We present the results of an experimental study of superconducting, disordered, thin films of amorphous indium oxide. These films can be driven from the superconducting phase to a reentrant insulating state by the application of a perpendicular magnetic field (B). We find that the high-B insulator exhibits activated transport with a characteristic temperature, TI. TI has a maximum value (TpI) that is close to the superconducting transition temperature (Tc) at B=0, suggesting a possible relation between the conduction mechanisms in the superconducting and insulating phases. Tp(I) and Tc display opposite dependences on the disorder strength.

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 measure microwave frequency (4-40 GHz) photoresistance at low magnetic field B, in high mobility 2D electron gas samples, excited by signals applied to a transmission line fabricated on the sample surface. Oscillatory photoresistance vs B is observed. For excitation at the cyclotron resonance frequency, we find an unprecedented, giant relative photoresistance (\Delta R)/R of up to 250 percent. The photoresistance is apparently proportional to the square root of applied power, and disappears as the temperature is increased.Comment: 4 pages, 3 figure

We present ac measurements of the diagonal conductivity o xx , in the integer quantum Hall regime for frequency /between 0.2 and 14 GHz and temperature r>:50 mK. Re(cr xx ) was obtained from the measured attenuation of a coplanar transmission line on the sample surface. For/S; 1 GHz, a xx peaks between IQHE minima broaden as / increases, roughly as (AB)