Two distinct superconducting phases in LiFeAs

Nag, Pranab Kumar; Schlegel, Ronny; Baumann, Danny; Grafe, H.-J.; Beck, Robert; Wurmehl, S.; Büchner, B.; Heß, Christian

doi:10.1038/srep27926

Cited by 19 publications

(21 citation statements)

References 47 publications

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“…This suggests these defects to be located not in the topmost layer but further below. These findings imply that for evolved spectroscopic studies of the pristine superconducting state of LiFeAs (), great care is required in selecting the position for STS as to ensure that no artifacts from such hidden “second‐layer” defect bound states affect the taken data.…”

Section: Resultsmentioning

confidence: 99%

“…In addition, evidence for a time‐reversal symmetry breaking state, partially depending on details of electronic doping is found in NMR/NQR (), STM/STS (), and high‐field magnetometry measurements (). Finally, compelling evidence for multiple superconducting transitions in the superconducting state has been reported from combined AC‐susceptibility/NMR measurements () as well as from STM/STS ().…”

Section: Introductionmentioning

confidence: 95%

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Defect states in LiFeAs as seen by low‐temperature scanning tunneling microscopy and spectroscopy

Schlegel

Nag

Baumann

et al. 2016

Physica Status Solidi (b)

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We present a microscopic investigation of frequently observed impurity‐induced states in stoichiometric LiFeAs using low‐temperature scanning tunneling microscopy and spectroscopy (STM/STS). Our data reveal seven distinct well‐defined defects which are discernible in topographic measurements. Depending on their local topographic symmetry, we are able to assign five defect types to specific lattice sites at the Li, Fe, and As positions. The most prominent result is that two different defect types have a remarkably different impact on the superconducting state. A specific and quite abundant Fe‐defect with D2‐symmetry generates significant impurity‐induced additional states primarily at positive bias voltage with pronounced peaks in the on‐site local density of states (LDOS) at about 4 and 12 mV. On the other hand, a D4‐symmetric As‐defect causes a significantly enhanced LDOS at both positive and negative bias voltages. We expect that these findings provide fresh input for further experimental and theoretical studies on elucidating the nature of superconductivity in LiFeAs.

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Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 95%

Defect states in LiFeAs as seen by low‐temperature scanning tunneling microscopy and spectroscopy

Schlegel

Nag

Baumann

et al. 2016

Physica Status Solidi (b)

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“…Quite clearly, the data show dominant signatures of inelastic tunneling: At energies close to the gap edges dI/dU of the superconducting state exhibits a depletion with respect to that of the normal state whereas it shows a step-like enhancement at about 14 meV and exceeds the dI/dU of the normal state beyond. This energy dependence leads to the known characteristic 'dip-hump' anomaly in normalized dI/dU data [9], which is often observed in various unconventional superconductors, including LiFeAs [3][4][5][6][7][8]40] (inset of Fig. 4(d)).…”

Section: B Comparison With Tunneling Spectroscopymentioning

confidence: 99%

Spectroscopic evidence of nematic fluctuations in LiFeAs

et al. 2019

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The role of nematic fluctuations in the pairing mechanism of iron-based superconductors is frequently debated. Here we present a novel method to reveal such fluctuations by identifying energy and momentum of the corresponding nematic boson through the detection of a boson-assisted resonant amplification of Friedel oscillations. Using Fourier-transform scanning tunneling spectroscopy, we observe for the unconventional superconductor LiFeAs strong signatures of bosonic states at momentum q ∼ 0 and energy Ω ≈ 8 meV. We show that these bosonic states survive in the normal conducting state, and, moreover, that they are in perfect agreement with well-known strong above-gap anomalies in the tunneling spectra. Attributing these small-q boson modes to nematic fluctuations we provide the first spectroscopic approach to the nematic boson in an unconventional superconductor.arXiv:1811.03489v1 [cond-mat.str-el]

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“…[13,14] (see right panel of Fig.4, where the black curve is at 2 K and should be compared to theoretical predictions in the left panel). In this case, just like in many high-temperature superconductors, the electronic spectrum in the normal state is non-flat.…”

Section: Figmentioning

confidence: 99%

“…Importantly, the polarization operator is not depending on the elastic tunneling elements t e , which only occur in the φ 4 terms and therefore the screening is in leading order not affected by the presence of the tunneling term in the Hamiltonian. The other additional contribution for the low-energy theory is the last term in (14). For the evaluation we need to inverse of the second equation in (12), which has the formal solution…”

Section: Derivation Of Low-energy Tunnel Hamiltonianmentioning

confidence: 99%

Tracing the Electronic Pairing Glue in Unconventional Superconductors via Inelastic Scanning Tunneling Spectroscopy

et al. 2017

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Scanning tunneling microscopy (STM) has been shown to be a powerful experimental probe to detect electronic excitations and further allows to deduce fingerprints of bosonic collective modes in superconductors. Here, we demonstrate that the inclusion of inelastic tunnel events is crucial for the interpretation of tunneling spectra of unconventional superconductors and allows to directly probe electronic and bosonic excitations via STM. We apply the formalism to the iron based superconductor LiFeAs. With the inclusion of inelastic contributions, we find strong evidence for a non-conventional pairing mechanism, likely via magnetic excitations.PACS numbers: 74.55.+v, 74.20.Mn, 74.20.Rp, 74.70.Xa Electron tunneling spectroscopy has turned out to be an outstanding tool for the investigation of superconductors. A classical example is the determination of the electron-phonon pairing interaction in conventional superconductors [1,2]. More recently, quasi-particles interference spectroscopy managed to exploit the local resolution of scanning tunneling microscopy (STM) to obtain momentum space information [3][4][5]. Both examples are based on elastic tunneling theory [6,7] where one interprets the low-temperature conductance to be proportional to the electronic density of states (DOS), including renormalizations of the DOS that occur for example within the strong coupling Eliashberg formalism [8]. An energy dependent coupling to phonons or electronic collective modes and the details of these bosonic spectral features lead to a renormalization of the electronic DOS in form of peak-dip features above the superconducting coherence peaks [1]. Such pronounced peak-dip features have also been observed in cuprate and iron-based superconductors [9][10][11][12][13][15][16][17][18][19][20][21][22][23][24][25]. A frequent interpretation is, based on elastic tunneling theory, in terms of a coupling of electrons to a sharp spin resonance mode with frequency ω res and with momentum at the antiferromagnetic ordering vector of the material [26][27][28].In an interacting system the injection of a real electron may cause both, the creation of a fermionic quasiparticle and the excitation of (bosonic) collective modes as depicted in Fig. 1. The strength of the interaction is usually crucial for the relative weight of the low-energy quasiparticle and the cloud of excitations associated with it. The excitation of the quasiparticle corresponds to the above discussed elastic tunneling, while the creation of real collective modes during the tunneling process corresponds to inelastic tunneling.In this paper we demonstrate that such inelastic tunneling events can lead to important and observable modifications of the STM spectrum in unconventional su- perconductors. In addition to fermionic exitations that are visible via elastic tunneling, inelastic tunneling spectroscopy can be used to identify the bosonic excitations of the system. We show that the fine-structures seen in LiFeAs are predominantly due to such inelastic tunneling processes and th...

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Two distinct superconducting phases in LiFeAs

Cited by 19 publications

References 47 publications

Defect states in LiFeAs as seen by low‐temperature scanning tunneling microscopy and spectroscopy

Defect states in LiFeAs as seen by low‐temperature scanning tunneling microscopy and spectroscopy

Spectroscopic evidence of nematic fluctuations in LiFeAs

Tracing the Electronic Pairing Glue in Unconventional Superconductors via Inelastic Scanning Tunneling Spectroscopy

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