Nodeless Superconductor Research Articles

Nodal topology in d -wave superconducting monolayer FeSe

A nodeless $d$-wave state is likely in superconducting monolayer FeSe on SrTiO$_3$. The lack of nodes is surprising but has been shown to be a natural consequence of the observed small interband spin-orbit coupling. Here we examine the evolution from a nodeless state to the nodal state as this spin-orbit coupling is increased from a topological perspective. We show that this evolution depends strongly on the orbital content of the superconducting degrees of freedom. In particular, there are two $d$-wave solutions, which we call orbitally trivial and orbitally nontrivial. In both cases, the nodes carry a $\pm 2$ topological winding number that originates from a chiral symmetry. However, the momentum space distribution of the positive and negative charges is different for the two cases, resulting in a different evolution of these nodes as they annihilate to form a nodeless superconductor. We further show that the orbitally trivial and orbitally nontrivial nodal states exhibit different Andreev flat band spectra at the edge.

Nodeless High-T_{c} Superconductivity in the Highly Overdoped CuO_{2} Monolayer.

We study the electronic structure and superconductivity in a CuO_{2} monolayer grown recently on the d-wave cuprate superconductor Bi_{2}Sr_{2}CaCu_{2}O_{8+δ}. Density functional theory calculations indicate a significant charge transfer across the interface such that the CuO_{2} monolayer is heavily overdoped into the hole-rich regime yet inaccessible in bulk cuprates. We show that both the Cu d_{x^{2}-y^{2}} and d_{3z^{2}-r^{2}} orbitals become important and the Fermi surface contains one electron and one hole pocket associated with the two orbitals, respectively. Constructing a minimal correlated two-orbital model for the e_{g} complex, we show that the spin-orbital exchange interactions produce a nodeless superconductor with extended s-wave pairing symmetry and a pairing energy gap comparable to the bulk d-wave gap, in agreement with recent experiments. The findings point to a direction of realizing new high-T_{c} superconductors in ozone grown transition-metal-oxide monolayer heterostructures.

Nodal versus nodeless superconductivity in isoelectronic LiFeP and LiFeAs

Nodal superconductivity is observed in LiFeP while its counterpart LiFeAs with similar topology and orbital content of the Fermi surfaces is a nodeless superconductor. We explain this difference by solving, in the two-Fe Brillouin zone, the frequency-dependent Eliashberg equations with the spin-fluctuation-mediated pairing interaction. Because of Fermi surface topology details, in LiFeAs all the $\mathrm{Fe}\text{\ensuremath{-}}{t}_{2g}$ orbitals favor a common pairing symmetry. By contrast, in LiFeP the ${d}_{xy}$ orbital favors a pairing symmetry different from ${d}_{xz/yz}$ and their competition determines the pairing symmetry and the strength of the superconducting instability: ${d}_{xy}$ orbital strongly overcomes the others and imposes the symmetry of the superconducting order parameter. The leading pairing channel is a ${d}_{xy}$-type state with nodes on both hole and electron Fermi surfaces. As a consequence, the ${d}_{xz/yz}$ electrons weakly pair, leading to a reduced transition temperature in LiFeP.

Electronic structure of Eu(Fe0.79Ru0.21)2As2 studied by angle-resolved photoemission spectroscopy

Eu(Fe0.79Ru0.21)2As2 is suggested to be a nodeless superconductor based on the empirical correlation between pnictogen height (hPn) and superconducting gap behavior, in contrast to BaFe2(As0.7P0.3)2 and Ba(Fe0.65Ru0.35)2As2. We studied the low-lying electronic structure of Eu(Fe0.79Ru0.21)2As2 with angle-resolved photoemission spectroscopy (ARPES). By photon energy dependence and polarization dependence measurements, we resolved the band structure in the three-dimensional momentum space and determined the orbital character of each band. In particular, we found that the -originated ζ band does not contribute spectral weight to the Fermi surface around Z, unlike BaFe2(As0.7P0.3)2 and Ba(Fe0.65Ru0.35)2As2. Since BaFe2(As0.7P0.3)2 and Ba(Fe0.65Ru0.35)2As2 are nodal superconductors and their hPn's are less than 1.33 Å, while the hPn of Eu(Fe0.79Ru0.21)2As2 is larger than 1.33 Å, our results provide more evidence for a direct relationship between nodes, orbital character and hPn. Our results help to provide an understanding of the nodal superconductivity in iron-based superconductors.

Topological superconductivity without proximity effect

Majorana fermions, strange particles that are their own antiparticles, were predicted in 1937 and have been sought after ever since. In condensed matter they are predicted to exist as vortex core or edge excitations in certain exotic superconductors. These are topological superconductors whose order parameter phase winds nontrivially in momentum space. In recent years, a new and promising route for realizing topological superconductors has opened due to advances in the field of topological insulators. Current proposals are based on semiconductor heterostructures, where spin-orbit-coupled bands are split by a band gap or Zeeman field and superconductivity is induced by proximity to a conventional superconductor. Topological superconductivity is obtained in the interface layer. The proposed heterostructures typically include two or three layers of different materials. In the current work we propose a device based on materials with inherent spin-orbit coupling and an intrinsic tendency for superconductivity, eliminating the need for a separate superconducting layer. We study a lattice model that includes spin-orbit coupling as well as on-site and nearest-neighbor interaction. Within this model we show that topological superconductivity is possible in certain regions of parameter space. These regions of nontrivial topology can be understood as a nodeless superconductor with $d$-wave symmetry which, due to the spin-orbit coupling, acquires an extra phase twist of $2\ensuremath{\pi}$.

Ca3Ir4Sn13: A weakly correlated nodeless superconductor

We report detailed Seebeck coefficient, Hall resistivity as well as specific heat measurement on Ca$_3$Ir$_4$Sn$_{13}$ single crystals. The Seebeck coefficient exhibits a peak corresponding to the anomaly in resistivity at $T^*$, and the carrier density is suppressed significantly below $T^*$. This indicates a significant Fermi surface reconstruction and the opening of the charge density wave gap at the supperlattice transition. The magnetic field induced enhancement of the residual specific heat coefficient $\gamma(H)$ exhibits a nearly linear dependence on magnetic field, indicating a nodeless gap. In the temperature range close to $T_c$ the Seebeck coefficient can be described well by the diffusion model. The zero-temperature extrapolated thermoelectric power is very small, implying large normalized Fermi temperature. Consequently the ratio $\frac{T_c}{T_F}$ is very small. Our results indicate that Ca$_3$Ir$_4$Sn$_{13}$ is a weakly correlated nodeless superconductor.

Anomaly of quasi-particle density of states in the vortex state of NbSe2

Abstract Magnetic field H dependence of the electronic specific heat coefficient γ was measured in various NbSe 2 crystals in the vortex states. With increasing residual resistivity ratio, γ ( H ) changed from conventional γ ∝ H behavior to the strongly sublinear function of H . The sublinear H dependence of γ is similar to that of nodal superconductors, while NbSe 2 is a nodeless superconductor. From measurements on the sample containing columnar defects, we found that the γ anomaly is brought by quasi-particle states outside of the vortex cores. This result suggests that excess quasi-particle states are generated by Doppler energy shift caused by supercurrent around vortices.

Absence of the zero bias peak in vortex tunneling spectra of high-temperature superconductors

The c-axis tunneling matrix of high-${T}_{c}$ superconductors is shown to depend strongly on the in-plane momentum of electrons and vanish along the four nodal lines of the ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$-wave energy gap. This anisotropic tunneling matrix suppresses completely the contribution of the most extended quasiparticles in the vortex core to the c-axis tunneling current and leads to a spectrum similar to that of a nodeless superconductor. Our results give a natural explanation of the absence of the zero-bias peak as well as other features observed in the vortex tunneling spectra of high-${T}_{c}$ cuprates.