High-energy spin and charge excitations in electron-doped copper oxide superconductors

Abstract

The evolution of electronic (spin and charge) excitations upon carrier doping is an extremely important issue in superconducting layered cuprates and the knowledge of its asymmetry between electron-and hole-dopings is still fragmentary. Here we combine X-ray and neutron inelastic scattering measurements to track the doping dependence of both spin and charge excitations in electron-doped materials. Copper L 3 resonant inelastic X-ray scattering spectra show that magnetic excitations shift to higher energy upon doping. Their dispersion becomes steeper near the magnetic zone centre and they deeply mix with charge excitations, indicating that electrons acquire a highly itinerant character in the doped metallic state. Moreover, above the magnetic excitations, an additional dispersing feature is observed near the G-point, and we ascribe it to particle-hole charge excitations. These properties are in stark contrast with the more localized spin excitations (paramagnons) recently observed in hole-doped compounds even at high doping levels.

Spin and charge excitations in artificial hole- and electron-doped infinite layer cuprate superconductors

“…The charge mode irradiates from the Γ point and moves rapidly towards higher energy with increasing q . As mentioned above, similar excitations have been observed in RIXS measurements on the prototypical e-doped cuprate NCCO 11,12 , with the same steep dispersion we find in SLCO, as shown in Figure 5 (b).…”

“…11,12 , at lower q (< 0.18 r.l.u.) the non-elastic spectral weight was decomposed into two non-resolution-limited components: a magnetic peak and a charge feature.…”

“…It has thus been shown that paramagnons persist throughout the phase diagram, from superconducting underand optimally-doped [7][8][9] to non-superconducting highly-overdoped 10 samples. Paramagnons have also been observed in the prototypical electron-doped (e-doped) cuprate compound Nd 2−x Ce x CuO 4 (NCCO) 11,12 . These findings have triggered several works 7,[13][14][15] , aiming to show that paramagnons are the driving excitations in Cooper pair formation.…”

“…These findings have triggered several works 7,[13][14][15] , aiming to show that paramagnons are the driving excitations in Cooper pair formation. However there is no obvious correlation between the critical temperature (T c ) and the paramagnon energy, and this may indicate that also other low energy excitations play a role, such as phonons (via electron-phonon coupling) and charge modes 11,12,16 . It is then worth studying the low energy excitations in cuprate compounds with different structure, in order to identify their correlations with high temperature superconductivity.…”

“…Experiments show that also the paramagnon excitations have a different doping evolution upon hole and electron doping: for h-doped compounds the dispersion is almost unaffected by doping, while for e-doped ones the dispersion is doping dependent, with hardening of the paramagnon mode up to 50% 11,12 . This asymmetry can be correctly captured by a relatively simple model including three-site exchange 19 , even though there is no perfect agreement with the experimental dispersion data 11,12 .…”