Numerical study of metastable States in ising spin glasses

We study numerically the structure of metastable states in the Sherrington-Kirkpatrick spin glass. We find that all nonparamagnetic stationary points of the free energy are organized into pairs, consisting of a minimum and an order 1 saddle, which coalesce in the thermodynamic limit. Within the annealed approximation, the entropic contribution of these states, that is the complexity, is compatible with the supersymmetry-breaking calculation performed by Bray and Moore [J. Phys. C 13, L469 (1980)]]. This result indicates that the supersymmetry is spontaneously broken in the Sherrington-Kirkpatrick model.     

Considerations on numerical analysis of QCD

We discuss the strategy to remove the quenched approximation and to minimize systematical and statistical errors occurring in the numerical simulation of lattice QCD.We suggest a way to compute the flavour singlet sector of the mass spectrum, and comment about the relation between the fluctuations of the mass values and the scattering amplitudes.     

Phase transition analysis in Z2 and U(1) lattice gauge theories

The transition region of Z₂ lattice gauge theory is investigated by inverting the strong coupling series of the average plaquette energy E_P(J). We find a clear evidence for a first-order transition and the existence of a metastable phase. In the U(1) case we confirm a second-order phase transition even if there is a little discrepancy on the critical point position as indicated by Monte Carlo simulations.     

On the breaking of Bjorken scaling

In the framework of coloured quael we obtain detailed predictions for the q² dependence of the structure functions of the proton and the neutron and the $\text{σ}{}_{\mathrm{<mtext>\nu </mtext>}}\text{/σ}{}_{\mathrm{\nu }}$ ratio.     

Conformal invariance in perturbation theory

Asymptotic conformal invariance is proved to be true at all orders in perturbation theory. The correct Ward Identities for broken conformal invariance are derived: they are the extension of the Callan Symanzik equation from scale to conformal transformations.     

Wide angle electron-positron bremsstrahlung at adone a new limit for the existence of a heavy electron

The cross section for the reaction e⁺+e^?→e⁺+e^?+γ has been measured at the Adone storage ring. The results in agreement with the QED predictions, establish a new limit for the mass and the coupling constant $\text{e}{}^1\text{e}\text{γ}$ of a heavy electron e¹.     

Novel dynamical effects and persistent memory in phase separated manganites

The time dependent response of the magnetic and transport properties of Fe-doped phase separated (PS) manganite La(0.5)Ca(0.5)MnO3 is reported. The nontrivial coexistence of ferromagnetic (FM) and non-FM regions induces a slow dynamics which leads to time relaxation and cooling rate dependence within the PS regime. This dynamics influences physical properties drastically. On one hand, metalliclike behavior, assumed to be a fingerprint of percolation, can be also observed before the FM phase percolates as a result of dynamical contributions. On the other hand, two novel effects for the manganites are reported, namely, the rejuvenation of the resistivity after aging and a persistent memory of low magnetic fields (<1 T), imprinted in the amount of the FM phase.     

The Bethe lattice spin glass revisited

So far the problem of a spin glass on a Bethe lattice has been solved only at the replica symmetric level, which is wrong in the spin glass phase. Because of some technical difficulties, attempts at deriving a replica symmetry breaking solution have been confined to some perturbative regimes, high connectivity lattices or temperature close to the critical temperature. Using the cavity method, we propose a general non perturbative solution of the Bethe lattice spin glass problem at a level of approximation which is equivalent to a one step replica symmetry breaking solution. The results compare well with numerical simulations. The method can be used for many finite connectivity problems appearing in combinatorial optimization. Received 27 September 2000