Quantum Hall effect induced by electron-electron interaction in disordered GaAs layers with a three-dimensional spectrum
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It is shown that the observed Quantum Hall Effect in epitaxial layers of heavily doped n-type GaAs with thickness (50-140 nm) larger the mean free path of the conduction electrons (15-30 nm) and, therefore, with a three-dimensional single-particle spectrum is induced by the electron-electron interaction. The Hall resistance R_xy of the thinnest sample reveals a wide plateau at small activation energy E_a=0.4 K found in the temperature dependence of the transverse resistance R_xx. The different minima in the transverse conductance G_xx of the different samples show a universal temperature dependence (logarithmic in a large range of rescaled temperatures T/T_0) which is reminiscent of electron-electron-interaction effects in coherent diffusive transport.The conductance of a tight-binding model on a Penrose tiling is calculated as a function of Fermi energy by the multichannel Landauer formula. The conductance shows spiky fine structures. The behavior of the conductance is compared with the density of states of the corresponding system. It is also found that the dependence of the conductance on the system size is anomalous and analogous to the universal conductance fluctuation.
Penrose tiling
Conductance quantum
Fermi energy
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We investigate conductance fluctuations in molecular junctions using a mechanically controllable break junction setup in a liquid environment. In contrast to conventional break junction measurements, time-dependent conductance signals were recorded while reducing the gap size between the two contact electrodes. Only small amplitude fluctuations of the conductance are observed when measuring in pure solvent. Conductance traces recorded in solutions containing alkanedithiols show significantly larger fluctuations which can take the form of random telegraph signals. Such signals emerge in a limited conductance range, which corresponds well to the known molecular conductance of the compounds investigated. These large-amplitude fluctuations are attributed to the formation and thermally driven breaking of bonds between a molecule and a metal electrode and provide a still poorly explored source of information on the dynamics of molecular junctions formation. The lifetimes of the high and low conductance states are found to vary between 0.1 ms and 0.1 s.
Break junction
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Conductance quantum
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The conductance and its fluctuations in mesoscopic NS (N is a normal metal and S is a superconductor) wires are studied numerically. It is shown that the conductance fluctuations in the NS wire is enhanced as compared with an ordinary normal metal wire (N wire) by about a factor two. For the conductance itself, an enhancement which has been expected is found to vanish when the length of the normal segment is longer than the mean free path.
Mesoscopic physics
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Conductance of single 1,4-benzenedithiol (BDT) molecules is investigated in a wide range (0–0.3)G0, exploiting mechanically controllable break junction technique. The authors observed a series of clear conductance steps both in low- (∼0.01G0) and high-conductance (∼0.1G0) regimes and corresponding two sets of peak structures in the conductance histograms. The two distinct conductance states are attributable to different Au–S bonding configurations of Au∕BDT∕Au junctions. The high-bias measurements reveal that the high-conductance state of single BDT molecules is stable up to 1.6V and prospective for molecular device applications.
Break junction
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We have studied electron transport properties of benzenedithiol and benzenedimethanethiol covalently bonded to gold electrodes by repeatedly creating a large number of molecular junctions. For each molecule, conductance histogram shows peaks at integer multiples of a fundamental conductance value, which is used to identify the conductance of a single molecule. The conductance values of a benzenedithiol and benzenedimethanethiol are 0.011 G0 and 0.0006 G0 (G0 = 2e2/h), respectively. The conductance peaks are broad, which reflects variations in the microscopic details of different molecular junctions. We have also studied electrochemical gate effect.
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Recent experiment found a quantum length dependence of oligothiophene molecule conductance at low bias [Xu et al., nano Lett. \textbf{5}, 1491 (2005)], the long molecule has large conductance. By means of a first-principles method we obtain both the quantum length dependence of conductance at low bias and the classical length dependence of conductance at high bias region for oligothiophene. In between there is an oscillated conductance behavior. The transport behaviors are determined by the distinct electronic structures of the molecular compounds. The various conductance length dependence may appear for the organic compounds. Our further investigation finds that the classical conductance length dependence in polyphenanthrene dithiolates and another unusual conductance length dependence in polyacene ditholates: the quantum length dependence of conductance is at the high bias and the classical length dependence of conductance is at the low bias.
Conductance quantum
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A systematic experimental study of the electrical conductance of single alkanedithiol molecules (HS-(CH(2))(N)-SH) between gold contacts in air for N = 3-12 is presented. For all of these molecules, three different fundamental conductance groups (low, medium and high conductance) were observed. For long molecules (N > 7) the conductance decays exponentially with molecular length for all three conductance groups, as it has been reported previously. In contrast, for short molecules (N < 8), it is shown that the decay of conductance with molecular length gets less pronounced for decreasing length, approaching length independent conductance values for N < 5 where the voltage dependence of the tunnelling current exhibits an anomalous behaviour. Possible reasons for these findings, including the influence of the image potential on the effective mass of the tunnelling electron (hole), are discussed.
Conductance quantum
Break junction
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Formal transport theory for heat conduction requires a constant thermal gradient over a small but finite distance if Q=−KdT/dx is to be a good approximation. The kinetic model is used to show that this relation holds within 2% if the gradient is constant over a minimum range of ±5Λ, where Λ is the mean free path of electrons in metals. The usual differential equation of heat conduction cannot be applied to the absorption of blackbody radiation by metals if the absorption coefficient is high, since it overestimates the effect of thermal conduction and does not give the correct temperature distribution within the first few mean free paths from the surface.
Black-body radiation
Constant (computer programming)
Thermal Radiation
Temperature Gradient
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