The dynamic evolution in ferroelastic crystals under external shear is explored by computer simulation of a two-dimensional model. The characteristic geometrical patterns obtained during shear deformation include dynamic tweed in the elastic regime as well as interpenetrating needle domains in the plastic regime. As a result, the statistics of jerk energy differ in the elastic and plastic regimes. In the elastic regime the distributions of jerk energy are sensitive to temperature and initial configurations. However, in the plastic regime the jerk distributions are rather robust and do not depend much on the details of the configurations, although the geometrical pattern formed after yield is strongly influenced by the elastic constants of the materials and the configurations we used. Specifically, for all geometrical configurations we studied, the energy distribution of jerks shows a power-law noise pattern $P\left(E\right)\sim E^{-\left(\gamma -1\right)}(\gamma -1=1.3-2)$ at low temperatures and a Vogel-Fulcher distribution $P$($E$) ∼ exp-($E/E_0$) at high temperatures. More complex behavior occurs at the crossover between these two regimes where our simulated jerk distributions are very well described by a generalized Poisson distributions $P\left(E\right)\sim E^{-(\gamma -1)}$ exp-($E/E_0$)${}^n$ with $n$ $=$ 0.4–0.5 and γ−1 ≈ 0 (Kohlrausch law). The geometrical mechanisms for the evolution of the ferroelastic microstructure under strain deformation remain similar in all thermal regimes, whereas their thermodynamic behavior differs dramatically: on heating, from power-law statistics via the Kohlrausch law to a Vogel-Fulcher law. There is hence no simple way to predict the local evolution of the twin microstructure from just the observed statistical behavior of a ferroelastic crystal. It is shown that the Poisson distribution is a convenient way to describe the crossover behavior contained in all the experimental data without recourse to specific scaling functions or temperature-dependent cutoff lengths.

Spin-transfer ferromagnetic resonance (ST-FMR) in symmetric magnetic tunnel junctions (MTJs) with a varied thickness of the MgO tunnel barrier ($0.75\text{nm}<t_{\mathrm{MgO}}<1.05\text{nm}$) is studied using the spin-torque diode effect. The application of an rf current into nanosized MTJs generates a dc mixing voltage across the device when the frequency is in resonance with the resistance oscillations arising from the spin-transfer torque. Magnetization precession in the free and reference layers of the MTJs is analyzed by comparing ST-FMR signals with macrospin and micromagnetic simulations. From ST-FMR spectra at different dc bias voltage, the in-plane and perpendicular torkances are derived. The experiments and free electron model calculations show that the absolute torque values are independent of tunnel barrier thickness. The influence of coupling between the free and reference layer of the MTJs on the ST-FMR signals and the derived torkances are discussed.

We construct energy-optimized resonating valence bond wave functions as a means to sketch out the zero-temperature phase diagram of the square-lattice quantum Heisenberg model with competing nearest- ($J_1)$ and next-nearest-neighbor ($J_2$) interactions. Our emphasis is not on achieving an accurate representation of the magnetically disordered intermediate phase (centered on a relative coupling $g=J_2/J_1\sim \frac{1}{2}$ and whose exact nature is still controversial) but on exploring whether and how the Marshall sign structure breaks down in the vicinity of the phase boundaries. Numerical evaluation of two- and four-spin correlation functions is carried out stochastically using a worm algorithm that has been modified to operate in either of two modes: one in which the sublattice labeling is fixed beforehand and another in which the worm manipulates the current labeling so as to sample various sign conventions. Our results suggest that the disordered phase evolves continuously out of the $(\pi ,\pi )$ Néel phase and largely inherits its Marshall sign structure; on the other hand, the transition from the magnetically ordered $(\pi ,0)$ phase is strongly first order and involves an abrupt change in the sign structure and spatial symmetry as the result of a level crossing.

We investigate the pressure phase diagram of FeTe, predicting structural and magnetic properties in the normal state at zero temperature within density functional theory. We carefully examined several possible different crystal structures over a pressure range up to $\approx 30$ GPa: simple tetragonal (PbO type), simple monoclinic, orthorhombic (MnP type), hexagonal (NiAs and wurzite type), and cubic (CsCl and NaCl type). We predict pressure to drive the system through different magnetic ordering (notably also some ferromagnetic phases), eventually suppressing magnetism at around 17 GPa. We speculate the ferromagnetic order to be the reason for the absence of a superconducting phase in FeTe at variance with the case of FeSe.

Boron is a common alloying element in modern steels with a significant influence on the mechanical properties already at concentrations of only a few parts per million. The effect of boron depends on its distribution in the microstructure. Here, we characterize the elemental factors that determine the boron distribution in $\alpha$-iron by density functional theory calculations. Boron as point defect has been considered in substitutional and interstitial sites. The calculated migration barriers for the substitutional and interstitial mechanisms show the first nearest-neighbor hops being preferred over second nearest-neighbor hops. A dissociative mechanism shows boron migrating via an interstitial mechanism to be likely trapped by vacancies. In order to characterize the interaction with other point defects, we determined the distance-dependent interaction energy of a boron defect with a vacancy, a second boron, and with hydrogen, carbon, nitrogen, oxygen, aluminum, silicon, phosphorus, and sulfur atoms. We find that substitutional boron binds strongly to interstitial point defects with dumbbell formation and weaker to substitutional point defects. Interstitial boron tends to repel substitutional and interstitial point defects. We find a similarity of substitutional boron and vacancies regarding their influence on elastic properties and their interaction with point defects in $\alpha$-iron.

The growth, structure, and magnetism of Fe films on the W(111) surface were investigated using low energy electron microscopy (LEEM) and diffraction (LEED), spin polarized LEEM (SPLEEM), and work function measurements. In contrast to an earlier report that Fe grows with fcc structure following initial pseudomorpic layer growth, we observe no evidence of the formation of fcc Fe over the entire thickness range studied, up to 18 monolayers (ML). Observations are instead consistent with the formation of a well-ordered, laterally (tensile) strained bcc Fe layer that gradually relaxes vertically and develops increasing disorder with increasing thickness. Ferromagnetic order appears at 6 ML, but surprisingly vanishes at 8 ML, and reappears just as suddenly at 9 ML during Fe deposition at room temperature. Ferromagnetism between 6 and 8 ML also vanishes at only 5 deg above room temperature. The magnetization direction of a monodomain structure remains constant before and after the ferromagnetic thickness gap at 8–9 ML until the formation of a multidomain structure at about 12 ML. Variations of exchange asymmetry in spin-polarized elastic electron scattering are also observed with increasing film thickness, particularly above 12 ML, that indicate changes in the spin-polarized electron band structure above the vacuum level. The evolution of magnetism and exchange asymmetry with increasing thickness and the appearance of the ferromagnetic gap are attributed to structural and morphological changes in the strained Fe layer, which eventually lead to a relaxed although highly disordered bcc Fe layer.

The magnetic excitations in the low-temperature amplitude-modulated magnetic structure of PrNi${}_2$Si${}_2$ have been investigated by inelastic neutron scattering. The dispersions and intensities of both longitudinal and transverse excitations are measured along the high-symmetry directions. The modulated magnitude of the ordered moments implies that the longitudinally polarized magnetic excitations are more intense and dispersive than the usual transverse spin waves. Several well-defined longitudinal amplitude modes are observed to coexist with the longitudinal phason mode. The experimental results are in good overall agreement with predictions from the random-phase approximation, using parameters already established from the macroscopic properties and the paramagnetic excitations. At low energies in the neighborhood of the magnetic zone center, the magnetic phason appears to hybridize with an unidentified dispersionless mode.

A detailed and rigorous theoretical investigation of the optical properties of a generic three-dimensional chiral structure of plasma spheres, without and under the action of an external static uniform magnetic field, is presented. Corresponding photonic band diagrams in conjunction with relevant transmission spectra, calculated by the full electrodynamic layer-multiple-scattering method properly extended to the case of gyrotropic spherical scatterers, are discussed in the light of the theory of nonsymmorphic space groups. This analysis provides a consistent interpretation of some remarkable features and effects like Dirac points, polarization-dependent transmission, as well as band splitting and non-reciprocal optical response that emerge as a result of time-reversal-symmetry breaking, induced by the external static magnetic field, and the lack of space-inversion symmetry in the crystal.

We present a detailed study of the photoinduced insulator-metal transition in VO${}_2$ with broadband time-resolved reflection spectroscopy. This allows us to separate the response of the lattice vibrations from the electronic dynamics and observe their individual evolution. When we excite VO${}_2$ above the photoinduced phase transition threshold, we find that the restoring forces that describe the ground-state monoclinic structure are lost during the excitation process, suggesting that an ultrafast change in the lattice potential drives the structural transition. However, by performing a series of pump-probe measurements during the nonequilibrium transition, we observe that the electronic properties of the material evolve on a different, slower time scale. This separation of time scales suggests that the early state of VO${}_2$, immediately after photoexcitation, is a nonequilibrium state that is not well defined by either the insulating or the metallic phase.

We investigate the scattering of electrons belonging to Shockley states of (111)-oriented noble metal surfaces using angle-resolved photoemission (ARPES) and scanning tunneling microscopy (STM). Both ARPES and STM indicate that monatomic steps on a noble metal surface may act either as strongly repulsive or highly transmissive barriers for surface electrons, depending on the coherence of the step lattice, and irrespectively of the average step spacing. By measuring curved crystal surfaces with terrace length ranging from 30 to 180 Å, we show that vicinal surfaces of Au and Ag with periodic step arrays exhibit a remarkable wave function coherence beyond 100 Å step spacings, well beyond the Fermi wavelength limit and independently of the projection of the bulk band gap on the vicinal plane. In contrast, the analysis of transmission resonances investigated by STM shows that a pair of isolated parallel steps defining a 58 Å wide terrace confines and decouples the surface state of the small terrace from that of the (111) surface. We conclude that the formation of laterally confined quantum well states in vicinal surfaces as opposed to propagating superlattice states depends on the loss of coherence driven by imperfection in the superlattice order.

Graphene grown catalytically on Ir(111) shows a large substrate-induced spin-orbit splitting of ${\Delta }_{\mathrm{so}}\sim 50\mathrm{meV}$ in spin- and angle-resolved photoemission. Its origin is hybridization as revealed by large gaps in the $\pi$ band. Reducing the growth temperature results in a very large splitting of the Dirac cone, which is found to be of geometric origin and allows for the structural identification of a rotated graphene phase. The spin-orbit splitting on this phase is considerably smaller, which implies that the magnitude of the extrinsic spin-orbit splitting in supported graphene can be controlled by its rotational displacement to the substrate.

The relation between electronic structure and hydrogen content at the edges of nanosized holes (nanoholes) in graphite single layer is studied by means of atomically resolved scanning tunneling microscopy and first-principles calculations. Repeatable production of nanoholes with predominantly zigzag hydrogenated edges was realized by bombardment of the top graphene layer of graphite with low-energy (100 eV) Ar${}^{+}$ ions and its further treatment (etching) in an atomic hydrogen environment. Two main types of nanohole zigzag edge, with a striking contrast to each other, are identified: (i) monohydrogenated zigzag edge supporting edge-localized $\pi$ state on its boundary; (ii) zigzag edge with repeating two monohydrogenated sites and one dihydrogenated site, without features of the edge-localized state and characterized by a prominent standing wave pattern. Absence of the localized state at a general (chiral) hydrogenated graphitic edge consisting of a mixture of zigzag and armchair fragments is also discussed.

Using ab initio calculations based on density functional theory, we investigate the effects of vacancies on the electronic and magnetic properties of zigzag SiC nanoribbons (Z-SiCNR). Single ($V_{\mathrm{C}}$ and $V_{\mathrm{Si}}$) and double ($V_{\mathrm{Si}}{V}_{\mathrm{Si}}$ and $V_{\mathrm{Si}}{V}_{\mathrm{C}}$) vacancies are observed to induce magnetism in Z-SiCNRs. The presence of a single $V_{\mathrm{Si}}$ does not affect the half-metallic behavior of pristine Z-SiCNRs; however, a single $V_{\mathrm{C}}$ leads to a transition from half-metallic to metallic behavior in Z-SiCNRs due to the edge Si $p$ orbitals and the atoms surrounding the vacancy. The interactions of vacancies with foreign impurity atoms (B and N) are also investigated, and it is observed that $V_{\mathrm{Si}}{N}_{\mathrm{C}}$ not only suppresses the oscillatory type magnetism of $V_{\mathrm{Si}}{V}_{\mathrm{C}}$ but also retains the half-metallic character of the pristine Z-SiCNRs. The defect formation energies of vacancies can be reduced by substitutional B and N atoms. We believe that ferromagnetism is expected if Z-SiCNRs are grown under suitable conditions.

Hybrid-exchange density functional theory has been used to model the electronic structure of LaCoO${}_3$. Based on a rhombohedral unit cell of $R\overline{3}c$ symmetry containing two Co atoms we find a mixed spin phase, comprising alternating low and high spin Co${}^{+3}$ ions, with a total energy at 0 K just 57 meV per formula unit above that of a nonmagnetic semiconducting ground state. In the mixed spin phase the high-spin Co${}^{+3}$ ions have spin moments of $3.1{\mu }_B$ and the state is insulating with a band gap of 2.2 eV. Our calculations suggest that the effective on-site Coulomb repulsion energy $U_{\mathrm{eff}}$ on Co${}^{+3}$ ions is spin dependent. The $U_{\mathrm{eff}}$ on Co${}^{+3}$ ions is 7.1 eV and 8.5 eV for the nonmagnetic ground state and for the magnetic high spin state, respectively. For the mixed spin state, two different $U_{\mathrm{eff}}$ are estimated for two Co${}^{+3}$ ions in the unit cell having different spin states, 8.0 eV for the high-spin Co${}^{+3}$ ion and 7.0 eV for the low-spin Co${}^{+3}$ ion. An estimate of the harmonic phonon free energy suggests that this mixed spin phase would become the more stable phase as the temperature increases, which is consistent with experimental evidence. An alternative intermediate spin state is higher in energy at all temperatures.

We study finite-frequency quantum noise and photon-assisted electron transport through a wide and ballistic graphene sheet between two metallic leads. The elementary excitations allow us to examine the differences between effects related to Fabry-Pérot-like interferences and signatures caused by correlations of coherently scattered particles in electron- and holelike parts of the Dirac spectrum. We identify different features in the current-current auto- and cross-correlation spectra and trace them back to the interference patterns of the product of transmission and reflection amplitudes, which define the integrands of the involved correlators. At positive frequencies, the correlator of the autoterminal noise spectrum with final and initial states associated to the measurement terminal is dominant. Phase jumps occur within the interference patterns of corresponding integrands, which also reveal the intrinsic energy scale of the two-terminal graphene setup. The excess noise spectra, as well as the cross-correlation ones, show large fluctuations between positive and negative values. Oscillatory signatures of the cross-correlation noise are due to an alternating behavior of the integrands.

We report a theoretical study of light diffraction by a spherical magnetic particle with a vortex magnetization distribution. It is shown that the intensity of the diffracted light involves a nonreciprocal contribution. This contribution depends on the vorticity of particle magnetization. It appears due to the excitation of an electric quadrupole, magnetic dipole, and the addition to the electric dipole moment in the particle, which depend on the particle magnetization vorticity. The dependence of the nonreciprocal contribution on particle parameters and geometry of diffraction is analyzed separately for the specified moments. The calculation of the effect for a cobalt particle and two linear polarizations of the incident light fits the data of recent experimental studies in the lattice of triangle magnetic particles by an order of magnitude.

The low-temperature phase diagram of ${}^4$He adsorbed on a single graphene sheet is studied by computer simulations of a system consisting of nearly a thousand helium atoms. In the first layer, two commensurate solid phases are observed with fillings $1/3$ and $7/16$, respectively, separated by a domain wall phase, as well as an incommensurate crystal at a higher coverage. No evidence of a thermodynamically stable superfluid phase is found for the first adlayer. Second-layer promotion occurs at a coverage of 0.111(4) Å${}^{-2}$. In the second layer, two phases are observed, namely a superfluid and an incommensurate solid, with no commensurate solid intervening between these two phases. The computed phase diagram closely resembles that predicted for helium on graphite.

The in-plane magnetic penetration depth ${\lambda }_m\left(T\right)$ was measured in a single crystal of SrPd${}_2$Ge${}_2$ superconductor in a dilution refrigerator down to $T=60$ mK and in magnetic fields up to $H_{dc}=1$ T by using a tunnel diode resonator. The London penetration depth $\lambda$ saturates exponentially approaching $T\to 0$ indicating fully gapped superconductivity. The thermodynamic Rutgers formula was used to estimate $\lambda \left(0\right)=426\pm 60$ nm which was used to calculate the superfluid density, ${\rho }_s\left(T\right)={\lambda }^2\left(0\right)/{\lambda }^2\left(T\right)$. Analysis of ${\rho }_s\left(T\right)$ in the full temperature range shows that it is best described by a single-gap behavior, perhaps with somewhat stronger coupling. In a magnetic field, the measured penetration depth is given by the Campbell penetration depth which was used to calculate the theoretical critical current density $j_c$. For $H\le 0.45$ T, the strongest pinning is achieved not at the lowest, but at some intermediate temperature, probably due to matching effect between temperature-dependent coherence length and relevant pinning length scale. Finally, we find compelling evidence for surface superconductivity. Combining all measurements, the entire $H$-$T$ phase diagram of SrPd${}_2$Ge${}_2$ is constructed with an estimated $H_{c2}\left(0\right)=0.4817$ T.

While most B2 materials are brittle, a new class of B2 (rare-earth) intermetallic compounds is observed to have large ductility. We analytically derive a necessary condition for ductility (dislocation motion) involving $\langle 111\rangle$ versus $\langle 001\rangle$ slip and the relative stability of various planar defects that must form. We present a sufficient condition for antiphase boundary bistability on $\left\{1\overline{1}0\right\}$ and $\left\{11\overline{2}\right\}$ planes that allows multiple slip systems. From these energy-based criteria, we construct two stability maps for B2 ductility that use only dimensionless ratios of elastic constants and defect energies, calculated via density functional theory. These two conditions fully explain and predict enhanced ductility (or lack thereof) for B2 systems. In the 23 systems studied, the ductility of YAg, ScAg, ScAu, and ScPd, ductile-to-brittle crossover for other rare-earth B2 compounds, and brittleness of all classic B2 alloys and ionic compounds are correctly predicted.

Using molecular-dynamics simulation and the Meyer-Entel interaction potential, we investigate the energetics and dynamics of the phase transformation in an Fe bicrystal containing a bcc/fcc interface. The interface obeys the Nishiyama-Wassermann orientation relationship. After relaxation we observe the emergence of threading dislocations at the phase boundary growing into the bcc side. We investigate the spontaneous temperature-induced phase transformation in the Fe bicrystal at temperatures of 100 K and 1300 K. Both homogeneous (at the lower temperature) and heterogeneous transformations are observed. The propagation velocity of the phase boundary amounts to 24 m/s at the low temperature; at the higher temperature, it is one order of magnitude larger. The dynamics of the phase transformation and of the propagating interface are discussed and compared to previous results.