• Quintero-Castro, D.L.; Lake, B.; Islam, A.T.M.N.; Wheeler, E.M.; Balz, C.; Mansson, M.; Rule, K.C.; Gvasaliya, S.; Zheludev, A.: Asymmetric Thermal Line Shape Broadening in a Gapped 3D Antiferromagnet: Evidence for Strong Correlations at Finite Temperature. Physical Review Letters 109 (2012), p. 127206/1-5

10.1103/PhysRevLett.109.127206
Open Access Version (externer Anbieter)

Abstract:
It is widely believed that magnetic excitations become increasingly incoherent as the 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 quasiparticles is not confined to special cases like the highly dimerized spin-1=2 chain but is found generally in dimerized antiferromagnets of all dimensionalities and perhaps gapped magnets in general. DOI: 10.1103/PhysRevLett.109.127206 PACS numbers: 75.10.Jm, 03.75.Gg, 03.75.Kk, 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 line shapes [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 spin¼5=2ðS¼5=2Þ, Heisenberg antiferromagnets (HAFs) Rb2MnF4 [3] and MnF2 [4] reveal symmetric Lorentzian line shapes 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