Gapless spin liquid in a square-kagome lattice antiferromagnet

正方カゴメ格子反強磁性体におけるギャップレススピン液体

藤原 理賀*; 森田 克洋*; Mole, R.*; 満田 節生*; 遠山 貴巳*; 矢野 真一郎*; Yu, D.*; 曽田 繁利*; 桑井 智彦*; 幸田 章宏*; 岡部 博孝*; Lee, H.*; 伊藤 晋一*; 羽合 孝文*; 益田 隆嗣*; 佐賀山 基*; 松尾 晶*; 金道 浩一*; 河村 聖子   ; 中島 健次   

Fujihara, Masayoshi*; Morita, Katsuhiro*; Mole, R.*; Mitsuda, Setsuo*; Toyama, Takami*; Yano, Shinichiro*; Yu, D.*; Sota, Shigetoshi*; Kuwai, Tomohiko*; Koda, Akihiro*; Okabe, Hirotaka*; Lee, H.*; Ito, Shinichi*; Hawai, Takafumi*; Masuda, Takatsugu*; Sagayama, Hajime*; Matsuo, Akira*; Kindo, Koichi*; Kawamura, Seiko; Nakajima, Kenji

Observation of a quantum spin liquid (QSL) state is one of the most important goals in condensed-matter physics, as well as the development of new spintronic devices that support next-generation industries. The QSL in two-dimensional quantum spin systems is expected to be due to geometrical magnetic frustration, and thus a kagome-based lattice is the most probable playground for QSL. Here, we report the first experimental results of the QSL state on a square-kagome quantum antiferromagnet, KCuAlBiO(SO)Cl. Comprehensive experimental studies via magnetic susceptibility, magnetisation, heat capacity, muon spin relaxation, and inelastic neutron scattering measurements reveal the formation of a gapless QSL at very low temperatures close to the ground state. The QSL behavior cannot be explained fully by a frustrated Heisenberg model with nearest-neighbor exchange interactions, providing a theoretical challenge to unveil the nature of the QSL state.