Article

# Four-Unit-Cell Superstructure in the Optimally Doped Y B a 2 C u 3 O 6.92 Superconductor

Department of Physics, University of Houston, Houston, Texas, United States

Physical Review Letters (Impact Factor: 7.51). 11/2004; 93(15):157008. DOI: 10.1103/PhysRevLett.93.157008 Source: PubMed

Get notified about updates to this publication Follow publication |

Data provided are for informational purposes only. Although carefully collected, accuracy cannot be guaranteed. The impact factor represents a rough estimation of the journal's impact factor and does not reflect the actual current impact factor. Publisher conditions are provided by RoMEO. Differing provisions from the publisher's actual policy or licence agreement may be applicable.

- [Show abstract] [Hide abstract]

**ABSTRACT:**We report on an infrared study of carrier dynamics within the ${\mathrm{CuO}}_{2}$ planes in heavily underdoped detwinned single crystals of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{y}$. In an effort to reveal the electronic structure near the onset of superconductivity, we investigate the strong anisotropy of the electromagnetic response due to an enhancement of the scattering rate along the $a$ axis. We propose that the origin of this anisotropy is related to a modulation of the electron density within the ${\mathrm{CuO}}_{2}$ planes. - [Show abstract] [Hide abstract]

**ABSTRACT:**Charge transfer and valence unstable materials exhibit numerous ground states as functions of doping, pressure, and temperature. Multiscale inhomogeneity is typical for most of these materials, e.g., relaxor ferroelectrics, manganites, cuprates, tungstates, and related compounds. Here, we show that the strong anharmonicity in all these systems allows for a distinct form of inherent, dynamic inhomogeneity dictated by “discrete breather” formation, where lattice, charge, and elasticity are intimately coupled. The spatial extent of the breather is governed by the strength of anharmonicity it carries and can vary considerably, defining length scales of inhomogeneity. The coupling of the breather to the host lattice prevents a lattice instability and induces a collapse of the elastic constants and an excess specific heat and entropy as soon as the discrete breather modes emerge from the continuum of the optic modes. Then a coexistence of local, dynamically distorted patterns with the regular lattice is manifested, defining a two-component system with very different dynamics and a strong analogy to glasses: A self-organization of multiple breathers into patterns takes place as a function of temperature and breather density. - [Show abstract] [Hide abstract]

**ABSTRACT:**A cluster variation method (CVM) has been used to calculate the phase diagram of oxygen ordering in YBa2Cu3O6+2c in terms of two dimensional Ising model that, besides of repulsive nearest-neighbor (NN) interaction V1 > 0 and two types of next-to-nearest neighbor (NNN) pair interactions V2 < 0 and V3 > 0, includes additional three-atom effective interaction Vth coupling two NNN oxygen atoms already bounded by V2 (through Cu(1) ion) with the third O atom that is connected with one of these two by the repulsive NNN Coulomb interaction V3 > 0. Phase diagrams were calculated for several different sets of interaction constants V1, V2, V3, and Vth together with the corresponding phase diagrams obtained for Vth = 0. The arrangement of main orthorhombic structures in (c, T) space and the nature of the phase transition curves between them is found to be in a very good agreement with those of the corresponding phase diagrams of the well-known ASYNNNI model, which had already proven itself as accurately reproducing the number of experimental findings on the structural phase diagram of YBa2Cu3O6+2c. On the other hand, unlike to the predictions of the ASYNNNI model (Vth = 0), it is shown that inclusion of three-atom interaction Vth results in significant increase of the number of isolated basal plane oxygen ions which are generally considered as not giving rise to the charge transfer process and the onset of superconductivity. The importance of obtained oxygen superstructures of low temperature oxygen ordering and its possible impact on existence of plateaus in Tc(c) dependence has been discussed.