Superconductivity in a Hole-Doped Mott-Insulating Triangular Adatom Layer on a Silicon Surface.

Adsorption of one-third monolayer of Sn on an atomically clean Si(111) substrate produces a two-dimensional triangular adatom lattice with one unpaired electron per site. This dilute adatom reconstruction is an antiferromagnetic Mott insulator; however, the system can be modulation doped and metallized using heavily doped p-type Si(111) substrates. Here, we show that the hole-doped dilute adatom layer on a degenerately doped p-type Si(111) wafer is superconducting with a critical temperature of 4.7±0.3 K. While a phonon-mediated coupling scenario would be consistent with the observed T_{c}, Mott correlations in the Sn-derived dangling-bond surface state could suppress the s-wave pairing channel. The latter suggests that the superconductivity in this triangular adatom lattice may be unconventional.

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.

Controllable dissociations of PH3 molecules on Si(001).

We demonstrate for the first time to our knowledge that controllable dissociation of PH3 adsorption products PHx (x = 2, 1) can be realized by STM (scanning tunneling microscope) manipulation techniques at room temperature. Five dissociative products and their geometric structures are identified via combining STM experiments and first-principle calculations and simulations. In total we realize nine kinds of controllable dissociations by applying a voltage pulse among the PH3-related structures on Si(001). The dissociation rates of the five most common reactions are measured by the I-t spectrum method as a function of voltage. The suddenly increased dissociation rate at 3.3 V indicates a transition from multivibrational excitation to single-step excitation induced by inelastic tunneling electrons. Our studies prove that selectively breaking the chemical bonds of a single molecule on semiconductor surface by STM manipulation technique is feasible.

Stabilization and manipulation of electronically phase-separated ground states in defective indium atom wires on silicon.

Exploration and manipulation of electronic states in low-dimensional systems are of great importance in the fundamental and practical aspects of nanomaterial and nanotechnology. Here, we demonstrate that the incorporation of vacancy defects into monatomic indium wires on n-type Si(111) can stabilize electronically phase-separated ground states where the insulating 8×2 and metallic 4×1 phases coexist. Furthermore, the areal ratio of the two phases in the phase-separated states can be tuned reversibly by electric field or charge doping, and such tunabilities can be quantitatively captured by first principles-based modeling and simulations. The present results extend the realm of electronic phase separation from strongly correlated d-electron materials typically in bulk form to weakly interacting sp-electron systems in reduced dimensionality.

Controlling Gold-Assisted Exfoliation of Large-Area MoS2 Monolayers with External Pressure.

Gold-assisted exfoliation can fabricate centimeter- or larger-sized monolayers of van der Waals (vdW) semiconductors, which is desirable for their applications in electronic and optoelectronic devices. However, there is still a lack of control over the exfoliation processes and a limited understanding of the atomic-scale mechanisms. Here, we tune the MoS2-Au interface using controlled external pressure and reveal two atomic-scale prerequisites for successfully producing large-area monolayers of MoS2. The first is the formation of strong MoS2-Au interactions to anchor the top MoS2 monolayer to the Au surface. The second is the integrity of the covalent network of the monolayer, as the majority of the monolayer is non-anchored and relies on the covalent network to be exfoliated from the bulk MoS2. Applying pressure or using smoother Au films increases the MoS2-Au interaction, but may cause the covalent network of the MoS2 monolayer to break due to excessive lateral strain, resulting in nearly zero exfoliation yield. Scanning tunneling microscopy measurements of the MoS2 monolayer-covered Au show that even the smallest atomic-scale imperfections can disrupt the MoS2-Au interaction. These findings can be used to develop new strategies for fabricating vdW monolayers through metal-assisted exfoliation, such as in cases involving patterned or non-uniform surfaces.

Controlled Switching of Bistable Nanophase Domains on a Silicon Surface.

Switching the crystalline phase of a material via electrostatic control is a proven strategy for developing memory devices such as memristors that are based on nonvolatile resistance switching phenomena. However, phase switching in atomic-scale systems is often difficult to control and poorly understood. Here, we explore nonvolatile switching of long 2.3 nm wide bistable nanophase domains in a Sn double-layer structure grown on Si(111), using a scanning tunneling microscope. We identified two mechanisms for this phase switching phenomenon. First, the electrical field across the tunnel gap continuously tunes the relative stability of the two phases and favors one over the other depending on the tunneling polarity. The second mechanism involves carrier injection into empty Sn orbitals. The coupling between these relatively long-lived hot electrons and surface phonons induces a lattice instability at sufficiently large tunneling current and provides access to a hidden metastable state of matter. This hidden state is nonvolatile but can be erased by choosing the appropriate tunneling conditions or raising the temperature. Similar mechanisms could possibly be exploited in phase-change memristor and field effect devices.

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.

Interface effects on the quantum well states of Pb thin films.

Using scanning tunneling spectroscopy, we have studied the interface effect on quantum well states of Pb thin films grown on various metal-terminated (Pb, Ag, and Au) n-type Si(111) surfaces and on two different p-type Si(111) surfaces. The dispersion relation E(k) of the electrons of the Pb film and the phase shift at the substrate interface were determined by applying the quantization rule to the measured energy positions of the quantum well states. Characteristic features in the phase shift versus energy curves were identified and were correlated to the directional conduction band of the silicon substrate and to the Schottky barrier formed between the metal film and the semiconductor. A model involving the band structure of the substrate, the Schottky barrier, and the effective thickness of the interface was introduced to qualitatively but comprehensively explain all the observed features of the phase shift at the substrate interface. Our physical understanding of the phase shift is critically important for using interface modification to control the quantum well states.

Assembling and disassembling Ag clusters on Si(111)-(7×7) by vertical atomic manipulation.

Atomic manipulation has been rarely used in the studies of complex structures and a low temperature requirement usually limits its application. Herein we have demonstrated a vertical manipulation technique to reproducibly and reversibly manipulating Ag atoms on an Si(111)-(7×7) surface by a scanning tunneling microscope tip at room temperature. Simple and complex Ag nanoclusters were assembled and disassembled with a precise control of single Ag atoms, which provided critical information on the size of these nanoclusters. The manipulation showed the growth processes of these Ag clusters and even partly unveiled their atomic structures. This technique can form a fundamental basis for further studies of the Ag/Si(111)-(7×7) system and for fabricating functional nanodevices in various metal-semiconductor systems.

Size-dependent superconducting state of individual nanosized Pb islands grown on Si(111) by tunneling spectroscopy.

By measuring the temperature-dependent tunneling spectroscopy of a set of flat-top Pb islands from 3.2 to 15 K, the limiting size of a nine-monolayer-thick Pb island with superconductivity above 3.2 K was determined to be ∼ 30 nm(2), in good agreement with the Anderson criterion. Further analysis indicates that the zero-temperature energy gap decreases significantly faster than the transition temperature when the Pb island size approaches this limit. This leads to a decrease of 2Δ(0)/k(B)T(C) from 4.5 to 3.3, thus showing that the Pb island superconductors undergo a change from strong to weak electron-phonon coupling.