Exciting transport properties of Ta₂NiSe₅

Ta₂NiSe₅ is a candidate excitonic insulator that crystalizes as stacked van de Waals layers. Its direct narrow-gap (~ 0.1 eV) promotes spontaneous exciton formation with electrons in Ta chains and holes in Ni chains. We report resistivity r, thermopower S, Hall effect r_xy, Nernst effect N and thermal conductivity k in Ta2NiSe5 from 10K to 400K, specific heat C_p from 2K to 395K. C_p shows an anomaly at 324 K like [1], but deviates from the Debye behavior at other temperatures. S has a maximum of 938 mV/K at 80 K and is zero for 10K<T<50K. At 90K<T<300K both S and r (similar to [2]) show activated behavior with activation energy ~0.1eV. Carrier concentration from r_xy freeze-out from 10²¹cm^-3 at 300K to 6.3x10¹⁵ at 40K. Both N and k are reported here for the first time and we describe the magnetic field dependence of these quantities.

[1] Lu & al. Nat Commun 8, 14408 (2017)
[2] Nakano & al. J. Phys. Soc. Jpn. 88, 113706 (2019)