Temperature renormalization of the magnetic excitations in S = 1/2 KCuCl3

Cavadini, N.; Rüegg, Ch.; Henggeler, W.; Fürrer, A.; Güdel, Hans‐Ulrich; Krämer, Karl; Mutka, H.

doi:10.1007/s100510070003

Cited by 35 publications

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“…Figure 1(b) shows its thermal evolution as obtained from the above models, and from the RPA treatment applied to KCuCl 3 in Ref. [14]. The Bose-MF approach shows a very strong renormalization at higher temperatures due to the possibility of unlimited boson occupation.…”

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“…Thermal renormalization of the dispersion, spectral weight, and damping of the triplet modes in KCuCl 3 , which shares the crystal structure and exchange coupling scheme of TlCuCl 3 , are adequately described within a selfconsistent RPA theory including higher-order corrections [14]. The considerably stronger interdimer interactions make this approach less successful for TlCuCl 3 .…”

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Quantum Statistics of Interacting Dimer Spin Systems

et al. 2005

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The compound TlCuCl 3 represents a model system of dimerized quantum spins with strong interdimer interactions. We investigate the triplet dispersion as a function of temperature by inelastic neutron scattering experiments on single crystals. By comparison with a number of theoretical approaches we demonstrate that the description of Troyer, Tsunetsugu, and Würtz [Phys. Rev. B 50, 13 515 (1994)] provides an appropriate quantum statistical model for dimer spin systems at finite temperatures, where many-body correlations become particularly important. DOI: 10.1103/PhysRevLett.95.267201 PACS numbers: 75.10.Jm, 05.30.Jp, 78.70.Nx The temperature is without doubt the key control parameter in solid-state physics, for both historical and technical reasons. Historically, the study of thermal fluctuations created the discipline of statistical mechanics. As one technical example, the liquefaction of helium in 1908 enabled the discovery of superconductivity in 1911 [1]. While control parameters such as the magnetic field or applied pressure may seem rather abstract by comparison, in certain classes of system they have a profound effect on the quantum mechanical fluctuations of the dominant degrees of freedom. In this respect, dimer spin systems such as BaCuSi 2 O 6 [2 -4], TlCuCl 3 [5-9], and Cs 3 Cr 2 Br 9 [10] have recently attracted considerable attention as they exhibit both field-and (in TlCuCl 3 ) pressure-induced magnetic quantum phase transitions. The ordered phases have also been described as a novel state resulting from BoseEinstein condensation of magnons [11,12].To date, the primary experimental and theoretical studies of these systems have focused on elucidating the parameter dependence of the ground and excited states at ''T 0 K' ' [3,4,6 -10]. However, the temperature dependence of the spin-spin correlation function S Q; ! in such dimer-based compounds is also of fundamental interest. At finite temperatures, thermal fluctuations populating the excited triplet states above the energy gap, 7:5 K, in the spin excitation spectrum of TlCuCl 3 become significant and compete with the intrinsic quantum fluctuations. The absence of a classical ordering transition in zero field and at ambient pressure, and the strong interdimer interactions, make TlCuCl 3 an ideal candidate for an experimental investigation of the quantum statistical description for such a spin system.In TlCuCl 3 , the antiferromagnetic (AF) dimer unit is formed by the two S 1=2 magnetic moments in a pair of Cu 2 ions, and has a singlet ground state (total spin S 0) with triplet (S 1) excited states which may be considered as bosonic quasiparticles. Only in the high-temperature limit, relative to the intradimer exchange constant J 56 K [5,13], is the system better described in terms of S 1=2 fermions whose interactions are weak compared to the thermal fluctuations, and AF dimer correlations are strongly suppressed. In the intermediate regime, < T < J, a singlet-triplet description remains valid but the number of excited triplets becomes significan...

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Quantum Statistics of Interacting Dimer Spin Systems

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“…The mechanism is assumed to be a reduction in lifetime due to thermal decoherence as the excitations scattering off each other [3,4,18,20,21]. The asymmetry observed in Sr 3 Cr 2 O 8 is therefore unprecedented because it suggests that far from becoming increasingly incoherent, the magnons interact strongly with each other and scatter in a coherent manner.…”

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“…Most importantly the lineshapes of the excitations become asymmetric with temperature being weighted towards the center of the band, quite unlike the symmetric Lorentzian lineshape expected from spin-wave theory. The RPA is able to model the reduction in the integrated intensity and renormalization for temperatures below 25 K. On the other hand the model described here predicts sharp excitations at all temperatures because it ignores correlations between the magnons, RPA can be modified to include damping effects [18,23,24]. However, it then predicts that, the excitations would broaden symmetrically with a Lorentzian lineshape for temperatures T < ∆/k B = 40 K. This contrasts strongly with our experimental observation that the lineshape becomes asymmetric for temperatures above 15 K.…”

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

et al. 2012

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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...

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“…From the analyses of the dispersion relations obtained by neutron inelastic scattering experiments, it was found that the origin of the spin gap is the strong antiferromagnetic interaction in the chemical dimer Cu 2 Cl 6 , and that the neighboring dimers are coupled by the interdimer interactions along the double chain and in the (1, 0, −2) plane [13,14,15,16,17]. On the other hand, NH 4 CuCl 3 is a gapless antiferromagnet with T N = 1.3 K [18].…”

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C63/65u- andC35/

et al. 2009

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63/65 Cu-and 35/37 Cl-NMR experiments were performed to investigate triplet localization in the S = 1/2 dimer compound NH 4 CuCl 3 , which shows magnetization plateaus at one-quarter and three-quarters of the saturation magnetization. In 63/65 Cu-NMR experiments, signal from only the singlet Cu site was observed, because that from the triplet Cu site was invisible due to the strong spin fluctuation of onsite 3d-spins. We found that the temperature dependence of the shift of 63/65 Cu-NMR spectra at the singlet Cu site deviated from that of macroscopic magnetization below T = 6 K. This deviation is interpreted as the triplet localization in this system. From the 35/37 Cl-NMR experiments at the 1/4-plateau phase, we found the two different temperature dependences of Cl-shift, namely the temperature dependence of one deviates below T = 6 K from that of the macroscopic magnetization as observed in the 63/65 Cu-NMR experiments, whereas the other corresponds well with that of the macroscopic magnetization in the entire experimental temperature region. We interpreted these dependences as reflecting the transferred hyperfine field at the Cl site located at a singlet site and at a triplet site, respectively. This result also indicates that the triplets are localized at low temperatures. 63/65 Cu-NMR experiments performed at high magnetic fields between the one-quarter and three-quarters magnetization plateaus have revealed that the two differently oriented dimers in the unit cell are equally occupied by triplets, the fact of which limits the theoretical model on the periodic structure of the localized triplets.

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Temperature renormalization of the magnetic excitations in S = 1/2 KCuCl3

Cited by 35 publications

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Quantum Statistics of Interacting Dimer Spin Systems

Quantum Statistics of Interacting Dimer Spin Systems

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

C63/65u- andC35/

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