Helical order: The spins of the electrons and nuclei (red arrows) take the form of a helix rotating along the axis of the quantum wire. The blue ribbon is a guide to the eye for the helix.
Illustration: B. Braunecker, P. Simon, and D. Loss, Phys. Rev. B 80, 165119 (2009)
In the past, this was possible only at much lower temperatures, typically in the microkelvin range. The coupling of nuclei and electrons creates a new state of matter whereby a nuclear spin order arises at a much higher temperature.
The results are consistent with a theoretical model developed in Basel a few years ago, as reported by the researchers in the scientific journal Physical Review Letters.
The researchers, led by Professor Dominik Zumbühl from the University of Basel’s Department of Physics, used quantum wires made from the semiconductor gallium arsenide. These are one-dimensional structures in which the electrons can move in only one spatial direction.
At temperatures above 10 kelvin, the quantum wires exhibited universal, quantized conductance, suggesting that the electron spins were not ordered. However, when the researchers used liquid helium to cool the wires to a temperature below 100 millikelvin (0.1 kelvin), the electronic measurements showed a drop in conductance by a factor of two, which would suggest a collective orientation of the electron spin. This state also remained constant when the researchers cooled the sample to even lower temperatures, down to 10 millikelvin.Electron-nuclear spin coupling
The reason why nuclear spin order is possible already at 0.1 kelvin is that the nuclei of the gallium and arsenic atoms in these quantum wires couple to the electrons, which themselves act back on the nuclear spins, which again interact with the electrons, and so on. This feedback mechanism strongly amplifies the interaction between the magnetic moments, thus creating the combined nuclear and electron spin magnetism. This order is further stabilized by the fact that the electrons in such quantum wires have strong mutual interactions, bumping into each other like railcars on a single track.Helical electron and nuclear spin order
The work of the Basel physicists opens up new avenues for mitigating these disruptive nuclear spin fluctuations: with the nuclear spin order achieved in the experiment, it may be possible to generate much more stable units of information in the quantum wires.
In addition, the nuclear spins can be controlled with electronic fields, which was not previously possible. By applying a voltage, the electrons are expelled from the semiconductor, which dissolves the electron-nucleus coupling and the helical order.International research partnership
The research was co-funded by the European Research Council, the Swiss National Science Foundation, the Basel Center for Quantum Computing and Quantum Coherence (Basel QC2 Center), the Swiss Nanoscience Institute and the NCCR Quantum Science & Technology (QSIT).Original Citations
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