Interfacial magnetoelectric coupling is a viable path to achieve electrical writing of magnetic information in spintronic devices. For the prototypical Fe/BaTiO₃ system, only tiny changes of the interfacial Fe magnetic moment upon reversal of the BaTiO₃ dielectric polarization have been predicted so far. Here, by using X-ray magnetic circular dichroism in combination with high-resolution electron microscopy and first principles calculations, we report on an undisclosed physical mechanism for interfacial magnetoelectric coupling in the Fe/BaTiO₃ system. At this interface, an ultrathin oxidized iron layer exists, whose magnetization can be electrically and reversibly switched on and off at room temperature by reversing the BaTiO₃ polarization. The suppression/recovery of interfacial ferromagnetism results from the asymmetric effect that ionic displacements in BaTiO₃ produces on the exchange coupling constants in the interfacial-oxidized Fe layer. The observed giant magnetoelectric response holds potential for optimizing interfacial magnetoelectric coupling in view of efficient, low-power spintronic devices.

Spin-orbit coupling is increasingly seen as a rich source of novel phenomena, as shown by the recent excitement around topological insulators and Rashba effects. We here show that the addition of ferroelectric degrees of freedom to a semiconductor featuring topologically nontrivial properties, such as SnTe, merges the intriguing field of spin-orbit-driven physics with nonvolatile functionalities appealing for spintronics. By using a variety of modeling techniques, we show that a strikingly rich sequence of phases can be induced in SnTe, when going from a room-temperature cubic phase to a low-temperature ferroelectric structure, ranging from a topological crystalline insulator to a time-reversal-invariant $Z_2$ topological insulator to a “ferroelectric Rashba semiconductor,” exhibiting a huge electrically controllable Rashba effect in the bulk band structure.

High temperature superconductivity has been found in many kinds of compounds built from planes of Cu and O, separated by spacer layers. Understanding why critical temperatures are so high has been the subject of numerous investigations and extensive controversy. To realize high temperature superconductivity, parent compounds are either hole doped, such as ${\mathrm{La}}_2{\mathrm{CuO}}_4$ (LCO) with Sr (LSCO), or electron doped, such as ${\mathrm{Nd}}_2{\mathrm{CuO}}_4$ (NCO) with Ce (NCCO). In the electron-doped cuprates, the antiferromagnetic phase is much more robust than the superconducting phase. However, it was recently found that the reduction of residual out-of-plane apical oxygen dramatically affects the phase diagram, driving those compounds to a superconducting phase. Here we use a recently developed first-principles method to explore how displacement of the apical oxygen (AO) in LCO affects the optical gap, spin and charge susceptibilities, and superconducting order parameter. By combining quasiparticle self-consistent GW (QS GW) and dynamical mean-field theory (DMFT), we show that LCO is a Mott insulator, but small displacements of the apical oxygen drive the compound to a metallic state through a localization-delocalization transition, with a concomitant maximum in $d$-wave order parameter at the transition. We address the question of whether NCO can be seen as the limit of LCO with large apical displacements, and we elucidate the deep physical reasons why the behavior of NCO is so different from the hole-doped materials. We shed new light on the recent correlation observed between $T_c$ and the charge transfer gap, while also providing a guide towards the design of optimized high-$T_c$ superconductors. Further, our results suggest that strong correlation, enough to induce a Mott gap, may not be a prerequisite for high-$T_c$ superconductivity.

We study, through the variational Monte Carlo technique, an extended Hubbard model away from half filled band density which contains two competing nearest-neighbor interactions: a superexchange $J$ favoring $d$-wave superconductivity and a repulsion $V$ opposing it. We find that the on-site repulsion $U$ effectively enhances the strength of $J$ while suppressing that of $V$, thus favoring superconductivity. This result shows that attractions which do not involve charge fluctuations are very well equipped against strong electron-electron repulsion so much to get advantage from it.

Angle-dispersive x-ray powder diffraction experiments have been performed on yttrium metal up to 183 GPa. We find that the recently discovered $oF16$ structure observed in the high-$Z$ trivalent lanthanides is also adopted by yttrium above 106 GPa, pressures where it has a superconducting temperature of $\sim 20$ K. We have also refined both tetragonal and rhombohedral structures against the diffraction data from the preceding “distorted-fcc” phase and we are unable to state categorically which of these is the true structure of this phase. Finally, analysis of yttrium's equation of state reveals a marked change in the compressibility upon adoption of the $oF16$ structure, after which the compression is that of a “regular” metal. Electronic structure calculations of $oF16$-Y confirm its stability over $oF8$ structure seen in Nd and Sm, and provide insight into the nature of the shift of orbital character from $s$ to $d$ under compression.

We present the implementation of dynamical mean-field theory (DMFT) in the CASTEP ab initio code. We explain in detail the theoretical framework for DFT+DMFT and we demonstrate our implementation for three strongly-correlated systems with $f$-shell electrons: $\gamma$-cerium, cerium sesquioxide ${\mathrm{Ce}}_2{\mathrm{O}}_3$, and samarium telluride SmTe by using a Hubbard I solver. We find very good agreement with previous benchmark DFT+DMFT calculations of cerium compounds, while for SmTe we show the improved agreement with the experimental structural parameters as compared with LDA. Our implementation works equally well for both norm-conserving and ultrasoft pseudopotentials, and we apply it to the calculation of total energy, bulk modulus, equilibrium volumes, and internal forces in the two cerium compounds. In ${\mathrm{Ce}}_2{\mathrm{O}}_3$ we report a dramatic reduction of the internal forces acting on coordinates not constrained by unit cell symmetries. This reduction is induced by the many-body effects, which can only be captured at the DMFT level. In addition, we derive an alternative form for treating the high-frequency tails of the Green function in Matsubara frequency summations. Our treatment allows a reduction in the bias when calculating the correlation energies and occupation matrices to high precision.