Doping dependence of the magnetic excitations in the cuprates and their relationship to high temperature superconductivity

Orateur : Mark P. M. DEAN (Brookhaven National Laboratory, USA)

High-resolution resonant inelastic x-ray scattering (RIXS) has recently emerged as a highly sensitive probe of magnetic excitations – it can even be used to measure magnetic excitations in isolated one-unit-cell-thick La2CuO4 layers [1]. In this talk, I will describe how RIXS has provided new insights into the magnetic excitations in the high-temperature superconducting cuprates. Exploiting the fact that RIXS is compatible with small sample volumes, we measured the magnetic excitations in Bi2Sr2CaCu2O8+δ a system with a very well characterized electronic structure. We show that a unified model can reconcile the magnetic excitations and the electronic structure with a single set of parameters [2]. In the heavily overdoped cuprates such as La2-xSrxCuO4 (x>0.3) superconductivity disappears despite the high electronic density of states. We used RIXS to measure the magnetic excitations across the whole La2-xSrxCuO4 phase diagram. The magnons resulting from local moment physics in La2CuO4 evolve smoothly into broadened, damped paramagnons in the overdoped state where itinerant quasi-particles dominate most properties of the cuprates. The fact that paramagnons persist relatively unchanged as superconductivity disappears is very difficult to reconcile with theories that suggest these high-energy paramagnons seen by RIXS are causing superconducting pairing. The lower energy magnetic excitations in other regions of the Brillouin zone, on the other hand, remain possible candidates for causing superconducting pairing [3]. Looking to the future, the availability of higher energy resolution and full angular freedoms in RIXS instrumentation will allow us to measure the relationship between static ordering such a charge stripes and magnetic excitations in a single experiment, as illustrated by our initial studies of La1.875Ba0.125CuO4 [4] [1] M. P. M. Dean et al. Nature Materials 11, 850–854 (2012) [2] M. P. M. Dean et al. Phys. Rev. Lett. 110, 147001 (2013) [3] M. P. M. Dean et al., Nature Materials 12, 1019–1023 (2013) [4] M. P. M. Dean et al., Phys. Rev. B 88, 020403(R) (2013)

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