We report electrical transport measurements on GaAs/AlGaAs based electron-hole bilayers. These systems are expected to make a transition from a pair of weakly coupled two-dimensional systems to a strongly coupled exciton system as the barrier between the layers is reduced. Once excitons form, phenomena such as Bose-Einstein condensation of excitons could be observed. In our devices, electrons and holes are confined in double quantum wells, and carriers in the devices are induced with top and bottom gates leading to variable density in each layer. Separate contact to each layer allows Coulomb drag transport measurements where current is driven in one layer while voltage is measured in the other. Coulomb drag is sensitive to interlayer coupling and has been predicted to provide a strong signature of exciton condensation. Drag measurement on EHBLs with a 30 nm barrier are consistent with drag between two weakly coupled 2D Fermi systems where the drag decreases as the temperature is reduced. When the barrier is reduced to 20 nm, we observe a consistent increase in the drag resistance as the temperature is reduced. These results indicate the onset of a much stronger coupling between the electrons and holes which leads to exciton formation and possibly phenomena related to exciton condensation.

Articles you may be interested inTwo-dimensional electron gases in strained quantum wells for AlN/GaN/AlN double heterostructure field-effect transistors on AlN High mobility two-dimensional hole system in Ga As ∕ Al Ga As quantum wells grown on (100) GaAs substrates Appl.Extremely high electron mobility of pseudomorphic In 0.74 Ga 0.26 As ∕ In 0.46 Al 0.54 As modulation-doped quantum wells grown on ( 411 ) A InP substrates by molecular-beam epitaxy Appl. Phys. Lett. 85, 4043 (2004); 10.1063/1.1807023Transport and quantum electron mobility in the modulation Si δ-doped pseudomorphic GaAs/In 0.2 Ga 0.8 As/Al 0.2 Ga 0.8 As quantum well grown by metalorganic vapor phase epitaxyThe authors present the fabrication details of completely undoped electron-hole bilayer devices in a GaAs/ AlGaAs double quantum well heterostructure with a 30 nm barrier. These devices have independently tunable densities of the two-dimensional electron gas and two-dimensional hole gas. The authors report four-terminal transport measurements of the independently contacted electron and hole layers with balanced densities from 1.2ϫ 10 11 cm −2 down to 4 ϫ 10 10 cm −2 at T = 300 mK. The mobilities can exceed 1 ϫ 10 6 cm 2 V −1 s −1 for electrons and 4 ϫ 10 5 cm 2 V −1 s −1 for holes.

The leader RNA sequence of human immunodeficiency virus type 1 (HIV-1) consists of a complex series of stem loop structures that are critical for viral replication. Three-dimensional structural analysis by NMR of one of these structures, the SL1 stem loop of the packaging signal region, revealed a highly conserved purine rich loop with a structure nearly identical to the Rev-binding loop of the Rev response element. Using band-shift assays, surface plasmon resonance, and further NMR analysis, we demonstrate that this loop binds Rev. HIV-1 appears to have a second Rev-binding site close to the major splice donor site that may have an additional role in the viral life cycle.Human immunodeficiency virus type 1 (HIV-1), 1 the causative agent of AIDS, is classified as a complex retrovirus that uses a group of regulatory and accessory proteins to control its life cycle and to influence cellular function. The unspliced genomic RNA of the virus has been shown to contain a large number of cis-acting sequences that influence transcription, splicing, intracellular transport, genome dimerization, and packaging. The leader region in particular is highly structured and has been extensively studied by a number of different groups to attempt to analyze both the structures and their functional significance. One function that has been consistently ascribed to the leader is the presence of a sequence that is used to identify the RNA genome for encapsidation into the viral particle. This is termed the packaging signal, abbreviated as the Greek letter . The encapsidation signal must distinguish the genomic RNA from viral as well as cellular messages. In HIV-1, using deletion mutagenesis, we identified a region involved in packaging (1) that was confirmed by other groups (2, 3). Since that time, a number of other regions inside and outside of the leader region have been implicated in packaging although largely in an enhancing role (4 -8). Secondary structure analysis of the region identified a series of conserved stem loops, and disruptive and compensatory mutation confirmed the importance of these structures in retaining packaging function ( Fig. 1) (9). NMR-derived three-dimensional structures of the SL3 region have been published with and without complexed viral nucleocapsid protein (10, 11). In addition, NMR analysis of the major splice donor loop (SL2) and a fourth loop of uncertain functional significance (SL4) have been published (12-14). The first stem loop in the packaging region (SL1) has been the subject of intense analysis because of its involvement in the dimerization process of the genomic RNA facilitated through the "kissing" interaction of the palindromic sequences at the tip (15). Proximal to the terminal loop of SL1, there is a second loop structure consisting of an AGG triplet opposite a single bulged G residue. This region is 100% conserved in all HIV-1 sequences so far identified and is thought to be important in the viral life cycle and, in particular, is thought to be critical for the occurrence of dimer linkag...

A low-temperature upturn of the Coulomb drag resistivity measured in an undoped electron-hole bilayer ͑uEHBL͒ device, possibly manifesting from exciton formation or condensation, was recently observed. The effects of density imbalance on this upturn are examined. Measurements of drag as a function of temperature in an uEHBL with a 20-nm-wide Al 0.90 Ga 0.10 As barrier layer at various density imbalances are presented. The results show drag increasing as the density of either two-dimensional system was reduced, both within and above the upturn temperature regime and with a stronger density dependence than weak-coupling theory predicts. A comparison of the data with numerical calculations of drag in the presence of electron-hole pairing fluctuations, which qualitatively reproduce the drag upturn behavior, is also presented. The calculations predict a peak in drag at matched densities, which is not reflected by the measurements.An exciton is a composite boson that forms in bulk semiconductors due to an attractive Coulomb interaction between its fermionic, constituent electron and hole. As such, excitons are expected under certain circumstances to condense at low temperature, where the lowest energy state becomes occupied by a macroscopic number of particles. While the bulk exciton condensate was later determined to be an insulator due to interband transitions which fix the phase of the order parameter, 1 the use of spatially separated electron-hole pairs or "indirect" excitons was predicted to mitigate this issue sufficient for a phase transition to occur. 2,3 Indirect excitons may be generated optically 4 or via field effect 5,6 in double quantum wells. The distinct advantages of field-effect devices, such as the uEHBL used in this study, are that the densities in each well can be adjusted and then maintained at constant values using gate voltages and the layers have separate electrical contacts to each. Together these allow for the interlayer Coulomb interaction between the electrons and holes to be probed directly using Coulomb drag measurements. Conceived of by Progrebinsky 7 and Price 8 and first demonstrated between two-dimensional electron gases ͑2DEGs͒ by Gramila et al.,9 in the Coulomb drag technique a current is driven in one layer of a bilayer device causing a longitudinal voltage to arise in the adjacent layer via interlayer scattering. The measured quantity is the drag resistivity D = V drag / I drive ͑L / W͒, where I drive is the current in the drive layer, V drag is the induced voltage in the drag layer, and L / W is the number of squares. In the "weakly coupled" limit, low temperature T and large interlayer separation d, the D is expected to have a T 2 dependence, due to phase space restrictions on the scattering set by the thermal broadening, and thereby decrease to zero as T → 0. 9,10 Deviation of D from this behavior, possibly due to enhanced interlayer coupling, would thus suggest a departure from Fermi-liquid physics.Seizing upon this possibility, Vignale and MacDonald 11 predicted that D in an ...