Research News
Zeroing In on Zero-Point Motion Inside a Crystal
A nanocrystal cooled to near absolute zero produces an unexpected light emission, which is shown to arise from quantum fluctuations in the crystal’s atomic lattice.
Media Whale Stock/stock.adobe.com The atoms inside a crystal never stop moving, even when the object’s temperature approaches absolute zero. This quantum effect is called zero-point motion.
Media Whale Stock/stock.adobe.com The atoms inside a crystal never stop moving, even when the object’s temperature approaches absolute zero. This quantum effect is called zero-point motion. ×
Zero-point motion is an irrepressible wiggling that becomes visible at temperatures near absolute zero. Evidence of this quantum motion has previously been uncovered for trapped particles and for small resonators. Now researchers studying nanocrystals have identified a low-temperature emission effect, which they show is related to zero-point motion within the crystal lattice [1]. The effect may be useful in cooling down nanocrystals to lower temperatures than previously possible.
Quantum physics often shows up at ultracold temperatures. Normally, as an object becomes colder, it moves less and less. However, the Heisenberg uncertainty principle dictates that the motion can’t go exactly to zero—there will always be fluctuations. These quantum fluctuations have been studied in microscopic systems, such as trapped atoms and molecules [2]. But they’ve also been observed in macroscopic objects. Previous experiments have identified signatures of zero-point motion in small mechanical resonators, such as drums and beams (see Viewpoint: Seeing the “Quantum” in Quantum Zero-Point Fluctuations).
Those investigations focused on the whole object as it moves back and forth like a vibrating spring. But there are also internal vibrations—the object’s atoms wiggle around in their lattice structure. Xiaoyong Wang from Nanjing University in China and his colleagues have detected a signature of zero-point motion in the lattice of a nanocrystal. “As far as we know, this is the first time that this effect has been seen in a solid material,” says team member Zhi-Gang Yu from Washington State University. “Even we were surprised to observe it.”
The observed signature appeared in photoluminescence measurements, in which an object is excited with a laser and then subsequently relaxes back to its initial state by emitting light. If the outgoing emission has a frequency that is higher than that of the laser, the process is called up-conversion. The opposite case—emission at lower frequency—is called down-conversion. Up-conversion is especially interesting to researchers because the object gives up some of its internal energy and thus becomes colder.
Wang and his colleagues explored up-conversion in nanocrystals made from a lead-halide perovskite (CSPbI 3 ). This semiconductor has several exciton states, which are formed when an electron hops from the valence band to a higher-energy conduction band. When the electron subsequently falls back to the valence band, light is emitted at the telltale exciton frequency.
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