Conductivity ofCuO3chains: Disorder versus electron-phonon coupling

Abstract

The optical conductivity of the CuO 3 -chains, a subsystem of the 1-2-3 materials, is dominated by a broad peak in the mid-infrared (ω ≈ 0.2eV), and a slowly falling high-frequency tail. The 1D t-J-model is proposed as the relevant low-energy Hamiltonian describing the intrinsic electronic structure of the CuO 3 -chains. However, due to charge-spin decoupling, this model alone cannot reproduce the observed σ (ω). We consider two additional scattering mechanisms: (i) Disregarding the not so crucial spin degrees of freedom, the inclusion of strong potential disorder yields excellent agreement with experiment, but suffers from the unreasonable value of the disorder strength necessary for the fit. (ii) Moderately strong polaronic electron-phonon coupling to the mode involving Cu(1)-O(4) stretching, can be modeled within a 1D Holstein Hamiltonian of spinless fermions. Using a variational approximation for the phonon Hilbert space, we diagonalize the Hamiltonian exactly on finite lattices. As a result of the experimental hole density ≈ 1/2, the chains can exhibit strong charge-density-wave (CDW) correlations, driven by phonon-mediated polaron-polaron interactions. In the vicinity of half filling, charge motion is identified as arising from moving domain walls, i. e., defects in the CDW. Incorporating the effect of vacancy disorder by choosing open boundary conditions, good agreement with the experimental spectra is found.In particular, a high-frequency tail arises as a consequence of the polaron-polaron interactions.

Optical absorption and single-particle excitations in the two-dimensional Holsteint−Jmodel

“…[35]) and Holstein models [59,60,50] on finite lattices employing ED techniques. First results, obtained recently for the 1D HtJM [32], indicate that EP coupling effects may be at least partly responsible for the experimentally observed frequency dependence of the optical conductivity in the CuO 3 chains of the 1:2:3 family of high-T c oxides.…”

“…For example, J/t ≃ 0.4 with t ≃ 0.3 eV is commonly used to model the La 2−x Sr x CuO 4 system. In (1), we have chosen the coupling to the hole,h i = 1 −ñ i is the local density operator of the spinless hole, as the dominant source of the EP coupling, because in the t-J model the unoccupied site corresponds to the Zhang-Rice singlet (formed from Cu 3d x 2 −y 2 and O 2p x,y hole orbits) for which the coupling should be much stronger than for the occupied (Cu 2+ ) site [31][32][33]. The hole-phonon coupling constant is denoted by ε p , andhω 0 is the bare phonon frequency (belowh = 1 and all energies are measured in units of t).…”

Thermodynamical properties of a spin-12Heisenberg chain coupled to phonons

“…As the dimension of the subspaces is quickly growing with the number of phonons the limits of a method requiring truncation is soon reached, this is relevant when using exact diagonalization which allows to treat only ≈ 16 spins and a few phonons per site. At zero temperature the truncation problem can be partly overcome by using coherent phonon states 4,20 or employing the DMRG technique, which allows to treat system sizes of up to 256 spins 17 . At finite temperatures, however, the truncation problem is even more important.…”

Fractionally charged excitations in the charge density wave state of a quarter-filled t-J chain with quantum phonons

“…On the contrary, in the uniform phase, strictly speaking A ≃ 0 and the charge correlation function decays as a power law. It has been argued that, although phononic quantum fluctuations are present, such state still belongs to the Luttinger Liquid universality class 14,16 . In that case, the extra Q = + e 2 charge is totally spread over the full chain.…”

“…This instability is in fact enhanced by the lattice (ω = 0) coupling and the 4k F CDW phase becomes stable even when V < 2 (and J finite) for K −1 exceeding a V -dependant critical value 12 . The numerical study of the model with quantum phonons using the infinite system DMRG method 13 requires an approximate (but reliable) treatment of the phonon degrees of freedom [14][15][16] . Indeed, an infinite number of phononic quantum states lives on each site.…”