Evidence of a New Type of Disordered Quantum Wigner Solid

Evidence of a New Type of Disordered Quantum Wigner Solid

Artistic representation of disordered anisotropic Wigner solid consisting of frozen electrons (fixed by the disorder) arranged in an anisotropic lattice. Credit: Hossain et al.

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

Princeton University researchers have been conducting research in this field of physics for several years now. Their most recent work, featured in Physical Assessment Letterscollected evidence of a new state predicted by Wigner, known as a Wigner disordered solid (WS).

“The phase predicted by Wigner, an ordered array of electrons (called the Wigner crystal or WS), has fascinated scientists for decades,” Mansour Shayegan, the study’s principal investigator, told Phys.org. “Its experimental realization is extremely challenging, as samples with very low densities and with suitable parameters (large effective mass and small dielectric constant) are needed to enhance the role of interaction.”

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

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

In their new study, the team used a clean and highly pure 2D AlAs (aluminium arsenide) sample with a anisotropic (ie different when measured in different directions) effective mass and Fermizee. In particular, their sample met the requirements for the realization of an anisotropic 2D WS very well.

“Our sample is an almost ideal platform for observing a quantum WS at zero magnetic field‘ said Shayegan. ‘Now the 2D electrons in AlAs appear to offer an extra bonus, namely an anisotropic energy band dispersion that leads to an anisotropic effective mass. What we found is that this anisotropy can manifest itself in the properties of the WS, such as the resistance and the pinning threshold in different directions in the plane.

The material Shayegan and his colleagues use in their experiments consists of a high-quality AlAs quantum source, with very few impurities and thus little disorder. In this quantum well, electrons are locked within 2 dimensions.

“We can use gate voltage to tune the density of the electrons in our sample,” Md Shafayat Hossain, lead author of the paper, told Phys.org. “We used a combination of electrical transport (ie, measurements of resistivity) and DC bias spectroscopy (ie, measurement of differential resistance as a function of source-drain DC bias) 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 a zero magnetic field, using an anisotropic material system. Ultimately, this allowed them to discover the effects of anisotropy on the elusive but fascinating WS state.

“Wigner’s observed solid shows different effective sliding possibilities in different directions,” Hossain said. “This is manifested through different de-pinning threshold voltages along different directions observed in our experiments.”

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

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

“Based on our experimental findings, the different electronic behavior along different directions of anisotropic WSs may also be of use in electronic devices,” Hossain said. “Such devices may respond differently depending on the direction of the applied voltage.”

Ultimately, 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 examine the microwave resonances of the state they have discovered, as these can provide more detail about the state and its anisotropy.

“For example, we will ask: does the WS exhibit resonances, similar to what has been seen in the case of magnetic field-induced WS, at very small fills (high magnetic fields)?” added Shayegan. “Observing 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 with respect to the orientation of the WS crystal would be fascinating and shed light on the role of anisotropy.”

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More information:
Md. S. Hossain et al, Anisotropic Two-Dimensional Disordered Wigner Solid, Physical Assessment 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 Assessment Letters (2002). DOI: 10.1103/PhysRevLett.82.1744

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Quote: Evidence of a new type of disordered quantum Wigner Solid (2022, August 3), retrieved August 4, 2022 from https://phys.org/news/2022-08-evidence-disordered-quantum-wigner-solid.html

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