Strange Quantum Oscillations in an Exotic Material Reveal a New Quantum Effect

Physicists in Brazil and the United States have observed an unexpected quantum effect in a material called zirconium pentatelluride (ZrTe5), where electrical resistance kept oscillating even under magnetic fields strong enough that conventional theory says such oscillations should stop. The findings, published in Nature Communications, come from experiments conducted near absolute zero, at about 0.7 kelvin (-272.45 degrees C), using magnetic fields reaching 60 tesla. The work involved researchers from the University of Sao Paulo (USP) in Brazil, Los Alamos National Laboratory, the University of Washington and other U.S. institutions.

ZrTe5 is a topological insulator, a material whose interior resists electrical conduction while its surface can still carry current, a property that comes from the topology, or overall quantum structure, of its electronic bands. ZrTe5 sits close to the boundary between different topological phases, making it especially sensitive to small changes in temperature, magnetic field, mechanical strain or composition, and a useful material for studying topological phase transitions.

Normally, a magnetic field forces electrons into discrete energy states called Landau levels, named after physicist Lev Landau. In pure metals, as these levels pass through the Fermi level, the boundary between occupied and unoccupied electron states, resistance rises and falls in a regular pattern known as Shubnikov-de Haas oscillations. These oscillations are expected to vanish once the magnetic field is strong enough to push all electrons into their lowest Landau level, a point called the quantum limit. In ZrTe5, the oscillations did not follow the usual periodic pattern and continued well beyond that expected limit.

The researchers attribute the effect to what they call 'reentrant Landau levels.' In ZrTe5, strong spin-orbit interaction means the cyclotron energy from electrons' orbital motion and the Zeeman effect from their spin cannot be treated independently; as the magnetic field changes, the energy levels evolve nonlinearly and can curve back to cross the Fermi level again, producing extra oscillations. 'In materials near topological phase transitions, electrons may cease to behave like ordinary particles within a metal,' said Caue Kaufmann Ribeiro, the paper's first author and a doctoral researcher under Julio Larrea Jimenez at USP's Physics Institute.

A central question was whether the effect came from many-body interactions among electrons or from the material's intrinsic topology. The researchers found that a single-particle model based on a three-dimensional Dirac Hamiltonian with strong spin-orbit coupling successfully reproduced the experimental results, indicating that electron interactions are not required to produce the phenomenon.