Study of Elastic and Structural Properties of BaFe2As2 Ultrathin Film Using Picosecond Ultrasonics

Di Cheng; Boqun Song; Jong-Hoon Kang; Chris Sundahl; Anthony L Edgeton; Liang Luo; Joong-Mok Park; Yesusa G Collantes; Eric E Hellstrom; Martin Mootz; Ilias E Perakis; Chang-Beom Eom; Jigang Wang

doi:10.3390/ma16217031

Study of Elastic and Structural Properties of BaFe₂As₂ Ultrathin Film Using Picosecond Ultrasonics

Materials (Basel). 2023 Nov 3;16(21):7031. doi: 10.3390/ma16217031.

Authors

Di Cheng^{1

2}, Boqun Song^{1

2}, Jong-Hoon Kang³, Chris Sundahl³, Anthony L Edgeton³, Liang Luo^{1

2}, Joong-Mok Park^{1

2}, Yesusa G Collantes⁴, Eric E Hellstrom⁴, Martin Mootz⁵, Ilias E Perakis⁵, Chang-Beom Eom³, Jigang Wang^{1

2}

Affiliations

¹ Department of Physics and Astronomy, Iowa State University, Ames, IA 50011, USA.
² Ames National Laboratory-USDOE, Ames, IA 50011, USA.
³ Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, WI 53706, USA.
⁴ Applied Superconductivity Center, National High Magnetic Field Laboratory, Florida State University, Tallahassee, FL 32310, USA.
⁵ Department of Physics, University of Alabama at Birmingham, Birmingham, AL 35294-1170, USA.

Abstract

We obtain the through-thickness elastic stiffness coefficient (C₃₃) in nominal 9 nm and 60 nm BaFe₂As₂ (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 T_s.

Keywords: BaFe2As2; intrinsic strain; longitudinal acoustic phonon; through-thickness elastic stiffness coefficient (C33); ultrathin film.

Abstract

Grants and funding