Electron Transport In Superconductor-Semiconductor Heterostructure Nanowires

As traditional computers approach the physical limits of performance,the study of quantum computers has begun to receive widespread attention.In quantum computing,the decoherence of qubits due to environmental perturbations is the biggest roadblock in experiments so far.Therefore,topological quantum computing based on topologically protected qubits has become an important direction in quantum computing research.Topological qubits are robust to local decoherence perturbations and can enable fault-tolerant topological quantum computing.Among various candidate platforms for topo-logical quantum computing,superconductor-semiconductor heterostructure nanowires are the main experimental research direction due to their simple theoretical model.The first experimental signature of the Majorana zero mode,the differential conduc-tance peak at zero bias,was observed in superconductor-semiconductor heterostructure nanowires as early as 2012.However,the experimentally observed zero-bias differen-tial conductance peak is significantly different from the theoretical prediction.Further theoretical studies also revealed that the zero-bias differential conductance peak is not a phenomenon specific to the Majorana zero mode.The presence of disorder or Kondo effect in the nanowires may lead to the zero-bias differential conductance peak.There-fore,the main research direction of the current Majorana nanowire experiments is to improve the quality of superconductor-semiconductor nanowires and to distinguish the origin of different types of zero-bias differential conductance peaks.To address the issue of nanowire device quality,we have effectively improved the quality of InAs/Al nanowire devices by reducing the diameter of InAs nanowires to suppress crystal defects.In addition,the use of thinner diameters is more con-ducive to achieving single subband occupancy,thereby approaching the Majorana one-dimensional electronic model.To effectively distinguish the zero-bias differential con-ductance peaks from different sources,we have implemented a dissipative electrode and have successfully observed the splitting of trivial zero-bias differential conduc-tance peaks at low temperatures based on a theory proposed in 2013.By systematically studying electron transport in thin InAs/Al nanowires and in thin InAs/Al nanowires with dissipative tunneling implementation,we draw the following main conclusions:1.Structural characterization of the thin InAs/Al nanowires reveals that twin defects or mixed crystal defects have been practically reduced or completely eliminated compared to the previous thicker InAs nanowires.At the same time,the Al films on the nanowire surface maintain a high degree of homogeneity even in the single crystal state at larger sizes.In electron transport studies of devices composed of thin InAs/Al nanowires,hard superconducting energy gaps,20)-periodic Cooper pair oscillations,zero-resistance su-percurrents,and zero-bias differential conductance peaks very close to the quantum conductance(20)~2/h)have been observed.A differential conductance plateau associ-ated with(quasi-)ballistic transport is observed in a 120 nm long Josephson junction,as well as an average transparency close to 1.These transport results indicate the high quality of thin InAs/Al nanowire devices.2.By reducing the nanowire diameter,the Gaussian-like monotonic decay of the superconducting critical current with increasing parallel magnetic field was observed for the first time in thin InAs/Al nanowires,and there was no superconducting critical current oscillation caused by interference between different conduction channels.3.By replacing the normal metal electrode with a high-resistance metal electrode,we have successfully introduced dissipative tunneling(dy-namical Coulomb blockade)in thin InAs/Al nanowires.For the first time,we found that the zero-bias differential conductance induced by various Andreev reflection processes has a power-law dependence on temperature with an exponent from 4to 8,whereis a dimensionless number characterizing the magnitude of the dissipation strength.4.In our experiments,we found that the zero-bias differential conductance peak of non-Majorana origin decreases with decreasing temperature under the effect of dissipative tunneling and eventually splits.5.By adding additional control gates and metal contact electrode at the third end of the InAs/Al nanowire,we achieved in situ tuning of the dissipation strength in such three-terminal nanowire devices.The in situ tuning of the dissipation strength was further used to split a zero-bias differential conductance peak whose value was fine-tuned to a quantum conductance.Overall,the results of this paper show that reducing the diameter of InAs/Al nanowires can effectively improve the quality of the devices.This implies that it is possible to find quantized Majorana zero-bias differential conductance peaks in thin InAs/Al nanowires.Furthermore,the experiment has demonstrated that dissipative tunneling at low temper-atures can suppress and split the zero-bias differential conductance peak induced by the trivial Andreev bound state.Taking it a step further,by improving the device structure,we have achieved in-situ tuning of the dissipation strength in nanowires,thus providing an efficient filtering tool for future Majorana bound state searches.