Functional renormalization group for nuclear structure theory

The atomic nucleus is the epitome of complexity : it is a strongly correlated system of nucleons (which are themselves composite degrees of freedom) coupled via the strong and electroweak interactions which features a wealth of emergent behaviors (deformation, supefluidity, clustering, ...). The long term endeavor of nuclear structure theory is to understand and predict how an arbitrary number of nucleons self-organize and become disorganized in nuclei. Among the various theoretical frames, the energy density functional (EDF) method, close, yet different from the density functional theory, provides the best compromise between the robustness of the description and its numerical complexity. However, the phenomenological ingredients entering the formulation of standard EDFs affect their predictive power.
The postdoctoral project aims at formulating the EDF approach from first principles, in order to benefit from a theoretical frame with both a maximal predictive power and a favorable numerical cost. The supervising team has identified the functional renormalization group (FRG) as the most relevant language for such a non empirical reformulation of the EDF method.
The present projet aims at formulating the EDF method from first principles via the FRG.

Evolution of ISAAC and Xpn codes for an extension of the QRPA method to the complete processing of odd nuclei; towards a database without interpolation for odd nuclei

The treatment of odd-isospin nuclei in microscopic approaches is currently limited to the so-called «blocking» approximation. In the Hartree-Fock Bogolyubov (HFB) approach, the ground state of an odd-mass nucleus is described as a one-particle excitation (qp) on its reference vacuum. Thus, in the QRPA approach, where the basic excitations are states «with 2 quasi-particles», the blocked qp is excluded from the valence space under the Pauli exclusion principle. As a result, the chosen qp is a spectator and is not involved in the QRPA collective states. If the single nucleon should have a significant contribution some levels will not be reproduced. The development in the QRPA codes (ISAAC and Xpn) of a procedure that allows all nucleons to participate in collective states is mandatory for a microscopic description of odd nuclei. Moreover, recent Xpn developments have allowed the description of forbidden ß- first decays improving the estimation of half-life time of fission fragments. This could be extended to address ß+ and electronic captures and could be adapted to large-scale calculations useful for nuclear astrophysics.

Calculation of the thermal conductivity of UO2 fuel and the influence of irradiation defects

Atomistic simulations of the behaviour of nuclear fuel under irradiation can give access to its thermal properties and their evolution with temperature and irradiation. Knowledge of the thermal conductivity of 100% dense oxide can now be obtained by molecular dynamics and the interatomic force constants[1] at the single crystal scale, but the effect of defects induced by irradiation (irradiation loop, cluster of gaps) or even grain boundaries (ceramic before irradiation) remain difficult to evaluate in a coupled way.
The ambition is now to include defects in the supercells and to calculate their effect on the force constants. Depending on the size of the defects considered, we will use either the DFT or an empirical or numerical potential to perform the molecular dynamics. AlmaBTE allows the calculation of phonon scattering by point defects [2] and the calculation of phonon scattering by dislocations and their transmission at an interface have also recently been implemented. Thus, the chaining atomistic calculations/AlmaBTE will make it possible to determine the effect of the polycrystalline microstructure and irradiation defects on the thermal conductivity. At the end of this post-doc, the properties obtained will be used in the existing simulation tools in order to estimate the conductivity of a volume element (additional effect of the microstructure, in particular of the porous network, FFT method), data which will finally be integrated into the simulation of the behavior of the fuel element under irradiation.
The work will be carried out at the Nuclear Fuel Department of the CEA, in a scientific environment characterised by a high level of expertise in materials modelling, in close collaboration with other CEA teams in Grenoble and in the Paris region who are experts in atomistic calculations. The results will be promoted through scientific publications and participation in international congresses.
[1] Bottin, F., Bieder, J., Bouchet, J. A-TDE

Tensor factorization for the nuclear many-body problem

Modeling silicon and germanium spin qubits

Silicon/Germanium spin qubits have attracted increasing attention and have made outstanding progress in the past two years. In these devices, the elementary information is stored as a coherent superposition of the spin states of an electron in a Si/SiGe heterostructure, or of a hole in a Ge/SiGe heterostructure. These spins can be manipulated electrically owing to the intrinsic (or to a synthetic) spin-orbit coupling, and get entangled through exchange interactions, allowing for the implementation of a variety of one- and two-qubit gates required for quantum computing and simulation. The aims of this postdoctoral position are to strengthen our understanding and support the development of electron and hole spin qubits based on Si/Ge heterostructures through analytical modeling as well as advanced numerical simulation. Topics of interests include spin manipulation & readout, exchange interactions in 1D and 2D arrays, coherence and interactions with other quasiparticles such as photons. The selected candidate will join a lively project bringing together > 50 people with comprehensive expertise covering the design, fabrication, characterization and modeling of spin qubits. He/She is expected to start early 2023, for up to three years.

Postdoctoral position on the modeling of silicon spin qubits

A post-doctoral position is opened at the Interdisciplinary Research Institute of Grenoble (IRIG) of the CEA Grenoble (France) on the theory and modeling of silicon spin quantum bits (qubits). The selected candidate is expected to start at the beginning of year 2022, for up to two years.
Quantum information technologies on silicon have raised an increasing interest over the last few years. Grenoble is pushing forward an original platform based on the “silicon-on-insulator” (SOI) technology. In order to meet the challenges of quantum information technologies, is essential to support the experimental activity with state-of-the-art modeling. For that purpose, CEA is actively developing the “TB_Sim” code. TB_Sim is able to describe very realistic qubit structures down to the atomic scale when needed using atomistic tight-binding and multi-bands k.p models for the electronic structure of the materials. The aims of this postdoctoral position are to strengthen our understanding of spin qubits, and to progress in the design of efficient and reliable Si and Si/Ge spin qubit devices and arrays using a combination of analytical models and advanced numerical simulations with TB_Sim. Topics of interest include spin manipulation & readout in electron and hole qubits, exchange interactions in 1D and 2D arrays of qubits and operation of multi-qubit gates, sensitivity to noise (decoherence) and disorder (variability). This work takes place in the context of the EU QLSI project and will be strongly coupled to the experimental activity in Grenoble and among the partners of CEA in Europe.

Development of a modular multi-detector instrumentation for the measurement of atomic and nuclear parameters

The LNE PLATINUM project (PLATFORM OF MODULAR NUMERICAL INSTRUMENTATION) aims to develop a modular platform, in order to test new instrumentation using two or more detectors in coincidence. The principle implemented in this project is based on the simultaneous detection of interactions taking place in two different detectors, by collecting information on the type of particle and its energy (spectroscopy). This principle is the basis for absolute measurements of activity or active continuous background reduction systems to improve detection limits. But it also allows the measurement of parameters characterizing the decay scheme, such as internal conversion coefficients, X-ray fluorescence yields or angular correlations between photons emitted in cascade.

Thanks to its expertise in atomic and nuclear data, the LNHB has noted for many years the incompleteness of decay schemes for certain radionuclides. These schemes, established at the time of evaluation from existing measured data, sometimes present inconsistencies or poorly known transitions, in particular in the presence of highly converted gamma transitions or very low intensity (for example, recent studies on 103Pa, 129I and 147Nd have revealed such inconsistencies). It therefore appears important for LNHB to better master the technique of coincidence measurement, taking advantage of the new possibilities in terms of data acquisition and time stamping to provide additional information on decay scheme and contribute to their improvement.

Modeling silicon-on-insulator quantum bit arrays

A post-doctoral position is open at the Interdisciplinary Research Institute of Grenoble (IRIG, formerly INAC) of the CEA Grenoble (France) on the theory and modeling of arrays of silicon-on-insulator quantum bits (SOI qubits). This position fits into an ERC Synergy project, quCube, aimed at developing two-dimensional arrays of such qubits. The selected candidate is expected to start between October and December 2019, for up to three years.
Many aspects of the physics of silicon qubits are still poorly understood, so that it is essential to support the experimental activity with state-of-the-art modeling. For that purpose, CEA is actively developing the “TB_Sim” code. TB_Sim relies on atomistic tight-binding and multi-bands k.p descriptions of the electronic structure of materials and includes, in particular, a time-dependent configuration interaction solver for the dynamics of interacting qubits.
The aims of this post-doctoral position are to improve our understanding of the physics of these devices and optimize their design, and, in particular,
- to model spin manipulation, readout, and coherence in one- and two-dimensional arrays of SOI qubits.
- to model exchange interactions in these arrays and assess the operation of multi-qubit gates.
The candidate will have the opportunity to interact with the experimental teams from CEA/IRIG, CEA/LETI and CNRS/Néel involved in quCube, and will have access to data on state-of-the-art devices.

Modeling silicon-on-insulator quantum bits

Quantum information technologies on silicon have raised an increasing interest over the last five years. CEA is pushing forward its own original platform based on the “silicon-on-insulator” (SOI) technology. The information is stored in the spin of carrier(s) trapped in quantum dots, which are etched in a thin silicon film and are controlled by metal gates. SOI has many assets: the patterning of the thin film can produce smaller, hence more scalable qubits; also, the use of the silicon substrate beneath as a back gate provides extra control over the quantum bits (qubits).
Many aspects of the physics of silicon spin qubits are still poorly understood. It is, therefore, essential to complement the experimental activity with state-of-the-art modeling. For that purpose, CEA is actively developing the "TB_Sim" code. The aims of this 2-year post-doctoral position are to model spin manipulation and readout in SOI qubits, and to model decoherence and relaxation at the atomistic scale using TB_Sim. This modeling work will be strongly coupled to the experimental activity in Grenoble. The candidate will have access to experimental data on state-of-the-art devices.