| 研究課題/領域番号 |
26790008
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| 研究機関 | 独立行政法人理化学研究所 |
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研究代表者 |
ディーコン ラッセル 独立行政法人理化学研究所, 石橋極微デバイス工学研究室, 研究員 (40552443)
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| 研究期間 (年度) |
2014-04-01 – 2017-03-31
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| キーワード | Nanowire / Josephson Junction / Superconductor / Spin Orbit Interaction |
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| 研究実績の概要 |
To realize hybrid Ge/Si core/shell nanowire and superconductor devices we developed a method of making superconducting contacts using sputtered Molybdenum Rhenium (MoRe) films. MoRe was found to have an advantage over Niobium and Niobium alloys in that it can be annealed without deterioration in superconducting properties. We quickly found that an annealing step was critical to achieve transparent superconducting contacts (and good ohmic contacts free of Schottky barriers at dilution fridge temperatures).
We characterized the spin orbit interaction in short nanowire junctions with normal contacts using weak anti localization and found short spin orbit lengths of order of 30nm. This short length indicates a strong spin orbit interaction and is therefore very promising for the future goals of this project.
In preparation for future measurements we hope to perform in the later years of this project we developed a measurement system to perform inverse ac-Josephson effect measurements on Josephson junctions. This system incorporates microwave lines with homebuilt electrical filters installed on a dilution refrigerator. Using this system we performed measurement of Shapiro steps in strained HgTe Josephson junctions searching for evidence of Majorana Fermions. We detect anomalous features in the Shapiro steps which indicate the presence of a 4 periodic or gapless Andreev bound states which are a good indication for Majorana modes (this work is now submitted and available as a preprint arXiv:1503.05591).
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| 現在までの達成度 (区分) |
現在までの達成度 (区分)
3: やや遅れている
理由
In the initial 1 1/2 years of this project we focus on two goals. Goal 1 to study spin physics in the Ge/Si core/shell nanowires by measurement of the spin-orbit interaction, g-factor anisotropy and detection of the helical state in an applied magnetic field. Goal 2 to achieve transparent superconducting contacts to the Ge/Si core/shell nanowires.
Working toward Goal 2 we have successfully measured a promising spin-orbit length through weak anti-localization but detection of the helical state remains elusive. Recent theoretical work indicates that the measurement of 1-D subbands in DC transport is complicated by scattering due to the abrupt gate potentials at the edges of the channel. This combined with backscattering from defects and Fabry Perot resonances can mask the signature of subbands in conventional transport measurements at low temperatures. Our goal of successful detection requires a change of approach and we are now looking at alternative measurement methods such as quantum capacitance measured with a cold capacitance bridge at the mixing chamber stage of a dilution fridge.
Our goal of achieving transparent superconducting contacts initially proved more difficult than anticipated as we were unable to achieve suitable devices using Aluminium contacts (despite a report in the literature of success with this approach). The novel approach of utilizing MoRe has however solved this issue and we have already succeeded to measure supercurrents in Ge/Si nanowire Josephson junctions indicating that this approach while not yet optimized will be suitable to achieve Goal 2.
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| 今後の研究の推進方策 |
In the following year we will continue to optimize the MoRe superconducting contacts and begin fabricating hybrid devices with normal-nanowire-superconductor junctions to study the Andreev bound states and look for Majorana features in tunneling spectroscopy. Our experience gained in the previous year studying inverse ac-Josephson effect in topological insulator Josephson junctions has indicated that this approach for the nanowire junctions maybe very difficult to achieve due to the low superconducting critical currents observed for such junctions. Therefore we will investigate and alternative detection scheme based on coupling of the Josephson junction devices to a superconducting transmission line cavity. In addition we will explore alternative methods for the detection of the signature of the helical state in the subbands of the one dimensional channel.
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