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1Wigner crystal state in two-dimensional semiconductor
A. Abramov1'2*, E. Chiglintsev2'3, A. Chernov2'3, V. Kravtsov1'2
1- School of Physics and Engineering, ITMO University, Saint-Petersburg, Russia 2- Russian Quantum Center, Skolkovo, Moscow, Russia
3- Moscow Institute of Physics and Technology (National Research University), Dolgoprudny, Russia
* artem.abramov@metalab.ifmo.ru
Two-dimensional materials are of great interest as a platform for the development of new optoelectronic devices. This is achieved by making the optical and transport properties of two-dimensional materials easily tunable by external influences such as material deformation, application of an external electromagnetic field, or doping. Therefore, the study of strongly correlated electronic states is important for understanding the processes occurring in two-dimensional materials. Electrons break the symmetry of continuous translation and form a periodic lattice (Wigner crystal) when Coulomb repulsion begins to dominate over their kinetic energy [1]. However, the creation of the necessary conditions for the formation of such a state has so far remained a difficult task. The main method of research was the measurement of the conductivity of electrons in semiconductors at one Landau level under the action of a strong magnetic field. Another way to observe a Wigner crystal is to create a Moiré potential [2]. The use of transition metal dichalcogenide monolayers for Wigner crystal observation is possible due to a high electron effective mass and reduced dielectric screening, which allows the observation of charge order even at zero magnetic field [3].
In this work, we demonstrate the Wigner crystal by measuring the optical reflectance spectra of a doped WSe2 monolayer at a temperature of 8 K. For this purpose, we assembled a structure consisting of a top and bottom graphene gate and a WSe2 monolayer separated by layers of dielectric hexagonal boron nitride. Monolayers of two-dimensional materials were obtained by mechanical exfoliation and then the structure was assembled by polycarbonate transfer on sapphire substrate. Then, the gold contacts were brought to the structure using high-resolution electronic lithography. The obtained sample was investigated by reflectance spectroscopy in a closed-loop helium cryostat. Illumination by the lamp light and collection of the reflected signal were performed using a 50x objective mounted on three-axis piezotranslator. The doping of the WSe2 monolayer was carried out by setting the voltage between the monolayer and one of the gates, while the other gate was grounded. The blue shift of the main exciton resonance X with energy of 1.72 eV and the appearance of trions with energies of 1.69-1.70 eV due to the increase in carrier density can be observed on the dependence of the reflection spectrum of the WSe2 monolayer. However, when carefully examining the region with low charge densities, we also observe another high-energy resonance, which, due to the low power of the oscillator, is visible only on the energydifferentiated reflection map. We associate this feature with high-energy excitonic umklapp resonances, which indicates the appearance of a Wigner crystal at a low electron density value [4].
The results of our research work are important for studying strong electron correlated states in structures based on two-dimensional materials.
[1] E. Wigner, On the interaction of electrons in metals, Phys. Rev. 46, 1002-1011 (1934).
[2] T. Smolenski, et al, Signatures of Wigner crystal of electrons in a monolayer semiconductor, Nature, 595, 53-57 (2021).
[3] E.C. Regan, et al, Mott and generalized Wigner crystal states in WSe2/WS2 moiré superlattices, Nature, 579, 359-363 (2021).
[4] Y. Shimazaki, et al, Optical signatures of periodic charge distribution in a Mott-like correlated insulator state. Physical Review X. 11, 021027 (2021).
