Pairing Symmetry Studied from Impurity Effects in the Undoped Superconductor T′-La_1.8Eu_0.2CuO₄

Abstract

We have investigated the effects of magnetic Ni and nonmagnetic Zn impurities on the superconductivity in undoped T'-La 1.8 Eu 0.2 CuO 4 (T'-LECO) with the Nd 2 CuO 4 -type structure, using the polycrystalline bulk samples, to clarify the pairing symmetry. It has been found that both suppression rates of the superconducting transition temperature T c by Ni and Zn impurities are nearly the same and are very similar to those in the optimally doped and overdoped regimes of hole-doped T-La 2−x Sr x CuO 4 with the K 2 NiF 4 -type structure. These results strongly suggest that the superconductivity in undoped T'-LECO is of the d-wave symmetry and is mediated by the spin fluctuation. * E-mail: tkawamata@teion.apph.tohoku.ac.jp J. Phys. Soc. Jpn. LETTERSIt has long been believed that the high-T c superconductivity appears through the hole and electron doping into the antiferromagnetic mother compound Ln 2 CuO 4 (Ln: lanthanide elements) with the K 2 NiF 4 -type (so-called T-type) and Nd 2 CuO 4 -type (so-called T'-type) structure, respectively. 1 In electron-doped T'-Ln 2−x Ce x CuO 4 , it has also been known that the reduction annealing of the as-grown samples to remove excess oxygen occupying the apical site just above Cu in the CuO 2 plane is necessary for the appearance of superconductivity at x 0.14. [2][3][4] Recently, in adequately reduced thin films of T'-Nd 2−x Ce x CuO 4 (T'-NCCO), however, superconductivity has been observed in a wide range of x and even in the undoped mother compound of x = 0. 5 Moreover, the superconductivity in undoped T'- The study of the effects of magnetic and nonmagnetic impurities on the superconductivity is useful to clarify the pairing symmetry of the superconductivity, because in a conventional s-wave superconductor, the SC transition temperature T c is suppressed by magnetic impurities more rapidly than by nonmagnetic impurities. In an unconventional superconductor with a sign-changing SC gap such as a d-wave superconductor, on the other hand, T c is rapidly suppressed by nonmagnetic impurities as well as by magnetic impurities. In fact, many studies of the impurity effect on T c have been carried out in high-T c cuprate superconductors. [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31] In hole-doped T-La 2−x Sr x CuO 4 (T-LSCO), the suppression of T c by nonmagnetic Zn is comparable or even slightly larger than the suppression by magentic Ni. 16-19, 22, 23, 27, 28 Such a tendency has also been observed in hole-doped Bi-2212 25, 29 and YBa 2 Cu 3 O 7 , 21,22,27 leading to the conclusion that the pairing symmetry of hole-doped high-T c cuprate superconductors is unconventional d-wave mediated by spin fluctuation. In electron-doped T'-NCCO, on the other hand, the T c suppression by Ni is much more rapid than by Zn. 18,20,24,32 In the electron- 20,23,30 respectively. It is found that both suppression rates of T c in T'-LECO by the Ni and Zn substitution are nearly the same. Surprisingly, this behavior is different from that of electron-doped T'-NCCO with x = 0.15 in...

Novel Electronic State and Superconductivity in the Electron-Doped High-Tc T’-Superconductors

“…Our crystal of Pr 1.3−x La 0.7 Ce x CuO 4+δ with x = 0.10 seems to be the case of the reduction of excess oxygen. Intriguing in our results is that the introduction of Cu vacancies into SC La 1.8 Eu 0.2 Cu 1−y O 4+δ does not change T c so much up to y = 0.015 [23], which seems to be incompatible with the Cu-deficiency model. In any case, this issue would be clarified by the recent high resolution neutron and X-ray diffraction experiments.…”

“…As Zn/Ni impurity effects on T c in La 1.8 Eu 0.2 Cu 1−y (Zn,Ni) y O 4+δ suggest that the electronic state of the parent compounds is similar to the overdoped regime of the hole-doped cuprates [23], it may be natural that further hole doping results in the decrease in T c .…”

“…insisted the occurrence of d-wave superconductivity in Nd 2−x Ce x CuO 4+δ with x = 0.15 and Pr 2−x Ce x CuO 4+δ with x = 0.15. To clarify the SC pairing symmetry of the undoped (Ce-free) superconductivity in the T'-cuprates, impurity effects on T c were precisely investigated in La 1.8 Eu 0.2 Cu 1−y (Zn,Ni) y O 4+δ [23]. Figure 7 superconductivity [57].…”

“…Yet to date, the mechanism of the undoped superconductivity is unclear. In this review article, we show our recent results relating to the undoped superconductivity in the T'cuprates [21][22][23]. First, we present the characteristics of the electron-doped T'-cuprates including the reduction annealing, conventional phase diagram and undoped superconductivity.…”

Electron-Doping Effect on Tc in the Undoped (Ce-Free) Superconductor T′-La1.8Eu0.2CuO4 Studied by the Fluorine Substitution for Oxygen

“…As to the mechanism of the superconductivity, d-wave paring mediated by spin fluctuations is suggested from experimental results of the impurity effects, 12,13) SR, 13,14) and NMR 15) in undoped (Ce-free) polycrystalline samples of T'-La 1.8 Eu 0.2 CuO 4 , in which nonmagnetic Eu 3+ with a small ionic radius is partially substituted for nonmagnetic La 3+ with a large ionic one to stabilize the T'-type structure during the reduction annealing. 10) As to the electronic state of the undoped (Ce-free) superconductor, there are two possibilities.…”