Magneto-Transport Study of Quantum Phases in Wide GaAs Quantum Wells

Abstract

In this thesis we study several quantum phases in very high quality two-dimensional electron systems (2DESs) confined to GaAs single wide quantum wells (QWs). In these systems typically two electric subbands are occupied. By controlling the electron density as well as the QW symmetry, we can fine tune the cyclotron and subband separation energies, so that Landau levels (LLs) belonging to different subbands cross at the Fermi energy <italic>EF</italic>. The additional subband degree of freedom enables us to study different quantum phases.

Magneto-transport measurements at fixed electron density n and various QW symmetries reveal a remarkable pattern for the appearance and disappearance of fractional quantum Hall (FQH) states at LL filling factors &nu = 10/3, 11/3, 13/3, 14/3, 16/3, and 17/3. These <italic>q/3</italic> states are stable and strong as long as <italic>EF</italic> lies in a ground-state (<italic>N</italic>=0) LL, regardless of whether that level belongs to the symmetric or the anti-symmetric subband.

We also observe subtle and distinct evolutions near filling factors &nu = 5/2 and 7/2, as we change the density n, or the symmetry of the charge distribution. The even-denominator FQH states are observed at &nu = 5/2, 7/2, 9/2 and 11/2 when <italic>EF</italic> lies in the <italic>N</italic>=1 LLs of the symmetric subband (the S1 levels). As we increase <italic>n</italic>, the &nu=5/2 FQH state suddenly disappears and turns into a compressible state once <italic>EF</italic> moves to the spin-up, <italic>N</italic>=0, anti-symmetric LL (the A0 level). The sharpness of this disappearance suggests a first-order transition from a FQH to a compressible state. Moreover, thanks to the renormalization of the susbband energy separation in a well with asymmetric change distribution, two LLs can get pinned to each other when they are crossing at <italic>EF</italic>. We observe a remarkable consequence of such pinning: There is a developing FQH state when the LL filling factor of the symmetric subband &nuS equals 5/2 while the antisymmetric subband has filling 1<&nuA<2.

Next, we study the evolution of the &nu=5/2 and 7/2 FQH states as we add a parallel magnetic field, <italic>B</italic>||, in the plane of the sample. The first-order transitions at &nu=5/2 and 7/2 are softened when <italic>B</italic>|| is applied, thanks to the mixing of the LLs from different subbands. Meanwhile, a small <italic>B</italic>|| also introduces a severe transport anisotropy at &nu=5/2 while the FQH state still remains reasonably strong.

Several other novel phenomena are also observed in wide QWs. In high (<italic>N</italic> &ge 2) LLs, our study reveals an unexpected rotation of the orientation of the stripe phase observed at a half-filled LL. This rotation is sensitive to the spin of the LL and the symmetry of the charge distribution in the QW. In the lowest LL, we observe a close competition between electron liquid and solid phases near filling factor &nu=1. In perticular, we observe a reentrant &nu=1 integer quantum Hall effect which signals the formation of a Wigner crystal state.