The nature of correlations in the insulating states of twisted bilayer graphene

The recently observed superconductivity in twisted bilayer graphene emerges from insulating states believed to arise from electronic correlations. While there have been many proposals to explain the insulating behaviour, the commensurability at which these states appear suggests that they are Mott insulators. Here we focus on the insulating states with $\pm 2$ electrons or holes with respect to the charge neutrality point. We show that the theoretical expectations for the Mott insulating states are not compatible with the experimentally observed dependence on temperature and magnetic field if, as frequently assumed, only the correlations between electrons on the same site are included. We argue that the inclusion of non-local (inter-site) correlations in the treatment of the Hubbard model can bring the predictions for the magnetic and temperature dependencies of the Mott transition to an agreement with experiments and have consequences for the critical interactions, the size of the gap, and possible pseudogap physics. The importance of the inter-site correlations to explain the experimental observations indicates that the observed insulating gap is not the one between the Hubbard bands and that antiferromagnetic-like correlations play a key role in the Mott transition.Comment: 8 pages (including appendix), 5 figure

Electron gas at the interface between two antiferromagnetic insulating manganites

We study theoretically the magnetic and electric properties of the interface between two antiferromagnetic and insulating manganites: La0.5Ca0.5MnO3, a strong correlated insulator, and CaMnO3, a band-insulator. We find that a ferromagnetic and metallic electron gas is formed at the interface between the two layers. We confirm the metallic character of the interface by calculating the in-plane conductance. The possibility of increasing the electron gas density by selective doping is also discussed.Comment: 6 pages, including 9 figure

Conductivity anisotropy in the antiferromagnetic state of iron pnictides

Recent experiments on iron pnictides have uncovered a large in-plane resistivity anisotropy with a surprising result: the system conducts better in the antiferromagnetic x direction than in the ferromagnetic y direction. We address this problem by calculating the ratio of the Drude weight along the x and y directions, Dx/Dy, for the mean-field Q=(\pi,0) magnetic phase diagram of a five-band model for the undoped pnictides. We find that Dx/Dy ranges between 0.3 < D_x/D_y < 1.4 for different interaction parameters. Large values of orbital ordering favor an anisotropy opposite to the one found experimentally. On the other hand D_x/D_y is strongly dependent on the topology and morfology of the reconstructed Fermi surface. Our results points against orbital ordering as the origin of the observed conductivity anisotropy, which may be ascribed to the anisotropy of the Fermi velocity.Comment: 4 pages, 3 pdf figures. Fig 1(b) changed, one equation corrected, minor changes in the text, references update

Orbital differentiation and the role of orbital ordering in the magnetic state of Fe superconductors

We analyze the metallic (pi,0) antiferromagnetic state of a five-orbital model for iron superconductors. We find that with increasing interactions the system does not evolve trivially from the pure itinerant to the pure localized regime. Instead we find a region with a strong orbital differentiation between xy and yz, which are half-filled gapped states at the Fermi level, and itinerant zx, 3z^2-r^2 and x^2-y^2. We argue that orbital ordering between yz and zx orbitals arises as a consequence of the interplay of the exchange energy in the antiferromagnetic x direction and the kinetic energy gained by the itinerant orbitals along the ferromagnetic y direction with an overall dominance of the kinetic energy gain. We indicate that iron superconductors are close to the boundary between the itinerant and the orbital differentiated regimes and that it could be possible to cross this boundary with doping.Comment: 6 pages, including 7 figures. As accepted in Phys. Rev.

Fourier Decomposition of RR Lyrae light curves and the SX Phe population in the central region of NGC 3201

CCD time-series observations of the central region of the globular cluster NGC~3201 were obtained with the aim of performing the Fourier decomposition of the light curves of the RR~Lyrae stars present in that field. This procedure gave the mean values, for the metallicity, of [Fe/H]$_{ZW}=-1.483 \pm 0.006$ (statistical) $\pm 0.090$ (systematical), and for the distance, $5.000 \pm 0.001$~kpc (statistical) $\pm 0.220$ (systematical). The values found from two RRc stars are consistent with those derived previously. The differential reddening of the cluster was investigated and individual reddenings for the RR Lyrae stars were estimated from their $V-I$ curves. We found an average value of $E(B-V)= 0.23 \pm 0.02$. An investigation of the light curves of stars in the {\it blue stragglers} region led to the discovery of three new SX~Phe stars. The period-luminosity relation of the SX~Phe stars was used for an independent determination of the distance to the cluster and of the individual reddenings. We found a distance of 5.0 kpcComment: To appear in Revista Mexicana de Astronom\'ia y Astrof\'isica, Octuber 2014 issue, Vol 50. 17 pages, 10 figure

Re-entrant ferromagnetism in a generic class of diluted magnetic semiconductors

Considering a general situation where a semiconductor is doped by magnetic impurities leading to a carrier-induced ferromagnetic exchange coupling between the impurity moments, we show theoretically the possible generic existence of three ferromagnetic transition temperatures, T_1 > T_2 > T_3, with two distinct ferromagnetic regimes existing for T_1 > T > T_2 and T < T_3. Such an intriguing re-entrant ferromagnetism, with a paramagnetic phase (T_2 > T > T_3) between two ferromagnetic phases, arises from a subtle competition between indirect exchange induced by thermally activated carriers in an otherwise empty conduction band versus the exchange coupling existing in the impurity band due to the bound carriers themselves. We comment on the possibility of observing such a re-entrance phenomenon in diluted magnetic semiconductors and magnetic oxides.Comment: 4 pages, 3 figure

On a Calderon-Zygmund commutator-type estimate

In this paper we extend a Calderon-Zygmund commutator-type estimate. This estimate enables us to prove an embedding result concerning weighted function spaces.Comment: 20 pages, no figure