Time evolution of the microwave second-order response of YBaCuO powder

Transient effects in the microwave second-order response of YBaCuO powder are investigated. The time evolution of the second harmonic signal has been measured for about 300 s after the sample had been exposed to variations of the DC magnetic field. We show that in different time scales the transient response has different origin. In the time scale of milliseconds the transient response of samples in the critical state is ascribable to processes of flux redistribution induced by the switching on/off of the microwave field. At longer times, the time evolution of the second harmonic signal can be ascribed to motion of fluxons induced by the variation of the DC magnetic field. In particular, diffusive motion of fluxons determines the response in the first 10 seconds after the stop of the magnetic field variation; magnetic relaxation over the surface barrier determines the response in the time scale of minutes.Comment: 16 pages, 7 figures, submited to Physica

The double cone: a mechanical paradox or a geometrical constraint?

In the framework of the Italian National Plan \u2018Lauree Scientifiche\u2019 (PLS) in collaboration with secondary schools, we have investigated the mechanical paradox of the double cone. We have calculated the geometric condition for obtaining an upward movement. Based on this result, we have built a mechanical model with a double cone made of aluminum and a couple of wooden rails

Introduzione al Laboratorio di Fisica: gli errori nelle misure sperimentali

L'attivit\ue0 di laboratorio deve fornire agli studenti la possibilita di effettuare osservazioni quantitative di grandezze fisiche facilmente individuabili e analizzabili. Per questo motivo, bisogna pensare esperimenti operativi, cioe esperimenti che possano essere eseguiti in laboratorio allo scopo di studiare la relazione tra le varie grandezze fisiche coinvolte e quindi determinare la relativa legge fisica. Questi esperimenti hanno valenze didattiche prevalenti per la misura, l'analisi dei dati, la formalizzazione della legge fisica e per gli aspetti formativi in generale. Gli esperimenti illustrativi, cioe esperimenti in cui viene visualizzato un determinato fenomeno fisico, non sono indicati per attivita di laboratorio finalizzata all'analisi dati; essi sono utili da mostrare in classe durante la lezione allo scopo di attirare l'attenzione degli studenti e stimolare la riflessione su una particolare fenomenologia, prima di trattare formalmente lo specifico argomento. Il materiale raccolto in questo volume proviene sostanzialmente dalle lezioni di Laboratorio di Fisica tenuti dall'autore per i Corsi di Studio della ex Facolta di Scienze Matematiche, Fisiche e Naturali confluita oggi nella Scuola delle Scienze di Base ed Applicate dell'Universita degli Studi di Palermo. Nella prima parte del libro, viene affrontato il problema della stima degli errori nelle misure dirette e viene trattata la propagazione degli errori nelle misure indirette. Nella seconda parte, vengono introdotti i metodi elementari di analisi statistica dei dati sperimentali e viene quindi introdotta la funzione di distribuzione di Gauss per gli errori casuali. La trattazione degli argomenti e focalizzata principalmente sul significato fisico delle nozioni introdotte piu che sul formalismo matematico-statistico, con un'esposizione non sempre completamente rigoros

Multidisciplinary learning at the University scientific museums: the Bunsen burner

Nell'articolo riportiamo un'attività di laboratorio svolta insieme a studenti di scuole superiori, al fine di aumentare l’interesse dei giovani studenti verso gli strumenti scientifici di interesse storico e stimolare il loro approccio agli studi scientifici. Viene proposta un'attività "hands on" che può essere facilmente condotta presso i musei scientifici universitari. Un gruppo di studenti selezionati ha frequentato, sotto la guida di ricercatori universitari, uno Stage estivo di una settimana organizzato presso la Collezione Storica degli Strumenti di Fisica e il Museo di Chimica dell'Università di Palermo. È stato proposto lo studio del becco Bunsen, di come funziona e come viene usato nei laboratori di chimica. In particolare, sono stati presi in considerazione alcuni esemplari appartenenti alle collezioni, che saranno illustrati nell'articolo. Alla fine dello Stage, gli studenti hanno descritto ai visitatori gli strumenti e illustrato le attività svolte.We report on a laboratory activity carried out together with secondary school (high-school) students, with the aim of increasing their interest toward historical scientific instruments and stimulate their approach to scientific knowledge. To this purpose, we propose a hands-on activity that can be fruitfully performed at the University scientific museums. We organized a one-week summer stage at the Historical Collection of Physics Instruments and at the Museum of Chemistry of the University of Palermo. A group of selected students attended it, under the tutoring of university researchers. They were showed some Bunsen burners belonging to the collections, how they are restored, how they work and how they were used in the chemical laboratories. At the end of the stage, students introduced museum visitors to these instruments, describing them and referring about the activities they had carried out

Field-induced suppression of the pi-band superconductivity and magnetic hysteresis in the microwave surface resistance of MgB_2 at temperatures near T_c

We report on the magnetic-field-induced variations of the microwave surface resistance, R_s, in a polycrystalline MgB_2 sample, at different values of temperature. We have detected a magnetic hysteresis in R_s, which exhibits an unexpected plateau on decreasing the DC magnetic field below a certain value. In particular, at temperatures near T_c the hysteresis manifests itself only through the presence of the plateau. Although we do not quantitatively justify the anomalous shape of the magnetic hysteresis, we show that the results obtained in the reversible region of the R_s(H) curve can be quite well accounted for by supposing that, in this range of magnetic field, the pi-gap is almost suppressed by the applied field and, consequently, all the pi-band charge carriers are quasiparticles. On this hypothesis, we have calculated R_s(H) supposing that fluxons assume a conventional (single core) structure and the flux dynamics can be described in the framework of conventional models. From the fitting of the experimental results, we determine the values of H_{c2}^pi(T) at temperatures near T_c. In our opinion, the most important result of our investigation is that, at least at temperatures near T_c, the value of the applied field that separates the reversible and irreversible regions of the R_s(H) curves is just H_{c2}^pi(T); a qualitative discussion of the possible reason of this finding is given.Comment: 20 pages, 8 embedded figures, 2 Appendices, accepted for publication in Supercond. Sci. Techno

Critical-state effects on microwave losses in type-II superconductors

We discuss the microwave energy losses in superconductors in the critical state. The field-induced variations of the surface resistance are determined, in the framework of the Coffey and Clem model, by taking into account the distribution of the vortex magnetic field inside the sample. It is shown that the effects of the critical state cannot generally be disregarded to account for the experimental data. Results obtained in bulk niobium at low temperatures are quantitatively justified.Comment: 4 pages, 4 embedded figures, to be published on Eur. Phys. J.

Microwave Second-Harmonic Response of Ceramic MgB2 Samples

Nonlinear microwave response of different ceramic MgB2 samples has been investigated by the technique of second-harmonic emission. The second-harmonic signal has been investigated as a function of temperature, DC magnetic field and input microwave power. The attention has mainly been devoted to the response at low magnetic fields, where nonlinear processes arising from motion of Abrikosov fluxons are ineffective. The results show that different mechanisms are responsible for the nonlinear response in the different ranges of temperature. At low temperatures, the nonlinear response is due to processes involving weak links. At temperatures close to Tc, a further contribution to the harmonic emission is present; it can be ascribed to modulation of the order parameter by the microwave field and gives rise to a peak in the temperature dependence of the harmonic signal.Comment: 16 pages, 8 figures, submitted to Physica

Fluxon dynamics by microwave surface resistance measurements in MgB2

Field-induced variations of the microwave surface resistance, Rs(H), have been investigated in high-density ceramic MgB2. At low temperatures, several peculiarities of the Rs(H) curves cannot be justified in the framework of models reported in the literature. We suggest that they are ascribable to the unconventional vortex structure in MgB2, related to the presence of two gaps. On the contrary, the results near Tc can be accounted for by the Coffey and Clem model, with fluxons moving in the flux-flow regime, provided that the anisotropy of the upper critical field is taken into due account.Comment: 6 pages, 4 figure

Depinning frequency in a heavily neutron-irradiated MgB2 sample

The magnetic-field-induced variations of the microwave surface resistance have been investigated in a heavily neutron-irradiated MgB2 sample, in which the irradiation has caused the merging of the two gaps into a single value. The experimental results have been analyzed in the framework of the Coffey and Clem model. By fitting the experimental data, we have determined the field dependence of the depinning frequency, omega_0, at different values of the temperature. Although the pinning is not particularly effective, the value of omega_0 obtained at low temperatures is considerably higher than that observed in conventional low-temperature superconductors.Comment: 15 pages, 6 figures; to be published in Physica

The Silver Voltameter: An Essential Instrument for the Definition of the Unit of Electric Current

We discuss the restoration of a silver voltameter belonging lo the Historical Collection of Physics Instruments of the University of Palermo. We stress the essential role this instrument had in the definition of the unit of electric current