Zero-Magnetic Field Fractional Quantum States.

Since the discovery of the fractional quantum Hall effect in 1982 there has been considerable theoretical discussion on the possibility of fractional quantization of conductance in the absence of Landau levels formed by a quantizing magnetic field. Although various situations have been theoretically envisaged, particularly lattice models in which band flattening resembles Landau levels, the predicted fractions have never been observed. In this Letter, we show that odd and even denominator fractions can be observed, and manipulated, in the absence of a quantizing magnetic field, when a low-density electron system in a GaAs based one-dimensional quantum wire is allowed to relax in the second dimension. It is suggested that such a relaxation results in formation of a zigzag array of electrons with ring paths which establish a cyclic current and a resultant lowering of energy. The behavior has been observed for both symmetric and asymmetric confinement but increasing the asymmetry of the confinement potential, to result in a flattening of confinement, enhances the appearance of new fractional states. We find that an in-plane magnetic field induces new even denominator fractions possibly indicative of electron pairing. The new quantum states described here have implications both for the physics of low dimensional electron systems and also for quantum technologies. This work will enable further development of structures which are designed to electrostatically manipulate the electrons for the formation of particular configurations. In turn, this could result in a designer tailoring of fractional states to amplify particular properties of importance in future quantum computation.

Familial hepatopulmonary syndrome in common variable immunodeficiency.

Common Variable Immunodeficiency (CVID) comprises a heterogeneous group of primary antibody deficiencies which lead to a range of complications, including infectious, neoplastic and inflammatory disorders. This report describes monozygotic twin brothers with CVID who developed cryptogenic liver disease and subsequently hepatopulmonary syndrome (HPS). This is the second report of the association of HPS and CVID. Its occurrence in two identical twins implicates a genetic basis.

Two-dimensional localization in GeSn.

Localization behaviour is a characteristic feature of thep-type GeSn quantum well (QW) system in a metal-insulator-semiconductor device. The transition to strongly localized behaviour is abrupt with thermally activated conductivity and a high temperature intercept of 0.12 ×e2h-1at a hole carrier density 1.55 × 1011cm-2. The activation energy for the conductivity in the localized state is 0.40 ± 0.05 meV compared to an activation energy of ∼0.1 meV for conductivity activation to a mobility edge at carrier densities >1.55 × 1011cm-2. Insulating behaviour can occur from a system that behaves as though it is in a minimum metallic state, albeit at high temperature, or from a conductivity greater than a minimum metallic state behaviour showing that local disorder conditions with local differences in the density of states are important for the onset of localization. In the presence of a high magnetic field, thermally activated conductivity is present down to Landau level filling factor <1/2but without a magnetic-field-dependent carrier density or a variable range hopping (VRH) transport behaviour developing even with conductivity ≪e2h-1. In the localized transport regime inp-type doped Ge0.92Sn0.08QWs the VRH mechanism is suppressed at temperatures >100 mK and this makes this two-dimensional system ideal for future many body localization studies in disordered hole gases that can be thermally isolated from a temperature reservoir.

Observation of charge transport by negatively charged excitons.

We report transport of electron-hole complexes in semiconductor quantum wells under applied electric fields. Negatively charged excitons (X-), created by laser excitation of a high electron mobility transistor, are observed to drift upon applying a voltage between the source and drain. In contrast, neutral excitons do not drift under similar conditions. The X- mobility is found to be as high as 6.5 x 10(4) cm2 V-1 s-1. The results demonstrate that X- exists as a free particle in the best-quality samples and suggest that light emission from opto-electronic devices can be manipulated through exciton drift under applied electric fields.

Conductance quantisation in patterned gate In0.75Ga0.25As structures up to 6 × (2e 2/h).

We present electrical measurements from In0.75Ga0.25As 1D channel devices with Rashba-type, spin-orbit coupling present in the 2D contact regions. Suppressed backscattering as a result of the time-reversal asymmetry at the 1D channel entrance results in enhanced ballistic transport characteristics with clear quantised conductance plateaus up to 6 × (2e 2/h). Applying DC voltages between the source and drain ohmic contacts and an in-plane magnetic field confirms a ballistic transport picture. For asymmetric patterned gate biasing, a lateral spin-orbit coupling effect is weak. However, the Rashba-type spin-orbit coupling leads to a g-factor in the 1D channel that is reduced in magnitude from the 2D value of 9 to ~6.5 in the lowest subband when the effective Rashba field and the applied magnetic field are perpendicular.

High mobility In0.75Ga0.25As quantum wells in an InAs phonon lattice.

InGaAs based devices are great complements to silicon for CMOS, as they provide an increased carrier saturation velocity, lower operating voltage and reduced power dissipation (International technology roadmap for semiconductors (www.itrs2.net)). In this work we show that In0.75Ga0.25As quantum wells with a high mobility, 15 000 to 20 000 cm2 V-1 s-1 at ambient temperature, show an InAs-like phonon with an energy of 28.8 meV, frequency of 232 cm-1 that dominates the polar-optical mode scattering from ∼70 K to 300 K. The measured optical phonon frequency is insensitive to the carrier density modulated with a surface gate or LED illumination. We model the electron scattering mechanisms as a function of temperature and identify mechanisms that limit the electron mobility in In0.75Ga0.25As quantum wells. Background impurity scattering starts to dominate for temperatures <100 K. In the high mobility In0.75Ga0.25As quantum well, GaAs-like phonons do not couple to the electron gas unlike the case of In0.53Ga0.47As quantum wells.

Self-organised fractional quantisation in a hole quantum wire.

We have investigated hole transport in quantum wires formed by electrostatic confinement in strained germanium two-dimensional layers. The ballistic conductance characteristics show the regular staircase of quantum levels with plateaux at n2e 2/h, where n is an integer, e is the fundamental unit of charge and h is Planck's constant. However as the carrier concentration is reduced, the quantised levels show a behaviour that is indicative of the formation of a zig-zag structure and new quantised plateaux appear at low temperatures. In units of 2e 2/h the new quantised levels correspond to values of n = 1/4 reducing to 1/8 in the presence of a strong parallel magnetic field which lifts the spin degeneracy but does not quantise the wavefunction. A further plateau is observed corresponding to n = 1/32 which does not change in the presence of a parallel magnetic field. These values indicate that the system is behaving as if charge was fractionalised with values e/2 and e/4, possible mechanisms are discussed.

Short-term evaluation of a foraging device for non-human primates.

In the USA, any institution involved in using non-human primates for research has had, for regulatory reasons, to address the psychological needs of these animals. Enriching the environment through the use of foraging devices has been one method and a study was designed to evaluate the short-term effect of a new foraging device on singly-housed cynomolgus monkeys. The study was divided into 3 one-week periods of observation: baseline, device filled with normal ration, and device filled with a novel food. Four behaviours were recorded: foraging, self-directed, hopper feeding, and other behaviours. During the observation periods the device was accepted in preference to the standard hopper style feeder and self-directed behaviours were significantly reduced compared with the baseline period. Changing to a novel food re-kindled interest in the device and reduced the extinguishing effect: i.e. decrease in interest or use of the device. Based on this study, the feeder has been included with several other devices in a rotation programme.