An MIT-led team demonstrates how to precisely control the properties of Weyl semimetals and other exotic materials. Quantum materials—those with electronic properties shaped by quantum mechanics, such as correlation and entanglement—can display exotic behaviors under specific conditions, including superconductivity, where they transmit electricity without resistance. To achieve optimal performance from these materials, precise tuning is essential, much like fine-tuning a race car.

A team led by Mingda Li, an associate professor in MIT’s Department of Nuclear Science and Engineering (NSE), has introduced a new, ultra-precise method for adjusting the characteristics of quantum materials, using Weyl semimetals as a prime example. This technique is versatile and can be applied to any inorganic bulk material or thin films, according to NSE postdoc Manasi Mandal, a lead author of a recent open-access paper published in Applied Physics Reviews.

Figure 1. New Method for Precision Tuning of Quantum Materials

The paper details an experiment with a specific Weyl semimetal, tantalum phosphide (TaP) crystal. Materials are categorized based on their electrical properties: metals conduct electricity readily, insulators impede it, and semimetals fall between these extremes, conducting electricity only within a narrow frequency band. Weyl semimetals are part of a broader class of topological materials, known for their unique electronic structures featuring Weyl nodes—singularities that resemble vortices. These nodes endow the materials with unusual and valuable electrical properties. One significant advantage of topological materials is their ability to maintain these properties despite imperfections or disturbances. Figure 1 shows New Method for Precision Tuning of Quantum Materials.

As Abhijatmedhi Chotrattanapituk, a PhD student in MIT’s Department of Electrical Engineering and Computer Science and co-lead author of the paper, notes, this robustness means that slight fabrication imperfections will not disrupt the material’s expected behavior.

Like Water in A Reservoir

The "tuning" required involves adjusting the Fermi level, which is the highest energy level occupied by electrons in a material. Mandal and Chotrattanapituk compare this adjustment to managing the water level in a dam. Just as the water level can be increased by adding water or decreased by removing it, the Fermi level can be adjusted by adding or removing electrons.

To fine-tune the Fermi level of the Weyl semimetal, Li’s team used a similar approach but added negative hydrogen ions (each consisting of a proton and two electrons) instead of actual electrons. This process, known as doping, involves substituting a hydrogen ion for a tantalum atom in the TaP crystal. When doping is optimized, the Fermi level aligns with the energy level of the Weyl nodes, maximizing the material’s desired quantum properties.

Weyl semimetals are particularly sensitive to doping because their Fermi level must be precisely aligned with the Weyl nodes to achieve optimal properties. This sensitivity is due to the unique geometry of the Weyl node. If the Fermi level is imagined as the water level in a reservoir, the reservoir in a Weyl semimetal resembles an hourglass, with the Weyl node located at the narrowest point. Just as adding too much or too little water would bypass the narrow neck of the hourglass, adding too many or too few electrons would miss aligning the Fermi level with the Weyl node.

Activate The Hydrogen

To achieve the necessary precision, the researchers used MIT’s two-stage “Tandem” ion accelerator at the Center for Science and Technology with Accelerators and Radiation (CSTAR). They exposed the TaP sample to high-energy ions from the powerful (1.7 million volt) accelerator beam. Hydrogen ions were selected for their small size, which causes less disruption to the material compared to larger dopants. “The advanced accelerator techniques enable unprecedented precision, allowing us to adjust the Fermi level to milli-electron volt accuracy,” says Kevin Woller, the principal research scientist leading the CSTAR lab. “Additionally, high-energy beams enable doping of bulk crystals, overcoming the limitations of thin films only a few tens of nanometers thick.”

The process involves bombarding the sample with hydrogen ions until the Fermi level is adjusted correctly. The challenge is determining the optimal exposure time and knowing when enough ions have been added. “The longer you run the machine, the higher the Fermi level becomes,” Chotrattanapituk explains. “However, we cannot measure the Fermi level while the sample is in the accelerator chamber.” The conventional method involves irradiating the sample for a set time, then removing it to measure the Fermi level, and reintroducing it if necessary—an impractical approach.

To streamline this process, the team developed a theoretical model to predict the number of electrons needed to achieve the desired Fermi level, which translates to the number of negative hydrogen ions required. This model also indicates how long the sample should remain in the accelerator.

Chotrattanapituk notes that their model aligns within a factor of 2 with established, more complex models that often require supercomputing resources. The team’s contributions include a novel accelerator-based precision doping technique and a theoretical model to guide the experiment, determining the amount of hydrogen needed based on ion beam energy, exposure time, and sample dimensions.

Refinements Lead to Promising Outcomes

Mandal notes that this approach could lead to significant practical advancements, as it can potentially adjust the Fermi level of a sample to the desired value within minutes—an improvement over conventional methods that sometimes take weeks and still may not achieve the required milli-eV precision.

Li believes that a precise and convenient method for fine-tuning the Fermi level could have wide-ranging applications. “For quantum materials, the Fermi level is crucial,” he says. “Many effects and behaviors we seek only appear when the Fermi level is correctly positioned.” For instance, accurately adjusting the Fermi level could increase the critical temperature at which materials become superconducting. Similarly, thermoelectric materials, which convert temperature differences into electrical voltage, become more efficient when the Fermi level is optimized. Precision tuning may also benefit quantum computing.

Thomas Zac Ward, a senior scientist at Oak Ridge National Laboratory, offered a positive assessment: “This work opens a new avenue for exploring the critical, yet still poorly understood, behaviors of emerging materials. Precisely controlling the Fermi level of a topological material is a significant milestone that could advance new quantum information and microelectronics device architectures.”

Source:MIT News

Cite this article:

Janani R (2024), A Novel Method for Optimizing Quantum Materials, AnaTechMaz, pp. 40