Combined use of a lock-in detector and a multichannel analyser for 1/f noise application to tunneling spectroscopy

Obtaining a good signal-to-noise ratio (S/N) is of great importance in tunneling spectroscopy, as it has often led to the discovery of new phenomena. The quantities of interest are the derivatives of the tunnel junction I-V curve ; the usual approach to obtain these characteristics is to apply a modulation voltage around a D. C. bias and to detect the proper harmonic with a lock-in detector. In order to improve the S/N ratio, one would think of increasing the detector time-constant (and total sweep time), but experimentalists know that, above a few seconds, such an increase is illusive and even misleading. This is because there are low frequency instabilities in the equipment, but also because there is noise present in the junction characteristic itself. The combined use of a lock-in detector and a multichannel analyser is proposed, in order to average a large number (n) of short sweeps instead of using one single long sweep. The S/N ratio is calculated for such an experiment, and it is found that it depends strongly upon the noise spectral density. For a 1/f spectrum a large improvement (n) is obtained with respect to the usual method, for the same total experimental duration. A system for adapting a commercial multichannel to the plotting of junction characteristics is described, and experiments are performed that demonstrate the advantages of this method. In conclusion, it is emphasized that the limit of the possible improvement of the S/N ratio in tunneling spectroscopy is removed and that the system can be applied to many other types of spectroscopy

Second and third derivative of a tunnel junction characteristic application to the observation of electronic excitations

Tunneling spectroscopy at high bias has proved to be useful as a tool for studying electronic excitations in junctions although one major difficulty encountered is the huge background in the second derivative of the tunneling characteristics I(V). This background however depends on which second derivative is used. We compare the sensitivity of the functions d 2V/dI2, d2I/dV 2 and 1/σ.dσ/dV to inelastic phenomena and conclude that the last one is rather better adapted. We describe a system for obtaining this function at the output of a lock-in detector. The use of higher derivatives to show up very weak, but rather abrupt, peaks in thé curves is discussed and we propose a circuit to plot a third derivative characteristic of tunnel junctions. In order to illustrate the possibilities of this equipment, we re-examine a situation where a peak due to an inelastic phenomenon was expected but not yet observed because of a large base-line and where the use of the third derivative technique enabled us to see it very distinctly.La spectroscopie par effet tunnel dans le domaine des fortes polarisations est un outil intéressant pour l'étude des excitations électroniques dans les jonctions. Mais les courbes d'effet tunnel présentent alors des fonds continus très importants et cela constitue une des difficultés essentielles de ces études. Toutefois ce fond continu dépend beaucoup de la dérivée seconde qui est utilisée. Nous comparons les aptitudes des fonctions d2V/dI2, d 2I/dV2 et 1/σ.dσ/dV quant à la détection d'un phénomène d'effet tunnel inélastique et concluons que la dernière est la mieux adaptée. Nous décrivons un montage qui permet d'obtenir cette fonction directement à la sortie d'une détection synchrone. Nous discutons ensuite la possibilité d'utiliser des dérivées d'ordre supérieur pour faire ressortir des structures très petites mais à variation relativement rapide et proposons un montage pour tracer une caractéristique de dérivée troisième de jonction tunnel. Pour illustrer les possibilités de cette méthode, nous nous intéressons à un cas où un pic d'effet tunnel inélastique attendu n'avait pas pu être observé jusqu'à présent du fait d'un fond continu trop important. La technique de la dérivée troisième nous permet de, l'observer distinctive-ment

Tunnelling investigation of the phonon spectrum of superconducting Be implications for an eventual enhancement of Tc

No abstract availabl

Tunnelling investigation of the phonon spectrum of superconducting Be implications for an eventual enhancement of Tc

Beryllium is an exciting superconductor in that the theoretical upper limit for its Tc is 140 K. We discuss here the possibility of increasing the critical temperature of the vapour quenched phase whose Tc is already 9.5 K. Tunnelling into films of this phase indicates that it is a weak coupling superconductor (λ < 0.6). Consequently, its Tc could be greatly enhanced, although experimental attemps have failed up to now.Le béryllium est un supraconducteur tout particulièrement intéressant car la théorie situe la limite supérieure de sa Tc à 140 K. Partant de la phase du Be dont la Tc est déjà de 9,5 K, nous discutons la possibilité d'augmenter cette température critique. Une étude par effet tunnel de cette phase nous conduit à penser qu'il s'agit bien d'un supraconducteur à couplage faible (λ<0,6) qui est donc susceptible de présenter des élévations importantes de sa Tc

Multiway Canonical Correlation Analysis of Brain Signals

Brain signals recorded with electroencephalography (EEG), magnetoencephalography (MEG) and related techniques often have poor signal-to-noise ratio due to the presence of multiple competing sources and artifacts. A common remedy is to average over repeats of the same stimulus, but this is not applicable for temporally extended stimuli that are presented only once (speech, music, movies, natural sound). An alternative is to average responses over multiple subjects that were presented with the same identical stimuli, but differences in geometry of brain sources and sensors reduce the effectiveness of this solution. Multiway canonical correlation analysis (MCCA) brings a solution to this problem by allowing data from multiple subjects to be fused in such a way as to extract components common to all. This paper reviews the method, offers application examples that illustrate its effectiveness, and outlines the caveats and risks entailed by the method

Fundamental Frequency Estimation of the Speech Signal Compressed by MP3 Algorithm Using PCC Interpolation

In this paper the fundamental frequency estimation results of the MP3 modeled speech signal are analyzed. The estimation of the fundamental frequency was performed by the Picking-Peaks algorithm with the implemented Parametric Cubic Convolution (PCC) interpolation. The efficiency of PCC was tested for Catmull-Rom, Greville and Greville two-parametric kernel. Depending on MSE, a window that gives optimal results was chosen