Unconventional Hall effect in pnictides from interband interactions

We calculate the Hall transport in a multiband systems with a dominant interband interaction between carriers having electron and hole character. We show that this situation gives rise to an unconventional scenario, beyond the Boltzmann theory, where the quasiparticle currents dressed by vertex corrections acquire the character of the majority carriers. This leads to a larger (positive or negative) Hall coefficient than what expected on the basis of the carrier balance, with a marked temperature dependence. Our results explain the puzzling measurements in pnictides and they provide a more general framework for transport properties in multiband materials.Comment: 5 pages, 2 figure

Optical excitation of phase modes in strongly disordered superconductors

According to the Goldstone theorem the breaking of a continuous U(1) symmetry comes along with the existence of low-energy collective modes. In the context of superconductivity these excitations are related to the phase of the superconducting (SC) order parameter and for clean systems are optically inactive. Here we show that for strongly disordered superconductors phase modes acquire a dipole moment and appear as a subgap spectral feature in the optical conductivity. This finding is obtained with both a gauge-invariant random-phase approximation scheme based on a fermionic Bogoliubov-de Gennes state as well as with a prototypical bosonic model for disordered superconductors. In the strongly disordered regime, where the system displays an effective granularity of the SC properties, the optically active dipoles are linked to the isolated SC islands, offering a new perspective for realizing microwave optical devices

Theory for Phase Transitions in Insulating Vanadium Oxide

We show that the recently proposed S=2 bond model with orbital degrees of freedom for insulating V$_{2}$O$_{3}$ not only explains the anomalous magnetic ordering, but also other mysteries of the magnetic phase transition. The model contains an additional orbital degree of freedom that exhibits a zero temperature quantum phase transtion in the Ising universality class.Comment: 5 pages, 2 figure

Interchain coherence of coupled Luttinger liquids at all orders in perturbation theory

We analyze the problem of Luttinger liquids coupled via a single-particle hopping \tp and introduce a systematic diagrammatic expansion in powers of \tp. An analysis of the scaling of the diagrams at each order allows us to determine the power-law behavior versus \tp of the interchain hopping and of the Fermi surface warp. In particular, for strong interactions, we find that the exponents are dominated by higher-order diagrams producing an enhanced coherence and a failure of linear-response theory. Our results are valid at any finite order in \tp for the self-energy.Comment: 4 pages, 3 ps figures. Accepted for publication in Phys. Rev. Let

Spectroscopic evidences of quantum critical charge fluctuations in cuprates

We calculate the optical conductivity in a clean system of quasiparticles coupled to charge-ordering collective modes. The absorption induced by these modes may produce an anomalous frequency and temperature dependence of low-energy optical absorption in some cuprates. However, the coupling with lattice degrees of freedom introduces a non-universal energy scale leading to scaling violation in low-temperature optical conductivity.Comment: Proceedings of M2S 2006. To appear in Physica

Absence of electron dephasing at zero temperature

Dephasing of electrons due to the electron-electron interaction has recently been the subject of a controversial debate, with different calculations yielding mutually incompatible results. In this paper we prove, by means of Ward identities, that neither a Coulomb interaction nor a short-ranged model interaction can lead to phase breaking at zero temperature in spatial dimensions d>2.Comment: 7 pp., LaTeX, no figs, final version as publishe

Networks from gene expression time series: characterization of correlation patterns

This paper describes characteristic features of networks reconstructed from gene expression time series data. Several null models are considered in order to discriminate between informations embedded in the network that are related to real data, and features that are due to the method used for network reconstruction (time correlation).Comment: 10 pages, 3 BMP figures, 1 Table. To appear in Int. J. Bif. Chaos, July 2007, Volume 17, Issue

X-Ray Resonant Scattering as a Direct Probe of Orbital Ordering in Transition-Metal Oxides

X-ray resonant scattering at the K-edge of transition metal oxides is shown to measure the orbital order parameter, supposed to accompany magnetic ordering in some cases. Virtual transitions to the 3d-orbitals are quadrupolar in general. In cases with no inversion symmetry, such as V$_2$O$_3$, treated in detail here, a dipole component enhances the resonance. Hence, we argue that the detailed structure of orbital order in V$_2$O$_3$ is experimentally accessible.Comment: LaTex using RevTex, 4 pages and two included postscript figure

Two-gap model for underdoped cuprate superconductors

Various properties of underdoped superconducting cuprates, including the momentum-dependent pseudogap opening, indicate a behavior which is neither BCS nor Bose-Einstein condensation (BEC) like. To explain this issue we introduce a two-gap model. This model assumes an anisotropic pairing interaction among two kinds of fermions with small and large Fermi velocities representing the quasiparticles near the M and the nodal points of the Fermi surface respectively. We find that a gap forms near the M points resulting into incoherent pairing due to strong fluctuations. Instead the pairing near the nodal points sets in with phase coherence at lower temperature. By tuning the momentum-dependent interaction, the model allows for a continuous evolution from a pure BCS pairing (in the overdoped and optimally doped regime) to a mixed boson-fermion picture (in the strongly underdoped regime).Comment: 5 pages, 1 enclosed figure. For further information see http://htcs.or