Name: Computational Challenges in Nuclear and Many-Body Physics
Start: 2014-09-15T09:00:00+02:00
End: 2014-10-10T18:00:00+02:00
Location: Nordita, Stockholm

Monday, 15 September 2014
08:45 Registration - Elizabeth Yang
Registration
- Elizabeth Yang
08:45 - 09:30
Room: Nordita building number 23 Performed by Nordita's secretary Elizabeth Yang. The registration will continue during the whole week. Therefore participants which do not reach to be registered during the period given here (i. e. 8:45 - 9:30) can do it afterwards, for instance within the lunch time.
09:30 Welcome address - Axel Brandenburg
Welcome address
- Axel Brandenburg
09:30 - 10:00
Room: FP41 Prof. Axel Brandenburg is a member of the Board and Deputy Director of Nordita.
10:00 Defects in Nuclear Pasta - Charles Horowitz
Defects in Nuclear Pasta
- Charles Horowitz
10:00 - 10:40
Room: FP41 Dense nuclear matter, near the base of the crust in neutron stars, is expected to have complex nuclear pasta shapes because of coulomb frustration. Competition between short-range nuclear attraction and long-range coulomb repulsion insures that many different shapes have very similar energies. We report large-scale molecular dynamics simulations of nuclear pasta and find long-lived topological defects. These defects could increase electron pasta scattering and reduce the electrical and thermal conductivities. A reduced thermal conductivity may be visible in X-ray observations of neutron star crust cooling. A reduced electrical conductivity could lead to the decay of magnetic fields.
10:40 Coffee break
Coffee break
10:40 - 11:00
Room: 132:028
11:00 Time-Dependent Dynamics of Fermionic Superfluids: from cold atomic gases, to nuclei and neutron stars - Aurel Bulgac
Time-Dependent Dynamics of Fermionic Superfluids: from cold atomic gases, to nuclei and neutron stars
- Aurel Bulgac
11:00 - 11:40
Room: FP41 The fascinating dynamics of superfluids, often referred to as quantum coherence revealed at macroscopic scale, has challenged both experimentalists and theorists for more than a century now, starting with electron superconductivity discovered in 1911 by Heike Kamerlingh Onnes. The phenomenological two-fluid model of Tizsa and its final formulation due to Landau, is ultimately a classical approach in which Planck’s constant never appears and it is unable to describe the generation and dynamics of the quantized vortices, which are the hallmark characteristics of superfluidity. Various quantum mechanical phenomenological models have been developed over the years by London, Onsager, Feynman, Ginzburg and Landau, Abrikosov, and many others, but truly microscopic approaches are very scarce. The Gross-Pitaevskii equation was for many years the only example, but it is applicable only to a weakly interacting Bose gas at zero temperature and it has been used to describe the large variety of experiments in cold atomic Bose gases. In the case of fermionic superfluids only a time-dependent mean filed approach existed for a long time, which is known to be quite inaccurate. With the emergence of the Density Functional Theory and its time-dependent extension it became relatively recently possible to have a truly microscopic approach of their dynamics, which proves to be extremely relabel in predicting and describing various experimental results in cold atomic fermionic gases, nuclei and which can be used as well to make predictions about the nature and dynamics of vortices in the neutron star crust. I will describe the time-dependent superfluid local density approximation, which is an adiabatic extension of the density functional theory to superfluid Fermi systems and their real-time dynamics. This new theoretical framework has been used to describe/predict a range of phenomena in cold atomic gases and nuclear collective motion: excitation of the Higgs modes in strongly interacting Fermi superfluids, generation of quantized vortices, crossing and reconnection of vortices, excitation of the superflow at velocities above the critical velocity, excitation of quantum shock waves, domain walls and vortex rings in superfluid atomic clouds, and excitation of collective states in nuclei. This approach is the natural framework to describe in a time-dependent framework various low energy nuclear reactions and in particular large amplitude collective motion and nuclear fission and the numerical implementation of this formalism requires the largest supercomputers available to science today.
11:40 Lattice tight-binding Bogoliubov-de Gennes approach to nonuniform superconductivity: Josephson junctions, vortices, and disorder - Annica Black-Schaffer
Lattice tight-binding Bogoliubov-de Gennes approach to nonuniform superconductivity: Josephson junctions, vortices, and disorder
- Annica Black-Schaffer
11:40 - 12:20
Room: FP41 I will present results on using a lattice tight-binding Bogoliubov-de Gennes formulation of nonuniform superconducting systems and solving self-consistently for the superconducting order parameter. Systems studied include Josephson junctions in graphene and spin-orbit coupled semiconductors, superconducting vortices in spin-orbit coupled semiconductors, and studies of the local effect of impurities and disordered edges in unconventional superconductors. While the method has limitations, especially with regards to system sizes possible to study, it offers a microscopically accurate description of the superconducting state, which can be crucial for a correct physical description of nonuniform superconducting systems.
12:20 Discussion
Discussion
12:20 - 13:00
Room: FP41

13:00 Lunch
Lunch
13:00 - 14:30
Room: 132:028
14:30 Nuclear- and particle-physics aspects of condensed-matter nanosystems - Constantine Yannouleas
Nuclear- and particle-physics aspects of condensed-matter nanosystems
- Constantine Yannouleas
14:30 - 15:10
Room: FP41 The physics of condensed-matter nanosystems exhibits remarkable analogies with atomic nuclei. Examples are: Plasmons corresponding to Giant resonances [1], electronic shells, de- formed shapes, and fission [2], beta-type decay, strongly correlated phenomena associated with symmetry breaking and symmetry restoration [3], etc. Most recently, analogies with relativistic quantum-field theories (RQFT) and high-energy particle physics are beeing explored in the field of graphene nanostructures [4]. The talk will review these analogies focusing in particular on the following three aspects: (1) The shell-correction method (SCM, commonly known as Strutinsky’s averaging method and introduced in the 1960’s in nuclear physics) was formulated [5] in the context of density functional theory (DFT). Applications of the DFT-SCM (and of a semiempirical variant, SE-SCM, closer to the nuclear Strutinsky approach) to condensed-matter finite systems will be discussed, including the charging and fragmentation of metal clusters, fullerenes, and metallic nanowires [5]. The DFT-SCM offers an improvement compared to the use of Thomas-Fermi gradient expansions for the kinetic energy density functional in the framework of orbital-free DFT. (2) A unified description of strongly correlated phenomena in finite systems of repelling particles [whether electrons in quantum dots (QDs) or ultracold bosons in rotating traps] has been achieved through a two-step method of symmetry breaking at the unrestricted Hartree- Fock (UHF) level and of subsequent symmetry restoration via post Hartree-Fock projection techniques [3]. The general principles of the two-step method can be traced to nuclear theory (Peierls and Yoccoz) and quantum chemistry (L ̈owdin). This method can describe a wide variety of novel strongly correlated phenomena, including: (I) Chemical bonding and dissociation in quantum dot molecules and in single elliptic QDs, with potential technological applications to solid-state quantum computing. (II) Particle localization at the vertices of concentric polygonal rings and formation of rotating (and other less symmetric) Wigner molecules in quantum dots and ultracold rotating bosonic clouds [6]. (III) At high magnetic field (electrons) or rapid rotation (neutral bosons), the method yields analytic trial wave functions in the lowest Landau level [7], which are an alternative to the fractional-quantum-Hall-effect (FQHE) composite-fermion and Jastrow-Laughlin approaches. (3) The physics of planar graphene nanorings with armchair edge terminations shows analo- gies with the physics described by the RQFT Jackiw-Rebbi model and the related Su-Schrieffer- Heeger model of polyacetylene [4]. This part of the talk will describe the emergence of exotic states and properties, like solitons, charge fractionization, and nontrivial topological insulators, in these graphene nanosystems. [1] C. Yannouleas, R.A. Broglia, M. Brack, and P.F. Bortignon, Phys. Rev. Lett. 63, 255 (1989); [2] C. Yannouleas, U. Landman, and R.N. Barnett, in Metal Clusters, edited by W. Ekardt (John-Wiley, New York, 1999) Ch. 4, p. 145; [3] C. Yannouleas and U. Landman, Rep. Prog. Phys. 70, 2067 (2007), and references therein; [4] I. Romanovsky, C. Yannouleas, and U. Landman, Phys. Rev. B 87, 165431 (2013); Phys. Rev. B 89, 035432 (2014). [5] C. Yannouleas and U. Landman, Phys. Rev. B 48, 8376 (1993); Ch. 7 in ”Recent Advances in Orbital-Free Density Functional Theory,” Y.A. Wang and T.A. Wesolowski Eds. (Word Scientific, Singapore, 2013) p. 203 (arXiv:1004.3536); [6] C. Yannouleas and U. Landman, Phys. Rev. Lett. 82, 5325 (1999); I. Romanovsky, C. Yannouleas, and U. Landman, Phys. Rev. Lett. 97, 090401 (2006). [7] C. Yannouleas and U. Landman, Phys. Rev. A 81, 023609 (2010); Phys. Rev. B 84, 165327 (2011).
15:10 Angular-momentum-projection method to approach nuclear many-body wave functions - Yang Sun
Angular-momentum-projection method to approach nuclear many-body wave functions
- Yang Sun
15:10 - 15:50
Room: FP41 In performing shell-model calculations for large nuclear systems, the central issue is how to truncate the shell-model space efficiently. It corresponds to a proper arrangement of the configuration space to separate the most important part from the rest of the space. There are different schemes for the shell-model truncation. Considering the fact that most nuclei in the nuclear chart are deformed, using a deformed basis supplemented by angular momentum projection is an efficient way. Shell-model Hamiltonian is then diagonalized in the projected basis. The method is in principle independent of how a deformed basis is prepared and how an effective interaction is chosen. This approach may be viewed as to bridge the two traditional nuclear physics methods: the deformed mean-field approximation and the conventional shell-model diagonalization, because it keeps all the advantages that a mean-field model has to incorporate important correlations, and has the properties of the conventional shell-model that configurations are mixed beyond the mean-filed states to include effects of residual interactions. In this talk, we present the above idea by taking the Projected Shell Model and its extensions as examples [1,2,3,4]. Given the strong demand for shell model calculations also from nuclear astrophysics, one needs such an approach that contains sufficient correlations and can generate wave functions in the laboratory frame, thus allowing exact calculations for transition probabilities, spectroscopic factors, and beta-decay and electron-capture rates, in heavy, deformed nuclei. This research is supported by the National Natural Science Foundation of China (No. 11135005) and by the 973 Program of China (No. 2013CB834401). [1] K. Hara, Y. Sun, Int. J. Mod. Phys. E4 (1995) 637. [2] Y. Sun and C.-L. Wu, Phys. Rev. C68 (2003) 024315. [3] Y. Sun, Int. J. Mod. Phys. E15 (2006) 1695. [4] Y. Sun, Rev. Mex. Fis. S54(3) (2008) 122.
15:50 Coffee break
Coffee break
15:50 - 16:10
Room: 132:028
16:10 Microscopic description of nuclear reactions within Coupled Cluster and Gamow Shell Model theories - Nicolas Michel
Microscopic description of nuclear reactions within Coupled Cluster and Gamow Shell Model theories
- Nicolas Michel
16:10 - 16:50
Room: FP41 Nuclei at drip-lines bear unique properties such as halos or resonant character at ground state level, inexistent in the valley of stability. While the latter consists of standard closed quantum systems, drip-line nuclei are open quantum systems, so that models describing their properties must include both nuclear inter-correlations and continuum degrees of freedom. Coupled Cluster and Gamow Shell Model theories, in both ab-initio and effective approaches, are tools of choice for that matter as nuclear correlations are present through configuration mixing while continuum degrees of freedom are imparted by the use of the Berggren basis. The latter methods, initially devised for structure calculations, can now be utilized to study reaction observables. Applications concern direct reactions on light and medium nuclei.
16:50 Round table
Round table
16:50 - 17:30
Room: FP41
Tuesday, 16 September 2014
09:00 Low and High-Energy Excitations of the Unitary Fermi Gas - Joseph Carlson
Low and High-Energy Excitations of the Unitary Fermi Gas
- Joseph Carlson
09:00 - 09:40
Room: FP41 I describe the use of Quantum Monte Carlo Methods to study low- and high-energy excitations of the Unitary Fermi Gas. We have employed Auxiliary Field Quantum Monte Carlo methods to study this regime of strong pairing in the inhomogeneous gas. The scale invariance of the system places strong constraints on the form of the density functional, unlike nuclear density functionals it can be described in only a very few constants. The derived functional can then be used to predict the properties of small trapped clusters, we find excellent agreement between microscopic calculations of these clusters and results predicted by the density functional. We have also studied the response of the unitary Fermi Gas at very large momentum transfer. Experimentally, the spin and density response are quite different even at very high momentum. We describe approaches to reproduce these responses and analogies to neutrino scattering in nuclei.
09:40 Quantum simulation of two-dimensional U(1) critical systems: A Higgs particle and the possible help of string theory for the optical conductivity - Lode Pollet
Quantum simulation of two-dimensional U(1) critical systems: A Higgs particle and the possible help of string theory for the optical conductivity
- Lode Pollet
09:40 - 10:20
Room: FP41 Quantum simulators are special purpose devices designed to provide physical insight in a specific quantum problem that is hard to study in the laboratory and impossible on a computer. However, before they can be used they require calibration. For cold atomic systems, quantum Monte Carlo simulations have played a key role there. They established a few years ago that the thermodynamic properties of the experimental system are in one-to-one agreement with the simulations of the corresponding model. The synergy between the two approaches has dramatically progressed since then, to each other’s benefice: In the main part of this talk, I will focus on the dynamical properties of a U(1) critical system in (2+1) dimensions focusing on the existence of the amplitude mode or Higgs particle, and on the optical conductivity, which we compare against predictions from the AdS/CFT correspondence. Finally, I will discuss some open problems for this approach to quantum simulation.
10:20 Coffee break
Coffee break
10:20 - 10:40
Room: 132:028
10:40 Recent Developments and Applications of the Auxiliary-Field Monte Carlo Method - Christopher Gilbreth
Recent Developments and Applications of the Auxiliary-Field Monte Carlo Method
- Christopher Gilbreth
10:40 - 11:20
Room: FP41 The auxiliary-field Monte Carlo (AFMC) method is a powerful technique to calculate thermal and ground-state properties of strongly correlated systems. In particular, it has been extensively applied to study the properties of nuclear and atomic systems. We discuss several recent developments and applications of the method to finite-size systems. (i) In finite systems, it is often necessary to use the canonical ensemble with fixed number of particles. However, the projection on an odd number of particles leads to a new sign problem at low temperatures that has severely limited the application of AFMC to such systems. We discuss a method to circumvent the odd-particle sign problem which allows accurate determination of the ground-state energy, and present its application to the calculation of nuclear pairing gaps from odd-even mass differences [1]. (ii) The level density is among the most important statistical nuclear properties, but its calculation in the presence of correlations is a difficult many-body problem. We discuss recent AFMC calculations of level densities in heavy nuclei. In particular, we present the first microscopic calculation of the collective enhancement factors, which describe the enhancement of level densities by collective states [2]. (iii) Low-temperature calculations require numerical stabilization of the long chains of matrix multiplications necessary to compute the propagator, and a corresponding stabilized method for particle-number projection. The latter is computationally expensive. We discuss an improved method of stabilizing canonical-ensemble calculations that exhibits better scaling and allows calculations for much larger systems [3]. (iv) Deformation is an important concept for the understanding of heavy nuclei. However, it is based on mean-field theory, which breaks rotational invariance, a cornerstone symmetry of finite nuclei. We discuss a method to analyze nuclear deformations at finite temperature using AFMC, which preserves the rotational invariance of the system [4]. In particular, we calculate the probability distribution of the quadrupole operator in heavy rare-earth nuclei, and show that it carries model-independent signatures of deformation. References: [1] A. Mukherjee and Y. Alhassid, Phys. Rev. Lett. 109, 032503 (2012). [2] C. Ozen, Y. Alhassid and H. Nakada, Phys. Rev. Lett. 110, 042502 (2013). [3] C. N. Gilbreth and Y. Alhassid, arXiv:1402.3585 (2014). [4] Y. Alhassid, C. N. Gilbreth and G. F. Bertsch, arXiv:1408.0081 (2014)
11:20 Towards the entanglement of strongly coupled fermions via lattice Monte Carlo - Joaquin Drut
Towards the entanglement of strongly coupled fermions via lattice Monte Carlo
- Joaquin Drut
11:20 - 12:00
Room: FP41 The calculation of the entanglement properties of strongly coupled many-body systems, in particular Renyi and von Neumann entropies, continues to be an active research area with many open questions. In this talk, I will outline the challenges and describe some of the advances, by my group and others, towards the characterization of entanglement in non-relativistic many-fermion systems using novel lattice Monte Carlo strategies.
12:00 Discussion
Discussion
12:00 - 13:00
Room: FP41

13:00 Lunch
Lunch
13:00 - 14:30
Room: 132:028
14:30 Pfaffians in nuclear structure theory - Luis Robledo
Pfaffians in nuclear structure theory
- Luis Robledo
14:30 - 15:10
Room: FP41 In those branches of physics involving quantum many body systems, mean field states are a good starting point for any theoretical study. One of the advantages of mean field states is the existence of generalized Wick theorems that simplify the evaluation of operator overlaps. Unfortunately, the number of terms to be considered increase with the double factorial of the number of creation and annihilation operators in the overlap. This and other problems that appear when the mean field states are of the Hartree Fock Bogoliubov (HFB) type can be easily handled introducing fermion coherent state techniques and the pfaffian. In my talk I will discuss this technique and the applications involving HFB states in the context of symmetry restoration and configuration techniqes common in low energy nuclear structure calculations.
15:10 Auxiliary-field calculations with or without the sign problem:from Fermi gases to molecules to solids - Shiwei Zhang
Auxiliary-field calculations with or without the sign problem:from Fermi gases to molecules to solids
- Shiwei Zhang
15:10 - 15:50
Room: FP41 I will describe recent progress in developing a general framework for accurate ground-state calculations of interacting electronic systems. This framework is based on the use of auxiliary-fields, and addresses the sign problem (which turns into a phase problem for realistic electron-electron interactions) by constraining the imaginary-time paths with an approximate sign (gauge) condition. The approach can be used to study either a fully materials-specific Hamiltonian or a Hubbard-like model --- or indeed any electronic Hamiltonian in between as the former is ``down-folded'' to the latter. As an example of materials-specific calculations, we determine the equation of state in a variety of solids, which systematically removes deficiencies of density-functional theory (DFT) results. As an example of model studies, the nature of magnetic and charge correlations in the doped Hubbard model are determined, in the context of models for high-temperature superconductivity. Its implications on the search for so-called FFLO phases with cold atoms will be discussed. We also present exact results on the properties of the two-dimensional ultracold Fermi gas. Calculations in systems with strong spin-orbit coupling will be discussed.
15:50 Coffe break
Coffe break
15:50 - 16:10
Room: FP41
16:10 The isospin- and angular-momentum-projected density functional theory and beyond: formalism and applications - Wojciech Satula
The isospin- and angular-momentum-projected density functional theory and beyond: formalism and applications
- Wojciech Satula
16:10 - 16:50
Room: FP41 Over the last few years we have developed the multi-reference density functional theory (DFT) involving the isospin- and angular-momentum projections of a single Slater determinant. The model, dubbed below static, was specifically designed to treat rigorously the conserved rotational symmetry and, at the same time, tackle the explicit breaking of the isospin symmetry resulting from a subtle balance between the long-range isospin-symmetry-breaking Coulomb field and short-range isospin-symmetry-conserving (predominantly) strong force. These unique features allowed us to calculate, in between, the isospin impurities in N~Z nuclei and isospin symmetry breaking corrections (ISB) to superallowed Fermi beta-decay matrix elements. Recently, we have extended the model to a variant (hereafter called dynamic) that allows for mixing of states that are projected from self-consistent Slater determinants representing low-lying (multi)particle-(multi)hole excitations. The states that are mixed have good angular momentum and, at the same time, include properly treated Coulomb isospin mixing. Hence, the extended model can be considered as a variant of the no core configuration-interaction approach, with two-body short-range (hadronic) and long-range (Coulomb) interactions treated on the same footing. It is based on a truncation scheme dictated by the self-consistent deformed Hartree-Fock (HF) solutions. The model can be used to calculate spectra, transitions, and beta-decay matrix elements in any nuclei, irrespective of their neutron- and proton-number parities. The aim of the talk is to introduce the theoretical frameworks of both the static and dynamic approaches and present selected applications. The applications will be focused on nuclei relevant to high-precision tests of the weak-interaction flavor-mixing sector of the Standard Model. In this context, we will present the results for ISB corrections to superallowed Fermi transitions and for the low-spin spectra in: 32S and 32Cl nuclei, in A=38 Ar, K, and Ca nuclei, and in 62Ga and 62Zn nuclei. In case of 62Zn the spectrum of 0+ states will be addressed. The 0+ states in this nucleus were reassigned in a recent experiment, and are now posing a challenge to theory.
16:50 Round table
Round table
16:50 - 17:30
Room: FP41
Wednesday, 17 September 2014
10:00 Clustering and response functions of light nuclei in explicitly correlated Gaussians - Yasuyuki Suzuki
Clustering and response functions of light nuclei in explicitly correlated Gaussians
- Yasuyuki Suzuki
10:00 - 10:40
Room: FP41 Explicitly correlated Gaussian basis is used for solving few-body problems in many fields. The basis functions are easily adaptable and flexible enough to describe complex few-body dynamics. We obtain a unified description of different types of structure and a fair account of correlated motion of interacting particles as well as the tail of the wave function. I present some examples that show the power of the correlated Gaussians: The bound and resonant states of 4He, the electric dipole response functions of 4He and 6He, and alpha-clustering in 16O in the framework of a 12C core plus four nucleon model. It is a challenge for future to extend the application of the correlated Gaussians to a study on a competition between single-particle motion and clustering around a non-inert core. Such a study will be important to evaluate the rate of the radiative capture reactions 12C(alpha, gamma)16O at low energy and to account for the low-lying spectrum of 212Po that shows the large alpha-decay width and the enhanced electric dipole transitions.
10:40 Superconductivity as a Universal Emergent Phenomenon in Diverse Physical Systems - Mike Guidry
Superconductivity as a Universal Emergent Phenomenon in Diverse Physical Systems
- Mike Guidry
10:40 - 11:20
Room: FP41 Superconductivity and superfluidity having generically recognizabl features are observed or suspected across a strikingly broad range of physical systems: traditional BCS superconductors, cuprate high temperature superconductors, iron-based high-temperature superconductors, organic superconductors, heavy-fermion superconductors, and superfluid helium-3 in condensed matter, in many aspects of low-energy nuclear structure physics, and in various exotic possibilities for gravitationally condensed objects such as neutron stars. Microscopically these systems differ fundamentally but the observed superconductivity and superfluidity exhibit two universal features: (1) They result from a condensate of fermion Cooper pairs, and (2) They represent emergent collective behavior that can have only an abstract dependence on the underlying microscopic physics. This universality can hardly be a coincidence but a unified understanding of superconductivity and superfluidity across these highly disparate fields seems impossible microscopically. A unified picture may be possible if superconductivity and superfluidity are viewed as resulting from physics that depends only on broad physical principles operating systematically at the emergent scale, with physics at the underlying microscopic scale entering only parametrically. I will give an overview of superconductivity and superfluidity found in various fermionic condensed matter, nuclear physics, and neutron star systems. I will then propose that all these phenomena result from the systematic occurrence of generic algebraic structures for the emergent effective Hamiltonian, with the underlying microscopic physics being largely irrelevant except for influencing parameter values.
11:20 Coffe break
Coffe break
11:20 - 11:40
Room: f
11:40 No-Core CI calculations of light nuclei: Emergence of rotational bands - Pieter Maris
No-Core CI calculations of light nuclei: Emergence of rotational bands
- Pieter Maris
11:40 - 12:20
Room: f The atomic nucleus is a self-bound system of strongly interacting nucleons. In No-Core Configuration Interaction (CI) calculations, the nuclear wavefunction is expanded in a basis of Slater Determinants of single-nucleon wavefunctions (Configurations), and the many-body Schrödinger equation becomes a large sparse matrix problem. The challenge is to reach numerical convergence to within quantifiable numerical convergence to within quantifiable numerical uncertainties for physical observables using finite truncations of the infinite-dimensional basis space. I discuss the (dis)advantages of different truncation schemes, as well as strategies for constructing and solving the resulting large sparse matrices of current multi-core computer architectures. Several of these strategies have been implemented in the code MFDn, a hybrid MPI/OpenMP Fortran code for ab-initio nuclear structure calculations that has been demonstrated to scale to over 200,000 cores. Finally, I present results for ground state energies, excitation spectra, and select electromagnetic observables for light nuclei in the A=6 to 14 range using realistic 2- and 3-body forces. In particular, I demonstrate that collective phenomena such as rotational band structures can emerge from these microscopic calculations.
12:20 Discussion
Discussion
12:20 - 13:00
Room: FP41

13:00 Lunch
Lunch
13:00 - 14:30
Room: Albanova Restaurant
14:30 A new approach for large-scale shell-model calculations and large-scale complex scaling calculations - Takahiro Mizusaki
A new approach for large-scale shell-model calculations and large-scale complex scaling calculations
- Takahiro Mizusaki
14:30 - 15:10
Room: FP41 In my presentation, I will present a new approach to numerically solve shell model calculations and complex scaling calculations, which have real energy eigenvalues and complex energy eigenvalues, respectively. For shell model calculations, I have already published in Ref.1 and this new approach works as well as the well-known Lanczos method. In an application concerning to isospin breaking [2], it is superior to the Lanczos method. I will show this new approach in detail. In the latter part of my presentation, I will show an extension of this work [3] to complex scaling calculations which is useful to describe resonance states. This approach will be able to open large-scale complex scaling calculations. This work is a result of collaboration with Prof. K. Kaneko, M. Honma, T. Sakurai, Y. Sun, S. Tazaki, G. de Angelis, T. Myo and K. Kato. Reference [1] T. Mizusaki, K. Kaneko, M. Honma, T. Sakurai, Phys. Rev. C82 024310 (2010). [2] T. Mizusaki, K, Kaneko, M. Honma, K. Sakurai, Acta Phsica Polonica B 42, 447 (2011). K. Kaneko, T. Mizusaki, Y. Sun, S. Tazaki, G. de Angelis, Phys. Rev. Lett. 109, 092504 (2012). [3] T.Mizusaki, T.Myo, K.Kato, to be submitted.
15:10 Ab-initio calculations of nuclei with many-body perturbation theory - Furong Xu
Ab-initio calculations of nuclei with many-body perturbation theory
- Furong Xu
15:10 - 15:50
Room: FP41 We start from a realistic nuclear force (N3LO [1] or JISP16 [2]), and use the similarity renormalization group (SRG) to renormalize the realistic nuclear force. With the softened NN force, we first perform the Hartree-Fock (HF) calculation, and then take the HF solution as the reference and basis for further corrections to the solution of the many-body system. The many-body perturbation theory (MBPT) [3] has been employed for the correction calculations. Corrections up to the third order in energy and up to second order in radius have been considered. As preliminary investigations, we have calculated 4He and 16O, obtaining quite good converged results in their binding energies and radii. We thank J. Vary for providing the JISP16 interaction and useful discussions. [1] D.R. Entem and R. Machleidt, Phys. Rev. C 68, 041001 (32003); [2] A.M. Shirokov, A.I. Mazur, S/A. Zaytsev, J.P. Vary and T.A. Weber, Phys. Rev. C 70, 044005 (2004); [3] I. Shavitt and R.J. Bartlett, Many-body methods in Chemistry and physics: MBPT and coupled-cluster theory (2009).
15:50 Coffe break
Coffe break
15:50 - 16:10
Room: FP41
16:10 Neutrino physics and nuclear structure for double-beta decay - Mihai Horoi
Neutrino physics and nuclear structure for double-beta decay
- Mihai Horoi
16:10 - 16:50
Room: FP41 Neutrinoless double-beta decay, if observed, would signal physics beyond the Standard Model that would be discovered at energies significantly lower than those at which the relevant degrees of freedom can be excited. Therefore, it could be difficult to use the neutrinoless double-beta decay observations to distinguish between several beyond Standard Model competing mechanisms that were propose to explain this process. Accurate nuclear structure calculations of the nuclear matrix elements (NME) necessary to analyze the decay rates could be helpful to narrow down the list of competing mechanisms, and to better identify the more exotic properties of the neutrinos. In my talk I will review the neutrino physics relevant for double-beta decay, I will analyze the status of the shell model calculation of the NME, and their relevance for discriminating thecontribution of possible competing mechanisms to the neutrinoless double-beta decay process. U.S. DoE grant DE-SC0008529 and U.S. NSF grants PHY-1068217 and PHY-1404442 are acknowledged.
16:50 Nucleon-pair Approximation of the shell model - Yu-Min Zhao
Nucleon-pair Approximation of the shell model
- Yu-Min Zhao
16:50 - 17:30
Room: FP41 Atomic nuclei are complex systems of protons and neutrons that strongly interact with each other via an attractive and short-range force, leading to a pattern of dominantly monopole and quadrupole correlations between like particles (i.e., proton-proton and neutron-neutron correlations) in low-lying states of atomic nuclei. Among many nucleon pairs, very few nucleon pairs such as proton and neutron pairs with spin zero, two, and occasionally isoscalar proton-neutron pairs with spin aligned, play a dominant role in low-energy nuclear structure. Therefore the nucleon-pair approximation provides us with an efficient truncation scheme of the full shell model configurations which are otherwise too large to handle for medium and heavy nuclei. Furthemore, the nucleon-pair approximation leads to simple pictures in physics, as the dimension of nucleon-pair subspace is small. In this talk I would like to give a brief review of its history, formulation, validity, applications, as well as its link to previous approaches. Numerical calculations of low-lying states for realistic atomic nuclei are demonstrated with examples. Applications of pair approximations to other problems are also discussed.
Thursday, 18 September 2014
09:00 Toward model-independent calculations of atomic nuclei - Thomas Pappenbrock
Toward model-independent calculations of atomic nuclei
- Thomas Pappenbrock
09:00 - 09:40
Room: FP41 This talk reviews recent results of coupled-cluster calculations for rare isotopes, optimization of interaction from chiral effective field theory, and finite size effects in the oscillator basis.
09:40 Highly Frustrated Spin-Lattice Models of Magnetism and Their Quantum Phase Transitions: A Microscopic Treatment via the Coupled Cluster Method - Raymond Bishop
Highly Frustrated Spin-Lattice Models of Magnetism and Their Quantum Phase Transitions: A Microscopic Treatment via the Coupled Cluster Method
- Raymond Bishop
09:40 - 10:20
Room: FP41 The coupled cluster method [1] (CCM) is one of the most pervasive, most powerful, and most successful of all ab initio formulations of quantum many-body theory. It has probably been applied to more systems in quantum field theory, quantum chemistry, nuclear, subnuclear, condensed matter and other areas of physics than any other competing method. The CCM has yielded numerical results which are among the most accurate available for an incredibly wide range of both finite and extended physical systems defined on a spatial continuum. These range from atoms and molecules of interest in quantum chemistry, where the method has long been the recognized "gold standard", to atomic nuclei; from the electron gas to dense nuclear and baryonic matter; and from models in quantum optics, quantum electronics, and solid-state optoelectronics to field theories of strongly interacting nucleons and pions. This widespread success for both finite and extended physical systems defined on a spatial continuum [2] has led to recent applications to corresponding quantum-mechanical systems defined on an extended regular spatial lattice. Such lattice systems are nowadays the subject of intense theoretical study. They include many examples of systems characterized by novel ground states which display quantum order in some region of the Hamiltonian parameter space, delimited by critical values which mark the corresponding quantum phase transitions. The quantum critical phenomena often differ profoundly from their classical counterparts, and the subtle correlations present usually cannot easily be treated by standard many-body techniques (e.g., perturbation theory or mean-field approximations). A key challenge for modern quantum many-body theory has been to develop microscopic techniques capable of handling both these novel and more traditional systems. Our recent work shows that the CCM is capable of bridging this divide. We have shown how the systematic inclusion of multispin correlations for a wide variety of quantum spin-lattice problems can be efficiently implemented with the CCM [3]. The method is not restricted to bipartite lattices or to non-frustrated systems, and can thus deal with problems where most alternative techniques, e.g., exact diagonalization of small lattices or quantum Monte Carlo (QMC) simulations, are faced with specific difficulties. In this talk I describe our recent work that has applied the CCM to strongly interacting and highly frustrated spin-lattice models of interest in quantum magnetism, especially in two spatial dimensions. I show how the CCM may readily be implemented to high orders in systematically improvable hierarchies of approximations, e.g., in a localized lattice-animal-based subsystem (LSUB$m$) scheme, by the use of computer-algebraic techniques. Values for ground-state (and excited-state) properties are obtained which are fully competitive with those from other state-of-the-art methods, including the much more computationally intensive QMC techniques in the relatively rare (unfrustrated) cases where the latter can be readily applied. I describe the method itself, and illustrate its ability to give accurate descriptions of the ground-state phase diagrams of a wide variety of frustrated magnetic systems via a number of topical examples of its high-order implementations, from among a very large corpus of results for spin lattices. The raw LSUB$m$ results are themselves generally excellent. I show explicitly both how they converge rapidly and can also be accurately extrapolated in the truncation index, $m \to \infty$, to the exact limit. [1] R.F. Bishop, in "Microscopic Quantum Many-Body Theories and Their Applications," (eds. J. Navarro and A. Polls), Lecture Notes in Physics Vol. 510, Springer-Verlag, Berlin (1998), 1. [2] R.F. Bishop, Theor. Chim. Acta 80 (1991), 95; R.J. Bartlett, J. Phys. Chem. 93 (1989), 1697. [3] D.J.J. Farnell and R.F. Bishop, in "Quantum Magnetism," (eds. U. Schollwöck, J. Richter, D.J.J. Farnell and R.F. Bishop), Lecture Notes in Physics Vol. 645, Springer-Verlag, Berlin (2004), 307.
10:20 Coffe break
Coffe break
10:20 - 10:40
Room: FP41
10:40 Open-shell systems via symmetry (broken and) restored coupled cluster theory - Thomas Duguet
Open-shell systems via symmetry (broken and) restored coupled cluster theory
- Thomas Duguet
10:40 - 11:20
Room: f Ab initio many-body methods have been developed over the past ten years to address closed-shell nuclei up to mass $\text{A}\sim 130$ on the basis of realistic two- and three-nucleon interactions. A current frontier relates to the extension of those many-body methods to the description of open-shell nuclei. Several routes are currently under investigation to do so among which one relies on the powerful concept of spontaneous symmetry breaking. Singly open-shell nuclei can be efficiently described via the breaking of U(1) gauge symmetry associated with particle-number conservation, as a way to account for their superfluid character. Doubly open-shell nuclei can be addressed by further breaking SU(2) symmetry associated with angular momentum conservation. Still, the description of finite quantum systems eventually requires the exact restoration of symmetry quantum numbers. In this context, we discuss two recent developments performed within the frame of single-reference coupled cluster theory. First, we present the Bogoliubov coupled cluster formalism, which consists of representing the exact ground-state wave function of the system as the exponential of a quasi-particle excitation cluster operator acting on a Bogoliubov reference state. Test calculations for the pairing Hamiltonian are presented along with realistic proof-of-principle calculations of even-even nuclei with $\text{A} \approx 20$. Second, we discuss a recent extension of symmetry-unrestricted coupled-cluster theory that allows for the exact restoration of the broken symmetry at any truncation order. The formalism, which encompasses both single-reference coupled cluster theory and projected Hartree-Fock theory as particular cases, permits the computation of usual sets of connected diagrams while consistently incorporating static correlations through the highly non-perturbative restoration of the symmetry. A key difficulty relates to the necessity to handle generalized energy {\it and} norm kernels for which naturally terminating coupled-cluster expansions are indeed obtained. The focus is on SU(2) and U(1) symmetries but the formalism can be extended to any (locally) compact Lie group and to discrete groups, such as most point groups.
11:20 Pairing and quarteting in proton-neutron systems - Nicolae Sandulescu
Pairing and quarteting in proton-neutron systems
- Nicolae Sandulescu
11:20 - 12:00
Room: FP41 The common treatment of proton-neutron pairing in N ≅ Z nuclei relies on Cooper pairs and mean-field BCS-type models. However, the nuclear interaction can induce, through the isospin conservation, quartet correlations of alpha type which might compete with the Cooper pairs. In fact, for any isovector pairing interactions the ground state of N=Z systems is accurately described not by Cooper pairs but in terms of collective quartets [1,2]. Cooper pairs and quartets can however coexist in isospin asymmetric systems with N>Z. In this case the ground state of the isovector pairing Hamiltonian can be described with high precision as a condensate of alpha-like quartets to which it is appended a condensate of Cooper pairs built with the excess neutrons [3,4]. Quartets appear to be the relevant degrees of freedom for treating not only the isovector pairing but also the competition between the isoscalar and the isovector proton-neutron pairing in N=Z nuclei [5]. These facts indicate that the many-body pairing problem in N ≅ Z nuclei can be more efficiently treated in calculation schemes based on alpha-type quartets rather than on Cooper pairs and BCS-type models. 1.N. Sandulescu, D. Negrea, J. Dukelski, C. W. Johnson, Phys. Rev. C85, 061303 (R) (2012); 2. N. Sandulescu, D. Negrea, C. W. Johnson, Phys. Rev. C86, 041302 (R) (2012); 3. M. Sambataro and N. Sandulescu, Phys Rev. C88, 061303 (R)(2013) ; 4. D. Negrea and N. Sandulescu, Phys. Rev. C (2014), in press; 5. M. Sambataro, N. Sandulescu and C. W. Johnson, in preparation
12:00 Discussion
Discussion
12:00 - 13:00
Room: FP41

13:00 Lunch
Lunch
13:00 - 15:00
Room: Albanova Restaurant
15:00 Nuclear physics: a laboratory for many-particle quantum mechanics (Colloquium) - George Bertsch
Nuclear physics: a laboratory for many-particle quantum mechanics (Colloquium)
- George Bertsch
15:00 - 16:00
Room: Oskar Klein room Nuclear structure physics has presented a fruitful testing ground for quantum many-body theory since its beginnings half a century ago. On the one hand, the observed phenomena have given rise to models that have been invaluable to interpret the underlying physics. On the other hand, the quest to make a predictive theory has given strong impetus to developing computational tools to solve the many-particle Schroedinger equation. I will review some of these theoretical highlights in nuclear structure, ranging from the modeling and computation of few-body systems to the many-particle finite systems represented by our heavy nuclei. Among the models I discuss are the unitary-limit fermionic Hamiltonian, the Nilsson model of nuclear deformations, and the Richardson-Gaudin model of pairing. Computational strategies that have been very successful in different contexts are the Monte-Carlo methods, the multi-configuration shell model, and the extensions of mean-field theory to restore broken symmetries.
Friday, 19 September 2014
09:00 Quantum tensor networks for simulating quantum spin systems - Frank Verstraete
Quantum tensor networks for simulating quantum spin systems
- Frank Verstraete
09:00 - 09:40
Room: FP41
09:40 Tensor methods and entanglement measurements for models with long-range interactions - Örs Legeza
Tensor methods and entanglement measurements for models with long-range interactions
- Örs Legeza
09:40 - 10:20
Room: FP41 Strongly correlated materials are typically rather difficult to treat theoretically. They have a complicated band structure, and it is quite difficult to determine which minimal model correctly describes their essential physical properties. Moreover, the value of the model parameters to be used for a given material is often the subject of debate. Unfortunately, analytic approaches often do not provide rigorous conclusions for the interesting parameter sets, therefore, numerical simulations are mandatory. Momentum-space formulations of local models such as the Hubbard model and problems in quantum chemistry are especially hard to treat using matrix- and tensor product-based algorithms because they contain non-local interactions. Quantum entropy-based measures can be used to map the entanglement structure in order to gain physical information and to optimize algorithms. In this tutorial contribution, we present an overview of the real space, momentum space and quantum chemistry versions of the DMRG/MPS and tree-TNS algorithms and their applications to various spin and fermionic lattice models, and to transition metal complexes. Data sparse representation of the wavefunction will be investigated through advances in entanglement localization providing optimized tensor topologies. Entropy generation by the RG procedure, the mutual information leading to a multiply connected network of lattice sites or orbitals, and reduction of entanglement by basis transformation will be discussed. Inclusion of the concepts of entanglement will be used to identify the wave vector of soft modes in critical models, to determine highly correlated molecular orbitals leading to an efficient construction of active spaces and for characterizing the various types of correlation effects relevant for chemical bonding. The state of the art matrix-product-based algorithms is demonstrated on polydiacetylene chains by reproducing experimentally measured quantities with high accuracy. [1] S. R. White, Phys. Rev. Lett. 69, 2863-2866 (1992). [2] S. R. White and R. L. Martin, J. Chem. Phys. 110, 4127-4130 (1999). [3] O. Legeza, R. M. Noack, J. SĂłlyom, and L. Tincani, in Computational Many-Particle Physics, eds. H. Fehske, R. Schneider, and A. Weisse 739, 653-664 (2008). [4] K. H. Marti and M. Reiher, Z. Phys. Chem. 224, 583-599 (2010). [5] G. K.-L. Chan and S. Sharma, Annu. Rev. Phys. Chem. 62, 465-481 (2011). [6] O. Legeza and J. SĂłlyom, Phys. Rev. B 68, 195116 (2003), ibid Phys Rev. B 70, 205118 (2004). [7] J. Rissler, R.M.Noack, and S.R. White, Chemical Physics, 323, 519 (2006). [8] K. Boguslawski, P. Tecmer, O. Legeza, and M. Reiher, J. Phys. Chem. Lett. 3, 3129-3135 (2012). [9] K. Boguslawski, P. Tecmer, G. Barcza, O. Legeza, and M. Reiher, J. Chem. Theory Comp. (2013). [10] F. Verstraete, J.I. Cirac, V. Murg, Adv. Phys. 57 (2), 143 (2008). [11] V. Murg, F. Verstraete, O. Legeza, and R. M. Noack, Phys. Rev. B 82, 205105 (2010). [12] V. Murg, F. Verstraete, R. Schneider, P. Nagy and O. Legeza, arxiv:1403.0981 (2014). [13] G. Barcza, O. Legeza, K. H. Marti, and M. Reiher, Phys. Rev. A 83, 012508 (2011). [14] G. Barcza, R. M. Noack, J. SĂłlyom, O. Legeza, arxiv:1406.6643 (2014)
10:20 Coffe break
Coffe break
10:20 - 10:40
Room: FP41
10:40 Integrable Richardson-Gaudin bases for pairing Hamiltonians - Stijn De Baerdemacker
Integrable Richardson-Gaudin bases for pairing Hamiltonians
- Stijn De Baerdemacker
10:40 - 11:20
Room: FP41 Configuration interaction methods for quantum many-body systems are generally represented within Fock space, the space spanned by all possible single-particle Slater determinant (SD) wave functions. For strongly-correlated quantum systems, the number of physically important SD basis states quickly reaches beyond the capacities of present (and future) computer hardware, due to the lack of correlations within these SD basis states. Bases spanned by Richardson-Gaudin (RG) eigenstates can be regarded as a generalisation of Fock space, because the strong correlations are inherently present within the basis states. In this contribution, it will be shown how configuration interaction methods can benefit from the use of RG bases over conventional Fock space.
11:20 Disorder in bilayer and double layer graphene - David Abergel
Disorder in bilayer and double layer graphene
- David Abergel
11:20 - 12:00
Room: FP41 We use a numerical application of Thomas-Fermi theory to describe the effects of fluctuations in the local charge density caused by charged impurities in bilayer and double layer graphene. In the bilayer, we show that the interplay between the non-linear screening of the disorder potential and a band gap causes the electron liquid to break into coexisting compressible and incompressible regions. For double layer graphene, we demonstrate that charged impurity disorder has a significant negative impact on the existence of theproposed excitonic condensate.
12:00 Discussion
Discussion
12:00 - 13:00
Room: FP41

13:00 Lunch
Lunch
13:00 - 14:30
Room: Albanova Restaurant
14:30 Aspects of time-dependence in many-body systems - Daniela Pfannkuche
Aspects of time-dependence in many-body systems
- Daniela Pfannkuche
14:30 - 15:10
Room: FP41 Dynamical processes set yet another degree of complexity to the many-body problem. With the advent of ultrafast measuring techniques modern experiments often leave the regime of adiabaticity or linear response. The evolution of quantum systems on short time scales subject to strong disturbances then becomes relevant. In this contribution, I will present two different examples that demonstrate aspects of non-equilibrium dynamics in many-body quantum systems. The first example considers the temporal evolution of a closed many-body-system represented by a simple molecule being exposed to a strong ionizing light pulse. Solving the time-dependent Schrödinger equation reveals different processes in the charge dynamics following ionization. It will be demonstrated that a proper choice of electron-hole excitations is crucial for capturing the essential physics. The second example features the non-equilibrium dynamics of an open quantum system. The spin of a magnetic ad-atom residing on the surface of a non-magnetic substrate is exposed to the tunneling current of a scanning tunneling microscope. Spin torque of the tunneling electrons induces spin-dynamics of the surface spin. A master-equation approach is used to solve von-Neumann's equation of motion for the reduced density matrix of the surface spin. Challenges in computing the short time dynamics of the system by different methods are being exhibited.
15:10 Path integral simulations of bosons with disorder - Mats Wallin
Path integral simulations of bosons with disorder
- Mats Wallin
15:10 - 15:50
Room: FP41 Monte Carlo simulation of worldlines of quantum particles in a path integral representation is a powerful tool mainly used for studying boson systems. Such approaches have been used to investigate properties of superfluid helium in confined geometries and localization of bosons in a random disorder potential. In particular we are interested in the role of correlations of the disorder distribution. I will introduce theoretical tools and techniques and discuss opportunities, difficulties and challenges in this field.
15:50 Coffe break
Coffe break
15:50 - 16:10
Room: FP41
16:10 Summary - Yoram Alhassid
Summary
- Yoram Alhassid
16:10 - 16:50
Room: FP41
Saturday, 20 September 2014

12:00 Boattrip with lunch
Boattrip with lunch
12:00 - 15:00
Room: Strandvägen,berth (kajplats) 16 We take a boat through the many islands in the Stockholm Archipelago. The final destination is the beautiful village of Vaxholm, which is the only town in the inner Stockholm archipelago and therefore known as its capital. Lunch will be served during the trip.
Sunday, 21 September 2014