| 1. |
- Broman, S. L., et al.
(author)
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Dihydroazulene Photoswitch Operating in Sequential Tunneling Regime: Synthesis and Single-Molecule Junction Studies
- 2012
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In: Advanced Functional Materials. - : Wiley. - 1616-3028 .- 1616-301X. ; 22:20, s. 4249-4258
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Journal article (peer-reviewed)abstract
- Molecular switches play a central role for the development of molecular electronics. In this work it is demonstrated that the reproducibility and robustness of a single-molecule dihydroazulene (DHA)/vinylheptafulvene (VHF) switch can be remarkably enhanced if the switching kernel is weakly coupled to electrodes so that the electron transport goes by sequential tunneling. To assure weak coupling, the DHA switching kernel is modified by incorporating p-MeSC6H4 end-groups. Molecules are prepared by Suzuki cross-couplings on suitable halogenated derivatives of DHA. The synthesis presents an expansion of our previously reported brominationeliminationcross-coupling protocol for functionalization of the DHA core. For all new derivatives the kinetics of DHA/VHF transition has been thoroughly studied in solution. The kinetics reveals the effect of sulfur end-groups on the thermal ring-closure of VHF. One derivative, incorporating a p-MeSC6H4 anchoring group in one end, has been placed in a silver nanogap. Conductance measurements justify that transport through both DHA (high resistivity) and VHF (low resistivity) forms goes by sequential tunneling. The switching is fairly reversible and reenterable; after more than 20 ON-OFF switchings, both DHA and VHF forms are still recognizable, albeit noticeably different from the original states.
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| 2. |
- Burnett, J., et al.
(author)
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PC2: Identifying noise processes in superconducting resonators
- 2013
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In: 2013 IEEE 14th InternationalSuperconductive Electronics Conference, ISEC 2013. - 9781467363716 ; , s. Art. no. 6604284-
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Conference paper (peer-reviewed)abstract
- Extensive studies of dielectric loss due to two level fluctuators (TLFs) in superconducting resonators have provided routes for low loss resonators. The research is motivated not only by the use of resonators as detectors and in quantum information processing, but more generally due to TLFs being a source of noise and decoherence in all quantum devices. In this work a frequency locked loop was used to measure frequency fluctuations at timescales in excess of 104 seconds, thereby accurately probing the TLF induced low- frequency noise of the resonator. Our measurement method lead to very high statistical confidence even for very long timescales, and here we can therefore present results explicitly identifying power dependent flicker frequency noise (S = 1/fa where a=1) persisting down to the mHz level.
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| 3. |
- Christensen, M. A., et al.
(author)
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Mixed valence radical cations and intermolecular complexes derived from indenofluorene-extended tetrathiafulvalenes
- 2014
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In: Journal of Materials Chemistry C. - : Royal Society of Chemistry (RSC). - 2050-7534 .- 2050-7526. ; 2:48, s. 10428-10438
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Journal article (peer-reviewed)abstract
- Engineering of mixed-valence (MV) radical cations and intermolecular complexes based on pi-extended tetrathiafulvalenes (TTFs) is central for the development of organic conductors. On another front, redox-controlled dimerization of radical cations has recently been recognized as an important tool in supramolecular chemistry. Here we show that pi-extended TTFs based on the indenofluorene core, prepared by Horner-Wadsworth-Emmons reactions, undergo reversible and stepwise one-electron oxidations and that the detectable, intermediate radical cation forms remarkably strong intermolecular MV ([neutral.cation]) and pi-dimer ([cation.cation]) complexes with near-infrared radical cation absorptions. The radical cation itself seems to be a so-called Class III MV species in the Robin-Day classification. The formation of MV dimers was corroborated by ESR spectroelectrochemical studies, revealing two slightly different ESR signals upon oxidation, one assigned to the MV dimer and the other to the cation monomer. Crystals of the radical cation with different anions (PF6- , BF4-, and TaF6-) were grown by electrocrystallization. Conductance studies revealed that the salts behave as semiconductors with the hexafluorotantalate salt exhibiting the highest conductance. Using a custom-built ESR spectrometer with sub-femtomole sensitivity, the magnetic properties of one crystal were investigated. While the spin-to-spin interaction between radical cations was negligible, a high cooperativity coupling to the microwave field was observed - as a result of an exceptionally narrow spin line width and high spin density. This could have great potential for applications in quantum computation where crystalline spin ensembles are exploited for their long coherence times.
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| 4. |
- de Graaf, Sebastian Erik, 1986, et al.
(author)
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A near-field scanning microwave microscope based on a superconducting resonator for low power measurements
- 2013
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In: Review of Scientific Instruments. - : AIP Publishing. - 1089-7623 .- 0034-6748. ; 84:2, s. 023706-
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Journal article (peer-reviewed)abstract
- We report on the design and performance of a cryogenic (300 mK) near-field scanning microwave microscope. It uses a microwave resonator as the near-field sensor, operating at a frequency of 6 GHz and microwave probing amplitudes down to 100 μV, approaching low enough photon population (N ∼ 1000) of the resonator such that coherent quantum manipulation becomes feasible. The resonator is made out of a miniaturized distributed fractal superconducting circuit that is integrated with the probing tip, micromachined to be compact enough such that it can be mounted directly on a quartz tuning-fork, and used for parallel operation as an atomic force microscope (AFM). The resonator is magnetically coupled to a transmission line for readout, and to achieve enhanced sensitivity we employ a Pound-Drever-Hall measurement scheme to lock to the resonance frequency. We achieve a well localized near-field around the tip such that the microwave resolution is comparable to the AFM resolution, and a capacitive sensitivity down to 6.4 × 10−20 F/rtHz, limited by mechanical noise. We believe that the results presented here are a significant step towards probing quantum systems at the nanoscale using near-field scanning microwave microscopy.
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| 5. |
- de Graaf, Sebastian Erik, 1986, et al.
(author)
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Accurate Real-Time Monitoring of Quality Factor and Center Frequency of Superconducting Resonators
- 2014
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In: IEEE Transactions on Applied Superconductivity. - : Institute of Electrical and Electronics Engineers (IEEE). - 1558-2515 .- 1051-8223. ; 24:6
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Journal article (peer-reviewed)abstract
- In this paper, we track both center frequency and quality factor of a superconducting microwave resonator with very high precision using a modified Pound-Drever-Hall frequency locking scheme. The quality factor measurements can be done with high enough bandwidth to study different processes in a superconducting resonator in real time. With this added functionality, the frequency readout remains unaffected, with typical accuracy around 10-30 Hz/root Hz, whereas the cavity loss rate can be determined to within 0.1% on a millisecond time scale (34 dB/root Hz). As we demonstrate, this can be particularly useful for electron spin resonance measurements, where we achieve a minimum number of detectable spins coupled to the resonator N = 5 . 10(5) spins/root Hz, based on quality factor measurements. It also enables the study of fast dynamic processes, for example, vortex dynamics, in these devices on a short time scale.
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| 6. |
- de Graaf, Sebastian Erik, 1986, et al.
(author)
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Charge Qubit Coupled to an Intense Microwave Electromagnetic Field in a Superconducting Nb Device: Evidence for Photon-Assisted Quasiparticle Tunneling
- 2013
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In: Physical Review Letters. - 1079-7114 .- 0031-9007. ; 111:13, s. Art. no. 137002-
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Journal article (peer-reviewed)abstract
- We study a superconducting charge qubit coupled to an intensive electromagnetic field and probe changes in the resonance frequency of the formed dressed states. At large driving strengths, exceeding the qubit energy-level splitting, this reveals the well known Landau-Zener-Stückelberg interference structure of a longitudinally driven two-level system. For even stronger drives, we observe a significant change in the Landau-Zener-Stückelberg pattern and contrast. We attribute this to photon-assisted quasiparticle tunneling in the qubit. This results in the recovery of the qubit parity, eliminating effects of quasiparticle poisoning, and leads to an enhanced interferometric response. The interference pattern becomes robust to quasiparticle poisoning and has a good potential for accurate charge sensing.
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| 7. |
- de Graaf, Sebastian Erik, 1986, et al.
(author)
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Galvanically split superconducting microwave resonators for introducing internal voltage bias
- 2014
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In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 104:5, s. art no 052601 -
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Journal article (peer-reviewed)abstract
- We present the design and performance of high-Q superconducting niobium nitride microwave resonators intended for use in hybrid quantum systems, coupling spin degrees of freedom to the cavity mode, both magnetically and electrically. We demonstrate a solution that allows to introduce static electric fields in the resonator without compromising the microwave performance. Quality factors above 10(5) remain unchanged in strong applied static electric fields above 10 MV/m and magnetic fields up to similar to 400 mT. By design, the configuration of the dc field matches that of the microwave field, especially advantageous for experiments on electrostatically controlled spin systems.
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| 8. |
- de Graaf, Sebastian Erik, 1986, et al.
(author)
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Magnetic field resilient superconducting fractal resonators for coupling to free spins
- 2012
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In: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 112:12, s. 123905-
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Journal article (peer-reviewed)abstract
- We demonstrate a planar superconducting microwave resonator intended for use in applications requiring strong magnetic fields and high quality factors. In perpendicular magnetic fields of 20 mT, the niobium resonators maintain a quality factor above 25 000 over a wide range of applied powers, down to single photon population. In parallel field, the same quality factor is observed above 160 mT, the field required for coupling to free spins at a typical operating frequency of 5 GHz. We attribute the increased performance to the current branching in the fractal design. We demonstrate that our device can be used for spectroscopy by measuring the dissipation from a pico-mole of molecular spins.
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| 9. |
- Lara Avila, Samuel, 1983, et al.
(author)
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Bianthrone in a Single-Molecule Junction: Conductance Switching with a Bistable Molecule Facilitated by Image Charge Effects
- 2010
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In: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 114:48, s. 20686-20695
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Journal article (peer-reviewed)abstract
- Bianthrone is a sterically hindered compound that exists in the form of two nonplanar isomers. Our experimental study of single-molecule junctions with bianthrone reveals persistent switching of electric conductance at low temperatures, which can be reasonably associated with molecular isomerization events. Temperature dependence of the switching rate allows for an estimate of the activation energy of the process, on the order of 120 +/- 50 meV. Quantum-chemical calculations of the potential energy relief of neutral bianthrone and its anion, including identification of transition states, yields the isolated molecule isomerization barriers too high vs the previous estimate, though compatible with previous experimental studies in solution. Nevertheless, we show that the attraction of the anion in the vicinity of the metal surface by its image charge can change the energetic landscape, in particular, by significantly reducing the barrier to values compatible with the observed switching behavior.
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| 10. |
- Lara Avila, Samuel, 1983, et al.
(author)
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Light-Triggered Conductance Switching in Single-Molecule Dihydroazulene/Vinylheptafulvene Junctions
- 2011
-
In: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 115:37, s. 18372-18377
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Journal article (peer-reviewed)abstract
- Derivatives of 1,1-dicyano-1,8a-dihydroazulene (DHA) undergo light-induced ring-opening to a corresponding vinylheptafulvene (VHF), which in turn is thermally reverted to DHA. Here we have fabricated single-molecule DHA/VHF junctions and measured light-triggered conductance switching of these junctions. The DHA/VHF system studied includes a substituent group at the seven-membered ring. Light-induced conversion of this DHA to VHF in the junction is supported by a reduced tunnelling gap in tunnelling density of states. In fact, the reduced tunnelling gap corresponds to the reduced HOMO-LUMO gap of VHF relative to that of DHA as measured by absorption spectroscopy and electrochemistry. For this comparison, electrochemical measurements were performed on both the parent DHA/VHF system and the functionalized system that was subject to transport measurements. In one junction, it was possible to switch back and forth between DHA and VHF three times, the forward reaction being induced by light and the back-reaction occurring after waiting a period of time.
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| 11. |
- Leppäkangas, Juha, 1979, et al.
(author)
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Effects of quasiparticle tunnelling in a circuit-QED realization of a strongly driven two-level system
- 2013
-
In: Journal of Physics B: Atomic, Molecular and Optical Physics. - : IOP Publishing. - 1361-6455 .- 0953-4075. ; 46:22, s. Art. no. 224019-
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Journal article (peer-reviewed)abstract
- We experimentally and theoretically study the frequency shift of a driven cavity coupled to a superconducting charge qubit. In addition to previous studies, here we also consider drive strengths large enough to energetically allow for quasiparticle creation. Quasiparticle tunnelling leads to the inclusion of more than two charge states in the dynamics. To explain the observed effects, we develop a master equation for the microwave dressed charge states, including quasiparticle tunnelling. A bimodal behaviour of the frequency shift as a function of gate voltage can be used for sensitive charge detection. However, at weak drives, the charge sensitivity is significantly reduced by nonequilibrium quasiparticles, which induce transitions to a non-sensitive state. Unexpectedly, at high-enough drives, the quasiparticle tunnelling enables a very fast relaxation channel to the sensitive state. In this regime, the charge sensitivity is thus robust against externally injected quasiparticles and the desired dynamics prevail over a broad range of temperatures. We find very good agreement between the theory and experiment over a wide range of drive strengths and temperatures.
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| 12. |
- Yager, Thomas, 1987, et al.
(author)
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Express Optical Analysis of Epitaxial Graphene on SiC: Impact of Morphology on Quantum Transport
- 2013
-
In: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 13:9, s. 4217-4223
-
Journal article (peer-reviewed)abstract
- We show that inspection with an optical microscope allows surprisingly simple and accurate identification of single and multilayer graphene domains in epitaxial graphene on silicon carbide (SiC/G) and is informative about nanoscopic details of the SiC topography, making it ideal for rapid and noninvasive quality control of as-grown SiC/G. As an illustration of the power of the method, we apply it to demonstrate the correlations between graphene morphology and its electronic properties by quantum magneto-transport.
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