High-energy magnetic excitations from heavy quasiparticles in CeCu$_2$Si$_2$

TL;DR

This study reveals that magnetic excitations in CeCu$_2$Si$_2$ are linked to heavy quasiparticles, showing a transition from three-dimensional low-energy to two-dimensional high-energy behavior, which is crucial for understanding its superconductivity.

Contribution

The paper provides the first detailed mapping of magnetic excitations in CeCu$_2$Si$_2$, demonstrating their origin from heavy electron quasiparticles and their evolution with energy.

Findings

High-energy magnetic excitations are nearly two-dimensional.

Low-energy excitations show three-dimensional characteristics.

Magnetic excitations are connected to heavy quasiparticles responsible for superconductivity.

Abstract

Magnetic fluctuations is the leading candidate for pairing in cuprate, iron-based and heavy fermion superconductors. This view is challenged by the recent discovery of nodeless superconductivity in CeCu$_2$Si$_2$, and calls for a detailed understanding of the corresponding magnetic fluctuations. Here, we mapped out the magnetic excitations in \ys{superconducting (S-type)} CeCu$_2$Si$_2$ using inelastic neutron scattering, finding a strongly asymmetric dispersion for $E\lesssim1.5$~meV, which at higher energies evolve into broad columnar magnetic excitations that extend to $E\gtrsim 5$ meV. While low-energy magnetic excitations exhibit marked three-dimensional characteristics, the high-energy magnetic excitations in CeCu$_2$Si$_2$ are almost two-dimensional, reminiscent of paramagnons found in cuprate and iron-based superconductors. By comparing our experimental findings with calculations in the random-phase approximation,we find that the magnetic excitations in CeCu$_2$Si$_2$ arise from quasiparticles associated with its heavy electron band, which are also responsible for superconductivity. Our results provide a basis for understanding magnetism and superconductivity in CeCu$_2$Si$_2$, and demonstrate the utility of neutron scattering in probing band renormalization in heavy fermion metals.