Spectroscopy of voltage dependence of oxygen movement inYBa2Cu3O7−δ

S. H. Huerth, H. D. Hallen, B. Moeckly
2003 Physical Review B (Condensed Matter)  
Injection of few-volt electrons at room temperature can displace oxygen atoms in the lattice of yttrium barium cuprate ͑YBCO͒. The metal cladding of a near-field scanning optical microscope ͑NSOM͒ probe tip is used as a tunnel electrode for locally injecting the electrons with controlled energies. The NSOM optical signal is used to detect changes in the local oxygen concentration. The data support bond breaking in a Franck-Condon-like effect causing enhanced diffusion of oxygen atoms in the
more » ... en atoms in the lattice. The voltage dependence is consistent with the band structure of YBCO. The movement of atoms induced by electron motionelectromigration or electron-induced motion ͑EIM͒-has been studied in many systems. [1] [2] [3] [4] [5] [6] There are a few possible processes by which EIM can occur. In the case of the classic electron-wind mechanism, 1,2,5,6 the atom motion is in the same direction as the current. We show here that this is not necessarily the case when few-eV electrons are used. In this case, a local electron interaction enhances the diffusion of an atom. Its motion is directed by the concentration gradient independent of the direction of electron injection. Naively, this localization of an electron interaction in a conductor seems unlikely, but it will occur when the electron can excite a carrier from a localized state on the atom rather than from the extended conduction states. The excitation places the atom in an unstable configuration, a Franck-Condon-like excitation, and the atom can move before relaxation if a vacancy is adjacent. Excitation from a localized state entails a threshold energy for the injected electron, which we measure for the case of oxygen motion in yttrium barium cuprate, YBa 2 Cu 3 O 7Ϫd ͑YBCO͒. It is interesting that the motion of oxygen described here shares many of the same qualitative features as the motion of vacancies in gold films with injection of few-eV tunnel electrons. 4,7-9 Both have energy thresholds, related to their respective band structures. Both exhibit atomic motion towards and away from the tunneling tip, underlining the diffusivity enhancement, not directed motion, of the effects. The EIM is limited to a single grain in both cases, indicating that few-eV electrons are scattered strongly at grain boundaries in quite disparate materials. There are differences, however. The topographies of the gold films change dramatically as the atoms move, whereas the oxygen atoms in YBCO move in a fixed lattice resulting in no detectable topographic change. 3 Only one componentthe oxygen-in YBCO moves, whereas all the gold atoms are subject to displacement. This paper identifies the EIM mechanism and illustrates the properties noted above for YBCO, with detailed studies of the dosage (currentϫtime) and spectroscopic ͑injected electron energy͒ dependence of oxygen motion. The EIM of oxygen in YBCO results in changes in its superconducting properties, since they depend on the oxygen concentration. 10, 11 The motion of the oxygen in YBCO is not surprising, since it is known to be mobile at elevated temperatures ͑450°C͒, as used for annealing. 12 We have shown in prior studies that it can be induced to move rapidly at room temperature by classic electromigration 10 or electroninduced migration. 3,13 To better understand the mechanism of oxygen movement in YBCO while avoiding complications due to grain boundaries, we use a near-field scanning optical microscope ͑NSOM͒. The metal cladding of the NSOM probe provides a scanning tunneling microscope ͑STM͒ tip to pull ͑inject͒ electrons from ͑into͒ an industrial quality sample of YBCO. This induces movement of oxygen. We have shown that a reflection-mode near-field scanning optical microscope can be used to image oxygen concentration variations in YBCO. 3 Optical NSOM images taken before and after EIM are compared to eliminate the native background oxygen concentration variations and determine that oxygen has moved in the lattice and where the movement occurred. The complete sample preparation description and a summary of the sample properties have been given. 3, 14 The sample set included laser ablated thin films and reactively coevaporated thin films. Topographic scans of the laser ablated samples showed a grain size on the order of 250 nm versus a grain size of 200 nm for the coevaporated thin films. The samples were studied using a near-field scanning optical microscope as diagrammed in Fig. 1 . Light from a Nanophase 532-nm, 1-ns, 6-mW, pulsed laser was coupled into an optical fiber terminated with an NSOM metal clad FIG. 1. Schematic of microscope setup.
doi:10.1103/physrevb.67.180506 fatcat:kfd6td2gavahfpcdxfymdypvmu