Using Disorder to Identify Bogoliubov Fermi-Surface States.

We argue that a superconducting state with a Fermi surface of Bogoliubov quasiparticles, a Bogoliubov Fermi surface (BG-FS), can be identified by the dependence of physical quantities on disorder. In particular, we show that a linear dependence of the residual density of states at weak disorder distinguishes a BG-FS state from other nodal superconducting states. We further demonstrate the stability of supercurrent against impurities and a characteristic Drude-like behavior of the optical conductivity. Our results can be directly applied to electron irradiation experiments on candidate materials of BG-FSs, including Sr_{2}RuO_{4}, FeSe_{1-x}S_{x}, and UBe_{13}.

Orbital Angular Momentum Induced Spin Polarization of 2D Metallic Bands.

The electrons in 2D systems with broken inversion symmetry are spin-polarized due to spin-orbit coupling and provide perfect targets for observing exotic spin-related fundamental phenomena. We observe a Fermi surface with a novel spin texture in the 2D metallic system formed by indium double layers on Si(111) and find that the primary origin of the spin-polarized electronic states of this system is the orbital angular momentum and not the so-called Rashba effect. The present results deepen the understanding of the physics arising from spin-orbit coupling in atomic-layered materials with consequences for spintronic devices and the physics of the superconducting state.

Superconductivity on Edge: Evidence of a One-Dimensional Superconducting Channel at the Edges of Single-Layer FeTeSe Antiferromagnetic Nanoribbons.

How superconductivity emerges from antiferromagnetic ordering is an essential question for Fe-based superconductors. Here, we explore the effect of dimensionality on the interplay between antiferromagnetic ordering and superconductivity by investigating nanoribbons of single-layer FeTe1-xSex films grown on SrTiO3(001) substrates by molecular beam epitaxy. Using scanning tunneling microscopy/spectroscopy, we find a one-dimensional (1D) superconducting channel 2 nm wide with a TC of 42 ± 4 K on the edge of FeTe1-xSex (x < 0.1) ribbons, coexisting with a non-superconducting ribbon interior that remains bicollinear antiferromagnetically ordered. Density functional theory calculations indicate that both Se and the presence of the edge destabilize the bicollinear antiferromagnetic magnetic order, resulting in a paramagnetic region near the edge with strong local checkerboard fluctuations that is conducive to superconductivity. Our results represent the highest TC achieved in 1D superconductors and demonstrate an effective route toward stabilizing superconductivity in Fe-based superconductors at reduced dimensions.

Evidence for d-Wave Superconductivity in Single Layer FeSe/SrTiO3 Probed by Quasiparticle Scattering Off Step Edges.

The de Gennes extrapolation length is a direction dependent measure of the spatial evolution of the pairing gap near the boundary of a superconductor and thus provides a viable means to probe its symmetry. It is expected to be infinite and isotropic for plain s-wave pairing, and finite and anisotropic for d-wave. Here, we synthesize single-layer FeSe films on SrTiO3(001) (STO) substrates by molecular beam epitaxy and measure the de Gennes extrapolation length by scanning tunneling microscopy/spectroscopy. We find a 40% reduction of the superconducting gap near specular [110]Fe edges, yielding an extrapolation length of 8.0 nm. However, near specular [010]Fe edges, the extrapolation length is nearly infinite. These findings are consistent with a phase changing pairing with 2-fold symmetry, indicating d-wave superconductivity. This is further supported by the presence of in-gap states near the specular [110]Fe edges, but not the [010]Fe edges. This work provides direct experimental evidence for d-wave superconductivity in single-layer FeSe/STO and demonstrates quasiparticle scattering at boundaries to be a viable phase sensitive probe of pairing symmetry in Fe-based superconductors.

Point-node gap structure of the spin-triplet superconductor UTe2.

Low-temperature electrical and thermal transport, magnetic penetration depth, and heat capacity measurements were performed on single crystals of the actinide superconductor UTe2 to determine the structure of the superconducting energy gap. Heat transport measurements performed with currents directed along both crystallographic a and b axes reveal a vanishingly small residual fermionic component of the thermal conductivity. The magnetic field dependence of the residual term follows a rapid, quasilinear increase consistent with the presence of nodal quasiparticles, rising toward the a-axis upper critical field where the Wiedemann-Franz law is recovered. Together with a quadratic temperature dependence of the magnetic penetration depth up to T/T c = 0.3, these measurements provide evidence for an unconventional spin-triplet superconducting order parameter with point nodes. Millikelvin specific heat measurements performed on the same crystals used for thermal transport reveal an upturn below 300 mK that is well described by a divergent quantum-critical contribution to the density of states (DOS). Modeling this contribution with a T -1/3 power law allows restoration of the full entropy balance in the superconducting state and a resultant cubic power law for the electronic DOS below T c , consistent with the point-node gap structure determined by thermal conductivity and penetration depth measurements.

Superconductivity without Inversion and Time-Reversal Symmetries.

Traditionally, in three dimensions, the only symmetries essential for superconductivity are time reversal (T) and inversion (I). Here, we examine superconductivity in two dimensions and find that T and I are not required, and having a combination of either symmetry with a mirror operation (M_{z}) on the basal plane is sufficient. By combining energetic and topological arguments, we classify superconducting states when T and I are not present, a situation encountered in several experimentally relevant systems, such as transition metal dichalcogenides or a two-dimensional Rashba system, when subject to an applied field, and in superconducting monolayer FeSe with Néel antiferromagnetic order. Energetic arguments suggest interesting superconducting states arise. For example, we find a unique pure intraband pairing state with Majorana chiral edge states in Néel-ordered FeSe. Employing topological arguments, we find when the only symmetry is the combination of I with M_{z}, the superconducting states are generically fully gapped and can have topologically protected chiral Majorana edge modes. In all other cases, there are no chiral Majorana edge states, but the superconducting bulk can have point nodes with associated topologically protected flatband Majorana edge modes. Our analysis provides guidance on the design and search for novel two-dimensional superconductors and superconducting heterostructures.

Beyond triplet: Unconventional superconductivity in a spin-3/2 topological semimetal.

In all known fermionic superfluids, Cooper pairs are composed of spin-1/2 quasi-particles that pair to form either spin-singlet or spin-triplet bound states. The "spin" of a Bloch electron, however, is fixed by the symmetries of the crystal and the atomic orbitals from which it is derived and, in some cases, can behave as if it were a spin-3/2 particle. The superconducting state of such a system allows pairing beyond spin-triplet, with higher spin quasi-particles combining to form quintet or septet pairs. We report evidence of unconventional superconductivity emerging from a spin-3/2 quasi-particle electronic structure in the half-Heusler semimetal YPtBi, a low-carrier density noncentrosymmetric cubic material with a high symmetry that preserves the p-like j = 3/2 manifold in the Bi-based Γ8 band in the presence of strong spin-orbit coupling. With a striking linear temperature dependence of the London penetration depth, the existence of line nodes in the superconducting order parameter Δ is directly explained by a mixed-parity Cooper pairing model with high total angular momentum, consistent with a high-spin fermionic superfluid state. We propose a k ⋅ p model of the j = 3/2 fermions to explain how a dominant J = 3 septet pairing state is the simplest solution that naturally produces nodes in the mixed even-odd parity gap. Together with the underlying topologically nontrivial band structure, the unconventional pairing in this system represents a truly novel form of superfluidity that has strong potential for leading the development of a new series of topological superconductors.

Coexistence of charge-density-wave and pair-density-wave orders in underdoped cuprates.

We analyze incommensurate charge-density-wave (CDW) and pair-density-wave (PDW) orders with transferred momenta (±Q,0)/(0,±Q) in underdoped cuprates within the spin-fermion model. Both orders appear due to an exchange of spin fluctuations before magnetic order develops. We argue that the ordered state with the lowest energy has nonzero CDW and PDW components with the same momentum. Such a state breaks C_{4} lattice rotational symmetry, time-reversal symmetry, and mirror symmetries. We argue that the feedback from CDW/PDW order on fermionic dispersion is consistent with ARPES data. We discuss the interplay between the CDW/PDW order and d_{x^{2}-y^{2}} superconductivity and make specific predictions for experiments.