ESR study on the excited state energy spectrum of SrCu$_2$(BO$_3$)$_2$ -a central role of multiple-triplet bound states

TL;DR

This ESR study reveals complex multiple-triplet bound states in SrCu$_2$(BO$_3$)$_2$, highlighting their significant role in its magnetic excitation spectrum and indicating proximity to a quantum critical point.

Contribution

The paper provides the first detailed ESR analysis of multiple-triplet bound states in SrCu$_2$(BO$_3$)$_2$, including the discovery of new bound states and their implications.

Findings

Identification of various multiple-triplet bound states including singlet, triplet, and quintet.

Observation of a new bound singlet state below the one-triplet energy gap.

Detection of a two-triplet bound state band with at least eight discrete levels.

Abstract

The excited state energy spectrum of a two-dimensional dimer system SrCu$_2$(BO$_3$)$_2$ has been investigated by high field ESR. Various types of multiple-triplet bound states-excited singlet, triplet and quintet states are identified, besides two non-degenerated one-triplet excitations. Extremely strong intensity is found for these higher order bound states, which is an important consequence of the strong suppression of the one-triplet process due to the orthogonality of dimers. A new bound singlet state is found slightly below the energy gap of the one-triplet state, indicating the proximity to a quantum critical point. A band of two-triplet bound state splits into at least eight sharp and discrete levels, which indicates the contribution of distant neighbor interactions. A bound quintet state is observed, which is the direct evidence of two-triplet object made up of four $S=1/2$ spins. The energy of the lowest quintet becomes lower than that of the one-triplet excitation before the closing of the lowest spin gap. It indicates that the two-triplet bound state touches to the ground singlet state before the one-triplet state does. The present investigation exhibits an essential role of various multiple-triplet bound states in the magnetic excitation spectrum of SrCu$_2$(BO$_3$)$_2$.