Anomalous population of 10^{10}He states in reactions with 11^{11}Li

Structure with the lowest energy observed in the $^{10}$He spectrum populated in the proton knockout reaction with $^{11}$Li beam has a peak at $1.2-1.5$ MeV. This peak is usually interpreted as a resonant $0^+$ ground state of $^{10}$He. Our theoretical calculations indicate that this peak is likely to be a pileup of $1^-$, $0^+$, and $2^+$ excitations with very similar shapes. %We predict a very specific nature of the $1^-$ excitation in $^{10}$He. Moreover, the ``soft'' $1^-$ excitation appears to be the lowest one in energy. Such an anomalous continuum response is traced to the halo structure of $^{11}$Li providing extreme low energy shift to all the expected continuum excitations. Competitions of the initial state structure (ISS) and the final state interaction (FSI) effects on the spectrum and three-body correlations in $^{10}$He are discussed. Analogous effect of the extreme low-energy shift could also be expected in other cases of $2n$ emitters populated in reactions with halo nuclei. Simplified example of the $^{10}$He spectrum in $\alpha$ knockout from $^{14}$Be, is given. We also discuss limits on the properties of $^{9}$He stemming from the observed $^{10}$He spectrum.Comment: 10 pages, 13 figure

Problems with interpretation of 10^{10}He ground state

The continuum of $^{10}$He nucleus is studied theoretically in a three-body $^{8}$He+$n$+$n$ model basing on the recent information concerning $^9$He spectrum [Golovkov, \textit{et al.}, Phys. Rev. C \textbf{76}, 021605(R) (2007)]. The $^{10}$He ground state (g.s.) candidate with structure $[p_{1/2}]^2$ for new g.s. energy of $^9$He is predicted to be at about $2.0-2.3$ MeV. The peak in the cross section associated with this state may be shifted to a lower energy (e.g. $\sim 1.2$ MeV) when $^{10}$He is populated in reactions with $^{11}$Li due to peculiar reaction mechanism. Formation of the low-energy ($E< 250$ keV) ``alternative'' ground state with structure $[s_{1/2}]^2$ is highly probable in $^{10}$He in the case of considerable attraction (e.g. $a<-5$ fm) in the s-wave $^9$He channel, which properties are still quite uncertain. This result either questions the existing experimental low-energy spectrum of $^{10}$He or place a limit on the scattering length in $^9$He channel, which contradicts existing data.Comment: 14 pages, 13 figures, 1 tabl

Optimized design of universal two-qubit gates

We construct optimized implementations of the CNOT and other universal two-qubit gates that, unlike many of the previously proposed protocols, are carried out in a single step. The new protocols require tunable inter-qubit couplings but, in return, show a significant improvements in the quality of gate operations. Our optimization procedure can be further extended to the combinations of elementary two-qubit as well as irreducible many-qubit gates.Comment: 6 pages, 2 figure

Two-proton radioactivity and three-body decay. V. Improved momentum distributions

Nowadays quantum-mechanical theory allows one to reliably calculate the processes of 2p radioactivity (true three-body decays) and the corresponding energy and angular correlations up to distances of the order of 1000 fm. However, the precision of modern experiments has now become sufficient to indicate some deficiency of the predicted theoretical distributions. In this paper we discuss the extrapolation along the classical trajectories as a method to improve the convergence of the theoretical energy and angular correlations at very large distances (of the order of atomic distances), where only the long-range Coulomb forces are still operating. The precision of this approach is demonstrated using the "exactly" solvable semianalytical models with simplified three-body Hamiltonians. It is also demonstrated that for heavy 2p emitters, the 2p decay momentum distributions can be sensitive to the effect of the screening by atomic electrons. We compare theoretical results with available experimental data.Comment: 13 pages, 18 figure

Transition from direct to sequential two-proton decay in ss-dd shell nuclei

Transitions among different mechanisms of two-proton decay are studied in general. The introduced improved direct-decay model generalizes the semi-analytical models used before and provides flawless phenomenological description of three-body correlations in $2p$ decays. This is demonstrated by examples of the low-lying $^{16}$Ne state decays. Different forms of transition dynamic are shown to be highly probable beyond the proton dripline for the $s$-$d$ shell nuclei. It is demonstrated that transition dynamic of $2p$ emitters can provide means for extraction of a width of the ground-state resonance of a core+$p$ subsystem of the core+$2p$ system. Practical applicability of the method is demonstrated by properties of the $^{14}$F ground state derived from the ^{15}\mbox{Ne}\rightarrow ^{\,13\!\!}\mbox{O}+2p decay data and of the $^{29}$Cl ground state derived from the ^{30}\mbox{Ar}\rightarrow ^{\,28\!\!}\mbox{S}+2p decay data.Comment: 8 pages, 10 figure

Two-proton events in the 17F(p,2p)16O reaction

In a recent experimental study (Gomez del Campo et al, PRL 86, 43 (2001)) of the reaction 17F(p,2p)16O, two-proton events were measured from excitations near a 1-, E*=6.15 MeV state in 18Ne. We calculate by means of R-matrix theory the resonant two-proton production cross section and branching ratios. We conclude that it is unlikely that two-proton production via population of the 1- state is sufficient to explain the observed two-proton events. Alternative sources of such events are discussed.Comment: 4 pages, 4 figures. Resubmission to Physical Review C (first received 6 March 2001

From Coulomb excitation cross sections to non-resonant astrophysical rates in three-body systems: 17^{17}Ne case

Coulomb and nuclear dissociation of $^{17}$Ne on light and heavy targets are studied theoretically. The dipole E1 strength function is determined in a broad energy range including energies of astrophysical interest. Dependence of the strength function on different parameters of the $^{17}$Ne ground state structure and continuum dynamics is analyzed in a three-body model. The discovered dependence plays an important role for studies of the strength functions for the three-body E1 dissociation and radiative capture. The constraints on the $[s^2]/[d^2]$ configuration mixing in $^{17}$Ne and on $p$-wave interaction in the $^{15}$O+$p$ channel are imposed based on experimental data for $^{17}$Ne Coulomb dissociation on heavy target.Comment: 12 pages, 13 figure

Phonon mediated tunneling into graphene

Recent scanning tunneling spectroscopy experiments [V. W. Brar et. al., Appl. Phys. Lett. 91, 122102 (2007), Y. Zhang et. al., arXiv:0802.4315 (2008)] on graphene reported an unexpected gap of about $\pm 60$meV around the Fermi level. Here, we give a theoretical investigation explaining the experimentally observed spectra and confirming the phonon mediated tunneling as the reason for the gap: We study the real space properties of the wave functions involved in the tunneling process by means of ab-initio theory and present a model for the electron-phonon interaction, which couples the graphene's Dirac electrons with quasi free electron states at the Brillouin zone center. The self-energy associated with this electron-phonon interaction is calculated and its effects on tunneling into graphene are discussed. In particular, good agreement of the tunneling density of states within our model and the experimental[V. W. Brar et. al., Appl. Phys. Lett. 91, 122102 (2007), Y. Zhang et. al., arXiv:0802.4315 (2008)] dI/dU spectra is found.Comment: 5 pages, 3 figure