Person: Song, Can-Li
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Song
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Can-Li
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Song, Can-Li
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Publication Imaging the Electron-Boson Coupling in Superconducting FeSe Films Using a Scanning Tunneling Microscope(American Physical Society (APS), 2014) Song, Can-Li; Wang, Yi-Lin; Jiang, Ye-Ping; Li, Zhi; Wang, Lili; He, Ke; Chen, Xi; Hoffman, Jenny; Ma, Xu-Cun; Xue, Qi-KunScanning tunneling spectroscopy has been used to reveal signatures of a bosonic mode in the local quasiparticle density of states of superconducting FeSe films. The mode appears below Tc as a “dip-hump” feature at energy Ω∼4.7kBTc beyond the superconducting gap Δ. Spectra on strained regions of the FeSe films reveal simultaneous decreases in Δ and Ω. This contrasts with all previous reports on other high-Tc superconductors, where Δ locally anticorrelates with Ω. A local strong coupling model is found to reconcile the discrepancy well, and to provide a unified picture of the electron-boson coupling in unconventional superconductors.Publication Dopant clustering, electronic inhomogeneity, and vortex pinning in iron-based superconductors(American Physical Society (APS), 2013) Song, Can-Li; Yin, Yi; Zech, Martin; Williams, Tess Lawanna; Yee, Michael Manchun; Chen, Gen-Fu; Luo, Jian-Lin; Wang, Nan-Lin; Hudson, E. W.; Hoffman, JennyWe use scanning tunneling microscopy to map the surface structure, nanoscale electronic inhomogeneity, and vitreous vortex phase in the hole-doped superconductor Sr0.75K0.25Fe2As2 with Tc=32 K. We find that the low-T cleaved surface is dominated by a half Sr/K termination with 1×2 ordering and ubiquitous superconducting gap, while patches of gapless, unreconstructed As termination appear rarely. The superconducting gap varies by σ/Δ¯=16% on a ∼3 nm length scale, with average 2Δ¯/kBTc=3.6 in the weak-coupling limit. The vortex core size provides a measure of the superconducting coherence length ξ=2.3 nm. We quantify the vortex lattice correlation length at 9 T in comparison to several iron-based superconductors. The comparison leads us to suggest the importance of dopant size mismatch as a cause of dopant clustering, electronic inhomogeneity, and strong vortex pinning.Publication Nanoscale surface element identification and dopant homogeneity in the high-\(T_{c}\) superconductor \(Pr_xCa_{1−x}Fe_2As_2\)(American Physical Society (APS), 2013) Zeljkovic, Ilija; Huang, Dennis; Song, Can-Li; Lv, Bing; Chu, Ching-Wu; Hoffman, JennyWe 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.Publication Bounds on nanoscale nematicity in single-layerFeSe/SrTiO3(American Physical Society (APS), 2016) Huang, Dennis; Webb, Tatiana; Fang, Shiang; Song, Can-Li; Chang, Cui-Zu; Moodera, Jagadeesh S.; Kaxiras, Efthimios; Hoffman, JennyWe 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 nematicityPublication Dumbbell Defects in FeSe Films: A Scanning Tunneling Microscopy and First-Principles Investigation(American Chemical Society (ACS), 2016) Huang, Dennis; Webb, Tatiana; Song, Can-Li; Chang, Cui-Zu; Moodera, Jagadeesh S.; Kaxiras, Efthimios; Hoffman, JennyThe 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.Publication Revealing the Empty-State Electronic Structure of Single-Unit-Cell FeSe/SrTiO3(American Physical Society (APS), 2015) Huang, Dennis; Song, Can-Li; Webb, Tatiana; Fang, Shiang; Chang, Cui-Zu; Moodera, Jagadeesh S.; Kaxiras, Efthimios; Hoffman, JennyWe 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.Publication Nanoscale Surface Element Identification and Dopant Homogeneity in the High-\(T_c\) Superconductor \(Pr_xC_{1-x}Fe_2As_2\)(American Physical Society, 2013) Zeljkovic, Ilija; Huang, Dennis; Song, Can-Li; Lv, Bing; Chu, Ching-Wu; Hoffman, JennyWe 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.Publication Suppression of Superconductivity by Twin Boundaries In FeSe(American Physical Society, 2012) Hoffman, Jenny; Song, Can-Li; Wang, Yi-Lin; Jiang, Ye-Ping; Wang, Lili; He, Ke; Chen, Xi; Ma, Xu-Cun; Xue, Qi-KunLow-temperature scanning tunneling microscopy and spectroscopy are employed to investigate twin boundaries in stoichiometric FeSe films grown by molecular beam epitaxy. Twin boundaries can be unambiguously identified by imaging the 90º change in the orientation of local electronic dimers from Fe site impurities on either side. Twin boundaries run at approximately 45º to the Fe-Fe bond directions, and noticeably suppress the superconducting gap, in contrast with the recent experimental and theoretical findings in other iron pnictides. Furthermore, vortices appear to accumulate on twin boundaries, consistent with the degraded superconductivity there. The variation in superconductivity is likely caused by the increased Se height in the vicinity of twin boundaries, providing the first local evidence for the importance of this height to the mechanism of superconductivity.