Experimental evidence for a glass forming stripe liquid in the magnetic ground state of La 1.65 Eu 0.2 Sr 0.15 CuO 4

We report measurements of the longitudinal ${(}^{139}{T}_{1}^{\ensuremath{-}1})$ and transverse ${(}^{139}{T}_{2}^{\ensuremath{-}1})$ decay rates of the magnetization of ${}^{139}\mathrm{La}$ nuclei performed in a high quality single crystal of ${\mathrm{La}}_{1.65}{\mathrm{Eu}}_{0.2}{\mathrm{Sr}}_{0.15}{\mathrm{CuO}}_{4}.$ We observe a dramatic slowing of the Cu 3d spins manifested as a sharp increase of both ${}^{139}{T}_{1}^{\ensuremath{-}1}$ and ${}^{139}{T}_{2}^{\ensuremath{-}1}$ below 30 K. We find that in this temperature range the fluctuations involve a unique time scale \ensuremath{\tau} which diverges as $(T\ensuremath{-}{T}_{\mathrm{A}}{)}^{\ensuremath{-}1.9}$ with ${T}_{\mathrm{A}}\ensuremath{\approx}5$ K. This behavior is distinct from the continuous freezing observed in underdoped ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4},$ which involves a distribution of energy barriers. By contrast, in ${\mathrm{La}}_{1.65}{\mathrm{Eu}}_{0.2}{\mathrm{Sr}}_{0.15}{\mathrm{CuO}}_{4},$ the freezing below $30\mathrm{K}$ is intrinsic to its magnetic ground state and the observed power law supports the existence of a glass forming ``charge stripe liquid.''