Browsing by Author "Kim, Hyun Ho"
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Item Magnetic-Field-Induced Quantum Phase Transitions in a van der Waals Magnet(2020) Li, Siwen; Ye, Zhipeng (TTU); Luo, Xiangpeng; Ye, Gaihua (TTU); Kim, Hyun Ho; Yang, Bowen; Tian, Shangjie; Li, Chenghe; Lei, Hechang; Tsen, Adam W.; Sun, Kai; He, Rui (TTU); Zhao, LiuyanExploring new parameter regimes to realize and control novel phases of matter has been a main theme in modern condensed matter physics research. The recent discovery of two-dimensional (2D) magnetism in nearly freestanding monolayer atomic crystals has already led to observations of a number of novel magnetic phenomena absent in bulk counterparts. Such intricate interplays between magnetism and crystalline structures provide ample opportunities for exploring quantum phase transitions in this new 2D parameter regime. Here, using magnetic field- and temperature-dependent circularly polarized Raman spectroscopy of phonons and magnons, we map out the phase diagram of chromium triiodide (CrI3) that has been known to be a layered antiferromagnet (AFM) in its 2D films and a ferromagnet (FM) in its three-dimensional (3D) bulk. However, we reveal a novel mixed state of layered AFM and FM in 3D CrI3 bulk crystals where the layered AFM survives in the surface layers, and the FM appears in deeper bulk layers. We then show that the surface-layered AFM transits into the FM at a critical magnetic field of 2 T, similar to what was found in the few-layer case. Interestingly, concurrent with this magnetic phase transition, we discover a first-order structural phase transition that alters the crystallographic point group from C3i (rhombohedral) to C2h (monoclinic). Our result not only unveils the complex single-magnon behavior in 3D CrI3, but it also settles the puzzle of how CrI3 transits from a bulk FM to a thin-layered AFM semiconductor, despite recent efforts in understanding the origin of layered AFM in CrI3 thin layers, and reveals the intimate relationship between the layered AFM-to-FM and the crystalline rhombohedral-to-monoclinic phase transitions. These findings further open opportunities for future 2D magnet-based magnetomechanical devices.Item Observation of the polaronic character of excitons in a two-dimensional semiconducting magnet CrI3(2020) Jin, Wencan; Kim, Hyun Ho; Ye, Zhipeng (TTU); Ye, Gaihua (TTU); Rojas, Laura (TTU); Luo, Xiangpeng; Yang, Bowen; Yin, Fangzhou; Horng, Jason Shih An; Tian, Shangjie; Fu, Yang; Xu, Gongjun; Deng, Hui; Lei, Hechang; Tsen, Adam W.; Sun, Kai; He, Rui (TTU); Zhao, LiuyanExciton dynamics can be strongly affected by lattice vibrations through electron-phonon coupling. This is rarely explored in two-dimensional magnetic semiconductors. Focusing on bilayer CrI3, we first show the presence of strong electron-phonon coupling through temperature-dependent photoluminescence and absorption spectroscopy. We then report the observation of periodic broad modes up to the 8th order in Raman spectra, attributed to the polaronic character of excitons. We establish that this polaronic character is dominated by the coupling between the charge-transfer exciton at 1.96 eV and a longitudinal optical phonon at 120.6 cm−1. We further show that the emergence of long-range magnetic order enhances the electron-phonon coupling strength by ~50% and that the transition from layered antiferromagnetic to ferromagnetic order tunes the spectral intensity of the periodic broad modes, suggesting a strong coupling among the lattice, charge and spin in two-dimensional CrI3. Our study opens opportunities for tailoring light-matter interactions in two-dimensional magnetic semiconductors.Item Raman fingerprint of two terahertz spin wave branches in a two-dimensional honeycomb Ising ferromagnet(2018) Jin, Wencan; Kim, Hyun Ho; Ye, Zhipeng (TTU); Li, Siwen; Rezaie, Pouyan (TTU); Diaz, Fabian (TTU); Siddiq, Saad (TTU); Wauer, Eric (TTU); Yang, Bowen; Li, Chenghe; Tian, Shangjie; Sun, Kai; Lei, Hechang; Tsen, Adam W.; Zhao, Liuyan; He, Rui (TTU)Two-dimensional (2D) magnetism has been long sought-after and only very recently realized in atomic crystals of magnetic van der Waals materials. So far, a comprehensive understanding of the magnetic excitations in such 2D magnets remains missing. Here we report polarized micro-Raman spectroscopy studies on a 2D honeycomb ferromagnet CrI3. We show the definitive evidence of two sets of zero-momentum spin waves at frequencies of 2.28 terahertz (THz) and 3.75 THz, respectively, that are three orders of magnitude higher than those of conventional ferromagnets. By tracking the thickness dependence of both spin waves, we reveal that both are surface spin waves with lifetimes an order of magnitude longer than their temporal periods. Our results of two branches of high-frequency, long-lived surface spin waves in 2D CrI3 demonstrate intriguing spin dynamics and intricate interplay with fluctuations in the 2D limit, thus opening up opportunities for ultrafast spintronics incorporating 2D magnets.