Contents - Max-Planck-Institut für Physik komplexer Systeme

from an itinerant perspective [4]. The results are shown
in Fig.2 together with experimental data. Beginning at
the ordering vector (π,0), the momentum versus energy
dispersion of the imaginary part of the spin susceptibility
in two perpendicular directions is plotted.
The spin wave velocities differ strongly in qx and qy
directions - an experimental fact that had been quite
puzzling in the beginning. In our itinerant description,
the anisotropy is nicely reproduced and a consequence
of the ellipticity of the electron pockets.
300
290
280
270
q 2
q 1
q (π/a)
1.0
-1.0
-1.0 0 1.0
q (π/a)
x
80 100 120 140
Figure 3: Real space image of our calculated QPI maps with a Fourier
transform in the inset. Our T-matrix calculation reproduces the
quasi-one-dimensional features observed in SI-STM experiments [3].
Quasiparticle Interference. In recent years SI-STM
has become a powerful experimental tool for elucidating
the nature of the many-body states in novel superconductors.
In the presence of perturbations internal
to the sample, such as nonmagnetic or magnetic impurities,
elastic scattering mixes two quasiparticle eigenstates
with momenta k1 and k2 on a contour of constant
energy. The resulting quasiparticle interference
(QPI) at wavevector q = k2 − k1 reveals a modulation
of the local density of states. The phase-sensitive
interference pattern in momentum space is what is visualized
by means of the SI-STM.
Experiments performed in the magnetic state of FPs revealed
quasi one-dimensional features in the tunneling
spectra [3] with strongly broken rotational symmetry.
Since the magnetic (π,0) state breaks the rotational
symmetry, we expected that our itinerant model
[1] Y. Kamihara et al., J. Am. Chem. Soc. 130 3296 (2008).
y0
[2] A.V. Chubukov, D.V. Efremov, and I. Eremin, Phys. Rev. B 78, 134512 (2008).
[3] T.-M. Chuang et al., Science 327, 181 (2010).
q 2
q 1
60
40
20
should be able to account for the SI-STM measurements.
Therefore, we introduced an impurity term in
our four band hamiltonian,
Himp =
[4] J. Knolle, I. Eremin, A. V. Chubukov, and R. Moessner, Phys. Rev. B 81, 140506 (2010).
[5] I. Eremin and A. V. Chubukov, Phys. Rev. B 81, 024511 (2010).
[6] J. Knolle, I. Eremin, A. Akbari, and R. Moessner, Phys. Rev. Lett. 104, 257001 (2010).
[7] T. Hanaguri, S. Niitaka, K. Kuroki, and H. Takagi, Science 328, 474 (2010).
[8] A. Akbari, J. Knolle, I. Eremin, and R. Moessner, Phys. Rev. B 82, 224506 (2010).
kk ′ ii ′ σσ ′
V ii′
kk ′δσσ ′ + Jii′ σσ
′S · σσσ ′
c †
ikσ ci ′ k ′ σ ′.
(1)
and performed a T-matrix calculation of the local density
of states [6]. The result is displayed in Fig.3. A
real space map of the QPI from the impurity is shown
together with the Fourier transformed quantity as an
inset. Our itinerant model reproduces the quasi onedimensional
features which are visible as stripe like
patterns in real space.
Probing the symmetry of the superconducting order
parameter requires phase sensitive measurements such
as QPI. The conjectured s ± symmetry of FPs is especially
difficult because it does not have nodes and only
changes sign between the FS pockets at the BZ center
and the pockets at the boundaries. Recent experiments
[7] have reported the results of SI-STM measurements
in the superconducting phase. They interpreted the
different scattering behaviour of various wave vectors
for magnetic and non-magnetc impurities as a proof
of the s ± symmetry. In an additional study [8] we
extended our QPI calculations to the superconducting
phase and confirmed their interpretation. Furthermore,
we looked at the effect of possible gap minima and
studied the QPI signatures of a coexistence phase in
which the itinerant electrons are antiferromagnetically
ordered and simultaneously superconducting.
In summary we have developed an itinerant description
of the newly discovered iron based superconductors.
Within our model we have calculated spin
waves as measured in inelastic neutron scattering experiments,
as well as quasiparticle interference signatures
that are believed to appear in SI-STM measurements.
The agreement of our theoretical treatment
and the available experiments leads to the conclusion
that basic physical properties of these new high-Tc materials
can be understood from an itinerant point of
view. In the future we like to investigate to what extent
stronger correlation effects and orbital degrees of
freedom become important.
2.27. Itinerant Magnetism in Iron Based Superconductors 95