Showing papers by "Sebastian Doniach published in 1988"
TL;DR: It is found that a nonzero $\ensuremath{\delta}$ results in a frustration of the antiferromagnetic (AF) order favored by the superexchange interaction.
Abstract: We have studied the single-band Hubbard model on a two-dimensional square lattice slightly below half filling. Using the canonical transformation and a form of variational wave function that allows an expansion in terms of hole concentration $\ensuremath{\delta}$ with respect to half filling, we find that a nonzero $\ensuremath{\delta}$ results in a frustration of the antiferromagnetic (AF) order favored by the superexchange interaction. Use of a variational Monte Carlo calculation enables us to construct a phase diagram in the $\frac{t}{U\ensuremath{-}\ensuremath{\delta}}$ plane in which the AF order disappears at $\ensuremath{\delta}\ensuremath{\approx}0.06$ for a typical value of $\frac{t}{U}=\frac{1}{4}$.
82 citations
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.
77 citations
TL;DR: Small-angle x-ray scattering is used to examine the solution structure of troponin C at pH 6.8 and the effect of Ca2+ on the structure, indicating rather that the molecule withCa2+ bound is only slightly more compact than the Ca2-free molecule.
47 citations
TL;DR: In this article, the ground state of a spin-1/2 Mott insulator doped with holes was investigated using a Jastrow variational wave function and a second-neighbor antiferromagnetic Heisenberg coupling.
Abstract: We investigate the nature of the ground state of quantum spin-(1/2) models on a 2D square lattice using a Jastrow variational wave function. The effects of quantum frustration in a spin-(1/2) Mott insulator doped with holes are represented by a second-neighbor antiferromagnetic Heisenberg coupling. We find disappearance of 2D antiferromagnetic long-range order above a critical hole concentration on comparing the ground-state energy for a Jastrow function with off-diagonal long-range order with that computed using a power law form. We also investigate the nature of the superfluidity of 4He monolayer on a periodic substrate. We find that in a limit where the substrate potential is sufficiently weak, represented by a 2D spin-(1/2) XY model, the effects of quantum fluctuations destroy the ODLRO of the 4He film.
10 citations
TL;DR: In this paper, a percolating network of highly hysteretic Josephson junctions is modeled as a piecewise-Ohmic device and the average voltage drop across a network on a large L × L grid is assumed to fall to zero as V ∼ (I − I c (L )) η as the applied current is reduced to the critical current I c(L ).
Abstract: We study a percolating network of highly hysteretic Josephson junctions in which each junction is modelled as a piecewise-Ohmic hysteretic device. At the percolation threshold, the average voltage drop across a network on a large L × L grid is assumed to fall to zero as V ∼ ( I − I c ( L )) η as the applied current is reduced to the critical current I c ( L ). Our finite-size Monte Carlo study gives the value η = 2.0±0.1. This value is remarkably close to the result η =2 obtained by a scaling analysis of a continuum version of the model.
6 citations
TL;DR: In this paper, the effect of moving holes in a Hubbard model close to half filling is considered as far as it affects the magnetic order in a superconducting ground state, and it is argued that, in the low hole-density limit, the problem can be formulated in terms of a spin Hamiltonian including both nearest neighbor and three site antiferromagnetic couplings, the latter originating in the motion of carriers.
Abstract: The effect of moving holes in a Hubbard model close to half filling is considered as far as it affects the magnetic order in a superconducting ground state. Starting from an effective Hamiltonian it is argued that, in the low hole-density limit, the problem can be formulated in terms of a spin Hamiltonian including both nearest neighbor and three site antiferromagnetic couplings, the latter originating in the motion of carriers. This term leads to frustration of the Ne´el antiferromagnetic ground state and destroys it for a critical hole concentration. Instead an RVB type state is stabilized as is seen from calculations on finite size systems. These conclusions are also supported by calculations in which the disordered state is represented in terms of a classical Coulomb gas following Kalmeyer and Laughlin.
5 citations