Magnetic Field-Induced "Mirage" Gap in an Ising Superconductor

Helical Topological Superconducting Pairing at Finite Excitation Energies

We propose helical topological superconductivity away from the Fermi surface in three-dimensional time-reversal-symmetric odd-parity multiband superconductors. In these systems, pairing between electrons originating from different bands is responsible for the corresponding topological phase transition. Consequently, a pair of helical topological Dirac surface states emerges at finite excitation energies. These helical Dirac surface states are tunable in energy by chemical potential and strength of band splitting. They are protected by time-reversal symmetry combined with crystalline twofold rotation symmetry. We suggest concrete materials in which this phenomenon could be observed.

Enhancement and switch effect of equal-spin Andreev reflection in ferromagnet/insulator/Ising superconductor junctions

We study the property of equal-spin Andreev reflection (ESAR) in the ferromagnet/insulator/Ising superconductor junction where Ising spin-orbit coupling is taken into account in the insulator. It is found that the ESAR exhibits a regular oscillation with the insulating barrier, the amplitude and period of which can be effectively controlled by the chemical potentials. Compared to that in the ferromagnet/Ising superconductor junction, the ESAR is greatly increased due to the resonant mode, suggesting an enhanced spin-triplet pairing. As an application, the proposed junction may work as a switch to turn on and off the ESAR. Furthermore, the insulating barrier does not change the magnetoanisotropic period of ESAR because of the invariant symmetry of the system, however, the magnetoanisotropy is strengthened.

Quasiparticle Interference as a Direct Experimental Probe of Bulk Odd-Frequency Superconducting Pairing

We show that quasiparticle interference (QPI) due to omnipresent weak impurities and probed by Fourier transform scanning tunneling microscopy and spectroscopy acts as a direct experimental probe of bulk odd-frequency superconducting pairing. Taking the example of a conventional s-wave superconductor under applied magnetic field, we show that the nature of the QPI peaks can only be characterized by including the odd-frequency pairing correlations generated in this system. In particular, we identify that the defining feature of odd-frequency pairing gives rise to a bias asymmetry in the QPI, present generically in materials with odd-frequency pairing irrespective of its origin.

Nonunitary superconductivity in complex quantum materials

We revisit the concept of nonunitary superconductivity and generalize it to address complex quantum materials. Starting with a brief review of the notion of nonunitary superconductivity, we discuss its spectral signatures in simple models with only the spin as an internal degree of freedom. In complex materials with multiple internal degrees of freedom, there are many more possibilities for the development of nonunitary order parameters. We provide examples focusing on d-electron systems with two orbitals, applicable to a variety of materials. We discuss the consequences for the superconducting spectra, highlighting that gap openings of band crossings at finite energies can be attributed to a nonunitary order parameter if this is associated with a finite superconducting fitness matrix. We speculate that nonunitary superconductivity in complex quantum materials is in fact very common and can be associated with multiple cases of recently reported time-reversal symmetry breaking superconductors.

Ising pairing in atomically thin superconductors

Ising-type pairing in atomically thin superconducting materials has emerged as a novel means of generating devices with resilience to a magnetic field applied parallel to the two-dimensional (2D) plane. In this mini-review, we canvas the state of the field by giving a historical account of 2D superconductors with strongly enhanced in-plane upper critical fields, together with the type-I and type-II Ising pairing mechanisms. We highlight the vital role of spin-orbit coupling in these superconductors and discuss other effects such as symmetry breaking, atomic thicknesses, etc. Finally, we summarize the recent theoretical proposals and highlight the open questions, such as exploring topological superconductivity in these systems and looking for more materials with Ising pairing.