Point-contact Junctions Research Articles

Tunneling Resistances Associated with Classical-to-Quantum Fluctuations in Single Point-Contact Junction

Tunneling Resistances Associated with Classical-to-Quantum Fluctuations in Single Point-Contact Junction

Non-Markovian dynamics of a nanomechanical resonator measured by a quantum point contact

We study the dynamics of a nanomechanical resonator (NMR) subject to a measurement by a low-transparency quantum point contact (QPC) or tunnel junction in the non-Markovian domain. We derive the non-Markovian number-resolved (conditional) and unconditional master equations valid to second order in the tunneling Hamiltonian without making the rotating-wave approximation and the Markovian approximation, generally made for systems in quantum optics. Our non-Markovian master equation reduces, in appropriate limits, to various Markovian versions of master equations in the literature. We find considerable difference in dynamics between the non-Markovian case and its Markovian counterparts. We also calculate the time-dependent transport current through the QPC, which contains information about the measured NMR system. We find an extra transient current term proportional to the expectation value of the symmetrized product of the position and momentum operators of the NMR. This extra current term, with a coefficient coming from the combination of the imaginary parts of the QPC reservoir correlation functions, has a substantial contribution to the total transient current in the non-Markovian case, but was generally ignored in the studies of the same problem in the literature. Considering the contribution of this extra term, we show that a significant qualitative and quantitative difference in the total transient current between the non-Markovian and the Markovian wideband-limit cases can be observed. Thus, it may serve as a witness or signature of the non-Markovian features in the coupled NMR-QPC system.

Nanomembrane-Based Mesoscopic Superconducting Hybrid Junctions

A new method for combining top-down and bottom-up approaches to create superconductor-normal metal-superconductor niobium-based Josephson junctions is presented. Using a rolled-up semiconductor nanomembrane as scaffolding, we are able to create mesoscopic gold filament proximity junctions. These are created by electromigration of gold filaments after inducing an electric field mediated breakdown in the semiconductor nanomembrane, which can generate nanometer sized structures merely using conventional optical lithography techniques. We find that the created point contact junctions exhibit large critical currents of a few milliamps at 4.2 K and an I(c)R(n) product placing their characteristic frequency in the terahertz region. These nanometer-sized filament devices can be further optimized and integrated on a chip for their use in superconductor hybrid electronics circuits.

Point-contact spectroscopic studies on normal and superconductingAFe2As2-type iron pnictide single crystals

Point-contact Andreev reflection spectroscopy is applied to investigatethe gap structure in iron pnictide single-crystal superconductors of theAFe2As2 (A = Ba, Sr) family (‘Fe-122’). The observed point-contact junction conductance curves,G(V), can be divided into two categories: one where Andreev reflection is present for both(Ba0.6K0.4)Fe2As2 andBa(Fe0.9Co0.1)2As2, and theother with a V2/3 background conductance universally observed, extending even up to 100 meV forSr0.6Na0.4Fe2As2 andSr(Fe0.9Co0.1)2As2.The latter is also observed in point-contact junctions on the nonsuperconducting parent compoundBaFe2As2 andsuperconducting (Ba0.6K0.4)Fe2As2 crystals. Mesoscopic phase-separated coexistence of magnetic and superconducting orders isconsidered to explain distinct behaviors in the superconducting samples. ForBa0.6K0.4Fe2As2, double peaks due to Andreev reflection with a strongly slopingbackground are frequently observed for point contacts on freshly cleavedc-axis surfaces. If normalized using a background baseline and analyzedusing the Blonder–Tinkham–Klapwijk model, the data show a gap size of ∼ 3.0–4.0 meVwith 2Δ0/kBTc ∼ 2.0–2.6, consistent with the smaller gap size reported for the LnFeAsO family (‘Fe-1111’). For theBa(Fe0.9Co0.1)2As2, theG(V) curves typically display a zero-bias conductance peak.

Studies of resistance switching effects in metal/YBa 2Cu 3O 7− x interface junctions

Current–voltage characteristics of planar junctions formed by an epitaxial c-axis oriented YBa 2Cu 3O 7− x thin film micro-bridge and Ag counter-electrode were measured in the temperature range from 4.2 K to 300 K. A hysteretic behavior related to switching of the junction resistance from a high-resistive to a low-resistive state and vice-versa was observed and analyzed in terms of the maximal current bias and temperature dependence. The same effects were observed on a sub-micrometer scale YBa 2Cu 3O 7− x thin film–PtIr point contact junctions using Scanning Tunneling Microscope. These phenomena are discussed within a diffusion model, describing an oxygen vacancy drift in YBa 2Cu 3O 7− x films in the nano-scale vicinity of the junction interface under applied electrical fields.

Study of the pairing symmetry in the electron-doped cuprate [formula omitted] by tunneling spectroscopy

We performed Point Contact Andreev Reflection Spectroscopy experiments on Pr 1 - x LaCe x CuO 4 - y crystals. The variety of the measured spectra are all explained in terms of a modified BTK model considering a d-wave symmetry of the superconducting order parameter. We give an estimation of the energy gap Δ = ( 3.5 ± 0.2 ) meV and of the ratio 2 Δ / k B T C ≃ 3.6 ± 0.3 . We measured the temperature evolution of the dI / dV vs. V characteristics and we extracted a conventional BCS temperature dependence of the superconducting energy gap. Finally, in order to explain the conductance spectra showing higher voltage gap-like features we consider a model in which the formation of a Josephson junction in series with the Point Contact junction is realized due to the tip pressure.

A tunneling spectroscopy study of the pairing symmetry in the electron-dopedPr1−xLaCexCuO4−y

We have performed scanning tunneling spectroscopy and point contactspectroscopy measurements on the electron-doped superconductorPr1−xLaCexCuO4−y (x = 0.12, K). We address the question of the symmetry of the order parameter andof the amplitude of the energy gap. We compare three possible scenarios,i.e. isotropic s-wave, ‘anisotropic’ s-wave, and d-wave. Evidence for a d-wavesymmetry of the order parameter is given. From the temperature evolution of thedI/dV versusV characteristics we extract a BCS-like temperature dependence of the superconducting energy gapΔ. Despitethe variety of measured spectra we give a consistent explanation for the whole set of data, indicatingΔ = (3.6 ± 0.2) meV and a ratio . In particular, point contact characteristics showing gap-like features at higher voltageshave been interpreted by considering the formation of an intergrain Josephson junction inseries with the point contact junction. Further confirmation of the correctness of themodel is given by the behavior of the critical current of the intergrain Josephsonjunction versus temperature which follows the Ambegaokar–Baratoff behavior.

πJunction to Probe Antiphases-Wave Pairing in Iron Pnictide Superconductors

Josephson junctions between a FeAs-based superconductor with antiphase s-wave pairing and a conventional s-wave superconductor are studied. The translational invariance in a planar junction between a single crystal pnictide and an aluminum metal greatly enhances the relative weight of electron pockets in the pnictide to the critical current. In a wide doping region of the pnictide, a planar and a point contact junction have opposite phases, which can be used to design a trijunction ring with pi phase to probe the antiphase pairing.

Tuning Nonlinear Charge Transport between Integer and Fractional Quantum Hall States

Controllable point junctions between different quantum Hall phases are a necessary building block for the development of mesoscopic circuits based on fractionally charged quasiparticles. We demonstrate how particle-hole duality can be exploited to realize such point-contact junctions. We show an implementation for the case of two quantum Hall liquids at filling factors nu=1 and nu*<or=1 in which both the fractional filling nu* and the coupling strength can be finely and independently tuned. A peculiar crossover from insulating to conducting behavior as nu* goes from 1/3 to 1 is observed. These results highlight the key role played on interedge tunneling by local charge depletion at the point contact.

Point-contact Andreev reflection tunneling spectroscopic (PCARTS) study of the superconducting gap structure in LuNi2B2C

We employ the point-contact Andreev reflection tunneling spectroscopy (PCARTS) technique in two sets of LuNi2B2C (Tc ∼16.0 K) single crystals along three different crystallographic orientations as a function of temperature. For one set of samples, our data taken on the [001], [110] and [100] faces show gap values at ∼2 K of ∼2.4 meV, 2.6 meV, and 2.3 meV, respectively, as analyzed by the one-gap Blonder-Tinkham-Klapwijk model. Another set of samples gives ∼2.5 meV, 2.8 meV, and 2.7 mev, respectively. Simulated gap values as a function of temperature follow the BCS-like temperature dependence in the weak-coupling limit. The small gap anisotropy observed in both sets of samples is explained by a model assuming an anisotropic s-wave with point-nodes for the gap structure and a large tunneling cone at the interface for the point-contact junctions.

Point-contact spectroscopy of competing/coexisting orders in Cd-doped CeCoIn5

Differential conductance spectra are taken from metallic point-contact junctions on Cd-doped CeCoIn5. For (100) junctions with nominal 10% doping, we observe signatures for subsequent antiferromagnetic and superconducting transitions in qualitative agreement with bulk measurements. In the superconducting state, two conductance channels compete, both with enhanced subgap conductance, one due to superconductivity and the other antiferromagnetism. The conductance data exhibit variances for different doping levels and crystallographic orientations and junction to junction. This issue will be addressed further in terms of surface inhomogeneity or intrinsic nonuniform phase formation.

On the origin of the conductance asymmetry in CeMIn5 (M=Co, Rh, Ir)

Asymmetric differential conductance has been frequently observed in heavy fermion point-contact junctions. We report such data obtained from the Ce-based 1–1–5 compounds CeMIn5 (M=Co, Rh, Ir). Apart from characteristics due to superconductivity or antiferromagnetism, a striking common feature is an asymmetry in the background conductance, which shows nontrivial temperature and voltage dependencies. These behaviors cannot be explained by the local heating model combined with large Seebeck effect in heavy fermions. We propose that a Fano-like interference may cause the asymmetry. The interference can occur between two conductance channels, one into the conduction band and the other into the heavy electron band formed by the hybridization of conduction electrons with localized f-electrons.

Point-contact Andreev reflection spectroscopy of heavy-fermion-metal/superconductor junctions

Our previous point-contact Andreev reflection studies of the heavy-fermion superconductor CeCoIn 5 using Au tips have shown two clear features: reduced Andreev signal and asymmetric background conductance. To explore their physical origins, we have extended our measurements to point-contact junctions between single crystalline heavy-fermion metals and superconducting Nb tips. Differential conductance spectra are taken on junctions with three heavy-fermion metals, CeCoIn 5, CeRhIn 5, and YbAl 3, each with different electron mass. In contrast with Au/CeCoIn 5 junctions, Andreev signal is not reduced and no dependence on effective mass is observed. A possible explanation based on a two-fluid picture for heavy fermions is proposed.

Conductance characteristics between a normal metal and a two-band superconductor carrying a supercurrent

The conductance is calculated for a point contact junction between a normal metal and a two-band superconductor with a supercurrent applied parallel to the junction interface. Predictions are made for the case of $\mathrm{Mg}{\mathrm{B}}_{2}$, which would allow for the determination of the two energy scales and interband coupling. In addition, the generic features of two-band superconductivity in this configuration are studied and contrasted with the one-band case. It is found that two peaks are possible in the zero bias conductance as a function of superfluid momentum ${q}_{s}$ for the case of two-band superconductivity, whereas only one could occur in the one-band case. We analyze the case of highly decoupled bands to examine further the potential signatures of interband coupling.

Electrical and optical properties of point-contacted a-Si:H/c-Si heterojunction solar cells with patterned SiO 2 at the interface

We report on the electrical and optical characteristics of a-Si:H/c-Si heterojunction solar cells with point-contact junction via patterned SiO 2 layer at the interface. The new structure showed improved electrical properties, having a smaller leakage current and a larger shunt resistance. The electrical conduction of the point-contacted samples followed the diffusion dominant process with bulk recombination, but the control samples without SiO 2 showed the space-charge region recombination dominant process. The point-contacted samples showed increased internal quantum efficiency in the bulk region, but decreased internal quantum efficiency in the surface region. As the distance between the holes decreased, the point-contacted solar cells showed an improved efficiency with a larger fill-factor but smaller open-circuit voltage and short-circuit current.

Two-band superconductor–normal metal point contact junction conductance with a transverse current

We calculate the conductance of a point contact junction between a normal metal and a two-band superconductor in the presence of a transverse supercurrent. We show that there is a region of parameter space which can be probed that is not accessible to the one-band case. It is proposed that this experiment may be used as a probe of interband coupling.

Point contact spectroscopy on ferromagnetic/superconducting heterostructures

The interplay between superconductivity and ferromagnetism is an interesting subject of experimental and theoretical research. The observation of this effect became possible only recently due to the great progress in the preparation of high-quality hybrid ferromagnet/superconductor (F/S) systems. In this paper we present conductance characteristics of point contact junctions realized between a normal Pt–Ir tip and YBa 2Cu 3O 7− x /La 0.7Ca 0.3MnO 3 (YBCO/LCMO) bilayers. The point contact characteristics show a zero bias conductance peak, as a consequence of the formation of Andreev bound states at the YBCO Fermi level. The temperature evolution of the spectra reveals a depressed zero bias peak and a reduced superconducting energy gap. We had already observed similar effects in the conductance spectra on polycrystalline RuSr 2GdCu 2O 8 pellets. By comparing spectra on both materials, we can thus conclude that these features are explainable in terms of spin polarization effects due to the ferromagnetic layer.

Dislocation‐induced deep levels in ELO InP revealed by point contact photocapacitance measurements

30K-photocapacitance and excitation photocapacitance methods were applied to reveal the dislocation-induced deep levels in coalescent epitaxial lateral overgrowth layers of InP. Point-contact Schottky barrier junctions with small junction areas were formed on dislocated and dislocation-free regions. In the dislocation-free layers, the dominant deep level was located at Ec-1.30 eV, whereas in the dislocated area, dominant deep levels were detected at Ec-0.86 eV and 1.05 eV. A neutralized state was also detected at 0.66 eV+Ev. Excitation photocapacitance results have shown that the defect configuration coordinate diagram of the dislocation-induced deep levels was considered with large Frank-Condon shifts of 0.28 eV. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Dislocation-induced deep electronic states in InP: Photocapacitance measurements

Photocapacitance and excitation photocapacitance methods were applied to reveal the dislocation-induced deep levels in coalescent epitaxial lateral overgrowth layers of InP. Point-contact Schottky barrier junctions with small junction areas were formed on dislocated and dislocation-free regions by using wedge wire-bonding of Au, and photocapacitance measurements were then carried out at $30\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. In the dislocation-free layers, the dominant deep level was located at $1.30\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ below the conduction band, whereas in the dislocated area, dominant deep levels were detected at $0.86\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ $(\ensuremath{\lambda}=1.44\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m})$ and $1.05\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ $(\ensuremath{\lambda}=1.18\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m})$ below the conduction band. A neutralized state was also detected at $0.66\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ above the valence band. From the detailed excitation photocapacitance results, it is shown that the defect configuration coordinate diagram of the dislocation-induced deep levels was considered with large Frank-Condon shifts $({d}_{\mathrm{FC}})$ of $0.28\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. This means that the atomic configurations around the deep levels are highly relaxed, as expected from the structures of the dislocation cores.

Point Contact Spectra on YBa2Cu3O7−x/La0.7Ca0.3MnO3 bilayers

We present conductance characteristics of point contact junctions realized between a normal Pt-Ir tip and YBa2Cu3O7−x/La0.7Ca0.3MnO3 (YBCO/LCMO) bilayers. The point contact characteristics show a zero bias conductance peak, as a consequence of the formation of Andreev bound states at the YBCO Fermi level. The temperature evolution of the spectra reveals a depressed zero bias peak and a reduced superconducting energy gap, both explainable in terms of spin polarization effects due to the LCMO layer.