Enhanced superconductivity in Sr2CuO4−v

Abstract

A critical review of previous investigations of the superconductivity with enhanced T c ~ 95K found in Sr 2 CuO 4-v shows that new physics occurs in a highly overdoped region of the cuprate phase diagram. Moreover, evidence is adduced from the literature that 30% of the oxygen sites in the CuO 2 layers are vacant, a conclusion which is at odds with the universally made assumption that superconductivity originates in stoichiometric CuO 2 layers. While further research is needed in order to identify the pairing mechanism(s) responsible for the enhanced T c , we suggest possible candidates. General ConsiderationsLiu et al. [1] and Yang et al. [2] have reopened the intriguing question concerning the origin of superconductivity in the cuprate superconductor Sr 2 CuO 3+d . This material had been synthesized at very high pressures in the presence of a strong oxidizing agent by several groups starting with the initial discovery of superconductivity by Hiroi et. al. [3], and followed by a series of careful experiments initiated by Han et al [4], and Laffez et al [5]. But those measurements were soon suspect due to the possibility of contamination by the oxidizing agent employed. As noted below the method employed [1] made three major improvements in the synthesis : 1) alleged impurity contamination was disproved, 2) the need for long range oxygen diffusion was removed, and 3) the synthesis provided a good estimate of oxygen concentration.Although the material is conventionally designated Sr 2 CuO 3+d we prefer to use the equivalent formula Sr 2 CuO 4-v because these superconductors have the same K 2 NiF 4 structure as the La 2 CuO 4 ["214"] cuprate family in which high temperature superconductivity was first

Optimum inhomogeneity of local lattice distortions in La 2 CuO 4+ y

“…The critical temperature scales with the cutoff according to a power law with an exponent 0.4 ± 0.05. This result points again toward the importance of connectivity and an optimum inhomogeneity for high critical temperature (12,26). It is also in qualitative agreement with the theoretical prediction of the increase of T c in a granular superconductor on an annealed complex network with a finite cutoff (55,56).…”

Distinct oxygen hole doping in different layers ofSr2CuO4−δ/La2CuO4superlattices

“…18 The absence of this feature in the cuprate NCCOC, which does not have the apical oxygen (Fig. 2), suggests that SHP is an apical oxygen state.…”

“…However, because of limits of bulk crystal growth, doping dependence studies have been limited to doping near x = 2, realized in bulk Sr 2 CuO 4−δ with a very large T c , [12][13][14][16][17][18]27 where the oxygen atoms are removed from the structure (δ > 0), and for relatively low x, near the superconducting dome, where there is evidence of a qualitative change with x in the doping process near x optimal . Specifically, the effective Cu ion magnetic moment in La 2−x Sr x CuO 4 and the magnetic exchange between Cu spins are strongly reduced 19 at x ∼ 0.2 and the T -independent Pauli paramagnetism is replaced at x ∼ 0.22 by T -dependent Curie paramagnetism with increased doping.…”

Fermi surface reconstruction of superoxygenated La2CuO4superconductors with ordered oxygen interstitials

“…17 Controlling their ordering by thermal treatments giant effects on superconductivity have been reported. [18][19][20] Electronic structure calculations show that the displacement of apical oxygen, due to oxygen defects, changes the spin-phonon coupling 21 and the electronic states near the Fermi level in cuprates. 22 The control of the functional electronic properties via dopant manipulation is of very high interest in manganites 23 and graphene.…”