Optical study on quantum phase transitions in two-dimensional transition metal dichalcogenides (TMDs)

It is especially interesting to research different types of interactions to understand the physics behind that mechanisms. For example spin interactions like Anti-ferromagnetic ordering or spin-liquid ordering, charge interactions as Mott insulators or electron-lattice-interactions like superconductivity.

Crystal Structure of 1T-TaS2
Crystal structure of 1T-TaS2.
Commensurate charge density wave formation in 1T-TaS2.
Gasparov et al, Phonon anomaly at the charge ordering transition in 1T-TaS2, Phys. Rev. B, (2002), DOI: 10.1103/PhysRevB.66.094301

Phase Diagram of 1T-TaS(2-x)Sex
Phase diagram of 1T-TaS2. Source:
Han et al., Exploration of metastability and hidden phases in correlated electron crystals visualized by femtosecond optical doping and electron crystallography. Science Advances, (2015), DOI: 10.1126/sciadv.1400173

A. TMDCs and 1T-TaS2

Over the past years transition metal dichalcogenides (TMDCs) has been researched inten-
sively for their unique properties. TMDCs have the general form MX2, where M denotes a transition metal element from group IV, V or VI and X denotes a chalcogen atom like S, Se or Te. They form sandwich like X-M-X structures. In this class of materials 1T-TaS2 is of specifc interest as it’s rich phase diagram is especially interesting because of the many different phases occurring as seen in the figure above. The material does not only show superconductivity but also different charge-density wave
phases, a Mott insulating phase and additionally TaS2 is also a promising candidate for a
quantum spin liquid. Below 180K the Ta atoms form a √ 13 x √ 13 long range superlattice with 13 Ta atoms, the famous Star of David pattern. A phase change from the nearly commensurate CDW (NCCDW) phase to the commensurate CDW (CCDW) phase happens and the 5d orbital splits into dz² , dx²-y² and dxy subbands. The different subbands are separated by energy gaps especially a Mott-gap and two CDW-gaps.

B. Charge-density waves and Mott insulator

Of particular interest for us are both charge-density waves (CDW) and the Mott insulating
phase. A CDW describes a change in periodicity of the atoms and thus a modulation of the charge density. Also the conduction band splits up and a gap emerges. A Mott insulator describes a strong correlated material which is insulating because of the strong coulomb repulsion, but should be metals. The Coulomb energy U is competing with the overlap integral t and if it is greater, the electrons become localised due to the increased potential wall, as the hopping between lattice sites is restricted.
Therefore, the conduction band splits up and a new gap appears.

C. Domains

With the formation of a superlattice di fferent domains are emerging. As the hopping is
increased in the domain walls another transition is happening inside the Mott gap.

This transition was analized optically.


Reflectivity data of a 1T-TaS2 sample
Optical reflectivity of the sample at different temperatures.
Optical conductivity of a 1T-TaS2 sample
Optical Conductivity data at different temperatures.
Optical conductivity of 1T-TaS2 at 10 K.
Optical conductivity of 1T-TaS2 at 10 K.

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