Possible Bose-Einstein condensate associated with an orbital degree of freedom in the Mott insulator $\mathrm{CaCr}{\mathrm{O}}_3$

Whether $\mathrm{CaCr}{\mathrm{O}}_3$ is a Mott insulator or a correlated metal is still controversial. We have performed measurements of magnetization, specific heat, and thermal conductivity on $\mathrm{CaCr}{\mathrm{O}}_3$ samples selected from many batches of high-pressure synthesis. The single-crystal $\mathrm{CaCr}{\mathrm{O}}_3$ sample exhibits an unprecedentedly sharp transition at a Néel temperature $T_N\approx 90\mathrm{K}$. 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 $\kappa$ exhibits an anomalous drop on cooling through $T_N$, which is opposite to all known influence on $\kappa$ 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 $T_N$ and the occupation of a pure, localized $xy$ orbital undergoing a Bose-Einstein condensate at $T_N$.

Figure 1

Temperature dependence of magnetization for single-crystal and polycrystalline samples of $\mathrm{CaCr}{\mathrm{O}}_3$. Curves labeled with A, B, C are for polycrystalline samples. A is a uniform polycrystalline sample; C is a polycrystalline sample with many embedded single crystal grains; B is between A and C. See the SEM results in SM for the detail.

Figure 2

(a) Temperature dependence of specific heat for a variety of $\mathrm{CaCr}{\mathrm{O}}_3$ samples; inset: a zoom-in plot of the transition region; (b) the magnetic contribution $C_m$ versus reduced temperature $(T-T_c)/T_c$ for single crystal $\mathrm{CaCr}{\mathrm{O}}_3$, Heisenberg magnets (A collection of many typical magnets), quantum spin system in ${\mathrm{Sr}}_3$Cr2${\mathrm{O}}_8$, and the λ transition in ${}^4\mathrm{He}$ for comparison. Inset: the plot of $log{C}_m$ versus ${log}_{10}|1-T/T_N|$ for ${\mathrm{CaCrO}}_3$.

Figure 3

(a) Temperature dependence of thermal conductivity for a variety of $\mathrm{CaCr}{\mathrm{O}}_3$ samples and $\mathrm{LaGa}{\mathrm{O}}_3$ crystal; inset: a zoom-in plot of the transition region of the $\mathrm{CaCr}{\mathrm{O}}_3$ samples; a solid line inside the data of $\mathrm{LaGa}{\mathrm{O}}_3$ represents the fitting result to the Boltzmann formula; (b) the inverse thermal conductivity versus temperature for the $\mathrm{CaCr}{\mathrm{O}}_3$ sample $C$ and $\mathrm{YV}{\mathrm{O}}_3, \mathrm{YCr}{\mathrm{O}}_3, \mathrm{LaTi}{\mathrm{O}}_3$, and ${\mathrm{LaGaO}}_3$ for comparison.