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Jul 20, 2013
07/13

by
W. Barford; S. Jadhav

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The Schwinger Boson mean field theories of the `t-J' model are extended by the consideration of anisotropic order parameters. This has two effects. First, a collinear phase, in which the spins are anti-ferromagnetically aligned in one direction and ferromagnetically aligned in the other, is found to be stable over a significant range of the phase diagram. Second, the (1,1) and (1,0) spiral phases become very close in energy. The inclusion of weak intra-sublattice coupling may therefore...

Source: http://arxiv.org/abs/cond-mat/9407007v1

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14

Jul 20, 2013
07/13

by
W. Barford; W. H. Beere; M. Steer

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A simple one dimensional model to simulate the establishment of the Bean critical state is introduced. It is shown that the dynamics of the flux lines as they enter the superconductor are dominated by `avalanches'. The distribution of distances moved by vortices in the avalanches obeys a power law with an exponent of -1. This suggests that the Bean state is a self-organised critical state. The density of flux lines is parabolic.

Source: http://arxiv.org/abs/cond-mat/9407008v1

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37

Sep 20, 2013
09/13

by
W. Barford; R. J. Bursill; M. Yu. Lavrentiev

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The two state molecular orbital (2-MO) model of the phenyl based semiconductors is used to calculate the low-lying spectra of the Ag and Bu states of poly(para-phenylene) (PPP). The model parameters are determined by fitting its predictions to exact Pariser-Parr-Pople model calculations of benzene and biphenyl, and it is solved using the density matrix renormalisation group method. It is shown that there exists a band of Bu (s-wave) excitons below the band states. In the long chain limit the...

Source: http://arxiv.org/abs/cond-mat/9802235v2

45
45

Sep 22, 2013
09/13

by
W. Barford; R. J. Bursill; M. Yu Lavrentiev

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We solve a model of interacting electrons coupled to longitudinal phonons using the density matrix renormalisation group method. The model is parametrised for polyenes. We calculate the ground state, and first excited odd-parity singlet and triplet states; and we investigate their energies, and bond length changes and fluctuations for up to 30 sites. The transition energy and the soliton width of the triplet state show deviations from the adiabatic approximation for chain lengths larger than...

Source: http://arxiv.org/abs/cond-mat/0003189v1

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45

Jul 20, 2013
07/13

by
M Harrison; W Barford

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Anisotropic London Theory is used to predict the Abriskosv flux lattice in planar crystals of YBaCuO. By taking into account the orientation of the flux lattice as a function of applied field it is shown that the vortex chain state is observed in Bitter pattern experiments.

Source: http://arxiv.org/abs/cond-mat/9407002v1

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41

Sep 22, 2013
09/13

by
C. Vermeulen; W. Barford; E. R. Gagliano

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Exact diagonalisations of the extended Hubbard model are performed. In the insulating regime it is shown that the nearest neighbour copper-oxygen repulsion, $V$, leads to Frenkel excitons in the charge transfer gap at values of $V$ of the order of copper-oxygen hybridisation, $t$. In the metallic regime it is shown that the static charge-transfer and density-density correlation functions diverge as a function of $V$, indicating a charge-tansfer instability and phase separation. This is...

Source: http://arxiv.org/abs/cond-mat/9503081v1

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122

Sep 18, 2013
09/13

by
R. J. Bursill; W. Barford; H. Daly

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A two state (2-MO) model for the low-lying long axis-polarised excitations of poly(p-phenylene) oligomers and polymers is developed. First we derive such a model from the underlying Pariser-Parr-Pople (P-P-P) model of pi-conjugated systems. The two states retained per unit cell are the Wannier functions associated with the valence and conduction bands. By a comparison of the predictions of this model to a four state model (which includes the non-bonding states) and a full P-P-P model...

Source: http://arxiv.org/abs/cond-mat/9901031v1

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35

Sep 18, 2013
09/13

by
Robert J. Bursill; W. Barford

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A density matrix renormalisation group scheme is developed, allowing for the first time essentially exact numerical solutions for the important excited states of a realistic semi-empirical model for oligo-phenylenes. By monitoring the evolution of the energies with chain length and comparing them to the experimental absorption peaks of oligomers and thin films, we assign the four characteristic absorption peaks of phenyl-based polymers. We also determine the position and nature of the nonlinear...

Source: http://arxiv.org/abs/cond-mat/0202520v1

14
14

Jul 20, 2013
07/13

by
W. Barford; M. W. Long

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We consider the effect of the nearest neighbour copper-oxygen repulsion, V, when coupled to the charge transfer resonances Cu2+ to Cu3+ and Cu2+ to Cu+ in the high temperature cuprate superconductors. This is done by deriving effective low energy Hamiltonians correct to second order in the copper-oxygen hybridisation. Only hole doping is considered. When Cu2+ to Cu3+ fluctuations dominate we derive an effective one-band model of `Zhang-Rice' singlets with a nearest neighbour repulsion between...

Source: http://arxiv.org/abs/cond-mat/9407065v1

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86

Jul 20, 2013
07/13

by
C. Vermeulen; W. Barford; E. R. Gagliano

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The effects of the nearest neighbour Coulomb repulsion, V, are considered in the one dimensional copper-oxide chain using the modified Lanczos method. Above a critical value of V we find that charge transfer excitons are the lowest lying energy exitations in the insulating phase. Close to the metal-insulator transition and $V=0$ there is good `particle-hole' symmetry, showing that the mapping from a two band model to a one band model is appropriate. The effect of the nearest neighbour repulsion...

Source: http://arxiv.org/abs/cond-mat/9407004v1

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153

Jul 20, 2013
07/13

by
C. Vermeulen; W. Barford; E. R. Gagliano

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The phase diagram of the copper-oxide chain as a function of density and the nearest-neighbour Coulomb interaction, V, is determined. Phase separation takes place above a critical value of $V$ when the Cu2+ to Cu+ valence fluctuations dominate. In the proximity of the phase separation boundary the superconducting correlations are the most divergent. We identify the parameter regions where the Luttinger Liquid theory applies and calculate the contours of the charge-charge correlation exponent...

Source: http://arxiv.org/abs/cond-mat/9407067v1

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57

Sep 24, 2013
09/13

by
Alan Race; W. Barford; Robert J. Bursill

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The Pariser-Parr-Pople Hamiltonian is used to calculate and identify the nature of the low-lying vertical transition energies of polydiacetylene. The model is solved using the density matrix renormalisation group method for a fixed acetylenic geometry for chains of up to 102 atoms. The non-linear optical properties of polydiacetylene are considered, which are determined by the third-order susceptibility. The experimental 1Bu data of Giesa and Schultz are used as the geometric model for the...

Source: http://arxiv.org/abs/cond-mat/0103238v1