Performed Scanning Tunneling Microscopy Research Articles

Coexistence of Superconductivity and Antiferromagnetism in Topological Magnet MnBi2Te4 Films.

The interface of two materials can harbor unexpected emergent phenomena. One example is interface-induced superconductivity. In this work, we employ molecular beam epitaxy to grow a series of heterostructures formed by stacking together two nonsuperconducting antiferromagnetic materials, an intrinsic antiferromagnetic topological insulator MnBi2Te4 and an antiferromagnetic iron chalcogenide FeTe. Our electrical transport measurements reveal interface-induced superconductivity in these heterostructures. By performing scanning tunneling microscopy and spectroscopy measurements, we observe a proximity-induced superconducting gap on the top surface of the MnBi2Te4 layer, confirming the coexistence of superconductivity and antiferromagnetism in the MnBi2Te4 layer. Our findings will advance the fundamental inquiries into the topological superconducting phase in hybrid devices and provide a promising platform for the exploration of chiral Majorana physics in MnBi2Te4-based heterostructures.

Enhanced π–π Stacking between Dipole-Bearing Single Molecules Revealed by Conductance Measurement

Dipoles are widely involved in π-π interactions and are central to many chemical and biological functions, but their influence on the strength of π-π interactions remains unclear. Here, we report a study of π-π interaction between azulene-based, polar single molecules and between naphthalene-based, nonpolar single molecules. By performing scanning tunneling microscopy break junction measurements of single-molecule conductance, we show that the π-stacked dimers formed by the azulene-based, polar aromatic structures feature higher electrical conductivity and mechanical stability than those formed by the naphthalene-based, nonpolar molecules. Mechanical control of π-π interactions in both rotational and translational motion reveals a sensitive dependence of the stacking strength on relative alignment between the dipoles. The antiparallel alignment of the dipoles was found to be the optimal stacking configuration that underpins the observed enhancement of π-π stacking between azulene-based single molecules. Density functional theory calculations further explained the observed enhancement of stacking strength and the corresponding charge transport efficiency. Our experimental and theoretical results show that the antiparallel alignment of the dipole moments significantly enhances the electronic coupling and mechanical stability of π-π stacking. In addition, in the formation of single-molecule junctions, the azulene group was experimentally and theoretically proved to form a Au-π contact with electrodes with high charge transport efficiency. This paper provides evidence and interpretation of the role of dipoles in π-π interactions at the single-molecule level and offers new insights into potential applications in supramolecular devices.

Surface and electronic structure at atomic length scales of the nonsymmorphic antiferromagnet Eu5In2Sb6

We performed scanning tunneling microscopy and spectroscopy (STM/STS) measurements to investigate the Zintl phase ${\mathrm{Eu}}_{5}{\mathrm{In}}_{2}{\mathrm{Sb}}_{6}$, a nonsymmorphic antiferromagnet. The theoretical prediction of a nontrivial Fermi surface topology stabilized by the nonsymmorphic symmetry motivated our research. On the cleaved (010) plane, we obtained striped patterns that can be correlated to the stacking of the [${\mathrm{In}}_{2}{\mathrm{Sb}}_{6}{]}^{10\ensuremath{-}}$ double chains along the crystallographic $c$ axis. The attempted cleavage along the $a$ axis revealed a more complex pattern. We combined the STS measurement on nonreconstructed (010) and (081) surfaces with DFT calculations to further elucidate the electronic structure of ${\mathrm{Eu}}_{5}{\mathrm{In}}_{2}{\mathrm{Sb}}_{6}$. From our investigations so far, direct experimental evidence of the predicted topological surface states remains elusive.

Probing tip-induced attractive deformation of graphite surfaces through wave function dissipation in field emission resonance

We studied wave function dissipation (WFD) in field emission resonance (FER) by performing scanning tunneling microscopy on the highly oriented pyrolytic graphite (HOPG) and Ag(111) surfaces under two conditions: (1) the same current and FER number; (2) the same tip structure but different currents. Under the first condition, we observed that the decay rate corresponding to the WFD exhibited a larger variation on the HOPG surface than it did on the Ag(111) surface. Under the second condition, the decay rate was nearly independent of the FER electric field for the Ag(111) surface; by contrast, it was linearly proportional to the FER electric field for the HOPG surface. These remarkable differences can be attributed to the factors that the tip-induced attractive deformation caused by the electrostatic force was considerably more prominent on the HOPG surface than on the Ag(111) surface and that the deformed HOPG top layer had a unique electronic structure similar to that of single-layer graphene.

Observation and Manipulation of a Phase Separated State in a Charge Density Wave Material.

The 1T polytype of TaS2 has been studied extensively as a strongly correlated system. As 1T-TaS2 is thinned toward the 2D limit, its phase diagram shows significant deviations from that of the bulk material. Optoelectronic maps of ultrathin 1T-TaS2 have indicated the presence of nonequilibrium charge density wave phases within the hysteresis region of the nearly commensurate (NC) to commensurate (C) transition. We perform scanning tunneling microscopy on exfoliated ultrathin flakes of 1T-TaS2 within the NC-C hysteresis window, finding evidence that the observed nonequilibrium phases consist of intertwined, irregularly shaped NC-like and C-like domains. After applying lateral electrical signals to the sample, we image changes in the geometric arrangement of the different regions. We use a phase separation model to explore the relationship between electronic inhomogeneity present in ultrathin 1T-TaS2 and its bulk resistivity. These results demonstrate the role of phase competition morphologies in determining the properties of 2D materials.

Controlling of the Dirac band states of Pb-deposited graphene by using work function difference

We have performed scanning tunneling microscopy (STM) and angle-resolved photoemission spectroscopy (ARPES) in Pb-deposited bilayer graphene (BLG) on the SiC(0001) substrate to investigate the dependence of the electronic structures on the Pb-deposition amount. We have observed that the Pb atoms form islands by STM and the π bands of the BLG shift toward the Fermi level by ARPES. This hole-doping-like energy shift is enhanced as the amount of Pb is increased, and we were able to tune the Dirac gap to the Fermi level by 4 ML deposition. Considering the band dispersion, we suggest that the hole-doping-like effect is related to the difference between the work functions of Pb islands and BLG/SiC; the work function of BLG/SiC is lower than that of Pb. Our results propose an easy way of band tuning for graphene with an appropriate selection of both the substrate and deposited material.

Visualization of localized perturbations on a (001) surface of the ferromagnetic semimetal EuB6

We performed scanning tunneling microscopy (STM) and spectroscopy on a (001) surface of the ferromagnetic semimetal ${\mathrm{EuB}}_{6}$. Large-amplitude oscillations emanating from the elastic scattering of electrons by the surface impurities are observed in topography and in differential conductance maps. Fourier transform of the conductance maps embracing these regions indicate a holelike dispersion centered around the $\overline{\mathrm{\ensuremath{\Gamma}}}$ point of the two-dimensional Brillouin zone. Using density functional theory slab calculations, we identify a spin-split surface state, which stems from the dangling ${p}_{z}$ orbitals of the apical boron atom. Hybridization with bulk electronic states leads to a resonance enhancement in certain regions around the $\overline{\mathrm{\ensuremath{\Gamma}}}$ point, contributing to the remarkably strong real-space response around static point defects, which are observed in STM measurements.

Surface Reconstruction of Germanium: Hydrogen Intercalation and Graphene Protection

Understanding the interfacial properties of functional nanomaterials on semiconductor surfaces is crucial for developing electronics, optoelectronics, and other devices. By using graphene on the germanium (110) surface as a model, we performed scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) to examine the surface reconstructions of Ge (110) under graphene. Two reconstructions, the (1 × 1) phase (R2) which was previously proposed to survive only at high temperature and a [225−1] superstructure (R1 phase), were determined based on atomically resolved STM images. The R2 phase will transform to R1 after annealing in UHV above 300 °C, while the R1 phase can reversibly change to the (1 × 1) phase after heating in hydrogen at 700 °C. Finally, we confirmed the presence of interfacial hydrogen that stabilized the (1 × 1) phase at the initial stage of graphene growth based on control experiments. The zigzag edge of graphene is perpendicular to the close-packed [1–10] direction of Ge(1...

Quasiparticle interference and charge order in a heavily overdoped non-superconducting cuprate

One of the key issues in unraveling the mystery of high TC superconductivity in the cuprates is to understand the normal state outside the superconducting dome. Here we perform scanning tunneling microscopy and spectroscopy measurements on a heavily overdoped, non-superconducting (Bi, Pb)2Sr2CuO6+δ cuprate. Spectroscopic imaging reveals dispersive quasiparticle interferences (QPIs) and the Fourier transforms uncover the evolution of momentum space topology. More interestingly, we observe nanoscale patches of static charge order with periodicity. Both the dispersive QPI and static charge order can be qualitatively explained by theoretical calculations, which reveal the unique electronic structure of strongly overdoped cuprate.

Identification of Lattice Oxygen in Few-Layer Black Phosphorous Exfoliated in Ultrahigh Vacuum and Largely Improved Ambipolar Field-Effect Mobilities by Hydrogenation and Phosphorization.

Black phosphorus (BP) has recently attracted considerable attention due to its unique structure and fascinating optical and electronic properties as well as possible applications in photothermal agents. However, its main drawback is rapid degradation in ambient environments of H2O and O2, which has led to much research on the improvement of its stability. Unfortunately, this research has not shown great improvement in carrier mobilities. Here, we perform scanning tunneling microscopy observations of few-layer BP (FLBP) sheets exfoliated in ultrahigh vacuum and reveal, for the first time, the existence of lattice oxygen introduced during crystal growth. As a proof-of-concept application, hydrogenation is conducted to remove the lattice oxygen atoms followed by phosphorization, which repairs the phosphorous vacancies caused by mechanical exfoliation and hydrogenation. The resulting FLBP sheets show high ambipolar field-effect mobilities of 1374 cm2 V-1 s-1 for holes and 607 cm2 V-1 s-1 for electrons at 2 K. After storage in air for 3 days, the hole and electron mobilities only decrease to 1181 and 518 cm2 V-1 s-1, respectively, and no structural degradation is observed. This work suggests an effective means to improve both the mobility and stability of BP sheets rendering practical application of FLBP sheets possible.

Cooperative Electron-Phonon Coupling and Buckled Structure in Germanene on Au(111).

Germanene, a single-atom-thick germanium nanosheet in a honeycomb lattice, was proposed to be a Dirac fermion material beyond graphene. We performed scanning tunneling microscopy and in situ Raman spectroscopy studies combined with first-principles calculations on the atomic structures and the electronic and phonon properties of germanene on Au(111). The low-buckled 1 × 1 germanene honeycomb lattice was determined to exist in an unexpected rectangular √7 × √7 superstructure. Through in situ Raman measurements, distinctive vibrational phonon modes were discovered in √7 × √7 germanene, revealing the special coupling between the Dirac fermion and lattice vibrations, namely, electron-phonon coupling (EPC). The significant enhancement of EPC is correlated with the tensile strain, which is evoked by the singular buckled structure of √7 × √7 germanene on the Au(111) substrate. Our results present clear evidence for the existence of epitaxial germanene and elucidate the exotic properties of germanene on Au(111).

Electronic structure of 2H-NbSe2 single-layers in the CDW state

A density functional theory study of NbSe2 single-layers in the normal non-modulated and the 3 × 3 CDW states is reported. We show that, in the single layer, the CDW barely affects the Fermi surface of the system, thus ruling out a nesting mechanism as the driving force for the modulation. The CDW stabilizes levels lying around 1.35 eV below the Fermi level within the Se-based valence band but having a substantial Nb–Nb bonding character. The absence of interlayer interactions leads to the suppression of the pancake-like portion of the bulk Fermi surface in the single-layer. We perform scanning tunneling microscopy simulations and find that the images noticeably change with the sign and magnitude of the voltage bias. The atomic corrugation of the Se sublayer induced by the modulation plays a primary role in leading to these images, but the electronic reorganization also has an important contribution. The analysis of the variation of these images with the bias voltage does not support a Fermi surface nesting mechanism for the CDW. It is also shown that underlying graphene layers (present in some of the recent experimental work) do not modify the conduction band, but do affect the shape of the valence band of NbSe2 single-layers. The relevance of these results in understanding recent physical measurements for NbSe2 single-layers is discussed.

Charge Puddles in Graphene near the Dirac Point.

The charge carrier density in graphene on a dielectric substrate such as SiO_{2} displays inhomogeneities, the so-called charge puddles. Because of the linear dispersion relation in monolayer graphene, the puddles are predicted to grow near charge neutrality, a markedly distinct property from conventional two-dimensional electron gases. By performing scanning tunneling microscopy and spectroscopy on a mesoscopic graphene device, we directly observe the puddles' growth, both in spatial extent and in amplitude, as the Fermi level approaches the Dirac point. Self-consistent screening theory provides a unified description of both the macroscopic transport properties and the microscopically observed charge disorder.

The Impurity Effects on the Superconducting Gap in the High- T c Superconductor Bi2Sr2CaCu2−x Fe x O 8 + δ Investigated by STM/STS

We performed scanning tunneling microscopy/ spectroscopy (STM/STS) experiments to investigate Fe substitution effects on the superconductivity (SC) of overdoped Bi2212. The gap-edge peaks in STS spectrum at T≪T c become inhomogeneous strongly in Fe-doped Bi2212. The spectral weight around zero bias, namely the density of states (DOS) at ∼E F is partly recovered by substitution of Fe in every position. The recovery of the DOS at ∼E F can be explained in terms of impurity effects on d-wave SC in the unitarity limit. However, the strong inhomogeneity of gap-edge peaks would be ascribed to different effects due to Fe-substitution.

Direct observation of single-electron solitons and Friedel oscillations in a quasi-one dimensional material with incommensurate charge-density waves

We have performed scanning tunneling microscopy experiments in the quasi-one dimensional charge density wave (CDW) system NbSe3, where we could image and study in detail individual solitons corresponding to the self-trapping of a single electron. Our analysis shows that the type of soliton we observed is an “Amplitude Soliton”, characterized by a vanishing CDW amplitude at the soliton center and by a π-shift of its phase along the chain direction. Pairs of solitons or multiple solitons were also observed. Such observations could be made only in the high-temperature CDW phase. Additionally, one-dimensional Friedel oscillations around charged defects could also be imaged and are found to be superimposed to the CDW. The distinction between amplitude solitons and Friedel oscillations can be made without any ambiguity.

Charge disproportionation in α-(BEDT-TTF)2x

We performed scanning tunneling microscopy (STM) at room temperature on a−b surface of α-(BETD-TTF)2I3 which undergoes a charge ordering transition at TCO=135K. BEDT-TTF molecular arrangement with α-type was clearly observed. Charge modulation with two-fold molecular periodicity was observed along both columns I and II. The auto correlation of the charge modulation along column II has extended in the area observed by STM. On the other hand, that along the column I is much weaker than column II although the charge modulation locally develop. The charge modulations along columns I and II form horizontal stripe structure with some inhomogeneity, whose structure is similar to the charge order below TCO135K. Our result suggests that horizontal charges stripe at room temperature is a precursor of the charge ordered state.

Microscopic evidence for strong periodic lattice distortion in two-dimensional charge-density wave systems

In the quasi-2D electron systems of the layered transition metal dichalcogenides (TMD) there is still a controversy about the nature of the transitions to charge-density wave (CDW) phases, i.e. whether they are described by a Peierls-type mechanism or by a lattice-driven model. By performing scanning tunneling microscopy (STM) experiments on the canonical TMD-CDW systems, we have imaged the electronic modulation and the lattice distortion separately in 2H-TaS$_2$, TaSe$_2$, and NbSe$_2$. Across the three materials, we found dominant lattice contributions instead of the electronic modulation expected from Peierls transitions, in contrast to the CDW states that show the hallmark of contrast inversion between filled and empty states. Our results imply that the periodic lattice distortion (PLD) plays a vital role in the formation of CDW phases in the TMDs and illustrate the importance of taking into account the more complicated lattice degree of freedom when studying correlated electron systems.

Modulation of Fermi velocities of Dirac electrons in single layer graphene by moiré superlattice

Study of electronic properties of graphene on an anisotropic crystal substrate including boron nitride (BN) has raised significant interest recently due to the application of graphene based vertical hetero-devices. We have performed scanning tunneling microscopy (STM) and scanning tunneling spectroscopy studies of single-layer graphene on hexagonal BN (h-BN) substrates with an applied perpendicular magnetic field at low temperature. Different periodic moiré superlattices can be resolved with STM, and their quantized Landau levels in high magnetic field are investigated. The renormalized Fermi velocities of massless Dirac fermions in graphene are revealed to show dependent on the moiré superlattice. Density functional theory calculation verifies that the interlayer interaction between graphene and h-BN is stronger with smaller twisting angle between lattices of graphene and h-BN, thus, leading to a reduction in the velocity of charge carriers. Our results should provide valuable insight of electronic properties and device performance of graphene on crystal substrates.

Superlattice Dirac points and space-dependent Fermi velocity in a corrugated graphene monolayer

Recent studies show that periodic potentials can generate superlattice Dirac points at energies in graphene (is the Fermi velocity of graphene and G is the reciprocal superlattice vector). Here, we perform scanning tunneling microscopy and spectroscopy studies of a corrugated graphene monolayer on Rh foil. We show that the quasi-periodic ripples of nanometer wavelength in the corrugated graphene give rise to weak one-dimensional (1D) electronic potentials and thereby lead to the emergence of the superlattice Dirac points. The position of the superlattice Dirac point is space-dependent and shows a wide distribution of values. We demonstrated that the space-dependent superlattice Dirac points is closely related to the space-dependent Fermi velocity, which may arise from the effect of the local strain and the strong electron-electron interaction in the corrugated graphene.

Artificially lattice-mismatched graphene/metal interface: Graphene/Ni/Ir(111)

We report the structural and electronic properties of an artificial graphene/Ni(111) system obtained by the intercalation of a monoatomic layer of Ni in graphene/Ir(111). Upon intercalation, Ni grows epitaxially on Ir(111), resulting in a lattice mismatched graphene/Ni system. By performing Scanning Tunneling Microscopy (STM) measurements and Density Functional Theory (DFT) calculations, we show that the intercalated Ni layer leads to a pronounced buckling of the graphene film. At the same time an enhanced interaction is measured by Angle-Resolved Photo-Emission Spectroscopy (ARPES), showing a clear transition from a nearly-undisturbed to a strongly-hybridized graphene $\pi$-band. A comparison of the intercalation-like graphene system with flat graphene on bulk Ni(111), and mildly corrugated graphene on Ir(111), allows to disentangle the two key properties which lead to the observed increased interaction, namely lattice matching and electronic interaction. Although the latter determines the strength of the hybridization, we find an important influence of the local carbon configuration resulting from the lattice mismatch.