我們的工作為研究韓國浦項大學的Sung-IK Lee 等人大約兩年前
提供的二硼化鎂（MgB2）薄膜樣品，在長時間的保存之下，樣品的
組成及性質的變化。

我們使用超導量子干涉儀在外加磁場-50000 Oe 到+50000 Oe 下
測量樣品的磁滯曲線，並且計算出臨界電流密度Jc。在溫度5K 時，
樣品的臨界電流密度約為6.4×10^6 A/cm2 ，將數據與韓國浦項大學的
Sung-IK Lee 等人發表的文獻中比較，發現在長時間放置之下，我們
測得的樣品的臨界電流密度降低了將近一個數量級。

由X光繞射儀中測得的數據發現，樣品有些微的氧化鎂（MgO）產生，
接著計算晶格常數，c 軸的長度些微縮短了。而在X 光近緣吸收光譜，
我們將同一片樣品在兩年前與最近測量的螢光產額（TFY）數據比較，
樣品上的MgO 與B2O3 的訊號都比兩年前增強了許多，且樣品的品質似乎
有些微的變差。

長時間置放後的樣品的臨界電流密度沒有因為包含雜質（MgO、B2O3）而上升，可能是由於這些雜質幾乎都在樣品的表面，這個與測量B2O3 的X 光近緣吸收光譜的電子產額（TEY）結果一致。

We report our work on the study of the change of MgB2 thin film after long-time storage(the ageing film). The film was prepared by Sung-IK et al. of Pohang University of Science and Technology, Korea about two years ago.

The hysteresis of the film in the magnetic field range of -50000 Oe to +50000 Oe was obtained by a superconducting quantum interference device (SQUID). The critical current density Jc was deduced by using Bean model and it was found to be 6.4×10^6 A/cm2 at 5 K. The critical current density Jc becomes about an order of magnitude smaller than the value published by Sung-IK Lee et al. in PRL87,087002(2001).

The X-ray diffraction measurements showed that the ageing film contains MgO. The lattice constant deduced from it indicates that the lattice constant c is a little shorter than that of the fresh film. The combined data of the X-ray diffraction and the X-ray near-edge absorption spectra suggests that the ageing film of MgB2 oxidized gradually in two years and some MgO and B2O3 had been produced.

The impurities of MgO and B2O3 in the ageing film do not increase the current density. It is possible that these impurities are mainly on the surface of the sample. This is consistent with the observations in the X-ray near-edge absorption spectra deduced from the sample current of B
K edge, which shows mainly the signals of B2O3.

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