Session A3: Fe-based Superconductors: Pairing Symmetry

Andreev reflection spectroscopy of iron-based superconductors

After a reign of over two decades by the cuprate superconductors, several new families of iron-based high-temperature superconductors have recently been discovered. Essential to a superconductor is the nature of the superconducting gap, its value, its structure, and its temperature dependence. Point contact Andreev reflection (PCAR) spectroscopy operating in the ballistic limit is one of few techniques that can quantitatively measure the gap of these new Fe-based superconductors and its temperature dependence. In SmFeAsO$_{1-x}$F$_{x}$ (0.15 $\le $ x $\le $ 0.30), we have determined a single gap 2$\Delta $/k$_{B}$T$_{C} \quad \approx $ 3.5-3.6 close to the BCS s-wave prediction and with a BCS-like temperature dependence.\footnote{T. Y. Chen, Z. Tesanovic, R. H. Liu, X. H. Chen, and C. L. Chien, Nature, \textbf{453}, 1224 (2008)} These results will be compared with various theoretical possibilities and those obtained by other measurements, such as ARPES and penetration depth. While the principles of the PCAR spectroscopy are well established, poor contact control and ballistic heating might lead to the appearance of spurious gaps and pseudogaps in PCAR measurements. In collaboration with T. Y. Chen, S. X. Huang and Z. Tesanovic at JHU and R. H. Liu and X. H. Chen at USTC.   

Pairing symmetry of iron-based superconductors revealed by ARPES

The recent discovery of superconductivity in iron-arsenic compounds with a transition temperature ($T_{c}$ ) as high as 56 K ended the monopoly of copper oxides in the family of high-$T_{c}$ superconductors. In this talk I will report our angle-resolved photoemission observation of the superconducting gap, including its momentum, temperature, and Fermi surface (FS) dependence in single crystals Ba$_{0.6}$K$_{0.4}$Fe$_{2}$As$_{2}$ ($T_{c}$ = 37 K). We found two nodeless and nearly isotropic superconducting gaps around their respective FS sheets: a large gap ($\Delta \quad \sim $ 12 meV) on the two small hole-like and electron-like FS sheets, and a small gap ($\sim $ 6 meV) on the large hole-like FS. The isotropic pairing interactions are strongly orbital dependent, as the ratio 2$\Delta $/$k_{B}T_{c}$ switches from weak to strong coupling on different bands. In addition, we have observed a dispersion kink that is likely related to a spin mode. These results reveal the importance of inter-band interactions in the pairing mechanism, and support the anti-phase $s$-wave pairing symmetry in the Fe-based superconductors.   

Magnetic penetration depth of oxy-ferropnictide superconductors

The determination of the symmetry of the superconducting order parameter is an important first step toward uncovering the mechanism of superconductivity in any material. In this regard, measurements of the magnetic penetration depth $\lambda$ have played an important role. Although not a true bulk probe, like specific heat, penetration depth measurements in the Meissner state probe a few thousand Angstroms below the crystal surface and so should be reasonably representative of the bulk. In this talk I will present data for the in-plane magnetic penetration depth of three different families oxy-ferropnictide superconductors, measured on single crystals using a sensitive radio frequency tunnel diode oscillator technique. Our results for samples of SmFeAsO$_{1-x}$F$_y$ ($x\simeq y \simeq 0.2$) with $T_c \simeq 45$\,K show that $\lambda(T)$ has an exponential temperature dependence suggesting that the Fermi surface is fully gapped. However, data for Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ ($T_c\simeq $22 K) show power-law behavior, $\Delta \lambda (T) \sim T^{1.7}$, at low temperature, possibly suggestive of line-nodes. Finally, data for LaFePO ($T_c\simeq 6$K), also show a power-law behavior for $\lambda(T)$, but in this case the temperature dependence is almost perfectly linear down to $T\simeq 100$mK. The results suggest that the gap symmetry may not be universal in all the oxy-ferropnictide superconductors.   

Antiferromagnetic Correlation and the Pairing Mechanism of the Cuprates and Iron Pnictides : a View From the Functional Renormalization Group Studies

We study the pairing symmetry of the iron pnictide superconductor using the functional renormalization group method. By comparing the results for the cuprates and the iron pnictides a coherent picture emerges. It suggests that antiferromagnetic correlation causes pairing for both materials. In collaboration with Fa Wang, Hui Zhai, Ying Ran, and Ashvin Vishwanath, University of California, Berkeley.   

Superconductivity, magnetism, and pairing symmetry in Fe-based superconductors

We analyze antiferromagnetism and superconductivity within the renormalization group(RG) technique in novel Fe-based superconductors using the itinerant model of small electron and hole pockets near (0; 0) and ($\pi$,$\pi$), respectively, originating from the two strongly hybridized orbitals. We find that, for this model, the bare interactions in the Cooper channel are repulsive, and superconductivity does not occur at the mean-field level. However, under RG the effective interaction in the superconducting channel changes sign and becomes attractive. Furthermore, the effective interactions in antiferromagnetic and superconducting channels logarithmically flow towards the same absolute values at low energies, i.e., both must be treated on equal footings. The magnetic instability comes first for equal sizes of the two pockets, but looses to superconductivity upon doping. The superconducting gap has no nodes, but changes sign between the two Fermi surfaces (extended s-wave symmetry). We argue that the T dependencies of the spin susceptibility and NMR relaxation rate for such state are exponential only at very low T, and can be well fitted by power-laws over a wide T range below Tc. We further show that below Tc excitonic resonance appears in the spin excitations spectrum. \\[4pt] [1] M. Korshunov, and I. Eremin, Phys. Rev. B 78, 140509(R) (2008) \\[0pt] [2] A.V. Chubukov, D. Efremov, I. Eremin, Phys. Rev. B 78, 134512 (2008). \\[0pt] [3] M.M. Korshunov and I. Eremin, Europhys. Lett. 83, 67003 (2008).