The recent discovery of ferromagnetic single-layer CrI ₃ creates ample opportunities for studying the fundamental properties and the spintronic applications of atomically thin magnets.

The electronic structure and properties of $\mathrm{Pu}{\mathrm{O}}_2$ and ${\mathrm{Pu}}_2{\mathrm{O}}_3$ have been studied from first principles by the all-electron projector-augmented-wave method. The local density $\text{approximation}+U$ and the generalized gradient $\text{approximation}+U$ formalisms have been used to account for the strong on-site Coulomb repulsion among the localized Pu $5f$ electrons. We discuss how the properties of $\mathrm{Pu}{\mathrm{O}}_2$ and ${\mathrm{Pu}}_2{\mathrm{O}}_3$ are affected by the choice of $U$ as well as the choice of exchange-correlation potential. Also, oxidation reaction of ${\mathrm{Pu}}_2{\mathrm{O}}_3$ , leading to formation of $\mathrm{Pu}{\mathrm{O}}_2$ , and its dependence on $U$ and exchange-correlation potential have been studied. Our results show that by choosing an appropriate $U$ , it is promising to correctly and consistently describe structural, electronic, and thermodynamic properties of $\mathrm{Pu}{\mathrm{O}}_2$ and ${\mathrm{Pu}}_2{\mathrm{O}}_3$ , which enable the modeling of redox process involving Pu-based materials possible.

We present a study of the structural phase transition and the mechanical and thermodynamic properties of UO${}_2$ by means of the local density approximation (LDA)+$U$ approach. A phase transition pressure of 40 GPa is obtained from theory at 0 K, and agrees well with the experimental value of 42 GPa. The pressure-induced enhancements of elastic constants, elastic moduli, elastic wave velocities, and Debye temperature of the ground-state fluorite phase are predicted. The phonon spectra of both the ground state fluorite structure and high-pressure cotunnite structure calculated by the supercell approach show that the cotunnite structure is dynamically unstable under ambient pressure. Based on the imaginary mode along the $\Gamma$ $-$ $X$ direction and soft phonon mode along the $\Gamma$ $-$ $Z$ direction, a transition path from cotunnite to fluorite has been identified. We calculate the lattice vibrational energy in the quasiharmonic approximation using both first-principles phonon density of state and the Debye model. The calculated temperature dependence of lattice parameter, entropy, and specific heat agrees well with experimental observations in the low temperature domain. The difference of the Gibbs free energy between the two phases of UO${}_2$ has predicted a boundary in the pressure-temperature phase diagram. The solid-liquid boundary is approximated by an empirical equation using our calculated elastic constants.

By growing monolayer FeSe on SrTiO₃(001) surface, researchers obtain the highest superconducting transition-temperature for iron-based superconductor. Here, we study the antiferromagnetic (AFM) checkerboard monolayer FeSe adsorbed on SrTiO₃(001) surface. We show that the system has a considerable charge transfer from SrTiO₃(001) substrate to FeSe monolayer, and so has a self-constructed electric field. The FeSe monolayer band structure near the Brillouin zone (BZ) center is sensitive to charge doping, and the spin-resolved energy bands at BZ corner are distorted to be flattened by the perpendicular electric field. It is revealed that the striking disappearance of the Fermi surface around the BZ center can be well explained by the AFM checkerboard phase with charge doping and electric field effects. We propose a tight-binding model Hamiltonian to take these key factors into account. We also show that this composite structure is an ideal electron-hole bilayer system, with electrons and holes respectively formed in FeSe monolayer and TiO₂ surface layer.

We have performed a systematic first-principles investigation to calculate the electronic structures, mechanical properties, and phonon-dispersion curves of NpO2. The local-density approximation+U and the generalized gradient approximation+U formalisms have been used to account for the strong on-site Coulomb repulsion among the localized Np\,5f electrons. By choosing the Hubbard U parameter around 4 eV, the orbital occupancy characters of Np\,5f and O\,2p are in good agreement with recent experiments [A. Seibert, T. Gouder, and F. Huber, J. Nucl. Mater. 389, 470 (2009)]. Comparing to our previous study of ThO2, we note that stronger covalency exists in NpO2 due to the more localization behavior of 5f electrons of Np in line with the localization-delocalization trend exhibited by the actinides series.

We study the topological edge states of the Haldane model with zigzag/armchair lattice edges. The Harper equation for solving the energies of the edge states is derived. The results show that there are two edge states in the bulk energy gap corresponding to the two zero points of the Bloch function on the complex-energy Riemann surface. The edge-state energy loops move around the hole of the Riemann surface in appropriate system parameter regimes. The quantized Hall conductance can be expressed by the winding numbers of the edge states, which reflects the topological feature of the Haldane model.