Local Tunneling Research Articles

Two-Dimensional Molecular Charge Density Waves in Single-Layer-Thick Islands of a Dirac Fermion System.

Charge density waves have been intensely studied in inorganic materials such as transition metal dichalcogenides; however their counterpart in organic materials has yet to be explored in detail. Here we report the finding of robust two-dimensional charge density waves in molecular layers formed by α-(BEDT-TTF)2-I3 on a Ag(111) surface. Low-temperature scanning tunneling microscopy images of a multilayer thick α-(BEDT-TTF)2-I3 on a Ag(111) substrate reveal the coexistence of 5a0 × 5a0 and R9° charge density wave patterns commensurate with the underlying molecular lattice at 80 K. Both charge density wave patterns remain in nanosize molecular islands with just a single constituent molecular-layer thickness at 80 and 5 K. Local tunneling spectroscopy measurements reveal the variation of the gap from 244 to 288 meV between the maximum and minimum charge density wave locations. Density functional theory calculations further confirm a vertical positioning of BEDT-TTF molecules in the molecular layer. While the observed charge density wave patterns are stable for the defect sites, they can be reversibly switched for one molecular lattice site by means of inelastic tunneling electron energy transfer with the electron energies exceeding 400 meV using a scanning tunneling microscope manipulation scheme.

Contact resistance and current crowding in tunneling type circular nano-contacts

Current transport and contact resistance in nanoscale electrical contacts are important to the overall device properties, especially for devices based on novel one-dimensional and two-dimensional materials or nanostructures. In this paper, we present a self-consistent method to model tunneling type circular thin film contacts. We solve the lumped circuit circular transmission line model (CTLM) with tunneling-induced specific contact resistivity which varies along the radial direction. The contacting members are separated by a thin insulating layer, where the radially dependent is calculated from local voltage dependent tunneling current density. The current and voltage distributions in such contacts and their overall contact resistance are studied in detail, for various input voltages, contact dimensions, and material properties (i.e. work function, sheet resistance of the contact members, and permittivity of the insulating layer). Our study shows that the contact resistance is voltage dependent, and the radial current distribution is strongly nonhomogeneous. The contact resistance and the current distribution can be controlled by engineering the contact layer properties and geometry radially. Although focused on Schrödinger tunneling type contacts in this work, our modified CTLM equations with radially varying are general, and may be readily used for other types of electrical contacts, such as ohmic and Schottky contacts.

Depletion of Two-Level Systems in Ultrastable Computer-Generated Glasses.

Amorphous solids exhibit quasiuniversal low temperature anomalies whose origin has been ascribed to localized tunneling defects. Using an advanced MonteCarlo procedure, we create insilico glasses spanning from hyperquenched to ultrastable glasses. Using a multidimensional path-finding protocol, we locate tunneling defects with energy splittings smaller than k_{B}T_{Q}, with T_{Q} the temperature below which quantum effects are relevant (T_{Q}≈1 K in most experiments). We find that as the stability of a glass increases, its energy landscape as well as the manner in which it is probed tend to deplete the density of tunneling defects, as observed in recent experiments. We explore the real-space nature of tunneling defects, and find that they are mostly localized to a few atoms, but are occasionally dramatically delocalized.

Scanning tunneling microscopy of the tetraphenylporphyrin thin films on a graphite single-crystal substrate

Features of the surface topography of a tetraphenylporphyrin film on a graphite single crystal (HOPG) substrate and the local current-voltage characteristics were studied using a scanning tunneling microscope under ambient conditions. STM images of surface were acquired on a scale from a few nanometers to 4 micrometers. Local tunnelling current-voltage characteristics in different ranges of bias voltage and in different directions of voltage scanning can have hysteresis due to the hopping electron conductivity in porphyrin.

Seismic vulnerability of circular tunnels in soft soil deposits: The case of Shanghai metropolitan system

This paper aims at the vulnerability assessment of circular tunnels in soft soil deposits of the metropolitan subway system of Shanghai, accounting for the effects of soil-structure-interaction, local soil conditions and tunnel burial depth. The study focuses on the seismic vulnerability in the transversal direction. A suite of twelve earthquake records is selected and scaled from 0.1 g to 1.0 g to perform non-linear incremental dynamic analysis (IDA), and thus to assess the tunnel response under increasing levels of ground shaking intensity. A visco-elasto-plastic model with Mohr-Coulomb yield criterion is adopted to capture the non-linear behaviour of soil deposits under the seismic excitation. Fragility curves are constructed as a function of free-field peak ground acceleration (PGA) and peak ground velocity (PGV) considering the main sources of uncertainties, with the damage measure (DM) being defined in terms of acting over capacity bending moment ratio of the tunnel lining. The derived fragility curves are compared with existing empirical and analytical ones. Furthermore, vulnerability curves are generated from the derived fragility curves. The significant role of local soil conditions and tunnel burial depth in the vulnerability of tunnels in soft and deep alluvial deposits is highlighted. The proposed fragility and vulnerability functions may be particularly used for the risk analysis of circular tunnels in soft deposits (Vs30 < 200 m/s) exposed to seismically-induced transient ground deformations in the transverse direction.

Indirect Detection of the Light Emission in the Local Tunnel Junction

Herein, I(V) characteristics of the tunnel junction between the scanning tunneling microscopy (STM) Pt/Ir probe and atomically flat Au film on mica using ultrahigh vacuum STM is investigated. To ensure cleanness and flatness of the Au films, optimization of the substrate annealing and Ar plasma treatment are performed. The obtained technological parameters allow to drastically improve the reproducibility of I(V) measurements. The analysis of I(V), d2I/dV2 (V), and Fowler–Nordheim plots is conducted, and the presence of the features in the bias region near 1.8 V in the form of peak and minimum, peak and anomalous extra minimum, respectively is demonstrated. The direct optical measurements confirm that the features on I(V) curves are associated with the generation of photons from the STM probe‐sample gap, governed by inelastic tunneling processes. The proposed I(V) analysis approach is used for indirect sensing and investigation of the light emission in the tunnel junction offering a powerful tool for the studies of the photonic sources with deeply subwavelength dimensions.

Understanding Interlayer Contact Conductance in Twisted Bilayer Graphene.

Bilayer or few-layer 2D materials showing novel electrical properties in electronic device applications have aroused increasing interest in recent years. Obtaining a comprehensive understanding of interlayer contact conductance still remains a challenge, but is significant for improving the performance of bilayer or few-layer 2D electronic devices. Here, conductive atomic force microscope (C-AFM) experiments are reported to explore the interlayer contact conductance between bilayer graphene (BLG) with various twisted stacking structures fabricated by the chemical vapor deposition (CVD) method. The current maps show that the interlayer contact conductance between BLG strongly depends on the twist angle. The interlayer contact conductance of 0° AB-stacking bilayer graphene (AB-BLG) is ≈4 times as large as that of 30° twisted bilayer graphene (t-BLG), which indicates that the twist angle-dependent interlayer contact conductance originates from the coupling-decoupling transitions. Moreover, the moiré superlattice-level current images of t-BLG show modulations of local interlayer contact conductance. Density functional theory calculations together with a theoretical model reproduce the C-AFM current map and show that the modulation is mainly attributed to the overall contribution of local interfacial carrier density and tunneling barrier.

Behavior of superconductivity in a Pb/Ag heterostructure

The superconducting proximity effect is a long-standing topic of great importance in condensed matter physics. A crucial but unresolved issue is which interfacial and material details determine the efficiency of the proximity effect. In this paper, we study an epitaxially grown superconductor/normal metal (SC-NM) heterostructure (Pb/Ag) and find a spatially constant superconducting gap determined by local tunneling spectroscopy and magnetoresponse measurements, despite the highly mismatched Fermi surfaces between individual Pb and Ag epitaxial layers and the large differences in the lattice constants and electronic densities of states in the separate components. The uniform superconducting gap is in contrast to the spatially varying pair potential with a discontinuity at the interface theoretically predicted for an ideal SC-NM junction and experimentally observed previously in several lateral SC-NM junctions. We experimentally verify that the transmission of electrons across the interface in the vertical Pb/Ag heterostructure is high enough that a new band structure emerges even at the single-particle level. Our experimental results call for further theoretical work in order to develop predictive power for the proximity effect starting from realistic, materially relevant microscopic models.

Tunnel Structural Heterogeneity in MnO2 Polymorphs: Origins and Effect on Ion Transport

The establishment of an accurate synthesis-structure-property relationship is the central scheme of materials science, which is particularly important for polymorphic materials possessing various structural forms with different properties. Manganese dioxide (MnO2) is a typical polymorphic material exhibiting various one-dimensional tunnel phases that are constructed by [MnO6] octahedra units, enabling its extensive applications in water desalination, (electro)catalysis and energy storage. Despite the long-range [MnO6] ordering confirmed by conventional diffraction tools, surprisingly, the electrochemical energy storage properties of a specific MnO2 tunnel phase still vary significantly in literature with unclear structural origins. Here, we demonstrate the existence of tunnel heterogeneity featuring localized tunnel intergrowths within single MnO2 nanoparticles via atomically-resolved imaging. Furthermore, combining both ex situ and in situ transmission electron microscopy, the tunnel heterogeneity within one MnO2 nanoparticle is demonstrated to significantly affect the energy storage kinetics in sodium ion battery even down to sub-nanometer scale. The origins of such tunnel structural heterogeneity in MnO2 are explored further, where a layer-to-tunnel (L-T) transition mechanism is identified to be responsible. The L-T transition is the essential step for the MnO2 layered precursors to gradually transform to tunnel polymorphs during the hydrothermal synthesis. The intermediate state during the L-T transition is successfully obtained and analyzed to extract the critical information regarding the atomic reconfiguration, compositional evolution and electronic structure change during this transition. It is found that the L-T transition is not homogeneous but rather topotactically happening from the layer edges into the body, which leads to the gradual splitting of 2D MnO2 layers into 1D nanowires possessing tunnel structure. The layers are transformed to 3×3 tunnels in multisteps via formation of intermediate tunnel phases exhibiting much larger openings. The transition starts by macroscopic layer distortion from adjacent Jahn-Teller active [Mn3+O6] octahedra, experiences Mn3+ disproportionation reaction and layer-interlayer Mn migration, expels doped Mg2+ out of the openings, and gradually builds the tunnels by reconstructing the Mn-O bonds. Findings in this work could guide the controlled synthesis and selective size-engineering of MnO2 tunnels for specific applications in various fields. We also expect it to call for renewed attention to the controlled synthesis of homogeneous tunnel-specific phases with predictable properties, and to yield a more precise structure-property relationship in polymorphic materials. Figure 1

Spatial Inhomogeneities in the Superconducting Gap of SrFe1.6Co0.4As2 Single Crystals

We report spatial inhomogeneity of the superconducting gap of optimally doped SrFe1.6Co0.4As2 single crystals using scanning tunneling microscopy and spectroscopy studies from 20 to 5 K. This composition becomes superconductor at TC$\sim $ 18 K and 60 % volume of the bulk becomes superconductor at 5 K. Local tunnel spectra at 5 K show coherence peaks with a depression near the Fermi energy. These spectra are asymmetric and amplitude of the coherence peaks vary with position. Gaps magnitude (Δ) is also not constant over the surface; we found a distribution of gap between 11.5 and 17.5 meV energy range along a line with an average value of $\bar {\Delta } =$ 13.7 meV and standard deviation σ = 1.4 meV. This leads to a fractional variation $\sigma /\bar {\Delta }~=~$ 10 %. The calculated reduced gap 2$\bar {\Delta }$/kBTC at 5 K shows a very high value of 17.68 as compared to the s/d-wave superconductor indicating a strong pairing strength for the SC order parameter. Above TC, spectra does not show any gap and the observed inhomogeneity is also less. By comparing our data with underdoped composition, we claim that spatial inhomogeneity in the SC gap is an intrinsic property of the Fe-As-based superconductors and phase boundary plays much important role for such inhomogeneity rather than the disorder or dopant.

Vortex-core properties and vortex-lattice transformation in FeSe

Low-temperature scanning tunneling microscopy and spectroscopy has been used to image the vortex core and the vortex lattice in FeSe single crystals. The local tunneling spectra acquired at the center of elliptical vortex cores display a strong particle-hole asymmetry with spatial oscillation, characteristic of the quantum-limit vortex core. Furthermore, a quasihexagonal vortex lattice at low magnetic field undergoes noticeable rhombic distortions above a certain field $\ensuremath{\sim}1.5$ T. This field ${H}^{*}$ also reveals itself as a kink in the magnetic field dependence of the specific heat. The observation of a nearly hexagonal vortex lattice at low field is very surprising for materials with an orthorhombic crystal structure and it is in apparent contradiction with the elliptical shape of the vortex cores. These observations can be directly connected to the multiband nature of superconductivity in this material, provided we attribute them to the suppression of superconducting order parameter in one of the energy bands. Above the field ${H}^{*}$ the superconducting coherence length for this band can well exceed the intervortex distance which strengthens the nonlocal effects. Therefore, in addition to multiple-band effects, other possible sources that can contribute to the observed evolution of the vortex-lattice structure include nonlocal effects which cause the field-dependent interplay between the symmetry of the crystal and vortex lattice or the magnetoelastic interactions due to the strain field generated by vortices.

Effects of Blade Loading Distribution on the Cavitation and Excitation Forces of a Tunnel Thruster

Compared with open propellers, the impeller of a tunnel thruster is more vulnerable to cavitation and structural vibration problems because the impeller is typically subject to severe non-uniformity of inflow produced by the blunt gearbox. Model tests and numerical simulations are carried out in the cavitation tunnel of Shanghai Jiao Tong University for a tunnel thruster using a “flat plate” impeller and a tip-unloaded one. The characteristics of fluctuating pressures on the tunnel wall and the resultant excitation forces are investigated. It is found that although unloading the blade tips of an impeller is effective in reducing the fluctuating pressures in a local tunnel wall area near the tips, the same trend does not apply to the case of the excitation forces. The results show that care should be taken when the experimentally measured fluctuating pressures are utilized as the input to the analysis of structural vibrations.

Tunneling into a Finite Luttinger Liquid Coupled to Noisy Capacitive Leads.

Tunneling spectroscopy of one-dimensional interacting wires can be profoundly sensitive to the boundary conditions of the wire. Here, we analyze the tunneling spectroscopy of a wire coupled to capacitive metallic leads. Strikingly, with increasing many-body interactions in the wire, the impact of the boundary noise becomes more prominent. This interplay allows for a smooth crossover from standard 1D tunneling signatures into a regime where the tunneling is dominated by the fluctuations at the leads. This regime is characterized by an elevated zero-bias tunneling alongside a universal power-law decay at high energies. Furthermore, local tunneling measurements in this regime show a unique spatial dependence that marks the formation of plasmonic standing waves in the wire. Our result offers a tunable method by which to control the boundary effects and measure the interaction strength (Luttinger parameter) within the wire.

Dynamics of spin–orbit-coupled cold atomic gases in a Floquet lattice with an impurity

In this work, we have studied the quantum tunneling of a single spin–orbit-coupled atom held in a periodically modulated optical lattice with an impurity. As a result we find that the dynamical localization takes place globally at the collapse points of quasienergy spectrum, even when the impurity potential is far off-resonant with the driving field. Meanwhile, two types of local second-order tunneling processes appear beyond expectation between the two nearest-neighbor sites of the impurity, with the spin unchanged and with the impurity site population negligible. Though tunneling behaviors of the two types seem to be the same, they are believed to involve two distinct mechanisms: one is related to spin-independent process, while the other is to spin-dependent tunneling process. The two types of second-order processes can be identified by means of resonant tunneling with or without spin flipping by tuning the impurity potential to be in resonance with the driving field. In the Floquet picture, the system with the localized impurity develops a fine structure of avoided crossings of quasienergies near the collapse point, which is crucial to understand the so-called second-order tunneling dynamics. These results are confirmed analytically on the basis of effective three-site model and multiple-time-scale asymptotic perturbative method, and may be exploited for engineering the spin-dependent quantum transport in realistic experiments.

Investigation of the light emission in the local tunnel junction and its dependence on the contact surface morphology

In this paper we investigate light emission in a tunnel junction between a thin gold film on a glass substrate and a gold-coated tungsten tip of a scanning-tunneling microscope probe. The experiments show that the size of grains in the gold film surface dramatically affects the intensity of emissions. We demonstrate that a decrease in the grain aspect ratio provides an increase in the quantum yield of the tunnel-junction emission.

Threshold effects in one-dimensional strongly interacting nonequilibrium systems

In this work we investigate the phenomena associated with the new thresholds in the spectrum of excitations arising when different one-dimensional strongly interacting systems are voltage biased and weakly coupled by tunneling. We develop the perturbation theory with respect to tunneling and derive an asymptotic behavior of physical quantities close to threshold energies. We reproduce earlier results for the electron relaxation at the edge of an integer quantum Hall system and for the non-equilibrium Fermi edge singularity phenomenon. In contrast to the previous works, our analysis does not rely on the free-fermionic character of local tunneling, therefore we are able to extend our theory to wider class of systems, without well-defined electron excitations, such as spinless Luttinger liquids and chiral quantum Hall edge states at fractional filling factors.

Bio-Memristor Based on Peptide and Peptide Composite with Gold Nanoparticles

The structure, morphology and electrical properties of thin dipeptide hexamethylenediamide bis (N-monosuccinylglutamlysin) (DPT) layers and a DPT composite with gold nanoparticles deposited on gold and HOPG substrates were studied by probe microscopy and spectroscopy. The chemical formula of DPT is: {HOOC–(CH2)2–CO-L-Glu-L-Lys-NH–(CH2)3}2, and it is a mimetic of nerve growth factor. The results demonstrate that the structure and morphology of DPT thin layers depend significantly on the molecule charge (neutral or anion) and the nature of the substrate–layer interface. It was possible to control the structure and properties of the formed solid layers by changing pH of aqua solution (the charge of the DPT molecule). Bipolar resistive switching was observed in thin DPT layers on graphite and gold surfaces. The crystallization of anions on the surface of gold led to the formation of a ferroelectric unlike graphite. A strong dependence of the morphology of DPT composite layers on the nature of the substrate and the state of its surface is revealed. It indicates the important role of interfacial interactions in the crystallization processes of the DPT layers. The electrical properties of layers also depend on the interaction of DPT with the substrate. An increase in the thickness of the layers significantly affects the morphology and value of the tunneling current. Similar to crystallization of DPT salt on a gold surface, crystallization of DPT composite with gold nanoparticles also leads to the formation of a ferroelectric. The differences found in the structure of DPT composite layers on graphite and gold surfaces can be explained by assuming that the structure of the second and all subsequent layers is completely determined by the structure of the first adsorption layer in DPT-substrate interface. So this layer serves as a template for the growth of all other layers. The results can find practical application in 3D printing technologies. The presence of negative differential conductivity on local tunnel current–voltage characteristics of peptide composites is of great practical importance when used as active elements for amplifying current and power, memory cells in organic electronics. Investigated DPT has rather good memristive characteristics, including good endurance, satisfying ON/OFF current ratio, long retention time and reproducible write-once read-many times (WORM) memory behavior. All this allows us to consider the DPT to be a perspective material of memristor organic electronics. Since it is also a drug, the polymorphism and its dependence on pH can also find application in the pharmaceutical industry.

RECENT ADVANCES IN ULTRAFAST TIME-RESOLVED SCANNING TUNNELING MICROSCOPY

Making smaller and faster functional devices has led to an increasing demand for a microscopic technique that allows the investigation of carrier and phonon dynamics with both high spatial and temporal resolutions. Traditional optical pump–probe methods can achieve femtosecond temporal resolution but fall short in the spatial resolution due to the diffraction limit. Scanning tunneling microscopy (STM), on the contrary, has realized atomic-scale spatial resolution relying on the high sensitivity of the tunneling current to the tip-sample distance. However, limited by the electronics bandwidth, STM can only push the temporal resolution to the microseconds scale, restricting its applications to probe various ultrafast dynamic processes. The combination of these two methods takes advantages of optical pump–probe techniques and highly localized tunneling currents of STM, providing one viable solution to track atomic-scale ultrafast dynamics in single molecules and low-dimensional materials. In this review, we will focus on several ultrafast time-resolved STM methods by coupling the tunneling junctions with pulsed electric waves, THz, near-infrared and visible laser. Their applications to probe the carrier dynamics, spin dynamics, and molecular motion will be highlighted. In the end, we will present an outlook on the challenges and new opportunities in this field.

Screening cloud and non-Fermi-liquid scattering in topological Kondo devices

The topological Kondo effect arises when conduction electrons in metallic leads are coupled to a mesoscopic superconducting island with Majorana fermions. Working with its minimal setup, we study the lead electron local tunneling density of states in its thermally smeared form motivated by scanning tunneling microscopy, focusing on the component $\rho_{2 k_F}$ oscillating at twice the Fermi wavenumber. As a function of temperature $T$ and at zero bias, we find that the amplitude of $\rho_{2 k_F}$ is nonmonotonic, whereby with decreasing $T$ an exponential thermal-length-controlled increase, potentially through an intermediate Kondo logarithm, crosses over to a $T^{1/3}$ decay. The Kondo logarithm is present only for tip-junction distances sufficiently smaller than the Kondo length, thus providing information on the Kondo screening cloud. The low temperature decay indicates non-Fermi-liquid scattering, in particular the complete suppression of single-particle scattering at the topological Kondo fixed point. For temperatures much below the Kondo temperature, we find that the $\rho_{2 k_F}$ amplitude can be described as a universal scaling function indicative of strong correlations. In a more general context, our considerations point towards the utility of $\rho_{2 k_F}$ in studying quantum impurity systems, including extracting information about the single-particle scattering matrix.

Ordering Heterogeneity of [MnO6] Octahedra in Tunnel-Structured MnO2 and Its Influence on Ion Storage

Summary [MnO6] octahedra are the structural units for a large family of manganese dioxides (MnO2) possessing one-dimensional tunnel structures with extensive applications in catalysis and energy storage. Despite the long-range [MnO6] ordering confirmed by conventional diffraction tools, surprisingly, the functional properties of a specific MnO2 tunnel phase still vary significantly in literature with unclear structural origins. Here, we demonstrate the existence of tunnel heterogeneity featuring localized tunnel intergrowths within single MnO2 nanoparticles via atomically resolved imaging. The degree of tunnel heterogeneity increases with the size increase of tunnels from β-MnO2 (1 × 1 tunnel) to α-MnO2 (2 × 2 tunnel), and to todorokite MnO2 (3 × 3 tunnel). Furthermore, the tunnel heterogeneity within one MnO2 nanoparticle significantly affects the energy storage kinetics even down to sub-nanometer scale. These findings are expected to call for renewed attention to the controlled synthesis of homogeneous tunnel-specific phases with predictable properties and to yield a more precise structure-property relationship in polymorphic materials.