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We present, for the first time, simultaneous determination of shear viscosity ($\eta$) and entropy density ($s$) and thus, $\eta/s$ for equilibrated nuclear systems from $A$ $\sim$ 30 to $A$ $\sim$ 208 at different temperatures. At finite temperature, $\eta$ is estimated by utilizing the $\gamma$ decay of the isovector giant dipole resonance populated via fusion evaporation reaction, while $s$ is evaluated from the nuclear level density parameter (${a}$) and nuclear temperature ($T$), determined precisely by the simultaneous measurements of the evaporated neutron energy spectra and the compound nuclear angular momenta. The transport parameter $\eta$ and the thermodynamic parameter $s$ both increase with temperature resulting in a mild decrease of $\eta$/$s$ with temperature. The extracted $\eta$/$s$ is also found to be independent of the neutron-proton asymmetry at a given temperature. Interestingly, the measured $\eta$/$s$ values are comparable to that of the high-temperature quark-gluon plasma, pointing towards the fact that strong fluidity may be the universal feature of the strong interaction of many-body quantum systems., Comment: Accepted for publication in Physical Review Letters, 6 pages, 4 figures

High energy $\gamma$-ray spectra measured by our group involving the compound nuclei (CN) $^{63}$Cu at excitation energy ($E^*$) $\sim$ 36 MeV with average angular momentum ($J$) = 12 - 17 $\hbar$, $^{97}$Tc at $E^* \sim$ 29 - 50 MeV with $J$ = 12 - 14 $\hbar$, $^{113}$Sb at $E^*$ = 109 MeV and 121 MeV with $J$ = 49 - 59 $\hbar$ and $^{201}$Tl at $E^*$ = 39.5, 47.5 MeV with $J$ = 18 - 24 $\hbar$ have been analyzed utilizing the level density prescriptions of (i)Ignatyuk, Smirenkin and Tishin (IST), (ii)Budtz-Jorgensen and Knitter (BJK), and (iii) Kataria, Ramamurthy and Kapoor (KRK). These three prescriptions have been tested for correct statistical model description of high energy $\gamma$-rays in the light of extracting the giant dipole resonance (GDR) parameters at low excitation energy and spin where shell effects might play an important role as well as at high excitation energy where shell effects have melted. Interestingly, only the IST level density prescription could explain the high energy $\gamma$-ray spectra with reasonable GDR parameters for all the four nuclei., Comment: 9 pages, 10 figure, Accepted for publication in Physical Review C

The universality of the Critical Temperature included Fluctuation Model (CTFM) in explaining the evolution of the giant dipole resonance (GDR) width as a function of angular momentum is examined in the light of recent experimental data on $^{144}$Sm and $^{152}$Gd. We compare both the data sets with the phenomenological formula based on the CTFM and the thermal shape fluctuation model (pTSFM). The CTFM describes both the data sets reasonably well using the actual ground state GDR width ($\Gamma_0$) values, whereas, the pTSFM describes the $^{144}$Sm data well but is unable to explain the $^{152}$Gd data using a single value of $\Gamma_0$ for two excitation energies. These interesting results clearly indicate that the phenomenological CTFM can be used universally to describe the evolution of the GDR width with both angular momentum and temperature in the entire mass region. Moreover, it should provide new insights into the modification of the TSFM., Comment: 6 pages, 7 Figures, Accepted for publication in Phys. Rev. C

The systematic study of the correlation between the experimental giant dipole resonance (GDR) width and the average deformation <\beta> of the nucleus at finite excitation is presented for the mass region A ~ 59 to 208. We show that the width of the GDR (\Gamma) and the quadrupole deformation of the nucleus do not follow a linear relation, as predicted earlier, due to the GDR induced quadrupole moment and the correlation also depends on the mass of the nuclei. The different empirical values of <\beta> extracted from the experimental GDR width match exceptionally well with the thermal shape fluctuation model. As a result, this universal correlation between <\beta> and \Gamma provides a direct experimental probe to determine the nuclear deformation at finite temperature and angular momentum in the entire mass region., Comment: 5 pages, 3 Figures, Published in Phys. Rev. C 87, 044325 (2013)