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Coexistence of metallic edge states and anti-ferromagnetic ordering in correlated topological insulators
We investigate the emergence of anti-ferromagnetic ordering and its effect on the helical edge states in a quantum spin Hall insulator, in the presence of strong Coulomb interaction. Using dynamical mean-field theory, we show that the breakdown of lattice translational symmetry favours the formation of magnetic ordering with non-trivial spatial modulation. The onset of a non-uniform magnetization enables the coexistence of spin-ordered and topologically non-trivial states. An unambiguous signature of the persistence of the topological bulk property is the survival of bona fide edge states. We show that the penetration of the magnetic order is accompanied by the progressive reconstruction of gapless states in sub-peripherals layers, redefining the actual topological boundary within the system. Correlation-driven Lifshitz transition and orbital order in a two-band Hubbard model
The role of orbital degrees of freedom and its interplay with the Mott physics in correlated materials has lately received a renewed interest, thanks to the experimental results obtained for different compounds. Motivated by this, we investigated the ground state properties of a simplified model which can capture the essential physics of these systems. We solved a two bands model with different bandwidths in the non-perturbative regime, using DMFT and analysed its behavior at weak- and strong-coupling by means of suitable expansions. At quarter filling, i.e. one electron per site, the presence of the interaction leads to the formation of an effective crystal field, not present in the original Hamiltonian, which ultimately favours the occupation of the wider band. For small bandwidth ratios an increase of the effective crystal field can drive a topological Lifshitz transition from a two- band to a one-band metal. For larger interactions the system undergoes a metal-insulator transition to a Mott state. The properties of such Mott insulator are significantly affected by the interplay of orbital and magnetic ordering, which we analyse in details. For larger values of the bandwidth ratio the system shows a direct first-order transition from the two- band metal to an orbitally ordered Mott state. Surprisingly, the metallic phase hosts a regime where the wide band gets more correlated than the narrow one, contrary to the orbital selective Mott scenario.  (credits image: Angelo Valli) Exploit quantum effects in nano-structures to manipulate the current flow.
We demonstrate that hexagonal graphene nanoflakes with zigzag edges display quantum interference (QI) patterns analogous to benzene molecular junctions. In contrast with graphene sheets, these nanoflakes also host magnetism. The cooperative effect of QI and magnetism enables spin-dependent quantum interference effects that result in a nearly complete spin polarization of the current and holds a huge potential for spintronic applications. We understand the origin of QI in terms of symmetry arguments, which show the robustness and generality of the effect. This also allows us to devise a concrete protocol for the electrostatic control of the spin polarization of the current by breaking the sublattice symmetry of graphene, by deposition on hexagonal boron nitride, paving the way to switchable spin filters. Such a system benefits from all of the extraordinary conduction properties of graphene, and at the same time, it does not require any external magnetic field to select the spin polarization, as magnetism emerges spontaneously at the edges of the nanoflake. Starting from Jan. 15th I will be visiting researcher in the group of L.de'Medici at the LPEM, EPSCI.
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Author:SISSA - CM Group
Office 417 - 4th floor Via Bonomea 265, 34136 Trieste, Italy tel.: +39 040 3787 505 fax: +39 040 3787 528 la differenza tra bufalo e locomotiva salta agli occhi,
la locomotiva ha la strada segnata, mentre il bufalo puo' scartare di lato, e cadere... Archives
May 2018
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