We study the evolution of a Mott-Hubbard insulator into a correlated metal upon doping in the two-dimensional Hubbard model using the Cellular Dynamical Mean Field Theory. Short-range spin correlations create two additional bands apart from the familiar Hubbard bands in the spectral function. Even a tiny doping into this insulator causes a jump of the Fermi energy to one of these additional bands and an immediate momentum dependent suppression of the spectral weight at this Fermi energy. The pseudogap is closely tied to the existence of these bands. This suggests a strong-coupling mechanism that arises from short-range spin correlations and large scattering rates for the pseudogap phenomenon seen in several cuprates.PACS numbers: 71.10. Fd, 71.27.+a, 71.30.+h, The issue of the origin of the pseudogap phenomenon observed in underdoped cuprates lies at the center of any theoretical explanation for high temperature superconductivity in the cuprates and is one of the most challenging questions in condensed matter physics. The suppression of low energy spectral weight in the normal state of these materials has been observed through various experimental probes [1]. In spite of many theoretical works to explain the observed anomalies, there is no consensus at present. The lack of controlled approximations to deal with the strong coupling physics and low dimensionality inherent to these systems continues to pose major stumbling blocks towards a complete theoretical understanding. Since the parent compounds of the cuprates are Mott-Hubbard insulators, an understanding of such an insulator and its evolution into a correlated metal upon doping is crucial.In this paper we study the two-dimensional Hubbard model on a square lattice at and near half-filling with Cellular Dynamical Mean-Field Theory (CDMFT) [2]. The CDMFT method is a natural generalization of the single site DMFT [3] to incorporate short-range spatial correlations. Since at and near half-filling short-range spin correlations are dominant at low energy, this method is expected to describe additional features caused by spin degrees of freedom in the single-particle spectrum. The CDMFT [4] has already passed several tests against exact results obtained by the Bethe Ansatz and the Density Matrix Renormalization Group (DMRG) techniques in one dimension, where the CDMFT scheme is expected to be in the worst case scenario. Long-range order involving several lattice sites such as d-wave superconductivity can be also described in CDMFT [5]. Several other cluster schemes have been proposed [6,7,8,9, 10] including Dynamical Cluster Approximation (DCA) [7], Cluster Perturbation Theory (CPT) [8] and its variational extension (V-CPT) [9]. The variational principle used in the last scheme allows one to consider CPT, V-CPT, and CDMFT within a unified framework.In the CDMFT construction [2, 4] the infinite lattice is tiled with identical clusters of size N c . In an effective action description, the degrees of freedom in a single cluster are treated exactly, while the remainin...

Proximity to a Mott insulating phase is likely to be an important physical ingredient of a theory that aims to describe high-temperature superconductivity in the cuprates. Quantum cluster methods are well suited to describe the Mott phase. Hence, as a step towards a quantitative theory of the competition between antiferromagnetism and d-wave superconductivity in the cuprates, we use Cellular Dynamical Mean Field Theory to compute zero temperature properties of the twodimensional square lattice Hubbard model. The d-wave order parameter is found to scale like the superexchange coupling J for on-site interaction U comparable to or larger than the bandwidth. The order parameter also assumes a dome shape as a function of doping while, by contrast, the gap in the single-particle density of states decreases monotonically with increasing doping. In the presence of a finite second-neighbor hopping t ′ , the zero temperature phase diagram displays the electron-hole asymmetric competition between antiferromagnetism and superconductivity that is observed experimentally in the cuprates. Adding realistic third-neighbor hopping t ′′ improves the overall agreement with the experimental phase diagram. Since band parameters can vary depending on the specific cuprate considered, the sensitivity of the theoretical phase diagram to band parameters challenges the commonly held assumption that the doping vs Tc/T max c phase diagram of the cuprates is universal. The calculated angle-resolved photoemission spectrum displays the observed electron-hole asymmetry. The tendency to homogeneous coexistence of the superconducting and antiferromagnetic order parameters is stronger than observed in most experiments but consistent with many theoretical results and with experiments in some layered high-temperature superconductors. Clearly, our calculations reproduce important features of d-wave superconductivity in the cuprates that would otherwise be considered anomalous from the point of view of the standard BardeenCooper-Schrieffer approach. At strong coupling, d-wave superconductivity and antiferromagnetism appear naturally as two equally important competing instabilities of the normal phase of the same underlying Hamiltonian.

The evolution from an anomalous metallic phase to a Mott insulator within the two-dimensional Hubbard model is investigated by means of the cellular dynamical mean-field theory. We show that approaching the density-driven Mott metal-insulator transition the Fermi surface is strongly renormalized and the quasiparticle description breaks down in a very anisotropic fashion. Regions where the quasiparticles are strongly scattered (hot spots) and regions where the scattering rate is relatively weak (cold spot) form irrespective of whether the parent insulator has antiferromagnetic long-range order, while their location is not universal and is determined by the interplay of the renormalization of the scattering rate and the Fermi surface shape.

The interplay between the structural and magnetic phase transitions occurring in the Fe-based pnictide superconductors is studied within a Ginzburg-Landau approach. We show that the magnetoelastic coupling between the corresponding order parameters is behind the salient features observed in the phase diagram of these systems. This naturally explains the coincidence of transition temperatures observed in some cases as well as the character (first or second-order) of the transitions. We also show that magnetoelastic coupling is the key ingredient determining the collinearity of the magnetic ordering, and we propose an experimental criterion to distinguish between a pure elastic from a spin-nematic-driven structural transition.PACS numbers: 74.70. Xa, 74.90.+n, 75.80.+q Introduction.-The discovery of an unconventional high-temperature superconductivity in the F-doped arsenic-oxide LaFeAsO 1 has given rise to a great interest on iron pnictide compounds. To date several families of Fe-based superconductors have been discovered, and the superconducting transition temperature has been raised to above 50 K.2 All of these systems display an intriguing competition between structural, magnetic and superconducting transitions.2,3 The parent compounds of the so-called 1111 and 111 families undergo a structural transition (ST) followed by a magnetic transition (MT) at a lower temperature, 4 whereas in the 122 and 11 cases these two transitions take place simultaneously. [5][6][7] In any case these orderings are quickly suppressed by doping or by applying pressure, which eventually gives rise to superconductivity. The role, if any, of magnetic and elastic degrees of freedom in inducing this superconductivity is currently an open question. There is already a growing body of theoretical works advocating for spin fluctuation mediated superconductivity, 8 but the isotope effect observed in both magnetism and superconductivity suggests that the elastic medium also plays a role.9 It is therefore compelling to understand the connection between the ST and the MT in these systems.

We investigate Weyl semimetals with tilted conical bands in a magnetic field. Even when the cones are overtilted (type-II Weyl semimetal), Landau-level quantization can be possible as long as the magnetic field is oriented close to the tilt direction. Most saliently, the tilt can be described within the relativistic framework of Lorentz transformations that give rise to a rich spectrum, displaying new transitions beyond the usual dipolar ones in the optical conductivity. We identify particular features in the latter that allow one to distinguish between semimetals of different types.