One- and Two-Particle Correlation Functions in the Cluster Perturbation Theory for Cuprates.

The physics of high-Tc superconducting cuprates is obscured by the effect of strong electronic correlations. One way to overcome this problem is to seek an exact solution at least within a small cluster and expand it to the whole crystal. Such an approach is at the heart of cluster perturbation theory (CPT). Here, we developed CPT for the dynamic spin and charge susceptibilities (spin-CPT and charge-CPT), with the correlation effects explicitly taken into account by the exact diagonalization. We applied spin-CPT and charge-CPT to the effective two-band Hubbard model for the cuprates obtained from the three-band Emery model and calculated one- and two-particle correlation functions, namely, a spectral function and spin and charge susceptibilities. The doping dependence of the spin susceptibility was studied within spin-CPT and CPT-RPA, that is, the CPT generalization of the random phase approximation (RPA). In the underdoped region, both our methods resulted in the signatures of the upper branch of the spin excitation dispersion with the lowest excitation energy at the (π,π) wave vector and no presence of low-energy incommensurate excitations. In the high doping region, both methods produced a low energy response at four incommensurate wave vectors in qualitative agreement with the results of the inelastic neutron scattering experiments on overdoped cuprates.

Electronic Structure and Minimal Models for Flat and Corrugated CuO Monolayers: An Ab Initio Study.

CuO atomic thin monolayer (mlCuO) was synthesized recently. Interest in the mlCuO is based on its close relation to CuO2 layers in typical high temperature cuprate superconductors. Here, we present the calculation of the band structure, the density of states and the Fermi surface of the flat mlCuO as well as the corrugated mlCuO within the density functional theory (DFT) in the generalized gradient approximation (GGA). In the flat mlCuO, the Cu-3dx2-y2 band crosses the Fermi level, while the Cu-3dxz,yz hybridized band is located just below it. The corrugation leads to a significant shift of the Cu-3dxz,yz hybridized band down in energy and a degeneracy lifting for the Cu-3dx2-y2 bands. Corrugated mlCuO is more energetically favorable than the flat one. In addition, we compared the electronic structure of the considered CuO monolayers with bulk CuO systems. We also investigated the influence of a crystal lattice strain (which might occur on some interfaces) on the electronic structure of both mlCuO and determined the critical strains of topological Lifshitz transitions. Finally, we proposed a number of different minimal models for the flat and the corrugated mlCuO using projections onto different Wannier functions basis sets and obtained the corresponding Hamiltonian matrix elements in a real space.

Direct Observation of the Spin Exciton in Andreev Spectroscopy of Iron-Based Superconductors.

Quasiparticle excitations provide viable information on the physics of unconventional superconductors. Higgs and Leggett modes are some of the classic examples. Another important bosonic excitation is the spin exciton originating from the sign-changing superconducting gap structure. Here we report a direct observation of the temperature-dependent spin exciton in the Andreev spectra of iron-based superconductors. Combined with the other experimental evidence, our observation confirms the extended s-wave (s±) order parameter symmetry and indirectly proves the spin-fluctuation mechanism of Cooper pairing.

Magnetic Collapse in Fe3Se4 under High Pressure.

Electronic structure and magnetic properties of Fe3Se4 are calculated using the density functional approach. Due to the metallic properties, magnetic moments of the iron atoms in two nonequivalent positions in the unit cell are different from ionic values for Fe3+ and Fe2+ and are equal to M1=2.071µB and M2=-2.042µB, making the system ferrimagnetic. The total magnetic moment for the unit cell is 2.135µB. Under isotropic compression, the total magnetic moment decreases non-monotonically and correlates with the non-monotonic dependence of the density of states at the Fermi level N(EF). For 7% compression, the magnetic order changes from the ferrimagnetic to the ferromagnetic. At 14% compression, the magnetic order disappears and the total magnetic moment becomes zero, leaving the system in a paramagnetic state. This compression corresponds to the pressure of 114 GPa. The magnetic ordering changes faster upon application of an isotropic external pressure due to the sizeable anisotropy of the chemical bondings in Fe3Se4. The ferrimagnetic and paramagnetic states occur under pressures of 5.0 and 8.0 GPa, respectively. The system remains in the metallic state for all values of compression.

Band Structure of Organic-Ion-Intercalated (EMIM)xFeSe Superconductor.

The band structure and the Fermi surface of the recently discovered superconductor (EMIM)xFeSe are studied within the density functional theory in the generalized gradient approximation. We show that the bands near the Fermi level are formed primarily by Fe-d orbitals. Although there is no direct contribution of EMIM orbitals to the near-Fermi level states, the presence of organic cations leads to a shift of the chemical potential. It results in the appearance of small electron pockets in the quasi-two-dimensional Fermi surface of (EMIM)xFeSe.

Anomalous in-plane electronic scattering in charge ordered Na0.41CoO2·0.6H2O.

We report electronic transport measurements on high quality floating zone grown Na(x)CoO2 and Na0.41CoO2·0.6H2O single crystals. We find an in-plane electronic scattering minimum near 11 K and a clear charge ordering at approximately 50 K. The electronic and magnetic properties in hydrated and nonhydrated Na0.41CoO2 samples are similar at higher temperature, but evolve in markedly different ways below ∼50 K, where a strong ferromagnetic tendency is observed in the hydrated sample. Model calculations show the relationship of this tendency to the structure of the Fermi surface. The results, particularly the clear differences between the hydrated and nonhydrated material show a substantially enhanced ferromagnetic tendency upon hydration. Implications for superconductivity are discussed.