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

Abstract

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 ($H_{c2}$), a measure of the strength of superconductivity, has been the subject of a long-term debate. We combined three complementary transport techniques on underdoped La${}_{1.8-x}$Eu${}_{0.2}$Sr${}_x$CuO${}_4$ with a "striped" charge order and a low $H=0$ transition temperature $T_c^0$, to establish the $T-H$ phase diagram and reveal the ground states in CuO${}_2$ planes: a superconductor, a wide regime of superconducting phase fluctuations (i.e. a vortex liquid), and a high-field normal state. The relatively high $H_{c2}$ is consistent with the opening of a superconducting gap above $T_c^0$, but only at $T\sim (2$-$3)T_c^0$, 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 $T\lesssim T_c^0$. 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.

A more comprehensive study, which also clarifies the findings from this paper, was presented in arXiv:1907.11706 and arXiv:1907.11708, which have been published in Science Advances and Nature Communications, respectively