加壓熱處理下二硒化鈮單晶的超導特性之研究

Abstract

本研究首先對於化學氣相傳導法(Chemical Vapor Transport, CVT)成長的單晶樣品，經過Se元素補充後尋求改善超導特性之最佳熱處理條件，發現在固定壓力6000 atm下使用了不同的退火溫度400 °C、500 °C、600 °C以及800 °C的最佳化退火溫度為600 °C。我們便進行對加壓前和加壓後的樣品進行電性的量測以及分析，以找出加壓退火後對超導特性的影響。透過R-T得到加壓熱處理前後的Tc，分別為Tc=6.45 K以及Tc=7.14 K，之後便經由R-T的量測得出Hc2,ab(0)在加壓熱處理前後為12.45 T和14.34 T，Hc2,c(0)在加壓熱處理前後分別為4.16 T和6.36 T，發現Hc2,ab>>Hc2,c跟Ising超導的性質符合，相干長度\xi_c分別為2.97 nm和3.33 nm、\xi_{ab}分別為8.9 nm和6.62 nm，∆Tc(Tconset-Tc(ρ=0))分別為0.44 K和0.17 K、RRR(R_{300\ K}/R_{T_c})分別為8和16.5，從U-H 分析兩樣品在磁通蠕動模型下的釘扎能，得出加壓熱處理後的樣品在兩方向上都比較大，I-V量測下的BKT相變在加磁場前後比較，判斷出具有二維特性，MR量測以及霍爾和載子濃度的量測下分析得下得出RRR(R_{300\ K}/R_{T_c})值對於磁阻為正相關，以及傳輸載子的轉變發生的更明顯，從在ab方向Hc3和Hc2的比較下發現大小差異不大，加壓後的僅在5 K時有Hc3=1.173 Hc2小於傳統理論Hc3=1.69 Hc2顯示出Ising超導的抑制。

In this study, we aimed to improve the superconducting properties of single-crystal samples grown using the Chemical Vapor Transport (CVT) method by supplementing Se elements and optimizing the annealing conditions. After conducting experiments at a fixed pressure of 6000 atm and different annealing temperatures 400 °C, 500 °C, 600 °C, and 800 °C, we found that the optimal annealing temperature for enhancing superconducting properties was 600 °C. We performed electrical measurements and analysis on the samples before and after pressurization to investigate the effects of pressurized annealing on superconducting characteristics. Through resistance-temperature (R-T) measurements, we determined the critical temperatures to be Tc=6.45 K before pressurization and Tc=7.14 K after pressurized annealing. Furthermore, the upper critical fields, Hc2,ab(0), were 12.45 T and 14.34 T, and Hc2,c(0) were 4.16 T and 6.36 T before and after pressurized annealing, respectively. We observed that Hc2,ab was significantly greater than Hc2,c, consistent with the properties of Ising superconductors. The coherence lengths, \xi_c and \xi_{ab}, were determined to be 2.97 nm and 3.33 nm, and 8.9 nm and 6.62 nm before and after pressurization, respectively. The difference in transition width, ∆Tc(Tconset-Tc(ρ=0)), was 0.44 K and 0.17 K, and the residual resistance ratios RRR(R_{300\ K}/R_{T_c}) were 8 and 16.5 before and after pressurized annealing. Using the U-H analysis, we found that the pressurized annealing resulted in higher pinning energies in both directions. Through I-V measurements, we compared the BKT transition before and after applying a magnetic field and determined that the samples exhibited 2D characteristics. Analyzing magnetoresistance and Hall measurements revealed a positive correlation between RRR values and magnetoresistance, indicating a more evident transition of transport carriers. By comparing Hc3 and Hc2 values in the ab direction, we observed that there was not a significant difference in magnitude. However, after pressurization, at 5 K, Hc3=1.173 Hc2, which was lower than the traditional theoretical value of Hc3=1.69 Hc2, suggesting the suppression from Ising superconductivity.