NMR profiling of quantum electron solids in high magnetic fields
This event is part of the Biophysics/Condensed Matter Seminar Series.
Abstract: We observe translational symmetry breaking in a two-dimensional electron system (2DES), confined to a GaAs quantum well, at filling factors v < 1/3 and v ~ 2 using resistively detected nuclear magnetic resonance (RD-NMR) spectroscopy at milliKelvin temperatures . It is widely believed that the fractional quantum Hall states terminate, at low filling factors, in an insulating phase comprising an electron solid or Wigner crystal (WC) . This has been supported by microwave pinning-mode resonance  measurements. However, microwave resonance experiments do not provide direct information on the microscopic structure of the 2DES. In this talk we show that RD-NMR is a local probe and can directly access the 2DES providing a novel investigation of the WC. We report striking anomalies in the RD-NMR spectral line shape for v < 1/3. These anomalies arise from modulations in the in-plane electron density, which we show are engendered by the formation of a Wigner solid. Using NMR as a local probe of the 2DES together with simulations based on microscopic wavefunctions, we further demonstrate that a careful analysis of the anomalous line shape reveals the presence of quantum mechanical correlations in the electron solid. In addition, we report evidence of the formation of correlated electron solids in the vicinity of the integer quantum Hall state at v ~ 2. At v slightly away from integer filling, the partial electron density forms a solid which we probe using RD-NMR. Further to providing evidence for electron correlations, our study shows the impact of Landau level index on WC behavior, highlighting the quantum mechanical nature of the solid. This study shows that NMR is a powerful means for probing the charge state of electrons in a semiconductor. In particular, it offers the significant advantage of high spatial resolution provided by nuclear spins, allowing one to obtain microscopic information about the electronic state on the nanometer scale.
 L. Tiemann(), T.D. Rhone(), N. Shibata, K. Muraki, Nature Physics 10, 648–652 (2014). [* - equal contribution]  Yu.E. Lozovik and V.I. Yudson, JETP Lett. 22, 11 (1975).  P.D. Ye et al., Phys. Rev. Lett. 89, 176802 (2002).