Spin-glass magnetism confined to individual weakly interacting vortices is detected in two different families of high-transition-temperature (T_c) superconductors, but only in samples on the low-doping side of the low-temperature normal state metal-to-insulator crossover (MIC). Our findings unravel the mystery of the MIC, but more importantly identify the true location of the field-induced quantum phase transition (QPT) in the superconducting state. The non-uniform appearance of magnetism in the vortex state favours a surprisingly exotic phase diagram, in which spatially inhomogeneous competing order is stabilized at the QPT, and an `avoided' quantum critical point (QCP) is realized at zero magnetic field.
@article{arxiv.cond-mat/0610051,
title = {Avoided Quantum Criticality in YBa_2Cu_3O_y and La_{2-x}Sr_xCuO_4},
author = {J. E. Sonier and F. D. Callaghan and Y. Ando and R. F. Kiefl and J. H. Brewer and C. V. Kaiser and V. Pacradouni and S. -A. Sabok-Sayr and X. F. Sun and S. Komiya and W. N. Hardy and D. A. Bonn and R. Liang},
journal= {arXiv preprint arXiv:cond-mat/0610051},
year = {2007}
}