Synopsis: Stripes can coexist with superconductivity

As researchers continue to fit some pieces into the long-standing puzzle of high-temperature superconductivity in copper-based oxides, they are also unearthing new questions. It is well known, for example, that adding holes to the insulating parent compounds of the superconducting cuprates can produce a one-dimensional modulation of charge and spin in the copper oxide planes. Given that spin and charge order are typically associated with an insulating ground state, it has been assumed that this “stripe order” must compete with, or even inhibit, superconductivity.

Now, based on a comprehensive set of measurements, John Tranquada and colleagues at Brookhaven National Laboratory, and collaborators at NIST (Gaithersburg) and in Germany, report in Physical Review B strong evidence for the coexistence of two-dimensional superconductivity with stripe order in single crystals of ${\text{La}}_2{\text{CuO}}_4$ doped with $\text{Ba}$. (Each $\text{Ba}$ introduces one hole per $\text{Cu}$ and stripe order is stabilized when there is exactly one extra hole for every eight $\text{Cu}$ atoms.) With x-ray and neutron scattering they identify the stripe ordering temperature $T_{\text{so}}$ (40 K), at which they also measure both a significant diamagnetic response and a sharp drop in the resistivity that eventually goes to zero at 16 K. Importantly, the two signatures of superconductivity—the Meissner effect and zero resistance—only occur in the $\text{CuO}$ planes. Also, this two-dimensional superconductivity occurs at a higher temperature than the bulk superconductivity that occurs in ${\text{La}}_2{\text{CuO}}_4$ when it is “optimally” doped. These intriguing discoveries are expected to have significant implications for an eventual theory of high-$T_c$ superconductivity. – Sarma Kancharla