鉍摻雜強無序超導體銀錫二硒之低溫磁傳輸研究

Abstract

本研究系統性探討Bi摻雜對強無序超導體AgSnSe₂ 的多晶樣品超導性質與低維行為之影響。透過X光繞射（XRD）與能量散射光譜（EDS）分析確認樣品之晶體結構與化學成分，並指出過量摻雜與成分不均為合成方法之侷限，可能導致非超導相生成而抑制超導性。磁化率量測顯示，適量Bi摻雜可有效提升超導臨界溫度，其中x = 0.1的樣品具有最高Tc。 由磁化強度–磁場（M–H）分析得到下臨界磁場Hc1（T），並利用髒極限模型外推Hc1（0），其值高於文獻報導之AgSnSe₂，顯示超導態穩定性提升。結合電阻–溫度（R–T）量測，在外加磁場垂直與平行於樣品平面的配置下，分別以Werthamer-Helfand-Hohenberg（WHH）模型估算上臨界磁場Hc2（0）與相干長度 ξ(0)，結果顯示明顯磁場各向異性，反映準二維超導特徵。進一步結合 H_c1 (T) 與髒極限模型，求得穿透深度 λ(0) 與Ginzburg–Landau參數 κ_GL，發現Bi摻雜後 κ_GL 由55降低至 45.9。 磁滯迴圈分析得到臨界電流密度與釘扎力，釘扎位能隨磁場變化呈現近似二維行為，且具有顯著各向異性。表面超導分析顯示H_c3/ H_c2比值低於Saint-James–de Gennes理論預測，且Bi摻雜樣品之比值進一步降低。BKT相變分析則確認系統中存在BKT相變，Bi摻雜樣品具有較高的〖 T〗_BKT ，且〖 T〗_BKT 對低磁場不敏感，顯示相位漲落受到有效抑制。磁阻分析中觀察到明顯弱反局域（weak anti-localization, WAL）效應，且 Bi摻雜樣品表現更為顯著，該效應隨溫度升高而逐漸減弱，反映其量子相干本質。 綜合磁性、輸運與臨界行為分析結果可知，Bi摻雜不僅引入強自旋軌道耦合，亦在強無序背景下顯著調控超導性質，使AgSnSe₂ 系統展現出明確的類二維超導特徵。包括各向異性的上臨界磁場、近似二維的釘扎能行為、Berezinskii–Kosterlitz–Thouless相變的穩定存在，以及隨溫度削弱的弱反局域效應，皆指向超導相干性在無序與spin–orbit coupling（SOC）競合下的重新組織。本研究顯示，Bi摻雜 AgSnSe₂ 可作為一個可調控的平台，用以探討強無序、低維超導與量子干涉效應之間的關聯，為進一步理解具拓樸潛力之超導材料奠定實驗基礎。

This study systematically investigates the effects of Bi substitution on the superconducting properties and low-dimensional behavior of the strongly disordered superconductor AgSnSe₂. The crystal structure and chemical composition of the samples were examined using X-ray diffraction（XRD）and energy-dispersive spectroscopy（EDS）.The results indicate that excessive Bi substitution and compositional inhomogeneity represent inherent limitations of the synthesis method, which may lead to the formation of non-superconducting secondary phases and consequently suppress superconductivity. Magnetization measurements reveal that an appropriate level of Bi substitution effectively enhances the superconducting critical temperature, with the sample at x = 0.1 exhibiting the highest Tc . The lower critical field H_c1 (T) was determined from magnetization–field（M–H）measurements and extrapolated to H_c1 (0) using the dirty-limit model. The obtained H_c1 (0) exceeds the value reported for pristine AgSnSe₂, indicating enhanced stability of the superconducting state.Combined with resistance–temperature（R–T）measurements under magnetic fields applied perpendicular and parallel to the sample plane, the upper critical field H_c2 (0) and coherence length ξ(0）were estimated using the Werthamer–Helfand–Hohenberg（WHH）model. The results demonstrate pronounced magnetic-field anisotropy, reflecting the quasi-two-dimensional nature of superconductivity in this system. Furthermore, by combining H_c1 (T) with the dirty-limit model, the London penetration depth λ(0)and the Ginzburg–Landau parameter κ_GL were determined, revealing that Bi substitution reduces κ_GL from 55 to 45.9. Analysis of magnetic hysteresis loops yields the critical current density and vortex pinning force. The field dependence of the pinning potential exhibits behavior close to that expected for a two-dimensional system and shows pronounced anisotropy. Surface superconductivity analysis reveals that the ratio H_c3/ H_c2 is lower than the value predicted by the Saint-James–de Gennes theory, with the ratio further reduced in Bi-substituted samples. Berezinskii–Kosterlitz–Thouless（BKT）transition analysis confirms the presence of a BKT transition in this system, with the Bi-substituted samples exhibiting a higher T_BKT. Moreover, T_BKT is found to be insensitive to low magnetic fields, indicating effective suppression of phase fluctuations. Magnetoresistance measurements reveal a pronounced weak anti-localization（WAL）effect, which is more prominent in Bi-substituted samples and gradually diminishes with increasing temperature, reflecting its quantum-coherent origin. Taken together, the magnetic, transport, and critical behavior analyses demonstrate that Bi substitution not only introduces strong spin–orbit coupling but also significantly modulates the superconducting physics of AgSnSe₂ in the presence of strong disorder, leading to clear quasi-two-dimensional superconducting characteristics. The observed anisotropic upper critical fields, quasi-two-dimensional vortex pinning behavior, robust BKT transition, and temperature-dependent suppression of the WAL effect collectively indicate a reorganization of superconducting coherence under the interplay between disorder and spin–orbit coupling. This study establishes Bi-substituted AgSnSe₂ as a tunable platform for exploring the relationship between strong disorder, low-dimensional superconductivity, and quantum interference effects, and provides an experimental foundation for further investigations into superconducting materials with potential topological relevance.