Electron Tunneling Measurements Research Articles

Tunneling spectroscopic signatures of charge doping and associated Mott transition in α−RuCl3 in proximity to graphite

The layered Mott insulator $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{RuC}{\mathrm{l}}_{3}$ has been extensively studied as a potential Kitaev quantum spin liquid candidate. Here, by constructing heterostructures with graphite, we employed electron tunneling measurements on few-layer $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{RuC}{\mathrm{l}}_{3}$ using scanning tunneling microscopy/spectroscopy. Characteristic tunneling spectra were detected on $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{RuC}{\mathrm{l}}_{3}$ layers in proximity to graphite. In the single-layer $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{RuC}{\mathrm{l}}_{3}$ in direct contact with graphite, distinct states in the Mott gap regime were observed. The in-gap states are demonstrated to be closely related to the electron orbitals in $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{RuC}{\mathrm{l}}_{3}$ and graphite, and to be sensitive to interfacial coupling, where a hybridization at the heterointerface is hypothesized. The in-gap states are also thought of as a charge reservoir for weakly doping the $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{RuC}{\mathrm{l}}_{3}$ upper layers. It demonstrated that the weak doping effect causes a considerable decrease in the Mott gap within the upper layers, suggesting that an unconventional Mott transition is occurring in these layers. The results show that the heterostructure comprising $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{RuC}{\mathrm{l}}_{3}$ and graphite is a good platform for investigating the doping physics in $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{RuC}{\mathrm{l}}_{3}$. Therefore, tunneling into such a doped system is a useful probe for studying otherwise insulating spin liquid candidates.

Microfabricated Nano-Gap Tunneling Current Zika Virus Sensors With Single Virus Detection Capabilities

Electron tunneling through thin ( ~ 1-50 nm), insulating materials can provide information regarding their band structure, density of states, surface and bulk defect densities and their energy levels and distributions. While electron tunneling has been used in thin inorganic insulating materials for nearly 7 decades, its applications in organic and biological materials, with exceptions of deoxyribonucleic acid and a few other materials, are not extensively explored. Here we report room temperature electron tunneling studies to differentiate between aptamers designed to attach to the capsid proteins on Zika viruses and the aptamer-Zika complexes. We show that the electron tunneling induced by the current versus voltage (I-V) measurements through these materials contain information that enable identification of the Zika virus without any need for DNA sequencing or additional molecular labels. The electron tunneling measurements were carried out using a conducting atomic force microscope, a home-made vertical tunneling junction and an electromigration-induced nanogap in a microfabricated 1 μm -wide gold line on silicon nitride bridges. The current through aptamers and aptamer-Zika complexes all have a threshold voltage just before the exponential increase in the junction current. Below this voltage, the leakage currents are different for the aptamer and aptamer-Zika samples. The tunnel nanogap device is suitable for detecting viruses that are nanometer-scale biomaterials.

Spectroscopic studies of atomic defects and bandgap renormalization in semiconducting monolayer transition metal dichalcogenides

Assessing atomic defect states and their ramifications on the electronic properties of two-dimensional van der Waals semiconducting transition metal dichalcogenides (SC-TMDs) is the primary task to expedite multi-disciplinary efforts in the promotion of next-generation electrical and optical device applications utilizing these low-dimensional materials. Here, with electron tunneling and optical spectroscopy measurements with density functional theory, we spectroscopically locate the mid-gap states from chalcogen-atom vacancies in four representative monolayer SC-TMDs—WS2, MoS2, WSe2, and MoSe2—, and carefully analyze the similarities and dissimilarities of the atomic defects in four distinctive materials regarding the physical origins of the missing chalcogen atoms and the implications to SC-mTMD properties. In addition, we address both quasiparticle and optical energy gaps of the SC-mTMD films and find out many-body interactions significantly enlarge the quasiparticle energy gaps and excitonic binding energies, when the semiconducting monolayers are encapsulated by non-interacting hexagonal boron nitride layers.

Observation of the 4\u03c0-periodic Josephson effect in indium arsenide nanowires

Quantum computation by non-Abelian Majorana zero modes (MZMs) offers an approach to achieve fault tolerance by encoding quantum information in the non-local charge parity states of semiconductor nanowire networks in the topological superconductor regime. Thus far, experimental studies of MZMs chiefly relied on single electron tunneling measurements, which lead to the decoherence of the quantum information stored in the MZM. As a next step towards topological quantum computation, charge parity conserving experiments based on the Josephson effect are required, which can also help exclude suggested non-topological origins of the zero bias conductance anomaly. Here we report the direct measurement of the Josephson radiation frequency in indium arsenide nanowires with epitaxial aluminium shells. We observe the 4π-periodic Josephson effect above a magnetic field of ≈200 mT, consistent with the estimated and measured topological phase transition of similar devices.

Electron tunneling measurements in atomic scale gap filled with liquid 4He below 4.2K

We report the tunneling spectroscopy investigation in an atomic scale gap filled with liquid 4He using mechanically controllable break junction (MCBJ) technique. In order to assure the filling of liquid 4He into the gap, we construct a cryostat with an inner chamber for the tunneling spectroscopy inside the vacuum jacket of the liquid 4He bath. MCBJ apparatus is installed in the inner chamber with a flexible bellows. After filling inner chamber with liquid 4He below 4.2 K, Au electrical electrodes were stretched by the mechanical force generated by a piezo device. We observed the increase of the tunnel conductance through liquid 4He compared to that in the vacuum environment.

Electron-tunneling measurements of low-Tcsingle-layer Bi2+xSr2−yCuO6+δ: Evidence for a scaling disparity between superconducting and pseudogap states

We experimentally study transport and intrinsic tunneling characteristics of a single-layer cuprate Bi(2+x)Sr(2-y)CuO(6+delta) with a low superconducting critical temperature Tc < 4 K. It is observed that the superconducting energy, critical field and fluctuation temperature range are scaling down with Tc, while the corresponding pseudogap characteristics have the same order of magnitude as for high-Tc cuprates with 20 to 30 times higher Tc. The observed disparity of the superconducting and pseudogap scales clearly reveals their different origins.

Microwave coupled electron tunneling measurement of Co nanoparticles

We study electron tunneling through Co nanoparticles in the presence of repeated microwave pulses at 4.2 K. While individual pulses are too weak to affect the magnetic switching field, repeated microwave pulses start to reduce the magnetic switching field at 10 μs spacing. We use I-V curve as a thermometer to show that the microwave pulses do not heat the sample, showing that magnetization in Co nanoparticles is directly excited by microwave pulses, and the relaxation time of the excitation energy is in the range of microsecond.

Built-in and induced polarization across LaAlO3/SrTiO3 heterojunctions

Ionic crystals terminated at oppositely charged polar surfaces are inherently unstable and expected to undergo surface reconstructions to maintain electrostatic stability. Essentially, an electric field that arises between oppositely charged atomic planes gives rise to a built-in potential that diverges with thickness. In ultra thin film form however the polar crystals are expected to remain stable without necessitating surface reconstructions, yet the built-in potential has eluded observation. Here we present evidence of a built-in potential across polar \lao ~thin films grown on \sto ~substrates, a system well known for the electron gas that forms at the interface. By performing electron tunneling measurements between the electron gas and a metallic gate on \lao ~we measure a built-in electric field across \lao ~of 93 meV/\AA. Additionally, capacitance measurements reveal the presence of an induced dipole moment near the interface in \sto, illuminating a unique property of \sto ~substrates. We forsee use of the ionic built-in potential as an additional tuning parameter in both existing and novel device architectures, especially as atomic control of oxide interfaces gains widespread momentum.

Electronic characterization of individual monolayer protected Au clusters by single electrontunneling force spectroscopy

Gold monolayer protected clusters (MPCs) exhibit strong quantum confinement effects andsize dependent electronic, optical and chemical properties. Chemical tuning ofthese properties can be achieved by established synthesis methods, providing anexcellent system for the study of the relationship between chemical and electronicstructure. In this paper, the first electronic spectra of individual Au MPCs (Au25) acquired by single electron tunneling force spectroscopy on non-conducting silicondioxide surfaces are reported. A HOMO–LUMO (highest occupied molecularorbital–lowest unoccupied molecular orbital) energy gap is observed in theAu25 spectra. Hysteretic charging of the particles is also observed while obtaining the energyspectra. The new single electron tunneling measurement methodology is described. Amodel explaining the measurements supports the existence of mid-HOMO–LUMO gapdefect states.

Valence-band electron-tunneling measurement of the gate work function: Application to the high-κ/polycrystalline-silicon interface

A technique is demonstrated to measure the band alignment between the silicon substrate and the gate electrode using the valence-band electron tunneling (VBET). When an n-channel metal-oxide-semiconductor field-effect transistor is biased in inversion the valence-band electron from the Si substrate can tunnel into the gate [A. Shanware, J. Shiely, H. Massoud, E. Vogel, K. Henson, A. Srivastava, C. Osburn, J. Hauser, and J. Wortman, Tech. Dig.-Int. Electron Devices Meet.1999, 815], depending on the overlapping of the density of states in the Si valence band and the gate. This technique is suitable to measure the band alignment between the silicon substrate and the gate electrode with any given gate dielectric, provided that both the gate and substrate leakages are dominated by direct tunneling. This technique has been applied to study the SiO2/polycrystalline-silicon (poly-Si) interface behavior in the presence of submonolayer traces of HfO2. The general applicability of VBET to arbitrary gate stacks is finally demonstrated with the HfSiON/poly-Si case.

Unusual oscillation in tunneling magnetoresistance near a quantum critical point in Sr3Ru2O7.

We performed single-electron tunneling measurements on bilayer ruthenate Sr3Ru2O7. We observe an unusual oscillation in tunneling magnetoresistance near the metamagnetic quantum phase transition. The characteristic features of this oscillation suggest that it is unrelated to traditional quantum oscillations caused by orbit quantization. In addition, tunneling spectra are found to change sharply in the low bias voltage range of V<2 mV near the transition field. These observations reveal that the Fermi surface of Sr3Ru2O7 changes in a surprising way as the system undergoes strong critical fluctuations.

Vibronic effects in off-resonant molecular wire conduction

A model for the calculation of the inelastic contribution to the low-bias electron transport in molecular junctions is presented. It is an extension to the inelastic case of the Green’s function approach to the calculation of the conduction of such systems. The model is suited for the calculation in the off-resonance regime (where molecular levels are far from the Fermi energy) and in the low bias limit, a typical situation encountered in inelastic electron tunneling measurements. The presentation of a general model is followed by the introduction of several approximations that make the calculation feasible for many systems of interest. Ab initio calculations of the vibronic coupling that leads to inelastic contribution to the conductance are performed for several molecules (butadiene, biphenyl, dipyrrole, and dithiophene), representative of possible molecular wires. The role of inelastic conduction is then quantified without empirical parameters and the vibrational modes that dominate the process are identified. The situations where the inelastic mechanism is particularly relevant are considered. The limits of this approach for the resonant case are also discussed.

Multiple-gap structure of the binary superconductor MgB2

Electron-tunneling measurements on the superconductor MgB2 with Tc=39 K have been carried out using a break-junction technique. We have observed the double-gap structures with several combinations of the gap sizes. The largest Δ=9 meV gap can be observed as a single gap, which gives very strong-coupling ratio 2Δ/kBTc=5.3–5.7.

Superconducting gap in polycrystalline MgB 2 studied by electron tunneling

Electron-tunneling measurements on the new superconductor MgB 2 with T c=39 K have been carried out using a break-junction technique. The measurements reveal the largest gap value of 2 Δ=17–19 meV at 4.2 K. This value leads to the ratio 2 Δ/ k B T c=5–5.6, indicating the strong-coupling superconductivity. We have also observed the reproducible gap values of 2 Δ≅4–4.5 meV and 7–9 meV. The smallest gap exists even beyond 30 K. We will discuss the implication of the data.

Energy gap in the Kondo semiconductor CeRhAs

Electron tunneling measurements have been carried out on the orthorhombic Kondo semiconductor CeRhAs using break junction. The measurements reveal the gap size 2Δ (4K)=0.5±0.1eV. This compound exhibits the susceptibility maximum at Tχ=510K, below which the gap is believed to open. The gap ratio 2Δ/kBTχ=12±3 is consistent with that for the isostructural CeNiSn and CeRhSb, thereby indicating the same mechanism of the gap creation among the compounds.

Is the chiral Luttinger liquid exponent universal?

We present experimental evidence from electron tunneling measurements that the chiral Luttinger liquid power-law exponent, α, for tunneling into the fractional quantum Hall edge deviates substantially from the universal behavior predicted by theory. Our results suggest that the existing standard analyses based on effective Chern–Simon field theories deserve careful reexamination when applied to the dynamics at the Hall fluid edge. To cite this article: A.M. Chang, C. R. Physique 3 (2002) 677–684.

Electron energy levels in superconducting and magnetic nanoparticles

We describe electron-tunneling measurements of the spectrum of discrete energy levels for electrons in metal nanoparticles, for metals with sufficiently strong electron interactions to form superconducting or magnetic states. For aluminum nanoparticles, superconducting pairing interactions produce an energy gap for tunneling, which affects even- and odd-electron spectra differently. As a function of an applied magnetic field, we observe that the correlated electron ground state is disrupted by breaking one Cooper pair at a time. Energy levels in ferromagnetic cobalt particles exhibit effects of strong exchange and magnetic anisotropy forces, with a non-linear, hysteretic dependence on an applied magnetic field.

Coulomb gap: how a metal film becomes an insulator

Electron tunneling measurements of the density of states (DOS) in ultrathin Be films reveal that a correlation gap mediates their insulating behavior. In films with sheet resistance R<5000 Omega the correlation singularity appears as the usual perturbative ln(V) zero bias anomaly (ZBA) in the DOS. As R is increased further, however, the ZBA grows and begins to dominate the DOS spectrum. This evolution continues until a nonperturbative |V| Efros-Shklovskii Coulomb gap spectrum finally emerges in the highest R films. Transport measurements of films which display this gap are well described by a universal variable range hopping law R(T) = (h/2e(2))exp(T0/T)(1/2).

Consistent refractive index parameters for ultrathin SiO2 films

Refined values are obtained in the present study for the refractive index parameter (n) for ultrathin SiO2 films (less than 8 nm in film thickness, Lox) and summarized as n=2.139−8.991×10−2×Lox+1.872×10−3×Lox2. This formula was obtained starting from variable angle of incidence ellipsometry data on ultrathin SiO2 films, and a previously derived refractive index parameter formula that was based on independently measured thickness’ using Fowler–Nordheim electron tunneling measurements. The new n values are useful for extracting more accurate ultrathin SiO2 film thicknesses from ellipsometry measurements.

Structural studies on silver cluster films deposited on softened PVP substrates

The deposition of metal cluster films on softenable polymer substrates is known to lead to subsurface particulate structure formation for many metal/polymer combinations. The nature of the subsurface film structure is critically dependent on deposition conditions such as polymer substrate temperature, film deposition rate, etc. and the influence of these parameters is now fairly well understood. In this paper, we show that the metal–polymer interaction is a very important parameter which can be used to control the particle size as well as the inter-cluster separation through an investigation of silver cluster films deposited on softened poly(2-vinylpyridine), PVP. The increased wettability of silver in the PVP matrix leads to the formation of relatively small islands separated by small gaps which is in contrast to silver films deposited on polystyrene which yielded highly agglomerated structures with large inter-island separations. The silver/softened PVP film structure has been characterized by scanning electron microscopy (SEM). Further evidence for significant overlap of the electronic properties of the clusters was obtained from electron tunneling measurements as well as by optical absorption spectroscopy. The depth of the silver clusters below the surface was estimated from angle dependent X-ray photoemission spectroscopy (XPS) to be ≈5 nm. The ability to control the particle size as well as to tune the interaction between clusters by varying the metal wettability in the polymer matrix is an aspect of the work which shows promise for further development.