RESUMEN
When intense lightwaves accelerate electrons through a solid, the emerging high-order harmonic (HH) radiation offers key insights into the material1-11. Sub-optical-cycle dynamics-such as dynamical Bloch oscillations2-5, quasiparticle collisions6,12, valley pseudospin switching13 and heating of Dirac gases10-leave fingerprints in the HH spectra of conventional solids. Topologically non-trivial matter14,15 with invariants that are robust against imperfections has been predicted to support unconventional HH generation16-20. Here we experimentally demonstrate HH generation in a three-dimensional topological insulator-bismuth telluride. The frequency of the terahertz driving field sharply discriminates between HH generation from the bulk and from the topological surface, where the unique combination of long scattering times owing to spin-momentum locking17 and the quasi-relativistic dispersion enables unusually efficient HH generation. Intriguingly, all observed orders can be continuously shifted to arbitrary non-integer multiples of the driving frequency by varying the carrier-envelope phase of the driving field-in line with quantum theory. The anomalous Berry curvature warranted by the non-trivial topology enforces meandering ballistic trajectories of the Dirac fermions, causing a hallmark polarization pattern of the HH emission. Our study provides a platform to explore topology and relativistic quantum physics in strong-field control, and could lead to non-dissipative topological electronics at infrared frequencies.
RESUMEN
The purpose of this study was to determine factors affecting retirement migration of older persons, using Haas and Serow's (1993) model as the framework. Data were based on 462 persons aged 50 and older who had migrated to Idaho and surrendered their out-of-state drivers' licenses during 1992 and 1993. Most moved from another state in the west and were motivated to move to Idaho by such factors as outdoor recreation and quality and pace of life. They had visited their new community at least five times before moving and developed social ties quickly upon arriving. Businesses and community planners can more effectively anticipate changes necessary to accommodate older migrants to their community.
