Frustrated electrons refuse to tunnel

Scientists of HFML have shown that decoherence between layers of a metallic system is linked to a loss of long-range magnetic order in the material. It is the first time that the cause of loss of interlayer coherence is experimentally shown. These findings are published in Nature Communications.

Many of the most topical and interesting metallic systems, such as the high temperature superconductors, have a layered crystal structure. As a result, an electrical current flows much more easily within the layers than between them.  In certain extreme cases, electrons are prevented from tunneling coherently (i.e. preserving their periodic wave-like motion) across adjacent layers; the electrons essentially become confined to individual layers and their motion between layers becomes diffusive. What induces this loss of interlayer coherence, however, has not been established in any material, though there has been much theoretical speculation as to the possible origin(s).

In a landmark study, a group of scientists at HFML and the IMM has discovered that a key signature of interlayer coherence in a frustrated triangular antiferromagnet PdCrO2 vanishes precisely at the temperature at which long-range magnetic order is lost. Moreover, through comparison with the isostructural non-magnetic PdCoO2, the group were able to demonstrate that it is the loss of long-range magnetic order (and the subsequent development of short-range magnetic fluctuations) that destroy interlayer coherence of the conduction electrons and not the loss of interlayer coherence that destroys the long-range magnetic order.

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Thus, while the spins on the Cr ions are ordered, the conduction electrons within the Pd-O plane are able to tunnel coherently from one layer to the next. However, above the ordering temperature, the now-fluctuating spins begin to scramble (scatter) the conduction electrons sufficiently to inhibit their ability to move coherently between the layers.

This study represents the first time that the cause of decoherence has been experimentally linked to a fundamental change in the material, in this case the loss of long-range magnetic order. By establishing this link, this study may have major implications for our understanding of interlayer coherence in a host of other low-dimensional metals that lie in close proximity to an ordered phase, be it of magnetic, electrostatic or orbital origin.

Related publication:

Simultaneous loss of interlayer coherence and long-range magnetism in quasi-two-dimensional PdCrO2, S. Ghannadzadeh, S. Licciardello, S. Arsenijevic, P. Robinson, H. Takatsu, M.I. Katsnelson & N.E. Hussey, Nature Communications 8 (2017)