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Raman spectroscopic and dynamic electrical investigation of multi-state charge-wave-density phase transitions in 1T-TaS2 Wen Wen, Yiming Zhu, Chunhe Dang, Wei Chen, and Liming Xie Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b04855 • Publication Date (Web): 21 Feb 2019 Downloaded from http://pubs.acs.org on February 23, 2019
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Nano Letters
Raman Spectroscopic and Dynamic Electrical Investigation of Multi-State Charge-Wave-Density Phase Transitions in 1T-TaS2 Wen Wen, †,‡ Yiming Zhu, † Chunhe Dang, †,‡ Wei Chen,┴ Liming Xie*,†,‡, §
†CAS
Key Laboratory of Standardization and Measurement for Nanotechnology, CAS
Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, P.R. China ‡
School of Nanoscience and Technology, University of Chinese Academy of
Sciences, Beijing 100049, P.R. China §
International College, University of Chinese Academy of Sciences, Beijing 100049,
P.R. China ┴Institute
of Semiconductors, Chinese Academy of Sciences, Beijing 100083, P. R.
China Email:
[email protected] Abstract Two-dimensional layered 1T-TaS2 exhibits rich charge-density-wave (CDW) states with distinct electronic structures and physical properties, leading to broad potential applications,
such
as
phase-transition
memories,
electrical
oscillators
and
photodetectors. Besides the various CDW ground states at different temperatures, multiple intermediate phases in 1T-TaS2 have been observed by applying optical and electrical stimulations. Here, we investigated the electric-field-driven multi-state CDW phase transition by Raman spectroscopy and voltage oscillations in 1T-TaS2. Strong correlation was observed between electrical conductivity and intensity of fold-back acoustic and optical phonon modes in 1T-TaS2. This indicates that the multi-state transitions arise from serial transitions, from the nearly commensurate (NC) CDW phase to out-of-equilibrium intermediate states, and finally to the incommensurate (IC) CDW phase. The dynamics of phase transition under electric field was investigated. As the electrical field increased, the dwell time of different 1 ACS Paragon Plus Environment
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CDW states changed. At temperatures lower down, the multi-state oscillations disappeared, due to higher energy barriers between the intermediate phases and/or lower thermal excitation energies at lower temperatures.
Keywords: charge-density wave, 1T-TaS2, multistate phase transition, intermediate state, Raman spectroscopy, electrical oscillation
Introduction Low-dimensional strongly correlated electron systems present rich physical properties, such as superconductivity, ferromagnetism and charge-density-wave (CDW) ordering, which is arising from the couplings between charge, spin and lattice.1-7 Two-dimensional 1T-TaS2 is an exemplary strongly correlated material known for its fertile CDW phases, which has promoted the fundamental insights into the macroscopic quantum states and device implementations.8-16 1T-TaS2 exhibits insulating Mott phase below 180 K, in which the periodically modulated electron density is commensurate with the underlying atomic lattice via the formation of 13-atom clusters (so-called David star, Figure 1a). Upon heating (from 180 to 350 K), nearly commensurate (NC) CDW phase, consisting of insulating commensurate domains and conductive incommensurate domain walls, is the equilibrium ground state (Figure 1b). As the temperature exceeds 350 K, the incommensurate (IC) CDW phase is established with disappeared commensurate clusters. Recently, studies have revealed some hidden intermediate states in 1T-TaS2 under optical pulse and electric field.17-26 For instance, an exotic hidden state with largely dropped electrical resistance was observed when the Mott insulating phase is excited by a femtosecond laser pulse.17-20 Under an in-plane electric field, multistep nonvolatile phase transition in 1T-TaS2 has also been observed due to intermediate states.21-25 The electrical driven phase transition in 1T-TaS2 holds promising applications in electronics. The ultrafast switching (
