Hydrogen adsorption and phase transitions in fullerite

Hydrogen desorption and adsorption properties of the fullerene materials C60, C70, and fullerite (a mixture of C60 and C70) were measured volumetrically using a Sievert's apparatus. Over several cycles of isotherm measurements at 77 K, the hydrogen storage capacities of one of the fullerite samples increased from an initial value of 0.4 wt % for the first cycle to a capacity of 4.4 wt % for the fourth cycle. Correspondingly, the surface area of this sample increased from 0.9 to 11 m^2/g, and there were changes in its x-ray powder diffraction pattern. In comparison, two other fullerite samples, prepared by a different procedure showed no such behavior. Pure C60 and pure C70 were also cycled and exhibited small and constant capacities of 0.7 and 0.33 wt %, respectively, as a function of number of cycles. The enhanced storage capacity of fullerite material is tentatively attributed to the presence of C60 oxide

Efficient Schemes for Reducing Imperfect Collective Decoherences

We propose schemes that are efficient when each pair of qubits undergoes some imperfect collective decoherence with different baths. In the proposed scheme, each pair of qubits is first encoded in a decoherence-free subspace composed of two qubits. Leakage out of the encoding space generated by the imperfection is reduced by the quantum Zeno effect. Phase errors in the encoded bits generated by the imperfection are reduced by concatenation of the decoherence-free subspace with either a three-qubit quantum error correcting code that corrects only phase errors or a two-qubit quantum error detecting code that detects only phase errors, connected with the quantum Zeno effect again.Comment: no correction, 3 pages, RevTe

Cosmological Lower Bound on Dark Matter Masses from the Soft Gamma-ray Background

Motivated by a recent detection of 511 keV photons from the center of our Galaxy, we calculate the spectrum of the soft gamma-ray background of the redshifted 511 keV photons from cosmological halos. Annihilation of dark matter particles into electron-positron pairs makes a substantial contribution to the gamma-ray background. Mass of such dark matter particles must be <~ 100 MeV so that resulting electron-positron pairs are on-relativistic. On the other hand, we show that in order for the annihilation not to exceed the observed background, the dark matter mass needs to be >~ 20 MeV. We include the contribution from the active galactic nuclei and supernovae. The halo substructures may increase the lower bound to >~ 60 MeV.Comment: 5 pages, 5 figures; accepted for publication in PRD, Rapid Communicatio