Observation of a BCS Spectral Function in a Conventional Superconductor by Photoelectron Spectroscopy

Reinert, F.; Nicolay, G.; Eltner, B.; Ehm, D.; Schmidt, S.; Hüfner, S.; Probst, Uwe; Bücher, E.

doi:10.1103/physrevlett.85.3930

Cited by 46 publications

(32 citation statements)

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“…The angular resolution was typically ≈ 0.3 • and the sample temperature could be varied from room temperature down to about 4.5 K (see Ref. [9,21] for details of the experimental setup). The samples were polycrystalline and single-crystalline pellets (Surface Preparation Laboratory, Netherlands), thermally connected to the sample holder by soldered indium.…”

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“…close to the Fermi level), the real part of Σ el−ph is linear in energy, and λ = −δℜeΣ el−ph (ω)/δω at ω = 0 is usually called the mass enhancement factor. In addition, λ defines the slope of the temperature dependence of the quasi-particle linewidth Γ el−ph = 2ℑmΣ el−ph ≈ 2πλk B T for temperatures above the Debye temperature Θ D .Photoemission spectroscopy (PES) is a versatile experimental method to study the electronic structure of solids and has been applied to many high-T c materials [5,6] and conventional superconductors [7,8,9,10,11]. Electronphonon coupling in metallic systems has been studied in detail by PES on low-dimensional electronic states at surfaces, e.g.…”

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Electron-Phonon Coupling and its Evidence in the Photoemission Spectra of Lead

et al. 2003

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We present a detailed study on the influence of strong electron-phonon coupling to the photoemission spectra of lead. Representing the strong-coupling regime of superconductivity, the spectra of lead show characteristic features that demonstrate the correspondence of physical properties in the normal and the superconducting state, as predicted by the Eliashberg theory. These features appear on an energy scale of a few meV and are accessible for photoemission only by using modern spectrometers with high resolution in energy and angle. The theoretical approach for the explanation of strongcoupling superconductors is based on the Eliashberg equations, with the coupling function α 2 F as the central property [3]. This so-called Eliashberg function can be calculated e.g. by first principle methods from the electronic wave functions, the phonon density of states, and the electron-phonon coupling between two Bloch states. However, α 2 F is a much more universal quantity and determines also the influence of the electron-phonon interaction on the normal state properties, e.g. the electrical resistivity, the electronic heat capacity, and -as a spectroscopic feature -the electron-phonon contribution Γ el−ph to the intrinsic quasi-particle linewidth, which can be determined e.g. by photoemission spectroscopy. In the Green's function method, the influence of electronphonon interaction is expressed as a contribution to the complex self energy of the conduction electrons Σ el−ph , which can be calculated from the Eliashberg function α 2 F [4]. The real part describes the induced band renormalization, where the imaginary part gives the quasiparticle line width, equivalent to the reciprocal hole lifetime τ . In particular, at very small energies (i.e. close to the Fermi level), the real part of Σ el−ph is linear in energy, and λ = −δℜeΣ el−ph (ω)/δω at ω = 0 is usually called the mass enhancement factor. In addition, λ defines the slope of the temperature dependence of the quasi-particle linewidth Γ el−ph = 2ℑmΣ el−ph ≈ 2πλk B T for temperatures above the Debye temperature Θ D .Photoemission spectroscopy (PES) is a versatile experimental method to study the electronic structure of solids and has been applied to many high-T c materials [5,6] and conventional superconductors [7,8,9,10,11]. Electronphonon coupling in metallic systems has been studied in detail by PES on low-dimensional electronic states at surfaces, e.g. 13,14,15,16,17,18] or quantum-well states [19,20]. PES investigations on electron-phonon effects in three-dimensional solids, however, are rare. One particularly interesting threedimensional model system is Pb, which among the conventional superconductors has quite un-conventional physical properties.In the case of Pb the two most relevant energy scales on which the electron-phonon features appear in the spectra, the Debye energy ω D = k B Θ D and the gap width ∆ 0 at T = 0, define prerequisites that can be met by PES experiments today ( ω D = 7.6 meV, ∆ 0 = 1.4 meV). The instrumental resolution must be at least of...

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Electron-Phonon Coupling and its Evidence in the Photoemission Spectra of Lead

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“…A similar peak above E F has been observed recently for V 3 Si below T c . [17] The gaps obtained from such fits to the spectra are plotted as a function of reduced temperature T/T c in Fig. 2b.…”

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Electron-phonon coupling induced pseudogap and the superconducting transition inBa0.67K0.33BiO
Chainani
¹
,
Yokoya
²
,
Shin
³

et al. 2001
Phys. Rev. B
30
3
19
0
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We study the single particle density of states (DOS) across the superconducting transition (Tc = 31 K) in single-crystal Ba0.67K0.33BiO3 using ultrahigh resolution angle-integrated photoemission spectroscopy. The superconducting gap opens with a pile-up in the DOS, ∆(5.3 K) = 5.2 meV and 2∆(0)/kB Tc = 3.9. In addition, we observe a pseudogap below and above Tc, occurring as a suppression in intensity over an energy scale up to the breathing mode phonon(∼ 70 meV). The results indicate electron-phonon coupling induces a pseudogap in Ba0. 67K0.33BiO3 . PACS numbers: 74.25.Jb, 74.25.Kc, 79.60.Bm From a variety of experiments and theory, it is clear that the high temperature cuprate superconductors belong to a special class of materials. Some of the important aspects of the cuprates are quasi twodimensionality, short-range antiferromagnetic correlations, an anisotropic pseudogap in the normal phase, and d x 2 −y 2 symmetry of the superconducting gap [1,2]. In contrast, the perovskite series Ba 1−x K x BiO 3 (BKBO) is three-dimensional, contains no transition metal ions, has no magnetic order but still exhibits the highest T c known for an oxide other than the cuprates [3]. The superconductivity in the BKBO series was preceded by the analogous BaBi 1−x Pb x O 3 (BPBO) series which shares the same parent BaBiO 3 (BBO) [4]. The role of electron-phonon coupling in the properties of the BKBO and BPBO series originates in the charge density wave (CDW) state of BBO.The parent BBO is expected to be a metal from band theory with a half-filled Bi 6s band, but the near perfect nesting possible in BBO causes a three-dimensional CDW gap to open up in the DOS [4,5]. The CDW in BBO is coupled to the breathing mode phonon which is due to the contraction and expansion of oxygen octahedra surrounding neighbouring Bi ions. Subtitution with K in the Ba-site results in a semiconductor-metal transition at a critical x c ∼ 0.3. While the CDW state is weakened by substitution, resulting in a systematic lowering of the CDW energy upon doping, the breathing mode phonon is observed even in the metallic phase [6]. Optical conductivity studies [7] suggest remnant local CDW order in the superconducting compositions with x = 0.33 and 0.4. EXAFS measurements[8] also show that there are two types of Bi ions, with short-and long-bond nearest neighbour Bi-O distances. On increasing x, the short and long Bi-O distances become equivalent around x = 0.4. This picture is derived from the formally Bi 3+ and Bi 5+ ions constituting the charge disproportionation in BBO[4], though X-ray and resonant photoemission spectroscopy studies have not shown clear evidence for the same in BBO and the doped compounds [9,10]. The results are understood as due to very small charge transfer between the unequal Bi sites [5]. The crystal structure also evolves with doping and at room temperature, for x = 0.0 -0.1 it is monoclinic, from x = 0.1 to 0.3 it is orthorhombic and above x = 0.3 up to 0.5( = the solubility limit), it is cubic [11].In this work we study the elect...

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“…Photoemission spectroscopy (PES) has revealed many key physical features of cuprate HTSCs such as the pseudo gap [7,8] and quasiparticles [9,10]. A recent remarkable progress in the energy resolution enables direct observation of a superconducting gap in metal superconductors [11,12].In this Letter, we report high-resolution PES results on MgB 2 to study the superconducting gap and its symmetry. By using a merit of high energy resolution (∆E < 5 meV), we have succeeded in directly observing opening/closure of the superconducting gap as a function of temperature, together with a narrow superconducting coherent peak located slightly below the Fermi level (E F ).…”

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High-Resolution Photoemission Study ofMgB2

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We have performed high-resolution photoemission spectroscopy on MgB2 and observed opening of a superconducting gap with a narrow coherent peak. We found that the superconducting gap is s-like with the gap value (∆) of 4.5±0.3 meV at 15 K. The temperature dependence (15 -40 K) of gap value follows well the BCS form, suggesting that 2∆/kBTc at T=0 is about 3. No pseudogap behavior is observed in the normal state. The present results strongly suggest that MgB2 is categorized into a phonon-mediated BCS superconductor in the weak-coupling regime.PACS numbers: 74.70. Ad, 74.25.Jb, 79.60.Bm The discovery of non-cuprate "high-temperature" superconductor MgB 2 [1] has evoked much attention in how this simple binary superconductor is similar to or different from the "conventional" high-temperature superconductors (HTSCs). The superconducting transition temperature (T c ) exceeds those of known metal superconductors and alkali-doped C 60 , being comparable to that of one of HTSCs (La 2−x Sr x CuO 4 ), and is located just on or a little above the theoretical upper limit predicted for the phonon-mediated superconductivity [2]. It is thus quite important to elucidate the mechanism and origin of this newly discovered "high-T c " superconductor in relation to the cuprate HTSCs. The superconducting gap and its symmetry has been investigated by tunneling [3][4][5] and NMR spectroscopy [6], but the results are not necessarily consistent with each other. Photoemission spectroscopy (PES) has revealed many key physical features of cuprate HTSCs such as the pseudo gap [7,8] and quasiparticles [9,10]. A recent remarkable progress in the energy resolution enables direct observation of a superconducting gap in metal superconductors [11,12].In this Letter, we report high-resolution PES results on MgB 2 to study the superconducting gap and its symmetry. By using a merit of high energy resolution (∆E < 5 meV), we have succeeded in directly observing opening/closure of the superconducting gap as a function of temperature, together with a narrow superconducting coherent peak located slightly below the Fermi level (E F ). We have performed numerical fittings to the PES spectra to investigate the size and symmetry of superconducting gap.The procedure to prepare polycrystalline MgB 2 samples has been described elsewhere [1]. The x-ray diffraction measurement shows that the samples are singlephased and the electrical resistivity and DC magnetization measurements confirm a sharp superconducting transition onset at 39.5 K. High-resolution PES measurements were performed using a SCIENTA SES-200 spectrometer with a high-flux discharge lamp and a toroidal grating monochromator. We set the total energy resolution at 4-7 meV to obtain a reasonable count rate near E F . The base pressure of spectrometer was 1.5x10 −11 Torr. Samples were scraped in-situ with a diamond file to obtain a clean surface for PES measurements. A measured PES spectrum represents the density of states (DOS) since the sample is a polycrystal. The temperature of sample was monito...

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Observation of a BCS Spectral Function in a Conventional Superconductor by Photoelectron Spectroscopy

Cited by 46 publications

References 12 publications

Electron-Phonon Coupling and its Evidence in the Photoemission Spectra of Lead

Electron-Phonon Coupling and its Evidence in the Photoemission Spectra of Lead

Electron-phonon coupling induced pseudogap and the superconducting transition inBa0.67K0.33BiO
Chainani
¹
,
Yokoya
²
,
Shin
³

et al. 2001
Phys. Rev. B
30
3
19
0
View full text Add to dashboard Cite

High-Resolution Photoemission Study ofMgB2

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Observation of a BCS Spectral Function in a Conventional Superconductor by Photoelectron Spectroscopy

Cited by 46 publications

References 12 publications

Electron-Phonon Coupling and its Evidence in the Photoemission Spectra of Lead

Electron-Phonon Coupling and its Evidence in the Photoemission Spectra of Lead

Electron-phonon coupling induced pseudogap and the superconducting transition inBa0.67K0.33BiOChainani1, Yokoya2, Shin3 et al. 2001Phys. Rev. B303190View full textAdd to dashboardCite

High-Resolution Photoemission Study ofMgB2

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Electron-phonon coupling induced pseudogap and the superconducting transition inBa0.67K0.33BiO
Chainani
¹
,
Yokoya
²
,
Shin
³

et al. 2001
Phys. Rev. B
30
3
19
0
View full text Add to dashboard Cite