35 research outputs found
A High-Resolution Combined Scanning Laser- and Widefield Polarizing Microscope for Imaging at Temperatures from 4 K to 300 K
Polarized light microscopy, as a contrast-enhancing technique for optically
anisotropic materials, is a method well suited for the investigation of a wide
variety of effects in solid-state physics, as for example birefringence in
crystals or the magneto-optical Kerr effect (MOKE). We present a microscopy
setup that combines a widefield microscope and a confocal scanning laser
microscope with polarization-sensitive detectors. By using a high numerical
aperture objective, a spatial resolution of about 240 nm at a wavelength of 405
nm is achieved. The sample is mounted on a He continuous flow cryostat
providing a temperature range between 4 K and 300 K, and electromagnets are
used to apply magnetic fields of up to 800 mT with variable in-plane
orientation and 20 mT with out-of-plane orientation. Typical applications of
the polarizing microscope are the imaging of the in-plane and out-of-plane
magnetization via the longitudinal and polar MOKE, imaging of magnetic flux
structures in superconductors covered with a magneto-optical indicator film via
Faraday effect or imaging of structural features, such as twin-walls in
tetragonal SrTiO. The scanning laser microscope furthermore offers the
possibility to gain local information on electric transport properties of a
sample by detecting the beam-induced voltage change across a current-biased
sample. This combination of magnetic, structural and electric imaging
capabilities makes the microscope a viable tool for research in the fields of
oxide electronics, spintronics, magnetism and superconductivity.Comment: 14 pages, 11 figures. The following article has been accepted by
Review of Scientific Instruments. After it is published, it will be found at
http://aip.scitation.org/journal/rs
Hot-spot formation in stacks of intrinsic Josephson junctions in Bi2Sr2CaCu2O8
We have studied experimentally and numerically temperature profiles and the
formation of hot spots in intrinsic Josephson junction stacks in Bi2Sr2CaCu2O8
(BSCCO). The superconducting stacks are biased in a state where all junctions
are resistive. The formation of hot spots in this system is shown to arise
mainly from the strongly negative temperature coefficient of the c-axis
resistivity of BSCCO at low temperatures. This leads to situations where the
maximum temperature in the hot spot can be below or above the superconducting
transition temperature Tc. The numerical simulations are in good agreement with
the experimental observations
Interaction of hot spots and THz waves in Bi_2Sr_2CaCu_2O_8 intrinsic Josephson junction stacks of various geometry
At high enough input power in stacks of Bi_2Sr_2CaCu_2O8 intrinsic Josephson
junctions a hot spot (a region heated to above the superconducting transition
temperature) coexists with regions still in the superconducting state. In the
``cold'' regions cavity resonances can occur, synchronizing the ac Josephson
currents and giving rise to strong coherent THz emission. We investigate the
interplay of hot spots and standing electromagnetic waves by low temperature
scanning laser microscopy and THz emission measurements, using stacks of
various geometries. For a rectangular and a arrow-shaped structure we show that
the standing wave can be turned on and off in various regions of the stack
structure, depending on the hot spot position. We also report on standing wave
and hot spot formation in a disk shaped mesa structure
Alquimia, Ocultismo, Maçonaria: o ouro e o simbolismo hermético dos cadinhos (Séculos XVIII e XIX)
Este artigo apresenta a arqueologia das enigmáticas marcas impressas na base de cadinhos dos sĂ©culos XVIII e XIX recuperados nas escavações da Casa da Moeda do Rio de Janeiro, na dĂ©cada de 1980, e a explanação do seu significado simbĂłlico Ă luz da alquimia, do ocultismo e da Maçonaria. Espraiando-se extraordinariamente mundo afora atravĂ©s de uma bem-sucedida estratĂ©gia de comunicação visual, a Maçonaria utilizou sĂmbolos hermĂ©ticos para a difusĂŁo de seus princĂpios nos mais diferentes suportes. Aparentemente estamos diante de um sinal de reconhecimento maçônico, o sinal exterior de uma organização oculta, sĂł partilhado por iniciados e incompreensĂvel para os demais, que contribuiu para difundir veladamente a doutrina maçônica por diferentes pontos do globo