Andreas Moor

Nematic versus ferromagnetic spin filtering of triplet Cooper pairs in superconducting spintronics

We consider two types of magnetic Josephson junctions (JJ). They are formed by two singlet superconductors S and magnetic layers between them so that the JJ is a heterostructure of the S-m/n/S-m type, where S-m includes two magnetic layers with noncollinear magnetization vectors. One layer is represented by a weak ferromagnet and another one-the spin filter-is either conducting strong ferromagnet (nematic or N-type JJ) or magnetic tunnel barrier with spin-dependent transparency (magnetic or M-type JJ). Due to spin filtering only a fully polarized triplet component penetrates the normal n wire and provides the Josephson coupling between the superconductors S. Although both filters let to pass triplet Cooper pairs with total spin S parallel to the filter axes, the behavior of nematic and magnetic JJs is completely different. Whereas in the nematic case the charge and spin currents, I-Q and I-sp, do not depend on mutual orientation of the filter axes, both currents vanish in magnetic JJ in the case of antiparallel filter axes, and change sign with reversal of the filter direction. The obtained expressions for I-Q and I-sp clearly show a duality between the superconducting phase phi and the angle alpha between the exchange fields in the weak magnetic layers.

Strong quantum interference in strongly disordered bosonic insulators.

We study the variable-range hopping of bosons in an array of sites with short-range interactions and a large characteristic coordination number. The latter leads to strong quantum interference phenomena yet allows for their analytical study. We develop a functional renormalization-group scheme that repeatedly eliminates high-energy sites properly renormalizing the tunneling between the low-energy ones. Using this approach we determine the temperature and magnetic field dependence of the hopping conductivity and find a large positive magnetoresistance. With increasing magnetic field the behavior of the conductivity crossovers from the Motts law to an activational behavior with the activation gap proportional to the magnetic field.

Amplitude Higgs Mode and Admittance in Superconductors with a Moving Condensate

We consider the amplitude (Higgs) mode in a superconductor with a condensate flow (supercurrent). We demonstrate that, in this case, the amplitude mode corresponding to oscillations δ|Δ|_{Ω}exp(iΩt) of the superconducting gap is excited by an external ac electric field E_{Ω}exp(iΩt) already in the first order in |E_{Ω}|, so that δ|Δ|_{Ω}∝(v_{0}E_{Ω}), where v_{0} is the velocity of the condensate. The frequency dependence δ|Δ|_{Ω} has a resonance shape with a maximum at Ω=2Δ. In contrast to the standard situation without the condensate flow, the oscillations of the amplitude δ|Δ(t)| contribute to the admittance Y_{Ω}. We provide a formula for admittance of a superconductor with a supercurrent. The predicted effect opens new ways of experimental investigation of the amplitude mode in superconductors and materials with superconductivity competing with other states.

Hidden Order as a Source of Interface Superconductivity

Interfacial superconductivity is observed in a variety of heterostructures composed of different materials including superconducting and nonsuperconducting (at appropriate doping and temperatures) cuprates and iron-based pnictides. The origin of this superconductivity remains in many cases unclear. Here, we propose a general mechanism of interfacial superconductivity for systems with competing order parameters. We assume that parameters characterizing the material allow formation of another order like charge- or spin-density wave competing and prevailing superconductivity in the bulk (hidden superconductivity). Diffusive electron scattering on the interface results in a suppression of this order and releasing the superconductivity. Our theory is based on the use of Ginzburg--Landau equations applicable to a broad class of systems. We demonstrate that the local superconductivity appears in the vicinity of the interface and the spatial dependence of the superconducting order parameter~

Realization of the π-state in junctions formed by multi-band superconductors with a spin-density-wave.

\Delta(x)

Inhomogeneous state in nonequilibrium superconductor/normal-metal tunnel structures: A Larkin-Ovchinnikov-Fulde-Ferrell-like phase for nonmagnetic systems

is described by the Gross--Pitaevskii equation. Solving this equation we obtain quantized values of temperature and doping levels at which~

Topological Defects in Systems with Two Competing Order Parameters: Application to Superconductors with Charge- and Spin-Density Waves

\Delta(x)

Static and dynamic properties of Josephson weak links with singlet and triplet coupling

appears. Remarkably, the local superconductivity shows up even in the case when the rival order is only slightly suppressed and may arise also on the surface of the sample (surface superconductivity).

Josephson effect in multiterminal superconductor-ferromagnet junctions coupled via triplet components

Using a simple model of multi-band superconductors, which can be applied in particular to Fe-based pnictides, we calculate the Josephson current IJ in a tunnel junction composed by such superconductors. We employ the tunneling Hamiltonian method and quasiclassical Green’s functions. We study both the case of coexistence of the superconducting (¢) and magnetic (SDW—spin density wave) order parameters and the case when only the superconducting order parameter exists. We show that the current IJ depends on the mutual orientation of magnetization of the SDW in the case of non-ideal nesting when the coexistence of superconducting and magnetic order parameters is possible as it takes place in Fe-based pnictides. It is foundthat the realization of the π-junction is possible in both cases. We compare our results for multi-band superconductors without the SDW with those obtained earlier and find that they coincide if the tunneling matrixelements arereal. Iftheseelements arecomplex, anewtermappearsintheformulafor theJosephson critical current.

Spin current in junctions composed of multi-band superconductors with a spin-density wave

We analyze non-equilibrium states in a tunnel superconductor-normal metal (NSN) structure in the presence of a tunnel current