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'Wrinkle engineering', as an effective approach to modify the properties of materials without introducing additional chemical doping, has been widely used in a variety of two-dimensional materials, including graphene, phosphorene and transitional metal dichalcogenides, etc. One-dimensional wrinkle or ripple arrays, which breaks the crystalline symmetry of materials, may reformate the nontrivial band structure and greatly affect the electronic properties of topological materials. Until now, such method has been rarely applied experimentally in tuning topological materials. Here, we fabricate highly uniform ripple arrays directly in few-quintuple-layers (QL) Bi2Te3 thin films on SrTiO3 substrate realized by in-situ strained epitaxial growth, investigated by detailed low temperature scanning tunneling microscope/spectroscopy. Well-defined one-dimensional (1D) ripple structures induced by the interfacial strain from the lattice mismatch between the Bi2Te3 films and the substrate has been observed. As the thickness of Bi2Te3 thin films increases, the release of the strain leads to the reduction of the averaged corrugation height and the periodicity of ripple array. The dI/dV spectra and spatially resolved spectroscopic survey acquired on the epitaxial layers with different thicknesses, reveal that both the energy gap and the Fermi velocity of the Dirac surface states are greatly enhanced on thinner films. Dirac fermion velocity vF is estimated about 7.8 × 105 m s−1 for 3 QL, which is greatly enhanced by compared with the bulk values (4.8 × 105 m s−1). An asymmetric dispersion behavior along different − directions is observed, highly related with the modulation of 1D corrugated ripples on the electronic structure. Moreover, an unconventional Bi (S4) termination mainly observed on thinner films shows a stress-related behavior, agreed with previous theoretical calculations. Our work proposes a promising route to fabricate a highly uniform ripple structure on nontrivial topological materials, which can effectively tune the electronic structure. Specially, it may have an immediate application for realizing a high-performance topological-based device with higher operating temperature and speed. [ABSTRACT FROM AUTHOR]

Electronic systems with quasi-one-dimensional (Q1D) Fermi surface tend to form either a charge-density-wave (CDW) or a spin-density-wave ground state at low temperatures due to one-dimensional instabilities. Among various CDW states, surface CDWs are different from that within the bulk due to the reduced dimensionality. Here we report the systematic investigation of charge density modulation on the surface of in situ cleaved TaTe4 crystal by means of low temperature scanning tunneling microscopy/spectroscopy, corroborated with density functional theory calculation. Well-defined Q1D modulation (4a, 6c) accompanied with a periodic lattice distortion is clearly observed on the (010) cleaved surface, distinct from that of its bulk CDW (2a × 2a × 3c). Tunneling spectroscopic measurements reveal a partially-opened energy gap about 23 meV around Fermi level. Such gap shows similar spatial variation with the periodicity of surface modulation and diminishes subsequently as temperature rises, which indicates a novel surface-related CDW gap states. The surface modulation vectors fit well with the Fermi surface nesting vectors, derived from the calculated Fermi surfaces. Surprisingly, such surface modulation can be suppressed greatly by applying vertical magnetic field and a critical field about 9.05 T can be estimated from field-dependent data. Our results demonstrate that this unique CDW modulation is strongly related to Fermi surface nesting mediated electron–electron coupling due to the reduced dimensionality of the surface, and can be readily tuned by relatively small magnetic field. [ABSTRACT FROM AUTHOR]

Electronic systems with quasi-one-dimensional (Q1D) Fermi surface tend to form either a charge-density-wave (CDW) or a spin-density-wave ground state at low temperatures due to one-dimensional instabilities. Among various CDW states, surface CDWs are different from that within the bulk due to the reduced dimensionality. Here we report the systematic investigation of charge density modulation on the surface of in situ cleaved TaTe4 crystal by means of low temperature scanning tunneling microscopy/spectroscopy, corroborated with density functional theory calculation. Well-defined Q1D modulation (4a, 6c) accompanied with a periodic lattice distortion is clearly observed on the (010) cleaved surface, distinct from that of its bulk CDW (2a × 2a × 3c). Tunneling spectroscopic measurements reveal a partially-opened energy gap about 23 meV around Fermi level. Such gap shows similar spatial variation with the periodicity of surface modulation and diminishes subsequently as temperature rises, which indicates a novel surface-related CDW gap states. The surface modulation vectors fit well with the Fermi surface nesting vectors, derived from the calculated Fermi surfaces. Surprisingly, such surface modulation can be suppressed greatly by applying vertical magnetic field and a critical field about 9.05 T can be estimated from field-dependent data. Our results demonstrate that this unique CDW modulation is strongly related to Fermi surface nesting mediated electron–electron coupling due to the reduced dimensionality of the surface, and can be readily tuned by relatively small magnetic field. [ABSTRACT FROM AUTHOR]

Although the MoS2/graphene 2D stack achieved a lot of attention in recent research on 2D semiconductor devices, the interface interaction between perfect MoS2 and graphene and its effects on the electronic properties are rarely studied. Here, we report our recent studies on Raman spectroscopy and photoluminescence of the MoS2/graphene heterostructure. A nearly perfect interface between these two materials was achieved with a dry transfer and forming gas annealing. Raman spectroscopy of the heterostructure shows that the strong interface interaction leads to blue-shifts of Raman modes in MoS2 and graphene. Moreover, indicated by the photoluminescene of the heterostructure, the photon excitation and electronic dynamics of MoS2 are significantly influenced by the additional graphene layer on top. Our results reveal that the interface of heterostructures plays an important role in determining the physical properties, and it is better to view the heterostructure as a newly emerging material rather than a simple physical combination of the two materials. [ABSTRACT FROM AUTHOR]

High pressure has been demonstrated to be a powerful approach of producing novel condensed-matter states, particularly in tuning the superconducting transition temperature ( T c ) of the superconductivity in a clean fashion without involving the complexity of chemical doping. However, the challenge of high-pressure experiment hinders further in-depth research for underlying mechanisms. Here, we have successfully synthesized continuous layer-controllable SnSe 2 films on SrTiO 3 substrate using molecular beam epitaxy. By means of scanning tunneling microscopy/spectroscopy (STM/S) and Raman spectroscopy, we found that the strong compressive strain is intrinsically built in few-layers films, with a largest equivalent pressure up to 23 GPa in the monolayer. Upon this, unusual 2 × 2 charge ordering is induced at the occupied states in the monolayer, accompanied by prominent decrease in the density of states (DOS) near the Fermi energy ( E F ), resembling the gap states of CDW reported in transition metal dichalcogenide (TMD) materials. Subsequently, the coexistence of charge ordering and the interfacial superconductivity is observed in bilayer films as a result of releasing the compressive strain. In conjunction with spatially resolved spectroscopic study and first-principles calculation, we find that the enhanced interfacial superconductivity with an estimated T c of 8.3 K is observed only in the 1 × 1 region. Such superconductivity can be ascribed to a combined effect of interfacial charge transfer and compressive strain, which leads to a considerable downshift of the conduction band minimum and an increase in the DOS at E F . Our results provide an attractive platform for further in-depth investigation of compression-induced charge ordering (monolayer) and the interplay between charge ordering and superconductivity (bilayer). Meanwhile, it has opened up a pathway to prepare strongly compressed two-dimensional materials by growing onto a SrTiO 3 substrate, which is promising to induce superconductivity with a higher T c .

Materials can exhibit exotic properties when they approach the two-dimensional (2D) limit. Because of promising applications in catalysis and energy storage, 2D transition-metal carbides (TMCs) have attracted considerable attention in recent years. Among these TMCs, ultrathin crystalline α-Mo 2 C flakes have been fabricated by chemical vapor deposition on Cu/Mo bilayer foils, and their 2D superconducting property was revealed by transport measurements. Herein, we studied the ultrathin α-Mo 2 C flakes by atomic-resolved scanning tunneling microscopy/spectroscopy (STM/S). Strain-related structural modulation and the coexistence of different layer-stacking modes are observed on the Mo-terminated surface of α-Mo 2 C flakes as well as various lattice defects. Furthermore, an enhanced superconductivity with shorter correlation length was observed by STS technique, and such superconductivity is very robust despite the appearance of the defects. A mechanism of superconducting enhancement is proposed based on the strain-induced strong coupling and the increased disordering originated from lattice defects. Our results provide a comprehensive understanding of the correlations between atomic structure, defects, and enhanced superconductivity of this emerging 2D material.

The freestanding PbSe monolayer has been predicted as a candidate of the two-dimensional topological crystalline insulator, which possesses the Dirac-cone-like edge states resided at the edge. Up to now, however, direct experimental evidence of topological PbSe monolayer has not yet been reported. Here, we report the epitaxial growth and scanning tunneling microscopy study of few-layers PbSe islands grown on SrTiO 3 substrate. From the investigation of different thickness, we discover the release of compressive strain and the reduction of bandgap as the thickness becomes thick. Following detailed spectroscopic measurements, a signature of Dirac-like edge states is observed at the edge of seventh-layer PbSe. In conjunction with first-principle calculations, we find that compressive-strain-induced buckling adjusts the topological band inversion and eventually leads to a phase transition from nontrivial two-dimensional topological crystalline insulator to trivial insulator, which match well with our experimental observations. Therefore, both theoretical calculations and experimental observations reveal that the strain can effectively affect the property of epitaxial PbSe, meanwhile demonstrate seventh-layer PbSe as a potential candidate of 2D TCI.