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Much of interest in quantum magnetic systems is driven by the fact that they serve as excellent realization of quantum critical phenomena. In these situations the finite temperature properties of the materials are defined by the quantum-disordered ground state. Often this concerns not just the thermodynamic properties, but also the details of the excitation spectra [1], which can be directly probed by inelastic neutron scattering. Despite quantum critical dynamics in 1D magnets is rather ubiquitous and has many faces, experimentally observing it is challenging and requires a careful choice of the model material and the measurement parameters. Nonetheless, today’s progress in sample synthesis and neutron instrumentation allows to verify many of the long-standing theoretical predictions.

The recent achievements include the “tunable” Tomonaga-Luttinger spin liquids (TLSL) in magnetized S=1/2 spin chains [2] and ladders [3]. In these systems the interactions between the effective fermionic quasiparticles vary as a function of external field [4], and sometimes can even be made attractive instead of a conventional repulsion. In addition to extended quantum critical phases such as TLSL, quantum critical dynamics can be also observed at isolated quantum critical points. For instance, “Ising model in transverse field” -type critical dynamics can be found in a dimerized S=1 chain [5]. The most recent example are the excitations at the saturation field of a quantum spin chain, representing z=2 critical point in one dimension [6]. Inherent to this QCP is the parameterless scaling, known as “zero-scale universality” [7], which, however, is found to be obscured at elevated temperatures.

[1] M. Vojta, Rep. Prog. Phys. 66, 2069 (2003)

[2] M. Haelg et al., Phys. Rev. B 92, 104416 (2015)

[3] K. Povarov et al., Phys. Rev. B 91, 020406 (2015)

[4] T. Giamarchi, Quantum physics in one dimension (Oxford University Press, 2003)

[5] M. Haelg et al., Phys. Rev. B 92, 014412 (2015)

[6] D. Blosser et al., arXiv:1707.05243, accepted for Phys. Rev. B

[7] S. Sachdev et al., Phys. Rev. B 50, 258 (1994)

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