T. Grenet

Resistivity, hall effect and thermopower in AIPdMn and AlCuFe quasicrystals

The long range ordered quasicrystals present anomalous electronic properties for metallic alloys. We present here for the first time a correlation between the temperature dependences of resistivity (p), Hall effect (RH) and thermoelectric power (S), in high structural, quality AlPdMn and AlCuFe quasicrystalline phases for various compositions. Strong composition effects are emphasized, for the high p values and the strong temperature dependences of the transport properties, as well as for sign reversals of both RH(T) and S(T). This is an indication for band structure effects in these phases. A simple two band model well accounts for these behaviours.

The new AlPdRe icosahedral phase: Towards universal electronic behaviour for quasicrystals?

Abstract Electronic properties have been measured for different new compositions close to Al 70.5 Pd 22 Re 7.5 of the icosahedral phase. They exhibit the same exceptional behaviours that were previously found for other stable quasicrystalline phases exept for the resistivity magnitude which is much higher that what is observed for the parent AlPdMn icosahedral phase. For high quality quasicrystals, the conductivity reaches the same order of magnitude as the one for doped semiconductors. The conductivity σ(T) increases rapidly with temperature. σ(T) curves appear to be parallel for all i-AlPdRe samples, although conductivity values strongly varies with the composition and the structural quality. High field magnetoconductivity indicates the presence of quantum interference effects including strong electron-electron interactions. All these features strengthen the concept of universal electronic behaviour for these complex phases, with particularly enhanced anomalous behaviour for i-AlPdRe.

Symmetrical field effect and slow electron relaxation in granular aluminium

Abstract:Conductivity and field effect measurements in thin insulating Al granular films are reported. The occurrence of a symmetrical field effect and of very slow conductance relaxations is demonstrated. They are identical to the electron glassy behaviours already reported in insulating indium oxide thin films. The results suggest that the phenomena are quite general. The study of structurally discontinuous samples should help to understand the origin and mechanism of the glassy behaviour.

Pseudogap in quasicrystals probed by tunneling spectroscopy

We present tunneling spectroscopy measurements at 4 K on three quasicrystalline phases i-AlPdMn, i-AlCuFe and i-AlPdRe. In the spectroscopic curves, dI/dU, we observe a structure centered about zero bias voltage and with a square root voltage dependence from 300 mV down to at least 5 mV. This structure can be interpreted as the preopening of a correlation gap in the electronic density of states (DOS), which is consistent with the proximity to a metal–insulator transition in the three phases.

Transport properties of Ln2-xCexCuO4-y single crystals under high pressure

Abstract The transport properties of the electron-doped cuprates Ln 2- x Ce x CuO 4 (Ln z.bbnd; Pr, Nd and Sm) single crystals are investigated under hydrostatic pressure up to 2 GPa. The effects of pressure on both the c -axis (ϱ c and the in-plane (ϱ ab resistivity are measured from room temperature down to the superconducting state. It is found that at 300 K, dln ϱ ab /d P ≈dln ϱ c /d P ≈ −3% GPa −1 . The pressure does not change the temperature dependence of the normal state resistivity, which at high temperature in both directions follows a ϱ ∼ A ( P ) T 2 law, but the coefficient A ( P ) decreases as pressure is increased. Moreover, we have found that for any lanthanide host (Pr, Sm and Nd) the onset of superconductivity does not change with pressure ( P

Granular Superconductivity in the Electron-Doped Compounds Ln2-xMxCuO4-y (Ln = Pr, Nd, Sm, Eu; M = Ce, Th)

We study the superconducting properties of the Electron.-doped compounds Ln2-xMxCuO4-y (Ln = Pr, Nd, Sm, Eu; M = Ce, Th). Three characteristic temperatures of the superconducting transition are defined, corresponding to: the macroscopic resistive transition midpoint (Tc mid), the diamagnetic signal onset (Tcχ), and the intragrain transition temperature (Tcg). Tcχ and Tcg are found to vary weakly in the series of the studied compounds, in contrast with Tc mid, which is strongly reduced in Eu1.85Ce0.15CuO4-y and Sm1.85Th0.15CuO4-y samples.

Electrical glassy behavior in granular aluminium thin films

Abstract We present new results obtained by field effect measurements on insulating granular Al thin films. First, reproducible and stable conductance fluctuations are seen in micron size samples as a function of gate voltage. The anomalous field effect and its slow relaxation already known to exist in macroscopic samples are shown to still exist in small samples and to have no influence on the fluctuations pattern. Secondly, “true” aging is demonstrated, i.e. the anomalous field effect relaxation depends on the time elapsed since the cooling, the longer this time the longer it takes for the system to react to a gate voltage change. Interpretations and implications of these findings are discussed.

Breakdown of metallic conductivity in quasicrystals

Abstract In the icosahedral phases i-AlCuFe, i-AlPdMn and i-AlPdRe, the electrical conductivity is in the same range as for doped semiconductors. Strong similarities are observed between the direct and tunneling conductivity for the i-AlPdRe phase and for disordered systems on both sides of the metal–insulator (MI) transition.

Charging of highly resistive granular metal films

We have used the Scanning Kelvin probe microscopy technique to monitor the charging process of highly resistive granular thin films. The sample is connected to two leads and is separated by an insulator layer from a gate electrode. When a gate voltage is applied, charges enter from the leads and rearrange across the sample. We find very slow processes with characteristic charging times exponentially distributed over a wide range of values, resulting in a logarithmic relaxation to equilibrium. After the gate voltage has been switched off, the system again relaxes logarithmically slowly to the new equilibrium. The results cannot be explained with diffusion models, but most of them can be understood with a hopping percolation model, in which the localization length is shorter than the typical site separation. The technique is very promising for the study of slow phenomena in highly resistive systems and will be able to estimate the conductance of these systems when direct macroscopic measurement techniques are not sensitive enough.

Ageing in granular aluminium insulating thin films

We present a new set of electrical field effect measurements on granular aluminium insulating thin films. We have explored how the conductance relaxations induced by gate voltage changes depend on the age of the system, namely the time elapsed since its quench at low temperature. A clear age dependence of the relaxations is seen, qualitatively similar to ageing effects seen in other well studied glassy systems such as spin glasses or polymers. We explain how our results differ from the previous ones obtained with different protocols in indium oxide and granular aluminium thin films. Our experimental findings bring new information on the dynamics of the system and put new constraints on the theoretical models that may explain slow conductance relaxations in disordered insulators.