Bulletin of the American Physical Society
88th Annual Meeting of the Southeastern Section of the APS
Volume 66, Number 16
Thursday–Saturday, November 18–20, 2021; University Center Club, Florida State University, Tallahassee, Florida
Session K02: Condensed Matter II |
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Chair: Kaya Wei, NHMFL Room: West Ballroom |
Friday, November 19, 2021 11:00AM - 11:30AM |
K02.00001: Probing spin-phonon coupling in magnetic materials using magneto-Raman spectroscopy Invited Speaker: Komalavalli Thirunavukkuarasu Combining spectroscopy with one or more external parameters such as low temperature, high pressure, and high magnetic fields, allows us to continuously tune correlations to probe properties of materials across their phase diagram. Recently, we employed magneto-Raman spectroscopy on two different kinds of magnetic materials. The first compound is SrCu$_{2}$(BO$_{3})_{2}$. It is a quasi-2D orthogonal spin dimer system with a spin singlet ground state and is a typical example for the Shastry-Sutherland model. It exhibits a sequence of magnetization plateau at magnetic fields higher than 20 T. The unique behavior results from interplay between geometrical frustration and quantum fluctuations. I will discuss the origin of the strong spin-lattice coupling in SrCu$_{2}$(BO$_{3})_{2}$ revealed by Raman studies at high magnetic fields up to 45T. The second compound is the multiferroic metal organic framework [(CH$_{3})_{2}$NH$_{2}$]Co(HCOO)$_{3}$ belonging to the family of MOFs comprised of methylammonium (A$=$ (CH$_{3})_{2}$NH$_{2})$ and metal (B$=$Co, Cu, Fe, Mn, Ni) cations with a formate (X$=$HCOO$_{3})$ anion. Several efforts have been made to understand the exchange interactions in these functional materials including magnetization at high magnetic fields up to 60 T and infrared spectroscopy at magnetic fields up to 35 T. In the infrared studies under applied magnetic fields, it was concluded that Co complex adopts a different mechanism for facilitating saturation of magnetic states by involving formate stretching distortions unlike other complexes in the family that use the formate bending mode. In this talk, I will discuss our Raman spectroscopy on [(CH$_{3})_{2}$NH$_{2}$]Co(HCOO)$_{3}$ at magnetic fields up to 31T to probe the magneto-elastic coupling. This work has been performed at the user facilities in the National High Magnetic Field Laboratory (NHMFL), Tallahassee. The NHMFL is supported by the National Science Foundation through NSF/DMR-1644779 and the state of Florida. The project is also funded by DoN HBCU/MI program award {\#} N000141713061. [Preview Abstract] |
Friday, November 19, 2021 11:30AM - 11:42AM |
K02.00002: High-Field EPR Study of the High- and Low-Spin States of a Mn$^{3+}$ Complex Exhibiting a Sharp Spin-Crossover Transition Brittany Grimm, Irina Kuehne, Conor Kelly, Grace Morgan, Stephen Hill Spin crossover (SCO) transitions occur in certain molecular complexes of octahedrally coordinated 3d$^{4}$ to 3d$^{7}$ transition metals and can be induced through variations in temperature, pressure, or via optical perturbations. In many manganese SCO complexes, it can be too costly energetically to convert all sites within a crystal from high-spin (HS) to low-spin (LS). Consequently, the transition often occurs gradually, with only a fraction of the sites converting, resulting in mixed LS/HS phases. It is challenging to characterize these mixed phases spectroscopically due to their inhomogeneous nature. The Mn$^{3+}$ complex considered in this investigation exhibits a complete 100{\%} HS ($S$~$=$~2) to 100{\%} LS ($S$~$=$~1) transition below a relatively sharp transition temperature (T$_{1/2} \quad =$ 51~K, with \textless 10~K hysteresis), allowing for a more straightforward characterization of both spin states. The magnetic properties of octahedrally coordinated Mn$^{3+}$ complexes are dominated by the combined influences of anisotropic crystal-field and spin-orbit interactions, which are often described in terms of 2$^{nd}$-order axial and rhombic zero-field splitting (ZFS) terms in an effective spin Hamiltonian. We employ high-field EPR in order to accurately characterize the ZFS interactions in both the HS and LS states, and rationalize these on the basis of the corresponding structures. [Preview Abstract] |
Friday, November 19, 2021 11:42AM - 11:54AM |
K02.00003: Martensitic transformation in V$_{3}$Si single crystal: $^{51}$V NMR evidence for coexistence of cubic and tetragonal phases Albert A. Gapud, Arneil P. Reyes The Martensitic transformation (MT) in A15 binary-alloy superconductor V$_{3}$Si is a second-order, displacive structural transition from cubic to tetragonal symmetry, at temperature T$_{m}$ a few K above the superconducting transition temperature T$_{c} \quad =$ 17 K. Though studied extensively, the MT has not yet been conclusively linked with a transition to superconductivity, and remains relevant, e.g. due to renewed interest in soft phonon modes, while V$_{3}$Si continues to be of interest, e.g. due to similarities with Fe-As superconductors. Previous NMR studies on the MT in V$_{3}$Si have mainly been on powder samples, and with little emphasis on temperature dependence during the transformation. Here we study a high-quality single crystal, where quadrupolar splitting and Knight shift of NMR spectra for $^{51}$V allowed us to distinguish between spectra from transverse chains of V as a function of temperature. This revealed evidence of the coexistence of untransformed cubic phase and transformed tetragonal phase over a few K below and above T$_{m}$, and that the Martensitic lengthening of one axis occurs predominantly in a plane perpendicular to the crystal growth axis, as twinned domains. More details on the effects on the electric field gradient and the hyperfine field due to spin/orbital susceptibility of electrons are also discussed. [Preview Abstract] |
Friday, November 19, 2021 11:54AM - 12:06PM |
K02.00004: Intrinsic optical absorption and d.c. conductivity in Dirac metals Adamya Goyal, Prachi Sharma, Dmitrii Maslov In an ideal Dirac metal, optical absorption is absent for frequencies below the Pauli threshold (twice the Fermi energy). In real systems, however, e.g., in doped graphene, both optical absorption [1] and Raman scattering [2] find a very broad transition region around the Pauli threshold. While a number of extrinsic damping mechanisms were proposed to explain this observation in the past, we argue that the effect can be explained by an intrinsic mechanism -- Auger-like recombination of optically excited minority carriers with equilibrium majority carriers. The idea goes back to a similar mechanism proposed for doped gapped semiconductors by Gavoret et al [3]. The width of the transition region in this mechanism is comparable to the Fermi energy. We also discuss certain electron-hole processes that give a \begin{figure}[htbp] \centerline{\includegraphics[width=0.15in,height=0.16in]{300920211.eps}} \label{fig1} \end{figure} scaling to the d.c. conductivity and could possibly be detected in 3-dimensional Dirac systems. \newline \newline [1] Li, Z.,~et al.~Nature Phys.~4, 532--535 (2008) \newline [2] E. Riccardi,~et al. Phys. Rev. Lett.~116, 066805 (2016) \newline [3] J. Gavoret,~et al. Journal de Physique, 1969, 30 (11-12), pp.987-997. [Preview Abstract] |
Friday, November 19, 2021 12:06PM - 12:18PM |
K02.00005: Hyperspectral mapping of a single grain of hybrid perovskite using cathodoluminescence microscopy Ethan Taylor, Vasudevan Iyer, Bibek Dhami, Clay Klein, Benjamin Lawrie, Kannatassen Appavoo Lead-halide perovskites have gained significant interest in optoelectronics over the last few years. While the performance of hybrid perovskite photovoltaics can now rival those fabricated from silicon, the role of grain boundaries is still under debate. While it is generally assumed that grain boundaries are detrimental due to an increase in non-radiative recombination, in some cases grain boundaries can enhance spatial separation of charge carriers and hence boost device performance. Here we map a single grain of hybrid perovskite using optical emission and cathodoluminescence microscopy to understand the role of grain boundaries on emission below the diffraction limit. The hyperspectral data obtained are decoded with non-negative matrix factorization, revealing components relating to primary band-edge emission, photon recycling, and defect emission. [Preview Abstract] |
Friday, November 19, 2021 12:18PM - 12:30PM |
K02.00006: Magnetic Ordering in GdAuAl$_4$Ge$_2$ and TbAuAl$_4$Ge$_2$: layered compounds with triangular lanthanide nets K. Feng, I. A. Leahy, K. Wei, W.L. Nelson, O. Oladehin, J.R. Galeano-Cabral, M. Lee, R. Baumbach We recently reported results for the weakly correlated $f$-electron metal CeAuAl$_4$Ge$_2$, where the atomic arrangement of the cerium ions creates the conditions for possible geometric frustration.[Z. Sheng,.et al, 2017] Although this compound does not show clear evidence for unusual behavior, this motivated us to investigate the broader lanthanide series $Ln$AuAl$_4$Ge$_2$ ($Ln$ = Pr - Tm) where magnetic interactions might be enhanced. We used an aluminum molten metal flux method to produce the single crystals throughout the series, and powder X-ray diffraction measurements show (1) the crystals form in the same rhombohedral structure as the Ce analogue and (2) the obtained lattice parameters ($a$ and $c$) and unit cell volume $v$ are consistent with a trivalent lanthanide contraction. Here we will focus on the $Ln$ = Gd and Tb examples, where magnetization, heat capacity, and electrical transport measurements reveal complex magnetic ordering at low temperatures. Evidence is also seen for magnetic frustration in the form of strong magnetic fluctuations (observed in the heat capacity) at temperatures well above the bulk ordering temperatures. Finally, we will present the temperature - magnetic field phase diagrams, which each feature several regions with distinct ordered states. [Preview Abstract] |
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