arXiv subjects

M. R. Graeber

Publications and source records attributed to M. R. Graeber.

5 recordsLinked to original sources

arXiv2007

Mapping electron delocalization by charge transport spectroscopy in an artificial molecule

In this letter we present an experimental realization of the quantum mechanics textbook example of two interacting electronic quantum states that hybridize forming a molecular state. In our particular realization, the quantum states themselves are fabricated as quantum dots in a molecule, a carbon nanotube. For sufficient quantum-mechanical interaction (tunnel coupling) between the two quantum states, the molecular wavefunction is a superposition of the two isolated (dot) wavefunctions. As a result, the electron becomes delocalized and a covalent bond forms. In this work, we show that electrical transport can be used as a sensitive probe to measure the relative weight of the two components in the superposition state as a function of the gate-voltages. For the field of carbon nanotube double quantum dots, the findings represent an additional step towards the engineering of quantum states.

cond-mat.mes-hall↗

arXiv2006

Defining and controlling double quantum dots in single-walled carbon nanotubes

We report the experimental realization of double quantum dots in single-walled carbon nanotubes. The device consists of a nanotube with source and drain contact, and three additional top-gate electrodes in between. We show that, by energizing these top-gates, it is possible to locally gate a nanotube, to create a barrier, or to tune the chemical potential of a part of the nanotube. At low temperatures we find (for three different devices) that in certain ranges of top-gate voltages our device acts as a double quantum dot, evidenced by the typical honeycomb charge stability pattern.

cond-mat.mes-hall↗

arXiv2006

Molecular states in carbon nanotube double quantum dots

We report electrical transport measurements through a semiconducting single-walled carbon nanotube (SWNT) with three additional top-gates. At low temperatures the system acts as a double quantum dot with large inter-dot tunnel coupling allowing for the observation of tunnel-coupled molecular states extending over the whole double-dot system. We precisely extract the tunnel coupling and identify the molecular states by the sequential-tunneling line shape of the resonances in differential conductance.

cond-mat.mes-hall↗

arXiv2004

Kondo resonance in a nanotube quantum dot coupled to a normal and a superconducting lead

We report on electrical transport measurements through a carbon nanotube quantum dot coupled to a normal and a superconducting lead. The ratio of Kondo temperature and superconducting gap $T_{K}/Δ$ is identified to govern the transport properties of the system. In the case of $T_{K}<Δ$ the conductance resonance splits into two resonances at $\pm Δ$. For the opposite scenario $T_{K}>Δ$ the conductance resonance persists, however the conductance is not enhanced compared to the normal state due to a relative asymmetry of the lead-dot couplings. Within this limit the data is in agreement with a simple model of a resonant SN-interface.

cond-mat.mes-hall↗

arXiv2004

Quantum Dot coupled to a normal and a superconducting lead

We report on electrical transport measurements in a carbon nanotube quantum dot coupled to a normal and a superconducting lead. Depending on the ratio of Kondo temperature $T_{K}$ and superconducting gap $Δ$ the zero bias conductance resonance either is split into two side-peaks or persists. We also compare our data with a simple model of a resonant level - superconductor interface.

cond-mat.mes-hall↗