Tobias Meng

Self-consistent description of Andreev bound states in Josephson quantum dot devices

We develop a general perturbative framework based on a superconducting atomic limit for the description of Andreev bound states (ABS) in interacting quantum dots connected to superconducting leads. A local effective Hamiltonian for dressed ABS, including both the atomic (or molecular) levels and the induced proximity effect on the dot is argued to be a natural starting point. A self-consistent expansion in single-particle tunneling events is shown to provide accurate results even in regimes where the superconducting gap is smaller than the atomic energies, as demonstrated by a comparison to recent numerical renormalization group calculations. This simple formulation may have bearings for interpreting Andreev spectroscopic experiments in superconducting devices, such as scanning tunnel microscope measurements on carbon nanotubes or radiative emission in optical quantum dots.

Non-Abelian parafermions in time-reversal-invariant interacting helical systems

The interplay between bulk spin-orbit coupling and electron-electron interactions produces umklapp scattering in the helical edge states of a two-dimensional topological insulator. If the chemical potential is at the Dirac point, umklapp scattering can open a gap in the edge state spectrum even if the system is time-reversal invariant. We determine the zero-energy bound states at the interfaces between a section of a helical liquid which is gapped out by the superconducting proximity effect and a section gapped out by umklapp scattering. We show that these interfaces pin charges which are multiples of e/2, giving rise to a Josephson current with 8 pi periodicity. Moreover, the bound states, which are protected by time-reversal symmetry, are fourfold degenerate and can be described as Z(4) parafermions. We determine their braiding statistics and show how braiding can be implemented in topological insulator systems.

Optical magnetic circular dichroism in threshold photoemission from a magnetite thin film

Threshold photoemission excited by polarization-modulated ultraviolet femtosecond laser light is exploited for phase-sensitive detection of magnetic circular dichroism (MCD) for a magnetite thin film. Magnetite (Fe(3)O(4)) shows a magnetic circular dichroism of ∼(4.5 ± 0.3) × 10(-3) for perpendicularly incident circularly polarized light and a magnetization vector switched parallel and antiparallel to the helicity vector by an external magnetic field. The asymmetry in threshold photoemission is discussed in comparison to the magneto-optical Kerr effect. The optical MCD contrast in threshold photoemission will provide a basis for future laboratory photoemission studies on magnetic surfaces.

Superconducting gap renormalization around two magnetic impurities: From Shiba to Andreev bound states

We study the renormalization of the gap of an s-wave superconductor in the presence of two magnetic impurities. For weakly bound Shiba states, we analytically calculate the part of the gap renormalization that is sensitive to the relative orientation of the two impurity spins. For impurities with a strong exchange coupling to the conduction electrons, we solve the gap equation self-consistently by numerics and find that the subgap Shiba state turns into a supragap Andreev state when the local gap parameter changes sign under the impurities.

Helical nuclear spin order in two-subband quantum wires

In quantum wires, the hyperfine coupling between conduction electrons and nuclear spins can lead to a (partial) ordering of both of them at low temperatures. By an interaction-enhanced mechanism, the nuclear spin order, caused by RKKY exchange, acts back onto the electrons and gaps out part of their spectrum. In wires with two subbands characterized by distinct Fermi momenta kF1 and kF2, the nuclear spins form a superposition of two helices with pitches {\pi}/kF1 and {\pi}/kF2, thus exhibiting a beating pattern. This order results in a reduction of the electronic conductance in two steps upon lowering the temperature.

Helical nuclear spin order in a strip of stripes in the quantum Hall regime

We investigate nuclear spin effects in a two-dimensional electron gas in the quantum Hall regime modeled by a weakly coupled array of interacting quantum wires. We show that the presence of hyperfine interaction between electron and nuclear spins in such wires can induce a phase transition, ordering electrons and nuclear spins into a helix in each wire. Electron-electron interaction effects, pronounced within the one-dimensional stripes, boost the transition temperature up to tens to hundreds of millikelvins in GaAs. We predict specific experimental signatures of the existence of nuclear spin order, for instance for the resistivity of the system at transitions between different quantum Hall plateaus.

Strongly anisotropic spin response as a signature of the helical regime in Rashba nanowires

Rashba nanowires in a magnetic field exhibit a helical regime when the spin-orbit momentum is close to the Fermi momentum,

Impurity-induced quantum phase transitions and magnetic order in conventional superconductors: Competition between bound and quasiparticle states

{k}_{F}\ensuremath{\approx}{k}_{SO}

Low-energy properties of fractional helical Luttinger liquids

. We show that this regime is characterized by a strongly anisotropic electron-spin susceptibility, with an exponentially suppressed signal along one direction in spin space, and that there are no low-frequency spin fluctuations along this direction. Since the spin response in the gapless regime

Missing shapiro steps and the 8PI-periodic Josephson effect in the interacting helical electron systems

{k}_{F}\ifmmode\neg\else\textlnot\fi{}\ensuremath{\approx}{k}_{SO}