鉍鍶鈣銅氧化物基垂直范德瓦爾斯接面元件之製作與常態輸運特性研究

Abstract

以 Bi2Sr2CaCu2O8+δ（鉍鍶鈣銅氧超導體，BSCCO）為代表的高 Tc 銅氧化物超導體具有高度各向異性的電子性質與非常規配對機制，因此成為研究層狀材料超導性的豐富平台。另一方面，由剝離二維晶體所形成的范德瓦耳（vdW）異質結構，提供了一種可靈活設計潔淨界面與新穎接面幾何的途徑。將 BSCCO 與常規 s 波超導體（例如二硒化鈮 NbSe2）沿 c 軸方向結合，被視為未來約瑟夫森實驗中，用以探測銅氧化物序參數中可能存在之各向同性成分的有前景策略。

本論文聚焦於 BSCCO 為基底的垂直范德瓦耳（vdW）接面之製作與正常態電性量測，作為推進上述約瑟夫森實驗之前的必要準備步驟。首先，我製作具 Au（金）電極的單片 BSCCO 元件，並以四探針傳輸量測其電性。所得 R–T 曲線呈現單一道超導轉變，其 Tc^90% ≈ 100 K；在溫度降至 2 K 的 I–V 量測中，可觀察到臨界電流量級約為 10^1 mA 的穩健超導態。這些單片元件可作為評估接觸品質與材料完整性的基準。

接著，我藉由堆疊兩片剝離之 BSCCO 薄片，實現 BSCCO–BSCCO c 軸接面。初始室溫接面電阻位於兆歐姆（MΩ）等級，顯示界面間存在厚或耦合不佳的勢壘。透過施加受控的電流退火處理，可將接面電阻降低數個數量級至千歐姆（kΩ）等級，並伴隨明顯的 I–V 特性改變。然而，在 78 K 進行的四探針量測中，於可及偏壓範圍內並未觀察到清楚的約瑟夫森超電流，顯示界面局部 Tc 可能遭到抑制，或勢壘仍然過於不透明。

最後，我展示了一個由 h-BN 薄片封蓋的 BSCCO–二硒化鈮（NbSe2）垂直接面。室溫二探針量測顯示，其接面電阻僅為數百歐姆，較先前 BSCCO–二硒化鈮（NbSe2）嘗試中所得到的數值明顯降低。此一改善可歸因於 h-BN 封蓋層提升了機械貼合度並減少界面縫隙，從而增進界面耦合與電流傳輸。

儘管本研究尚未在量測中觀察到具相位相干性的約瑟夫森行為，但所建立的元件結構與製程流程已顯著提升 BSCCO 為基底之范德瓦耳（vdW）接面的透明度與可重現性。這些結果為未來在 BSCCO–二硒化鈮（NbSe2）異質結構上進行低溫穿隧與約瑟夫森實驗提供了實務上的堅實基礎。

High-Tc cuprate superconductors such as Bi2Sr2CaCu2O8+δ (BSCCO) exhibit highly anisotropic electronic properties and unconventional pairing, making them a rich platform for studying superconductivity in layered materials. In parallel, van der Waals (vdW) heterostructures based on exfoliated two-dimensional (2D) crystals have emerged as a flexible route to engineer clean interfaces and novel junction geometries. Combining BSCCO with a conventional s-wave superconductor such as NbSe2 along the c-axis is a promising strategy for future Josephson experiments aimed at probing possible isotropic components in the cuprate order parameter.

In this thesis, I focus on the fabrication and normal-state electrical characterization of BSCCO-based vertical vdW junctions as a necessary step toward such experiments. First, single-flake BSCCO devices are fabricated with Au contacts and characterized using four-probe transport measurements. The R–T curve shows a single superconducting transition with Tc^90% ≈ 100 K, and I–V measurements down to 2 K confirm a robust superconducting state with critical currents of order 10^1 mA. These devices serve as benchmarks for contact quality and material integrity.

Second, BSCCO–BSCCO c-axis junctions are realized by stacking two exfoliated BSCCO flakes. Initial room-temperature junction resistances are in the megaohm (MΩ) range, indicating a thick or poorly coupled interfacial barrier. By applying controlled current annealing, the junction resistance is reduced by several orders of magnitude to the kilo-ohm (kΩ) range, with corresponding changes in the I–V characteristics. A four-probe measurement at 78 K reveals no clear Josephson supercurrent within the accessible bias range, suggesting that either the local Tc at the interface is suppressed or the barrier remains too opaque.

Finally, I demonstrate a BSCCO–NbSe2 vertical junction capped by an h-BN flake. Room-temperature two-probe measurements yield junction resistances of a few hundred ohms, substantially lower than in earlier BSCCO–NbSe2 attempts. This improvement is attributed to enhanced mechanical conformity and reduced interfacial gaps provided by the h-BN capping layer.

Although phase-coherent Josephson behavior is not observed in the present measurements, the devices and protocols established here significantly improve the transparency and reproducibility of BSCCO-based vdW junctions. These results provide a practical foundation for future low-temperature tunneling and Josephson experiments on BSCCO–NbSe2 heterostructures.