We present the first experimental determination of the time auto-correlation of magnetization in the non-stationary regime of a spin glass, and its quantitative comparison with the corresponding response, the magnetic relaxation function. These measurements were performed in a new experimental setup working as an absolute thermometer. Clearly, we observe a non-linear fluctuation-dissipation relation between correlation and relaxation: at large correlation (short observation times) fluctuation dissipation theorem is obeyed, while the fluctuation dissipation relation is driven by an effective temperature higher than the bath temperature in the aging regime (small correlation and large observation times). In the aging regime, the relaxation vs. correlation curves depend weakly on the waiting time. According to theoretical developments on mean field models, and lately on short range ones, in the limit of very large waiting times, the relation between relaxation and correlation in the aging regime becomes temperature independent for a given system. A scaling procedure allows us to extrapolate to the limit of long waiting times by separating stationary and non-stationary regimes and to check the validity of the temperature independence of the fluctuation dissipation relation in the non-stationary regime.
The large increase in the flux-flow voltage noise, commonly observed in the vicinity of the peak-effect in superconductors, is ascribed to a novel noise mechanism. The mechanism consists of random injection of the strongly pinned metastable disordered vortex phase through the sample edges and its subsequent random annealing into the weakly pinned ordered phase in the bulk. This results in large critical current fluctuations causing strong vortex velocity fluctuations. The
excess noise due to this dynamic admixture of two vortex phases is found to display pronounced reentrant behavior. In the Corbino geometry the injection of the metastable phase is prevented and, accordingly, the excess noise disappears. The excess flux-flow noise in the peak-effect regime is dominated by vortex velocity fluctuations while the density fluctuations, frequently considered in the conventional flux-flow noise models, are negligibly weak. Strong nongaussian fluctuations are associated with S-shaped current-voltage characteristics. The spectral properties of the noise reflect the form of the frequency characteristics of the dynamically coexisting
vortex phases which is equivalent to the first order filter response. The cutoff frequency in the spectra corresponds to the time-of-flight of vortices through the disordered part of the sample.
We report on studies aimed at understanding and improving the intrinsic noise of high-performance sensors using a 2D electron gas channel confined by a quantum well in the pseudomorphic AlGaAs/InGaAs/GaAs heterostructure. MIS gated and ungated Hall sensors shaped as a Greek cross with dimensions ranging from 100 μm down to submicrometer range have been investigated. At room temperature the predominant low frequency Hall voltage noise originates from the ensemble of trapping/detrapping events occurring within the continuum of GaAs surface states. Its power spectral density can be deduced from independent measurements of the interface trap density-of-states by applying Shockley-Read-Hall dynamics and the Fluctuation-Dissipation Theorem. In fact, theoretical spectra calculated without any adjustable fitting parameter coincide closely with the experimentally measured ones. At cryogenic temperature this interface traps noise freezes out, thus revealing a much weaker intrinsic background noise with 1/f spectrum. For small sensors the intrinsic 1/f noise converts into one or a few lorentzians due to the action of individual random telegraph signals (RTS). For Hall crosses with an intersection of 4x4μm<sup>2</sup>, we find statistically less than 1 fluctuator per each decade of time constant at 77 K. Due to the random distribution of the elementary fluctuators, some of these small Hall crosses may show less low-frequency noise than much larger 60x60μm<sup>2</sup> sensors.
Conference Committee Involvement (2)
Fluctuations and Noise in Materials
26 May 2004 | Maspalomas, Gran Canaria Island, Spain