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Electrons flow like a fluid in a metal superconductor

08 Oct 2021 Isabelle Dumé
new electron-phonon liquid
A small crystal of the new material. (Courtesy: Fazel Tafti, Boston College)

A team of researchers in the US has discovered that electrons in a transition metal superconductor called ditetrelide flow like a fluid rather than behaving as individual particles. The finding, which is connected to the physics of electron–phonon liquids, could shed fresh light on the fundamental properties of these technologically important materials and their potential applications.

Electrons usually travel through metals via diffusion, getting scattered by phonons (quasiparticles that arise from vibrations of the crystal lattice) along the way. A recently developed theory, however, suggests that under certain conditions, a coupled electron–phonon liquid can form in which the electrons transition from a diffusive (particle-like) flow to a hydrodynamic (fluid-like) one. In this case, the electrons would flow inside the metal like water flows through a pipe.

The theory also predicts that these electron-phonon liquids should form when certain other interactions (including Umklapp electron–electron scattering) are suppressed, allowing electrons to transfer momentum to the material’s crystal lattice. The electrical and thermal conductivities of such liquids should be higher than those of conventional (Fermi) liquids, in which electrons propagate through metals with weak electron–electron correlations. Until the current study, however, such liquids had not been seen in the laboratory for lack of suitable materials to experiment on.

Three different experimental techniques

Researchers led by Fazel Tafti of Boston College have now found evidence for an electron–phonon liquid in niobium germanide (NbGe2), a superconducting metal also known as ditetrelide. The team studied the behaviour of electrons in this material using three different techniques. The first, quantum oscillations, revealed that the effective mass of electrons was three times higher than the expected value, implying that electron–phonon interactions are present. Second, electrical resistivity measurements revealed a discrepancy between the experimental data and the values expected for standard Fermi liquids. Finally, Raman scattering showed a change in the vibration of the NbGe2 crystal thanks to the fluid-like flow of electrons. In addition, the team found that the Raman spectra of the phonons at different temperatures fit best in a model that takes into account phonon–electron coupling.

The researchers, who report their work in Nature Communications, say that it would now be interesting to conduct more direct experiments on NbGe2 to verify the hydrodynamic behaviour of its electrons. “Our work implies that the heavier-than-expected effective electron mass comes from strong electron–phonon interactions and we have demonstrated this for the first time in a metal superconductor,” says team member Hung-Yu Yang. To back up this finding, Yang suggests that one possible test would be to shrink the sample size to the nanoscale and see if it behaves differently, just as it becomes more difficult for water to flow through a pipe as the pipe gets narrower. “Another direction would be to find more electron–phonon liquid candidates using the design principle we have proposed,” he tells Physics World.

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