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Quantum states in low-dimensional spin systems are characterized by their characteristic spin dynamics. In this seminar, I will talk about terahertz spectroscopy of quantum spin dynamics in a spin-1/2 Heisenberg-Ising antiferromagnetic chain as a function of temperature and magnetic field. At zero field, confined spinon excitations are observed below the Néel temperature [1]. In a longitudinal magnetic field, high-energy string excitations are observed as well as low energy spin excitations [2]. In contrast, in a transverse magnetic field, emergent fermionic excitations are observed, when the confinement is suppressed and an order-disorder phase transition is induced by the transverse field [3]. These experimental results are understood by comparison to precise calculations of dynamic structure factors using Bethe ansatz and infinite time evolving block decimation, respectively, for the longitudinal and transverse fields [2,3,4]. In addition, the results in transverse fields provided strong indication of possible existence of an experimentally well-defined quantum critical point of the transverse-field Ising-chain [5]. The existence of such a quantum critical point indeed has been revealed by our recent thermodynamic measurements [6], the quantum criticality of which is consistent with a recent analytical study of the transverse-field Ising-chain [7]. [1] ZW et al. Phys. Rev. B 91, 140404(R) (2015). [2] ZW et al. Nature 554, 219–223 (2018). [3] ZW et al. Phys. Rev. B 94, 125130 (2016). [4] Wang Yang et al. arXiv:1702.01854 (2017) [5] S. K. Niesen et al. Phys. Rev. B 87, 224413 (2013). [6] ZW et al. Phys. Rev. Lett. 120, 207205 (2018). [7] Jianda Wu et al. Phys. Rev. B 97, 245127 (2018).

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Zhe Wang, Helmholtz-Zentrum Dresden-Rossendorf (HZDR)
Seminar Room of the Institute of Physics II
Contact: T. Lorenz