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| 2. |
- Churchill, H. O. H., et al.
(författare)
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Superconductor-nanowire devices from tunneling to the multichannel regime: Zero-bias oscillations and magnetoconductance crossover
- 2013
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Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 87:24
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Tidskriftsartikel (refereegranskat)abstract
- We present transport measurements in superconductor-nanowire devices with a gated constriction forming a quantum point contact. Zero-bias features in tunneling spectroscopy appear at finite magnetic fields and oscillate in amplitude and split away from zero bias as a function of magnetic field and gate voltage. A crossover in magnetoconductance is observed: Magnetic fields above similar to 0.5 T enhance conductance in the low-conductance (tunneling) regime but suppress conductance in the high-conductance (multichannel) regime. We consider these results in the context of Majorana zero modes as well as alternatives, including the Kondo effect and analogs of 0.7 structure in a disordered nanowire.
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| 4. |
- Deng, Mingtang, et al.
(författare)
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Anomalous Zero-Bias Conductance Peak in a Nb-InSb Nanowire-Nb Hybrid Device.
- 2012
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Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 12:12, s. 6414-6419
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Tidskriftsartikel (refereegranskat)abstract
- Semiconductor InSb nanowires are expected to provide an excellent material platform for the study of Majorana fermions in solid state systems. Here, we report on the realization of a Nb-InSb nanowire-Nb hybrid quantum device and the observation of a zero-bias conductance peak structure in the device. An InSb nanowire quantum dot is formed in the device between the two Nb contacts. Due to the proximity effect, the InSb nanowire segments covered by the superconductor Nb contacts turn to superconductors with a superconducting energy gap Δ(InSb) ∼ 0.25 meV. A tunable critical supercurrent is observed in the device in high back gate voltage regions in which the Fermi level in the InSb nanowire is located above the tunneling barriers of the quantum dot and the device is open to conduction. When a perpendicular magnetic field is applied to the devices, the critical supercurrent is seen to decrease as the magnetic field increases. However, at sufficiently low back gate voltages, the device shows the quasi-particle Coulomb blockade characteristics and the supercurrent is strongly suppressed even at zero magnetic field. This transport characteristic changes when a perpendicular magnetic field stronger than a critical value, at which the Zeeman energy in the InSb nanowire is E(z) ∼ Δ(InSb), is applied to the device. In this case, the transport measurements show a conductance peak at the zero bias voltage and the entire InSb nanowire in the device behaves as in a topological superconductor phase. We also show that this zero-bias conductance peak structure can persist over a large range of applied magnetic fields and could be interpreted as a transport signature of Majorana fermions in the InSb nanowire.
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| 5. |
- Deng, Mingtang
(författare)
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Charge Transport in Semiconductor Nanowire Quantum Devices: From Single Quantum Dots to Topological Superconductors
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis focuses on charge transport in semiconductor InSb nanowire quantum devices, including the electron transport, the hole transport, and the Cooper pair transport. Devices in which InSb semiconductor nanowire quantum dots are coupled with normal metals, superconductors or the proximity effect induced topological superconductors are fabricated and measured. Firstly, we have fabricated and measured normal metal contacted InSb nanowire devices. In each of these devices, a quantum dot is formed in the InSb nanowire between the contacts. We report on the magnetotransport measurements performed to these quantum dot devices, and reveal several novel transport features. First, we demonstrate the ambipolar quantum dot devices in which the quantum dots can be tuned from the n-type regime to the p-type regime. The transport measurements in both of the n-type regime and the p-type regime are performed. We also show that two methods can be used to estimate the effective g-factor of the quantum dot, but they can give very different estimation in the presence of a Kondo effect. In the p-type regime of an ambipolar quantum dot, we observe conductance peaks in the stability diagram which can be attributed to the quasi-1D lead states. Secondly, we have fabricated and characterized the superconductor coupled InSb nanowire quantum dots. We probe the density of states of the quasi-particles in the superconductor contacts, via a weakly coupled InSb quantum dot. In the strongly coupled InSb nanowire-superconductor junctions, dissipationless Josephson currents are observed. A SQUIDS device is also fabricated and measured, in which an anomalous modulation of the Josephson current in the magnetic field is observed. In the medium coupling regime, we observe the signatures of the multiple Andreev reflections, the sub-gab bound states, and the Josephson current, interplaying with the Kondo effect. By adjusting the gate voltages, we can control the dot-lead coupling strength and asymmetry. Here, we report the quantum phase transition induced by tuning the dot-lead coupling and the quantum phase transition induced by a magnetic field. We have also found the coupling asymmetry is very important for the observation of the Josephson current. In the magnetic field, the evolution of the Kondo effect enhanced Josephson current is found to be strongly dependent on the energy ratio of Kondo energy and superconducting gap. Finally, an anomalous low-field suppression of the zero-bias conductance peak in the Kondo regime is observed. In the last part of the thesis, we report on our efforts to search for Majorana fermions in solid state systems. Nb-InSb nanowire quantum dot-Nb hybrid devices were fabricated and the transport measurements were performed at low temperatures for these devices. We have observed anomalous zero-bias conductance peaks emerging in finite magnetic fields in the Nb-InSb nanowire quantum dot-Nb hybrid devices as a signature of the Majorana bound states in such hybrid devices. We have also found that the zero-bias conductance peak are independent of the even-odd parity of the quasi-particle number in the quantum dots and are associated with interesting fine structures. As a validation, a Au-InSb nanowire quantum dot-Nb device is fabricated and measured. Here, signatures of Majorana bound states, i.e., the zero-bias conductance peaks in finite magnetic fields are also observed. In addition, we analyze several other mechanisms that can lead to the emergence of zero-bias conductance peaks in finite magnetic fields, and discuss the results in comparison with the signatures of Majorana bound states.
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| 6. |
- Deng, Mingtang, et al.
(författare)
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Parity independence of the zero-bias conductance peak in a nanowire based topological superconductor-quantum dot hybrid device.
- 2014
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Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 4
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Tidskriftsartikel (refereegranskat)abstract
- We explore the signatures of Majorana fermions in a nanowire based topological superconductor-quantum dot-topological superconductor hybrid device by charge transport measurements. At zero magnetic field, well-defined Coulomb diamonds and the Kondo effect are observed. Under the application of a finite, sufficiently strong magnetic field, a zero-bias conductance peak structure is observed. It is found that the zero-bias conductance peak is present in many consecutive Coulomb diamonds, irrespective of the even-odd parity of the quasi-particle occupation number in the quantum dot. In addition, we find that the zero-bias conductance peak is in most cases accompanied by two differential conductance peaks, forming a triple-peak structure, and the separation between the two side peaks in bias voltage shows oscillations closely correlated to the background Coulomb conductance oscillations of the device. The observed zero-bias conductance peak and the associated triple-peak structure are in line with Majorana fermion physics in such a hybrid topological system.
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| 9. |
- Fan, Dingxun, et al.
(författare)
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Formation of long single quantum dots in high quality InSb nanowires grown by molecular beam epitaxy
- 2015
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Ingår i: Nanoscale. - : Royal Society of Chemistry (RSC). - 2040-3372 .- 2040-3364. ; 7:36, s. 14822-14828
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Tidskriftsartikel (refereegranskat)abstract
- We report on realization and transport spectroscopy study of single quantum dots (QDs) made from InSb nanowires grown by molecular beam epitaxy (MBE). The nanowires employed are 50-80 nm in diameter and the QDs are defined in the nanowires between the source and drain contacts on a Si/SiO2 substrate. We show that highly tunable QD devices can be realized with the MBE-grown InSb nanowires and the gate-to-dot capacitance extracted in the many-electron regimes is scaled linearly with the longitudinal dot size, demonstrating that the devices are of single InSb nanowire QDs even with a longitudinal size of similar to 700 nm. In the few-electron regime, the quantum levels in the QDs are resolved and the Lande g-factors extracted for the quantum levels from the magnetotransport measurements are found to be strongly level-dependent and fluctuated in a range of 18-48. A spin-orbit coupling strength is extracted from the magnetic field evolutions of a ground state and its neighboring excited state in an InSb nanowire QD and is on the order of similar to 300 mu eV. Our results establish that the MBE-grown InSb nanowires are of high crystal quality and are promising for the use in constructing novel quantum devices, such as entangled spin qubits, one-dimensional Wigner crystals and topological quantum computing devices.
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