Dr Benjamin Huddart

Abstract:

We report the results of muon-spin spectroscopy (μ+SR) measurements on the staggered molecular spin chain [pym-Cu(NO3)2(H2O)2] (pym = pyrimidine), a material previously described using sine-Gordon field theory. Zero-field μ+SR reveals a long range magnetically ordered ground state below a transition temperature TN=0.23(1) K. Using longitudinal-field (LF) μ+SR we investigate the dynamic response in applied magnetic fields 0 < B < 500 mT and find evidence for ballistic spin transport. Our LF μ+SR measurements on the chiral spin chain [Cu(pym)(H2O)4]SiF6⋅H2O instead demonstrate one-dimensional spin diffusion, and the distinct spin transport in these two systems suggests that additional anisotropic interactions play an important role in determining the nature of spin transport in S=1/2 antiferromagnetic chains.

Abstract:

We present an investigation of the effect of randomizing exchange coupling strengths in the S=1/2 square lattice quasi-two-dimensional quantum Heisenberg antiferromagnet (QHAF) (QuinH)2Cu(ClxBr1-x)4·2H2O (QuinH = Quinolinium, C9H8N+), with 0≤x≤1. Pulsed-field magnetization measurements allow us to estimate an effective in-plane exchange strength J in a regime where exchange fosters short-range order, while the temperature TN at which long-range order (LRO) occurs is found using muon-spin relaxation, allowing us to construct a phase diagram for the series. We evaluate the effectiveness of disorder in suppressing TN and the ordered moment size, and we find an extended disordered phase in the region 0.4≲x≲0.8 where no magnetic order occurs. The observed critical substitution levels are accounted for by an energetics-based competition between different local magnetic orders. Furthermore, we demonstrate experimentally that the ground-state disorder is driven by quantum effects of the exchange randomness, which is a feature that has been predicted theoretically and has implications for other disordered quasi-two-dimensional QHAFs.