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Site-specific and doping-dependent electronic structure of YBa2Cu~O"probed by 0 1s and Cu 2p I-ray-absorption spectroscopy

We measured the peak effect in the magnetic hysteresis loop of an untwinned YBa&Cu307 q crystal in the vicinity of the critical temperature and studied its temperature, time, and angular dependence. The location of the peak is determined by the component of the field along the c direction. The pinning force density F~(B) is scaled into a single curve for all measured isotherms (75-87 K) and time scales (4-1500 sec), and its functional form reflects the observed peak effect. The scaling field is related to the irreversibility field of oxygen-deficient regimes. The results indicate that pinning in this system is due to inhomogeneity of the oxygen deficiency and that the peak is related to the formation of a percolationlike network of reversible regimes.

The response of the YBazCuiOv-s lattice to superconducting order is studied using an ultrahighresolution capacitance dilatometer. The onset of superconductivity is accompanied by highly anisotropic jumps of the expansivities in the a-b plane. This leads to a reduction of the orthorhombic splitting below Tc, which suggests that superconductivity favors a symmetric ib=a) CuOz plane. Little effect is seen along the c axis. Superconducting fluctuations are for the first time clearly observed in the lattice expansion.PACS numbers: 74.30. Ek, 65.70.+y, 74.70.Vy There is a great deal of interest in studying the coupling of superconductivity to the structural parameters in YBa2Cu307-5, as the numerous structural studies using x-ray [1-5] and neutron diffraction [6-9] demonstrate. These methods, however, usually suffer from insufficient resolution to observe the very small effects due to superconductivity, and this has led to many contradictory results. Detailed information about the temperature dependence of the structure provides important clues about the nature of superconductivity in these materials. In general, the crystal structure of a superconductor adjusts itself below Tc in order to minimize the total free energy of the lattice and the superconducting electrons. The onset of these adjusting strains results in second-order jumps of the expansivity at T^, which, together with the specificheat discontinuity, provide information about the uniaxial pressure dependence of Tc, The lattice parameters are also expected to show fluctuation effects near Tc similar to specific-heat fluctuations [10], but this has not been demonstrated. Modern dilatometers are several orders of magnitude more sensitive than either x-ray or neutron diffraction and can be very useful for studying these eff'ects. In a previous dilatometric investigation of twinned YBa2Cu307-5 crystals, we showed that the expansivity jumps at the onset of superconductivity occur only within the a-b plane and that the positive hydrostatic dTc/dp is due almost entirely to the uniaxial pressure components within the a-b plane [11]. This has been confirmed by recent anisotropic pressure measurements [12].In this Letter we present ultrahigh-resolution thermalexpansion data from 5 to 300 K of untwinned YBa2Cu3-Oi-s crystals. It will be shown that a very large anisotropy exists within the a-b plane, which suggests that a nondistorted Cu02 plane ib^^a)is favorable for superconductivity.YBa2Cu307-5 crystals were grown in a Y203-stabilized Zr02 crucible and were subsequently detwinned under uniaxial pressure (^0.