Session T37: Focus Session: Fe-based Superconductors: Spectroscopic Probes

ARPES studies of underdoped (Ba,K)Fe2As2 iron-based superconductors

Phase competition is a topic of high interest in the high temperature superconductivity (HTSC) field as HTSC occurs in proximity to competing phases in both cuprates and iron pnictides. In the pnictides, phase competition to superconductivity takes form in both a tetragonal to orthorhombic structural transition and a collinear spin-density wave transition. In this talk, I will present our ARPES studies of underdoped (Ba,K)Fe2As2, in which distinct spectroscopic signatures associated with all three transitions (structural, SDW, and superconductivity) are observed. The interaction of these three order parameters will be discussed.   

Orbital Dependent Band Renormalization in Fe$_{1+y}$Te$_{1-x}$Se$_{x}$

One of the important factors in understanding the Fe-based superconductor is their multi-orbital nature. In this study we present ARPES results on the iron chalcogenide Fe$_{1+y}$Te$_{1-x}$Se$_{x}$ (known as the 11 system), the structurally simplest member in Fe-based superconductors. Our result shows that as Te substitutes Se, the Fe dxy orbital has seen a significant increase in the band renormalization while the other orbitals stay unchanged. Our discovery indicates that different orbitals in Fe-based superconductors have different correlation levels, evolve distinctively with crystal parameters and may play different roles in the emergence of superconductivity.   

Iron Selenide thin films studied with ARPES

Dimensionality and length scales play an important role in material properties and their phases. Recently, superconductivity was discovered in a thin film of Iron Selenide just 1 unit cell thick. We have grown Iron Selenide films of different thickness with molecular beam epitaxy and measured these films in situ with angle-resolved photoemission spectroscopy (ARPES). The ability to measure films in situ eliminates the need for Se capping and provides high quality ARPES data. We will discuss these results, among them what changes can be observed in the band structure between films of different thicknesses.   

Laser ARPES study of optimally doped FeTe$_{0.6}$Se$_{0.4}$

We have studied the electronic structure of optimally doped FeTe$_{0.6}$Se$_{0.4}$ ($T_c$ = 14.5 K), using laser-excited angle-resolved photoemission spectroscopy (laser ARPES). We observe sharp superconducting coherence peaks in the hole band slightly shifted from the $\Gamma$ point at $T$ = 2.5 K. In contrast to earlier ARPES studies but consistent with thermodynamic results, the momentum dependence shows a $\cos(4\varphi)$ modulation of the SC-gap anisotropy. In addition, we found an electron band at the $\Gamma$ point, lying just above $E_F$. This electron band also shows a sharp superconducting coherence peak with gap formation below $T_c$. The hole and electron bands show significantly different values of superconducting gap $\Delta$ and Fermi energy $\epsilon_F$ , while the associated Bogoliubov quasiparticle dispersions get merged. The results suggest composite superconductivity in an iron-based superconductor, consisting of strong-coupling Bose-Einstein condensation (BEC) in the electron band while the hole band superconductivity lies closer to the weak-coupling Bardeen-Cooper-Schrieffer (BCS) limit.   

Thermodynamic signatures of quantum criticality in BaFe$_2$(As$_{(1-x)}$P$_x$)$_2$

Iron based superconductors are one of many classes of material where superconductivity occurs in the vicinity of a magnetic quantum critical point (QCP). The degree to which the QCP drives or otherwise influences the high temperature superconductivity is however still a matter of debate. In this context it is useful to determine experimentally, the degree to which the quasiparticle effective mass diverges at the QCP and how this is reflected in various physical properties. Here we will report measurements of the specific heat $\gamma$ and the de Haas-van Alphen effect which quantify these effects. Far from the QCP the enhancement of the mass as measured by $\gamma$, dHvA and the magnetic penetration depth $\lambda$ are all consistent. However, very close to the QCP significant differences are found which likely result from finite temperature and/or multi-band effects.   

Anisotropic superconducting gap distribution in the presence of spin density wave in Co-doped NaFeAs

The coexisting regime of spin density wave (SDW) and superconductivity in the iron pnictides represents a novel ground state. We have performed high resolution angle-resolved photoemission measurements on NaFe$_{1-x}$Co$_{x}$As ($x=0.0175$) in this regime and revealed its distinctive electronic structure, which provides some microscopic understandings of its behavior. The SDW signature and the superconducting gap are observed on the same bands, illustrating the intrinsic nature of the coexistence. However, because the SDW and superconductivity are manifested in different parts of the band structure, their competition is non-exclusive. Particularly, we found that the gap distribution is anisotropic and nodeless, in contrast to the isotropic superconducting gap observed in an SDW-free NaFe$_{1-x}$Co$_{x}$As (x=0.045), which puts strong constraints on theory.   

Evidence of competing $s$ and $d$-wave pairing channels in iron-based superconductors

Superconductivity is determined by the interactions that drive Cooper pairing. However, experimental access to the pairing potential $V_{\mathbf{k},\mathbf{k}'}$ becomes increasingly complicated upon going from conventional metals to complex systems such as the cuprates, some heavy fermion compounds or the iron-based superconductors. We show that electronic Raman scattering affords a window into the essential properties of $V_{\mathbf{k},\mathbf{k}'}$ of iron-based superconductors. In ${\rm Ba_{0.6}K_{0.4}Fe_2As_2}$ we observe band dependent energy gaps along with excitonic Bardasis-Schrieffer modes characterizing, respectively, the dominant and subdominant pairing channel. The $d_{x^2-y^2}$ symmetry of all excitons allows us to identify the subdominant channel to originate from the interaction between the electron bands. Consequently, the dominant channel driving superconductivity results from the interaction between the electron and hole bands and has the full lattice symmetry. The results in ${\rm Rb_{0.8}Fe_{1.6}Se_2}$ along with earlier ones in ${\rm Ba(Fe_{0.939}Co_{0.061})_2As_2}$ highlight the influence of the Fermi surface topology on the pairing interactions.   

A de Haas-van Alphen study of the Fermi surface of LiFeP

We report de Haas-van Alphen (dHvA) measurements of the Fermi surface of the 111 iron based superconductor LiFeP with $T_c\approx5$ K. Comparison of our experimental results to density functional theory band-structure calculations show good agreement. As in other iron-based superconductors we find that the electron and hole bands are quasi-nested. The effective masses, determined individually for the different Fermi surface sheets (orbits) generally show significant enhancement. The smallest hole pocket sheet is an exception to this and shows a very small enhancement. This difference in the many body interaction suggest a suppression of electron-hole scattering for this sheet which may result from its different orbital character. This might be the reason why LiFeP has nodes in its superconducting gap whereas its sister compound LiFeAs does not.   

Optical conductivity and Raman scattering of iron superconductors

Raman and optical conductivity are very useful techniques to analyze the electronic properties of strongly correlated electron systems. Optical conductivity experiments have provided very valuable information on the reorganization of the spectral weight and the opening of gaps in many materials. In cuprates the use of different polarizations in Raman scattering has allowed to disentangle the different physics of the nodal and the antinodal electronic states. The multiband character of iron superconductors complicates the analysis of their Raman and optical conductivity spectra. We discuss how to analyze the optical conductivity and Raman spectrum of multi-orbital systems using velocity and Raman vertices in a similar way Raman vertices were used to disentangle nodal and antinodal regions in cuprates. We apply this method to iron superconductors in the magnetic and non-magnetic state, including the orbital differentiation regime. We also show that the Drude weight anisotropy in the magnetic state is sensitive to small changes in the lattice structure.   

Non-Resonant Raman Scattering in an effective single orbital model of Iron based superconductors

We investigate non-resonant Raman response of the 122-type Iron pnictide and chalcogenide superconductors using the framework of an effective single orbital model that was recently proposed to capture the essential electronic and magnetic properties of Iron based superconductors. We compute the momentum matrix elements and the resulting Raman vertices exactly (within the tight- binding approximation) for different polarization geometries of the hole/electron doped 122 pnictide and electron overdoped 122 chalcogenide. Our calculations, performed with a simple coskxcosky form for the gap, find good agreement with data reported by Kretzschmar et. al and Muschler et.al   

Nonlinear optical study of surface electrons on Ba(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$

We report second harmonic generation (SHG) measurements on single crystals of Ba(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$. SHG from Ba(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$ is dominated by surface contributions due to the broken inversion symmetry at the surface. By varying the polarization of incident ultrafast laser pulses, we demonstrate that SHG reveals the tetragonal crystal structure of Ba(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$ at ambient conditions. We will discuss prospects of using SHG as a probe of the surface electrons, the in-plane anisotropy, and the dichotomy between surface and bulk superconductivity in iron-based superconductors.   

Infrared Faraday measurements on Ba(Fe$_{\mathrm{1-x}}$Co$_{\mathrm{x}}$)$_{2}$As$_{2}$ superconductors

We report infrared Faraday measurements on electron-doped Ba(Fe$_{\mathrm{1-x}}$Co$_{\mathrm{x}}$)$_{2}$As$_{2}$ superconducting films, which are grown by pulsed laser deposition. The complex Faraday angle $\theta_{\mathrm{F}}$ is proportional to the difference of the sample's response to right and left circularly polarized light, making it a highly sensitive tool to probe electronic structure, electron-electron correlations, and magnetic ordering. We measure $\theta _{\mathrm{F}}$ for normal and superconducting states in the 110-1400 meV range at temperatures down to 10K and magnetic fields up to 7T. This work is supported by NSF-DMR1006078.   

Shallow pockets and very strong coupling superconductivity in FeSe$_x$Te$_{1-x}$

The celebrated BCS theory has been successful in explaining metallic superconductors, yet many believe that it must be modified to deal with the newer high temperature superconductors. A possible extension is provided by the BCS-BEC theory, describing a smooth evolution from a system of weakly-interacting pairs to a BEC of molecules of strongly-bounded fermions. Despite its appeal, spectroscopic evidence for the BCS-BEC crossover was never observed in solids. Here we report electronic structure measurements in FeSe$_x$Te$_{1-x}$ showing that these materials are in the BCS-BEC crossover regime. Above $T_c$ we find multiple bands with remarkably small values for the Fermi energy $\varepsilon_F$. Yet, in the superconducting state, the gap $\Delta$ is comparable to $\varepsilon_F$. The ratio $\Delta/\varepsilon_F\approx 0.5$ is much larger than found in any previously studied superconductor, resulting in an anomalous dispersion of the coherence peak very similar to that found in cold Fermi gas experiments, in agreement with the predictions of the BCS-BEC crossover theory.