Andreev reflexion at nanocontacts of superconducting AuIn2 with copper

We investigated superconducting AuIn₂ by point-contact spectroscopy. Andreev reflexion was observed with nearly full excess current at contact sizes down to 0.15 nm. This implies, together with a large superconducting coherence length of 11 μm, almost ideal retroreflectivity. At the onset of lateral confinement of the ballistic electrons the Andreev-derived spectra broaden dramatically due to inelastic scattering at defects or the strain field near the contact. Lateral confinement does not suppress Andreev reflexion.     

Investigating the effects of the interface defects on the gate leakage current in MOSFETs

The effects of the interface defects on the gate leakage current have been numerically modeled. The results demonstrate that the shallow and deep traps have different effects on the dependence relation of the stress-induced leakage current on the oxide electric field in the regime of direct tunneling, whereas both traps keep the same dependence relation in the regime of Fowler-Nordheim tunneling. The results also shows that the stress-induced leakage current will be the largest at a moderate oxide voltage for the electron interface traps but it increases with the decreasing oxide voltage for the hole interface traps. The results illustrate that the stress-induced leakage current strongly depends on the location of the electron interface traps but it weakly depends on the location of the hole interface traps. The increase in the gate leakage current caused by the electron interface traps can predict the increase, then decrease in the stress-induced leakage current, with decreasing oxide thickness, which is observed experimentally. And the electron interface trap level will have a large effect on the peak height and position.     

Persistent currents in superconducting quantum interference devices

Starting from the reduced dynamical model of a two-junction quantum interference device, it shown that a quantum analog of the system can be exhibited. This quantum model extends the well-known properties of the device when its characteristic dimensions are of the order of mesoscopic length scales. By finding eigenvalues of the corresponding Hamiltonian operator, the persistent currents flowing in the ring have been obtained. The resulting quantum analog of the overdamped two-junction quantum interference device can be seen as a supercurrent qubit operating in the limit of negligible capacitance and finite inductance.     

Incomplete Andreev reflection in a clean SFS junction

We study the stationary Josephson effect in a ballistic superconductor/ferromagnet/superconductor junction for arbitrarily large spin polarizations. Due to the exchange interaction in the ferromagnet, the Andreev reflection is incomplete. We describe how this effect modifies the Josephson current in the crossover from a superconductor/normal metal/superconductor junction to a superconductor/half metal/superconductor junction.     

Electric transport character in the La0.7Ce0.3MnO3-SrTiO3-Nb heterojunction

The electric transport character in heterojunction composed of a La_0.7Ce_0.3MnO₃ film and a 0.5 wt% Nb-doped SrTiO₃ substrate (LCEM/STON) is investigated. It is found that the energy band gap (E_g) between LCEM and STON decreases with increasing temperatures. The most striking observation of present work is that there exists a variation of reverse transport mechanism from ionization to tunneling at the temperature of 175 K. We attribute the temperature dependence of reverse transport mechanism to co-work of E_g and the ferromagnetic (FM) insulting phase in the heterojunction. These results are helpful in configuring artificial devices using manganites.     

Subharmonic gap structure in superconducting scanning tunneling microscope junctions

We observe a subharmonic gap structure (SGS) and the Josephson effect in superconducting scanning tunneling microscope junctions with resistances below 100 kΩ. The magnitude of the n=2 SGS is shown to scale with the square of the junction normal state conductance, in agreement with theory. We show by analyzing the Josephson effect in these junctions that the superconducting phase dynamics are strongly affected by thermal fluctuations. We estimate the linewidth of the Josephson oscillations due to phase fluctuations, a quantity that may be important in modern theories of the subgap structure. While phase fluctuations may smear the SGS current onsets, we conclude that the sharpness of these onsets in our data is not limited by fluctuations.     

Excess current in point contacts on two-band superconductor MgB2 in magnetic field

A series of I(V) characteristics and bias-dependent differential resistance dV/dI(V) curves of point contacts made between a single crystal of two-band superconductor MgB₂ and Cu were measured in magnetic fields up to 9 T. The magnetic field dependences of the excess current in the I(V) curves were obtained and analyzed using Koshelev and Golubov's [Phys. Rev. Lett. 90, 177002 (2003)] theoretical results for the mixed state of a dirty two-band superconductor. Introducing a simple model for the excess current in the point contact in the mixed state, our data can be qualitatively described using the theoretical magnetic field dependence of the superconducting order parameter of the σ and π-bands and the averaged electronic density of states in MgB₂.     

Nonequilibrium Kondo effect and Andreev reflection through double quantum dots with spin-flip scattering

The Kondo effect and the Andreev reflection tunneling through a normal (ferromagnet)-double quantum dots-superconductor hybrid system is examined in the low temperature by using the nonequilibrium Green's function technique in combination with the slave-boson mean-field theory. The interplay of the Kondo physics and the Andreev bound state physics can be controlled by varying the interdot hopping strength. The Andreev differential conductance is mainly determined by the competition between Kondo states and Andreev states. The spin-polarization of the ferromagnetic electrode increases the zero-bias Kondo peak. The spin-flip scattering influences the Kondo effect and the Andreev reflection in a nontrivial way. For the ferromagnetic electrode with sufficiently large spin polarization, the negative Andreev differential conductance is found when the spin flip strength in the double quantum dots is sufficiently strong.     

Critical current oscillations in S/F heterostructures in the presence of s–d scattering

Josephson current is investigated in the superconductor/ferromagnet/superconductor junction. It was shown that the current exhibited damping oscillations as a function of the ferromagnetic layer thickness. Previous theories based on Usadel or Eilenberger equations have predicted that the damping length and oscillation period divided by 2π were the same for weak ferromagnetic spacer. This contradicts past experiments. A new calculation of the Josephson current is proposed. The Gorkov equations are solved taking into account s–d scattering in ferromagnet. It is shown that the oscillation period depends only on the exchange magnetic field in the spacer, whereas the damping length is connected to the ferromagnetic mean free path. The concordance with the former experiment allows one to conclude that s–d scattering as a pair-breaking mechanism plays a significant role in the proximity effect in S/F heterostructures.     

Phase Locking and Chaos in a Josephson Junction Array Shunted by a Common Resistance

The dynamics of a Josephson junction array shunted by a common resistance are investigated by using numerical methods. Coexistence of phase locking and chaos is observed in the system when the resistively and capacitively shunted junction model is adopted. The corresponding parameter ranges for phase locking and chaos are presented. When there are three resistively shunted junctions in the array, chaos is found for the first time and the parameter range for chaos is also presented. According to the theory of Chernikov and Schmidt, when there are four or more junctions in the array, the system exhibits chaotic behavior. Our results indicate that the theory of Chernikov and Schmidt is not exactly appropriate.     

Superconducting proximity effect in the presence of strong spin scattering

We report measurements of the four terminal temperature dependent resistance of narrow Au wires implanted with 100 ppm Fe impurities in proximity to superconducting Al films. The wires show an initial decrease in resistance as the temperature is lowered through the superconducting transition of the Al films, but then show an increase in resistance as the temperature is lowered further. In contrast to the case of pure Au wires in contact with a superconducting film, the resistance at the lowest temperatures rises above the normal state resistance. Analysis of the data shows that, in addition to contributions from magnetic scattering and electron-electron interactions, the temperature dependent resistivity shows a substantial contribution from the superconducting proximity effect, which exists even in the presence of strong spin scattering.     

On the conductance oscillations in a mesoscopic proximity system

A fork-shapedN-branch Andreev interferometer is considered and the conductance of the system is calculated as a function of the phase difference in the superconducting order parameters across the interferometer. The results obtained from the quasiclassical, and the scattering-matrix theories are quantitatively compared, which were in good qualitative agreement showing a typical 2π-periodic interference pattern with certain fine structures.     

Rectifying effect in a Josephson junction ratchet array

We have studied numerically a rectifying effect in an underdamped Josephson junction ratchet array driven by dc and ac current. The array consists of both alternating potential barriers and alternating inter-capacitances along the direction of vortex flow. The guide banks of high critical currents are assigned for all the longitudinal junctions to prevent the percolative pattern of vortex motion. In some junction parameters, we see a rectifying effect which indicates a finite value of the time-averaged voltage at zero dc bias. The directional dependence of the vortex motion becomes fairly large when the junction parameters lie in an optimal range which gives rise to a Shapiro step at zero dc bias. Such a rectifying effect survives for small thermal fluctuation, but eventually disappear beyond a certain critical temperature.     

Cooper-Pair Number--Phase Quantization for Inductance Coupling Circuit Including Josephson Junctions

Based on the entangled state representation and a bosonic phase operator formalism, we tackle with Cooperpair number-phase quantization for the inductance coupling circuit including Josephson junctions, and then investigate how Josephson current equations change due to the presence of the coupling inductance and obtain bosonic operator Faraday formula, as well as the corresponding number-phase uncertainty relation.     

Josephson and Andreev transport through quantum dots

In this article, we review the state of the art on the transport properties of quantum dot systems connected to superconducting and normal electrodes. The review is mainly focused on the theoretical achievements, although a summary of the most relevant experimental results is also given. A large part of the discussion is devoted to the single-level Anderson-type models generalized to include superconductivity in the leads, which already contains most of the interesting physical phenomena. Particular attention is paid to the competition between pairing and Kondo correlations, the emergence of π-junction behavior, the interplay of Andreev and resonant tunneling, and the important role of Andreev bound states that characterized the spectral properties of most of these systems. We give technical details on the several different analytical and numerical methods which have been developed for describing these properties. We further discuss the recent theoretical efforts devoted to extend this analysis to more complex situations like multidot, multilevel or multiterminal configurations in which novel phenomena is expected to emerge. These include control of the localized spin states by a Josephson current and also the possibility of creating entangled electron pairs by means of non-local Andreev processes.     

Properties of Josephson junctions in the inhomogeneous magnetic field of a system of ferromagnetic particles

The effect of an array of ferromagnetic nanoparticles on the field-dependent critical current of the short overlap Josephson junction is experimentally studied. Large reversible variations of the maximum critical current are observed depending on the magnetic state of the particles. The pronounced commensurability effects are detected which are proved by the additional peaks of magnetic field induced diffraction pattern.     

Scaling of decay lengths for surface states in the gaps of one-dimensional semi-infinite superlattices

We look at some one-dimensional semi-infinite superlattices with an underlying Hamiltonian that is of the nearest neighbour, tight binding type. A real space rescaling procedure which is exact in one dimension is applied to obtain the location of the subbands. It has been found that these subbands never overlap in 1D, and we interpret this as a band repulsion effect. Relevance in the case of a disordered system where this band repulsion crosses over to the well-known level repulsion is discussed. Then with a proper matching at the boundary we solve for the sets of denumerably infinite number of decaying solutions (the surface states) in the gaps. These types of states have been proposed quite some time ago. We look at detail theirexact analytical solutions in 1D and find that their decay lengths near the band edges diverge as |E–E _b|^–v, wherev=1/2 andE _b is the nearest band edge. The decay lengths and their divergence exponent match extremely well with those obtained from transfer matrix method. Some recent experiments on quantum well structures seem to have observed such states.     

Effects of interactions and disorder in two dimensions

The interplay between Coulomb interactions and randomness has been a long-standing problem in condensed matter physics. Recent thermodynamic and transport experiments have shown that in clean two-dimensional electron systems, strong interactions between carriers lead to Pauli spin susceptibility growing critically at low electron densities. In the immediate vicinity of the metal-insulator transition (MIT), both the resistance and the effective interactions become temperature dependent and exhibit a fan-like spread as the MIT is crossed. A resistance-interaction flow diagram clearly reveals a quantum critical point.     

Effect of Rashba spin-orbit coupling on the spin-polarized transport in ferromagnet/semiconductor double tunnel junctions

Considering the Rashba spin-orbit interaction in the semiconductor, we study theoretically the spin-polarized transport in a two-dimensional ferromagnetic semiconductor double tunnel junctions by a quantum-mechanical approach. It is found that the transmission coefficient shows typical resonant transmission properties and the Rashba spin-orbit coupling has great different influences on the transmission coefficients of electrons with spin-up and down and tunnelling magnetoresistance (TMR). More importantly, the TMR is significantly enhanced by increasing the spin-orbit coupling, which is very useful for the designing of magnetic digital and memory sensor.