Superconductivity in AuNiGe Ohmic contacts to a GaAs-based high mobility two-dimensional electron gas

To cool a high mobility two-dimensional electron gas (2DEG) at a GaAs-AlGaAs heterojunction to milliKelvin temperatures, we have fabricated low resistance Ohmic contacts based on alloys of Au, Ni, and Ge. The Ohmic contacts have a typical contact resistance of R C ≈ 0.8 ω at 4.2 K, which drops to 0.2 ω below 0.9 K. Scanning electron microscope images establish that the contacts have the same inhomogeneous microstructure that has been observed in previous studies. Measurements of the contact resistance R C, the four-terminal resistance along the top of a single contact, and the vertical resistance RV all show that there is a superconductor in the Ohmic contact, which can be turned completely normal with a magnetic field of 0.15 T. We briefly discuss how this superconductivity may be affecting the electrical transport measurements of 2DEGs, especially how it may hinder the cooling of electrons in a 2DEG below 0.1 K

A complete laboratory for transport studies of electron-hole interactions in GaAs/AlGaAs ambipolar bilayers

We present GaAs/AlGaAs double quantum well devices that can operate as both electron-hole (e-h) and hole-hole (h-h) bilayers, with separating barriers as narrow as 5 nm or 7.5 nm. With such narrow barriers, in the h-h configuration, we observe signs of magnetic-field-induced exciton condensation in the quantum Hall bilayer regime. In the same devices, we can study the zero-magnetic-field e-h and h-h bilayer states using Coulomb drag. Very strong e-h Coulomb drag resistivity (up to 10% of the single layer resistivity) is observed at liquid helium temperatures, but no definite signs of exciton condensation are seen in this case. Self-consistent calculations of the electron and hole wavefunctions show this might be because the average interlayer separation is larger in the e-h case than the h-h case

Mechanically robust cylindrical metal terahertz waveguides for cryogenic applications

As the ambition behind THz quantum cascade laser based applications continues to grow, abandoning free-space optics in favor of waveguided systems promises major improvements in targeted, easy to align, and robust radiation delivery. This is especially true in cryogenic environments, where illumination is traditionally challenging. Although the field of THz waveguides is rapidly developing, most designs have limitations in terms of mechanical stability at low temperatures, and are costly and complicated to fabricate to lengths > 1 m. In this work, we investigate readily available cylindrical metal waveguides which are suitable for effective power delivery in cryogenic environments, and explore the optimal dimensions and materials available. The materials chosen were extruded un-annealed and annealed copper, as well as stainless steel, with bore diameters of 1.75, 2.5, and 4.6 mm. Measurements were performed at three different frequencies, 2.0, 2.85 and 3.2 THz, with optimal transmission losses 1, and forms a comprehensive investigation of cryogenically compatible THz waveguides and optical couplers, paving the way for a new generation of systems to utilize THz QCLs for a host of low-temperature investigations

An Antenna-Coupled Dual-Gated Electron Channel as Direct Detector of 2 THz Radiation

An antenna-coupled dual-gated two-dimensional electron gas (2DEG) based on a GaAs-AlGaAs heterostructure shows a pronounced response to 2 THz radiation. The device is shown to be a direct detector, and its photoresponse arises without any source-drain bias. The detection is based on a novel mechanism that yields a substantially stronger photoresponse than predicted by the classical plasma-wave self-mixing and other mechanisms

Atypical Balance between Occipital and Fronto-Parietal Activation for Visual Shape Extraction in Dyslexia

Reading requires the extraction of letter shapes from a complex background of text, and an impairment in visual shape extraction would cause difficulty in reading. To investigate the neural mechanisms of visual shape extraction in dyslexia, we used functional magnetic resonance imaging (fMRI) to examine brain activation while adults with or without dyslexia responded to the change of an arrow’s direction in a complex, relative to a simple, visual background. In comparison to adults with typical reading ability, adults with dyslexia exhibited opposite patterns of atypical activation: decreased activation in occipital visual areas associated with visual perception, and increased activation in frontal and parietal regions associated with visual attention. These findings indicate that dyslexia involves atypical brain organization for fundamental processes of visual shape extraction even when reading is not involved. Overengagement in higher-order association cortices, required to compensate for underengagment in lower-order visual cortices, may result in competition for top-down attentional resources helpful for fluent reading.Ellison Medical FoundationMartin Richmond Memorial FundNational Institutes of Health (U.S.). (Grant UL1RR025758)National Institutes of Health (U.S.). (Grant F32EY014750-01)MIT Class of 1976 (Funds for Dyslexia Research

Research data supporting "Orientation of hole quantum Hall nematic phases in an out-of-plane electric field"

Magnetoresistance data for a 2D hole system (2DES) in an out-of-plane magnetic field as a function of the crystallographic direction, out-of-plane electric field, temperature and hole density. Data were taken at the Cavendish Laboratory, University of Cambridge, in 2013