Novel Bi-Polaronic Ground State in K/Si(111):B

Abstract

We report here on low energy electron diffraction (LEED), Auger spectroscopy, scanning tunneling microscopy (STM) and angle-resolved photoemission (ARPES) studies of ultrathin films of K/Si(111)-√ 3 × √ 3R30:B. A potassium-induced surface state is evidenced being maximum at the saturation coverage. ARPES data clearly evidence the folding of the K-induced surface band near the expected kF attesting of a large Mott gap ∆ = 500 meV and a very narrow measured bandwidth W = 140 meV. This clearly signs the strongly correlated nature of this material. Nevertheless, by decreasing the temperature, a novel insulator to insulator surface phase transition is observed characterized by a doubling of the unit cell together with a stabilization of the surface band. In addition, the linear temperature dependence of the ARPES linewidth suggests these materials are characterized by a strong electron-phonon coupling. Therefore, such a phase transition can be explained if the electron-phonon coupling is large enough to compensate the strong on-site repulsion defining a bi-polaronic insulating ground state instead of a Mott-Hubbard one.

Switching the Charge State of Individual Surface Atoms at Si(111)-√3 × √3:B Surfaces

“…On the other hand, the e–ph coupling (E p ) is very strong to give a negative U eff (= −0.79 eV), favoring the formation of a two-electron bound state. These U and E p values are, however, similar to those of potassium (K)-deposited √3 surface: Cardenas et al observed a strong temperature dependent Franck–Condon broadening of the occupied surface band of K-deposited √3-surface near −0.8 eV and, from the model fitting of such behavior, determined the e–ph coupling parameters, i.e., g 2 ≈ 8 and ℏω = 55 meV. These, in turn, give E p ≈ 0.44 eV and U ≈ 0.16 eV, being close to our values.…”

“…These, in turn, give E p ≈ 0.44 eV and U ≈ 0.16 eV, being close to our values. A similar e–ph coupling behavior between bare and K-deposited √3-surfaces may be ascribed to rather a simple role of potassium; that is, it transfers its 4s electrons to the √3-surface without significant interactions …”

“…A similar e−ph coupling behavior between bare and K-deposited √3-surfaces may be ascribed to rather a simple role of potassium; that is, it transfers its 4s electrons to the √3-surface without significant interactions. 24 Since a two-electron bound state has the electronic level within the valence band continuum below E F (B1 feature in Figure 3e) with a locally stable total energy (H-configuration in Figure 3c), it is not subject to the radiative decay process. Also, the red curve in Figure 3c indicates that nonradiative decay process, that is, the electron release from this state to the delocalized one through the configurational changes is blocked by an energy barrier of ∼100 meV.…”

“…As mentioned above, the H-configuration is a two-electron bound state, whose formation is a direct consequence of the strong e–ph coupling at the √3-surface. Indeed this e–ph coupling has been suggested as a cause for forming the bipolaronic insulating states at the √3-surface after the electrons are donated from alkali metal deposits. − In general, the electronic system with a strong e–ph coupling as well as an electron–electron interaction is effectively described by the Hubbard–Holstein (HH) Hamiltonian, for which the former is represented by a polaron binding energy ( E p = g 2 ℏω) with ℏω being a typical phonon energy and the latter by an on-site Coulomb repulsion ( U ) . If the e–ph coupling is so strong that U eff (= U – 2 E p ) has a negative value, this Hamiltonian favors the formation of a two-electron bound state (i.e., bipolaron state) and develops two prominent spectral peaks centered at E = E F ± ( U – 4 E p )/2 .…”