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1. Super-resolved time–frequency measurements of coupled phonon dynamics in a 2D quantum material

   https://doi.org/10.1038/s41598-022-22055-w  

   Gentry, Christian; Liao, Chen-Ting; You, Wenjing; Ryan, Sinéad A.; Varner, Baldwin Akin; Shi, Xun; Guan, Meng-Xue; Gray, Thomas; Temple, Doyle; Meng, Sheng; et al (November 2022, Scientific Reports)

   Abstract Methods to probe and understand the dynamic response of materials following impulsive excitation are important for many fields, from materials and energy sciences to chemical and neuroscience. To design more efficient nano, energy, and quantum devices, new methods are needed to uncover the dominant excitations and reaction pathways. In this work, we implement a newly-developed superlet transform—a super-resolution time-frequency analytical method—to analyze and extract phonon dynamics in a laser-excited two-dimensional (2D) quantum material. This quasi-2D system, 1T-TaSe₂, supports both equilibrium and metastable light-induced charge density wave (CDW) phases mediated by strongly coupled phonons. We compare the effectiveness of the superlet transform to standard time-frequency techniques. We find that the superlet transform is superior in both time and frequency resolution, and use it to observe and validate novel physics. In particular, we show fluence-dependent changes in the coupled dynamics of three phonon modes that are similar in frequency, including the CDW amplitude mode, that clearly demonstrate a change in the dominant charge-phonon couplings. More interestingly, the frequencies of the three phonon modes, including the strongly-coupled CDW amplitude mode, remain time- and fluence-independent, which is unusual compared to previously investigated materials. Our study opens a new avenue for capturing the coherent evolution and couplings of strongly-coupled materials and quantum systems. 

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2. Coherent modulation of the electron temperature and electron–phonon couplings in a 2D material

   https://doi.org/10.1073/pnas.1917341117  

   Zhang, Yingchao; Shi, Xun; You, Wenjing; Tao, Zhensheng; Zhong, Yigui; Cheenicode Kabeer, Fairoja; Maldonado, Pablo; Oppeneer, Peter M.; Bauer, Michael; Rossnagel, Kai; et al (April 2020, Proceedings of the National Academy of Sciences)

   Ultrashort light pulses can selectively excite charges, spins, and phonons in materials, providing a powerful approach for manipulating their properties. Here we use femtosecond laser pulses to coherently manipulate the electron and phonon distributions, and their couplings, in the charge-density wave (CDW) material 1T-TaSe₂. After exciting the material with a femtosecond pulse, fast spatial smearing of the laser-excited electrons launches a coherent lattice breathing mode, which in turn modulates the electron temperature. This finding is in contrast to all previous observations in multiple materials to date, where the electron temperature decreases monotonically via electron–phonon scattering. By tuning the laser fluence, the magnitude of the electron temperature modulation changes from ∼200 K in the case of weak excitation, to ∼1,000 K for strong laser excitation. We also observe a phase change of π in the electron temperature modulation at a critical fluence of 0.7 mJ/cm², which suggests a switching of the dominant coupling mechanism between the coherent phonon and electrons. Our approach opens up routes for coherently manipulating the interactions and properties of two-dimensional and other quantum materials using light. 

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3. The phononic and charge density wave behavior of entire rare-earth tritelluride series with chemical pressure and temperature

   https://doi.org/10.1063/5.0110395  

   Yumigeta, Kentaro; Attarde, Yashika; Kopaczek, Jan; Sayyad, Mohammed_Y; Shen, Yuxia; Blei, Mark; Rajaei_Moosavy, Seyed_Tohid; Qin, Ying; Sailus, Renee; Tongay, Sefaattin (November 2022, APL Materials)

   Here, we present comprehensive phononic and charge density wave properties (CDW) of rare-earth van der Waals tritellurides through temperature dependent angle-resolved Raman spectroscopy measurements. All the possible rare-earth tritellurides (RTe3) ranging from R = La–Nd, Sm, Gd–Tm were synthesized through a chemical vapor transport technique to achieve high quality crystals with excellent CDW characteristics. Raman spectroscopy studies successfully identify the emergence of the CDW state and transition temperature (TCDW), which offers a non-destructive method to identify their CDW response with micron spatial resolution. Temperature dependent Raman measurements further correlate how the atomic mass of metal cations and the resulting chemical pressure influence its CDW properties and offer detailed insight into the strength of CDW amplitude mode-phonon coupling during the CDW transition. Angle-resolved Raman measurements offer the first insights into the CDW-phonon symmetry interplay by monitoring the change in the symmetry of phonon mode across the CDW transition. Overall results introduce the library of RTe3 CDW materials and establish their characteristics through the non-destructive angle-resolved Raman spectroscopy technique. 

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4. Creation of a novel inverted charge density wave state

   https://doi.org/10.1063/4.0000132  

   Zhang, Yingchao; Shi, Xun; Guan, Mengxue; You, Wenjing; Zhong, Yigui; Kafle, Tika_R; Huang, Yaobo; Ding, Hong; Bauer, Michael; Rossnagel, Kai; et al (January 2022, Structural Dynamics)

   Charge density wave (CDW) order is an emergent quantum phase that is characterized by periodic lattice distortion and charge density modulation, often present near superconducting transitions. Here, we uncover a novel inverted CDW state by using a femtosecond laser to coherently reverse the star-of-David lattice distortion in 1T-TaSe2. We track the signature of this novel CDW state using time- and angle-resolved photoemission spectroscopy and the time-dependent density functional theory to validate that it is associated with a unique lattice and charge arrangement never before realized. The dynamic electronic structure further reveals its novel properties that are characterized by an increased density of states near the Fermi level, high metallicity, and altered electron–phonon couplings. Our results demonstrate how ultrafast lasers can be used to create unique states in materials by manipulating charge-lattice orders and couplings. 

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5. Direct determination of mode-projected electron-phonon coupling in the time domain

   https://doi.org/10.1126/science.aaw1662  

   Na, M. X.; Mills, A. K.; Boschini, F.; Michiardi, M.; Nosarzewski, B.; Day, R. P.; Razzoli, E.; Sheyerman, A.; Schneider, M.; Levy, G.; et al (December 2019, Science)

   Ultrafast spectroscopies have become an important tool for elucidating the microscopic description and dynamical properties of quantum materials. In particular, by tracking the dynamics of nonthermal electrons, a material’s dominant scattering processes can be revealed. Here, we present a method for extracting the electron-phonon coupling strength in the time domain, using time- and angle-resolved photoemission spectroscopy (TR-ARPES). This method is demonstrated in graphite, where we investigate the dynamics of photoinjected electrons at the $\overline{\mathrm{K}}$point, detecting quantized energy-loss processes that correspond to the emission of strongly coupled optical phonons. We show that the observed characteristic time scale for spectral weight transfer mediated by phonon-scattering processes allows for the direct quantitative extraction of electron-phonon matrix elements for specific modes. 

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