Evidence for a new type of disordered Wigner quantum solid

Artistic representation of a disordered anisotropic Wigner solid composed of frozen (disorderly pinned) electrons arranged in an anisotropic lattice. Credit: Hossain et al.

For several decades, physicists have been trying to determine the ground states of 2D electron systems at extremely low densities and temperatures. The first theoretical predictions of these ground states were made by physicists Felix Bloch in 1929 and Eugene Wigner in 1934, who both suggested that interactions between electrons could lead to ground states that had never been observed before.

Researchers at Princeton University have been conducting studies in this area of ​​physics for several years now. Their most recent work, featured in Physical examination lettersgathered evidence for a new state that had been predicted by Wigner, known as the disordered Wigner solid (WS).

“Wigner’s predicted phase, an ordered lattice of electrons (the so-called Wigner crystal or WS), has fascinated scientists for decades,” Mansour Shayegan, lead researcher of the study, told Phys.org. “Its experimental realization is extremely difficult, because it requires samples with very low densities and with appropriate parameters (large effective mass and small dielectric constant) to reinforce the role of the interaction.”

To successfully produce a WS or quantum WS in the lab, researchers need extremely pure, high-quality samples. This means that the substances they use in their experiments must contain a minimum number of impurities, because these impurities can attract electrons and cause them to rearrange themselves randomly.

Since it is very difficult to meet the production requirements of these states, previous studies of quantum WS systems, in which electron-electron interactions dominate over the so-called Fermi energy, have been incredibly rare. The first quantum WS was observed in 1999 by Jongsoo Yoon of Princeton University and some of the researchers involved in the recent study, using a 2D GaAs/AlGaAs heterostructure.

In their new study, the team used a clean and very pure 2D AlAs (aluminum arsenide) sample with an anisotropic effective mass (i.e. different when measured in different directions) and a Fermi sea. In particular, their sample responded very well to the requirements for the realization of an anisotropic 2D WS.

“Our sample is an almost ideal platform for observing a zero magnetic field quantum WS,” Shayegan said. “Now it turns out that the 2D electrons in AlAs provide an added bonus, namely an anisotropic energy band dispersion which leads to an anisotropic effective mass. What we have discovered is that this anisotropy can manifest in the properties of the WS such as its resistance and unanchoring threshold along different directions in the plane.

The material used by Shayegan and his colleagues in their experiments consists of a high-quality AlAs quantum well, with very few impurities and therefore little disorder. In this quantum well, the electrons are confined in 2 dimensions.

“We can use the gate voltage to tune the electron density in our sample,” Md Shafayat Hossain, lead author of the paper, told Phys.org. “We used a combination of electrical transport (i.e. resistivity measurements) and DC bias spectroscopy (i.e. a measurement of differential resistance as a function of source-DC bias). drain) to study the anisotropic 2D disordered Wigner solid.”

Measurements of the resistivity and differential resistance of the team’s sample showed that they had in fact observed a new quantum WS at zero magnetic field, using a system of anisotropic materials. Ultimately, this allowed them to discover the effects of anisotropy on the elusive but fascinating WS state.

“The observed Wigner solid shows different effective sliding abilities in different directions,” Hossain said. “This is manifested by different stall threshold voltages in different directions seen in our experiments.”

The anisotropic WS state observed by this team of researchers is probably an entirely new quantum state. This means that so far very little is known about its properties and characteristics.

In the future, these recent discoveries could thus inspire new theoretical and experimental studies aimed at better understanding this newly identified quantum state with intrinsic anisotropy (i.e. with different values ​​when measured in different directions ). These studies could, for example, try to determine the characteristic shape of the state network.

“Based on our experimental findings, the different electronic behaviors in different directions of anisotropic WSs may also be useful in electronic devices,” Hossain said. “Such devices could react differently depending on the direction of the applied voltage.”

Eventually, the anisotropic WS discovered by this team of researchers could pave the way for the development of new types of anisotropic quantum devices. In their next work, Shayegan, Hossain and their colleagues will probe the microwave resonances of the state they discovered, as these could provide more details about the state and its anisotropy.

“For example, we will ask: does the WS exhibit resonances, similar to what has been observed in the case of magnetic field-induced WSs, at very small fills (high magnetic fields)?” Shayegan added. “Observing the resonances would be very useful as they would provide strong evidence for the WS phase. Also, observing resonances whose frequencies depend on the orientation of the applied electric field relative to the orientation of the WS crystal would be fascinating and would shed light on the role of anisotropy.”

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More information:
Md. S. Hossain et al, Disordered two-dimensional anisotropic Wigner solid, Physical examination letters (2022). DOI: 10.1103/PhysRevLett.129.036601

Jongsoo Yoon et al, Wigner crystallization and metal-insulator transition of two-dimensional holes in GaAs at B=0, Physical examination letters (2002). DOI: 10.1103/PhysRevLett.82.1744

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