In previous work [1], which we recently reviewed in [2,3,4], we discovered a critical point in the behavior of hysteretic systems. Adding disorder to the system, we found a second order transition from hysteresis loops with a macroscopic jump or burst (roughly as seen in the supercooling of liquids) to smoothly varying hysteresis loops (as seen in most magnets). We have studied the critical point in the nonequilibrium zero temperature random field Ising model (RFIM) (which is a simple model for magnets, that has aplications far beyond magnetic hysteresis and associated Barkhausen Noise), using mean field theory, renormalization group techniques, and numerical simulations in 2,3,4, and 5 dimensions. In a large region near the critical disorder the model exhibits power law distributions of noise (avalanches), universal behavior, and a diverging length scale [5,6,7].

We experimentally investigate the average shapes of magnetization avalanches with the goal to understand the origin of their time asymmetry. We performed measurements on amorphous and polycrystalline magnetic materials, showing that the asymmetry can depend on several parameters, such as the applied field rate, and the duration and size of the avalanche itself. We suggest, as a possible explanation, to investigate the effect of the eddy currents on the measured signal.

We report on the voltage fluctuations in exchange-biased, micron-scale magnetic tunnel junctions. We find that the spectral power density is 1/f-like at low frequencies and becomes frequency independent at high frequencies. The frequency-independent background noise is due to thermal and shot noise mechanisms. The nature of the 1/f noise has its origin to two different mechanisms. In the magnetic hysteresis loops this noise power is strongly field-dependent and is due to thermal magnetization fluctuations in both the 'free' and 'fixed' magnetic layers. We attribute these magnetic fluctuations to thermally excited hopping of magnetic domain walls between pinning sites. A second mechanism for the 1/f noise, connected with defects in the tunnel barrier but not related to the overall magnetization fluctuations, was found at fields for which the magnetic structure in the free and fixed layers is well aligned. This noise is associated with electron trapping processes having thermally activated kinetics and a broad distribution of activation energies. Below ~ 25 K the noise power is temperature independent suggesting that the kinetics are dominated by tunneling. Our results show that the thermal stability of the both magnetic layers and the quality of the tunnel barrier are important factors in reducing the low-frequency noise in magnetic tunnel junctions.

Experiments of spin injection on magnetic nanostructures show that the magnetization can be switched with high current pulses, and without the need of a magnetic field. The mechanism responsible for this effect is still not known, but some experiments show that it is related to fluctuations and noise in magnetic systems. The response of a magnetic nanostructure shows typical processes of activation out of a metastable state and two level fluctuations. An effective temperature is measured is measured through the effective barrier variation. The effective temperature depends on the magnetic configuration and to the current direction. This approach leads to conclude that the transfer of energy is due to a transfer of spin from the current to the magnetization. A new mechanism is proposed as a consequence of the s-d relaxation at the interface in 3d ferromagnets. This mechanism couples the electronic transport (Boltzmann equations) to the dynamics of the magnetization (Landau-Lifshitz-Gilbert equation).

We have investigated the conductance fluctuation in a shape memory alloy (NiTi-Nitinol) near its martensite transition in order to study how the noise evolves near a phase transition that is driven by long range elastic interactions. The alloy has a martensite start and completion temperatures of 333 K and 325 K respectively and change of around 6%-8% in resistance at the transition. The conductance fluctuation (noise) measurements were carried out using a 5-probe ac technique in the temperature interval 200 K to 400 K in wire samples with typical thickness 5 μm and width of 120 μm. The samples were annealed at high temperature to remove any effect of previous temperature or strain cycle. We made the following observations: (1) There is a large change in the measured noise spectral power S(f) at the transition. (2) Away from transition temperature S(f) follows a 1/f-type behavior. This changes drastically near the transition, where large spectral density appears for f <1 Hz, and this strong dependence tends to saturate around 1 mHz. (3) Close to the transition, the probability density function (PDF) deviates from a Gaussian distribution, and shows a significant contribution from a power-law distribution. (4) When the transition is tuned by an applied constant stress (staying close to but below the transition temperature range), a similar strongly frequency-dependent low frequency S(f) appears, accompanied by a non-Gaussian PDF with a power-law statistics.

In this work we review the investigations of conductance fluctuations in doped silicon at low temperatures (2K < T < 20K) as it is tuned through the metal-insulator transition by changing the carrier concentration n. Spectral power, S(f), of the conductance fluctuation retains a generic 1/f^α dependence. In the metallic regime (n>n_c) the doped Si is like a weakly-localized electron system and the conductance fluctuation is governed by the mechanism of Universal conductance fluctuations. The relative variance of fluctuation follows the temperature dependence ∝ T^-β, where β≈1/2. However, the noise diverges by orders of magnitude as n decreases through the critical concentration n_c and the fluctuation also becomes strongly temperature dependent with β>> 1. At the transition (n/n_c≈1) the fluctuation becomes strongly non-Gaussian below 20K as observed through the second spectrum S⁽²⁾(f). At T=4.2K, we find that after subtracting the Gaussian background , S⁽²⁾(f)∝ 1/f^p where p is small (< 0.5) for metallic samples (n/n_c≥ 1.5) and it grows to > 1 for samples close to the transition n/n_c ≈1. The growth of non-Gaussianity is accompanied by a growth in low frequency spectral weight as seen through a significant enhancement of α from close to 1 (n>n_c) to nearly 1.4 for n/n_c ≈1. The growth of non-Gaussian fluctuation of extremely large magnitude with significant low frequency component points to a correlated low frequency dynamics of charge fluctuation near the insulator-metal transition. This has been interpreted as the onset of a glassy freezing of the electronic system across the transition.

Glassy behavior is a generic feature of electrons close to disorder-driven metal-insulator transitions. Deep in the insulating phase, electrons are tightly bound to impurities, and thus classical models for electron glasses have long been used. As the metallic phase is approached, quantum fluctuations become more important, as they control the electronic mobility. In this paper we review recent work that used extended dynamical mean-field approaches to discuss the influence of such quantum fluctuations on the glassy behavior of electrons, and examine how the stability of the glassy phase is affected by the Anderson and the Mott mechanisms of localization.

We investigate the dynamics of the 2D Coulomb glass model by Monte Carlo simulation. The system has a zero-temperature freezing transition signaled by an exponential divergence of the relaxation rate. At low enough temperatures the system may remain off-equilibrium on experimental timescales. In this regime the dynamics of charge fluctuations and the response of the system to random perturbations of the local potential exhibit aging. The fluctuation dissipation theorem is violated and aging is also observed in the electron diffusion. The relevance of these findings for recent transport experiments in Anderson-insulating films is pointed out.

Studies of low-frequency resistance noise show that the glassy freezing of the two-dimensional electron system (2DES) in Si in the vicinity of the metal-insulator transition (MIT) persists in parallel magnetic fields B of up to 9 T. At low B, both the glass transition density n_g and n_c, the critical density for the MIT, increase with B such that the width of the metallic glass phase (n_c < n_s < n _g) increases with B. At higher B, where the 2DES is spin polarized, n_c and n_g no longer depend on B. Our results demonstrate that charge, as opposed to spin, degrees of freedom are responsible for glassy ordering of the 2DES near the MIT.

The 1/f resistance noise of a two-dimensional (2D) hole system in a high mobility GaAs quantum well has been measured on both sides of the 2D metal-insulator transition (MIT) at zero magnetic field (B = 0), and deep in the insulating regime. The two measurement methods used are described: I or V fixed, and measurement of resp. Vor I fluctuations. The normalized noise magnitude S_R/R² increases strongly when the hole density is decreased, and its temperature (T) dependence goes from a slight increase with T at the largest densities, to a strong decrease at low density. We find that the noise magnitude scales with the resistance, S_R/R² ~ R^2.4. Such a scaling is expected for a second order phase transition or a percolation transition. The possible presence of such a transition is investigated by studying the dependence of the conductivity as a function of the density. This dependence is consistent with a critical behavior close to a critical density p* lower than the usual MIT critical density p_c.

The unusual glassy dynamics exhibited by the spin fluctuations in the stripe-ordered cuprates can be quantitatively measured by La nuclear magnetic resonance. We analyze the spin lattice relaxation data in the low temperature tetragonal structural phase of La_1.8-xEu_0.2Sr_xCuO₄ and find that there is a distribution of local fluctuations times, with a Vogel-Fulcher temperature dependence. Furthermore, the data are consistent with a stretched exponential form for the dynamical spin correlation function, typical of glassy systems.

The spin glass transition temperature of the non-superconducting series La_2-xSr_xCu_0.95Zn_0.05O₄ (x=0.10-0.22) is found to decrease with increasing x and vanish at a critical doping x_c=0.19. This value of x_c coincides with the doping at which the superfluid density and critical current density peak and normal state pseudogap extrapolates to zero in the superconducting analogues of this high-T_c family. The presence and disappearance of T_g for x<0.19 and at x=0.19, are discussed in terms of a competing order and a quantum glass transition, respectively.

Flux-noise and ac susceptibility measurements have been performed on epitaxial YBa₂Cu₃O₇ films. The validity of the fluctuation-dissipation-theorem is verified by the proportionality between flux noise and ac susceptibility results. The nominal zero-field flux-noise spectrum (S_φ(f)) can be characterized as follows: Below a temperature dependent frequency f₀(T), the flux-noise spectrum is frequency independent. For frequencies f > f₀(T), the flux-noise spectrum follows f^-x, with x ≈ 1.7-1.8. f₀ decreases by 4 orders of magnitude in a temperature interval of ▵T ≈ 0.5 K. Moreover, the flux-noise spectrum scales as fS_φ = g(f/f₀(T)), where g(·) is a scaling function. The influence of weak perturbing magnetic fields was investigated. While the general characteristics of the flux-noise spectrum remain, the characteristic frequency f₀ increases by 3-4 orders of magnitude increasing the field from zero to 1 Oe. Flux-noise measurements performed using a compensating field to reduce the residual field in the experimental setup reveal new features of the measured noise spectra. The flux-noise is dominated by thermally generated vortex-antivortex pair fluctuations at temperatures below the mean-field transition temperature, while with decreasing temperature there is a transition to vortex fluctuations being dominated by field generated vortices.

We analyze the noise fluctuations near charge ordering transitions in disordered materials. In particular we consider a two-dimensional (2D) system at finite temperature with a fixed number of defect sites and increase the number of mobile charges. We find that for a high density of charges the system forms a high mobility liquid with low spectral power and a white noise characteristic. As the density of charges is lowered we find a coexistence of liquid regions with locally frozen regions. In this regime the noise power is high and has a 1/f^α form with α ~ 0.5 to 1.5. For the low charge density regime we find a disordered frozen regime where all the charges are localized. We also find that for increasing temperature the noise power drops. We compare our results to recent noise experiments on the 2D metal-insulator transition and the vortex-liquid to vortex glass transition found in superconductors.

The electric thermal noise has been measured in two aging materials, a colloidal suspension (Laponite) and a polymer (polycarbonate), presenting very slow relaxation towards equilibrium. The measurements have been performed during the transition from a fluid-like to a solid-like state for the gel and after a quench for the polymer. For both materials we have observed that the electric noise is characterized by a strong intermittency, which induces a large violation of the Fluctuation Dissipation Theorem (FDT) during the aging time, and may persist for several hours at low frequency. The statistics of these intermittent signals and their dependance on the quench speed for the polymer or on sample concentration for the gel are studied. The results are in a qualitative agreement with recent models of aging, that predict an intermittent dynamics.

We use Time Resolved Correlation (TRC), a recently introduced light scattering method, to study the dynamics of a variety of jammed, or glassy, soft materials. The output of a TRC experiment is c_I(t,r), the time series of the degree of correlation between the speckle patterns generated by the light scattered at time t and t + r. We characterize the fluctuations of c_I by calculating their Probability Density Function, their variance as a function of the lag r, and their time autocorrelation function. The comparison between these quantities for a Brownian sample and for jammed materials indicate unambiguously that the slow dynamics measured in soft glasses is temporally heterogeneous. The analogies with recent experimental, numerical and theoretical work on temporal heterogeneity in glassy dynamics are briefly discussed.

I briefly review a recent series of papers aiming at obtaining a coarse-grained theoretical description of the physics of supercooled liquids approaching their glass transition. After a suitable coarse-graining, the liquid is mapped onto a mobility field, the properties of which being amenable to detailed analytic treatments. The statistical properties of the mobility field then determine those of the liquid. Thermodynamic, spatial, topographic, dynamic properties of the liquid can then be quantitatively described within a single framework, and derive from the existence of an underlying dynamic critical point located at zero-temperature, where timescales and lengthscales diverge.

We present some of our work (done in collaboration with D.S. Fisher) on Kinetically Constrained Lattice Gases focusing on the Kob and Andersen's simple lattice models (KA) for the dynamics of structural glasses. In these systems, although the particles have only hard core interactions, the imposed constraint that they cannot move if surrounded by too many others causes slow dynamics. A very rich dynamical behavior emerges from the simple kinetic rules introduced by KA. Finite dimensional KA models "almost" have a dynamical phase transition: such a transition exists on Bethe lattices and it is replaced by a crossover in finite dimensions. The dominant processes that destroy this putative "mean-field" transition involve cooperative rearrangements of regions, called defects, whose size diverges in the high density limit leading to a physical behavior reminiscent of fragile glass-forming liquids.

One of the striking properties of a glassy system is that many material properties depend on its age, i.e., the time since the system entered its glassy phase. In this this talk we shall review some recent progress (work in collaboration with H. E. Castillo, P. Charbonneau, J. L. Iguain, M. P. Kennett, D. R. Reichman and M. Sellitto) in understanding local aging, through the study of local observable quantities, which reveal that there are spatial heterogeneities and fluctuations in the aging process of macroscopic systems. We show that a number of universal properties are shared by many non-equilibrium systems, both with and without quenched disorder, such as the 3D Edwards-Anderson model and some kinetically constrained non-interacting 2D and 3D spin models, for example. Similar scaling relations are found for mesoscopic sample-to-sample fluctuations of global quantities in small size systems. We discuss how the emergence of a symmetry in aging systems, time-reparametrization invariance, could be responsible for the observed universal behavior of the local and mesoscopic non-equilibrium fluctuations.

Using large scale molecular dynamics simulations, we study the non-linear rheology of a model glass (a 80:20 binary Lennard-Jones mixture). In a glassy state, this model exhibits shear banding at low imposed shear rates. This behavior seems to be closely related to the existence of a static yield stress, σ_y, and to the observation that, under imposed shear rate, σ(γ→0) < σ_y. We measure the static yield stress via simulations under imposed stress and investigate its dependence on temperature. For temperatures far below T_c (the mode coupling critical temperature of the model), σ_y hardly decreases upon heating whereas it changes much faster as T_c is approached. Furthermore, at extremely low shear rates, large stress fluctuations are observed which is reminiscent of the stick-slip motion observed in friction simulations. Our results, which open the possibility of exploring complex rheological behavior using simulations, are compared to recent experiments on various soft glasses.

We investigate local fluctuations in the out of equilibrium dynamics of a simple glass former, a binary Lennard-Jones system, after a quench to the glass state. Motivated by recent experiments that probe fluctuations in glassy materials at distance scales of the order of tens of nanometers, we define the local (i.e. averaged over a small region in the sample) two-time mean square displacement Δ_r(t,t_w) and two-time intermediate scattering function C_r(t,t_w). We study the probability distributions for the values of these local two-time functions as a function of both time variables. We discuss the connection between the scaling behavior of the probability distributions and a soft mode controlling the slow dynamics.

Stochastic surface growth driven by surface tension (Edwards-Wilkinson model) is investigated. The much studied stationary state, characterized by Gaussian distributed Fourier modes with power-law dispersion, is reexamined here to include extremal value statistics. We calculate the probability distribution of the largest Fourier intensity and find that, generically, it does not obey any of the known extreme statistics limit distributions, apart from special border cases where the Fisher-Tippett-Gumbel (FTG) distribution emerges. If a gap is, however, introduced in the dispersion then necessarily the FTG distribution is recovered.

Shot-noise in the electrical current through a 'device' is caused by random processes that determine the electron transport from source to drain. Two sources can be distinguished: on the hand, electrons may randomly emanate from the contacts (source and drain), because the relevant states in the reservoirs fluctuate. On the other hand, the transmission through the device is non-deterministic (non-classical). As we demonstrate in this article the former dominates noise in the vacuum tube, whereas the latter applies to coherent mesoscopic devices, which have been studied in great detail during the last decade.

We present the first measurements of the third moment of the voltage fluctuations in a conductor. This technique can provide new and complementary information on the electronic transport in conducting systems. The measurement was performed on non-superconducting tunnel junctions as a function of voltage bias, for various temperatures and bandwidths up to 1GHz. The data demonstrate the significant effect of the electromagnetic environment of the sample.

The Kubo fluctuation-dissipation theorem relates the current fluctuations of a system in an equilibrium state with the linear AC-conductance. This theorem holds also out of equilibrium provided that the system is in a stationary state and that the linear conductance is replaced by the (dynamic) conductance with respect to the nonequilibrium state. We provide a simple proof for that statement and then apply it in two cases. We first show that in an excess noise measurement at zero temperature, in which the impedance matching is maintained while driving a mesoscopic sample out of equilibrium, it is the nonsymmetrized noise power spectrum which is measured, even if the bare measurement, i.e. without extracting the excess part of the noise, obtains the symmetrized noise. As a second application we derive a commutation relation for the two components of fermionic or bosonic currents which holds in every stationary state and which is a generalization of the one valid only for bosonic currents. As is usually the case, such a commutation relation can be used e.g. to derive Heisenberg uncertainty relationships among these current components.

We discuss how threshold detectors can be used for a direct measurement of the full counting statistics (FCS) of current fluctuations and how to implement Josephson junctions in this respect. We propose a scheme to characterize the full counting statistics from the current dependence of the escape rate measured. We illustrate the scheme with explicit results for tunnel, diffusive and quasi-ballistic mesoscopic conductors.

We formulate a general path integral approach which describes statistics of current fluctuations in mesoscopic coherent conductors at arbitrary frequencies and in the presence of interactions. Applying this approach to the non-interacting case, we analyze the frequency dispersion of the third cumulant of the current operator S₃ at frequencies well below both the inverse charge relaxation time and the inverse electron dwell time. This dispersion turns out to be important in the frequency range comparable to applied voltages. For comparatively transparent conductors it may lead to the sign change of S₃. We also analyze the behavior of the second cumulant of the current operator S₂ (current noise) in the presence of electron-electron interactions. In a wide range of parameters we obtain explicit universal dependencies of S₂ on temperature, voltage and frequency. We demonstrate that Coulomb interaction decreases the Nyquist noise. In this case the interaction correction to the noise spectrum is governed by the combination Σ_nT_n(T_n-1), where T_n is the transmission of the n-th conducting mode. The effect of electron-electron interactions on the shot noise is more complicated. At sufficiently large voltages we recover two different interaction corrections entering with opposite signs. The net result is proportional to Σ_nT_n(T_n-1)(1-2T_n), i.e. Coulomb interaction decreases the shot noise at low transmissions and increases it at high transmissions.

Results of investigations of the equilibrium 1/f^φ noise and non-linearity of the current-voltage characteristics (CVC) of thin metal films and different type of contacts are presented. Contacts metal-metal, contacts of Ta film-based resistors, ohmic contacts to semiconductor, and micro-welded contacts of Cu film-gold wire were studied. The experiments were conducted under the DC, and AC current, and under the influence of series short rectangular current pulses. The theoretical substantiation of experimental results is give here. The use of equilibrium and non-equilibrium 1/f^φ noise and non-linearity of CVC for nondestructive quality control of thin-film conductors, film resistors, different type contacts, which determines reliability of integrated circuits, large-scale integrated circuits (LSICs) and modern electronic devices with long terms of active functioning, is presented. The levels of second harmonic of contacts and third harmonic of resistive film are sensitive parameters of the film resistors quality. LSICs with a high level of current fluctuations in the supply circuit, observed in the static mode, contain local latent defects such as ohmic contact fault in contact windows, break of metallization on a step of oxide, Al-film degradation due to electrocorrosion or EM damages, etc.

The Hooge parameters of compound semiconductors are found to be in inverse proportional to the mean free paths of materials. The newly developed model of 1/f phonon energy partition fluctuation in thermal equilibrium predicts the value of Hooge parameter as α_H = a/λ, the ratio of the lattice constant a and the mean free path λ. Several reported experimental results on α_H for very pure semiconductors are found on the a/λ line. Experimental verification is given by measuring noise in InGaAs/InAlAs heterostructure, where optical phonon effects can be observed due to negligible impurity scattering. The Hooge parameter of about 1 in p-InGaAs and 10^-3 to 10^-5 in n-InGaAs reflects the two order difference in the mobility and corresponding λ values.

The noise of the current of a driven classical one-dimensional charge density wave system is studied in the weak pinning regime using the overdamped equation of motion and the Wavelet Transform Modulus Maxima method. Above the zero temperature depinning transition at low temperatures, the power spectrum of the current noise S(f) scales with frequency f as S(f) ~ f^-γ, where γ≈1, which suggests the existence of flicker. Experimental measurements for quasi-one-dimensional charge density wave materials are in agreement with our findings, providing the first evidence of 1/f behaviour obtained from first principles.

We use large-scale atomistic simulations to study the work-hardening process that occurs when two metals slide against one another. Dislocations form at the interface between the work pieces and then migrate into the bulk. We examine the relationship between the velocity noise signature at the atomistic level and the number of dislocations present. We compare these signatures to those observed in a system of a single particle dragged through a lattice, where local melting can occur.

We have investigated the resistance fluctuations in Si:P as a function of doping level n, across the Metal-Insulator transition at low temperatures. The fluctuation size increases sharply with decrease in the doping level, and shows indications of correlations (presented elsewhere in this conference). The measured jumps in voltage in a current biased sample due to resistance fluctuations were stored digitally and the fluctuation size statistics were estimated in the form of a Probabilty Density Function (PDF). On the metallic side, the PDF's were found to have more or less a Gaussian shape, as expected from an ensemble of small uncorrelated fluctuators. However, we find marked deviation of the PDF from a Gaussian behavior as the system crosses into the insulating side. The deviation starts to occur at the tail of the distribution, and grows in size with decreasing doping levels. The deviating part of the tail could be fitted with a log-normal expression. On the insulating side, this growth of a log-normal tail is also seen to occur as the temperature is lowered. The observations have been analysed using existing theories.

We have studied the conductance fluctuations in silver nanowires in the temperature range 4K to 375K. The nanowires with an average diameter of 15nm were electrochemically deposited using polycarbonate membrane as template. Principal motivation is to study low frequency defect relaxations in the nanowires that give rise to conductance fluctuations with a spectral power S(f) ∝ 1/f^α. The Ag nanowires, stabilized at 400K with a current of few mA, show metallic temperature dependence. The S(f) was measured with a psuedo 4 probe ac technique with rms current of few tens of μA. We find that S_V(f) (which is ∝1/f^α) shows a rapid rise at around 220K as T is increased along with an enhancement in the exponent α. The exponent α≈1-1.1 for T<220 and it increases to ≈1.4 at T=375K. In the same temperature range S(f) rises by an order of magnitude. We analyze the data using a model assuming that there are two components to the 1/f^α fluctuations--one arising from relaxation of local defects give α≈1. The other arises from the long-range diffusion of defects characterized by α≈3/2. It is seen that for T < 220K the noise arises mainly from local defect relaxation and the temperature dependence of a follows the Dutta-Horn model. Above this temperature the contribution from long-range diffusion dominates with the noise becoming thermally activated with an activation energy (~300meV). Interestingly the activation energy is similar to but somewhat higher than that seen in micron sized films.

We have studied the conductance fluctuation in metal film which is under electromigration stressing. The apparatus used by us allows measurement of noise with an ac 5-probe technique with a superimposed dc stressing current (typically 2MA/cm²). This allows measurement of noise in the film at different stages of the electromigration process till the film is damaged completely. We study both the spectral power S_V(f) and also the probability density function (PDF) from the time series. The electromigration stressing was done to elevated temperature on Al and Cu metal lines grown by RF magnetron sputtering. Principal motivation of the investigation is to study low frequency defect relaxations in the metal film due to electromigration that give rise to conductance fluctuations with a spectral power S_V(f)∝ 1/f^α. SV(f) (both magnitude as well as the spectral power quantified through α) shows changes continuously and some times non-monotonically during the electromigration process and it is large just before the damage of the film. It was also observed that the PDF width increases significantly during the course of the em stressing and it changes from a Gaussian to a non-Gaussian PDF.

In this work, we have characterized three NbN_x thin films deposited on sapphire substrate and compared their noise properties. The three films were measured in the same conditions. In the first time, the films were characterized with an impedance analyzer from 20 Hz to 1 MHz. The films are then considered as a RC dipole with a resistor R in parallel with a capacitor C. With the Nyquist formula, we calculate the noise voltage spectral density S_vTh of the RC dipole considering that only the resistor R exhibits thermal noise in unbiased samples. In the second time, noise measurements were made with the samples biased. Thanks to a four contacts configuration, we checked that contact noise do not contribute to our measurements. The difference between the measured noise and the calculated thermal noise S_vTh shows an extra 1/f noise without GR noise contributions. The 1/f noise in the three films extra noise is compared. These results are also compared to the noise measured on NbN thin films deposited on silicon substrate [1].

In this work, we have characterized YBaCuO high Tc superconducting thin films deposited on (001) MgO substrates and compared their noise properties. The films were sputtered on substrates which were annealed at different temperatures prior to deposition. The noise measurements were performed under the same conditions: 1) Without bias, the films are at equilibrium and exhibit only thermal noise proportional to the real part of the impedance for the voltage fluctuations or proportional to the real part of the admittance for the current fluctuations. 2) With bias, the films exhibit 1/f noise due to the conductivity fluctuations. The extra noise is compared with Hooge's empirical relation. The normalized noise spectral density (S_v / V²) measured at 300 K as a function of the substrate annealing temperature displays a bell-shaped dependence with a maximum at a critical temperature.

We present a set of experimental results concerning the current noise produced during the resistive transition in a MgB₂ polycrystalline thin film. Measurements of the power spectrum of the current noise, observed when the temperature is slowly changed across its critical value, are given and discussed. During the transition a large electrical noise component is generated, having a power spectrum of the 1/fⁿ type (n ≈ 3) over a quite wide range of frequencies. This noise may be considered as generated by the abrupt creation of resistive strips across the specimen constituted by grains which have undergone the resistive transition. A computer model, based on this assumption, has been developed to simulate the resistive transition and to evaluate the noise power spectrum. The specimen has been represented as a two dimensional film characterized by randomly oriented grains having slightly different critical currents characterized by a gaussian distribution. When the temperature is incresed and reaches its critical value, resistive strips are formed according to a percolative process, giving rise to resistance steps which are at the origin of the noise. The theoretical results obtained by this model, concerning both the shape and intensity of the noise power spectrum, are compared with the experimental ones directly measured on the specimen.

Resistance fluctuations of low strain thin film La_0.7Ca_0.3MnO₃ grown on NdGaO₃ are examined. The appearance of two state resistance fluctuations are found to be correlated with the onset of remanent magnetization and not with the onset of percolation-like conduction. Their behavior with current, applied magnetic field, and temperature provide information on the nature of the fluctuation. In contrast to the magnetization at the putative T_c of the thin film, the resistance fluctuations display memory of the applied magnetic filed history. An explanation involving a strain enhanced AF interaction is posited.