We report the observation, for the first time, of a fractional quantum Hall state at v -1/2 Landaulevel filling in a low disorder, double-layer electron system realized in a 680-A-wide GaAs/AlGaAs single quantum well. A nearly vanishing diagonal resistance and a Hall resistance quantized at 2h/e 2 to within 0.3% are observed at -15 T and -26 mK. The activated temperature dependence of the diagonal resistance minimum yields a quasiparticle excitation energy gap of 230 mK.In a standard hierarchy picture [1], assuming that the two-dimensional (2D) electrons are all spin polarized, the fractional quantum Hall (FQH) states can only be realized at Landau-level filling factors (v) with odd denominators. This restriction to odd denominators is a result of the Fermi statistics which requires antisymmetry under particle exchange. If the condition of a spin-polarized ground state is relaxed, FQH states at even-denominator v become possible [2]. This is the explanation given by Haldane and Rezayi [3] for the experimental observation of the v -5/2 FQH state [4] which has been seen only at relatively low magnetic fields where the Zeeman energy is small [5]. For other even-denominator fillings, and especially for v = l/2, although many transport anomalies have been reported for the 2D electron systems in GaAs/Al v Gaiv As heterostructures [6-8], no evidence of a FQH state has yet been found [9]. Another candidate for the possible observation of an even-denominator FQH state [10] is a double-layer electron system (DLES) where the layer index can be treated as a pseudospin to take into account the additional degrees of freedom. Recent numerical studies [11,12] show that the FQH state at v=l/2 can be stabilized in a DLES with an appropriate CMB, where d is the separation between the layers and Ig^ih/eB)^2is the magnetic length. Experimentally, a DLES can be realized in a double quantum-well structure [13] where two sheets of electrons are separated by a high band-gap barrier, or in a wide, single, quantum well [14] where the electrons are separated by their own electrostatic repulsion. Experiments in these systems have shown that strong magnetic fields can destroy or weaken [13,14] the integral quantum Hall (IQH) states at odd v corresponding to the symmetric-antisymmetric energy gap (ASAS); this phenomenon has been attributed to the competition between the interwell and intrawell Coulomb interactions [15,16]. However, no observation of a v-\/2 FQH state has been reported in a DLES prior to this work.In this Letter, we present IQH and FQH effect data in a high-quality DLES realized in a wide, single, GaAs quantum well. We observe a well-developed FQH state at v=l/2 with a nearly vanishing diagonal resistance (R xx ) and a Hall resistance (R xx ) quantized at 2h/e 2 to within 0.3%. The temperature-activated behavior of R xx allows us to determine the energy gap for the quasiparti-

We report observations of an electric-field threshold conduction and of related ac voltage (broad-band noise) generation in low-disorder two-dimensional electron systems in the extreme magnetic quantum limit. We interpret these phenomena as definitive evidence for formation of a pinned quantum Wigner crystal and determine its melting phase diagram from the disappearance of threshold and noise behavior at higher temperatures.

We study the effect of subband energy separation and asymmetry on the quasiparticle excitation gaps of the v = 1/2 and coexisting fractional quantum Hall (FQH) states in wide single quantum wells. A new even-denominator FQH state at v = 3/2 and a dramatic subband-mixing-driven phase transition from a one-to two-component FQH state at v = 2/3 are observed. Our results reveal the two-component origin of the v = 1/2 FQH state, and allow us to construct an experimental phase diagram for electron states at half 611ing.

As the electronic charge distribution in a wide quantum well is tuned from a single-layer through an interacting bilayer configuration to weakly-coupled parallel layers, we observe an insulating phase concurrently manifesting a dramatic evolution. The data reveal that interlayer interactions, playing a crucial role, are able to stabilize a {\it correlated bilayer} electron insulator, thus providing tantalizing evidence of a pinned bilayer Wigner solid phase crystallizing at total filling factor $\nu$ as large as 0.54 ($\nu > \frac14$ in each layer).Comment: 10 pages, REVTeX, submitted to Phys. Rev. Lett.; 4 figures (1 color) and text available for viewing (HTML) or download (Postscript) at http://www.ee.princeton.edu/~hari

We report low temperature (T ) heat capacity (C) data on a multiplequantum-well GaAs/AlGaAs sample in the quantum Hall regime. Relative to its low field magnitude, C exhibits up to ∼ 10 5 -fold enhancement near ν=1where Skyrmions are the ground state of the confined two-dimensional electrons. We attribute the large C to a Skyrmion-induced, strong coupling of the nuclear spin system to the lattice. The data are consistent with the Schottky nuclear heat capacity of Ga and As atoms in the quantum wells, except at very low T where C vs T exhibits a remarkably sharp peak suggestive of a phase transition in the electronic system. PACS numbers: 65.40.Hq, 73.20.Dx, 73.40.Hm Typeset using REVT E X 1 Heat capacity is one of the most fundamental physical properties as it directly probes thermodynamic quantities such as entropy [1]. In the case of two-dimensional electron systems (2DESs) heat capacity can be a powerful probe of single-and many-body properties such as the Landau quantized density of states and the quantum Hall effect (QHE), both integral and fractional [2,3]. Measurements of 2DES heat capacity, however, are among the most challenging experiments because of the very small electron contribution. As a result, in contrast to the overwhelming number of magnetotransport and magnetooptical experiments reported for 2DESs in the QHE regime [4], very few heat capacity measurements have been reported so far [5,6]. In this Letter we report heat capacity measurements of a modulation-doped GaAs/Al x Ga 1−x As multiple-quantum-well heterostructure down to very low temperature (T ≥ 27 mK) and Landau level filling factor (ν > ∼ 0.5). As a function of increasing magnetic field, B, in addition to oscillations associated with the 2DESs' oscillating density of states at the Fermi level, we observe a dramatic increase of the low-T heat capacity (C) in the range 0.5 < ∼ ν < ∼ 1.5. For 0.7 < ∼ ν < ∼ 0.85, C exhibits a striking T -dependence, including a remarkably sharp peak suggestive of a phase transition at very low T . We interpret these unexpected observations in terms of the Schottky model [7] for the nuclear-spin heat capacity of Ga and As atoms which couple to the lattice via the 2DESs' low energy excitation spin textures (Skyrmions). The origin of the peak at very low T is discussed in relation with a phase transition in the electronic system. The experiments were performed on a multiple-quantum-well heterostructure grown by molecular-beam epitaxy and consisting of 100 GaAs quantum wells separated by Al 0.3 Ga 0.7 As barriers. The wells and barriers are 250 and 1850Å thick respectively, and the barriers are δ-doped with donors (Si) near their centers. Electrical resistivity data on samples from the same wafer exhibit well-developed fractional QHE and attest to the high quality of the sample [8]. A 7 × 7 mm 2 piece of the wafer was thinned to 65 µm and two carbon paint resistors were deposited on the substrate side and connected to the heat sink withNbTi wires. One carbon resistor was used as a thermometer while the other se...