[en] The spin stiffness of frustrated spin-1/2 Heisenberg models in one and two dimensions is computed by exact diagonalizations on small clusters that implement spin-dependent twisted boundary conditions. Finite-size extrapolation to the thermodynamic limit yields a value of 0.14±0.01 for the spin stiffness of the unfrustrated planar antiferromagnet. We also present a general discussion of the linear-response theory for spin twists, which ultimately leads to the moment sum rule

[en] For the one-dimensional (1D) Hubbard model, we extended the bosonization technique, away from half filling, in such a way that a general formula is obtained for the zero temperature equal-time single-particle Green function with validity over the whole doping range. With our method, we can calculate, for the first time, the one-body Green function and thus, the spectral function in both the Tomonaga-Luttinger and Luther-Emery universality classes. The method also gives a tool to determine the coefficients of the correlation functions which cannot be determined with previously used methods. ((orig.))

[en] An unrestricted Hartree-Fock (UHF) scheme is used to obtain approximate solutions of the Hubbard model on a Bethe lattice of infinite connectivity. The UHF energy yields a correct upper bound of the total energy over the whole phase diagram. In addition to the disappearance of antiferromagnetic long-range order away from half-filling, our results give a line separating the magnetic region of the phase diagram from the nonmagnetic one

[en] Short communication

[en] An efficient method for production of cold antihydrogen (bar H) is proposed. Alkali-metal atoms laser excited to a Rydberg state are charge exchanged with cold trapped positrons, producing Rydberg states of positronium. In a second Rydberg-state charge exchange, the positronium atoms give up their Rydberg positrons to cold trapped antiprotons, producing Rydberg states of bar H. These bar H atoms soon decay down to the ground state, and, because they are cold, could be trapped in a magnetic trap. The efficiency of the process results from the extremely large cross sections for Rydberg charge exchange. Classical trajectory Monte Carlo calculations indicate an bar H instantaneous production rate of 10⁶/s. copyright 1998 The American Physical Society

[en] High-sensitivity femtosecond transient absorption is applied to directly measure the population-depopulation dynamics of the lowest (1S ) and the first excited (1P ) electron states in CdSe nanocrystals (NC close-quote s) of different radii with 1S emdash 1P energy separation up to 16longitudinal optical phonon energies. Instead of the drastic reduction of the energy relaxation rate expected due to a phonon bottleneck, we observe a fast subpicosecond 1P -to-1S relaxation, with the rate enhanced in NC close-quote s of smaller radius. This indicates the opening of new confinement-enhanced relaxation channels which likely involve Auger-type electron-hole energy transfer. copyright 1998 The American Physical Society

[en] A hybrid theory which combines the full nonlocal open-quotes exactclose quotes exchange interaction with the local spin-density approximation of density-functional theory is shown to lead to marked improvement in the description of antiferromagnetically coupled systems. Semiquantitative agreement with experiment is found for the magnitude of the coupling constant in La₂CuO ₄ , KNiF₃ , and K₂NiF ₄ . The magnitude of the unpaired spin population on the metal site is in excellent agreement with experiment for La₂CuO ₄ . copyright 1997 The American Physical Society

[en] The electronic transmission across metal/conjugated-oligomer/metal structures in the presence of lattice fluctuations is studied for short oligomer chains. The lattice fluctuations are approximated by static white noise disorder. Resonant transmission occurs when the energy of an incoming electron coincides with a discrete electronic level of the oligomer. The corresponding transmission peak diminishes in intensity with increasing disorder strength. Because of disorder there is an enhancement of the electronic transmission for energies that lie within the electronic gap of the oligomer. If fluctuations are sufficiently strong, a transmission peak within the gap is found at the midgap energy E=0 for degenerate conjugated oligomers (e.g., trans-polyacetylene) and E≠0 for AB-type degenerate oligomers. These results can be interpreted in terms of soliton-antisoliton states created by lattice fluctuations. copyright 1997 The American Physical Society

[en] We study the singularities in x-ray absorption spectra of one-dimensional Hubbard and t-J models. We use boundary conformal field theory and the Bethe ansatz solutions of these models with both periodic and open boundary conditions to calculate the exponents describing the power-law decay near the edges of x-ray absorption spectra in the case where the core-hole potential has bound states. copyright 1997 The American Physical Society

[en] The diffusion and coarsening of two-dimensional homoepitaxial islands on Cu(100) and Ag(100) surfaces have been studied at room temperature with time-sequenced scanning tunneling microscopy. Quantitative analyses of the dependence of island diffusion coefficient D vs the island side length L , D∝L^-α , yield noninteger scaling exponents which are consistent with island coalescence. Moreover, the near absence of island decay shows that the island diffusion occurs via mass transport along the island periphery. copyright 1997 The American Physical Society