Using the constrained-path Monte Carlo method, a two-orbital model for the pnictide superconductors is studied at half filling and in both the electron-and hole-doped cases. At half filling, a stable (π, 0)/(0, π) magnetic order is explicitly observed, and the system tends to be in an orthomagnetic order rather than the striped antiferromagnetic order when increasing the Coulomb repulsion U . In the electron-doped case, the (π, 0)/(0, π) magnetic order is enhanced upon doping and suppressed eventually, and a s± pairing state dominates all the possible nearest-neighbor-bond pairings. Whereas in the hole-doped case, the magnetic order is straightforwardly suppressed and two nearly degenerate A1g and B1g intraband pairings become the dominant ones.
A recently introduced one-dimensional three-orbital Hubbard model displays orbital-selective Mott phases with exotic spin arrangements such as spin block states [J. Rincón et al., Phys. Rev. Lett. 112, 106405 (2014)PRLTAO0031-900710.1103/PhysRevLett.112.106405]. In this publication we show that the constrained-path quantum Monte Carlo (CPQMC) technique can accurately reproduce the phase diagram of this multiorbital one-dimensional model, paving the way to future CPQMC studies in systems with more challenging geometries, such as ladders and planes. The success of this approach relies on using the Hartree-Fock technique to prepare the trial states needed in CPQMC. We also study a simplified version of the model where the pair-hopping term is neglected and the Hund coupling is restricted to its Ising component. The corresponding phase diagrams are shown to be only mildly affected by the absence of these technically difficult-to-implement terms. This is confirmed by additional density matrix renormalization group and determinant quantum Monte Carlo calculations carried out for the same simplified model, with the latter displaying only mild fermion sign problems. We conclude that these methods are able to capture quantitatively the rich physics of the several orbital-selective Mott phases (OSMP) displayed by this model, thus enabling computational studies of the OSMP regime in higher dimensions, beyond static or dynamic mean-field approximations.
We introduce a two-orbital Hamiltonian on a square lattice that contains on-site attractive interactions involving the two eg orbitals. Via a canonical mean-field procedure similar to the one applied to the well-known negative-U Hubbard model, it is shown that the new model develops d-wave (B1g) superconductivity with nodes along the diagonal directions of the square Brillouin zone. This result is also supported by exact diagonalization of the model in a small cluster. The expectation is that this relatively simple attractive model could be used to address the properties of multiorbital d-wave superconductors in the same manner that the negative-U Hubbard model is widely applied to the study of the properties of s-wave single-orbital superconductors. In particular, we show that by splitting the eg orbitals and working at three-quarters filling, such that the x 2 − y 2 orbital dominates at the Fermi level but the 3z 2 − r 2 orbital contribution is nonzero, the d-wave pairing state found here phenomenologically reproduces several properties of the superconducting state of the high Tc cuprates.
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