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    D. Mahalu

    Charged excitations in the fractional quantum Hall effect are known to carry fractional charges, as theoretically predicted and experimentally verified. Here we report on the dependence of the tunneling quasiparticle charge, as determined... more
    Charged excitations in the fractional quantum Hall effect are known to carry fractional charges, as theoretically predicted and experimentally verified. Here we report on the dependence of the tunneling quasiparticle charge, as determined via highly sensitive shot noise measurements, on the measurement conditions, in the odd denominators states v=1/3 and v=7/3 and in the even denominator state v=5/2. In particular, for very weak backscattering probability and sufficiently small excitation energies (temperature and applied voltage), tunneling charges across a constriction were found to be significantly higher than the theoretically predicted fundamental quasiparticle charges.
    Electron pairing is a rare phenomenon appearing only in a few unique physical systems; for example, superconductors and Kondo-correlated quantum dots. Here, we report on an unexpected electron pairing in the integer quantum Hall effect... more
    Electron pairing is a rare phenomenon appearing only in a few unique physical systems; for example, superconductors and Kondo-correlated quantum dots. Here, we report on an unexpected electron pairing in the integer quantum Hall effect regime. The pairing takes place within an interfering edge channel in an electronic Fabry-Perot interferometer at a wide range of bulk filling factors, between 2 and 5. We report on three main observations: high-visibility Aharonov-Bohm conductance oscillations with magnetic flux periodicity equal to half the magnetic flux quantum; an interfering quasiparticle charge equal to twice the elementary electron charge as revealed by quantum shot noise measurements, and full dephasing of the pairs' interference by induced dephasing of the adjacent inner edge channel-a manifestation of inter-channel entanglement. Although this pairing phenomenon clearly results from inter-channel interaction, the exact mechanism that leads to electron-electron attraction ...
    Abstract The dilute magnetic semiconductor Cd0.95Mn0.05Se was investigated as a photoelectrode in a photoelectrochemical cell. It is found that a short photoelectrochemical etching produces the highest photoresponse of this material. The... more
    Abstract The dilute magnetic semiconductor Cd0.95Mn0.05Se was investigated as a photoelectrode in a photoelectrochemical cell. It is found that a short photoelectrochemical etching produces the highest photoresponse of this material. The effective energy gap is found to be higher than expected by about 130 meV. This observation can be attributed to the onset of the internal d-d transitions in the Mn in or to some nonuniformities in the alloy composition.
    ABSTRACT
    Contains table of contents for Section 2, research goals and objectives, summary of recent work and a list of publications.
    Quantum critical systems derive their finite temperature properties from the influence of a zero temperature quantum phase transition. The paradigm is essential for understanding unconventional high-Tc superconductors and the non-Fermi... more
    Quantum critical systems derive their finite temperature properties from the influence of a zero temperature quantum phase transition. The paradigm is essential for understanding unconventional high-Tc superconductors and the non-Fermi liquid properties of heavy fermion compounds. However, the microscopic origins of quantum phase transitions in complex materials are often debated. Here we demonstrate experimentally, with support from numerical renormalization group calculations, a universal crossover from quantum critical non-Fermi liquid behavior to distinct Fermi liquid ground states in a highly controllable quantum dot device. Our device realizes the non-Fermi liquid two-channel Kondo state, based on a spin-1/2 impurity exchange-coupled equally to two independent electronic reservoirs. Arbitrarily small detuning of the exchange couplings results in conventional screening of the spin by the more strongly coupled channel for energies below a Fermi liquid scale T*. We extract a quad...
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    ABSTRACT Charged excitations in the fractional quantum Hall effect are known to carry fractional charges, as theoretically predicted and experimentally verified. Here we report on the dependence of the tunneling quasiparticle charge, as... more
    ABSTRACT Charged excitations in the fractional quantum Hall effect are known to carry fractional charges, as theoretically predicted and experimentally verified. Here we report on the dependence of the tunneling quasiparticle charge, as determined via highly sensitive shot noise measurements, on the measurement conditions, in the odd denominators states v=1/3 and v=7/3 and in the even denominator state v=5/2. In particular, for very weak backscattering probability and sufficiently small excitation energies (temperature and applied voltage), tunneling charges across a constriction were found to be significantly higher than the theoretically predicted fundamental quasiparticle charges.
    Layered dichalcogenide semiconductors (LS) are studied intensively for photovoltaic applications. It is generally accepted that the smooth van der Waals face (⊥c) is most suitable for this purpose. Recombination centers have been... more
    Layered dichalcogenide semiconductors (LS) are studied intensively for photovoltaic applications. It is generally accepted that the smooth van der Waals face (⊥c) is most suitable for this purpose. Recombination centers have been associated with surface steps (exposing ||c facets) or near-surface dislocations. Efforts to passivate the recombination centers had, until now, only limited success. In contrast to this approach, a method of surface preparation of LS, leading to rough surfaces that exhibit high photoactivity with high reproducibility, is presented. The physical principles underlying this technique are discussed in detail.
    We study a novel surface superlattice structure formed by a honeycomb gate pattern on GaAs/AlGaAs heterostructures with a high-mobility two-dimensional electron gas. The hexagonal gate arrays, fabricated by electron beam lithography, had... more
    We study a novel surface superlattice structure formed by a honeycomb gate pattern on GaAs/AlGaAs heterostructures with a high-mobility two-dimensional electron gas. The hexagonal gate arrays, fabricated by electron beam lithography, had pitches as small as 150 nm, corresponding to hexagons with 87 nm between adjacent vertices. Magnetoresistance measurements show features which reflect the presence of the induced hexagonal superlattice potential.
    ABSTRACT
    Neutral atoms can be trapped and manipulated with surface mounted microscopic current carrying and charged structures. We present a lithographic fabrication process for such atom chips based on evaporated metal films. The size limit of... more
    Neutral atoms can be trapped and manipulated with surface mounted microscopic current carrying and charged structures. We present a lithographic fabrication process for such atom chips based on evaporated metal films. The size limit of this process is below 1$\mu$m. At room temperature, thin wires can carry more than 10$^7$A/cm$^2$ current density and voltages of more than 500V. Extensive test measurements for different substrates and metal thicknesses (up to 5 $\mu$m) are compared to models for the heating characteristics of the microscopic wires. Among the materials tested, we find that Si is the best suited substrate for atom chips.
    The fractional quantum Hall effect, where plateaus in the Hall resistance at values of coexist with zeros in the longitudinal resistance, results from electron correlations in two dimensions under a strong magnetic field. Current flows... more
    The fractional quantum Hall effect, where plateaus in the Hall resistance at values of coexist with zeros in the longitudinal resistance, results from electron correlations in two dimensions under a strong magnetic field. Current flows along the edges carried by charged excitations (quasi particles) whose charge is a fraction of the electron charge. While earlier research concentrated on odd denominator
    WSe2 is a layered-type semiconductor which exhibits strong anisotropy in its electronic properties. The photoresponse of this material is adversely affected by the presence of morphological defects at and near the surface. Passivation of... more
    WSe2 is a layered-type semiconductor which exhibits strong anisotropy in its electronic properties. The photoresponse of this material is adversely affected by the presence of morphological defects at and near the surface. Passivation of recombination centers on the surface of n- and p-type WSe2 crystals using (photo)electrochemical etching was investigated. Measurements of the transmissivity-reflectivity, photocurrent, surface photovoltage, Hall effect and
    ABSTRACT We study conductance through a system of weakly coupled narrow wires formed in the plane of a GaAs/AlxGa1-xAs heterojunction, where a voltage Vds is applied perpendicular to the wires, in the plane. We find an abrupt onset of... more
    ABSTRACT We study conductance through a system of weakly coupled narrow wires formed in the plane of a GaAs/AlxGa1-xAs heterojunction, where a voltage Vds is applied perpendicular to the wires, in the plane. We find an abrupt onset of current as Vds is increased beyond a certain critical value. In contrast, when Vds is swept down there is a gradual decrease in current which extends well below the critical Vds. The abrupt onset and the hysteresis are interpreted in terms of a bistability of the system, where the conducting state is associated with dynamic depopulation of the wires. Additional features in the conductance are also observed and discussed.
    Multi-valued logic gates, which can handle quaternary numbers as inputs, are developed by exploiting the ballistic transport properties of quantum point contacts in series. The principle of a logic gate that finds the minimum of two... more
    Multi-valued logic gates, which can handle quaternary numbers as inputs, are developed by exploiting the ballistic transport properties of quantum point contacts in series. The principle of a logic gate that finds the minimum of two quaternary number inputs is demonstrated. The device is scalable to allow multiple inputs, which makes it possible to find the minimum of multiple inputs in a single gate operation. Also, the principle of a half-adder for quaternary number inputs is demonstrated. First, an adder that adds up two quaternary numbers and outputs the sum of inputs is demonstrated. Second, a device to express the sum of the adder into two quaternary digits [Carry (first digit) and Sum (second digit)] is demonstrated. All the logic gates presented in this paper can in principle be extended to allow decimal number inputs with high quality QPCs.
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    ... where Vsd is the applied dc excitation voltage, gi=gi−gi−1, with gj = je2/h, t i−i−1 = g−gi−1 gi is the transmission probability of the ith channel, and g is the two-terminal ... We thank Ady Stern, Yuval Gefen, Bernd Rosenow, Dario... more
    ... where Vsd is the applied dc excitation voltage, gi=gi−gi−1, with gj = je2/h, t i−i−1 = g−gi−1 gi is the transmission probability of the ith channel, and g is the two-terminal ... We thank Ady Stern, Yuval Gefen, Bernd Rosenow, Dario Ferraro, and Maura Sassetti for helpful discussions. ...
    ABSTRACT Quantum point contact (QPC) with an extra metallic gate in between the split gates of a conventional QPC was fabricated and studied. Clear conductance quantization was observed at 4.2 K when a proper positive voltage was set to... more
    ABSTRACT Quantum point contact (QPC) with an extra metallic gate in between the split gates of a conventional QPC was fabricated and studied. Clear conductance quantization was observed at 4.2 K when a proper positive voltage was set to the middle gate of the QPC. The maximum energy spacing between the ground and the first exited state of the QPC was around 7 meV which is at least a few times larger than that of conventional QPCs. Using same approach, a possibility of making a relatively clean and long 1D wire has been tested.
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