An article presenting results of cooperation of P. Chudzinski, M. Berben, Xiaofeng Xu, N. Wakeham, B. Bernáth, C. Duffy, R. D. H. Hinlopen, Yu-Te Hsu, S. Wiedmann, P. Tinnemans, Rongying Jin, M. Greenblatt, N. E. Hussey was published in Science.
“Emergent symmetry in a low-dimensional superconductor on the edge of Mottness”
“Imagine a magic-trick where a completely uninteresting ungraceful object turns into a beautiful, perfectly symmetric sphere. This is a kind of trick that we have had the pleasure to see in a less known material, LiMoO. Our magician was nature, we were only a spectators that had to look carefully enough to appreciate the show.
The measurement we have done was magneto-resistance (MR) as a function of temperature. It had been known already for more than three decades that resistivity in this material is extremely anisotropic, with a mysterious upturn happening around 30K. Yet, quite astonishingly, our MR(T) revealed perfectly linear behaviour, for all measured directions, down to temperatures of 2K.
Previous results on this material proved that back-scattering amplitude G in LiMoO strongly depends on magnetic field. This has helped us understand what is going on: it turns out that what MR(T) measures is how G is changing when high energy carriers are gradually averaged out. But this conjecture has very profound implications – a perfect linearity of MR(T) implies that the electrons in the material are precisely neatly balanced between being localized (insulator) and itinerant (metal). This is just like the perfect symmetry of a sphere, but for the charge motion degrees of freedom. Curiously, there is absolutely no reason why such perfect symmetry should be there. The nature-magician seemed to play an outstanding trick on us.
In the second part, we show how the measurement of an additional quantity, Hall effect RH(T), can help us understand what is going on behind the scenes. The RH(T) is again a power law, but with an exponent that is a bit awkward. It can be reconciled with other transport probes only if we assume that G is not uniform but vary in space. When the system is in the metallic phase, then carriers can explore different values of G, and upon the averaging procedure G grows quickly; while when G grows too much and the material becomes an insulator then carriers cannot travel any more and the growth of G stops. Thus, the system has a hidden, self-stabilizing mechanism inside.
To put our research in a broader context one may realize that states with inhomogeneous two-body interactions are right now becoming serious contenders to describe parents states in various unconventional superconductors, notably the famous cuprates. Quite nicely, our humble LiMoO also becomes a superconductor, just below 2K, so that our beautiful “sphere” may be likely a parent state for something even more remarkable – a superconductor in the close vicinity to Mott-charge order. This raises hope that our Cinderella-material, will actually be the one that can pave the way to understand quite a few outstanding mysteries puzzling us in the heart of contemporary condensed matter physics”.
