Shielding property for thermal equilibrium states in the quantum Ising model.

We show that Gibbs states of nonhomogeneous transverse Ising chains satisfy a shielding property. Namely, whatever the fields on each spin and exchange couplings between neighboring spins are, if the field in one particular site is null, then the reduced states of the subchains to the right and to the left of this site are exactly the Gibbs states of each subchain alone. Therefore, even if there is a strong exchange coupling between the extremal sites of each subchain, the Gibbs states of the each subchain behave as if there is no interaction between them. In general, if a lattice can be divided into two disconnected regions separated by an interface of sites with zero applied field, then we can guarantee a similar result only if the surface contains a single site. Already for an interface with two sites we show an example where the property does not hold. When it holds, however, we show that if a perturbation of the Hamiltonian parameters is done in one side of the lattice, then the other side is completely unchanged, with regard to both its equilibrium state and dynamics.

Endoscopic treatment of Zenker's diverticulum.

BACKGROUND: The most important aspect of the surgical management of Zenker's diverticulum is probably the cricopharyngeal myotomy. Endoscopic diverticulotomy can be performed with a needle-knife papillotome, which allows simultaneous myotomy of the upper esophageal sphincter. METHODS: Since 1978, 47 patients (28 men and 19 women 51 to 81 years of age) underwent endoscopic diverticulotomy. Most patients underwent more than one treatment session (mean value 2.2). The procedure was performed with sedation. Tubes were not used, and oral intake of food was begun the first day after the operation. RESULTS: Forty-five (95.74%) patients had no dysphagia or only occasional, mild dysphagia during the postoperative course. No fistula, no recurrent laryngeal paralysis, and no deaths occurred. CONCLUSION: Endoscopic diverticulotomy seems to be a good choice of therapy at least for patients with associated diseases that increase surgical risk.