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Register a new userTransmission of phase information between electrons and holes in graphene
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We have studied quantum interference between electrons and holes in a split-ring gold interferometer with graphene arms, one of which contained a pn junction. The carrier type, the pn junction and the phase of the oscillations in a magnetic field were controlled by a top gate placed over one of the arms. We observe clear Aharonov-Bohm oscillations at the Dirac point and away from it, regardless of the carrier type in each arm. We also find clear oscillations when one arm of the interferometer contains a single pn junction, allowing us to study the interplay of Aharonov-Bohm effect and Klein tunneling.
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The relativistic nature of Dirac electrons and holes in graphene profoundly affects the way they interact with impurities. Signatures of the relativistic behavior have been observed recently in scanning tunneling measurements on individual impurities, but the conductance measurements in this regime are typically dominated by electron and hole puddles. Here we present measurements of quantum interference noise and magnetoresistance in graphene pn junctions. Unlike the conductance, the quantum interference noise can provide access to the scattering at the Dirac point:it is sensitive to the motion of a single impurity, it depends strongly on the fundamental symmetries that describe the system and it is determined by the phase-coherent phenomena which are not necessarily obscured by the puddles. The temperature and the carrier density dependence of resistance fluctuations and magnetoresistance in graphene p-n junctions at low temperatures suggest that the noise is dominated by the quantum interference due to scattering on impurities and that the noise minimum could be used to determine the point where the average carrier density is zero. At larger carrier densities, the amplitude of the noise depends strongly on the sign of the impurity charge, reflecting the fact that the electrons and the holes are scattered by the impurity potential in an asymmetric manner.
The relativistic nature of charge carriers in graphene is expected to lead to an angle- dependent transmission through a potential barrier, where Klein tunneling involves annihilation of an electron and a hole at the edges of the barrier. The signatures of Klein tunneling have been observed in gated graphene devices, but the angle dependence of the transmission probability has not been directly observed. Here we show measurements of the angle-dependent transmission through quasi-ballistic graphene heterojunctions with straight and angled leads, in which the barrier height is controlled by a shared gate electrode. Using a balanced differential measurement technique, we isolate the angle-dependent contribution to the resistance from other angle-insensitive, gate-dependent and device-dependent effects. We find large oscillations in the transmission as a function of the barrier height in the case of Klein tunneling at a 45 deg angle, as compared to normal incidence. Our results are consistent with the model that predicts oscillations of the transmission probability due to interference of chiral carriers in a ballistic barrier. The observed angle dependence is the key element behind focusing of electrons and the realization of a Veselago lens in graphene.
We present temperature-dependent magneto-transport experiments around the charge neutrality point in graphene and determine the amplitude of potential fluctuations $s$ responsible for the formation of electron-hole puddles. The experimental value $s approx 20$ meV is considerably larger than in conventional semiconductors which implies a strong localization of charge carriers observable up to room temperature. Surprisingly, in the quantized regime, the Hall resistivity overshoots the highest plateau values at high temperatures. We demonstrate by model calculations that such a peculiar behavior is expected in any system with coexisting electrons and holes when the energy spectrum is quantized and the carriers are partially localized.
We reveal a dramatic departure of electron thermodiffusion in solids relative to the commonly accepted picture of the ideal free-electron gas model. In particular, we show that the interaction with the lattice and impurities, combined with a strong material dependence of the electron dispersion relation, leads to counterintuitive diffusion behavior, which we identify by comparing a single-layer two-dimensional electron gas (2DEG) and graphene. When subject to a temperature gradient $ abla T$, thermodiffusion of massless Dirac electrons in graphene exhibits an anomalous behavior with electrons moving along $ abla T$ and accumulating in hot regions, in contrast to normal electron diffusion in a 2DEG with parabolic dispersion, where net motion against $ abla T$ is observed, accompanied by electron depletion in hot regions. These findings have fundamentally importance for the understanding of the spatial electron dynamics in emerging material, establishing close relations with other branches of physics dealing with electron systems under nonuniform temperature conditions.
Three-dimensional topological insulators harbour metallic surface states with exotic properties. In transport or optics, these properties are typically masked by defect-induced bulk carriers. Compensation of donors and acceptors reduces the carrier density, but the bulk resistivity remains disappointingly small. We show that measurements of the optical conductivity in BiSbTeSe$_2$ pinpoint the presence of electron-hole puddles in the bulk at low temperatures, which is essential for understanding DC bulk transport. The puddles arise from large fluctuations of the Coulomb potential of donors and acceptors, even in the case of full compensation. Surprisingly, the number of carriers appearing within puddles drops rapidly with increasing temperature and almost vanishes around 40 K. Monte Carlo simulations show that a highly non-linear screening effect arising from thermally activated carriers destroys the puddles at a temperature scale set by the Coulomb interaction between neighbouring dopants, explaining the experimental observation semi-quantitatively. This mechanism remains valid if donors and acceptors do not compensate perfectly.
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