Atomic Junctions Research Articles

Formation of Single Cu Atomic Chain in Nitrogen Atmosphere

We study the conductance and geometry of the Cu atomic junction in the presence of N2, through combination of experimental measurement and theoretical calculation. A mechanically controllable break-junction measurement at low temperature reveals N2 molecules stabilize a Cu atomic junction, and reduce its conductance value. The length analysis about the Cu atomic junction indicates that it is elongated with the length of a few atoms, although it is not elongated without molecules. We investigate Cu atomic junction’s geometry by calculating the conductance and total energies of the several models by changing the separation between two Cu electrodes. Through combination of experimental and theoretical study, we show that the Cu linear atomic chain is formed with the support of N2 molecule, and N2 molecule attached on the Cu linear atomic chain. The formation of Cu linear atomic chain is explained that attached N2 molecules reduce the surface energy of the Cu atomic junction or N2 molecule directly supports the Cu–Cu bond.

Mapping the details of contact effect of modulated Au-octanedithiol-Au break junction by force-conductance cross-correlation.

We have measured the force and conductance of Au-octanedithiol-Au junctions using a modified conducting atomic force microscopy break junction technique with sawtooth modulations. Force-conductance two-dimensional cross-correlation histogram (FC-2DCCH) analysis for the single-molecule plateaus is demonstrated. Interestingly, four strong correlated regions appear in FC-2DCCHs consistently when modulations with different amplitudes are applied, in sharp contrast to the results under no modulation. These regions reflect the conductance and force changes during the transition of two molecule/electrode contact configurations. As the modulation amplitude increases, intermediate transition states of the contact configurations are discerned and further confirmed by comparing individual traces. This study unravels the relation between force and conductance hidden in the data of a modulated single-molecule break junction system and provides a fresh understanding of electron transport properties at molecule/electrode interfaces.

Transport characteristics of a singleC60-molecule junction revealed by multiple Andreev reflections

The I-V characteristics for atomic junction between two superconductors exhibit steps at voltages 2$\mathrm{\ensuremath{\Delta}}/n$ (with n=1, 2, 3, \dots{}) associated with multiple Andreev reflections (MAR). These features have been used in the past to determine the fundamental transport characteristics of atomic junctions. In this work, the authors present a development of the MAR technique by utilizing a superconducting Nb STM tip as one of the leads to study the transport properties of C${}_{60}$ fullerene molecule. By analyzing the tunneling spectra and high-resolution molecular imaging, they were able to determine the number of conduction channels for C${}_{6}0$ as well as the origin and the dependence of the transmission coefficients on the contact geometry.

High probability of single molecule junction formation with Ag electrodes

We use a scanning tunneling microscope-based break-junction technique to compare the probability of the formation of a single-molecule junction for a series of amine-terminated oligophenyl and alkane species using either Ag or Au as electrodes. For all molecules, we find that there is a significantly higher probability of junction formation when using Ag electrodes than with Au electrodes. We also find that longer molecules have a higher probability than shorter molecules to form a junction for both Ag and Au electrodes. For all molecules, the measured molecular junction length that is formed with the Ag electrodes was longer than that formed with Au electrodes. Furthermore, we can make a single atomic oxygen junction and can measure its conductance using Ag electrodes. These observations are attributed to a narrower gap of the Ag electrodes compared to that of the Au electrodes after the metal contact ruptures. Since there is a high probability of a molecular junction forming when using Ag electrodes, we can therefore perform a statistical analysis within the context of the material properties that are suitable for future molecular electronics.

Shot noise variation within ensembles of gold atomic break junctions at room temperature

Atomic-scale junctions are a powerful tool to study quantum transport, and are frequently examined through the mechanically controllable break junction technique. The junction-to-junction variation of atomic configurations often leads to a statistical approach, with ensemble-averaged properties providing access to the relevant physics. However, the full ensemble contains considerable additional information. We report a new analysis of shot noise over entire ensembles of junction configurations using scanning tunneling microscope-style gold break junctions at room temperature in ambient conditions, and compare these data with simulations based on molecular dynamics, a sophisticated tight-binding model, and nonequilibrium Green's functions. The experimental data show a suppression in the variation of the noise near conductances dominated by fully transmitting channels, and a surprising participation of multiple channels in the nominal tunneling regime. Comparison with the simulations, which agree well with published work at low temperatures and ultrahigh vacuum conditions, suggests that these effects likely result from surface contamination and disorder in the electrodes. We propose additional experiments that can distinguish the relative contributions of these factors.

Non-Equilibrium Transport Theory Applied to Nano Electronics Problems

Atomistic non-equilibrium transport theory has been developed a lot in a bit different community than the nano-electronics, i.e., the atomic and molecular junctions community.[1] The NEGF (non-equilibrium Green function) method has been combined with first principle electronic structure methods, which has become a standard tool in nano-electronics, too, nowadays. After making a brief overview of the latest progresses obtained in non-equilibrium transport theory including the heat problem and the noise problem, we will discuss some results obtained in our nano-electronics research. The subject of our discussions includes channel and electrode materials dependence of transport properties of some resistive RAM junctions. [2]Ref.)[1] Yoshihiro Asai, “Nonequilibrium phonon effects on transport properties through atomic and molecular bridge junctions”, Phys. Rev. B 78, 045434 (2008).[2] Takehide Miyazaki, Hisao Nakamura, Kengo Nishio, Hisashi Shima, Hiroyuki Akinaga, and Yoshihiro Asai, ”First-Principles Transport Modeling for Metal/Insulator/Metal Structures”, JPS Conf. Proc. , 012075 (2014).ACKNOWLEDGMENTSThe authors would like to thank to Drs. Takehide MIYAZAKI, Kengo NISHIO, Hisashi SHIMA, and Hiroyuki AKINAGA for their collaborations and discussions.

Orbital stability of standing waves of two-component Bose–Einstein condensates with internal atomic Josephson junction

In this paper, we prove existence, symmetry and uniqueness of standing waves for a coupled Gross–Pitaevskii equations modeling component Bose–Einstein condensates BEC with an internal atomic Josephson junction. We will then address the orbital stability of these standing waves and characterize their orbit.

Single-charge detection by an atomic precision tunnel junction

We demonstrate sensitive detection of single charges using a planar tunnel junction 8.5 nm wide and 17.2 nm long defined by an atomically precise phosphorus doping profile in silicon. The conductance of the junction responds to a nearby gate potential and also to changes in the charge state of a quantum dot patterned 52 nm away. The response of this detector is monotonic across the entire working voltage range of the device, which will make it particularly useful for studying systems of multiple quantum dots. The charge sensitivity is maximized when the junction is most conductive, suggesting that more sensitive detection can be achieved by shortening the length of the junction to increase its conductance.

Optimal l ∞ error estimates of finite difference methods for the coupled Gross-Pitaevskii equations in high dimensions

Due to the difficulty in obtaining the a priori estimate, it is very hard to establish the optimal point-wise error bound of a finite difference scheme for solving a nonlinear partial differential equation in high dimensions (2D or 3D). We here propose and analyze finite difference methods for solving the coupled Gross-Pitaevskii equations in two dimensions, which models the two-component Bose-Einstein condensates with an internal atomic Josephson junction. The methods which we considered include two conservative type schemes and two non-conservative type schemes. Discrete conservation laws and solvability of the schemes are analyzed. For the four proposed finite difference methods, we establish the optimal convergence rates for the error at the order of O(h2 +τ2) in the l∞-norm (i.e., the point-wise error estimates) with the time step τ and the mesh size h. Besides the standard techniques of the energy method, the key techniques in the analysis is to use the cut-off function technique, transformation between the time and space direction and the method of order reduction. All the methods and results here are also valid and can be easily extended to the three-dimensional case. Finally, numerical results are reported to confirm our theoretical error estimates for the numerical methods.

Potential-Dependent Restructuring and Chemical Noise at Au–Ag–Au Atomic Scale Junctions

The effect of electrochemical potential on the behavior of electrochemically deposited Au-Ag-Au bimetallic atomic scale junctions (ASJs) is addressed here. A common strategy for ASJ production begins with overgrown nanojunctions and uses electromigration to back-thin the junction. Here, these steps are carried out with the entire junction under electrochemical potential control, and the relationship between junction stability and applied potential is characterized. The control of electrochemical potential provides a reliable method of regulating the size of nanojunctions. In general, more anodic potentials decrease junction stability and increase the rate at which conductance decays. Conductance behavior under these labile conditions is principally determined by Ag oxidation potential, electrochemical potential-induced surface stress, and the nature of the adsorbate. Junctions fabricated at more cathodic potentials experience only slight changes in conductance, likely due to surface atom diffusion and stress-induced structural rearrangement. Electrochemical potential also plays a significant role in determining adsorption-desorption kinetics of surface pyridine at steady state at Au-Ag-Au ASJs, as revealed through fluctuation spectroscopy. Average cutoff frequencies increase at more anodic potentials, as does the width of the cutoff frequency distribution measured over 80 independent runs. Three reversible reactions--pyridine adsorption, Ag atom desorption, and Ag-pyridine complex dissolution--can occur on the surface, and the combination of the three can explain the observed results.

Thermoelectric voltage measurements of atomic and molecular wires using microheater-embedded mechanically-controllable break junctions.

We developed a method for simultaneous measurements of conductance and thermopower of atomic and molecular junctions by using a microheater-embedded mechanically-controllable break junction. We find linear increase in the thermoelectric voltage of Au atomic junctions with the voltage added to the heater. We also detect thermopower oscillations at several conductance quanta reflecting the quantum confinement effects in the atomic wire. Under high heater voltage conditions, on the other hand, we observed a peculiar behaviour in the conductance dependent thermopower, which was ascribed to a disordered contact structure under elevated temperatures.

Investigation on the Pyrazine Molecular Junction Studied by Conductance Measurement and Near Edge X-ray Absorption Fine Structure

A pyrazine molecular junction was investigated using mechanically controllable break junction (MCBJ) technique at 10 K. The conductance measurements revealed the single pyrazine molecular junctions showed two distinct conductance values of 0.27 ± 0.04 and 1.0 ± 0.2 G 0 (2e 2/h). The conductance value of the single pyrazine molecular junction was comparable with that of the metal atomic junction. The interface between pyrazine molecule and Pt surface was investigated by near edge X-ray absorption fine structure (NEXAFS). The broadening of the π* peak in N K-edge NEXAFS spectra suggested that the pyrazine molecule connected to Pt surface via a nitrogen atom. Based on the measurements of the conductance and NEXAFS, we could propose the structural models of two distinct conductance states for the single pyrazine molecular junction.

Thermoelectricity in atom-sized junctions at room temperatures

Atomic and molecular junctions are an emerging class of thermoelectric materials that exploit quantum confinement effects to obtain an enhanced figure of merit. An important feature in such nanoscale systems is that the electron and heat transport become highly sensitive to the atomic configurations. Here we report the characterization of geometry-sensitive thermoelectricity in atom-sized junctions at room temperatures. We measured the electrical conductance and thermoelectric power of gold nanocontacts simultaneously down to the single atom size. We found junction conductance dependent thermoelectric voltage oscillations with period 2e2/h. We also observed quantum suppression of thermovoltage fluctuations in fully-transparent contacts. These quantum confinement effects appeared only statistically due to the geometry-sensitive nature of thermoelectricity in the atom-sized junctions. The present method can be applied to various nanomaterials including single-molecules or nanoparticles and thus may be used as a useful platform for developing low-dimensional thermoelectric building blocks.

Dynamic response of silicon nanostructures at finite frequency: An orbital-free density functional theory and non-equilibrium Green's function study

Orbital-free density functional theory (OFDFT) replaces the wavefunction in the kinetic energy by an explicit energy functional and thereby speeds up significantly the calculation of ground state properties of the solid state systems. So far, the application of OFDFT has been centered on closed systems and less attention is paid on the transport properties in open systems. In this paper, we use OFDFT and combine it with non-equilibrium Green's function to simulate equilibrium electronic transport properties in silicon nanostructures from first principles. In particular, we study ac transport properties of a silicon atomic junction consisting of a silicon atomic chain and two monoatomic leads. We have calculated the dynamic conductance of this atomic junction as a function of ac frequency with one to four silicon atoms in the central scattering region. Although the system is transmissive with dc conductance around 4 to 5 e2/h, capacitive-like behavior was found in the finite frequency regime. Our analysis shows that, up to 0.1 THz, this behavior can be characterized by a classic RC circuit consisting of two resistors and a capacitor. One resistor gives rise to dc resistance and the other one accounts for the charge relaxation resistance with magnitude around 0.2 h/e2 when the silicon chain contains two atoms. It was found that the capacitance is around 5 aF for the same system.

NOTE ON THE GROUND STATES OF TWO-COMPONENT BOSE-EINSTEIN CONDENSATES WITH AN INTERNAL ATOMIC JOSEPHSON JUNCTION

In this paper, we consider two-component Bose-Einstein condensates with an internal atomic Josephson junction in the general case, i.e., 0 < p < <TEX>$\frac{2}{(d-2)^+}$</TEX>. We prove existence and uniqueness results for the ground states, and obtain some properties of the ground states with large parameters.

Nonlinearity enhanced interfacial thermal conductance and rectification

We study the nonlinear interfacial thermal transport across atomic junctions by the quantum self-consistent mean-field (QSCMF) theory based on the nonequilibrium Green's function approach; the QSCMF theory we propose is very precise and matches well with the exact results from quantum master equation. The nonlinearity at the interface is studied by effective temperature-dependent interfacial coupling calculated from the QSCMF theory. We find that nonlinearity can provide an extra channel for phonon transport in addition to the phonon scattering which usually blocks heat transfer. For weak linearly coupled interface, the nonlinearity can enhance the interfacial thermal transport; with increasing nonlinearity or temperature, the thermal conductance shows nonmonotonical behavior. The interfacial nonlinearity also induces thermal rectification, which depends on the mismatch of the two leads and also the interfacial linear coupling.

The Higher Andreev State

Physics In superconductors, materials capable of perfect conduction of electricity, the associated supercurrent is carried by pairs of electrons known as Cooper pairs. If a superconductor is placed in contact with a nonsuperconducting material, a single electron impinging on the interface from the nonsuperconducting side is reflected as a hole and forms a Cooper pair on the superconductor side—a process known as Andreev reflection. For a nonsuperconducting material sandwiched between two superconductors (as is the case in devices such as tunnel Josephson junctions), the reflections on the two interfaces will cause the formation of standing waves—Andreev bound states (ABSs)—which are responsible for carrying the supercurrent across the weak link. The ABSs have a discrete spectrum, but usually only the lowest energy state is accessed in experiments. Bretheau et al. directly detected an excited ABS in a less common form of a Josephson junction, where the weak link is an atomic contact. Their apparatus consisted of an atomic contact junction in parallel with a tunnel junction, and with another tunnel junction acting as a source and detector of microwaves. By measuring the current and voltage dependence of the latter, and by varying the phase difference across the atomic contact, they were able to precisely map out the energy needed to excite the ground ABS. It is expected that the existence of two ABSs can be used as a basis for a quantum bit or as a stage for exploring fundamental mesoscopic phenomena. Nature 499 , 312 (2013).

Optimal Point-Wise Error Estimate of a Compact Difference Scheme for the Coupled Gross–Pitaevskii Equations in One Dimension

The coupled Gross---Pitaevskii (CGP) equation studied in this paper is an important mathematical model describing two-component Bose---Einstein condensate with an internal atomic Josephson junction. We here analyze a compact finite difference scheme which conserves the total mass and energy in the discrete level for the CGP equation. In general, due to the difficulty caused by compact difference on nonlinear terms, optimal point-wise error estimates without any restrictions on the grid ratios of compact difference schemes for nonlinear partial differential equations are very hard to be established. To overcome the difficulty caused by the compact difference operator, we introduce a new norm and an interesting transformation by which the difference scheme is transformed into a special equivalent vector form, we then use the energy method and some important lemmas on the equivalent system to obtain the optimal convergent rate, without any restrictions on the grid ratio, at the order of $$O(h^{4}+\tau ^2)$$ O ( h 4 + ? 2 ) in the maximum norm with time step $$\tau $$ ? and mesh size $$h$$ h . Finally, numerical results are reported to test the theoretical results and simulate the dynamics of the CGP equation.

Light emission and finite-frequency shot noise in molecular junctions: From tunneling to contact

Scanning tunneling microscope induced light emission from an atomic or molecular junction has been probed from the tunneling to contact regime in recent experiments. There, the intensity of the light emission shows strong correlation with the current/charge fluctuations at optical frequencies. We show that this is consistent with the established theory in the tunneling regime, by writing the finite-frequency shot noise as a sum of inelastic transitions between different electronic states. Based on this, we develop a practical scheme to perform calculations on realistic structures using Green's functions. The photon emission yields obtained re-produce the essential feature of the experiments.

Heat dissipation in atomic-scale junctions

Atomic and single-molecule junctions represent the ultimate limit to the miniaturization of electrical circuits. They are also ideal platforms for testing quantum transport theories that are required to describe charge and energy transfer in novel functional nanometre-scale devices. Recent work has successfully probed electric and thermoelectric phenomena in atomic-scale junctions. However, heat dissipation and transport in atomic-scale devices remain poorly characterized owing to experimental challenges. Here we use custom-fabricated scanning probes with integrated nanoscale thermocouples to investigate heat dissipation in the electrodes of single-molecule ('molecular') junctions. We find that if the junctions have transmission characteristics that are strongly energy dependent, this heat dissipation is asymmetric--that is, unequal between the electrodes--and also dependent on both the bias polarity and the identity of the majority charge carriers (electrons versus holes). In contrast, junctions consisting of only a few gold atoms ('atomic junctions') whose transmission characteristics show weak energy dependence do not exhibit appreciable asymmetry. Our results unambiguously relate the electronic transmission characteristics of atomic-scale junctions to their heat dissipation properties, establishing a framework for understanding heat dissipation in a range of mesoscopic systems where transport is elastic--that is, without exchange of energy in the contact region. We anticipate that the techniques established here will enable the study of Peltier effects at the atomic scale, a field that has been barely explored experimentally despite interesting theoretical predictions. Furthermore, the experimental advances described here are also expected to enable the study of heat transport in atomic and molecular junctions--an important and challenging scientific and technological goal that has remained elusive.