Tunneling Conductance in Quantum-Wire/Ferromagnetic-Insulator/d-Wave Superconductor Junction

We have studied the quasiparticle transport in quantum-wire /ferromagnetic-insulator/d wave superconductor Junction (q/FI/d) in the framework of the Blonder-Tinkham-Klapwijk model. We calculate the tunneling conductance in q/FI/d as a function of the bias voltage at zero temperature and finite temperature based on Bogoliubov-de Gennes equations. Different from the case in normal-metal/insulator/d wave superconductor Junctions, the zero-bias conductance peaks vanish for the single-mode case. The tunneling conductance spectra depend on the magnitude of the exchange interaction at the ferromagnetic-insulator.     

Differential conductance of normal metal-insulator-d-wave superconductor junctions with interface roughness

Within the framework of Blonder-Tinkham-Klapwijk (BTK) model and taking into account the interface roughness, we calculate the differential conductance of normal metal-insulator-d-wave superconductor junctions. We find that the tunneling spectrum depends strongly on both the incident angle of electrons and the crystalline axis orientation of the superconductor, and exhibits the zero-bias anomaly under suitable arrangements. Interestingly, even in the absence of the insulating layer, the tunneling spectrum of normal metal-d-wave superconductor junctions differs significantly from that for normal metal-s-wave superconductor due to the interface roughness. Moreover, both the tunnel conductance peak at the energy gap and the zero-bias conductance peak are suppressed by the interface roughness. Our results can explain many experimental measurements on the tunneling spectra of high-T_c superconductors.     

Tunneling density of states of high T(c) superconductors: d-wave BCS model versus SU(2) slave-boson model

Motivated by recent experimental measurements of the tunneling characteristics of high T(c) materials using scanning tunneling spectroscopy, we have calculated the I-V and differential conductance curves in the superconducting state at zero temperature. Comparing BCS-like d-wave pairing and the SU(2) slave-boson approach, we find that the slave-boson model can explain the asymmetric background observed in experiments. The slave-boson model also predicts that the height of the conductance peak relative to the background is proportional to the hole doping concentration x, at least for underdoped samples. We also observe the absence of the van Hove singularity, and comment on possible implications.     

Doping dependent tunneling conductance in SDW ordered copper oxide superconductors

It is observed that doping suppresses the long range anti-ferromagnetic order and induces superconducting phase for a suitable doping. In order to study this effect, we present a model study of the doping dependence of the tunneling conductance in high-T_c systems. The system is described by the Hamiltonian consisting of spin density wave (SDW) and s-wave type superconducting interaction in presence of varying impurity concentrations. The gap equations are calculated by using Green’s functions technique of Zubarev. The gap equations and the chemical potential are solved self-consistently. The imaginary part of the electron Green’s functions shows the quasi-particle density of states which represent the tunneling conductance observed by the scanning tunneling microscopy (STM). We investigate the effect of impurity on the gap equations as well as on the tunneling conductance. The results will be discussed based on the experimental observations.     

Tunneling between two-dimensional electron systems in a strong magnetic field

We calculate the tunnel current between two parallel two-dimensional electron systems in a strong perpendicular magnetic field. We model the strongly correlated electron systems by Wigner crystals, and describe their low-energy dynamics in terms of magnetophonons. The effects of the magnetophonons on the tunneling processes can be described by an exactly solvable independent-boson model. A tunneling electron shakes up magnetophonons, which results in a conductance peak that is displaced away from zero voltage and broadened compared with the case of no magnetic field. At low temperatures and low enough voltages the tunneling conductance is strongly suppressed, and the I–V characteristics exhibit a power-law behavior. The zero-voltage conductance is thermally activated with an activation temperature 10 K. The results are in very good agreement with experiment.     

Low energy dynamics of two-dimensional lateral tunnel junctions

We report on the low temperature tunneling characteristics of two-dimensional lateral tunnel junctions (2DLTJs) consisting of two coplanar two-dimensional electron systems separated by an in-plane tunnel barrier. The tunneling conductance of the 2DLTJ exhibits a characteristic dip at small voltages—consistent with the phenomenon of zero-bias anomaly in low-dimensional tunnel junctions—and a broad conductance peak at the Coulombic energy scale. The conductance peak remains robust under magnetic fields well into the quantum Hall regime. We identify the broad conductance maxima as the signature of the pseudogap in the tunneling density of states below the characteristic Coulomb interaction energy of the 2DLTJ.     

LINEAR TUNNELING CONDUCTANCE AND INELASTIC-TUNNELING MODEL

The inelastic-tunneling model can be used to explain the linear conductance background observed in tunneling in the high-Tc oxide super conductors. Introducing a parameter into the tunneling Hamiltanian to describe the strength of on-site Coulomb repulsion, we have derived an expression for the inelastic-tunneling conductance. We further discuss what form of the spin-fluctuation propagators in the inelastic-tunneling model can produce the linear tunneling conductance which would agree well with the experimental data.     

Spin-polarized quasiparticle transport in ferromagnet/insulator/p-wave superconductor junction

We have studied the tunneling conductance in ferromagnet/insulator/p-wave superconductor junctions, taking into account the rough interface scattering effect. We find that there exist zero-bias conductance peaks and single-minimum structure in tunneling spectroscopy. As the exchange energy increases, the Andreev reflection is always suppressed and the differential conductance decreases. The differential conductance depends on the barrier strength and the roughness at the interface. Supported by the Natural Science Foundation of Jiangsu Higher Education Institutions, China (Grant No. 06KJB140009)     

正常金属/铁磁绝缘层/p波超导体结的隧道谱

利用Blonder-Tinkham-Klapwijk理论,计算了正常金属/铁磁绝缘层/p波超导体结的隧道谱.结果表明:(1)在正常金属/铁磁绝缘层/p波超导体结的隧道谱中存在零偏压电导峰、零偏压电导凹陷;(2)在Px波结的隧道谱中,磁散射能导致零偏压电导峰的劈裂,而界面的粗糙散射却可以阻止其劈裂;(3)界面的势垒散射,磁散射及其与粗糙散射的共同作用对px、py波结零偏压电导的影响是不同的.     

Intrinsic asymmetries in spin-valves: a tight binding model study

We have calculated the I–V curves, dynamical conductance, and tunneling magnetoresistance (TMR) of 1D magnetic tunneling junction through singleband tight binding model calculations based on the non-equilibrium Green's function approach. The difference in density of state of two ferromagnetic leads and the bias dependence of the propagator cause intrinsic asymmetries in TMR and dynamical conductance at finite bias. Besides, we have displayed that large TMR can be obtained even at high bias for half metallic leads.     

正常金属-d波超导隧道结中杂质和界面散射对微分电导的影响

考虑到正常金属区域的杂质散射和界面粗糙的散射,运用Bogoliubov-de Gennes (BdG)方程和Bolonder-Tinkham-Klapwijk(BTK)理论模型,计算正常金属-d波超导隧道结的微分电导.计算发现：隧道谱强烈地依赖电子的入射角和超导体晶轴方位,并能展示零偏压电导峰的存在;此外,杂质散射能使隧道谱的凹陷分裂出两个小凹陷,而界面粗糙能抹平和压低零偏压电导峰和能隙电导峰.这些结果都将很好地解释高T_c超导隧道谱的实验现象. 关键词：     

Tunneling conductance on surface of topological insulator ferromagnet/insulator/(s- or d-wave) superconductor junction: Effect of magnetically-induced relativistic mass

We investigate the tunneling conductance on the surface of topological insulator ferromagnet (F)/insulator (I)/superconductor (S) junction where superconducting type is either s- or d-wave paring. Topological insulators (TI) are insulating in bulk but conducting on the surface with the Dirac-fermion-like carriers. In contrast to the Dirac fermions in graphene, relativistic mass of the Dirac fermions in TI can be easily caused by applying magnetic field perpendicular to its surface. In this work, we emphatically focus on the effect of the magnetically-induced relativistic mass on the tunneling conductance of a TI-based F/I/S junction. We find that, due to the effect of spinless fermions as carriers in TI, the behavior of the tunneling conductance in a TI-based NIS junction resembles that in a nonmagnetic graphene-based NIS junction. In case of the d-wave paring F/I/S junction, increasing magnetically-induced relativistic mass changes the zero bias conductance dip (peak) to a zero bias conductance peak (dip). This behavior cannot be observed in a graphene-based F/I/S junction.     

Current-distance-voltage characteristics of electron tunneling through an electrochemical STM junction

We have studied the electron tunneling process through an electrochemical scanning tunneling microscopic (STM) junction formed by a gold tip and a gold electrode immersed in an inert NaClO₄ solution. Current-distance-voltage characteristics of the tunneling process are examined by simultaneous measurement of tunneling current, voltage, and distance. The results indicate that the tunneling voltage across the junction changes with tunneling distance; however, tunneling conductance is an inverse exponential function of distance over the entire investigated range of tunneling current, voltage, and distance. The results provide clear evidence for the validity of a one-dimensional tunneling model for the aqueous tunneling process. Implications of the observation are mentioned with regard to the distance-dependent STM imaging and the origin of a low tunneling barrier height.     

Tunneling transport in topological insulator normal metal/insulator/d-wave superconductor junctions

We investigate the tunneling conductance in a normal metal/insulator/d-wave superconductor (NM/I/d-wave SC) junction with a barrier of thickness d and with an arbitrary gate voltage V(0) applied across the barrier region, formed on the surface of a topological insulator, using the Dirac-Bogoliubov-de Gennes equation and Blonder-Tinkham-Klapwijk?(BTK) formalism. We find that the tunneling conductance as a function of both d and V(0) displays an oscillatory behavior whose amplitude decreases with increase of V(0). We also find that when the Andreev resonant condition is met, the tunneling conductance approaches a maximum value of 2G(0), independent of the gate voltage V(0).     

Hall resistivity of granular metals

We calculate the Hall conductivity sigma(xy) and resistivity rho(xy) of a granular system at large tunneling conductance g(T)>1. We show that in the absence of Coulomb interaction the Hall resistivity depends neither on the tunneling conductance nor on the intragrain disorder and is given by the classical formula rho(xy)=H/(n*ec), where n* differs from the carrier density n inside the grains by a numerical coefficient determined by the shape of the grains. The Coulomb interaction gives rise to logarithmic in temperature T correction to rho(xy) in the range Gamma less or similar T less or similar min(g(T)E(c), E(Th)), where Gamma is the tunneling escape rate, E(c) is the charging energy, and E(Th) is the Thouless energy of the grain.     

Single electron tunneling at large conductance: The semiclassical approach

We study the linear conductance of single electron devices showing Coulomb blockade phenomena. Our approach is based on a formally exact path integral representation describing electron tunneling nonperturbatively. The electromagnetic environment of the device is treated in terms of the Caldeira-Leggett model. We obtain the linear conductance from the Kubo formula leading to a formally exact expression which is evaluated in the semiclassical limit. Specifically we consider three models. First, the influence of an electromagnetic environment of arbitrary impedance on a single tunnel junction is studied focusing on the limits of large tunneling conductance and high to moderately low temperatures. The predictions are compared with recent experimental data. Second, the conductance of an array of N tunnel junctions is determined in dependence on the length N of the array and the environmental impedance. Finally, we consider a single electron transistor and compare our results for large tunneling conductance with experimental findings. Received 2 February 2000     

Double point contact in quantum Hall line junctions

We show that multiple point contacts on a barrier separating two laterally coupled quantum Hall fluids induce Aharonov-Bohm (AB) oscillations in the tunneling conductance. These quantum coherence effects provide new evidence for the Luttinger liquid behavior of the edge states of quantum Hall fluids. For a two point contact, we identify coherent and incoherent regimes determined by the relative magnitude of their separation and the temperature. We analyze both regimes in the strong and weak tunneling amplitude limits as well as their temperature dependence. We find that the tunneling conductance should exhibit AB oscillations in the coherent regime, both at strong and weak tunneling amplitudes with the same period but with different functional form.     

STM spectroscopy of an organic superconductor

Electron tunneling spectroscopy of the organic superconductor κ-(BEDT-TTF)₂Cu(NCS)₂using low temperature scanning tunneling microscope (STM) is reported. The tunneling differential conductance in the superconducting phase was obtained in theb–cplane of a single crystal, by varying the tip position on the sample surface. The differential conductance is reduced near zero bias voltage and enhanced at the gap edge, associated with the superconducting gap structure below[formula] K. The gap width differs slightly from sample to sample, while the overall functional shape of the conductance is sample-independent. The tunneling conductance is reduced to almost zero near zero bias voltage, while it is finite inside the gap edge. The curve obtained cannot be fit to the BCS density of states withs-wave pairing symmetry, even if the life-time broadening of one-electron levels is taken into account. Finite conductance inside the gap edge suggests anisotropy of the gap. However, the conductance curve obtained is not explained by a simpled-wave symmetry for Δ(k). The reduced conductance near zero bias voltage suggests a finite gap. An anisotropic model with a finite gap, in which Δ(k) varies depending on the direction ink-space, is examined. The tunneling conductance in the low-energy region is almost fit by the model with Δ_min = 2 meV and Δ_max = 6 meV. The finite conductance is explained by introducing a small effect of life time broadening. We conclude that the gap is anisotropic and is finite (at least Δ_min = 2 meV) on the entire Fermi surface.     

A theoretical calculation of tunneling spectroscopy of an electron waveguide with or without a scatterer

A theoretical study on the tunneling spectroscopy of an electron waveguide recently observed by Eugster and del Alamo is presented. A narrow electron waveguide coupled with another much wider one by a thin barrier between them is taken as a theoretical model for the leaky electron waveguide implemented by Eugster et al., and the transport properties of electrons are studied comprehensively through the wavefunction of the system. The results demonstrate that the conductance for the current tunneling out the barrier oscillates strongly with the width of the narrow electron waveguide, in line with its conductance steps. The theory is in good agreement with the experiments and confirm that the oscillations of the tunneling current can be considered as a spectroscopy of the 1D DOS (one dimensional electron density of states) in the electron waveguide as proposed by Eugster et al. In order to study the effects of scatterers on the transport properties of the leaky electron waveguide, a δ-function is used to simulate the scattering potential The results show that the presence of even a single scatterer located in the waveguide will lead to obvious distortion of the shape of conductance steps, and will greatly influence the oscillations of the tunneling current observed in clean waveguides. However the effects of scatterers located outside the tunneling barrier on either the conductance steps or the oscillations of the tunneling current are negligible.     

POSITRON ANNIHILATION STUDY OF YBa2(Cu1-xSnx)3Oy SUPERCONDUCTORS

The inelastic-tunneling model can be used to explain the linear conductance background observed in tun-neling in the high-T_c oxide super conductors. Introducing a parameter into the tunneling Hamiltanian to describe the strength of on-site Coulomb repulsion, we have derived an expression for the inelastic-tunneling conductance. We further discuss what form of the spin-fluctuation propagators in the inelastic-tunneling model can produce the linear tunneling conductance which would agree well with the experimental data.