Sign-changing Pairing Research Articles

Sign changing pairing in single layer FeSe/SrTiO3 revealed by nonmagnetic impurity bound states

The discovery of high-temperature superconductivity in single layer FeSe epitaxially grown on SrTiO3(001) substrates has instigated extensive debate over whether its pairing symmetry is conventional sign-preserving s-wave or unconventional sign-changing. Here, we probe the pairing state for single layer FeSe/SrTiO3 grown by molecular beam epitaxy using scanning tunneling microscopy/spectroscopy. We observe robust in-gap bound states induced by non-magnetic Fe-vacancy defects within the Fe-plane, which exhibit strong spatial electron-hole asymmetry with two-fold symmetry in hole states and four-fold in electron states. The bound states exhibit no energy shift or splitting under an applied magnetic field, consistent with a sign-changing order parameter. This is further confirmed by defect bound state quasiparticle interference that shows a sign-changing behavior with a pair of corresponding peaks at the positive and negative energies near the impurity bound states. Our findings provide unambiguous evidence for a sign-changing pairing symmetry for single layer FeSe/SrTiO3.

Impurity-induced bound states inside the superconducting gap of FeSe

We investigate the local density of states in the vicinity of a native dumbbell defect arising from an Fe vacancy in FeSe single crystals. The tunneling spectra close to the impurity display two bound states inside the superconducting gap, equally spaced with respect to zero energy but asymmetric in amplitude. Using spin-polarized density functional theory (DFT) calculations on realistic slab models with Fe vacancy, we show that such a defect does not induce a local magnetic moment. Therefore, the dumbbell defect is considered as non-magnetic. Thus, the in-gap bound states emerging from a non-magnetic defect-induced pair-breaking suggest a sign changing pairing state in this material.

Robustness of a quasiparticle interference test for sign-changing gaps in multiband superconductors

Recently, a test for a sign-changing gap function in a candidate multiband unconventional superconductor involving quasiparticle interference data was proposed. The test was based on the antisymmetric, Fourier transformed conductance maps integrated over a range of momenta $\bf q$ corresponding to interband processes, which was argued to display a particular resonant form, provided the gaps changed sign between the Fermi surface sheets connected by $\bf q$. The calculation was performed for a single impurity, however, raising the question of how robust this measure is as a test of sign-changing pairing in a realistic system with many impurities. Here we reproduce the results of the previous work within a model with two distinct Fermi surface sheets, and show explicitly that the previous result, while exact for a single nonmagnetic scatterer and also in the limit of a dense set of random impurities, can be difficult to implement for a few dilute impurities. In this case, however, appropriate isolation of a single impurity is sufficient to recover the expected result, allowing a robust statement about the gap signs to be made.

Effect of nonmagnetic impurities ons±superconductivity in the presence of incipient bands

Several Fe chalcogenide superconductors without hole pockets at the Fermi level display high temperature superconductivity, in apparent contradiction to naive spin fluctuation pairing arguments. Recently, scanning tunneling microscopy measurements have measured the influence of impurities on some of these materials, and claimed that non-magnetic impurities do not create in-gap states, leading to the conclusion that the gap must be $s_{++}$, i.e. conventional $s$ wave with no gap sign change. Here we critique this argument, and give various ways sign-changing gaps can be consistent with the absence of such bound states. In particular, we calculate the bound states for an $s_\pm$ system with a hole pocket below the Fermi level, and show that the nonmagnetic impurity bound state energy generically tracks the gap edge $E_{min}$ in the system, thereby rendering it unobservable. A failure to observe a bound state in the case of a nonmagnetic impurity can therefore not be used as an argument to exclude sign-changing pairing states.

Zero-bias conductance peak in detached flakes of superconducting 2H-TaS2probed by scanning tunneling spectroscopy

We report an anomalous tunneling conductance with a zero bias peak in flakes of superconducting 2H-TaS$_2$ detached through mechanical exfoliation. To explain the observed phenomenon, we construct a minimal model for a single unit cell layer of superconducting 2H-TaS$_2$ with a simplified 2D Fermi surface and sign-changing Cooper pair wavefunction induced by Coulomb repulsion. Superconductivity is induced in the central $\Gamma$ pocket, where it becomes nodal. We show that weak scattering at the nodal Fermi surface, produced by non-perturbative coupling between tip and sample, gives Andreev states that lead to a zero bias peak in the tunneling conductance. We suggest that reducing dimensionality down to a few atom thick crystals could drive a crossover from conventional to sign changing pairing in the superconductor 2H-TaS$_2$.

Instabilities near the onset of spin density wave order in metals

We discuss the low-energy theory of two-dimensional metals near the onset of spin density wave order. It is well known that such a metal has a superconducting instability induced by the formation of spin-singlet pairs of electrons, with the pairing amplitude changing sign between regions of the Fermi surface connected by the spin density wave ordering wavevector. Here, we review recent arguments that there is an additional instability that is nearly as strong: towards the onset of a modulated bond order that is locally an Ising-nematic order. This new instability is a consequence of an emergent ‘pseudospin’ symmetry of the low-energy theory—the symmetry maps the sign-changing pairing amplitude to the bond order parameter.

Anisotropic fluctuations and quasiparticle excitations inFeSe0.5Te0.5

We present data for the temperature dependence of the magnetic penetration depth $\ensuremath{\lambda}(T)$, heat capacity $C(T)$, resistivity $\ensuremath{\rho}(T)$, and magnetic torque $\ensuremath{\tau}$ for highly homogeneous single-crystal samples of ${\text{Fe}}_{1.0}{\text{Se}}_{0.44(4)}{\text{Te}}_{0.56(4)}$. $\ensuremath{\lambda}(T)$ was measured down to 200 mK in zero field. We find $\ensuremath{\lambda}(T)$ follows a power law $\ensuremath{\Delta}\ensuremath{\lambda}\ensuremath{\sim}{T}^{n}$ with $n=2.2\ifmmode\pm\else\textpm\fi{}0.1$. This is similar to some 122 iron arsenides and likely results from a sign-changing pairing state combined with strong scattering. Magnetic fields of up to $B=55$ or 14 T were used for the $\ensuremath{\tau}(B)$ and $C(T)/\ensuremath{\rho}(T)$ measurements, respectively. The specific heat, resistivity, and torque measurements were used to map out the $(H,T)$ phase diagram in this material. All three measurements were conducted on exactly the same single-crystal sample so that the different information revealed by these probes is clearly distinguished. Heat-capacity data strongly resemble those found for the high-${T}_{c}$ cuprates, where strong fluctuation effects wipe out the phase transition at ${H}_{c2}$. Unusually, here we find the fluctuation effects appear to be strongly anisotropic.