Mid-Atlantic Section Fall Meeting 2020
Volume 65, Number 20
Friday–Sunday, December 4–6, 2020;
Virtual
Session E04: Quantum Materials & Optics II
11:30 AM–1:30 PM,
Saturday, December 5, 2020
Chair: Wendell T. Hill III, University of Maryland, College Park
Abstract: E04.00003 : Phonon Anomalies in Strained SrMnO$_{\mathrm{3}}$ Films Studied by THz and Infrared Ellipsometry
12:42 PM–1:18 PM
Preview Abstract
Abstract
Author:
Premysl Marsik
(University of Fribourg)
I will present the case for investigating the optical response of thin,
strained and ultrathin films with spectroscopic ellipsometry in the
far-infrared and THz range. Spectroscopic ellipsometry is usually associated
with film thickness measurements with commercially available tools operating
around visible part of electromagnetic spectrum. At the University of
Fribourg, we have built ellipsometers based on Fourier-transformed infrared
(FTIR) and Time Domain Terahertz (TD-THz) spectroscopies.
I will briefly introduce the techniques and discuss specifics of
ellipsometry on ultrathin films - ultrathin in the sense of film thickness
being much smaller than the wavelength.
SrMnO$_{\mathrm{3}}$ in bulk is a cubic perovskite with antiferromagnetic
ordering under T$_{\mathrm{N}}=$230-260 K. Under epitaxial strain it is
possible to stabilize polar order caused by off-center displacement of the
central magnetic Mn$^{\mathrm{4+}}$ ion. Such multiferroic state is expected
to show large magnetoelectric coupling. The strong interaction between the
spin ordering and lattice phonons has been demonstrated on bulk
Sr$_{\mathrm{1-x}}$Ba$_{\mathrm{x}}$MnO$_{\mathrm{3}}$ (x $=$ 0-0.3).
Our study was motivated by the possibility to see changes of the phonon
spectra upon entering the ferroelectric state.
We prepared a series of 30 nm thick SrMnO$_{\mathrm{3}}$ films by pulsed
laser deposition (PLD) with varying epitaxial strain driven by lattice
mismatch of SrMnO$_{\mathrm{3}}$ with respect to substrate. The substrates
were chosen accordingly: LaAlO$_{\mathrm{3}}$ causing small compressive
strain (-0.3{\%}), tetragonal SrLaGaO$_{\mathrm{4}}$ (001) with moderate
1.1{\%} tensile strain and cubic LSAT with tensile strain of 1.8{\%} that
should be sufficient for the ferroelectric instability.
In the temperature dependent (10-400 K), THz-FIR ellipsometric spectra we
observe the three characteristic phonons of cubic perovskite, with dominant
low-energy mode that shows softening with increasing strain and anomaly at
temperature of the antiferromagnetic transition.