Correlated Decay of Triplet Excitations in the Shastry-Sutherland CompoundSrCu2(BO3)2

Abstract

The temperature dependence of the gapped triplet excitations (triplons) in the 2D Shastry-Sutherland quantum magnet SrCu 2 ðBO 3 Þ 2 is studied by means of inelastic neutron scattering. The excitation amplitude rapidly decreases as a function of temperature, while the integrated spectral weight can be explained by an isolated dimer model up to 10 K. Analyzing this anomalous spectral line shape in terms of damped harmonic oscillators shows that the observed damping is due to a two-component process: one component remains sharp and resolution limited while the second broadens. We explain the underlying mechanism through a simple yet quantitatively accurate model of correlated decay of triplons: an excited triplon is long lived if no thermally populated triplons are nearby but decays quickly if there are. The phenomenon is a direct consequence of frustration induced triplon localization in the Shastry-Sutherland lattice. Quantum spin systems display a wide range of intriguing many-body quantum effects. A particularly active field is the study of interacting dimer systems. Two antiferromagnetically coupled spins forming a dimer have a singlet ground state with an energy gap to excited triplet states. In extended systems where dimers couple to each other, the ground state often remains a spin singlet and gapped. The excitations are known as triplons and can be described in terms of quasiparticles as hard-core bosons [1][2][3][4]. Because of coupling between dimers, triplons usually become mobile and can hop to neighboring dimer sites. For particular cases, the thermodynamic finite temperature behavior of the mobility of these hard-core bosons has been treated using statistical models reproducing triplon band renormalization and damping observed in experiments [5][6][7][8][9]. In general, the increase of the thermal population of bosons produces an increased repulsion and a reduced mobility, which is observed as a reduction of the dispersion bandwidth. The theoretical treatment of finite temperature damping, reflecting the lifetime of the boson and the related spectral functions, remains an ongoing challenge [10][11][12][13].The compound SrCu 2 ðBO 3 Þ 2 constitutes an important example for testing our understanding of quantum spin systems, as it is a close realization of the frustrated but "exactly solvable" 2D Shastry-Sutherland model [14,15] (see Refs. [16,17] for a review). In SrCu 2 ðBO 3 Þ 2 , triplons are prevented from hopping already at zero temperature by frustrated interdimer interactions. Despite the strong (frustrated) coupling, the triplon dispersion is very shallow. Theoretical studies show that hopping is allowed only from the sixth order in the inter-to intradimer coupling ratio J 0 =J or in the presence of Dzyaloshinskii-Moriya (DM) terms. It has, however, been shown that multiple triplons forming a bound state are more mobile than a single triplon due to correlated hopping processes, appearing already in second order in J 0 =J [16,18]. In high magnetic fields, triplons in SrCu 2 ðBO 3 Þ 2 crystalliz...

Thermodynamic properties of the Shastry-Sutherland model throughout the dimer-product phase

“…Excellent magnetic susceptibility [8,12] and specific-heat [13] measurements made at this time were used to estimate the coupling ratio as J/J D 0.635, thereby placing SrCu 2 (BO 3 ) 2 rather close to the dimer-plaquette QPT. Detailed experiments performed in the intervening two decades have revealed a spectacular series of magnetization plateaus [8,[14][15][16][17][18][19][20][21], as well as a curious redistribution of spectral weight at temperatures very low on the scale of the triplon gap [11,22]. Of most interest to our current study is the result [23] that an applied pressure makes it possible to increase the ratio J/J D to the extent that, at approximately 1.9 GPa, the material is pushed through the QPT into a plaquette phase.…”

Thermodynamic properties of highly frustrated quantum spin ladders: Influence of many-particle bound states

“…11), with the temperature scale remaining higher and the maximum appearing more rounded. SrCu 2 (BO 3 ) 2 is also known from two-magnon Raman [99,100] and inelastic neutron scattering measurements [101,102] to show a highly anomalous thermal evolution of the spectral weight, and this may also be interpreted [103] on the basis of the spectrum in Subsec. II A.…”

Multi-triplet bound states and finite-temperature dynamics in highly frustrated quantum spin ladders

“…16 allows a qualitative analogy with the situation in SrCu 2 (BO 3 ) 2 . This material is known to be located close to a first-order quantum phase transition out of its rungsinglet phase (to a plaquette valence-bond state), and to have an anomalously low value, T 1/2 /∆ 0 = 7 K/35 K = 0.2 [25], of the ratio between the half-height temperature of the one-triplon intensity and the size of the onetriplon gap. The conventional discussion of this material is phrased using 1/j , with the phase transition occurring at 1/j c = 0.675 [96] and the best estimates of 1/j (= 0.635 [22,97]) falling within 5% of this value.…”

“…A triplet excitation of any single dimer experiences complete frustration (up to sixth order in a perturbative expansion [21,22]) and therefore the excitation band measured at low temperatures is almost completely flat, at an energy of approximately 3.0 meV ( 35 K) [23]. Remarkably, if the temperature is increased to only 6 K, this mode suffers a serious loss of intensity, and by 12 K it is essentially indiscernible, its spectral weight spread over the entire energy spectrum [24,25]. Some phenomenological proposals [24,25] and one numerical study [26] have tried to account for this anomalously strong decay of the one-triplet excitation, but with limited success.…”

“…Remarkably, if the temperature is increased to only 6 K, this mode suffers a serious loss of intensity, and by 12 K it is essentially indiscernible, its spectral weight spread over the entire energy spectrum [24,25]. Some phenomenological proposals [24,25] and one numerical study [26] have tried to account for this anomalously strong decay of the one-triplet excitation, but with limited success.…”