Ab Initio Description of Open-Shell Nuclei: Merging No-Core Shell Model and In-Medium Similarity Renormalization Group

We merge two successful ab initio nuclear-structure methods, the no-core shell model (NCSM) and the multi-reference in-medium similarity renormalization group (IM-SRG) to define a new many-body approach for the comprehensive description of ground and excited states of closed and open-shell nuclei. Building on the key advantages of the two methods---the decoupling of excitations at the many-body level in the IM-SRG and the access to arbitrary nuclei, eigenstates, and observables in the NCSM---their combination enables fully converged no-core calculations for an unprecedented range of nuclei and observables at moderate computational cost. We present applications in the carbon and oxygen isotopic chains, where conventional NCSM calculations are still feasible and provide an important benchmark. The efficiency and rapid convergence of the new approach make it ideally suited for ab initio studies of the complete spectroscopy of nuclei up into the medium-mass regime.Comment: 5 pages, 4 figures, v2: update to published versio

Long-lived electron emission reveals localized plasmon modes in disordered nanosponge antennas

We report long-lived, highly spatially localized plasmon states on the surface of nanoporous gold nanoparticles-nanosponges-with high excitation efficiency. It is well known that disorder on the nanometer scale, particularly in two-dimensional systems, can lead to plasmon localization and large field enhancements, which can, in turn, be used to enhance nonlinear optical effects and to study and exploit quantum optical processes. Here, we introduce promising, three-dimensional model systems for light capture and plasmon localization as gold nanosponges that are formed by the dewetting of gold/ silver bilayers and dealloying. We study light-induced electron emission from single nanosponges, a nonlinear process with exponents of n approximate to 5...7, using ultrashort laser pulse excitation to achieve femtosecond time resolution. The long-lived electron emission process proves, in combination with optical extinction measurements and finite-difference time-domain calculations, the existence of localized modes with lifetimes of more than 20 fs. These electrons couple efficiently to the dipole antenna mode of each individual nanosponge, which in turn couples to the far-field. Thus, individual gold nanosponges are cheap and robust disordered nanoantennas with strong local resonances, and an ensemble of nanosponges constitutes a meta material with a strong polarization independent, nonlinear response over a wide frequency range

Bogoliubov many-body perturbation theory for open-shell nuclei

A Rayleigh–Schrödinger many-body perturbation theory (MBPT) approach is introduced by making use of a particle-number-breaking Bogoliubov reference state to tackle (near-)degenerate open-shell fermionic systems. By choosing a reference state that solves the Hartree–Fock–Bogoliubov variational problem, the approach reduces to the well-tested Møller–Plesset, i.e., Hartree–Fock based, MBPT when applied to closed-shell systems. Due to its algorithmic simplicity, the newly developed framework provides a computationally simple yet accurate alternative to state-of-the-art non-perturbative manybody approaches. At the price of working in the quasi-particle basis associated with a single-particle basis of sufficient size, the computational scaling of the method is independent of the particle number. This paper presents the first realistic applications of the method ranging from the oxygen to the nickel isotopic chains on the basis of a modern nuclear Hamiltonian derived from chiral effective field theory

Hidden spin-isospin exchange symmetry

The strong interactions among nucleons have an approximate spin-isospin exchange symmetry that arises from the properties of quantum chromodynamics in the limit of many colors, $N_c$. However this large-$N_c$ symmetry is well hidden and reveals itself only when averaging over intrinsic spin orientations. Furthermore, the symmetry is obscured unless the momentum resolution scale is close to an optimal scale that we call $\Lambda_{{\rm large-}N_c}$. We show that the large-$N_c$ derivation requires a momentum resolution scale of $\Lambda_{{\rm large-}N_c} \sim 500$ MeV. We derive a set of spin-isospin exchange sum rules and discuss implications for the spectrum of $^{30}$P and applications to nuclear forces, nuclear structure calculations, and three-nucleon interactions.Comment: 5 pages (main) + 3 pages (supplemental materials), 1 figure (main) + 4 figures (supplemental materials), final version to appear in Phys. Rev. Let

An Ab-Initio Approach to Pairing Phenomena Using Modern Effective Interactions

The Unitary Correlation Operator Method (UCOM) and the Similarity Renormalization Group (SRG) allow the derivation of `tamed' phase-shift equivalent nucleon-nucleon interactions which are a suitable starting point for a wide array of many-body methods, from simple mean-field approaches like Hartree-Fock to the exact No-Core Shell Model. While the UCOM and the SRG are conceptually very different, we explicitly show that the generators of both types of unitary transformations have the same structure, and therefore treat the same kind of physics, i.e., the short-range central and tensor correlations induced by realistic nucleon-nucleon interactions. Mean-field calculations with the correlated interaction V_UCOM yield bound nuclei over the whole mass chart, and by including long-range correlations which are not explicitly described by the UCOM transformation in many-body perturbation theory, very good agreement with experimental binding energies is achieved. In conventional approaches, this is only possible by using phenomenological interactions which are explicitly tailored to mean-field calculations and therefore unable to describe nucleon scattering phase shifts. To extend our calculations to open-shell nuclei and allow for the treatment of pairing phenomena, we develop a fully consistent Hartree-Fock-Bogoliubov (HFB) approach in this work. Exact and approximate projection techniques are generalized to a simultaneous restoration of the neutron and proton number symmetries, which are broken by the introduction of quasiparticles in the HFB method. The use of V_UCOM in this framework enables us to study the pairing properties of nuclei from first principles, and provides insight into the effect of short-range correlations on the pair formation. We present results from the application of the HFB method with and without projection to the study of the tin isotopic chain. While the effect of three-nucleon forces on the binding energies can be minimized by an appropriately chosen UCOM transformation, the HF and HFB ground states calculated with such a two-body interaction exhibit too-small radii and a low level density, which are caused by the strong non-locality of the corresponding V_UCOM. Naturally, the low level density is found to be a strong impediment to pairing. In exploratory HF calculations, a three-nucleon contact force was able to improve the radii and level densities. Since the use of such a force in HFB calculations is more demanding, we approximate it by a zero-range density-dependent two-body interaction in order to assess the impact of three-nucleon effects in our HFB framework. The ground states obtained from the HFB method serve as the basis for a fully self-consistent Quasiparticle Random Phase Approximations (QRPA), which can be used to study pairing effects on collective excitations. We present selected results on electromagnetic resonances in the tin isotopes, in particular the pygmy dipole resonance in the neutron-rich isotopes Sn-130 and Sn-132. In addition, we apply the charge-exchange version of our QRPA to the isobaric analog and Gamow-Teller resonances in Zr-90

fully consistent framework for HF(B), (Q)RPA etc.:

fully consistent framework for HF(B), (Q)RPA etc.: complete interaction in particle-hole & particle-particle channel