Browsing by Author "Ye, Zhipeng (TTU)"
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Item Fermion–boson many-body interplay in a frustrated kagome paramagnet(2020) Yin, J. X.; Shumiya, Nana; Mardanya, Sougata; Wang, Qi; Zhang, Songtian S.; Tien, Hung Ju; Multer, Daniel; Jiang, Yuxiao; Cheng, Guangming; Yao, Nan; Wu, Shangfei; Wu, Desheng; Deng, Liangzi; Ye, Zhipeng (TTU); He, Rui (TTU); Chang, Guoqing; Liu, Zhonghao; Jiang, Kun; Wang, Ziqiang; Neupert, Titus; Agarwal, Amit; Chang, Tay Rong; Chu, Ching Wu; Lei, Hechang; Hasan, M. ZahidKagome-nets, appearing in electronic, photonic and cold-atom systems, host frustrated fermionic and bosonic excitations. However, it is rare to find a system to study their fermion–boson many-body interplay. Here we use state-of-the-art scanning tunneling microscopy/spectroscopy to discover unusual electronic coupling to flat-band phonons in a layered kagome paramagnet, CoSn. We image the kagome structure with unprecedented atomic resolution and observe the striking bosonic mode interacting with dispersive kagome electrons near the Fermi surface. At this mode energy, the fermionic quasi-particle dispersion exhibits a pronounced renormalization, signaling a giant coupling to bosons. Through the self-energy analysis, first-principles calculation, and a lattice vibration model, we present evidence that this mode arises from the geometrically frustrated phonon flat-band, which is the lattice bosonic analog of the kagome electron flat-band. Our findings provide the first example of kagome bosonic mode (flat-band phonon) in electronic excitations and its strong interaction with fermionic degrees of freedom in kagome-net materials.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 Magnons and magnetic fluctuations in atomically thin MnBi2Te4(2022) Lujan, David; Choe, Jeongheon; Rodriguez-Vega, Martin; Ye, Zhipeng (TTU); Leonardo, Aritz; Nunley, T. Nathan; Chang, Liang-Juan; Lee, Shang-Fan; Yan, Jiaqiang; Fiete, Gregory A; He, Rui (TTU); Li, XiaoqinElectron band topology is combined with intrinsic magnetic orders in MnBi2Te4, leading to novel quantum phases. Here we investigate collective spin excitations (i.e. magnons) and spin fluctuations in atomically thin MnBi2Te4 flakes using Raman spectroscopy. In a twoseptuple layer with non-trivial topology, magnon characteristics evolve as an external magnetic field tunes the ground state through three ordered phases: antiferromagnet, canted antiferromagnet, and ferromagnet. The Raman selection rules are determined by both the crystal symmetry and magnetic order while the magnon energy is determined by different interaction terms. Using non-interacting spin-wave theory, we extract the spin-wave gap at zero magnetic field, an anisotropy energy, and interlayer exchange in bilayers. We also find magnetic fluctuations increase with reduced thickness, which may contribute to a less robust magnetic order in single layers.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.Item Tunable layered-magnetism-assisted magneto-Raman effect in a two-dimensional magnet CrI3(2020) Jin, Wencan; Ye, Zhipeng (TTU); Luo, Xiangpeng; Yang, Bowen; Ye, Gaihua (TTU); Yin, Fangzhou; Ho Kim, Hyun; Rojas, Laura (TTU); Tian, Shangjie; Fu, Yang; Yan, Shaohua; Lei, Hechang; Sun, Kai; Tsen, Adam W.; He, Rui (TTU); Zhao, LiuyanWe used a combination of polarized Raman spectroscopy experiment and model magnetism-phonon coupling calculations to study the rich magneto-Raman effect in the two-dimensional (2D) magnet CrI3. We reveal a layered-magnetism-assisted phonon scattering mechanism below the magnetic onset temperature, whose Raman excitation breaks time-reversal symmetry, has an antisymmetric Raman tensor, and follows the magnetic phase transitions across critical magnetic fields, on top of the presence of the conventional phonon scattering with symmetric Raman tensors in N-layer CrI3. We resolve in data and by calculations that the first-order Ag phonon of the monolayer splits into an N-fold multiplet in N-layer CrI3 due to the interlayer coupling (N≥2) and that the phonons within the multiplet show distinct magnetic field dependence because of their different layered-magnetism-phonon coupling. We further find that such a layered-magnetism-phonon coupled Raman scattering mechanism extends beyond first-order to higher-order multiphonon scattering processes. Our results on the magneto-Raman effect of the first-order phonons in the multiplet and the higherorder multiphonons in N-layer CrI3 demonstrate the rich and strong behavior of emergent magneto-optical effects in 2D magnets and underline the unique opportunities of spin-phonon physics in van der Waals layered magnets.