Collective Magnetic Excitations in SrCu_2(BO_3)_2

TL;DR

This study investigates low-temperature Raman spectra of SrCu_2(BO_3)_2, revealing phononic and magnetic excitations, and analyzes their symmetries, resonance, and coupling mechanisms to understand the material's magnetic properties.

Contribution

The paper provides a detailed symmetry analysis and models of magnetic excitations, including the effects of intra-dimer DM interactions, advancing understanding of spin gap modes.

Findings

Identification of quasi-degenerate phononic modes with different symmetries.

Confirmation of local one-triplet modes at 24 cm-1 in the spin gap.

Explanation of magnetic mode resonance behavior under external magnetic fields.

Abstract

We study low temperature Raman data on phononic and magnetic excitations in SrCu_2(BO_3)_2. Regarding the former, in the 0 to 350 cm-1 range we find several pairs of quasi-degenerate modes which have different symmetries. Group theoretical analysis suggests that the existence of these modes is related to quite different atomic vibrational pattern, i.e. in-plane and c-axis. Collective magnetic excitations are studied in terms of symmetry, resonance and coupling mechanisms in zero and applied magnetic fields. The analysis of a 4-spin cluster allows us to understand the group symmetries of the zero field Brillouin zone center spin gap branches around 24 cm-1 confirming the picture of local elementary one-triplet modes. By considering an additional intra-dimer DM interaction we are also able to understand the observed selection rules and intensity variations of the spin gap branches in external magnetic fields applied parallel or perpendicular to the dimer planes. We identified two effective magnetic light scattering Hamiltonians responsible for the coupling to the magnetic modes which allowed us to explain their resonance behavior.