Table of contents

Direct growth of graphene on Co₃O₄(111) at 1000 K was achieved by molecular beam epitaxy from a graphite source. Auger spectroscopy shows a characteristic sp² carbon lineshape, at average carbon coverages from 0.4 to 3 ML. Low energy electron diffraction (LEED) indicates (111) ordering of the sp² carbon film with a lattice constant of 2.5(±0.1) Å characteristic of graphene. Sixfold symmetry of the graphene diffraction spots is observed at 0.4, 1 and 3 ML. The LEED data also indicate an average domain size of ∼1800 Å, and show an incommensurate interface with the Co₃O₄(111) substrate, where the latter exhibits a lattice constant of 2.8(±0.1) Å. Core level photoemission shows a characteristically asymmetric C(1s) feature, with the expected π to π* satellite feature, but with a binding energy for the 3 ML film of 284.9(±0.1) eV, indicative of substantial graphene-to-oxide charge transfer. Spectroscopic ellipsometry data demonstrate broad similarity with graphene samples physically transferred to SiO₂ or grown on SiC substrates, but with the π to π* absorption blue-shifted, consistent with charge transfer to the substrate. The ability to grow graphene directly on magnetically and electrically polarizable substrates opens new opportunities for industrial scale development of charge- and spin-based devices.

Self-consistency-based Kohn–Sham density functional theory (KS-DFT) electronic structure calculations with Gaussian basis sets are reported for a set of 17 protein-like molecules with geometries obtained from the Protein Data Bank. It is found that in many cases such calculations do not converge due to vanishing HOMO–LUMO gaps. A sequence of polyproline I helix molecules is also studied and it is found that self-consistency calculations using pure functionals fail to converge for helices longer than six proline units. Since the computed gap is strongly correlated to the fraction of Hartree–Fock exchange, test calculations using both pure and hybrid density functionals are reported. The tested methods include the pure functionals BLYP, PBE and LDA, as well as Hartree–Fock and the hybrid functionals BHandHLYP, B3LYP and PBE0. The effect of including solvent molecules in the calculations is studied, and it is found that the inclusion of explicit solvent molecules around the protein fragment in many cases gives a larger gap, but that convergence problems due to vanishing gaps still occur in calculations with pure functionals. In order to achieve converged results, some modeling of the charge distribution of solvent water molecules outside the electronic structure calculation is needed. Representing solvent water molecules by a simple point charge distribution is found to give non-vanishing HOMO–LUMO gaps for the tested protein-like systems also for pure functionals.

We summarize the theory of van der Waals (dispersion) forces, with emphasis on recent microscopic approaches that permit the prediction of forces between solids and nanostructures right down to intimate contact and binding. Some connections are pointed out between microscopic theory and macroscopic Lifshitz theory.

Plasmonic nano-antennas constitute a central research topic in current science and engineering with an enormous variety of potential applications. Here we review the recent progress in the niche of plasmonic nano-antennas operating in the near infrared part of the spectrum which is important for a variety of applications. Tuning of the resonance into the near infrared regime is emphasized in the perspectives of fabrication, measurement, modeling, and analytical treatments, concentrating on the vast recent achievements in these areas.

By performing density functional theory calculations we show that it is possible to make the electronic bandgap in bilayer graphene supported on hexagonal boron nitride (h-BN) substrates tunable. We also show that, under applied electric fields, it is possible to insert states from h-BN into the bandgap, which generate a conduction channel through the substrate making the system metallic. In addition, we verify that the breakdown voltage strongly depends on the number of h-BN layers. We also show that both the breakdown voltage and the bandgap tuning are independent of the h-BN stacking order.

We study a method to generate pure spin current in monolayer graphene over a wide range of Fermi energy by adiabatic quantum pumping. The device consists of three gate electrodes and two ferromagnetic strips, which induce a spin-splitting in the graphene through the proximity effect. A pure spin current is generated by applying two periodic oscillating gate voltages. We find that the pumped pure spin current is a sensitive oscillatory function of the Fermi energy. Large spin currents can be found at Fermi energies where there are Fabry–Perot resonances in the barriers. Furthermore, we analyze the effects of the parameters of the system on the pumped currents. Our predicted pumped spin current can be of the order of 100 nA which is measurable using the current technology. The proposed method is useful in the realization of graphene spintronic devices.

The properties of the body-centered cubic γ phase of uranium (U) are calculated using atomistic simulations. First, a modified embedded-atom method interatomic potential is developed for the high temperature body-centered cubic (γ) phase of U. This phase is stable only at high temperatures and is thus relatively inaccessible to first principles calculations and room temperature experiments. Using this potential, equilibrium volume and elastic constants are calculated at 0 K and found to be in close agreement with previous first principles calculations. Further, the melting point, heat capacity, enthalpy of fusion, thermal expansion and volume change upon melting are calculated and found to be in reasonable agreement with experiment. The low temperature mechanical instability of γ U is correctly predicted and investigated as a function of pressure. The mechanical instability is suppressed at pressures greater than 17.2 GPa. The vacancy formation energy is analyzed as a function of pressure and shows a linear trend, allowing for the calculation of the extrapolated zero pressure vacancy formation energy. Finally, the self-defect formation energy is analyzed as a function of temperature. This is the first atomistic calculation of γ U properties above 0 K with interatomic potentials.

The electronic structure, elastic constants and lattice dynamics of the B₂ type intermetallic compound LaAg are studied by means of density functional theory calculations with the generalized gradient approximation for exchange and correlation. The calculated equilibrium properties and elastic constants agree well with available experimental data. From the ratio between the bulk and shear moduli, LaAg is found to be ductile, which is unusual for B₂ type intermetallics. The computed band structure shows a dominant contribution from La 5d states near the Fermi level. The phonon dispersion relations, calculated using density functional perturbation theory, are in good agreement with available inelastic neutron scattering data. Under pressure, the phonon dispersions develop imaginary frequencies, starting at around 2.3 GPa, in good accordance with the martensitic instability observed above 3.4 GPa. By structural optimization the high pressure phase is identified as orthorhombic B₁₉.

Pb₂MnW_1−xRe_xO₆ samples have been synthesized and their structure determined by powder x-ray diffraction. These samples undergo a first order structural phase transition between 413 and 445 K depending on the composition. Above this temperature, the samples are cubic. Below the transition temperature, solid solutions are found for x ≤ 0.2 and x ≥ 0.5. The W-rich samples adopt an orthorhombic cell whereas the Re-rich compounds are monoclinic. In the intermediate region, 0.2 < x < 0.5, both phases coexist. X-ray absorption spectra did not reveal significant changes in the local structure for Pb, Mn or Re atoms across the structural phase transition. All the atoms exhibit distorted environments in the whole series. In the case of Pb and W(Re) atoms, the local distortion remains in the high temperature phase. Samples with x ≤ 0.2 also show a sharp discontinuity in the dielectric permittivity at the phase transition temperature indicating the presence of a concomitant electrical ordering in the bulk grains. Such an anomaly in the dielectric constant is not observed for the x ≥ 0.5 samples, compatible with the lack of dipole ordering for this composition range. The different electrical behaviours also explain the differences in the entropy content for the two types of transition.

First principles density functional theory calculations were performed to study the effects of strain, edge passivation, and surface functional species on the structural and electronic properties of armchair graphene nanoribbons (AGNRs), with a particular focus on the work function. The work function was found to increase with uniaxial tensile strain and decrease with compression. The variation of the work function under strain is primarily due to the shift of the Fermi energy with strain. In addition, the relationship between the work function variation and the core level shift with strain is discussed. Distinct trends of the core level shift under tensile and compressive strain were discovered. For AGNRs with the edge carbon atoms passivated by oxygen, the work function is higher than for nanoribbons with the edge passivated by hydrogen under a moderate strain. The difference between the work functions in these two edge passivations is enlarged (reduced) under a sufficient tensile (compressive) strain. This has been correlated to a direct–indirect bandgap transition for tensile strains of about 4% and to a structural transformation for large compressive strains at about  − 12%. Furthermore, the effect of the surface species decoration, such as H, F, or OH with different covering density, was investigated. It was found that the work function varies with the type and coverage of surface functional species. Decoration with F and OH increases the work function while H decreases it. The surface functional species were decorated on either one side or both sides of AGNRs. The difference in the work functions between one-sided and two-sided decorations was found to be relatively small, which may suggest an introduced surface dipole plays a minor role.

The luminescence properties of K_1/2Bi_1/2TiO₃:Pr³⁺ and Na_1/2Bi_1/2TiO₃:Pr³⁺ powders are investigated in the temperature range 10–600 K. The experimental data are interpreted on the basis of metal-to-metal charge transfer processes and by considering Bi³⁺-to-Pr³⁺ sensitization effects.

The evolution of the ground state properties of FeSb₂ has been investigated via temperature (4.2–300 K), magnetic field (0–12 T) and pressure (0–8.8 GPa) dependent electrical resistivity studies. The temperature dependence of the resistivity follows activated behavior in the high temperature (HT) regime (T > 60 K), while variable range hopping (VRH) dictates the transport in the intermediate temperature (IT) regime (10 K > T > 45 K) and power law behavior is observed in the low temperature (LT) regime (T < 10 K). The pressure profoundly affects the resistivity in all the temperature regimes. The energy gap (Δ) extracted in the HT regime initially increases with pressure and then decreases, while the VRH parameter T₀ deduced in the IT regime is seen to decrease monotonically and vanish beyond 5 GPa leading to an insulator to metal transition (MIT) on account of delocalization of the electronic states in the gap. The analysis of the logarithmic derivative of the conductivity indicates the MIT to occur at ∼6 GPa. The magnetoresistivity is found to be positive. The analysis of the resistivity behavior under pressure and magnetic field indicates that the former induces delocalization, while the latter tends to assist localization of the defect states inside the gap of FeSb₂.

Soft resonant x-ray Bragg diffraction (SRXD) at the Ho M_4,5 edges has been used to study Ho 4f multipoles in the combined magnetic and orbitally ordered phase of HoB₂C₂. A full description of the energy dependence for both σ and π incident x-rays at two different azimuthal angles, as well as the ratio I_σ/I_π as a function of azimuthal angle for a selection of energies, allows a determination of the higher order multipole moments of rank 1 (dipole) to 6 (hexacontatetrapole). The Ho 4f multipole moments have been estimated, indicating a dominant hexadecapole (rank 4) order with an almost negligible influence from either the dipole or the octupole magnetic terms. The analysis incorporates both the intra-atomic magnetic and quadrupolar interactions between the 3d core and 4f valence shells as well as the interference of contributions to the scattering that behave differently under time reversal. Comparison of SRXD, neutron diffraction and non-resonant x-ray diffraction shows that the magnetic and quadrupolar order parameters are distinct. The $(00\frac{1}{2})$ component of the magnetic order exhibits a Brillouin type increase below the orbital ordering temperature T_Q, while the quadrupolar order increases more sharply. We conclude that the quadrupolar interaction is strong, but quadrupolar order only occurs when the magnetic order gives rise to a quasi-doublet ground state, which results in a lock-in of the orbitals at T_Q.

Motivated by recent experiments on Yb-doped CeCoIn₅, we study the effect of correlated disorder in a Kondo lattice. Correlations between the impurities are considered at the two-particle level. We use a mean-field theory approximation for the Anderson lattice model to calculate how the emergence of coherence in the Kondo lattice is impacted by correlations between impurities. We show that the rate at which disorder suppresses coherence temperature depends on the length of the impurity correlations. As the impurity concentration increases, we generally find that the suppression of coherence temperature is significantly reduced. The results are discussed in the context of available experimental data.

An implementation of full self-consistency over the electronic density in the DFT + DMFT framework on the basis of a plane wave–projector augmented wave (PAW) DFT code is presented. It allows for an accurate calculation of the total energy in DFT + DMFT within a plane wave approach. In contrast to frameworks based on the maximally localized Wannier function, the method is easily applied to f electron systems, such as cerium, cerium oxide (Ce₂O₃) and plutonium oxide (Pu₂O₃). In order to have a correct and physical calculation of the energy terms, we find that the calculation of the self-consistent density is mandatory. The formalism is general and does not depend on the method used to solve the impurity model. Calculations are carried out within the Hubbard I approximation, which is fast to solve, and gives a good description of strongly correlated insulators. We compare the DFT + DMFT and DFT + U solutions, and underline the qualitative differences of their converged densities. We emphasize that in contrast to DFT + U, DFT + DMFT does not break the spin and orbital symmetry. As a consequence, DFT + DMFT implies, on top of a better physical description of correlated metals and insulators, a reduced occurrence of unphysical metastable solutions in correlated insulators in comparison to DFT + U.

It is known that solutions of Richardson equations can be represented as stationary points of the 'energy' of classical free charges on the plane. We suggest considering the 'probabilities' of the system of charges occupying certain states in the configurational space at the effective temperature given by the interaction constant, which goes to zero in the thermodynamical limit. It is quite remarkable that the expression of 'probability' has similarities with the square of the Laughlin wavefunction. Next, we introduce the 'partition function', from which the ground state energy of the initial quantum-mechanical system can be determined. The 'partition function' is given by a multidimensional integral, which is similar to the Selberg integrals appearing in conformal field theory and random-matrix models. As a first application of this approach, we consider a system with the constant density of energy states at arbitrary filling of the energy interval where potential acts. In this case, the 'partition function' is rather easily evaluated using properties of the Vandermonde matrix. Our approach thus yields a quite simple and short way to find the ground state energy, which is shown to be described by a single expression all over from the dilute to the dense regime of pairs. It also provides additional insight into the physics of Cooper-paired states.

We generalized the semiclassical path integral method originally used in the D'yakonov–Perel' mechanism to study the spin relaxation of the Elliott–Yafet mechanism in low-dimensional systems. In quantum wells, the spin properties calculated by this method confirmed the experimental results. In two-dimensional narrow wires, size and impurity effects on the Elliott–Yafet relaxation were predicted, including the wire-width-dependent relaxation time, the polarization evolution on the sample boundaries, and the relaxation behavior during the diffusive–ballistic transition. These properties were compared with those of the D'yakonov–Perel' relaxation calculated under similar conditions. For ballistic narrow wires, we derived an exact relation between the Elliott–Yafet relaxation time and the wire width, which confirmed the above simulations.

A detailed investigation of the paramagnetic to ferromagnetic transition in (La_1−xEu_x)_0.67Ca_0.33MnO₃ having small Eu³⁺-content (0 ≤ x ≤ 0.2) has been carried out through resistivity and magnetization measurements. X-ray diffraction patterns of the compounds reveal a single phase (La_1−xEu_x)_0.67Ca_0.33MnO₃ (0 ≤ x ≤ 0.2) of an orthorhombic crystal structure after annealing the precursor at 800 °C for 2 h in air. With increasing Eu³⁺-content, the second-order transition (at x = 0 and 0.1) changes to first-order at x = 0.2. The experimental results demonstrate thermomagnetic irreversibility of the transition for x = 0.2 composition. This arises between the supercooling and superheating regimes where both the ferromagnetic and paramagnetic phases coexist.

The two layered hexagonal hydroxides of Ni are β-Ni(OH)₂ and α-Ni(OH)₂; β-Ni(OH)₂ is now known to be an antiferromagnet whereas the nature of the magnetism in α-Ni(OH)₂ is not yet well established. Here, the magnetic properties of α-Ni(OH)₂ with lattice parameters a = 3.02 Å and c = 8.6 Å, and flower-like morphology with petal thickness of  ≃ 50 Å are reported. Temperature (2–300 K) and magnetic field (up to 65 kOe) dependence of the magnetization and ac susceptibility at f = 0.1–1000 Hz were measured. Analysis of the data yields ferromagnetic ordering in the system with T_C ≃ 16 K. In addition, a nanosize related blocking temperature T_B = 8 K and spin-glass-like ordering of the surface spins near 3.5 K are inferred from the ac frequency and dc magnetic field dependence of these transitions. Fitting to the high temperature series and quasi-2D nature of the system is used to determine J₁/k_B = 4.38 K (J₂/k_B = 0.14 K) for the intraplane (interplane) exchange coupling between the Ni²⁺ ions.

The long-range magnetic ordering of PrMn₂O₅ has been studied on polycrystalline samples from neutron diffraction and specific heat measurements. The onset of antiferromagnetic ordering is observed at T_N ≈ 25 K. In the temperature interval 18 K < T < 25 K the magnetic structure is defined by the propagation vector k₁ = (1/2,0,0). Below 18 K, some additional magnetic satellites appear in the NPD patterns, which are indexed with k₂ = (0,0,1/2). Therefore, below 18 K the magnetic structure consists of two independent magnetic domains, defined by the propagation vectors k₁ and k₂. The magnetic structure of the k₁-domain is given by the basis vectors (C_x,0,0) and (C_x',0,0) for Mn(4h) and Mn(4f), respectively. In the k₂-domain, the magnetic structure is defined by the basis vectors (0,0,G_z) and (F_x',G_y',0) for Mn(4h) and Mn(4f), respectively. At T = 1.5 K, for the magnetic phase associated with k₁, the magnetic moments of the Mn atoms at the 4h and 4f sites are 1.82(7) and 1.81(6) μ_B, respectively; for the magnetic phase associated with k₂, the magnetic moments for the Mn(4h) and Mn(4f) atoms are 0.59(5) and 2.62(5) μ_B, respectively.

L1₀ FePt is an important material for the fabrication of high density perpendicular recording media, but the ultrahigh coercivity of L1₀ FePt restricts its use. Tilting of the magnetic easy axis and the introduction of a soft magnetic underlayer can solve this problem. However, high temperature processing and the requirement of epitaxial growth conditions for obtaining an L1₀ FePt phase are the main hurdles to be overcome. Here, we introduce a bilayered magnetic structure ((111) L1₀ FePt/glassy Fe₇₁Nb₄Hf₃Y₂B₂₀/SiO₂/Si) in which the magnetic easy axis of L1₀ FePt is tilted by ∼36° from the film plane and epitaxial growth conditions are not required. The soft magnetic underlayer not only promotes the growth of L1₀ FePt with the preferred orientation but also provides an easy cost-effective micro/nanopatterning of recording bits. A detailed magnetic characterization of the bilayered structure in which the thickness of (111) L1₀ FePt with the soft magnetic Fe₇₁Nb₄Hf₃Y₂B₂₀ glassy underlayer varied from 5 to 60 nm is carried out in an effort to understand the magnetization switching mechanism. The magnetization switching behavior is almost the same for bilayered structures in which FePt layer thickness is >10 nm (greater than the domain wall thickness of FePt). For FePt film ∼10 nm thick, magnetization reversal takes place in a very narrow field range. Magnetization reversal first takes place in the soft magnetic underlayer. On further increase in the reverse magnetic field, the domain wall in the soft magnetic layer compresses at the interface of the hard and soft layers. Once the domain wall energy becomes sufficiently large to overcome the nucleation energy of the domain wall in L1₀ FePt, the magnetization of the whole bilayer is reversed. This process takes place quickly because the domain walls in the hard layer do not need to move, and the formation of a narrower domain wall may not be favorable energetically. Our results showed that the present bilayered structure is very promising for the fabrication of tilted bit-patterned magnetic recording media.

The magnetic properties of Ho₂Sn₂O₇ have been investigated and compared to other spin ice compounds. Although the lattice has expanded by 3% relative to the better studied Ho₂Ti₂O₇ spin ice, no significant changes were observed in the high temperature properties, T ≳ 20 K. As the temperature is lowered and correlations develop, Ho₂Sn₂O₇ enters its quantum phase at a slightly higher temperature than Ho₂Ti₂O₇ and is more antiferromagnetic in character. Below 80 K a weak inelastic mode associated with the holmium nuclear spin system has been measured. The hyperfine field at the holmium nucleus was found to be ≈700 T.

We report the structural and magnetic properties of the La_1−xTb_xMn_1/2Sc_1/2O₃ series. LaMn_1/2Sc_1/2O₃ shows a long range ferromagnetic ordering in agreement with a fully polarized Mn-sublattice. The substitution of La with Tb produces structural changes which affect the magnetic properties. This substitution leads to a contraction in the unit cell volume that mainly reduces the M–O–M bond angle (M = Mn, Sc). The bending of this angle decreases the Mn–O–Mn superexchange interaction and enhances the competition between nearest-neighbour and next-nearest-neighbour interactions. Accordingly, the magnetic ground state changes from ferromagnetic to a glassy magnetic state. Large thermal irreversibility between zero-field-cooled and field-cooled conditions is observed for all samples. The study of the dynamic magnetic properties has been performed using the frequency dependent real part of the ac magnetic susceptibility. The use of both the Vogel–Fulcher law and the conventional critical slowing down law yields similarly good accuracies in the fits. The relaxation times obtained from both laws concur with the existence of a cluster-glass for x ≥ 0.5 samples.

Some amendments need to be made to the calculations in the original article. The necessary amendments are described in the attached PDF.