Single-atom Junctions Research Articles

Shot-noise measurements of single-atom junctions using a scanning tunneling microscope.

Current fluctuations related to the discreteness of charge passing through small constrictions are termed shot noise. This unavoidable noise provides both advantages-being a direct measurement of the transmitted particles' charge-and disadvantages-a main noise source in nanoscale devices operating at low temperature. While better understanding of shot noise is desired, the technical difficulties in measuring it result in relatively few experimental works, especially in single-atom structures. Here, we describe a local shot-noise measurement apparatus and demonstrate successful noise measurements through single-atom junctions. Our apparatus, based on a scanning tunneling microscope, operates at liquid helium temperatures. It includes a broadband commercial amplifier mounted in close proximity to the tunnel junction, thus reducing both the thermal noise and input capacitance that limit traditional noise measurements. The full capabilities of the microscope are maintained in the modified system, and a quick transition between different measurement modes is possible.

Long-Lived Gold Single-Atom Junctions Formed by a Flexible Probe for Scanning Tunneling Microscopy Applications

Flexible probes have shown promise in improving resolution and sensitivity in various scanning probe microscopy based imaging and spectroscopy techniques. However, it has not been utilized in scann...

Synthesis and Characterization of Well-Defined, Tadpole-Shaped Polystyrene with a Single Atom Junction Point

An efficient method for synthesis of well-defined, well-characterized, tadpole-shaped polystyrene with a single atom junction point that is optimal for the study of dynamics has been developed using anionic polymerization, silicon chloride linking chemistry, and metathesis ring closure. The difunctional macromolecular linking agent, ω-methyldichlorosilylpolystyrene, was formed by reacting sec-butyllithium-initiated poly(styryl)lithium with excess (30×) methyltrichlorosilane to eliminate formation of linear dimer and three-arm star polystyrene. The asymmetric, three-arm, star precursor was formed by linking excess α-4-pentenylpoly(styryl)lithium (α-PSLi) with the macromolecular linking agent, and the excess α-PSLi functionalized with ethylene oxide before termination with methanol to facilitate chromatographic separation. Cyclization of the three-arm, star precursor to form tadpole-shaped polystyrene was effected in methylene chloride at high dilution using the Grubbs first generation catalyst, bis(tricycl...

Electronic conduction during the formation stages of a single-molecule junction

Single-molecule junctions are versatile test beds for electronic transport at the atomic scale. However, not much is known about the early formation steps of such junctions. Here, we study the electronic transport properties of premature junction configurations before the realization of a single-molecule bridge based on vanadocene molecules and silver electrodes. With the aid of conductance measurements, inelastic electron spectroscopy and shot noise analysis, we identify the formation of a single-molecule junction in parallel to a single-atom junction and examine the interplay between these two conductance pathways. Furthermore, the role of this structure in the formation of single-molecule junctions is studied. Our findings reveal the conductance and structural properties of premature molecular junction configurations and uncover the different scenarios in which a single-molecule junction is formed. Future control over such processes may pave the way for directed formation of preferred junction structures.

Quantized thermal transport in single-atom junctions.

Thermal transport in individual atomic junctions and chains is of great fundamental interest because of the distinctive quantum effects expected to arise in them. By using novel, custom-fabricated, picowatt-resolution calorimetric scanning probes, we measured the thermal conductance of gold and platinum metallic wires down to single-atom junctions. Our work reveals that the thermal conductance of gold single-atom junctions is quantized at room temperature and shows that the Wiedemann-Franz law relating thermal and electrical conductance is satisfied even in single-atom contacts. Furthermore, we quantitatively explain our experimental results within the Landauer framework for quantum thermal transport. The experimental techniques reported here will enable thermal transport studies in atomic and molecular chains, which will be key to investigating numerous fundamental issues that thus far have remained experimentally inaccessible.

Electronic states in an atomistic carbon quantum dot patterned in graphene

We reveal the emergence of metallic Kondo clouds in an atomistic carbon quantum dot, realized as a single-atom junction in a suitably patterned graphene nanoflake. Using density functional dynamical mean-field theory (DFDMFT) we show how correlation effects lead to striking features in the electronic structure of our device, and how those are enhanced by the electron-electron interactions when graphene is patterned at the atomistic scale. Our setup provides a well-controlled environment to understand the principles behind the orbital-selective Kondo physics and the interplay between orbital and spin degrees of freedom in carbon-based nanomaterials, which indicate new pathways for spintronics in atomically patterned graphene.

Ballistic Anisotropic Magnetoresistance of Single-Atom Contacts.

Anisotropic magnetoresistance, that is, the sensitivity of the electrical resistance of magnetic materials on the magnetization direction, is expected to be strongly enhanced in ballistic transport through nanoscale junctions. However, unambiguous experimental evidence of this effect is difficult to achieve. We utilize single-atom junctions to measure this ballistic anisotropic magnetoresistance (AMR). Single Co and Ir atoms are deposited on domains and domain walls of ferromagnetic Fe layers on W(110) to control their magnetization directions. They are contacted with nonmagnetic tips in a low-temperature scanning tunneling microscope to measure the junction conductances. Large changes of the magnetoresistance occur from the tunneling to the ballistic regime due to the competition of localized and delocalized d-orbitals, which are differently affected by spin-orbit coupling. This work shows that engineering the AMR at the single atom level is feasible.

Conductance of hydrogen-incorporated Fe single-atom junction

Conductance of hydrogen-incorporated Fe single-atom junction

Experimental visualization of scattering at defects in electronic transport through a single atomic junction

For electronic transport at the nanoscale, coherent scattering at defects plays an important role. Therefore, the capability of visualizing the influence of defects on the conductivity of single atomic junctions may benefit the development of future nano-electronics. Here, we report imaging the coherent scattering from a defect with well-controlled geometry by quantum point contact microscopy recently developed by us. An \ensuremath{\sim}10$%$ modulation in transport conductance of a single atomic junction is observed, with a phase shift of nearly \ensuremath{\pi} compared to the tunneling conductance. With the well-defined scattering geometry, we performed a theoretical calculation of the conductance and found the result consistent with the experiment.

Fluctuated atom-sized junctions in a liquid environment

Durability of atom-sized junctions in a liquid environment was investigated and compared with that in a vacuum. The single atom junction lifetime was measured in an organic solvent under various stretching speed vd ranging from 10 to 0.001 nm/s. We found the natural lifetime of about 1 s for Au single-atom chains formed in a non-polar organic solvent at vd ≤ 0.01 nm/s, which was an order of magnitude shorter than that in a vacuum. The decreased junction lifetime indicates contact instability induced by thermal collisions of solvent molecules that exert pressure on the nanocontacts.

Tunneling magnetoresistance and exchange interaction in single-atom contacts

The tunneling magnetoresistance of single-atom junctions is shown to depend on the electrode separation owing to exchange forces and the resulting geometric relaxations. An analytical model is proposed to extract relaxations from the magnetoresistances measured with a scanning tunneling microscope. Exchange forces and relaxations calculated within density functional theory demonstrate the validity of the model for a range of distances between tip and sample which extends from tunneling close to the point of maximal attraction.

Quantum conductance of a single magnetic atom: Anab initiostudy

Our ab initio study explains recent puzzling experiments on the conductance of a single Co adatom deposited either on Cu(111) or on ferromagnetic Co islands and contacted with both magnetic and nonmagnetic electrodes [N. N\'eel, J. Kr\"oger, and R. Berndt, Phys. Rev. Lett. 102, 086805 (2009)]. We provide clear evidence that the conductance of a single atomic junction in the contact regime is close to ${G}_{0}/2$ (${G}_{0}$ is the quantum of conductance) for ferromagnetic electrodes and to ${G}_{0}$ for nonmagnetic ones. Spin-dependent calculations reveal that a conductance of ${G}_{0}/2$ originates from a combination of partially open majority and minority channels. The bonding between the Co adatom and the Co island reduces significantly the contribution of the minority $d$ electrons to the conductance, leading to the observation of half-integer conductance.

Single-atom contacts with a scanning tunnelling microscope

The tip of a cryogenic scanning tunnelling microscope is used to controllably contact single atoms adsorbed on metal surfaces. The transition between tunnelling and contact is gradual for silver, while contact to adsorbed gold atoms is abrupt. The single-atom junctions are stable and enable spectroscopic measurements of, e.g., the Abrikosov–Suhl resonance of single Kondo impurities.

Scanning Tunneling Microscopic Investigations into the Conductance of Single-Atom Junctions

Experiments using a scanning tunneling microscope to controllably fabricate single-atom contacts are reviewed. An important feature of this approach is that the electronic and geometric properties of the junctions can be characterized prior to and after contact formation. Investigations on magnetic contacts have led to the observation of unusual conductance values. Spectroscopy of the electronic states of single atoms in contact with the tip is demonstrated through the observation of the Kondo effect and of surface state localization at adsorbed atoms. Time-resolved two-level fluctuations of a single-atom tunneling contact reveal heating of the junction in an otherwise cryogenic environment.

Kondo Effect in Single Atom Contacts: The Importance of the Atomic Geometry

Co single atom junctions on copper surfaces are studied by scanning tunneling microscopy and ab initio calculations. The Kondo temperature of single cobalt atoms on the Cu(111) surface has been measured at various tip-sample distances ranging from tunneling to the point contact regime. The experiments show a constant Kondo temperature for a whole range of tip-substrate distances consistently with the predicted energy position of the spin-polarized d levels of Co. This is in striking difference to experiments on Co/Cu(100) junctions, where a substantial increase of the Kondo temperature has been found. Our calculations reveal that the different behavior of the Co adatoms on the two Cu surfaces originates from the interplay between the structural relaxations and the electronic properties in the near-contact regime.

Multiple Andreev reflection in single-atom niobium junctions

Single-atom junctions between superconducting niobium leads are produced using the mechanically controllable break junction technique. The current-voltage characteristics of these junctions are analyzed using an exact formulation for a superconducting quantum point contact. For tunneling between two single atoms with a sufficiently large vacuum barrier, it is found that a single channel dominates the current, and that the current-voltage characteristic is described by the theory, without adjustable parameters. For a contact of a single Nb atom it is shown that five conductance channels contribute to the conductance, in agreement with the number expected based on the number of valence orbitals for this d metal. For each of the channels the transmission probability is obtained from the fits, and the limits of accuracy of these numbers are discussed.

Coulomb blockade oscillation in a single atomic junction

The first observation of the Coulomb blockade effect in the smallest possible system with a single atom as the central island of a double-barrier tunnel junction is reported. Our system consists of a single tungsten atom as the central island and a tungsten STM tip and a silicon reconstructed surface as the two electrodes. The use of a single atom as the central island makes the change in the electrostatic potential due to a variation of number of electrons in the island of the order of 1 eV and thus the Coulomb blockade effect is made more controllable and stable even at room temperature. A specific shape of the tip apex forms a tunnel junction between an apex atom and the rest of the tip with the energy-level broadening of the apex atom smaller than the change in the charging energy due to the change in the number of electrons in the single tungsten atom. This theoretical prediction was confirmed by the experimental results of I-V measurements with an STM tip made from a W(111) single-crystal wire where the change in the charging energy is 1.1 eV.

Transport through a single-atom junction

We have performed first-principles density functional calculations in conjunction with a three-dimensional evaluation of the quantum scattering matrix for single-atom Si and Al tunnel junctions. We predict the equilibrium conductance and resonance transmission properties. Our results unambiguously show that when the atom is somewhat isolated from the electrodes, the differential electric current should show resonances due to atomic valence orbitals. The conductance resonance peaks may or may not retain a universal value, depending on whether or not there is a partial overlap of the resonance transmissions. Our results also clearly demonstrate the formation of a wire when the atomic junction becomes less and less isolated from the electrodes.

Theory of Transport Through a Single Atomic Junction

A theory of transport properties through a single atomic junction consisting of a single atom and two electrodes is presented. Such an atomic junction is realized with a system of a scanning tunneling microscope (STM) tip, a single atom located at the tip apex and a sample surface. Electron tunneling between an atom and an electrode through a vacuum potential barrier is governed simultaneously by the Coulomb potential (Coulomb blockade effect) and Pauli exclusion principle due to an atomic discrete energy spectrum. Measurement of conductance oscillation due to the Coulomb blockade effect and Pauli principle in such a system allows one to determine the highest occupied energy level and its degeneracy of the atom as well as the local work function of the tip apex and the band gap of the sample surface.

Conductance simulation through single atom junctions at the scanning tunnelling microscope

Using a Keldish-Green Function formalism we simulate the current flowing through transition and noble metal atoms wires. Such a kind of wire is nowadays experimentally studied with several methods involving mechanically controllable break junction or a scanning tunnelling microscope operating in close contact regime. Our aim is to reproduce the experimental differences found between several materials. Results for Pt, Ni and Au and their comparison with the experiments are discussed.