Zero-point fluctuations and the quenching of the persistent current in normal metal rings

The ground state of a phase-coherent mesoscopic system is sensitive to its environment. We investigate the persistent current of a ring with a quantum dot which is capacitively coupled to an external circuit with a dissipative impedance. At zero temperature, zero-point quantum fluctuations lead to a strong suppression of the persistent current with decreasing external impedance. We emphasize the role of displacement currents in the dynamical fluctuations of the persistent current and show that with decreasing external impedance the fluctuations exceed the average persistent current.     

Manifestly non-Gaussian fluctuations in superconductor-normal metal tunnel nanostructures

We propose a mesoscopic setup which exhibits strong and manifestly non-Gaussian fluctuations of energy and temperature when suitably driven out of equilibrium. The setup consists of a normal metal island (N) coupled by tunnel junctions (I) to two superconducting leads (S), forming a SINIS structure, and is biased near the threshold voltage for quasiparticle tunneling, eV≈2Δ. The fluctuations can be measured by monitoring the time-dependent electric current through the system. This makes the setup suitable for the realization of feedback schemes which can be used to stabilize the temperature to the desired value.     

Electrical resistance and 1/f fluctuations in thin titanium films

The thickness of a metallic layer has been determined and the resistivity and spectral density of voltage fluctuations in thin titanium films with initial thicknesses of 5–100 nm, which were obtained by magnetron sputtering and intended for promising elements of the neutron optics, have been investigated. It has been found that a continuous metallic layer necessary for functioning is retained even in thinnest samples, and excess fluctuations of the layer resistance with the 1/f-type spectrum are observed. It has been shown that the method of measuring film resistivity can be used as effective express-method of determining the thickness of metallic nanolayers.     

Electron theory of long-wavelength concentration fluctuations in liquid metal alloys

Summary It is shown that long-wavelength concentration fluctuations in a binary liquid metal alloy are determined by a pair interchange free energyw, which is exactly given by the sum of an elastic strain term and of the concentration-concentration direct correlation function. The latter is evaluated in an alloy of homovalent components by electron screening theory in the pseudopotential approach of Shaw and Harrison and shown to be entirely determined by nonlocal terms reflecting charge transfer to the more electronegative alloy component. Numerical calculations for the liquid Na−K alloy show strong cancellation between the two contributions tow at all concentrations, in qualitative agreement with experiment.     

Electrical conductivity of metal powders under pressure

A model for calculating the electrical conductivity of a compressed powder mass consisting of oxide-coated metal particles has been derived. A theoretical tool previously developed by the authors, the so-called ‘equivalent simple cubic system’, was used in the model deduction. This tool is based on relating the actual powder system to an equivalent one consisting of deforming spheres packed in a simple cubic lattice, which is much easier to examine. The proposed model relates the effective electrical conductivity of the powder mass under compression to its level of porosity. Other physically measurable parameters in the model are the conductivities of the metal and oxide constituting the powder particles, their radii, the mean thickness of the oxide layer and the tap porosity of the powder. Two additional parameters controlling the effect of the descaling of the particle oxide layer were empirically introduced. The proposed model was experimentally verified by measurements of the electrical conductivity of aluminium, bronze, iron, nickel and titanium powders under pressure. The consistency between theoretical predictions and experimental results was reasonably good in all cases.     

1/f noise generated by equilibrium temperature fluctuations in metal films

A model of flicker noise generated by quasi-equilibrium temperature fluctuations for metal films on dielectric substrates is examined. A hyperbolic thermal-conductivity equation is solved that describes temperature-fluctuation propagation at a finite velocity with allowance for energy dissipation in the specimen. A formula is derived for the energy spectrum of flicker-voltage fluctuations at frequencies of 10^–5–10⁵ Hz.Moscow Institute of Electronic Engineering. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 37, No. 2, pp. 161–182, February, 1994.     

Aharonov-Bohm cages in 2D normal metal networks

We report on magnetoresistance transport measurements performed on a bipartite tiling of rhombus in the GaAs/GaAlAs system. We observe for the first time large amplitude h/e oscillations in this network as compared to the one measured in square lattices of similar size. These oscillations are the signature of a recently predicted localization phenomenon induced by Aharonov-Bohm interferences in this peculiar network.     

Pseudogap and spin fluctuations in the normal state of the electron-doped cuprates

We present reliable many-body calculations for the t-t(')-t(')-U Hubbard model that explain in detail the results of recent angle-resolved photoemission experiments on electron-doped high-temperature superconductors. The origin of the pseudogap is traced to two-dimensional antiferromagnetic spin fluctuations whose calculated temperature-dependent correlation length also agrees with recent neutron scattering measurements. We make specific predictions for photoemission, for neutron scattering, and for the phase diagram.     

Aharonov-Bohm effect in normal metal quantum coherence and transport

We review some of the recent surprising theoretical and experimental results obtained on the transport properties of small disordered metal samples. Even in the presence of disorder, the quantum mechanical interference of electron wavefunctions can still be observed. The Aharonov-Bohm effect is a particularly clear demonstration of this. In doubly connected structures (such as loops of wire) threaded by a magnetic flux, the electrical conductance oscillates because of the Aharonov-Bohm effect. In fact, because the electron trajectories are diffusive (i.e. random walks), even a lone wire (a singly connected structure) will exhibit a random pattern of conductance fluctuations as a function of the magnetic field because of the same interference effects. All that is required for the observation of these interferences is that the electrons retain ‘phase memory’ duing the period of transit through the sample. The length over which memory is maintained (the phase coherence length) can be much larger than the random walk step length (the mean free path). We focus mainly on effects observed in the limit where the phase coherence length of the electrons is comparable to or larger than the sample size. We explain how the interferences are averaged as the system size grows larger than the phase coherence length. We also remark on surprising aspects of the fluctuations such as those resulting from the non-local character of the wavefunction; some of the results are forbidden classically.     

Inducing odd-frequency triplet superconducting correlations in a normal metal

This work discusses theoretically the interplay between the superconducting and ferromagnetic proximity effects, in a diffusive normal metal strip in contact with a superconductor and a nonuniformly magnetized ferromagnetic insulator. The quasiparticle density of states of the normal metal shows clear qualitative signatures of triplet correlations with spin one (TCS1). When one goes away from the superconducting contact, TCS1 focus at zero energy under the form of a peak surrounded by dips, which show a typical spatial scaling behavior. This effect can coexist with a focusing of singlet correlations and triplet correlations with spin zero at finite but subgap energies. The simultaneous observation of both effects would enable an unambiguous characterization of TCS1.     

Thermoelectric effects in normal metal/superconductor interface structures

The thermopower of Andreev interferometers, which are doubly connected loops in which one arm is a superconductor and one arm is a normal metal, oscillates as a function of magnetic field with a fundamental period corresponding to a flux quantum h / 2 e through the area of the loop. While the magnetoresistance of an Andreev interferometer is symmetric with respect to the magnetic field, the thermopower can be either symmetric or antisymmetric, depending on the topology of the sample. The temperature dependence of the thermopower oscillations is nonmonotonic. This nonmonotonic behavior does not appear to be related to the reentrance observed by many groups in the conductance of normal-metal/superconductor (NS) structures.     

Electrical control of antiferromagnetic metal up to 15 nm

Manipulation of antiferromagnetic (AFM) spins by electrical means is on great demand to develop the AFM spintronics with low power consumption. Here we report a reversible electrical control of antiferromagnetic moments of FeMn up to 15 nm, using an ionic liquid to exert a substantial electric-field effect. The manipulation is demonstrated by the modulation of exchange spring in [Co/Pt]/FeMn system, where AFM moments in FeMn pin the magnetization rotation of Co/Pt. By carrier injection or extraction, the magnetic anisotropy of the top layer in FeMn is modulated to influence the whole exchange spring and then passes its influence to the [Co/Pt]/FeMn interface, through a distance up to the length of exchange spring that fully screens electric field. Comparing FeMn to IrMn, despite the opposite dependence of exchange bias on gate voltages, the same correlation between carrier density and exchange spring stiffness is demonstrated. Besides the fundamental significance of modulating the spin structures in metallic AFM via all-electrical fashion, the present finding would advance the development of low-power-consumption AFM spintronics.     

Electrical properties of doped 3-tetradecylpolypyrrole/metal devices

The electrical properties of devices made of doped 3-tetradecylpolypyrrole (PPy-C₁₄) thin films sandwiched between indium-tin-oxyde (ITO) and gold metal electrodes are reported. The current density–voltage (J–V) curves are asymmetric and nonlinear implying a non Ohmic rectifying contact. Using standard thermionic emission theory (Schottky) J–V characteristics were satisfactorily fitted with a saturation current of J₀=1.5×10^-5 A cm^-2, a barrier height of ϕ_b=0.7 eV, and an ideality factor of n=5.3. Characteristics from the plot of J–V versus 1/T show that the activation energy of the thermionic emission process is higher below the glass transition temperature of PPy-C₁₄ (T_g=45 °C) than above, which seems to indicate that the hopping conduction process is enhanced at T>T_g. The carrier concentration has been calculated from capacitance–voltage (C-V) measurements (N=1.9×10¹⁷ cm^-3) allowing estimation of the carrier mobility μ=2.6×10^-2 cm² V^-1 s^-1. PACS 73.61.Ph; 73.40.Sx; 73.30.+y