Cuprate Planes Research Articles

Recent neutron results on magnetic superconductors and related systems

We briefly summarize the presentation at the conference discussing our recent neutron scattering results on magnetic superconductors. We give some historical perspective, and then focus on the coexistence and competition between the magnetic and superconducting order parameters in the cuprate superconductors. We also discuss our very recent work on the Na x CoO 2 system, where the Co ions carry S = 1 2 as in the square–planar cuprates, but with the Co ions occupying a triangular lattice.

Circulating-current states and ring-exchange interactions in cuprates

We consider the consequences for circulating-current states of a cyclic, four-spin, ``ring-exchange'' interaction of the type shown recently to be significant in cuprate systems. The real-space Hartree-Fock approach is used to establish the existence of charge-current and spin-current phases in a generalized Hubbard model for the CuO_2 planes in cuprates. We compare the results of the Hartree-Fock approximation with the correlated states renormalized by Gutzwiller projection factors which allows us to gauge the qualitative effects of projection to no double site occupancy. We find that charge flux states may be competitive in cuprates, whereas spin flux states are suppressed in the strongly correlated regime. We then include the ring-exchange interaction and demonstrate its effect on current-carrying states both at and away from half-filling.

Synthesis and Crystal Structure of Na7Cu3O8, Containing a New Type of Oxocuprate(III) Oligoanion

AbstractNa7Cu3O8 was prepared through oxidation of a Na3CuO2/NaCuO mixture (2:1) in dried oxygen at 450 °C. Single crystals have been grown by annealing of Na7Cu3O8, in the presence of Na2O2, at 450 °C for 2000 h in silver crucibles, which were sealed in glass ampoules under dried Ar. According to the X‐ray analysis of the crystal structure ($P {\bar 1}$, Z = 1, a = 5.5891(2), b = 6.0945(2), c = 7.8890(3) Å, α = 110.059(2), β = 108.669(2), γ = 90.237(2)°) a new Cu3O87‐ oxocuprate anion, consisting of three edge sharing CuO4 squares, is the prominent structural feature. These anions are aligned parallel to the space diagonal of the unit cell and can be regarded as infinite chains from which every fourth copper atom has been removed. This new representative of an oxocuprate(III) anion gives support to the expectation that the gap between dimeric and infinite edge sharing units of square planar cuprate anions can be closed, in principle.

Electric field effect on insulating cuprate planes

We have studied field effect doping of nearly insulating $p$-type ${\mathrm{CuO}}_{2}$ planes in single crystal transistor heterostructures. By using a high ${\ensuremath{\epsilon}}_{r}$ epitaxial ${\mathrm{SrTiO}}_{3}$ dielectric layer, a wide range of doping control is obtained, from $\ensuremath{-}0.40$ to 0.15 $\text{carriers}∕\mathrm{Cu}$ (or $\ensuremath{\sim}{10}^{14}\phantom{\rule{0.3em}{0ex}}\text{carriers}∕{\mathrm{cm}}^{2}$). While a considerable field effect is observed for carrier depletion, the induced holes are completely localized even up to carrier density levels far beyond the bulk insulator-to-superconductor transition value. This implies that large induced carrier density and single crystalline interface is not a sufficient condition for electric field induced insulator-to-superconductor transition for cuprates. We show that the induced carriers are almost confined to the top single ${\mathrm{CuO}}_{2}$ plane and propose that two-dimensional confinement introduces this localization. Understanding and overcoming this localization behavior is a serious challenge to any attempt to use electric field to induce superconductivity in insulating cuprates.

Magnetic resonance of Cu and of Gd in insulating GdSr2Cu2NbO8 and in superconducting GdSr2Cu2RuO8

Weak-ferromagnetic (or antiferromagnetic) resonance of Cu and electron spin resonance (ESR) of Gd are observed both in insulating GdSr2Cu2NbO8 and in superconducting GdSr2Cu2RuO8. The Cu resonance implies that the CuO2 planes are magnetic and indicates that the superconducting layer of GdSr2Cu2RuO8 is SrO (not CuO2), as in its related superconducting compound without cuprate planes, doped Sr2YRuO6.

Circulating-current states and ring-exchange interactions in cuprates

We consider circulating charge- and spin-current states in a generalised Hubbard model for the CuO 2 planes in cuprates. We investigate the parameter regimes for these phases in microscopic calculations on small clusters. We demonstrate that a positive ring-exchange interaction enhances (suppresses) spin- (charge-)flux phases.

Density functional application to strongly correlated electron systems

The local spin density approximation plus onsite Coulomb repulsion approach (LSDA+ U) to density functional theory is carefully reanalyzed. Its possible link to single-particle Green's function theory is occasionally discussed. A simple and elegant derivation of the important sum rules for the on-site interaction matrix elements linking them to the values of U and J is presented. All necessary expressions for an implementation of LSDA+ U into a non-orthogonal basis solver for the Kohn–Sham equations are given, and implementation into the full-potential local-orbital solver (Phys. Rev. B 59 (1999) 1743) is made. Results of application to several planar cuprate structures are reported in detail and conclusions on the interpretation of the physics of the electronic structure of the cuprates are drawn.

Muon spin rotation in GdSr2Cu2RuO8: Implications

Muon spin rotation measurements are reported for GdSr2Cu2RuO8, a material with an onset temperature for superconductivity of about 45 K (which is virtually the same for its superconducting sister compounds Sr2YRu1− u Cu u O6 and Gd2− z Ce z Sr2Cu2RuO10). The data indicate two magnetic ordering transitions, at about 15 K and about 130 K, in addition to the Gd ordering transition known to occur at about 2.6 K. We tentatively attribute the 130 K transition to Ru and the 15 K transition to Cu ordering, effectively ruling out any superconducting mechanism based on fluctuating magnetic moments, which are frozen below about 15 K. If there is only one mechanism of high-temperature superconductivity, then the three facts that (i) all three sister compounds have essentially the same onset T c for superconductivity and (ii) all three of these compounds contain SrO layers but (iii) only two of the three compounds, GdSr2Cu2RuO8 and Gd2− z Ce z Sr2Cu2RuO10 (and not Sr2YRu1− u Cu u O6) contain cuprate planes imply that the superconducting layers of all three compounds must be the common SrO layers, and not the cuprate planes (which do not occur in Cu-doped Sr2YRuO6). Otherwise the coincidence of onset temperatures must be an accident, and there must be at least two mechanisms of high-T c superconductivity: one for doped Sr2YRuO6 and another for cuprate-plane superconductivity.

SrO and BaO high-temperature superconductivity

Sr2YRu1−uCuuO6 (for 0.05⩽u⩽0.15) has only two types of layers: magnetic YRuO4 layers doped on Ru sites with Cu, and non-magnetic (SrO)2 layers. It superconducts (without cuprate planes) in its SrO layers with an onset temperature of ≈49 K, and with an absolute critical temperature of 23 K, which coincides with the Néel temperature TN at which the Ru moments stop fluctuating. GdSr2Cu2RuO8 and Gd2−zCezSr2Cu2RuO10 also start to superconduct near 49 K, because the SrO layers, not the cuprate planes, superconduct. Four superconductors have been successfully predicted based on this idea. PrBa defects destroy PrBa2Cu3O7’s superconductivity (in the BaO layers). Our muon data show that YBa2Cu3O7 exhibits s-wave or extended s-wave pairing, not d-wave pairing.

Subsolidus phase relations and crystal structures of R–Ca–Cu–O ( R=Nd, Sm, Gd, Tm) systems

The subsolidus phase relations of R 2O 3–CaO–CuO ternary systems ( R=Nd, Sm, Gd, Tm) have been investigated by X-ray powder diffraction. All samples were synthesized at about 950° in air. There exists a ternary compound Ca 14− x R x Cu 24O 41 ( x = 4 for R=Nd, Gd and x = 5 for R = Sm) and a ternary solid solution Ca 2+ x R 2− x Cu 5O 10 ( R=Nd, Sm, Gd, Tm) with a wide composition range Δ x of about 0.6. The compound Ca 14− x R x Cu 24O 41 possesses a layered orthorhombic structure and is isostructural to Sr 14− x Ca x Cu 24O 41. The lattice parameters a and c of the compound are basically independent of the ionic radius of R, while the lattice parameter b and unit-cell volume V decrease substantially with the decrease of the ionic radii of R. The Ca 2+ x R 2− x Cu 5O 10 solid solution is isostructural to Ca 2+ x Y 2− x Cu 5O 10, the structure of which is based on an orthorhombic “NaCuO 2-type” subcell containing infinite one-dimensional chains of edge-shared square planar cuprate groups crosslinked by the layered cations Ca and R that locate in the inter-chain tunnels.

Zigzag filamentary theory of broken symmetry of neutron and infrared vibronic spectra of YBa2Cu3O6+x

Abstract Filamentary high-temperature superconductor (HTSC) theory differs fundamentally from continuous HTSC theories because it emphasizes self-organized discrete dopant networks and does not make the effective-medium approximation. Analysis of neutron and infrared (especially with c-axis polarization) vibrational spectra, primarily for YBa2Cu3O6+x, within the filamentary framework, shows that the observed vibronic anomalies near 400 cm−1 (50meV) are associated with curvilinear filamentary paths. These paths pass through the cuprate chains and planes, as well as resonant tunnelling centres in the BaO layers. The analysis and the data confirm earlier filamentary structural models containing ferroelastic domains of diameter 3–4 nm in the CuO2 planes; it is these nanodomains that are responsible for the discrete glassy nature of both electronic and vibrational properties. Chemical trends in vibronic energies and oscillator strengths, for both neutron and photon scattering, that were anomalous in continuum ...

Zigzag filamentary theory of broken symmetry of neutron and infrared vibronic spectra of YBa 2 Cu 3 O 6+x

Filamentary high-temperature superconductivity (HTSC) theory differs fundamentally from continuous HTSC theories because it emphasizes self-organized, discrete dopant networks and does not make the effective medium approximation (EMA). Analysis of neutron and infrared (especially with c-axis polarization) vibrational spectra, primarily for YBa2Cu3O(6+x), within the filamentary framework, shows that the observed vibronic anomalies near 400 cm-1 (50 meV) are associated with curvilinear filamentary paths. these paths pass through cuprate chains and planes, as well as resonant tunneling centers in the BaO layers. The analysis and the data confirm earlier filamentary structural models containing ferroelastic domains of 3-4 nm in the CuO2 planes; it is these nanodomains that are responsible for the discrete glassy nature of both electronic and vibronic properties. Chemical trends in vibronic energies and oscillator strengths, both for neutron and photon scattering, that were anomalous in continuum models, are readily explained by the filamentary model.

Néel transition, spin fluctuations, and pseudogap in underdoped cuprates by a Lorentz invariant four-fermion model in 2+1 dimensions

We show that the N\'eel transition and spin fluctuations near the N\'eel transition in planar cuprates can be described by an SU(2) invariant relativistic four-fermion model in 2+1 dimensions. Features of the pseudogap phenomenon are naturally described by the appearance of an anomalous dimension for the spinon propagator.

Explanation of high-temperature superconductivity without cuprate planes

Abstract The superconducting hole condensate resides in the SrO planes, or in the BaO planes, or in the interstitial regions of high-temperature perovskite superconductors, depending upon the crystal structure. Supporting evidence includes the following: firstly, PrBa2Cu3O7 superconducts in its BaO layers, and not in its cuprate planes; secondly, the similarity of the layer charges for YBa2Cu3Ox and PrBa2Cu3Ox indicates that both materials superconduct in their BaO layers; thirdly, Gd2-zCezSr2Cu2TiO10, Pr2-zCezSr2Cu2NbO10 and Eu2-zCezSr2Cu2TiO10 all superconduct in their SrO layers, and not in their cuprate planes; and fourthly, the superconducting hole condensate for Cu-doped Sr2YRuO6 (a material with no cuprate planes to better than 1%) must reside in its SrO layers.

Explanation of high-temperature superconductivity without cuprate planes

Explanation of high-temperature superconductivity without cuprate planes

Frustrated phase separation in the overdoped regime of '123' high-Tc superconductors

Recent inelastic neutron scattering measurements revealed the crystal-field (CF) spectra of the overdoped R1 − yCayBa2Cu3O≈7 (R = Ho, Er; 0 < y < 0.25) high-Tc superconductors to consist of two spectral components associated with the optimally doped and the overdoped domains. Increase of the Ca concentration does not affect the local charge density of domains, but changes the spectral weight of the components. In the sense of this “two-phase” picture, which was established earlier for the underdoped region, there is a smooth crossover between the under- and overdoped parts of the phase diagram. The charge origin of the observed superposition effect is confirmed by the systematic analysis of the concentration dependences of the CF parameters for both Ho- and Er-based compounds. Therefore, the local charge inhomogeneity in the CuO2 planes of high-Tc cuprates is a characteristic feature of the doping process, which depends neither on the way to introduce doping nor on the doping level.

Magnetic resonance and Mössbauer studies of GdSr 2Cu 2RuO 8

GdSr 2Cu 2RuO 8 superconducts at an onset temperature T c≈45 K, and exhibits a low-field magnetic resonance, which the evidence suggests is due to magnetically ordered Cu. Since Bernhard et al. [Phys. Rev. B 59 (1999) 1409] have shown that the magnetism and the superconductivity are homogeneous, it follows that if the cuprate planes are homogeneously magnetic, the superconductivity must be located, not in the cuprate planes, but in the SrO layers. Alternatively there are magnetically ordered Cu impurities on Ru sites, with a locally inhomogeneous magnetization.

The electronic structure of cuprates from high energy spectroscopy

Abstract We report studies of the electronic structure and elementary excitations of doped and undoped cuprate chains, ladders and planes. Using high energy spectroscopies such as X-ray absorption, core level photoemission and angle resolved photoemission spectroscopy, important information regarding the charge distribution and hole dynamics can be obtained. The comparison of the experimental data with suitable theoretical models sets constraints on the parameters entering into the model calculations, and offers insight into the important physical quantities governing the electronic structure of these materials. Recurring themes include the importance of the dimensionality of the Cu–O network (1D→2D) and the crucial role played by the spin-background in determining the dynamics of low-lying excitations in these strongly correlated systems.

Mixed state of itinerant and localized charges in layered cuprates

Under the translation invariance of a ground state in a 2D electron–hole system, the mixed state of itinerant and localized charges can be realized if the number of the localized electrons is equal to that of the localized holes. If the chemical potential of the localized charge is around or less than 1 eV, the localized charges are aligned in one direction to form the charge distribution of a stripe configuration, which is observed on CuO 2 planes in many layered cuprates.

Magnetism and superconductivity in Sr YRu Cu O and magnetism in Ba GdRu Cu O

We report magnetization, surface resistance ( ), and electron spin resonance (ESR) for non-superconducting Ba2GdRu1-uCuuO6, and find that all three magnetic ions (Gd, Ru, and Cu) are ordered at low temperatures. Both ESR (Gd sublattice) and weak ferromagnetic resonance (dopant Cu) are observed, while no magnetic resonance due to either paramagnetic or ordered Ru is detected. In addition, for superconducting ( K) Sr2YRu1-uCuuO6, resistivity, muon spin rotation ( SR), and 99Ru Mossbauer absorption are reported. None of the O6 materials (e.g., Sr2YRu1-uCuuO6) have cuprate planes, although Cu is employed as a dopant. In Sr2YRu1-uCuuO6, the Ru moments order at a temperature ( 23 K) below that for the resistive onset of superconductivity, while the Cu orders at a higher temperature, 86 K. Therefore at low temperatures, this material exhibits magnetic order, coexisting with diamagnetism. The only non-magnetic layers in the superconducting O6 structure, the SrO layers, carry holes and exhibit diamagnetic screening characteristic of type-II superconductivity.