Many unusual properties of strongly correlated materials have been attributed to the proximity of quantum phase transitions (QPTs), where different types of orders compete and coexist, and may even give rise to novel phases. In two-dimensional (2D) systems, the nature of the magnetic-field-tuned QPT from a superconducting to a normal state has been widely studied, but it remains an open question. Underdoped copper-oxide high-temperature superconductors are effectively 2D materials and thus present a promising new platform for exploring this long-standing problem. Although in cuprates the normal state is commonly probed by applying a perpendicular magnetic field (H) to suppress superconductivity, the identification and understanding of the H-induced normal state 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 recent experimental advances in identifying and characterizing a full sequence of ground states as a function of H in underdoped cuprates. In both the absence and the presence of charge order, the results demonstrate the key role of disorder in the H-tuned suppression of 2D superconductivity, giving rise to an intermediate regime with large quantum phase fluctuations, in contrast to the conventional scenario. Most strikingly, the interplay of the “striped” charge order with high-temperature superconductivity leads to the emergence of an unanticipated, insulatinglike ground state with strong superconducting phase fluctuations, suggesting an unprecedented freezing (i.e. “the hidden order”) of Cooper pairs. Possible scenarios will be discussed, including the implications of the results for understanding the physics of the cuprate pseudogap regime, as well as other 2D superconductors.