Experiments using laser light and pieces of gray material the size of fingernail clippings may offer clues to a fundamental scientific riddle: What is the relationship between the everyday world of classical physics and the hidden quantum realm that obeys entirely different rules?

"We found a particular material that is straddling these two regimes," said Peter Armitage, an associate professor of physics at the Johns Hopkins University, who led the study. Six scientists from Johns Hopkins and Rutgers University were involved in the work on materials called topological insulators, which can conduct electricity at their surface but not within their bulk. The scientists report their findings in a paper in Science.

Topological insulators were predicted in the 1980s, first observed in 2007 and have been studied intensively since. Made from any number of hundreds of elements, these materials have the capacity to show quantum properties that usually appear only at the atomic level, but here appear in a material visible to the naked eye.

This study establishes these materials as a distinct state of matter "that exhibits macroscopic quantum mechanical effects," Armitage said. "Usually, we think of quantum mechanics as a theory of small things, but in this system quantum mechanics is appearing on macroscopic length scales. The experiments are made possible by unique instrumentation developed in my laboratory."

In the experiments reported in Science, dark gray material samples made of the elements bismuth and selenium – each a few millimeters long and of different thicknesses – were hit with light waves at terahertz frequencies. The researchers measured the reflected light as it moved through the material samples, and found fingerprints of a quantum state of matter.

"Usually, we think of quantum mechanics as a theory of small things, but in this system quantum mechanics is appearing on macroscopic length scales."Peter Armitage, Johns Hopkins University

Specifically, they found that the light waves rotated by a specific amount as they were transmitted through the material, which is related to physical constants that are usually only measurable in atomic scale experiments. The amount of rotation fitted the predictions of what would be possible in this quantum state.

These results add to scientists' understanding of topological insulators, but may also contribute to the larger subject that Armitage calls "the central question of modern physics". What is the relationship between the macroscopic classical world and the atomic-scale quantum world from which it arises?

Since the early 20th century, scientists have struggled with the question of how one set of physical laws governing objects above a certain size can co-exist alongside a different set of laws governing the atomic and subatomic scale. How does classical mechanics emerge from quantum mechanics, and where is the threshold that divides the realms?

Those questions remain to be answered, but topological insulators could be part of the solution.

"It's a piece of the puzzle," said Armitage, who worked on the experiments along with Liang Wu, a graduate student at Johns Hopkins when the work was done, and Maryam Salehi, Nikesh Koirala, Jisoo Moon and Sean Oh from Rutgers University.

This story is adapted from material from Johns Hopkins University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.