Researchers at the UCLA Samueli School of Engineering and Auburn University achieved a scientific first: They measured atomic vibrations that carry heat and sound waves, and connected them to a quantum property called spin, which is a way of characterizing a subatomic particle's built-in magnetism.
The team looked at thin sheets — only a few atoms tall — made of the semiconductor nickel phosphorous trisulfide in a repeating crystal pattern. At room temperature, the spin of the material's electrons is random. But cooled below about 180 degrees below zero Fahrenheit, the electrons' spin organizes into a zigzag pattern with the poles of adjacent electrons pointing in opposite directions. By contrast, the poles point in the same direction in a magnet.
The study found that quantum spins and the vibrations, called acoustic phonons, had a stronger correlation below -180 F, when the electrons' spin was organized. Around the temperature at which the change occurs, acoustic phonons on the surface of the semiconductor slowed down.
Taken together, the findings indicate that the magnetic order of the material affects how vibrations travel through its crystal lattice.
BACKGROUND
The processing behind today's popular artificial intelligence applications requires lots of energy to power servers and water to cool them down. At the same time, the accumulation of heat on computer chips may soon hamper the capacity to add more and more features year after year, which has been at the heart of progress in computing since the 1950s.
Controlling quantum spin and its interaction with phonons could help overcome these limitations. The possibilities include systems that process data using spin, phonons or magnetic waves. Platforms such as these are expected to use energy more efficiently and dissipate heat more effectively than current computers, which use the flow of electrons for processing.
METHOD
The team used a technique with the unique ability to visualize acoustic phonons at the Brillouin – Mandelstam Inelastic Light Scattering Spectroscopy (BMS) Facility located in the California NanoSystems Institute at UCLA (CNSI) building. The instrumentation was custom-built by Alexander Balandin , holder of the Fang Lu Endowed Chair in Engineering at UCLA Samueli and a distinguished professor of materials science and engineering, and his colleagues. The researchers also confirmed the mechanisms they observed using computer simulations.
IMPACT
At left is an electron microscope image of nickel phosphorous trisulfide's crystal lattice. At right is a graph measuring vibrations from heat and sound. (Image courtesy: Balandin Lab / UCLA)
New avenues for technology open up when the counterintuitive quantum behaviors of electrons and other subatomic particles are linked to events on the macroscale such as acoustic phonons. For instance, the touchscreen on a smartphone or tablet relies on quantum tunneling, in which particles seem to pop up on the other side of what should be an impassable barrier.
Measuring spin and phonons is a necessary step toward turning them into foundations for energy-efficient devices resilient to overheating. While magnetic hard drives that store data using "spintronics" have been available to consumers for many years, processing data using spin is still out of reach.
Looking for materials suitable for this sort of processing, scientists have mostly focused on ones that conduct or insulate electricity. Using semiconductors such as nickel phosphorous trisulfide could provide the flexibility to combine electrons, spins and phonons to perform computing more efficiently than traditional microelectronics.
The findings may also benefit the development of future quantum computers, which are expected to yield unbreakable cybersecurity and model nature more accurately than traditional computers can. A huge challenge in this field is that the quantum states needed to realize this potential are fragile. Phonons easily disrupt them, so breaking the link between phonons and spin would be a major boon.
Beyond the implications for computing, this study demonstrates an entirely new capability for measurement. By allowing scientists to access information about acoustic phonons and their relationship with quantum properties, Brillouin – Mandelstam spectroscopy may play a role in new fundamental physics discoveries and new technologies we can't imagine today.
AUTHORS
The study's co-corresponding authors are Balandin, a CNSI member, the director of the BMS Facility and the research thrust lead in future electronics materials in the Semiconductor Hub at UCLA Samueli; and Fariborz Kargar of Auburn. Zahra Ebrahim Nataj, a UCLA doctoral candidate, is the study's first author. Other authors are affiliated with UCLA and Auburn.
DISCLOSURES
There are no disclosures associated with this study.
JOURNAL
The study was published in Nature Communications .
FUNDING
The research was supported by the National Science Foundation and Auburn University's Easley High Performance Computing Cluster.
