Table of contents

After the revival of the magnetoelectric effect which took place in the early 2000s, the interest in multiferroic materials displaying simultaneous presence of spontaneous long-range magnetic and dipolar order has motivated an exponential growth of research activity, from both the experimental and theoretical perspectives. Within this context, and relying also on the rigorous formulation of macroscopic polarization as provided by the Berry-phase approach, it has been possible to identify new microscopic mechanisms responsible for the appearance of ferroelectricity. In particular, it has been realized that electronic spin, charge and orbital degrees of freedom may be responsible for the breaking of the space-inversion symmetry, a necessary condition for the appearance of electric polarization, even in centrosymmetric crystal structures. In view of its immediate potential application in magnetoelectric-based devices, many efforts have been made to understand how magnetic orderings may lead to ferroelectric polarization, and to identify candidate materials. On the other hand, the role of charge and orbital degrees of freedom, which have received much less attention, has been predicted to be non-negligible in several cases. Here, we review recent theoretical advances in the field of so-called electronic ferroelectricity, focusing on the possible mechanisms by which charge- and/or orbital-ordering effects may cause the appearance of macroscopic polarization. Generally, a naive distinction can be drawn between materials displaying almost localized electrons and those characterized by a strong covalent character and delocalized electrons. As for the latter, an intuitive understanding of basic mechanisms is provided in the framework of tight-binding model Hamiltonians, which are used to shed light on unusual charge/orbital effects in half-doped manganites, whereas the case of magnetite will be thoroughly discussed in light of recent progress pointing to an electronic origin of its proposed ferroelectric and magnetoelectric properties.

Atomistic modelling of magnetic materials provides unprecedented detail about the underlying physical processes that govern their macroscopic properties, and allows the simulation of complex effects such as surface anisotropy, ultrafast laser-induced spin dynamics, exchange bias, and microstructural effects. Here we present the key methods used in atomistic spin models which are then applied to a range of magnetic problems. We detail the parallelization strategies used which enable the routine simulation of extended systems with full atomistic resolution.

A review on silica clathrate compounds, which are variants of pure silica zeolites with relatively small voids, is presented. Zeolites have found many uses in industrial and domestic settings as materials for catalysis, separations, adsorption, ion exchange, drug delivery, and other applications. Zeolites with pure silica frameworks have attracted particular interest because of their high thermal stability, well-characterized framework structures, and simple chemical compositions. Recent advances in new synthetic routes have extended the structural diversity of pure silica zeolite frameworks. Thermochemical analyses and computational simulations have provided a basis for applications of these materials and the syntheses of new types of pure silica zeolites. High-pressure and high-temperature experiments have also revealed diverse responses of these framework structures to pressure, temperature, and various guest species. This paper summarizes the framework topologies, synthetic processes, energetics, physical properties, and some applications of silica clathrate compounds.

The optical constants together with the magneto-optical Voigt constants of several phthalocyanine (Pc) and methoxy functionalized tetraphenylporphyrin (TMPP) thin films prepared on silicon substrates are presented. The materials investigated are MePc with Me = Fe, Co, Ni, Cu, Zn and MeTMPP with Me = Cu, Ni. We also compared our results to the metal-free H₂Pc, H₂TPP and H₂TMPP. The experimental results will be supported by electronic structure calculations based on density functional theory (DFT) and interpreted using the perimeter model initially proposed by Platt. The model allows for qualitative understanding of the forbidden character of transitions in planar, aromatic molecules, and is able to qualify differences between Pc and TMPP type materials.

Recent experiments on current-induced domain-wall motion in chiral domain walls reveal important contributions both from spin–orbit torques (SOTs) and from the Dzyaloshinskii–Moriya interaction (DMI). We derive a Berry phase expression for the DMI and show that within this Berry phase theory DMI and SOTs are intimately related, in a way formally analogous to the relation between orbital magnetization (OM) and anomalous Hall effect (AHE). We introduce the concept of the twist torque moment, which probes the internal twist of wavepackets in chiral magnets in a similar way as the orbital moment probes the wavepacket's internal self-rotation. We propose to interpret the Berry phase theory of DMI as a theory of spiralization in analogy to the modern theory of OM. We show that the twist torque moment and the spiralization together give rise to a Berry phase governing the response of the SOT to thermal gradients, in analogy to the intrinsic anomalous Nernst effect. The Berry phase theory of DMI is computationally very efficient because it only needs the electronic structure of the collinear magnetic system as input. As an application of the formalism we compute the DMI in Co/Pt(111), O/Co/Pt(111) and Al/Co/Pt(111) magnetic bi- and trilayers and show that the DMI is highly anisotropic in these systems.

Spin-transport calculations present certain difficulties which are sometimes overlooked when using density-functional theory (DFT) to analyze and predict the behavior of molecular-based devices. We analyze and give examples of some caveats of spintronic calculations using DFT. We first describe how the broken-symmetry problem of DFT can cause serious problems in the evaluation of the spin polarization of electron currents. Next, we signal the low-energy scale of magnetic excitations, which makes them ubiquitous at already rather small biases. The existence of excitations in spin transport has catastrophic consequences in the reliability of the usual transport calculations. Finally, we compare DFT and configuration-interaction calculations of a ferrocene-based double decker that has been heralded as a possible spin-filter, and we cast a word of caution when we show that DFT is qualitatively wrong in the description of both the ground state and the excited states of ferrocene double deckers.

Junctions comprised of ferromagnets and nonmagnetic materials are one of the key building blocks in spintronics. With the recent breakthroughs of spin injection in ferromagnet/graphene junctions it is possible to consider spin-based applications that are not limited to magnetoresistive effects. However, for critical studies of such structures it is crucial to establish accurate predictive methods that would yield atomically resolved information on interfacial properties. By focusing on Co(0001)/graphene junctions and their electronic structure, we illustrate the inequivalence of different spin polarizations. We show atomically resolved spin polarization maps as a useful approach to assess the relevance of Co(0001)/graphene for different spintronics applications.

We present a computational materials design for defect-induced ferrimagnetic MnO. The magnetic properties of MnO containing Mn vacancies were investigated using first-principle calculations. For these electronic structure calculations, we employed a pseudo-self-interaction-corrected local density approximation (PSIC-LDA). We used the Korringa–Kohn–Rostoker coherent potential approximation (KKR-CPA) to create a random distribution of atoms at the assigned sites. Having described the magnetic properties with a classic Heisenberg model, we calculated the effective exchange coupling constants by applying the magnetic force theorem to two magnetic sites embedded in the CPA medium. We estimated the Curie temperatures from the calculated exchange interactions. This study found that the Mn vacancies induced ferrimagnetic ground states in MnO, and that the Curie temperature could reach room temperature at Mn vacancy concentrations above 20%. These findings suggest a new route for designing ferrimagnetic materials from anti-ferromagnetic host materials.

The realization of metal–molecule junctions for future electronic devices relies on our ability to assemble these heterogeneous objects at a molecular level and understand their structure and the behavior of the electronic states at the interface. Delocalized interface states near the metal Fermi level are a key ingredient for tailoring charge injection, and such a delocalization depends on a large number of chemical, structural and morphological parameters, all influencing the spatial extension of the electron wavefunctions. Our large-scale dynamical simulations, combined with experiments, show that a double-decker organometallic compound (ferrocene) can be deposited on a Cu(111) surface, providing an ideal system to investigate the adsorption, the interface states and localized spin states at a metal–organometallic interface. Adsorbed ferrocene is shown to have a peculiar pattern and realizes a 2D-like interface state strongly resembling Shockley's surface state of Cu. By a subsequent deposition of single metal atoms on the adsorbed ferrocene, we analyze the sensitivity of the interface state to local modifications of the interface potential. This provides an insight into adsorption, spin configuration and charge redistribution processes, showing how to tune the electron behavior at a metal–molecule interface.

A many-body potential model for the description of actinide oxide systems, which is robust at high temperatures, is reported for the first time. The embedded atom method is used to describe many-body interactions ensuring good reproduction of a range of thermophysical properties (lattice parameter, bulk modulus, enthalpy and specific heat) between 300 and 3000 K for AmO₂, CeO₂, CmO₂, NpO₂, ThO₂, PuO₂ and UO₂. Additionally, the model predicts a melting point for UO₂ between 3000 and 3100 K, in close agreement with experiment. Oxygen–oxygen interactions are fixed across the actinide oxide series because it facilitates the modelling of oxide solid solutions. The new potential is also used to predict the energies of Schottky and Frenkel pair disorder processes.

The ²²⁹thorium isotope presents an extremely low-energy isomer state of the nucleus which is expected around 7.8 eV, in the vacuum ultraviolet (VUV) regime. This unique system may bridge between atomic and nuclear physics, enabling coherent manipulation and precision spectroscopy of nuclear quantum states using laser light. It has been proposed to implant ²²⁹thorium into VUV transparent crystal matrices to facilitate laser spectroscopy and possibly realize a solid-state nuclear clock. In this work, we validate the feasibility of this approach by computer modelling of thorium doping into calcium fluoride single crystals. Using atomistic modelling and full electronic structure calculations, we find a persistent large band gap and no additional electronic levels emerging in the middle of the gap due to the presence of the dopant, which should allow direct optical interrogation of the nuclear transition.

Based on the electronic structure, we estimate the thorium nuclear quantum levels within the solid-state environment. Precision laser spectroscopy of these levels will allow the study of a broad range of crystal field effects, transferring Mössbauer spectroscopy into the optical regime.

We have created an improved xenon interatomic potential for use with existing UO₂ potentials. This potential was fit to density functional theory calculations with the Hubbard U correction (DFT + U) using a genetic algorithm approach called iterative potential refinement (IPR). We examine the defect energetics of the IPR-fitted xenon interatomic potential as well as other, previously published xenon potentials. We compare these potentials to DFT + U derived energetics for a series of xenon defects in a variety of incorporation sites (large, intermediate, and small vacant sites). We find the existing xenon potentials overestimate the energy needed to add a xenon atom to a wide set of defect sites representing a range of incorporation sites, including failing to correctly rank the energetics of the small incorporation site defects (xenon in an interstitial and xenon in a uranium site neighboring uranium in an interstitial). These failures are due to problematic descriptions of Xe–O and/or Xe–U interactions of the previous xenon potentials. These failures are corrected by our newly created xenon potential: our IPR-generated potential gives good agreement with DFT + U calculations to which it was not fitted, such as xenon in an interstitial (small incorporation site) and xenon in a double Schottky defect cluster (large incorporation site). Finally, we note that IPR is very flexible and can be applied to a wide variety of potential forms and materials systems, including metals and EAM potentials.

We study the relationship between bipartite entanglement, subsystem particle number and topology in a half-filled free fermion system. It is proposed that the spin-projected particle numbers can distinguish the quantum spin Hall state from other states, and can be used to establish a new topological index for the system. Furthermore, we apply the new topological invariant to a disordered system and show that a topological phase transition occurs when the disorder strength is increased beyond a critical value. It is also shown that the subsystem particle number fluctuation displays behavior very similar to that of the entanglement entropy. This provides a lower-bound estimation for the entanglement entropy, which can be utilized to obtain an estimate of the entanglement entropy experimentally.

Physical properties of polycrystalline CeCrGe₃ and LaCrGe₃ have been investigated by x-ray absorption spectroscopy, magnetic susceptibility χ(T), isothermal magnetization M(H), electrical resistivity ρ(T), specific heat C(T) and thermoelectric power S(T) measurements. These compounds are found to crystallize in the hexagonal perovskite structure (space group P6₃/mmc), as previously reported. The ρ(T), χ(T) and C(T) data confirm the bulk ferromagnetic ordering of itinerant Cr moments in LaCrGe₃ and CeCrGe₃ with T_C = 90 K and 70 K respectively. In addition, a weak anomaly is also observed near 3 K in the C(T) data of CeCrGe₃. The T dependences of ρ and finite values of Sommerfeld coefficient γ obtained from the specific heat measurements confirm that both the compounds are of metallic character. Further, the T dependence of ρ of CeCrGe₃ reflects a Kondo lattice behavior. An enhanced γ of 130 mJ mol⁻¹ K⁻² together with the Kondo lattice behavior inferred from the ρ(T) establish CeCrGe₃ as a moderate heavy fermion compound with a quasi-particle mass renormalization factor of ∼45.