銅氧化物金屬與絕緣態的相變

Abstract

我們已系統性地將YBa2Cu3Ox及YBa2Cu2.9Zn0.1Ox薄膜由滿氧的狀態(x =7) 轉變成絕緣狀態(x =6.1)。在這過程中，我們小心地只是將氧從銅氧鍵慢慢抽離。絕緣樣品的電阻率在1.5 K 到300 K的溫間測量，比較載子數(氧含量)及雜質(摻Zn)對傳輸的影響。經過處理顯示YBa2Cu3O6.2及YBa2Cu2.9Zn0.1O6.3 從極低溫至200K的電阻率隨溫度的變化與Mott所預測強侷限性中的改變區域跳躍2D系統異常地符合。同時，我們在溫度低於20 K時，對YBa2Cu3O6.2及YBa2Cu2.9Zn0.1O6.3薄膜延c軸加0T至8T的磁場，觀察樣品的磁阻，發現異常的正磁阻的現象。

We have systematically converted superconducting YBa2Cu3Ox and YBa2Cu2.9Zn0.1Ox (x=7) to insulating samples (x=6.1). In the process, oxygen of CuO chains was carefully removed. Resistivity of the insulating samples was measured between 1.5 and 300 K. Effects of the carrier concentration and Zn doping have been studied. Measurements of □(T) below 200 K reveal a two-dimensional Mott variable range hopping (VRH). Furthermore, we have measured magnetoresisitivity of YBa2Cu3O6.2 and YBa2Cu2.9Zn0.1O6.3 at low temperatures with magnetic fields up to 8 T along the c-axis. In small magnetic fields, an intriguing positive magnetoresistivity was observed in both samples. This phenomenon can not be well explained in the context of Mott VRH.