XAFS study of the low-temperature tetragonal phase ofLa2−xBaxCuO

Abstract

Angular dependent x-ray absorption fine structure ͑XAFS͒ measurements of all La, Ba, and Cu sites determined the origin and nature of the intrinsic disorder present in the low temperature tetragonal ͑LTT͒ structural ground state of La 2Ϫx Ba x CuO 4 with xϭ0.125,0.15 at Tϭ10 K. Ba doping induces major local distortions that extend as far as 5 Å from its lattice site. Despite the low Ba content the large distortions significantly affect the XAFS of the majority La and Cu sites. The distribution of the local LTT tilt angle of CuO 6 octahedra was directly determined from single and multiple scattering contributions to the XAFS of La and Cu sites. This angle, which regulates the strength of electron-lattice and spin-spin coupling in this material, was found to exhibit large, random fluctuations with rms ͓͗(Ϫ͗͘) 2 ͔͘ 1/2 Ϸ2͗͘, ͗͘ being the average tilt angle determined by diffraction techniques. These fluctuations originate in the large, random, static disorder introduced with Ba doping. Their presence has implications for the plausibility of a mobility gap causing T c suppression at xϭ0.125 in only the LTT phase, and for the correlation length of the postulated charge and spin stripes.

Flux phase as a dynamic Jahn-Teller phase: Berryonic matter in the cuprates

“…Moreover, there is no change in the local structure when warming into the HTT phase. EXAFS results are consistent 41 . In LSCO, Božin, et al 43,44 find a crossover from local LTO order very near half filling, to a clear mixing of local LTO and LTT order in the doped materials.…”

“…It is a common finding that the local structure in the cuprates differs from the average structural order determined by neutron or x-ray diffraction 38 , and it has been suggested that the observed LTO phase in LSCO is really dynamically disordered, with the local order being closer to LTT 15 . There have been numerous hints of local LTT order [39][40][41][42][43][44] , but the problem is complicated by the simultaneous occurence of stripe order. The issue of the relation between stripe order and the LTT distortion is addressed in the Appendix.…”

“…This strongly supports a model of the stripe phase as a form of nanoscale phase separation, with the charged-stripe phase stabilized by an LTT-like phase 14,15 . [40][41][42][43][44] have explored the possibility 15 that the high-temperature tetragonal (HTT) and LTO phases can be 'dynamic LTT' phases (i.e., with local LTT order). Billinge, et al 40 analyzed neutron diffraction pair distribution functions (PDF's) of LBCO, and found that in the LTO phase the local order has LTT symmetry, crossing over to average LTO structure on a 10Å length scale.…”

Magnetic field induced anisotropy ofLa139spin-lattice relaxation rates in stripe orderedLa1.875Ba0.125

“…[34][35][36][37][38] In the average structure model there is no tilt of the CuO 6 octahedra in the HTT phase, and the HTT → LTO transition is determined basically by the tilt angle of the octahedra. At the LTO → LTT transition, the tilt axis rotates by 45…”

In-Plane Oxygens in High-Temperature Superconducting Cuprates

“…While this observation led some to doubt [4] the coherent Peierls-like picture, it can only emphasize the critical sensitivity of the SC to a perturbation of the oxygen degeneracy. Significantly, the conductivity remains metallic, indeed with strong SC fluctuations above T c around 1/8 doping [5].…”