| 1. |
- di Marco, Angelo, 1984, et al.
(författare)
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Effect of photon-assisted Andreev reflection in the accuracy of a SINIS turnstile
- 2015
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Ingår i: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969. ; 92:9
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Tidskriftsartikel (refereegranskat)abstract
- We consider a hybrid single-electron transistor constituted by a gate-controlled normal-metal island (N) connected to two voltage-biased superconducting leads (S) by means of two tunnel junctions (SINIS), operated as a turnstile. We show that the exchange of photons between this system and the high-temperature electromagnetic environment where it is embedded enhances Andreev reflection, thereby limiting the single-electron tunneling accuracy.
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| 2. |
- Mannila, E. T., et al.
(författare)
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A superconductor free of quasiparticles for seconds
- 2021
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Ingår i: Nature Physics. - : Springer Science and Business Media LLC. - 1745-2473 .- 1745-2481.
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Tidskriftsartikel (refereegranskat)abstract
- Superconducting devices, based on the Cooper pairing of electrons, play an important role in existing and emergent technologies, ranging from radiation detectors1,2 to quantum computers3. Their performance is limited by spurious quasiparticle excitations formed from broken Cooper pairs4–12. Efforts to achieve ultra-low quasiparticle densities have reached time-averaged numbers of excitations on the order of one in state-of-the-art devices2,12–15. However, the dynamics of the quasiparticle population as well as the timescales for adding and removing individual excitations remain largely unexplored. Here, we experimentally demonstrate a superconductor completely free of quasiparticles for periods lasting up to seconds. We monitor the quasiparticle number on a mesoscopic superconductor in real time by measuring the charge tunnelling to a normal metal contact. Quiet, excitation-free periods are interrupted by random-in-time Cooper pair breaking events, followed by a burst of charge tunnelling within a millisecond. Our results demonstrate the possibility of operating devices without quasiparticles with potentially improved performance. In addition, our experiment probes the origins of nonequilibrium quasiparticles in our device. The decay of the Cooper pair breaking rate over several weeks following the initial cooldown rules out processes arising from cosmic or long-lived radioactive sources16–19.
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| 3. |
- Nguyen, H. Q., et al.
(författare)
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Electrostatic control of quasiparticle poisoning in a hybrid semiconductor-superconductor island
- 2023
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Ingår i: Physical Review B. - 2469-9950. ; 108:4
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Tidskriftsartikel (refereegranskat)abstract
- The performance of superconducting devices is often degraded by the uncontrolled appearance and disappearance of quasiparticles, a process known as poisoning. We demonstrate the electrostatic control of quasiparticle poisoning in the form of single-charge tunneling across a fixed barrier onto a Coulomb island in an InAs/Al hybrid nanowire. High-bandwidth charge sensing was used to monitor the charge occupancy of the island across Coulomb blockade peaks, where tunneling rates were maximal, and Coulomb valleys, where tunneling was absent. Electrostatic gates changed the on-peak tunneling rates by two orders of magnitude for a barrier with fixed normal-state resistance, which we attribute to the gate dependence of the size and softness of the induced superconducting gap on the island, corroborated by separate density-of-states measurements. Temperature and magnetic field dependence of tunneling rates are also investigated.
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| 4. |
- Aref, Thomas, 1980, et al.
(författare)
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Andreev tunneling in charge pumping with SINIS turnstiles
- 2011
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Ingår i: Europhysics Letters. - : IOP Publishing. - 0295-5075 .- 1286-4854. ; 96:3
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Tidskriftsartikel (refereegranskat)abstract
- We present measurements on hybrid single-electron turnstiles with superconducting leads contacting a normal island (SINIS). We observe Andreev tunneling of electrons influencing the current plateau characteristics of the turnstiles under radio-frequency pumping. The data is well accounted for by numerical simulations. We verify the dependence of the Andreev tunneling rate on the turnstile's charging energy. Increasing the charging energy effectively suppresses the Andreev current.
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| 5. |
- Aref, Thomas, 1980, et al.
(författare)
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Characterization of aluminum oxide tunnel barriers by combining transport measurements and transmission electron microscopy imaging
- 2014
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 116:7
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Tidskriftsartikel (refereegranskat)abstract
- We present two approaches for studying the uniformity of a tunnel barrier. The first approach is based on measuring single-electron and two-electron tunneling in a hybrid single-electron transistor. Our measurements indicate that the effective area of a conduction channel is about one order of magnitude larger than predicted by theoretical calculations. With the second method, transmission electron microscopy, we demonstrate that variations in the barrier thickness are a plausible explanation for the larger effective area and an enhancement of higher order tunneling processes.
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| 6. |
- Barker, D., et al.
(författare)
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Individually addressable double quantum dots formed with nanowire polytypes and identified by epitaxial markers
- 2019
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 114:18
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Tidskriftsartikel (refereegranskat)abstract
- Double quantum dots (DQDs) hold great promise as building blocks for quantum technology as they allow for two electronic states to coherently couple. Defining QDs with materials rather than using electrostatic gating allows for QDs with a hard-wall confinement potential and more robust charge and spin states. An unresolved problem is how to individually address these QDs, which is necessary for controlling quantum states. We here report the fabrication of DQD devices defined by the conduction band edge offset at the interface of the wurtzite and zinc blende crystal phases of InAs in nanowires. By using sacrificial epitaxial GaSb markers selectively forming on one crystal phase, we are able to precisely align gate electrodes allowing us to probe and control each QD independently. We hence observe textbooklike charge stability diagrams, a discrete energy spectrum, and electron numbers consistent with theoretical estimates and investigate the tunability of the devices, finding that changing the electron number can be used to tune the tunnel barrier as expected by simple band diagram arguments.
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| 7. |
- Dalelkhan, B., et al.
(författare)
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Ambipolar transport in narrow bandgap semiconductor InSb nanowires
- 2020
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Ingår i: Nanoscale. - : Royal Society of Chemistry (RSC). - 2040-3364 .- 2040-3372. ; 12:15, s. 8159-8165
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Tidskriftsartikel (refereegranskat)abstract
- We report on a transport measurement study of top-gated field effect transistors made out of InSb nanowires grown by chemical vapor deposition. The transistors exhibit ambipolar transport characteristics revealed by three distinguished gate-voltage regions: In the middle region where the Fermi level resides within the bandgap, the electrical resistance shows an exponential dependence on temperature and gate voltage. With either more positive or negative gate voltages, the devices enter the electron and hole transport regimes, revealed by the resistance decreasing linearly with decreasing temperature. From the transport measurement data of a 1 μm-long device made from a nanowire of 50 nm in diameter, we extracted a bandgap energy of 190-220 meV. The off-state current of this device is found to be suppressed within the measurement noise at a temperature of T = 4 K. A shorter, 260 nm-long device is found to exhibit a finite off-state current and a circumference-normalized on-state hole current of 11 μA μm-1 at VD = 50 mV which is the highest for such a device to our knowledge. The ambipolar transport characteristics make the InSb nanowires attractive for CMOS electronics, hybrid electron-hole quantum systems and hole based spin qubits.
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| 8. |
- Göransson, D. J.O., et al.
(författare)
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Coulomb blockade from the shell of an InP-InAs core-shell nanowire with a triangular cross section
- 2019
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 114:5
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Tidskriftsartikel (refereegranskat)abstract
- We report on growth of InP-InAs core-shell nanowires and demonstration of the formation of single quantum structures, which show the Coulomb blockade effect, over entire lengths of the nanowires. The core-shell nanowires are grown by a selective area growth technique via metal-organic vapor phase epitaxy. The as-grown core-shell nanowires are found to be of wurtzite crystals. The InP cores have a hexagonal cross section, while the InAs shells are grown preferentially on specific { 1 1 ¯ 00} facets, leading to the formation of the core-shell nanowires with an overall triangular cross section. The grown core-shell nanowires are transferred onto a Si/SiO 2 substrate and then contacted by several narrow metal electrodes. Low-temperature transport measurements show the Coulomb-blockade effect. We analyze the measured gate capacitance and single electron charging energy of the devices and demonstrate that a quantum structure which shows the Coulomb blockade effect of a many-electron quantum dot is formed over the full length of a single core-shell nanowire and consists of the entire InAs shell in the nanowire.
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| 9. |
- Hofmann, A., et al.
(författare)
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Phonon spectral density in a GaAs/AlGaAs double quantum dot
- 2020
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Ingår i: Physical Review Research. - 2643-1564. ; 2:3
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Tidskriftsartikel (refereegranskat)abstract
- We study phonon emission in a GaAs/AlGaAs double quantum dot by monitoring the tunneling of a single electron between the two dots. We prepare the system such that a known amount of energy is emitted in the transition process. The energy is converted into lattice vibrations, and the resulting tunneling rate depends strongly on the phonon scattering and its effective phonon spectral density. We are able to fit the measured transition rates and see imprints of interference of phonons with themselves causing oscillations in the transition rates.
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| 10. |
- Mannila, E. T., et al.
(författare)
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Detecting parity effect in a superconducting device in the presence of parity switches
- 2019
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Ingår i: Physical Review B. - 2469-9950. ; 100:2
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Tidskriftsartikel (refereegranskat)abstract
- We present a superconducting device showing a clear parity effect in the number of electrons, even when there is, on average, a single nonequilibrium quasiparticle present and the parity of the island switches due to quasiparticles tunneling in and out of the device at rates on the order of 100 Hz. We detect the switching by monitoring in real time the charge state of a superconducting island connected to normal leads by tunnel junctions. The quasiparticles are created by Cooper pairs breaking on the island at a rate of a few kilohertz. We demonstrate that the pair breaking is caused by the backaction of the single-electron transistor used as a charge detector. With sufficiently low probing currents, our superconducting island is free of quasiparticles 97% of the time.
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