Two-gap 超導體:Ax(NH3)Fe2Se2 (A=Ba, Sr or Ca)的超導性質研究

Abstract

高純度超導體Ax(NH3)Fe2Se2 (A =鋇, 鍶 or 鈣)是經由氨熱法將鹼土金族鑲入belta-FeSe裡，製作出來的樣本是純超導，在過去245-phase的反鐵磁相會跟超導相一起共存，這是一個很頭痛的問題，因為反鐵磁相會影響超導性質的基本量測。 利用氨熱法製作的樣本之基礎超導性質像超導臨界溫度Tc (Tc = 39 K for A =鋇, 44 K for鍶 和43 K for鈣)，然而Tc的增高引起了大家對於Ax(NH3)Fe2Se2 (A = alkali 或 alkaliearth)產生了高度的興趣，不只是好奇為何Tc會增高，還包含了一連串超導性質與機制。 而經由X-ray的測量後，我們發現Bax(NH3)Fe2Se2與Srx(NH3)Fe2Se2的c-axis的變化，進而讓鐵硒原本的三維費米面轉化為近似二維費米面，除此之外，這篇論文還會提供基本的超導性質像超導低臨界場Bc1(0 K) (30 G for A =鋇 和24 G for A = 鍶) 和超導高臨界場Bc2(0 K) (13.4 T for A =鋇 和60.7 T for A =鍶)隱喻了Ax(NH3)Fe2Se2 (A =鋇, 鍶 or 鈣)是two-gap model、超導相干長度xi(0) (4.96 nm for A = Ba和2.33 nm for A = Sr)、Ginzburg-Landau parameters kappa(102.4 for A = Ba和266 for A = Sr)、超導穿透深度penetration depths lambda(0) (508.2 nm for A = Ba和620 nm for A = Sr)以及超導能隙分別為delta1 = 6.47 meV 和 delta2 = 1.06 meV。

High purity samples of Ax(NH3)Fe2Se2 (A = barium, strontium and calcium) superconductors were successfully synthesized by intercalating alkaliearth metals into tetragonal belta-FeSe by liquid ammonia. The ammonothermal method employed is known for capable of preparing single phase alkali/alkaliearth-intercalated iron-basic superconductors. The coexistence of antiferromagnitism in the alkali-containing iron-selenium superconductors synthesized by conventional thermal process is a huge barrier interfering detail studies of superconductivity. Beside the pure phase for superconductivity discussion, the enhancement of critical temperature Tc (Tc = 39 K for A = Ba, 44 K for Sr and 43 K for Ca) after alkali/alkaliearth intercalation into belta-FeSe attracts even more interests. In this work, the details of belta-FeSe preparation, ammonothermal intercalation process, lattice structure and magnetic properties measurements are described and used for physical parameters derivation. The elongated c-axis and almost unchanged a-axis of Bax(NH3)Fe2Se2 and Srx(NH3)Fe2Se2, comparing with -FeSe, suggested an unchanged intra-Fe2Se2-layer structure and the Tc enhancement is due to a 3D to 2D-like Fermi surface transformation. The superconducting properties coherent lengths xi(0) (4.96 nm for A = Ba and 2.33 nm for A = Sr), Ginzburg-Landau parameters kappa(102.4 for A = Ba and 266 for A = Sr) and penetration depths lambda(0) (508.2 nm for A = Ba and 620 nm for A = Sr) obtained from the extrapolated lower and upper critical fields Bc1(0 K) (30 G for A = Ba and 24 G for A = Sr) and Bc2(0 K) (13.4 T for A = Ba and 60.7 T for A = Sr) indicates that both compounds are typical type-II superconductors. The temperature dependence of 1/lambda2(T) of Bax(NH3)Fe2Se2 deduced from the low field magnetic susceptibility shows a two-gap s-wave behaviour with superconducting gaps of delta1 = 6.47 meV and delta2 = 1.06 meV.