Possible Bose-Einstein condensate associated with an orbital degree of freedom in the Mott insulator CaCr O3

Abstract

Whether CaCrO3 is a Mott insulator or a correlated metal is still controversial. We have performed measurements of magnetization, specific heat, and thermal conductivity on CaCrO3 samples selected from many batches of high-pressure synthesis. The single-crystal CaCrO3 sample exhibits an unprecedentedly sharp transition at a Néel temperature TN≈90K. The critical behavior of specific heat cannot be rationalized by the renormalization group theory for a second-order magnetic transition. More surprisingly, the thermal conductivity κ exhibits an anomalous drop on cooling through TN, which is opposite to all known influence on κ from either spin or orbital ordering. We have argued, on the basis of anomalies found in all three measurements and structural data, for the coexistence of itinerant π-bonding electrons in a c-axis band and localized xy electrons in xy orbitals responsible for type-C antiferromagnetic order below TN and the occupation of a pure, localized xy orbital undergoing a Bose-Einstein condensate at TN.