Cheng Di, Song Boqun, Kang Jong-Hoon, Sundahl Chris, Edgeton Anthony L, Luo Liang, Park Joong-Mok, Collantes Yesusa G, Hellstrom Eric E, Mootz Martin, Perakis Ilias E, Eom Chang-Beom, Wang Jigang
Department of Physics and Astronomy, Iowa State University, Ames, IA 50011, USA.
Ames National Laboratory-USDOE, Ames, IA 50011, USA.
Materials (Basel). 2023 Nov 3;16(21):7031. doi: 10.3390/ma16217031.
We obtain the through-thickness elastic stiffness coefficient () in nominal 9 nm and 60 nm BaFeAs (Ba-122) thin films by using picosecond ultrasonics. Particularly, we reveal the increase in elastic stiffness as film thickness decreases from bulk value down to 9 nm, which we attribute to the increase in intrinsic strain near the film-substrate interface. Our density functional theory (DFT) calculations reproduce the observed acoustic oscillation frequencies well. In addition, temperature dependence of longitudinal acoustic (LA) phonon mode frequency for 9 nm Ba-122 thin film is reported. The frequency change is attributed to the change in Ba-122 orthorhombicity (a-b)/(a+b). This conclusion can be corroborated by our previous ultrafast ellipticity measurements in 9 nm Ba-122 thin film, which exhibit strong temperature dependence and indicate the structural phase transition temperature .
我们通过皮秒超声技术获得了标称厚度为9纳米和60纳米的BaFeAs（Ba-122）薄膜的全厚度弹性刚度系数（）。特别地，我们揭示了随着薄膜厚度从块体值减小到9纳米，弹性刚度增加，我们将其归因于薄膜-衬底界面附近本征应变的增加。我们的密度泛函理论（DFT）计算很好地再现了观测到的声振荡频率。此外，还报道了9纳米Ba-122薄膜纵向声学（LA）声子模频率的温度依赖性。频率变化归因于Ba-122正交性（a-b）/（a+b）的变化。这一结论可以通过我们之前在9纳米Ba-122薄膜中的超快椭圆率测量得到证实，该测量显示出强烈的温度依赖性并表明了结构相变温度。
