The two dimensional electron gas subjected to a perpendicular magnetic field
is a model system that supports a variety of electronic phases. Perhaps the most
well-known are the fractional quantum Hall states, but in recent years there has
been an upsurge of interest in the charge ordered phases commonly referred to as
electron solids. These solids are a consequence of electron-electron interactions in a
magnetic field. While some solid phases form in the lowest Landau level, the charged
ordered phases are most abundant in the higher Landau levels. Examples of such
phases include the Wigner solids, electronic bubble phases and stripe or nematic
phases. Open questions surround the exact role of disorder, confinement potential,
temperature and the Landau level index in determining the stability and competition
of these phases with other ground states. <div>The interface of GaAs/AlGaAs remains the cleanest host for the two-dimensional
electron gas due to the extremely high quality of materials available and the advancement in molecular beam epitaxy growth techniques. As a result, exceptionally high
electron mobilities in this system have been instrumental in the discovery of numerous
electron solids. </div><div>In this Thesis, I discuss the discovery and properties of several electron solids
that develop in such state-of-the-art two dimensional electron gases. These electron
solids often develop at ultra low temperatures, in the milliKelvin temperature range.
After an introduction to the physics of the quantum Hall effect in two dimensions, in
chapter 3, I discuss electron solids developing in the N=1 Landau level. While these
solids have been known for some time, details of the competition of these phases
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with the nearby fractional quantum Hall states remains elusive. A number of reports
observe new fractional quantum Hall states at filling factors where electron solids
are found in other experiments. We undertook a systematic study to answer some
of these unsettled questions. We see evidence for incipient fractional quantum Hall
states at 2+2/7 and 2+5/7 at intermediate temperatures which are overtaken by
the electronic bubble phases at lower temperatures. Several missing fractional states
including those at filling factors 2+3/5, 2+3/7, 2+4/9 highlight the relative stability
of the electronic solids called the bubble phases in the vicinity in our sample. </div><div>In chapter 4, I discuss a newly seen electron crystal which manifests itself in
transport measurements as a reentrant integer quantum Hall state. Reentrant integer
behavior is common in high Landau levels, but so far it was not observed in the
lowest Landau level in narrow quantum well samples. In contrast to high Landau
levels, where such reentrant integer behavior was associated with electronic bubbles,
we believe that the same signature in the N=0 Landau level is due to an electronic
Wigner crystal. The filling factors at which we observe such reentrance reveal that
it is a crystal of holes, rather than electrons. The discovery of this reentrant integer
state paints a complex picture of the interplay of the Wigner crystal and fractional
quantum Hall states. </div><div>Finally, in chapter 5, I discuss the observation of a novel phenomenon, that of
reentrant fractional quantum Hall effect. In the lowest Landau level, we observe a
fractional quantum Hall state, but as the field is increased, we see a deviation and
then a return to quantization in the Hall resistance. Such a behavior indicates a novel
electron solid. In contrast to the collective localization of electrons evidenced by the
reentrant integer quantum Hall effect, such reentrance to a fractional Hall resistance
clearly points to the involvement of composite fermion quasiparticles. This property thus distinguishes the ground state we observed as a solid formed of composite
fermions. Such a solid phase is evidence for exotic electron-electron correlations at
play which are clearly different from those in the traditional Wigner solid of electrons.<br></div>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/8980331 |
| Date | 15 August 2019 |
| Creators | Vidhi Shingla (7023347) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/Magnetotransport_Studies_of_Diverse_Electron_Solids_in_a_Two-Dimensional_Electron_Gas/8980331 |
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