Direct observation of the multiple spin gap excitations in two-dimensional dimer system SrCu2(BO3)2

TL;DR

This study uses submillimeter wave ESR to observe multiple spin gap excitations in SrCu2(BO3)2, revealing field-dependent splitting and strong inter-dimer coupling, advancing understanding of its magnetic excitation spectrum.

Contribution

First direct observation of multiple spin gap excitations in SrCu2(BO3)2, elucidating anisotropic exchange effects and the localized nature of excitations in this two-dimensional dimer system.

Findings

Lowest spin gap energy is 722 GHz with zero-field splitting.

Second spin gap excitation observed at 1140 GHz, indicating strong inter-dimer coupling.

Multiple gapped ESR modes suggest localized excited states.

Abstract

Various spin gap excitations have been observed in the two-dimensional dimer system SrCu_2(BO_3)_2 by means of submillimeter wave ESR. The zero-field energy gap of the lowest spin gap excitation shows a splitting into two triplet modes and the energy splitting clearly depends on the magnetic field orientation when a field is rotated in the {\mib {ac}}-plane. A zero-field splitting is also found between the S(_z)=+1 and S(_z)=-1 branches of each triplet. These behaviors are qualitatively explained by considering the anisotropic exchange coupling of inter-dimer and intra-dimer, respectively. The averaged value of the lowest spin gap energy is determined to be 722 \pm 2 GHz(34.7 K). We have also found the second spin gap excitation at 1140 GHz(54.7 K), which indicates that the inter-dimer coupling is significantly strong. Besides these modes, a number of gapped ESR absorption are found and we propose that these multiple magnetic excitations are caused by the localized nature of the excited state in the present system.