Physicists Find Quantum Oscillations That Refuse to Disappear in Exotic Material

Physicists have found an unexpected form of quantum oscillation in a three-dimensional topological insulator, a material whose interior behaves as an electrical insulator while its surface conducts electricity. The study, published in Nature Communications, was led by researchers from the University of São Paulo (USP) in Brazil, Los Alamos National Laboratory and the University of Washington, and focused on a material called zirconium pentatelluride, or ZrTe5.

The team combined electrical transport experiments in magnetic fields as strong as 60 tesla and temperatures near 0.7 kelvin (-272.45°C) with theoretical calculations. In a magnetic field, electrons are restricted to specific energy values called Landau levels, and each time one crosses the Fermi level — the boundary between occupied and unoccupied electron states — it produces an oscillation in electrical resistance. These oscillations normally follow a predictable pattern and should vanish once the field pushes the material beyond a point called the quantum limit. In ZrTe5, the oscillations did not follow this pattern and continued well beyond the quantum limit.

First author Cauê Kaufmann Ribeiro, who carried out much of the experimental work during an internship at the National High Magnetic Field Laboratory in Los Alamos, said the material's electrons behave like quasiparticles similar to Dirac fermions, or relativistic particles, whose spin plays a central role. "When we apply strong magnetic fields, the interaction between spin and the magnetic field profoundly alters the energy levels of the electrons," he said, adding that some Landau levels can "return" and cross the Fermi level again — a behaviour the team calls reentrant Landau levels.

The researchers tested whether the effect came from collective interactions among many electrons or from the material's intrinsic topological properties, and found that a single-particle model was enough to reproduce their experimental results. "What we saw is that the effect doesn't stem from many-body interactions, but rather from a nontrivial topology of the electronic bands," said Julio Larrea Jiménez, a professor at USP's Physics Institute and Ribeiro's doctoral advisor.