Squeezing lepton pairs out of broken symmetries

We discuss two possible signatures of symmetry breaking that can appear in dilepton spectra, as measured in relativistic heavy ion collisions. The first involves scalar-vector meson mixing and is related to the breaking of Lorentz symmetry by a hot medium. The second is related to the breaking of Furry's theorem by a charged quark-gluon plasma. Those signals will be accessible to upcoming measurements to be performed at the GSI, RHIC, and the LHC.Comment: 5 pages, 4 figures, talk given at the INPC 2001 (International Conference on Nuclear Physics), 30 July - 3 August 2001, Berkeley, C

Local electromagnetic properties of magnetic pnictides: A comparative study probed by NMR measurement

75As and 31P NMR studies are performed in PrCoAsO and NdCoPO respectively. The Knight shift data in PrCoAsO indicate the presence of an antiferromagnetic interaction between the 4f moments along the c axis in the ferromagnetic state of Co 3d moments. We propose a possible spin structure in this system. The 75As quadrupolar coupling constant, $\nu_Q$ increases continuously with the decrease of temperature and is found to vary linearly with the intrinsic spin susceptibility, Kiso. This indicates a possibility of the presence of a coupling between charge density and spin density fluctuations. Further, 31P NMR Knight shift and spin lattice relaxation rate (1/T1) in the paramagnetic state of NdCoPO indicate that the differences between LaCoPO and NdCoPO with SmCoPO are due to the decrement of inter layer separation and not due to the moments of 4f electrons. Nuclear spin lattice relaxation time (T1) in NdCoPO shows weak anisotropy at 300 K. Using self consistent renormalization (SCR)theory of itinerant ferromagnet, it is shown that in the ab plane, the spin fluctuations are three dimensional ferromagnetic in nature. From SCR theory the important spin fluctuation parameters ($T_0$, $T_A$, $\bar{F}_1$) are evaluated. The similarities and dissimilarities of the NMR results in As and P based systems, with different rare earths have also been discussed.Comment: 18 pages,13 figures, accepted for publication in J. Phys. Condens. Matte

Long-range and short-range magnetic correlations, and microscopic origin of net magnetization in the spin-1 trimer chain compound CaNi3P4O14

Spin-spin correlations and microscopic origin of net magnetization in the spin-1 trimer chain compound CaNi3P4O14 have been investigated by powder neutron diffraction. The present study reveals a 3D long-range magnetic ordering below 16 K where the magnetic structure consists of ferromagnetic trimers that are coupled ferromagnetically along the spin-chain. The moment components along the a and c axes arrange antiferromagnetically. Our study establishes that the uncompensated moment components along the b axis result in a net magnetization per unit cell. The magnetic structure, determined in the present study, is in agreement with the results of recent first principles calculation; however, it is in contrast to a fascinating experimental prediction of ferrimagnetic ordering based on the periodicity of the exchange interactions in CaNi3P4O14. Our study also confirms the presence of broad diffuse magnetic scattering, due to 1D short-range spin-spin correlations, over a wide temperature range below ~50 K down to a temperature well below the Tc. Total neutron scattering analysis by the RMC method reveals that the dominating spin-spin correlation above Tc is ferromagnetic and along the b axis. The nearest neighbour spin-spin correlations along the a and c axes are found to be weakly antiferromagnetic. The nature of the trimer spin structure of the short-range state is similar to that of the 3D long-range ordered state. The present investigation of microscopic nature of the magnetic ground state also explains the condition required for the 1/3 magnetization plateau to be observed in the trimer spin-chains. In spite of the S=1 trimer chain system, the present compound CaNi3P4O14 is found to be a good realization of 3D magnet below the Tc=16 K with full ordered moment values of ~2 mu_B/Ni2+ (1.98 and 1.96 mu_B/Ni2+ for two Ni sites, respectively) at 1.5 K.Comment: 10 pages, 8 figure

Universality in Fluid Domain Coarsening: The case of vapor-liquid transition

Domain growth during the kinetics of phase separation is studied following vapor-liquid transition in a single component Lennard-Jones fluid. Results are analyzed after appropriately mapping the continuum snapshots obtained from extensive molecular dynamics simulations to a simple cubic lattice. For near critical quench interconnected domain morphology is observed. A brief period of slow diffusive growth is followed by a linear viscous hydrodynamic growth that lasts for an extended period of time. This result is in contradiction with earlier inclusive reports of late time growth exponent 1/2 that questions the uniqueness of the non-equilibrium universality for liquid-liquid and vapor-liquid transitions.Comment: 6 pages, 5 figure