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

   https://doi.org/10.1038/s41598-023-37989-y  

   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 (July 2023, Scientific Reports)

   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. Real-space visualization of a defect-mediated charge density wave transition

   https://doi.org/10.1073/pnas.2402129121  

   Hart, James L; Pan, Haining; Siddique, Saif; Schnitzer, Noah; Mallayya, Krishnanand; Xu, Shiyu; Kourkoutis, Lena F; Kim, Eun-ah; Cha, Judy J (August 2024, Proceedings of the National Academy of Sciences)

   We study the coupled charge density wave (CDW) and insulator-to-metal transitions in the 2D quantum material 1T-TaS₂. By applying in situ cryogenic 4D scanning transmission electron microscopy with in situ electrical resistance measurements, we directly visualize the CDW transition and establish that the transition is mediated by basal dislocations (stacking solitons). We find that dislocations can both nucleate and pin the transition and locally alter the transition temperatureT_cby nearly ~75 K. This finding was enabled by the application of unsupervised machine learning to cluster five-dimensional, terabyte scale datasets, which demonstrate a one-to-one correlation between resistance—a global property—and local CDW domain-dislocation dynamics, thereby linking the material microstructure to device properties. This work represents a major step toward defect-engineering of quantum materials, which will become increasingly important as we aim to utilize such materials in real devices. 

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4. 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 (RTe 3 ) 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 (T CDW ), 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 RTe 3 CDW materials and establish their characteristics through the non-destructive angle-resolved Raman spectroscopy technique. 

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5. Advances in Rare‐Earth Tritelluride Quantum Materials: Structure, Properties, and Synthesis

   https://doi.org/10.1002/advs.202004762  

   Yumigeta, Kentaro; Qin, Ying; Li, Han; Blei, Mark; Attarde, Yashika; Kopas, Cameron; Tongay, Sefaattin (March 2021, Advanced Science)

   Abstract A distinct class of 2D layered quantum materials with the chemical formula ofRTe₃(R= lanthanide) has gained significant attention owing to the occurrence of collective quantum states, superconductivity, charge density waves (CDW), spin density waves, and other advanced quantum properties. To study the Fermi surface nesting driven CDW formation, the layeredRTe₃family stages an excellent low dimensional genre system. In addition to the primary energy gap feature observed at higher energy, optical spectroscopy study on someRTe₃evidence a second CDW energy gap structure indicating the occurrence of multiple CDW ordering even with light and intermediateRTe₃compounds. Here, a comprehensive review of the fundamentals ofRTe₃layered tritelluride materials is presented with a special focus on the recent advances made in electronic structure, CDW transition, superconductivity, magnetic properties of these unique quantum materials. A detailed description of successful synthesis routes including the flux method, self‐flux method, and CVT along with potential applications is summarized. 

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