12 research outputs found
Fluctuation-driven superconductivity in SrRuO from weak repulsive interactions
We provide results for the leading superconducting instabilities for a model
pertaining to SrRuO obtained within spin-fluctuation mediated
superconductivity in the very weak-coupling limit. The theory incorporates
spin-orbit coupling (SOC) effects both in the band structure and in the pairing
kernel in the form of associated magnetic anisotropies. The leading
superconducting phase is found to be and a nodal -wave state.
However, the odd-parity helical solution can become leading either for small
SOC and Hund's coupling in the weak -limit, or in the opposite limit
with large SOC and at larger values of the Hubbard-. The odd-parity
chiral solution is never found to be leading. Finally we discuss the form of
the resulting superconducting spectral gaps in the different explored parameter
regimes.Comment: 5 pages, 5 figure
Orbital-dependent self-energy effects and consequences for the superconducting gap structure in multi-orbital correlated electron systems
We perform a theoretical study of the effects of electronic correlations on
the superconducting gap structure of multi-band superconductors. In particular,
by comparing standard RPA-based spin-fluctuation mediated gap structures to
those obtained within the FLEX formalism for an iron-based superconductor, we
obtain directly the feedback effects from electron-electron interactions on the
momentum-space gap structure. We show how self-energy effects can lead to an
orbital inversion of the orbital-resolved spin susceptibility, and thereby
invert the hierarchy of the most important orbitals channels for
superconducting pairing. This effect has important consequences for the
detailed gap variations on the Fermi surface. We expect such self-energy
feedback on the pairing gap to be generally relevant for superconductivity in
strongly correlated multi-orbital systems.Comment: 8 pages, 5 figure
Superconducting gap symmetry from Bogoliubov quasiparticle interference analysis on {Sr}{RuO}
The nature of the superconducting order parameter in {Sr}{RuO} has
generated intense interest in recent years. Since the superconducting gap is
very small, high resolution methods such as scanning tunneling spectroscopy
might be the best chance to directly resolve the gap symmetry. Recently, a
Bogoliubov quasiparticle interference imaging (BQPI) experiment has suggested
that the gap symmetry is appropriate for {Sr}{RuO}. In
this work, we use a material-specific theoretical approach based on Wannier
functions of the surface of {Sr}{RuO} to calculate the continuum
density of states as detected in scanning tunneling microscopy experiments. We
examine several different proposed gap order parameters, and calculate the
expected BQPI pattern for each case. Comparing to the available experimental
data, our results suggest that a gap order parameter is the most
probable state, but the measured BQPI patterns still display features
unaccounted for by the theory for any of the states currently under discussion.Comment: 14 pages, 9 figure
Pairing in the Two-Dimensional Hubbard Model from Weak to Strong Coupling
The Hubbard model is the simplest model that is believed to exhibit
superconductivity arising from purely repulsive interactions, and has been
extensively applied to explore a variety of unconventional superconducting
systems. Here we study the evolution of the leading superconducting
instabilities of the single-orbital Hubbard model on a two-dimensional square
lattice as a function of onsite Coulomb repulsion and band filling by
calculating the irreducible particle-particle scattering vertex obtained from
dynamical cluster approximation (DCA) calculations, and compare the results to
both perturbative Kohn-Luttinger (KL) theory as well as the widely used random
phase approximation (RPA) spin-fluctuation pairing scheme. Near half-filling we
find remarkable agreement of the hierarchy of the leading pairing states
between these three methods, implying adiabatic continuity between weak- and
strong-coupling pairing solutions of the Hubbard model. The -wave
instability is robust to increasing near half-filling as expected. Away
from half filling, the predictions of KL and RPA at small for transitions
to other pair states agree with DCA at intermediate as well as recent
diagrammatic Monte Carlo calculations. RPA results fail only in the very dilute
limit, where it yields a ground state instead of a -wave state
established by diagrammatic Monte Carlo and low-order perturbative methods, as
well as our DCA calculations. We discuss the origins of this discrepancy,
highlighting the crucial role of the vertex corrections neglected in the RPA
approach. Overall, comparison of the various methods over the entire phase
diagram strongly suggests a smooth crossover of the superconducting interaction
generated by local Hubbard interactions between weak and strong coupling.Comment: 9 pages, 5 figure
Theory of Strain-Induced Magnetic Order and Splitting of and in SrRuO
The internal structure of the superconducting state in SrRuO remains
elusive at present, and exhibits evidence for time-reversal symmetry breaking.
Recent muon spin relaxation measurements under uniaxial strain have revealed an
increasing splitting between the superconducting critical temperature and
the onset of time-reversal symmetry breaking with applied strain
[Grinenko et al., arXiv:2001.08152]. In addition, static magnetic order is
induced by the uniaxial strain beyond 1 GPa, indicating that unstrained
SrRuO is close to a magnetic quantum critical point. Here, we perform a
theoretical study of the magnetic susceptibility and the associated pairing
structure as a function of uniaxial strain. It is found that the recent muon
relaxation data can be qualitatively explained from the perspective of
spin-fluctuation mediated pairing and the associated strain-dependence of
accidentally degenerate pair states in unstrained SrRuO. In addition,
while unstrained SrRuO features mainly magnetic
fluctuations, uniaxial strain promotes magnetism.Comment: 8 pages, 4 figure
Gradual emergence of superconductivity in underdoped LSCO
We present triple-axis neutron scattering studies of low-energy magnetic
fluctuations in strongly underdoped LaSrCuO with ,
and , providing quantitative evidence for a direct competition
between these fluctuations and superconductivity. At dopings and
, three-dimensional superconductivity is found, while only a very weak
signature of two-dimensional superconductivity residing in the CuO planes
is detectable for . We find a surprising suppression of the low-energy
fluctuations by an external magnetic field at all three dopings. This implies
that the response of two-dimensional superconductivity to a magnetic field is
similar to that of a bulk superconductor. Our results provide direct evidence
of a very gradual onset of superconductivity in cuprates.Comment: 5 pages, 4 figures, and supplementary materia
Superconductivity from repulsive interactions on the kagome lattice
The discovery of superconductivity in layered vanadium-based kagome metals AV3Sb5 (A: K, Rb, Cs) has added a new family of materials to the growing class of possible unconventional superconductors. However, the nature of the superconducting pairing in these materials remains elusive. We present a microscopic theoretical study of the leading superconducting instabilities on the kagome lattice based on spin- and charge-fluctuation mediated Cooper pairing. The applied methodology includes effects of both on-site and nearest-neighbor repulsive Coulomb interactions. Near the upper van Hove filling -- relevant for the AV3Sb5 materials -- we find a rich phase diagram with several pairing symmetries being nearly degenerate. In particular, while a substantial fraction of the phase diagram is occupied by a spin-singlet order parameter transforming as a two-dimensional irreducible representation of the point group, several nodal spin-triplet pairing states remain competitive. We compute the band and interaction parameter-dependence of the hierarchy of the leading superconducting instabilities, and determine the detailed momentum dependence of the resulting preferred gap structures. Crucially, for moderate values of the interaction parameters, the individual pairing states depend strongly on momentum and exhibit multiple nodes on the Fermi surface. We discuss the properties of these superconducting gap structures in light of recent experimental developments of the AV3Sb5 materials