STM observation of ZrTe3 single crystals annealed at different temperatures
*Sora Kobayashi1, Shun Ohta1, Satoshi Demura2, Atsushi Nomura1, Hideaki Sakata1

ZrTe3, a transition metal trichalcogenide, undergoes one-dimensional charge density wave (CDW) and superconducting (SC) transitions successively [1]. These transitions are strongly affected by microscopic structures in the crystal [2,3,4]. For example, a disorder of atomic position is changed by the difference in the synthesis temperature, leading to the change in the CDW and SC transition temperature [2,3]. In the single crystal synthesized at 660℃, CDW appears along the a-axis at 72K, and filamentary SC appears along the a-axis at 2K. In single crystals synthesized at 750℃, CDW transition temperature decreases down to 61K and bulk SC appears at 4K [2,3]. Another example is the lattice defects. The defects cause Friedel oscillation that affects the formation of CDW. Such Friedel oscillation has been observed by soft X-ray diffraction experiments [4]. These reports indicate the importance of microscopic structural information on CDW and SC transitions in ZrTe3.

Here, we performed STM measurements, which clarify the microscopic structure, on ZrTe3 single crystals prepared at different temperatures. Single crystals of ZrTe3 were synthesized from Zr and Te sponge shot by chemical vapor transport method with iodine as the transport agent at 750℃(source)/650℃(sink). To prepare samples with different CDW and SC transition temperatures, synthesized single crystals are annealed at 650℃ or 850℃. The observed CDW and SC transition temperatures in the samples annealed at 650℃ and 850℃ were consistent with the results reported in the samples synthesized at each temperature, respectively [3].

STM measurements at 4K revealed that the sample annealed at 850℃ has a higher number of defects than that annealed at 650℃. This clarified the annealing at different temperatures causes the change in the microscopic structure. The observed CDW in the samples annealed at 650℃ showed long-range order, whereas the samples annealed at 850℃ showed rather disordered CDW, indicating the defects pinned the CDW. This seems to cause a decrease in the CDW transition temperature in the samples annealed at 850℃.

The effect of defects on CDW was also confirmed in STM measurements at 80K, which is higher than the CDW transition temperature. Even above the CDW transition temperature, the modulation structure was found to exist and strongly be pinned at defects.

[1] D.J. Eaglesham et al., Journal of Physics C: Solid State Physics 17 (1984) L697. 
[2] X. Zhu et al., Physical Review B 87, 024508 (2013).
[3] A. Nomura et al., Europhysics Letters 133, 37003 (2021).
[4] L. Yue et al., Nature Communications 11.1 (2020).

Keywords: ZrTe3, superconductivity, CDW, STM