Second anomaly in the specific heat of β-pyrochlore oxide superconductor KOs₂O₆

Resistivity and specific heat have been measured on a single-crystalline sample of a β-pyrochlore oxide superconductor, KOs₂O ₆. It is found that a second peak in specific heat, which may evidence an unknown phase transition, appears at around T_p ∼ 7.5 K below the superconducting transition temperature T_c = 9.53 K. Applying magnetic fields up to 14 T, T_c is reduced gradually down to 7.1 K, while T_p is slightly raised and becomes even higher than T_c at 14 T, which implies that the second anomaly is not associated directly with the superconductivity. It is demonstrated, however, that there is significant communication between the two anomalies, suggesting that they come from the same electrons. It is also reported that the Sommerfeld coefficient γ in KOs₂O₆ is possibly much larger than those in other members of β-pyrochlore oxide superconductors, RbOs₂O ₆ (T_c = 6.3 K) and CsOs₂O₆ (T _c = 3.3 K).