1,108 research outputs found
On the "spin-freezing" mechanism in underdoped superconducting cuprates
The letter deals with the spin-freezing process observed by means of NMR-NQR
relaxation or by muon spin rotation in underdoped cuprate superconductors. This
phenomenon, sometimes referred as coexistence of antiferromagnetic and
superconducting order parameters, is generally thought to result from randomly
distributed magnetic moments related to charge inhomogeneities (possibly
stripes) which exhibit slowing down of their fluctuations on cooling below
T . Instead, we describe the experimental findings as due to fluctuating,
vortex-antivortex, orbital currents state coexisting with d-wave
superconducting state. A direct explanation of the experimental results, in
underdoped YCaBaCuO and LaSrCuO,
is thus given in terms of freezing of orbital current fluctuations
Quasiparticle interference from different impurities on the surface of pyrochlore iridates: signatures of the Weyl phase
Weyl semimetals are gapless three-dimensional topological materials where two
bands touch at an even number of points in the bulk Brillouin zone. These
semimetals exhibit topologically protected surface Fermi arcs, which pairwise
connect the projected bulk band touchings in the surface Brillouin zone. Here,
we analyze the quasiparticle interference patterns of the Weyl phase when
time-reversal symmetry is explicitly broken. We use a multi-band -electron
Hubbard Hamiltonian on a pyrochlore lattice, relevant for the pyrochlore
iridate RIrO (where R is a rare earth). Using exact
diagonalization, we compute the surface spectrum and quasiparticle interference
(QPI) patterns for various surface terminations and impurities. We show that
the spin and orbital texture of the surface states can be inferred from the
absence of certain backscattering processes and from the symmetries of the QPI
features for non-magnetic and magnetic impurities. Furthermore, we show that
the QPI patterns of the Weyl phase in pyrochlore iridates may exhibit
additional interesting features that go beyond those found previously in TaAs.Comment: 15 pages, 16 figure
Superconducting phase diagram of itinerant antiferromagnets
We study the phase diagram of the Hubbard model in the weak-coupling limit
for coexisting spin-density-wave order and spin-fluctuation-mediated
superconductivity. Both longitudinal and transverse spin fluctuations
contribute significantly to the effective interaction potential, which creates
Cooper pairs of the quasi-particles of the antiferromagnetic metallic state. We
find a dominant -wave solution in both electron- and hole-doped
cases. In the quasi-spin triplet channel, the longitudinal fluctuations give
rise to an effective attraction supporting a -wave gap, but are overcome by
repulsive contributions from the transverse fluctuations which disfavor
-wave pairing compared to . The sub-leading pair instability is
found to be in the -wave channel, but complex admixtures of and are
not energetically favored since their nodal structures coincide. Inclusion of
interband pairing, in which each fermion in the Cooper pair belongs to a
different spin-density-wave band, is considered for a range of electron dopings
in the regime of well-developed magnetic order. We demonstrate that these
interband pairing gaps, which are non-zero in the magnetic state, must have the
same parity under inversion as the normal intraband gaps. The self-consistent
solution to the full system of five coupled gap equations give intraband and
interband pairing gaps of structure and similar gap magnitude. In
conclusion, the gap dominates for both hole and electron doping
inside the spin-density-wave phase.Comment: 14 pages, 9 figure
Interplay of magnetic and structural transitions in Fe-based pnictide superconductors
The interplay between the structural and magnetic phase transitions occurring
in the Fe-based pnictide superconductors is studied within a Ginzburg-Landau
approach. We show that the magnetoelastic coupling between the corresponding
order parameters is behind the salient features observed in the phase diagram
of these systems. This naturally explains the coincidence of transition
temperatures observed in some cases as well as the character (first or
second-order) of the transitions. We also show that magnetoelastic coupling is
the key ingredient determining the collinearity of the magnetic ordering, and
we propose an experimental criterion to distinguish between a pure elastic from
a spin-nematic-driven structural transition.Comment: 5 pages, 3 figures. v2: Fig. 1 improved, references added
Polaron Effects on Superexchange Interaction: Isotope Shifts of , , and in Layered Copper Oxides
A compact expression has been obtained for the superexchange coupling of
magnetic ions via intermediate anions with regard to polaron effects at both
magnetic ions and intermediate anions. This expression is used to analyze the
main features of the behavior of isotope shifts for temperatures of three types
in layered cuprates: the Neel temperatures (), critical temperatures of
transitions to a superconducting state (), and characteristic temperatures
of the pseudogap in the normal state ().Comment: 4 pages, 1 figur
Pairing symmetry of the one-band Hubbard model in the paramagnetic weak-coupling limit: a numerical RPA study
We study the spin-fluctuation-mediated superconducting pairing gap in a
weak-coupling approach to the Hubbard model for a two dimensional square
lattice in the paramagnetic state. Performing a comprehensive theoretical study
of the phase diagram as a function of filling, we find that the superconducting
gap exhibits transitions from p-wave at very low electron fillings to
d_{x^2-y^2}-wave symmetry close to half filling in agreement with previous
reports. At intermediate filling levels, different gap symmetries appear as a
consequence of the changes in the Fermi surface topology and the associated
structure of the spin susceptibility. In particular, the vicinity of a van Hove
singularity in the electronic structure close to the Fermi level has important
consequences for the gap structure in favoring the otherwise sub-dominant
triplet solution over the singlet d-wave solution. By solving the full gap
equation, we find that the energetically favorable triplet solutions are chiral
and break time reversal symmetry. Finally, we also calculate the detailed
angular gap structure of the quasi-particle spectrum, and show how
spin-fluctuation-mediated pairing leads to significant deviations from the
first harmonics both in the singlet d_{x^2-y^2} gap as well as the chiral
triplet gap solution.Comment: 11 pages 11 figure
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
Electronic theory for superconductivity in SrRuO: triplet pairing due to spin-fluctuation exchange
Using a two-dimensional Hubbard Hamiltonian for the three electronic bands
crossing the Fermi level in SrRuO we calculate the band structure and
spin susceptibility in quantitative agreement with
nuclear magnetic resonance (NMR) and inelastic neutron scattering (INS)
experiments. The susceptibility has two peaks at {\bf Q}
due to the nesting Fermi surface properties and at {\bf q}
due to the tendency towards ferromagnetism. Applying spin-fluctuation exchange
theory as in layered cuprates we determine from ,
electronic dispersions, and Fermi surface topology that superconductivity in
SrRuO consists of triplet pairing. Combining the Fermi surface topology
and the results for we can exclude and wave
symmetry for the superconducting order parameter. Furthermore, within our
analysis and approximations we find that -wave symmetry is slightly favored
over p-wave symmetry due to the nesting properties of the Fermi surface.Comment: 5 pages, 5 figures, misprints correcte
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