In graphene, a kink was observed at energies of 200 meV below the Fermi level ${\epsilon }_F$ by angle-resolved photoemission spectroscopy (ARPES). It was claimed that the kink structure can be explained by the electron-phonon interaction [Phys. Rev. Lett. 99, 236802 (2007)], but their theoretical results were not consistent with the ARPES data as well as the first-principles results. We revisit this approach and find a different quasiparticle dispersion which agrees with the ARPES data and the first-principle calculations.

The Shubnikov–de Haas effect is used to explore the conduction-band edge of high mobility SrTiO${}_3$ films doped with La. The results largely confirm the earlier measurements by Uwe et al. [Jpn. J. Appl. Phys. 24 (Suppl. 24-2), 335 (1985)]. The band edge dispersion differs significantly from the predictions of ab initio electronic structure theory.

Deposition of Mg on Si(111)7 × 7 produces an epitaxial magnesium silicide layer. Under identical annealing conditions, the thickness of this Mg${}_2$Si(111) layer increases with deposition amount, reaching a maximum of 4 monolayer (ML) and decreasing to ∼3 ML at higher Mg coverage. Excess Mg coalesces into atomically flat, crystalline Mg(0001) films. This surprising growth mode can be attributed to the accidental commensurability of the Mg(0001), Si(111), and Mg${}_2$Si(111) interlayer spacing and the concurrent minimization of in-plane Si mass transfer and domain-wall energies. The commensurability of the interlayer spacing defines a highly unique solid-phase epitaxial growth process capable of producing trilayer structures with atomically abrupt interfaces and atomically smooth surface morphologies.

We use a combined ab initio calculations and $\mathbf{k}·\mathbf{p}$ theory based approach to derive a low-energy effective Hamiltonian for monolayer ${\mathrm{MoS}}_2$ at the $K$ point of the Brillouin zone. It captures the features which are present in first-principles calculations but not explained by the theory of Xiao et al. [Phys Rev Lett 108, 196802 (2012)], namely the trigonal warping of the valence and conduction bands, the electron-hole symmetry breaking, and the spin splitting of the conduction band. We also consider other points in the Brillouin zone which might be important for transport properties. Our findings lead to a more quantitative understanding of the properties of this material in the ballistic limit.

Dirac fermions exist in many solid state systems including graphene, silicene, and other two-dimensional membranes that belong to group VI dichalcogenides, as well as on the surface of some insulators where such states are protected by topology. Coupling of those fermions to phonons introduces new structures in their dispersion curves and, in the case of massive Dirac fermions, can shift and modify the gap. We show how these changes are present in the angular-resolved photoemission spectroscopy of the dressed charge carrier dispersion curves and scanning tunneling microscopy measurements of their density of states. In particular, we focus on the region around the band gap. In this region, the charge carrier spectral density no longer consists of a dominant quasiparticle peak and a smaller incoherent phonon related background. The quasiparticle picture has broken down, and this leads to important modification in both dispersion curves and density of states.

The effect of octahedral tilting of the $A$SnO${}_3$ ($A$ $=$ Ca, Sr, Ba) parent compound and bicolor $A$SnO${}_3$/$A^{\prime }$SnO${}_3$ superlattice ($A$, $A^{\prime }$ $=$ Ca, Sr, Ba) is predicted from density-functional theory. In the $A$SnO${}_3$ parent compound, the structural phase transition as a function of the $A$-site cation size is correlated with the magnitude of the two octahedral tilting modes ($a^{-}$$a^{-}$$c^0$ tilting and $a^0$$a^0$$c^{+}$ tilting). The magnitude of octahedral tilting modes in the superlattices is analyzed quantitatively and is associated with that of constituent parent materials. $A$SnO${}_3$/$A^{\prime }$SnO${}_3$ superlattices show the hybrid improper ferroelectricity resulting from the coupling of two octahedral tilting modes ($a^{-}$$a^{-}$$c^0$ tilting and $a^0$$a^0$$c^{+}$ tilting), which are also responsible for the structural phase transition from the tetragonal phase to the orthorhombic phase. Ferroelectricity due to $A$-site mirror symmetry breaking is a secondary order parameter for the orthorhombic phase transition in the bicolor superlattice and is related to the ${\Gamma }_{5-}$ symmetry mode. The coupling between tilting modes and the ferroelectric mode in the bicolor superlattice of $A$SnO${}_3$/$A^{\prime }$SnO${}_3$ is analyzed by group theory and symmetry mode analysis.

We performed a systematic structural search of high-pressure carbon allotropes for unit cells containing from 6 to 24 atoms using the minima hopping method. We discovered a series of new structures that are consistently lower in enthalpy than the ones previously reported. Most of these include ($5+7$)- or ($4+8$)-membered rings and can therefore be placed in the families proposed by H. Niu et al. [Phys. Rev. Lett. 108, 135501 (2012)]. However, we also found three more families with competitive enthalpies that contain ($5+5+8$)-membered rings, $s{p}^2$ motives, or buckled hexagons. These structures are likely to play an important role in dislocation planes and structural defects of diamond and hexagonal diamond.

The structure-property correlation in the lead-free piezoelectric ($1-x$)(Na${}_{0.5}$Bi${}_{0.5}$)TiO${}_3$-($x$)BaTiO${}_3$ has been systematically investigated in detail as a function of composition ($0<x\le 0.11$), temperature, electric field, and mechanical impact by Raman scattering, ferroelectric, piezoelectric measurement, x-ray, and neutron powder diffraction methods. Although x-ray diffraction study revealed three distinct composition ranges characterizing different structural features in the equilibrium state at room temperature: (i) monoclinic ($Cc$)+rhombohedral ($R$3$c$) for the precritical compositions, $0\le x\le 0.05$, (ii) cubiclike for $0.06\le x\le 0.0675$, and (iii) morphotropic phase boundary (MPB) like for $0.07\le x<0.10$, Raman and neutron powder diffraction studies revealed identical symmetry for the cubiclike and the MPB compositions. The cubiclike structure undergoes irreversible phase separation by electric poling as well as by pure mechanical impact. This cubiclike phase exhibits relaxor ferroelectricity in its equilibrium state. The short coherence length (∼50 Å) of the out-of-phase octahedral tilts does not allow the normal ferroelectric state to develop below the dipolar freezing temperature, forcing the system to remain in a dipolar glass state at room temperature. Electric poling helps the dipolar glass state to transform to a normal ferroelectric state with a concomitant enhancement in the correlation length of the out-of-phase octahedral tilt.

We investigate the effect of spatially correlated disorder on two-dimensional topological insulators and on the quantum spin Hall effect which the helical edge states in these systems give rise to. Our work expands the scope of previous investigations which found that uncorrelated disorder can induce a nontrivial phase called the topological Anderson insulator (TAI). In extension of these studies, we find that spatial correlations in the disorder can entirely suppress the emergence of the TAI phase. We show that this phenomenon is associated with a quantum percolation transition and quantify it by generalizing an existing effective medium theory to the case of correlated disorder potentials. The predictions of this theory are in good agreement with our numerics and may be crucial for future experiments.

The static and dynamic behavior of Ni${}_{80}$Fe${}_{20}$ anti-ring arrays has been systematically investigated using broadband ferromagnetic resonance (FMR) spectroscopy, magneto-optical Kerr effect (MOKE), and magnetic force microscopy (MFM). The unit cell of this periodic lattice represents a hole in a continuous film, with an elliptical nanodot placed in the middle of the hole. The FMR responses display multiple absorption peaks due to the superposition of the absorptions from the anti-dot and the central nanomagnet (dot) regions of the anti-ring structures. Interestingly, for fixed anti-ring geometry, the static and dynamic behavior is markedly sensitive to the thickness of the structure due to complicated spatial distributions of the demagnetizing field. Direct MFM imaging reveals that at remanence, the central nanomagnets are in a single-domain state, for small sample thicknesses, whereas for larger thicknesses, they display a multidomain or vortex state. This observation is in good agreement with both FMR and MOKE data. All our results agree well with micromagnetic simulations.

Time-resolved x-ray diffraction is a very powerful tool for visualizing transient one-dimensional crystalline strains, ranging from crystal growth to shockwave production. In this work, we use picosecond x-ray diffraction to visualize transient strain formation from nanometer-scaled laser-excited gold films into crystalline substrates. We show that there is a direct correspondence between the measured time-resolved x-ray diffraction pattern and the transient acoustic wave, providing a straightforward method to make a reconstruction of the transient strain. In addition, we discuss real-world experimental constraints that place limits on the validity of the reconstructed transient acoustic wave.

We study the ground state phase diagram of a frustrated spin tube in a strong external magnetic field. This model can be viewed as two coupled zigzag spin chains, or as a two-leg spin ladder with frustrating next-nearest-neighbor couplings along the legs, and its study is motivated by the physics of such materials as sulfolane-${\mathrm{Cu}}_2{\mathrm{Cl}}_4$ and ${\mathrm{BiCu}}_2{\mathrm{PO}}_6$. In magnetic fields right below the saturation, the system can be effectively represented as a dilute gas of two species of bosonic quasiparticles that correspond to magnons with inequivalent incommensurate momenta at two degenerate minima of the magnon dispersion. Using the method previously proposed and tested for frustrated spin chains, we calculate effective interactions in this two-component Bose gas. On this basis, we establish the phase diagram of nearly saturated frustrated spin tube, which is shown to include the two-component Luttinger liquid, two types of vector chiral phases, and phases whose physics is determined by the presence of bound magnons. We study the phase diagram of the model numerically by means of the density matrix renormalization group technique, and find a good agreement with our analytical predictions.

Spin pumping is the most important magnetic relaxation channel in ultrathin ferromagnetic layers in contact with normal metals (NMs). Recent experiments indicate that in thick films of insulating yttrium iron garnet (YIG) there is a large broadening of the ferromagnetic resonance (FMR) lines with deposition of a thin Pt layer which cannot be explained by the known damping processes. Here we present a detailed study of the magnetic relaxation due to spin pumping in bilayers made of a ferromagnetic material (FM) and a NM. Two alternative approaches are used to calculate the transverse and longitudinal relaxation rates used in the Bloch-Bloembergen formulation of damping. In one we consider that the dynamic exchange coupling at the interface transfers magnetic relaxation from the heavily damped conduction electron spins in the NM layer to the magnetization of the FM layer while the other utilizes spin currents and the concept of the spin-mixing conductance at the interface. While in thin FM films, the relaxation rates vary with the inverse of the FM layer thickness; in thick films, they become independent of the thickness because in the FM/NM structure the FMR excitation has a surface mode character. Regardless of the thickness range the longitudinal relaxation rate is twice the transverse rate resulting in damping of the magnetization with constant amplitude characterizing a Gilbert process. The enhanced spin-pumping damping explains the experimental observations in YIG/Pt bilayers.

Motivated by recent experiments on all-optical magnetization switching (AOS), we analyze theoretically optical spin pumping in ferromagnetic metals. It is shown that spin polarizations, induced by linearly and circularly polarized pump pulses, can be comparable in magnitude. We assume that the photoinduced spin polarization influences ultrafast light-induced magnetization dynamics in ferromagnets. In particular, it plays a crucial role in AOS. We analyze the light-induced magnetization dynamics in a system of localized and itinerant spins coupled by s-d exchange interaction, assuming that light heats up the itinerant electrons and, in addition, changes their spin polarization. When the pumped spin polarization has an appropriate sign and magnitude, it triggers magnetization switching. We find that temporal evolution of the magnetization is determined by a complicated interplay between heating, the photoinduced spin polarization, and spin relaxation.

Stability conditions for a spin valve with a composite free layer consisting of two antiferromagnetically coupled films—known as synthetic antiferromagnet or ferrimagnet—is studied theoretically by means of the linearized coupled Landau-Lifshitz-Gilbert equations. The Lyapunov and Routh-Hurwitz methods have been used to examine stability of the free layer subject to external in-plane magnetic field and electric current flowing perpendicularly to the layers’ plane. A simple formula for the critical current density, valid for a variety of composite free layer structures, also has been derived. The analytical results are compared with those obtained from numerical simulations in which we took into account the spin transfer torque in the diffusive spin-dependent transport model. An excellent agreement between the analytical and numerical results has been achieved.

We develop a formalism to calculate the $L$-edge resonant x-ray scattering (RIXS) spectra from transition-metal compounds. Using a multiorbital tight-binding model, we derive useful formulas to calculate the spectra by collecting up the ladder diagrams on the basis of the Keldysh scheme, without relying on the fast collision approximation. They are feasible in the weak and intermediate coupling regimes of itinerant electron systems, where the charge and the magnetic excitations are mixed together. We examine the difference between the present formulas and those of the fast collision approximation. Finally, to demonstrate how our theory works, we employ the formulas to study the RIXS spectra on a simple model, the single-orbital Hubbard model on the square lattice at half filling. The intensities originated from the magnon and from the continuous states are obtained on the equal footing in the antiferromagnetic ground state.

We present the experimental phase diagram of LiHo${}_x$Er${}_{1-x}$F${}_4$, a dilution series of dipolar-coupled model magnets. The phase diagram was determined using a combination of ac susceptibility and neutron scattering. Three unique phases in addition to the Ising ferromagnet LiHoF${}_4$ and the $XY$ antiferromagnet LiErF${}_4$ have been identified. Below $x=0.86$, an embedded spin-glass phase is observed, where a spin glass exists within the ferromagnetic structure. Below $x=0.57$, an Ising spin glass is observed consisting of frozen needlelike clusters. For $x\sim 0.3–0.1$, an antiferromagnetically coupled spin glass occurs. A reduction of $T_C\left(x\right)$ for the ferromagnet is observed which disobeys the mean-field predictions that worked for LiHo${}_x$Y${}_{1-x}$F${}_4$.

Imaging of flux vortices in high quality MgB${}_2$ single crystals has been successfully performed in a commercial field-emission gun-based transmission electron microscope. In cryo-Lorentz microscopy, the sample quality and the vortex lattice can be monitored simultaneously, allowing one to relate microscopically the surface quality and the vortex dynamics. Such a vortex motion ultimately determines the flow resistivity ${\rho }_f$, the knowledge of which is indispensable for practical applications such as superconducting magnets or wires for magnetic resonance imaging. The observed patterns have been analyzed and compared with other studies by cryo-Lorentz microscopy or Bitter decoration. We find that the vortex lattice arrangement depends strongly on the surface quality obtained during the specimen preparation, and tends to form a hexagonal Abrikosov lattice at a relatively low magnetic field. Stripes or gossamerlike patterns, recently suggested as potential signatures of an unconventional behavior of MgB${}_2$, were not observed.

We predict and study theoretically a new nonlinear electromagnetic phenomenon in a sample of a layered superconductor of finite length placed in a waveguide with ideal walls. Two geometries are considered here: the superconducting layers parallel or perpendicular to the waveguide axis. We show that the transmittance of the superconductor slab can vary over a wide range, from nearly zero to one, when changing the amplitude of the incident wave. Thus, one can induce the total transmission or reflection of the incident wave by simply changing its amplitude. Moreover, the dependence of the superconductor transmittance on the incident wave amplitude has a hysteretic behavior with jumps. The considered phenomenon of self-induced transparency can be observed even at small amplitudes, if the wave frequency $\omega$ is close to the cutoff frequency for linear Josephson plasma waves.

The origin of giant electromechanical response in a mixed-phase rhombohedral-tetragonal BiFeO${}_3$ thin film is probed using subcoercive scanning probe microscopy based multiple-harmonic measurements. Significant contributions to the strain arise from a second-order harmonic response localized at the phase boundaries. Strain and dissipation data, backed by thermodynamic calculations, suggest that the source of the enhanced electromechanical response is the motion of phase boundaries. These findings elucidate the key role of labile phase boundaries, both natural and artificial, in achieving thin films with giant electromechanical properties.