Control of dopant crystallinity in electrochemically treated cuprate thin films

A. Frano, M. Bluschke, Z. Xu, B. Frandsen, Y. Lu, M. Yi, R. Marks, A. Mehta, V. Borzenets, D. Meyers, M. P. M. Dean, F. Baiutti, J. Maier, G. Kim, G. Christiani, G. Logvenov, E. Benckiser, B. Keimer, and R. J. Birgeneau

Phys. Rev. Materials 3, 063803 – Published 27 June 2019

Abstract

We present a methodology based on ex situ (postgrowth) electrochemistry to control the oxygen concentration in thin films of the superconducting oxide ${La}_{2} {CuO}_{4 + y}$ grown epitaxially on substrates of isostructural ${LaSrAlO}_{4}$ . The superconducting transition temperature, which depends on the oxygen concentration, can be tuned by adjusting the pH level of the base solution used for the electrochemical reaction. As our main finding, we demonstrate that the dopant oxygens can either occupy the interstitial layer in an orientationally disordered state or organize into a crystalline phase via a mechanism in which dopant oxygens are inserted into the substrate, changing the lattice symmetry of both the substrate and the epitaxial film. We discuss this mechanism, and we describe the resulting methodology as a platform to be explored in thin films of other transition-metal oxides.

Received 14 January 2019
Revised 16 May 2019

DOI:https://doi.org/10.1103/PhysRevMaterials.3.063803

Physics Subject Headings (PhySH)

Dopants Electrochemistry

Cuprates Glassy systems Oxides Thin films

Crystal structures X-ray diffraction

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

A. Frano^1,2,*, M. Bluschke^3,4, Z. Xu², B. Frandsen², Y. Lu³, M. Yi², R. Marks⁵, A. Mehta⁵, V. Borzenets⁵, D. Meyers⁶, M. P. M. Dean⁶, F. Baiutti³, J. Maier³, G. Kim³, G. Christiani³, G. Logvenov³, E. Benckiser³, B. Keimer³, and R. J. Birgeneau^2,†

¹Department of Physics, University of California, San Diego, California 92093, USA
²Department of Physics, University of California, Berkeley, California 94720, USA
³Max Planck Institute for Solid State Research, Heisenbergstrasse 1, 70569 Stuttgart, Germany
⁴Helmholtz-Zentrum Berlin für Materialien und Energie, Wilhelm-Conrad-Röntgen-Campus BESSY II, Albert-Einstein-Strasse 15, 12489 Berlin, Germany
⁵Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA
⁶Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, New York 11973, USA

^*afrano@ucsd.edu
^†robertjb@berkeley.edu

Article Text (Subscription Required)

Click to Expand

References (Subscription Required)

Click to Expand

Issue

Vol. 3, Iss. 6 — June 2019

Reuse & Permissions

Access Options

Article Available via CHORUS

Download Accepted Manuscript

Author publication services for translation and copyediting assistance advertisement

Images

Figure 1
(a),(b) Schematic representation of the structural unit cells and oxygen ordering patterns in the ${La}_{2} {CuO}_{4} / {LaSrAlO}_{4}$ film-substrate system. (a) For electrolyte solutions with $pH ≲ 11$ dopant oxygens penetrate the film alone, separating along the $c$ -axis into evenly spaced two-dimensional layers of interstitial oxygens. Within these layers the dopant oxygens are orientationally disordered (depicted schematically by dotted yellow lines). (b) For $pH ≳ 12.5$ dopant oxygens penetrate the substrate effecting a structural distortion (dark purple superstructure) that is incorporated into the film promoting crystallization of the dopant oxygens within the interstitial layers. Dotted yellow lines indicate the dopant-oxygen induced superstructure cell. (c) Schematic of the setup used to electrochemically intercalate oxygen into films of ${La}_{2} {CuO}_{4}$ grown on LSAO. (d) At the working electrode side to which ${OH}^{-}$ ions are attracted, the film and the substrate are oxidized. During the short duration of our treatment, the oxygen ions likely penetrate through the film into the substrate. (e) Magnetization normalized to the maximum diamagnetic signal as a function of temperature for samples electrochemically doped in an alkaline solution ( $H_{2} O + NaOH$ ) with the indicated pH levels.
Reuse & Permissions
Figure 2
For a sample doped in a $pH = 10$ solution: (a) X-ray diffraction $L$ -scan over the (0,1,6) reflex of ${La}_{2} {CuO}_{4 + y}$ displaying two satellite peaks that correspond to staging with period $\sim 6 .$ (b) Integrated intensity of the satellite peaks as a function of temperature, where the shaded regions in panel (a) denote the integration ranges. (c) Reciprocal space map of the experimentally accessible ( $H, K$ ) plane for $L = 5.8$ taken at $T = 10$ K. The dotted circle indicates in-plane $Q$ -vectors of equal length.
Reuse & Permissions
Figure 3
For a sample doped in a $pH = 14$ solution: (a) X-ray diffraction reciprocal space map of the ( $H, K$ )-plane at $L = 5.75$ and 5.25 (outline in blue) measured at $T = 10$ K. A cluster of eight peaks around commensurate Bragg positions as well as a remnant of the diffraction ring near (0,1,5.75) can be observed. (b) X-ray-diffraction scans along $L$ at in-plane ( $H, K$ ) values corresponding to two of the oxygen superstructure peaks; blue squares at ( $\frac{27}{50}, \frac{31}{50}$ ) and red circles at ( $\frac{27}{50}, \frac{19}{50}$ ). Vertical lines are drawn at every stage-4 position. (c) Scans along $L$ at ( $H, K) = (\frac{1}{2}, \frac{1}{2}$ ), where the peaks identified by a $*$ symbol occur at half-integer values of $L$ in reciprocal-lattice units of the substrate.
Reuse & Permissions
Figure 4
(a) Reciprocal space scans of one representative stage 4 in-plane superstructure peak for different temperatures, as labeled. (b) The integrated intensity of the stage 4 peak plotted as a function of temperature. (c) A representation in real space of the in-plane unit cells associated with the two superstructure domains. The black dots represent the planar Cu ions. A black square denotes the size of the orthorhombic in-plane unit cell. For simplicity, the orthorhombic distortion is not shown. (d) In reciprocal space, each of the domains creates a four-peak multiplet (color-coded according to the left panel) as observed in the data.
Reuse & Permissions

Physical Review Materials