[en] The effects of nonpenetrating electron irradiation on thermally grown silicon dioxide films were studied using MOS structures. The principal samples had dry-grown oxides with thicknesses of 4000 A and 4700 A on n-type silicon substrates. The samples were irradiated through the field plate by an electron beam in the 1 to 5 kV range. The I-V characteristics with field plate negative showed a square-law region typical of a trap-dominated space-charge-limited electron current. The sample current was insensitive to changes in temperature but was directly proportional to the magnitude of the electron beam current, indicating that detrapping of carriers was not thermal but was induced by beam-generated photons. The concentration of electron traps in the oxide required in the foregoing interpretation (approximately greater than 10¹⁷ cm^-3) was far in excess of that measured in nonirradiated control samples (less than 10¹⁵ cm^-3); it, therefore, appears that irradiation of the silicon dioxide by the nonpenetrating beam resulted in the generation of substantial concentrations of electron traps in the oxide well beyond the range of the primary electrons. The traps could be discharged by annealing the sample at 300 to 350⁰C. The traps themselves remained, however, and could be recharged by electrons that were internally photoinjected from either the substrate or the field plate. The photoinduced I-V characteristics of negatively charged samples were approximately symmetrical and showed a dead space about the origin, indicating that the centroid of the trapped charge was in the bulk of the oxide and not at the oxide-silicon interface. Attempts at photodepopulation of the traps, using photon energies up to 4 eV, were unsuccessful, indicating that the traps were deep

[en] The dispersion of a single hole in the t-J model obtained by the exact result of 32 sites and the results obtained by self-consistent Born approximation and the Green function Monte Carlo method can be simply derived by a mean-field theory with d-wave resonating-valence-bond (d-RVB) and antiferromagnetic order parameters. In addition, it offers a simple explanation for the difference observed between those results. The presence of the extended van Hove region at (π,0) is a consequence of the d-RVB pairing instead of the antiferromagnetic order. Results including t^' and t^double-prime are also presented and explained consistently in a similar way. copyright 1997 The American Physical Society

[en] The boundary of phase separation of the two-dimensional t-J model is investigated by the power-Lanczos method and Maxwell construction. The method is similar to a variational approach and it determines the lower bound of the phase-separation boundary with J_c/t=0.6±0.1 in the limit n_e∼1. In the physically interesting regime of high-T_c superconductors where 0.3< J/t<0.5 there is no phase separation. copyright 1997 The American Physical Society

[en] The mechanism of high-temperature superconductivity (HTS) and the correlation between the antiferromagnetic long-range order (AFLRO) and superconductivity (SC) phases are the central issues of the study of HTS theory. SC and AFLRO of the hole-doped two-dimensional extended t- J model are studied by the variational Monte Carlo method. The results show that SC is greatly enhanced by the long-range hopping terms t' and t'' for the optimal and overdoped cases. The phase of coexisting SC and AFM in the t-J model disappears when t' and t'' are included. It is concluded that the extended t-J model provides a more accurate description for HTS than the traditional t-J model does. The momentum distribution function n(k) and the shape of Fermi surface play critical roles for establishing the phase diagram of HTS materials

[en] The power-Lanczos method, a Green's function Monte-Carlo simulation, is used to calculate the energies of the two-dimensional t–J model up to 122 sites. The phase boundary of phase separation of the model is then determined by the Maxwell construction. Our conclusion is that there is no phase separation for J⩽0.4.

[en] We present an exact diagonalization study of the low-energy singlet and triplet states for both the one-dimensional (1D) and 2D t-J models. A scan of the parameter ratio J/t shows that for most low-energy states in both 1D and 2D the excitation energy takes the form E(t,J)=a·t+b·J. In 1D this is the natural consequence of the factorization of the low-energy wave functions, i.e., spin-charge separation. Examination of the low-energy eigenstates in 2D shows that most of these are collective modes, which for larger J correspond to a periodic modulation of the hole density. The modulation is well reproduced by treating holes as hard-core bosons with an attractive interaction. copyright 1997 The American Physical Society