Antiferromagnetism and high-temperature superconductivity

A model is presented describing superconductivity in close association with antiferromagnetism in a narrow-band system with electron correlations. We employ the Hubbard-Peierls Hamiltonian for weak to intermediate ratios between the on-site Coulomb repulsion and the electronic band width. Depending on the band structure and the electron number density antiferromagnetism arises, which for a nearly half-filled band gives rise to a Mott-Hubbard gap and resultant band splitting. The related density of energy states exhibits a singularity at the upper and lower edge of the lower and upper of the two split bands, respectively, if the lattice possesses the property of perfect nesting. The electron-phonon interaction is recast, by help of the method of canonical transformations, into a new form implying an attractive interaction between the quasiparticles in a Debye shell near the Fermi surface of each subband. A large effective interaction constant can be achieved and a BCS-type model for superconduction applies to each of the split bands if they are partially filled. Pairing arises between quasiparticles with parallel effective spin, i.e. in triplet states. The resulting gap equation is discussed in detail and the thermodynamic potential of the superconducting antiferromagnet is derived.     

Antiferromagnetism of the α-Mn system

The variation of the Néel temperature (T_N) of antiferromagnetic alloys of Mn with 3-d elements has been studied by electrical resistivity measurements. Binary Mn-rich alloys Fe, Co, Ni, Ti, Rh, Os, Ir, and Ru were investigated. The results show a strong and systematic dependence on the number of d-electrons of the impurity element. Elements with more d electrons than Mn raise T_N, while those with fewer d electrons depress T_N. The change of T_N per atomic percent impurity also exhibits a striking, non-monotonic dependence on the impurity used. The dependence shows inversion symmetry with impurities around Mn.To further elucidate the properties of the Mn-rich antiferromagnetic system, a study was made of ternary Mn-rich alloys in the MnFeCr, MnCoNi, and MnVTi systems. The results from the MnFeCr system are consistent with the rigid-band model, the average number of d-electrons per atom being of primary importance determiningh the antiferromagnetism in this system.Our goal in these studies has been to provide a firm experimental data baseupon which future theoretical work can be built. A more detailed understanding of the very intriguing α-Mn magnetic system now awaits further theoretical investigation.     

Antiferromagnetism in the Hubbard model

The energy spectrum of the two-sublattice Hubbard model is obtained in the static-fluctuation approximation. It is shown how the structure of the energy spectrum is modified as the parameters of the Hubbard model are varied. The ground state of the simple Hubbard model of dimension d=2 is the dielectric antiferromagnetic state. The author derives a consistency equation for the magnetization, which has an antiferromagnetic solution. Fiz. Tverd. Tela (St. Petersburg) 39, 1594–1599 (September 1997)     

Antiferromagnetism and lattice-distortion in the Peierls-Hubbard model

The ground state of the Peierls-Hubbard model is investigated. It is found to be dimerized or antiferromagnetic, depending on the strength of the effective electron-phonon coupling and the electron-electron interaction.Lattice distortion and magnetization do not coexist in the ground state.     

Antiferromagnetism in solid oxygen

Solid oxygen is known to have spontaneous antiferromagnetic order in its lowest temperature phase. Muons implanted in \alpha\mbox- O₂ in zero applied magnetic field take up positions where the local field is non‐zero and precess at a frequency proportional to the sublattice magnetisation. The reduction of the magnetisation as the temperature increases toward T_N is similar to that expected for the case of an anisotropic AFM coupling among spins of O_2 molecules. A fit to a simple non‐interacting, harmonic spin wave model yields an anisotropy‐induced gap in the spin wave density of states E_g/k_B=80.5(6) K, implying a large anisotropy constant K\approx 16J. This revised version was published online in July 2006 with corrections to the Cover Date.     

High-Temperature Heat Capacity and Thermal Extension of FeTa2O6

Physics of the Solid State - The heat capacity of synthesized iron tantalate FeTa2O6 was measured in the temperature range of 323–1103 K by the ratio method using a thermal analyzer combining...     

Incommensurate Antiferromagnetism in the Extended t-J Model

The effect of the extra second neighbor hopping t‘ on the incommensurate spin correlation in the t-J modelin the underdoped regime is studied within the fermion-spin theory. It is shown that although the extra second neighborhopping t‘ is systematically accompanied with the increasing of the weight of the incommensurate peaks in the dynamicalspin structure factor, for the physical reasonable small value of t‘ the qualitative behavior of the incommensurate spincorrelation in the t-t‘-J model is the same as in the case of t-J model.     

Antiferromagnetism of ThCr2Si2

Neutron diffraction measurements indicate that the magnetic moments of chromium atoms in the ThCr₂Si₂ compound show long-range order. The Cr magnetic moment equal to 1.20(25)μ_B at 1.5 K lie in the basal plane and form magnetic structure AFl-type.     

Antiferromagnetism of quasi two-dimensional manganese stearate

The magnetic properties of manganese stearate, a highly two-dimensional magnetic material, are found to be changed considerably when it is synthesized by a new procedure. Whereas earlier samples behaved like weak-ferromagnets with ordering temperatures about 5 K, the new material is an antiferromagnet with T_n = 10 K. The reason may be a change in the crystal structure.     

Antiferromagnetism in hydrated 123 compounds

Copper nuclear quadrupole resonance and zero-field nuclear magnetic resonance (ZFNMR) studies of YBa₂Cu₃O_6.5 show that a magnetic phase appears in underdoped 123 superconductors treated in ambient moist air. The studies give convincing evidence that the “empty” CuO chains play the role of easy water insertion channels. The reaction occurs first in ordered regions of the crystallites. The final product of the reaction is a nonsuperconducting antiferromagnetic compound characterized by at least two types of magnetically ordered copper ions, with ZFNMR spectra in the frequency ranges 46–96 and 96–135 MHz, respectively. Even for powder samples fixed in an epoxy resin, this reaction is found to have partially occurred after a few years. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 10, 739–744 (25 May 1999) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Antiferromagnetism and Kondo screening on a honeycomb lattice

Magnetic adatoms in the honeycomb lattice have received tremendous attention due to the interplay between Ruderman–Kittel–Kasuya–Yosida interaction and Kondo coupling leading to very rich physics. Here we study the competition between the antiferromagnetism and Kondo screening of local moments by the conduction electrons on the honeycomb lattice using the determinant quantum Monte Carlo method. While changing the interband hybridization V, we systematically investigate the antiferromagnetic-order state and the Kondo singlet state transition, which is characterized by the behavior of the local moment, antiferromagnetic structure factor, and the short range spin-spin correlation. The evolution of the single particle spectrum are also calculated as a function of hybridization V, we find that the system presents a small gap in the antiferromagnetic-order region and a large gap in the Kondo singlet region in the Fermi level. We also find that the localized and itinerant electrons coupling leads to the midgap states in the conduction band in the Fermi level at very small V. Moreover, the formation of antiferromagnetic order and Kondo singlet are studied as on-site interaction U or temperature T increasing, we have derived the phase diagrams at on-site interaction U(or temperature T) and hybridization V plane.     

Antiferromagnetism and the node structure of the superconducting order parameter of UPd Al

The node structure of the superconducting order parameter of the heavy-fermion system is analyzed within the weak-coupling theory. A pairing interaction induced by the exchange of antiferromagnetic spin excitations is assumed as suggested by recent inelastic neutron scattering experiments and tunneling spectroscopy. The multi-sheeted Fermi surface is taken into account. Based on a model susceptibility for the simple antiferromagnetic structure of , line nodes result at the rim of the magnetic Brillouin zone. Received 29 July 1999