Gustav Baardsen

• Palmer, Teresa Lynne; Baardsen, Gustav & Skartlien, Roar (2017). Reduction of the effective shear viscosity in polymer solutions due to crossflow migration in microchannels: Effective viscosity models based on DPD simulations. Journal of Dispersion Science and Technology.  ISSN 0193-2691.  39(2), s 190- 206 . doi: 10.1080/01932691.2017.1306784 Fulltekst i vitenarkiv.
• Hagen, Gaute; Papenbrock, Thomas; Ekström, Jan Andreas; Wendt, Kyle A.; Baardsen, Gustav; Gandolfi, Stefano; Hjorth-Jensen, Morten & Horowitz, Charles J. (2014). Coupled-cluster calculations of nucleonic matter. Physical Review C. Nuclear Physics.  ISSN 0556-2813.  89:014319 . doi: 10.1103/PhysRevC.89.014319 Vis sammendrag

  Background: The equation of state (EoS) of nucleonic matter is central for the understanding of bulk nuclear properties, the physics of neutron star crusts, and the energy release in supernova explosions. Because nuclear matter exhibits a finely tuned saturation point, its EoS also constrains nuclear interactions. Purpose: This work presents coupled-cluster calculations of infinite nucleonic matter using modern interactions from chiral effective field theory (EFT). It assesses the role of correlations beyond particle-particle and hole-hole ladders, and the role of three-nucleon forces (3NFs) in nuclear matter calculations with chiral interactions. Methods: This work employs the optimized nucleon-nucleon (NN) potential NNLOopt at next-to-next-to leading order, and presents coupled-cluster computations of the EoS for symmetric nuclear matter and neutron matter. The coupled-cluster method employs up to selected triples clusters and the single-particle space consists of a momentum-space lattice. We compare our results with benchmark calculations and control finite-size effects and shell oscillations via twist-averaged boundary conditions. Results: We provide several benchmarks to validate the formalism and show that our results exhibit a good convergence toward the thermodynamic limit. Our calculations agree well with recent coupled-cluster results based on a partial wave expansion and particle-particle and hole-hole ladders. For neutron matter at low densities, and for simple potential models, our calculations agree with results from quantum Monte Carlo computations. While neutron matter with interactions from chiral EFT is perturbative, symmetric nuclear matter requires nonperturbative approaches. Correlations beyond the standard particle-particle ladder approximation yield non-negligible contributions. The saturation point of symmetric nuclear matter is sensitive to the employed 3NFs and the employed regularization scheme. 3NFs with nonlocal cutoffs exhibit a considerably improved convergence than their local cousins. We are unable to find values for the parameters of the short-range part of the local 3NF that simultaneously yield acceptable values for the saturation point in symmetric nuclear matter and the binding energies of light nuclei.

• Baardsen, Gustav; Ekström, Jan Andreas; Hagen, Gaute & Hjorth-Jensen, Morten (2013). Coupled-cluster studies of infinite nuclear matter. Physical Review C. Nuclear Physics.  ISSN 0556-2813.  88(5) . doi: 10.1103/PhysRevC.88.054312 Vis sammendrag

  Background: Coupled-cluster (CC) theory is a widely used many-body method for studying strongly correlated many-fermion systems. It allows for systematic inclusions of complicated many-body correlations beyond a mean field. Recent applications to finite nuclei have shown that first-principles approaches like CC theory can be extended to studies of medium-heavy nuclei, with excellent agreement with experiment. However, CC calculations of properties of infinite nuclear matter are rather few and date back more than 30 yr. Purpose: The aim of this work is thus to develop the relevant formalism for performing CC calculations in nuclear matter and neutron-star matter, including thereby important correlations to infinite order in the interaction and testing modern nuclear forces based on chiral effective field theory. Our formalism includes also the exact treatment of the so-called Pauli operator in a partial-wave expansion of the equation of state. Methods: Nuclear- and neutron-matter calculations are done using a coupled particle-particle and hole-hole ladder approximation. The coupled ladder equations are derived as an approximation of CC theory, leaving out particle-hole and nonlinear diagrams from the CC doubles amplitude equation. This study is a first step toward CC calculations for nuclear and neutron matter. Results: We present results for both symmetric nuclear matter and pure neutron matter employing state-of-the-art nucleon-nucleon interactions based on chiral effective field theory. We employ also the newly optimized chiral interaction [ Ekström et al. Phys. Rev. Lett. 110 192502 (2013)] to study infinite nuclear matter. The ladder approximation method and corresponding results are compared with conventional Brueckner-Hartree-Fock theory. The ladder approximation is derived and studied using both exact and angular-averaged Pauli exclusion operators, with angular-averaged input momenta for the single-particle potentials in all calculations. The inclusion of an exact treatment of the Pauli operators in a partial-wave expansion yields corrections of the order of 1.7%–2% of the total energy in symmetric nuclear matter. Similarly, the inclusion of both hole-hole and particle-particle ladders result in corrections of the order 0.7%–2% compared to the approximation with only particle-particle ladders. Including these effects, we get at most almost a 6% difference between our CC calculation and the standard Brueckner-Hartree-Fock approach. Conclusions: We have performed CC calculations of symmetric nuclear matter and pure neutron matter including particle-particle and hole-hole diagrams to infinite order using an exact Pauli operator and angular-averaged single-particle energies. The contributions from hole-hole diagrams and exact Pauli operators add important changes to the final energies per particle.

• Ekström, Jan Andreas; Baardsen, Gustav; Forssen, C; Hagen, Gaute; Hjorth-Jensen, Morten; Jansen, Gustav R.; Machleidt, Ruprecht; Nazarewicz, witek; Papenbrock, Thomas; Sarich, Jason & Wild, Stefan (2013). Optimized Chiral Nucleon-Nucleon Interaction at Next-to-Next-to-Leading Order. Physical Review Letters.  ISSN 0031-9007.  110(19) . doi: 10.1103/PhysRevLett.110.192502 Vis sammendrag

  We optimize the nucleon-nucleon interaction from chiral effective field theory at next-to-next-to-leading order (NNLO). The resulting new chiral force NNLOopt yields χ2≈1 per degree of freedom for laboratory energies below approximately 125 MeV. In the A=3, 4 nucleon systems, the contributions of three-nucleon forces are smaller than for previous parametrizations of chiral interactions. We use NNLOopt to study properties of key nuclei and neutron matter, and we demonstrate that many aspects of nuclear structure can be understood in terms of this nucleon-nucleon interaction, without explicitly invoking three-nucleon forces.

Se alle arbeider i Cristin

• Rebolini, Elisa; Baardsen, Gustav; Hansen, Audun Skau; Leikanger, Karl Roald & Pedersen, Thomas Bondo (2017). Local coupled cluster methods for periodic systems.
• Leikanger, Karl R.; Rebolini, Elisa; Hansen, Audun Skau; Baardsen, Gustav & Pedersen, Thomas Bondo (2016). Hartree-Fock calculations using a priori Wannier orbitals for solids.
• Baardsen, Gustav (2014). Coupled-cluster theory for infinite matter.

Se alle arbeider i Cristin