Showing papers by "Marc Kastner published in 2003"

Singlet triplet transition in a single-electron transistor at zero magnetic field

Abstract: We report sharp peaks in the differential conductance of a single-electron transistor (SET) at low temperature for gate voltages at which charge fluctuations are suppressed. For odd numbers of electrons we observe the expected Kondo peak at zero bias. For even numbers of electrons we generally observe Kondo-like features corresponding to excited states. For the latter, the excitation energy often decreases with gate voltage until a new zero-bias Kondo peak results. We ascribe this behavior to a singlet-triplet transition in zero magnetic field driven by the change of shape of the potential that confines the electrons in the SET.

Effect of a magnetic field on long-range magnetic order in stage-4 and stage-6 superconducting La 2 CuO 4+y

Abstract: We have measured the enhancement of the static incommensurate spin-density wave (SDW) order by an applied magnetic field in stage-4 and stage-6 samples of superconducting ${\mathrm{La}}_{2}{\mathrm{CuO}}_{4+y}.$ We show that the stage-6 ${\mathrm{La}}_{2}{\mathrm{CuO}}_{4+y}$ ${(T}_{c}=32\mathrm{K})$ forms static long-range SDW order with the same wave vector as that in the previously studied stage-4 material. We have measured the field dependence of the SDW magnetic Bragg peaks in both stage-4 and stage-6 materials at fields up to 14.5 T. A recent model of competing SDW order and superconductivity describes these data well.

Observation of Bimolecular Carrier Recombination Dynamics in Close-Packed Films of Colloidal CdSe Nanocrystals

Abstract: We study the intensity dependence of the photoconductivity in close-packed films of CdSe quantum dots in which the photoconductivity is characterized by a low density of trapped charges. We find that the intensity dependence of the photocurrent is nonlinear, which is indicative of bimolecular charge carrier recombination dynamics. We present a physical model for the intensity dependence of the photocurrent that is used to fit the data and extract a ratio of the density of carriers to the density of trapped charges. As our model predicts, we show that the current vs intensity plot becomes more linear when the carrier density is decreased.

Imaging the charge transport in arrays of CdSe nanocrystals

Abstract: A method to image charge is used to measure the diffusion coefficient of electrons in films of CdSe nanocrystals at room temperature. This method makes possible the study of charge transport in films exhibiting extremely high resistances or very small diffusion coefficients.

Anomalous Transport in Quantum Dot Arrays

Abstract: A novel model of transport is proposed to explain power law current transients and memory phenomena observed in partially ordered arrays of semiconducting nanocrystals. The model describes electron transport by a stationary Levy process of transmission events and thereby requires no time dependence of system properties. The waiting time distribution with a characteristic long tail gives rise to a nonstationary response in the presence of a voltage pulse. We report on noise measurements that agree well with the predicted non-Poissonian fluctuations in current, and discuss possible mechanisms leading to this behavior.