Abstract
Charge density wave (CDW) formation, a key physics issue for materials, arises from interactions among electrons and phonons that can also lead to superconductivity and other competing or entangled phases. The prototypical system TiSe2, with a particularly simple (2 × 2 × 2) transition and no Kohn anomalies caused by electron-phonon coupling, is a fascinating but unsolved case after decades of research. Our angle-resolved photoemission measurements of the band structure as a function of temperature, aided by first-principles calculations, reveal a hitherto undetected but crucial feature: a (2 × 2) electronic order in each layer sets in at ~232 K before the widely recognized three-dimensional structural order at ~205 K. The dimensional crossover, likely a generic feature of such layered materials, involves renormalization of different band gaps in two stages.
Highlights
The rich physics of Charge density wave (CDW) is exemplified by the varied properties of a large number of transition-metal dichalcogenides[1,2,3,4,5,6,7,8,9,10], which are of interest for their potential as alternates of graphene/graphite for electronic applications
(D) Projected two-dimensional Brillouin zones in a vertical plane. (E) angle-resolved photoemission spectroscopy (ARPES) intensity maps at various energies taken with 58 eV photons for the normal phase at 300 K and the CDW phase at 10 K
Similar features are evident for the CDW phase at 10 K; a weak replica bayti(n2y ×c 2ir ×cle 2)apfopledainrsgaotfMth e=SΓe⁎4aptbtahnedFseirsmcliealervlyels,ewenhiccehntceormedeastfMro m= Γth⁎e(kTx i=3 0d and ky = 1.03 Å–1). conduction band minimum
Summary
The rich physics of CDW is exemplified by the varied properties of a large number of transition-metal dichalcogenides[1,2,3,4,5,6,7,8,9,10], which are of interest for their potential as alternates of graphene/graphite for electronic applications. Its crystal structure (Fig. 1A) consists of Se-Ti-Se trilayers loosely bonded together by van der Waals forces[11] It undergoes a second-order CDW transition at TC ~ 205 K to a commensurate (2 × 2 × 2) superlattice[7,9]. Referring to the pictures of the Brillouin zones for both the normal and CDW phases shown in Fig. 1B–D, the top of the valence band, located at the Γpoint, is of the Se 4p character, and the bottom of the conduction band, located at the L point, is of the Ti 3d character. The mathematical formulation is similar for the two cases, but the underlying physics is different In the latter case, the transition is purely a band-structure effect, requiring no higher-order electronic interactions; the coupled electron-lattice system evolves, or distorts, to minimize the total energy. The system remains an indirect-gap semiconductor because the valence band maximum shifts from Γto A*
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
