Low-temperature Scanning Tunneling Research Articles

Direct Observations of Spontaneous In-Plane Electronic Polarization in 2D Te Films.

Single-element polarization in low dimensions is fascinating for constructing next-generation nanoelectronics with multiple functionalities, yet remains difficult to access with satisfactory performance. Here, spectroscopic evidences are presented for the spontaneous electronic polarization in tellurium (Te) films thinned down to bilayer, characterized by low-temperature scanning tunneling microscopy/spectroscopy. The unique chiral structure and centrosymmetry-breaking character in 2DTe gives rise to sizable in-plane polarization with accumulated charges, which is demonstrated by the reversed band-bending trends at opposite polarization edges in spatially resolved spectra and conductance mappings. The polarity of charges exhibits intriguing influence on imaging the moiré superlattice at the Te-graphene interface. Moreover, the plain spontaneous polarization robustly exists for various film thicknesses, and can universally preserve against different epitaxial substrates. The experimental validations of considerable electronic polarization in Te multilayers thus provide a realistic platform for promisingly facilitating reliable applications in microelectronic devices.

Superconductivity and charge density wave in Cu0.06TiSe2: A low-temperature STM/STS investigation

As one of the earliest discovered two-dimensional materials possessing charge density wave (CDW), TiSe2 has attracted wide attention due to its superconductivity induced by Cu intercalation. Until now, the relationship between superconductivity and CDW remains unclear, largely due to insufficient research at extremely low temperatures and magnetic fields. In this study, spatially resolved electronic density of states (DOS) of Cu0.06TiSe2 is investigated using low-temperature scanning tunneling microscopy/spectroscopy measurements. It is found that short-ranged commensurate CDW coexists with a homogeneous superconductivity exhibiting an anisotropic s-wave gap with an amplitude of 0.5 meV. Compared to the parent compound TiSe2, the spectra of Cu0.06TiSe2 exhibit a clear electron doping effect, as evidenced by a 70 meV shift of Fermi energy. Interestingly, the DOS is found to be strongly modified near the Fermi energy, despite its overall rigid band nature. These findings suggest that it is the remnant electron–hole coupling that sustains the short-ranged CDW, while the doping enhanced DOS facilitates superconductivity. This reveals a momentum space competition between the two microscopically coexistent orders.

Moiré Enhanced Two-Band Superconductivity in a MnTe/NbSe2 Heterojunction.

Recent advances in creating moiré periods of two-dimensional heterostructures enable diverse and compatible tunability to modulate the conventional proximity effect involving superconductivity, magnetism, and topology. Here, by constructing a MnTe/NbSe2 heterojunction via molecular beam epitaxy growth, we report on a moiré-enhanced multiband superconductivity by low-temperature scanning tunneling microscopy/spectroscopy measurements. We observe a distinct double-gap superconducting spectrum on monolayer MnTe that is absent on the NbSe2 substrate. The subgap character exhibits a moiré-related oscillation in real space, which can be well described by an effective two-band model. The restored two-gap feature and its rapid suppression under a small magnetic field are speculated to be mediated by the moiré superlattice, which is closely related to the enhanced interband coupling strength of quasiparticle scattering. Our work paves the way for engineering proximitized properties of heterostructures by a moiré landscape with spatial modulations.

Identifying s-wave pairing symmetry in single-layer FeSe from topologically trivial edge states

Determining the pairing symmetry of single-layer FeSe on SrTiO3 is the key to understanding the enhanced pairing mechanism. It also guides the search for superconductors with high transition temperatures. Despite considerable efforts, it remains controversial whether the symmetry is the sign-preserving s- or the sign-changing s±-wave. Here, we investigate the pairing symmetry of single-layer FeSe from a topological point of view. Using low-temperature scanning tunneling microscopy/spectroscopy, we systematically characterize the superconducting states at edges and corners of single-layer FeSe. The tunneling spectra collected at edges and corners show a full energy gap and a substantial dip, respectively, suggesting the absence of topologically non-trivial edge and corner modes. According to our theoretical calculations, these spectroscopic features can be considered as strong evidence for the sign-preserving s-wave pairing in single-layer FeSe.

Substrate Effect on Band Bending of MoSe2 Monolayer Near Mirror‐Twin Domain Boundaries

AbstractBand bending near mirror twin domain boundaries (MTBs) in a MoSe2 monolayer grown on different substrates, i.e., highly oriented pyrolytic graphite (HOPG), graphene‐on‐SiC, and crystalline Au(110), is investigated by low temperature scanning tunneling microscopy/spectroscopy. Upshift bending of the valence band edge near MTB is observed on both graphene and HOPG substrates, whereas a downshift bending is found on Au(110). For the former, the magnitudes of bending are different. This is explained based on the static charge model, where an accumulative charge exists at MTBs due to both the electrical dipole discontinuity across the MTB as well as charge transfer between the substrate and MoSe2 epilayer. The relevance of the static electric model is further affirmed by noting a geometric effect on the band bending, where it is asymmetric across the vertex of an MTB loop.

Atomically constructing a van der Waals heterostructure of CrTe2/Bi2Te3 by molecular beam epitaxy

A 2D heterostructure with proximity coupling of magnetism and topology can provide enthralling prospects for hosting new quantum states and exotic properties that are relevant to next-generation spintronic devices. Here, we synthesize a delicate van der Waals (vdW) heterostructure of CrTe2/Bi2Te3 at the atomic scale via molecular beam epitaxy. Low-temperature scanning tunneling microscopy/spectroscopy measurements are utilized to characterize the geometric and electronic properties of the CrTe2/Bi2Te3 heterostructure with a compressed vdW gap. Detailed structural analysis reveals complex interfacial structures with diversiform step heights and intriguing moiré patterns. The formation of the interface is ascribed to the embedded characteristics of CrTe2 and Bi2Te3 by sharing Te atomic layer upon interfacing, showing intercoupled features of electronic structure for CrTe2 and Bi2Te3. Our study demonstrates a possible approach to construct artificial heterostructures with different types of ordered states, which may be of use for achieving tunable interfacial Dzyaloshinsky–Moriya interactions and tailoring the functional building blocks in low dimensions.

Discrete scale invariance of the quasi-bound states at atomic vacancies in a topological material

Recently, log-periodic quantum oscillations have been detected in the topological materials zirconium pentatelluride (ZrTe5) and hafnium pentatelluride (HfTe5), displaying an intriguing discrete scale invariance (DSI) characteristic. In condensed materials, the DSI is considered to be related to the quasi-bound states formed by massless Dirac fermions with strong Coulomb attraction, offering a feasible platform to study the long-pursued atomic-collapse phenomenon. Here, we demonstrate that a variety of atomic vacancies in the topological material HfTe5 can host the geometric quasi-bound states with a DSI feature, resembling an artificial supercritical atom collapse. The density of states of these quasi-bound states is enhanced, and the quasi-bound states are spatially distributed in the "orbitals" surrounding the vacancy sites, which are detected and visualized by low-temperature scanning tunneling microscope/spectroscopy. By applying the perpendicular magnetic fields, the quasi-bound states at lower energies become wider and eventually invisible; meanwhile, the energies of quasi-bound states move gradually toward the Fermi energy (EF). These features are consistent with the theoretical prediction of a magnetic field-induced transition from supercritical to subcritical states. The direct observation of geometric quasi-bound states sheds light on the deep understanding of the DSI in quantum materials.

Evolution of one-dimensional electronic dynamics in Nb2n+1SinTe4n+2

The electronic dynamics in ${\mathrm{Nb}}_{2n+1}{\mathrm{Si}}_{n}{\mathrm{Te}}_{4n+2}$ with varying component $n$ are systematically investigated by low-temperature scanning tunneling microscopy/spectroscopy (STM/STS). Our quasiparticle interferences reveal a nondecay standing wave at the Fermi level, proving the one-dimensional (1D) nature of the charge carriers (holes) in the materials. The energy and component dependence of phase relaxation length ${l}_{\ensuremath{\phi}}$ and lifetime $\phantom{\rule{4pt}{0ex}}\ensuremath{\tau}$ of 1D holes are unveiled and found to be consistent with the Tomonaga-Luttinger liquid theory. Our work demonstrates ${\mathrm{Nb}}_{2n+1}{\mathrm{Si}}_{n}{\mathrm{Te}}_{4n+2}$ as a platform to manipulate 1D physics and offers an alternative perspective for studying electronic dynamics of non-Fermi liquids through STM experiments.

Robustness of the unidirectional stripe order in the kagome superconductor CsV3Sb5

V-based kagome materials AV3Sb5 (A = K, Rb, Cs) have attracted much attention due to their novel properties such as unconventional superconductivity, giant anomalous Hall effect, charge density wave (CDW) and pair density wave. Except for the 2a 0 × 2a 0 CDW (charge density wave with in-plane 2 × 2 superlattice modulation) in AV3Sb5, an additional 1 × 4 (4a 0) unidirectional stripe order has been observed at the Sb surface of RbV3Sb5 and CsV3Sb5. However, the stability and electronic nature of the 4a 0 stripe order remain controversial and unclear. Here, by using low-temperature scanning tunneling microscopy/spectroscopy (STM/S), we systematically study the 4a 0 stripe order on the Sb-terminated surface of CsV3Sb5. We find that the 4a 0 stripe order is visible in a large energy range. The STM images with positive and negative bias show contrast inversion, which is the hallmark for the Peierls-type CDW. In addition, below the critical temperature about 60 K, the 4a 0 stripe order keeps unaffected against the topmost Cs atoms, point defects, step edges and magnetic field up to 8 T. Our results provide experimental evidences on the existence of unidirectional CDW in CsV3Sb5.

Planar Heterojunction of Ultrathin CrTe3 and CrTe2 van der Waals Magnet.

The fabrication of planar heterojunctions with magnetic van der Waals ultrathin crystals is essential for constructing miniaturized spintronic devices but is yet to be realized. Here, we report the growth of CrTe3 and CrTe2 ultrathin films with molecular beam epitaxy and characterize their morphological and electronic structure through low-temperature scanning tunneling microscopy/spectroscopy. The former is identified as a Mott insulator, and the latter has shown a robust magnetic order previously. Through vacuum annealing, CrTe3 can be transformed into CrTe2, whose relative ratio is controlled via the annealing time. This renders the feasibility of constructing CrTe3-CrTe2 planar heterojunctions, which express atomically sharp interfaces and smooth band bending. We also identified a superstructure conceivably formed via hybrid units of CrTe3 and CrTe2, whose electronic structure exhibits stunning tunability with the length of the superstructure. Our study sets a foundation for the development of magnetic tunneling junctions for building spintronic circuits and engineering electronic states in artificial superlattice structures.

Observation of 1D Fermi arc states in Weyl semimetal TaAs.

Fermi arcs on Weyl semimetals exhibit many exotic quantum phenomena. Usually found on atomically flat surfaces with approximate translation symmetry, Fermi arcs are rooted in the peculiar topology of bulk Bloch bands of 3D crystals. The fundamental question of whether a 1D Fermi arc can be probed remains unanswered. Such an answer could significantly broaden potential applications of Weyl semimetals. Here, we report a direct observation of robust edge states on atomic-scale ledges in TaAs using low-temperature scanning tunneling microscopy/spectroscopy. Spectroscopic signatures and theoretical calculations reveal that the 1D Fermi arcs arise from the chiral Weyl points of bulk crystals. The crossover from 2D Fermi arcs to eventual complete localization on 1D edges was arrested experimentally on a sequence of surfaces. Our results demonstrate extreme robustness of the bulk-boundary correspondence, which offers topological protection for Fermi arcs, even in cases in which the boundaries are at the atomic-scale. The persistent 1D Fermi arcs can be profitably exploited in miniaturized quantum devices.

Magnetic Doping Induced Superconductivity-to-Incommensurate Density Waves Transition in a 2D Ultrathin Cr-Doped Mo2C Crystal.

In the vicinity of a competing electronic order, superconductivity emerges within a superconducting dome in the phase diagram, which has been demonstrated in unconventional superconductors and transition-metal dichalcogenides (TMDs), suggesting a scenario where fluctuations or a partial melting of a parent order are essential for inducing superconductivity. Here, we present a contrary example, the two-dimensional (2D) superconductivity in transition-metal carbide can be readily turned into charge density wave (CDW) phases via dilute magnetic doping. Low temperature scanning tunneling microscopy/spectroscopy (STM/STS), transport measurements, and density functional theory (DFT) calculations were employed to investigate Cr-doped superconducting Mo2C crystals in the 2D limit. With ultralow Cr doping (2.7 atom %), the superconductivity of Mo2C is heavily suppressed. Strikingly, an incommensurate density wave (IDW) and a related partially opened gap are observed at a temperature above the superconducting regime. The wave vector of IDW agrees well with the calculated Fermi surface nesting vectors. By further increasing the Cr doping level to 9.4 atom %, a stronger IDW with a smaller periodicity and a larger partial gap appear concurrently. The resistance anomaly implies the onset of the CDW phase. Spatial-resolved and temperature-dependent spectroscopy reveals that such CDW phases exist only in a nonsuperconducting regime and could form long-range orders uniformly. The results provide the understanding for the interplay between charge ordered states and superconductivity in 2D transition-metal carbide.

In-gap states induced by distortion in α-bismuthene

Ultrathin Bi(110) films have attracted significant attention recently as potential two-dimensional topological insulators. Their topological band structure induces one-dimensional (1D) helical edge states inside the bulk band gap; however, modification of the density of states resulting from Bi atomic distortion has also been suggested. Here we report the distortion-modified electronic states of α-bismuthene films grown on graphene/6H-SiC(0001) by using low-temperature scanning tunneling microscopy/spectroscopy. We found that the edges along the zigzag boundary between α-bismuthene and graphene are reconstructed and distorted and that the edge states are topographically dependent. The possible trivial origin of edge states induced by distortion complicates the investigation of topological nature in α-bismuthene. Steps from the substrate, stripes, and domain boundaries also induce additional site-dependent in-gap states. Additionally, we located an inversion domain boundary across which atoms are configured in the opposite order. This defect has extended spatial distribution and 1D parabolic dispersion, and hence we suggest it to be a very good platform for further topological exploration in α-bismuthene.

Promoting a Weak Coupling of Monolayer MoSe2 Grown on (100)-Faceted Au Foil.

As a two-dimensional semiconductor with many physical properties, including, notably, layer-controlled electronic bandgap and coupled spin-valley degree of freedom, monolayer MoSe2 is a strong candidate material for next-generation opto- and valley-electronic devices. However, due to substrate effects such as lattice mismatch and dielectric screening, preserving the monolayer's intrinsic properties remains challenging. This issue is generally significant for metallic substrates whose active surfaces are commonly utilized to achieve direct chemical or physical vapor growth of the monolayer films. Here, we demonstrate high-temperature-annealed Au foil with well-defined (100) facets as a weakly interacting substrate for atmospheric pressure chemical vapor deposition of highly crystalline monolayer MoSe2. Low-temperature scanning tunneling microscopy/spectroscopy measurements reveal a honeycomb structure of MoSe2 with a quasi-particle bandgap of 1.96 eV, a value comparable with other weakly interacting systems such as MoSe2/graphite. Density functional theory calculations indicate that the Au(100) surface exhibits the preferred energetics to electronically decouple from MoSe2, compared with the (110) and (111) crystal planes. This weak coupling is critical for the easy transfer of monolayers to another host substrate. Our study demonstrates a practical means to produce high-quality monolayers of transition-metal dichalcogenides, viable for both fundamental and application studies.

Characterization of structure and superconducting properties of low-temperature phase of Pb-Bi alloy films

Lead-bismuth (Pb-Bi) alloys, as a superconducting material, have been widely studied at their superconducting transition temperatures and the critical magnetic fields for different composition ratios. Most of experimental studies focused on the stable ε phase formed at high temperatures, but less on the Pb-Bi alloys grown at low temperatures. So far, the structural and superconducting properties of the low-temperature Pb-Bi phases are far from good understanding. Here, we report our investigation of structural and superconducting properties of a low-temperature phase of Pb-Bi alloy. The Pb-Bi alloy films with a nominal thickness of about 6 nm are prepared by co-depositing Bi and Pb on Bi(111)/Si(111)-(7 × 7) substrates at a low temperature of 100 K followed by annealing at a treatment of 200 K for 2 h. The structural and superconducting properties of the Pb-Bi alloy films are characterized in situ by using low-temperature scanning tunneling microscopy/spectroscopy (STM/STS). It is observed that the spatially separated phases of nearly pure Bi(111) domains and Pb<sub>1–<i>x</i></sub>Bi<i><sub>x</sub></i> alloy domains are formed in the films, where these phases can be identified by their distinct differences in the atomic structure and the distributions of step heights in the atomically resolved STM images, as well as by their distinguished STS spectra. The Pb<sub>1–<i>x</i></sub>Bi<i><sub>x</sub></i> alloy phase presents the structure of Pb(111), in which about <i>x</i> ≈ 0.1 Bi is substituted for Pb. The STS spectra show that the Pb<sub>1–<i>x</i></sub>Bi<i><sub>x</sub></i> alloy phase is superconducting, with a transition temperature <i>T</i><sub>c</sub> = 7.77 K derived from the variable-temperature measurements. This transition temperature is higher than that in pure Pb film (6.0–6.5 K), which can be well explained by the Mattias rules, with considering the fact that the average number of valance electrons increases after Bi atoms with five valance-electrons have been substituted for Pb atoms with four valance-electrons. The analysis shows that the ratio <inline-formula><tex-math id="M1">\begin{document}$ 2\Delta (0)/{k_{\rm{B}}}{T_{\rm{C}}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20210482_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20210482_M1.png"/></alternatives></inline-formula> is about 4.94 with the superconducting gap <inline-formula><tex-math id="M2">\begin{document}$ \varDelta (0) = 1.66$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20210482_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20210482_M2.png"/></alternatives></inline-formula> meV at 0 K, indicating that the Pb<sub>1–<i>x</i></sub>Bi<i><sub>x</sub></i> alloy is a strongly-coupled superconductor. The non-superconducting Bi(111) and the superconducting Pb<sub>1–<i>x</i></sub>Bi<i><sub>x</sub></i> alloy domains form an in-plane superconductor-normal metal-superconductor (S-N-S) Josephson junction. The proximity effect in the Bi(111) domains is measured at different N-S junctions, which suggests that the lateral superconducting penetration length in Bi(111) might be affected by the area of the quasi-two-dimensional interface. The superconducting gap in the Bi(111) region with a narrow width of 23 nm in an S-N-S Josephson junction is found to be greatly enhanced due to the existence of multiple Andreev reflections. Since Bi can host potential topological properties, the lateral Bi(111)-Pb<sub>1–<i>x</i></sub>Bi<i><sub>x</sub></i> heterostructures, because of the existing proximity effect, could have potential applications in exploring the novel topological and superconducting phenomena.

Nano-scale periodic structures and gap distribution in Pb-doped Bi2223 cuprate superconductors observed by STM/STS

The first systematic study of the nano-scale periodic local density of states (LDOS) and the inhomogeneous gap distributions for the Pb doped Bi2Sr2Ca2Cu3O10 + y (PbBi2223) microcrystals was carried out using the low-temperature scanning tunneling microscopy/spectroscopy. The spatially inhomogeneous gap distributions with patchy structures of several nm size reveal the average gap magnitude Δave ≃ 46 meV with the standard deviation σ = 8 meV. Those features are similar to properties of the widely investigated two-layer Bi-based cuprates. The energy dispersive types of the LDOS modulations are identified as being due to the quasiparticle interference. The relevant scattering vectors seem to testify that the PbBi2223 superconductors are ones, which was shown earlier to be most probable for Bi2212. Three types of energy non-dispersive modulations manifest themselves. Among them, the modulation in the diagonal direction with the period of ∼2.1a0 [ (2π/a0)] seems to be observed for the first time.

Charge Density Modulation and the Luttinger Liquid State in MoSe2 Mirror Twin Boundaries.

A mirror twin-domain boundary (MTB) in monolayer MoSe2 represents a (quasi) one-dimensional metallic system. Its electronic properties, particularly the low-energy excitations in the so-called 4|4P-type MTB, have drawn considerable research attention. Reports of quantum well states, charge density waves, and the Tomonaga-Luttinger liquid (TLL) have all been made. Here, by controlling the lengths of the MTBs and employing different substrates, we reveal by low-temperature scanning tunneling microscopy/spectroscopy, Friedel oscillations and quantum confinement effects causing the charge density modulations along the defect. The results are inconsistent with charge density waves. Interestingly, for graphene-supported samples, TLL in the MTBs is suggested, whereas that grown on gold, an ordinary Fermi liquid, is indicated.

Large-Cavity Coronoids with Different Inner and Outer Edge Structures.

Coronoids, polycyclic aromatic hydrocarbons with geometrically defined cavities, are promising model structures of porous graphene. Here, we report the on-surface synthesis of C168 and C140 coronoids, referred to as [6]- and [5]coronoid, respectively, using 5,9-dibromo-14-phenylbenzo[m]tetraphene as the precursor. These coronoids entail large cavities (>1 nm) with inner zigzag edges, distinct from their outer armchair edges. While [6]coronoid is planar, [5]coronoid is not. Low-temperature scanning tunneling microscopy/spectroscopy and noncontact atomic force microscopy unveil structural and electronic properties in accordance with those obtained from density functional theory calculations. Detailed analysis of ring current effects identifies the rings with the highest aromaticity of these coronoids, whose pattern matches their Clar structure. The pores of the obtained coronoids offer intriguing possibilities of further functionalization toward advanced host–guest applications.

Investigating molecular orbitals with submolecular precision on pristine sites and single atomic vacancies of monolayer h-BN

Understanding the influence of adsorption sites to the electronic properties of adsorbed molecules on two-dimensional (2D) ultrathin insulator is of essential importance for future organic-inorganic hybrid nanodevices. Here, the adsorption and electronic states of manganese phthalocyanine (MnPc) on a single layer of hexagonal boron nitride (h-BN) have been comprehensively studied by low-temperature scanning tunneling microscopy/spectroscopy and tight binding calculations. The frontier orbitals of the MnPc can change drastically by reversible manipulation of individual MnPc molecules onto and away from the single atomic vacancies at the h-BN surface. Particularly, the change of the molecular electronic configuration can be controlled depending on whether the atomic vacancy is below the metal center or the ligand of the MnPc. These findings give new insight into defect-engineering of the organic-inorganic hybrid nanodevices down to submolecular level.

Atomic Structure and Electronic Properties of Intermetallic CaBi2 Thin Films.

Intermetallic bismuth-based compounds have attracted great interest as promising candidates for novel topological superconductivity. Among them, CaBi2 is a newly discovered member for which the atomic structure and electronic properties have never been systematically explored. Using low-temperature scanning tunneling microscopy/spectroscopy (STM/S), we systematically characterized the atomic structure and electronic properties of CaBi2(010) thin films grown by molecular beam epitaxy (MBE) and found that their growth follows a Stranski-Krastanov mode. A nonreconstructed IBi layer and a (1 × 2) reconstructed IICa layer were found to be the most common surfaces. Nonreconstructed IIIBi and VCa layers were further exposed with reduced bismuth growth flux. All of these constituent layers exhibit unique features in the STS spectra, indicating that unique electronic properties exist in each specific constituent layer. Our findings provide for deeper understanding of the physical properties of this compound and suggest further studies of the two-dimensional (2D) layered materials family.