40 research outputs found
Metal-Insulator transitions in the periodic Anderson model
We solve the Periodic Anderson model in the Mott-Hubbard regime, using
Dynamical Mean Field Theory. Upon electron doping of the Mott insulator, a
metal-insulator transition occurs which is qualitatively similar to that of the
single band Hubbard model, namely with a divergent effective mass and a first
order character at finite temperatures. Surprisingly, upon hole doping, the
metal-insulator transition is not first order and does not show a divergent
mass. Thus, the transition scenario of the single band Hubbard model is not
generic for the Periodic Anderson model, even in the Mott-Hubbard regime.Comment: 5 pages, 4 figure
Interplay between destructive quantum interference and symmetry-breaking phenomena in graphene quantum junctions
We study the role of electronic spin and valley symmetry in the quantum interference (QI) patterns of the transmission function in graphene quantum junctions. In particular, we link it to the position of the destructive QI antiresonances. When the spin or valley symmetry is preserved, electrons with opposite spin or valley display the same interference pattern. On the other hand, when a symmetry is lifted, the antiresonances are split, with a consequent dramatic differentiation of the transport properties in the respective channel. We demonstrate rigorously this link in terms of the analytical structure of the electronic Green function, which follows from the symmetries of the microscopic model, and we confirm the result with numerical calculations for graphene nanoflakes. We argue that this is a generic and robust feature that can be exploited in different ways for the realization of nanoelectronic QI devices, generalizing the recent proposal of a QI-assisted spin-filtering effect [A. Valli et al., Nano Lett. 18, 2158 (2018)10.1021/acs.nanolett.8b00453]
Inducing and controlling magnetism in the honeycomb lattice through a harmonic trapping potential
We study strongly interacting ultracold spin-1/2 fermions in a honeycomb lattice in the presence of a harmonic trap. Tuning the strength of the harmonic trap we show that it is possible to confine the fermions in artificial structures reminiscent of graphene nanoflakes in solid state. The confinement on small structures induces magnetic effects which are absent in a large graphene sheet. Increasing the strength of the harmonic potential we are able to induce different magnetic states, such as a N\ue9el-like antiferromagnetic or ferromagnetic state, as well as mixtures of these basic states. The realization of different magnetic patterns is associated with the terminations of the artificial structures, in turn controlled by the confining potential
Analyticity of the SRB measure for a class of simple Anosov flows
We consider perturbations of the Hamiltonian flow associated with the
geodesic flow on a surface of constant negative curvature. We prove that, under
a small perturbation, not necessarely of Hamiltonian character, the SRB measure
associated to the flow exists and is analytic in the strength of the
perturbation. An explicit example of "thermostatted" dissipative dynamics is
constructed.Comment: 23 pages, corrected typo
Correlation-driven Lifshitz transition and orbital order in a two-band Hubbard model
We study by dynamical mean-field theory the ground state of a quarter-filled Hubbard model of two bands with different bandwidths. At half-filling, this model is known to display an orbital selective Mott transition, with the narrower band undergoing Mott localization while the wider one being still itinerant. At quarter-filling, the physical behavior is different and to some extent reversed. The interaction generates an effective crystal field splitting, absent in the Hamiltonian, that tends to empty the narrower band in favor of the wider one, which also become more correlated than the former at odds with the orbital selective paradigm. Upon increasing the interaction, the depletion of the narrower band can continue till it empties completely and the system undergoes a topological Lifshitz transition into a half-filled single-band metal that eventually turns insulating. Alternatively, when the two bandwidths are not too different, a first order Mott transition intervenes before the Lifshitz's one. The properties of the Mott insulator are significantly affected by the interplay between spin and orbital degrees of freedom
Exciton condensation in strongly correlated quantum spin Hall insulators
Time reversal symmetric topological insulators are generically robust with
respect to weak local interaction, unless symmetry breaking transitions take
place. Using dynamical mean-field theory we solve an interacting model of
quantum spin Hall insulators and show the existence, at intermediate coupling,
of a symmetry breaking transition to a non-topological insulator characterised
by exciton condensation. This transition is of first order. For a larger
interaction strength the insulator evolves into a Mott one. The transition is
continuous if magnetic order is prevented, and notably, for any finite Hund's
exchange it progresses through a Mott localization before the condensate
coherence is lost. We show that the correlated excitonic state corresponds to a
magneto-electric insulator which allows for direct experimental probing.
Finally, we discuss the fate of the helical edge modes across the excitonic
transition.Comment: 8 pages, 5 figure
Asymmetry between the electron- and hole-doped Mott transition in the periodic Anderson model
We study the doping driven Mott metal-insulator transition (MIT) in the
periodic Anderson model set in the Mott-Hubbard regime. A striking asymmetry
for electron or hole driven transitions is found. The electron doped MIT at
larger U is similar to the one found in the single band Hubbard model, with a
first order character due to coexistence of solutions. The hole doped MIT, in
contrast, is second order and can be described as the delocalization of
Zhang-Rice singlets.Comment: 18 pages, 19 figure
Coexistence of metallic edge states and antiferromagnetic ordering in correlated topological insulators
We investigate the emergence of antiferromagnetic 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 favors the formation of magnetic ordering with nontrivial spatial modulation. The onset of a nonuniform magnetization enables the coexistence of spin-ordered and topologically nontrivial 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 subperipheral layers, redefining the actual topological boundary within the system
Augmented hybrid exact-diagonalization solver for dynamical mean field theory
We present a new methodology to solve the Anderson impurity model, in the
context of dynamical mean-field theory, based on the exact diagonalization
method. We propose a strategy to effectively refine the exact diagonalization
solver by combining a finite-temperature Lanczos algorithm with an adapted
version of the cluster perturbation theory. We show that the augmented
diagonalization yields an improved accuracy in the description of the spectral
function of the single-band Hubbard model and is a reliable approach for a full
d-orbital manifold calculation