Session K45: MYRIAM SARACHIK SYMPOSIUM: Wigner Crystals and the Quantum Wigner Transition - New Developments

Opening Remarks-APS PresidentFrances Hellman and DCMP Past Chair Eva Andrei

Myriam Sarachik [1933-2021] was a pioneer who did groundbreaking work in condensed matter physics and was deeply dedicated to promoting inclusivity and championing human rights of scientists. Among her many recognitions, she received the 2020 APS Medal for Exceptional Achievement in Research for her "fundamental contributions to the physics of electronic transport in solids and molecular magnetism." Ever delighting in new physics, her recent excitement included witnessing the latest developments in the realm of Wigner crystals; this symposium is dedicated to her.    

Optical detection of charge order in a Wigner crystal

In this talk, I will describe recent experiments in atomically-thin transition metal dichalcogenides (TMDs) where Coulomb interactions between electrons dominate over their kinetic energy. Our measurements provide a direct evidence that the electrons at densities < 3 · 10¹¹ cm^-2 in a pristine MoSe₂ monolayer form a Wigner crystal even at B = 0 [1]. This is revealed by our low-temperature (T = 80 mK) magneto-optical spectroscopy experiments that utilize a newly developed technique allowing to unequivocally detect charge order in an electronic Mott-insulator state [2]. This method relies on the modification of excitonic band structure arising due to the periodic potential experienced by the excitons interacting with a crystalline electronic lattice. Under such conditions, optically-inactive exciton states with finite momentum matching the reciprocal Wigner lattice vector k = k­_W get Bragg scattered back to the light cone, where they hybridize with the zero-momentum bright exciton states. This leads to emergence of a new, umklapp peak in the optical spectrum heralding the presence of periodically-ordered electronic lattice.   

Microscopic Visualization of Correlated Electrons: from Moiré Flat Band to Generalized Wigner Crystal

Van der Waals heterostructures of atomically thin transition metal dichalcogenides provide an exciting new platform to design and fabricate novel electronic and optical devices. Through the precise control of the stacking order and the twist angle between two adjacent layers, the moiré superlattice can lead to tunable narrow electronic minibands, where long-range Coulomb interactions play a critical role in determining strongly correlated electron states. This has led to the observation of the Mott insulating state at half filling, as well as the generalized Wigner crystal states at fractional fillings. However, the direct microscopic understanding of these emerging quantum phases has long been hindered by many experimental challenges. In this talk, I will present a series of technical advancements in scanning tunneling microscopy which allow us to directly visualize the correlated phases in the closely aligned WS₂/WSe₂ moiré superlattices.   

Bilayer Wigner Crystals on an Atomically Thin Canvas

One of the first theoretically predicted manifestations of strong interactions in many-electron systems was the Wigner crystal, in which electrons crystallize into a regular lattice. Quantum melting of the Wigner crystal is predicted to produce exotic intermediate phases and quantum magnetism because of the intricate interplay of Coulomb interactions and kinetic energy. However, studying two-dimensional Wigner crystals in the quantum regime has often required a strong magnetic field or a moiré superlattice potential, thus limiting access to the full phase diagram of the interacting electron liquid. In my talk, I will discuss our recent observation of bilayer Wigner crystals without magnetic fields or moiré potentials in an atomically thin transition metal dichalcogenide heterostructure, which consists of two MoSe₂ monolayers separated by hexagonal boron nitride. We observe optical signatures of robust correlated insulating states at symmetric (1:1) and asymmetric (3:1, 4:1, and 7:1) electron doping of the two MoSe₂ layers at cryogenic temperatures. We attribute these features to bilayer Wigner crystals composed of two interlocked commensurate triangular electron lattices, stabilized by inter-layer interaction. The Wigner crystal phases are remarkably stable and undergo quantum and thermal melting transitions at electron densities of up to 6 × 10¹² per square centimeter and temperatures of up to about 40 Kelvin. I will end the talk by discussing ongoing experimental studies of the spin orders near the quantum phase transition points in mono- and bilayer Wigner crystals.   

Nonlinear transport of Wigner crystal on liquid helium in microchannel devices

Electrons trapped on the surface of liquid helium present the cleanest Coulomb system that can be found in nature. The ground state of the system is a classical 2D Wigner crystal, which is realized already at moderately low temperatures around 1K and without magnetic field. The surface of liquid helium substrate is free from disorder. On the other hand, the Wigner crystal can strongly interact with the quantized field of capillary waves on the helium surface, ripplons. This provides a unique opportunity to study the coupled dynamics of a charged system and medium excitations in a disorder-free environment.

It was demonstrated that confining electrons in capillary-condensed microchannel devices provides important advatages for studying such dynamics via the transport measurements [1]. In particular, the time-resolved transport measurements revealed an interesting stick-slip dynamics and provided an insight into dynamic phases of the driven system [2]. A novel regime of ripplo-polaronic charge transport of Wigner crystal was demonstrated in a three-terminal microchannel device [3]. Finally, such studies could be interesting in the general context of friction whose microscopic mechanism presents a challenging question.   

One dimensional quantum Wigner crystals and SU(4) magnetism

The quantum crystal of electrons, predicted by Jenő Wigner almost 90 years ago, is one of the most elusive states of matter. In the very dilute limit, where the crystal forms, disorder effects and inhomogeneity very easily destroy this fragile state of matter, which is therefore very hard to be observed in its pristine form. It is also a major challenge for theorists to produce quantitative results in this strongly interacting, dilute limit, where the melting of the crystal occurs. We present corroborated experimental  and theoretical results, which lead to a recent, direct observation of the spatial crystal structure of one dimensional Wigner quantum crystals in carbon nanotubes [1]. In the experiments, we a non-invasive single electron (hole) probe is employed, and we compare the experimental results with self-consistent DMRG simulations. We also investigate the tunneling of the crystal in a double well potential, and demonstrate by comparing the experimental observations with instanton as well as DMRG computations  that tunneling of the quantum crystal is a collective phenomenon, which involves all electrons or holes forming the crystal [1,2]. Electrons (holes) in carbon nanotube also possess SU(4) spins, which yields an SU(4) antiferromagnetic Wigner crystals phase at low temperatures. We argue that earlier transport experiments [Nat. Phys. 4, 314 (2008)] may be explained in terms of segregated magnetic phases [3]. Transport spectroscopy of Wigner molecules should reveal the magnetic excitation spectrum and thus the SU(4) symmetry of the exchange interaction.