Imagine a road with two lanes in each direction. One lane is reserved for slow cars and the other for fast cars. For electrons moving along a quantum wire, researchers from Cambridge and Frankfurt have found that there are also two “pathways”, but electrons can take both at the same time!
The current in a wire is carried by the flow of electrons. When the wire is very narrow (one-dimensional, 1D), the electrons cannot pass each other, because they strongly repel each other. Rather, current, or energy, is carried by compression waves as one particle pushes the next.
It has long been known that there are two types of excitation for electrons, because in addition to their charge they have a property called twirl. Spin and charge excitations move at fixed but different speeds, as predicted by the Tomonaga-Luttinger model decades ago. However, theorists are unable to calculate precisely what happens beyond small disturbances because the interactions are too complex. The Cambridge team measured these velocities as their energies are varied, and found that a very simple picture emerges (now published in the journal Scientists progress). Each type of excitation can have low or high kinetic energy, like cars on a road, with the well-known formula E=1/2mV2, which is a parabola. But to turn and charge the masses m are different and, since charges repel each other and therefore cannot occupy the same state as another charge, there are twice as many impulses for charge as for spin. Results measure energy as a function of magnetic field, which is equivalent to momentum or velocity vshowing these two energy parabolas, which can be seen in places up to five times the highest energy occupied by electrons in the system.
“It’s as if the cars (like the loads) were going in the slow lane but their passengers (like the spins) were going faster, in the fast lane,” explained Pedro Vianez, who carried out the measurements for his PhD. . at the Cavendish Laboratory in Cambridge. “Even when cars and passengers slow down or speed up, they still remain separated!”
“What is remarkable here is that we are no longer talking about electrons but rather about (quasi) composite particles of spin and charge – commonly called spinons and holons, respectively. For a long time it was believed that they became unstable at such high energies, but what is observed indicates the exact opposite – they seem to behave in a very similar way to normal, free and stable electrons, each with its own mass, except that they are not, in actually, electrons, but excitations of a whole sea of charges or spins!” said Oleksandr Tsyplyatyev, the theorist who led the work at Goethe University 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 pick out new features. We fabricated a series of semiconductor arrays of wires ranging from 1 to 18 microns in length (i.e. up to a thousandth of a millimeter or about 100 times thinner than a human hair), with as few as 30 electrons in a wire and measured them at 0.3 K (or in other words, -272.85 ⚬C, ten times colder than Cosmos).”
Details of the experimental technique
The 1D wires tunnel electrons into an adjacent two-dimensional electron gas, which acts as a spectrometer, producing a map of the relationship between energy and momentum. “This technique is in all respects very similar to the angular resolution photoemission spectroscopy (ARPES), which is a commonly used method for determining the band structure of materials in condensed matter physics. The main difference is that, rather than probing at the surface, our system is buried a hundred nanometers below,” Vianez said. This allowed the researchers to achieve unprecedented resolution and control for this type of spectroscopy experiment.
These results now open the question of whether this spin-charge separation of the whole electron sea remains robust beyond 1D, for example in high-temperature superconducting materials. It can also now be applied to logic devices that exploit spin (spintronics), which offer a drastic reduction (by three orders of magnitude!) of the energy consumption of a transistor, simultaneously improving our understanding of quantum matter and providing a new tool for engineering quantum materials.
Pedro MT Vianez et al, Observing separate spin and charge Fermi seas in a strongly correlated one-dimensional conductor, Scientists progress (2022). DOI: 10.1126/sciadv.abm2781
University of Cambridge
Quote: Electrons move through fast and slow lanes at the same time (June 17, 2022) retrieved June 18, 2022 from https://phys.org/news/2022-06-electrons-fast-lanes.html
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