Theory of Neutron Scattering for Gapless Neutral Spin-1 Collective Mode in Graphite

TL;DR

This paper provides a detailed RPA analysis of a gapless, neutral spin-1 collective mode in graphite, exploring its neutron scattering signatures and the effects of Coulomb interactions, with implications for experimental detection.

Contribution

It offers a comprehensive RPA-based theoretical analysis of the neutron scattering cross section for the spin-1 collective mode in graphite, including effects of Coulomb interactions and small-q behavior.

Findings

Neutron scattering intensity vanishes as q^3 near K and Gamma points.

The collective mode can be identified with spin triplet excitons with small spatial extent.

Long-range Coulomb interactions do not qualitatively alter the collective mode.

Abstract

Using tight binding band picture for 2D graphite, and the Hubbard interaction, recently we obtained a gapless, neutral spin-1 collective mode in graphite \cite{SZS}. In this paper we present a detailed RPA analysis of the Neutron Scattering cross section for this collective mode. Near $K-$point and very close to $\Gamma-$point, the intensity of neutron scattering peaks vanishes as $q^3$. This is shown using a simple Dirac cone model for the graphite band structure, which captures the small$-q$ behavior of the system. As we move away from the $\Gamma-$ and $K-$points in the Brillouin zone of the collective mode momenta, we can identify our collective mode quanta with spin triplet excitons with the spatial extent of the order of a few to a couple of lattice parameter $a$, with more or less anisotropic character, which differs from point to point. We also demonstrate that the inclusion of the long range tail of the Coulomb interaction in real graphite, does not affect our spin-1 collective mode qualitatively. This collective mode could be probed at different energy scales by thermal, hot and epithermal neutron scattering experiments. However, the smallness of the calculated scattering intensity, arising from a reduced form factor of carbon $2p_z$ orbital makes the detection challenging.