S. R. Haynes

Spin configurations and quasiparticle fractional charge of fractional QHE ground states in the N=0 Landau level

Abstract Spin configurations of fractional states v= p q in the N=0 Landau level are examined by in-situ rotation studies of an exceptionally high quality sample. The 2 3 and 4 3 parent states are found to be unpolarised at low B, as is the 8 5 daughter state (of the polarised 5 3 parent), and field-induced phase transitions for these states are observed. The validity of the probe σ c xx =σ xx ( 1 T =0) ≏ ( e q ) 2 /h of the quasiparticle fractional charge e ∗ =± e q is confirmed by extensive integer QHE activation studies, from which a universal prefactor σcxx close to e 2 h is identified.

Optical spectroscopy of correlated phases of degenerate two-dimensional electrons

Photoluminescence measurements of correlated states of two-dimensional electrons in GaAs/GaAlAs heterojunctions at low temperatures and in high magnetic fields are discussed in terms of many-body effects in the recombination process. The formation of incompressible states is studied by observing radiative transitions from densely and sparsely populated subbands. Optical evidence for the formation of an electron solid in the extreme quantum limit is presented.

Experimental tests of fractional quantum hall effect theory

Abstract The predicted effect of disorder on the hierarchical model of the FQHE, the extension of the FQHE to the N =1 Landau level and the Laughlin scaling diagram are examined in a range of experiments. A new method of obtaining the ground state energy gaps by analysis of the widths of fractional resistivity minima is outlined and preliminary data are presented that point to a method for the experimental determination of quasi-particle charge.

Optical Detection of Integer and Fractional QHE in GaAs: Extension to the Electron Solid

Ten years on from the discovery of the integer QHE (IQHE) [1], and the subsequent discovery of condensation of electrons into an incompressible liquid — the fractional QHE (FQHE) [2], experiments are still largely restricted to electrical transport studies. Whilst there is a report of an optical measurement of the FQHE in a Si structure at 1.5K [3], optical experiments that probe the QHE in GaAs have remained a major challenge [4,5]. An important advance was made by the observation of anomalies in the energy of photoluminescence near v = 2/3 and v = 1 from a GaAs multiple quantum well at 0.4K [6]. In this paper we report a definitive detection of both the integer and fractional QHE in GaAs, using intrinsic bandgap photoluminescence, by a study of integer states from v = 1 to 10 and of the v = 2/3 hierarchy out to the 5/9 daughter state, in an ultra-high mobility single heterojunction at 120mK [7]. At higher fields the photoluminescence spectra are mapped out at 80mK through FQHE states of the v = 1/3 and 1/5 hierarchies. Of particular interest is a new photoluminescence peak that becomes clearly resolved in the region v < 1/5. The appearance of this peak at v ≃ 1/5 correlates with the rapid onset of a substantial non-linear out-of-phase conduction that has recently been shown to arise from threshold behaviour that may be associated with crystallisation [8, 9]. It also correlates in filling factor with the onset of a resonant radio-frequency absorption in this sample which maps out a phase boundary in the B-T plane [8].

Luminescence from degenerate two-dimensional electrons at an ultrahigh mobility heterojunction

A study of the temperature dependence of luminescence from a two-dimensional electron system confined at an ultrahigh mobility single heterojunction reveals the importance of both many-body effects and of a thermally excited population in higher levels in determining the intensity of luminescence from states near the Fermi level.

The Spin Configuration of Fractional QHE Ground States in the N=0 Landau Level

Current opinion is that FQHE ground states seen in the N=0 Landau level are fully spin polarized. No one has seriously questioned this, despite theoretical evidence to the contrary /1/ and observation of anomalies in the 1+(2>v>1) fractions /2/. The purpose of the present work is to demonstrate that the spin behaviour is not so simple: fully polarized ground states are not favourable throughout the full range of filling factor and magnetic field. Furthermore, the measured activation energies have a large Zeeman component in place of the expected Coulomb gap.

Optical Measurements of Correlated States of Two Dimensional Electrons in GaAs at Low Temperatures and High Magnetic Fields

Photoluminescence measurements of two-dimensional electrons in GaAs/GaAlAs heterojunctions in the integer and fractional quantum Hall regimes are reviewed. Intensity and energy anomalies observed experimentally are discussed in terms of many-body effects in the recombination process. Optical evidence for the formation of an electron solid in the extreme quantum limit is presented.

Optical Detection of the Integer and Fractional Quantum Hall Effects in GaAs at Millikelvin Temperatures

We describe a definitive optical measurement, using band gap photoluminescence, of the integer and fractional quantum Hall effects in GaAs by a comprehensive study of integer states from ν = 1 to 10 and fractional states of the ν = 2/3 hierarchy out to the 5/9 daughter state, in an ultra-high mobility single heterojunction at 120 mK.

Experimental Determination of Fractional Charge e/q in the FQHE and Its Application to the Destruction of States

We have recently demonstrated [1] that the LAUGHLIN/ HALDANE prediction e*=±e/q for the charge e* of quasiparticles excited across the energy gap Δ of FQHE ground states at v=p/q [2,3], a new fundamental quantum of nature, is consistent with experiment. The experimental probe of e* is σxx c=σxx(1/T=0) obtained from extrapolated values of ρxx c determined from activation data defined by σxx=σ xx c e−∆/kT. In a high quality, low density GaAs-GaAIAs heterojunction G139 (n= 1 x 1011 cm-2, µ= 1 x 106 cm2/Vs, 1600 A spacer) we found that ρ xx c =c(e/q)2/h (independent of p) for some 13 fractional states at v=p/q with q=3, 5, 7 and 9, where the numerical constant c= 0.91 ±0.11. This result is not only consistent with e*=±e/q but strongly indicative of the existence of a minimum quasiparticle conductivity [1]. In this paper, we report a study of σxx c and Δ in a higher density heterojunction G156 (n= 1 x 1011 cm−2, µ= 1.6 x 106 cm2/Vs, 400A spacer) at two angles of the B-field to the sample plane normal, θ=0° and 47°. Our objectives are; (i) to examine the probe of e* for fractional states with q=3, 5 and 7 in a different structure and geometry, (ii) to compare normal and tilted field activation energies to examine the nature of the quasiparticle energy gaps and (iii) to apply the e* measurements for θ=0°and 47° to investigate what occurs when fractional states which are unpolarised at low field are ‘destroyed’ by the increased total field induced by tilt. Details of the FQHE ground state spin configurations for the N=0 Landau level are presented separately in these proceedings [4]. Here we consider aspects relevant to the quasiparticle charge.