64 research outputs found
Imaging Orbital-selective Quasiparticles in the Hund's Metal State of FeSe
Strong electronic correlations, emerging from the parent Mott insulator
phase, are key to copper-based high temperature superconductivity (HTS). By
contrast, the parent phase of iron-based HTS is never a correlated insulator.
But this distinction may be deceptive because Fe has five active d-orbitals
while Cu has only one. In theory, such orbital multiplicity can generate a
Hund's Metal state, in which alignment of the Fe spins suppresses inter-orbital
fluctuations producing orbitally selective strong correlations. The spectral
weights of quasiparticles associated with different Fe orbitals m should
then be radically different. Here we use quasiparticle scattering interference
resolved by orbital content to explore these predictions in FeSe. Signatures of
strong, orbitally selective differences of quasiparticle appear on all
detectable bands over a wide energy range. Further, the quasiparticle
interference amplitudes reveal that , consistent with
earlier orbital-selective Cooper pairing studies. Thus, orbital-selective
strong correlations dominate the parent state of iron-based HTS in FeSe.Comment: for movie M1, see
http://www.physik.uni-leipzig.de/~kreisel/osqp/M1.mp4, for movie M2, see
http://www.physik.uni-leipzig.de/~kreisel/osqp/M2.mp4, for movie M3, see
http://www.physik.uni-leipzig.de/~kreisel/osqp/M3.mp4, for movie M4, see
http://www.physik.uni-leipzig.de/~kreisel/osqp/M4.mp4, for movie M5, see
http://www.physik.uni-leipzig.de/~kreisel/osqp/M5.mp
Multi-Atom Quasiparticle Scattering Interference for Superconductor Energy-Gap Symmetry Determination
Complete theoretical understanding of the most complex superconductors
requires a detailed knowledge of the symmetry of the superconducting energy-gap
, for all momenta on the Fermi surface
of every band . While there are a variety of techniques for determining
, no general method existed to measure the signed
values of . Recently, however, a new technique based
on phase-resolved visualization of superconducting quasiparticle interference
(QPI) patterns centered on a single non-magnetic impurity atom, was introduced.
In principle, energy-resolved and phase-resolved Fourier analysis of these
images identifies wavevectors connecting all k-space regions where
has the same or opposite sign. But use of a single
isolated impurity atom, from whose precise location the spatial phase of the
scattering interference pattern must be measured is technically difficult. Here
we introduce a generalization of this approach for use with multiple impurity
atoms, and demonstrate its validity by comparing the
it generates to the determined from single-atom
scattering in FeSe where energy-gap symmetry is established. Finally,
to exemplify utility, we use the multi-atom technique on LiFeAs and find
scattering interference between the hole-like and electron-like pockets as
predicted for of opposite sign
Imaging atomic-scale effects of high-energy ion irradiation on superconductivity and vortex pinning in Fe(Se,Te)
Maximizing the sustainable supercurrent density, Jc, is crucial to high
current applications of superconductivity and, to achieve this, preventing
dissipative motion of quantized vortices is key. Irradiation of superconductors
with high-energy heavy ions can be used to create nanoscale defects that act as
deep pinning potentials for vortices. This approach holds unique promise for
high current applications of iron-based superconductors because Jc
amplification persists to much higher radiation doses than in cuprate
superconductors without significantly altering the superconducting critical
temperature. However, for these compounds virtually nothing is known about the
atomic scale interplay of the crystal damage from the high-energy ions, the
superconducting order parameter, and the vortex pinning processes. Here, we
visualize the atomic-scale effects of irradiating FeSexTe1-x with 249 MeV Au
ions and find two distinct effects: compact nanometer-sized regions of crystal
disruption or 'columnar defects', plus a higher density of single atomic-site
'point' defects probably from secondary scattering. We show directly that the
superconducting order is virtually annihilated within the former while
suppressed by the latter. Simultaneous atomically-resolved images of the
columnar crystal defects, the superconductivity, and the vortex configurations,
then reveal how a mixed pinning landscape is created, with the strongest
pinning occurring at metallic-core columnar defects and secondary pinning at
clusters of pointlike defects, followed by collective pinning at higher fields.Comment: Main text (14 pages, 5 figures) and supplementary information (6
pages, 7 figures
Discovery of orbital-selective Cooper pairing in FeSe
For movie S1, see http://www.physik.uni-leipzig.de/~kreisel/oscp/S1.mp4, for movie S2, see http://www.physik.uni-leipzig.de/~kreisel/oscp/S2.mp4 and for movie S3, see http://www.physik.uni-leipzig.de/~kreisel/oscp/S3.mp4 Funding: Moore Foundation’s EPiQS Initiative through Grant GBMF4544 (JCSD)The superconductor iron selenide (FeSe) is of intense interest owing to its unusual nonmagnetic nematic state and potential for high-temperature superconductivity. But its Cooper pairing mechanism has not been determined. We used Bogoliubov quasiparticle interference imaging to determine the Fermi surface geometry of the electronic bands surrounding the Γ = (0, 0) and X = (π/aFe, 0) points of FeSe and to measure the corresponding superconducting energy gaps. We show that both gaps are extremely anisotropic but nodeless and that they exhibit gap maxima oriented orthogonally in momentum space. Moreover, by implementing a novel technique, we demonstrate that these gaps have opposite sign with respect to each other. This complex gap configuration reveals the existence of orbital-selective Cooper pairing that, in FeSe, is based preferentially on electrons from the dyz orbitals of the iron atoms.PostprintPeer reviewe
Multi-atom quasiparticle scattering interference for superconductor energy-gap symmetry determination
Complete theoretical understanding of the most complex superconductors requires a detailed knowledge of the symmetry of the superconducting energy-gap Δ, for all momenta k on the Fermi surface of every band α. While there are a variety of techniques for determining |Δ|, no general method existed to measure the signed values of Δ. Recently, however, a technique based on phase-resolved visualization of superconducting quasiparticle interference (QPI) patterns, centered on a single non-magnetic impurity atom, was introduced. In principle, energy-resolved and phase-resolved Fourier analysis of these images identifies wavevectors connecting all k-space regions where Δ has the same or opposite sign. But use of a single isolated impurity atom, from whose precise location the spatial phase of the scattering interference pattern must be measured, is technically difficult. Here we introduce a generalization of this approach for use with multiple impurity atoms, and demonstrate its validity by comparing the Δ it generates to the Δ determined from single-atom scattering in FeSe where s± energy-gap symmetry is established. Finally, to exemplify utility, we use the multi-atom technique on LiFeAs and find scattering interference between the hole-like and electron-like pockets as predicted for Δ of opposite sign
Highly efficient polymer solar cells cast from non-halogenated xylene/anisaldehyde solution
Several high performance polymer:fullerene bulk-heterojunction photo-active layers, deposited from the non-halogenated solvents o-xylene or anisole in combination with the eco-compatible additive p-anisaldehyde, are investigated. The respective solar cells yield excellent power conversion efficiencies up to 9.5%, outperforming reference devices deposited from the commonly used halogenated chlorobenzene/1,8-diiodooctane solvent/additive combination. The impact of the processing solvent on the bulk-heterojunction properties is exemplified on solar cells comprising benzodithiophene-thienothiophene co-polymers and functionalized fullerenes (PTB7:PC71BM). The additive p-anisaldehyde improves film formation, enhances polymer order, reduces fullerene agglomeration and shows high volatility, thereby positively affecting layer deposition, improving charge carrier extraction and reducing drying time, the latter being crucial for future large area roll-to-roll device fabrication. © The Royal Society of Chemistry 2015
Severe dirac mass gap suppression in Sb 2 Te 3-based quantum anomalous Hall materials
The quantum anomalous Hall (QAH) effect appears in ferromagnetic topological insulators (FMTIs) when a Dirac mass gap opens in the spectrum of the topological surface states (SSs). Unaccountably, although the mean mass gap can exceed 28 meV (or ∼320 K), the QAH effect is frequently only detectable at temperatures below 1 K. Using atomic-resolution Landau level spectroscopic imaging, we compare the electronic structure of the archetypal FMTI Cr0.08(Bi0.1Sb0.9)1.92Te3 to that of its nonmagnetic parent (Bi0.1Sb0.9)2Te3, to explore the cause. In (Bi0.1Sb0.9)2Te3, we find spatially random variations of the Dirac energy. Statistically equivalent Dirac energy variations are detected in Cr0.08(Bi0.1Sb0.9)1.92Te3 with concurrent but uncorrelated Dirac mass gap disorder. These two classes of SS electronic disorder conspire to drastically suppress the minimum mass gap to below 100 μeV for nanoscale regions separated by <1 μm. This fundamentally limits the fully quantized anomalous Hall effect in Sb2Te3-based FMTI materials to very low temperatures
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