Single crystal growth and precise phase diagram of superconducting UTe

超伝導体UTeの単結晶育成と正確な超伝導相図

酒井 宏典   ; 常盤 欣文  ; Opletal, P.  ; 木俣 基*; 淡路 智*; 佐々木 孝彦*; 青木 大*; 神戸 振作  ; 徳永 陽  ; 芳賀 芳範   

Sakai, Hironori; Tokiwa, Yoshifumi; Opletal, P.; Kimata, Motoi*; Awaji, Satoshi*; Sasaki, Takahiko*; Aoki, Dai*; Kambe, Shinsaku; Tokunaga, Yo; Haga, Yoshinori

最近、我々は溶融塩フラックス法により超伝導UTeの純良単結晶育成に成功した。これらの純良単結晶は、残留抵抗比が大きく、超伝導転移温度以下の残留比熱も小さい。今回、純良単結晶を用いて、磁化困難軸である軸方向に磁場をかけた時の超伝導相図を、電気抵抗率と交流帯磁率の同時測定により、正確に決定した。その結果、低磁場超伝導相と高磁場超伝導相が、全く異なる磁場角度依存性を示すことを明らかにした。また、高磁場超伝導相が現れる特徴磁場が、試料純良性によらず、約15Tと一定であること、低磁場超伝導相内にピン留め力の小さな中間超伝導相が新たに現れることなどを見出した。

We have recently developed and optimized a molten salt flux method for growing high-purity single crystals of the superconductor UTe. The obtained crystals show a remarkably large residual resistivity ratio to the room temperature value and a small residual electronic contribution in specific heat well below . Using the high-quality single crystal of UTe, we have explored the superconducting phase diagram under magnetic fields along the hard magnetic -axis. Simultaneous electrical resistivity and AC magnetic susceptibility measurements discern low- and high-field SC (LFSC and HFSC, respectively) phases with contrasting field-angular dependence. Crystal quality increases the upper critical field of the LFSC phase, but the characteristic field of 15 T, at which the HFSC phase appears, is always the same through the various crystals. A phase boundary signature is also observed inside the LFSC phase near the characteristic field of 15 T, indicating an intermediate SC phase characterized by small flux pinning forces.