Abstract
研究 YPrBaCuO的Tc隨鐠含量增加而降低的方法，一般都是測量其電阻、 磁矩、比熱、紅外線吸收光譜及拉曼散射光譜，而本文主要討論YPBCO/金 屬平面結 的穿隧特性，以便了解高溫超導機制。我們使用雷射蒸鍍法在 MgO基板上製作YPBCO薄膜，製作出來之 YBCO薄膜Tc0高達91K，由RHEED繞 射條紋可知YBCO(100)或(010)方向 平行MgO(100)。而YPBCO 薄膜之Tc隨 pr增加而減小，當X>=0.5 為非超導。 我們共製作了自然位障的YPBCO/Pb ，,YPBCO╱Au ，YPBCO/A1及 YPBCO/In等四種結，利用電橋電路及AC解調 之方法量取結之I-V,(dI dV)-V,(dI/dV)-V曲線及其隨溫度變化情形。 YPBCO/Pb結的特性：(a)4.2k時均能觀測鉛的超導能隙及電子聲子強耦合 作用，且鉛之能隙不隨X而改變，(b)X<0.2時YPBCO超導及電子聲子耦合作 用均非常清晰，C軸能隙/KbTc=1.5,a.b軸能隙/KbTc=4.9 (c)YPBCO/Pb應 為YPBCO/N/I/Pb,YPBCO/N/Pb及YPBCO/N/I/Pb等三種結並聯所組成，(d) YPBCO/Pb結之(X<0.2)時能隙不隨X而改變，而X>0.2 (dI/dV)-V上超導結 構非常微弱，幾不可見，(e)YPBCO/Pb之(dI/dV)- V 曲線於溫度為鉛之 Tc 時突然上升,而溫度介於鉛之Tc與YPBCO之Tc時，其曲線斜率為正。 YPBCO/Au 結於X=0.0及0.1時，其(dI/dV)-V曲線在零偏壓生發生極大值, 其起因可能是Andreev反射。YPBCO/A1結在零偏壓時(dV/dI)。非常大，此 現象只能以Giaever-Zeller之二階段穿隧模式解釋。YPBCO/In 亦可見 YPBCO 之超導結構。所有結的(dI/dV)。 -T曲線於YPBCO 之Tc時均突然上 升，視結電阻與薄膜電阻比值決定上升大小，此乃因電流重新分佈之故。 Tunneling characteristics of planar tunneling junctions on Y1- xPrxBaCuO films are presented.The Y1-xPrxBaCuO films with x=0-0.45 were prepared by laser ablation on MgO(100) substrates Pb,Au,In or Al counter-electrode was deposited on top of native barrier. The tunneling characteristics at various temperatures were measured. The gap-like structures of YBCO as well as Pb energy gap and phonon structures were observed in YBCO/Pb junction at 4.2 K.Less pronounced structures were observed in the case of YPBCO junctions. Junctions with In and Au counter- electrodes have similar characteristics as that of Pb,except that Au junctions with small resistance develop an Andreev reflection at zero bias. For YPBCO/Al junctions, a large resistence peak around zero bias at low temperature was observed.