Quantum heat dynamics toggled by magnetic fields

The quantum material ZrTe₅ belongs to the class of so-called topological semimetals. In physics, the term "topological" describes special materials that, due to their unique electronic structure, have extremely robust ("topologically protected") conduction properties. In such materials, quantum effects can lead to unconventional and often bizarre phenomena that could play a crucial role in advancing future quantum technologies. Notably, both research and industry are currently investing considerable effort into developing quantum computers, with topological materials emerging as a promising avenue for their realization. Like ZrTe₅: it combines a rare set of non-trivial electronic properties, making it potentially relevant for high-precision electronics applications and magnetic-field sensor technologies.

"When a normal metal such as silver or copper is placed in strong magnetic fields at temperatures close to absolute zero, that is −273.15 °C, its heat conduction is expected to oscillate − a striking example of quantum mechanical dynamics of electrons in metals. This effect arises due to the existence of the so-called Fermi surface, a boundary between occupied and unoccupied energy states of electrons in a metal," Dr. Stanisław Gałeski, currently assistant professor at Radboud University and visiting scientist at the Dresden High Magnetic Field (HLD) laboratory at HZDR, explains. "On the other hand, in semimetals, there are very few electrons available to transport heat, and as such, heat conduction is widely believed to be dominated by phonons − emergent particles that represent crystal lattice vibrations. As such, quantum oscillations should not be detectable in the transport of heat," Gałeski sums up more traditional expectations. However, several recent experiments have found giant quantum oscillations in the heat conduction of semimetals, questioning the mechanism of heat transport.

Counterintuitive mechanism, surprising behavior

The present study demonstrates that this phenomenon stems from a very counterintuitive mechanism for the transport of heat under strong magnetic fields in semimetals. "It turned out that indeed thermal transport is by far dominated by lattice vibrations. However, due to the presence of strong magnetic fields, the electron energies become confined into discrete energy levels. This process dramatically enhances the interaction between the electrons and phonons. Consequently, phonons inherit some of the properties of the electrons and exhibit quantum oscillations in conduction themselves," HLD´s Dr. Toni Helm outlines the process.

"We have corroborated the existence of this unconventional phenomenon through the study of thermal conductivity and ultrasonic attenuation in semimetallic ZrTe₅ in strong magnetic fields and temperatures only a fraction of a degree above absolute zero. In our experiment, we have detected clear thermal quantum oscillations with a frequency characteristic of the electronic sub-system. However, the temperature dependence of their amplitude clearly follows the characteristic behavior of the phonons − a clear indication that the proposed mechanism is at play," Gałeski recalls.

Remarkably, this principle is not limited to ZrTe₅ but applies to all semimetals with low charge-carrier density − regardless of whether they are topological or not. Famous examples include graphene and bismuth. The study suggests that the thermal conductivity of lattice vibrations can serve as a sensitive tool to study subtle quantum effects that might be barely detectable through other means.