Metastable resistance-anisotropy orientation of two-dimensional electrons in high Landau levels

In half-filled high Landau levels, two-dimensional electron systems possess collective phases which exhibit a strongly anisotropic resistivity tensor. A weak, but as yet unknown, rotational symmetry-breaking potential native to the host semiconductor structure is necessary to orient these phases in macroscopic samples. Making use of the known external symmetry-breaking effect of an in-plane magnetic field, we find that the native potential can have two orthogonal local minima. It is possible to initialize the system in the higher minimum and then observe its relaxation toward equilibrium.     

Density-induced interchange of anisotropy axes at half-filled high Landau levels

We observe density-induced 90 degrees rotations of the anisotropy axes in transport measurements at half-filled high Landau levels in the two dimensional electron system, where stripe states are proposed ( nu = 9/2, 11/2, etc.). Using a field effect transistor, we find the transition density to be 2.9x10(11) cm(-2) at nu = 9/2. Hysteresis is observed in the vicinity of the transition. We construct a phase boundary in the filling factor magnetic field plane in the regime 4.4相似文献   

Electrical conductivity of a two-dimensional model for a structurally anisotropic composite

The electrical conductivity of a two-dimensional structurally anisotropic model for a composite is considered. The model represents an isotropic matrix with a system of nonconducting inclusions in the form of infinitely thin straight line segments (scratches). The scratches make an angle θ or −θ with a preferred axis (for definiteness, axis y) at the same probability, and their centers are chaotically distributed. An approximate effective medium method is used to obtain a general expression for the effective conductivity tensor $ \hat \sigma _e $ \hat \sigma _e of this model that is valid over a wide concentration range. In this approximation, both components of tensor are $ \hat \sigma _e $ \hat \sigma _e shown to vanish at the same percolation threshold, which is expressed explicitly. The conductivity of the model in a critical region is considered in terms of the similarity hypothesis.     

In-plane magnetic field-induced anisotropy and orientation energy of stripe phases at half-filled high Landau levels

We report on detailed Hartree–Fock calculations of the unidirectional charge density wave 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 two-dimensional electron systems we predict tilt-induced stripe states with variable anisotropy energy sign.     

Optical conductivity of the half-filled hubbard chain

We combine well-controlled analytical and numerical methods to determine the optical conductivity of the one-dimensional Mott-Hubbard insulator at zero temperature. A dynamical density-matrix renormalization group method provides the entire absorption spectrum for all but very small coupling strengths. In this limit we calculate the conductivity analytically using exact field-theoretical methods. Above the Lieb-Wu gap the conductivity exhibits a characteristic square-root increase. For small to moderate interactions, a sharp maximum occurs just above the gap. For larger interactions, another weak feature becomes visible around the middle of the absorption band.     

Excitonic effects in the recombination radiation spectra of completely filled Landau levels of two-dimensional electrons

The recombination radiation spectra of two-dimensional electrons in an asymmetrically doped GaAs/AlGaAs quantum well are investigated at different temperatures and laser-excitation energies. At low temperatures and in high magnetic fields the recombination lines of the electrons from completely filled Landau levels are split into narrow sublevels. It is shown that this fine structure of the Landau levels is due to the presence of excitonic effects in the initial and final states of the photoexcited system. It is demonstrated that the recombination process is accompanied by the excitation of intersubband and cyclotron magnetoplasma modes. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 1, 38–43 (10 January 1997)     

Commensurate-incommensurate transition in two-coupled chains of nearly half-filled electrons

We investigate the physical properties of two coupled chains of electrons, with a nearly half-filled band, as a function of the interchain hopping t _⊥ and the doping. We show that upon doping, the system undergoes a metal-insulator transition well described by a commensurate-incommensurate transition. By using bosonization and renormalization we determine the full phase diagram of the system, and the physical quantities such as the charge gap. In the commensurate phase two different regions, for which the interchain hopping is relevant and irrelevant exist, leading to a confinement-deconfinement crossover in this phase. A minimum of the charge gap is observed for values of t _⊥ close to this crossover. At large t _⊥ the region of the commensurate phase is enhanced, compared to a single chain. At the metal-insulator transition the Luttinger parameter takes the universal value K _ρ ^* = 1, in agreement with previous results on special limits of this model. Received 31 July 2000     

Effects of rotation on Landau states of electrons on a spherical shell

Based on a previously observed analogy between electromagnetic and non-inertial effects, we investigate the competition between magnetic field and rotation in the quantum motion of an electron constrained to the surface of a sphere. We solve numerically the Schrödinger equation of the problem for the energy eigenvalues and the eigenfunctions and compare the effects of the magnetic field and rotation. We obtain that, for a weak magnetic field, an electron can not distinguish between magnetic field and rotation, since they lead to equivalent behavior. But this is no longer true for strong magnetic fields. However, surprisingly, even though the rotation and magnetic fields play different roles in the electronic properties of the system, when together, each influence of the magnetic field on the energy levels can be separately balanced by rotation. We also show that no matter the intensity of the magnetic field, it is always possible to destroy the Landau levels in the sphere by rotation.     

Rayleigh scattering of light by two-dimensional electrons in a high magnetic field

Resonance Rayleigh scattering of light by a two-dimensional electron system in the ultraquantum limit is investigated. The scattering process under study involves electronic states belonging to the two spin sublevels of the zero Landau level. It is shown that the main contribution to resonance Rayleigh scattering originates from the fluctuations of the random potential in the quantum well hosting the two-dimensional system.     

Anisotropic states of two-dimensional electrons in high magnetic fields

We study the collective states formed by two-dimensional electrons in Landau levels of index n > or = near half filling. By numerically solving the self-consistent Hartree-Fock (HF) equations for a set of oblique two-dimensional lattices, we find that the stripe state is an anisotropic Wigner crystal (AWC), and determine its precise structure for varying values of the filling factor. Calculating the elastic energy, we find that the shear modulus of the AWC is small but finite (nonzero) within the HF approximation. This implies, in particular, that the long-wavelength magnetophonon mode in the stripe state vanishes q(3/2) like as in an ordinary Wigner crystal, and not like q(5/2) as was found in previous studies where the energy of shear deformations was neglected.     

Landau levels of holes in a two-dimensional channel

A method for calculating the Landau levels in a two-dimensional channel is proposed. This method is aimed at construction of an effective two-dimensional Hamiltonian of holes H(k_x, k_y, B). The Hamiltonian is expanded in powers of the magnetic field B and noncommuting momenta k_x and k_y. The results indicate an interconnection between effective masses and g-factors of the two-dimensional holes. It is possible to account for the effects of band warping in a nonperturbative manner. The shift of phase in the Shubnikov-de Haas oscillations is explained.     

Kinetics of two-dimensional electrons on a curved surface

A number of effects associated with the curvature of the surface on which a two-dimensional (2D) electron gas is placed are studied. The most significant effect in an external magnetic field (which for 2D electrons becomes effectively nonuniform) is the lifting of the degeneracy of the Landau levels. The intensity and shape of the cyclotron resonance line (inhomogeneously broadened) for different polarizations and the corrections to the Hall constant are found for the example of a circular cylinder. A picture of the quantization of the conductance that is qualitatively different from the case of a flat strip is obtained for a quasi-one-dimensional quantum wire in the form of a hollow cylinder. It is shown that in contradistinction to the planar case the spectrum of 2D electrons on the curved surface is sensitive to the sign of the spin-orbit coupling constant (for a fixed sign of the curvature). For hetero-junctions, for example, this opens up new possibilities for extracting information about their “hidden parameters.” Pis’ma Zh. éksp. Teor. Fiz. 64, No. 6, 421–426 (25 September 1996)