9,363 research outputs found
Disorder-induced pseudodiffusive transport in graphene nanoribbons.
We study the transition from ballistic to diffusive and localized transport in graphene nanoribbons in the presence of binary disorder, which can be generated by chemical adsorbates or substitutional doping. We show that the interplay between the induced average doping (arising from the nonzero average of the disorder) and impurity scattering modifies the traditional picture of phase-coherent transport. Close to the Dirac point, intrinsic evanescent modes produced by the impurities dominate transport at short lengths giving rise to a regime analogous to pseudodiffusive transport in clean graphene, but without the requirement of heavily doped contacts. This intrinsic pseudodiffusive regime precedes the traditional ballistic, diffusive, and localized regimes. The last two regimes exhibit a strongly modified effective number of propagating modes and a mean free path which becomes anomalously large close to the Dirac point
Strong Tunneling and Coulomb Blockade in a Single-Electron Transistor
We have developed a detailed experimental study of a single-electron
transistor in a strong tunneling regime. Although weakened by strong charge
fluctuations, Coulomb effects were found to persist in all samples including
one with the effective conductance 8 times higher than the quantum value (6.45
k). A good agreement between our experimental data and
theoretical results for the strong tunneling limit is found. A reliable
operation of transistors with conductances 3-4 times larger than the quantum
value is demonstrated.Comment: revtex, 4 page
Conductance of the Single Electron Transistor for Arbitrary Tunneling Strength
We study the temperature and gate voltage dependence of the conductance of
the single electron transistor focusing on highly conducting devices. Electron
tunneling is treated nonperturbatively by means of path integral Monte Carlo
techniques and the conductance is determined from the Kubo formula. A
regularized singular value decomposition scheme is employed to calculate the
conductance from imaginary time simulation data. Our findings are shown to
bridge between available analytical results in the semiclassical and
perturbative limits and are found to explain recent experimental results in a
regime not accessible by earlier methods.Comment: 4 pages, 2 figure
Weak localization, Aharonov-Bohm oscillations and decoherence in arrays of quantum dots
Combining scattering matrix theory with non-linear -model and Keldysh
technique we develop a unified theoretical approach enabling one to
non-perturbatively study the effect of electron-electron interactions on weak
localization and Aharonov-Bohm oscillations in arbitrary arrays of quantum
dots. Our model embraces (i) weakly disordered conductors (ii) strongly
disordered conductors and (iii) metallic quantum dots. In all these cases at the electron decoherence time is found to saturate to a finite value
determined by the universal formula which agrees quantitatively with numerous
experimental results. Our analysis provides overwhelming evidence in favor of
electron-electron interactions as a universal mechanism for zero temperature
electron decoherence in disordered conductors.Comment: 19 pages, 13 figures, invited paper, published in a special issue of
Fiz. Nizk. Temp. (Kharkov) dedicated to Prof. Igor Kuli
Coulomb blockade in one-dimensional arrays of high conductance tunnel junctions
Properties of one-dimensional (1D) arrays of low Ohmic tunnel junctions (i.e.
junctions with resistances comparable to, or less than, the quantum resistance
k) have been studied experimentally
and theoretically. Our experimental data demonstrate that -- in agreement with
previous results on single- and double-junction systems -- Coulomb blockade
effects survive even in the strong tunneling regime and are still clearly
visible for junction resistances as low as 1 k. We have developed a
quasiclassical theory of electron transport in junction arrays in the strong
tunneling regime. Good agreement between the predictions of this theory and the
experimental data has been observed. We also show that, due to both heating
effects and a relatively large correction to the linear relation between the
half-width of the conductance dip around zero bias voltage, , and the
measured electronic temperature, such arrays are inferior to those
conventionally used in the Coulomb Blockade Thermometry (CBT). Still, the
desired correction to the half-width, , can be determined
rather easily and it is proportional to the magnitude of the conductance dip
around zero bias voltage, . The constant of proportionality is a
function of the ratio of the junction and quantum resistances, ,
and it is a pure strong tunneling effect.Comment: LaTeX file + five postscript figure
Electron transport through interacting quantum dots
We present a detailed theoretical investigation of the effect of Coulomb
interactions on electron transport through quantum dots and double barrier
structures connected to a voltage source via an arbitrary linear impedance.
Combining real time path integral techniques with the scattering matrix
approach we derive the effective action and evaluate the current-voltage
characteristics of quantum dots at sufficiently large conductances. Our
analysis reveals a reach variety of different regimes which we specify in
details for the case of chaotic quantum dots. At sufficiently low energies the
interaction correction to the current depends logarithmically on temperature
and voltage. We identify two different logarithmic regimes with the crossover
between them occurring at energies of order of the inverse dwell time of
electrons in the dot. We also analyze the frequency-dependent shot noise in
chaotic quantum dots and elucidate its direct relation to interaction effects
in mesoscopic electron transport.Comment: 21 pages, 4 figures. References added, discussion slightly extende
Persistent current noise and electron-electron interactions
We analyze fluctuations of persistent current (PC) produced by a charged
quantum particle moving in a ring and interacting with a dissipative
environment formed by diffusive electron gas. We demonstrate that in the
presence of interactions such PC fluctuations persist down to zero temperature.
In the case of weak interactions and/or sufficiently small values of the ring
radius PC noise remains coherent and can be tuned by external magnetic flux
piercing the ring. In the opposite limit of strong interactions and/or
large values of fluctuations in the electronic bath strongly suppress
quantum coherence of the particle down to and induce incoherent
-independent current noise in the ring which persists even at
when the average PC is absent.Comment: 12 pages, 8 figure
Charge Fluctuations in the Single Electron Box
Quantum fluctuations of the charge in the single electron box are
investigated. Based on a diagrammatic expansion we calculate the average island
charge number and the effective charging energy in third order in the tunneling
conductance. Near the degeneracy point where the energy of two charge states
coincides, the perturbative approach fails, and we explicitly resum the leading
logarithmic divergencies to all orders. The predictions for zero temperature
are compared with Monte Carlo data and with recent renormalization group
results. While good agreement between the third order result and numerical data
justifies the perturbative approach in most of the parameter regime relevant
experimentally, near the degeneracy point and at zero temperature the
resummation is shown to be insufficient to describe strong tunneling effects
quantitatively. We also determine the charge noise spectrum employing a
projection operator technique. Former perturbative and semiclassical results
are extended by the approach.Comment: 20 pages, 15 figure
Strong Charge Fluctuations in the Single-Electron Box: A Quantum Monte Carlo Analysis
We study strong electron tunneling in the single-electron box, a small
metallic island coupled to an electrode by a tunnel junction, by means of
quantum Monte Carlo simulations. We obtain results, at arbitrary tunneling
strength, for the free energy of this system and the average charge on the
island as a function of an external bias voltage. In much of the parameter
range an extrapolation to the ground state is possible. Our results for the
effective charging energy for strong tunneling are compared to earlier -- in
part controversial -- theoretical predictions and Monte Carlo simulations
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