True First-Order Surface Phase Transition without Nanoscale Phase Separation.

Nanoscale phase separation is common in many materials ranging from correlated electron systems to semiconductor surfaces undergoing phase transitions. On solid surfaces, nanoscale phase separations are known to occur over an extended temperature range during temperature-driven first-order surface phase transitions, precluding the true first-order transitions in thermodynamics. Here, we report the case of a surface phase transition very close to a true first-order transition. An array of indium wires on Si(111) undergoes a first-order charge-density-wave (CDW) transition with surprisingly little or no phase separation when prepared free of indium adatom impurities. The lack of phase separation was attributed to the small difference in strain with the substrate between the two competing normal and CDW phases. Indium adatom impurities cause phase separation and blur the transition, making it gradual and incomplete. These experimental observations provide insight into the surface phase transition at the nanoscale level.

√7 × âˆš3 surface with a double layer of In on Si(1 1 1) exhibiting both hexagonal and rectangular features.

Using a scanning tunneling microscope (STM), we demonstrate that the In-induced hexagonal (√7 × âˆš3-hex) and rectangular (√7 × âˆš3-rect) √7 × âˆš3 phases on Si(111) are from the same surface with a double layer of In. The double-layer In thickness was derived from observations that a √7 × âˆš3-hex island was formed on the √7 × âˆš3-'striped' phase, which is believed to have a single layer of In atoms. Bias-dependent STM images were obtained from the same √7 × âˆš3 domain and exhibited both √7 × âˆš3-hex and √7 × âˆš3-rect features, which led to the conclusion that both √7 × âˆš3 STM features originate from the same structure. These findings are in stark contrast to the prevailing idea that there are two √7 × âˆš3 surfaces with different structures and In coverage. We also observed a long-range Moiré-like superstructure in the √7 × âˆš3 surface and attribute it to the mismatch of the lattices of the surface layer of In and the Si(1 1 1) substrate.

Intertwined Solitons and Impurities in a Quasi-One-Dimensional Charge-Density-Wave System: In/Si(111).

Recent studies on a quasi-one-dimensional (quasi-1D) charge-density-wave (CDW) system, In atomic wires on Si(111), have brought intriguing issues on topological solitons in quasi-1D systems: the existence of few-atom-sized solitons, chirality of solitons, and the realization of logic exploiting the chirality switching. Using scanning tunneling microscopy and first-principles calculations, we show that the previously reported "short" phase-flip defects are In adatoms and thus have nonsolitonic nature, resolving the controversy over the existence of highly localized solitons. The observed "long" phase-flip and phase-slip defects are genuine solitons with and without chirality, respectively. While achiral solitons (phase-slip defects) can exist on the pristine CDW (8×2) surface, chiral solitons (phase-flip defects) cannot due to their breakage of 8×2 ordering. The chiral solitons can exist only when they are trapped by In adatoms and constitute a part of a closed-loop domain wall. The intertwinement of chiral solitons and In adatoms implies the limitations of the previously proposed logic utilizing soliton chirality, but it provides an opportunity to realize this by controlling the In-adatom defect.

Hidden phase in a two-dimensional Sn layer stabilized by modulation hole doping.

Semiconductor surfaces and ultrathin interfaces exhibit an interesting variety of two-dimensional quantum matter phases, such as charge density waves, spin density waves and superconducting condensates. Yet, the electronic properties of these broken symmetry phases are extremely difficult to control due to the inherent difficulty of doping a strictly two-dimensional material without introducing chemical disorder. Here we successfully exploit a modulation doping scheme to uncover, in conjunction with a scanning tunnelling microscope tip-assist, a hidden equilibrium phase in a hole-doped bilayer of Sn on Si(111). This new phase is intrinsically phase separated into insulating domains with polar and nonpolar symmetries. Its formation involves a spontaneous symmetry breaking process that appears to be electronically driven, notwithstanding the lack of metallicity in this system. This modulation doping approach allows access to novel phases of matter, promising new avenues for exploring competing quantum matter phases on a silicon platform.

Realization of a Hole-Doped Mott Insulator on a Triangular Silicon Lattice.

The physics of doped Mott insulators is at the heart of some of the most exotic physical phenomena in materials research including insulator-metal transitions, colossal magnetoresistance, and high-temperature superconductivity in layered perovskite compounds. Advances in this field would greatly benefit from the availability of new material systems with a similar richness of physical phenomena but with fewer chemical and structural complications in comparison to oxides. Using scanning tunneling microscopy and spectroscopy, we show that such a system can be realized on a silicon platform. The adsorption of one-third monolayer of Sn atoms on a Si(111) surface produces a triangular surface lattice with half filled dangling bond orbitals. Modulation hole doping of these dangling bonds unveils clear hallmarks of Mott physics, such as spectral weight transfer and the formation of quasiparticle states at the Fermi level, well-defined Fermi contour segments, and a sharp singularity in the density of states. These observations are remarkably similar to those made in complex oxide materials, including high-temperature superconductors, but highly extraordinary within the realm of conventional sp-bonded semiconductor materials. It suggests that exotic quantum matter phases can be realized and engineered on silicon-based materials platforms.

Reinvestigation of the alkali-metal-induced Ge(111)3 × 1 reconstruction on the basis of boundary structure observations.

We have investigated the surface atomic structure of boundary area of Li- and Na-induced Ge(111)3 × 1 reconstruction using scanning tunneling microscope. On Li/Ge(111)3 × 1, the 3 × 1 phase was found to be terminated with a single row in the filled-state image and with dimer-like features in the empty-state image. The images of both interior and boundary of the Li/Ge(111)3 × 1 surface are compatible with the honeycomb-chain-channel (HCC) model, which has substrate atoms with double bonds and is well established as the structure of AM/Si(111)3 × 1 surfaces. In contrast, termination with zigzag double rows at the domain boundary edges was observed in the filled-state images of the Na/Ge(111)3 × 1 phase, which is not reconcilable with the HCC structure. The filled-state STM feature of the boundary region of the Na/Ge(111)3 × 1 phase supports a structural model not having Ge = Ge double bonds, which was proposed to interpret its empty-state images. The trend of bondings between atoms in the surface layer of the AM-induced 3 × 1 reconstruction of Si and Ge is discussed in terms of electronegativity differences.

Room-temperature growth of Mg on Si(111): stepwise versus continuous deposition.

Using low-energy electron diffraction and scanning tunnelling microscopy, we studied the formation of Mg silicide and metallic Mg islands on a Si(111)-7 × 7 surface at room temperature as a function of Mg coverage. We found that the mechanism by which Mg islands grew on the Si(111)-7 × 7 surface, and the morphology of the islands that formed, depended on whether the Mg deposition was performed in a stepwise or continuous manner. When Mg was deposited in a stepwise manner, with 1 h between deposition events, an amorphous Mg silicide overlayer formed on the Si(111)-7 × 7 surface during the initial stage of deposition (up to 2.0 ML Mg coverage), as shown by the observation of δ7 × 7 and 1 × 1 low-energy electron diffraction patterns. Upon further stepwise Mg deposition, round-shaped Mg islands grew on the amorphous Mg silicide layer, as shown by scanning tunnelling microscopy and the emergence of a 1 × 1 low-energy electron diffraction pattern. If, on the other hand, the Mg was deposited continuously in a single step, hexagonal Mg islands formed on the flat Mg silicide layers, and a [Formula: see text] and 1 × 1 mixed phase was observed. Moreover, using scanning tunnelling spectroscopy, we confirmed the semiconducting and metallic nature of the Mg silicide layer and hexagonal Mg islands on the Si(111)-7 × 7 surface depending on their Mg coverage, respectively.

Real-space observation of nanoscale inhomogeneities and fluctuations in a phase transition of a surface quasi-one-dimensional system: In/Si(111).

We report direct visualizations of the fluctuation and condensation phenomena in a phase transition of a one-dimensional (1D) In/Si(111) system using scanning tunneling microscopy. The high-temperature (HT) and low-temperature (LT) phases are found to coexist on the nanometer scale near Tc. Above Tc, 1D LT-phase stripes fluctuate in the HT phase and coalesce into 2D islands with decreasing temperature. They condense to make the LT phase below Tc. Small areas of the HT phase also exist below Tc. The observed temperature-dependent evolution of the nanoscale inhomogeneities is consistent with the theoretical predictions for a second-order phase transition.

Intertwined electronic and structural phase transitions in the In/Si(111) interface.

The structural (4 x 1) to (8 x 2) transition and the electronic metal to semimetal transition at the In/Si interface are studied with scanning tunneling microscopy and spectroscopy. Both transitions are gradual, resulting in a complex domain structure in the transition temperature regime. At these intermediate temperatures, the metallic (4 x 1) and semimetallic (8 x 2) domains coexist with each other and with new nanophases. By probing the two intertwined but distinguishable transitions at the atomic level, the interaction between different phases is visualized directly.

Two-dimensional carbon incorporation into Si(001): C amount and structure of Si(001)-c(4 x 4).

The C amount and the structure of the Si(001)-c(4 x 4) surface is studied using scanning tunneling microscopy (STM) and ab initio calculations. The c(4 x 4) phase is found to contain 1/8 monolayer C (1 C atom in each primitive unit cell). From the C amount and the symmetry of high-resolution STM images, it is inferred that the C atoms substitute the fourth-layer site below the dimer row. We construct a structure model relying on ab initio energetics and STM simulations. Each C atom induces an on-site dimer vacancy and two adjacent rotated dimers on the same dimer row. The c(4 x 4) phase constitutes the subsurface Si(0.875)C(0.125) delta layer with two-dimensionally ordered C atoms.

Facile fabrication of 2-dimensional arrays of sub-10 nm single crystalline Si nanopillars using nanoparticle masks.

A simple procedure for the fabrication of sub-10 nm scale Si nanopillars in a 2-D array using reactive ion etching with 8 nm Co nanoparticles as etch masks is demonstrated. The obtained Si nanopillars are single crystalline tapered pillar structures of 5 nm (top) x 8 nm (bottom) with a density of approximately 4 x 10(10) pillars cm(-2) on the substrate, similar to the density of Co nanoparticles distributed before the ion etching process. The uniform spatial distribution of the Si nanopillars can also be patterned into desired positions. Our fabrication method is straightforward and requires mild process conditions, which can be extended to patterned 2-D arrays of various Si nanostructures.

Initial stage of carbon incorporation into si(001) and one-dimensional ordering of embedded carbon.

We investigate the initial stage of the C incorporation into Si(001) using thermal dissociation of C2H2. The scanning tunneling microscopy shows that C-induced dimer vacancies (DVs) with depressed adjacent dimers are generated on the surface and aligned in the dimer direction, forming the 2xn structure. The ab initio pseudopotential calculations reveal that, with the presence of a DV in the surface, the alpha site in the fourth subsurface layer directly below the DV is the most favorable for the incorporated C atoms. The embedded C atoms align one dimensionally due to the interaction which is attractive in neighboring dimer rows but repulsive in the same dimer row.