二硫化鈮單晶的超導特性之研究

Abstract

本研究系統性探討了厚度對層狀二維材料 NbS2 超導特性的影響。實驗採用固態合成法製備 NbS2 單晶，進行交流磁化率量測，並通過機械剝離法獲得不同厚度 S40(厚度約 40 μm) 以及 S5(厚度約 5 μm) 樣品，隨後對其進行電性量測。 研究發現，厚度對 NbS2 的超導性質產生些微影響：較厚樣品 S40 的超導轉變溫度為 Tc = 6.074 K，而較薄樣品 S5 的 Tc = 6.151 K，顯示較薄樣品呈現略高的超導轉變溫度。上臨界磁場 Hc2 在 ab 面內方向兩個樣品保持相差甚微（約 14 T），均超過其 Pauli 限制場 HPauli ≈ 11 T，可能存在 Fulde–Ferrell–Larkin–Ovchinnikov 超導態；但在 c 軸方向則由 2.37 T 增至3.10 T，較厚樣品表現出更強的各向異性特徵。相干長度 ξc 與 ξab 隨厚度變化甚微，且 ξc 始終遠小於 ξab。 磁通釘扎能分析揭示了明顯的維度效應：較薄樣品 S5 在兩個磁場方向均呈現U (H) ∝ ln H 的二維釘扎特徵，而 S40 的兩個方向在低磁場皆下具有較高釘扎能，突顯了厚度對維度轉變的調控作用。Berezinskii–Kosterlitz–Thouless（BKT）相變量測進一步證實了較薄樣品 S5 具更明顯的二維超導行為，與磁通釘扎結果一致。 表面超導態的對比研究顯示，S40 與 S5 的 Hc3/Hc2 比值分別為 1.17 及 1.28，均低於 Ginzburg–Landau 理論預測的 1.69。相比之下，塊體樣品 S0 的比值約為1.34，更接近理論預期，說明樣品厚度的減少削弱了表面超導態的穩定性。

This study systematically investigates the effect of thickness on the superconducting properties of the layered two-dimensional material NbS2. Single crystals of NbS2 were synthesized via a solid‐state reaction, and their AC magnetic susceptibility was measured. Mechanically exfoliated samples of two different thicknesses—S5 (approximately 40 μm) and S5 (approximately 5 μm)—were obtained, followed by electrical transport measurements. We found that thickness has a slight impact on the superconductivity of NbS2: the thicker sample S40 exhibits a superconducting transition temperature of Tc = 6.074 K, whereas the thinner sample S5 shows Tc = 6.151 K, indicating a marginally higher transition temperature in the thinner specimen. The upper critical field Hc2 within the ab plane remains nearly identical for both samples (around 14 T), exceeding their Pauli limit field HPauli ≈ 11 T, which suggests the possible realization of an Fulde–Ferrell–Larkin–Ovchinnikov superconducting state. Along the c axis, however, Hc2 increases from 2.37 T to 3.10 T, demonstrating stronger anisotropy in the thicker sample. The coherence lengths ξc and ξab vary only slightly with thickness, with ξc consistently much smaller than ξab. Analysis of the vortex pinning potential reveals a pronounced dimensional effect: the thinner sample S5 displays two‐dimensional pinning behavior U (H) ∝ ln H in both field orientations, while S40 exhibits higher pinning energies at low fields in both directions,underscoring the role of thickness in tuning the dimensional crossover. Measurements of the Berezinskii–Kosterlitz–Thouless (BKT) transition further confirm that the thinner sample S5 manifests more pronounced two‐dimensional superconducting behavior, consistent with the vortex pinning results. A comparative study of surface superconductivity shows that the ratios Hc3/Hc2 for S40 and S5 are 1.17 and 1.28, respectively—both below the Ginzburg–Landau theoretical prediction of 1.69. In contrast, the bulk sample S0 has a ratio of approximately 1.34, closer to the theoretical value, indicating that reducing sample thickness weakens the stability of the surface superconducting state.