Bias asymmetric subgap states mimicking Majorana signatures (original) (raw)
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Energy gaps are ubiquitous in the solid state. While in superconductors the Bardeen-Cooper-Schrieffer gap comes from Cooper pairing mediated by the crystal lattice, in Mott insulators the Coulomb gap results instead from electron-electron interactions. These gaps can be populated by subgap states due to various mechanisms. Here we demonstrate the existence of \textit{Coulombic} states, a new type of subgap states arising in a device with an energy gap opened by both Cooper pairing and Coulomb repulsion. The hybrid gap is provided by a superconducting island in Coulomb blockade, while the states arise due to electron transfer between the island and a nearby quantum dot. The Coulomb interaction in the device produces an unusual excitation spectrum exhibiting broken electron-hole energy symmetry, discontinuity of spectral curves, strongly renormalized g-factors and high-degeneracy points. The repercussions of the new states for the search of Majorana states in topological superconducti...
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Realizations of Majorana fermions in solid state materials have attracted great interests recently in connection to topological order and quantum information processing. We propose a novel way to create Majorana fermions in superconductors. We show that an incipient non-collinear magnetic order turns a spin-singlet superconductor with nodes into a topological superconductor with a stable Majorana bound state in the vortex core; at a topologically-stable magnetic point defect; and on the edge. We argue that such an exotic non-Abelian phase can be realized in extended t-J models on the triangular and square lattices. It is promising to search for Majorana fermions in correlated electron materials where nodal superconductivity and magnetism are two common caricatures.
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We design and investigate an experimental system capable of entering an electron transport blockade regime in which a spin-triplet localized in the path of current is forbidden from entering a spin-singlet superconductor. To stabilize the triplet a double quantum dot is created electrostatically near a superconducting lead in an InAs nanowire. The superconducting lead is a molecular beam epitaxy grown Al shell. The shell is etched away over a wire segment to make room for the double dot and the normal metal gold lead. The quantum dot closest to the normal lead exhibits Coulomb diamonds, the dot closest to the superconducting lead exhibits Andreev bound states and an induced gap. The experimental observations compare favorably to a theoretical model of Andreev blockade, named so because the triplet double dot configuration suppresses Andreev reflections. Observed leakage currents can be accounted for by finite temperature. We observe the predicted quadruple level degeneracy points of high current and a periodic conductance pattern controlled by the occupation of the normal dot. Even-odd transport asymmetry is lifted with increased temperature and magnetic field. This blockade phenomenon can be used to study spin structure of superconductors. It may also find utility in quantum computing devices that utilize Andreev or Majorana states.
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Physical Review Letters
We design and investigate an experimental system capable of entering an electron transport blockade regime in which a spin-triplet localized in the path of current is forbidden from entering a spin-singlet superconductor. To stabilize the triplet a double quantum dot is created electrostatically near a superconducting lead in an InAs nanowire. The superconducting lead is a molecular beam epitaxy grown Al shell. The shell is etched away over a wire segment to make room for the double dot and the normal metal gold lead. The quantum dot closest to the normal lead exhibits Coulomb diamonds, the dot closest to the superconducting lead exhibits Andreev bound states and an induced gap. The experimental observations compare favorably to a theoretical model of Andreev blockade, named so because the triplet double dot configuration suppresses Andreev reflections. Observed leakage currents can be accounted for by finite temperature. We observe the predicted quadruple level degeneracy points of high current and a periodic conductance pattern controlled by the occupation of the normal dot. Even-odd transport asymmetry is lifted with increased temperature and magnetic field. This blockade phenomenon can be used to study spin structure of superconductors. It may also find utility in quantum computing devices that utilize Andreev or Majorana states.
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We study proximity-induced superconductivity on the surface of a topological insulator (TI), focusing on unconventional pairing. We find that the excitation spectrum becomes gapless for any spin-triplet pairing, such that both subgap bound states and Andreev reflection is strongly suppressed. For spin-singlet pairing, the zeroenergy surface state in the d xy -wave case becomes a Majorana fermion, in contrast to the situation realized in the topologically trivial high-T c cuprates. We also study the influence of a Zeeman field on the surface states. Both the magnitude and direction of this field is shown to strongly influence the transport properties, in contrast to the case without TI. We predict an experimental signature of the Majorana states via conductance spectroscopy.
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Near a Mott transition, strong electron correlations may enhance Cooper pairing. This is demonstrated in the Dynamical Mean Field Theory solution of a twofold-orbital degenerate Hubbard model with an inverted on-site Hund rule exchange, favoring local spin-singlet configurations. Close to the Mott insulator, which here is a local version of a valence bond insulator, a pseudogap non-Fermiliquid metal, a superconductor, and a normal metal appear, in striking similarity with the physics of cuprates. The strongly correlated s-wave superconducting state has a larger Drude weight than the corresponding normal state. The role of the impurity Kondo problem is underscored.
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Majorana bound states (MBSs) offer a promising route to fault-tolerant quantum computation, because of their non-Abelian anyonic exchange statistics. They emerge as protected boundary modes of one dimensional topological superconductors (TSCs). Due to the finite size of these TSCs the wave functions of the two MBSs can spread across the whole TSC which leads to the possibility to access both MBSs at the same end of the TSC. First, we consider a spinless metallic lead-TSC-quantum dot setup in which the Majorana system is described with a Kitaev chain. Here, we show that a pair of Fano resonances arises as a function of dot level energy in the differential conductance. In an analytical low-energy description, we show that in the case of isolated MBS, i.e. only one MBS is contacted by the lead and the second MBS is only contacted by the quantum dot, these Fano resonances are invariant under a sign change of the dot level energy. This symmetry, however, is broken as soon as we allow the...