Spin dynamics in the low-temperature tetragonal phase of $\cong \frac{1}{8}$ doped single crystal ${\mathrm{La}}_{1.67}{\mathrm{Eu}}_{0.2}{\mathrm{Sr}}_{0.13}{\mathrm{CuO}}_4$

We present ${}^{139}\mathrm{La}$ and ${}^{63}\mathrm{Cu}$ NMR relaxation measurements in single crystal ${\mathrm{La}}_{1.67}{\mathrm{Eu}}_{0.2}{\mathrm{Sr}}_{0.13}{\mathrm{CuO}}_4.$ A strong peak in the ${}^{139}\mathrm{La}$ spin-lattice relaxation rate observed in the spin ordered state is well described by the BPP mechanism [Bloembergen, Purcell, and Pound, Phys. Rev. 73, 679 (1948)] and arises from continuous slowing of electronic spin fluctuations with decreasing temperature; these spin fluctuations exhibit $\mathrm{XY}$-like anisotropy in the ordered state. The spin pseudogap is significantly enhanced by the static charge-stripe order in the LTT phase.