Signatures of strong correlation effects in resonant inelastic x-ray scattering studies on cuprates

Abstract

Recently, spin excitations in doped cuprates have been measured using resonant inelastic x-ray scattering. The paramagnon dispersions show the large hardening effect in the electron-doped systems and seemingly doping independence in the hole-doped systems, with the energy scales comparable to that of the antiferromagnetic (AFM) magnons. This anomalous hardening effect and the lack of softening were partially explained by using the strong-coupling t − J model but with a three-site term [Nat. Commun. 5, 3314 (2014)], although the hardening effect is already present even without the latter. By considering the t − t − t − J model and using the slave-boson mean-field theory, we obtain, via the spin-spin susceptibility, the spin excitations in qualitative agreement with the experiments. The doping-dependent bandwidth due to the strong correlation physics is the origin of the hardening effect. We also show that dispersions in the AFM regime, different from those in the paramagnetic (PM) regime, hardly vary with dopant density. These excitations are mainly collective in nature instead of particle-hole-like. We further discuss the interplay and different contributions of these two kinds of excitations in the PM phase and show that the dominance of the collective excitation increases with decreasing dopant concentrations.

Doping dependence of the magnetic excitations in La2−xSrxCuO4

“…Itinerant quasi-particle pictures, in which correlations are treated within several different approximate schemes, are also popular. These have a significant advantage in terms of allowing predictions with high Q-resolution [23,29,[34][35][36][37]. They also capture the fact that the nodal excitations are more strongly doping dependent than the anti-nodal excitations, as observed here.…”

“…Calculations of the Hubbard model indeed capture the persistent high energy magnetic excitations in good accord with experiments [50,54]. However, such calculations are typically confined to small lattices and there is significant interest in conceptualizing magnetism in terms of either local moment [55,56] or (renormalized) itinerant quasiparticle methods [23,[34][35][36][37]. A local moment based approach is well justified in La 2 CuO 4 and has been used a lot in terms of describing stripe physics in the underdoped regime [55].…”

“…This is notably different from the low-energy AFM correlations around (0.5, 0.5) which are strongly modified in the overdoped regime. Qualitatively similar differences between the nodal and anti-nodal directions are captured by calculations of the magnetic excitation spectrum based on itinerant quasi-particles, which is discussed in terms of conceptualizing magnetism in the overdoped cuprates [23,[34][35][36][37].…”

Quantum spin fluctuations and evolution of electronic structure in cuprates

“…Therefore, all local observables of impurity model ( 4) can be calculated using simpler local problems written in the EDMFT (10) and DMFT (11) forms. It turns out that this approximation is very attractive for numerical calculations, since the inclusion of the spin channel in the impurity problem (4) does not require additional implementation, since the simplified actions (10) and (11) contain only the charge degrees of freedom.…”

“…This resonant AFM mode is present in cuprates within a broad range of temperatures and doping values, and is even proposed as a possible pairing mediator for superconductivity [3,10]. Various model calculations associate this mode either with paramag-netic fluctuations of correlated itinerant electrons [11,12] or with particle-hole excitations that depend on the band structure of different cuprate compounds [13,14]. However, there is no conventional understanding of the most distinctive feature of the AFM resonance -why does it remain unchanged in the broad range of doping values?…”