Kavli Affiliate: Feng Wang
| First 5 Authors: Su-Di Chen, Su-Di Chen, , ,
| Summary:
In clean two-dimensional (2D) systems, electrons are expected to
self-organize into a regular lattice, a Wigner crystal, when their mutual
Coulomb repulsion overwhelms kinetic energy. Understanding the Wigner crystal
at zero magnetic field is a long-sought goal in physics, thanks to its
fundamental simplicity and possible connection to the density-driven
metal-insulator transition. To date, evidence for such a crystal has been
reported across various platforms. However, the AC conductivity of a zero-field
Wigner crystal, a key observable characterizing its electrodynamics, has never
been measured. Here, we develop an ultrasensitive on-chip terahertz (THz)
spectroscopy technique to probe the AC conductivity in electrostatically gated
monolayer MoSe2 encapsulated in hexagonal boron nitride. We observe a sub-THz
resonance corresponding to the pinning mode of a zero-field Wigner crystal,
whose frequency is orders of magnitude higher than those under high magnetic
fields. Using the pinning mode as an indicator, we reveal that moderate
disorder notably stabilizes the Wigner crystal. With increasing density towards
melting, we find that the pinning mode of the Wigner crystal coexists with a
growing Drude component characteristic of an electron liquid, and the
competition between these two components in the conductivity spectra leads to
the insulator-metal transition of the 2D electron system. Our findings not only
elucidate the low-energy electrodynamics of a zero-field Wigner crystal, but
also establish on-chip THz spectroscopy as a powerful probe for correlated
quantum phases in two-dimensional materials.
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