Session B22: Focus Session: Fe-based Superconductors- ARPES and Fermi Surfaces

Distinct Fermi Surface Topology in A$_x$Fe$_{2-y}$Se$_2$ Revealed by ARPES

The discovery of superconductivity with a transition temperature above 30 K in A$_x$Fe$_{2-y}$Se$_2$ (A=alkali metal or Thallium) triggered a new wave of research on the iron-based superconductors. The new A$_x$Fe$_{2-y}$Se$_2$ superconductor exhibits many unique characteristics which make it a new platform to uncover the pairing mechanism of the iron-based superconductors. In this talk, we will show the electronic structures of A$_x$Fe$_{2-y}$Se$_2$ by means of ARPES, including Fermi Surface topology, superconducting gap structure and electron dynamics. The observed Fermi surface topology with only electron-like pockets is distinct from other iron-based superconductors and challenges the pairing mechanism based on scattering between electron-like and hole-like Fermi surface sheets. Nearly isotropic superconducting gap without nodes is revealed for all Fermi surface sheets which favors s-wave pairing symmetry. Other related topics, such as Fe vacancy order and phase separation, will also be discussed from viewpoint of our ARPES results. \\[4pt] [1] D. Mou, S. Liu, J. He, et. al, Phys. Rev. Lett. 106, 107001 (2011).\\[0pt] [2] L. Zhao, D. Mou, S. Liu, et al., Phys. Rev. B 83, 140508(R) (2011).   

Role of Degeneracy, Hybridization, and Nesting in the Properties of Multi-Orbital Systems

To understand the role that degeneracy, hybridization, and nesting play in the magnetic and pairing properties of multiorbital Hubbard models we here study numerically two types of two-orbital models, both with hole-like and electron-like Fermi surfaces (FS's) that are related by nesting vectors ($\pi$, 0) and (0, $\pi$) [1]. In one case the bands that determine the FS's arise from strongly hybridized degenerate dxz and dyz orbitals, while in the other the two bands are determined by non-degenerate and non-hybridized s-like orbitals. In the weak coupling regime it is shown that only the model with hybridized bands develops metallic magnetic order, while the other model exhibits an ordered excitonic orbital-transverse spin state that is insulating and does not have a local magnetization. Thus this state would be observed by ARPES experiments, but not by neutron scattering. However, both models display similar insulating magnetic stripe ordering in the strong coupling limit when Coulomb interactions create strong hybridization of the orbitals.\\[4pt] [1] A. Nicholson, et al., Phys. Rev. B 84, 094519 (2011).   

Thermoelectric power of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$, $0\le x\le 0.025$: tracking changes in Fermi surface topology

Temperature-dependent, in-plane, thermoelectric power (TEP) data will be presented for single crystals of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$, ($0\le x \le 0.025$). Previously reported TEP data for this system showed a big jump in the TEP data from x=0.02 to x=0.024 suggesting a Lifshitz transition, a result which was later confirmed by ARPES measurement. Given that TEP and ARPES delineated a rather large region for the Lifshitz transition to occur, and the underdoped side of the phase diagram is poorly explored, newly careful measurements of TEP on tightly spaced concentrations of Co, $0\le x \le 0.025$, were carried out. The data show clear evidence of a Lifshitz transition, but instead of a discontinuous jump in TEP between $0\le x \le 0.025$, there is a more gradual evolution in the S(T) plots as x is increased.   

Angle-resolved photoemission spectroscopy study of Ba(Fe$_{1-x}$Ru$_{x})_{2}$As$_{2}$

Ru-doped BaFe$_{2}$As$_{2}$ compounds were discovered to show superconductivity in a relatively wide doping range. We have performed angle-resolved photoemission spectroscopy measurements on a series of Ru-doped BaFe$_{2}$As$_{2}$ samples. We observed that band dispersions become more three-dimensional and Fermi velocities increase significantly with Ru doping. We will report these results and discuss implications to its superconductivity.   

What rome does the Fermi surface play in tuning the properties of iron arsenic superconductors?

External control parameters such as pressure or chemical substitution are the key to extend the phase space and achieve high temperature (T$_{c})$ superconductivity in the FeAs family. These materials show interesting properties where it is important to understand the role of Fermi surfaces (FS's) in the mechanism of yielding higher T$_{c}$. Here, we use angle-resolved photoemission to study the electronic structure of the Ba(Fe$_{1-x}$Ru$_{x})_{2}$As$_{2}$ as a function of Ru concentration ($x)$. We find that the substitution of Ru for Fe is isoelectronic, i. e., it does not change the value of the chemical potential. More interestingly, there are no measured significant changes in the shape of the FS or in the Fermi velocity over a wide range [1]. We contrast this unusual behavior with the Co substitution, where even small substitutions induce large changes not only in the size of the FS pockets but also in the FS topology [2]. Given that the suppression of the antiferromagnetic and structural phase has been associated with the emergence of the superconducting state, Ru substitution must achieve this via a mechanism that does not involve changes of the Fermi surface. We speculate that this mechanism relies on magnetic dilution that leads to the reduction of the effective Stoner enhancement. \\[4pt] [1] R. S. Dhaka, \textit{et al.}, PRL, (2011). \\[0pt] [2] Chang Liu, \textit{et al.}, Nature Physics, \textbf{6}, 419 (2010).   

Fermi surface in BaFe$_2$As$_2$ via SdH measurements on detwinned crystals

We have completely determined the Fermi surface in the antiferromagnetic orthorhombic phase of BaFe$_2$As$_2$ by measuring Shubnikov-de Haas oscillations in detwinned single crystals (T. Terashima et al., PRL 107, 176402 (2011)). The determined Fermi surface consists of one hole and two electron pockets, and the carrier compensation is satisfied, the carrier number being 0.024 holes and electrons per primitive unit cell. The Fermi surface can well be accounted for by an LSDA band-structure calculation using the experimental crystal structure. The mass enhancements $m^*/m_{band}$ are found to be 2--3. The Sommerfeld coefficient estimated from the determined Fermi surface and effective masses agrees well with an experimental value. Previous ARPES reports are not very consistent with our determined Fermi surface.   

Fermi surface topology and low-lying electronic structure of a new iron-based superconductor Ca$_{10}$(Pt$_{3}$As$_{8})$(Fe$_{2}$As$_{2})_{5}$

We report a first study of low energy electronic structure and Fermi surface topology for the recently discovered iron-based superconductor Ca$_{10}$(Pt$_{3}$As$_{8})$(Fe$_{2}$As$_{2})_{5}$ (the 10-3-8 phase, with Tc $\sim $ 8 K), via angle resolved photoemission spectroscopy (ARPES). Despite its triclinic crystal structure, ARPES results reveal a fourfold symmetric band structure with the absence of Dirac-cone-like Fermi dots (related to magnetism) found around the Brillouin zone corners in other iron-based superconductors. Considering that the triclinic lattice and structural supercell arise from the Pt$_{3}$As$_{8}$ intermediary layers, these results indicate that those layers couple only weakly to the FeAs layers in this new superconductor at least near the surface, which has implications for the determination of its potentially novel pairing mechanism.   

Recent advance in ARPES data analysis of dispersive features in Fe-based superconductors

Angle-resolved photoemission spectroscopy (ARPES) is a powerful tool to image the electronic band dispersion of materials, especially in multi-band systems such as the Fe-based superconductors. Here we present a new method to visualize ARPES data based on the mathematical concept of curvature, which improves the advantages and the reliability of the second derivative method in tracking the positions of extrema from the experimental data. We apply it to the Fe-based superconductors. We reveal clear kink features and FS contours, making it easier to capture the essential physics from the data. \\[4pt] [1] P. Zhang, P. Richard, T. Qian, Y.-M. Xu, X. Dai and H. Ding, \emph{A precise method for visualizing dispersive features in image plots}, Rev. Sci. Instrum. \textbf{82}, 043712 (2011).   

Fermi Surfaces of Iron-Pnictide High-T$_{\rm c}$ Superconductors from the Limit of Local Magnetic Moments

We study a 2-orbital t-J model for an isolated square lattice of iron atoms, which stack up to form an iron-pnictide high-T$_{\rm c}$ superconductor. The two orbitals in question are the degenerate $d\pm = 3d_{(x\pm iy)z}$ ones, which maximize the Hund's Rule coupling. First-neighbor and second-neighbor hopping (t) and Heisenberg exchange (J) are included. A Schwinger-boson-slave-fermion mean-field analysis yields a hidden half metal state in which holes hop through a $\nwarrow_{d+}\searrow_{d-}$ spin background without much hopping across orbitals. This state is characterized by an inner and an outer Fermi surface pocket centered at the $\Gamma$ point. The Fermi surface pockets resemble those predicted by band structure calculations that include all five $3d$ orbitals. By sweeping the Hund's coupling, we also identify a quantum-critical point (QCP) where zero-energy spin-wave excitations exist at the momenta associated with commensurate spin-density-wave (cSDW) order. These low-energy spin-waves result in nested Fermi-surface pockets centered at cSDW momenta. Exact diagonalization of one hole in the 2-orbital t-J model over a 4$\times$4 square lattice yields low-energy spectra that are consistent with the nested Fermi surfaces that are predicted to exist at the QCP.   

dHvA measurements of the Fermi surface of LiFeP and its relation to the nodal gap structure

The iron-pnictides are highly unusual in that there appears to be considerable variation in the structure of the superconducting gap across the different materials. The 111 compounds, LiFeX (X=As, P) superconduct at ambient pressure in their undoped stoichiometric form. LiFeP (($T_c$==5K) was found to have superconducting gap nodes whereas LiFeAs ($T_c$=17K) does not. Linking these differences in gap structure to Fermi surface features could provide a key test of microscopic theories which seek to explain superconductivity in iron pnictides. Here we report de Haas-van Alphen effect data which determine, almost completely, the {\it bulk} Fermi surface of LiFeP. The topology of the Fermi surface, which consists of quasi nested electron and hole sheets, is in good agreement with DFT band structure calculations when allowance for small band energy shifts is made. We find that one hole sheet has a anomalously small mass enhancement (compared to the others) which suggest it interacts weakly. This is probably because of its mixed orbital character rather than for any geometrical reason. We suggest that this could be the driver for node formation in this material.   

Evidence of Strong Coupling in Antiferromagnetic Ordered Iron Chalcogenide Fe$_{1.02}$Te Observed by Photoemission

The role of many-body effects is one of the central questions for unconventional superconductivity. For the recently discovered iron-based superconductors, the strength of electronic correlations is still an unsettled issue. For one of them, iron chalcogenides, a strong correlation scenario has both been proposed by theory and suggested by experiments. However, the metallic behavior in the antiferromagnetic ordered state in Fe$_{1.02}$Te seems to deviate from such scenario. Our discovery of evidence of strong coupling in electronic bandstructure probed by angle resolved photoemission (ARPES) reconciles this contrast. Our finding also highlights the non-trivial enrichment of many-body effects when multiple ingredients of interactions reinforce each other.   

Three-dimensionality and orbital characters of Fermi surface in Tl$_{0.5}$Rb$_{0.3}$Fe$_{1.63}$Se$_{2}$

We report a comprehensive study of the tridimensional electronic bands in the recently discovered Iron-selenide superconductor Tl$_{0.5}$Rb$_{0.3}$Fe$_{1.63}$Se$_2$ (T$_c$$\sim$32 K) with angle-resolved photoemission spectroscopy (ARPES). We determined the orbital characters and the $k_z$ dependence of the low-energy electronic structure by tuning the polarization and the photon energy of the incident photons. We observed a small 3D electron pocket near the Brillouin Zone (BZ) center and a 2D like electron pocket near the zone boundary. The photon energy dependence, the polarization analysis and the LDA calculations suggest a significant contribution from the Se 4$p_z$, Fe 3$d_{xy}$ and the Fe 3$d_{z^2}$ orbitals. Comparing with iron-pnictide superconductors, the emergence of Se 4$p_z$ states may be the cause of the different magnetic properties between iron-chalcogenides and iron-pnictides.   

ARPES measurements of superconducting gaps in iron-chalcogenide superconductors

The size and momentum dependence of the superconducting gap are crucial to the determination of the mechanism leading to Cooper pairing. Previous ARPES results on iron-pnictides superconductors reveal nearly-isotropic superconducting gaps with size varying from one Fermi surface to another. Here we show that this scheme is also valid in the iron-chalcogenide superconductors. We demonstrate that the superconducting gaps can be fitted by a single function derived from local pairing scenarios. Our finding of an apparent universality in iron-based superconductivity is a serious challenge to weak coupling approaches and rather favors pairing from local antiferromagnetic exchange interactions.