OREANDA-NEWS. A research team comprising members from Nippon Telegraph and Telephone Corp. (NTT) and the Japan Science and Technology Agency has succeeded in observing the crystallization of electrons in a semiconductor device under conditions of low temperature and high magnetic field using nuclear magnetic resonance (NMR).

The research team exploits the fact that the effective magnetic field electron spins exert on nuclear spins spatially varies when the electrons form a crystal lattice and the measurement was achieved by using a pristine semiconductor heterostructure and highly sensitive resistively detected NMR.

While it is established that such an electronic state, predicted 80 years ago and known as the "Wigner crystal", exists from the resonance absorption of electromagnetic waves, this is the first time that its microscopic structure has been revealed. The outcome of this study shows that NMR is a powerful means for probing not only the spin but also the charge or orbital state of electrons in a semiconductor, which may lead to elucidation of other exotic phases or the development of materials with novel physical properties. Furthermore, it enables us to quantify the variation of electron density on a nanometer scale induced by randomly distributed impurities. This will be a useful technique for characterizing nanometer-scale electronic devices.

These results will be published in the UK science journal "Nature Physics" on 20th, July 2014.

A team comprising members from NTT Basic Research Laboratories (NTT-BRL) and JST performed NMR measurements on a pristine gallium arsenide (GaAs)/aluminum gallium arsenide (AlGaAs) heterostructure containing a high-mobility two-dimensional electron system and obtained resonance spectra of arsenic nuclei constituting the GaAs layer that directly indicate the crystallization of electrons at low temperature and high magnetic field.

The resonance frequency of nuclear spins is subject to a tiny shift caused by an effective magnetic field exerted by the electrons surrounding the nuclei. From this shift, known as the Knight shift, NMR obtains information about electron spins. In this study, researchers exploited the fact that the Knight shift is proportional to the local electron density at the location of the nuclei to demonstrate the spatial variation of local electron density that results directly from electron crystallization. Furthermore, comparison with computer simulations based on microscopic wave functions of electron solids revealed the microscopic structure of the electron solid.