Superconductivity induced by structural reorganization in the electron-doped cuprate ${\mathrm{Nd}}_{2\ensuremath{-}x}{\mathrm{Ce}}_{x}{\mathrm{CuO}}_{4}$

Superconductivity induced by structural reorganization in the electron-doped cuprate ${Nd}_{2 - x} {Ce}_{x} {CuO}_{4}$

Anita Guarino, Carmine Autieri, Pasquale Marra, Antonio Leo, Gaia Grimaldi, Adolfo Avella, and Angela Nigro

Phys. Rev. B 105, 014512 – Published 19 January 2022

Abstract

Electron-doped and hole-doped superconducting cuprates exhibit a symmetric phase diagram as a function of doping. This symmetry is, however, only approximate. Indeed, electron-doped cuprates become superconductors only after a specific annealing process: This annealing affects the oxygen content by only a tiny amount but has a dramatic impact on the electronic properties of the sample. Here we report the occurrence of superconductivity in oxygen-deficient ${Nd}_{2 - x} {Ce}_{x} {CuO}_{4}$ thin films grown in an oxygen-free environment after annealing in pure argon flow. As verified by x-ray diffraction, annealing induces an increase of the interlayer distance between ${CuO}_{2}$ planes in the crystal structure. Since this distance is correlated to the concentration of oxygens in apical positions, and since oxygen content cannot substantially increase during annealing, our experiments indicate that the superconducting phase transition has to be ascribed to a migration of oxygen ions to apical positions during annealing. Moreover, as we confirm via first-principles density functional theory calculations, the changes in the structural and transport properties of the films can be theoretically described by a specific redistribution of the existing oxygen ions at apical positions with respect to ${CuO}_{2}$ planes, which remodulates the electronic band structure and suppresses the antiferromagnetic order, allowing the emergence of hole superconductivity.

Received 20 December 2020
Revised 24 December 2021
Accepted 6 January 2022

DOI:https://doi.org/10.1103/PhysRevB.105.014512

Physics Subject Headings (PhySH)

Impurities in superconductors Superconducting phase transition

Cuprates High-temperature superconductors Thin films

Annealing Crystallography Density functional theory Epitaxy Methods in superconductivity Resistivity measurements

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Anita Guarino ^1,2,*, Carmine Autieri ^3,2,*, Pasquale Marra ^4,5,†, Antonio Leo ^1,2,6, Gaia Grimaldi ^1,2, Adolfo Avella ^1,2,7,‡, and Angela Nigro ^1,2

¹Dipartimento di Fisica “E. R. Caianiello,” Universitá degli Studi di Salerno, 84084 Fisciano (Salerno), Italy
²Consiglio Nazionale delle Ricerche CNR-SPIN, UOS Salerno, 84084 Fisciano (Salerno), Italy
³International Research Centre Magtop, Institute of Physics, Polish Academy of Sciences, Aleja Lotników 32/46, 02668 Warsaw, Poland
⁴Graduate School of Mathematical Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro, Tokyo, 153-8914, Japan
⁵Department of Physics, and Research and Education Center for Natural Sciences, Keio University, 4-1-1 Hiyoshi, Yokohama, Kanagawa, 223-8521, Japan
⁶NANO_MATES Research Center, Universitá degli Studi di Salerno, 84084 Fisciano (Salerno), Italy
⁷Unitá CNISM di Salerno, Universitá degli Studi di Salerno, 84084 Fisciano (Salerno), Italy

^*These authors contributed equally to this work.
^†pmarra@ms-u.tokyo.ac.jp
^‡a.avella@unisa.it

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Vol. 105, Iss. 1 — 1 January 2022

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Figure 1
The NCCO parent compound (undoped ${Nd}_{2} {CuO}_{4}$ ) crystallizes in the tetragonal $T^{'}$ structure, with ${CuO}_{2}$ layers sandwiched between charge reservoir ${Nd}_{2 - x} {Ce}_{x} O_{2}$ layers, copper ions surrounded by square-planar arrangements of oxygen ions in the $a b$ plane, and oxygen ions located within the ${CuO}_{2}$ and charge reservoir layers. In doped compounds, oxygen ions may partially occupy apical sites above or below the Cu atoms, e.g., one apical oxygen for every two copper atoms, or every copper atom ( $T^{*}$ structure). The concentration of apical oxygens is correlated to the elongation of the $c$ -axis parameter.
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Figure 2
(a) Resistivity as a function of temperature for type A samples plotted on a log-log scale and normalized to the resistivity minimum. (b) Resistance as a function of temperature for type B samples plotted on a log-log scale. Continuous lines $R (T) \propto T^{- α}$ are the best fit to the data. (c) Normalized resistivity as a function of temperature for type A and B samples after annealing showing the superconducting transition at $T_{c} ≲ 24 K$ . (d) Residual resistivity ratio $R R R$ for as-grown samples (both types) and (e) superconducting critical temperature $T_{c}$ of samples after annealing as a function of the $c$ -axis parameter. The dashed gray line is the average value of $c_{SC}$ and the continuous smooth curve is a guide for the eye. The annealing process was optimized for the sample reaching $T_{c} \approx 24 K$ .
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Figure 3
DFT electronic band structure of NCCO for (a) $T^{*}$ structure with $x = 0.125$ , (b) $T^{*}$ structure with $x = 0.25$ , (c) $T^{'}$ structure with $x = 1 / 6 \approx 0.17$ , and (d) $T_{SC} = 2 T^{*} + T^{'}$ structure with $x = 1 / 6$ , where we highlighted the dominant $T^{'}$ and $T^{*}$ bands with larger contributions. The Fermi level is set to zero. The flat bands at $0.5 - 1.5 eV$ above the Fermi level are the cerium $4 f$ bands. All other lower-energy bands are copper $3 d$ bands.
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Figure 4
XRD patterns in units of counts per second for an as-grown type A sample, an as-grown type B sample, and a superconducting sample. The (001) and (002) peaks are due to the presence of the ${SrTiO}_{3}$ substrate. Insets show the linear fit of $2 sin θ / λ$ as a function of the Miller index $l$ . The line slopes give the $c$ -axis lattice parameter values.
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Figure 5
Angular positions $2 θ$ of the (004), (103), and (114) peaks in panels (a), (b), and (c) respectively, for a type A (red circles) and a type B (blue squares) as-grown sample. Dashed lines correspond to the $2 θ$ value measured in both samples after the annealing process. (d) $ω$ -Scans around the (004) reflections for a type A as-grown sample (red circles), a type A superconducting sample (purple circles), a type B as-grown sample (blue squares), and a type B superconducting sample (green squares), normalized to the maximum value.
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Figure 6
(a) Band structure of the ${Nd}_{2} {CuO}_{4}$ . (a) $T^{'}$ phase with antiferromagnetic order, (b) $T^{'}$ phase with nonmagnetic atoms, (c) $T^{*}$ phase with antiferromagnetic order. The value of the Coulomb repulsion is $U_{Cu} = 3.2 eV$ .
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Physical Review B

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Superconductivity induced by structural reorganization in the electron-doped cuprate ${Nd}_{2 - x} {Ce}_{x} {CuO}_{4}$

Anita Guarino, Carmine Autieri, Pasquale Marra, Antonio Leo, Gaia Grimaldi, Adolfo Avella, and Angela Nigro

Phys. Rev. B 105, 014512 – Published 19 January 2022

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Physical Review B

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Superconductivity induced by structural reorganization in the electron-doped cuprate Nd2−xCexCuO4

Anita Guarino, Carmine Autieri, Pasquale Marra, Antonio Leo, Gaia Grimaldi, Adolfo Avella, and Angela Nigro

Phys. Rev. B 105, 014512 – Published 19 January 2022

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Superconductivity induced by structural reorganization in the electron-doped cuprate ${Nd}_{2 - x} {Ce}_{x} {CuO}_{4}$