Disproportionation phenomena on free and strained Sn/Ge(111) and Sn/Si(111) surfaces

Quantum Spin Hall States in Stanene/Ge(111)

For topological insulators to be implemented in practical applications, it is a prerequisite to select suitable substrates that are required to leave insulators’ nontrivial properties and sizable opened band gaps (due to spin-orbital couplings) unaltered. Using ab initio calculations, we predict that Ge(111) surface qualified as a candidate to support stanene sheets, because the band structure of √3 × √3 stanene/Ge(111) (2 × 2) surface displays a typical Dirac cone at Γ point in the vicinity of the Fermi level. Aided with the result of Z₂ invariant calculations, a √3 × √3 stanene/Ge(111) (2 × 2) system has been proved to sustain the nontrivial topological phase, with the prove being confirmed by the edge state calculations of stanene ribbons. This finding can serve as guidance for epitaxial growth of stanene on substrate and render stanene feasible for practical use as a topological insulator.

Triangular Mott-Hubbard insulator phases of Sn/Si(111) and Sn/Ge(111) surfaces

The ground state of Sn/Si(111) and Sn/Ge(111) surface alpha phases is reexamined theoretically, based on ab initio calculations where correlations are approximately included through the orbital dependence of the Coulomb interaction (in the local density+Hubbard U approximation). The effect of correlations is to destabilize the vertical buckling in Sn/Ge(111) and to make the surface magnetic, with a metal-insulator transition for both systems. This signals the onset of a stable narrow gap Mott-Hubbard insulating state, in agreement with very recent experiments. Antiferromagnetic exchange is proposed to be responsible for the observed Gamma-point photoemission intensity, as well as for the partial metallization observed above 60 K in Sn/Si(111). Extrinsic metallization of Sn/Si(111) by, e.g., alkali doping, could lead to a novel 2D triangular superconducting state of this and similar surfaces.

Chemically resolved structure of the surface

The structure and chemical states of the Sn/Ge(111) surface are characterized by x-ray standing waves combined with photoemission. For the room temperature square root 3xsquare root 3 phase two chemical components, approximately 0.4 eV apart, are observed for both Sn 3d and 4d core levels. Our model-independent, x-ray standing wave analysis shows unambiguously that the two components originate from Sn adatoms located at two different heights separated vertically by 0.23 A, in favor of a model composed of a fluctuating Sn layer. Contrary to the most accepted scenario, the stronger Sn 3d and 4d components, which appear at the lower binding-energy sides and account for 2/3 of the Sn adatoms, are identified to be associated with the higher Sn position, manifesting their filled valence state character.

Direct observation of a (3 x 3) phase in alpha-Pb/Ge(111) at 10 K

We have investigated the recently reported structural phase transition at low temperature (LT) for alpha-Pb/Ge(111) [from a (3 x 3) symmetry to a disordered phase] using scanning tunneling microscopy (STM). By tracking exactly the same surface regions with atomic resolution while varying the sample temperature from 40 to 140 K, we have observed that substitutional point defects are not mobile, in clear contrast to previous assumptions. Moreover, STM data measured at the lowest temperatures ever reported for this system (10 K) show that while filled-state images display the apparent signature of a glassy phase with no long-range order, in empty-state images honeycomb patterns with (3 x 3) periodicity, and not distinguishable from data measured at much higher temperatures, are clearly resolved. These new observations cast serious doubts on the nature and/or on the existence of a disordered phase at LT.

Novel electronically driven surface phase predicted in C/Si(111)

We predict a novel electronically driven phase for the recently created C/Si(111) surface at 1/3 monolayer coverage. Whereas the isoelectronic surface Sn/Ge(111) is a 3 x 3 distorted metal and Si/SiC(0001) is an undistorted magnetic Mott insulator, the new phase combines both features. Two of three adatoms in C/Si(111) should form a distorted (3 x 3) honeycomb sublattice, the third an undistorted insulating and magnetic triangular sublattice. The generally conflicting elements, namely, band energy, favoring distortion, and strong electron correlations favoring a Mott state, actually conspire in this case. This kind of state represents the surface analog of the Fazekas-Tosatti state in the charge density wave compound 1T-TaS2.

Direct observation of sn adatoms dynamical fluctuations at the Sn/Ge(111) surface

The well-known low-temperature phase transition sqrt[3]xsqrt[3] to 3x3 for the 1/3 monolayer of Sn adatoms on the Ge(111) surface has been studied by scanning tunneling microscopy. The STM tip was used as a probe to record the tunneling current as a function of time on top of the Sn adatoms. The presence of steps on the current-time curves allowed the detection of fluctuating Sn atoms along the direction vertical to the substrate. We discuss the effect of temperature and surface defects on the frequency of the motion, finding consistency with the dynamical fluctuations model.

Surface phase transitions induced by electron mediated adatom-adatom interaction

We propose that the indirect adatom-adatom interaction mediated by the conduction electrons of a metallic surface is responsible for the sqrt[3]xsqrt[3]<==>3x3 structural phase transitions observed in Sn/Ge (111) and Pb/Ge (111). When the indirect interaction overwhelms the local stress field imposed by the substrate registry, the system suffers a phonon instability, resulting in a structural phase transition in the adlayer. Our theory is capable of explaining all the salient features of the sqrt[3]xsqrt[3]<==>3x3 transitions observed in Sn/Ge (111) and Pb/Ge (111), and is in principle applicable to a wide class of systems whose surfaces are metallic before the transition.

Phonon softening, chaotic motion, and order-disorder transition in Sn/Ge(111)

The phonon dynamics of the Sn/Ge(111) interface is studied using high-resolution helium atom scattering and first-principles calculations. At room temperature we observe a phonon softening at the Kmacr; point in the (sqrt[3]xsqrt[3])R30 degrees phase, associated with the stabilization of a (3x3) phase at low temperature. That phonon band is split into three branches in the (3x3) phase. We analyze the character of these phonons and find out that the low- and room-temperature modes are connected via a chaotic motion of the Sn atoms. The system is shown to present an order-disorder transition.