Local Measurement of the Eliashberg Function of Pb Islands: Enhancement of Electron-Phonon Coupling by Quantum Well States

Schackert, Michael; Märkl, Tobias; Jandke, Jasmin; Hölzer, Martin; Ostanin, S.; Gross, E. K. U.; Ernst, A.; Wulfhekel, Wulf

doi:10.1103/physrevlett.114.047002

Cited by 39 publications

(45 citation statements)

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“…Complete details on the sample preparation and measurement are provided in the methods and supplemental information [38]. The asym- [41,42].…”

Section: Resultsmentioning

confidence: 99%

“…This unconventional regime is evident when compared with conventional superconductors over a broad range of e-ph coupling (data compiled from Refs. 20, 21, 42, and 52).with 2∆ 0 /k B T c = 4.5 and λ = 1.3), which is accounted for by including retardation corrections within ME[20,42]. Due to the signatures of strong e-ph coupling observed in the normal state, one might expect SrTiO 3 to exhibit significant departures from weakcoupling BCS theory, particularly in the underdoped region where large phonon renormalizations are found.…”

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confidence: 99%

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Polaronic behavior in a weak-coupling superconductor

Swartz

Inoue

Merz

et al. 2018

Proc. Natl. Acad. Sci. U.S.A.

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The nature of superconductivity in the dilute semiconductor SrTiO has remained an open question for more than 50 y. The extremely low carrier densities ([Formula: see text]-[Formula: see text] cm) at which superconductivity occurs suggest an unconventional origin of superconductivity outside of the adiabatic limit on which the Bardeen-Cooper-Schrieffer (BCS) and Migdal-Eliashberg (ME) theories are based. We take advantage of a newly developed method for engineering band alignments at oxide interfaces and access the electronic structure of Nb-doped SrTiO, using high-resolution tunneling spectroscopy. We observe strong coupling to the highest-energy longitudinal optic (LO) phonon branch and estimate the doping evolution of the dimensionless electron-phonon interaction strength ([Formula: see text]). Upon cooling below the superconducting transition temperature ([Formula: see text]), we observe a single superconducting gap corresponding to the weak-coupling limit of BCS theory, indicating an order of magnitude smaller coupling ([Formula: see text]). These results suggest that despite the strong normal state interaction with electrons, the highest LO phonon does not provide a dominant contribution to pairing. They further demonstrate that SrTiO is an ideal system to probe superconductivity over a wide range of carrier density, adiabatic parameter, and electron-phonon coupling strength.

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“…Complete details on the sample preparation and measurement are provided in the methods and supplemental information [38]. The asym- [41,42].…”

Section: Resultsmentioning

confidence: 99%

mentioning

confidence: 99%

Polaronic behavior in a weak-coupling superconductor

Swartz

Inoue

Merz

et al. 2018

Proc. Natl. Acad. Sci. U.S.A.

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“…[ 136–141 ] In contrast, IETS of lattice vibrations has very scarcely been reported so far. Phonons of graphite, [ 142 ] Bi 2 Sr 2 CaCu 2 O 8 +δ , [ 143 ] Au(111), [ 144,145 ] Ag(100), [ 146 ] thin Pb films on Cu(111), [ 147 ] and Cu(110) [ 148 ] were previously measured using STM‐IETS. Only recently, a theoretical work has described the STM‐IETS of surface phonons of Cu(110) using a nonequilibrium Green's function method.…”

Section: Methodsmentioning

confidence: 99%

Local Probes of Graphene Lattice Dynamics

2020

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Inelastic electron tunneling spectroscopy with a scanning tunneling microscope is a powerful method to excite and detect vibrational quanta with atomic resolution. The focus of this review article is on the local spectroscopy of graphene phonons. The experimental observation of their spectroscopic signatures together with theoretical modeling highlight the importance of the graphene–surface as well as the graphene–tip hybridization, the electron–phonon coupling strength, phonon‐mediated tunneling, local doping profiles, and the phonon density of state for the measured signal. Meanwhile, a comprehensive understanding of the underlying mechanisms has been attained and justifies an overview on available findings.

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“…A tip-electron tunnels elastically into a high-energy off-shell state far away from the Fermi surface from where the electron scatters inelastically via the emission/absorption of a boson to a state near the Fermi-surface. Inelastic tunneling has been observed for conventional superconductors in the normal state [30,31] where it was shown, that tunneling electrons excite bulk phonons since the measured second derivative of the tunneling turned out to be proportional to the Eliashberg function (d 2 I/dU 2 ∝ α 2 F (ω)) which is given by the electron-phonon coupling constant α times the phonon spectrum F (ω). This inelastic contribution has recently been shown to be of importance even in the superconducting state of Pb-films (almost to the same extent) [33].…”

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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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Local Measurement of the Eliashberg Function of Pb Islands: Enhancement of Electron-Phonon Coupling by Quantum Well States

Cited by 39 publications

References 28 publications

Polaronic behavior in a weak-coupling superconductor

Polaronic behavior in a weak-coupling superconductor

Local Probes of Graphene Lattice Dynamics

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

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