Actinides: Correlated Electrons and Nuclear Materials

Ramada Jarvis Hotel, Manchester
14-16 June 2010

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PLEASE NOTE THAT THIS WORKSHOP IS NOW FULL AND REGISTRATION HAS NOW CLOSED

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Organisers:
Leon Petit (STFC Daresbury Laboratory)
Bernard Amadon (CEA-DIF Bruyères le Châtel)

Registration

Registration closed.

Workshop Motivation

The motivation for this workshop is to bring together physicists of the electronic structure of actinides and nuclear materials science, i.e. two communities whose interest centers on the same nuclear/actinide materials, but that focus on different aspects of the physics and chemistry of these materials. The overarching goal of this workshop is to take up important aspects of the f-electron challenge and to advance the fundamental understanding of actinide materials, and then, see the application of these studies to nuclear fuels and waste as well as their interaction with the environment. We could thus benefit from the interaction between fundamental and applied physics. The issues that will be addressed range from novel oxide fuels and transmutation matrices, to the behaviour of advanced waste forms under extreme conditions. By confronting the quantum atomistic simulations to real world problems we aim to achieve improved understanding of the possibilities and challenges with regards to first principles design of nuclear materials. Plenty of time will be available for in depth discussions to draw up a roadmap on how the theoretical and experimental investigations of the electronic structure could most favorably be applied to the nuclear cycle and to a more thorough comprehension of the physics of electrons in actinides."

Participants

The workshop is open to students and scientists in the field of correlated electrons/nuclear materials who wishes to participate. The workshop venue will be the Ramada Jarvis Hotel, Manchester. The number of expected speakers is between 25 and 30. The total number of participants is expected to be around 50.

Speakers

• Mike Brooks (European Commission JRC-Karlsruhe)
• Jean-Paul Crocombette (CEA, Centre de Saclay)
• Michel Freyss (CEA, Centre de Cadarache)
• Thomas Gouder (European Commission JRC-Karlsruhe)
• Claude Guet (CEA)
• Ladislav Havela (Charles University)
• Gerald Jomard (CEA, Bruyeres-le-Chatel)
• Nik Kaltsoyannis (University College London)
• Jindrich Kolorenc (University of Hamburg)
• Rudy Konings (European Commission JRC-Karlsruhe)
• Eugene Kotomin (University of Latvia)
• Gerry Lander (Institut Laue-Langevin)
• Chris Marianetti (Columbia University)
• Richard Martin (Los Alamos National Laboratory)
• Peter Oppeneer (Uppsala University)
• Asok Ray (University of Texas at Arlington)
• Malcolm Stocks (Oak Ridge National Laboratory)
• Axel Svane (Aarhus University)
• Gerrit van der Laan (STFC Diamond Light Source)
• Younsuk Yun (Paul Scherrer Institut)
• Kevin Moore (Lawrence Livermore National Laboratory)
• Paolo Santini (University of Parma)
• Sung Woo Yu (Lawrence Livermore National Laboratory)
• Michael Manley (Lawrence Livermore National Laboratory)
• Alexey Lukoyanov (Russian Academy of Sciences)
• Dzidka Szotek (STFC Daresbury Laboratory)
• Fei Zhou (University of California)
• John Purton (STFC Daresbury Laboratory)

Programme

The workshop will be located at the Ramada Jarvis Hotel in Manchester, UK. The workshop will start in the morning at 09:00 on 14 June and finish in the evening at 17:00 on 16 June. The registration deadline is 30 April 2010.

PROGRAMME

REGISTRATION: FOURTH FLOOR FOYER OUTSIDE PARK SUITE
SUNDAY, 13 JUNE 1700-1900 AND MONDAY 14 JUNE 0800

Travel and Accommodation

Both invited speakers and delegates will need to make their own travel arrangements. Rooms are available at the Ramada Jarvis Hotel for both invited speakers and delegates. The rate for delegates is 60 GBP per night. Invited speakers rooms will be paid for by the organizing committee. Conference fees and local costs (meals and conference dinner) for both the delegates and the invited speakers will be paid for by the organizing committee.

If you are a delegate and you have requested accommodation you will need to arrange to pay STFC for your hotel booking. There are three methods of payment available to you...

(1) Credit Card Using PayPal
If you would like to pay by credit card using the PayPal system please contact Shirley Miller who will send you a PayPal payment request by email.

(2) Bank Transfer
If you are paying by bank transfer you MUST reference "CECAM ACTINIDES / YOUR NAME" on the transfer form so that we can track your payment. Failure to do so may result in your payment being misplaced in our bank account and your registration showing an outstanding amount, which could affect your registration for the workshop. Please use the following bank account details to complete your transfer...

Bank Name and Address: Lloyds Bank PLC, Market Place, Didcot, Oxfordshire OX11 7LQ
Account Name: Science & Technology Facilities Council
Sort Code: 30-93-93

Sterling Account No: 00143698 (for payment in sterling or other currency)
IBAN Number "STERLING": GB17LOYD 30939300 143698

Euro Account No: 59005079 (for payment in EURO's only)
IBAN Number "EURO": GB42LOYD 309393 59005079
Swift Code: LOYDGB2L
BIC: LOYDGB21097

(3) Cheque
Cheques should be made payable to "STFC" and sent to Shirley Miller at the following address...

STFC Daresbury Laboratory
Daresbury Science & Innovation Campus
Daresbury
Warrington
WA4 4AD
UK

If you require an invoice in order to make the payment please contact Shirley Miller who will arrange for an invoice to be sent.

Scientific Summary

The local-spin-density (LSD) and semi-local generalized gradient approximations (GGA) to density-functional theory have proven very useful and accurate in describing bonding properties of solids with weakly correlated electrons.[1,2] The LSD approximation however ignores exchange and correlation effects beyond those of the homogeneous electron gas. The lanthanide 4f-orbitals tend to be very localized in space. A purely band picture description thus fails to fully capture the physics of rare earth materials. For 5f orbitals, the situation is complicated because electrons are less localized than in the lanthanides but still correlated[3]: Depending on the specific actinide, the chemical environment, and/or the external conditions (pressure), the f-electrons behave as either localized, or delocalized. Furthermore, spin-orbit coupling effects are important in these materials leading to a complex magnetism, with an important orbital contribution.[5]

In spite of this, studies have been carried out in the LSD, assuming magnetic ordering, which leads to surprisingly good results for e.g. delta Pu.[6] Furthermore LSD/GGA approximation are being applied to bulk actinide oxides and surfaces, as well as in simulations of radiation defects or impurity migration in nuclear materials.[7,8] Considerable new insights into the impurity/vacancy formations as well as structural changes associated with relaxation has been achieved, providing, among other, valuable starting points for multiscale modeling of nuclear fuels.[9]

To address the underlying issues of electron-electron correlations, in the last few years a number of highly sophisticated modeling approaches have been developed. A first approach is to describe explicitly the interaction between electrons with a Hubbard like term in the Hamiltonian.[10] The mean field solution (LDA+U), gives a better description of Mott insulators. The dynamical mean field theory (DMFT) goes beyond the mean field approximation by taking into account fluctuations between electronic configurations on one site.[11] The DMFT is capable of reproducing both the Hubbard bands and the quasi particle peak at the Fermi level, and represents currently the most sophisticated approach to the strongly correlated electron problem. It can in particular describe both delocalized and localized correlated electron systems making it an adequate tool for the description of actinides. The self-interaction corrected (SIC) local spin density approximation takes an altogether different approach, as it corrects the LSD approximation for the unphysical interaction of an electron with itself.[12] The groundstate of the system is determined from total energy considerations and it has the advantage of being a parameter free theory. In the hybrid functional approach, the exchange is taken to be a combination of Hartree-Fock exact exchange with LSD exchange and correlations.[13] This can mimic the physical effect of a screened exchange, as a consequence, the self-interaction is partially removed.

The development of these new methodologies is still in progress, but has already led to new insight into the electronic structure of actinide materials. Striking examples are the impressive number of theoretical studies on the electronic, magnetic, and spectroscopic properties of actinide metals,[5] especially delta-Pu,[14] the study of phonons and heat conductivities of UO2/PuO2 with DMFT,[15] the investigation of phase transitions under pressure or temperature, the electronic structure of actinide oxides with hybrid functionals[13] and LDA+U,[16] the PuO2 oxidation with SIC-LSD.[17] The capability of reproducing the spectroscopies of strongly correlated materials enables the direct comparison between electronic structure and experimental measurements.[18] Here the potential application of the GW approximation, which incorporates many body effects to first order, is bound to lead to considerable progress when combined with the described beyond-LSD methodologies. Because of the difficulty of these calculations, it is only very recently that GW calculations on f-electrons systems have appeared.[19]

Despite considerable progress, understanding the experimental evidence emerging from studies of correlated electron systems remains a challenge. Despite considerable efforts, the absence of magnetism in delta-Pu for example still remains unclear, as is the case with the so-called hidden order in URu2Si2.[20] Many of the actinide compounds display superconductivity, which so far remains largely inaccessible to first principles methodologies. X-ray absorption and XPS studies show compounds situated at the boundary of localization/delocalization transition, and from EELS studies it emerges that a number of actinide metals are governed by intermediate coupling.[21] Also, recent experimental developments call for an improved theoretical description of higher order multipole ordering.[22]

Nevertheless the achieved accuracy of the current methodologies makes it a timely enterprise to investigate the impact of electron-electron correlations on the physics and chemistry of the nuclear materials. In combination with the already existing modeling approaches the above mentioned electronic structure codes will provide guidelines for producing improved fuels and less toxic wasteforms. Among many others, topics that need to be addressed range from the composition dependent properties of mixed oxide fuels[23] and the chemistry of minor actinides in inert matrix fuels,[24] to the influence of the actinide oxidation state on the mobility of radionuclides[25] and the potential performance of novel actinide carbides and nitrides fuels in GEN-IV reactors.

Moreover, the surface behavior in contact with the environment is strongly influenced by the f-electron structure, as are the properties of nuclear fuels such as heat conductivities. The study of surface interaction with molecules[26] is particularly challenging because it requires handling in the same theoretical framework the f-electrons and molecular physics. A number of results have recently being obtained in these fields: the stability of point and external defects in actinides oxides has been the subject of various studies using DFT[7] and DFT+U,[27] and interesting results have been obtained in the description of surfaces and thermal properties.

Understanding the actinides is a prerequisite for understanding the behaviour of nuclear materials, and implies understanding the fundamental physics of correlated electrons. On the other hand, the large amount of experimental data that is derived from experimental investigation of nuclear fuels under operating conditions, or nuclear waste under storage conditions, can give valuable additional feedback with regards to understanding actinide physics, and ultimately provide new insight into the complex nature of correlated electrons.

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[19] A. N. Chantis, R. C. Albers, A. Svane, and N. E. Christensen, Phil. Mag. 89, 1801 (2009).

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[22] N. Magnani, S. Carretta, R. Caciuffo, P. Santini, G. Amoretti, A. Hiess, J. Rebizant, and G. H. Lander, Phys. Rev. B 78, 104425 (2008).

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[27] H. Y. Geng, Y. Chen, Y. Kaneta, M. Iwasawa, T. Ohnuma, M. Kinoshita, Phys. Rev. B 77, 104120 (2008).