Sensitive detection of photoexcited carriers by resonant tunneling through a single quantum dot

We show that the resonant tunnel current through a single energy level of an individual quantum dot within an ensemble of dots is strongly sensitive to photoexcited holes that become bound in the close vicinity of the dot. The presence of these holes lowers the electrostatic energy of the quantum dot state and switches the current carrying channel from fully open to fully closed with a high on/off ratio (> 50). The device can be reset by means of a bias voltage pulse. These properties are of interest for charge sensitive photon counting devices.Comment: 5 pages, 4 figure

Visualization of wave function of quantum dot at fermi-edge singularity regime

We consider electron tunneling spectroscopy through an InAs quantum dot in a magnetic field applied perpendicular to the tunneling direction. We examine in details the anisotropic behavior of the amplitude and shape of the resonant peaks of I-V curves and concluded that (i) magnetotunneling spectroscopy at FES regime allows establishing position of resonant level in QD with high accuracy. (ii) The distinguishable shape of FES peak allows extracting the amplitude with much better accuracy. (iii) FES exponent dependence on magnetic field gives additional information about potential distribution outside QD.Foundation for Science and Technology (FCT

Nonlinear electron transport in normally pinched-off quantum wire

Nonlinear electron transport in normally pinched-off quantum wires was studied. The wires were fabricated from AlGaAs/GaAs heterostructures with high-mobility two-dimensional electron gas by electron beam lithography and following wet etching. At certain critical source-drain voltage the samples exhibited a step rise of the conductance. The differential conductance of the open wires was noticeably lower than e^2/h as far as only part of the source-drain voltage dropped between source contact and saddle-point of the potential relief along the wire. The latter limited the electron flow injected to the wire. At high enough source-drain voltages the decrease of the differential conductance due to the real space transfer of electrons from the wire in GaAs to the doped AlGaAs layer was found. In this regime the sign of differential magnetoconductance was changed with reversing the direction of the current in the wire or the magnetic field, whet the magnetic field lies in the heterostructure plane and is directed perpendicular to the current. The dependence of the differential conductance on the magnetic field and its direction indicated that the real space transfer events were mainly mediated by the interface scattering.Comment: LaTeX 2e (epl.cls) 6 pages, 3 figure

Spin splitting of X-related donor impurity states in an AlAs barrier

We use magnetotunneling spectroscopy to observe the spin splitting of the ground state of an X-valley-related Si-donor impurity in an AlAs barrier. We determine the absolute magnitude of the effective Zeeman spin splitting factors of the impurity ground state to be g$_{I}$= 2.2 $\pm$ 0.1. We also investigate the spatial form of the electron wave function of the donor ground state, which is anisotropic in the growth plane

A magnetically-induced Coulomb gap in graphene due to electron-electron interactions

Insights into the fundamental properties of graphene's Dirac-Weyl fermions have emerged from studies of electron tunnelling transistors in which an atomically thin layer of hexagonal boron nitride (hBN) is sandwiched between two layers of high purity graphene. Here, we show that when a single defect is present within the hBN tunnel barrier, it can inject electrons into the graphene layers and its sharply defined energy level acts as a high resolution spectroscopic probe of electron-electron interactions in graphene. We report a magnetic field dependent suppression of the tunnel current flowing through a single defect below temperatures of $\sim$ 2 K. This is attributed to the formation of a magnetically-induced Coulomb gap in the spectral density of electrons tunnelling into graphene due to electron-electron interactions

Acoustic Phonon-Assisted Resonant Tunneling via Single Impurities

We perform the investigations of the resonant tunneling via impurities embedded in the AlAs barrier of a single GaAs/AlGaAs heterostructure. In the $I(V)$ characteristics measured at 30mK, the contribution of individual donors is resolved and the fingerprints of phonon assistance in the tunneling process are seen. The latter is confirmed by detailed analysis of the tunneling rates and the modeling of the resonant tunneling contribution to the current. Moreover, fluctuations of the local structure of the DOS (LDOS) and Fermi edge singularities are observed.Comment: accepted in Phys. Rev.

Observation of spin and valley splitting of Landau levels under magnetic tunneling in graphene/boron nitride/graphene structures

Resonance magnetic tunneling in heterostructures formed by graphene single sheets separated by a hexagonal boron nitride barrier and bounded by two gates has been investigated in a strong magnetic field, which has allowed observing transitions between spin- and valley-split Landau levels with various indices belonging to different graphene sheets. An unexpected increase with the temperature in the interlayer tunneling conductance owing to transitions between the Landau levels in strong magnetic fields cannot be explained by existing theories

On cycles in the transcription network of Saccharomyces cerevisiae

<p>Abstract</p> <p>Background</p> <p>We investigate the cycles in the transcription network of <it>Saccharomyces cerevisiae</it>. Unlike a similar network of <it>Escherichia coli</it>, it contains many cycles. We characterize properties of these cycles and their place in the regulatory mechanism of the cell.</p> <p>Results</p> <p>Almost all cycles in the transcription network of <it>Saccharomyces cerevisiae </it>are contained in a single <it>strongly connected component</it>, which we call LSCC (L for "largest"), except for a single cycle of two transcription factors. The fact that LSCC includes almost all cycles is well explained by the properties of a random graph with the same in- and out-degrees of the nodes.</p> <p>Among different physiological conditions, cell cycle has the most significant relationship with LSCC, as the set of 64 transcription interactions that are active in all phases of the cell cycle has overlap of 27 with the interactions of LSCC (of which there are 49).</p> <p>Conversely, if we remove the interactions that are active in all phases of the cell cycle (25% of interactions to transcription factors), the LSCC would have only three nodes and 5 edges, many fewer than expected. This subgraph of the transcription network consists mostly of interactions that are active only in the stress response subnetwork.</p> <p>We also characterize the role of LSCC in the topology of the network. We show that LSCC can be used to define a natural hierarchy in the network and that in every physiological subnetwork LSCC plays a pivotal role.</p> <p>Conclusion</p> <p>Apart from those well-defined conditions, the transcription network of <it>Saccharomyces cerevisiae </it>is devoid of cycles. It was observed that two conditions that were studied and that have no cycles of their own are <it>exogenous</it>: diauxic shift and DNA repair, while cell cycle and sporulation are <it>endogenous</it>. We claim that in a certain sense (slow recovery) stress response is <it>endogenous </it>as well.</p