Similarities and differences between electron- and hole-doped cuprate superconductors unveiled by inelastic X-ray scattering

Superconductivity is a state of matter where electrical resistance disappears completely. This phenomenon is particularly fascinating and mysterious in high-T_c superconducting cuprates, whose complicated electronic and magnetic structure has attracted great interest since their discovery more than a quarter century ago.

The parent compound of cuprate superconductors is an antiferromagnetic insulator. The material becomes a metal when either electrons or holes become mobile charge carriers through the effect of doping. The superconductivity occurs in the doped metallic state and the antiferromagnetic interaction, which causes the spin order in the parent compound, is also intimately related to the superconductivity. Therefore, both spin and charge degrees of freedom should be clarified comprehensively for a definitive understanding of the electron dynamics in the cuprates and differences or similarities between electron- and hole-doped compounds have been central in the study of the cuprate superconductors over the last few decades.

In this study, we have performed Cu L₃-edge resonant inelastic X-ray scattering (RIXS) experiments of the electron-doped superconductor Nd_2-xCe_xCuO₄ at beamline ID08 (now ID32) using the AXES spectrometer. Very recently, high-resolution RIXS at the transition-metal L-edge has become a complementary technique to the conventional inelastic neutron scattering (INS) for measuring momentum-resolved spin excitations [1]. In addition, RIXS possesses sensitivity to charge excitations, which are hardly observed by neutron based techniques.

Fig. 11: Cu L3-edge RIXS intensity map of the parent antiferromagnetic insulator Nd2CuO4 and the electron-doped superconductor Nd1.85Ce0.15CuO4. Blue and red circles respectively indicate the peak positions of the spin and charge excitations obtained by fitting the spectra. The spectra were measured with a π polarised incident beam.

Figure 11 shows RIXS intensity maps as a function of momentum in the CuO₂ plane (q_||) and energy. In the parent compound Nd₂CuO₄, spin-wave excitations with sinusoidal dispersion are predominantly observed. Charge excitations are missing in this energy region because the large charge transfer gap is about 2 eV. When electrons are added by partial substitution of trivalent Nd with tetravalent Ce, the spectral weight clearly moves to higher energy accompanied by the broadening of the width, as seen in the spectra of superconducting Nd_1.85Ce_0.15CuO₄. We also confirmed the high-energy shift of the spin excitations near the magnetic zone centre by inelastic X-ray scattering (INS). In addition, we found a distinct mode whose fast dispersion connects well to that of particle-hole excitations observed in the Cu K-edge RIXS spectra. We therefore attribute the dispersive mode to the same charge origin.

From the Cu L₃-edge RIXS, complemented by INS and Cu K-edge RIXS, we have clarified the doping evolution of the spin and charge excitations in the electron-doped cuprate superconductors. Although the spin excitations preserve their strength in the superconducting state, both in the electron- and hole-doped superconductors, they differ substantially in their dispersion relation. In Figure 12, we summarise the spin and charge excitations in the parent antiferromagnetic insulator and the electron- and hole-doped superconductor. A high-energy shift of the spin excitations and their mixture with the charge excitations indicate that the electron dynamics of the electron-doped cuprates has a highly itinerant character in the sub-eV energy scale. In contrast, the spectral distribution of the spin excitations also broadens but keeps its energy position almost unchanged upon hole doping [2], which means that a more localised picture is suitable. Our findings impose constraints on theoretical models and a comprehensive description of the electronic excitations in the electron- and hole-doped cuprates is a prerequisite for complete understanding of the superconductivity.

Fig. 12: Schematic of spin and charge excitations in the copper oxides. In the present work, the parent antiferromagnetic insulator and an electron-doped superconductor were studied. Excitations in hole-doped superconductors have been reported in Refs. [2,3].

Finally, we note that while polarisation of the scattered photons was not resolved in this work, its discrimination, which is very useful for distinction between spin and charge excitations, is becoming available [4]. Furthermore, this type of experiments will soon become even more enlightening thanks to the ERIXS spectrometer installed at the new ID32 beamline, where a simultaneous increase of energy resolution (x5) and intensity (x3) is expected with respect to the experimental conditions of this work.

Principal publication and authors
K. Ishii (a), M. Fujita (b), T. Sasaki (b),  M. Minola (c), G. Dellea (c),  C. Mazzoli (c), K. Kummer (d),  G. Ghiringhelli (c), L. Braicovich (c),  T. Tohyama (e), K. Tsutsumi (b),  K. Sato (b), R. Kajimoto (f), K. Ikeuchi (g), K. Yamada (h), M. Yoshida (a,i),  M. Kurooka (i) and J. Mizuki (a,i), Nature Communications 5, 3714 (2014).
(a) Japan Atomic Energy Agency, Sayo (Japan)
(b) Tohoku University, Sendai (Japan)
(c) Politecnico di Milano, Milano (Italy)
(d) ESRF
(e) Kyoto University, Kyoto (Japan)
(f) J-PARC Center, Tokai (Japan)
(g) Comprehensive Research Organization for Science and Society, Tokai (Japan)
(h) High Energy Accelerator Research Organization, Tsukuba (Japan)
(i) Kwansei Gakuin University, Sanda (Japan)

References
[1] L.J.P. Ament et al., Phys. Rev. Lett. 103, 117003 (2009).
[2] M. Le Tacon et al., Nature Phys. 7, 725-730 (2011).
[3] S. Wakimoto et al., Phys. Rev. B 87, 104511 (2013).
[4] L. Braicovich et al., Rev. Sci. Instrum. 85, 115104 (2014).