The phononic and charge density wave behavior of entire rare-earth tritelluride series with chemical pressure and temperature

Abstract

Here, we present comprehensive phononic and charge density wave properties (CDW) of rare-earth van der Waals tritellurides through temperature dependent angle-resolved Raman spectroscopy measurements. All the possible rare-earth tritellurides (RTe3) ranging from R = La–Nd, Sm, Gd–Tm were synthesized through a chemical vapor transport technique to achieve high quality crystals with excellent CDW characteristics. Raman spectroscopy studies successfully identify the emergence of the CDW state and transition temperature (TCDW), which offers a non-destructive method to identify their CDW response with micron spatial resolution. Temperature dependent Raman measurements further correlate how the atomic mass of metal cations and the resulting chemical pressure influence its CDW properties and offer detailed insight into the strength of CDW amplitude mode-phonon coupling during the CDW transition. Angle-resolved Raman measurements offer the first insights into the CDW-phonon symmetry interplay by monitoring the change in the symmetry of phonon mode across the CDW transition. Overall results introduce the library of RTe3 CDW materials and establish their characteristics through the non-destructive angle-resolved Raman spectroscopy technique.

Experimental and Theoretical Studies of the Surface Oxidation Process of Rare‐Earth Tritellurides

“…Previously, the temperature variation of Raman modes was obtained by several authors to track the anti‐crossing interaction between coupled phonon and amplitude mode and to determine the T CDW . [ 12,31,40 ] As shown in Figure 6c, such anti‐crossing behavior was observed for both pristine and aged DyTe 3 samples, where the frequency of CDW amplitude (at 68 cm −1 ) and phonon (at 55 cm −1 ) mode reduces as temperature increases starting from 80 K. Subsequently, at around 140 K **nature of both branches changes, and the top one is more phononic, whereas the bottom resembles amplitude mode. It can be seen that amplitude mode softens much quicker compared to phonon mode since the temperature diminishes the CDW order abruptly.…”

“…Here in Figure 3a, freshly exfoliated LaTe 3 sheets exhibit three prominent optical modes located at 88, 97, and 105 cm −1 , whereas the low-frequency Raman peak (amplitude mode at 70 cm −1 ) is related to the CDW phase since LaTe 3 forms that phase at room temperature. [12,31] In situ Raman measurements show that 10 days after preparing fresh thin crystal by exfoliation, new peaks start to emerge at around 128 and 145 cm −1 . At the same time fundamental optical modes and CDW amplitude mode gradually reduce in Raman intensity across ≈2 months.…”

“…NdTe 3 , SmTe 3 , GdTe 3 , and DyTe 3 crystals were realized using the CVT technique from elemental precursors of metal lanthanide and tellurium powders (99.999%, Alfa Aesar). [ 31 ] The typical growth procedure involved sealing these precursors with 20 mg of iodine (I 2 ) as a transporting agent in a 2 mm thick quartz ampoule and subsequently pumping it down to pressure better than 10 −5 Torr. Crystal growth was realized at 830 °C with Δ T = 10 °C thermal gradient for 1–2 weeks to produce millimeter to centimeter‐sized crystals.…”

Solution Synthesis of NdTe3 Magnetic Nanosheets

“…In the lanthanide polytelluride family, as the tellurium becomes more anionic, the magnetic coupling weakens, as a result of increased ionicity and the opening of a bandgap. 24 For example, the related reduced polytelluride, NdTe 1.89 , is a paramagnetic semiconductor with a theta of −4.9K. 78 This effect might have been expected for the thick NdTe 3 with the terminating layer [Nd 3+ Te 2− ] + [Cl] − , which behaved closer to the bulk but may explain the weaker peak at the Neél temperature.…”

“…21,22 The telluride layers also host charge density waves (CDWs), which are formed through the condensation of the pseudo-square lattice into telluride oligomers. 23 The CDW is stable to unusually high temperatures, ranging from >500 K for LaTe 3 24 down to 244 K for TmTe 3 , 25 where the transition temperature varies regularly with lanthanide ionic radii. The structural and electronic anisotropy is echoed in the magnetic properties, where the magnetic moment aligns in the [LnTe] + plane for Ln = Ce, Nd, and Sm.…”