A Quantum Metal Just Changed What We Know About Heat

In an ultra-cold, high-magnetic-field setting, ZrTe₅ defies expectations by showing quantum heat oscillations. Researchers found that phonons, normally dominant in semimetals, behave more like electrons due to magnetic effects, reshaping our understanding of heat transport.

Heat conduction is one of the most basic physical properties of matter and plays a vital role in many engineering applications. Scientists have a solid understanding of how traditional materials, like metals and insulators, conduct heat under normal conditions. But under extreme environments, such as near absolute zero temperatures combined with strong magnetic fields, the rules begin to change. In these conditions, quantum effects can take over, leading to unexpected behavior, especially in quantum materials.

A team of researchers from the Helmholtz-Zentrum Dresden-Rossendorf (HZDR), the University of Bonn, and the Centre national de la recherche scientifique (CNRS) recently tested this by exposing the quantum material zirconium pentatelluride (ZrTe5), a semimetal, to high magnetic fields at ultra-low temperatures. What they discovered was surprising: dramatically enhanced heat oscillations that appear to be driven by a previously unknown mechanism.

The result challenges a long-standing assumption in physics: that magnetic quantum oscillations—a hallmark of quantum behavior—should not appear in the heat transport of semimetals. But in this case, they clearly did. The findings, published in PNAS, open up new questions about how heat moves through quantum materials—and what other surprises may be hidden in the cold.

Quantum Materials and Topological Properties

The quantum material ZrTe₅ belongs to a class of materials known as topological semimetals. In physics, “topological” refers to materials with unique electronic structures that give rise to extremely stable, or topologically protected, conduction properties. These materials can exhibit unusual and often counterintuitive quantum behaviors, phenomena that are increasingly seen as promising for the development of next-generation quantum technologies.

As both researchers and industry race to develop quantum computers, topological materials like ZrTe₅ are attracting growing interest. ZrTe₅ is especially notable for its rare combination of non-trivial electronic properties, which could make it useful in high-precision electronics and magnetic-field sensors.

Unexpected Heat Behavior in Extreme Conditions

“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, recent experiments have defied that expectation, revealing giant quantum oscillations in the heat conduction of semimetals like ZrTe₅. These surprising results are prompting scientists to rethink the mechanisms behind heat transport in quantum materials.

A Counterintuitive Mechanism for Heat Flow

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 to 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.

Evidence of Quantum Oscillations in Phonons

“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.

Beyond ZrTe₅: Broader Implications for Semimetals

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.

Reference: “Giant quantum oscillations in thermal transport in low-density metals via electron absorption of phonons” by Baptiste Bermond, Rafał Wawrzyńczak, Sergei Zherlitsyn, Tommy Kotte, Toni Helm, Denis Gorbunov, Genda Gu, Qiang Li, Filip Janasz, Tobias Meng, Fabian Menges, Claudia Felser, Joachim Wosnitza, Adolfo Grushin, David Carpentier, Johannes Gooth and Stanisław Gałeski, 5 March 2025, Proceedings of the National Academy of Sciences.
DOI: 10.1073/pnas.2408546122

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