Influence of the Accumulation Layer on the Spectral Properties of Full-Shell Majorana Nanowires
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Influence of the Accumulation Layer on the Spectral Properties of Full-Shell Majorana Nanowires A. A. Kopasova, * and A. S. Mel’nikova, b a
Institute for Physics of Microstructures, Russian Academy of Sciences, GSP-105, Nizhny Novgorod, 603950 Russia b Lobachevsky State University of Nizhny Novgorod, Nizhny Novgorod, 603950 Russia *e-mail: [email protected] Received March 26, 2020; revised March 26, 2020; accepted April 2, 2020
Abstract—We study the influence of the accumulation layer on the spectral properties of semiconducting nanowires fully covered by a superconducting shell within the framework of the Bogoliubov–de Gennes equations. It is shown that both the decrease in the thickness of the layer and the increase in the ratio of the Fermi velocities in the shell and the core result in the enhancement of the spectral gap and restrict the parameter range corresponding to the topologically nontrivial phase. The presence of the accumulation layer can also lead to reentrant magnetic flux dependencies of the gap, which can be experimentally observed in measurements of charge transport through the nanowire in the Coulomb blockade conditions. Keywords: semiconducting nanowires, Majorana nanowires, superconducting proximity effect, induced superconductivity, bound states in vortices DOI: 10.1134/S1063783420090164
1. INTRODUCTION Semiconducting nanowires with strong spin-orbit interaction, large g-factor and the induced superconductivity provide a perspective platform for the search of the Majorana states in condensed matter systems [1–7]. This is related to the fact that these wires in strong magnetic fields VZ m*α2 (the Kitaev limit) exhibit the so-called spinless p-wave superconductivity, which supports the Majorana states at the wire edges [8]. Here VZ = μBgH/2 is the Zeeman splitting, μB is the Bohr magneton, H is the external magnetic field directed along the wire axis, m* is the effective mass for the electrons in the semiconductor, and α is the spin-orbit coupling constant. Topological phase transition usually occurs through the closing of the gap in the excitation spectrum of the wire Eg with a further gap reopening in the topologically nontrivial phase [9, 10]. The gap in the topologically nontrivial regime, which protects the Majorana states, is proportional to the induced gap parameter Δind and is determined by the transparency of the interface between the semiconductor (SM) and the superconductor (SC). Recently, a significant progress has been made in fabrication of hybrid structures made of InAs nanowires covered by a thin layer of superconducting Al [11–14]. In particular, the induced gap at zero magnetic field Δind ~ 0.2 meV is of the order of the superconducting gap in Al (ΔAl = 0.34 meV), which indicates a highly transparent SM/SC interface. Theoretical description
of a highly transparent SM/SC interface requires taking into account the effects of hybridization between the states in the semiconducting core and in the metal shell [15–18]. One can expect these effects to be sensitive t
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