Session W13: Focus Session: Iron Pnictides and Other Novel Superconductors XIV: ARPES

Thursday, March 19, 2009

The electronic structure in the normal state of Co-doped BaAs$_{2}$Fe$_{2}$ superconductors has been measured by Angle Resolved photoemission (ARPES). Co doping on the Fe site results in electron doping [A. S. Sefat et al., Phys. Rev. Lett. 101, 117004 (2008)]. The data qualitatively reveal that Co-doping results in raising the chemical potential, as expected with electron doping. The Fermi surface topology and the possible relevance to the mechanism of spin fluctuation will also be discussed.   

Thursday, March 19, 2009

The iron-based layered superconductors have galvanized explosive interest in the field of high temperature superconductivity since its discovery early this year. With transition temperatures as high as 55K, this new family of compounds not only ended the monopoly of copper oxides in the high T$_{c}$ field, but also provides us a new direction to better understand the phenomenon of high temperature superconductivity. Here we present recent angle-resolved photoemission results on these iron-based layered superconductors, including direct measurements of the electronic band structures and Fermi surface topology. This new class of superconductors is different from the cuprates in that they have a high density of states near the Fermi level and have multiple bands that cross the Fermi level, which make ARPES an ideal technique to study them because of its unique capability to resolve and capture the rich information on the electronic structure in momentum space.   

Thursday, March 19, 2009

Recently superconductivity has been discovered in many iron pnictides when they are properly doped with charge carriers. Thus it is important to understand the undoped parent compounds that also have a puzzling collinear antiferromagnetic ground state. We have performed a systematic angle-resolved photoemission study on some of the parent compounds, mostly on SrFe$_{2}$As$_{2}$ and BaFe$_{2}$As$_{2}$, to investigate their electronic structure and Fermi surface. We will report our experimental results and the comparisons to first-principle band calculations.   

Thursday, March 19, 2009

The electronic states near the Fermi level are the key ingredient to understand the superconducting mechanism of iron-based superconductor. Although electrons in the iron orbitals have been found to play a key role to the occurrence of the superconductivity, the microscopic origin of high-$T_{c}$ superconductivity is still unclear. To address this important issue, we report our recent high-resolution ARPES results on hole-doped Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$, and demonstrate the Fermi-surface-sheet and momentum dependence of the superconducting gap as well as the doping evolution of the Fermi surface and band structure.   

Thursday, March 19, 2009

High transition temperature superconductivity has been discovered recently in many doped iron pnictides which join the cuprates in the family of high-Tc superconductors. It is very important to understand the nature of the superconducting gap and its doping dependence, as in the case of the cuprates, in order to understand this new class of superconductors. A systematic angle-resolved photoemission spectroscopy (ARPES) study has been performed on the iron pnictide Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ at different K concentrations, to determine its doping dependence of the superconducting gap. We will report our ARPES results and their implications.   

Thursday, March 19, 2009

We present a systematic angle-resolved photoemission spectroscopic study of the high-T$c$ superconductor class (Sr/Ba)$_{1-x}$(K/Na)$_x$Fe$_2$As$_2$. By utilizing a photon-energy-modulation contrast and scattering geometry we report the Fermi surface and the momentum dependence of the superconducting gap, $\Delta(\overrightarrow{k})$. A prominent quasiparticle dispersion kink reflecting strong scattering processes is observed in a binding-energy range of 25-55 meV in the superconducting state, and the coherence length or the extent of the Cooper pair wave function is found to be about 20 $\AA$, which is uncharacteristic of a superconducting phase realized by the BCS-phonon-retardation mechanism. The observed 40$\pm$15 meV kink likely reflects contributions from the frustrated spin excitations in a J$_1$-J$_2$ magnetic background and scattering from the soft phonons. Results taken collectively provide direct clues to the nature of the pairing potential including an internal phase-shift factor in the superconducting order parameter which leads to a Brillouin zone node in a strong-coupling setting.   

Thursday, March 19, 2009

We use angle resolved photoemission spectroscopy (ARPES) to study the momentum dependence of the superconducting gap in NdFeAsO$_{1-x}$F$_x$ single crystals. We find that the $\Gamma$ hole pocket is fully gapped below the superconducting transition temperature. The value of the superconducting gap is 15 $\pm$ 3 meV and its anisotropy around the hole pocket is smaller than 20$\%$ of this value. This is consistent with an isotropic or anisotropic s-wave symmetry of the order parameter or exotic d-wave symmetry with nodes located off the Fermi surface sheets. This is a significant departure from the situation in the cuprates, pointing to possibility that the superconductivity in the iron arsenic based system arises from a different mechanism.   

Thursday, March 19, 2009

The band structure of the parent compounds of iron-arsenic superconductors BaFe2As2, CaFe2As2 and SrFe2As2 are investigated by angle-resolved photoemission spectroscopy. The dispersion of predominant Fe 3d bands has been successfully resolved and compared with theoretical calculations. Although the overall band structure is in line with nonmagnetic DFT computations, the Fe 3d band dispersions strongly deviate from calculations, and the Fermi surface topology differs from theoretical results. These results suggest that some significant correlations have not been correctly involved in the current understanding of these new materials. The kz dependence of the band structure has also been studied for these quasi-2D materials.   

Thursday, March 19, 2009

The discovery of superconductivity at 26 K in LaFeAsO$_{1-x}$F$_{x}$ has triggered intensive researches on the high-temperature ($T_{c})$ superconductivity of iron pnictides and opened a new avenue for high-$T_{c}$ material research beside cuprates. To elucidate the mechanism of high-$T_{c}$ superconductivity in terms of the electronic structure, previous angle resolved photoemission spectroscopy (ARPES) studies have been performed on both hole and electron-doped compounds in the optimally- an non(under)-doped region. On the other hand, little is known about the electronic states in the overdoped region. We report ARPES measurements on heavily overdoped pnictides. Our results indicate that the electronic states around the M point play an important role in the high-$T_{c}$ superconductivity of these materials and suggests that the interband scattering via the antiferromagnetic wave vector essentially controls the $T_{c}$ value in the overdoped region.   

Thursday, March 19, 2009

State-of-art high resolution angle-resolved photoemission spectroscopic studies have been carried out on the electron doped BaFe$_{2-x}$Co$_x$As Superconductor (Tc=26K). Electronic band structure, Fermi surface topology and superconducting gap evolution would be reported in this presentation. Nature of the spin sensity wave (SDW) state would be discussed from a band nesting point of view.   

Thursday, March 19, 2009

The recent discovery of Fe-based superconductors with critical temperatures up to 56 K raises the prospect of unconventional superconducting pairing mechanism. While the electronic pairing in conventional superconductors is mediated by phonons, its nature in the Fe-based high-$T_{c}$ superconductors is unknown. A direct signature of an electron-mode coupling is an anomaly in the electronic energy dispersion (kink). For example, previous angle-resolved photoelectron spectroscopy (ARPES) studies revealed a kink in cuprates, which is believed to be linked to the pairing. We report an ARPES observation of a kink around 25 meV in the dispersion of superconducting Ba$_{0.6}$K$_{0.4}$Fe$_{2}$As$_{2}$ that nearly vanishes above $T_{c}$. The energy scale of the related mode (13$\pm $2 meV) and its strong dependence on orbital and temperature indicates that it is unlikely related to phonons. Moreover, the momentum locations of the kink can be connected by the antiferromagnetic wavevector. Our results point towards an electronic origin of the mode and the superconducting pairing in the Fe-based superconductors, and strongly support the anti-phase s-wave pairing symmetry.   

Thursday, March 19, 2009

CaFe$_{2}$As$_{2}$ is a parent compound of a new family of FeAs based high-$T_{c}$ superconductors. It undergoes a first-order structural transition from low-$T$ orthorhombic to high-$T$ tetragonal phase [Ni \textit{et al.}, Phys. Rev. B \textbf{78}, 014523]. Moderate pressure lowers the transition temperature, and turns on the superconductivity [Torikachvili \textit{et al.}, Phys. Rev. Lett. \textbf{101}, 057006]. Study on its electronic properties is of crucial importance for understanding the pairing mechanism of the FeAs based superconductors. Here we present angle-resolved photoemission spectroscopy (ARPES) results on both the orthorhombic and the tetragonal phase of CaFe$_{2}$As$_{2}$. In the orthorhombic phase, we find strong $k_{z}$ dispersion on the Fermi surfaces, showing a three dimensional electronic structure. We also find dramatic difference of the Fermi surface structure between the orthorhombic and the tetragonal phase.   

Thursday, March 19, 2009

Fermi surface of the parent compound of iron-based superconductor BaFe2As2 is studied by angle-resolved photoemission spectroscopy using VUV-laser. This compound shows structural and magnetic phase transition around TN = 140 K [1]. We found the transformation of Fermi surface across TN. We will discuss its origin comparing with the first principle band calculation. \\[4pt] [1] M. Rotter et al., Phys. Rev B 78, 020503 (2008).   

Thursday, March 19, 2009

Much excitement has surrounded the recent discovery of the doped iron pnictides which exhibit high temperature superconductivity. These new materials are generally grouped into either 1111 or 122 compounds by the stoichiometric formula of the parent compounds. Understanding how doping with in-plane elements with different valence affects their superconducting properties is a vital element of working toward a complete picture of these interesting new compounds. To that end, we have conducted a high-resolution ARPES study of two 122 compounds: Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ and Ba(Fe$_{1-x}$Ni$_{x})_{2}$As$_{2 }$and will report our results.