R2：様式甲／Form Kou ２－１
学位論文の要旨
Abstract of Thesis 研究科
School
自然科学
専 攻
Division
地球生命物質科学
学生番号
Student No.
５１４２９２５１
氏 名
Name 王 雅囡
学位論文題目 Title of Thesis（学位論文題目が英語の場合は和訳を付記）
Study on preparation of new superconducting materials and their detailed characterization in a wide pressure range
（新しい超伝導物質の作製と広い圧力領域での詳細な特性評価に関する研究）
学位論文の要旨 Abstract of Thesis
Superconductivity of titanium-based pnictide oxides, Ba1-xAxTi2Sb2O (A: Na, K and Rb) and BaTi2(Sb1-yBiy)2O, has been discovered during the past decade [1-4]. Here, the superconductivity emerged through the suppression of the charge density wave (CDW) / spin density wave (SDW) transition. From the above results, the titanium-based pnictide oxide superconductors may provide a new platform to study and discuss the correlation between superconductivity and nonmagnetic / magnetic ordered state. In this doctoral thesis, the titanium-based pnictide oxide superconductors were prepared, and their superconducting properties were fully investigated in a wide pressure range.
On the other hand, the topological materials such as topological insulators and Weyl / Dirac semimetals have been extensively studied for the expectation not only to new physics but also to diverse applications [5,6]. In this doctoral thesis, the superconducting properties of possible Dirac semimetals, Pt1-xPdxTe2 and BaBi3, were investigated owing to much interest in topologically nontrivial nature in superconductivity. Exactly speaking, the analogues of the above materials such as PtSe2, PtTe2, PdTe2, and A3Bi (A: Na, K and Rb) are recognized as Dirac semimetals [7-12]. In chapters 6 and 7, the superconducting properties of Pt1-xPdxTe2 and BaBi3 in a wide pressure range are reported in detail.
This doctoral thesis comprises eight chapters. In chapter 1, the author describes the introduction to superconductor research, and in chapter 2 the motivation and purpose of this study are described. The results obtained in this study and detailed discussion based on the results are described in chapters 3 – 7. Finally, the conclusion of this study is given in chapter 8.
In chapter 3, the preparation of titanium-based pnictide oxide compounds, Ba1-xAxTi2Sb2O (A: K and Rb), and their superconducting properties are reported, demonstrating that these materials are type-II superconductors. Here, the exact stoichiometry was determined to be Ba0.70(5)K0.30(5)Ti1.7(3)Sb2.0(3)O and Ba0.63(4)Rb0.37(4)Ti1.9(4)Sb1.8(4)O, on the basis of energy dispersive X- ray (EDX) spectroscopy. Moreover, the temperature (T) dependence of electrical resistance (R) was measured at 0 – 3.09 GPa. The superconducting transition temperature, Tc, of Ba1-xRbxTi2Sb2O decreased with an increase in pressure up to 3.09 GPa. This study is the pre-stage study of chapter 4.
In chapter 4, the preparation of new superconducting titanium-based pnictide oxide compound,
R2：様式甲／Form Kou ２－２ Name 王 雅囡
Ba1-xCsxTi2Sb2O, is reported, and its superconducting properties are demonstrated in a wide pressure range. The exact stoichiometry of Ba1-xCsxTi2Sb2O prepared in this study was determined to be Ba0.59(3)Cs0.41(3)Ti1.8(4)Sb1.7(3)O, on the basis of EDX spectroscopy. The value of Tc was 4.4 K for Ba0.59(3)Cs0.41(3)Ti1.8(4)Sb1.7(3)O at ambient pressure. The crystal structure was determined to be tetragonal (space group of P4/mmm (No. 123)). The CDW / SDW transition was observed at 44 K at ambient pressure, but the transition was suppressed by applying pressure. Very interesting Tc – pressure (p) behavior was observed; the Tc – p behavior was obtained from the R – T plots at different pressures. The value of Tc decreased with an increase in pressure up to 4.0 GPa, but it increased above 4.0 GPa, indicating that it might not be a simple BCS type superconductor. No structural phase transitions were observed at 0 – 23.4 GPa. Therefore, the above Tc – p behavior may be related to the electronic transition such as Lifshitz transition.
In chapter 5, the preparation of superconducting BaTi2Bi2O is reported. The value of Tc was 4.33 K. The crystal structure and superconducting properties of BaTi2Bi2O are demonstrated in a wide pressure range. The crystal structure was determined to be tetragonal (space group of P4/mmm (No.
123)) on the basis of the X-ray diffraction (XRD) pattern at ambient pressure, and the XRD pattern was well analyzed by Le Bail fitting. The superconducting properties at ambient pressure were elucidated by the measurements of temperature dependence of magnetic susceptibility (M / H) at different applied magnetic fields, H’s, and the M – H plots at different temperatures. The pressure dependent XRD patterns showed no structural phase transitions up to 19.8 GPa. The R – T plots were recorded at different pressures to determine the value of Tc. The value of Tc increased monotonously with an increase in pressure up to 4.0 GPa, and it saturated above 4.0 GPa. The reduced critical field (h*) – normalized temperature (t (= T / Tc)) plot at 7.28 GPa suggested the deviation from the simple s- wave dirty limit model, suggesting the unconventionality in superconductivity.
In chapter 6, the preparation and characterization of a possible type-II Dirac semimetal, Pt1-xPdxTe2
(x = 0.25), are demonstrated. The value of Tc was ~3.2 K at ambient pressure. The exact stoichiometry was Pt0.74(2)Pd0.26(2)Te1.82(6) which was determined by EDX spectroscopy. All XRD patterns were well analyzed with the space group of P3m1 (trigonal structure (No. 164)) by Le Bail method, indicating no structural phase transitions at 0 – 29.1 GPa. The value of Tc was almost constant at 0 – 8.18 GPa.
Interestingly, the high Tc value of 7.2 K was recorded above 4 GPa for one sample of Pt1-xPdxTe2
prepared in this study. The exact characterization of 7.2 K superconducting phase may be important.
This is the future task.
In chapter 7, the author reports preparation of superconducting BaBi3, and its superconducting properties are demonstrated in a wide pressure range. The unconventional superconductivity was suggested from h* – t plot at ambient pressure. The value of the upper critical field, Hc2, was determined to be 1.27 T, using the formula, 𝐻 0 0.80𝑇 . The Tc – p plot of BaBi3 was fully investigated up to 10.5 GPa, and the value of Tc was almost constant against pressure at 0 – 10.5 GPa. Therefore, applying higher pressure for BaBi3 may be significant to explore the interesting electronic transitions and new superconducting phases.
Thus, the author discovered unconventional superconducting properties in titanium-based pnictide oxides and possible Dirac semimetals. Some interesting Tc – p behavior found in the above materials could not be associated with the structural variation against pressure. Therefore, it may be correlated with the electronic transition. Also, Cooper pair symmetry for some superconducting phases may not be ascribable to the simple s-wave dirty limit mode, suggestive of unconventional superconductivity.
R2：様式甲／Form Kou ２－３ Name 王 雅囡
The origin of unconventional superconductivity remains to be clarified, but through this study, the author clearly demonstrated good target materials for studying unconventional superconductivity as well as the strange Tc – p behavior which cannot be explained within the framework of a simple BCS theory.
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