Spin-dependent tunneling between individual superconducting bound states

Magnetic impurities on superconductors induce discrete bound levels inside the superconducting gap, known as Yu-Shiba-Rusinov (YSR) states. YSR levels are fully spin polarized such that the tunneling between YSR states depends on their relative spin orientation. Here, we use scanning tunneling spectroscopy to resolve the spin dynamics in the tunneling process between two YSR states by experimentally extracting the angle between the spins. To this end, we exploit the ratio of thermally activated and direct spectral features in the measurement to directly extract the relative spin orientation between the two YSR states. We find freely rotating spins down to 7 mK, indicating a purely paramagnetic nature of the impurities. Such a noncollinear spin alignment is essential not only for producing Majorana bound states but also as an outlook manipulating and moving the Majorana state onto the tip.

Scanning tunneling spectroscopy at the single atom scale

Markus Ternes

This thesis reports measurements at the single atom scale by using low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS). Different sample systems where analyzed with normal conducting and superconducting tips. Chapter 2 presents the theoretical aspects which have to be taken into account for a detailed analysis and a consistent interpretation of the STS measurements. In chapter 3 the creation of a hexagonally ordered superlattice of single Ce adatoms on Ag(111) is reported and understood within a scattering model of the surface state electrons with the adatoms. Furthermore, the change in the local density of states of the surface state in ordered and slightly disordered superlattices is measured and theoretically explained within a tight-binding model which allows to understand the creation and stability of the superlattice by an energy gain of the participating surface-state electrons. Because Ce atoms have a non-vanishing magnetic moment which is expected to interact with the continuous states of the supporting surface leading to a Kondo resonance, chapter 4 presents measurements on single Ce adatoms on different Ag surfaces. This chapter shows the difficulties to interpret the obtained data. For instance, bistable Ce adatoms are detected on Ag(100) which show drastical changes in their apparent height and spectral signature depending on the tunneling conditions. The possible physical processes behind these phenomena are discussed. While the results presented in the first chapters were obtained with a normal conducting tip, chapter 5 intensively discusses the opportunities superconducting tips offer in low-temperature STS measurements. Novel insight in and thorough understanding of Andreev reflection processes are obtained by using the unique possibility of having different superconducting gaps in the tip and the sample. Detailed analyses of the supercurrent at low tunneling resistances reveal tunneling currents which are not described within the standard resistivity shunted junction model, and are presumably due to self-induced tunneling or due to an additional quasiparticle tunneling channel which only exist in asymmetric junctions. Furthermore, the influence of single magnetic Co atoms inbetween the superconducting tunnel junction on the obtained spectrum is discussed.

EPFL2006

Dipole Moment of Water in Highly Vibrationally Excited States: Analysis of Photofragment Quantum-Beat Spectroscopy Measurements Using a Local-Mode Hamiltonian

Andrea Callegari, Patrice Theulé

We present here the analysis of experimental Stark effect measurements made using photofragment quantum beat spectroscopy on the 14,0->, 15,0->, 18,0+> and 14,0-> 12 > vibrational states of H2O [Callegari, A.; et al. Science 2002, 297, 993.]. To link the measured Stark coefficients with the dipole surface, we analyze our results using a coupled anharmonic oscillator model, which takes into account the local-mode nature of higly excited OH stretching vibrations in water, and the tunneling between the two equivalent bonds. The large inertial frame tilt associated with the local-mode bond stretching results in a complex interaction between rotational-, vibrational-, and tunneling-motion, all of which become deeply entangled in the Stark coefficients. A perturbational approach makes it possible to analyze the problem at increasingly higher levels of approximation and to disentangle the different contributions, according to the different time scales involved. This simple model reproduces most experimental values to within a few percent, even for these highly vibrationally excited levels, and gives valuable insight into the complex rotational and vibrational motions that link the dipole moment surface with the Stark coefficients.

2009

Tunneling spectra simulation of interacting Majorana wires

Ronny Thomale

Recent tunneling experiments on InSb hybrid superconductor-semiconductor devices have provided hope for a stabilization of Majorana edge modes in a spin-orbit quantum wire subject to a magnetic field and superconducting proximity effect. Connecting the experimental scenario with a microscopic description poses challenges of a different kind, such as accounting for the effect of interactions on the tunneling properties of the wire. We develop a density matrix renormalization group (DMRG) analysis of the tunneling spectra of interacting Majorana chains, which we explicate for the Kitaev chain model. Our DMRG approach allows us to calculate the spectral function down to zero frequency, where we analyze how the Majorana zero-bias peak is affected by interactions. For topological phase transitions between the topological and trivial superconducting phase in the Majorana wire, the bulk gap closure generically affects the proximity peaks and the Majorana peak.

Amer Physical Soc2013

Scanning tunneling spectroscopy at the single atom scale

Markus Ternes

EPFL2006