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 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 magnetotransport measurements in a weakly coupled double-layer electron system realized in a wide quantum well. This system has the unique property that the distance and the coupling between the layers can be changed continuously by varying the electron density in the well. We observe the absence of quantum Hall states at odd filling factors. Our results complement earlier experimental work and are consistent with a recent theoretical model proposed for the magnetic-field-driven destruction of the quantum Hall effect in double quantum wells.The two-dimensional electron system (2DES) has provided the means for the observation of many new physical phenomena, such as the integral ' and fractional quantum Hall effects (IQHE and FQHE). Recently, there is much interest in the fabrication and physics of structures which contain two or more layers of electrons in close proximity so that the interlayer Coulomb interactions are strong. ' Theoretically, the possibility of new collective states such as the FQHE at even-denominator Landaulevel fillings (v), or Wigner crystallization in such multilayer structures has been proposed.There has also been some experimental work in these systems. The observation of the IQHE in multilayer systems with significant interlayer tunneling has been reported. ' Boebinger et al. recently studied the IQHE in strongly coupled, high-quality double quantum wells (DQW) in which the symmetric to antisymmetric energy gap (AsAs) is expected to give rise to the IQHE at odd v. They observed a remarkable effect, namely the absence of certain IQHE states at low-odd v for sufficiently small AsAs and large interlayer distance (d). The origin of this phenomenon is not clear yet although recent calculations relate it to the Coulomb-driven destruction of hsAs in a strong magnetic field. ' According to these calculations, if the magnetic field (8) is sufficiently strong, hsAs collapses and the DQW system makes a transition to a different ground state with weak interwell, but strong intrawell correlations.In this Rapid Communication, we report magnetotransport measurements in a novel high-quality double-layer system realized in a wide, single, GaAs quantum well (Fig. 1). The idea is that when electrons are introduced in a wide quantum well, the electrostatic repulsion between the electrons forces them into a stable configuration in which two 2DES's are formed at the well's sidewalls. A major advantage of this system over a conventional DQW is the minimization of alloy scattering since the barrier between the two 2DES's is GaAs rather than Al Gal -"-As. Also, both hsAs and d can be changed by varying the electron density (n, ) in the well (Figs. 1 and 2). Our study of this system in a regime~here the two 2DES's are weakly coupled reveals the absence of IQHE states at odd v. Our data can be qualitatively explained by the theories proposed by MacDonald, Platzman, and Boebinger and Brey. 0.3-I (a) empty 0.2------electron distribution Ec 0-I I I~I 0 3 -(b) n, = 5.6x10 cm 02-o 0.1

Low-frequency current fluctuations are monitored and the mechanism of electric noise investigated in layered 2H-type α-molybdenum ditelluride transistors. The charge transport mechanism of electric noise in atomically thin transition-metal dichalcogenides is studied under different environments; the development of a new sensing functionality may be stimulated.