Electron-phonon coupling in a system with broken symmetry: Surface ofBe(0001)

Abstract

The momentum-resolved Eliashberg function (ELF), , , for the Be(0001) zone-center surface state was extracted from the high-quality angle-resolved photoemission spectroscopy (ARPES) data at the Fermi energy in the Γ direction, displaying ten peaks. A comparison of the peaks in the ELF to the bulk phonon density of states (DOS) and the bulk and surface phonon dispersion allows for an identification of the origin of all but two of the peaks. The five high energy peaks (> 52 meV) are associated with the coupling of the surface state to bulk phonon modes. The peaks at 44.5 meV and at 49.0 meV have contributions from both the bulk and surface phonons. The most intense peak at 37.5 meV is evidently having contribution from EPC of the surface state to the surface Rayleigh phonon mode. Surprisingly, the two lowest energy modes, which must be associated with surface Rayleigh phonon, cannot be attributed to a high phonon DOS at the surface nor to any Fermi surface nesting. After detail analysis, the three lowest energy peaks are associated with momentum dependence in the EPC matrix, reflected in the phonon line-width changes. As a result of the broken symmetry at the surface, coupling of the initial surface state due to the presence of the surface phonons contributes ~48.5±12.5% of the spectral weight in the ELF and ~6 6.5±10.5% to the mass enhancement ( ).

Hourglass Dirac chain metal in rhenium dioxide

“…It has been argued that they could produce a huge surface density of states, which may offer a route toward high-temperature superconductivity 60 . The recent work by Li et al 26 attributed the unusually high surface density of states on the Be (0001) surface to the drumhead surface states, which combined with the strong electron–phonon coupling found on that surface 61 may lead to a surface superconductivity (yet to be confirmed by experiment). Interestingly, the giant enhancement of the Friedel oscillation on the Be (0001) surface was also found to be due to these nontrivial surface states 26 .…”

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“…The underlying driving force for the lattice expansion has again been attributed to unusual electronic states at the surfaces [35,36]. Thirdly, the large electron-phonon coupling (λ = 1.18 ) [30] and giant Friedal oscillations [31] have been found at its (0001) surface. This fact along with the high density of states at the Fermi level was suggested to be a candidate to show surface superconductivity [29].…”

“…The understanding of the surface states (SF-band 1 in Fig. 3f and Ring B in Fig .4a) has also been a long-standing question since the 1980s [26][27][28][29][30][31][32][33][34]. It was suspected to be correlated with unusually large outward relaxation of the topmost surface atoms, so-called p to s electron demotion as well as the core-level shifts [26][27][28][32][33][34].…”

“…We find that the presence of the topologically protected (0001) surface states around the Γ point originates from this DNL state. Furthermore, realization of the existence of the DNL in principle offers new opportunities and insights in rationalizing the long-standing problem [26][27][28][29][30][31][32][33][34] of the pronounced surface states in Be.…”

“…1980s [26][27][28][29][30][31][32][33][34]. It was suspected to be correlated with unusually large outward relaxation of the topmost surface atoms, so-called p to s electron demotion as well as the core-level shifts [26][27][28][32][33][34].…”