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Introduction of Carbon Nanotubes
Published in Abhay Kumar Singh, Tien-Chien Jen, Chalcogenide, 2021
Abhay Kumar Singh, Tien-Chien Jen
Quantized conductance in a single MWCNT multiples [Go(2e2/h)] and ballistic transport has been observed at room temperature, by a Scanning Probe Microscope (SPM) [228]. However, the 2G conduction could be measured in the absence of a magnetic field and spin degeneracy may be resolved through electron lattice structure coupling. Another experiment of the MWNT was grown in situ on a tungsten contact to get better contact resistance and probed with a W tip, a conductance of up to 490 Go [229]. A characteristic of multi-channel quasi-ballistic transport was observed. Usually in measurements nanotubes placed on/below metal electrode contacts on substrates suffer from non-reliable Ohmic contacts, which is a recurring theme in electrical characterization. While, in the STM lithography measurement at low temperature (~20 mK), the electron interference typically effects disordered conductors that are present in the transport characteristics [230]. A logarithmic decrease of the conductance with temperature up to saturation was observed, this could be correlated to two-dimensional weak localization effects [231]. Also the evidence of a localization phenomenon in a negative Magneto-Resistance (MR) at a low value (<20 nm) for the lϕ was also noticed [232]. Additional, in the doped MWNTs, the Fermi energy (EF) may shift by changing the total number of participating conduction channels. These localization effects have also been confirmed in MWCNTs [233].
SGM measurements on a disordered InGaAs QPC
Published in J Kono, J Léotin, Narrow Gap Semiconductors, 2006
N. Aoki, C. R. da Cunha, R. Akis, D. K. Ferry, Y. Ochiai
The typical magnetoconductance curve across the QPC without approaching the tip is shown in Fig. 4c. A clear weak localization peak [12] is observed and the full width at half maximum (FWHM) is about 30 mT that corresponds to an interference area of 420 nm in diameter, very close to the one found from the image correlation. The overall effect can be understood as follows: the tip images regions where the same conditions for quantum interference are maintained, whose typical size is approximately 400 nm diameter. The way it performs this task is by locally lifting the background energy. The magnetic field has a similar effect as it breaks these interferences and destroys weak localization. The average image correlation function gives the interference area considering disturbances at different points. These last two have values very close to each other, implying that the tip potential is only capable of changing the local quantum interference patterns, but leaves the overall effect intact. Thus, the images taken with SGM should correspond to regions of interference within the quantum constriction.
Role of Temperature
Published in David K. Ferry, An Introduction to Quantum Transport in Semiconductors, 2017
By fitting to the shape of the weak localization contribution to the conductance in the above equation, one can estimate the values for the coherence length and the phase-breaking time from these measurements. For example, in Fig. 5.8, the conductance measured in a GaAlAs/GaAs heterostructure at low temperature is plotted [8]. There is basically a logarithmic rise of the conductance as the magnetic field is raised, but at low magnetic field, there tends to be a saturation. This increase in conductance with magnetic field is opposite to the normal magnetoresistance in such a structure and is the signal of the weak localization. The shape of these curves can be fit, and the main parameter is the phase-breaking time.
Overcoming intra-molecular repulsions in PEDTT by sulphate counter-ion
Published in Science and Technology of Advanced Materials, 2021
Dominik Farka, Theresia Greunz, Cigdem Yumusak, Christoph Cobet, Cezarina Cela Mardare, David Stifter, Achim Walter Hassel, Markus C. Scharber, Niyazi Serdar Sariciftci
The positive MC in PEDTT:sulf was rather weak, with an increase of a mere 0.03%. Consequently, it could be argued that only a small fraction of the material actually underwent an increase in conductivity, indicating a portion of the material was in the metallic state, i.e. highly doped and devoid of disorder, embedded in non-metallic material [62]. This positive change was most pronounced for measurements at 1.9 K and 10 K. In the latter, two peaks were observed: 0.12 and 0.28 T. Whether this originated from a double resonance, i.e. two different conduction mechanisms being involved, spin-orbit effects [58,59], or from an overlap of two separate phases with different conduction mechanisms, remains subject to speculation. For temperatures between these, the positive MC was even less pronounced. The measurements at 3.9 K were especially puzzling, as no positive effect was observed. As the positive effect comes from weak localization, while the negative originates from electron–electron (e–e) interactions, the former must have been suppressed by either spin-orbit effects or by scattering on magnetic impurities [60,62]. Judging by the way the experiment was conducted; the latter seems like the more likely explanation.
A set of basis functions to improve numerical calculation of Mie scattering in the Chandrasekhar-Sekera representation
Published in Waves in Random and Complex Media, 2021
Alexandre Souto Martinez, José Renato Alcarás, Tiago José Arruda
With the scattering plane representation, one can express directly the photon ‘history’ from the source to the detector. Nevertheless, for multiple scattering it presents some inconveniences. From one scattering event to another, the electric field needs to be written in local basis. The scattering event is written in the preceding basis, which depends on the photon history as illustrated by Figure 1. For instance, to write the scattered field along a scattering sequence in a Monte Carlo scheme, the azimuthal and scattering angles and all the distances along the sequence need to be recorded to perform the right basis transformation back to the laboratory (fixed) frame. This record is necessary when the reversed scattering sequences must be considered. This representation becomes specially cumbersome for the calculation of the coherent backscattering enhancement (weak localization of light) [13–16,59].
Structure of graphene and its disorders: a review
Published in Science and Technology of Advanced Materials, 2018
Gao Yang, Lihua Li, Wing Bun Lee, Man Cheung Ng
In an idealistic planar graphene model without disorders, the Fermi energy level lies at the Dirac point, where the valence band and conduction band intersect, and the dispersion relation around the Dirac point is isotropic and linear [236]. However, the electronic homogeneity of graphene would be violated by the introduction of disorders into the graphene structure. These disorders are able to alter the bond length of the interatomic bonds and lead to the re-hybridization of and orbitals. Moreover, all defects may cause the scattering of electron waves and change the electron trajectories [237,238]. As a result, the electronic structure in the vicinity of these disorders differs from that in a perfect lattice. More specifically, intrinsic ripples are expected to influence the electrical properties of graphene by changing band gap [239], creating polarized carrier puddles [240] and inducing pseudo-magnetic fields [241]. Whereas wrinkles and crumples result in several electronic phenomena, such as electron-hole puddles [189,242], carrier scattering [195,243], band gap opening [244], suppression of weak localization [245] and quantum corrections [246].