#
APS March Meeting 2010

## Volume 55, Number 2

##
Monday–Friday, March 15–19, 2010;
Portland, Oregon

### Session J1: The Gap Structure of the Fe Superconducters

11:15 AM–2:15 PM,
Tuesday, March 16, 2010

Room: Oregon Ballroom 201

Sponsoring
Unit:
DCMP

Chair: Douglas Scalapino, University of California, Santa Barbara

Abstract ID: BAPS.2010.MAR.J1.3

### Abstract: J1.00003 : Anisotropic London Penetration Depth in Iron-based Pnictide Superconductors

12:27 PM–1:03 PM

Preview Abstract
Abstract

####
Author:

Ruslan Prozorov

(Ames Lab and Iowa State University)

The temperature dependent London penetration depth, $\lambda
\left( T \right)$, is linked directly to the structure of the
superconducting gap, thus providing valuable insight into the
pairing mechanism. I will summarize measurements of the
penetration depth in single crystals of iron-based pnictide
superconductors comparing the ``1111'', ``11'' and ``122''
families. Compatibility of our results with other gap sensitive
probes, such as ARPES and thermal conductivity, will be
addressed. A detailed discussion of the doping dependent
penetration depth will be given for the well-characterized
``122'' family,
(Ba$_{1-y}$K$_{y})$(Fe$_{1-x}$T$_{x})_{2}$As$_{2}$ (T=Co, Ni, Pd,
Rh). Overall, $\lambda \left( T \right)$ exhibits a power law
variation at low
temperatures, $\lambda \left( T \right)=\lambda \left( 0
\right)+bT^n$ (down to 80 mK in the case of FeNi-122). The
exponent $n$ is typically less than 2.8, which is clearly
different from $n\approx 4$ that parameterizes the exponential
behavior expected for conventional fully gapped s-wave
superconductors. The low-temperature parameters, $\lambda \left(
0 \right)$, $b$ and $n$ depend on the doping level and the
orientation of a magnetic field with respect to the crystal axes.
This evolution is best observed in the out-of-plane penetration
depths, $\lambda _c \left( T \right)$, which at
least in the FeNi-122 system, changes from a high power in the
underdoped regime to $T-$linear in overdoped samples.
Simultaneously, the in-plane penetration depth, $\lambda _{ab}
\left( T \right)$, evolves towards a sub-quadratic behavior with
$n\approx 1.7$. Furthermore, analysis of the superfluid density
in the full temperature range is consistent with two-gap
superconductivity. However, the temperature dependencies of the
anisotropies, ${\lambda _c } \mathord{\left/ {\vphantom {{\lambda
_c } {\lambda _{ab} }}} \right. \kern-\nulldelimiterspace}
{\lambda _{ab} }$ and ${\xi _{ab} } \mathord{\left/ {\vphantom
{{\xi _{ab} } {\xi _c }}} \right.
\kern-\nulldelimiterspace} {\xi _c }$, are opposite compared to
another two-gap superconductor, MgB$_{2}$. Consistency of these
results with theories that explain the power law behavior to be
due to scattering in a two-dimensional $s_\pm $ model will be
discussed.
Overall, our results suggest that the superconducting gap in
iron-based pnictide superconductors develops nodal structure in
the overdoped regime with nodes located at finite $k_z $ wave
vectors on a three-dimensional Fermi surface.
\\[4pt]
\textbf{References:} C. Martin \textit{et al.}, Phys. Rev. Lett.
\textbf{102}, 247002 (2009); R. T. Gordon \textit{et al.}, Phys.
Rev. Lett. \textbf{102}, 127004 (2009); R. T. Gordon \textit{et
al.}, Phys. Rev. B \textbf{79}, 100506(R) (2009)

To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2010.MAR.J1.3