| |
|
PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 470 February 10, 2000 by Phillip F. Schewe and Ben Stein
|
Previous February 2000 Main page |
|
A NEW FORM OF NUCLEAR MATTER has been detected at the
CERN lab in Geneva. Results from seven different experiments,
conducted at CERN over a span of several years, were announced
at a series of seminars today. In the experiments a high energy
beam of lead ions (160 GeV/nucleon, times 208 nucleons, for a
total energy of about 33 TeV) smashes into fixed targets of lead or
gold atoms. The center-of-mass energy of these collisions, the true
energy available for producing new matter, is about 3.5 TeV.
From the debris that flies out of the smashups, the CERN scientists
estimate that the "temperature" of the ensuing nuclear fireball
might have been as high as 240 MeV (under these extreme
conditions energy units are substituted for degrees kelvin), well
above the temperature where new nuclear effects are expected to
occur. In the CERN collisions the effective, momentary, nuclear
matter density was calculated to be 20 times normal nuclear
density. It is not quite certain whether the novel nuclear state is
some kind of denser arrangement of known nuclear matter or a
manifestation of the much-sought quark-gluon plasma (QGP), in
which quarks, and the gluons which normally bind the quarks into
clumps of two quarks (mesons) or three quarks (baryons), spill
together in a seething soup analogous to the condition of ionized
atoms in a
plasma. Such a nuclear plasma might have existed in the very
early universe only microseconds after the big bang. Evidence
for the transition from a hadron phase (baryons and mesons) into a
QGP phase was expected to consist of (1) an enhanced production
of strange mesons, (2) a decrease in the production of heavy psi
mesons (each consisting of a charm and anticharm quarks), and (3)
an increase in the creation of energetic photons and
lepton-antilepton pairs. Just this sort of (indirect) evidence (at least
of types 1 and 2) has now turned up in the CERN data. (CERN
press release, www.cern.ch) To demonstrate the existence of QGP
more directly, one would like the plasma state to last longer, and
one should observe the sorts of particle jets and gamma rays that
come with still higher-energy fireballs. That energy (about 40
TeV, center-of-mass) will be available in the next few months at
the Relativistic Heavy Ion Collider undergoing final preparations
at Brookhaven.
|
accelerators atomic physics cosmology nuclear physics particle physics quarks education |
|
D-WAVE SQUID. The working fluid of superconductors consists of
pairs of electrons (or pairs of the holes left behind in a crystal when an
electron moves somewhere else). These Cooper pairs form a coherent
state with specific symmetry properties. For example, in most low
temperature superconductors, the pairs are fairly isotropic; if you imagine
one electron at the origin of some coordinate system, the likelihood of
finding a second electron is pretty much the same in all directions. Thus
the Cooper pair is essentially spherical and the pair is said to possess "s-
wave" symmetry. In high-temperature superconductors, the symmetry is
thought to resemble a four-leave clover, referred to as a "d-wave." A
fundamental consequence of the d-wave symmetry is a phase-change of
pi between neighboring lobes of the clover in the quantum wave function
describing the Cooper pair. All of this can be important in the design of
superconducting quantum interference devices, or SQUIDs, which consist
of a superconducting loop interrupted in two places by thin insulating
junctions, through which the Cooper pairs must tunnel. SQUIDs are
highly sensitive to applied magnetic fields and are used in a variety of
magnetometer applications (in biology, geology, new materials research,
etc.). Furthermore, SQUIDs form the building blocks of superconducting
electronics. A group at Augsburg University in Germany (Robert Schulz,
011-49-821-598-3650, robert.schulz@physik.uni-augsburg.de) has
developed a SQUID that exploits the special nature of the d-wave
symmetry of the high-Tc superconductors. Using specially prepared
tetracrystalline crystals as substrates, they devised and built a SQUID in
which the symmetry properties give rise to a pi phase-change over one of
the two junctions (see the figure at www.aip.org/physnews/graphics). For
this reason, the Augsburg researchers call their device a pi-SQUID. The
pi-SQUID is a realization of the recently proposed complementary
Josephson electronics and its operation provides strong evidence for the d-
wave symmetry in the high-Tc superconductors. Such devices present a
novel approach for the fabrication of quantum computers. (Schulz et al.,
Applied Physics Letters, 7 Feb; Select Article.)
|
computers crystals/solids low temperature quantum theory superconductivity |
| Previous February 2000 Main page | |
Articles copyright ©American Institute of Physics, index and html by the University of Exeter. Please mail comments/problems to John Rowe |