In single-molecule transistors, we observe inelastic cotunneling features that correspond energetically to vibrational excitations of the molecule, as determined by Raman and infrared spectroscopy. ...This is a form of inelastic electron tunneling spectroscopy of single molecules, with the transistor geometry allowing in situ tuning of the electronic states via a gate electrode. The vibrational features shift and change shape as the electronic levels are tuned near resonance, indicating significant modification of the vibrational states. When the molecule contains an unpaired electron, we also observe vibrational satellite features around the Kondo resonance.
We have observed the Autler-Townes doublet in a superconducting AI/AIO sub(X)/AI transmon qubit that acts as an artificial atom embedded in a three-dimensional Cu microwave cavity at a temperature of ...22 mK. Using pulsed microwave spectroscopy, the three lowest transmon levels are isolated, eliminating unwanted effects of higher qubit modes and cavity modes. The long coherence time (~ 40 mu s) of the transmon enables us to observe a clear Autler-Townes splitting at drive amplitudes much smaller than the transmon level anharmonicity (177 MHz). Three-level density matrix simulations with no free parameters provide excellent fits to the data. At maximum separation, the fidelity of a dark state achieved in this experiment is estimated to be 99.6%- 99.9%.
Fabrication of coplanar waveguide resonators with internal quality factors near 106 remains challenging. Here, high-purity superconductors are implemented through metamorphic epitaxial aluminum that ...is grown via molecular beam epitaxy on silicon and sapphire substrates. X-ray diffraction and scanning transmission electron microscopy indicate an abrupt highly ordered interface that results in crystal relaxation within a few monolayers of the substrate interface and no measurable interfacial contamination. Quarter-wave coplanar waveguide resonators are fabricated using optical lithography and measured at temperatures below 100 mK. Post measurement characterization with charge contrast imaging in a scanning electron microscope identifies processing artifacts at the waveguide sidewalls, on the exposed substrate area and on the exposed aluminum surface. Of primary importance are processing induced corrosion defects on aluminum sidewalls, nanoparticle contamination, and photoresist residue that is difficult to remove without affecting the superconductor material. Likely correlations between these artifacts and the measured quality factor are discussed in context of device to device variations in resonator performance.
We report Kondo resonances in the conduction of single-molecule transistors based on transition metal coordination complexes. We find Kondo temperatures in excess of 50 K, comparable to those in ...purely metallic systems. The observed gate dependence of the Kondo temperature is inconsistent with observations in semiconductor quantum dots and a simple single-dot-level model. We discuss possible explanations of this effect, in light of electronic structure calculations.
We report electronic transport measurements of single-molecule transistor devices incorporating bipyridyl-dinitro oligophenylene-ethynylene dithiol (BPDN-DT), a molecule known to exhibit conductance ...switching in other measurement configurations. We observe hysteretic conductance switching in 8% of devices with measurable currents and find that dependence of the switching properties on gate voltage is rare when compared to other single-molecule transistor devices. This suggests that polaron formation is unlikely to be responsible for switching in these devices. We discuss this and alternative switching mechanisms.
We report on the spectrum of a superconducting transmon device coupled to a planar superconducting resonator in the strong dispersive limit where discrete peaks, each corresponding to a different ...number of photons, are resolved. A thermal population of 5.474 GHz photons at an effective resonator temperature of T = 120 mK results in a weak n = 1 photon peak along with the n = 0 photon peak in the qubit spectrum in the absence of a coherent drive on the resonator. Two-tone spectroscopy using independent coupler and probe tones reveals an Autler-Townes splitting in the thermal n = 1 photon peak. The observed effect is explained accurately using the four lowest levels of the dispersively dressed qubit-resonator system and compared to results from numerical simulations of the steady-state master equation for the coupled system.
We present resistively detected NMR measurements in induced and modulation-doped electron quantum point contacts, as well as induced hole quantum point contacts. While the magnitude of the resistance ...change and associated NMR peaks in n-type devices is in line with other recent measurements using this technique, the effect in p-type devices is too small to measure. This suggests that the hyperfine coupling between holes and nuclei in this type of device is much smaller than the electron hyperfine coupling, which could have implications in quantum information processing.
Electromigrated nanoscale junctions have proven very useful for studying electronic transport at the single-molecule scale. However, confirming that conduction is through precisely the molecule of ...interest and not some contaminant or metal nanoparticle has remained a persistent challenge, typically requiring a statistical analysis of many devices. We review how transport mechanisms in both electronic and optical measurements can be used to infer information about the nanoscale junction configuration. The electronic response to optical excitation is particularly revealing. We briefly discuss surface-enhanced Raman spectroscopy on such junctions, and present new results showing that currents due to optical rectification can provide a means of estimating the local electric field at the junction due to illumination.
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