Unveiling the phase diagram of a striped cuprate at high magnetic fields: Hidden order of Cooper pairs

The interplay of charge orders with superconductivity in underdoped cuprates at high magnetic fields (H) is an open question, and even the value of the upper critical field (Hc2), a measure of the strength of superconductivity, has been the subject of a long-term debate. We combined three complementary transport techniques on underdoped La1.8−xEu0.2SrxCuO4 with a "striped" charge order and a low H=0 transition temperature T0c, to establish the T−H phase diagram and reveal the ground states in CuO2 planes: a superconductor, a wide regime of superconducting phase fluctuations (i.e. a vortex liquid), and a high-field normal state. The relatively high Hc2 is consistent with the opening of a superconducting gap above T0c, but only at T∼(2-3)T0c, an order of magnitude below the pseudogap temperature. Within the vortex liquid, an unanticipated, insulatinglike region, but with strong superconducting correlations, begins to emerge already at T≲T0c. The results suggest that the presence of stripes plays a crucial role in the freezing of Cooper pairs in this novel state. Our findings provide a fresh perspective on the pairing strength in underdoped cuprates, and introduce a new avenue for exploring the interplay of various orders.