Speaker
Description
The antiferromagnetic spin-$1/2$ spin chain with Heisenberg-Ising (XXZ) anisotropy is a rich source of novel phenomena. Good physical realizations are the compounds SrCo$_2$V$_2$O$_8$ and BaCo$_2$V$_2$O$_8$ where the Co$^{2+}$ ions have effective spin-$1/2$ and are coupled by antiferromagnetic interactions into chains while collinear long-range magnetic order occurs below $T_N\approx 5$ K due to weak interchain coupling. In a longitudinal magnetic field applied along the easy axis, the magnetic order is suppressed and using inelastic neutron scattering and optical spectroscopy we find the evidence for complex bound states of magnetic excitations, known as Bethe strings. Furthermore, the characteristic energy, scattering intensity and linewidth of the observed string states exhibit excellent agreement with precise Bethe ansatz calculations. Our results confirm the existence of the long-sought Bethe string excitations predicted almost a century ago. Application of transverse magnetic field along the direction perpendicular to the easy axis induces a quantum phase transition where the antiferromagnetic order is destroyed at a three-dimensional quantum critical point. The evolution of the excitations is investigated as a function of field revealing a complex series of modes and continuum scattering which are compared to theory. At a particular field still within the antiferromagnetically ordered phase we find a sequence of excitations whose energies match the predictions based on the E$8$ Lie group symmetry.
Acknowledgements
Funding was provided by Helmholtz Gemeinschaft.
