Abstract

In two-dimensional (2D) systems, the nature of the magnetic-field-tuned transition from a superconducting to a normal state remains an open question. In underdoped cuprate superconductors, which are quasi-2D materials, the normal state is commonly probed by applying a perpendicular magnetic field (H) to suppress superconductivity. However, the identification and understanding of the H-induced normal state in cuprates has been a challenge because of the complex interplay of disorder, temperature and quantum fluctuations, and the near-universal existence of charge-density-wave correlations. This talk will describe a comprehensive study of linear and nonlinear transport down to temperatures (T) as low as 0.016 K and H up to 35 T in La1.7Eu0.2Sr0.1CuO4, a superconductor with a “striped” charge order present already at H=0. The results reveal for the first time a full sequence of ground states as a function of H in a striped cuprate: a superconductor, a wide regime of superconducting phase fluctuations or a vortex liquid, and a high-field normal state. Most strikingly, an unanticipated, domelike (T, H) region of insulating, hysteretic behavior emerges below ~0.05 K within the vortex liquid, indicating the ordering of localized Cooper pairs within the charge stripes in the planes. This finding also supports proposals that superconductivity in “striped” La-based cuprates takes the form of a pair density wave, a new phase of matter.