Singling out the effect of quenched disorder in the phase diagram of cuprates

Abstract

We investigate the specific influence of structural disorder on the suppression of antiferromagnetic order and on the emergence of cuprate superconductivity. We single out pure disorder, by focusing on a series of YzEu1−zBa2Cu3O6+y samples at fixed oxygen content y = 0.35, in the range 0 ≤ z ≤ 1. The gradual Y/Eu isovalent substitution smoothly drives the system through the Mott-insulator to superconductor transition from a full antiferromagnet with Néel transition TN = 320 K at z = 0 to a bulk superconductor with superconducting critical temperature Tc = 18 K at z = 1, YBa2Cu3-O6.35. The electronic properties are finely tuned by gradual lattice deformations induced by the different cationic radii of the two lanthanides, inducing a continuous change of the basal Cu(1)-O chain length, as well as a controlled amount of disorder in the active Cu(2)O2 bilayers. We check that internal charge transfer from the basal to the active plane is entirely responsible for the doping of the latter and we show that superconductivity emerges with orthorhombicity. By comparing transition temperatures with those of the isoelectronic clean system we determine the influence of pure structural disorder connected with the Y/Eu alloy.Disorder has an important role in the physics of cuprates, since it induces nucleation of the Anderson localization transition from the non-Fermi-liquid superconductor phase to the antiferromagnetic (AF) ordered charge transfer insulator phase. Very strong coupling has been directly investigated by means of Cu substitutional impurities, often employed also directly as a probe. 1 An inevitable source of weaker disorder is due to the randomly localized ionic charges and the accompanying local structural distortion in the buffer layer that provides chemical doping, controlling the electronic transition. It is often referred to as quenched disorder. In this respect the two most investigated cuprate oxides La 2−x Sr x CuO 4 and YBa 2 Cu 3 O 6+y differ in the distance between the buffer and the active layers. In La 2−x Sr x -CuO 4 the doping is tuned by heterovalent La/Sr cation substitution, close to Cu. In YBa 2 Cu 3 O 6+y , considered the closest case to the clean limit cuprate for its low degree of quenched disorder and high optimal T c , two inequivalent Cu ions are present and doping is controlled by oxygen stoichiometry in a basal buffer layer, containing fragments of Cu(1)O chains (hereafter, chains), whereas magnetism and superconductivity take place in more distant Cu(2)O 2 bilayers (hereafter planes).Theoretically, disorder was addressed explicitly by Alvarez et al. 2 , in a phenomenological model of itinerant electrons on a Cu(2)O 2 square lattice showing that disorder does not lead to a universal behavior. For instance towards the clean limit, as in YBa 2 Cu 3 O 6+y , it may give rise e.g. to coexistence of AF and superconducting order, 3 or to an instability against charge density waves formation, 4,5 whereas quenched disorder opens a hole-density window where none of the two competing orders domina...

Special issue on novel superconducting and magnetic materials

“…The key role of doping and disorder is reflected in the present collection by several contributions both of computational 18,19 and experimental type [20][21][22] . In passing, the effect of doping on superconductivity has attracted also the curiosity of Sandro, see for instance the DFT-based study of Al and Li doping of Mgb 2 23 .…”