Magnetic materials have always been excellent model systems for
experimental studies of phase transitions. While various thermodynamic
quantities are accessible through bulk measurements, by means of neutron
scattering also dynamical correlations can be probed. More recently such
systems have also allowed studies of quantum phase transitions. By
applying for example hydrostatic pressure or an external magnetic field
they can often be tuned through a variety of zero temperature
transitions.
This talk focuses on magnetic field induced quantum phase transitions in
spin chains and spin ladders. These transitions are theoretically well
understood [1]. At the same time excellent materials are known realizing
such structures. This opens the possibility to experimentally test
concrete predictions. A series of experiments on the spin chain material
Cu(pyrazine)NO3, revealed striking scaling behavior of various
thermodynamic quantities near the critical point [2]. More recently, by
inelastic neutron scattering, also dynamic correlations could be
measured near this critical point in a spin chain [3] and in a spin
ladder compound [4]. On the one hand, these experiments showed
spectacular agreement between the measured critical dynamic structure
factor and theoretical predictions. At the same time, they also revealed
unexpected non-universal excitations, not covered by any universal
theory.
[1] S. Sachdev et al. Phys. Rev. B 50, 258 (1994)
[2] O. Breunig et al. Sci. Adv. vol. 3, no. 12 (2017)
[3] D. Blosser et al. Phys. Rev. B 96, 134406 (2017)
[4] D. Blosser et al. Phys. Rev. Lett. 121, 247201 (2018)