Experimental studies of structure-property relationship in nine- and four-layer BaRuO$_{3}$

BaRuO$_3$ is a fascinating material as it possesses four different crystalline forms that possess rather different physical properties. We report results of our low-temperature magnetotransport measurements on single crystals of nine-layer rhombohedra (9R) and four-layer hexagonal (4H) BaRuO$_3$, two of the four different crystalline structures adopted by this material. Structurally a very short Ru-Ru distance was found in both 4H and 9R BaRuO$_3$, leading to metal-metal bonding, while 9R BaRuO$_3$ features three but 4H BaRuO$_3$ two face-sharing RuO$_6$ octahedra. For both 9R and 4H BaRuO$_3$, the magnetoresistance was found to become significant only below 30 K. More importantly, the magnetoresistance of 9R BaRuO$_3$ was found to be negative, while that of 4H BaRuO$_3$ positive. We suggest that the difference in the sign of magnetoresistance is associated with the difference in the two crystalline forms and electronic states of BaRuO$_3$, with only 9R BaRuO$_3$ featuring the previously proposed energy gap on certain part of the Fermi surface. Specific heat measurements are also being pursued to seek for additional insight into the physics of BaRuO$_3$. The work is supported by DOD ARO and NSF.