Magnetic and ligand field properties of copper at the interfaces of (CaCuO${}_{2}$)${}_{n}$/(SrTiO${}_{3}$)${}_{n}$ superlattices

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Magnetic and ligand field properties of copper at the interfaces of (CaCuO $_{2}$ ) $_{n}$ /(SrTiO $_{3}$ ) $_{n}$ superlattices

M. Minola, D. Di Castro, L. Braicovich, N. B. Brookes, D. Innocenti, M. Moretti Sala, A. Tebano, G. Balestrino, and G. Ghiringhelli

Phys. Rev. B 85, 235138 – Published 20 June 2012

Abstract

Using high-resolution resonant inelastic x-ray scattering (RIXS) excited at the Cu $L_{3}$ edge we have investigated the ligand field ( $d d$ ) and magnetic excitations in (CaCuO $_{2}$ ) $_{n}$ /(SrTiO $_{3}$ ) $_{n}$ superlattices ( $n = 2, 3$ ) and compared them to those of a 14-nm-thick CaCuO $_{2}$ film. The $d d$ excitation spectrum reveals a pyramidal coordination of Cu ions at the CaCuO $_{2}$ /SrTiO $_{3}$ interfaces. In the three samples, spin excitations are in the form of dispersing magnons, with similar spectral intensity but reduced dynamics in SLs with respect to pure CaCuO $_{2}$ . By fitting the dispersions within linear spin wave theory we have obtained the leading term of the in-plane superexchange parameters: $J = 127$ meV, 138 meV, and 157 meV for $n = 2$ , 3 SLs, and CaCuO $_{2}$ , respectively. These results demonstrate that the antiferromagnetic order is preserved in these SLs down to very small cuprate layer thickness and despite the chemical and structural alterations at the interface. This finding opens the way to the production of artificial Cu-based high-temperature superconductors where the charge reservoir layer is constituted by the interface itself.

Received 23 December 2011

DOI:https://doi.org/10.1103/PhysRevB.85.235138

Authors & Affiliations

M. Minola¹, D. Di Castro², L. Braicovich³, N. B. Brookes⁴, D. Innocenti², M. Moretti Sala^3,*, A. Tebano², G. Balestrino², and G. Ghiringhelli³

¹CNISM and Dipartimento di Fisica, Politecnico di Milano, I-20133, Italy
²CNR-SPIN and Dipartimento di Ingegneria Civile e Ingegneria Informatica, Università di Roma “Tor Vergata”, I-00133 Roma, Italy
³CNR-SPIN and Dipartimento di Fisica, Politecnico di Milano, I-20133, Italy
⁴European Synchrotron Radiation Facility, F-38043 Grenoble, France

^*Present address: European Synchrotron Radiation Facility, F-38043 Grenoble, France.

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Vol. 85, Iss. 23 — 15 June 2012

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Images

Figure 1
(a) Schematic view of (CaCuO $_{2}$ ) $_{n}$ /(SrTiO $_{3}$ ) $_{n}$ structure. Each block contains $n$ unit cells of CaCuO $_{2}$ (SrTiO $_{3}$ ) and, therefore, $n$ CuO $_{2}$ (TiO $_{2}$ ) planes. (b) Layout of the experimental setup and indication of the reciprocal space region (magenta thick line) spanned inside the first Brillouin zone.Reuse & Permissions
Figure 2
(a) RIXS raw spectra at Cu $L_{3}$ edge of bulk CaCuO $_{2}$ (black filled squares) and (CaCuO $_{2}$ ) $_{n}$ /(SrTiO $_{3}$ ) $_{n}$ SLs with $n = 3$ (red filled triangles), $n = 2$ (blue open circles) at $q_{∥}$ $=$ 0.375 r.l.u. and $T = 20$ K using $π$ incident polarization. Intensities are normalized to the $d d$ area put to 100. The shaded area under the gray solid line represents the difference between the bulk CaCuO $_{2}$ and the SL with $n = 2$ multiplied by 2. The ticks at 2.65 and 2.2 eV indicate, respectively, the center of mass of the resulting positive and negative peaks. Note how the spectral weight of the $d_{z^{2}}$ peak moves toward lower energy loss passing from bulk CaCuO $_{2}$ to the SL. (b) Decomposition of SL $n = 2$ spectrum in the low energy region: elastic (A) and magnon (B) peaks; optical phonon (C) and multiple magnons (D) spectral features.Reuse & Permissions
Figure 3
Schematic representation of the possible interfaces in (CaCuO $_{2}$ ) $_{n}$ /(SrTiO $_{3}$ ) $_{n}$ SLs starting from a TiO $_{2}$ layer. Thick lines highlight bondings while thin lines guide the eye. Note that Cu ions at the interface can either maintain the infinite layer coordination (a) or gain an apical oxygen (thin dashed line) from a SrO plane (b).Reuse & Permissions
Figure 4
XAS spectra at Cu $L_{3}$ edge of bulk CaCuO $_{2}$ (black filled squares) and (CaCuO $_{2}$ ) $_{n}$ /(SrTiO $_{3}$ ) $_{n}$ SLs with $n = 3$ (red filled triangles), $n = 2$ (blue open circles) measured at $T = 20$ K using $σ$ incident polarization. Intensities are normalized to the main peak.Reuse & Permissions
Figure 5
Selected RIXS spectra for bulk CaCuO $_{2}$ (black filled squares) and (CaCuO $_{2}$ ) $_{n}$ /(SrTiO $_{3}$ ) $_{n}$ SL with $n = 2$ (blue open circles), after the substraction of the elastic peak, measured at different $q_{∥}$ . Solid lines (thinner for bulk CaCuO $_{2}$ ) with the same color code represent the magnon peak simulated by a resolution limited Gaussian lineshape. Ticks and vertical lines are guides for the eye to follow the magnon dispersion.Reuse & Permissions
Figure 6
Magnon dispersion determined by RIXS for CaCuO $_{2}$ (black filled squares) and (CaCuO $_{2}$ ) $_{n}$ /(SrTiO $_{3}$ ) $_{n}$ SLs with $n = 2$ (blue open circles), $n = 3$ (red filled triangles) at $T = 20$ K. Solid lines with the same color code represent the relative fittings according to the linear spin wave theory for $q_{∥} ⩽$ 0.375 (see text for details).Reuse & Permissions

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