Composite fermions (CFs), exotic particles formed by pairing an even number of flux quanta to each electron, provide a fascinating description of phenomena exhibited by interacting two-dimensional electrons at high magnetic fields. At and near Landau level filling ν=1/2, CFs occupy a Fermi sea and exhibit commensurability effects when subjected to a periodic potential modulation. We observe a pronounced asymmetry in the magnetic field positions of the commensurability resistance minima of CFs with respect to the field at ν=1/2. This unexpected asymmetry is consistent with the CFs' Fermi wave vector being determined by the minority carriers in the lowest Landau level, and suggests a breaking of the particle-hole symmetry for CFs near ν=1/2.

In high-quality two-dimensional electrons confined to GaAs quantum wells, near Landau level filling factors ν = 1/2 and 1/4, we observe signatures of the commensurability of the electron-flux composite fermion cyclotron orbits with a unidirectional periodic density modulation. Focusing on the data near ν = 1/2, we directly and quantitatively probe the shape of the composite fermions' cyclotron orbit, and therefore their Fermi contour, as a function of magnetic field (B || ) applied parallel to the sample plane. The composite fermion Fermi contour becomes severely distorted with increasing B || and appears to be elliptical, in sharp contrast to the electron Fermi contour which splits as the system becomes bilayer-like at high B || . We present a simple, qualitative model to interpret our findings.

We fabricated a highly efficient, broadband light emitting diode driven by an optical near field generated at the inhomogeneous domain boundary of a dopant in a homojunction bulk Si crystal and evaluated its performance. To fabricate this device, a forward current was made to flow through a Si p-n junction to anneal it. During this process, the device was irradiated with near-infrared light, producing stimulated-emission light using a two-step phonon-assisted process triggered by the optical near field, and the annealing rate was controlled in a self-organized manner. The device emitted light in a wide photon energy region of 0.73-1.24 eV (wavelength 1.00-1.70 μm). The total power of the emitted light with 11 W of electrical input power was as high as 1.1 W. The external power conversion efficiency of the emitted light was 1.3%, the differential external power conversion efficiency was 5.0%, the external quantum efficiency was 15%, and the differential external quantum efficiency was 40%. The dependency of the emitted light power density on the injected current density clearly showed a characteristic reflecting the two-step phonon-assisted transition process.T. Kawazoe ( ) · M.A. Mueed · M. Ohtsu

Via measurements of commensurability features near the Landau filling factor ν=1/2, we probe the shape of the Fermi contour for hole-flux composite fermions confined to a wide GaAs quantum well. The data reveal that the composite fermions are strongly influenced by the characteristics of the Landau level in which they are formed. In particular, their Fermi contour is warped when their Landau level originates from a hole band with significant warping.

We demonstrate a severe Fermi contour anisotropy induced by the application of a parallel magnetic field to high-mobility electrons confined to a 30-nm-wide (001) GaAs quantum well. We study commensurability oscillations, namely geometrical resonances of the electron orbits with a unidirectional, surface-strain-induced, periodic potential modulation, to directly probe the size of the Fermi contours along and perpendicular to the parallel field. Their areas are obtained from the Shubnikov-de Haas oscillations. Our experimental data agree semi-quantitatively with the results of parameter-free calculations of the Fermi contours but there are significant discrepancies.

We report ballistic transport commensurability minima in the magnetoresistance of ν = 3/2 composite fermions (CFs). The CFs are formed in high-quality two-dimensional electron systems confined to wide GaAs quantum wells and subjected to an in-plane, unidirectional periodic potential modulation. We observe a slight asymmetry of the CF commensurability positions with respect to ν = 3/2, which we explain quantitatively by comparing three CF density models and concluding that the ν = 3/2 CFs are likely formed by the minority carriers in the upper energy spin state of the lowest Landau level. Our data also allow us to probe the shape and size of the CF Fermi contour. At a fixed electron density of 1.8 × 10 11 cm −2 , as the quantum well width increases from 30 to 60 nm, the CFs show increasing spin-polarization. We attribute this to the enhancement of the Zeeman energy relative to the Coulomb energy in wider wells where the latter is softened because of the larger electron layer thickness. The application of an additional parallel magnetic field (B || ) leads to a significant distortion of the CF Fermi contour as B || couples to the CFs' out-of-plane orbital motion. The distortion is much more severe compared to the ν = 1/2 CF case at comparable B || . Moreover, the applied B || further spin-polarizes the ν = 3/2 CFs as deduced from the positions of the commensurability minima. arXiv:1411.2994v1 [cond-mat.mes-hall]

There has been a surge of recent interest in the role of anisotropy in interaction-induced phenomena in two-dimensional (2D) charged carrier systems. A fundamental question is how an anisotropy in the energy-band structure of the carriers at zero magnetic field affects the properties of the interacting particles at high fields, in particular of the composite fermions (CFs) and the fractional quantum Hall states (FQHSs). We demonstrate here tunable anisotropy for holes and hole-flux CFs confined to GaAs quantum wells, via applying in situ in-plane strain and measuring their Fermi wavevector anisotropy through commensurability oscillations. For strains on the order of 10 −4 we observe significant deformations of the shapes of the Fermi contours for both holes and CFs. The measured Fermi contour anisotropy for CFs at high magnetic field (αCF) is less than the anisotropy of their low-field hole (fermion) counterparts (αF), and closely follows the relation: αCF = √ αF.The energy gap measured for the ν = 2/3 FQHS, on the other hand, is nearly unaffected by the Fermi contour anisotropy up to αF ∼ 3.3, the highest anisotropy achieved in our experiments.High-mobility, two-dimensional (2D), charged carriers at high perpendicular magnetic fields B and low temperatures exhibit rich many-body physics driven by Coulomb interaction. Examples include the fractional quantum Hall state (FQHS), Wigner crystal, and stripe phase [1,2]. Recently, the role of anisotropy has become a focus of new studies . This interest has been amplified by the recognition that, although the FQHSs at fillings 1/q (q = odd integer) are well described by Laughlin's wave function with a rotational symmetry [24], there is a geometric degree of freedom associated with the anisotropy of the 2D carrier system [8].The fundamental issue we address here is how the anisotropy of the energy-band structure of the low-field carriers transfers to the interacting particles at high B and, in particular, to the FQHSs and composite fermions (CFs). The latter are electron-flux quasi-particles that form a Fermi sea at a half-filled Landau level [2,25], and provide a simple explanation for the nearby FQHSs [26].

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