z=2 quantum critical dynamics in spin chain and spin ladder compounds

By means of high resolution inelastic neutron scattering, we investigate finite temperature critical dynamics near the magnetic field induced quantum critical point with dynamical exponent z=2 in one dimension.

In the S=1/2 Heisenberg spin chain compound K₂CuSO₄Cl₂ at saturation, we find correlations characteristic of the z=2, d=1 quantum critical point. At the same time we find a novel thermally activated longitudinal mode that remains underdamped across most of the Brillouin zone. By comparison to finite temperature density matrix renormalization group (DMRG) calculations we quantitatively explain the experimental data [1].

In the strong rung quantum spin ladder (C₅H₁₂N)₂CuBr₄ (BPCB), at the same z=2, d=1 critical point, as expected we find a similar low energy excitation spectrum. However, making use of the additional symmetry of the spin ladder, here we can separate universal critical and non-universal structure factor contributions. We find universal finite-temperature scaling of the transverse local dynamic structure factor in spectacular quantitative agreement with long-standing theoretical predictions. Already at rather low temperatures, we again observe strong non-universal longitudinal fluctuations [2].

[1] Phys. Rev. B 96, 134406 (2017)
[2] arXiv:1806.10392