Temperature-dependent excitonic superfluid plasma frequency evolution in an excitonic insulator, Ta2NiSe5

Abstract

An interesting van der Waals material, Ta2NiSe5 has been known one of strong excitonic insulator candidates since it has very small or zero bandgap and can have a strong exciton binding energy because of its quasi-one-dimensional crystal structure. Here we investigate a single crystal Ta2NiSe5 using optical spectroscopy. Ta2NiSe5 has quasi-one-dimensional chains along the a-axis. We have obtained anisotropic optical properties of a single crystal Ta2NiSe5 along the a- and c-axes. The measured a- and c-axis optical conductivities exhibit large anisotropic electronic and phononic properties. With regard to the a-axis optical conductivity, a sharp peak near 3050 cm−1 at 9 K, with a well-defined optical gap ( 1800 cm−1) and a strong temperature-dependence, is observed. With an increase in temperature, this peak broadens and the optical energy gap closes around ∼325 K (). The spectral weight redistribution with respect to the frequency and temperature indicates that the normalized optical energy gap () is . The temperature-dependent superfluid plasma frequency of the excitonic condensation in Ta2NiSe5 has been determined from measured optical data. Our study may pave new avenues in the future research on excitonic insulators.

Mapping the unoccupied state dispersions in Ta2NiSe5 with resonant inelastic x-ray scattering

“…This value corresponds to the size of the charge gap in Ta 2 NiSe 5 at 30 K, as determined from our RIXS study, and is in good agreement with the charge gap estimated from scanning tunneling spectroscopy measurements (at 78 K) [12] or from a symmetric band behavior around the Fermi level in photoemission [6]. At first glance, it is substantially larger than the gap of 0.16 to 0.22 eV obtained from recent optical studies that probe Q = 0 interband transitions [8,9,13]. However, applying the same analysis method and extrapolating the low-energy cutoff of the RIXS spectrum down to the intensity baseline [see the inset of Fig.…”

“…Below T c = 328 K, the Ta-Ni separation shortens due to a second-order crystallographic phase transition without any signature of a CDW, and the unit cell changes from orthorhombic to monoclinic symmetry [1]. At T c , the valence band maximum measured by ARPES at continuously shifts to higher binding energy up to about 0.18 eV [6,7], and the optical band gap increases in a similar way up to about 0.16 to 0.22 eV [8,9]. The nature of the low-energy electronic structure above T c is currently heavily debated and argued to be a semimetal [10][11][12], a zero-gap semiconductor [13], or a semiconductor [6,7,14].…”

Ultrafast melting and recovery of collective order in the excitonic insulator Ta2NiSe5

“…Contrary to previous reports 23 , the Δ R / R signal does not depend on the pump polarisation, but it significantly changes with the orientation of the probe polarisation (see Supplementary Note 1 ). We determine that the response to a polarisation oriented parallel to the atomic chains is more directly related to the status of the condensate (as suggested in previous equilibrium measurements 32 , 33 ) and limit our measurements to this probe configuration. This behaviour, together with the wavelength dependence described below, is in close analogy with that reported for superconducting systems 34 – 36 , and can be ascribed to the difference in the initial and final states for the probe-induced transitions as a function of polarisation and wavelength.…”

Ta2NiSe5 nanosheets as a novel broadband saturable absorber for solid-state pulse laser generation

“…This twisted chain structure leads to great anisotropic properties of Ta 2 NiSe 5 in the layer plane. Besides, a recent study also shows that Ta 2 NiSe 5 is a zero-gap and direct bandgap semiconductor, with a transition to an excitonic insulator (EI) occurring near 326 K [23]. Below this temperature, flat band at the top of the valence band occur, which is ascribed to the EI effect [24].…”