Master's Thesis

Quantum phase transitions in two-dimensional transition metal dichalcogenides (TMDs)

Optical measurements, 2D materials, Quantum phase transition, Mott transition


Two-dimensional (2D) layered materials including graphene, transition metal dichalcogenides (TMDs), and black phosphorus are attracting great attention [1], because they display intriguing physical and chemical properties, ranging from charge density wave (CDW), quantum Hall effect to superconductivity due to the complex interplay of charge, spin and orbital degrees of freedom. The ability to control these properties plays a key role in technological developments, such as new generation of photonic and memory devices.

Among these 2D families, transition metal dichalcogenides (TMDs) 1T-TaS2 are of particular interest due to its multi-charge-wave-density ground states [2], which can be continuously manipulated via external stimuli, such as temperature, thickness, intense electric field, carrier doping and hydrostatic pressure. The fundamental understanding of the physical mechanisms underlying the phase transitions between these ground states, i.e., quantum phase transitions, would be an important step for designing new functional materials.

In this bachelor/master project, we will perform broadband Fourier-transform infrared spectroscopy (FTIR) studies on a series of Se-doped sample 1T-TaS2-xSex [3] to investigate the mechanism of insulator-metal phase transition. Specifically, we want to clarify the following questions:

  • How do the Mott-gap and charge transfer gap evolve upon chemical doping and decreasing temperature?
  • What is the effect of disorder on the low-energy density of state (DOS)?
  • Can we identify the symmetry of the CDW order using polarized light?
  • What are the dynamic properties of those itinerant electrons in the metallic state?


[1] Novoselov, K. S. et al. 2D materials and van der Waals heterostructures. Science 353, aac9439 (2016).

[2] T. Han. et al.  Exploration of metastability and hidden phases in correlated electron crystals visualized by femtosecond optical doping and electron crystallography Sci. Adv. 1, e1400173 (2015).

[3] Y. Liu. et al. Superconductivity induced by Se-doping in layered charge-density-wave system 1T-TaS2-xSex Appl. Phys. Lett. 102, 192602 (2013)

[4] J. Gao. et al. Pseudogap and weak multifractality in disordered Mott charge-density-wave insulator arXiv:1904.04508v1.(2019)


Optical measurements, 2D materials, Quantum phase transition, Mott transition


Tobias Biesner

Tobias Biesner

Quantum Spin Liquids

Martin Dressel
Prof. Dr. rer. nat.

Martin Dressel

Head of Institute

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