14 research outputs found
Thermally activated charge carriers and mid-infrared optical excitations in quarter-filled CDW systems
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.Comment: 12 pages, 9 figure
Effective numbers of charge carriers in doped graphene: The generalized Fermi liquid approach
The single-band current-dipole Kubo formula for the dynamical conductivity of
heavily doped graphene from Kup\v{c}i\'{c} [Phys. Rev. B 91, 205428 (2015)] is
extended to a two-band model for conduction 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 vanishes at 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 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 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.Comment: 13 pages, 11 figure
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 on the
improved description of the conduction electron coupling to the external
electromagnetic fields, distinguishing further the direct and indirect
(assisted) scattering on the quasi-static disorder. The two scattering channels
are distinguished in terms of the energy and momentum conservation laws. The
theory is illustrated on the Emery three-band model for the normal state of the
underdoped high- cuprates which includes the incoherent electron
scattering on the disorder associated with the quasi-static fluctuations around
the static antiferromagnetic (AF) ordering. It is shown, for the first time
consistently, that the incoherent indirect processes dominate the low-frequency
part of the Raman spectra, while the long-range screening which is dynamic
removes the long-range forces in the channel. In the mid-infrared
frequency range the coherent AF processes are dominant. In contrast to the
nonresonant response, which is large by itself, the resonant interband
transitions enhance both the and Raman spectra to comparable
values, in good agreement with experimental observation. It is further argued
that the AF correlations give rise to the mid-infrared peak in the
Raman spectrum, accompanied by a similar peak in the optical conductivity. The
doping behavior of these peaks is shown to be correlated with the linear doping
dependence of the Hall number, as observed in all underdoped high-
compounds.Comment: 18 pages, 14 figures; to appear in Phys. Rev.
Transport, magnetic and superconducting properties of RuSr2RCu2O8 (R= Eu, Gd) doped with Sn
Ru{1-x}Sn{x}Sr2EuCu2O8 and Ru{1-x}Sn{x}Sr2GdCu2O8 have been comprehensively
studied by microwave and dc resistivity and magnetoresistivity and by the dc
Hall measurements. The magnetic ordering temperature T_m is considerably
reduced with increasing Sn content. However, doping with Sn leads to only
slight reduction of the superconducting critical temperature T_c accompanied
with the increase of the upper critical field B_c2, indicating an increased
disorder in the system and a reduced scattering length of the conducting holes
in CuO2 layers. In spite of the increased scattering rate, the normal state
resistivity and the Hall resistivity are reduced with respect to the pure
compound, due to the increased number of itinerant holes in CuO2 layers, which
represent the main conductivity channel. Most of the electrons in RuO2 layers
are presumably localized, but the observed negative magnetoresistance and the
extraordinary Hall effect lead to the conclusion that there exists a small
number of itinerant electrons in RuO layers that exhibit colossal
magnetoresistance.Comment: 10 pages, 9 figure