Person:

Hoffman, Jenny

We use scanning tunneling microscopy to determine the surface structure and dopant distribution in (Pr_xCa_{1-x}Fe_2As_2), the highest-(T_c) member of the 122 family of iron-based superconductors. We identify the cleaved surface termination by mapping the local tunneling barrier height, related to the work function. We image the individual Pr dopants responsible for superconductivity, and show that they do not cluster, but in fact repel each other at short length scales. We therefore suggest that the low volume fraction high-(T_c) superconducting phase is unlikely to originate from Pr inhomogeneity.

We use scanning tunneling microscopy to determine the surface structure and dopant distribution in (Pr_xCa_{1−x}Fe_2As_2), the highest-Tc member of the 122 family of iron-based superconductors. We identify the cleaved surface termination by mapping the local tunneling barrier height, related to the work function. We image the individual Pr dopants responsible for superconductivity, and show that they do not cluster, but in fact repel each other at short length scales. We therefore suggest that the low volume fraction high-Tc superconducting phase is unlikely to originate from Pr inhomogeneity.

We use scanning tunneling spectroscopy to investigate the filled and empty electronic states of superconducting single-unit-cell FeSe deposited on SrTiO3(001). We map the momentum-space band structure by combining quasiparticle interference imaging with decay length spectroscopy. In addition to quantifying the filled-state bands, we discover a Γ-centered electron pocket 75 meV above the Fermi energy. Our density functional theory calculations show the orbital nature of empty states at Γ and explain how the Se height is a key tuning parameter of their energies, with broad implications for electronic properties.

We use scanning tunneling microscopy to determine the surface structure and dopant distribution in PrxCa1−xFe2As2, the highest-Tc member of the 122 family of iron-based superconductors. We identify the cleaved surface termination by mapping the local tunneling barrier height, related to the work function. We image the individual Pr dopants responsible for superconductivity, and show that they do not cluster, but in fact repel each other at short length scales. We therefore suggest that the low volume fraction high-Tc superconducting phase is unlikely to originate from Pr inhomogeneity.

We use scanning tunneling microscopy (STM) and quasiparticle interference (QPI) imaging to investigate the low-energy orbital texture of single-layer FeSe/SrTiO3. We develop a T -matrix model of multiorbital QPI to disentangle scattering intensities from Fe 3dxz and 3dyz bands, enabling the use of STM as a nanoscale detection tool of nematicity. By sampling multiple spatial regions of a single-layer FeSe/SrTiO3 film, we quantitatively exclude static xz/yz orbital ordering with domain size larger than δr2 = 20 nm × 20 nm, xz/yz Fermi wave vector difference larger than δk = 0.014 π, and energy splitting larger than δE = 3.5 meV. The lack of detectable ordering pinned around defects places qualitative constraints on models of fluctuating nematicity

The properties of iron-based superconductors (Fe-SCs) can be varied dramatically with the introduction of dopants and atomic defects. As a pressing example, FeSe, parent phase of the highest-Tc Fe-SC, exhibits prevalent defects with atomic-scale “dumbbell” signatures as imaged by scanning tunneling microscopy (STM). These defects spoil superconductivity when their concentration exceeds 2.5%. Resolving their chemical identity is a prerequisite to applications such as nanoscale patterning of superconducting/ nonsuperconducting regions in FeSe as well as fundamental questions such as the mechanism of superconductivity and the path by which the defects destroy it. We use STM and density functional theory to characterize and identify the dumbbell defects. In contrast to previous speculations about Se adsorbates or substitutions, we find that an Fe-site vacancy is the most energetically favorable defect in Se-rich conditions and reproduces our observed STM signature. Our calculations shed light more generally on the nature of Se capping, the removal of Fe vacancies via annealing, and their ordering into a √5 × √5 superstructure in FeSe and related alkali-doped compounds.