Probing Electromigration of Oxygen Vacancies in YBa$_2$Cu$_3$O$_{7-δ}$ Devices by Multimodal X-ray Techniques

Abstract

Control of oxygen vacancies by electrical currents in complex oxides such as YBa$_2$Cu$_3$O$_{7-δ}$ (YBCO) has attracted considerable interest due to the relative simplicity of its implementation and its potential for both fundamental studies and the tuning of superconducting device properties. However, the structural evolution and depth-dependent effects associated with current-based techniques remain largely unexplored, particularly with respect to the connection between optical signatures and the spatial distribution of oxygen vacancies. Here, we combine nanoprobe X-ray Diffraction (NanoXRD), Cu K-edge X-ray Absorption Near-Edge Structure (XANES), X-ray Photoelectron Spectroscopy (XPS), electrical transport, and optical measurements to reveal modifications induced in YBCO microbridges by pulsed electromigration. We observe a c-axis expansion correlated with spectroscopic features of oxygen depletion in the Cu-O chains, and we confirm that oxygen redistribution, crystallographic changes, and copper coordination evolve consistently across techniques. Notably, the spatial profile of unit-cell expansion closely follows the optical contrast observed after electromigration, demonstrating that the different signatures capture the same underlying oxygen reordering. We further show that optical microscopy cannot reliably capture bipolar electromigration involving strong resistance modifications, as surface deoxygenation appears largely irreversible. Taken together, our findings provide a significant step toward a microscopic understanding of current-assisted oxygen migration in YBCO and establish a framework for effectively exploiting vacancy control in high-temperature superconducting devices.

Figures (5)

• Figure 1: (a) Schematic of the triple-constriction device with the corresponding electrical circuit layout. Arrows labeled $\mathrm{O^{2-}}$ and $\mathrm{V_O^{2+}}$ indicate the expected drift directions of oxygen anions and oxygen vacancies, respectively, under the applied current polarity. (b) Schematic of the electromigration pulse-probe current protocol (current versus time). (c) Differential optical images of sample S1 at successive stages of electromigration, after EM1, EM2, and EM3. The enhanced contrast (yellowish regions) highlights the progressive rightward propagation of the oxygen-vacancy front. (d) Pulsed electromigration resistance traces normalized by initial values $R_0$ for each one of the three bridges of sample S1, obtained under ambient conditions at room temperature for three consecutive EM runs.
• Figure 2: (a) Profile of the $c$-axis lattice parameter variation measured by NanoXRD for sample S1. The upper panel shows a false-colored optical image of the device, aligned with the NanoXRD sampling positions. The plotted curve represents the average of four parallel line scans performed along the device length, as illustrated in the inset; the four individual scans, which have been arbitrarily shifted along the y-axis for clarity, are displayed in the lower panel. The right y-axis indicates the corresponding hole doping $p$. (b) XANES spectra acquired from the regions marked by the colored arrows in (a). The inset highlights the pre-edge region, used for the qualitative assessment of oxygen content.
• Figure 3: (a) Optical image and NanoXRD profile of sample S2. The pronounced modulation of the $c$-axis lattice parameter evidences oxygen redistribution, with the right y-axis indicating the corresponding hole doping $p$. Local hole doping spans approximately from the underdoped to the overdoped regimes. (b) Cu K-edge XANES spectra acquired at the indicated positions highlight the correlation between structural modifications and copper coordination, manifested by an increased pre-edge intensity.
• Figure 4: (a) Profile of the $c$-axis lattice-parameter variation measured by NanoXRD for sample S3. The upper panel shows a false-colored optical image of the device, aligned with the NanoXRD sampling positions. No increase in reflectivity is detected. The right y-axis indicates the corresponding hole doping $p$. The inset shows resistance versus temperature curves measured in the central constriction before and after EM.(b) Cu K-edge XANES spectra acquired from the regions marked by the colored arrows in (a), highlighting the correlation between structural modifications and copper coordination, manifested by changes in the pre-edge intensity.
• Figure 5: O 1$s$ and Cu 2$p$ XPS spectra for different regions of sample S1. Panels (a) and (b) show the O 1$s$ spectra, distinguishing surface and lattice oxygen, while panels (c) and (d) show the Cu 2$p$ spectra. Panels (a) and (c) correspond to region C, which exhibits increased optical reflectivity, and panels (b) and (d) correspond to region D, which shows no optical contrast. The pie charts indicate the relative fractions of lattice oxygen and copper in each region, revealing enhanced surface deoxygenation in the high-reflectivity area.