[Paper Review] Spontaneous gap opening and potential excitonic states in an ideal Dirac semimetal Ta$_2$Pd$_3$Te$_5$

Peng Zhang, Yuyang Dong+20 more|arXiv (Cornell University)|Dec 22, 2023

Topological Materials and Phenomena41 references4 citations

TL;DR

This study identifies Ta₂Pd₃Te₅ as a promising candidate for an excitonic insulator, demonstrating spontaneous gap opening at 350 K with no entangled phase transitions. The gap grows with decreasing temperature, is suppressed by potassium doping, and features in-gap flat bands—key signatures of excitonic order in a quasi-one-dimensional Dirac semimetal with near-ideal Dirac dispersion and vanishing carrier density.

ABSTRACT

The opening of an energy gap in the electronic structure generally indicates the presence of interactions. In materials with low carrier density and short screening length, long-range Coulomb interaction favors the spontaneous formation of electron-hole pairs, so-called excitons, opening an excitonic gap at the Fermi level. Excitonic materials host unique phenomenons associated with pair excitations. However, there is still no generally recognized single-crystal material with excitonic order, which is, therefore, awaited in condensed matter physics. Here, we show that excitonic states may exist in the quasi-one-dimensional material Ta$_2$Pd$_3$Te$_5$, which has an almost ideal Dirac-like band structure, with Dirac point located exactly at Fermi level. We find that an energy gap appears at 350 K, and it grows with decreasing temperature. The spontaneous gap opening is absent in a similar material Ta$_2$Ni$_3$Te$_5$. Intriguingly, the gap is destroyed by the potassium deposition on the crystal, likely due to extra-doped carriers. Furthermore, we observe a pair of in-gap flat bands, which is an analog of the impurity states in a superconducting gap. All these observations can be properly explained by an excitonic order, providing Ta$_2$Pd$_3$Te$_5$ as a new and promising candidate realizing excitonic states.

Motivation & Objective

• To identify a single-crystal material hosting pure excitonic order without coexisting phase transitions.
• To determine whether the observed energy gap in Ta₂Pd₃Te₅ arises from excitonic pairing rather than structural distortions or charge-density waves.
• To establish experimental signatures of excitonic order, including gap evolution, doping response, and in-gap states.
• To distinguish excitonic gap formation from lattice effects using high-resolution electron diffraction and specific heat measurements.

Proposed method

• Angle-resolved photoemission spectroscopy (ARPES) to map the electronic band structure and detect gap opening at the Fermi level.
• Low-temperature ARPES measurements to track the temperature dependence of the gap, showing growth with decreasing temperature.
• Potassium deposition to introduce extrinsic carriers and test gap suppression, a hallmark of excitonic states.
• High-resolution electron diffraction to detect lattice distortions and rule out structural transitions as the origin of the gap.
• Specific heat measurements to assess the thermodynamic signature of the transition, with analysis of electronic vs. lattice contributions.
• Theoretical modeling to predict the formation of in-gap flat bands due to non-magnetic impurities in excitonic insulators, consistent with observed ARPES features.

Experimental results

Research questions

• RQ1 Does the energy gap in Ta₂Pd₃Te₅ arise from excitonic order rather than structural or charge-density wave transitions?
• RQ2 How does the gap evolve with temperature, and is it consistent with a spontaneous symmetry-breaking transition?
• RQ3 Can the gap be suppressed by extrinsic doping, as expected for excitonic states?
• RQ4 Are in-gap flat bands observed, as predicted for excitonic insulators with impurities?
• RQ5 Is the lattice distortion associated with the gap formation large enough to produce a detectable specific heat anomaly?

Key findings

• An energy gap opens spontaneously at 350 K in Ta₂Pd₃Te₅, growing with decreasing temperature, indicating a temperature-driven electronic transition.
• The gap is suppressed by potassium deposition, consistent with the destruction of excitonic order by extra carriers.
• No significant lattice distortion is detected via XRD or electron diffraction, ruling out structural transitions as the origin of the gap.
• A pair of in-gap flat bands is observed, resembling impurity states in superconductors and predicted for excitonic insulators.
• Specific heat measurements show no discernible jump at 350 K, consistent with a small electronic contribution and the absence of a strong lattice anomaly.
• The absence of coexisting CDW or structural transitions, combined with the gap's behavior under doping and in-gap states, strongly supports excitonic order as the dominant mechanism.