Table of contents

By mapping the low-energy electronic dynamics using angle resolved photoemission spectroscopy (ARPES), we have shed light on essential electronic characteristics of the (3 × 3) silicene phase on Ag(111) surfaces. In particular, our results show a silicene-derived band with a clear gap and linear energy–momentum dispersion near the Fermi level at the Γ symmetry point of the (3 × 3) phase at several distinctive Brillouin zones. Moreover, we have confirmed that the large buckling of ∼0.7 Å of this silicene structure induces the opening of a gap close to the Fermi level higher than at least 0.3 eV, in agreement with recent reported photoemission results. The two-dimensional character of the charge carriers has also been revealed by the photon energy invariance of the gapped silicene band, suggesting a limited silicene–silver hybridization, in disagreement with recent density-functional theory (DFT) predictions.

Density functional theory calculations have been performed to provide details of the structural and charge-transfer details related to the solid solution of hydrogen in (δ)-plutonium. We follow the Flanagan model that outlines the process by which hydrogen interacts with a metal to produce hydride phases, via a sequence of surface, interstitial and defect-bound (trapped) states. Due to the complexities of the electronic structure in plutonium solid-state systems, we take the pragmatic approach of adopting the 'special quasirandom structure' to disperse the atomic magnetic moments. We find that this approach produces sound structural and thermodynamic properties in agreement with the available experimental data. In δ-Pu, hydrogen has an exothermic binding energy to all of the states relevant in the Flanagan model, and, furthermore, is anionic in all these states. The charge transfer is maximized (i.e. most negative for hydrogen) in the hydride phase. The pathway from surface to hydride is sequentially exothermic, in the order surface < interstitial < grain boundary < vacancy < hydride (hydride being the most exothermic state). Thus, we find that there is no intermediate state that involves an endothermic increase in energy, consistent with the general experimental observations that the hydriding reaction in plutonium metal can proceed with zero apparent activation barrier.

Bond-order potentials (BOPs) are derived from the tight-binding approximation and provide a linearly-scaling computation of the energy and forces for a system of interacting atoms. While the numerical BOPs involve the numerical integration of the response (Green's) function, the expressions for the energy and interatomic forces are analytical within the formalism of the analytic BOPs. In this paper we present a detailed comparison of numerical and analytic BOPs. We use established parametrizations for the bcc refractory metals W and Mo and test structural energy differences; tetragonal, trigonal, hexagonal and orthorhombic deformation paths; formation energies of point defects as well as phonon dispersion relations. We find that the numerical and analytic BOPs generally are in very good agreement for the calculation of energies. Different from the numerical BOPs, the forces in the analytic BOPs correspond exactly to the negative gradients of the energy. This makes it possible to use the analytic BOPs in dynamical simulations and leads to improved predictions of defect energies and phonons as compared to the numerical BOPs.

During deposition on a stepped surface the growth mode depends on the conditions such as temperature T, deposition rate F and width of the terraces w. In this work we studied the influence of all the above mentioned characteristics using the kinetic Monte Carlo (kMC) technique. We concentrated on the conditions on the terrace at the moment of the first nucleation. The critical density of monomers for nucleation η_m decreases with the width of the terrace and the nucleation starts at surprisingly low densities of monomers. We tested several definitions of the critical width for nucleation w_c used in various articles in the past and we compared our results with results of the analytical steady-state mean-field model (Ranguelov and Altman 2007 Phys. Rev. B 75 245419). To check how the simplified assumption about the steady-state regime during deposition influences the resulting dependence of w_c ≃ (D/F)^κ, we set and also solved a time-dependent analytical model. This analytical model as well as kMC predict that w_c ≃ (D/F)^1/5.

kMC simulation also shows that the Ehrlich–Schwöbel barrier has only limited influence on the nucleation on the stepped surface at conditions close to the nucleation regime. For all widths of terraces there is a critical value of the Ehrlich–Schwöbel barrier $\Delta {E}_{\mathrm{ES}}^{\mathrm{c}}/{k}_{\mathrm{B}}T\sim 7.3$ ($\Delta {E}_{\mathrm{ES}}^{\mathrm{c}}\sim 0.1 1$ eV at T = 175 K), and only below this critical value does the Ehrlich–Schwöbel barrier affect the final value of the density of nuclei.

The results of the kMC are summarized in a semi-empirical analytical formula which describes the dependence of the step-flow growth and nucleation on the terrace width w, diffusion coefficient D and deposition rate F. In our simulations we tested two models of the stepped surface with different thicknesses of the step, both with an Ehrlich–Schwöbel barrier on the edge of the terrace.

We investigate the energetic stability and dissociation dynamics of water adsorption at the LaAlO₃ surface of the n-type LaAlO₃/SrTiO₃ (LAO/STO) interface and its effect on the electronic properties of the interface by carrying out first-principles electronic structure calculations. In an ambient atmosphere at room temperature the configuration of 1 monolayer (ML) of water molecules including 3/4 ML of dissociated water molecules adsorbed at the surface is found to be most stable, whereas the configuration of 1 ML of dissociated water molecules is metastable. Water molecule dissociation induces an up-shift of the valence band maximum (VBM) of the LAO surface, reducing the gap between the VBM of the LAO surface and the conduction band minimum of the STO. For the LAO/STO interface with three LAO unit-cell layers, once the coverage of dissociated water molecules reaches 1/2 ML the gap is closed, the interface becomes metallic and the carrier density at the LAO/STO interface increases with increasing coverage of dissociated water molecules. Our findings suggest two ways to control the conductivity at the LAO/STO interface: (I) an insulator–metal transition by adsorbing an amount of water at the bare surface; (II) a carrier density change by the transition between the most stable and the metastable adsorption configurations for 1 ML coverage in an ambient atmosphere at room temperature.

The effects of S atom surface adsorption and substitution on the helical surface states of Sb₂Te₃ are studied by the density-functional theory with spin–orbit coupling being taken into account self-consistently. It is found that S atoms play the role of surface passivation when adsorbed on both surfaces of a 6QL Sb₂Te₃ film in symmetrical configuration. For symmetrical surfaces with both the top and bottom surfaces of a thin film with adsorbed S atoms, the linear dispersion of the surface states is found to be preserved and the topological surface states survive. The spatial distribution of charge density of the surface state at the $\bar {\Gamma }$ point is also symmetric. For a film with asymmetric S atom adsorption, i.e., only one of the surfaces has adsorbed S atoms, the surface band structure is found to be very different. The degeneracy of the surface states from the two sides of a film is broken. The gap opens slightly at $\bar {\Gamma }$ and the spatial distribution of charge density of the surface state at the $\bar {\Gamma }$ point is also modified greatly. The Fermi level is robust against S impurity adsorption on the surface of Sb₂Te₃. Compared with S substitution, the effect of single surface S adsorption on electron structures is more prominent. This supports the idea that the topological insulator surface electronic states are dominated by its structural symmetry and the effect of the asymmetric environment of topological insulator Sb₂Te₃ films should thus be considered.

We theoretically study the phonon-drag contribution to the thermoelectric power and the hot-electron energy-loss rate in a Rashba spin–orbit coupled two-dimensional electron system in the Bloch–Gruneisen (BG) regime. We assume that electrons interact with longitudinal acoustic phonons through a deformation potential and with both longitudinal and transverse acoustic phonons through a piezoelectric potential. The effect of the Rashba spin–orbit interaction on the magnitude and temperature dependence of the phonon-drag thermoelectric power and hot-electron energy-loss rate is discussed. We numerically extract the exponent of temperature dependence of the phonon-drag thermopower and the energy-loss rate. We find that the exponents are suppressed due to the presence of the Rashba spin–orbit coupling.

The structural behaviour under compression of different lanthanide (La, Gd, Ho, Yb) and actinide (Am) monochalcogenides is studied by means of in situ high-pressure x-ray diffraction. All the investigated compounds crystallize at ambient conditions within a cubic (B1) NaCl-type structure but show different behaviours at high pressures. LaTe and AmTe undergo B1 to B2 (CsCl-type structure) phase transitions, starting at 9 GPa and 12 GPa, respectively. The high-pressure phase of AmTe exhibits an electronic transition, identified by an anomaly in the compression curve which is accompanied by a sample colour change. The other three monochalcogenides studied here show clear evidence of decomposition and amorphization under pressure and are, to the best of our knowledge, the first in the LnTe series to show a pressure-induced amorphization. The bulk moduli of all B1-type structure compounds are calculated using the third-order Birch–Murnaghan equation of state.

The ferromagnetic heavy fermion compound Ce₃RhSi₃ was studied by means of electrical resistivity, Hall effect, thermoelectric power and Nernst coefficient measurements. Below T ≈ 30 K, all the transport characteristics were found to behave anomalously as functions of temperature and magnetic field. In particular, the Hall and Nernst coefficients at low temperatures exhibit pronounced and strongly field-dependent maxima, likely possessing the same microscopic origin, which however cannot be captured by available theoretical models.

The interplay between superconductivity and Eu²⁺ magnetic ordering in Eu(Fe_1−xIr_x)₂As₂ is studied by means of electrical transport and magnetic measurements. For the near optimally doped sample Eu(Fe_0.86Ir_0.14)₂As₂, we witnessed two distinct transitions: a superconducting transition below 22.6 K which is followed by a resistivity reentrance caused by the ordering of the Eu²⁺ moments. Further, the low field magnetization measurements show a prominent diamagnetic signal due to superconductivity, which is remarkable in the presence of a large-moment magnetically ordered system. The electronic structure for 12.5% Ir doped EuFe_1.75Ir_0.25As₂ is investigated along with the parent compound EuFe₂As₂. As compared to EuFe₂As₂, the doped compound has an effectively lower value of density of states throughout the energy scale with a more extended bandwidth and stronger hybridization involving Ir. Shifting of the Fermi energy and a change in band filling in EuFe_1.75Ir_0.25As₂ with respect to the pure compound indicate electron doping in the system.

The understanding of radiation-induced strengthening in ferritic FeCr-based steels remains an essential issue in the assessment of materials for fusion and fission reactors. Both early and recent experimental works on Fe–Cr alloys reveal Cr segregation on radiation-induced nanostructural features (mainly dislocation loops), whose impact on the modification of the mechanical response of the material might be key for explaining quantitatively the radiation-induced strengthening in these alloys. In this work, we use molecular dynamics to study systematically the interaction of dislocations with 1/2〈111〉 and 〈100〉 loops in all possible orientations, both enriched by Cr atoms and undecorated, for different temperatures, loop sizes and dislocation velocities. The configurations of the enriched loops have been obtained using a non-rigid lattice Monte Carlo method. The study reveals that Cr segregation influences the interaction mechanisms with both 1/2〈111〉 and 〈100〉 loops. The overall effect of Cr enrichment is to penalize the mobility of intrinsically glissile 1/2〈111〉 loops, modifying the reaction mechanisms as a result. The following three most important effects associated with Cr enrichment have been revealed: (i) absence of dynamic drag; (ii) suppression of complete absorption; (iii) enhanced strength of small dislocation loops (2 nm and smaller). Overall the effect of the Cr enrichment is therefore to increase the unpinning stress, so experimentally 'invisible' nanostructural features may also contribute to radiation-induced strengthening. The reasons for the modification of the mechanisms are explained and the impact of the loading conditions is discussed.

We investigated magnetic phase transitions, magnetic anisotropy, and magnetic domains in Pd_1−xFe_x alloys with different Fe concentrations x = 2.2–7.2%. The Curie temperature depends linearly on the Fe concentration in the regime studied. The magnetization is dominantly in-plane with a small out-of-plane remanent contribution. Resonant magnetic small angle scattering with circularly polarized x-rays tuned to the L₃ resonance edge of Fe revealed a small angle scattering ring corresponding to magnetic domain fluctuations on a length scale of 100 nm. These fluctuations are isotropically distributed in the film plane and appear to have an out-of-plane component. On increasing the transverse coherence of the incident beam, the scattering ring decomposes in a speckle pattern, indicative of magnetic correlations on a length scale smaller than the x-ray coherence length of about 4 μm.

Magnetic shape memory alloys exhibit a hierarchically twinned microstructure, which has been examined thoroughly in epitaxial Ni–Mn–Ga films. Here we analyze the consequences of this 'twin within twins' microstructure on the magnetic domain pattern. Atomic and magnetic force microscopy are used to probe the correlation between the martensitic microstructure and magnetic domains. We examine the consequences of different twin boundary orientations with respect to the substrate normal as well as variant boundaries between differently aligned twinned laminates. A detailed micromagnetic analysis is given which describes the influence of the finite film thickness on the formation of magnetic band domains in these multiferroic materials.

We have carried out density functional theory based calculations for understanding the structural, electronic and magnetic properties of pristine and transition metal (TM) doped ZnTe nanowires. Pristine ZnTe nanowires (NWs) turn out to be semiconducting in nature, with the band gap varying with the diameter of the NWs. In Mn-doped ZnTe NWs, the Mn atoms retain a magnetic moment of 5 μ_B each and couple anti-ferromagnetically. A half metallic ferromagnetic state, although energetically not favorable, is observed arising from a strong hybridization between the d-states of Mn atoms and p-states of Te atoms. Further studies of V- and Sc-doped ZnTe NWs reveal the systems to be anti-ferromagnetic.