Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session M50: Is Ta2NiSe5 an Excitonic Insulator?Invited Live
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Sponsoring Units: DCMP Chair: Girsh Blumberg, Rutgers University, New Brunswick |
Wednesday, March 17, 2021 11:30AM - 12:06PM Live |
M50.00001: Almost zero-gap gap semiconductor/semimetal to insulator transition in layered chalcogenide Ta2NiSe5 Invited Speaker: Hidenori Takagi The excitonic insulator is a long conjectured correlated electron phase of narrow gap semiconductors and semimetals, driven by weakly screened electron-hole interactions. Having been proposed more than 50 years ago, conclusive experimental evidence for its existence remains elusive. A new generation of candidate excitonic insulator, the layered chalcogenide Ta2NiSe5, was very recently proposed [1,2]. Band structure calculations show that the valence band comprises a hybridized Ni 3d/Se 4p state, while the conduction band contains the Ta 5d state. Since both the top of valence band and the bottom of conduction band are about touching at the Γ point, Ta2NiSe5 is a direct and almost zero-gap semiconductor/semimetal, which gives rise to an ideal playground for excitonic physics. An almost zero-gap semiconductor-to-insulator transition occurs at TC ∼ 328 K, accompanied with a second-order structural transition without any superlattice formation. The optical gap formed below TC is as large as ~0.2 eV, close to the exciton binding energy of 0.2-0.3 eV observed in the sister compound Ta2NiS5, with one electron energy gap of ~ a half eV[3].. The 0.2 eV excitation gap is observed also in the local DOS dI/dV measured by STM-STS at 4.2 K. We find a drastic collapse of the 0.2 eV gap with approaching the STM-tip to the sample surface, which we argue to indicate the many body nature of the gap. Those results provide strong support for the formation of excitonic insulating phase in Ta2NiSe5 below Tc. |
Wednesday, March 17, 2021 12:06PM - 12:42PM Live |
M50.00002: Band hybridization at the semimetal-semiconductor transition of Ta2NiSe5 enabled by mirror-symmetry breaking Invited Speaker: Matthew D Watson We present a fresh look at the experimental electronic structure of the excitonic insulator candidate Ta2NiSe5. Our angle-resolved photoemission spectroscopy measurements unambiguously establish the normal state as a semimetal with a significant band overlap of > 100 meV. Our temperature-dependent measurements indicate how these low-energy states hybridize when cooling through the well-known 327 K phase transition in this system. From our calculations and polarization-dependent photoemission measurements, we demonstrate the importance of a loss of mirror symmetry in enabling the band hybridization, driven by a shear-like structural distortion which reduces the crystal symmetry from orthorhombic to monoclinic. Our supporting ab-initio calculations in the monoclinic show the opening of an energy gap comparable with the experimental results, pointing to the key role of the lattice distortion in enabling the phase transition of Ta2NiSe5. We will show the links between the experimental electronic structure and other recent experimental results on this material. We will also briefly present a comparison with the latest experimental results in the related material TiSe2. |
Wednesday, March 17, 2021 12:42PM - 1:18PM Live |
M50.00003: Spatial mapping of collective modes and time-resolved tracking of excitonic order melting in Ta2NiSe5 at room temperature Invited Speaker: Hope Bretscher The ternary compound Ta2NiSe5 has recently been proposed and investigated as a possible candidate for the long sought-after excitonic insulator (EI) phase in bulk materials. This fascinating phase of matter has attracted widespread attention as it could provide a toolset for the study of many-body physics phenomena and for the exploration of macroscopic coherent behaviors at room temperature. However, many questions remain open about the microscopic nature of the symmetry breaking process occurring in Ta2NiSe5 and other proposed EI candidates. In this talk, I will present our investigation of this material by imaging the spatial propagation of the condensate’s collective modes and detecting the effect of the destruction of the excitonic order on a femtosecond timescale. Through these attempts, we attempt to fill in some of the missing knowledge using ultrafast spectroscopic techniques. |
Wednesday, March 17, 2021 1:18PM - 1:54PM Live |
M50.00004: Electronic phase diagram of the excitonic insulator candidates Ta2Ni(Se1-xSx)5 probed by Raman scattering Invited Speaker: Pavel Volkov Excitonic insulator (EI) is a phase driven by Coulomb attraction between electrons and holes leading to a proliferation of particle-hole pairs. EIs break the lattice symmetries, raising the question of whether a particular transition is excitonic or structural. Recently, the transition origin in a candidate material Ta2NiSe5 has become a subject of interest, with both excitonic [1] and lattice [2] mechanisms proposed. |
Wednesday, March 17, 2021 1:54PM - 2:30PM Live |
M50.00005: The spontaneous symmetry breaking in Ta2NiSe5 is structural in nature Invited Speaker: Nuh Gedik The excitonic insulator is an electronically driven phase of matter that emerges upon the spontaneous formation and Bose condensation of excitons. Detecting this exotic order in candidate materials is a subject of paramount importance, as the size of the excitonic gap in the band structure establishes the potential of this collective state for superfluid energy transport. However, the identification of this phase in real solids is hindered by the coexistence of a structural order parameter with the same symmetry as the excitonic order. Only a few materials are currently believed to host a dominant excitonic phase, Ta2NiSe5 being the most promising. Here, we test this scenario by using an ultrashort laser pulse to quench the broken-symmetry phase of this transition metal chalcogenide. Tracking the dynamics of the material's electronic and crystal structure after light excitation reveals surprising spectroscopic fingerprints that are only compatible with a primary order parameter of phononic nature. We rationalize our findings through state-of-the-art calculations, confirming that the structural order accounts for most of the electronic gap opening. Our findings conclusively rule out any substantial excitonic character in this instability of Ta2NiSe5. |
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