弱鐵磁超導性 RuSr2RCu2O8 (R = Gd, Eu, Sm, and Nd) 之光譜性質研究

Abstract

我們研究 RuSr2RCu2O8 (R = Gd, Eu, Sm, and Nd) 多晶樣品的光譜性質。RuSr2GdCu2O8 樣品的室溫拉曼散射光譜與之前的論文 [Physica C 341, 2209 (2000)] 結果相當吻合，以離子半徑較大的稀土元素 (R = Eu, Sm, and Nd) 取代 Gd 離子時，可觀察到 A1g(5) 及 A1g(6) 振動模的頻率位置隨著置換離子半徑增大出現紅移現象且半高寬增大，印證了中心離子變大造成單位晶胞體積增加的趨勢。

其次，我們研究這些樣品的變溫全頻反射光譜。室溫時，由光譜圖中可觀察到 RuSr2RCu2O8 (R = Gd, Eu, and Sm) 三個樣品在低頻處有居德響應，表示樣品呈現金屬性。隨著溫度的降低，RuSr2GdCu2O8 樣品的 638 cm-1 振動模及 RuSr2EuCu2O8 樣品的 656 cm-1 振動模頻率位置出現藍移現象且半高寬變窄。相反地，RuSr2NdCu2O8 樣品的 346、583 及 648 cm-1 振動模隨溫度降低出現紅移現象，我們猜測這些聲子頻率的位移可能是因為電子自旋與聲子耦合的影響，或是在降溫後樣品結構上出現局部相轉變有關。在超導相變溫度以下時，我們利用偏總和定則，計算出有效電荷數的減少量，並進一步估算出 RuSr2GdCu2O8 與 RuSr2EuCu2O8 的倫敦穿透深度 (λL) 約為 9410 Å 與 23805 Å。

We study the optical properties of weak-ferromagnetic superconductor system RuSr2RCu2O8 (R = Gd, Eu, Sm, and Nd). The room-temperature Raman spectrum of RuSr2GdCu2O8 is similar to the reported results [Physica C 341, 2209 (2000)]. When replaced with larger rare earth ions (R = Eu, Sm, and Nd) on Gd, the frequencies of A1g(5) and A1g(6) Raman-active phonons show a shift of the peak position to lower frequencies and their linewidths are broadening, which is mainly due to the expansion effects of the unit cell volume.

The optical reflectance spectra of RuSr2RCu2O8 (R = Gd, Eu, Sm, and Nd) were measured between 10 and 340 K. At room temperature, the spectra of RuSr2RCu2O8 (R = Gd, Eu, and Sm) show the characteristics of metallic behavior at low frequencies with Drude response. With decreasing temperature, the phonon modes at about 638 cm-1 in RuSr2GdCu2O8 and 656 cm-1 in RuSr2EuCu2O8 harden and become narrowing. On the contrary, the frequencies of infrared-active phonon modes at about 346, 583, and 648 cm-1 in RuSr2NdCu2O8 shift toward lower frequency, which may be attributed to a strong spin-phonon interaction or a local lattice distortion. In the superconducting state, a partial sum-rule evaluation of the effective number of carriers from the optical conductivity indicates that the values of London penetration depth (λL) of RuSr2GdCu2O8 and RuSr2EuCu2O8 are about 9410 and 23805 Å, respectively.