研究課題
For investigation of spin-related phenomenon in Ge/Si nanowires we performed an study of double gated devices to search for electrical tunability of the spin-orbit interaction, a feature which has been theoretically predicted for the system. Our results currently indicate negligible electrical tuning. We are now preparing a second indirect confirmation of the strong spin-orbit interaction in the system by fabricating double quantum dot devices to measure electrical dipole spin resonance. In this direction we have developed techniques to couple nanowire devices to coplanar waveguide resonators, allowing charge readout to easily reach low electron numbers and successfully applied this to InSb nanowire devices (to appear in Applied Physics Letters).Toward the development of a method for the detection of the Majorana Fermion we have built upon our successful detection of fractional ac-Josephson effect in strained HgTe Josephson junctions in FY2013-2014. In FY 2014-2015 we studied the ac-Josephson effect in HgTe quantum wells in the quantum spin hall insulator (QSHI) regime and detect clear suppressions of odd Shapiro steps indicating the presence of Majorana bound states (preprint arXiv: 1601.08055). These studies indicate that the detection for nanowire devices using Shapiro response is very challenging and so we explored an alternative detection scheme. We couple the HgTe QSHI Josephson junctions to a transmission line and succeed to detect the Josephson emission (preprint arXiv:1603.09611) with a feature at half the Josephson energy indicating the presence of the Majorana.
4: 遅れている
In FY2012-2015 of this project the initial targets were to achieve devices in Ge/Si core/shell nanowires with well characterized spin-orbit interaction, ballistic transport and transparent superconducting contacts. Of these targets we have performed measurements of the spin-orbit interaction which indicate that it is strong but not as tunable as hoped. We have succeeded to produce suitable superconducting contacts using MoRe but with only a low yield of usable devices. Unfortunately we have been unable to achieve ballistic transport due to poor device mobilities.Toward the goal of detection we have built a measurement system which has been applied to HgTe based devices. These efforts have been very successful in detecting the presence of the Majorana modes and have highlighted additional challenges that will have to be overcome if applied to nanowire devices, in particular supercurrents are typically an order of magnitude smaller and the parameter space is significantly larger (high magnetic fields and multiple electrical gates). Finally efforts to detect the Josephson emission (which appears the best method for nanowire devices) for InSb nanowire devices have so far been hampered by excessive heating of the junctions due to noise from the cold HEMT amplifier used to readout the emission. The solution to this issue maybe to isolate the junction from the HEMT using a Josephson parametric amplifier or to develop an on-chip detection scheme using a capacitively coupled Josephson junction (which is likely incompatible with the large magnetic fields needed for the Ge/Si system).
In FY2015-2016 we will continue efforts to improve the quality of our Ge/Si nanowire devices toward realizing the Majorana Fermion. Current ideas include fabrication with cleaner dielectrics such as exfoliated hexagonal Boron Nitride. In addition we will aim to perform measurement of EDSR in double quantum dot devices to confirm and exploit the presence of the strong spin orbit interaction.Toward the detection of Majorana Fermions in hybrid devices we will continue the successful work on HgTe, aiming to fabricate devices in which single Josephson junctions are coupled to microwave resonator cavities allowing spectroscopy of the Andreev bound states. To overcome the technical barriers for integrating nanowires into our existing detection scheme we will expore the integration of a quantum noise limited Josephson parametric amplifiers for the readout of Josephson emission and attempt to apply this to either InSb or Ge/Si devices. These amplifiers will decouple the junction from the high thermal noise generated by the measurement electronics. By developing a travelling wave parametric amplifier we expect to be able to operate even in an applied magnetic field necessary for the formation of the Majorana in a nanowire.Finally as a demonstration of the successful elements developed in FY2012-2015 we will fabricate hybrid devices of Ge/Si nanwores with normal and supercodnucting contacts to search for signature of the Majorana Fermion in tunneling spectroscopy.
すべて 2016 2015
すべて 雑誌論文 (2件) (うち国際共著 1件、 査読あり 2件、 オープンアクセス 1件、 謝辞記載あり 1件) 学会発表 (7件)
Nature Communications
巻: 7 ページ: 10303
doi:10.1038/ncomms10303
Applied Physics Letters
巻: 107 ページ: 222602
10.1063/1.4936888
