In the world of semiconductor materials, the frequent collisions between particles have long been seen as the main obstacle hindering the flow of electric current. However, the latest research by scientists has revealed that the resistivity caused by particle collisions does not increase endlessly. Under the rules of quantum mechanics, there exists an “ultimate limit” that cannot be surpassed.
Published in “Physical Review Letters,” the study was conducted by a multinational team consisting of researchers from the University of Toronto, École Normale Supérieure in Paris, and Lehigh University in Pennsylvania. Using an “optical lattice” created by interweaving lasers, the team trapped ultra-cold potassium atoms close to absolute zero, constructing a highly controllable “quantum simulator” to perfectly replicate the microscopic processes of electron traveling and colliding in a solid lattice.
In traditional metal materials, electron scattering (collisions) between electrons dissipates the kinetic energy of the current and converts energy into waste heat. This dissipative effect not only limits the efficiency of electronic devices but also causes up to 8% power losses in long-distance power grids.
For a long time, condensed matter physicists have been exploring whether the resistivity from collisions will infinitely increase when the interaction between electrons is extremely intense.
However, the complex structure of real solid materials, intertwined with factors like impurities and lattice vibrations (phonons), makes it extremely difficult for scientists to isolate the resistivity effects solely generated by “pure particle collisions.”
To solve this challenge, Professor Joseph Thywissen’s research team at the University of Toronto opted to use the perfect alternative of physics – ultra-cold gaseous potassium atoms.
The researchers cooled potassium atoms to temperatures just slightly above absolute zero, measured in nanokelvin (nK), and confined the atoms in a grid-like chessboard structure created by cross lasers. Under these extremely controlled conditions, the optical lattice simulated the crystal periodic potential inside metals, with potassium atoms perfectly playing the role of “electrons.”
An astonishing quantum phenomenon emerged in the experiment: although the physical size of these potassium atoms was only a few nanometers, under strong interactions, quantum effects significantly enhanced their “effective scattering size.”
Professor Thywissen vividly likened it to “a group of ducks swimming in water full of bubbles, where the collision frequency is determined not by their own size but by the bubbles surrounding them.” This quantum enhancement effect dramatically increased the probability of collisions between atoms located at the same lattice point, thereby significantly raising the system’s resistivity.
Even more surprisingly, when the research team raised the magnetic field to further enhance the strength of interactions between atoms, the upward trend in resistivity of the system abruptly stopped, entering a stable “saturation platform.”
Theoretical analysis showed that this saturation phenomenon stemmed from the fundamental scattering unitarity bound in quantum mechanics – when the probability of particle collisions reaches the maximum probability allowed by quantum mechanics, even with continued strengthening of the mutual attractive or repulsive forces between particles, the rate of momentum loss per unit time no longer increases.
This means that the resistivity caused by electron collisions in low-density metals is constrained by some intrinsic quantum limit and cannot increase limitlessly.
This research not only provides a clear physical explanation for the microscopic transport mechanisms in metal materials but also demonstrates the powerful capability of ultra-cold atomic quantum simulators in exploring extreme material states.
“Our experimental results offer a clear microscopic image for understanding the operational mechanism of resistivity in low-density metals,” summarized Professor Thywissen. “At the same time, this opens a brand new door for future research on strongly correlated atomic systems and various complex quantum materials like high-temperature superconductors and two-dimensional quantum materials.” ◇