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.     

Theory of tunneling conductance of STS around magnetic impurity in superconductors

Local density of states of quasiparticles around the magnetic impurity in superconductors are calculated on the basis of a pair potential the spatial dependence of which is determined self-consistently using negative Hubbard model. The spatial dependence of the tunneling conductance observed by Scanning tunneling spectroscopy (STS) strongly depends on the magnitude of the impurity potentials.     

Zero bias conductance peak in InAs nanowire coupled to superconducting electrodes

We report the occurrence of the zero-bias conductance peak (ZBCP) in an InAs nanowire coupled to PbIn superconductors with varying temperature, bias voltage, and magnetic field. The ZBCP is suppressed with increasing temperature and bias voltage above the Thouless energy of the nanowire. Applying a magnetic field also diminishes the ZBCP when the resultant magnetic flux reaches the magnetic flux quantum h/2e. Our observations are consistent with theoretical expectations of reflectionless tunneling, in which the phase coherence between an electron and its Andreev-reflected hole induces the ZBCP as long as time-reversal symmetry is preserved.     

Fluctuations in high field superconductors

We present calculations for the influence of fluctuations in high field superconductors where the critical field is limited by Pauli paramagnetism. Due to the fact that the critical field at the second order phase transition point as function of temperature may have a maximum atT≠0 the additional conductivity due to fluctuations may have a nonmonotonic temperature dependence. This way we can account for recent experimental findings by Tedrow, Meservey and Schwartz. We also calculate the additional tunneling density of states due to fluctuations. Under proper conditions it exhibits a maximum at zero frequency like in the gapless regime. Finally we show that our findings of a nonmonotonic resistivity should also apply to superconductors containing magnetic impurities such as La_3-x Gd _x In in an external field.     

Impurity states and interlayer tunneling in high temperature superconductors

We argue that the scanning tunneling microscope (STM) images of resonant states generated by doping Zn or Ni impurities into Cu-O planes of BSCCO are the result of quantum interference of the impurity signal coming from several distinct paths. The impurity image seen on the surface is greatly affected by interlayer tunneling matrix elements. We find that the optimal tunneling path between the STM tip and the metal (Cu, Zn, or Ni) d(x(2)-y(2)) orbitals in the Cu-O plane involves intermediate excited states. This tunneling path leads to the fourfold nonlocal filter of the impurity state in Cu-O plane that explains the experimental impurity spectra. Applications of the tunneling filter to the Cu vacancy defects and "direct" tunneling into Cu-O planes are also discussed.     

Upper critical field of high Tc superconductors

The upper critical field H_c2 of the high T_c superconductors such as La-Sr-Cu-O and La-Ba-Cu-O systems has been calculated by using the modified Suhl's et al. two-band model. A modification to Gor'kov theory is found for the ratio of K₁(T) at T=0 and T=T_c. The results may explain the discrepancy between the BCS theory and experimental data.     

Tunneling conductance in layered oxide superconductors

The copper oxide superconductors are considered to have stacking structures of strongly and weakly super-conducting layers. Using this model, the tunneling conductance in a junction of the superconductor and a normal metal is calculated as a function of bias voltage. The calculation predicts that a characteristic fine structure appears inside the superconducting gap in the tuneling spectrum at low temperatures.     

Point-contact tunneling involving low-dimensional spin-triplet superconductors

We modify and extend previous microscopic calculations of tunneling in superconducting junctions based on a nonequilibrium Green function formalism to include the case of spin-triplet pairing. We show that distinctive features are present in the I-V characteristics of different kinds of junctions, in particular, when the effects of magnetic fields are taken into account, that permit to identify the type of pairing. We discuss the relevance of these results in the context of quasi-one-dimensional organic superconductors such as (TMTSF)2PF6 and layered compounds like Sr2RuO(4).     

Intrinsic Josephson tunneling for basic studies of high-temperature superconductors

High-temperature superconductors (HTS) are highly anisotropic with layers of CuO₂ separated by layers of charge reservoirs. The superconducting CuO₂ planes are weakly coupled by Josephson tunneling of Cooper pairs in the c-axis direction. It may be possible to use the intrinsic tunneling to realize a stack of intrinsic Josephson junctions at the surface of a single crystal (or epitaxial film) of HTS. These will form excellent tools in the study of superconducting properties of HTS.     

Electrical conductance from the Fowler-Nordheim tunneling

Electrical conductance, including its normalized version, is discussed quantitatively in the context of the Fowler-Nordheim tunneling by considering ballistic electron transport through a generic insulating layer. This discussion is applicable to several nanostructures as, for example, nanowires as well as to specific problems in electron optics.     

Quantum tunneling of flux lines in superconductors

We discuss the different regimes which can be expected for quantum creep of flux lines by analysing the experimental results obtained for the relaxation rate as T→0 for Y₁Ba₂Cu₃O₇, Bi₂Sr₂Ca₁Cu₂O₈ and (ET)₂Cu(NCN)₂Br single crystals. We address the dimensionality of the pinned flux line, the possible bundle regime at high fields for the organic superconductor and the relative invariability of the quantum rate upon introduction of columnar defects.     

Spin polarized tunneling at finite bias

A mesoscopic spin valve is used to determine the dynamic spin polarization of electrons tunneling out of and into ferromagnetic (FM) transition metals at finite voltages. The dynamic polarization of electrons tunneling out of the FM slowly decreases with increasing bias but drops faster and even inverts with voltage when electrons tunnel into it. A free-electron model shows that in the former case electrons originate near the Fermi level of the FM with large polarization whereas in the latter, electrons tunnel into hot electron states for which the polarization is significantly reduced. The change in sign is ascribed to the matching of the electron wave function inside and outside the tunnel barrier.     

Nonequilibrium conductance of a nanodevice for small bias voltage

Using nonequilibrium renormalized perturbation theory, we calculate the retarded and lesser self-energies, the spectral density ρ(ω) near the Fermi energy, and the conductance G through a quantum dot as a function of a small bias voltage V, in the general case of electron-hole asymmetry and intermediate valence. The linear terms in ω and V are given exactly in terms of thermodynamic quantities. When the energies necessary to add the first electron (Ed) and the second one (Ed + U) to the quantum dot are not symmetrically placed around the Fermi level, G has a term linear in V if, in addition, either the voltage drop or the coupling to the leads is not symmetric. The effects of temperature are discussed. The results simplify for a symmetric voltage drop, a situation usual in experiment.