LiLi Wang started by motivating
her experimental work as an effort to create higher temperature
superconductors. Displaying a figure showing the discovered superconducting
Tc’s over time, she pointed out that the high Tc materials exceeded
expectations based on the established electron phonon interaction based models
of superconductivity. Predictions
of those models were distilled by McMillan by a formula that suggested a
maximum for Tc limited by the electron-phonon coupling. LiLi noted that the high Tc materials
have lower carrier densities and higher Debye temperatures than the pre-high Tc
superconducting materials.Thus, materials with these two characteristics are
interesting to pursue. LiLi next described two inspirations for increasing
Tc: Ginzburg predicted that
superconducting states could form at the surfaces of materials. These superconducting energy gap of
these states would have to be larger than the bulk gap. The second inspiration came from
experiments showing superconductivity at the interface of two different
semi-conductors by Fogel and coworkers.
Armed with this motivation LiLi
and her coworkers started searching for superconductivity in metal films on
ceramic dielectric substrates with high dielectric constants. LiLi presented STM and transport
results on FeSe and FeTeSe on STO substrates. FeSe superconducts with a bulk Tc of 9 K at ambient pressure
and 37 K at high pressure. Te
substitution can raise the bulk Tc to 15K at ambient pressure.
LiLi presented a comparison of
FeSe films grown on STO and grown on graphene to understand how the substrate
modified the FeSe film properties.
On graphene, FeSe forms islands that are weakly mechanically coupled to
the graphene. The square lattice
spacing a was 0.38 nm, which is close to the bulk value. On STO, the FeSe forms a nearly uniform
epitaxial film with a=0.39 nm, which is the STO lattice constant.
The FeSe films on graphene show a BCS like quasiparticle
tunneling characteristic as measured by in situ STM. Unlike BCS, however, the sub gap conductance has a linear
energy dependence. The
superconducting Tc as determined from the temperature dependence of the zero
bias conductance decreased as 1/d where d is the FeSe film thickness to
disappear at an extrapolated thickness of dc=0.7 nm.
By contrast, FeSe films on STO
only showed a superconducting quasiparticle tunneling characteristic when they
were 1 UC thick. Thicker films
showed no sign of superconductivity.
Remarkably, the conductance showed a large gap with vertical edges, two
peaks at positive and negative polarities and a very low zero bias
conductance. The presence of two
peaks suggests that there are two energy gaps in these films with 2\Delta=25
and 40 meV. The gap closes when
the temperature is raised up to 68 K implying a substantially enhanced Tc in
these FeSe films. ARPES data from
another group also exhibits a large gap (\Delta=15-19 meV) that forms on the
surface of 4 electron pockets centered on the M points in the Brillouin
zone. The gap looks
isotropic.
To corroborate the high Tc
values, LiLi developed a method to cap the FeSe for ex situ transport
measurements. Films capped with a
10UC FeTe and 30 nm thick amorphous Si film exhibited a resistively measured
midpoint Tc of 32 K and a inductively measured Tc of 21 K. Both of these values are lower than the
in situ STM measurements reveal.
Next, LiLi presented STM
measurements on FeTe1-x films to see how substitution affected the
properties. The gap in the
tunneling conductance varied negligibly with Te additions up to 90%
substitution. 100% substitution
quenched the gap structure.
At the end of her talk, LiLi
rhetorically asked what may make Tc so high in these 1 UC FeSe films. Is it a charge doping effect? This
seems possible because gating can affect Tc and recent experiments by a Japanese
group showed that K atoms on the surface of FeSe films could enhance their
superconducting properties. Is it
a surface enhanced electron phonon coupling effect? If so, then adding an STO layer atop the FeSe films might
enhance the Tc further. LiLi
suggested that a STO/FeSe/STO sandwich may be a delectable treat for science!
Blogged by Jim Valles
No comments:
Post a Comment