When exposed to stress and strain, materials can display a wide range of different properties. Using sound waves, scientists have begun to explore fundamental stress behaviors in a crystalline material that could form the basis for quantum information technologies. These technologies involve materials that can encode information in a number of states simultaneously, allowing for more efficient computation.
Scientists used X-rays to observe spatial changes in a silicon carbide crystal when using sound waves to strain buried defects inside it. Because these defects are well isolated within the crystal, they can act as a single molecular state and as carriers of quantum information. When the electrons trapped near the defects change between spin states, they emit energy in the form of photons. Depending on which state the electrons are in, they emit either more or fewer photons in a technique known as spin-dependent readout.
In the experiment, the researchers sought to assess the relationship between the sound energy used to produce the strain on the defects in the crystal lattice and the spin transitions indicated by the emitted photons. While the defects in the crystal naturally fluoresce, the additional strain causes the ground spin of the electron to change state, resulting in a coherent manipulation of the spin state that can be measured optically.
The electrodes used to generate the sound waves are roughly five microns in width — far larger than the defects themselves that consist of two missing atoms known as a divacancy complex. The sound wave strains the defects by alternately pushing and pulling on them, causing the electrons to change their spins.
To characterize the lattice and defects, researchers used the Hard X-ray Nanoprobe beamline at Argonne National Lab. Through a newly developed technique called stroboscopic Bragg diffraction microscopy, scientists imaged the lattice around the defects at many different points throughout the strain cycle.
Stroboscopic Bragg diffraction involves synchronizing the frequency of the acoustic wave to the frequency of the electron pulses, freezing the wave in time. This allowed them to create a series of images of the strain experienced by the lattice at each point on the wave. The sound waves cause the lattice to curve and exactly how much the lattice curves can be measured by going through a specific point of the lattice at a specific point in time.
The use of stroboscopic Bragg diffraction allows scientists to determine the direct correlation between the dynamic strain and the quantum behavior of the defect. In silicon carbide, this relationship is fairly well understood but in other materials, the technique could reveal surprising relationships between strain and other properties.