Uncovering Concealed Local States in a Quantum Material

Quantum material portray exotic behaviors resulting from quantum mechanics, or how matter acts on the small scale of atoms and subatomic particles. The technologically significant properties of quantum materials result from intricate interactions of electron charge, orbital, and spin and their connection to the material’s crystal framework. For instance, in some materials, electrons can move openly with no resistance; this phenomenon, called superconductivity, could be used to transmit power more efficiently. Usually, properties arise at low temperatures, where crystals present low (broken) structural symmetry.

” Unsurprisingly, this low-temperature regimen is well researched,” stated Emil Bozin, a physicist in the X-ray Scattering Group of the Condensed Matter Physics and Materials Science (CMPMS) Department at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory. “Meanwhile, the high-temperature regime stays mainly unexplored because it is connected with a relatively high symmetry, which is considered to be dull.”

But Bozin and associates have just lately discovered high-temperature local symmetry breakdown situations. These local states function as orbital degeneracy lifting (ODL) “precursors” to what happens at low temperatures because they are connected to electronic orbitals, which are regions of an atom where electrons are likely to be located. When orbitals have the same amount of energy, this is known as orbital degeneracy. From this degeneracy, it may be inferred that some orbitals will undoubtedly have relative higher energies than another.

According to Bozin, “We assume that such local states still serve as enablers of the material qualities of interest that arise at much-lower temperatures.”

The researchers first observed these local states in 2019 in a material (copper iridium sulfide) with a metal-insulator transition as well as in an iron-based superconductor. The group representing – Brookhaven Lab; DOE’s Oak Ridge National Lab; University of Tennessee, Knoxville; and Columbia College – has discovered them in an insulator including sodium, titanium, oxygen, and silicon. This insulating material is one of the minerals developing the Earth’s upper mantle. Past the geological interest, it is a candidate for quantum spin liquids (QSLs), an unfamiliar state of matter in which electron spins stay fluid-like to the lowest temperature, constantly changing. QSLs might give a material platform for quantum computing, spintronics (electronics based on electron spin rather than charge), superconductivity, and other technologies.

Weiguo Yin, a physicist of the CMPMS Division Rigid Matter Theory Group, said that the findings “indicate that this ODL precursor habits at high temperature may be quite common and needs to be taken into account in theoretical studies to fully understand the functionality of quantum substances.”

In order to probe the material’s atomic structure, the group examined how the material scattered neutrons and X-rays. Both probes are required due to their different sensitivities to specific elements based on atomic weight. Unlike X-rays, neutrons can discern light elements, such as oxygen. Using the neutron and X-ray scattering patterns, the local arrangement of atoms can be reasoned through the atomic pair distribution function (PDF), describing ranges between various atoms in an example. Utilizing software, researchers can then discover the structural model that best fits the experimental atomic PDF function.

Their analysis showed signatures of local symmetry breaking much over the temperature at which the material undertakes a structural transition to make titanium dimers (two molecules connected). When the material is heated, these dimers appear to go away, but truly, they stay, evolving into a dual ODL state.

“The high-temperature, high-crystallographic-symmetry state supposes the presence of orbital degeneracy, yet orbital depravity might not be vigorously beneficial,” stated Bozin. “As we see here, the dimers obtain replaced, as well as what remains is an in your area misshaped crystal framework. This distortion lifts the depravity of 2 orbitals and allows the system to go into a lower-energy state.”

Next off, the group plans to tailor orbital properties in this material, for instance, by replacing titanium with ruthenium, which will undoubtedly transform the electron count and is expected to supply a much better QSL. They will also observe whether the ODL precursors exist in other products and how they are connected to phenomena of interest, such as superconductivity. Notably, they would like to explore systems with various levels of spin-orbit coupling, which is an alternative mechanism for ODL.

The identification of these orbital predecessors, according to Simon Billinge, a physicist in the CMPMS Division X-ray Scattering Group and teacher of material science and engineering as well as of applied math and physics at Columbia University, helps us better understand the competition between various low temperatures quantum states and will allow us to level the playing field to obtain materials with desired low temperatures properties.

Read the original article on Brookhaven National Laboratory.

Reference: R. J. Koch et al, Dual Orbital Degeneracy Lifting in a Strongly Correlated Electron System, Physical Review Letters (2021). DOI: 10.1103/PhysRevLett.126.186402