Phase transitions in superconductors
1 July 1999
According to Ehrenfest's phase transition classification scheme,
at the first-order transition materials change their state in a
sudden jump, releasing (or absorbing) some amount of heat in
doing so. In second-order transitions, while the state of a
material changes in a continuous way, thermodynamic derivatives,
e. g., specific heat, may experience jumps: these are transitions
many metals and alloys go through when becoming superconducting.
If specific heat remains unaffected, the transition is said to be
third or higher order. The first observation of such a transition
has been reported by D Hall and R G Goodrich who detected no
specific heat change at the superconducting transition when
studying the temperature dependence of the critical
(superconductivity destroying) magnetic fields in Ba0.6K0.4BiO3.
Together with P Kumar, D Hall and R G Goodrich were able to
develop a phase transition theory which agrees quite well the
experimental data. Along with Ba0.6K0.4BiO3, other
superconductors of similar structure will hopefully provide a
full understanding of the nature of higher-order transitions.
Phys. Rev. Lett. 82 4532 (1999)
New transuranic elements
1 July 1999
Elements 116 and 118 have been obtained for the first time at
Berkeley Lab's Cyclotron by bombarding targets of lead with an
about 450 MeV krypton ion beam. Within less than a millisecond
after its formation, the element 118 nucleus emits an α - particle
to become an isotope of element 116 with mass number 289. Element
116 is in turn radioactive and α - decays into element 114, after
which a chain of successive decays follows until element 106
forms. During 11 days of experiments, three such decay chains
were observed and hence 3 atoms of element 118 emerged. Although
transuranic elements are unstable and decay very quickly into
other elements, an `island of stability' - i. e., a region of
relatively long half-decay times - is predicted for elements with
about 114 protons and 173 neutrons in their nuclei. Decay time
measurements along the decay chain confirmed the stability island
prediction. The work was initiated by Polish theorists who
demonstrated the production of superheavy elements to be in
principle possible with the experimental facilities available.
Dispersion of light in vacuum
1 July 1999
The speed of light is frequency independent to within a factor of
6×10-21, B Schaefer of Yale University has established based on gamma-
ray burst spectra. Frequency dependence would imply that light
waves of different frequency reach the Earth at different times,
which they do not. The previous most accurate estimate, derived
from the spectrum of the Crab pulsar, was 5×10-17. The speed of light
would be frequency dependent if, for example, the photon mass
were nonzero as some theories suggest. B Schaefer's result now
puts an upper limit of mγ<10-44g on this mass.
Source: Physics News Update, Number 432;
Phys. Rev. Lett. 82 4964 (1999)
1 July 1999
The Hubble Space Telescope project on determining the universe’s
rate of expansion has been completed. In the 18 faraway galaxies
studied in the course of the project, nearly 800 Cepheid variable
stars were discovered which, due to their luminosity being stable
to pulse period changes, are good `standard candles’ for distance
determination purposes. Knowing the distance to and the redshift
of a galaxy yields the universe’s expansion rate, i. e., Hubble’s constant, whose value was found to be 70 km s-1Mpc-1 to within
10%. If the universe has a critical density, this value implies the universe’s age to be t0=12×1012years. For the open model, it may be somewhat older, as will also be the case for a nonzero Λ-term.
The Extracts from the Internet is a section of Uspekhi Fizicheskih Nauk (Physics Uspekhi) the monthly rewiew journal of the current state of the most topical problems in physics and in associated fields. The presented News is devoted to the fundamental discoveries of physics and astrophysics.
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