Anharmonic phonon excitations in subbarrier fusion reactions

Recently measured high precision data of fusion excitation function have enabled a detailed study on the effects of nuclear collective excitations on fusion reactions. Using such highly accurate data of the $^{16}$O + $^{144,148}$Sm reactions, we discuss the anharmonic properties of collective phonon excitations in $^{144,148}$Sm nuclei. It is shown that subbarrier fusion reactions are strongly affected by the anharmonic effects and thus offer an alternative method to extract the static quadrupole moments of phonon states in a spherical nucleus.Comment: 10 pages, To be published in the Proceedings of the Tours Symposium on Nuclear Physics III, Tours, France, September 1997 (American Institute of Physics

Possible Localized Modes in the Uniform Quantum Heisenberg Chains of Sr2CuO3

A model of mobile-bond defects is tentatively proposed to analyze the "anomalies" observed on the NMR spectrum of the quantum Heisenberg chains of Sr2CuO3. A bond-defect is a local change in the exchange coupling. It results in a local alternating magnetization (LAM), which when the defect moves, creates a flipping process of the local field seen by each nuclear spin. At low temperature, when the overlap of the LAM becomes large, the defects form a periodic structure, which extends over almost all the chains. In that regime, the density of bond-defects decreases linearly with T.Comment: 4 pages + 3 figures. To appear in Physical Review

NMR evidence for the persistence of spin-superlattice above the 1/8 magnetization plateau in SrCu2(BO3)2

We present 11B NMR studies of the 2D frustrated dimer spin system SrCu2(BO3)2 in the field range 27-31 T covering the upper phase boundary of the 1/8 magnetization plateau, identified at 28.4 T. Our data provide a clear evidence that above 28.4 T the spin-superlattice of the 1/8 plateau is modified but does not melt even though the magnetization increases. Although this is precisely what is expected for a supersolid phase, the microscopic nature of this new phase is much more complex. We discuss the field-temperature phase diagram on the basis of our NMR data.Comment: 5 pages, 4 figures, published versio

Addendum: Attenuation of the intensity within a superdeformed band

We investigate a random matrix model [Phys. Rev. C {\bf 65} 024302 (2002] for the decay-out of a superdeformed band as a function of the parameters: $\Gamma^\downarrow/\Gamma_S$, $\Gamma_N/D$, $\Gamma_S/D$ and $\Delta/D$. Here $\Gamma^\downarrow$ is the spreading width for the mixing of an SD state $|0>$ with a normally deformed (ND) doorway state $|d>$, $\Gamma_S$ and $\Gamma_N$ are the electromagnetic widths of the the SD and ND states respectively, $D$ is the mean level spacing of the compound ND states and $\Delta$ is the energy difference between $|0>$ and $|d>$. The maximum possible effect of an order-chaos transition is inferred from analytical and numerical calculations of the decay intensity in the limiting cases for which the ND states obey Poisson and GOE statistics. Our results show that the sharp attenuation of the decay intensity cannot be explained solely by an order-chaos transition.Comment: 4 pages, 4 figures, submitted to Physical Review

Energy averages over regular and chaotic states in the decay out of superdeformed bands

We describe the decay out of a superdeformed band using the methods of reaction theory. Assuming that decay-out occurs due to equal coupling (on average) to a sea of equivalent chaotic normally deformed (ND) states, we calculate the average intraband decay intensity and show that it can be written as an ``optical'' background term plus a fluctuation term, in total analogy with average nuclear cross sections. We also calculate the variance in closed form. We investigate how these objects are modified when the decay to the ND states occurs via an ND doorway and the ND states' statistical properties are changed from chaotic to regular. We show that the average decay intensity depends on two dimensionless variables in the first case while in the second case, four variables enter the picture.Comment: 8 pages, 1 figure, presented at FUSION03, Matsushima, Miyagi, Japan, Nov 12-15, 2003, to appear in Progress of Theoretical Physics; corrected typo

Enhanced low-energy spin dynamics with diffusive character in the iron-based superconductor (La0.87Ca0.13)FePO: Analogy with high Tc cuprates (A short note)

In a recent NMR investigation of the iron-based superconductor (La0.87Ca0.13)FePO [Phys. Rev. Lett. 101, 077006 (2008)] Y. Nakai et al. reported an anomalous behavior of the nuclear spin-lattice relaxation of 31P nuclei in the superconducting state: The relaxation rate 1/T1 strongly depends on the measurement frequency and its T dependence does not show the typical decrease expected for the superconducting state. In this short note, we point out that these two observations bear similarity with the situation is some of the high Tc cuprates.Comment: To appear in J. Phys. Soc. Jpn. (Short Note

Field Dependence of Electronic Specific Heat in Two-Band Superconductors

The vortex structure is studied in light of MgB$_2$ theoretically based on a two-band superconducting model by means of Bogoliubov-de Gennes framework. The field dependence of the electronic specific heat coefficient $\gamma (H)$ is focused. The exponent $\alpha$ in $\gamma (H)\propto H^{\alpha}$ is shown to become smaller by adjusting the gap ratio of the two gaps on the major and minor bands. The observed extremely small value $\alpha\sim 0.23$ could be explained reasonably well in this two-band model with the gap ratio $\sim 0.3$.Comment: 5 pages, 4 figures, to be published in J. Phys. Soc. Jp

17O NMR study of q=0 spin excitations in a nearly ideal S=1/2 1D Heisenberg antiferromagnet, Sr2CuO3, up to 800 K

We used 17O NMR to probe the uniform (wavevector q=0) electron spin excitations up to 800 K in Sr2CuO3 and separate the q=0 from the q=\pm\pi/a staggered components. Our results support the logarithmic decrease of the uniform spin susceptibility below T ~ 0.015J, where J=2200 K. From measurement of the dynamical spin susceptibility for q=0 by the spin-lattice relaxation rate 1/T_{1}, we demonstrate that the q=0 mode of spin transport is ballistic at the T=0 limit, but has a diffusion-like contribution at finite temperatures even for T << J.Comment: Submitted to Phys. Rev. Lett. 4 pages, 4 figure