Superconductivity, the state of matter in which electrons can move without friction, is a relevant sci-entific driver since its discovery in 1911. It is especially true for high temperature superconductivity discovered in 1986 which has outdated a scientific misbelief that the critical temperature, which sep-arates the normal state from the superconducting one, cannot be higher than 30 K. Nowadays the record critical temperature is at 133 K, and it is believed that if the underlying mechanism of this phenomenon is understood, we could be able to create room temperature superconducting materi-als, which would be economically enormously beneficial. My master thesis project addressed several issues related to the normal state property of one promi-nent member of high temperature copper-oxide superconductors, the Bi2Sr2CaCu2O8. (abbreviated as Bi-2212). Its critical temperature could be tuned from 91 K to 0 K by substitution of Ca2+ with Y3+ or Pr3+. I have measured two transport coefficients as function of temperature: electrical resistivity (ρ) and thermoelectric power also called as Seebeck coefficient (S or TEP). The electrical resistivity has been abundantly studied in the literature and it marks very well the various phase transitions and cross-overs in the electronic structure of the compound as a function of temperature and doping. It is possible to observe: superconducting transition, metal-insulator transition, pseudogap, super-conducting fluctuations and Fermi liquid behavior. The Seebeck coefficient has been less studied and there is no real understanding of its temperature dependence, and the major interactions which de-termine its behavior. My major findings are: S is not influenced by the pseudogap visible in ρ(T); strong electron-phonon interactions (polaronic character) at low doping determine the temperature dependence and magnitude of S; and point defects (introduced by electron irradiation) do not affect it. The role of magnetic interactions which evolve with decreasing Tc has been suggested in shaping thermoelectric power curves. In order to explore the possibility of this material for applications other than superconducting, com-posite structures were tested with TiO2 and CH3NH3PbI3 for memristor and photoresponsive devices.
La superconduttività, stato della materia che presenta zero resistività, è dal tempo della sua scoperta, nel 1911, fonte di intensa ricerca scientifica. Questo risulta particolarmente vero nel caso della superconduttività ad alta temperatura, scoperta nel 1986, che ha smentito errate conclusioni scientifiche, come la convinzione che tale fenomeno non fosse possibile sopra i 30K. Il record attuale è di 133K, e si crede che la completa comprensione del meccanismo sotteso a questo fenomeno possa permettere la sintesi di materiali superconduttori a temperatura ambiente, con una considerevole rilevanza economica. Il mio progetto di tesi è stato indirizzato verso lo studio dello stato non superconduttivo di uno tra composti più rilevanti della famiglia degli ossidi di rame superconduttori ad alta temperatura, il Bi2Sr2CaCu2O8 (abbreviato come Bi-2212). La temperatura critica di questo composto può essere variata da 91K a 0K, attraverso la sostituzione del Ca2+ con Y3+ o Pr3+. Su questo materiale ho misurato due proprietà di trasporto in funzione della temperatura: resistività elettrica (ρ) e coefficiente termoelettrico o di Seebeck (S). La prima è stato abbondantemente studiato in letteratura, e permette di identificare, in funzione del doping e della temperatura, le diverse transizioni di fase e cross-over nella struttura elettronica di questo composto. Sono identificabili: transizione superconduttiva, transizione metallo-isolante, pseudogap, fluttuazioni superconduttive e stati di liquido di Fermi. S al contrario è stato meno studiato e manca una chiara interpretazione della sua dipendenza con la temperatura e delle interazioni che ne stabiliscono l’andamento. Le mie principali osservazioni sono: S non è influenzato dallo stato di pseudogap, riconoscibile in ρ(T); forti interazioni elettrone-fonone (di natura polaronica) a basso livello di doping influenzano il valore di S; difetti puntuali (introdotti per irraggiamento con elettroni) non hanno effetto su tale coefficiente. E’ stato proposto un ruolo delle interazioni magnetiche, più intense al diminuire della Tc, nel determinare l’andamento di S. Altre possibili applicazioni e fenomeni legati a questo materiale sono state investigate creando strutture composite con TiO2 o CH3NH3PbI3 per memristor o dispositivi fotosensibili.
Study of transport coefficients in cuprate high temperature superconductor Bi-2212
MARTINO, EDOARDO
2014/2015
Abstract
Superconductivity, the state of matter in which electrons can move without friction, is a relevant sci-entific driver since its discovery in 1911. It is especially true for high temperature superconductivity discovered in 1986 which has outdated a scientific misbelief that the critical temperature, which sep-arates the normal state from the superconducting one, cannot be higher than 30 K. Nowadays the record critical temperature is at 133 K, and it is believed that if the underlying mechanism of this phenomenon is understood, we could be able to create room temperature superconducting materi-als, which would be economically enormously beneficial. My master thesis project addressed several issues related to the normal state property of one promi-nent member of high temperature copper-oxide superconductors, the Bi2Sr2CaCu2O8. (abbreviated as Bi-2212). Its critical temperature could be tuned from 91 K to 0 K by substitution of Ca2+ with Y3+ or Pr3+. I have measured two transport coefficients as function of temperature: electrical resistivity (ρ) and thermoelectric power also called as Seebeck coefficient (S or TEP). The electrical resistivity has been abundantly studied in the literature and it marks very well the various phase transitions and cross-overs in the electronic structure of the compound as a function of temperature and doping. It is possible to observe: superconducting transition, metal-insulator transition, pseudogap, super-conducting fluctuations and Fermi liquid behavior. The Seebeck coefficient has been less studied and there is no real understanding of its temperature dependence, and the major interactions which de-termine its behavior. My major findings are: S is not influenced by the pseudogap visible in ρ(T); strong electron-phonon interactions (polaronic character) at low doping determine the temperature dependence and magnitude of S; and point defects (introduced by electron irradiation) do not affect it. The role of magnetic interactions which evolve with decreasing Tc has been suggested in shaping thermoelectric power curves. In order to explore the possibility of this material for applications other than superconducting, com-posite structures were tested with TiO2 and CH3NH3PbI3 for memristor and photoresponsive devices.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/112407