Tag: tantalum

Not All Gaps Are Created Equal

Not All Gaps Are Created Equal

In a charge density wave (CDW), conduction electrons in a metal (typically a low-dimensional material) arrange themselves in a regular pattern, sometimes accompanied by lattice distortions. One material that undergoes a CDW transition is a compound of tantalum, selenium, and iodine [(TaSe4)2I]. Its quasi-one-dimensional structure consists of TaSe4 molecular chains interspersed with I– ionic chains.

The CDW transition in (TaSe4)2I is of particular interest because it’s potentially a mechanism that could lead to a spontaneous transformation into an “axion” state of matter. Axions are hypothetical particles that were proposed as a way to solve a well-known problem in particle physics. But the concept has crossed over to condensed matter physics, as a way to describe emergent properties in topological materials.

Prevailing theories predict that the CDW transition in (TaSe4)2I—a type of topological material known as a Weyl semimetal—should lead to a dispersion gap at points where linear Weyl bands intersect, but this has never been confirmed experimentally through angle-resolved photoemission spectroscopy (ARPES), because of difficulties arising from a very weak ARPES intensity near the Fermi level.

To address this, a team led by Meng-Kai Lin (National Central University, Taiwan) and Tai-Chang Chiang (University of Illinois at Urbana-Champaign) re-examined the electronic structure of (TaSe4)2I at Advanced Light Source Beamline 10.0.1 and other facilities, using high-statistics ARPES to bring out subtle features in the data.

Read more on ALS website

Image: Left: ARPES map for (TaSe4)2I in the normal phase at room temperature. Right: Corresponding ARPES map in the CDW phase at a low temperature.

Magnesium Protects Tantalum

Magnesium Protects Tantalum

UPTON, NY—Scientists at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory have discovered that adding a layer of magnesium improves the properties of tantalum, a superconducting material that shows great promise for building qubits, the basis of quantum computers. As described in a paper just published in the journal Advanced Materials, a thin layer of magnesium keeps tantalum from oxidizing, improves its purity, and raises the temperature at which it operates as a superconductor. All three may increase tantalum’s ability to hold onto quantum information in qubits.

This work builds on earlier studies in which a team from Brookhaven’s Center for Functional Nanomaterials (CFN), Brookhaven’s National Synchrotron Light Source II (NSLS-II), and Princeton University sought to understand the tantalizing characteristics of tantalum, and then worked with scientists in Brookhaven’s Condensed Matter Physics & Materials Science (CMPMS) Department and theorists at DOE’s Pacific Northwest National Laboratory (PNNL) to reveal details about how the material oxidizes.

Those studies showed why oxidation is an issue.

“When oxygen reacts with tantalum, it forms an amorphous insulating layer that saps tiny bits of energy from the current moving through the tantalum lattice. That energy loss disrupts quantum coherence—the material’s ability to hold onto quantum information in a coherent state,” explained CFN scientist Mingzhao Liu, a lead author on the earlier studies and the new work.

While the oxidation of tantalum is usually self-limiting—a key reason for its relatively long coherence time—the team wanted to explore strategies to further restrain oxidation to see if they could improve the material’s performance.

“The reason tantalum oxidizes is that you have to handle it in air and the oxygen in air will react with the surface,” Liu explained. “So, as chemists, can we do something to stop that process? One strategy is to find something to cover it up.”

All this work is being carried out as part of the Co-design Center for Quantum Advantage (C2QA), a Brookhaven-led national quantum information science research center. While ongoing studies explore different kinds of cover materials, the new paper describes a promising first approach: coating the tantalum with a thin layer of magnesium.

Read more on BNL website

Image: Chenyu Zhou, a research associate in the Center for Functional Nanomaterials (CFN) at Brookhaven National Laboratory and first author on the study, with Mingzhao Liu (CFN), Yimei Zhu (CMPMS), and Junsik Mun (CFN and CMPMSD), at the DynaCool Physical Property Measurement System (PPMS) in CFN. The team used this tool to make tantalum thin films with and without a protective magnesium layer so they could determine whether the magnesium coating would minimize tantalum oxidation.

Credit: Jessica Rotkiewicz/Brookhaven National Laboratory