RESUMO
Many properties of solids result from the fact that in a periodic crystal structure, electronic wave functions are delocalized over many lattice sites. Electrons should become increasingly localized when a strong electric field is applied. So far, this Wannier-Stark regime has been reached only in artificial superlattices. Here we show that extremely transient bias over the few-femtosecond period of phase-stable mid-infrared pulses may localize electrons even in a bulk semiconductor like GaAs. The complicated band structure of a three-dimensional crystal leads to a strong blurring of field-dependent steps in the Wannier-Stark ladder. Only the central step emerges strongly in interband electro-absorption because its energetic position is dictated by the electronic structure at an atomic level and therefore insensitive to the external bias. In this way, we demonstrate an extreme state of matter with potential applications due to e.g., its giant optical non-linearity or extremely high chemical reactivity.
RESUMO
A new method to determine antibiotic bone concentrations is presented. The bone fragments, obtained during hip joint replacement operations in 10 patients were investigated for antibiotic activity after i.v. application of 2 g of oxacillin and 4 g of mezlocillin. We were able to show that both penicillins do not penetrate into the inorganic bone part (hydroxylapatite). Therefore the concentrations measured are related to the organic bone part and are expressed in units per volume rather than in units per weight. With this method the differences between spongious and cancellous bone observed so far are diminished to values without any statistical difference. After 2 g of oxacillin and 4 g of mezlocillin the concentrations are 13.1 mg/l and 34.0 mg/l, respectively. The serum concentrations measured are compared to the bone concentrations. The terminal serum half-life is 63 min for oxacillin and 80 min for mezlocillin when the drugs are given simultaneously.