Table of contents

We show through first-principles calculations that the electronic properties of Pt₄ clusters can be tuned by adsorption on substrates with different electronic valence characters. Pt clusters exhibit a metallic character on γ-Al₂O₃(111) and insulator properties on CaZrO₃(001). The noted difference indicates the role of the electronic valence states of the substrate atoms that directly bond with Pt.

The interaction between a single magnetic atom and the metal environment (including a magnetic field) is analyzed by introducing an ionic Hamiltonian combined with an effective crystal-field term, and by using a Green-function equation of motion method. This approach describes the inelastic electron tunneling spectroscopy and the Kondo resonances as due to atomic spin fluctuations associated with electron co-tunneling processes between the leads and the atom. We analyze in the case of Fe on CuN the possible spin fluctuations between states with S = 2 and 3/2 or 5/2 and conclude that the experimentally found asymmetries in the conductance with respect to the applied bias, and its marked structures, are well explained by the 2↔3/2 spin fluctuations. The case of Co is also considered and shown to present, in contrast with Fe, a resonance at the Fermi energy corresponding to a Kondo temperature of 6 K.

Recent experiments with nitrogen as a seeding gas in fusion plasma devices together with the option of using beryllium as an armor material in the future ITER tokamak (International Thermonuclear Experimental Reactor) have raised new interest in the interactions of beryllium surfaces with nitrogen (atomic or molecular). The strong reactivity of nitrogen implies the formation of beryllium nitrite and, in conjunction with oxygen and other possible impurities, experimentalists have to consider the probability of generating various complex moieties such as imine, amine or oxyamine, and amide radicals. This chemistry would obviously dramatically perturb the plasma, and quantum investigations can be of great predictive help.

Nitrogen adsorption on beryllium basal surfaces is investigated through quantum density functional theory. Different situations are examined: molecular or atomic nitrogen reactions; nitride radical adsorption or formation on surfaces; hydrogen retention on surfaces; combined nitrogen/oxygen reactivity and hydrogen retention. A tentative comparison with experiment is also proposed.

It is argued that to arrive at a quantitative description of the surface tension of a liquid drop as a function of its inverse radius, it is necessary to include the bending rigidity k and Gaussian rigidity $\bar {k}$ in its description. New formulae for k and $\bar {k}$ in the context of density functional theory with a non-local, integral expression for the interaction between molecules are presented. These expressions are used to investigate the influence of the choice of Gibbs dividing surface, and it is shown that for a one-component system, the equimolar surface has a special status in the sense that both k and $\bar {k}$ are then the least sensitive to a change in the location of the dividing surface. Furthermore, the equimolar value for k corresponds to its maximum value and the equimolar value for $\bar {k}$ corresponds to its minimum value. An explicit evaluation using a short-ranged interaction potential between molecules shows that k is negative with a value around minus 0.5–1.0 k_BT and that $\bar {k}$ is positive with a value that is a bit more than half the magnitude of k. Finally, for dispersion forces between molecules, we show that a term proportional to log(R)/R² replaces the rigidity constants and we determine the (universal) proportionality constants.

We study the adhesion between smooth polydimethylsiloxane (PDMS) rubber balls and smooth and rough poly(methyl methacrylate) (PMMA) surfaces, and between smooth silicon nitride balls and smooth PDMS surfaces. From the measured viscoelastic modulus of the PDMS rubber we calculate the viscoelastic contribution to the crack-opening propagation energy γ_eff(v,T) for a wide range of crack tip velocities v and for several temperatures T. The Johnson–Kendall–Roberts (JKR) contact mechanics theory is used to analyze the ball pull-off force data, and γ_eff(v,T) is obtained for smooth and rough surfaces. We conclude that γ_eff(v,T) has contributions of similar magnitude from both the bulk viscoelastic energy dissipation close to the crack tip, and from the bond-breaking process at the crack tip. The pull-off force on the rough surfaces is strongly reduced compared to that of the flat surface, which we attribute mainly to the decrease in the area of contact on the rough surfaces.

Graphene based spintronic devices require an understanding of the growth of magnetic metals. Rare earth metals have large bulk magnetic moments so they are good candidates for such applications, and it is important to identify their growth mode. Dysprosium was deposited on epitaxial graphene, prepared by thermally annealing 6H-SiC(0001). The majority of the grown islands have triangular instead of hexagonal shapes. This is observed both for single layer islands nucleating at the top of incomplete islands and for fully completed multi-height islands. We analyze the island shape distribution and stacking sequence of successively grown islands to deduce that the Dy islands have fcc(111) structure, and that the triangular shapes result from asymmetric barriers to corner crossing.

In this paper, we present the first non-contact atomic force microscopy (nc-AFM) of a silicene on a silver (Ag) surface, obtained by combining non-contact atomic force microscopy and scanning tunneling microscopy (STM). STM images over large areas of silicene grown on the Ag(111) surface show both (√13 × √13)R13.9° and (4 × 4) superstructures. For the widely observed (4 × 4) structure, the observed nc-AFM image is very similar to the one recorded by STM. The structure resolved by nc-AFM is compatible with only one out of two silicon atoms being visible. This indicates unambiguously a strong buckling of the silicene honeycomb layer.

With the goal of achieving an understanding of the properties of bimetallic alloy clusters having atoms of two isoelectronic elements, we have studied the structural, electronic and magnetic properties of Mn_mTc_n, Mn_mRe_n and Ti_mZr_n clusters with m + n = 13 (n = 0, 1, 4, 6, 9, 12, 13), using first-principles density functional calculations. Mn_mTc_n and Mn_mRe_n represent clusters of isoelectronic series with a half-filled d shell, while Ti_mZr_n represents an isoelectronic cluster series of early transition metals. Mn-rich alloy clusters are found to prefer compact structures and isoelectronic Tc-rich or Re-rich alloy clusters are found to adopt open structures. In contrast, Ti_mZr_n clusters are all found to stabilize in compact structures, irrespective of being Ti-rich or Zr-rich. This change in behavior between two isoelectronic series is found to be driven by differences in hybridization effects, due to differences in the evolution of the relative energy positions of the d level with respect to the s and p levels upon moving from 3d to 4d or 5d elements. This effect further competes with the magnetization effect to decide the morphology of the alloy clusters. Focusing on the magnetic properties of the studied clusters, we find that the single Tc atom substituted alloy cluster exhibits markedly improved magnetic properties compared to that of pure Mn clusters.

First-principles calculations based on density functional theory were performed to investigate the co-doping effects of Sm and Gd in ceria on its oxygen ion conduction. The focus of this study is on the interactions between the cation dopants and an oxygen vacancy within the two adjacent tetrahedral sites of fluorite structure surrounding the oxygen migration path. Vacancy formation energies, dopant–vacancy association energies, and migration energies were calculated to elucidate the doping effects on oxygen ion conduction. The migration energies show remarkable dependences on the ionic radii of the cations located at the edges of the migration path. A simple relation between migration energy and vacancy formation energy is proposed. This work provides an informative insight into vacancy diffusion that could be useful in optimizing doping materials for improving oxygen ion conductivity in doped ceria.

The vacuum referred binding energies of electrons in divalent and trivalent lanthanide impurity states and host band states in the rare earth (RE = La, Gd, Y, Lu, Sc) orthophosphates REPO₄, orthoborates REBO₃, aluminum perovskites REAlO₃, and sesqui-oxides RE₂O₃ have been determined by combining the recently developed chemical shift model with spectroscopic data from the archival literature. The main trends in impurity and host band level locations with changing type of RE, which determines the site size, and with changing P, B, Al, or RE cation, which determines the strength of bonding with the oxygen ligands, are identified. Sc³⁺-based compounds are characterized by a relatively low energy for the conduction band bottom, or equivalently a high electron affinity, which is attributed to a relatively strong electron bonding in the 3d-shell of Sc²⁺.

The effect of C substitution in the AlLiB₁₄ lattice is examined using first-principles methods. The inter-icosahedra B site is found to be the most favorable B site for C substitution and the formation energy is predicted to be 1.7 eV in B-rich conditions. Substituting C does not affect the band gap, nor does it introduce defect states to the gap. An ideal brittle cleavage model is used to study the impact of C doping on the mechanical properties of AlLiB₁₄, and it is concluded that introducing C to the crystal decreases the ideal fracture strength by 3.3 GPa, which is about a 12% reduction in overall strength.

Our first-principles calculations show that the ordering of stoichiometric cation vacancies in Ga₂Se₃ has a large influence on the bandgap, up to 0.55 eV. Therein, the zigzag-line vacancy-ordered Ga₂Se₃ has the maximum bandgap (∼2.56 eV direct bandgap), and the straight-line vacancy-ordered Ga₂Se₃ has the minimum bandgap (∼1.99 eV indirect bandgap) at 0 K, according to scGW calculations. The bandgap difference (0.55 eV) is almost the same for normal density functional theory (DFT) calculations, hybrid DFT calculations and GW calculations. The calculation results are consistent with the experimental bandgap range of 2.0–2.6 eV at room temperature. Also, hydrostatic pressure (<9 GPa) tends to increase the bandgap, consistent with the experiments in the literature.

Employing high resolution photoemission spectroscopy, we studied the temperature evolution of the electronic structure of EuFe₂As₂, a unique pnictide, where antiferromagnetism of the Eu layer survives within the superconducting phase due to 'FeAs' layers, achieved via substitution and/or pressure. High energy and angle resolution helped to reveal the signature of peak–dip features, having significant p orbital character and spin density wave transition induced band folding in the electronic structure. A significant spectral weight redistribution is observed below 20 K manifesting the influence of antiferromagnetic order on the conduction electrons.

We investigate the multiquantum vortex states in a type-II superconductor in both 'clean' and 'dirty' regimes defined by impurity scattering rate. Within a quasiclassical approach we calculate self-consistently the order parameter distributions and electronic local density of states (LDOS) profiles. In the clean case we find the low temperature vortex core anomaly predicted analytically by Volovik (1993 JETP Lett.58 455) and obtain the patterns of LDOS distributions. In the dirty regime multiquantum vortices feature a peculiar plateau in the zero energy LDOS profile, which can be considered as an experimental hallmark of multiquantum vortex formation in mesoscopic superconductors.

The magnetic ground state of Sr₂CrO₄ with a distorted honeycomb lattice was investigated by means of measurements of the magnetic susceptibility, high-field magnetization process, and electron spin resonance (ESR). Antiferromagnetic ordering was observed clearly below T_N = 3.2 K, while the magnetic ground state had been thought to be a spin-singlet state. A two-sublattice model with biaxial anisotropy was applicable for the observed ESR modes. Plateau-like behavior and a sharp kink were detected in the magnetization curve.

The effect of external strain on the evolution of magnetic multi-vortices in nanoscale ferromagnetic platelets is investigated by a phase field model that explicitly includes the coupling between the magnetization and deformation. Phase field simulations show that a compressive strain makes the magnetic vortex–antivortex pair stable in rectangular ferromagnetic platelets, which is unstable in the absence of an external magnetic field and strain. The magnetic clockwise (CW) and counterclockwise (CCW) vortex pairs disappear in ferromagnetic platelets under an external magnetic field through the annihilation of the vortex and antivortex, or through expulsion when external strain is absent. In the presence of tensile strain, the expulsion of CW and CCW vortices is suppressed in ferromagnetic platelets. However, external strain has less effect on the annihilation of CW and CCW vortices. For ferromagnetic platelets with triple vortices, both tensile strain and a magnetic field induce the annihilation and expulsion of vortices. The effect of strain on the evolution of magnetic vortices suggests a new way to control them by strain engineering.

A mean field model is presented for the configuration dependent effective demagnetizing and anisotropy fields in assemblies of exchange decoupled magnetic particles of arbitrary shape which are expressed in terms of the demagnetizing factors of the particles and the volumetric shape containing the assembly. Perpendicularly magnetized two-dimensional (2D) assemblies have been considered, for which it is shown that the demagnetizing field is lower than the continuous thin film. As an example of these 2D systems, arrays of bistable cylindrical nanowires have been characterized by remanence curves as well as ferromagnetic resonance, serving to show the correspondence of these measurements with the model and also to validate the mean field approach. Linear chains of cylinders and spheres have been analyzed, leading to simple expressions to describe the easy axis rotation induced by the interaction field in chains of low aspect ratio cylindrical particles, and the dipolar magnetic anisotropy observed in the linear chain of spheres. These examples serve to underline the dependence on the dipolar interaction field and effective demagnetizing factor of the contributions that arise from the shape of the outer volume.

Monte Carlo modeling suggests that the magnetothermal features of the Fe₂P-structured FeCrAs-like compound offer a promising route for the design of magnetocaloric materials. The prototype structure is modeled as antiferromagnetically coupled layered Heisenberg systems mimicking the distorted Kagome/triangular stacked architecture of FeCrAs iron-pnictide. The magnetic entropy change ΔS_m(T) presents a plateau-like behavior which can be tailored by tuning either the J_Cr−Fe/J_Cr−Cr exchange energy ratio or the magnetic field. The plateau is defined by cooperative spin ordering within a ferrimagnetic region which exists between two critical temperatures separating at the lower bound (${T}_{\mathrm{c}}^{a}$) a canted antiferromagnetic phase and at the upper bound (${T}_{\mathrm{c}}^{d}$) the thermally disordered phase. The refrigerant capacity and adiabatic change of temperature are $A(H)({T}_{\mathrm{c}}^{d}-{T}_{\mathrm{c}}^{a})$ and A(H)T_p/C_m respectively, with ${T}_{\mathrm{c}}^{a}\lt {T}_{\mathrm{p}}\lt {T}_{\mathrm{c}}^{d}$, A(H) an increasing positive function of the field defining the height of the plateau and C_m the magnetic specific heat, whose critical behavior is related to the ${T}_{\mathrm{c}}^{a,d}$ values.