Cyclotron resonance and mass enhancement by electron correlation in   KFe$_2$As$_2$
  Cyclotron resonance (CR) measurements for the Fe-based superconductor KFe$_2$As$_2$ are performed. One signal for CR is observed, and is attributed to the two-dimensional $\alpha$ Fermi surface at the $\Gamma$ point. We found a large discrepancy in the effective masses of CR [(3.4$\pm$0.05)$m_e$ ($m_e$ is the free electron mass)] and de-Haas van Alphen (dHvA) results, a direct evidence of mass enhancement due to electronic correlation. A comparison of the CR and dHvA results shows that both intra- and interband electronic correlations contribute to the mass enhancement in KFe$_2$As$_2$. 
Motoi Kimata, Taichi Terashima, Nobuyuki Kurita, Hidetaka Satsukawa,   Atsushi Harada, Kouta Kodama, Kanji Takehana, Yasutak...
cond-mat.mtrl-sci cond-mat.supr-con