Charge-density-wave formation inTa2NiSe7studied by scanning tun

Abstract

the various linear chain materials when they are in the CDW phase.The crystals of TaaNiSey grow as needles up to 20 mm in length and have monoclinic symmetry with lattice parameters of ao = 13.827 A, bo = 3AS2 A, co = 18.577 A, and j8 = 108.8°. The crystal structure has been analyzed in detail by Sunshine and Ibers [7]. The chains are oriented along the b axis and there are four formula units Scanning tunneling microscope images of Ta2NiSe7 taken at 77 and 4.2 K show a complex electronic rearrangement occurring below the charge-density-wave (CDW) onset temperature of 52.5 K. A large energy gap of 41.0 ±3.0 meV is observed and a CDW amplitude modulation of wavelength ~2^o is detected. The electronic transfer between chains appears to drive the CDW formation.PACS numbers: 61.16. Di, 71.45.Lr, 73.20.Dx, 73.40.Gk A scanning tunneling microscope (STM) study of the compound Ta2NiSe7 shows that electronic modifications characteristic of a charge-density wave (CDW) are observed at 4.2 K. The electronic spectrum measured at 4.2 K shows a large energy gap of ACDW =41.0 ± 3.0 meV and the local density of states (LDOS) shows a modulation with a wavelength of --2bo, consistent with the wave vector of q = (0,0.483,0) measured by Fleming et al.[1] using x-ray and electron diffraction. The STM scans at 4.2 K also show a significant rearrangement of electron density on the chains compared to the uniform density observed at 77 K.The CDW transition occurs at 52.5 K and the resulting electronic rearrangement produces STM scans which show a very complex LDOS within the unit cell characterized by one chain with very strong intensity, and one much weaker chain. However, the CDW modulation along the chains as detected by the STM at 4.2 K is very weak, in contrast to the strong electronic rearrangement between chains. This suggests that the CDW may in fact be driven by the initial electron transfer between chains rather than a simple nesting of the high-temperature Fermi surface, a significantly different sequence than observed in other linear chain CDW materials such as NbSe3.Ta2NiSe7 is a linear chain compound similar in some respects to FeNbaSeio, although the latter is driven into a metal-insulator transition [2] by the formation of a CDW while TaiNiSev remains a semimetal below the CDW transition. The tunneling parameters used for Ta2NiSe7 are very similar to those used [3-6] for the semimetal NbSea and electronic structural changes appear to dominate the STM scans of both materials below the CDW transitions. However, in contrast to NbSes, the CDW amplitude modulation along the chain is extremely weak in Ta2NiSe7 and the STM image is dominated by a very strong charge transfer between the two chains in the unit cell. This gives rise to a strong difference in STM response for the two chains in the unit cell, even though the chains are equivalent with respect to height and atomic structure. Above the CDW transition at 52.5 K they appear equivalent in both STM and AFM images (see Fig. 2 for STM image). In this respect the STM results on...

Angle-dependent magnetoresistance as a sensitive probe of the charge density wave in quasi-one-dimensional semimetal Ta2NiSe7

“…As shown in Fig.1(c, d), Ta2NiSe7 shows a metallic behavior with the current along the chain direction (b axis) in the temperature range of 2~300 K for all magnetic fields between 0 and 14 T. A clear kink at around 61.8 K corresponds to the CDW transition temperature (TCDW). The TCDW of our samples are among the highest in the literature 23,[31][32][33]. MR for magnetic field applied along b (MR 1∥3 ) and c directions (MR 143 ) are both positive, as shown in the insets of Fig.1…”

Band dependence of charge density wave in quasi-one-dimensional Ta2NiSe7 probed by orbital magnetoresistance

“…1(b). T CDW , defined by the peak temperature in dR/dT , is at around 62.5 K. This value is nearly 10 K higher than the 52.5 K reported in earlier literatures 11,14,15 and comparable to the highest reported so far 17 . The large difference in T CDW is attributed to different sample qualities due to the existence of impurities.…”

“…was suggested not to be the Fermi surface nesting effect, but rather, through the charge transfer between the two Ta2 chains 14 . However, X-ray diffraction experiment pointed out that all Ta2 atoms are equivalent in symmetry in the CDW state 15 .…”

“…The consequence, such as the thermal hysteresis of the CDW transition 19 , was indeed found in Ta 2 NiSe 7 11 . CDW gap only removes part of the Fermi surface, so that the system remains semimetallic below CDW transition temperature (T CDW ) 13,14 .…”

Anisotropic Electron–Phonon Interactions and Crystallographic Orientation Identification in Ta2NiSe7 Thin Flakes

“…Ternary chalcogenides Ta 2 NiSe 7 is a typical quasi-one-dimensional vdW layered charge density wave (CDW) materials with an incommensurate structural distortion into CDW order at 52.5 K . Ta 2 NiSe 7 belongs to the monoclinic space group of C 2/ m , and the lattice constants are a = 13.827 Å, b = 3.482 Å, c = 18.577 Å, β = 108.8°, respectively.…”