Abstract
The lower critical field for highly underdoped with between 8.9 and 22 K has been determined by measurements of magnetization curves with applied field parallel to the axis. is linear in temperatures below about , and is proportional to , clearly violating the proportionality between and . Moreover, the slope decreases steeply toward zero as approaches zero, indicating that the effective charge of the quasiparticles vanishes as the doping is decreased toward the insulating phase.
- Received 1 September 2004
DOI:https://doi.org/10.1103/PhysRevLett.94.117001
©2005 American Physical Society