The orbital-selective Mott phase (OSMP) of multiorbital Hubbard models has been extensively analyzed before using static and dynamical mean-field approximations. In parallel, the properties of Block states (antiferromagnetically coupled ferromagnetic spin clusters) in Fe-based superconductors have also been much discussed. The present effort uses numerically exact techniques in one-dimensional systems to report the observation of Block states within the OSMP regime, connecting two seemingly independent areas of research, and providing analogies with the physics of Double-Exchange models. PACS numbers: 71.30.+h, 71.27.+a, 71.10.Fd, 71.10.Fd Introduction. The combined interplay of charge, spin, lattice, and orbital degrees of freedom have led to an enormous variety of emergent phenomena in strongly correlated systems. A prototypical example is the half-filled single-orbital metal-insulator transition, that is realized in materials such as La 2 CuO 4 , a parent compound of the Cu-based high temperature superconductors. If several active orbitals are also considered in the study of this transition, an even richer phase diagram is anticipated, where states such as band insulators, correlated metals, and orbital-selective Mott phases (OSMP) can be stabilized. In particular, the study of the OSMP and its associated orbital-selective Mott transition has attracted considerable attention in recent years [1][2][3].The OSMP is a state where even though Mott insulator (MI) physics occurs, it is restricted to a subset of all the active orbitals present in the problem. This state has narrow-band localized electrons related to the MI orbitals, coexisting with wide-band itinerant electrons at the other orbitals [4][5][6]. To stabilize the OSMP, a robust Hund interaction J is needed. In general, the hybridization within orbitals γ, V γ,γ ′ , and crystal fields, ∆ γ , work against J since they favor low-spin ground states. Therefore, if J ≫ ∆ γ , V γ,γ ′ , the OSMP is expected to be stable and display robust local moments [6].Several studies focused on the effects of interactions, filling fractions, etc., on the stability of orbital-selective phases [3]. This previous theoretical work was performed within meanfield approximations (such as Dynamical Mean Field Theory [4][5][6][7][8], slave-spins [6,[8][9][10][11]13]). Using these methods the OSMP stability conditions have been established. However, to our knowledge, there have been no detailed studies of the influence of full quantum fluctuations on this phase and therefore, and more importantly for our purposes, of their low-temperature electronic and magnetic properties.Recently, these issues received considerable attention in the Fe-based superconductors community. In this context, multiorbital models containing Hubbard U and J interactions, as well as crystal-field splittings, are widely employed, and the existence of OSMP regimes has been extensively investi-

Recent neutron scattering experiments addressing the magnetic state of the two-leg ladder selenide compound BaFe2Se3 have unveiled a dominant spin arrangement involving ferromagnetically ordered 2×2 iron-superblocks, that are antiferromagnetically coupled among them (the "block-AFM" state). Using the electronic five-orbital Hubbard model, first principles techniques to calculate the electronic hopping amplitudes between irons, and the real-space Hartree-Fock approximation to handle the many-body effects, here it is shown that the exotic block-AFM state is indeed stable at realistic electronic densities close to n ∼ 6.0. Another state (the "CX" state) with parallel spins along the rungs and antiparallel along the legs of the ladders is close in energy. This state becomes stable in other portions of the phase diagrams, such as with hole doping, as also found experimentally via neutron scattering applied to KFe2Se3. In addition, the present study unveils other competing magnetic phases that could be experimentally stabilized varying either n chemically or the electronic bandwidth by pressure. Similar results were obtained using two-orbital models, studied here via Lanczos and DMRG techniques. A comparison of the results obtained with the realistic selenides hoppings amplitudes for BaFe2Se3 against those found using the hopping amplitudes for pnictides reveals several qualitative similarities, particularly at intermediate and large Hubbard couplings.

Coherent electronic transport through individual molecules is crucially sensitive to quantum interference. We investigate the zero-bias and zero-temperature conductance through pi-conjugated annulene molecules weakly coupled to two leads for different source-drain configurations, finding an important reduction for certain transmission channels and for particular geometries as a consequence of destructive quantum interference between states with definite momenta. When translational symmetry is broken by an external perturbation we find an abrupt increase of the conductance through those channels. Previous studies concentrated on the effect at the Fermi energy, where this effect is very small. By analyzing the effect of symmetry breaking on the main transmission channels we find a much larger response thus leading to the possibility of a larger switching of the conductance through single molecules.