Variable-range-hopping conductivity of half-doped bilayer manganite LaSr${}_2$Mn${}_2$O${}_7$

Abstract

We report measurements of in-plane ${\rho }_{ab}$ and out-of-plane ${\rho }_c$ resistivities on a single crystal of the half-doped bilayer manganite LaSr${}_2$Mn${}_2$O${}_7$. In the temperature $T$ range 220 to 300 K, the resistive anisotropy ${\rho }_c/{\rho }_{ab}=A+B/T$ ($A$ and $B$ constants), which provides evidence for the variable-range-hopping conduction in the presence of a Coulomb gap. This hopping mechanism also accounts for the quadratic magnetic field $H$ and ${\sin }^2\varphi$ dependences of the negative magnetoresistivity $\ln [{\rho }_i(T,H,\varphi )/{\rho }_i(T,H=0)]$ ($i=ab,c$), where $\varphi$ is the in-plane angle between the magnetic field and the current.

5 pages, 5 figures, to appear in Physical Review B