We show that the manifestation of quantum interference in graphene is very different from that in conventional two-dimensional systems. Due to the chiral nature of charge carriers, it is sensitive not only to inelastic, phase-breaking scattering, but also to a number of elastic scattering processes. We study weak localization in different samples and at different carrier densities, including the Dirac region, and find the characteristic rates that determine it. We show how the shape and quality of graphene flakes affect the values of the elastic and inelastic rates and discuss their physical origin.

We have performed the first experimental investigation of quantum interference corrections to the conductivity of a bilayer graphene structure. A negative magnetoresistance--a signature of weak localization--is observed at different carrier densities, including the electroneutrality region. It is very different, however, from the weak localization in conventional two-dimensional systems. We show that it is controlled not only by the dephasing time, but also by different elastic processes that break the effective time-reversal symmetry and provide intervalley scattering.

We have fabricated graphene devices with a top gate separated from the graphene layer by an air gap-a design which does not decrease the mobility of charge carriers under the gate. This gate is used to realise p-n-p structures where the conducting properties of chiral carriers are studied. The band profile of the structures is calculated taking into account the specifics of the graphene density of states and is used to find the resistance of the p-n junctions expected for chiral carriers. We show that ballistic p-n junctions have larger resistance than diffusive ones. This is caused by suppressed transmission of chiral carriers at angles away from the normal to the junction.

Near infrared pump-probe spectroscopy has been used to measure the ultrafast dynamics of photoexcited charge carriers in monolayer and multilayer graphene. We observe two decay processes occurring on 100 fs and 2 ps timescales. The first is attributed to the rapid electron-phonon thermalisation in the system. The second timescale is found to be due to the slow decay of hot phonons. Using a simple theoretical model we calculate the hot phonon decay rate and show that it is significantly faster in monolayer flakes than in multilayer ones. In contrast to recent claims, we show that this enhanced decay rate is not due to the coupling to substrate phonons, since we have also seen the same effect in suspended flakes. Possible intrinsic decay mechanisms that could cause such an effect are discussed. The symmetric, linear electronic band structure of graphene gives rise to some very unusual physical properties, such as quantised transmission 1 , extremely high thermal conductivity 2 and high carrier mobility.3 An important underlying feature is the very strong electronphonon coupling that exists in graphene, which is revealed by the presence of Kohn anomalies.4 In graphite, it is known that strongly coupled optical phonons have high quantum energies of up to 0.2 eV and are excited only by electrons of elevated energy.4-6 To progress towards applications in real (high-current) circuits and devices, it is crucial to understand how graphene behaves under such high energy, non-equilibrium conditions. Despite the surge of interest in this material and its potential applications, investigations into the kinetic properties of "hot" charge carriers remain rather limited. 5,7Hot electron relaxation in large area, epitaxially grown graphene layers using pump-probe spectroscopy has been studied previously. [8][9][10][11][12][13][14][15][16] These measurements point to biexponential decay dynamics characterised by a fast ∼100 fs component and a slow ∼2 ps component. There is, however, significant variation in the reported timescales. Epitaxial graphene exhibits inhomogeneity in layer thickness on the micron scale 12 and can result in significant variability in relaxation dynamics from sample to sample.8 Pump-probe measurements have also been performed on mechanically exfoliated graphene, 17,18 which is homogeneous over much greater length scales. It was concluded that the slow relaxation process was caused by the coupling to phonons in the substrate. 17In this paper we use near infrared pump-probe spectroscopy to investigate the relaxation dynamics of hot carriers in mechanically exfoliated graphene. Similar to previous results we find that the relaxation occurs on two timescales, one fast (∼100 fs) and the other slow (∼2 ps). By measuring the relaxation in monolayer and multilayer graphene flakes we show a clear correlation between the slow decay rate and flake thickness, with the fastest rate observed for monolayer graphene. This slow decay rate is found to occur in both supported and suspended flakes. Therefore, in contrast to r...