Physics News 470, February 10, 2000 by Phillip F. Schewe and Ben Stein

PHYSICS NEWS UPDATE The American Institute of Physics Bulletin of Physics News Number 470 February 10, 2000 by Phillip F. Schewe and Ben Stein

February 2000
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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, 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.

atomic physics
nuclear physics
particle physics
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, 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 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.)

low temperature
quantum theory
February 2000
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