In superconducting spintronics, it is essential to generate spin-triplet Cooper pairs on demand. Up to now, proposals to do so concentrate on hybrid structures in which a superconductor (SC) is combined with a magnetically ordered material (or an external magnetic field). We, instead, identify a novel way to create and isolate spin-triplet Cooper pairs in the absence of any magnetic ordering. This achievement is only possible because we drive a system with strong spin-orbit interaction-the Dirac surface states of a strong topological insulator (TI)-out of equilibrium. In particular, we consider a bipolar TI-SC-TI junction, where the electrochemical potentials in the outer leads differ in their overall sign. As a result, we find that nonlocal singlet pairing across the junction is completely suppressed for any excitation energy. Hence, this junction acts as a perfect spin-triplet filter across the SC, generating equal-spin Cooper pairs via crossed Andreev reflection.

We study the differential conductance of a time-reversal symmetric Weyl semimetalsuperconductor (N-S) junction with an s-wave superconducting state. We find that there exists an extended regime where the zero-bias differential conductance acquires the universal value e 2 /h per unit channel, independent of the pairing and chemical potentials on each side of the junction, due to a perfect cancellation of Andreev and normal reflection contributions. This universal conductance can be attributed to the interplay of the unique spin/orbital-momentum locking and s-wave pairing that couples Weyl nodes of the same chirality. We expect that the universal conductance can serve as a robust and distinct signature for time-reversal symmetric Weyl fermions, and be observed in the recently discovered time-reversal symmetric Weyl semimetals. PACS numbers:where k = (k x , k y , k z ) is the wave vector, the four-T is written in terms of annihilation operators c s,σ,k with spin indices σ =↑, ↓ and orbital indices s = A, B. Here, σ i (i = 0, x, y, z) are the 2 × 2 identity and Pauli matrices for the spin-1/2 space, and s i (i = 0, x, y, z) for the orbital space. κ 0 , α and β are real model parameters. The model (1) breaks inversion symmetry, i.e., s z H(k)s z = H(−k) by the β term, but preserves timereversal symmetry as shown by σ y H * (k)σ y = H(−k). Suppose 0 < β < κ 0 , the model (1) has four Weyl nodes at ±Q ± where Q ± = (β, 0, ±k 0 ) and k 0 = [κ 2 0 − β 2 ] 1/2 . Near the Weyl nodes, we can linearize the model (1) and rewrite it as a sum of four effective Hamiltonians, arXiv:1711.07882v1 [cond-mat.supr-con]

We study the transport properties and superconducting proximity effect in NSN junctions formed by a time-reversal symmetry broken Weyl semimetal (WSM) in proximity to an s-wave superconductor. We find that the differential conductances and induced pairing amplitudes strongly depend on the angle between the junction direction in real space and the axis separating the Weyl nodes in momentum space. We identify the influence of a chiral chemical potential, i.e., the electron population imbalance between Weyl nodes of opposite chirality, on the transport characteristics of the junction. Remarkably, we observe a net spin polarization of Cooper pairs that are generated via Andreev reflection in the two WSM regions. The spin polarization is opposite in the two WSM regions and highly sensitive to the chirality imbalance and excitation energy. arXiv:1903.06229v2 [cond-mat.supr-con]

We evaluate the microscopically relevant parameters for electrical transport of hybrid superconductorsemiconductor interfaces. In contrast to the commonly used geometrically constricted metallic systems, we focus on materials with dissimilar electronic properties like low-carrier density semiconductors combined with superconductors, without imposing geometric confinement. We find an intrinsic mode-selectivity, a directional momentum-filter, due to the differences in electronic band structure, which creates a separation of electron reservoirs each at the opposite sides of the semiconductor, while at the same time selecting modes propagating almost perpendicular to the interface. The electronic separation coexists with a transport current dominated by Andreev reflection and low elastic backscattering, both dependent on the gate-controllable electronic properties of the semiconductor.