Cheering up a frustrated quantum system

September 24, 2020

One of the holy-grail questions in condensed matter physics is how superconductivity — the property of many electrons to go into a quantum soup state that can carry electricity without losses — emerges at relatively high temperatures in certain materials, and how these temperatures could be boosted even further. Now a research team at the University of Oxford and the MPSD is reporting in Physical Review Letters that a dynamical version of superconductivity, which is generated by periodically shaking the material, is intimately tied to strong electronic correlations and geometric frustration.

Geometric frustration is a property of interacting quantum systems with a spin degree of freedom that can be likened to little compass needles. In a so-called antiferromagnet, when these spins are next to each other, they try to point in opposite directions in order to lower their energy — one of them to the north, the other one to the south. Now suppose that you force a third spin close into a triangle with the other two. How should it orient its spin? If it points north, the first spin will be unhappy; if it points south, the second one will be. This dilemma is called frustration and is at the root of many intriguing properties in quantum materials. 

Electrons with spin (arrows) can hop between sites on a triangular lattice. When they occupy the same lattice site, they pay an energy price U due to their mutual Coulomb repulsion. When the system is irradiated with a laser pulse (red wiggly lines) at the right color such that the lattice is shaken, the U becomes time-dependent (U(t)), which leads to superconducting pairing between the electrons.

Now a theory team led by Dieter Jaksch at Oxford University and Michael Sentef at the MPSD reveals a new avenue towards exploiting magnetic frustration. When the frustrated material is stimulated by a short laser pulse of the right color, it can become superconducting at temperatures much higher than those at which superconductivity is observed in the same material without laser stimulation. „We were stunned when we first saw this effect in our numerical results“, says Joseph Tindall, a PhD student in Jaksch’s group and lead author of the study.

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