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論文

Nature of structural instabilities in superconducting Sr$$_3$$Ir$$_4$$Sn$$_{13}$$

金子 耕士; Cheung, Y. W.*; Hu, Y.*; 今井 正樹*; 谷奥 泰明*; 金川 響*; 村川 譲一*; 森山 広大*; Zhang, W.*; Lai, K. T.*; et al.

JPS Conference Proceedings (Internet), 30, p.011032_1 - 011032_6, 2020/03

https://doi.org/10.7566/JPSCP.30.011032

A quantum critical point appears as a second-order phase transition which takes place at zero temperature. In contrast to heavy-fermion systems in which magnetism often plays a vital role, recent studies revealed that structural instabilities can drive a system to a quantum critical point as well. In quasi-skutterudite (Ca,Sr)$$_3T_4$$Sn$$_{13}$$ ($$T$$=Rh, Ir), Sr$$_3$$Ir$$_4$$Sn$$_{13}$$ exhibits superconductivity around $$T_{rm sc}$$$${sim}$$5 K and a structural transition at $$T^*{simeq}$$147 K. Applying physical or chemical pressure on Sr$$_3$$Ir$$_4$$Sn$$_{13}$$ suppresses $$T^*$$ rapidly, and a quasi-linear $$T$$ dependence of electrical resistivity, signature of non-Fermi liquid behavior, was observed where $$T^*$$ extrapolates to 0 K. The isomorphs (Ca$$_x$$Sr$$_{1-x}$$)$$_3$$Rh$$_4$$Sn$$_{13}$$ exhibits similar behavior, where the criticality can be reached by $$x{sim}$$0.9 without external pressure. Neutron scattering experiments in Sr$$_3$$Ir$$_4$$Sn$$_{13}$$ evidences the second order nature of the structural transition at $$T^*$$ by the observation of a continuous evolution of superlattice peak below $$T^*$$ and a gradual increase of critical scattering upon approaching to $$T^*$$ by cooling. Increase of $$x$$ in (Ca$$_x$$Sr$$_{1-x}$$)$$_3$$Rh$$_4$$Sn$$_{13}$$ toward the quantum critical point leads to the systematic variation of the critical exponents of the order parameter. In addition, this substitution induces the phonon softening around the M point towards zero energy revealed by inelastic X-ray scattering experiment. We will present systematic variations in both elastic and inelastic channels upon approaching to the quantum critical point.

論文

Evidence of a structural quantum critical point in (Ca$$_x$$Sr$$_{1-x}$$)$$_3$$Rh$$_4$$Sn$$_{13}$$ from a lattice dynamics study

Cheung, Y. W.*; Hu, Y. J.*; 今井 正樹*; 谷奥 泰明*; 金川 響*; 村川 譲一*; 森山 広大*; Zhang, W.*; Lai, K. T.*; 吉村 一良*; et al.

Physical Review B, 98(16), p.161103_1 - 161103_5, 2018/10

https://doi.org/10.1103/PhysRevB.98.161103
 被引用回数：20 パーセンタイル：66.05(Materials Science, Multidisciplinary)

Approaching a quantum critical point has been an effective route to stabilize superconductivity. While the role of magnetic QCPs has been extensively discussed, similar exploration of a structural QCP is scarce. Using inelastic X-ray scattering, we examine the phonon spectrum of the nonmagnetic quasi-skutterudite (Ca$$_x$$Sr$$_{1-x}$$)$$_3$$Rh$$_4$$Sn$$_{13}$$, which represents a precious system to explore the interplay between structural instabilities and superconductivity by tuning the Ca concentration x. We unambiguously detect the softening of phonon modes around the M point on cooling towards the structural transition. Intriguingly, at x = 0:85, the soft mode energy squared at the M point extrapolates to zero at -5.7 K, providing the first compelling microscopic evidence of a structural QCP in (Ca$$_x$$Sr$$_{1-x}$$)$$_3$$Rh$$_4$$Sn$$_{13}$$. The enhanced phonon density-of-states at low energy provides the essential ingredient for realizing strong-coupling superconductivity near the structural QCP.

論文

Maximizing $$T_c$$ by tuning nematicity and magnetism in FeSe$$_{1-x}$$S$$_x$$ superconductors

松浦 康平*; 水上 雄太*; 新井 佑基*; 杉村 優一*; 前島 尚行*; 町田 晃彦*; 綿貫 徹*; 福田 竜生; 矢島 健*; 廣井 善二*; et al.

Nature Communications (Internet), 8, p.1143_1 - 1143_6, 2017/10

https://doi.org/10.1038/s41467-017-01277-x
 被引用回数：89 パーセンタイル：91.56(Multidisciplinary Sciences)

A fundamental issue concerning iron-based superconductivity is the roles of electronic nematicity and magnetism in realising high transition temperature ($$T_c$$). To address this issue, FeSe is a key material, as it exhibits a unique pressure phase diagram involving nonmagnetic nematic and pressure-induced antiferromagnetic ordered phases. However, as these two phases in FeSe have considerable overlap, how each order affects superconductivity remains perplexing. Here we construct the three-dimensional electronic phase diagram, temperature ($$T$$) against pressure ($$P$$) and iso-valent S-substitution ($$x$$), for FeSe$$_{1-x}$$S$$_x$$. By simultaneously tuning chemical and physical pressures, against which the chalcogen height shows a contrasting variation, we achieve a complete separation of nematic and antiferromagnetic phases. In between, an extended nonmagnetic tetragonal phase emerges, where $$T_c$$ shows a striking enhancement. The completed phase diagram uncovers that high-$$T_c$$ superconductivity lies near both ends of the dome-shaped antiferromagnetic phase, whereas $$T_c$$ remainslow near the nematic critical point.

論文

Second-order structural transition in (Ca$$_{0.5}$$Sr$$_{0.5}$$)$$_3$$Rh$$_4$$Sn$$_{13}$$

Cheung, Y. W.*; Hu, Y. J.*; Goh, S. K.*; 金子 耕士; 筒井 智嗣; Logg, P. W.*; Grosche, F. M.*; 金川 響*; 谷奥 泰明*; 今井 正樹*; et al.

Journal of Physics; Conference Series, 807(3), p.032002_1 - 032002_4, 2017/04

BB2016-0329.pdf:0.86MB

https://doi.org/10.1088/1742-6596/807/3/032002
 被引用回数：5 パーセンタイル：82.37(Physics, Condensed Matter)

(Ca$$_{0.5}$$Sr$$_{0.5}$$)$$_3$$Rh$$_4$$Sn$$_{13}$$ is a member of the substitution series (Ca$$_{x}$$Sr$$_{1-x}$$)$$_3$$Rh$$_4$$Sn$$_{13}$$ which has recently been argued to feature a structural quantum critical point at $$x_c$$ = 0.9. In the stoichiometric compound Sr$$_{3}$$Rh$$_{4}$$Sn$$_{13}$$, the structural transition at $$T^*$$ $$approx$$ 138 K has been shown to be a second-order phase transition. Moving towards xc, we examine the character of the structural transition in (Ca$$_{0.5}$$Sr$$_{0.5}$$)$$_3$$Rh$$_4$$Sn$$_{13}$$ (i.e. $$x$$ = 0.5, $$T^*$$ $$approx$$ 55 K) using electrical resistivity, heat capacity and X-ray scattering. The absence of the thermal hysteresis in specific heat around $$T^*$$, and the continuous evolution of the superlattice reflection detected by X-ray diffraction are consistent with the scenario that the structural transition associated with a modulation vector $$q$$ = (0.5, 0.5, 0) in (Ca$$_{0.5}$$Sr$$_{0.5}$$)$$_3$$Rh$$_4$$Sn$$_{13}$$ remains second-order on approaching the quantum critical point.

論文

Second-order structural transition in the superconductor La$$_3$$Co$$_4$$Sn$$_{13}$$

Cheung, Y. W.*; Zhang, J. Z.*; Zhu, J. Y.*; Yu, W. C.*; Hu, Y. J.*; Wang, D. G.*; 大友 優香*; 岩佐 和晃*; 金子 耕士; 今井 正樹*; et al.

Physical Review B, 93(24), p.241112_1 - 241112_5, 2016/06

AA2016-0058.pdf:0.95MB

https://doi.org/10.1103/PhysRevB.93.241112
 被引用回数：16 パーセンタイル：57.39(Materials Science, Multidisciplinary)

The quasi-skutterudite superconductor La$$_3$$Co$$_4$$Sn$$_{13}$$ undergoes a phase transition at $$T^{*}$$ = 152 K. By measuring the temperature dependence of heat capacity, electrical resistivity, and the superlattice reflection intensity using X-ray, we explore the character of the phase transition at $$T^{*}$$. Our lattice dynamic calculations found imaginary phonon frequencies around the $$M$$ point, when the high temperature structure is used in the calculations, indicating that the structure is unstable at the zero temperature limit. The combined experimental and computational results establish that $$T^{*}$$ is associated with a second-order structural transition with $$q$$ = (0.5, 0.5, 0) (or the $$M$$ point). Further electronic band structure calculations reveal Fermi surface sheets with low-curvature segments, which allow us to draw qualitative comparison with both Sr$$_3$$Ir$$_4$$Sn$$_{13}$$ and Sr$$_3$$Rh$$_4$$Sn$$_{13}$$ in which similar physics has been discussed recently.

口頭

Complementary use of neutron and X-ray scattering to explore structural quantum criticality in superconducting stannides

金子 耕士; 松田 雅昌*; 筒井 智嗣; Cheung, Y. W.*; Hu, Y. J.*; Goh, S. K.*; 松元 卓也*; 今井 正樹; 谷奥 泰明*; 金川 響*; et al.

no journal, , 

スクッテルダイトに類似した篭状構造をもつSr$$_3$$Ir$$_4$$Sn$$_{13}$$は、常圧で$$T_{rm c}$$=5Kの超伝導に加え、$$T^*{sim}$$150K近傍にも転移を示す。高温の転移点である$$T^*$$は、物理・化学的圧力の印加により急激に減少し、転移が消失する近傍で、線形的な電気抵抗等、量子臨界点を示唆する振る舞いが現れる。$$T^*$$の転移は非磁性の構造相転移だと考えられていることから、この物質では、従来の磁気的とは異なる、非磁性起源の量子臨界点の出現が期待されている。今回、この現象の鍵となる$$T^*$$での転移について、X線及び中性子散乱を用いてその詳細を調べた。その結果、$$T^*$$以下で格子系起源の超格子反射が出現することを観測し、その温度変化では、反射強度が$$T^*$$以下から連続的に成長していること、ヒステリシスを示さないことを明らかにした。さらに高温から$$T^*$$に向けて、臨界散乱的な強度の増加も見られることから、$$T^*$$の構造相転移は2次であると結論づけられる。この結果は、加圧によって現れる量子臨界点への連続性と矛盾しない。本発表では、上記回折実験に加え、$$T^*$$転移近傍におけるダイナミクスの結果についても合わせて報告する。

口頭

篭状構造をもつ超伝導体Sr$$_3$$Ir$$_4$$Sn$$_{13}$$における相転移

金子 耕士; 松田 雅昌*; 筒井 智嗣*; Goh, S. K.*; 松元 卓也*; 今井 正樹*; 吉村 一良*

no journal, , 

篭状構造をもつSr$$_3$$Ir$$_4$$Sn$$_{13}$$は、常圧では$$T_{rm c}$$=5 Kの超伝導に加え、$$T^*{sim}$$150 K近傍にも転移を示す。高温の転移点である$$T^*$$は、物理・化学的圧力の印加により急激に減少し、転移が消失する近傍で、量子臨界点を示唆する振る舞いが現れる。$$T^*$$の転移は非磁性の構造相転移だと考えられていることから、この物質では、従来の磁気的とは異なる、非磁性起源の量子臨界点の出現が期待されている。今回、この$$T^*$$での転移について、X線及び中性子散乱を用いてその詳細を調べた。$$q$$=(1/2, 1/2, 0)で表される逆格子点において、$$T^*$$以下で格子系起源の超格子反射が出現することを観測した。温度変化では、反射強度が$$T^*$$以下から連続的に成長していることに加え、ヒステリシスを示さないことを明らかにした。さらに高温から$$T^*$$に向けて、臨界散乱的な強度の増加も見られることから、$$T^*$$の構造相転移は2次であると結論づけられる。この結果は、加圧によって現れる量子臨界点への連続性と、矛盾しない。本発表では、上記回折実験に加え、$$T^*$$転移近傍におけるダイナミクスの結果についても合わせて報告する。

口頭

LaFeAsO$$_{1-y}$$の最適ドープ試料における磁場侵入長測定から見た超伝導ギャップ構造

水上 雄太*; 川本 雄太*; Goh, S. K.*; 石角 元志*; 石田 茂之*; 鬼頭 聖*; 伊豫 彰*; 永崎 洋*; 社本 真一; 芝内 孝禎*; et al.

no journal, , 

今回われわれは母物質LaFeAsOにおいて酸素欠損させることにより得られる超伝導体LaFeAsO$$_{1-y}$$の最適ドープ試料(T$$_{rm c}$$$$sim$$25K)を用いて、準粒子の低エネルギー励起を直接観測可能な磁場侵入長測定を500mK(T/(T$$_{rm c}$$$$sim$$0.02)まで行った。講演では磁場侵入長の温度依存性の詳細を報告し、超伝導ギャップ構造について議論したい。

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