Effects of in-plane oxygens on the magnetic response in cuprates

A brief analysis of ARPES, Raman, and neutron data is used to show the importance of the oxygen degree of freedom for the metallic phase of cuprate high-T_c superconductors. It is based on published results and a number of new calculations, relevant to Raman and neutron scattering in the metallic underdoped and optimally doped regime. They are placed in the context of a recently developed theoretical understanding of the microscopic interplay between metallicity and local valence-bond fluctuations in the cuprates. It is argued that the oxygen degree of freedom is dominantly responsible for the metallicity in the normal state. The coexistence of metallic oxygen-dominated states with localized copper-dominated states is a key experimental constraint on the microscopic understanding of the normal-state pseudogap proposed here, especially of its strong k-dependence.     

A precursor superconductivity approach to magnetic field effects in the pseudogap phase

We demonstrate that the observed dependences of T_c and T^∗ on small magnetic fields can be readily understood in a precursor superconductivity approach to the pseudogap phase. In this approach, the presence of a pseudogap at T_c (but not at T^∗) and the associated suppression of the density of states lead to very different sensitivities to pair-breaking perturbations for the two temperatures. Our semi-quantitative results address the puzzling experimental observation that the coherence length ξ is weakly dependent on hole concentration x throughout most of the phase diagram. We present our results in a form which can be compared with the recent experiments of Shibauchi et al. and argue that orbital effects contribute in an important way to the H dependence of T^∗.     

A model of the T-dependent pseudogap and its competition with superconductivity in copper oxides

Results for pseudogaps are obtained from a band model, where the stability of the gap depends on the amplitudes of vibrational displacements, or magnetic moments, and their coupling to electrons. A one-particle gap is favored by normal thermal excitations of phonons or spin waves. Another gap can be generated by spontaneous waves at lower temperature, if the electronic energy gain overcomes the elastic/magnetic energy needed for increased amplitudes of the oscillations. This state is characterized by charge or spin density waves. The pseudogap has many features in common with the superconducting gap, and the model lends support to the interpretation that the pseudogap is a precursor of, and competes with, superconducting pairing.     

Correlation effects obtained from optical spectra of Fe-pnictides using an extended Drude-Lorentz model analysis

We introduce an analysis model, an extended Drude–Lorentz model, and apply it to Fe-pnictide systems to extract their electron–boson spectral density functions (or correlation spectra). The extended Drude–Lorentz model consists of an extended Drude mode for describing correlated charge carriers and Lorentz modes for interband transitions. The extended Drude mode can be obtained by a reverse process starting from the electron–boson spectral density function and extending to the optical self-energy and, eventually, to the optical conductivity. Using the extended Drude–Lorentz model, we obtained the electron–boson spectral density functions of K-doped BaFe₂As₂ (Ba-122) at four different doping levels. We discuss the doping-dependent properties of the electron–boson spectral density function of K-doped Ba-122. We also can include pseudogap effects in the model using this approach. Therefore, this approach is very helpful for understanding and analyzing measured optical spectra of strongly correlated electron systems, including high-temperature superconductors (cuprates and Fe-pnictides).     

Electronic phase separation and the inhomogeneities in Bi2212

Recent works on scanning tunneling microscopy (STM) have shown how the measured local gaps vary in a Bi2212 family of samples with different hole doping levels and also, how they vary with the temperature. Here we use the Cahn-Hilliard (CH) equation of phase segregation in alloys to describe the phase separations of holes in HTS. This method allows us to determine how the local Landau free energy changes in a given sample as a function of temperature, assuming that the line of anomalies or signals related with the upper pseudogap is the phase separation temperature T_ps. The free energy and the hole density form regions of low and high values separated by a potential barrier, which we propose to be the origin of the superconducting attraction.     

Intermediate coupling model of cuprates: Adding fluctuations to a weak coupling model of pseudogap and superconductivity competition

We demonstrate that many features ascribed to strong correlation effects in various spectroscopies of the cuprates are captured by a calculation of the self-energy incorporating effects of spin and charge fluctuations. The self-energy is calculated over the full doping range from half-filling to the overdoped system. In the normal state, the spectral function reveals four subbands: two widely split incoherent bands representing the remnant of the two Hubbard bands, and two additional coherent, spin- and charge-dressed in-gap bands split by a spin-density wave, which collapses in the overdoped regime. The resulting coherent subbands closely resemble our earlier mean-field results. Here we present an overview of the combined results of our mean-field calculations and the newer extensions into the intermediate coupling regime.     

Paraconductivity of doped LaOFeAs superconductors

The paraconductivity of both electron- and hole-doped LaOFeAs superconductors is studied within the existed fluctuation mechanisms. It is found that the FC data at the temperature close to T_c can be explained with the three-dimensional (3D) Aslamazov-Larkin (AL) theory. The Gaussian-Ginzburg-Landau (GGL) approach under various cutoff conditions can only account for the fluctuation conductivity (FC) data in the 3D AL regime. While the approach taking into account the pseudogap effect can describe the paraconductivity very well in the whole temperature region. This result suggests that the pseudogap state of doped LaOFeAs system is probably due to the formation of paired fermions in the form of strongly bound bosons (local pairs) at T_c<T<T^∗.     

Pseudogap formation and quantum phase transition in strongly-correlated electron systems

Pseudogap formation is a ubiquitous phenomenon in strongly-correlated superconductors, for example cuprates, heavy-fermion superconductors, and iron pnictides. As the system is cooled, an energy gap opens in the excitation spectrum before entering the superconducting phase. The origin of formation and the relevancy to the superconductivity remain unclear, which is the most challenging problem in condensed matter physics. Here, using the cuprate as a model, we demonstrate that the formation of pseudogap is due to a massive gauge interaction between electrons, where the mass of the gauge boson, determining the interaction length scale, is the consequence of the remnant antiferromagnetic fluctuation inherited from the parent compounds. Extracting from experimental data, we predict that there is a quantum phase transition belonging to the 2D XY universality class at the critical doping where pseudogap transition vanishes.