Evidence for an Anisotropic State of Two-Dimensional Electrons in High Landau Levels

Abstract

Magnetotransport experiments on high mobility two-dimensional electron gases in GaAs͞AlGaAs heterostructures have revealed striking anomalies near half filling of several spin-resolved, yet highly excited, Landau levels. These anomalies include strong anisotropies and nonlinearities of the longitudinal resistivity r xx which commence only below about 150 mK. These phenomena are not seen in the ground state or first excited Landau level but begin abruptly in the third level. Although their origin remains unclear, we speculate that they reflect the spontaneous development of a generic anisotropic many-electron state. [S0031-9007(98) A magnetic field applied perpendicular to the plane of a two-dimensional electron gas (2DEG) resolves the energy spectrum into discrete Landau levels (LLs). As the field increases, the Fermi level drops down through the Landau ladder in a series of steps until, at high field, it resides in the lowest (N 0) level. In this situation the kinetic energy of the electrons is quenched and electron-electron interactions dominate the physics with the fractional quantized Hall effect (FQHE) as the most spectacular consequence [1]. After more than 15 years of study, much is known about electron correlations in this lowest LL case. The same cannot be said when the Fermi level is in a higher Landau level. In the second LL (N 1), the FQHE is virtually absent; only fragile and poorly understood states at Landau filling fractions n 7͞3, 5͞2, and 8͞3 are seen in the best samples. In the third and higher LLs (N $ 2) still less is known, although there have been interesting suggestions of charge density waves in the clean limit [2,3]. At very high N, and therefore very low magnetic field, the Landau level splitting becomes insignificant and the 2DEG assumes the character of a weakly disordered Fermi liquid.In this paper we report the observation of several dramatic anomalies in the low temperature magnetotransport of clean 2DEGs when the Fermi level lies near the middle of a spin-resolved highly excited Landau level. These effects, which commence only below about 150 mK, abruptly begin and are strongest in the third (N 2) LL, but persist up to about N 6. Including strong anisotropies and intriguing nonlinearities of the resistivity, these effects suggest a considerably more interesting tableau at high N than independent electrons moving in a disordered Landau band.The samples used in this study are GaAs͞AlGaAs heterojunctions grown by molecular beam epitaxy (MBE). Data from six samples (A through F) will be discussed. Samples A, B, and C were taken from one MBE wafer, D and E from a second, and F from a third. Each wafer was rotated during growth to ensure high homogeneity of the electron density n s . These densities (in units of 10 11 cm 22 ) are close to n s 2.67 for samples A, B, and C; n s 2.27 for samples D and E; and n s 1.52 for sample F. The low temperature mobility of each is m $ 9 3 10 6 cm 2 ͞V s. Each sample was cleaved (along ͗110͘ directions) into a 5 3 5 mm square from its parent ͗001͘ wa...

Fractional quantum Hall effect in a tilted magnetic field

“…Upon closer examination, we find that the momenta of the quasidegenerate states are of the form (k x , 0), indicating linear order along x-direction. This is known to be one of the signatures of the stripe phase [42][43][44][45][46] .…”

An investigation of orientational symmetry-breaking mechanisms in high Landau levels

“…Longitudinal resistance data were obtained by measuring the resulting voltage drop between corner contacts. Each of these samples clearly exhibits the characteristic low temperature resistance anisotropies near half-filling of the N ≥ 2 LLs first reported by Lilly, et al [2].…”

“…At lower magnetic fields the higher LLs become occupied, and in the third or higher (N ≥ 2) LL no FQHE states have been observed. Recent experiments have nevertheless uncovered extraordinary transport signatures unique to the high LL regime, pointing to the existence of a new class of many-body states distinct from the incompressible quantum fluids responsible for the FQHE [2,3]. The most striking of these signatures is the rapid development, at very low temperatures, of strong anisotropies in the longitudinal resistance of the 2D electron system.…”

Spin and Charge Ordering in the Quantum Hall Regime

“…These states are therefore also called reentrant integer quantum Hall states (RIQHS). In the case of ≥ 2, these RIQHS occur at elevated temperatures around partial filling factors˜= 1 ⁄4 and˜= 3 ⁄4, whered enotes the filling factor of the highest occupied Landau level branch [95][96][97]. If the electron temperature is low enough, a seamless transition to the IQHE is observable in Fig.…”