Finite Zero-bias Conductance Research Articles

STM spectroscopy on superconducting FeSe1−xTex single crystals at 300 mK

We report cryogenic scanning tunneling spectroscopy measurements on single crystals of superconducting FeSe1−xTex, at doping levels of x=0.5 and 0.7, with critical temperatures Tc≈12K. Atomically resolved topographic images were obtained, showing large-scale density-of-state clustering which appears to have no periodicity and to vary with the doping. Conductance spectra taken at 300mK showed a generally asymmetric V-shaped background, along with a sharp dip structure within ∼±2–4mV. These spectra appeared to vary over ∼nm length scale, and not correlated with the topography. The overall spectral evolution versus temperature is consistent with the dip structure arising from a superconducting energy gap which closes above Tc, and with the spectral background having a non-superconducting origin. The persistence of finite zero-bias conductance down to 300mK, well below Tc, indicates the presence of low-energy quasiparticles on parts of the Fermi surface. We discuss our data in light of some other recent spectroscopic measurements of FeSe1−xTex, and in terms of its characteristic band structure.

Polaronic pseudogap in the metallic phase ofLa0.625Ca0.375MnO3 thin films

The electronic density of states (DOS) ofLa0.625Ca0.375MnO3 (LCMO) strain-free epitaxial thin films with an insulator–metal transition temperature (TIM) of 250 K was probed using variable-temperature scanning tunneling microscopyand spectroscopy. We find a depression in the DOS with a finite zero biasconductance (ZBC) signifying a pseudogap in the 78–310 K temperature range.With cooling, the ZBC is found to increase, indicating an increased DOS nearEF. We interpret the pseudogap as a signature of Jahn–Teller polarons while the ZBC change,in agreement with the bulk insulator–metal transition, optical Drude peak andphotoemission experiments, indicates the presence of free carriers at the Fermi energy inthe metallic phase. The free carriers are discussed in terms of correlated polaronic states.

Temperature dependent tunneling study of La0.625Ca0.375MnO3 thin films

The tunneling measurements on La0.625Ca0.375MnO3 strain free epitaxial thin films with an insulator-metal transition temperature (TIM) of 250 K were performed using variable temperature scanning tunneling microscopy and spectroscopy (STM/S). We find a depression in the DOS with a finite zero bias conductance (ZBC) signifying the presence of a pseudogap (PG) at all temperatures. With cooling, the ZBC is found to increase together with an increase in the PG energy of about 0.2 eV near the transition. The PG is a signature of the polarons while the increasing ZBC, in agreement with the bulk insulator-metal transition indicates the presence of free carriers at the Fermi energy.

Electronic Transport in Single Molecule Junctions: Control of the Molecule-Electrode Coupling through Intramolecular Tunneling Barriers

We report on single molecule electron transport measurements of two oligophenylenevinylene (OPV3) derivatives placed in a nanogap between gold (Au) or lead (Pb) electrodes in a field effect transistor device. Both derivatives contain thiol end groups that allow chemical binding to the electrodes. One derivative has additional methylene groups separating the thiols from the delocalized pi-electron system. The insertion of methylene groups changes the open state conductance by 3-4 orders of magnitude and changes the transport mechanism from a coherent regime with finite zero-bias conductance to sequential tunneling and Coulomb blockade behavior.

Effect of unitary impurities on non-STM types of tunneling in high-Tcsuperconductors

Based on an extended Hubbard model, we present calculations of both the local (i.e., single-site) and spatially averaged differential tunneling conductance in d-wave superconductors containing nonmagnetic impurities in the unitary limit. Our results show that a random distribution of unitary impurities of any concentration can at most give rise to a finite zero-bias conductance (with no peak there) in spatially averaged non-STM type of tunneling. This is in spite of the fact that local tunneling in the immediate vicinity of an isolated impurity does show a conductance peak at zero bias. We also find that to give rise to even a small zero-bias conductance peak in the spatially averaged type of tunneling the impurities must form dimers, trimers, etc., along the [110] directions. In addition, we find that the most recently observed novel pattern of the tunneling conductance around a single impurity by Pan et al. [Nature (London) 403, 746 (2000)] can be explained in terms of a realistic model of the tunneling configuration which gives rise to the experimental results reported there. The key feature in this model is the blocking effect of the BiO and SrO layers which exist between the tunneling tip and the ${\mathrm{CuO}}_{2}$ layer being probed.

D-wave Bose–Einstein condensate and tunnelling in superconducting cuprates

Starting from the Hamiltonian, which describes holes in a doped Mott insulator with the strong electron–phonon and Coulomb interactions we show that bipolaron formation leads to a d-wave charged Bose–Einstein condensate in cuprates. It is the bipolaron energy dispersion rather than a particular pairing interaction which is responsible for the d-wave symmetry. Single-particle spectral density is derived taking into account realistic band structure and disorder in cuprates. The tunnelling and photoemission (PES) spectra are described, including the temperature independent gap observed both in the superconducting and normal states, the emission/injection asymmetry, the finite zero-bias conductance, the spectral shape in the gap region and its temperature and doping dependence, the dip–hump incoherent asymmetric features at high voltage (tunnelling) and large binding energy (PES). The interaction responsible for the high value of T c is elucidated which is the Fröhlich electron–phonon interaction.

Influence of magnetic field and Pincus states in normal-metal-superconductor tunnel junctions.

Normal-metal---superconductor tunneling, when applied to metallic superconductors with well-known bulk properties, yields useful information on barrier properties. In particular, it is quite common to observe deviations from the predictions of a homogeneous barrier model, Nb-oxide barriers being the most-investigated example. In discussing these shortcomings, high local nearly metallic transparency instead of a tunneling probability of order ${10}^{\ensuremath{-}8}$ is an often applied concept. We here present experimental results on Ag/Nb and Ag/Ta tunneling junctions which, in addition to a commonly observed finite zero-bias conductance, show a pronounced, characteristic magnetic field dependence. A theory is developed on the basis of the Bogoliubov-de Gennes equations which takes into account the influence of the shielding current as well as tunneling of quasiparticles with nonperpendicular incidence, typical for inhomogeneous tunneling barriers with high local transparency. The field effect due to quasiparticles with non-normal incidence is nearly antisymmetric with respect to the gap voltage. Below $\frac{\ensuremath{\Delta}}{e}$ the effect is positive on account of resonant transmission to, the Pincus-type bound states appearing for quasiparticle momenta antiparallel to the shielding current. Above $\frac{\ensuremath{\Delta}}{e}$, a compensating negative contribution is obtained. The results are quantitatively discussed with respect to their dependence on barrier transparency and angular spreading of quasiparticle injection. We find good agreement with the experiments for a broad angular distribution of injected quasiparticles, typical for inhomogeneous barriers with high local transparency. Furthermore differences by an order of magnitude between Nb-and Ta-based junctions can be fully accounted for. On the other hand, strong confinement of quasiparticle injection in forward direction which is typical for an ideal, homogeneous barrier, yields marked deviations from experiment. Thus, we can conclude a broad angular distribution of quasiparticle transmission, characteristic for high local transparency.

Macroscopic structural coherence in two-component superconductivity

In two-component theory 1,2 pairing arises from localized negative-U states and mobility arises from extended single particle states. A small hybridization of localized and extended states enables mobility and pairing to provide a high T c. RPA analysis of the “normal” state implies uncondensed charged pairs carry current, while long lived single particle excitations are neutral electron-hole hybrids. At T c pairs condense and single particle states undergo Cooper pairing. In the superconducting state pair-pair excitations exist in the BCS-like fermionic gap. Signatures of this theory range from distinctive T c, Δ, H c, ξ, conductance anomalies in sound and bulk modulii at T c, linear temperature dependence of normal state resistivity, 2e charge carriers in the normal state, linear voltage dependence in normal-state-tunneling conductance, and finite zero-bias conductance in superconducting state tunneling. Quantitative comparisons with superconducting properties of YBa 2Cu 3O 7 were presented. 1 A distinctive signature is the prediction of dynamical structural correlations which are local above T c and macroscopic below T c. Experiments provide direct evidence for such dynamical correlations: neutron diffraction “thermal ovals”, 3 channeling experiment cross section changes as a function of temperature near T c, 4,5 pair-distribution-function neutron diffraction including enelastic and elastic scattering showing direct evidence for dynamic correlations which change at T c, 6 and EXAFS showing a large dynamical displacement of oxygen atoms tunneling between sites separated by 0.13Å, 7 In two-component theory strong lattice coupling is consistent with low isotope shifts since tunneling occurs by a virtual Frank-Condon transition. 1,2 Predictions for the dynamical structure factor are presented.

Two-component superconductivity. I. Introduction and phenomenology.

A two-component theory of superconductivity is developed where one electronic component provides mobility and the other provides pairing. For the Cu-O-based high-${\mathrm{T}}_{\mathrm{c}}$ materials the two components are identified with mobile electronic states associated Cu-O planes, and localized negative-U states associated with oxygen vacancies in the Cu-O planes. An explicit comparison of phenomenology with BCS theory is performed including comparison with experiments on ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$. The discussion includes quantitative comparison of the superconducting properties ${\mathrm{T}}_{\mathrm{c}}$, \ensuremath{\Delta}, ${\mathrm{H}}_{\mathrm{c}}$, and \ensuremath{\xi}. Long-wave collective excitations, normal-state properties including resistance and tunneling, and the isotope shift are described. Unusual properties are predicted including neutral-fermion excitations, a spreading of the fermionic gap onset, a separation between the resistive transition ${\mathrm{T}}_{\mathrm{c}}^{\ensuremath{'}}$ and the evaporation of the condensate ${\mathrm{T}}_{\mathrm{c}}$, anomalies in sound and bulk modulii at ${\mathrm{T}}_{\mathrm{c}}$, linear temperature dependence of normal-state resistivity, linear voltage dependence in normal-state tunneling conductance, and finite zero-bias conductance in superconducting-state tunneling. A new signature of structural coherence obtained by channeling experiments is indicated.

Observations of quasi-particle tunneling and Josephson behavior in Y1Ba2Cu3O7−x/ native barrier/Pb thin-film junctions

Low-leakage, thin-film planar tunnel junctions made of Y1Ba2Cu3O7−x/ native barrier/Pb were fabricated. The Y1Ba2Cu3O7−x films were prepared by in situ molecular beam epitaxy aided with an activated oxygen source. The as-grown, smooth superconducting perovskite film surface exhibits quasi-particle tunneling characteristics very similar to the etched bulk single-crystal data. The results in agreement are a linear dependence of the normal-state conductance on voltage, a gap-like structure at ∼20 mV, asymmetric modulations up to 50 mV, and a finite zero-bias conductance at low temperature. Junctions of lower resistance show, at temperatures below Tc of Pb, the development of a supercurrent at zero bias and associated hysteretic subgap structure, with a typical IcR ∼0.5 mV. Josephson-like behavior occurred in response to applied magnetic field and microwaves.

Electron Tunneling into Single Crystals of YBa2Cu3O7-δ

AbstractWe have fabricated tunnel junctions between chemically etched single crystals of YBa2Cu3O7-δ and evaporated metal counterelectrodes which exhibit reproducible characteristics. Above the bulk critical temperature of YBa2Cu3O7-δ, Tc, the conductance, G(V), has a linear dependence with voltage and has some asymmetry. Below Tc, additional structure associated with the superconductivity appears in G(V). At T>>Tc there is a reproducible finite zero bias conductance.