利用飛秒期激發─探測技術研究高溫超導體在低溫下之超快動力學

Abstract

利用自行建立的飛秒級被動式鎖模鈦一藍寶石雷射，配合激發 一探測系統以及一套開放式冷卻系統，針對不同氧含量的鋇銅氧 高溫超導薄膜，我們已成功的量測在不同溫度下反射率△R/R的瞬態 時間弛緩現象。 在120K到20K溫度改變下，利用 1.6eV的探測能量對不同 氧含量。鋇銅氧薄膜量測微分反射率之瞬時變化，實驗結果發現對 滿氧薄膜的量測與早期利用2eV的探測能量所得結果不同，而對氧 含量為6.8、6.7以及6．0的薄膜其結果則相似，此差異可解釋為: 樣品受光激發後，在超導態費米能階附近態密度的急劇變化，能否 被探測到而定。我們亦對量測結果曲線做適配，得到超導態的弛緩 時間較正常態要長，以及超導能瞭或準粒子隨溫度之變化不如 B CS理論預測那麼快速。 另外對 鋇銅氧的量測得到在室溫時△R/R隨時間弛緩的曲線其 符號為正，到溫度為 240K後開始有轉變為負號的趨勢，直到220K以 下則完全呈負號，我們對此的解釋為電荷轉移能隙的寬度會隨溫度 下降而變窄所導致的結果。

We have preformed a series of transient reflectivity(△R/R) measurements on oxygen-controlled YBCO thin films at different temperatures. A home-made femtosecond pump-probe system including a passively mode-lock Ti:sapphire laser and a continuous transfer cold finger cryostat were used for these measurement. The △R/R curves versus delay time of the oxygen-rich YBa2Cu3Ox (x=6.95) film for temperature ranging from 120K to 20Kwere different from those reports previously. The difference may due to the fact that they used different photon energy (~2eV) for the measurements and the drastically changes of density of states near Fermi level caused by pump beam could be probed. We also find the relaxation time has an abrupt change and become longer at superconducting state for oxygen-rich film while the relaxation time increases slightly with decreasing temperature for oxygen-deficiency (x=6.0) film. Additionally, we also measured the transient differential reflectivity of PBCO thin film. The sign of △R/R at room temperature is positive but it has a tendency to become negative at 240K and finally becomes negative below 220K. The change in sign if △R/R may due to the width of charge-transfer gap decreases when the temperature decreases.