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Recently reported giant anisotropy in the longitudinal resistivity of a 2D electron system with valence Landau level index N ≥ 2 has been interpreted as a signal of unidirectional charge density wave (UCDW) ground states. We report on detailed Hartree-Fock calculations of the UCDW orientation energy induced by a tilted magnetic field. We find that for current experimental samples stripes are oriented perpendicular to the in-plane field, consistent with experiment. For wider 2D electron systems we predict tilt-induced stripe states with variable anisotropy energy sign.Several groups [1][2][3][4][5] have reported the observation of strong anisotropies and nonlinearities in the low temperature magnetotransport of clean 2D electron systems over ranges of Landau level (LL) filling factor surrounding ν = n + 1/2 for n ≥ 4, i.e., for valence LL orbital index N ≥ 2. Although the origin of these anomalies has not been firmly established, the anisotropy is probably associated with the UCDW states which have been predicted to occur under precisely these circumstances [6,7]. Recently Pan et al. [4] and Lilly et al. [2] have discovered that the isotropic gapped ν = 5/2 quantum Hall state gives way to the anisotropic state for sufficiently large inplane magnetic fields. Shayegan and Manoharan [5] have observed that in a 2D hole system the ν = 5/2 state is already anisotropic even without in-plane field, indicating that lower electron density (more LL mixing) can stabilize the CDW. Both of these observations are consistent with an anisotropic spontaneously-broken orientationalsymmetry state, like the UCDW state. Several recent theoretical papers [8][9][10] have explored the properties of these 'liquid crystal' states for perpendicular field.In this paper we evaluate the dependence of UCDW state's energy on its orientation relative to the in-plane field component, when the magnetic field is tilted away from the 2D electron system normal. Theoretical studies along similar lines have recently been carried out by two other groups [11,12]. We find that screening due to polarization of remote LL's plays an essential role for the preferred orientation of the stripes. Using a realistic model for the sample of Lilly et al. (a single GaAs/Al x Ga 1−x As heterojunction with density N e = 2.67 × 10 11 cm −2 ) we quantitatively determine the anisotropy energy and find that the stripes prefer to be aligned perpendicular to the in-plane field for the whole range of studied field-tilt angles and filling factors, consistent with experimental observations. The same conclusion was found to apply for the lower density (N e = 2.2 × 10 11 cm −2 ) sample of Pan et al.. To explore the dependence of this result on system geometry we have repeated these calculations for a parabolic confinement quantum well models with variable subband separation. These calculations reveal a mechanism for tilt-induced UCDW states in samples with more than one occupied subband for which the perpendicular field state is expected to be isotropic. We find that stripe orient...

We show that the sign of magnetic anisotropy energy in quantum Hall ferromagnets is determined by a competition between electrostatic and exchange energies. Easy-axis ferromagnets tend to occur when Landau levels whose states have similar spatial profiles cross. We report measurements of integer QHE evolution with magnetic-field tilt. Reentrant behavior observed for the ν = 4 QHE at high tilt angles is attributed to easy-axis anisotropy. This interpretation is supported by a detailed calculation of the magnetic anisotropy energy. 73.40Hm, 75.10Lp, 75.30Gw Typeset using REVT E X 1

Cyclotron resonance has been measured in far-infrared transmission of GaAs/Al x Ga 1−x As heterostructures with an etched trigonal lateral superlattice intended to mimic graphene with lattice constant of the order of 100 nm (about 1000 times larger than that of natural graphene). Nonlinear dependence of the resonance position on magnetic field was observed, as well as its splitting into several modes. Our explanation, based on a perturbative calculation, describes the observed phenomena as a weak effect of the lateral potential on the twodimensional electron gas. Using this approach, we found a correlation between parameters of the lateral patterning and the created effective potential and obtained thus insights into how the electronic miniband structure has been tuned. The miniband dispersion was calculated using a simplified model and allowed us to formulate four basic criteria that have to be satisfied to reach graphene-like physics in such systems.