鉈鉛 (1212) 系統的稀土磁有序及超導性研究

Abstract

在本論文中，我們詳細地討論了有關於具有 TlBa2CaCu2O7 (1212) 結構 之 TlBa2RCu2O7 (R=Pr,Nd,Sm,Eu,Gd,Dy,Ho,Er, Tm,Yb,Lu)的低溫磁性質 。稀土離子R 對Ca離子之置換使超導體中載子濃度降低，造成TlBa2(Ca, R)Cu2O7之超導性由overdop到 underdop，以至於形成非超導之絕緣態， 此金屬-絕緣相轉變約發生在稀土離子一半取代Ca之時。在TlBa2EuCu2O7 材料中，異常的磁化率行為可以Van-Vleck的順磁性解釋之，即一非磁性 之基態加上非常接近之其它激發態 此外，在TlBa2RCu2O7(R=Nd,Sm, Dy=0.94,Ho,Er,Tm)中，除了Pr和Gd外沒有其它稀土離子之反鐵磁溫度超 過2K。在磁化率比熱及中子繞射之研究中證實TlBa2PrCu2O7中Pr具有相當 高的反鐵磁溫度(8K)，遠高於TlBa2GdCu2O7的2.2 K為了與PrBa2Cu3O7比 較，載子與不同之鹼土族置換效應將在本論文中討論，為此我們製做了 TlSr2RCu2O7 (R = Pr, Gd) 及 (Pb,Cu)Sr2RCu2O7 (R = Pr, Gd)，我們 發現在1212系統中，對 Pr之反鐵磁溫度而研，載子濃度是一次要因素， 但若Ba被Sr取代將造成反鐵磁溫度之劇烈變化。在Rietveld Refinement 之研究中我們發現Sr之置換造成c 軸之縮短但Pr-O之距離反而增長。此結 果強烈地證明Pr之異常反鐵磁溫度及PrBa2Cu3O7之非超導性乃由Pr(4f)和 O(2p)之軌道雜化所造成。另外在TlBa2GdCu2O7上，由於Gd-Gd ion 之強 烈偶極矩做用力，使得在這些兩層銅氧面之超導相關材料中Gd之反鐵磁溫 度不受結構或載子濃度之影響，這近乎不變之 Gd 反鐵磁溫度支持了Gd之 反鐵磁機制來自於一個二維之高度異向性偶極矩做用力，另外比熱上在反 鐵磁溫度下之肩型變化，乃由於強烈之自旋擾動所造成之低溫能級分裂所 致。 Low temperature magnetic study for the insulating TlBa2RCu2O7-y (R=Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu and Y) system with the TlBa2CaCu2O7-y type (1212) structure are reported. For tlBa2EuCu2O7 compound, anomalous temperature dependent of magnetic susceptibility can be described by a nonmagnetic Eu3+ ground state (J = 0) with closely spaced magnetic excited state (J > 0). No rare earth except, Pr and Gd, magnetic transitions above 2 K are observed form magnetic rare earth compounds TlBa2 RCu2O7 (R = Nd, Sm, Dy = 0.94 K, Ho, Er, Tm, Yb). Magnetic susceptibility heat capacity, neutron and x-ray diffraction measurements indicate an anomalously high antiferromagnetic Neel temperature TN(Pr) = 8K with an onset around 10 K for TlBa2 PrCu2O7 as compared to TN(Gd) = 2.2 K for TlBa2GdCu2O7. Detail neutron diffraction data show complex magnetic behavior around TN(Pr). In comparing with the anomalous Pr ordering in PrBa2Cu3 O7 system, charge carrier, and different ion size of alkali earth ion substitution effect are considered. For that, TlSr2 RCu2O7 (R = Pr and Gd) and (Pb,Cu)Sr2RCu2O7 (R = Pr and Gd) compounds are synthesized. In this 1212 system , charge carrier has minor effect on TN, but Sr substitution for Ba significantly reduced the magnetic Pr ordering temperature. The Rietveld x-ray refinement results show that Pr-O bond length is enlarged as Ba is replaced by Sr , alothough the c-axis parameter become shoter. These results strongly support that the hybridization of Pr(4f) and O(2p) orbitals are the mechanism of Pr ordering and superconductivity supression in PrBa2Cu3O7-y. The strong dipolar interaction of Gd-Gd ions maintains the TN(Gd) near 2.2 K not affected by the structure change and any charge carrier effect. This near constant ordering temperature in the double-CuO2 layer Gd containing cuprates suggests that the interaction is highly anisotropic (quasi-2D). A broad heat capacity shoulder below TN(Gd) indicates strong spin fluctuation of the large Gd spins