An EPR investigtion on Pseudogap in SmBa2Cu3O6+d superconductor

The energy gap is the energy needed to break a Cooper pair into two free electrons and it is one of the defining characteristics of a superconductor. However, in the mid 1990s a similar gap (called pseudogap) has been discovered at the temperature T* > Tc in the so-called underdoped materials, i.e. with carrier concentration not high enough to give the best superconducting properties to the oxide [1, 2]. Many theories have been proposed on pseudogap and superconductivity mechanism, among which that of preformed pairing. Following this theory, electrons would bind into pairs at T*> Tc, but would remain unable to superconduct because they move incoherently, being unable to establish “phase coherence” with all the other pairs. Phase coherence and superconduction would occur only at T = Tc. Therefore, the spin pairing at T* could be only the first step towards superconductivity. However, other researchers have claimed the presence of a pseudogap in a nonsuperconducting material such as a strontium doped lanthanum manganite, which became ferromagnetic at enough low temperature and displayed colossal magnetoresistance [3]. Therefore, pseudogap formation at T = T* could be a general phenomenon not necessarily involving superconductivity at any T < T*. Indeed, in the present investigation we attribute a broadening at T* @ 180 K of the exchange-narrowed Lorentzian-shaped EPR line of SmBa2Cu3-xAlxO6.98 (x = 0.33) to pseudogap formation in this non-superconducting material. An analogous linebroadening is observed with the x = 0.15 sample characterised by Tc @ 47 K [4] but not with the x = 0 sample, which is the best-performing superconductor of this series, having Tc @ 90 K. References [1] T.Morinari, Superconductor Science and Technology, 16 (2003) 624. [2] M.Vershinin, S.Misra, S.Ono, Y.Abe, Y.Ando, A.Yazdani, Science, 303 (2004) 1995. [3] N.Mannella, W.L.Yang, X.J.Zhou, H.Zheng, J.F.Mitchell, J.Zaanen, T.P.Devereaux, N.Nagaosa, Z.Hussain, Z.-X.Shen, Nature, 438 (2005) 474. [4] M.Scavini, M.Daldosso, S.Cappelli, C.Oliva, M.Brunelli, C.Ferrero, A.Lascialfari, Europhysics Lett., 76 (2006) 443.