Physicists have been trying to determine the ground states of 2D electron systems at extremely low densities and temperatures for several decades. The first theoretical predictions for these ground states were put forward by physicists Felix Bloch in 1929 and Eugene Wigner in 1934, both of whom suggested that interactions between electrons could lead to ground states that had never been observed before.
Researchers at Princeton University have been studying this area of physics for many years. His most recent work, featured in physical review papercollected evidence of a new state predicted by Wigner, known as a disordered Wigner solid (WS).
“The phases predicted by Wigner, an ordered array of electrons (the so-called Wigner crystals, or WS), have fascinated scientists for decades,” Mansoor Shaygan, the study’s principal investigator, told Phys.org. “Its experimental realization is extremely challenging, as it requires samples with very low densities and appropriate parameters (large effective mass and small dielectric constant) to enhance the role of the interaction.”
To successfully produce 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 should contain the least amount of impurities, as these impurities can attract electrons and cause them to rearrange themselves randomly.
Since the requirements for the production of these states are very challenging to meet, previous studies investigating quantum WS systems, in which electron–electron interactions dominate at the so-called Fermi energy, have been incredibly rare. The first quantum WS was observed in 1999 by Jongsu Yoon at Princeton University and by some of the researchers involved in the recent study using GaAs/AlGaAs 2D heterostructures.
In their new study, the team used a clean and highly pure 2D AlAs (aluminum arsenide) sample. anistropic (ie, different when measured in different directions) effective mass and the Fermi ocean. Notably, their sample satisfied the requirements for the realization of anisotropic 2D WS very well.
“Our sample is an almost perfect platform for observing quantum WS at zero Magnetic FieldNow, it turns out that the 2d electrons in ALA provide an added 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 properties of WS such as its resistance and de-pinning threshold along different in-plane directions.
The materials used by Shaygan and his colleagues in their experiments contain a high quality ALAS quantum well with very few impurities and thus low disorder. In this quantum well, electrons are confined in 2 dimensions.
“We can use the gate voltage to tune the density of electrons in our sample,” Shafayet Hussain, MD, lead author of the paper, told Phys.org. “We used a combination of electrical transport (i.e., measurement of resistivity) and DC bias spectroscopy (ie, measurement of differential resistance as a function of source-drain DC bias) to study anisotropic 2D disordered Wigner solids. “
The team’s measurements of the sample’s resistivity and differential resistance showed that they had indeed observed a new quantum WS at zero magnetic field using an anisotropic material system. Ultimately, this allowed them to uncover the effects of anisotropy on the elusive but fascinating WS state.
“The observed Wigner solids show different effective sliding capabilities in different directions,” Hussain said. “This is manifested through the different de-pinning threshold voltages along the different directions observed in our experiments.”
The anisotropic WS state observed by this team of researchers is likely to be a new quantum state entirely. This means that very little is known about its properties and characteristics as of now.
In the future, these recent findings may thus inspire new theoretical and experimental studies, which aim to attribute this newly identified quantum state to an intrinsic anisotropy (i.e., with different values when measured in different directions). To understand better. For example, these studies may attempt to determine the state-specific lattice shape.
“Based on our experimental findings, different electronic behaviors of anisotropic WS with different directions can also be used in electronic devices,” Hussain said. “Such devices may respond differently depending on the direction of the applied voltage.”
Ultimately, the anisotropic WS uncovered by this team of researchers may pave the way for the development of new types of anisotropic quantum devices. In their next works, Shaygan, Hussein and their colleagues will investigate the state’s microwave resonances, as these may provide more information about the state and its anisotropy.
“For example, we would ask: does WS show resonance, as seen in the case of magnetic-field-induced WS, at very small fillings (high magnetic fields)?” Shaygan added. “Observing the resonances would be very helpful because they would provide strong evidence for the WS phase. Furthermore, observing resonances whose frequencies depend on the orientation of the applied electric field with respect to the orientation of the WS crystal would be attractive, and the light Will shed on the role of anisotropy.”
Md. S. Hossain et al, Anisotropic Two-Dimensional Disorder Wigner Solid, physical review paper (2022). DOI: 10.1103/PhysRevLet.129.036601
Jongsu Yoon et al., Wigner crystallization and transition of two-dimensional holes in the metal-insulator GaAs b = 0, physical review paper (2002). DOI: 10.1103/PhysRevLet.82.1744
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Citation: Evidence for a new type of disordered quantum Wigner solid (2022, August 3) Retrieved 4 August 2022 from https://phys.org/news/2022-08-evidence-disordered-quantum-wigner-solid.html .
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