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1. Raman Spectroscopy of Quasi-One-Dimensional NbTe Weyl Semimetal Nanowires

   Zahra Ebrahimnatajmalekshah, Fariborz Kargar (April 2023, Materials Research Society (MRS) Spring Meeting (2023))

   Abstract Body: Recently a new research field of quasi-one-dimensional (1D) van der Waals quantummaterials has emerged from earlier work on low-dimensional systems [1-2]. The quasi-1D van der Waalsmaterials have 1D motifs in their crystal structure [1]. Many of these materials reveal strongly correlatedphenomena such as charge density waves (CDW) [1-2]. The CDW phase is a periodic modulation of theelectronic charge density, accompanied by distortions in the underlying crystal lattice. Potential uses for CDWmaterials include memory storage and oscillators [3]. Raman spectroscopy can identify the CDW transitions todifferent phases via the appearance of phonon peaks due to emerging superstructure or the disappearance ofcertain peaks due to the loss of translation symmetry in the crystal lattice [3]. In this presentation, we report theresults of the angle and temperature-dependent Raman scattering spectroscopy investigation of themechanically exfoliated nanowires of the quasi-1D Nb van der Waals material. It is known that Nb forms in atetragonal crystal structure with space group 124 (P4/mcc). Recently, this material attracted attention as aCDW material with multiple phase transitions, some of them, possibly, near room temperature. Littleinformation is known on the Raman characteristics of this material. Our Raman data for different polarizationangles show strong anisotropy in the response depending on the crystal direction. The most pronouncedRaman peaks reveal strong temperature dependence. The results of the measurements will be compared withthe theoretical predictions. Our data is important for further investigation of this quasi-1D CDW material forpossible applications in phase-change memory and reconfigurable devices. A.A.B. acknowledges the support of the Vannevar Bush Faculty Fellowship (VBFF) from the Office of NavalResearch (ONR) contract N00014-21-1-2947 “One-Dimensional Quantum Materials” and the National ScienceFoundation (NSF) program Designing Materials to Revolutionize and Engineer our Future (DMREF) via aproject DMR-1921958 “Data-Driven Discovery of Synthesis Pathways and Distinguishing ElectronicPhenomena of 1D van der Waals Bonded Solids”. A. D. and S. K. acknowledge support through the MaterialGenome Initiative funding allocated to the National Institute of Standards and Technology. [1] A. A. Balandin, F. Kargar, T. T. Salguero, and R. Lake, “One-dimensional van der Waals quantummaterials", Mater. Today, 55, 74 (2022). [2] A. A. Balandin, R. K. Lake, and T. T. Salguero, "One-dimensional van der Waals materials - Advent of a newresearch field" Appl. Phys. Lett., 121, 040401 (2022). [3] A. A. Balandin, S. V. Zaitzev-Zotov, and G. Grüner, "Charge-density-wave quantum materials and devices—New developments and future prospects", Appl. Phys. Lett., 119, 170401 (2021). [4] R. Samnakay, et al., “Zone-folded phonons and the charge-density-wave transition in 1T-TaSe2 thin films,” Nano Lett., 15, 2965 (2015). 

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2. Raman spectroscopy of phonon states in NbTe ₄ and TaTe ₄ quasi‐one‐dimensional van der Waals crystals

   https://doi.org/10.1002/jrs.6661  

   Nataj, Zahra_Ebrahim; Kargar, Fariborz; Krylyuk, Sergiy; Debnath, Topojit; Taheri, Maedeh; Ghosh, Subhajit; Zhang, Huairuo; Davydov, Albert_V; Lake, Roger_K; Balandin, Alexander_A (February 2024, Journal of Raman Spectroscopy)

   Abstract We report the results of polarization‐dependent Raman spectroscopy of phonon states in single‐crystalline quasi‐one‐dimensional NbTe₄and TaTe₄van der Waals materials. The measurements were conducted in the wide temperature range from 80 to 560 K. Our results show that although both materials have identical crystal structures and symmetries, there is a drastic difference in the intensity of their Raman spectra. While TaTe₄exhibits well‐defined peaks through the examined wavenumber and temperature ranges, NbTe₄reveals extremely weak Raman signatures. The measured spectral positions of the phonon peaks agree with the phonon band structure calculated using the density‐functional theory. We offer possible reasons for the intensity differences between the two van der Waals materials. Our results provide insights into the phonon properties of NbTe₄and TaTe₄van der Waals materials and indicate the potential of Raman spectroscopy for studying charge‐density‐wave quantum condensate phases. 

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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. Controlling the Charge Density Wave Transition in Monolayer TiSe ₂ : Substrate and Doping Effects

   https://doi.org/10.1002/qute.201800070  

   Kolekar, Sadhu; Bonilla, Manuel; Diaz, Horacio_Coy; Hashimoto, Makato; Lu, Donghui; Batzill, Matthias (October 2018, Advanced Quantum Technologies)

   Abstract TiSe₂is an exciting material because it can be tuned between superconducting and charge density wave (CDW) transitions. In the monolayer limit, TiSe₂exhibits a sizable energy gap in the CDW phase that makes it a promising quantum material. It is shown that interfacing a single layer of TiSe₂with dissimilar van der Waals materials enables control of its properties. Using angle‐resolved photoemission spectroscopy, the energy gap opening is analyzed as a function of temperature for TiSe₂monolayers supported on different van der Waals substrates. A substantial increase in the CDW transition temperature of ≈45 K is observed on MoS₂compared to graphite (highly oriented pyrolytic graphite) substrates. This control of the CDW in monolayer TiSe₂is suggested to arise from varying charge screening of the unconventional CDW of TiSe₂by the substrate. In addition, the suppression of CDW order and a complete closing of the energy gap by electron doping of monolayer TiSe₂is demonstrated. Regulating the many‐body physics phenomena in monolayer TiSe₂lays the foundation of modifying TiSe₂in, for example, artificial van der Waals heterostructures and thus creates a new approach for utilizing the quantum states of TiSe₂in device applications. 

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5. Two-dimensional charge order stabilized in clean polytype heterostructures

   https://doi.org/10.1038/s41467-021-27947-5  

   Sung, Suk Hyun; Schnitzer, Noah; Novakov, Steve; El Baggari, Ismail; Luo, Xiangpeng; Gim, Jiseok; Vu, Nguyen M.; Li, Zidong; Brintlinger, Todd H.; Liu, Yu; et al (December 2022, Nature Communications)

   Abstract Compelling evidence suggests distinct correlated electron behavior may exist only in clean 2D materials such as 1T-TaS 2 . Unfortunately, experiment and theory suggest that extrinsic disorder in free standing 2D layers disrupts correlation-driven quantum behavior. Here we demonstrate a route to realizing fragile 2D quantum states through endotaxial polytype engineering of van der Waals materials. The true isolation of 2D charge density waves (CDWs) between metallic layers stabilizes commensurate long-range order and lifts the coupling between neighboring CDW layers to restore mirror symmetries via interlayer CDW twinning. The twinned-commensurate charge density wave (tC-CDW) reported herein has a single metal–insulator phase transition at ~350 K as measured structurally and electronically. Fast in-situ transmission electron microscopy and scanned nanobeam diffraction map the formation of tC-CDWs. This work introduces endotaxial polytype engineering of van der Waals materials to access latent 2D ground states distinct from conventional 2D fabrication. 

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