Session K14: Fe-based Superconductors -- ARPES and STM

Emergent Electronic Orders in Hole-doped BaFe2As2

The strong coupling of magnetism with nematicity has been found to be an identifying characteristic of the parent ground state of many iron pnictide superconducting families. Recently, the surprising discovery of the reentrant C₄ phase in many hole-doped pnictides has raised the interesting possibility that there exists a parameter regime where such coupling is not necessarily the lowest energy ground state. In such a reentrant C₄ phase, the spins remain ordered and reconstructs into a double-Q magnetic order while the lattice reverts back to tetragonal. In this talk, I will discuss our experimental explorations of this reentrant C₄ phase and electronic phases in its vicinity in an effort to gain insights into the microscopic origin of this reentrant C₄ phase.   

ARPES Study on the Electronic Structure and Superconducting Gap of Reentrant C4 Phase in (Sr,Na)Fe2As2

The relationship between magnetism, nematicity and superconductivity is important in understanding the superconductivity mechanism of the iron-based superconductors. Recently, reentrant C₄ tetragonal magnetic phase has been identified as a universal behavior within a narrow doping range in the hole-doped 122 iron-based superconductors in the nematic phase. Here we report high resolution laser-based angle-resolved photoemission (ARPES) measurements on the (Sr_,Na)Fe₂As₂ superconductors which exhibits reentrant C₄ magnetic phase. We measured the electronic structure distortion caused by the reentrant C₄ magnetic phase and the superconducting gap structure. Our results provide new insight in understanding the reentrant C₄ magnetic phase and superconductivity in the iron-based superconductors.   

STM/S study of the magnetic doping effect in BaFe2As2 superconductors

Study of the magnetic effect and spin excitation in the Fe-based superconductors (FeSC) is crucial for understanding the origin of the superconductivity (SC) in the system. It’s believed that the anti-ferromagnetism (AF) order should be suppressed by particular dopants to induce SC in FeSC. With Ni and Co doping into Fe sites, the c type AFM ground state in BaFe₂As₂ is suppressed and altered into SC electronic phase. However, in Cr doped is of great interest to reveal the differences of the magnetic doping effect in BaFe₂As₂. Using scanning tunneling microscopy/spectroscopy, we studied the electronic properties in the Cr, Co and Ni doped BaFe₂As₂ crystals within AF, SC and AF/SC coexistence electronic phase, respectively. Superconducting phase separation and nanoscale inhomogeneity were found in Ba(Fe_0.92Ni_0.08)₂As₂ and Ba(Fe_0.96Co_0.04)₂As₂ superconductor crystals. Furthermore, electronic structures nearby defects of Cr, Ni and Co doped BaFe₂As₂, were compared systematically. Our results will shed light on the microscopic understanding of relation between the superconductivity and magnetism in the FeSC. This research was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility.   

STM study of the electronic structure and superconductivity in heavily hole-doped RbFe2As2

The heavily hole-doped iron-pnictide superconductors AFe₂As₂ (A=K, Rb, Cs) exhibit unusual properties such as only hole band presented at E_F, large Sommerfeld coefficient and high sensitivity to nonmagnetic impurities, which indicate they have strong electron correlation and possibly different pairing mechanism with respect to other iron-based superconductors. Here we present an ultra-low temperature (20 mK) STM study on RbFe₂As₂ single crystal. We observed a typical “V” shaped SC gap which can only be fit with a nodal gap function. The gap is found to be suppressed near the surface Rb vacancies and atomic step edges along As-As direction, which consists with a d-wave pairing. Furthermore, quasi-particle interference (QPI) measurements reveal that the band structure has significant two-fold symmetry, which breaks the four-fold symmetry of the lattice. Particularly, such C4-C2 symmetry breaking happens along (π, π) direction rather than (π, 0). Our observation suggests d-wave pairing coexists with a new type of electronic nematicity in RbFe₂As₂.   

A route for simultaneous increase of Tc and Jc in iron-based superconductors by low-energy proton irradiation

We have grown iron-chalcogenide FeSe_0.5Te_0.5 (FST) superconducting films on various single crystal and flexible metal substrates [Applied Physics Letters 95, 052504 (2009); Rep. Prog. Phys. 74, 124510 (2011)]. The FST films on CeO₂ buffer layer exhibit enhanced transition temperature T_c, which is about 30% higher than that found in the bulk materials, and carry high critical current density J_c [Nat. Commun. 4, 1347 (2013)]. In this talk, I will present a new route for simultaneous increase of T_c and J_c in FST films by low-energy proton irradiation [Nat. Commun. 7, 13036 (2016).]. We provide direct atomic-scale imaging of cascade defects and mapping of the surrounding nanoscale strain field produced by low-energy proton irradiation. T_c is enhanced due to the nanoscale compressive strain and proximity effect, while J_c is doubled at zero field and 4.2 K through strong vortex-pinning by the cascade defects and surrounding nanoscale strain. We observed an increase of J_c at 12 K by one order of magnitude after the irradiation at magnetic fields over 15 T for H//ab and over 6 T for H//c. This study opens up the possibility to achieve significant enhancement of J_c without T_c reduction through the design of vortex pinning landscape by ion irradiation in iron-based superconductors

  

Scanning tunneling microscopy and spectroscopy of BaNi2As2 at 4.8K

Materials of the form AM₂P₂, with alkali or alkaline earth metal A, transition metal M, and pnictide P, have garnered interest of late due largely to their rich structural phase transitions and unusual electronic properties. We have performed low-temperature scanning tunneling microscopy topography and scanning tunneling spectroscopy (STS) of in-situ cleaved BaNi₂As₂, under ultra-high vacuum (UHV) conditions. A variety of features are observed on the surface at 4.8K, including atomically-resolved islands of √2x√2 surface reconstruction, and 2x1 reconstruction. We will discuss our high-resolution STS results in light of recent theoretical calculations.   

Scanning Tunneling Microscopy and Spectroscopy of Surface Impurities in the Unconventional Superconductor FeSe1-xTex

Bulk FeSe_1-xTe_x hosts unconventional superconductivity and nematicity, and may have non-trivial topological surface states. Resonant states from impurities provide a powerful local probe of electronic structure, and can be used to search for possible Majorana zero modes (MZMs) and give important insight into electronic order. Here we study the impurity effect in FeSe_1-xTe_x with low-temperature scanning tunneling microscopy and spectroscopy. We image Fe-site and Se/Te-site surface impurities, all of which create low-energy resonant states inside the superconducting gap. A detailed comparison of these in-gap impurity states can provide insight into the superconducting order parameter, and is necessary to further clarify whether these resonances are MZMs. We also track nematicity in this material using quasiparticle interference (QPI) obtained at energies larger than the superconducting gap.   

Vector field STM study of iron-based superconductor

Magnetic field studies of iron-based superconductors have been largely limited to c-axis applied fields. Using a combination of a vector magnetic field and high resolution scanning tunneling microscopy/spectroscopy, we study the three dimensional field based phase diagram of vortex electronic matter in a correlated iron-based superconductor. We find that the lattice structure of the vortices is strongly correlated with the magnitude and direction of the magnetic field in an anisotropic manner. Probing the quasiparticle excitations and interferences reveals an unusual magnetic pair-breaking effect with distinct scattering vectors. These results will help us understand the interplay between Cooper pairing and emergent three dimensional vortex matter in iron-based superconductors.   

Stripes developed at the strong limit of nematicity in FeSe films

Superconductivity in one monolayer FeSe film on SrTiO₃ has attracted enormous attentions. However, the electronic behaviors of FeSe films on SrTiO₃ show extraordinary dichotomy as the superconductivity is suppressed and strong nematicity develops for the film thicker than one monolayer. On the other hand, long-range magnetism is absent, despite large local magnetic moment has been found in bulk FeSe. Here we report on low-temperature scanning tunneling microscopy studies of the multilayer FeSe films grown by molecular beam epitaxy. We find a stripe-type charge ordering instability, which develops beneath the nematic state. The emergence of stripe-type ordering indicates a magnetic fluctuation, competing with the ordinary collinear antiferromagnetic order in FeSe films. The existence of stripes in iron-based superconductor that resemble the stripe order in cuprates provides a platform to reveal the complex interactions between nematicity, magnetism and superconductivity in high-temperature superconductors.   

Scanning Tunneling Spectroscopy, Angle-resolved Photoemission Spectroscopy and Double-coil Mutual Inductance Investigation of Single Crystalline FeSe Superconductors

We report investigation of single crystalline FeSe superconductors by combining scanning tunneling microscopy/spectroscopy (STM/S), angle-resolved photoemission spectroscopy (ARPES) and double-coil mutual inductance measurement, which were all applied in-situ to a sample cleaved in an ultra-high-vacuum (UHV) chamber. Using STM/S, we clearly observed the 1×1 atomic topography and the superconducting gap of this single crystal. In addition, ARPES was used to measure the band dispersion of FeSe. Finally, using a home built double-coil mutual inductance system, operating in UHV, we measured its superfluid density. These measurements shed light on the origin of superconductivity in FeSe. Additionally, using FeSe as a superconducting platform, we are investigating the proximity effect with other 2D materials.   

Low-temperature scanning tunneling microscopy and spectroscopy of FeSe0.6Te0.4 single crystals

FeSe has the simplest crystal structure among Fe-based superconductors with a superconducting transition at T_C ~ 9 K and a nematic transtion at T_S ~ 90 K. By replacing 60% of Se with Te, FeSe_1-xTe_x (x=0.6), T_C increases up to 14 K and the superconducting gap structure changes from nodal to full-gap with the disappearance of the nematic phase transition. This system is therefore an ideal playground to understand the relationship between superconductivity and electronic nematicity in Fe-based superconductors. However, a difficulty in single-crystal growth in the phase-separation region (0<x<0.5) so far prevent a full understanding of the system [1]. We successfully grew single crystals in this region and performed electronic transport and low-temperature scanning tunneling microscopy and spectroscopy measurements on these crystals. The details will be discussed in the presentation. [1] M. Fang et al., PRB 78, 224503 (2008).   

Electronic structure and magnetic phase transition of hexagonal FeSe thin films studied by photoemission spectroscopy

Hexagonal FeSe thin films were grown on SrTiO₃ substrates and the temperature and thickness dependence of their electronic structures were studied. The hexagonal FeSe is found to be metallic, with a Fermi surface consisting of six elliptical electron pockets. With decreased temperature, parts of the bands shift downward to high binding energy while some bands shift upward to E_F. The shifts of these bands begin around 300 K and saturate at low temperature, indicating a magnetic phase-transition temperature of about 300 K. With increased film thickness, the Fermi surface topology and band structure show no obvious change. Our paper reports the electronic structure of hexagonal FeSe for the first time, and shows that the possible magnetic transition is driven by large-scale electronic structure reconstruction.   

Strongly enhanced temperature dependence of the chemical potential in FeSe

FeSe is a prime candidate to study the effects of high-temperature superconductivity and nematic ordering in the Fe-based superconductors, as it has a highly tunable superconducting onset and experiences a nematic phase transition without any accompanying antiferromagnetic phase. Given the theoretical interest in understanding these phenomena, an accurate model of the electronic structure is highly desirable.
To overcome the known limitations of ab-initio modelling in FeSe, we present a new set of hopping parameters for a 10-orbital tight binding model fitted directly to our latest angle-resolved photoemission spectroscopy (ARPES) data for the tetragonal phase of FeSe. This provides a quantitatively accurate description of the low-energy dispersions observed in FeSe.
Using this parameter set we predict a large rigid shift of the chemical potential as a function of temperature. We confirm this experimentally using high-resolution ARPES at the Diamond Light Source. This unexpectedly strong chemical potential shift has important implications for future theoretical models of superconductivity and of nematic ordering in FeSe.