High magnetic field allows for observation of Bethe strings
An international group of physicists reported the first experimental observation of Bethe strings in condensed matter systems. Using the unique facility of terahertz spectroscopy in magnetic fields up to 30 Tesla at HFML, they resolved string states in the antiferromagnetic Heisenberg-Ising spin-chain SrCo2V2O8. The obtained results pave the way towards the deterministic manipulation of complex magnetic many-body states in solid-state materials and shed light on the study of quantum quench dynamics, the Hubbard model, and string excitations in string theory.
Almost a century ago, complex bound states of magnetic excitations called “string states” were predicted in one-dimensional quantum magnets. However, despite many efforts, the experimental realisation of such strings had not been achieved until now.
SrCo2V2O8 is a one-dimensional compound, where the Co2+ ions form a chain of spins (S=1/2) (Fig.1a top). Below 5 K, the antiferromagnetic exchange interaction between the spins keeps them antiparallel to one another, and stabilizes the Neel-ordered phase (Fig.1a middle), where the excitation spectrum can be described by confined spinons. The externally applied magnetic field induces a quantum phase transition at 4 T. At higher fields, the interplay between the exchange interaction, anisotropy and magnetic field brings the system to the quantum critical regime (Fig. 1b), before a fully field-polarised state is reached at Bs = 28.7 T (Fig.1a bottom).
In the quantum critical regime many-body two-string and three-strings are identified (Fig. 1 e-f). Additionally, paired psinon-psinon and psinon-antipsinon excitations (Fig. 1 c-d) have been observed. The magnetic field shifts the energies of the different excitations differently which allows for clear distinction between them (Fig. 2).
Experimental observation of Bethe strings, Z. Wang, J. Wu, W. Yang, A. K. Bera, D. Kamenskyi, A. T. M. N. Islam, S. Xu, J. M. Law, B. Lake, C. Wu and A. Loidl, Nature 554, 219 (2018)