Researchers at UC Santa Barbara have found that electron-phonon interactions—usually a supply of resistance—can truly improve electrical conductivity in two-dimensional semiconductors by enabling hydrodynamic power circulate.
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What’s lengthy been seen as a hindrance to electrical conduction in semiconductors may very well be an asset, no less than in two-dimensional supplies.
The analysis crew discovered that in atomically skinny semiconductors, these interactions can preserve momentum and power when electrons and phonons are handled as a single system. Their findings, printed in Bodily Overview Letters, level to new design prospects for environment friendly 2D digital units.
“That is in sharp distinction to three-dimensional programs the place you could have loads of momentum loss processes,” stated Liao, who focuses on thermal and power science.
Diffusion and Hydrodynamic Circulation
On the coronary heart of the research are two key modes of power transport: diffusion and hydrodynamic circulate. Diffusion describes particles shifting randomly beneath a gradient, like smoke spreading by way of air. Hydrodynamic circulate, in contrast, is extra orderly, with particles shifting collectively, like water by way of a pipe.
“These are two very completely different bodily processes,” Liao stated.
In hydrodynamic circulate, power is transported extra effectively than by way of diffusion as a result of the whole momentum of particles is conserved throughout collisions. Though particles might collide, they trade momentum and proceed shifting collectively.
“However once we consider warmth conduction in materials, it isn’t carried by ‘actual’ particles,” Liao continued.
As an alternative, it’s dealt with by phonons—quasiparticles that symbolize the collective vibrations of atoms in a cloth. Phonons are inclined to diffuse by way of supplies through microscopic collisions that usually don’t preserve momentum, making this course of comparatively inefficient for transporting power.
These phonon-electron interactions are what trigger electrons to decelerate or lose momentum, which can also be why conductors have decrease electrical resistance at decrease temperatures: much less thermal power means fewer disruptive collisions.
Nevertheless, Liao and Quan level out that the physics in two dimensions works otherwise.
They’ve some uncommon properties. For instance, in these supplies, corresponding to graphene, when the phonons scatter with one another, it’s identified that their momentum is basically conserved. That is because of the completely different dimensionality that imposes some constraint on how they’ll work together with one another.
Bolin Liao, Mechanical Engineer, College of California, Santa Barbara
In simulations of 2D semiconductors with sturdy electron-phonon interactions, the researchers discovered that when cost and warmth carriers are handled as a unified system, their interactions give rise to a collective hydrodynamic circulate.
“They begin to transfer collectively like molecules in a fluid circulate. They drift along with the identical velocity, like fluid circulate by way of a pipe,” Liao stated.
This course of, so-called “coupled electron-phonon hydrodynamics,” displays how this mixed system flows like a classical liquid. On this course of, Liao stated, fluid circulate, warmth diffusion, and even electrical conduction “can change into very comparable.”
We are able to present that in the event you take into consideration this hydrodynamic conduct, the cost transport on two-dimensional materials may be very environment friendly, far more environment friendly than individuals would anticipate from simply how continuously they collide with the warmth carriers.
Bolin Liao, Mechanical Engineer, College of California, Santa Barbara
These findings carry essential implications for the design of 2D semiconductors, notably within the pursuit of environment friendly electrical conductivity at room temperature. Whereas one conventional strategy to enhancing effectivity is decreasing the temperature to scale back collisions, Liao affords another.
Our new thought right here is as an alternative of making an attempt to scale back how typically they collide, we are able to simply engineer the fabric to verify many of the collision processes are momentum-conserving.
Bolin Liao, Mechanical Engineer, College of California, Santa Barbara
On this state of affairs, even when cost carriers lose some momentum by way of collisions with warmth particles, conserving the system’s general momentum nonetheless results in low power dissipation and extremely environment friendly transport.
To exhibit their idea, the researchers studied atomically skinny molybdenum disulfide (MoS₂), displaying that cost mobility improved by almost sevenfold when hydrodynamic conduct was taken under consideration.
“That may be a very important enhancement,” Liao stated.
Relatively than counting on ultra-low temperatures, leveraging a cloth’s capability to help hydrodynamic electron-phonon interactions “may be very promising for microelectronic purposes.”
This analysis is supported by the Air Pressure Workplace of Scientific Analysis beneath this system led by Ali Sayir.
Journal Reference:
Quan, Y., et al. (2025) Coupled Electron-Phonon Hydrodynamics in Two-Dimensional Semiconductors. Bodily Overview Letters. doi.org/10.1103/physrevlett.134.226301.
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