March 1, 2024

Research offers direct insight into tantalum oxidation that impedes qubit coherence

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Left: This scanning transmission electron microscope (STEM) image of a tantalum (Ta) film surface shows an amorphous oxide above the regularly arranged atoms of crystalline Ta metal. Right: STEM imaging combined with computational modeling revealed details of the interface between these layers, including the formation of the amorphous oxide (top layer) and a suboxide layer that retains crystalline characteristics (second layer) above the regularly arranged tantalum atoms. Credit: Brookhaven National Laboratory

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Left: This scanning transmission electron microscope (STEM) image of a tantalum (Ta) film surface shows an amorphous oxide above the regularly arranged atoms of crystalline Ta metal. Right: STEM imaging combined with computational modeling revealed details of the interface between these layers, including the formation of the amorphous oxide (top layer) and a suboxide layer that retains crystalline characteristics (second layer) above the regularly arranged tantalum atoms. Credit: Brookhaven National Laboratory

Scientists at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory and DOE’s Pacific Northwest National Laboratory (PNNL) used a combination of scanning transmission electron microscopy (STEM) and computational modeling to get a more detailed look and a deeper understanding of tantalum oxide. When this amorphous layer of oxide forms on the surface of tantalum – a superconductor that shows great promise for making the “qubit” building blocks of a quantum computer – it can impede the material’s ability to retain quantum information.

Learning how the oxide forms could offer clues as to why this happens – and potentially point to ways to prevent the loss of quantum coherence. The research was recently published in the journal ACS Nano.

The paper builds on previous research by a team at Brookhaven’s Center for Functional Nanomaterials (CFN), Brookhaven’s National Synchrotron Light Source II (NSLS-II), and Princeton University, conducted as part of the Center for Co- design for Quantum Advantage (CtwoQA), a national quantum information science research center led by Brookhaven, in which Princeton is a key partner.

“In this work, we used X-ray photoemission spectroscopy at NSLS-II to infer details about the type of oxide that forms on the surface of tantalum when it is exposed to oxygen in the air,” said Mingzhao Liu, a scientist at CFN and one of the main authors of the study. “But we wanted to understand more about the chemistry of this very thin oxide layer by making direct measurements,” he explained.

So in the new study, the team partnered with scientists in Brookhaven’s Department of Condensed Matter Physics and Materials Science (CMPMS) to use advanced STEM techniques that allowed them to directly study the ultrathin oxide layer. They also worked with PNNL theorists who performed computational modeling that revealed the most likely arrangements and interactions of atoms in the material as it underwent oxidation.

Together, these methods helped the team build an atomic-level understanding of the ordered crystal lattice of tantalum metal, the amorphous oxide that forms on its surface, and intriguing new details about the interface between these layers.

“The key is to understand the interface between the surface oxide layer and the tantalum film because this interface can profoundly impact qubit performance,” said study co-author Yimei Zhu, a physicist at CMPMS, echoing the wisdom of the award winner. Nobel laureate Herbert Kroemer, who famously stated: “The interface is the device.”

Emphasizing that “quantitatively probing a mere interface one or two atomic layers thick represents a formidable challenge,” Zhu noted, “we were able to directly measure the atomic structures and bonding states of the oxide layer and tantalum film as well as identify those at the interface using advanced electron microscopy techniques developed at Brookhaven.”

“Measurements reveal that the interface consists of a ‘suboxide’ layer nested between the periodically ordered tantalum atoms and the fully disordered amorphous tantalum oxide. Within this suboxide layer, only a few oxygen atoms are integrated into the tantalum crystal lattice. ” said Zhu. .

Combined structural and chemical measurements offer a crucially detailed perspective on the material. Density functional theory calculations helped scientists validate and gain deeper insight into these observations.

“We simulated the effect of gradual surface oxidation, gradually increasing the number of oxygen species at the surface and in the underground region,” said Peter Sushko, one of the PNNL theorists.

By evaluating the thermodynamic stability, structure and changes in electronic properties of tantalum films during oxidation, scientists concluded that although the fully oxidized amorphous layer acts as an insulator, the suboxide layer retains characteristics of a metal.

“We have always thought that if tantalum is oxidized, it will become completely amorphous, without any crystalline order,” Liu said. “But in the suboxide layer, the tantalum sites are still quite ordered.”

With the presence of fully oxidized tantalum and a suboxide layer, scientists wanted to understand which part is most responsible for the loss of coherence in qubits made from this superconducting material.

“It is likely that the oxide has multiple functions,” Liu said.

First, he noted, the fully oxidized amorphous layer contains many lattice defects. That is, the locations of the atoms are not well defined. Some atoms can change into different configurations, each with a different energy level. Although these changes are small, each consumes a small amount of electrical energy, which contributes to the qubit’s energy loss.

“This so-called two-level system loss in an amorphous material brings parasitic and irreversible losses to quantum coherence – the material’s ability to retain quantum information,” Liu said.

But because the suboxide layer is still crystalline, “it might not be as bad as people thought,” Liu said. Perhaps the more fixed atomic arrangements in this layer minimize the loss of the two-level system.

On the other hand, he noted, because the suboxide layer has some metallic characteristics, it can cause other problems.

“When you place a normal metal next to a superconductor, it can contribute to breaking electron pairs that move through the material without resistance,” he noted. “If the pair splits into two electrons again, you have loss of superconductivity and coherence. And that’s not what you want.”

Future studies may reveal more details and strategies to prevent the loss of superconductivity and quantum coherence in tantalum.

More information:
Junsik Mun et al, Probing Oxidation-Driven Amorphized Surfaces in a Ta(110) Film for Superconducting Qubit, ACS Nano (2023). DOI: 10.1021/acsnano.3c10740

Diary information:
ACS Nano

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