“Bad metals”
and “good
superconductors”
Surprisingly, the best superconductors
discovered to date are often not very good
metals. This is true for the family holding
the highest superconducting transition
temperatures, the cuprates, but also for
some iron superconductors discovered more
recently. For physicists a “bad metal” is a
complex object that is not well described by
current theories. At the CASSIOPEE beamline,
we have used Angle Resolved Photoemission
to reveal some unexpected characteristic
of the electronic structure of one of this bad
metallic state, namely the formation of a
“pseudogap”.
In the newly discovered iron
superconductors, the origin of
superconductivity still raises many
questions. There is a close proximity
between superconducting and magnetic
phases. This reminds the situation
in another family of high temperature
superconductors, cuprates, and
suggests that, instead of destroying
superconductivity, as it usually does,
magnetism may help in certain cases
to reach superconductivity at quite high
temperatures. Understanding this in more
details may help to create new types
of superconductors, possibly at even
higher temperatures.
To go further, it is necessary to understand
better the nature of magnetism itself and
the role of magnetic correlations in the
metallic state. In cuprates, magnetism sets
in within an insulating background, with
one electron localized at each copper site,
which then order antiferromagnetically.
The Coulomb repulsion is so strong in
these systems that it would cost too much
energy to have two electrons on the same
site and the system chooses instead to
localize one electron per site (this state
is called the Mott insulator).
In iron superconductors, the situation
is quite different. Magnetism sets in within
a metallic background and there are six
electrons per site. However, some of these
phases are “bad metals” meaning
it becomes very difficult for electrons
to move from site to site. Why would “bad
metals” become “good superconductors”?
This apparent contradiction may be at the
heart of the formation of high temperature
superconductivity.
In a recent study, we investigated how
this bad metallic behavior manifests itself
in the electronic structure of Fe
1.06
Te. We
imaged the electronic structure with angle
resolved photoemission spectroscopy
at the CASSIOPEE beamline of the
SOLEIL synchrotron, both in the metallic
paramagnetic phase (i.e. magnetically
disordered, which occurs for temperatures
T above 76K) and in the magnetically
ordered phase (T<76K). Figure
➊
(a )
shows the evolution of one electronic band
in this compound. The Fermi level (the
occupied state having the highest energy)
is shown by the white line and the red
intensity corresponds to states occupied
with electrons. Normally, one gets a metal
if there are bands crossing the Fermi
level, meaning there are partially filled
bands where electrons are free to move.
This is the case for low temperatures: at
20K, in Fig.
➊
(a4 )
, we see electrons up
to the Fermi level and the spectra taken
at this point (Fig. 1b) shows a “Fermi
step” characteristic of the metal. At high
temperatures, the intensity is small at
the Fermi level and the peak in Fig.
➊
(b )
moves away from it. This situation can
be called “pseudogap” as it is intermediate
between the one expected for a metal
(no gap) and an insulator (full gap). The
fact that a good metallic state is recovered
in the magnetic state is a very direct
indication of the role of magnetic disorder
to create the bad metallic state. The idea
is that there is a local tendency at each
Fe site to align the spins of the electrons
in different orbitals and form
a local magnetic moment. In the magnetic
phase, these moments order in a way
optimizing electronic conduction in certain
directions and magnetic fluctuations
are frozen. In the paramagnetic phase,
the disordered moments interact with
conducting electrons and hinder their
motion. There is then in the heart of the
paramagnetic metallic phase, strong
magnetic correlations between electrons
that impact the nature of the metallic
state. How it creates a pseudogap remains
New iron superconductors
Magnetism, superconductivity and metallicity
Anomalous metallic state in Fe1.06Te
SURFACES, INTERFACES AND NANOSYSTEMS
20
SOLEIL
HIGHLIGHTS
2013
