Asymmetric Thermal Line Shape Broadening in a Gapped 3D Antiferromagnet: Evidence for Strong Correlations at Finite Temperature

Abstract

It is widely believed that magnetic excitations become increasingly incoherent as temperature is raised due to random collisions which limit their lifetime. This picture is based on spin-wave calculations for gapless magnets in 2 and 3 dimensions and is observed experimentally as a symmetric Lorentzian broadening in energy. Here, we investigate a three-dimensional dimer antiferromagnet and find unexpectedly that the broadening is asymmetric -indicating that far from thermal decoherence, the excitations behave collectively like a strongly correlated gas. This result suggests that a temperature activated coherent state of quasi-particles is not confined to special cases like the highlydimerized spin-1/2 chain but is found generally in dimerized antiferromagnets of all dimensionalities and perhaps gapped magnets in general.PACS numbers: 75.10. Jm, 03.75.Kk, 03.75.Gg, 78.70.Nx In the conventional picture of temperature effects in magnetism, the excitations are long-lived at low temperatures and their lifetime decreases as temperature is increased. The accepted reason for this is that thermally activated excitations collide with each other limiting their lifetimes which results in a Lorentzian energy broadening of the lineshapes [1,2]. This model works well for gapless magnets with long-range magnetic order. Experimental investigations of the two-dimensional (2D) and three-dimensional (3D), gapless spin−5/2 (S−5/2), Heisenberg antiferromagnets (HAF) Rb 2 MnF 4 [3] and MnF 2 [4] reveal symmetric Lorentzian lineshapes which broaden with temperature in excellent agreement with spin-wave calculations [1,2]. The basic assumption behind this description is that the spin-waves interact only weakly and the states available to them cover an extensive region of phase space. As the population of excited spin-waves increases with temperature, the rate of collisions between them also increases and they fluctuate within this large range of states in an uncorrelated manner. The damping is then due simply to loss of coherence associated with the reduced lifetime of the excitations.The concept of thermal decoherence and the associated Lorentzian linewidth broadening have become so entrenched in current thinking that it is generally assumed to apply to all magnetic systems [5,6]. However for some magnets where there are strong interactions between the excitations and the phase space is restricted, it is not obvious that this reasoning should apply. Gapped antiferromagnets such as Haldane chains, spin ladders and dimer magnets which have a singlet ground state and triplet excitations are potential candidates. For the S-1/2, dimer HAFs a dominant antiferromagnetic exchange interaction J 0 , pairs the spins into dimers. The ground state is a singlet and the excited state is a triplet gapped by ≈J 0 while interdimer interactions allow the triplets to become dispersive and mobile. In the case of a large gap compared to the excitation bandwidth, the distribution of dimer excitations is severely limited. In addition, the dimers are subjec...

Correlated Decay of Triplet Excitations in the Shastry-Sutherland CompoundSrCu2(BO3)2Zayed1, Rüegg2, Strässle3 et al. 2014Phys. Rev. Lett.192190View full textAdd to dashboardCite

“…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.…”

“…3(b)] for a triplon radius R ¼ [9,13]. While these systems show the finite lifetime of their quasiparticles at finite temperatures due to scattering on thermally excited triplets and hence an effect that can be understood by a density-of-states argument, the spectral line shapes reported here for the frustrated Shastry-Sutherland compound SrCu 2 ðBO 3 Þ 2 appear to be very different with two components and over all pagoda shapes, which is very distinct from (asymmetric) Lorentzian models proposed normally for T ≪ Δ [10].…”

Effects of interactions on dynamic correlations of hard-core bosons at finite temperatures

“…In contrast to most previous studies, the Brückner approach has the asset that it is not restricted to one dimension or small system size, but in return it relies on a small parameter, namely exp(−β∆) where β is the inverse temperature and ∆ the energy gap, so that it is particularly reliable at low temperatures. It is important to dispose of methods which are applicable for all dimensions because the thermal broadening of hard-core bosonic line shapes is observed also in three dimensions [18]. A good description has been obtained by an expansion in the inverse coordination number [12].…”

Time-dependent correlations in quantum magnets at finite temperature

“…However, recent investigations of one-dimensional [9,10] and three-dimensional [11] materials of coupled spin dimers * benedikt.fauseweh@tu-dortmund.de † felix.groitl@psi.ch ‡ goetz.uhrig@tu-dortmund.de using inelastic neutron scattering (INS) show that the lineshape in the frequency domain develops an asymmetric tail. It has been argued that such tails may be found in a broad range of quantum systems.…”