Bulletin of the American Physical Society
Mid-Atlantic Section Fall Meeting 2020
Volume 65, Number 20
Friday–Sunday, December 4–6, 2020; Virtual
Session E04: Quantum Materials & Optics II |
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Chair: Wendell T. Hill III, University of Maryland, College Park |
Saturday, December 5, 2020 11:30AM - 12:06PM |
E04.00001: Terahertz spectroscopic studies of noncentrosymmetric magnets. Invited Speaker: Diyar Talbayev Magnetism is a rich and fascinating subject in condensed matter physics. We will present spectroscopic studies of magnetic insulators using terahertz (THz) frequency light. This frequency range is important in magnetism because it often hosts the spin resonances and spin waves found in many classes of magnetically ordered crystals. Combined with the ability to collect spectroscopic information at low temperatures and high magnetic fields, THz spectroscopy can serve as a tool for the determination of microscopic magnetic interactions that govern a material's magnetic properties. We will also introduce polarimetry of light and nonlinear optics as a way to interrogate the symmetries inherent in magnetic crystals. When magnetism occurs in noncentrosymmetric crystals, new kinds of optical phenomena often emerge, such as nonreciprocity. In nonreciprocal propagation, a crystal can be transparent to a light wave traveling in one direction and be completely opaque to the same wave traveling in the exact opposite direction. We will consider the necessary symmetry conditions and microscopic origins of the nonreciprocity. [Preview Abstract] |
Saturday, December 5, 2020 12:06PM - 12:42PM |
E04.00002: Unusual electronic and vibrational behavior in the high thermopower antimonides FeSb$_2$ and CrSb$_2$ Invited Speaker: Christopher Homes The antimonides FeSb$_2$ and CrSb$_2$ both exhibit an extraordinarily high thermoelectric power factor at low temperature; however, the origin of this behavior is still a subject of some debate, having been variously attributed to either electronic correlations or the phonon drag effect. % The optical properties of a material provide information about both the electronic and vibrational properties of a material; accordingly, the temperature dependence of the complex optical properties of single crystals of FeSb$_2$ and CrSb$_2$ have been measured along the principle optical axes over a wide frequency range. % At room temperature, the low-frequency optical conductivity of FeSb$_2$ is anisotropic, with the conductivity along the $b$ axis significantly larger than along either the $a$ or $c$ axes. Below $\simeq 100$ K the low-frequency conductivity decreases dramatically, but a step-like feature emerges along the $b$ axis at $\simeq 75$ meV in agreement with first principle calculations, suggesting one-dimensional behavior and signalling the presence of electronic correlations. The infrared-active modes are extremely narrow; curiously, the character of the lattice modes changes between $\simeq 100 - 200$ K, possibly indicating a weak structural distortion.\footnote{C. C. Homes, Q. Du, C. Petrovic, W. H. Brito, S. Choi, and G. Kotliar, Sci. Rep. \textbf{8}, 11292 (2018).} % The high thermopower observed in CrSb$_2$ is attributed to the phonon drag effect. The behavior of the optical conductivity is similar to that of FeSb$_2$, but there is no anisotropy or evidence of low-dimensional behavior. No phonon anomalies are observed in this material; however, just below $T_{\rm N}\simeq 273$ K a peculiar electronic mode appears at $\simeq 50$ meV, which rapidly softens and decreases in intensity at low temperature. [Preview Abstract] |
Saturday, December 5, 2020 12:42PM - 1:18PM |
E04.00003: Phonon Anomalies in Strained SrMnO$_{\mathrm{3}}$ Films Studied by THz and Infrared Ellipsometry Invited Speaker: Premysl Marsik 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. [Preview Abstract] |
Saturday, December 5, 2020 1:18PM - 1:30PM |
E04.00004: Backfolded acoustic phonons as ultrasonic probes in metal-oxide superlattices Fryderyk Lyzwa, Andrew Chan, Jarji Khmaladze, Katrin Fürsich, Bernhard Keimer, Christian Bernhard, Matteo Minola, Benjamin P. P. Mallett Ultrasound has been widely used as an incisive probe of internal interfaces. For thin-film structures however, it is largely ineffective because the signal is dominated by the substrate. Using confocal Raman spectro-microscopy, we show that multiple reflection of sound waves at internal interfaces of a metal-oxide superlattice generates standing waves that are insensitive to the substrate. These backfolded acoustic phonon modes are sensitive to atomic-scale thickness variations of the sublayers and thus serve as a powerful characterization tool for metal-oxide superlattices. [Preview Abstract] |
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