Showing papers on "Antiferromagnetism published in 1988"
TL;DR: A comprehensive account of the macroscopic and microscopic physical properties of chromium (and where appropriate those of its dilute alloys) that relate to its antiferromagnetism is given in this paper.
Abstract: A comprehensive account is given of the macroscopic and microscopic physical properties of chromium (and where appropriate those of its dilute alloys) that relate to its antiferromagnetism. Neutron scattering is treated in great detail, first in the historical introduction, then as an experimental probe of both the magnetic structure and the excitations of the incommensurate spin-density-wave state and (with the assistance of x rays) of the concomitant charge-density wave and strain wave. Neutron scattering is considered as a tool to explore not only the disappearance of long-range order with increasing temperature through the growth of excitations as the weak first-order N\'eel transition is approached, but also the persistence of these spin fluctuations well into the paramagnetic state---processes that are still little understood. The article surveys, without mathematical details, model systems designed to reproduce the magnetic and thermodynamic properties of Cr. The energy-band structure calculations are given a more comprehensive review. Special attention is paid to calculations of the wave-vector-dependent susceptibility that reproduce the observed wave vector of the spin-density wave, and to a recent finite-temperature calculation that gives almost the right N\'eel temperature. The review of Fermi-surface studies emphasizes those designed to relate the spin-density wave vector (and its pressure dependence) to the nesting vector of the Fermi surface. An account is given of the spectroscopic determination of the energy gap(s), whose theoretical analysis is still unclear, and of experiments aimed at determining physical properties that throw light on the origin of the weak first-order N\'eel transition. The article describes the use of magnetic anomalies in the elastic moduli to determine the volume dependence of the exchange interaction responsible for antiferromagnetism in Cr. The experimental features of the spin-flip transition are reviewed, although a theory of this phenomenon is wanting. The experimental study of microscopic structure by the use of hyperfine-interaction properties is surveyed. An account is given of both experimental and theoretical studies of the surface of Cr and of Cr films and sandwiches. Finally, "technical antiferromagnetism" is discussed: the effect of severe internal strain in producing a commensurate antiferromagnetic state, wave-vector Q domains, polarization S domains (for which the experimental evidence is scanty), and ultrasonic attenuation as a tool to study them.
866 citations
TL;DR: In this paper, the existence of long-range, three-dimensional, antiferromagnetic order of the Cu spins was confirmed by the use of uniaxially oriented powders.
Abstract: Neutron-diffraction experiments on ceramic powders of YBa/sub 2/Cu/sub 3/O/sub 6+//sub x/ (nonsuperconducting, with x = 0.0 and 0.15) have confirmed the existence of long-range, three-dimensional, antiferromagnetic order of the Cu spins. The structure determination was aided by the use of uniaxially oriented powders. The ordering wave vector within a CuO/sub 2/ plane is (1/21/2) and the planes are coupled antiferromagnetically along the c axis. The Neel tempreature is 400 +- 10 K for the x = 0.15 +- 0.05 sample and approx. >500 K for x = 0.0.
431 citations
TL;DR: The temperature-concentration phase diagram of doped La/sub 2/CuO/sub 4/.
Abstract: We discuss the temperature-concentration phase diagram of doped ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$. The addition of holes introduces a local ferromagnetic exchange coupling between Cu spins. The resulting frustration destroys the 3D N\'eel state characterizing pure ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$, and generates a new 3D spin-glass phase. In the paramagnetic phase, strongly correlated Cu spins in the planes are canted by the holes, yielding an oscillating dipole-dipole attraction between holes. The possible relevance to superconductivity is discussed.
430 citations
TL;DR: Inelastic neutron scattering demonstrates that the fluctuating moments are strongly correlated; the characteristic energy for fluctuations with wave vectors close to the ordering wave vector is 0.2 meVapprox.
Abstract: Neutron diffraction shows that superconducting ${\mathrm{UPt}}_{3}$ can be an antiferromagnet with an ordered moment of (0.02\ifmmode\pm\else\textpm\fi{}0.01)${\mathrm{\ensuremath{\mu}}}_{\mathrm{B}}$ and a N\'eel temperature (${\mathrm{T}}_{\mathrm{N}}$) of 5 K. The squared order parameter is linear function of temperature from ${\mathrm{T}}_{\mathrm{N}}$ to the superconducting transition temperature, ${\mathrm{T}}_{\mathrm{c}}$=0.5 K, below which it ceases to evolve. Inelastic neutron scattering demonstrates that the fluctuating moments are strongly correlated; the characteristic energy for fluctuations with wave vectors close to the ordering wave vector is 0.2 meV\ensuremath{\approxeq}4${\mathrm{kT}}_{\mathrm{c}}$..AE
337 citations
TL;DR: In this paper, the authors present spin-wave calculations for a frustrated antiferromagnetic Hamiltonian on a square lattice, and show that quantum fluctuations can destabilize the classical ordered ground state, even at large values of frustration.
Abstract: We present conventional spin-wave calculations for a frustrated antiferromagnetic Hamiltonian on a square lattice. We find that quantum fluctuations can destabilize the classical ordered ground state, even at large $S$, for large enough values of frustration. This instability is likely to generate a spin liquid, providing the first example of a resonating-valence-bond ground state in two dimensions.
300 citations
TL;DR: Enhancement of the conductivity in the ferromagnetic phase demonstrates a strong connection between the magnetism and charge transport.
Abstract: Measurements of the magnetic moment of antiferromagnetic ${\mathrm{La}}_{2}$Cu${\mathrm{O}}_{4}$ at high fields reveal a new phase boundary originating from a previously undetected canting of the ${\mathrm{Cu}}^{2+}$ spins out of the Cu${\mathrm{O}}_{2}$ planes. This canting, together with the exponential temperature dependence of the two-dimensional correlation length, accounts quantitatively for the susceptibility peak at the N\'eel temperature. Enhancement of the conductivity in the ferromagnetic phase demonstrates a strong connection between the magnetism and charge transport.
285 citations
TL;DR: In this article, the authors formulate a density functional theory (DF) to describe non-collinear magnetism and present self-consistent, spin-polarised energy-band calculations based on the local approximation to DF theory.
Abstract: The authors formulate a density functional theory (DF) to describe non-collinear magnetism. Self-consistent, spin-polarised energy-band calculations based on the local approximation to DF theory are presented in which the magnetisation associated with different atoms in a unit cell is allowed to point along different, non-collinear directions. Non-self consistent calculations employing non-collinear quantisation axes have been presented before; the present calculations are, they believe, distinguished by: first, being self-consistent; second, providing the total energy; and third, providing the spin-quantisation axes. In their first applications they deal with the non-collinear antiferromagnets gamma -FeMn, RhMn3, and PtMn3 and show that their total energies are minimised in the tetrahedral (FeMn) or triangular (RhMn3, PtMn3) magnetic structures first proposed by Kouvel and Kasper (1963).
280 citations
TL;DR: In this article, a single-crystal neutron diffraction measurements have been used to study the long-range magnetic ordering in cupric oxide, CuO, and good agreement is obtained for the commensurate phase with a multipole model for the copper magnetisation density and spins of 0.65(3) mu B directed parallel to b.
Abstract: Single-crystal neutron diffraction measurements have been used to study the long-range magnetic ordering in cupric oxide, CuO. An incommensurate antiferromagnetic structure forms below the Neel temperature on 230(1) K, with a propagation vector (0.506(1)a*-0.483(1)c*) which remains constant down to a magnetic phase transition at 213(1) K. Below the latter temperature, the structure remains antiferromagnetic with a commensurate propagation vector (1/2 0-1/2), and this structure persists to the lowest temperature reached in the investigation (20 K). The arrangement of the copper moments in both phases is such that the n-glide perpendicular to the b axis of the monoclinic cell, space group C2/c, does not reverse the direction of the spin. The two magnetic sublattices related to the C-face-centring scatter in phase quadrature and the relative directions of the spin on them could not be determined. Good agreement is obtained for the commensurate phase with a multipole model for the copper magnetisation density and spins of 0.65(3) mu B directed parallel to b. The lower sublattice magnetisation in the incommensurate phase precluded a meaningful multipole fit, but a reasonable agreement is obtained with a model in which the spins rotate in the a-c plane following an elliptical envelope with major axis directed 33(2) degrees to c in beta obtuse and a maximum moment of 0.38(2) mu B at 215 K. The paramagnetic scattering at ambient temperature and 550 K was measured to try to find the origin of the peak in the susceptibility. No significant paramagnetic scattering could be obtained from a powdered sample although the sensitivity of detection was some five times that required to observe the scattering from a Cu2+ ion in an ideal paramagnet.
244 citations
TL;DR: The ground state of a quantum vacancy in a 2D antiferromagnet is found to involve a long-range dipolar distortion of the staggered magnetization, and an effective Hamiltonian for vacancies interacting with long-wavelength spin waves is derived.
Abstract: The ground state of a quantum vacancy in a 2D antiferromagnet is found to involve a long-range dipolar distortion of the staggered magnetization. An effective Hamiltonian of vacancies interacting with long-wavelength spin waves is derived. The implications for the anitferromagnetic long-range order are discussed.
240 citations
TL;DR: In this article, a more flexible semiclassical theory is developed based on a 1 n expansion, which predicts the occurrence of a nematic and a semi-ordered phase, in addition to the usual ferromagnetism and antiferromagnetic behavior.
210 citations
TL;DR: The extended McConnell model was developed and provides the synthetic chemist with guidance for making new molecular materials to study cooperative magnetic coupling in systems and a model of configuration mixing of the lowest charge-transfer excited state with the ground state was developed.
Abstract: Quantitative bulk ferromagnetic behavior has been established for the molecular/organic solid [Fe(III)(C(5)Me(5))(2)].(+)[TCNE].(-). Above 16 K the dominant magnetic interactions are along a 1-D chain and, near T(c), 3-D bulk effects as evidenced by the value of the critical exponents dominate the susceptibility. The extended McConnell model was developed and provides the synthetic chemist with guidance for making new molecular materials to study cooperative magnetic coupling in systems. Assuming the electron-transfer excitation arises from the POMO, ferromagnetic coupling by the McConnell mechanism requires stable radicals (neutral, cations/anions, or ions with small diamagnetic counterions) with a non-half-filled POMO. The lowest excited state formed via virtual charge transfer (retro or forward) must also have the same spin multiplicity and mix with the ground state. These requirements limit the structure of a radical to D(2d) or C>/=(3) symmetry where symmetry breaking distortions do not occur. Intrinsic doubly and triply degenerate orbitals are not necessary and accidental degeneracies suffice. To achieve bulk ferromagnetism, ferromagnetic coupling must be established throughout the solid and a microscopic model has been discussed. These requirements are met by [Fe(III)(C(5)Me(5))(2)].(+)[TCNE].(-). Additionally this model suggests that the Ni(III) and Cr(III) analogs should be antiferromagnetic and ferrimagnetic, respectively, as preliminary data suggest. Additional studies are necessary to test and further develop the consequences of these concepts. Some molecular/organic solids comprised of linear chains of alternating metallocenium donors (D) and cyanocarbon acceptors (A) with spin state S = 1/2 (...D.(+)A.(-)D.(+)A.(-)...) exhibit cooperative magnetic phenomena, that is, ferro-, antiferro-, ferri-, and metamagnetism. For [Fe(III)(C(5)Me(5))(2)].(+)[TCNE](-). (Me = methyl; TCNE = tetracyanoethylene), bulk ferromagnetic behavior is observed below the Curie temperature of 4.8 K. A model of configuration mixing of the lowest charge-transfer excited state with the ground state was developed to understand the magnetic coupling as a function of electron configuration and direction of charge transfer. This model predicts that ferromagnetic coupling requires stable radicals with a non-half-filled degenerate valence orbital and a charge-transfer excited state with the same spin multiplicity that mixes with the ground state. Ferromagnetic coupling must dominate in all directions to achieve a bulk ferromagnet. Thus, the primary, secondary, and tertiary structures are crucial considerations for the design of molecular/organic ferromagnets.
TL;DR: In this article, it was shown that fcc Fe(100) stabilized at a lattice constant close to that of Cu has an antiferromagnetic ground state, and showed that it is magnetically ordered and shows a small average hyperfine field, e.g., 1.1 T at 29 K for 10 monolayers.
Abstract: Ultrathin films (10 to 17 monolayers) of fcc Fe(100) grown epitaxially on Cu(100) are unambiguously found to be paramagnetic at 295 K by $^{57}\mathrm{Fe}$ conversion-electron M\"ossbauer spectroscopy in UHV. Below ${T}_{\mathrm{N}}=65\ifmmode\pm\else\textpm\fi{}5$ K, fcc-Fe films are magnetically ordered and show a small average hyperfine field, e.g., 1.1 T at 29 K for 10 monolayers, indicating a small atomic magnetic moment. This demonstrates that fcc Fe(100) stabilized at a lattice constant close to that of Cu has an antiferromagnetic ground state.
TL;DR: In this article, positive-muon spin-rotation and relaxation measurements of the oxygen-deficient perovskite YBa2Cu3O(x) have revealed local antiferromagnetic order for x = 6.0-6.4 with a Neel temperature TN that decreases rapidly with increasing oxygen content.
Abstract: Positive-muon spin-rotation and -relaxation measurements of the oxygen-deficient perovskite YBa2Cu3O(x) have revealed local antiferromagnetic order for x = 6.0-6.4 with a Neel temperature TN that decreases rapidly with increasing oxygen content x. For slowly annealed samples with x = 6.35-6.5 the superconducting transition temperature Tc increases smoothly with x from 25 K at x = 6.348 to 60 K at x = 6.507. Two such samples with x = 6.348 and x = 6.400 appear to 'switch' from superconductivity to antiferromagnetic order at lower temperatures.
TL;DR: In this paper, a full-potential linearized augmented plane wave method for the whole transition-metal series (V,Cr,Mn,Fe,Co,Ni) as overlayers on the Pd(001) surface is presented.
Abstract: We report systematic calculations based on the full-potential linearized augmented-plane-wave method for the whole transition-metal series (V,Cr,Mn,Fe,Co,Ni) as overlayers on the Pd(001) surface An energy analysis shows that Fe, Co, and Ni overlayers favor the ferromagnetic p(1 x 1) configuration, but V, Cr, and Mn, the antiferromagnetic c(2 x 2) superstructure We conjecture that this result is a general trend which should also be found on the (001) surfaces of Pt and the noble metals
TL;DR: In this paper, the existence of Neel order in the ground state of the 3D spin-1/2 Heisenberg antiferromagnet on the cubic lattice was shown.
Abstract: The methods of Dyson, Lieb, and Simon are extended to prove the existence of Neel order in the ground state of the 3D spin-1/2 Heisenberg antiferromagnet on the cubic lattice. We also consider the spin-1/2 antiferromagnet on the cubic lattice with the coupling in one of the three lattice directions taken to be r times its value in the other two lattice directions. We prove the existence of Neel order for 0.16 ⩽ r ⩽ 1. For the 2D spin-1/2 model we give a series of inequalities which involve the two-point function only at short distances and each of which would by itself imply Neel order.
TL;DR: In this article, the magnetic susceptibility measurements of the magnetoplumbite-like compound SrCr 8 Ga 4 O 19 showed that no magnetic phase transition occurs up to T = 4.2K in spite of very high antiferromagnetic interactions making apparent a very high degree of magnetic frustration (O/Tg 117).
TL;DR: The energy of one and two holes in a Hubbard antiferromagnet for nonzero exchange, J, in the Ising limit is calculated within the Brinkman-Rice approximation.
Abstract: The energy of one and two holes in a Hubbard antiferromagnet for nonzero exchange, J, in the Ising limit is calculated within the Brinkman-Rice approximation. Only the p- and d-symmetry states bind with an energy of order J. The implications for superconductivity and antiferromagnetism of doped Hubbard insulators is discussed.
TL;DR: In this article, the phase 4H-SrMnO3 is paramagnetic at 290 K, contrary to earlier reports that the susceptibility maximum at ∼350 K is thought to represent only short-range coupling between Mn4+ ions in the Mn2O9 pairs of face-sharing octahedra in the structure.
01 Jan 1988
TL;DR: In this paper, the authors proved the existence of N6el order in the ground state of the 3D spin-l/2 Heisenberg antiferromagnet on the cubic lattice with the coupling in one of the three lattice directions taken to be r times its value in the other two directions.
Abstract: The methods of Dyson, Lieb, and Simon are extended to prove the existence of N6el order in the ground state of the 3D spin-l/2 Heisenberg antiferromagnet on the cubic lattice. We also consider the spin-l/2 antiferromagnet on the cubic lattice with the coupling in one of the three lattice directions taken to be r times its value in the other two lattice directions. We prove the existence of N6el order for 0.16 ~< r ~< 1. For the 2D spin-l/2 model we give a series of inequalities which involve the two-point function only at short distances and each of which would by itself imply N6el order.
TL;DR: In this article, the authors proposed a theoretical model which allows to explain high T c superconductivity in perovskites and showed that when doping with holes immobile ferromagnetic clusters are formed which at higher hole concentrations percolate to metallic lines.
Abstract: We propose a theoretical model which allows to explain high- T c superconductivity in perovskites. It starts from a Hubbard-type Hamiltonian and from a description of the antiferromagnetic state. We show that when doping with holes immobile ferromagnetic clusters are formed which at higher hole concentrations percolate to metallic lines. The superconductivity occurring along these one-dimensional lines is caused by deformation coupling to Jahn-Teller active optical vibrations. Only a small isotope effect is predicted. The model also explains the spin-glass behaviour for intermediate concentrations and the destruction of superconductivity at high concentrations.
TL;DR: In this article, the magnetic properties of equiatomic uranium transition metal ternary compounds of general formula UT(Si, Ge) have been investigated in the orthorhombic CeCu 2 -type structure.
TL;DR: In this article, the properties of the ground state in chain-like systems, including the competition between ferromagnetic and antiferromagnetic exchange interactions, are discussed, and it is pointed out that the characteristic singlet state expressed by the combinations of RVB distributed uniformly to spin lattice points is given as the exact solution on some condition of the competition.
Abstract: The properties of the ground state in chain-like systems, including the competition between ferromagnetic and antiferromagnetic exchange interactions, are discussed. It is, in particular, pointed out that the characteristic singlet state expressed by the combinations of RVB distributed uniformly to spin lattice points is given as the exact solution on some condition of the competition. Besides analytical discussion, the numerical calculation for the spin number N ≦16 is also made so as to make clear the behavior of a multiple degeneracy for the ground state and of low-lying excitation levels.
TL;DR: The observed magnetic intensities at 9 K are consistent with ground-state spin configurations of a model which takes account of frustration, and a second magnetic phase transition associated with appearance of superlattice peaks at about 40 K, which is well below the Neel temperature 405 +- 5 K.
Abstract: Neutron diffraction experiments on a tetragonal single crystal of YBa/sub 2/Cu/sub 3/O/sub 6/..mu../sub x/ (with xapprox.0.35) reveal a second magnetic phase transition associated with appearance of superlattice peaks at (12 12l)(l = half-integer)= at about 40 K, which is well below the Neel temperature 405 +- 5 K. This transition is associated with the ordering of the Cu/sup 2 +/ moments on the oxygen-deficient layers. The observed magnetic intensities at 9 K are consistent with ground-state spin configurations of a model which takes account of frustration
TL;DR: The zero-field antiferromagnetic nuclear resonance (AFNR) of Cu in the high- T c related oxide YBa 2 Cu 3 O 6 has been observed at 89.89±0.05 MHz for 63 Cu and 96.29± 0.3 K as mentioned in this paper.
Abstract: The zero-field antiferromagnetic nuclear resonance (AFNR) of Cu in the high- T c related oxide YBa 2 Cu 3 O 6 has been observed at 89.89±0.05 MHz for 63 Cu and 96.29±0.05 MHz for 65 Cu with well articulated quadrupole splittings at 1.3 K. These frequencies correspond to a hyperfine field of 79.65±0.05 KOe. From the analysis of the AFNR spectrum and the observation of NQR at 30.11±0.2 MHz for 65 Cu and 27.88±0.02 MHz for 63 Cu, both of which have no Zeeman splitting, it is concluded that the antiferromagnetic mounents reside only on the CuO 2 plane sites and the direction of moments is perpendicular to the c -axis. This is consistent with the spin structure proposed by neutron diffraction studies. The nuclear relaxation behavior is also discussed briefly.
IBM1
TL;DR: Afin de faire une exploration complementaire de la phenomenologie de l'anisotropie d'echange, et dans le but d'obtenir des observations experimentales additionnelles qui pourraient aider a elucider la nature microscopique of l'interaction d'interface, on realise une etude de R.M.F.
Abstract: Ferromagnetic resonance (FMR) spectra of Permalloy thin films exchange-coupled to iron-manganese films are analyzed. Studies were made on bilayer, ferromagnetic-antiferromagnetic (FA) and trilayer (AFA) structures, as a function of both F and A layer thicknesses in the range 20--800 A\r{}. Data are presented at a frequency of 9.3 GHz for both in-plane and perpendicular directions of the applied field, and at 34.1 GHz, in-plane. Analysis of these data enables extraction of the magnetization, gyromagnetic ratio, and an exchange shift due to spin-wave stiffness and perpendicular-surface anisotropy, as a function of layer thickness. The azimuthal dependence of the in-plane resonance is used to determine the magnitude of the exchange anisotropy (bias field). The magnetization and gyromagnetic ratio show little dependence on the thickness of either the F or A layer down to 50 A\r{}, implying that the interfaces are sharp on a scale of a few lattice constants. Within this interfacial region the magnetization is reduced as a result of interaction with the antiferromagnet. We suggest that the perpendicular-surface anisotropy is created by exchange coupling to the antiferromagnet whose easy axes are not in the plane of the interface. Finally, we suggest a model for exchange anisotropy in which the antiferromagnetic domain pattern is not totally locked, but adjusts in response to the ferromagnetization. Such a model qualitatively explains the bias field exerted by the antiferromagnetic layer deposited before the ferromagnet, the field-training effect, the FMR linewidth, and the magnitude of the bias field.
TL;DR: In this paper, the magnetic structure of the incommensurate spiral phase of holmium has been studied using elastic neutron scattering for temperatures between 30 and 18K, the temperature region just above the transition to a ferromagnetic cone phase with a wavevector of 1/6 c.
Abstract: The magnetic structure of the incommensurate spiral phase of holmium has been studied using elastic neutron scattering for temperatures between 30 and 18K, the temperature region just above the transition to a ferromagnetic cone phase with a wavevector of 1/6 c*. The results show that the structure consists of commensurate regions separated by spin discommensurations or spin slips. In the commensurate regions, pairs of spins (one on each sublattice) deviate by about 10 degrees from each of the easy directions within the plane, and there is evidence for a c axis antiferromagnetic component which changes domain type from one commensurate region to another. The spin slips, which occur on alternate sublattices, have one plane of spins aligned with the planar easy directions, and are found to disturb the commensurate regions for about three planes on either side of each spin slip.
TL;DR: In this paper, the magnetic ordering in the high-temperature superconducting compound GdBa/sub 2/Cu/sub 3/O/sub 7 was investigated and it was shown that the ordered magnetic moment per Gd atom lies along the c axis of the orthorhombic cell.
Abstract: Neutron diffraction has been employed to study the magnetic ordering in the high-temperature superconducting compound GdBa/sub 2/Cu/sub 3/O/sub 7/. The compound is found to undergo an antiferromagnetic transition with the Gd magnetic moments ordering at 2.22 +- 0.07 K. The ordering is shown to be three dimensional with a doubling of the orthorhombic unit in all three directions. An analysis of the intensities of the observed magnetic Bragg reflections shows that the ordered magnetic moment per Gd atom is 7.4 +- 0.6..mu../sub B/ and lies along the c axis of the orthorhombic cell. It is unlikely that dipolar ordering can explain the occurrence of this magnetic phase transition where the system locks into a three-dimensional ordered arrangement at the Neel temperature, since the separation of the Gd atoms along the c axis is approximately three times that along the a,b axes.
TL;DR: In this article, temperature dependence of resistivity (up to 24 Kbar) and of E.S.R. (at 1 bar) measurements suggest that centered monoclinic TTF[Pd(Dmit) 2 ] 2 could be an anisotropic and weak antiferromagnetic fluctuating metal.
TL;DR: A mean-field theory of the Hubbard model is investigated in the large on-site correlation limit, allowing for the possibility of a commensurate spin-density wave together with s- and d-wave-like superconductivity.
Abstract: We investigate a mean-field theory of the Hubbard model in the large on-site correlation limit, allowing for the possibility of a commensurate spin-density wave together with s- and d-wave-like superconductivity. As a function of t/U and band filling, the resulting phase diagram exhibits phases which are simultaneously superconducting and antiferromagnetic. We show that the superconducting order parameter goes to zero in the limit of a 1/2-filled Hubbard band. We comment on the possible relevance of these results to the newly discovered high-${T}_{c}$ superconductors.
TL;DR: The magnon pairing mechanism is derived to explain the high-temperature superconductivity of both the La2-xSrxCu1O4 and Y1Ba2Cu3O7 systems and it is found that both the CuO sheets and theCuO chains can contribute to the supercurrent.
Abstract: The magnon pairing mechanism is derived to explain the high-temperature superconductivity of both the L_(a2-x)Sr_xCu_1O_4 and Y_1Ba_2Cu_3O_7 systems. Critical features include (i) a one- or two-dimensional lattice of linear Cu-O-Cu bonds that contribute to large antiferromagnetic (superexchange) coupling of the Cu^(II)(d^9) orbitals; (ii) holes in the oxygen pπ bands [rather than Cu^(III)(d^8)] leading to high mobility hole conduction; and (iii) strong ferromagnetic coupling between oxygen pπ holes and adjacent Cu^(II)(d^9) electrons. The ferromagnetic coupling of the conduction electrons with copper d spins induces the attractive interaction responsible for the superconductivity, leading to triplet-coupled pairs called "tripgems." The disordered Heisenberg lattice of antiferromagnetically coupled copper d spins serves a role analogous to the phonons in a conventional system. This leads to a maximum transition temperature of about 200 K. For La_(1.85)Sr_(0.15)Cu_1O_4, the energy gap is in excellent agreement with experiment. For Y_1Ba_2Cu_3O_7, we find that both the CuO sheets and the CuO chains can contribute to the supercurrent.