SmN is a ferromagnetic semiconductor with the unusual property of an orbital-dominant magnetic moment that is largely canceled by an antiparallel spin contribution, resulting in a near-zero net moment. However, the ferromagnetic ground state is not fully understood, with existing density functional theory calculations providing values of the $4f$ magnetic moments at odds with experimental data. To clarify the situation, we employ an effective $4f$ Hamiltonian incorporating spin-orbit coupling, exchange, the crystal field, and $J$ mixing to calculate the ground state $4f$ moments. Our results are in excellent agreement with experimental data, revealing moderate quenching of both spin and orbital moments to magnitudes of $\sim 2$ ${\mu }_B$ in bulk SmN, enhanced to an average of $\sim 3$ ${\mu }_B$ in SmN layers within a SmN/GdN superlattice.

Skyrmion lines in metallic chiral magnets carry an emergent magnetic field experienced by the conduction electrons. The inflow and outflow of this field across a closed surface is not necessarily equal, thus it allows for the existence of emergent monopoles. One example is a segment of skyrmion line inside a crystal, where a monopole and antimonopole pair is connected by the emergent magnetic flux line. This is a realization of Dirac stringlike excitations. Here we study the dynamics of monopoles in chiral magnets under an electric current. We show that in the process of creation of skyrmion lines, skyrmion line segments are first created via the proliferation of monopoles and antimonopoles. Then these line segments join and span the whole system through the annihilation of monopoles. The skyrmion lines are destroyed via the proliferation of monopoles and antimonopoles at high currents, resulting in a chiral liquid phase. We also propose to create the monopoles in a controlled way by applying an inhomogeneous current to a crystal. Remarkably, an electric field component in the magnetic field direction proportional to the current squared in the low current region is induced by the motion of distorted skyrmion lines, in addition to the Hall and longitudinal voltage. The existence of monopoles can be inferred from transport or imaging measurements.

We propose a powerful extension to combined molecular and spin dynamics that fully captures the coupling between the atomic and spin subsystems via spin-orbit interactions. Its foundation is the inclusion of the local magnetic anisotropies that arise as a consequence of the lattice symmetry breaking due to phonons or defects. We demonstrate that our extension enables the exchange of angular momentum between the atomic and spin subsystems, which is critical to the challenges arising in the study of fluctuations and nonequilibrium processes in complex, natural, and engineered magnetic materials.

A three-dimensional kinetic Monte Carlo (KMC) model has been developed and used to simulate the microstructure and growth morphology of cubic transition metal nitride (TMN) thin films deposited by reactive magnetron sputtering. Results are presented for the case of stoichiometric TiN, chosen as a representative TMN prototype. The model is based on a NaCl-type rigid lattice and includes deposition and diffusion events for both N and Ti species. It is capable of reproducing voids and overhangs, as well as surface faceting. Simulations were carried out assuming a uniform flux of incoming particles approaching the surface at normal incidence. The ballistic deposition model is parametrized with an interaction parameter $r_0$ that mimics the capture distance at which incoming particles may stick on the surface, equivalently to a surface trapping mechanism. Two diffusion models are implemented, based on the different ways to compute the site-dependent activation energy for hopping atoms. The influence of temperature (300–500 K), deposition flux (0.1–100 monolayers/s), and interaction parameter $r_0$ (1.5–6.0 Å) on the obtained growth morphology are presented. Microstructures ranging from highly porous, [001]-oriented straight columns with smooth top surface to rough columns emerging with different crystallographic facets are reproduced, depending on kinetic restrictions, deposited energy (seemingly captured by $r_0)$, and shadowing effect. The development of facets is a direct consequence of the diffusion model which includes an intrinsic (minimum energy-based) diffusion anisotropy, although no crystallographic diffusion anisotropy was explicitly taken into account at this stage. The time-dependent morphological evolution is analyzed quantitatively to extract the growth exponent $\beta$ and roughness exponent $\alpha$, as indicators of kinetic roughening behavior. For dense TiN films, values of $\alpha \approx 0.7$ and $\beta =0.24$ are obtained in good agreement with existing experimental data. At this stage a single lattice is considered but the KMC model will be extended further to address more complex mechanisms, such as anisotropic surface diffusion and grain boundary migration at the origin of the competitive columnar growth observed in polycrystalline TiN-based films.

The evolution of the domain structure with the thickness of bcc Fe films deposited on the Ag(116) vicinal surface is studied by spin-polarized low-energy electron microscopy. We show that a spin reorientation transition proceeds via two mechanisms: continuous rotation of magnetization within the vertical plane perpendicular to the steps and discontinuous reorientation of the in-plane component of magnetization, leading to splitting of the domains. In contrast to previously investigated systems with stripe domains, we reveal that in the case of a vicinal ferromagnetic surface, the domain width increases while changing the orientation of the magnetization from a canted out-of-plane state into an in-plane state. A theoretical model developed in this work successfully describes the domain structure behavior observed in our experiments and can be equally applied to other ferromagnetic films grown on vicinal surfaces.

Phase transitions in the Hubbard model and ionic Hubbard model at half-filling on the honeycomb lattice are investigated in the strong-coupling perturbation theory which corresponds to an expansion in powers of the hopping $t$ around the atomic limit. Within this formulation we find analytic expressions for the single-particle spectrum, whereby the calculation of the insulating gap is reduced to a simple root finding problem. This enables high-precision determination of the insulating gap that does not require any extrapolation procedure. The critical value of Mott transition on the honeycomb lattice is obtained to be $U_c\approx 2.38t$. Studying the ionic Hubbard model at the lowest order, we find two insulating states, one with Mott character at large $U$ and another with single-particle gap character at large ionic potential $\mathrm{\Delta }$. The present approach gives a critical gapless state at $U=2\mathrm{\Delta }$ at lowest order. By systematically improving on the perturbation expansion, the density of states around this critical gapless phase reduces.

We propose an analytical approach for calculation of the homogenized dielectric functions ${\epsilon }_{\parallel }\left(\omega \right)$ and ${\epsilon }_{\perp }\left(\omega \right)$ of one-dimensional periodic metal-dielectric structure. The obtained formulas are valid at high frequencies near the points of topological transition from an elliptic to hyperbolic regime. The proposed method of high-frequency homogenization takes into account rapidly varying electromagnetic fields within the metallic component of a unit cell, in particular, the evanescent character of the plasmonic mode and oscillatory behavior of the waveguidelike modes. Our results show good correspondence to the exact solution of the Rytov's dispersion equation and significant deviation from the widely used quasistatic formulas obtained by spatial averaging along the direction of periodicity $z$ of $\epsilon \left(z\right)$ and $1/\epsilon \left(z\right)$. The quasistatic approach ignores $z$ dependence of the fields that leads to its limited applicability near the frequency of topological transition.

In the case of graphene, hydrogenation removes the conductivity due to the bands forming the Dirac cone by opening up a band gap. This type of chemical functionalization is of the utmost importance for electronic applications. As predicted by theoretical studies, a similar change in the band structure is expected for silicene, the closest analog to graphene. We here report a study of the atomic and electronic structures of hydrogenated silicene with hydrogen on one side, the so-called half-silicane. The (“$2\sqrt{3}\times 2\sqrt{3}$”) phase of silicene on Ag(111) was used in this Rapid Communication since it can be formed homogeneously across the entire surface of the Ag substrate. Low-energy electron diffraction and scanning tunneling microscopy data clearly show that hydrogenation changes the structure of silicene on Ag(111) resulting in a (1 × 1) periodicity with respect to the silicene lattice. The hydrogenated silicene also exhibits a quasiregular ($2\sqrt{3}\times 2\sqrt{3}$)-like arrangement of vacancies. Angle-resolved photoelectron spectroscopy revealed two dispersive bands which can be unambiguously assigned to half-silicane. The common top of these bands is located at ∼0.9 eV below the Fermi level. We find that the experimental bands are closely reproduced by the theoretical band structure of free-standing silicene with H adsorbed on the upper hexagonal sublattice.

Structural and magnetic properties of $\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{M}{\mathrm{n}}_{1-x}\mathrm{C}{\mathrm{r}}_x{\mathrm{O}}_3\left(0<x\le 0.7\right)$ have been studied in order to determine the effect of substitution of $\mathrm{C}{\mathrm{r}}^{3+}$ for $\mathrm{M}{\mathrm{n}}^{3+}$. The data consist of neutron and x-ray powder-diffraction and magnetization measurements. We previously suggested these systems transition from ferromagnetic to antiferromagnetic ordering with the intermediate concentrations containing coexisting ferromagnetic and antiferromagnetic domains. Upon further detailed examination, we find that the neutron data can be fit using a single homogeneous long-range magnetically ordered state and compositionally dependent charge ordering. The magnetic structures are controlled by the competition between Mn-Mn, Mn-Cr, and Cr-Cr interactions (double exchange and superexchange). The metal to semimetal and semimetal to insulator transitions can be quantitatively described as due to the localization effect of superexchange. The presence of charge ordered states in the insulating region arises from the favorable energetics of $\mathrm{M}{\mathrm{n}}^{4+}-\mathrm{O}-\mathrm{C}{\mathrm{r}}^{3+}$ superexchange bonds relative to $\mathrm{M}{\mathrm{n}}^{3+}-\mathrm{O}-\mathrm{C}{\mathrm{r}}^{3+}$ bonds.

We study the possibility of fractional quantum Hall effects in HgTe quantum wells using exact diagonalization. Our results show that Laughlin states, the Moore-Read state, and the Read-Rezayi $Z_3$ state can all be supported. However, near the level crossing point (of the single-particle spectrum) the gap can be destroyed by Landau level mixing, and the Moore-Read state and the Read-Rezayi state dominate over their respective competing states only for wide wells. For smaller well widths the Moore-Read state crosses over to the composite fermion Fermi sea, while the Read-Rezayi state loses its dominance over the hierarchy state.

The quasiparticle band gap is one of the most important materials properties for photovoltaic applications. Often the band gap of a photovoltaic material is determined (and can be controlled) by various factors, complicating predictive materials optimization. An in-depth understanding of how these factors affect the size of the gap will provide valuable guidance for new materials discovery. Here we report a comprehensive investigation on the band gap formation mechanism in organic-inorganic hybrid perovskites by decoupling various contributing factors which ultimately determine their electronic structure and quasiparticle band gap. Major factors, namely, quasiparticle self-energy, spin-orbit coupling, and structural distortions due to the presence of organic molecules, and their influences on the quasiparticle band structure of organic-inorganic hybrid perovskites are illustrated. We find that although methylammonium cations do not contribute directly to the electronic states near band edges, they play an important role in defining the band gap by introducing structural distortions and controlling the overall lattice constants. The spin-orbit coupling effects drastically reduce the electron and hole effective masses in these systems, which is beneficial for high carrier mobilities and small exciton binding energies.

We introduce a topological quantum number—coined dynamical topological order parameter (DTOP)—that is dynamically defined in the real-time evolution of a quantum many-body system and represented by a momentum space winding number of the Pancharatnam geometric phase. Our construction goes conceptually beyond the standard notion of topological invariants characterizing the wave function of a system, which are constants of motion under coherent time evolution. In particular, we show that the DTOP can change its integer value at discrete times where so called dynamical quantum phase transitions occur, thus serving as a dynamical analog of an order parameter. Interestingly, studying quantum quenches in one-dimensional two-banded Bogoliubov–de Gennes models, we find that the DTOP is capable of resolving if the topology of the system Hamiltonian has changed over the quench. Furthermore, we investigate the relation of the DTOP to the dynamics of the string order parameter that characterizes the topology of such systems in thermal equilibrium.

We demonstrate a new level of precision in measuring melting temperatures at high pressure using laser flash heating in a diamond cell followed by an analysis using scanning electron microscopy and focused ion beam milling. The new measurements on tantalum put unprecedented constraints on its highly debated melting slope, calling for a reevaluation of theoretical, shock compression, and diamond cell approaches to determine melting at high pressure. X-ray analysis of the recovered samples confirmed the absence of chemical reactions, which likely played a significant role in previous experiments.

Domain walls in magnetic multilayered systems can exhibit a very complex and fascinating behavior. For example, the magnetization of thin films of hard magnetic materials is in general perpendicular to the thin-film plane, thanks to the strong out-of-plane anisotropy, but its direction changes periodically, forming an alternating spin-up and spin-down stripe pattern. The latter is stabilized by the competition between the ferromagnetic coupling and dipole-dipole interactions, and disappears when a moderate in-plane magnetic field is applied. It has been suggested that such a behavior may be understood in terms of a self-induced stripe glassiness. In this paper we show that such a scenario is compatible with the experimental findings. The strong out-of-plane magnetic anisotropy of the film is found to be beneficial for the formation of both stripe-ordered and glassy phases. At zero magnetic field the system can form a glass only in a narrow interval of fairly large temperatures. An in-plane magnetic field, however, shifts the glass transition towards lower temperatures, therefore enabling it at or below room temperature. In good qualitative agreement with the experimental findings, we show that a moderate in-plane magnetic field of the order of 50 mT can lead to the formation of defects in the stripe pattern, which sets the onset of the glass transition.

The dynamics of all four combinations of possible polarity and circularity states in a stack of two vortices is investigated by time-resolved scanning transmission x-ray microscopy. The vortex stacks are excited by unidirectional magnetic fields leading to a collective oscillation. Four different modes are observed that depend on the relative polarizations and circularities of the stacks. They are excited to a driven oscillation. We observe a repulsive and attractive interaction of the vortex cores depending on their relative polarizations. The nonlinearity of this core interaction results in different trajectories that describe a two-body problem.

Interacting Bose-Fermi systems play a central role in condensed matter physics. Here, we analyze a novel Bose-Fermi mixture formed by a cavity exciton-polariton condensate interacting with a two-dimensional electron system. We show that that previous predictions of superconductivity [F. P. Laussy, Phys. Rev. Lett. 104, 106402 (2010)] and excitonic supersolid formation [I. A. Shelykh, Phys. Rev. Lett. 105, 140402 (2010)] in this system are closely intertwined, resembling the predictions for strongly correlated electron systems such as high-temperature superconductors. In stark contrast to a large majority of Bose-Fermi systems analyzed in solids and ultracold atomic gases, the renormalized interaction between the polaritons and electrons in our system is long-ranged and strongly peaked at a tunable wave vector, which can be rendered incommensurate with the Fermi momentum. We analyze the prospects for experimental observation of superconductivity and find that critical temperatures on the order of a few kelvins can be achieved in heterostructures consisting of transition metal dichalcogenide monolayers that are embedded in an open cavity structure. All-optical control of superconductivity in semiconductor heterostructures could enable the realization of new device concepts compatible with semiconductor nanotechnology. In addition the possibility to interface quantum Hall physics, superconductivity, and nonequilibrium polariton condensates is likely to provide fertile ground for investigation of completely new physical phenomena.

A weak parallel Zeeman field combined with spin-orbit coupling can induce the supercurrent in an $s$-wave two-dimensional superconductor. At the same time, the thermodynamic equilibrium state of such a system is characterized by the helix phase in which the order parameter varies in space as $exp(i\mathbf{Q}·\mathbf{r})$. In this state, the electric current that is induced by the Zeeman interaction is exactly counterbalanced by the current produced by the gradient of the order parameter. We studied the interplay of the helix state and magnetoelectric current in the case of a varying in-space Zeeman field, as it might be realized in hybrid heterostructures with magnetic and superconducting layers. The theoretical analysis was based on Usadel equations for Green functions in a dirty superconductor. It is shown that even weak inhomogeneity produces a strong long-range effect on the magnetoelectric current and the order-parameter phase. Consequently, depending on the macroscopic shape of such an inhomogeneity, either the helix state with the zero supercurrent, or a locally uniform state with the finite supercurrent, is realized. A mixture of these two extreme situations is also possible. It is also shown that the current can be induced at a large distance from a ferromagnetic island embedded into a superconductor. Quantum effects associated with the magnetoelectric effect are briefly discussed for multiply connected systems. The theory proposes an alternative point of view on the interplay of the magnetoelectric effect and helix phase in spin-orbit coupled superconductors. It also suggests an interesting method that enables us to couple superconducting and magnetic circuits.

We have investigated atomic dynamics of caged compound ${\mathrm{LaRu}}_2{\mathrm{Zn}}_{20}$ by the measurements of specific heat $C$ and inelastic x-ray scattering (IXS). The lattice part of the specific heat $C_{\mathrm{lat}}$ divided by $T^3,C_{\mathrm{lat}}/T^3$, shows a broad peak at around 15 K, which is reproduced by two Einstein modes with characteristic temperatures of ${\theta }_{\mathrm{E}1}=35$ K and ${\theta }_{\mathrm{E}2}=82$ K, respectively. IXS measurements along the [111] and [110] directions reveal weakly dispersive phonon modes at 3 meV (35 K) and 7 meV (80 K), respectively, whose values agree with the values of ${\theta }_{\mathrm{E}}$'s. The first-principles calculation has assigned the 3 meV phonon modes as the optical modes of Zn atoms located at the middle of two La atoms, which is likely to lead to the structural instability.

The high-pressure behavior of $\mathrm{C}{\mathrm{e}}_2{\mathrm{O}}_3$ was studied using angle-dispersive x-ray diffraction to 70 GPa and compared with that of $\mathrm{Ce}{\mathrm{O}}_2$. Up to the highest pressure $\mathrm{C}{\mathrm{e}}_2{\mathrm{O}}_3$ remains in the hexagonal phase (space group 164, $P\overline{3}2/m1)$ typical for the lanthanide sesquioxides. A theoretically predicted phase instability for 30 GPa is not observed. The isothermal bulk modulus and its pressure derivative for the quasihydrostatic case are $B_0=111\pm 2\mathrm{GPa},B_0^{\prime }=4.7\pm 0.3,$ and for the case without pressure-transmitting medium $B_0=104\pm 4\mathrm{GPa},B_0^{\prime }=6.5\pm 0.4$. Starting from ambient-pressure magnetic susceptibility measurements for both oxides in highly purified form, we find that the Ce atom in $\mathrm{C}{\mathrm{e}}_2{\mathrm{O}}_3$ behaves like a trivalent $\mathrm{C}{\mathrm{e}}^{3+}$ ion ($2.57{\mu }_{\mathrm{B}}$ per Ce atom) in contrast to previously published data. Since x-ray emission spectroscopy of the $L\gamma$ $\left(4d_{3/2}\to 2p_{1/2}\right)$ transition is sensitive to the $4f$ -electron occupancy, we also followed the high-pressure dependence of this line for both oxides up to 50 GPa. No change of the respective line shape was observed, indicating that the $4f$-electron configuration is stable for both materials. We posit from this data that the $4f$ electrons do not drive the volume collapse of $\mathrm{Ce}{\mathrm{O}}_2$ from the high-symmetry, low-pressure fluorite structure to the lower-symmetry orthorhombic phase.