Key takeaways
- The noise that disrupts communication devices and sensors results from electrons being scattered by defects and temperature-driven vibrations in the materials they travel through.
- A UCLA-led research team demonstrated that minuscule wires made from two unconventional materials can potentially reduce noise below the lowest level possible in traditional electronics.
- These findings not only hold the potential to improve current technologies but also suggest possibilities in quantum computing.
That low-frequency fuzz that can bedevil cellphone calls has to do with how electrons move through and interact in materials at the smallest scale. The electronic flicker noise is often caused by interruptions in the flow of electrons by various scattering processes in the metals that conduct them.
The same sort of noise hampers the detecting powers of advanced sensors. It also creates hurdles for the development of quantum computers — devices expected to yield unbreakable cybersecurity, process large-scale calculations and simulate nature in ways that are currently impossible.
A much quieter, brighter future may be on the way for these technologies, thanks to a new study led by UCLA. The research team demonstrated prototype devices that, above a certain voltage, conducted electricity with lower noise than the normal flow of electrons.
These experimental devices used unconventional materials to form nanowires, ribbons so thin that it would take a thousand or more to match the width of a strand of hair. In contrast to conventional electronics — in which noise levels tend to remain constant — the nanowires displayed a surprising property: Noise dropped as the electrical current increased.
The behavior of the materials was driven by a quantum phenomenon in which electrons move in concert with phonons, temperature-driven vibrations that can cause flicker noise. Importantly, one of the materials in the study dampened noise at room temperature and above.
"Normally we think about phonons as the bad guys that are scattering electrons," said corresponding author Alexander Balandin, holder of the Fang Lu Endowed Chair in Engineering at the UCLA Samueli School of Engineering, distinguished professor of materials science and engineering and a member of the California NanoSystems Institute at UCLA (CNSI). "In this particular case, we found the phonons allowed electrons to jointly move along. This weird, unique property with respect to noise could allow us to improve signal-to-noise ratio."
The study was published in the journal Nature Communications.
Electrons surf the wave
When voltage is applied to a metallic wire, electrons travel under the action of the electric field, constantly being bumped off-path by phonons and various defects in materials, which results in noisy current. The researchers took advantage of an additional mode for electrons to move, under very specific circumstances induced by the counterintuitive rules of quantum mechanics. In this mode, electrons tend to clump together in periodic patterns that are enabled by interactions with phonons and largely synchronized with phonons.
By analogy, electrons can be pictured as surfers traveling the ocean of a conducting material, with waves of phonons flowing through it.
In the usual mode, electrons act like newbie surfers, occasionally getting knocked off their boards by phonon waves. Electrons in the quantum-based mode are like expert surfers, catching phonon waves and using their energy to move along smoothly.
With the motion of phonons and electrons so closely connected, the materials that unlock expert-surfer mode are called "strongly correlated materials."
"We exploited this regime with collective, correlated motion of electrons to get a benefit in terms of noise reduction," said Balandin, vice chair for graduate education in UCLA's materials science and engineering department and the director of the Brillouin – Mandelstam Inelastic Light Scattering Spectroscopy (BMS) Facility in CNSI.
Turning down the noise requires adjustments to theory
The investigators fabricated nanowires from materials whose atomic bonds were strong in only one direction, then hooked them up to tiny electrodes. One of these so-called quasi-one-dimensional nanowires was made of a compound based on the element tantalum, a blue-gray metal used in electronic components. The other quasi-1D material was based on that element's chemical cousin, niobium.
In the tantalum-based material, noise dropped as current increased until it fell below the limit of practical measurement, at temperatures around -100 degrees Fahrenheit. The niobium-based material displayed similar behavior. In that 1D nanowire, noise fell well below the level made by normal electrons, then remained constant, in experiments conducted at room temperature and even above it.
The researchers were surprised. In earlier experiments with other strongly correlated materials or different applied voltages, the noise levels always rebounded to the level of normal electrons. So the team fashioned additional devices to double-check their results. They also developed new theoretical models that accounted for the unique behavior.
"Strongly correlated materials are transforming materials science," Balandin said. "It seems a lot of properties were lost in the simplified description that we had before, so we needed to revise our theoretical models and interpretations. If we can describe the materials more accurately, it can help us to elicit and understand new properties."
Implications for future technologies
The findings indicate the potential for practical applications in future ultralow-noise communication and sensor technologies. They also hold promise for stabilizing the finicky fundamental components of quantum computers — and do so above the extremely cold temperatures necessary today.
Balandin envisions a future in which strongly correlated materials can be used as conductors for connecting components on computer chips. He thinks these materials may even support a fundamental change in circuit architecture.
"All good things come to an end," he said. "With the demand for high-end, high-power computation for artificial intelligence, we have to look at materials that, 10-plus years from now, can give us an alternative means for sending electrical signals and processing them."
The researchers plan to further investigate the materials from this study, while also seeking other materials that carry charge density waves even more efficiently at room temperature.
"Perhaps there are materials that are even better," Balandin said. "The search is on."
Study authors and funding sources
UCLA postdoctoral researcher Subhajit Ghosh is the first author of the study. Other co-authors are Zahra Nataj, also of UCLA; Nicholas Sesing and Tina Salguero of the University of Georgia; Sergey Rumyantsev of the Polish Academy of Sciences; and Roger Lake of UC Riverside.
The study received funding from the U.S. Office of Naval Research and the European Research Council.
The nanowires were fabricated at the UCLA NanoLab and tested in the UCLA Phonon Optimized Engineered Materials Laboratory. Additional analysis was performed at two CNSI technology centers, the Electron Imaging Center for Nanosystems and the Nano and Pico Characterization Laboratory, as well as the BMS Facility, a CNSI partner.
