3 research outputs found
Structural collapse and superconductivity in rare earth-doped CaFe2As2
Aliovalent rare earth substitution into the alkaline earth site of CaFe2As2
single-crystals is used to fine-tune structural, magnetic and electronic
properties of this iron-based superconducting system. Neutron and single
crystal x-ray scattering experiments indicate that an isostructural collapse of
the tetragonal unit cell can be controllably induced at ambient pressures by
choice of substituent ion size. This instability is driven by the interlayer
As-As anion separation, resulting in an unprecedented thermal expansion
coefficient of K. Electrical transport and magnetic
susceptibility measurements reveal abrupt changes in the physical properties
through the collapse as a function of temperature, including a reconstruction
of the electronic structure. Superconductivity with onset transition
temperatures as high as 47 K is stabilized by the suppression of
antiferromagnetic order via chemical pressure, electron doping or a combination
of both. Extensive investigations are performed to understand the observations
of partial volume-fraction diamagnetic screening, ruling out extrinsic sources
such as strain mechanisms, surface states or foreign phases as the cause of
this superconducting phase that appears to be stable in both collapsed and
uncollapsed structures.Comment: 15 pages, 18 figure
Spectroscopic scanning tunneling microscopy insights into Fe-based superconductors
In the first three years since the discovery of Fe-based high Tc
superconductors, scanning tunneling microscopy (STM) and spectroscopy have shed
light on three important questions. First, STM has demonstrated the complexity
of the pairing symmetry in Fe-based materials. Phase-sensitive quasiparticle
interference (QPI) imaging and low temperature spectroscopy have shown that the
pairing order parameter varies from nodal to nodeless s\pm within a single
family, FeTe1-xSex. Second, STM has imaged C4 -> C2 symmetry breaking in the
electronic states of both parent and superconducting materials. As a local
probe, STM is in a strong position to understand the interactions between these
broken symmetry states and superconductivity. Finally, STM has been used to
image the vortex state, giving insights into the technical problem of vortex
pinning, and the fundamental problem of the competing states introduced when
superconductivity is locally quenched by a magnetic field. Here we give a
pedagogical introduction to STM and QPI imaging, discuss the specific
challenges associated with extracting bulk properties from the study of
surfaces, and report on progress made in understanding Fe-based superconductors
using STM techniques.Comment: 36 pages, 23 figures, 229 reference