探討超導性質在NbN 微米尺寸單晶和銅插層於(CuI)0.002Bi2Te2.7Se0.3

Abstract

本博士論文由以下兩個研究主題所組成：第一部份為微米尺寸ε-NbN的單晶不具有超導特性;第二部份為以高壓誘發Cux(CuI)0.002Bi2Te2.7Se0.3晶體中的超導特性。 第一部份：微米尺寸ε-NbN的單晶不具有超導特性 研究高品質單晶的特性對於解決存在某些材料中具爭議性的物理特性非常重要。然而，在單晶樣品在實驗測量上往往不是合宜的尺寸。NbN就是實例之一，由於其多樣化的物理特性及各種結構相，它吸引了科學和工程方面的興趣。到目前為止，NbN的超導特性僅在立方δ-和四方γ-結構相中觀察到，但在六方ε-相中並未觀察到。最近由Zou等人的研究指出在六方ε-相為基底的多相粉末樣品中量測出超導轉變溫度(Tc)約在11.6 K的超導性質。為了解決這個問題，這項工作使用電子背散射衍射(Electron Back-Scattering Diffraction, EBSD) 技術對於來自Alfa Aesar和Goodfellow的NbN微米尺寸粉末樣品進行結構相分析及測量其電子傳輸特性。我們的結果明確地證實NbN其六方ε-相不具有超導特性。 第二部份：以高壓誘發Cux(CuI)0.002Bi2Te2.7Se0.3晶體中的超導特性 我們成功地觀察到在拓樸化合物Cux(CuI)0.002Bi2Te2.7Se0.3晶體中以壓力誘發的超導特性。測量其與溫度相關的電阻從0.8到36 GPa，在 3.8 GPa 時檢測到Tc於2 K的超導特性。進一步增加壓力後，在15 GPa時達到最高的Tc為9.3 K；超過15 GPa後，Tc隨著壓力的增加而降低。除此之外，霍爾效應測量指出載子濃度從 4.2 GPa 時的約1017 cm-3到 15 GPa 時的約1022 cm-3顯著上升了大約5個數量級，同時對應了10K的電阻於壓力4.2 GPa之下降轉變。在同步輻射收集了在1.97到44.99 GPa高壓結構研究，在室溫中分別於13.9和25.3 GPa的壓力下觀察到兩次結構相轉變。所有與壓力相關的載流子濃度、殘餘電阻率和Tc都強烈表明在常壓晶相中存在電子相轉變。在不同壓力下的(101)面應變分析中可對應到載子濃度與超導Tc的行為。我們的觀察指出隱藏於常壓晶相的電子相轉變可能是由Cux(CuI)0.002Bi2Te2.7Se0.3晶體內的各向異性內應力引發的。

This doctoral thesis is composed of two following research subjects: I. The absence of superconductivity in micrometer-sized ε-NbN single crystals. II. The pressure induced superconductivity in Cux(CuI)0.002Bi2Te2.7Se0.3 crystals. I. The absence of superconductivity in micrometer-sized ε-NbN single crystals: It is important to study the properties of high quality single crystal in order to resolve the issue of an interesting material that certain debatable fundamental properties exist. However, it is unfortunate that sizable single crystal for experimental measurements is not always available. NbN is one of the examples; it has attracted scientific and engineering interests due to its diverse physical properties and a variety of structural phases. Until now superconductivity is only observed in cubic δ- and tetragonal γ- NbN but not in hexagonal ε-NbN. Recently, Zou et al. reported the observation of superconductivity with Tc ~ 11.6 K in a hexagonal ε-NbN based on the measurement on a multi-phase powder specimen. In order to resolve the issue, the work used the Electron Back-Scattering Diffraction (EBSD) technique to characterize phases of micron-size NbN crystals from commercial powders and measure their transport properties. Our results unambiguously confirm that the hexagonal ε-NbN phase is not superconducting. II. The pressure induced superconductivity in Cux(CuI)0.002Bi2Te2.7Se0.3 crystal: We report a successful observation of pressure-induced superconductivity in a topological compound Cux(CuI)0.002Bi2Te2.7Se0.3. The temperature dependent electrical resistance was measured from 0.8 through 36 GPa. Superconductivity was detected at 3.8 GPa with Tc = 2 K. Upon further pressure increase, where they reach maximum Tc = 9.3 K at 15 GPa. Over 15 GPa, superconducting transition temperature decreased as pressure increased. Furthermore, the Hall effects measurements indicated carrier concentration up-turned dramatically about five orders in magnitude of power from ~ 1017 cm-3 at 4.2 GPa to ~ 1022 cm-3 at 15 GPa, which respond to the downturn transition at 4.2 GPa of resistance at 10 K. The high-pressure structure investigations with synchrotron radiation were collected at various pressures from 1.97 to 44.99 GPa. We observed two times of structure phase transitions at room temperature at the pressures of 13.9 and 25.3 GPa, respectively. All of the pressure dependent carrier concentration, residual resistivity, and superconductivity Tc are strongly suggested that there is an electronic phase transition embedded in the ambient pressure crystal phase. The strain analysis on (101) plane which is corresponding to the carrier concentration and superconductivity Tc. Our observations suggest that the hidden electronic phase transition may induced by an anisotropic inner stress in the Cux(CuI)0.002Bi2Te2.7Se0.3.