Real time decoherence of Landau and Levitov quasi-particles in quantum Hall edge channels

Quantum Hall edge channels at integer filling factor provide a unique test-bench to understand decoherence and relaxation of single electronic excitations in a ballistic quantum conductor. In this Letter, we obtain a full visualization of the decoherence scenario of energy (Landau) and time (Levitov) resolved single electron excitations at filling factor $\nu=2$. We show that the Landau excitation exhibits a fast relaxation followed by spin-charge separation whereas the Levitov excitation only experiences spin-charge separation. We finally suggest to use Hong-Ou-Mandel type experiments to probe specific signatures of these different scenarios.Comment: 14 pages, 8 figure

Separation of neutral and charge modes in one dimensional chiral edge channels

Coulomb interactions have a major role in one-dimensional electronic transport. They modify the nature of the elementary excitations from Landau quasiparticles in higher dimensions to collective excitations in one dimension. Here we report the direct observation of the collective neutral and charge modes of the two chiral co-propagating edge channels of opposite spins of the quantum Hall effect at filling factor 2. Generating a charge density wave at frequency f in the outer channel, we measure the current induced by inter-channel Coulomb interaction in the inner channel after a 3-mm propagation length. Varying the driving frequency from 0.7 to 11 GHz, we observe damped oscillations in the induced current that result from the phase shift between the fast charge and slow neutral eigenmodes. We measure the dispersion relation and dissipation of the neutral mode from which we deduce quantitative information on the interaction range and parameters.Comment: 23 pages, 6 figure

Supercollision cooling in undoped graphene

Carrier mobility in solids is generally limited by electron-impurity or electron-phonon scattering depending on the most frequently occurring event. Three body collisions between carriers and both phonons and impurities are rare; they are denoted supercollisions (SCs). Elusive in electronic transport they should emerge in relaxation processes as they allow for large energy transfers. As pointed out in Ref. \onlinecite{Song2012PRL}, this is the case in undoped graphene where the small Fermi surface drastically restricts the allowed phonon energy in ordinary collisions. Using electrical heating and sensitive noise thermometry we report on SC-cooling in diffusive monolayer graphene. At low carrier density and high phonon temperature the Joule power $P$ obeys a $P\propto T_e^3$ law as a function of electronic temperature $T_e$. It overrules the linear law expected for ordinary collisions which has recently been observed in resistivity measurements. The cubic law is characteristic of SCs and departs from the $T_e^4$ dependence recently reported for metallic graphene below the Bloch-Gr\"{u}neisen temperature. These supercollisions are important for applications of graphene in bolometry and photo-detection

Conserved spin and orbital phase along carbon nanotubes connected with multiple ferromagnetic contacts

We report on spin dependent transport measurements in carbon nanotubes based multi-terminal circuits. We observe a gate-controlled spin signal in non-local voltages and an anomalous conductance spin signal, which reveal that both the spin and the orbital phase can be conserved along carbon nanotubes with multiple ferromagnetic contacts. This paves the way for spintronics devices exploiting both these quantum mechanical degrees of freedom on the same footing.Comment: 8 pages - minor differences with published versio

Electron quantum optics : partitioning electrons one by one

We have realized a quantum optics like Hanbury Brown and Twiss (HBT) experiment by partitioning, on an electronic beam-splitter, single elementary electronic excitations produced one by one by an on-demand emitter. We show that the measurement of the output currents correlations in the HBT geometry provides a direct counting, at the single charge level, of the elementary excitations (electron/hole pairs) generated by the emitter at each cycle. We observe the antibunching of low energy excitations emitted by the source with thermal excitations of the Fermi sea already present in the input leads of the splitter, which suppresses their contribution to the partition noise. This effect is used to probe the energy distribution of the emitted wave-packets.Comment: 5 pages, 4 figure

Hot electron cooling by acoustic phonons in graphene

We have investigated the energy loss of hot electrons in metallic graphene by means of GHz noise thermometry at liquid helium temperature. We observe the electronic temperature T / V at low bias in agreement with the heat diffusion to the leads described by the Wiedemann-Franz law. We report on $T\propto\sqrt{V}$ behavior at high bias, which corresponds to a T4 dependence of the cooling power. This is the signature of a 2D acoustic phonon cooling mechanism. From a heat equation analysis of the two regimes we extract accurate values of the electron-acoustic phonon coupling constant $\Sigma$ in monolayer graphene. Our measurements point to an important effect of lattice disorder in the reduction of $\Sigma$, not yet considered by theory. Moreover, our study provides a strong and firm support to the rising field of graphene bolometric detectors.Comment: 5 figure