Institute for Solid State Physics, University of Tokyo, 5-1-5 Kashiwa, Chiba 277-8581, Japan.
College of Science, Ibaraki University, Mito 310-8512, Japan.
Phys Rev Lett. 2018 Jun 22;120(25):257206. doi: 10.1103/PhysRevLett.120.257206.
The specific heat of the Kondo insulator YbB_{12} has been measured up to 60 T. The Sommerfeld coefficient γ significantly increases at around 50 T, where the insulator metal transition occurs with a steep increase of the magnetization. γ reaches 67 mJ/(mol K^{2}) at high fields, which directly indicates that the quasiparticles gain a heavy thermodynamic effective mass and transform into a Kondo metal under magnetic fields. The field-induced Kondo metal has a rather high Kondo temperature around 200 K. The strong Kondo coupling proves that the energy gap collapse does not correspond to the breakdown of the Kondo bound state. The steep increase of the magnetization at the transition manifests the sharp density of states at the Fermi energy formed via the Kondo resonance.
高温超导体 YBCO 的比热已经被测量到 60T。在约 50T 处，Kondo 绝缘体到金属的转变发生，同时磁化强度发生陡增，Sommerfeld 系数γ也显著增加。在高场下，γ达到 67mJ/(mol·K2)，这直接表明准粒子获得了重的热力学有效质量，并在磁场下转变为 Kondo 金属。磁场诱导的 Kondo 金属在约 200K 具有相当高的 Kondo 温度。强的 Kondo 耦合证明了能隙坍塌并不对应于 Kondo 束缚态的破坏。在转变处磁化强度的陡增表现出通过 Kondo 共振形成的费米能附近的尖锐态密度。
