Enhancedd-wave pairing in the two-dimensional Hubbard model with periodically modulated hopping amplitudes.

Utilizing determinant quantum Monte Carlo algorithm, the evolution of thed-wave pairing in the Hubbard model on the square lattice tuned by the periodically modulated hopping amplitudes is studied. The hopping amplitudes are homogeneous in thexˆ-direction, while in theyˆ-direction the hopping amplitudes are modulated with periodP, wherety=t+dt,ty'=t-(P-1)dt, and the modulation periodPequals 2, 3 and 4 lattice spacings. The latter two modulation periods are motivated by the observation of period-3 and period-4 stripe order in cuprate superconductors. For all the periodsP, we find that the modulated hopping inhomogeneity enhances thed-wave pairing and an optimal inhomogeneity exists.

Monolayer NbF4: a 4d1-analogue of cuprates.

The electronic structure and possible electronic orders in monolayer NbF4 are investigated by density functional theory and functional renormalization group. Because of the niobium-centered octahedra, the energy band near the Fermi level is found to derive from the 4dxy orbital, well separated from the other bands. Local Coulomb interaction drives the undoped system into an antiferromagnetic insulator. Upon suitable electron/hole doping, the system is found to develop [Formula: see text] -wave superconductivity with sizable transition temperature. Therefore, the monolayer NbF4 may be an exciting 4d1 analogue of cuprates, providing a new two-dimensional platform for high-Tc superconductivity.

Predicting diamond-like Co-based chalcogenides as unconventional high temperature superconductors.

We predict Co-based chalcogenides with a diamond-like structure can host unconventional high temperature superconductivity (high-Tc). The essential electronic physics in these materials stems from the Co layers with each layer being formed by vertex-shared CoA4 (A=S, Se, Te) tetrahedra complexes, a material genome proposed recently by us to host potential unconventional high-Tc close to a d7 filling configuration in 3d transition metal compounds. We calculate the magnetic ground states of different transition metal compounds with this structure. It is found that (Mn, Fe, Co)-based compounds all have a G-type antiferromagnetic (AFM) insulating ground state while Ni-based compounds are paramagnetic metal. The AFM interaction is the largest in the Co-based compounds as the three t2g orbitals all strongly participate in AFM superexchange interactions. The abrupt quenching of the magnetism from the Co to Ni-based compounds is very similar to those from Fe to Co-based pnictides in which a C-type AFM state appears in the Fe-based ones but vanishes in the Co-based ones. This behavior can be considered as an electronic signature of the high-Tc gene. Upon doping, as we predicted before, this family of Co-based compounds favor a strong d-wave pairing superconducting state.

A possible family of Ni-based high temperature superconductors.

We suggest that a family of Ni-based compounds, which contain [Ni2M2O]2- (M = chalcogen) layers with an antiperovskite structure constructed by mixed-anion Ni complexes, NiM4O2, can be potential high temperature superconductors (high-Tc) upon doping or applying pressure. The layer structures have been formed in many other transitional metal compounds such as La2B2Se2O3 (B = Mn, Fe, Co). For the Ni-based compounds, we predict that the parental compounds host collinear antiferromagnetic states similar to those in iron-based high temperature superconductors. The electronic physics near Fermi energy is controlled by two egd-orbitals with completely independent in-plane kinematics. We predict that the superconductivity in this family is characterized by strong competition between extended s-wave and d-wave pairing symmetries.

A possible new family of unconventional high temperature superconductors.

We suggest a new family of Co/Ni-based materials that may host unconventional high temperature superconductivity (high-Tc). These materials carry layered square lattices with each layer being formed by vertex-shared transition metal tetrahedra cation-anion complexes. The electronic physics in these materials is determined by the two dimensional layer and is fully attributed to the three near degenerated t2gd-orbitals close to a d7 filling configuration in the d-shell of Co/Ni atoms. The electronic structure meets the necessary criteria for unconventional high Tc materials proposed recently by us to unify the two known high-Tc families, cuprates and iron-based superconductors. We predict that they host superconducting states with a d-wave pairing symmetry with Tc potentially higher than those of iron-based superconductors. These materials, if realized, can be a fertile new ground to study strongly correlated electronic physics and provide decisive evidence for superconducting pairing mechanism.