Cryogenic Scanning Tunneling Microscope Research Articles

Single-electron tunneling effects in granular metal films.

Cryogenic scanning tunneling microscopy is used to study local electrical-transport properties of thin granular Au/${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$ films in the vicinity of the percolation threshold. The current-voltage characteristics are found to vary dramatically from one tip position to another over distances of the order of a few nanometers. These characteristics often exhibit interesting Coulomb-staircase structures having unusual variations in step widths and heights due to complex tunneling paths. A triple-barrier tunnel-junction model accounts quantitatively for the experimental results.

Integrated cryogenic scanning tunneling microscopy and sample preparation system

A scanning tunneling microscope (STM) operable from room temperature to 1.5 K with an integrated sample preparation chamber has been constructed and successfully tested. Both the sample and the tunneling tip can be cleaned and/or modified in a UHV processing chamber which is connected at the top of a liquid-helium flow cryostat. The STM is mounted to a threaded rod inside the cryostat so that it can be translated vertically. This allows a newly prepared sample/tip to be transferred onto the STM and lowered into a cryogenic environment without breaking vacuum, minimizing surface contamination which causes significant tunneling problems at cryogenic temperatures.

Cryogenic scanning tunneling microscope with a magnetic coarse approach

A compact, rigid, and reliable cryogenic scanning tunneling microscope (CSTM) with a vertical electromagnetic coarse approach system was developed. This device can be used for topographic and local tunneling spectroscopy studies at liquid nitrogen and helium temperatures. Minimal step sizes of 28 nm for the electromagnetic translation device were achieved. The additional possibility of a coarse approach operation in the inertial slip-stick mode, without electromagnets, was successfully tested, making this STM compatible with external magnetic fields. A simple technique for characterizing the STM rigidity has been developed. Preliminary data, taken with this instrument are presented, demonstrating the achievement, at liquid helium temperature, of atomic resolution for topographic studies, and also the possibility of measuring simultaneously superconducting energy gap spectra.

Polarization charge relaxation and the Coulomb staircase in ultrasmall double-barrier tunnel junctions

Experimental results are reported on the Coulomb staircase in a double-barrier tunnel junction formed by the tip of a cryogenic scanning tunneling microscope, an ultrasmall Au particle (4 nm in diameter), a ZrO 2 tunnel oxide-barrier, and a Au covered substrate. Two discrepancies with the orthodox model (global rule) are frequently found: an enhanced asymptotic separation of the current-voltage characteristic, and an anomalous suppression of the first current steps in the region around zero voltage. These observations are tentatively attributed to the effect of slow dielectric relaxation of polarization charge induced in the tunnel oxide. This notion is supported, although some discrepancies remain, by a calculation of Coulomb staircases for large relaxation times according to the local rule.

Observation of bound quasiparticle states in thin Au islands by scanning tunneling microscopy.

Cryogenic scanning tunneling microscopy has been used to measure the structure and the local electronic density of states of thin gold islands in metallic contact with a bulk superconducting ${\mathrm{NbSe}}_{2}$ crystal. Peaks are observed in the tunneling conductance below the energy gap corresponding to discrete bound quasiparticle states in the gold. The energies and spatial dependence of the bound states imply that the superconducting pair potential falls off abruptly at the Au-${\mathrm{NgSe}}_{2}$ interface.

The direct observation of structural development during vanadium pentoxide gelation

The sequence of structural evolution in the gelation of vanadic acid to form vanadium pentoxide gels was studied using cryogenic transmission electron microscopy (cryo-TEM) and scanning tunneling microscopy (STM). Small whiskers form from initially homogeneous solutions, and then grow into crystalline ribbon-like colloidal particles. It is proposed that the whiskers form from polymerization of dioxovanadium cations. The ribbons then grow by continued addition of dioxovanadium cations which are supplied by the decomposition of decavanadate ions. In solution, the ribbon-like particles have dimensions of approximately 25 nm × 3 nm × over 1 μm. These ribbons are flexible perpendicular to the plane of the ribbon. Upon drying, a flat rigid mass of ribbon-like particles is formed. The ribbons examined by STM showed striations 3 nm wide, a value that corresponds with the width of the unit cell proposed by J. Legendre and J. Livage [J. Colloid. and Interf. Sci. 94, 75 (1983)].

Surface oxide effects on tunneling spectroscopy measured with a cryogenic scanning tunneling microscope

We describe a simple, versatile cryogenic (∼4.2 K) scanning tunneling microscope (CSTM) system optimized for spectroscopic studies. We find that spectroscopic data obtained at low temperatures are a function of the local surface characteristics of both the tip probe and sample. The superconducting density of states of lead is masked for tips with a heavy surface oxide (etched tungsten, stainless steel), yet easily observed with nonoxidized tips (gold, platinum). We show direct evidence that small metallic inclusions embedded in some oxides can produce single electron capacitive charging effects that dominate current-voltage characteristics.

Analytic solution for the current-voltage characteristic of two mesoscopic tunnel junctions coupled in series.

We present a theoretical analysis for a system composed of two mesoscopic tunnel junctions coupled in series. We show that the current-voltage characteristic for this system can be obtained analytically. The usefulness of the model is demonstrated through the fit of experimental data acquired with a cryogenic (4.2 K) scanning tunneling microscope. A simple extension of the model predicts additional structure in the system characteristics when discrete middle electrode states are present.

Single-electron and oxide-impurity effects in junctions formed by a cryogenic scanning tunneling microscope.

The authors show that the I-V characteristics measured at 4 K of single tunnel junctions between the tip of a scanning tunneling microscope and a metallic sample are sensitive to the materials used. The superconducting energy gap of lead is observed for tips without surface oxides, but eliminated for tips with thick oxides. Probing bulk etched tungsten, observation of the Coulomb staircase suggests capacitive effects due to a small metallic-oxide impurity.

Tunneling spectroscopy study of YBa2Cu3O7 thin films using a cryogenic scanning tunneling microscope.

We have measured reproducible tunneling spectra on ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ (${\mathit{T}}_{\mathit{c}}$\ensuremath{\sim}85 K) thin films (thickness \ensuremath{\sim}2 \ensuremath{\mu}m) with a cryogenic scanning tunneling microscope. We find that the I-V curves are generally of three types. The most common type, featured in a large majority of the data, shows a region of high conductance at zero bias. The amplitude of this region is inversely proportional to the tunneling resistance between the tip and sample. It is possible that this can be explained in terms of Josephson effects within the films, although an alternative is given based on electronic self-energy corrections. Data showing capacitive charging steps are analyzed in terms of two ultrasmall tunnel junctions in series.. Theoretical fits to the data give specific values of the junction parameters that are consistent with the assumed geometry of the tip probing an individual grain of the film. The third type of I-V curves exhibits negative differential resistance. We conclude that this phenomenon is probably due to tunneling to localized states in the surface oxide. We also present and discuss data with energy-gap-like behavior; the best example gives \ensuremath{\Delta} to be about 27 mV.

Scanning tunneling microscope observations of the Coulomb blockade

The smooth Coulomb blockade (CB) I–V curve is sometimes observed in cryogenic scanning tunneling microscopy using a voltage biased tip to scan a metal film. Our experiments always give some evidence of the Coulomb staircase, characteristic of two tunnel capacitors in series (STM tip, small particle, and substrate) accompanying the smooth CB I–V curves. We suggest that a small (100 Å) particle occasionally encountered between tip and surface, providing a capacitance of order 10 -18 F unaffected by voltage biased leads, is the origin of both effects.

Wide range temperature compensated cryogenic scanning tunneling microscope

We describe a compact scanning tunneling microscope using piezoelectric bimorph elements to achieve a large scanning area of 20 μm2 at 4.2 K. This instrument provides inherent temperature compensation, compatibility with a high magnetic field environment, and a novel means of rough sample–tip (z) adjustment.