Optical conductivity of unconventional charge density wave systems: Role of vertex corrections

The optical conductivity of a d-CDW conductor is calculated for electrons on a square lattice and a nearest-neighbor charge-charge interaction using the lowest-order conserving approximation. The spectral properties of the Drude-like peak at low frequencies and the broad hump due to transitions across the gap at large frequencies are discussed, also as a function of temperature and of the second-nearest neighbor hopping term t'. We find that vertex corrections enhance the d.c. conductivity, make the Drude peak narrower and provide a smooth transition from a renormalized regime at low to the bare theory at high frequencies. It is also shown that vertex corrections enhance the temperature dependence of the restricted optical sum leading to a non-negligible violation of the sum rule in the d-CDW state.Comment: 10 pages, 6 figure

Charge susceptibility in the t-J model

Momentum and doping dependence of the static charge susceptibility \chi(q) in the t-t'-J model is investigated. Correlations lead to a strongly momentum dependent renormalization of \chi(q). The charge susceptibility near (\pi,\pi) region of the Brillouin zone is strongly suppressed as the hole density \delta is decreased. However, contrary to naive expectations, \chi(q) around q = (\pi,0) and (0,\pi) remains large and practically unchanged at \delta \sim 0.1-0.5. This effect is consistent with a tendency towards low-energy charge fluctuations with the wave vectors along the \Gamma-X direction, reported in earlier studies. Our main finding is that the above trends are amplified by J-driven pairing effects, indicating that the pseudogap formation may promote the charge inhomogeneity. The next-nearest hopping t' leads to weakening of the above momentum-selective renormalizations of \chi(q). We analyze the effects of long-range Coulomb interaction, taking into account a layered structure of cuprates. As an application, the results are discussed in the context of bond-stretching phonon softening in hole-doped cuprates. In particular, a peculiar doping and momentum dependence of the electron-phonon coupling constant is found.Comment: 12 pages, 11 figures, references added, explanations provided on the meaning and limitations of our formalis

The Hall conductivity in unconventional charge density wave systems

Charge density waves with unconventional order parameters, for instance, with d-wave symmetry (DDW), may be relevant in the underdoped regime of high-T_c cuprates or other quasi-one or two dimensional metals. A DDW state is characterized by two branches of low-lying electronic excitations. The resulting quantum mechanical current has an inter-branch component which leads to an additional mass term in the expression for the Hall conductivity. This extra mass term is parametrically enhanced near the ``hot spots'' of fermionic dispersion and is non-neglegible as is shown by numerical calculations of the Hall number in the DDW state.Comment: 4 pages, 4 figure