77 research outputs found
Universal pair-breaking in transition metal-substituted iron-pnictide superconductors
The experimental transport scattering rate was determined for a wide range of
optimally doped transition metal-substituted FeAs-based compounds with the
ThCr2Si2 (122) crystal structure. The maximum transition temperature Tc for
several Ba-, Sr-, and Ca-based 122 systems follows a universal rate of
suppression with increasing scattering rate indicative of a common
pair-breaking mechanism. Extraction of standard pair-breaking parameters puts a
limit of \sim26 K on the maximum Tc for all transition metal-substituted 122
systems, in agreement with experimental observations, and sets a critical
scattering rate of 1.5x10^14 s^-1 for the suppression of the superconducting
phase. The observed critical scattering rate is much weaker than that expected
for a sign-changing order parameter, providing important constraints on the
nature of the superconducting gap in the 122 family of iron-based
superconductors.Comment: 4 pages, 3 figure
Quenched Fe Moment in the Collapsed Tetragonal Phase of CaPrFeAs
We report As NMR studies on single crystals of rare-earth doped iron
pnictides superconductor CaPrFeAs (=0.075 and
0.15). The As spectra show a chemical pressure effect with doping and a
first order structure transition to the collapsed tetragonal phase upon
cooling. A sharp drop of the Knight shift is seen below the structural
transition, whereas is strongly enhanced at low-temperatures. These
evidences indicate quenching of Fe local magnetism and short-range ordering of
Pr moment in the collapsed tetragonal phase. The quenched Fe moment
through structure collapse suggests a strong interplay of structure and
magnetism, which is important for understanding the nature of the collapsed
tetragonal phase.Comment: 5 pages, 5 figure
Superconductivity at 23 K in Pt doped BaFe2As2 single crystals
We report superconductivity in single crystals of the new iron-pnictide
system BaFe1.9Pt0.1As2 grown by a self-flux solution method and characterized
via x-ray, transport, magnetic and thermodynamic measurements. The magnetic
ordering associated with a structural transition at 140 K present in BaFe2As2
is completely suppressed by substitution of 5% Fe with Pt and superconductivity
is induced at a critical temperature Tc=23 K. Full diamagnetic screening in the
magnetic susceptibility and a jump in the specific heat at Tc confirm the bulk
nature of the superconducting phase. All properties of the superconducting
state including transition temperature Tc, the lower critical field Hc1=200 mT,
upper critical field Hc2~65 T, and the slope dHc2/dT are comparable in value to
the those found in other transition-metal-substituted BaFe2As2 series,
indicating the robust nature of superconductivity induced by substitution of
Group VIII elements.Comment: 6 pgs, 4 figs, and 1 tbl, slightly revised, updated reference
Role of electron-electron interactions in the charge dynamics of rare-earth-doped CaFe2As2
We have investigated the charge dynamics and the nature of many-body interactions in La- and Pr-doped CaFe2As2. From the infrared part of the optical conductivity, we discover that the scattering rate of mobile carriers above 200 K exhibits saturation at the Mott-Ioffe-Regel limit of metallic transport. However, the dc resistivity continues to increase with temperature above 200 K due to the loss of Drude spectral weight. The loss of Drude spectral weight with increasing temperature is seen in a wide temperature range in the uncollapsed tetragonal phase, and this spectral weight is recovered at energy scales about one order of magnitude larger than the Fermi energy scale in these semimetals. The phenomena noted above have been observed previously in other correlated metals in which the dominant interactions are electronic in origin. Further evidence of significant electron-electron interactions is obtained from the presence of quadratic temperature and frequency-dependent terms in the scattering rate at low temperatures and frequencies in the uncollapsed tetragonal structures of La-doped and Pr-doped CaFe2As2. For temperatures below the structure collapse transition in Pr-doped CaFe2As2 at similar to 70 K, the scattering rate decreases due to weakening of electronic correlations, and the Drude spectral weight decreases due to modification of the low-energy electronic structure
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
Thermodynamics and structure of self-assembled networks
We study a generic model of self-assembling chains which can branch and form
networks with branching points (junctions) of arbitrary functionality. The
physical realizations include physical gels, wormlike micells, dipolar fluids
and microemulsions. The model maps the partition function of a solution of
branched, self-assembling, mutually avoiding clusters onto that of a Heisenberg
magnet in the mathematical limit of zero spin components. The model is solved
in the mean field approximation. It is found that despite the absence of any
specific interaction between the chains, the entropy of the junctions induces
an effective attraction between the monomers, which in the case of three-fold
junctions leads to a first order reentrant phase separation between a dilute
phase consisting mainly of single chains, and a dense network, or two network
phases. Independent of the phase separation, we predict the percolation
(connectivity) transition at which an infinite network is formed that partially
overlaps with the first-order transition. The percolation transition is a
continuous, non thermodynamic transition that describes a change in the
topology of the system. Our treatment which predicts both the thermodynamic
phase equilibria as well as the spatial correlations in the system allows us to
treat both the phase separation and the percolation threshold within the same
framework. The density-density correlation correlation has a usual
Ornstein-Zernicke form at low monomer densities. At higher densities, a peak
emerges in the structure factor, signifying an onset of medium-range order in
the system. Implications of the results for different physical systems are
discussed.Comment: Submitted to Phys. Rev.
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
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