Electron Tunneling Experiments Research Articles

Nonequilibrium theory of tunneling into a localized state in a superconductor

A single static magnetic impurity in a fully-gapped superconductor leads to formation of an intragap quasiparticle bound state. At temperatures much below the superconducting transition, the energy relaxation and spin dephasing of the state are expected to be exponentially suppressed. The presence of such a state can be detected in electron tunneling experiments as a pair of conductance peaks at positive and negative biases. Here we show, that for an arbitrarily weak tunneling strength, the peaks have to be symmetric with respect to the applied bias. This is in contrast to the standard result that the tunneling conductance is proportional to the local (in general particle-hole asymmetric) density of states. The asymmetry can be recovered is one allows for either a finite density of impurity states, or that impurities are coupled to another, non-superconducting, equilibrium bath.

Electron tunneling experiments on La-substituted Kondo-semiconductor CeRhAs

Polycrystalline Ce 1− x La x RhAs is investigated by means of break-junction tunneling. On Ce substituted by La ( x = 0.01 ), a pronounced hump structure is developed at the bias ±0.25 V with a shallow dip on it. The hump emergence is consistent with a drastic reduction in the resistivity. These facts give direct evidence for the appearance of mid-gap states near the Fermi energy by a small La substitution for Ce in CeRhAs.

Phonon behaviors and electronic structures of the filled skutterudite YbyCo4Sb12 compounds: An electron tunneling study

Electron tunneling experiments were performed on YbyCo4Sb12–Al oxide–Al junctions for y=0–0.25 at 4.2 K. In the second derivative tunneling spectrum of CoSb3 compound (y=0), three peaks were observed at around 5, 20, and 33 mV, which are closely related to an optical phonon mode with a rigid rectangle, Sb–Sb bond bending and bond stretching, and a large Co atomic motion, respectively. Appearance of the strong peak at 7 mV observed in Yb-filled samples corresponds to a rattled phonon mode of Yb ions. The peak energy due to the Sb–Sb bonds is unchanged, whereas the one due to Co motions shifts to lower with increasing Yb concentration. This fact indicates that the filled Yb ions strongly interact to the host framework Co atoms, which were clearly observed in the change of tunneling conductance.

The Effect of Magnetic Atom Substitution on the Tunneling Conductance in Skutterudite CoSb3 Semiconductors

Electron tunneling experiments were performed on n-Co 0.9 Ni 0.1 Sb 3 -Al oxide-Al junctions measured at 4.2 K. A V-shape tunneling conductance curve with small zero bias offset is observed, which can be associated with disorder states. Comparing with the results on p-Co 0.9 Fe 0.1 Sb 3 samples, it has been clarified that the Fe substitution leads to a stronger three-dimensional disorder state as compared to the case of Ni substitution. Magnetic susceptibility measurements suggest that the observed three-dimensional disorder state is mainly magnetic in nature.

Electron tunneling experiments on skutterudite Co1−xFexSb3 semiconductors

Electron tunneling experiments were performed on p-Co1−xFexSb3–Al-oxide–Al junctions for x=0 and x=0.1 at 4.2 K. A U-shaped tunneling conductance curve obtained for polycrystalline p-CoSb3 clearly shows an energy-band gap of ∼50 meV. For p-Co0.9Fe0.1Sb3 skutterudites, a strong zero-bias conductance anomaly is observed. This anomaly may be due to a structural disorder arising from defects such as vacancies and interstitial Fe atoms.

Shallow electron traps at the 4H–SiC/SiO2 interface

Low-temperature electrical measurements and photon-stimulated electron tunneling experiments reveal the presence of a high density of interface states at around 0.1 eV below the conduction band of 4H–SiC at its interface with thermally grown SiO2. These states, related to defects in the near-interfacial oxide layer, trap a considerable density of electrons from the SiC, and are likely responsible for the severe degradation of the electron mobility observed in the surface channel of 4H–SiC/SiO2 devices. The negative impact of the observed defects can be minimized by using SiC modifications (e.g., 6H, 15R, 3C) with a larger conduction band offset with the oxide than 4H–SiC leading to a largely reduced density of electrons trapped in the oxide.

Inelastic electron tunneling experiments in Bi1−xSbx alloys

Inelastic electron tunneling experiments have been performed in Bi0.94Sb0.06–Al oxide-Al junctions. A strong peak observed at 1.9 mV in the second derivative tunnel spectra is closely related to the estimated effective Debye temperature, besides intervalley acoustic and optical phonon structures. This structure clearly corresponds to the electron–intravalley acoustic phonon coupling.

Mercury−Mercury Tunneling Junctions. 1. Electron Tunneling Across Symmetric and Asymmetric Alkanethiolate Bilayers

Electron tunneling experiments involving Hg−Hg junctions incorporating two alkanethiolate monolayers are described. Formation of a symmetric junction (Hg−SCn−CnS−Hg) is accomplished by bringing in contact two small (3 × 10-3 cm2) mercury drop electrodes in a 5−20% (v/v) hexadecane solution of an alkanethiol. Formation of asymmetric junctions (Hg−SCn−CmS−Hg) and junctions containing n-alkane-3-thiopropanamide bilayers are also described. Tunneling currents in the junctions were measured for voltage biases extending to ±1.5 V. The currents decrease exponentially with the junction thickness yielding the tunneling decay constant, β = 0.89 ± 0.1 per CH2. The decay constant exhibits only a weak dependence on the voltage bias suggesting that electron tunneling follows a through-bond mechanism. Tunneling currents in the n-alkane-3-thiopropanamide bilayer junctions were larger than those in alkanethiolate junctions with the same number of atoms suggesting that introduction of an amide group increases the strength of the electronic coupling through these types of σ-bonded systems.

Coulomb gap in a doped semiconductor near the metal-insulator transition: Tunneling experiment and scaling ansatz

Electron tunneling experiments are used to probe Coulomb correlation effects in the single-particle density-of-states (DOS) of boron-doped silicon crystals near the critical density of the metal-insulator transition (MIT). At low energies, a DOS measurement distinguishes between insulating and metallic samples with densities 10 to 15 % on either side of the MIT. However, at higher energies the DOS of both insulators and metals show a common behavior, increasing roughly as the square-root of energy. The observed characteristics of the DOS can be understood using a classical treatment of Coulomb interactions combined with a phenomenological scaling ansatz to describe the length-scale dependence of the dielectric constant as the MIT is approached from the insulating side.

Inelastic electron tunneling into p-Bi2Te3 crystals

Inelastic electron tunneling experiments have been performed in p-Bi2Te3–Al oxide-Al junctions. Strong peaks due to electron-acoustic phonon coupling are observed in the second derivative tunnel spectrum. These phonon structures are closely related to the effective Debye temperature. This gives direct evidence that the electron-acoustic phonon scattering plays a dominant role in p-Bi2Te3.

Tunneling study of inhomogeneous Sb films

Electron tunneling experiments have been performed to study the oneparticle density of states in inhomogeneous Sb films. A zero bias anomaly can be well described in terms of the theory of weakly disordered metals. On the other hand, the background conductance at higher voltages exhibits roughly a parabolic shape, which is typical in a metal-oxide-metal tunnel junction. The feature of the band structure in the semimetal disappears completely. This result indicates that an inhomogeneous Sb film constitutes a metallic, degenerate electron gas system at the junction interface. A metastable metallic phase may be stabilized due to a higher disorder.

Attenuation of microwave filters for single-electron tunneling experiments

Two types of microwave filters, metallic powder filters and a filter using a resistive coaxial cable, were tested. Attenuation of the metallic powder filters using copper or stainless steel (SUS304L) was measured at 300 K, 77 K, and 4.2 K, and it was found that SUS304L powder of nominal 30 /spl mu/m grain size gives best result as the microwave filter for single electron tunneling (SET) experiments. Attenuation of a coaxial filter, or Thermocoax with SMA connectors attached at both ends is larger at low temperatures than that at room temperatures, and the temperature dependence of the attenuation does not agree with Zorin's model. The filter arrangement in a cryostat designed for SET experiments, is also reported.

Indication for an antiferromagnetic exchange interaction in high-Tc superconductors.

Recently observed dip structures in angle-resolved photoemission spectroscopy and electron-tunneling experiments on high-${T}_{c}$ superconductors are investigated choosing an antiferromagnetic exchange interaction with a resonant peak structure in frequency as observed in recent neutron-scattering experiments. In our calculations we see similar structures in the spectral weight, in the density of states, and in the $c$-axis optical conductivity, which provides a possible explanation for this unusual behavior.

Electron tunneling in oxide superconductors

Electron tunneling experiments in very well-characterized polycrystalline samples of Ba[sub 1[minus]x]K[sub x]BiO[sub 3] and Bi[sub 2]Sr[sub 2]Ca[sub 2]Cu[sub 3]O[sub 10[minus][delta]] show very clear features of the energy gap. The energy gap 2[Delta] and the ratio 2[Delta]/K[sub B]T[sub c] were determined. In the Ba[sub 1[minus]x]K[sub x]BiO[sub 3] compound we also study the behavior of the tunneling conductance in the normal state. 9 refs., 6 figs.

Electron tunneling experiments near the metal-insulator transition

Tunneling measurements of the electronic density of states N( E) on amorphous Ge 1−xCr x and Si 1−xCr x near the metal-insulator transition are reported. N( E) shows a square-root law in the energy dependence on the metallic side. The results are interpreted in terms of the electron-electron interaction in disordered materials. The range of the E 1/2 dependence is the same for all chromium concentrations in each material. An E 1/2 dependence can be seen for higher energies on the insulating side as well. Temperature dependent contributions to N( E) should be relevant only for energies smaller than 5 k BT .

Many-body effects in quantum dots under magnetic fields

A theoretical analysis of single electron charging processes in a quantum dot with under a perpendicular magnetic field is presented in a broad range of the number of electrons. Exact diagonalizations and an unrestricted Hartree-Fock approximation have been used in the low and intermediate field regime to study the addition spectra and the ω → 0 spectral weight function, which are the magnitudes directly connected with single electron capacitance spectroscopy and tunneling experiments. In the high field regime the quantum dot undergoes a set of integer charge edge excitations. The results of an exact calculation suggest a simple approach for the study of these excitations.

Superconductivity inBa1−xKxBiO3

Superconductivity in ${\mathrm{Ba}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{K}}_{\mathit{x}}$${\mathrm{BiO}}_{3}$ is studied within the framework of Eliashberg theory using a model of the electron-phonon interaction, ${\mathrm{\ensuremath{\alpha}}}^{2}$F(\ensuremath{\omega}). The model is based upon molecular-dynamics calculations of the phonon density of states, which are in good agreement with inelastic-neutron-scattering experiments. The function \ensuremath{\alpha}(\ensuremath{\omega}) is constructed using information from electron-tunneling experiments. Our model is constructed with the premise that the electron-phonon coupling constant \ensuremath{\lambda}\ensuremath{\approxeq}1 and that strong electron-phonon coupling exists for high-energy (30--60 meV) phonon modes. Superconducting properties of the material at zero and finite temperatures are calculated and compared with experiments.

Analogies between electron and photon tunneling: A proposed experiment to measure photon tunneling times

The phenomenon of tunneling is a well-known fundamental consequence of quantum mechanics. All particles can in principle tunnel. In particular, both electrons and photons can tunnel through classically forbidden regions of space known as “barriers”. However, there have been numerous controversies over how long it takes a particle to cross a barrier. Exploiting an analogy between electrons and photons, we suggest an experiment to infer the characteristics of an electron's barrier-traversal time by measuring the time it takes a photon to traverse a similar barrier. Electron tunneling experiments are in general much more difficult to perform than analogous optical ones. With an optical technique one can construct optical barriers on the scale of microns, in contrast to theångstrom-scale barriers required for electron tunneling. Our experiment may help settle the controversies over tunneling times. By means of a newly developed quantum optical technique, we should be able to measure the tunneling times of individual photons with sub-picosecond resolution. In our experiments we are using a two-photon light source, in which a pair of tightly correlated photons is generated by the process of spontaneous parametric down-conversion. Hong, Ou and Mandel have already achieved a sub-picosecond comparison between the arrival times of two such photons at a beam splitter placed at the intersection of their paths. In our geometry, one member of the photon pair tunnels through a barrier, while the other does not. Then coincidence detection of this photon pair constitutes the detection of an individual tunneling event. The particle aspect of photon tunneling can thus be clearly observed. We propose to use the Hong-Ou-Mandel technique to measure the tunneling time. We have chosen for our tunneling barrier two glass prisms placed in close proximity, utilizing the phenomenon of frustrated total internal reflection.

Electron tunneling experiments on amorphous TixTe1− x films

Systematic electron tunneling measurements were performed on Al/Al-oxide/ oxide Tl x Te 1− x junctions. Amorphous Tl x Te 1− x films of different concentrations x were prepared by quench-condensation at 4.2 K. The metal-insulator trasition is observed at a critical concentration x c ≈ 0.66. The tunnel conductivity is interpreted in terms of the theory of electron-electron interaction in disordered systems.

Tunneling spectroscopy in Bi2Sr2CaCu2O8: Is the energy gap anisotropic?

Electron tunneling experiments have been performed on single crystals of the superconductor Bi 2 Sr 2 CaCu 2 O 8 using point contact and break junction configurations. For tunnel current injected in the the a-b plane and c-axis directions, the respective current-voltage characteristics suggest zero-temperature energy gap values of Δ ab ≈ 23meV and Δ c ≈ 12meV, corresponding to 2 Δ ab /k B T c =6.2±0.3 and 2 Δ c /k B T c =3.3±0.3. The observed gap temperature dependence is in good agreement with BCS (mean field) behavior.