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THE NANO-TEXTURED PHASE OF 1T-TaS2 PROBED BY OPTICAL CONDUCTIVITY

Nano-textured phase of 1T-TaS2 is a peculiar near commensurate charge density wave (NCCDW) phase in which nanometer-sized, and roughly hexagonal, commensurate charge density wave (CCDW) domains coexist with, and are separated by, roughly triangular pieces of metallic phase. [1] Cooling removes this phase in pure 1T-TaS2 around 180 K, as the system enters commensurate-CDW/Mott state. However, the transition may be suppressed by pressure or by intercalation, leaving the sample in non-metallic NCCDW state down to very low temperatures, where it turns superconductive.[2] Surprisingly, the temperature of the superconducting transition is constant in pressure, even though the resistivity changes by orders of magnitude, [3] suggesting that superconductivity arises in triangular metallic parts, while DC conductivity is dominated by weak links between them. In order to examine more closely the evolution of the NCCDW phase in temperature, one would like to be able to observe separately the two components of the NCCDW phase, which is obviously impossible through dc transport measurements. Therefore, we turn to optical measurements hoping for separate signatures of insulating and the conductive regions at different frequencies. Here, we report the reflectance of pure and intercalated 1T-TaS2 samples, recently measured in the temperature range from 23 K to 290 K, over a frequency range of 30 cm-1 - 37000 cm-1 as well as the optical conductivity derived through Kramers-Kronig analysis.[4] We observe a single, wide metallic contribution, extending to 400 cm-1, on the top of which the phonon contribution shows above 40 cm-1. This suggests that the largest contribution to optical conductivity also comes from connections between conducting areas. This work was supported by the DAAD (Deutscher Akademischer Austausch Dienst) and Unity through Knowledge Fund, under Grant No. 65/10. [1] A. Spijkerman, Phys. Rev. B 56 (1997) 13757. [2] P. Xu, Phys. Rev. B 81 (2010) 172503. [3] B. Sipos, Nature Mater. 7 (2008) 960. [4] K. Velebit, et. al., in preparation

1T-TaS2; optical conductivity

The paper was presented as an invited talk by Kristijan Velebit. This work was supported in part by the UKF project (65/10) “New electronic states driven by frustration in layered materials”.