We present results of a systematic Quantum-Monte-Carlo study for the single-band Hubbard model. Thereby we evaluated single-particle spectra (PES & IPES), two-particle spectra (spin & density correlation functions), and the dynamical correlation function of suitably defined diagnostic operators, all as a function of temperature and hole doping. The results allow to identify different physical regimes. Near half-filling we find an anomalous 'Hubbard-I phase', where the band structure is, up to some minor modifications, consistent with the Hubbard-I predictions. At lower temperatures, where the spin response becomes sharp, additional dispersionless 'bands' emerge due to the dressing of electrons/holes with spin excitatons. We present a simple phenomenological fit which reproduces the band structure of the insulator quantitatively. The Fermi surface volume in the low doping phase, as derived from the single-particle spectral function, is not consistent with the Luttinger theorem, but qualitatively in agreement with the predictions of the Hubbard-I approximation. The anomalous phase extends up to a hole concentration of ≈ 15%, i.e. the underdoped region in the phase diagram of high-Tc superconductors. We also investigate the nature of the magnetic ordering transition in the single particle spectra. We show that the transition to an SDWlike band structure is not accomplished by the formation of any resolvable 'precursor bands', but rather by a (spectroscopically invisible) band of spin 3/2 quasiparticles. We discuss implications for the 'remnant Fermi surface' in insulating cuprate compounds and the shadow bands in the doped materials.71.10. Hf,71.10.Fd,75.40.Gb

Abstract. -An extensive calorimetric study of the normal-and superconducting-state properties of Ba(Fe1−xCox)2As2 is presented for 0 < x < 0.2. The normal-state Sommerfeld coefficient increases (decreases) with Co doping for x < 0.06 (x > 0.06), which illustrates the strong competition between magnetism and superconductivity to monopolize the Fermi surface in the underdoped region and the filling of the hole bands for overdoped Ba(Fe1−xCox)2As2. All superconducting samples exhibit a residual electronic density of states of unknown origin in the zero-temperature limit, which is minimal at optimal doping but increases to the normal-state value in the strongly under-and over-doped regions. The remaining specific heat in the superconducting state is well described using a two-band model with isotropic s-wave superconducting gaps.Introduction. -Despite the fact that the theoretical background has been available since the late 50's with the pioneering papers of Suhl et al.[1] and Moskalenko [2], multiband superconductivity (MBSC) emerged as an unanimously accepted phenomenon only after the discovery of the MgB 2 superconductor in 2001 [3]. Rapidly, calorimetric signatures, like the excess specific heat observed at low temperature (with respect to the single-band BCS curve) [4], the initial rapid rise of the mixed-state heat capacity with magnetic field [5], and the anomalous positive curvature of the upper critical field [6] provided the most convincing evidence of its existence and are now textbook hallmarks of the existence of two gaps. Later, significant interband contributions to the Eliashberg function, reminiscent of MBSC, were experimentally detected using tunneling experiments [49]. Since then, the occurrence of MBSC has been discussed for many different compounds including heavy fermions [7], cobaltates [8], chalcogenides [9], A15 compounds [50], and the recently discovered iron-pnictide family. The aforementioned characteristic signatures of MBSC are less pronounced in iron pnictide superconductors since the interband coupling, pro-

We study the spontaneous symmetry breaking of the excitonic insulator state induced by the Coulomb interaction U in the two-dimensional extended Falicov-Kimball model. Using the variational cluster approximation (VCA) and Hartree-Fock approximation (HFA), we evaluate the order parameter, single-particle excitation gap, momentum distribution functions, coherence length of excitons, and single-particle and anomalous excitation spectra, as a function of U at zero temperature. We find that in the weak-to-intermediate coupling regime, the Fermi surface plays an essential role and calculated results can be understood in close correspondence with the BCS theory, whereas in the strong-coupling regime, the Fermi surface plays no role and results are consistent with the picture of BEC. Moreover, we find that HFA works well both in the weak-and strong-coupling regime, and that the difference between the results of VCA and HFA mostly appears in the intermediate-coupling regime. The reason for this is discussed from a viewpoint of the self-energy. We thereby clarify the excitonic insulator state that typifies either a BCS condensate of electron-hole pairs (weak-coupling regime) or a Bose-Einstein condensate of preformed excitons (strong-coupling regime).

We report on the determination of the electronic heat capacity of a slightly overdoped ͑x = 0.075͒ Ba͑Fe 1−x Co x ͒ 2 As 2 single crystal with a T c of 21.4 K. Our analysis of the temperature dependence of the superconducting-state specific heat provides strong evidence for a two-band s-wave order parameter with gap amplitudes 2⌬ 1 ͑0͒ / k B T c = 1.9 and 2⌬ 2 ͑0͒ / k B T c = 4.4.

Basing on t-J model we calculate the k-dependence of a single hole photoproduction probability for CuO2 plane at zero doping. We also discuss the radiation of spin-waves which can substantially deform the shape of photoemission spectra.Comment: latex 8 pages, 3 figure