236 research outputs found
Magnetoconductance of the Corbino disk in graphene
Electron transport through the Corbino disk in graphene is studied in the
presence of uniform magnetic fields. At the Dirac point, we observe conductance
oscillations with the flux piercing the disk area , characterized by
the period , where () is the outer
(inner) disk radius. The oscillations magnitude increase with the radii ratio
and exceed 10% of the average conductance for in the case
of the normal Corbino setup, or for in the case of the
Andreev-Corbino setup. At a finite but weak doping, the oscillations still
appear in a limited range of , away from which
the conductance is strongly suppressed. At large dopings and weak fields we
identify the crossover to a normal ballistic transport regime.Comment: RevTeX, 5 pages, 3 figures. New version with minor revisions and
references added; to be published in Phys. Rev.
Propagation of surface initiated rolling contact fatigue cracks in bearing Steel
Surface initiated rolling contact fatigue, leading to a surface failure known as pitting, is a life limiting failure mode in many modern machine elements, particularly rolling element bearings. Most research on rolling contact fatigue considers total life to pitting. Instead, this work studies the growth of rolling contact fatigue cracks before they develop into surface pits in an attempt to better understand crack propagation mechanisms. A triple-contact disc machine was used to perform pitting experiments on bearing steel samples under closely controlled contact conditions in mixed lubrication regime. Crack growth across the specimen surface is monitored and crack propagation rates extracted. The morphology of the generated cracks is observed by preparing sections of cracked specimens at the end of the test. It was found that crack initiation occurred very early in total life, which was attributed to high asperity stresses due to mixed lubrication regime. Total life to pitting was dominated by crack propagation. Results provide direct evidence of two distinct stages of crack growth in rolling contact fatigue: stage 1, within which cracks grow at a slow and relatively steady rate, consumed most of the total life; and stage 2, reached at a critical crack length, within which the propagation rate rapidly increases. Contact pressure and crack size were shown to be the main parameters controlling the propagation rate. Results show that crack propagation under rolling contact fatigue follows similar trends to those known to occur in classical fatigue. A log-log plot of measured crack growth rates against the product of maximum contact pressure and the square root of crack length, a parameter describing the applied stress intensity, produces a straight line for stage 2 propagation. This provides the first evidence that growth of hereby-identified stage 2 rolling contact fatigue cracks can be described by a Paris-type power law, where the rate of crack growth across the surface is proportional to the contact pressure raised to a power of approximately 7.5
Entanglement and transport through correlated quantum dot
We study quantum entanglement in a single-level quantum dot in the
linear-response regime. The results show, that the maximal quantum value of the
conductance 2e^2/h not always match the maximal entanglement. The pairwise
entanglement between the quantum dot and the nearest atom of the lead is also
analyzed by utilizing the Wootters formula for charge and spin degrees of
freedom separately. The coexistence of zero concurrence and the maximal
conductance is observed for low values of the dot-lead hybridization. Moreover,
the pairwise concurrence vanish simultaneously for charge and spin degrees of
freedom, when the Kondo resonance is present in the system. The values of a
Kondo temperature, corresponding to the zero-concurrence boundary, are also
provided.Comment: Presented on the International Conference "Nanoelectronics '06", 7-8
January 2006, Lancaster, U
Graphene Rings in Magnetic Fields: Aharonov-Bohm Effect and Valley Splitting
We study the conductance of mesoscopic graphene rings in the presence of a
perpendicular magnetic field by means of numerical calculations based on a
tight-binding model. First, we consider the magnetoconductance of such rings
and observe the Aharonov-Bohm effect. We investigate different regimes of the
magnetic flux up to the quantum Hall regime, where the Aharonov-Bohm
oscillations are suppressed. Results for both clean (ballistic) and disordered
(diffusive) rings are presented. Second, we study rings with smooth mass
boundary that are weakly coupled to leads. We show that the valley degeneracy
of the eigenstates in closed graphene rings can be lifted by a small magnetic
flux, and that this lifting can be observed in the transport properties of the
system.Comment: 12 pages, 9 figure
Impurity and edge roughness scattering in armchair graphene nanoribbons: Boltzmann approach
The conductivity of armchair graphene nanoribbons in the presence of
short-range impurities and edge roughness is studied theoretically using the
Boltzmann transport equation for quasi-one-dimensional systems. As the number
of occupied subbands increases, the conductivity due to short-range impurities
converges towards the two-dimensional case. Calculations of the
magnetoconductivity confirm the edge-roughness-induced dips at cyclotron radii
close to the ribbon width suggested by the recent quantum simulations
Anomalously large conductance fluctuations in weakly disordered graphene
We have studied numerically the mesoscopic fluctuations of the conductance of
a graphene strip (width W large compared to length L), in an ensemble of
samples with different realizations of the random electrostatic potential
landscape. For strong disorder (potential fluctuations comparable to the
hopping energy), the variance of the conductance approaches the value predicted
by the Altshuler-Lee-Stone theory of universal conductance fluctuations. For
weaker disorder the variance is greatly enhanced if the potential is smooth on
the scale of the atomic separation. There is no enhancement if the potential
varies on the atomic scale, indicating that the absence of backscattering on
the honeycomb lattice is at the origin of the anomalously large fluctuations.Comment: 5 pages, 8 figure
Symmetry Classes in Graphene Quantum Dots: Universal Spectral Statistics, Weak Localization, and Conductance Fluctuations
We study the symmetry classes of graphene quantum dots, both open and closed,
through the conductance and energy level statistics. For abrupt termination of
the lattice, these properties are well described by the standard orthogonal and
unitary ensembles. However, for smooth mass confinement, special time-reversal
symmetries associated with the sublattice and valley degrees of freedom are
critical: they lead to block diagonal Hamiltonians and scattering matrices with
blocks belonging to the unitary symmetry class even at zero magnetic field.
While the effect of this structure is clearly seen in the conductance of open
dots, it is suppressed in the spectral statistics of closed dots, because the
intervalley scattering time is shorter than the time required to resolve a
level spacing in the closed systems but longer than the escape time of the open
systems.Comment: 4 pages, 4 figures, RevTex, submitted to Phys. Rev. Let
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