Andreev Bound States in High Temperature Superconductors

Andreev bound states (ABS) at the surface of superconductors are expected for any pair potential showing a sign change in different k-directions with their spectral weight depending on the relative orientation of the surface and the pair potential. We report on the observation of ABS in HTS employing tunneling spectroscopy on bicrystal grain boundary Josephson junctions (GBJs). The tunneling spectra were studied as a function of temperature and applied magnetic field. The tunneling spectra of GBJ formed by YBCO, BSCCO, and LSCO show a pronounced zero bias conductance peak that can be interpreted in terms of Andreev bound states at zero energy that are expected at the surface of HTS having a d-wave symmetry of the order parameter. In contrast, for the most likely s-wave HTS NCCO no zero bias conductance peak was observed. Applying a magnetic field results in a shift of spectral weight from zero to finite energy. This shift is found to depend nonlinearly on the applied magnetic field. Further consequences of the Andreev bound states are discussed and experimental evidence for anomalous Meissner currents is presented.Comment: 17 pages, 10 figures, to appear in Eur. Phys. J.

Postbuckling of a Circular Plate - Comparing Different Solutions

Azisymmetric problems have been often investigated in the past. Since the problem is one-dimensional, the boundary problem is suitable for analytical investigations and acts as a benchmark for numerical methods. The postbuckling of an elastic circular plate under azisymmetric loading is investigated. An analytical description is given. Solutions by means of the perturbation method and the finite element method (axisymmetric shell element) are introduced. Numerical results are presented

Remarks on Raasch’s Hook

Finite Element’s designers have always been seeking for benchmarks to judge the capability and potentiality of a numerical method. Considering shell elements many benchmark tests have been established over the years. The Raasch challenge problem, a clamped curved hook with a tip in-plane shear load, acts as a very interesting benchmark of shell elements. The structure consists of two cylindrical shells with different curvatures. In this paper the problem is also modelled as a curved beam with a rectangular cross-section. The beam model is investigated analytically. Thus an analytical expression for the tip deflection can be obtained. Further on numerical calculations with 4-node-shell elements based on a director theory are carried out and verify the elements applicability

Termination dependent topological surface states of the natural superlattice phase Bi4_4Se3_3

We describe the topological surface states of Bi$_4$Se$_3$, a compound in the infinitely adaptive Bi$_2$-Bi$_2$Se$_3$ natural superlattice phase series, determined by a combination of experimental and theoretical methods. Two observable cleavage surfaces, terminating at Bi or Se, are characterized by angle resolved photoelectron spectroscopy and scanning tunneling microscopy, and modeled by ab-initio density functional theory calculations. Topological surface states are observed on both surfaces, but with markedly different dispersions and Kramers point energies. Bi$_4$Se$_3$ therefore represents the only known compound with different topological states on differently terminated surfaces.Comment: 5 figures references added Published in PRB: http://link.aps.org/doi/10.1103/PhysRevB.88.08110

Li₀.₆[Li₀.₂Sn₀.₈S₂] – a layered lithium superionic conductor

One of the key challenges of energy research is finding solid electrolytes with high lithium conductivities comparable to those of liquid electrolytes. In this context, developing new structural families of potential Li+ ion conductors and identifying structural descriptors for fast Li+ ion conduction to occur is key to expand the scope of viable Li+ ion conductors. Here, we report that the layered material Li0.6[Li0.2Sn0.8S2] shows a Li+ ion conductivity comparable to the currently best lithium superionic conductors (LISICONs). Li0.6[Li0.2Sn0.8S2] is composed of layers comprising edge-sharing Li/SnS6 octahedra, interleaved with both tetrahedrally and octahedrally coordinated Li+ ions. Pulsed field gradient (PFG) NMR studies on powder samples show intragrain (bulk) diffusion coefficients DNMR on the order of 10−11 m2 s−1 at room temperature, which corresponds to a conductivity σNMR of 9.3 × 10−3 S cm−1 assuming the Nernst–Einstein equation, thus putting Li0.6[Li0.2Sn0.8S2] en par with the best Li solid electrolytes reported to date. This is in agreement with impedance spectroscopy on powder pellets, showing a conductivity of 1.5 × 10−2 S cm−1. Direct current galvanostatic polarization/depolarization measurements on such samples show negligible electronic contributions (less than 10−9 S cm−1) but indicate significant contact resistance (d.c. conductivity in a reversible cell is 1.2 × 10−4 S cm−1 at 298 K). Our results suggest that the partial occupation of interlayer Li+ positions in this layered material is beneficial for its transport properties, which together with tetrahedrally coordinated Li sites provides facile Li+ ion diffusion pathways in the intergallery space between the covalent Sn(Li)S2 layers. This work therefore points to a generic design principle for new layered Li+ ion conductors based on the controlled depletion of Li+ ions in the interlayer space

Field-induced quasi-particle tunneling in the nodal-line semimetal HfSiS revealed by de Haas-van Alphen quantum oscillations

We present a de Haas–van Alphen quantum oscillation study of the Dirac nodal-line semimetal HfSiS up to 32 T to unravel the structure of the high-frequency magnetic breakdown spectrum that was previously obscured in transport experiments. Despite a threefold enhanced gap between adjacent electron and hole pockets relative to the sister compound ZrSiS, a large number of large-area magnetic breakdown orbits enclosing the nodal-loop are identified. All breakdown orbits are assigned by extracting their cyclotron masses. Moreover, one additional low-frequency magnetic breakdown orbit, previously absent in ZrSiS, is observed and attributed to the larger spin-orbit interaction in HfSiS

Electron-Hole Tunneling Revealed by Quantum Oscillations in the Nodal-Line Semimetal HfSiS

We report a study of quantum oscillations in the high-field magnetoresistance of the nodal-line semimetal HfSiS. In the presence of a magnetic field up to 31 T parallel to the c axis, we observe quantum oscillations originating both from orbits of individual electron and hole pockets, and from magnetic breakdown between these pockets. In particular, we reveal a breakdown orbit enclosing one electron and one hole pocket in the form of a “figure of eight,” which is a manifestation of Klein tunneling in momentum space, although in a regime of partial transmission due to the finite separation between the pockets. The observed very strong dependence of the oscillation amplitude on the field angle and the cyclotron masses of the orbits are in agreement with the theoretical predictions for this novel tunneling phenomenon

Treatment of transient phenomena in analysis of slag-metal-gas reaction kinetics

Equations commonly used in describing reaction kinetics are examined and the problem of applying such equations to transient processes is discussed. Three examples of transient phenomena are examined in detail. It is shown that for carbon injection into slag, the reaction can be described by employing data for carbon oxidation in CO/CO2 by assuming reaction conditions approximately halfway between those in equilibrium with the slag and those in equilibrium with carbon. It is demonstrated that, when the time averaged interfacial area is employed, the rate of reaction between slag and iron-aluminum alloys can be described by a single first order rate equation, accommodating a 300% change in interfacial area. Creation of surface area in oxygen steelmaking is discussed and a method to determine the size distribution of droplets that are generated is proposed. It is concluded that changes in conditions during reaction complicate the analysis of kinetics. However, it should be possible to develop quantitative kinetic models to describe real processes