121 research outputs found
Knight Shift and Leading Superconducting Instability From Spin Fluctuations in Sr2RuO4
Recent nuclear magnetic resonance studies [A. Pustogow {\it et al.},
arXiv:1904.00047] have challenged the prevalent chiral triplet pairing scenario
proposed for SrRuO. To provide guidance from microscopic theory as to
which other pair states might be compatible with the new data, we perform a
detailed theoretical study of spin-fluctuation mediated pairing for this
compound. We map out the phase diagram as a function of spin-orbit coupling,
interaction parameters, and band-structure properties over physically
reasonable ranges, comparing when possible with photoemission and inelastic
neutron scattering data information. We find that even-parity pseudospin
singlet solutions dominate large regions of the phase diagram, but in certain
regimes spin-orbit coupling favors a near-nodal odd-parity triplet
superconducting state, which is either helical or chiral depending on the
proximity of the band to the van Hove points. A surprising
near-degeneracy of the nodal - and -wave solutions leads
to the possibility of a near-nodal time-reversal symmetry broken
pair state. Predictions for the temperature dependence
of the Knight shift for fields in and out of plane are presented for all
states.Comment: 5 pages (3 figures) + supplementary informatio
Distinct nature of static and dynamic magnetic stripes in cuprate superconductors
We present detailed neutron scattering studies of the static and dynamic
stripes in an optimally doped high-temperature superconductor,
LaCuO. We find that the dynamic stripes do not disperse towards the
static stripes in the limit of vanishing energy transfer. We conclude that the
dynamic stripes observed in neutron scattering experiments are not the
Goldstone modes associated with the broken symmetry of the simultaneously
observed static stripes, but rather that the signals originate from different
domains in the sample. These domains may be related by structural twinning, or
may be entirely different phases, where the static stripes in one phase are
pinned versions of the dynamic stripes in the other. Our results explain
earlier observations of unusual dispersions in underdoped
LaSrCuO () and LaBaCuO ().
Our findings are relevant for all compounds exhibiting magnetic stripes, and
may thus be a vital part in unveiling the nature of high temperature
superconductivity
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
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
Risiko ved transport af farligt gods
Projektet âTransport af farligt godsâ, er nĂŚsten afsluttet. Slutrapporten publiceres i 1999. Den indeholder to risikomodeller for transporter af farligt gods: en model for jernbanetrafik og en model for vejtrafik. Disse modeller angiver risikoen for udslip af forskellige størrelser ved givne transporter. Endvidere angives modeller for konsekvenserne af givne udslip, dels hu-mane konsekvenser i form af fN-kurver for dødsfald, dels miljøkonsekvenser i form af jord-og grundvandsforurening og i form af forurening af overfladevand. Modellerne indlĂŚgges i et GIS, som kan anskueliggøre konsekvensernes geografiske fordeling totalt eller vise forskelle ved forskelligt rutevalg for en given transport
Glassy low-energy spin fluctuations and anisotropy gap in La<sub>1.88</sub>Sr<sub>0.12</sub>CuO<sub>4</sub>
We present high-resolution triple-axis neutron scattering studies of the
high-temperature superconductor La1.88Sr0.12CuO4 (Tc=27 K). The temperature
dependence of the low-energy incommensurate magnetic fluctuations reveals
distinctly glassy features. The glassiness is confirmed by the difference
between the ordering temperature TN ~ Tc inferred from elastic neutron
scattering and the freezing temperature Tf ~ 11 K obtained from muon spin
rotation studies. The magnetic field independence of the observed excitation
spectrum as well as the observation of a partial suppression of magnetic
spectral weight below 0.75 meV for temperatures smaller than Tf, indicate that
the stripe frozen state is capable of supporting a spin anisotropy gap, of a
magnitude similar to that observed in the spin and charge stripe ordered ground
state of La1.875Ba0.125CuO4. The difference between TN and Tf implies that the
significant enhancement in a magnetic field of nominally elastic incommensurate
scattering is caused by strictly in-elastic scattering -- at least in the
temperature range between Tf and Tc -- which is not resolved in the present
experiment. Combining the results obtained from our study of La1.88Sr0.12CuO4
with a critical reappraisal of published neutron scattering work on samples
with chemical composition close to p=0.12, where local probes indicate a sharp
maximum in Tf(p), we arrive at the view that the low-energy fluctuations are
strongly dependent on composition in this regime, with anisotropy gaps
dominating only sufficiently close to p=0.12 and superconducting spin gaps
dominating elsewhere.Comment: 8 pages, 4 figure
Field-induced electronic phase separation in a cuprate high temperature superconductor
We present a combined neutron diffraction (ND) and high-field muon spin
rotation (SR) study of the magnetic and superconducting phases of the
high-temperature superconductor LaSrCuO (~K). We observe a linear dependence of the ND signal from the modulated
antiferromagnetic order (m-AFM) on the applied field. The magnetic volume
fraction measured with SR increases linearly from 0\% to 40\% with
applied magnetic field up to 8~T. This allows us to conclude, in contrast to
earlier field-dependent neutron diffraction studies, that the long-range m-AFM
regions are induced by an applied field, and that their ordered magnetic moment
remains constant
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