Correlated States of 2D Electrons near the Landau Level Filling <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>ν</mml:mi><mml:mo>=</mml:mo><mml:mn>1</mml:mn><mml:mo stretchy="false">/</mml:mo><mml:mn>7</mml:mn></mml:math>

Abstract

The ground state of two-dimensional electron systems (2DESs) at low Landau level filling factors ($\ensuremath{\nu}\ensuremath{\lesssim}1/6$) has long been a topic of interest and controversy in condensed matter. Following the recent breakthrough in the quality of ultrahigh-mobility GaAs 2DESs, we revisit this problem experimentally and investigate the impact of reduced disorder. In a GaAs 2DES sample with density $n=6.1\ifmmode\times\else\texttimes\fi{}{10}^{10}/{\mathrm{cm}}^{2}$ and mobility $\ensuremath{\mu}=25\ifmmode\times\else\texttimes\fi{}{10}^{6}\text{ }\text{ }{\mathrm{cm}}^{2}/\mathrm{V}\text{ }\mathrm{s}$, we find a deep minimum in the longitudinal magnetoresistance (${R}_{xx}$) at $\ensuremath{\nu}=1/7$ when $T\ensuremath{\simeq}104\text{ }\text{ }\mathrm{mK}$. There is also a clear sign of a developing minimum in ${R}_{xx}$ at $\ensuremath{\nu}=2/13$. While insulating phases are still predominant when $\ensuremath{\nu}\ensuremath{\lesssim}1/6$, these minima strongly suggest the existence of fractional quantum Hall states at filling factors that comply with the Jain sequence $\ensuremath{\nu}=p/(2mp\ifmmode\pm\else\textpm\fi{}1)$ even in the very low Landau level filling limit. The magnetic-field-dependent activation energies deduced from the relation ${R}_{xx}\ensuremath{\propto}{e}^{{E}_{A}/2kT}$ corroborate this view and imply the presence of pinned Wigner solid states when $\ensuremath{\nu}\ensuremath{\ne}p/(2mp\ifmmode\pm\else\textpm\fi{}1)$. Similar results are seen in another sample with a lower density, further generalizing our observations.

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