Activation mechanisms in sodium-doped Silicon MOSFETs

We have studied the temperature dependence of the conductivity of a silicon MOSFET containing sodium ions in the oxide above 20 K. We find the impurity band resulting from the presence of charges at the silicon-oxide interface is split into a lower and an upper band. We have observed activation of electrons from the upper band to the conduction band edge as well as from the lower to the upper band. A possible explanation implying the presence of Hubbard bands is given.Comment: published in J. Phys. : Condens. Matte

Disorder and electron interaction control in low-doped silicon metal-oxide-semiconductor field effect transistors

We fabricated silicon metal-oxide-semiconductor field effect transistors where an additional sodium-doped layer was incorporated into the oxide to create potential fluctuations at the Si-SiO2 interface. The amplitude of these fluctuations is controlled by both the density of ions in the oxide and their position relative to the Si-SiO2 interface. Owing to the high mobility of the ions at room temperature, it is possible to move them with the application of a suitable electric field. We show that, in this configuration, such a device can be used to control both the disorder and the electron-electron interaction strength at the Si-SiO2 interface.Comment: 3 pages, 2 figure

Evidence for multiple impurity bands in sodium-doped silicon MOSFETs

We report measurements of the temperature-dependent conductivity in a silicon metal-oxide-semiconductor field-effect transistor that contains sodium impurities in the oxide layer. We explain the variation of conductivity in terms of Coulomb interactions that are partially screened by the proximity of the metal gate. The study of the conductivity exponential prefactor and the localization length as a function of gate voltage have allowed us to determine the electronic density of states and has provided arguments for the presence of two distinct bands and a soft gap at low temperature.Comment: 4 pages; 5 figures; Published in PRB Rapid-Communication

Variation of the hopping exponent in disordered silicon MOSFETs

We observe a complex change in the hopping exponent value from 1/2 to 1/3 as a function of disorder strength and electron density in a sodium-doped silicon MOSFET. The disorder was varied by applying a gate voltage and thermally drifting the ions to different positions in the oxide. The same gate was then used at low temperature to modify the carrier concentration. Magnetoconductivity measurements are compatible with a change in transport mechanisms when either the disorder or the electron density is modified suggesting a possible transition from a Mott insulator to an Anderson insulator in these systems.Comment: 6 pages, 5 figure

Non-invasive detection of the evolution of the charge states of a double dot system

Coupled quantum dots are potential candidates for qubit systems in quantum computing. We use a non-invasive voltage probe to study the evolution of a coupled dot system from a situation where the dots are coupled to the leads to a situation where they are isolated from the leads. Our measurements allow us to identify the movement of electrons between the dots and we can also identify the presence of a charge trap in our system by detecting the movement of electrons between the dots and the charge trap. The data also reveals evidence of electrons moving between the dots via excited states of either the single dots or the double dot molecule.Comment: Accepted for publication in Phys. Rev. B. 4 pages, 4 figure

0.7 structure and zero bias anomaly in ballistic hole quantum wires

We study the anomalous conductance plateau around G=0.7(2e(2)/h) and the zero bias anomaly in ballistic hole quantum wires with respect to in-plane magnetic fields applied parallel B-parallel to and perpendicular B-perpendicular to to the quantum wire. As seen in electron quantum wires, the magnetic fields shift the 0.7 structure down to G=0.5(2e(2)/h) and simultaneously quench the zero bias anomaly. However, these effects are strongly dependent on the orientation of the magnetic field, owing to the highly anisotropic effective Lande g-factor g(*) in hole quantum wires. Our results highlight the fundamental role that spin plays in both the 0.7 structure and zero bias anomaly

The Asteroseismic Poltential of TESS: Exoplanet-Host Stars

New insights on stellar evolution and stellar interior physics are being made possible by asteroseismology. Throughout the course of the Kepler mission, asteroseismology has also played an important role in the characterization of exoplanet-host stars and their planetary systems. The upcoming NASA Transiting Exoplanet Survey Satellite (TESS) will be performing a near all-sky survey for planets that transit bright nearby stars. In addition, its excellent photometric precision, combined with its fine time sampling and long intervals of uninterrupted observations, will enable asteroseismology of solar-type and red-giant stars. Here we develop a simple test to estimate the detectability of solar-like oscillations in TESS photometry of any given star. Based on an all-sky stellar and planetary synthetic population, we go on to predict the asteroseismic yield of the TESS mission, placing emphasis on the yield of exoplanet-host stars for which we expect to detect solar-like oscillations. This is done for both the target stars (observed at a 2-minute cadence) and the full-frame-image stars (observed at a 30-minute cadence). A similar exercise is also conducted based on a compilation of known host stars. We predict that TESS will detect solar-like oscillations in a few dozen target hosts (mainly subgiant stars but also in a smaller number of F dwarfs), in up to 200 low-luminosity red-giant hosts, and in over 100 solar-type and red-giant known hosts, thereby leading to a threefold improvement in the asteroseismic yield of exoplanet-host stars when compared to Kepler's.Science and Technology Facilities Council (Great Britain

Experimental Evidence for Coulomb Charging Effects in an Open Quantum Dot at Zero Magnetic Field

We have measured the low-temperature transport properties of an open quantum dot formed in a clean one-dimensional channel. For the first time, at zero magnetic field, continuous and periodic oscillations superimposed upon ballistic conductance steps are observed when the conductance through the dot $G$ exceeds $2e^2/h$. We ascribe the observed conductance oscillations to evidence for Coulomb charging effects in an open dot. This is supported by the evolution of the oscillating features for $G>2e^2/h$ as a function of both temperature and barrier transparency. Our results strongly suggest that at zero magnetic field, current theoretical and experimental understanding of Coulomb charging effects overlooks charging in the presence of fully transmitted 1D channels.Comment: To appear in Phys. Rev. Lett. 81 (Oct 19 issue