Polaron contribution to the infrared optical response ofLa2−xSr

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

“…6 (b) support the claims, however, that the MIR band in the cuprates has a mainly "electronic" origin, i.e., the lattice polaron effects are rather weak. The opposite is true for their isostructural counterpart, the nickelate system, where the MIR absorption band has been ascribed by many investigators to "polaronic" origin [16,18]. Within the HtJM such a situation can be modeled by the parameter set used in Fig.…”

“…6 (b) and (c), which correspond to the weak and strong EP coupling situations realized in the cuprate (La 2−x Sr x CuO 4 ) and nickelate (La 2−x Sr x NiO 4 ) systems, respectively. The "effective" EP interaction ε p /t in the nickelates is estimated to be about one order of magnitude larger than in the cuprates simply because of the much smaller transfer amplitude (t ≃ 0.08 eV [16]). According to the internal structure of the low-spin state, the hopping transport of spin-1/2 composite holes in a spin-1 background is rather complex; implying, within an effective single-band description, a strong reduction of the transition matrix elements [64].…”

“…Here the anomalous broad optical absorption detected for the La 2−x Sr x [Cu, Ni]O 4 compounds in several (normal-state) infrared experiments has attracted a lot of attention. In particular for the nickelate system, where the existence of self-trapped charge carriers is well established, the mid-infrared absorption band has been interpreted successfully in terms of photon-assisted hopping of small-size lattice polarons [16,17]. On the other hand, from a comparison of the optical absorption of several perovskites (La 1.67 Sr 0.33 NiO 4 , Sr 1.5 La 0.5 MnO 4 , and Nd 1.96 Ce 0.04 CuO 4 ), it has been argued that in the low-doping cuprates the lattice polarons appear to be rather large [18,19].…”

“…If we fix the energy scale by t ∼ 0.08 eV (which is the estimate for the nearest neighbour transfer integral in La 2−x Sr x NiO 4 given in Ref. [16]), the maximum in the optical absorption is again located at about 0.5 eV. The whole spectrum clearly shows lattice polaron characteristics, where it seems that the lattice hole polarons are of small-to-intermediate size [66].…”

Small polarons and magnetic antiphase boundaries inCa2−xNaxCuO2ClPatterson1 2008Phys. Rev. B18090View full textAdd to dashboardCite

“…The universal nature of stripes in cuprates is illustrated by the fact that common stripe features are observed by scanning tunneling conductance measurements in both Na-CCOC and Bi 2 Sr 2 Dy 0.2 Ca 0.8 Cu 2 O 8+␦ ͑Dy-Bi2212͒. 25 Further experimental evidence for polaronic states in cuprates is provided by the observation of strong electronphonon coupling: Very broad, Gaussian photoemission line shapes are widely observed in cuprates and attributed to Franck-Condon broadening; 3,6 pinning of charge carriers by a crystallographic distortion in a low temperature tetragonal phase of La 2−x Ba x CuO 4 ͑LBCO͒, with 0.11Ͻ x Ͻ 0.14, can be lifted and superconductivity restored by application of hydrostatic pressure; 1 new features in the midinfrared optical absorption spectrum of doped transition metal oxides, including cuprates, 32,33 were attributed to polarons and modeled using a model Hamiltonian approach. 16 A phonon anomaly was observed in LBCO ͑x = 0.12͒ at a wave vector, which corresponds to the 4a 0 stripe spacing.…”