Anomalous Enhancement of the Electron Dephasing Rate from Magnetoresistance Data in ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$Cu${\mathrm{O}}_6$

The low-temperature magnetoresistance (MR) in ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$Cu${\mathrm{O}}_6$ is highly anisotropic. By fitting the orbital MR with the weak-localization expression, we derive a carrier dephasing rate $\frac{1}{{\tau }_{\phi }}$ that varies as $T^{-\frac{1}{3}}$. This implies that the energy levels are greatly broadened at low temperatures. Both the energy scale derived from $\frac{1}{{\tau }_{\phi }}$ vs $T$ and the behavior of the longitudinal MR suggest a strong coupling between the holes and the Cu spins. We discuss implications of the large dephasing rate, and contrast the MR with that of conventional disordered metals.