Graph on S composition and Te composition
Graph on S composition and Te composition

Nematicity: a pathway towards better and stronger superconductors?

A longstanding collaboration between HFML-FELIX and the Universities of Tokyo and Kyoto in Japan has recently discovered that superconductivity in the iron chalcogenide family FeSe1-xTex is optimized just at a particular value of x (corresponding to the amount of Te substitution on the Se site) where a certain long-range order is suppressed to absolute zero, a special point in the temperature-Te concentration phase diagram referred to as a ‘quantum critical point’ or QCP. The importance of this result is that the order in question – a pure ‘electron nematic’ order – is rather unique and this result constitutes the first experimental evidence that superconductivity can be optimized at purely nematic QCP. The article describing their discovery has just been published in Physical Review X.

High-temperature superconductivity (the ability to conduct electricity without resistance at elevated temperatures) is one of the biggest unsolved problems in condensed matter physics, due to its unconventional superconducting pairing mechanism, which goes beyond the standard electron-phonon interaction. Many materials with such an unconventional state commonly host an additional antiferromagnetic (AFM) phase that competes and/or coexists with the superconductivity. Interestingly, the superconducting transition temperature Tc is very often enhanced in the part of the phase diagram where this AFM phase transition is suppressed down to zero-temperature, i.e. in vicinity of the AFM QCP. Such a correlation has led to a strong belief that AFM quantum critical fluctuations play a decisive role in the SC pairing of unconventional superconductors.

Graph on S composition and Te composition
Combined Tc vs. x phase diagram for FeSe1-xSx (left-hand side) and FeSe1-xTex (right-hand side) for different values of H.

Similar Tc enhancement near pure nematic QCP

The key finding from the HFML-FELIX-Japan collaboration is that it has revealed a similar enhancement of Tc also occurs in vicinity of a pure nematic QCP. The electron nematic phase is a peculiar state that breaks the rotational symmetry while preserving the translational symmetry of a material. Nematicity is also seen in liquid crystals whereby oblate molecules stack preferentially along one particular orientation below an ordering temperatures. To investigate the interplay between superconductivity and quantum critical fluctuations of the nematic order, the team conducted experiments using high magnetic fields to see how the behaviour of electron pairs changes near the QCP. Specifically, they measured a property called the upper critical field (Hc2 – the maximum field strength to which superconductivity can survive) in a series of high-quality FeSe1-xTex single crystals. As they increased the magnetic field, they observed that the superconducting phase of FeSe1-xTex became smaller, forming a narrow region around the nematic QCP.

Their study suggests that the interaction between electron pairs becomes significantly enhanced due to fluctuations associated with the nematic QCP. Importantly, unlike superconductivity mediated by magnetism, these findings suggest a new pathway, involving the nematic QCP, towards improved superconducting performance. The essential argument is the following: Although the electron–phonon interaction acts on the entire (Fermi) sea of electrons, it is inherently weak. By contrast, AFM interactions tend to be much stronger, but they influence only a restricted region of this sea. In principle, nematicity offers the best of both worlds – with strong interactions acting on the entire Fermi sea. The results published in Physical Review X give us promising hints that nematicity can indeed offer a pathway towards better and stronger superconductors!


Read the full article on the website of Physical Review X.

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