Abstract
Transition metal dichalcogenides are lamellar materials which can exhibit unique and remarkable electronic behavior due to effects of electron-electron and electron-phonon coupling. Among these materials, 1T-tantalum disulfide (1T-TaS2) has spurred considerable interest, due to its multiple first order phase transitions between different charge density wave (CDW) states. In general, the basic effects of charge density wave formation in 1T-TaS2 can be attributed to in plane re-orientation of Ta-atoms during the phase transitions. Only in recent years, an increasing number of studies has also emphasized the role of interlayer interaction and stacking order as a crucial aspect to understand the specific electronic behavior of 1T-TaS2, especially for technological systems with a finite number of layers. Obviously, continuously monitoring the out of plane expansion of the sample can provide direct inside into the rearrangement of the layer structure during the phase transition. In this letter, we therefore investigate the c-axis lattice discontinuities of 1T-TaS2 by atomic force microscopy (AFM) method under ultra-high vacuum conditions. We find that the c-axis lattice experiences a sudden contraction across the nearly-commensurate CDW (NC-CDW) phase to commensurate CDW (C-CDW) phase transition during cooling, while an expansion is found during the transition from the C-CDW phase to a triclinic CDW phase during heating. Thereby our measurements reveal, how higher order C-CDW phase can favor a more dense stacking. Additionally, our measurements also show subtler effects like e.g. two expansion peaks at the start of the transitions, which can provide further insight into the mechanisms at the onset of CDW phase transitions.
Highlights
During the past decades, it was found, that first order phase transitions in quasi-two-dimensional (2D) materials can lead to a variety of interesting phenomena, such as superconductivity, occurrence of charge density waves, or friction anomalies[1,2,3,4,5,6]
Cooling down from temperatures below this transition temperature, 1T-TaS2 experiences a number of reversible phase transitions: At 543 K the material transforms from a metallic phase to an incommensurate charge density wave (CDW) (IC-CDW) where Ta atoms shift slightly from their original positions and form an incommensurate CDW phase
347 K, a nearly-commensurate CDW (NC-CDW) phase is formed with partial structures of reorientation being commensurate to the original lattice
Summary
It was found, that first order phase transitions in quasi-two-dimensional (2D) materials can lead to a variety of interesting phenomena, such as superconductivity, occurrence of charge density waves, or friction anomalies[1,2,3,4,5,6]. Www.nature.com/scientificreports and stacking order on the electronic properties[19,20,21,22,23,24], an effect that can be especially relevant for nanodevices with only few layers of 1T-TaS216 In this context, accurately measuring the z-axis lattice discontinuity across a phase transition, can be considered as a fundamental approach providing direct insight into the mechanisms of the first order phase transitions but these out of plane expansion effects should reflect changes in the stacking order of the material. The c-axis lattice (i.e. the z-direction perpendicular to the layers) experiences a sudden expansion during the transition from the nearly commensurate CDW structure to the commensurate CDW structure upon cooling and a sudden contraction during the transition from the commensurate CDW structure to the triclinic CDW structure upon heating, a behavior that is rationalized by thermodynamic considerations based on the Clausius-Clapeyron equations and experimental results of the entropy change and the pressure dependence of the transition temperature
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