Academic literature on the topic 'Y-Bi2212'

The spiral growth mechanisms associated with the growth conditions of Bi 2 Sr 2 CaCu 2 O y (Bi-2212) crystals grown at a KCI surface have been studied. The spiral growth mechanisms and the effects of growth conditions on the formation of spirals in YBa 2 Cu 3 O y (Y-123) thin films and single crystal have also been studied. Screw dislocation formation mechanism in the Bi2212 is different from that in Y-123. A model for a "vapour" deposition process with discontinuous growth conditions is put forward. It is proposed that spiral growth in Bi-2212 is similar to that in Y-123 thin films produced by Pulsed Laser Deposition. It is concluded that the formation of screw dislocations and discontinuous growth conditions are responsible for the coexistence between spiral and two-dimensional nucleation growth.

We show an evidence for the superconducting stripes (superstripes) in the Bi2212 system by joint x-ray diffraction and angle resolved photoemission. The kink observed at k y =0.4π in the energy distribution curves is shown to be related to a modulation of the Cu displacement out of the oxygen plane with a wavevector Q ~(0.4π, 0.4π) that modulates the next-nearest neighbor hopping integral t ′. The resulting Fermi surface reveals broken segments around the M points due to the modulation of the t ′, associated with modulation of the electron-lattice coupling λ(ε) that depends on the micro strain ε of the CuO 2 plane. The present findings further enlightens the fact that the micro-strain, controlling the electron-lattice coupling λ(ε) is a critical parameter for the superstripes.

We had performed a study on magnetoresistivity Hall, Nernst and Seebeck effects in the mixed and normal state for "Bi2212" bulk with Gd (0≤x≤0.50) and Zn (0≤y≤0.03) prepared by the conventional solid state reaction method in magnetic fields between 0 and 5 T and in the temperature range 5–300 K. The critical temperatures, the Hall concentration, the Nernst and Seebeck coefficients depend strongly on the Zn and Gd contents in the samples. Also, we have found an anomalous suppression of superconductivity at x=0.30–0.35 and y=0.025–0.030, when the hole concentration per Cu is p~1/8 and the transport properties exhibit less metallic behavior than usual. There is a possibility that a kind of order of holes and/or spins is stabilized owing to pinning by Zn, as in the La-based cuprate.

We discuss a few possibilities of high- T c superconductivity with more than one orbital symmetry contributing to the pairing. First, we show that the high energies of orbital excitations in various cuprates suggest a simplified model with a single orbital of x 2 − y 2 symmetry doped by holes. Next, several routes towards involving both e g orbital symmetries for doped holes are discussed: (i) some give superconductivity in a CuO 2 monolayer on Bi2212 superconductors, Sr 2 CuO 4 − δ , Ba 2 CuO 4 − δ , while (ii) others as nickelate heterostructures or Eu 2 − x Sr x NiO 4 , could in principle realize it as well. At low electron filling of Ru ions, spin-orbital entangled states of t 2 g symmetry contribute in Sr 2 RuO 4 . Finally, electrons with both t 2 g and e g orbital symmetries contribute to the superconducting properties and nematicity of Fe-based superconductors, pnictides or FeSe. Some of them provide examples of orbital-selective Cooper pairing.

We apply the spin-wave theory with the additional constraint of zero staggered magnetization to investigate the two-dimensional t–J model in the paramagnetic state in the ranges of hole concentrations 0.02 ≲ x ≲ 0.17 and temperatures T ≲ 100 K. In this region the hole spectrum is nonmetallic and contains a pseudogap with properties similar to those observed in Bi2212 photoemission. The calculated spin correlation length, susceptibility, spin-lattice relaxation times at the Cu and O sites and Cu spin-echo decay time are in qualitative and in some cases in quantitative agreement with experiment in underdoped YBa2Cu3O 6+y. The temperature dependences of these quantities are typical for the quantum disordered regime with a pseudogap in the spectrum of magnetic excitations. In the Eliashberg formalism the hole–magnon interaction was found to be unable alone to give rise to superconductivity. With inclusion of a moderate interaction with apex oxygen vibrations, high-T c 's are obtained for even frequency dx2-y2 pairing.

2010/2011
La superconduttività nei superconduttori ad alta temperature critica basati su ossidi di Rame (noti anche come cuprati), a partire dalla propria scoperta (risalente al 1986), ha stimolato una mole senza precedenti di sforzi teorici e sperimentali. Ad oggi, diversi aspetti rimangono senza risposta. Tra questi è possibile citare: i) la natura della ‘glue’ che porta alla formazione delle coppie di Cooper; ii) la natura della pseudo gap; iii) la natura dell’interplay tra fenomeni a scale di energia molto diverse (quella del condensato e quella legata a stati Rame-Ossigeno ad alta (~1 eV) energia). Nel mio progetto di Dottorato ho affrontato questi problemi con una tecnica innovativa, completamente ottica, risolta in tempo: la spettroscopia risolta in tempo. Lo sviluppo di questa nuova tecnica spettroscopica è stato affrontato durante la prima parte del progetto di ricerca. Attraverso impulsi di luce ultracorti (<100 fs) e ad ampio contenuto spettrale (complessivamente, il range spettrale compreso tra 500 e 2500 nm è stato analizzato), è stata studiata l’evoluzione temporale della funzione dielettrica di campioni superconduttivi di Y-Bi2212 (Bi2Sr2Y0.08Ca0.92CuO8+), portati in una condizione di ‘fuori-equilibrio’ mediante un impulso di pompa. I campioni analizzati coprono diversi livelli di doping (under doping, optimally doping, overdoping). Una debole eccitazione dei campioni assicura che soltanto la risposta intrinseca di ciascuna delle fasi rilevanti del sistema (stato normale, pseudo gap, stato superconduttivo) sia analizzata. Facendo uso di un modello basato su una funzione dielettrica differenziale, e grazie all’ampio intervallo spettrale analizzato, è stato possibile associare senza ambiguità una origine fisica ai segnali ottici risolti in tempo. In particolare, l’analisi della riflettività transiente misurata a temperatura ambiente (T=300 K), effettuata simultaneamente nei domini del tempo (con un modello a quattro temperature) e delle frequenze (con un modello ‘Extended Drude’), ha permesso di concludere che il pairing nei cuprati superconduttori è principalmente di natura elettronica. Ovvero, che le eccitazioni bosoniche di origine elettronica (di cui sono possibili candidati fluttuazioni antiferromagnetiche di spin oppure correnti circolanti) sono i fattori più importanti per la formazione dello stato superconduttivo ad alta temperatura critica in sistemi basati su ossidi di Rame. Nella regione di pseudo gap, le evidenze sperimentali indicano uno scenario in cui l’accoppiamento elettrone-bosone acquista una dipendenza dalla temperatura, a causa della comparsa di un nuovo modo di eccitazione. Questo fatto suggerisce che la pseudogap sia una vera fase termodinamica della materia. Molti sforzi sono stati dedicati allo studio delle proprietà ottiche fuori equilibrio nella fase superconduttiva. La comparsa dello stato superconduttivo è accompagnato da una forte modificazione di stati Rame-Ossigeno ad alta energia, coinvolti in transizioni ottiche ad 1.5 e 2 eV. Questa osservazione è in accordo, e conferma, evidenze sperimentali precedenti, riguardanti un interplay tra la fisica del condensato superconduttivo e la fisica degli stati ad alta energia, inteso come un trasferimento di peso spettrale tra queste due regioni, che avviene attraverso la transizione nello stato superconduttivo. L’alta sensibilità della tecnica risolta in tempo, rispetto alle spettroscopie convenzionali all’equilibrio, ha permesso di dimostrare senza ambiguità che l’intero trasferimento di peso spettrale evidenziato dalla spettroscopia ottica all’equilibrio è dovuto ad una modificazione di due transizioni ottiche, ad 1.5 e 2 eV. La variazione di energia cinetica associata al trasferimento di peso spettrale cambia segno in prossimità del livello di doping ottimale, necessario ad ottenere la massima temperatura critica del materiale. Ciò suggerisce che la superconduttività sia guidata da meccanismi diversi, nelle regioni di under- ed over- doping del diagramma delle fasi di un superconduttore basato su ossidi di Rame. Attraverso la razionalizzazione delle evidenze sperimentali ottenute attraverso la spettroscopia ottica risolta in tempo, applicata a materiali underdoped, optimally doped ed underdoped, e nelle fasi normale, di pseudogap e superconduttiva, è stato possibile formulare un diagramma delle fasi per il materiale Y-Bi2212, interamente basato su evidenze derivanti da una tecnica ottica fuori equilibrio. Tale diagramma delle fasi è governato da un punto critico quantistico che si trova a T=0, all’interno del dome superconduttivo. La ‘critical line’ di questo diagramma delle fasi delimita una regione di spazio delle fasi Doping-Temperatura nella quale l’accoppiamento elettrone-bosone acquista una dipendenza dalla temperatura.
Superconductivity in copper-oxide based high temperature superconductors (also known as cuprates) stimulated, since its discovery (dated 1986), an unprecedented amount of both theoretical and experimental efforts. Many aspects, such as: i) the glue leading to Cooper Pair formation; ii) the nature of the pseudogap and iii) the interplay between physics at different energy scales (the superconducting condensate and the Cu-O high-energy states) remain elusive. With my PhD Project I tackled these problems with a novel, all-optical, time-resolved experimental technique, the time-resolved spectroscopy. The development of this novel spectroscopic technique has been faced during the first part of my project. By means of ultrashort (<100 fs) and broadband (the spectral range 500-2500 nm has been investigated) probe pulses, the temporal evolution of the dielectric function of Y-Bi2212 (Bi2Sr2Y0.08Ca0.92CuO8+) superconducting samples (available with different doping levels: underdoping, optimally doping and overdoping) brought out of equilibrium by a pump pulse has been studied. A weak sample excitation ensured only the intrinsic response of each of the relevant phases of the material (normal state, pseudogap, superconducting phase) were probed. By means of a differential dielectric function approach, and thanks to the broad spectral range I investigated, the actual connection between the observed time-resolved optical signal, and the physics beyond it, has been unambiguously established. In particular, the analysis of the room-temperature (T=300 K) time-resolved data simultaneously in the temporal (with a four-temperature model) and in the spectral (with an Extended Drude model) domains, allowed to conclude that the pairing in cuprate superconductors is mainly of electronic origin. That is, bosonic excitations of electronic origin (possible candidates are antiferromagnetic spin fluctuation and current loops) are the most important factor for the formation of the superconductivity state at high temperatures in copper-oxide based systems. In the pseudogap region, the experimental evidence points toward a scenario in which the electron-boson coupling acquires a temperature-dependence, due to the appearance of an excitation mode. This suggests the pseudogap is indeed a phase of matter. Many efforts has been devoted to study the non-equilibrium optical properties in the superconducting phase. The onset of superconductivity is accompanied by a strong modification of high energy Cu-O states, involved in optical transitions at 1.5 and 2 eV. This finding supports previous evidences of an interplay between the condensate and the high-energy physics, in the sense of a transfer of spectral weight among the two regions, across the superconducting transition. The high sensibility of our technique with respect to the conventional, equilibrium spectroscopies, allowed to unambiguously demonstrate that the whole spectral weight transfer is due to a modification of the 1.5 and 2 eV optical transitions. The carriers kinetic energy modification associated to the spectral weight transfer changes sign near the optimal doping level required to attain the maximum critical temperature. This suggests that superconductivity is driven by different mechanisms, in the underdoped and in the overdoped regions of the phase diagram of copper-oxide based superconductors. By rationalizing the experimental findings obtained by the time-resolved spectroscopy in underdoped, optimally doped and overdoped samples, in the normal state, in the pseudogap and in the superconducting state, a phase diagram for the Y-Bi2212 cuprate superconductor, based entirely on non-equilibrium evidences, has been proposed. It is governed by a quantum critical point located inside the superconducting dome. The critical line delimits a region in which the electron-boson coupling is temperature-dependent.
XXIV Ciclo
1983