Experimental consequences of disorder at an antiferromagnetic quantum phase transition

Disorder is known to have a profound influence on classical phase transitions, and it is anticipated to be even more important for quantum phase transitions. However, experimental investigation of the influence of disorder on phase transitions normally requires numerous samples over the range of disorder. Here, we investigate the field-driven quantum phase transition in the antiferromagnet LiErF₄. The isotopic distribution of natural Er permits us to probe the transition in the clean and dirty regimes in the same sample. ¹⁶⁷Er, with non-zero nuclear spin on 23% of the Er sites, induces random mass disorder in the dirty (low-temperature) regime. We use specific heat and ac magnetic susceptibility experiments to identify a crossover between the two regimes at T = 150 mK. The critical behavior is consistent with a violation of the Harris criterion in the clean regime and a change of universality class in the dirty regime.