Electrons occupy fast and slow bands at the same time

Electrons occupy fast and slow bands at the same time

Figure 1. Parabolas for spin (green) and charge (purple) excitation. The insert shows the charging line in more detail. Credit: Research Team, Cavendish Laboratory, Department of Physics, University of Cambridge

Imagine a path with two lanes in each direction. One lane is for slow cars and the other for fast ones. For quantum-conducting electrons, researchers from Cambridge and Frankfurt have found that there are two “bands”, but electrons can accept both at the same time!

The current in the conductor is carried by the flow of electrons. When the wire is very narrow (one-dimensional, 1D), then the electrons cannot overtake because they are strongly repelled. Instead, current or energy is transferred by compression waves when one particle presses the next.

It has long been known that there are two types of excitation of electrons, because in addition to their charge, they have a property called rotation. Rotating and charging excitations travel at fixed but different speeds, as predicted by the Tomonaga-Luttinger model many decades ago. However, theorists are not able to calculate what exactly happens outside of only small disturbances, because the interactions are too complex. The Cambridge team measured these velocities because their energies were different, and found that a very simple picture emerged (now published in the journal Scientific achievements). Each type of excitation can have low or high kinetic energy, like cars on the road, with the well-known formula E = 1/2 mv2which is a parabola. But for rotating and loading tables m are different and because the charges are repelled and therefore cannot take the same state as another charge, there is twice the range of the charge pulse than for the rotation. The results measure the energy as a function of the magnetic field, which is equivalent to the momentum or velocity vshowing these two energy parabolas, which can be seen in places up to five times the highest energy occupied by the electrons in the system.

Electrons occupy fast and slow bands at the same time

Figure 2. Spin (green) and charger (‘holon’, purple) excitation in 1D wire. Credit: Research Team, Cavendish Laboratory, Department of Physics, University of Cambridge

“It’s as if cars (as charges) are moving in the slow lane, but their passengers (as turns) are moving faster, in the fast lane,” explained Pedro Vianes, who performed the measurements for his doctorate. at the Cavendish Laboratory in Cambridge. “Even when cars and passengers slow down or accelerate, they still remain separated!”

What is remarkable here is that we are no longer talking about electrons, but instead of composite (quasi) particles of spin and charge – usually called spinons and holons, respectively. It has long been thought that they become unstable at such high energies, but what has been observed shows just the opposite – they seem to behave in a way very similar to normal, free, stable electrons, each with its own mass, except that they are not actually electrons, but excitations of a whole sea of ​​charges or spins! ” said Alexander Tsiplyatiev, the theorist who led the work at Goethe University in Frankfurt.

“This paper marks 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’ve always been curious to see what happens if we bring the system to higher energies, so we’ve gradually improved our measurement resolution to choose new features. We made a series of semiconductor arrays of wires with a length of 1 to 18 microns (ie up to one thousandth of a millimeter or approximately 100 times thinner than a human hair), with only 30 electrons in a wire and measure them at 0.3 K (or in other words, -272.85 C, ten times colder than outer space). “

Electrons occupy fast and slow bands at the same time

Figure 3a. Scanning electron micrographs of a device showing the various ports used to define 1D wires (Part 1). Credit: Research Team, Cavendish Laboratory, Department of Physics, University of Cambridge

Details of the experimental technique

Electrons tunnel 1D wires into an adjacent two-dimensional electron gas, which acts as a spectrometer, creating a map of the relationship between energy and momentum. “This technique is very similar to the corner technique in every way photoemission spectroscopy (ARPES), which is a commonly used method for determining the band structure of materials in condensed matter physics. The key difference is that instead of probing the surface, our system is buried a hundred nanometers below it, “Vianes said. This allowed researchers to achieve unprecedented resolution and control for this type of spectroscopic experiment.

Electrons occupy fast and slow bands at the same time

Figure 3b. Scanning electron micrographs of a device showing the various ports used to define 1D wires (Part 2). Credit: Research Team, Cavendish Laboratory, Department of Physics, University of Cambridge

Conclusion

These results now raise the question of whether this spin-charge separation of the entire electron sea remains stable beyond 1D, for example in high-temperature superconducting materials. It can now also be applied to logic devices that use spin (spintronics), which offer a drastic reduction (by three orders of magnitude!) Of energy transistor consumption, while improving our understanding of quantum matter, and offers a new tool for designing quantum materials.


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


More info:
Pedro MT Vianez et al, Observation of individual rotating and charging seas of Fermi in a highly correlated one-dimensional conductor, Scientific achievements (2022). DOI: 10.1126 / sciadv.abm2781

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

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