Aging and memory properties of topologically frustrated magnets

The model 2d kagome system (H3O)Fe3(SO4)2(OH)6 and the 3d pyrochlore Y2Mo2O7 are two well characterized examples of low-disordered frustrated antiferromagnets which rather then condensing into spin liquid have been found to undergo a freezing transition with spin glass-like properties. We explore more deeply the comparison of their properties with those of spin glasses, by the study of characteristic rejuvenation and memory effects in the non-stationary susceptibility. While the pyrochlore shows clear evidence for these non-trivial effects, implying temperature selective aging, that is characteristic of a wide hierarchical distribution of equilibration processes, the kagome system does n not show clearly these effects. Rather, it seems to evolve towards the same final state independently of temperature.Comment: submitted for the proceedings of the 46th MMM conference (Seattle, 2001

Abundance of Delta Resonances in 58Ni+58Ni Collisions between 1 and 2 AGeV

Charged pion spectra measured in 58Ni-58Ni collisions at 1.06, 1.45 and 1.93 AGeV are interpreted in terms of a thermal model including the decay of Delta resonances. The transverse momentum spectra of pions are well reproduced by adding the pions originating from the Delta-resonance decay to the component of thermal pions, deduced from the high transverse momentum part of the pion spectra. About 10 and 18% of the nucleons are excited to Delta states at freeze-out for beam energies of 1 and 2 AGeV, respectively.Comment: 14 pages, LaTeX with 3 included figures; submitted to Physics Letters

Entropy production in the Au + Au reaction between 150 and 800 A MeV

The entropy per nucleon (S/A) has been extracted for the Au [(150—800)A MeV] + Au reaction by using the phase I setup of the 4 pi facility at GSI, Darmstadt. The entropy has been obtained from the comparison of various observables characterizing the dM/dZ fragment multiplicity distributions, extending up to Z~15, with those calculated with the quantum statistical model. It is the first time that S/A values are determined by considering the full ensemble of charged products detected in the reaction. Consistent values of S/A are found from different methods. These entropy values are shown to be fairly independent of the volume of the ``participant'' region considered. They are somewhat lower than those extracted in earlier works but are in good agreement with hydrodynamic calculations and suggest a low viscosity for the hot and dense nuclear matter

Experimentelle Untersuchung des Mechanismus von Schwerionenreaktionen im Energiebereich 0.1 bis 2 GeV/Nukleon und Detektorentwicklungen

SIGLEAvailable from TIB Hannover: F96B1015+a / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekBundesministerium fuer Forschung und Technologie (BMFT), Bonn (Germany)DEGerman

Interplay of collective flow phenomena and velocity correlations of intermediate-mass fragments in collisions of Au + Au at E = 100 - 400A.MeV

Velocity correlations of intermediate mass fragments (IMFs), produced in collisions of Au+Au at 100, 150, 250, and 400 A.MeV beam energy, are extracted from measurements with the FOPI (phase I) detector system at SIS in GSI Darmstadt. The IMF correlation functions of peripheral and semi-central events are found to be strongly affected by the collective sideward motion of nuclear matter. The sideflow causes an enhancement of correlations at small relative velocities. This enhancement results from the mixing of differently azimuthally oriented events; it vanishes if the events are rotated into a unique reaction plane. Selecting violent central collisions, the comparison of the data with a Coulomb dominated final-state interaction model points to a radius of the expanding and multifragmenting source of R_s#approx =#13 fm for 100 A.MeV which appears shrinking by 20% when increasing the projectile neergy to 400 MeV per nucleon. The deduced source radii are found to depend on the radial explosion energy used in the model. The inclusion of such collective expansion is necessary for a reasonable description of the experimental single-particle spectra of the IMFs. The unique Coulomb suppression of small relative IMF velocities, found for the given beam energy range, is attributed to rather constant averaged next-neighbour distances left angle d_I_M_F right angle =8.6#+-#0.2 fm of the IMF charge centers within the source at break-up time. (orig.)Available from TIB Hannover: RR 1847(94-56) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman