}^{4+}\left(4d^4\right)$ ions for $5d\mathrm{I}{\mathrm{r}}^{4+}\left(5d^5\right)$ ions in $\mathrm{S}{\mathrm{r}}_2\mathrm{Ir}{\mathrm{O}}_4$ directly adds holes to the $t_{2g}$ bands, reduces the SOC, and thus rebalances the competing energies in single-crystal $\mathrm{S}{\mathrm{r}}_2\mathrm{I}{\mathrm{r}}_{1-x}\mathrm{R}{\mathrm{u}}_x{\mathrm{O}}_4$. A profound effect of Ru doping driving a rich phase diagram is a structural phase transition from a distorted $I{4}_1/acd$ to a more ideal $I4/mmm$ tetragonal structure near $x=0.50$ that accompanies a phase transition from an antiferromagnetic-insulating state to a paramagnetic-metal state. We also make a comparison with Rh-doped $\mathrm{S}{\mathrm{r}}_2\mathrm{Ir}{\mathrm{O}}_4$, highlighting important similarities and differences.

We study experimentally the influence of temperature on the emission characteristics of quantum dot-micropillars in the strong coupling regime of cavity quantum electrodynamics (cQED). In particular, we investigate its impact on the vacuum Rabi splitting (VRS) and we address the important question of the temperature stability of the coherent coupling regime in a semiconductor system, which is relevant in view of both fundamental study and future applications. To study the temperature dependence we investigate an unprecedentedly large number of strong coupling cases (89) in a wide temperature range from 10 up to 50 K, which constitutes a good basis for statistical analysis. The experiment indicates a statistically significant increase of the VRS with temperature in contrast to an expected decrease of the VRS due to the dephasing induced by acoustic phonons. From the theoretical point of view, the phonon-induced renormalization of the VRS is calculated using a real-time path-integral approach for strongly confined quantum dots (QDs), which allows for a numerical exact treatment of the coupling between the QD and a continuum of longitudinal acoustic phonons. The absence of the expected decrease of the VRS with temperature in our experimental data can be attributed to a unique optical property of laterally extended $\mathrm{I}{\mathrm{n}}_{0.4}\mathrm{G}{\mathrm{a}}_{0.6}\mathrm{As}$ QDs used in this study. Their electronic structure facilitates an effective temperature-driven increase of the oscillator strength of the excitonic state by up to 40% in the given temperature range. This leads to enhanced light-matter interaction and overcompensates the phonon-related decrease of the VRS. The observed persistence of strong coupling in the presence of phonon-induced decoherence demonstrates the appealing possibility to counteract detrimental phonon effects in the cQED regime via engineering the electronic structure of QDs.

The dynamics of a classical heavy particle moving in a quantum environment is determined by a Langevin equation which encapsulates the effect of the environment-induced reaction forces on the particle. For an open quantum system, these include a Born-Oppenheimer force, a dissipative force, and a stochastic force due to shot and thermal noise. Recently, it was shown that these forces can be expressed in terms of the scattering matrix of the system by considering the classical heavy particle as a time-dependent scattering center, allowing to demonstrate interesting features of these forces when the system is driven out of equilibrium. At the same time, it is well known that small changes in a scattering potential can have a profound impact on a fermionic system due to the Anderson orthogonality catastrophe. In this work, by calculating the Loschmidt echo, we relate Anderson orthogonality effects with the mesoscopic reaction forces for an environment that can be taken out of equilibrium. In particular, we show how the decay of the Loschmidt echo is characterized by fluctuations and dissipation in the system and discuss different quench protocols.

Density functional theory is used to study structural energetics of Pb vacancy cluster formation on ${\mathrm{C}}_{60}/\mathrm{Pb}/\mathrm{Si}\left(111\right)$ to explain the unusually fast and error-free transformations between the “Devil's Staircase” (DS) phases on the Pb/Si(111) wetting layer at low temperature $(\sim 110\mathrm{K})$. The formation energies of vacancy clusters are calculated in ${\mathrm{C}}_{60}/\mathrm{Pb}/\mathrm{Si}\left(111\right)$ as Pb atoms are progressively ejected from the initial dense Pb wetting layer. Vacancy clusters larger than five Pb atoms are found to be stable with seven being the most stable, while vacancy clusters smaller than five are highly unstable, which agrees well with the observed ejection rate of ∼5 Pb atoms per ${\mathrm{C}}_{60}$. The high energy cost (∼0.8 eV) for the small vacancy clusters to form indicates convincingly that the unusually fast transformation observed experimentally between the DS phases, upon ${\mathrm{C}}_{60}$ adsorption at low temperature, cannot be the result of single-atom random walk diffusion but of correlated multi-atom processes.

We investigate the quantum phase transition in an $S=1/2$ dimerized Heisenberg antiferromagnet in three spatial dimensions. By performing large-scale quantum Monte Carlo simulations and detailed finite-size scaling analyses, we obtain high-precision results for the quantum critical properties at the transition from the magnetically disordered dimer-singlet phase to the antiferromagnetically ordered Néel phase. This transition breaks $\text{O}\left(N\right)$ symmetry with $N=3$ in $D=3+1$ dimensions. This is the upper critical dimension, where multiplicative logarithmic corrections to the leading mean-field critical properties are expected; we extract these corrections, establishing their precise forms for both the zero-temperature staggered magnetization $m_s$ and the Néel temperature $T_N$. We present a scaling ansatz for $T_N$, including logarithmic corrections, which agrees with our data and indicates exact linearity with $m_s$, implying a complete decoupling of quantum and thermal fluctuation effects even arbitrarily close to the quantum critical point. We also demonstrate the predicted $N$-independent leading and subleading logarithmic corrections in the size dependence of the staggered magnetic susceptibility. These logarithmic scaling forms have not previously been identified or verified by unbiased numerical methods, and we discuss their relevance to experimental studies of dimerized quantum antiferromagnets such as ${\mathrm{TlCuCl}}_3$.

The conventional spin-diffusion equation, based on the presence of spin-split local chemical potentials, has successfully described spin accumulation attendant to diffusive transport in spintronics. A recent experiment shows that spin accumulation far exceeds the limit set by such spin-diffusive theory when the mean free path is longer than the spin dephasing length. By introducing the momentum and spin dependent chemical potential, we develop a generalized spin transport equation that is capable of addressing spin transport in systems where ballistic processes are embedded in the overall diffusive conductor. We find that the ballistic spin injection through a barrier into a diffusive nonmagnetic layer with strong spin-orbit coupling can enhance spin accumulation by an order of magnitude when compared to the conventional theory.

We investigate the dependence of the photoinduced inverse spin-Hall effect (ISHE) signal in a set of Pt/Ge(001) junctions as a function of the Pt thickness $t_{\mathrm{Pt}}$ in the 1–20-nm range at room temperature. The spin-polarized electrons, photoexcited through optical orientation at the direct gap of bulk Ge, diffuse towards the Pt layer, where spin-dependent scattering yields a transverse electromotive field ${\mathbf{E}}_{\mathrm{ISHE}}$. We show that the experimental data can be properly interpreted in the frame of the spin diffusion model, provided that we consider a variation of the spin diffusion length $L_{\mathrm{s}}$ in platinum when the Pt film thickness is increased. Indeed, for $t_{\mathrm{Pt}}>10$ nm we estimate $L_{\mathrm{s}}=8.2\pm 3.5$ nm, whereas for $t_{\mathrm{Pt}}<10$ nm the spin diffusion length decreases to $L_{\mathrm{s}}=3.5\pm 2.1$ nm. Moreover, the experimental results clearly indicate that for $t_{\mathrm{Pt}}<10$ nm the spin Hall angle $\gamma$ is three times larger than the value corresponding to thick Pt layers, suggesting a drastic change of the spin-dependent scattering mechanism for very thin Pt films.

We present the results of elastic and inelastic neutron scattering measurements on nonsuperconducting $\mathrm{Ba}\left(\mathrm{Fe}{}_{0.957}\mathrm{Cu}{}_{0.043}\right){}_2\mathrm{As}{}_2$, a composition close to a quantum critical point between antiferromagnetic (AFM) ordered and paramagnetic phases. By comparing these results with the spin fluctuations in the low-Cu composition as well as the parent compound ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ and superconducting $\mathrm{Ba}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Ni}}_x\right)}_2{\mathrm{As}}_2$ compounds, we demonstrate that paramagnon-like spin fluctuations are evident in the antiferromagnetically ordered state of $\mathrm{Ba}{\left({\mathrm{Fe}}_{0.957}{\mathrm{Cu}}_{0.043}\right)}_2{\mathrm{As}}_2$, which is distinct from the AFM-like spin fluctuations in the superconducting compounds. Our observations suggest that Cu substitution decouples the interaction between quasiparticles and the spin fluctuations. We also show that the spin-spin correlation length $\xi \left(T\right)$ increases rapidly as the temperature is lowered and find $\omega /T$ scaling behavior, the hallmark of quantum criticality, at an antiferromagnetic quantum critical point.

The knowledge of the spin diffusion length, ${\lambda }_A$, is a prerequisite for the estimation of the spin Hall angle of given materials. We investigate the spin-current absorption of materials with a short ${\lambda }_A$ using AuW stripes inserted in Cu-based lateral spin valves. Width variations of the AuW stripe lead to drastic changes of the spin absorption which cannot be explained by a conventional analysis. We show that the spin-current polarization and the spin-accumulation attenuation in Cu in the vicinity of the spin absorber must be precisely taken into account for an accurate estimation of ${\lambda }_A$. We propose an analytical extension of the standard diffusion model of spin transport and spin absorption based on the existence of an effective spin diffusion length for Cu being in direct contact with AuW. The calculations are supported by numerical investigations which allow us to extract proper values of ${\lambda }_A$.

Phase transitions in isotropic quantum antiferromagnets are described by an O(3) nonlinear quantum field theory. In three dimensions, the fundamental property of this theory is logarithmic scaling of the coupling constant. At the quantum critical point the coupling asymptotically vanishes, and the quasiparticles become free. This logarithmic decay of the coupling constant has never been observed. In this Rapid Communication, we derive finite-temperature properties of the field theory and use our results to analyze the existing data on the real antiferromagnet ${\mathrm{TlCuCl}}_3$. Including finite temperatures in the theory, we find that agreement between theory and experiment is sufficiently sensitive to unambiguously identify the asymptotic decay of the coupling constant. We also comment on the unique possibility to study Landau pole physics in quantum magnets.

We investigated the effect of electric current on the magnetic-field-driven magnetic domain wall (DW) creep velocity in ultrathin Co with perpendicular magnetic anisotropy deposited on a Pt underlayer. The DW velocity was considerably modulated by the current, and its field dependence deviated from the scaling law with the critical exponent of 1/4, which is generally valid in ferromagnetic metals. This characteristic feature of the DW motion can be explained by considering the perpendicular effective field generated by spin-orbit torque at the Pt/Co interface. From the relation between the injected current and the modified creep velocity, the determination of the effective field was demonstrated.

Four-dimensional ultrafast transmission electron microscopy measurements reveal a rich variety of structural dynamic phenomena at a phase transition in the charge density wave (CDW) of $1T\text{−}{\mathrm{TaSe}}_2$. Under photoexcitation, remarkable changes in both the CDW intensity and orientation are clearly observed, associated with the transformation from a commensurate (C) phase into an incommensurate (IC) phase in a time scale of picoseconds. Moreover, the transient states reveal a notable “structural isosbestic point” at a wave vector of $q_{\mathrm{iso}}$ where the C and IC phases yield their diffracting efficiencies in equal ratio. This observation reveals that the crystal planes parallel to $q_{\mathrm{iso}}$ adopt a common dynamic feature in the CDW phases throughout whole nonequilibrium transition. The second-order characteristics observed in this nonequilibrium phase transition have also been analyzed based on time-resolved structural data.

Magnetization dynamics are strongly influenced by damping, namely, the loss of spin angular momentum from the magnetic system to the lattice. An “effective” damping constant ${\alpha }_{\mathrm{eff}}$ is often determined experimentally from the spectral linewidth of the free induction decay of the magnetization after the system is excited to its nonequilibrium state. Such an ${\alpha }_{\mathrm{eff}}$, however, reflects both intrinsic damping as well as inhomogeneous broadening that arises, for example, from spatial variations of the anisotropy field. In this paper, we compare measurements of the magnetization dynamics in ultrathin nonepitaxial films having perpendicular magnetic anisotropy using two different techniques, time-resolved magneto-optical Kerr effect (TRMOKE) and hybrid optical-electrical ferromagnetic resonance (OFMR). By using an external magnetic field that is applied at very small angles to the film plane in the TRMOKE studies, we develop an explicit closed-form analytical expression for the TRMOKE spectral linewidth and show how this can be used to reliably extract the intrinsic Gilbert damping constant. The damping constant determined in this way is in excellent agreement with that determined from the OFMR method on the same samples. Our studies indicate that the asymptotic high-field approach that is often used in the TRMOKE method to distinguish the intrinsic damping from the effective damping may result in significant error, because such high external magnetic fields are required to make this approach valid that they are out of reach. The error becomes larger at lower intrinsic damping constants and thus may account for the anomalously high damping constants that are often reported in TRMOKE studies. In conventional ferromagnetic resonance (FMR) studies, inhomogeneous contributions can be readily distinguished from intrinsic damping contributions by studying the magnetic field dependence of the FMR linewidth. Using an analogous approach, we show how reliable values of the intrinsic damping can be extracted from TRMOKE in two distinct magnetic systems with significant perpendicular magnetic anisotropy: ultrathin CoFeB layers and Co/Ni/Co trilayers.

Growth of magnetron sputtered Fe films on clean single crystalline MgO (001) substrate has been studied using in situ grazing incidence small angle x-ray scattering (GISAXS) and grazing incidence nuclear resonant scattering (GINRS) measurements. While GISAXS provides information about morphological changes, GINRS provides information about structural and magnetic properties, thus making it possible to correlate the evolution of magnetic properties with that of morphology and structure of the film. The film exhibits a Volmer-Weber type growth, with percolation transition occurring around 2 nm film thickness. Presence of a finite quadrupole splitting, as seen in GINRS measurements, suggests a significant distortion from cubic symmetry up to a film thickness of 3.5 nm, which can be attributed to hybridization between Fe $3d$ and O $2p$ orbitals at the interface as well as in-plane tensile strain induced as a result of coalescence of islands. Initially Fe islands exhibit superparamagnetic relaxation, while finite magnetic moment appears upon formation of macroscopic percolation islands. The film exhibits a weak perpendicular magnetic anisotropy (PMA), which vanishes concurrently with disappearance of structural distortion, suggesting that the observed PMA at least partly originates from inherent strain in the film. No presence of any known oxide of Fe was detected at the interface. More precise information about topological and magnetic structure of the interfaces between Fe and MgO layers is obtained using combined x-ray reflectivity and nuclear resonance reflectivity measurements on a ${}^{57}\mathrm{Fe}/\mathrm{MgO}$ multilayer. Measurements show that about two monolayers of Fe at the interface have a reduced hyperfine field, providing evidence for hybridization with O atoms, as predicted by theory.