It is an old dream of solid-state physics: “spin liquids” are a hypothetical state of matter with exotic magnetic properties. They could be used to transmit information without even a single electron having to move from its site. This could offer great advantages for electronic components and even quantum computers.
For years, there have been indications that such spin liquids might exist in real materials, but no convincing experimental proof has been reported to date. Now, however, new measurements show that spin liquids cannot even form in a material that was previously considered the prime candidate. This means that new routes have to be taken. The results were published in the scientific journal "Science".
Spin instead of charge
Whether it's a power cable, a light bulb or a semiconductor chip: Our technologies are based on the transport of electrical charge. Electrons are moving, information is being transmitted by electrical signals - "charge" versus "no charge", or "current" versus "no current".
"This does not happen without loss," says Prof. Andrej Pustogow from the Institute of Solid State Physics at TU Wien. "There is always a conversion of electrical energy into heat, or a loss of information."
However, in addition to its electrical charge, the electron also has a second important property that can be utilized: The electron spin. Similar to a compass needle, the spin of an electron can be aligned with a magnetic field. For quantum physical reasons, the spin can only align in two different states: Up or down.
"In a solid, an interaction can occur between the spins of neighbouring electrons," says Andrej Pustogow. "One electron can pass on the orientation of its spin to the next one, and thus information is transmitted through the material without any particle ever moving from its site." Analogous to a metallic conductor in which electronic charges move, this can be referred to as a "spin metal" in which the orientation of the spins can propagate freely, but the charge is stuck like in an insulator.
Rigid arrangement makes movement impossible
However, this is only possible if there is no fixed spin configuration. If, for example, a stable, regular pattern is formed that consists of alternating spin-up electrons and spin-down electrons, then turning a spin around is energetically very unfavourable and the spin structure remains rigid. For decades, scientists have therefore been thinking about the possibility of creating so called “spin liquids”, in which the spins do not form a fixed pattern, even at lowest temperatures, but fluctuate and remain disordered so that the spin orientation can move around freely.
"You can imagine, for example, a material in which the electrons are arranged in a triangular pattern," says Andrej Pustogow. "If two electrons in this triangle occupy opposite spin states, the spin of the third electron must match one of these spins." A state in which all neighbours have opposite spin at the same time cannot be formed in such a triangular structure.
"Since 2003, an organic compound with spins on an almost perfect triangular lattice has been considered the prime candidate for a spin liquid," says Andrej Pustogow. But this hope has not been fulfilled: A breakthrough has now been achieved in a study published in the journal “Science”, which resulted from work done before Pustogow's appointment at TU Wien together with international colleagues led by Martin Dressel (Uni Stuttgart).
The crystal lattice plays a crucial role
The team was able to show that the spins neither arrange themselves in a rigid up-down pattern nor in a dynamic state, as had been predicted for a spin liquid. "Instead, the electrons come together in pairs that move closer together, which probably also distorts the lattice," explains Andrej Pustogow. This breaks the symmetry of the material - and thus also shatters the hope of being able to use such materials as spin liquids, because neither the charges nor the spins move here.
"Of course, this doesn't mean that spins cannot be used to transfer or store information," says Andrej Pustogow. "But our results show that a dead end has been reached in the search for spin liquids. Only when one understands these effects one can start systematically searching for suitable new materials." Perhaps the charge-transporting metals and semiconductors of state-of-the-art microelectronics will be joined by one or two "uncharged" guests in the near future.
B. Miksch et al., Gapped magnetic ground state in quantum spin liquid candidate k-(BEDT-TTF)2Cu2(CN)3, Science 372, 6539.
Ass. Prof. Dr. Andrej Pustogow
Institute for Solid State Physics
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