(<Emphasis Type="Italic">p</Emphasis>,π<Superscript>±</Superscript>) correlations in central heavy-ion collisions at 1 ÷ 2 AGeV

The proton–charged pion correlated emission is studied in the reactions Au (1.06 AGeV) + Au, Ni (1.06 and 1.93 AGeV) + Ni and Ni (1.97 AGeV) + Cu within the BUU approach. The associated invariant mass distributions are shifted to smaller energies with respect to the free Δ(1232) mass distribution due to kinematical reasons. We find that the existing and partly conflicting experimental data do not allow to draw definite conclusions on the in-medium modification of the Δ(1232). Received: 18 October 1999 / Revised version: 4 February 2000     

Inclusive meson production in peripheral collisions of ultrarelativistic heavy ions

There exist several proposals to use Weizsäcker-Williams photons generated by ultrarelativistic heavy ions to produce exotic particles in γγ fusion reactions. To estimate the background conditions for such reactions we analyze various mechanisms of meson production in very peripheral collisions of ultrarelativistic heavy ions at RHIC and LHC energies. Besides the γγ fusion they include also electromagnetic γA interactions and strong nucleon-nucleon interactions in grazing AA collisions. All these processes are characterised by low multiplicities of produced particles. The γA an d AA events are simulated by corresponding Monte Carlo codes, RELDIS and FRITIOF. In each of these processes a certain fraction of pions is produced close to the mid-rapidity regionthat gives a background for the γγ events. The possibility of selecting the mesons produced in the γγ fusion events via different p _t cut procedures is demonstrated.     

Anelastic relaxation and around the critical Sr content x = 0.02

Anelastic relaxation and ¹³⁹LaNQR relaxation rates in La_2–xSr_xCuO₄ for Sr content around 2 and 3 percent are discussed in terms of spin and lattice excitations and of the related ordering processes. It is argued how the phase diagram of La_2–xSr_xCuO₄ at the boundary between the antiferromagnetic (AF) and the spin-glass phase (x = 0.02) could be more complicate than previously thought, with a transition to a quasi-long range ordered state at K, as indicated by neutron scattering data. On the other hand, the ¹³⁹LaNQR spectra are compatible with a transition to an AF phase around K, in agreement with the phase diagram commonly accepted in literature. In this case the peaks in NQR and anelastic relaxation rates around 150 K and 80 K respectively in La_1.98Sr_0.02CuO₄, yield the first evidence of freezing process involving simultaneously lattice and spin excitations, possibly corresponding to motion of charged stripes. Received 18 May 2000 and Received in final form 11 July 2000     

Effect of relevant umklapp process on the two-leg Hubbard ladder with a half-filled band under pressure

By means of perturbative renormalization approach we study the effect of relevant umklapp process on dimensional crossover caused by interladder one particle hopping in weakly coupled two-leg Hubbard ladders with a half filled-band. We found that a crossover takes place at a finite value which increases as the amplitude of umklapp process increases. For the system undergoes a phase transition to the spin density wave phase (SDW) via the two particle hopping process, while for the system undergoes a crossover to the two dimensional Fermi liquid phase via one particle hopping process. Received 25 December 1998     

Anomalous magnetic behavior of superconducting thin films in small magnetic fields [4] close to the transition temperature

Using a specially designed SQUID magnetometer we measured the temperature dependence of the critical current density in a ring patterned thin film for magnetic fields parallel to the c-axis. In addition, the temporal relaxation of the remanent state as prepared by field cooling in an external field of 100 Oe at different temperatures is determined. The j c ( T ) data show a field-dependent anomalous kink close to T_c pointing to reduced dissipation with increasing temperature allowing to construct a corresponding H-T borderline. A similar behavior is observed for the normalized relaxation rate S ( T ) as extracted from the temporal behavior of the remanent state, which, at low temperatures, exhibits the expected increase for increasing T-values, while an anomalous decrease of S ( T ) is found for temperatures above 85 K. While the low-T regime is attributed to creep of 2D pinned single vortex lines, the high-T behavior is suggested to be dominated by collective motion with a more sluggish dynamics. This change in dynamics is also reflected by the activation barriers for flux creep U ( j ), which show a corresponding crossover in μ from 0.06 to 0.99. An additional scaling analysis of the E-j characteristics for according to vortex glass theory reveals quasi-2D collective creep behavior with . Received: 8 April 1998 / Revised: 15 July 1998 / Accepted: 2 September 1998     

Strange-quark collectivity of the φ-meson at RHIC

Based on A Multi-Phase Transport (AMPT) model, the elliptic flow v₂ of φ-mesons which is reconstructed from K ⁺ K ^- at the Relativistic Heavy-Ion Collider (RHIC) energy has been studied. The results show that the reconstructed v₂ of the φ-meson can keep the earlier information before φ decays and it seems to obey the number of constituent-quark scaling as other mesons and baryons. This result indicates that the φ v₂ mostly reflects the parton level collectivity developed during the early stage of the collisions and the strange and light up/down quarks have similar collectivity properties before the hadronization.     

Transverse energy and charged-particle multiplicity at various centralities at RHIC: Statistical model estimates⋆

The transverse energy and the charged-particle multiplicity at midrapidity are evaluated in a single-freeze-out model for different centrality bins at RHIC at = 130 and 200GeV. The predictions of the model are done at the freeze-out parameters determined earlier from measured particle yields and p_T spectra. The results agree qualitatively well with the experimental data.     

Pinning mechanism in carbon nanotube doping of MgB2 superconductor

MgB_1.9C_0.1 samples are synthesized under the ambient pressure (AP) and high pressure (HP), respectively. The further studies demonstrate different field-dependence of the critical current density J_c(H) in each sample. In the view of two-gap superconductivity in these samples, δT_c pinning (resulting from the spatial fluctuations of the transition temperature) is dominant in the AP sample, while in the HP sample, both δT_c and δl pinning (due to the mean-free-path fluctuations) act together and their contributions vary with temperature. Besides the improvement of H_c2(0), due to the different pinning mechanism, J_c(H) of the HP sample shows a slower decay with the increasing fields than that of the AP sample in high fields, which suggests a possible method of retarding the rapid decay of J_c(H) under elevated fields.