Electrons occupy the fast and slow lanes at the same time

Figure 1. Parabolas for the spin (green) and charge (magenta) excitations. Inset shows loading line in more detail. Credit: research team, Cavendish Laboratory, Department of Physics, University of Cambridge

Imagine a road with two lanes in each direction. One lane is for slow cars and the other for fast ones. For electrons moving along a quantum wire, researchers in Cambridge and Frankfurt have found that there are also two “orbits”, but electrons can take both at the same time!

Current in a wire is carried by the flow of electrons. If the wire is very narrow (one-dimensional, 1D), electrons cannot pass each other, because they repel each other strongly. Current, or energy, is instead carried by compression waves as one particle pushes on the next.

It has long been known that there are two types of excitation for electrons as they have a property called spin in addition to their charge. Spin and charge excitations travel at fixed, but different velocities, as predicted many decades ago by the Tomonaga-Luttinger model. However, theorists are unable to calculate what exactly happens outside of small perturbations because the interactions are too complex. The Cambridge team measured these velocities because their energies vary, and found that a very simple picture emerges (now published in the journal scientific progress† Any type of excitation can have low or high kinetic energy, like cars on a road, with the well-known formula E=1/2 mv2, which is a parabola. But to keep the mass spinning and charging m are different, and since charges repel and thus cannot occupy the same state as another charge, there is twice as wide a range of momentum for charge as for spin. The results measure energy as a function of magnetic field, which is equal to momentum or velocity vshowing these two energy parabolas, which can be seen at places up to five times the highest energy occupied by electrons in the system.

Electrons occupy the fast and slow lanes at the same time

Figure 2. Spin (green) and charge (‘holon’, magenta) excitations in a 1D wire. Credit: research team, Cavendish Laboratory, Department of Physics, University of Cambridge

“It’s as if the cars (like payloads) are moving in the slow lane, but their passengers (like spins) are moving faster, in the fast lane,” explains Pedro Vianez, who conducted the measurements for his Ph.D. at the Cavendish laboratory in Cambridge. “Even when the cars and passengers slow down or speed up, they stay separated!”

What’s remarkable here is that we’re no longer talking about electrons, but instead about compound (quasi-)particles of spin and charge — commonly called spinons and holons, respectively. High energies, but what is observed points to the exact opposite – they seem to behave in a way very similar to normal, free, stable electrons, each with their own mass, except that they are in fact not electrons, but excitations of a whole sea of ​​charges or spins!” said Oleksandr Tsyplyatyev, the theorist who led the work at Goethe University in Frankfurt.

“This paper represents the culmination of more than a decade of experimental and theoretical work on the physics of one-dimensional systems,” said Chris Ford, who led the experimental team. “We were always curious to see what would happen if we took the system to higher energies, so we gradually improved our measurement resolution to discover new features. We fabricated a series of semiconductor arrays of wires from 1 to 18 microns in length. (that is, to a thousandth of a millimeter or about 100 times thinner than a human hair), with only 30 electrons in a wire, and they measured at 0.3 K (or in other words, -272.85 C, ten times colder than space).”

Electrons occupy the fast and slow lanes at the same time

Figure 3a. Scanning electron micrographs of a device, showing the different ports used to define the 1D wires (part 1). Credit: research team, Cavendish Laboratory, Department of Physics, University of Cambridge

Details about experimental technique

Electrons tunnel from the 1D wires to an adjacent two-dimensional electron gas, which acts as a spectrometer and produces a map of the relationship between energy and momentum. “This technique is very similar in all respects to angle-resolved photoemission spectroscopy (ARPES), a widely used method for determining the band structure of materials in condensed matter physics. The main difference is that, rather than at the surface our system is buried a hundred nanometers beneath it,” says Vianez. This allowed the researchers to achieve resolution and control unprecedented for this type of spectroscopy experiment.

Electrons occupy the fast and slow lanes at the same time

Figure 3b. Scanning electron micrographs of a device, showing the different ports used to define the 1D wires (part 2). Credit: research team, Cavendish Laboratory, Department of Physics, University of Cambridge

Conclusion

These results now open the question of whether this spin-charge separation of the entire electron sea remains robust beyond 1D, for example in superconducting materials at high temperature. It can now also be applied to logic devices using spin (spintronics), which provide a drastic reduction (by three orders of magnitude!) in the power consumption of a transistor, while improving our understanding of quantum matter and providing a new tool for constructing quantum materials.


Quantum simulator shows how parts of electrons move at different speeds in 1D


More information:
Pedro MT Vianez et al, Observing separate spin and charge Fermi seas in a highly correlated one-dimensional conductor, scientific progress (2022). DOI: 10.1126/sciaadv.abm2781

Provided by the University of Cambridge

Quote: Electrons take the fast and slow lanes simultaneously (2022, June 17) retrieved June 18, 2022 from https://phys.org/news/2022-06-electrons-fast-lanes.html

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