No metal without charge

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This research article published in Science has been featured in press releases of TU Wien (English version) and Uni Stuttgart (English version), as well as The Academic Times, Physics World and other media.

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Since school days we know: current comes out of an electrical outlet – and consists of moving charges. The digital age would be impossible without the manifold applications of electricity in lighting, computer technology and increasingly also electric vehicles. As the name indicates, in all these cases electrons – the negatively charged components of atoms (Fig. 1a) – are moving through conductive components made of metals or semiconductors. Charge transport does not happen without losses and always results in a conversion of electrical energy in heat, and a loss of information. A way out would be superconductors, where current flows without resistance, but to date there are no suitable materials for room-temperature applications. Pustogow Spectroscopy Lab at the Institute of Solid State Physics at TU Wien investigates whether energy and information can be transmitted through solids without the necessity to move charges, avoiding the associated drawbacks.

The quantum nature of electrons involves, additionally to their electric charge, a magnetic moment which is known as ‘spin’ (analogue to the induced magnetic field of a rotating charge). One can consider a tiny compass needle (indicated by arrows in Fig. 1) attached to each charge that lines up with an external magnetic field. A spin is the smallest unit of entropy and information, and the total spin of a system is conserved, hence it cannot be simply ‘lost’. In particular, the electron’s magnetic moment interacts much weaker with the crystal lattice than its charge – but can one also selectively transport spins through a solid?

A key requirement are magnetic materials where the orientation of spins primarily depends on that of their nearest-neighbours: in an antiferromagnet the spins are arranged antiparallel to each other in a periodic “up-down” pattern, sketched in Fig. 1b. While this works out nicely on a square lattice, a triangular arrangement of the spins poses an issue: along one of the three directions the unwanted (energetically unfavourable) parallel alignment cannot be avoided, known as geometrical frustration. Such a pair of parallel spins is called ‘spinon’ (Fig. 1c). Yet, how is this related to electrical current?

Current flow in metals is realized by electrons hopping from occupied to unoccupied sites (Fig. 2a). If almost all sites are occupied with electrons, charge transport can be described by the propagation of ‘holes’ (Fig. 2b). Since they move in the opposite direction compared to electrons, holes can be described as positively charged quasiparticles. However, as illustrated in Fig. 2c, also spinons can travel through a crystal by simultaneously flipping a pair of spin-up and spin-down. This happens without changing the total spin and energy of the system – and without moving a single charge! Analogue to current in a metal, this could be considered as a ‘spin metal’ or, similar to electrons in a Fermi liquid, as a ‘spin liquid’. Crucially, uncharged spinons cannot be scattered as easily as charged electrons; in some cases, these spin excitations are even ‘topologically protected’. Realizing a spin liquid would be highly intriguing towards possible applications in novel electronic components and even quantum computation.

Effective transport of spin excitations through a solid requires a large number of spinons which, however, are energetically unfavourable and therefore unstable in antiferromagnets. To that end, condensed-matter research has focused on the search for materials where antiferromagnetism is suppressed through geometrical frustration. Since 2003, the organic compound κ-(BEDT-TTF)₂Cu₂(CN)₃, where the spins are arranged on an almost perfect triangular lattice, has been considered the prime candidate for a spin liquid. A breakthrough has now been achieved in a study published in the journal “Science”, which resulted from work done before Andrej Pustogow's appointment at TU Wien together with international colleagues led by Martin Dressel (Uni Stuttgart). Here, Pustogow identified the spin gap by assessing newly available electron-spin-resonance results, inferring that mobile spinons play no role in this material. “It seems that Mother nature does not like frustration”, says Pustogow, “but rather prefers to arrange the spins in pairs at the cost of a lattice distortion.” Once the spins pair up, there remain no mobile spinons. This way, the hope for a spin liquid with uncharged spinons propagating through the crystal like electrons in a metal had to be abandoned in the case of κ-(BEDT-TTF)₂Cu₂(CN)₃.

Original article:

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Gapped magnetic ground state in quantum-spin-liquid candidate κ-(BEDT-TTF)₂Cu₂(CN)₃,
B. Miksch, A. Pustogow, M. Javaheri Rahim, A. A. Bardin, K. Kanoda, J. A. Schlueter, R. Hübner, M. Scheffler, and M. Dressel
Science 372, 276-279 (2021),
DOI:10.1126/science.abc6363, arXiv:2010.16155.

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