Double-barrier Tunnel Junctions Research Articles

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.

Conductance through a one-atom point contact

Linear conductance and current–voltage characteristics in a one-atom point contact are calculated. A one-atom point contact is modeled by a double-barrier tunnel junction with a central island which is either a single atom or an atomic cluster. A finite electron reflection and induced charge at both sides of the tapered constriction alter the conductance from the quantum unit of conductance in a perfectly smooth point contact. The conductance is expressed in terms of a difference ΔWbetween the work function of the electrodes and the atomic energy level, a charging energyUand an energy level broadening ħΓ in our simple model of a one-atom point contact.

Resonant Tunneling Through Discrete Electronic Levels of a C60Molecule in the Presence of Charging Effects

We have conducted tunneling spectroscopy studies for isolated C60 molecules in the double barrier tunnel junction configuration. The tunneling current-voltage (I-V) and dI/dV vs. V spectra of these molecules exhibit rich structures resulting from both resonant tunneling through the discrete levels and single electron charging effects. In particular, we observe degeneracy lifting within the molecular orbitals, probably due to the Jahn-Teller effect and local electric fields. Theoretical fits, performed using the orthodox model for single-electron tunneling modified to account for the discrete level spectrum of C60, agree well with our data.

Modification of current-voltage characteristics of double-barrier tunnel junctions under the influence of quasiparticle extraction

Enhancement of the magnitude of the value corresponding to the superconducting energy gap of Al${}^{\ensuremath{'}}$ in Nb/Al-AlO${}_{x}$-Al${}^{\ensuremath{'}}$-AlO${}_{x}$-Al/Nb and Nb/Al/Nb/Al-AlO${}_{x}$-Al${}^{\ensuremath{'}}$-AlO${}_{x}$-Al/Nb/Al/Nb devices by tunnel extraction of quasiparticles has been investigated experimentally. The enhancement is observed both at the gap-difference and gap-sum voltage. In addition to the gap-difference and gap-sum features, there is a feature in the $I$-$V$ characteristic which corresponds to the sum of the energy gaps of the external electrodes. The dc and ac Josephson currents appear in the devices below the same critical temperature ${T}_{\mathrm{c}\mathrm{}\mathrm{eq}},$ which is found to be ${T}_{\mathrm{c}\mathrm{}\mathrm{eq}}=2.35$ and 2.60 K for the devices of the first and second types, respectively. Below ${T}_{\mathrm{c}\mathrm{}\mathrm{eq}},$ an enhancement of the dc component of ac Josephson current has been observed at $V>0$, as compared with the magnitude of the dc Josephson current at $V=0$.

Shot-Noise Suppression in the Single-Electron Tunneling Regime.

Electrical current fluctuations through tunnel junctions are studied with a scanning-tunneling microscope. For single-tunnel junctions classical Poisson shot noise is observed, indicative for uncorrelated tunneling of electrons. For double-barrier tunnel junctions, formed by a nanoparticle between tip and surface, the shot noise is observed to be suppressed below the Poisson value. For strongly asymmetric junctions, where a Coulomb staircase is observed in the current-voltage characteristic, the shot-noise suppression is periodic in the applied voltage. This originates from correlations in the transfer of electrons imposed by single-electron charging effects.

Low temperature scanning tunneling microscopy studies of granular metal films

Cryogenic scanning tunneling microscopy is used to study local electrical transport properties of thin granular Au/Al2O3 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. The characteristics often exhibit single electron tunneling effects such as the Coulomb blockade and the Coulomb staircase. This behavior is similar to that observed for tunneling into a single isolated nanometer size metallic particle which was explained in terms of a double-barrier tunnel junction model. Some of the characteristics show, however, novel Coulomb-staircase structures having unusual variations in step widths and heights due to complex tunneling paths. A triple-barrier tunnel junction model, where the electron tunnels through two metallic particles along its path, accounts quantitatively for the experimental results.

Double-barrier tunnel junctions for quasiparticle mixers

Double-barrier niobium tunnel junctions with the middle electrode thinner than the London penetration depth were fabricated. Their I-V curves, annealing behavior, response to external magnetic field, and response to 230 and 350 GHz irradiation in mixer experiments is discussed. Junctions with an integrated tuning circuit designed for the 230 GHz frequency range gave double-sideband receiver noise temperatures of 100 K. Vertically stack arrays can be attractive for various applications. Compared to single-barrier devices possible drawbacks were noted. The I-V curve indicates heating and/or nonequilibrium effects. Double-barrier devices became nonuniform after thermal annealing at 200 °C. The response to the external magnetic field is more complex and considerably larger flux densities are needed to suppress Josephson effects.

Direct observation of the electron spectral function in the integer and fractional quantum Hall regimes by resonant tunneling.

Low-temperature current-voltage characteristics of a double-barrier resonant tunnel junction are used to directly measure the two-dimensional (2D) electron spectral function. Our data provide quantitative determination of the 2D density of states, including chemical potential and linewidth oscillations as a function of magnetic field up to 51 T. We also measure a chemical potential jump at 1/3 fractional filling of the lowest Landau level.

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.

Single-electron tunneling in double-barrier junctions by scanning tunneling microscopy

Ultrasmall (≲5 nm in lateral diameter) double-barrier tunnel junctions have been realized using a scanning tunneling microscope and an optimized metal-particle-oxide-metallic substrate system. The effects of single-electron charging (single-electron tunneling) on the current-voltage characteristics are studied experimentally at 4.2 K. The circumstances under which a Coulomb staircase is observable for this kind of model systems is investigated systematically, using the orthodox theory of single-electron tunneling to analyze the data.

Enhancement of superconductivity far above the critical temperature in double-barrier tunnel junctions.

Motivated by the observation of a superconducting energy gap far above the equilibrium critical temperature ${\mathit{T}}_{\mathit{c}}$ in an Al film forming the center electrode of a Nb/${\mathrm{AlO}}_{\mathit{x}}$/Al/${\mathrm{AlO}}_{\mathit{x}}$/Nb structure we analyze the mechanism of gap enhancement in symmetric double-barrier superconducting tunnel junctions. It is found that such structures are very effective in creating a nonthermal distribution of quasiparticles in the middle electrode. At certain bias conditions this leads, according to the BCS gap equation, to the appearance of a nonzero superconducting energy gap even at temperatures up to several times the equilibrium ${\mathit{T}}_{\mathit{c}}$. So the double-barrier arrangement offers the remarkable possibility of making a material become superconducting by applying a voltage or passing a current. Calculated current-voltage characteristics exhibit current steps at voltages eV=2(${\mathrm{\ensuremath{\Delta}}}_{\mathrm{Nb}}$-${\mathrm{\ensuremath{\Delta}}}_{\mathrm{Al}}$) and eV=2(${\mathrm{\ensuremath{\Delta}}}_{\mathrm{Nb}}$+${\mathrm{\ensuremath{\Delta}}}_{\mathrm{Al}}$) in agreement with measured curves. Calculations of the thermodynamic stability of the nonequilibrium superconducting state indicate the possibility of hysteresis effects around these current steps.

Single-electron tunneling up to room temperature

Ultrasmall (≲5 nm in lateral diameter) double-barrier tunnel junctions have been realized using metal particles (2-5 nm in diameter) sandwiched in between a metallic substrate and the metallic tip of a scanning tunneling microscope (STM). The particles were either grown by e-beam evaporation or prepared using colloid chemistry. Two electrical transport effects, in good agreement with the semi-classical theory of single-electron tunneling, have been found at room temperature: the Coulomb gap and the Coulomb staircase. The interpretation in terms of single-electron tunneling is further supported by liquid helium temperature STM measurements on identical samples.

Quantum well surface-plasmon oscillator

We identify a new type of electromagnetic oscillator which utilizes the negative differential conductivity of a quantum well tunnel junction to drive surface-plasmon oscillations on the outside boundaries of the junction. We calculate the complex propagation constant as a function of frequency for the antisymmetric gap-mode surface plasmon on a state of the art GaAs/AlGaAs double-barrier tunnel junction. The surface-plasmon gain coefficient is found to be positive for frequencies up to 5 GHz. Single-mode oscillation of the surface plasmon at frequencies near 5 GHz is predicted on a tunnel junction with a lateral dimension equal to about 150 μm.