Ivan Kupčić

Electronic Raman scattering in a multiband model for cuprate superconductors

Charge-charge, current-current, and Raman correlation functions are derived in a consistent way using the unified response theory. The theory is based

Intraband memory function and memory-function conductivity formula in doped graphene

The generalized self-consistent field method is used to describe intraband relaxation processes in a general multiband electronic system with presumably weak residual electron-electron interactions. The resulting memory-function conductivity formula is shown to have the same structure as the result of a more accurate approach based on the quantum kinetic equation. The results are applied to heavily doped and lightly doped graphene. It is shown that the scattering of conduction electron by phonons leads to the redistribution of the intraband conductivity spectral weight over a wide frequency range, however, in a way consistent with the partial transverse conductivity sum rule. The present form of the intraband memory function is found to describe correctly the scattering by quantum fluctuations of the lattice, at variance with the semiclassical Boltzmann transport equations, where this scattering channel is absent. This is shown to be of fundamental importance in quantitative understanding of the reflectivity data measured in lightly doped graphene as well as in different low-dimensional strongly correlated electronic systems, such as the cuprate superconductors.

Multiband Responses in High-T c Cuprate Superconductors

We report on the interplay of localized and extended degrees of freedom in the metallic state of high-temperature superconductors in a multiband setting. Various ways in which the bare magnetic response may become incommensurate are measured against both phenomenological and theoretical requirements. In particular, the pseudogap temperature is typically much higher than the incommensurability temperature. When microscopic strong-coupling effects with real-time dynamics between copper and oxygen sites are included, they tend to restore commensurability. Quantum transport equations for low-dimensional multiband electronic systems are used to explain the linear doping dependence of the dc conductivity and the doping and temperature dependence of the Hall number in the underdoped LSCO compounds. Coulomb effects of dopands are inferred from the doping evolution of the Hartree–Fock model parameters.

Experimental electronic structure and Fermi-surface instability of the correlated 3d sulphide BaVS3: High-resolution angle-resolved photoemission spectroscopy

The correlated

In-plane optical features of the underdoped La2CuO4 based compounds: theoretical multiband analysis

3d

Effective numbers of charge carriers in doped graphene: Generalized Fermi liquid approach

sulphide BaVS_3 exhibits an interesting coexistence of one-dimensional and three-dimensional properties. Our experiments determine the electronic band structure and shed new light on this puzzle. High-resolution angle-resolved photoemission measurements in a 4-eV wide range below the Fermi energy level uncover and investigate the coexistence of a_{; ; ; ; ; 1g}; ; ; ; ; wide-band and e_g narrow-band d-electrons that lead to the complicated electronic properties of this material. We explore the effects of strong correlations and the Fermi surface instability associated with the metal-insulator transition.The correlated 3d sulphide BaVS_3 is a most interesting compound because of the apparent coexistence of one-dimensional and three-dimensional properties. Our experiments explain this puzzle and shed new light on its electronic structure. High-resolution angle-resolved photoemission measurements in a 4eV wide range below the Fermi level explored the coexistence of weakly correlated a_1g wide-band and strongly correlated e_g narrow-band d-electrons that is responsible for the complicated behavior of this material. The most relevant result is the evidence for a_1g--e_g inter-band nesting condition.

In-Plane Oxygens in High-Temperature Superconducting Cuprates

The three-component ab-plane optical conductivity of the high-Tc cuprates is derived using the gauge invariant response theory, and compared to the data previously obtained from the optical reflectivity measurements in the La2CuO4 based families. The valence electrons are described by the Emery three-band model with the antiferromagnetic correlations represented by an effective single-particle potential. In the 0<δ<0.3 doping range, it is shown that the total spectral weight of the three-band model is shared between the intra- and interband channels nearly in equal proportions. At optimum doping, the low-frequency conductivity has a (non-Drude) nearly single-component form, which transforms with decreasing doping into a two-component structure. The mid-infrared spectral weight is found to be extremely sensitive to the symmetry of the effective single-particle potential, as well as to the doping level. The gauge invariant form of the static and elastic Raman vertices is determined, allowing explicit verification of the effective mass theorem and the related conductivity sum rules.

Thermally activated charge carriers and mid-infrared optical excitations in quarter-filled CDW systems

The single-band current-dipole Kubo formula for the dynamical conductivity of heavily doped graphene from Kupcic [Phys. Rev. B 91, 205428 (2015)] is extended to a two-band model for conduction pi electrons in lightly doped graphene. Using a posteriori relaxation-time approximation in the two-band quantum transport equations, with two different relaxation rates and one quasi-particle lifetime, we explain a seemingly inconsistent dependence of the dc conductivity of ultraclean and dirty lightly doped graphene samples on electron doping, in a way consistent with the charge continuity equation. It is also shown that the intraband contribution to the effective number of conduction electrons in the dc conductivity vanishes at T=0 K in the ultraclean regime, but it remains finite in the dirty regime. The present model is shown to be consistent with a picture in which the intraband and interband contributions to the dc conductivity are characterized by two different mobilities of conduction electrons, the values of which are well below the widely accepted value of mobility in ultraclean graphene. The dispersions of Dirac and pi plasmon resonances are reexamined to show that the present, relatively simple expression for the dynamical conductivity tensor can be used to study simultaneously single-particle excitations in the dc and optical conductivity and collective excitations in energy loss spectroscopy experiments.

Experimental electronic structure and Fermi-surface instability of the correlated 3d sulphide BaVS_3

The role of the oxygen degree of freedom in the cuprates’ superconducting planes is analyzed in detail. Structural and photoemission results are reviewed to show that the most sparse description of the in-plane electronic states requires explicit control of the oxygens. For metallic states, the relative contributions of oxygen and copper vary along the Fermi surface (FS), with the arc metallicity dominantly oxygen-derived. For the magnetic responses, we find that the observed incommensurability arises naturally if one keeps separate the roles of the two sites. For the charge order in LBCO, we propose a scenario, based on magnetic interactions in the plane. We stress the need for further experimental investigations of the evolution of the intracell charge distribution with doping, and for a better theoretical understanding of the large particle-hole-symmetry breaking required for successful phenomenologies, but difficult to reconcile with ab initio calculations.

Experimental Electronic Structure and Interband Nesting in BaVS_3

Abstract.The optical properties of the quarter-filled single-band CDW systems have been reexamined in the model with the electron-phonon coupling related to the variations of electron site energies. It appears that the indirect, electron-mediated coupling between phase phonons and external electromagnetic fields vanishes for symmetry reasons, at variance with the infrared selection rules used in the generally accepted microscopic theory. It is shown that the phase phonon modes and the electric fields couple directly, with the coupling constant proportional to the magnitude of the charge-density wave. The single-particle contributions to the optical conductivity tensor are determined for the ordered CDW state and the related weakly doped metallic state by means of the Bethe-Salpeter equations for elementary electron-hole excitations. It turns out that this gauge-invariant approach establishes a clear connection between the effective numbers of residual, thermally activated and bound charge carriers. Finally, the relation between these numbers and the activation energy of dc conductivity and the optical CDW gap scale is explained in the way consistent with the conductivity sum rules.