No Arabic abstract
Electric field gradients (EFGs) were calculated for the $CeIn_3$ compound at both $^{115}In$ and $^{140}Ce$ sites. The calculations were performed within the density functional theory (DFT) using the augmented plane waves plus local orbital (APW+lo) method employing the so-called LDA+U scheme. The $CeIn_3$ compound were treated as nonmagnetic, ferromagnetic, and antiferromagnetic cases. Our result shows that the calculated EFGs are dominated at the $^{140}Ce$ site by the Ce-4f states. An approximately linear relation is intuited between the main component of the EFGs and total density of states (DOS) at Fermi level. The EFGs from our LDA+U calculations are in better agreement with experiment than previous EFG results, where appropriate correlations had not been taken into account among 4f-electrons. Our result indicates that correlations among 4f-electrons play an important role in this compound and must be taken into account.
We investigated the effect of spin polarization on the structural properties and gradient of electric field (EFG) on Sn, In, and Cd impurity in RSn$_3$ (R=Sm, Eu, Gd) and RIn$_3$ (R=Tm, Yb, Lu) compounds. The calculations were performed self-consistently using the scalar-relativistic full potential linearized augmented plane wave method. The local density approximations (LDA) and generalized gradient approximation without spin polarization (GGA) and with spin polarization (GGA+SP) to density functional theory were applied. In addition to that we performed some calculations within open core treatment (GGA+open core). It is clearly seen that GGA+SP is successful in predicting the larger lattice parameter and the dramatic drop of EFG for R=(Eu, Yb) relative to other rare earth compounds. This is an indication that spin splitting generated by spin polarization without any modification, is capable of treating properly the highly correlated f electrons in these systems.
Graphene-based moir{e} systems have attracted considerable interest in recent years as they display a remarkable variety of correlated phenomena. Besides insulating and superconducting phases in the vicinity of integer fillings of the moir{e} unit cell, there is growing evidence for electronic nematic order both in twisted bilayer graphene and twisted double-bilayer graphene (tDBG), as signaled by the spontaneous breaking of the threefold rotational symmetry of the moir{e} superlattices. Here, we combine symmetry-based analysis with a microscopic continuum model to investigate the structure of the nematic phase of tDBG and its experimental manifestations. First, we perform a detailed comparison between the theoretically calculated local density of states and recent scanning tunneling microscopy data [arXiv:2009.11645] to resolve the internal structure of the nematic order parameter in terms of the layer, sublattice, spin, and valley degrees of freedom. We find strong evidence that the dominant contribution to the nematic order parameter comes from states at the moir{e} scale rather than at the microscopic scale of the individual graphene layers, which demonstrates the key role played by the moire degrees of freedom and confirms the correlated nature of the nematic phase in tDBG. Secondly, our analysis reveals an unprecedented tunability of the orientation of the nematic director in tDBG by an externally applied electric field, allowing the director to rotate away from high-symmetry crystalline directions. We compute the expected fingerprints of this rotation in both STM and transport experiments, providing feasible ways to probe it. Rooted in the strong sensitivity of the flat bands of tDBG to the displacement field, this effect opens an interesting route to the electrostatic control of electronic nematicity in moir{e} systems.
The electronic structure of the honeycomb lattice iridates Na2IrO3 and Li2IrO3 has been investigated using resonant inelastic x-ray scattering (RIXS). Crystal-field split d-d excitations are resolved in the high-resolution RIXS spectra. In particular, the splitting due to non-cubic crystal fields, derived from the splitting of j_eff=3/2 states, is much smaller than the typical spin-orbit energy scale in iridates, validating the applicability of j_eff physics in A2IrO3. We also find excitonic enhancement of the particle-hole excitation gap around 0.4 eV, indicating that the nearest-neighbor Coulomb interaction could be large. These findings suggest that both Na2IrO3 and Li2IrO3 can be described as spin-orbit Mott insulators, similar to the square lattice iridate Sr2IrO4.
We review the basic ideas of the dynamical mean field theory (DMFT) and some of the insights into the electronic structure of strongly correlated electrons obtained by this method in the context of model Hamiltonians. We then discuss the perspectives for carrying out more realistic DMFT studies of strongly correlated electron systems and we compare it with existent methods, LDA and LDA+U. We stress the existence of new functionals for electronic structure calculations which allow us to treat situations where the single--particle description breaks down such as the vicinity of the Mott transition.
The hyperfine interaction between the quadrupole moment of atomic nuclei and the electric field gradient (EFG) provides information on the electronic charge distribution close to a given atomic site. In ferroelectric materials, the loss of inversion symmetry of the electronic charge distribution is necessary for the appearance of the electric polarization. We present first-principles density functional theory calculations of ferroelectrics such as BaTiO3, KNbO3, PbTiO3 and other oxides with perovskite structures, by focusing on both EFG tensors and polarization. We analyze the EFG tensor properties such as orientation and correlation between components and their link with electric polarization. This work supports previous studies of ferroelectric materials where a relation between EFG tensors and polarization was observed, which may be exploited to study ferroelectric order when standard techniques to measure polarization are not easily applied.