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University of Cambridge
History in background

History in background

The extraordinary tapestry of materials woven from the elements of the periodic table continues to astonish with novel physical properties and functionality. Yet, for some large classes of materials with remarkable properties, our ability to understand these still remains at the post facto level of descriptive science. Physical properties associated with magnetoresistive or thermoelectric oxides, high-temperature superconductors, carbon nanostructures, heavy fermion metals, spin injection, and many others, appeared largely unheralded, and the models used to describe them are mostly phenomenological. Furthermore, complex materials, or materials under conditions far from equilibrium, as often encountered in biological sciences, also raise outstanding challenges. This contrasts with the great success we have with the simplest inorganic semiconductors such as Si or GaAs, where the level of understanding is such that fully principled theories and models can be used to design devices with complex functionality.

There are three distinct paths that now offer the possibility to move forward in fundamental understanding and eventual design: novel theoretical methodologies, improved materials control, and advanced experimental techniques. All of these agendas are being actively pursued, and widely. Since these different strands rarely cohabit geographically, our proposed network will exploit and enhance the high level of local investment with a european dimension, furthermore linking to already existing global programmes.

Methodologies for theory: strongly correlated systems, excited states and out-of-equilibrium situations. Over the last decade, new ab initio computational methodologies have been evolving, including developments in and beyond Density Functional Theory (DFT), Dynamical Mean Field Theory (DMFT), Quantum Monte Carlo (QMC) methods and, importantly, hybrid combinations of such methods. These new theoretical and computational tools are currently reaching the stage where they can are successfully applied to realistic materials, as exemplified below.

Science of materials and solid-state chemistry. Over the same period, our control of materials has advanced enormously. Oxide materials partly propelled by work on magnetism, superconductivity, ferroelectricity, and thermoelectricity are enormously advanced. The control of nanomaterials, including carbon-based nanostructures, inorganic semiconductor heterostructures, organic/inorganic composites, and biomaterials has reached an advanced art. New functionalities are emerging from these developments, but also new levels of complexity and new challenges for theoretical modelling and understanding.

Experimental probes. The richness of materials behaviour is revealed by an array of experimental tools, few available in a single lab. For example, spectroscopic probes on the nanoscale (such as STM) have revealed surprisingly rich behaviour. The remarkable advances of photoemission spectroscopy (ARPES) have revealed many unconventional properties in cuprate superconductors. Extreme conditions of field, temperature, and pressure can be used to tune between phases of matter. Time-dependent spectroscopies (e.g pump-probe) with fast laser sources can probe the dynamics of chemical pathways or phase transformations.

Eventually, combining these different scientific paths in a trans-disciplinary framework should allow understanding, and perhaps even designing, classes of materials with new or improved functionalities or applications. There are numerous opportunities to be considered, include spintronic devices (e.g oxide-based), resistive memories, new thermoelectrics, phosphors for solid-state lighting, environmentally friendly pigments, improved solar cells, quantum coherent devices, biosensors, etc

For this to happen however, scientists mastering these very diverse techniques at the highest possible level of excellence must be in close contact, across disciplinary boundaries, and younger scientists must be offered in-depth training in these techniques. The proposed programme precisely pursues these objectives, using+ three types of activities detailed below (conferences and workshops, training sessions for young scientists, and collaborative research visits aimed at defining collective strategies for functional materials exploration and exploitation).

Objectives and envisaged achievements. Our programme will foster vigorous research collaborations and training events, focused on the following specific scientific challenges.

New theoretical methods for strong correlations. Correlated electron materials present unique electronic properties such as magnetism, superconductivity, thermoelectricity, peculiar optical properties, metal-insulator transitions and spin-sensitivity of transport properties. However, these materials pose an outstanding challenge to theoretical methods because these effects are caused by strong electron-electron interactions and subtle quantum correlations, and cannot be understood without a proper description of excited states. As such, methods such a density-functional based first principle simulations which are very successful for so many materials, face serious limitations when dealing with strongly-correlated materials. In the past decade, in a truly interdisciplinary effort, many-body techniques have been increasingly combined with first-principles simulation methods to tackle realistic correlated materials. Examples are Quantum Monte Carlo, many body perturbation theory methods (GW) and, importantly, Dynamical Mean-Field Theory (DMFT). However, much remains to be done along at least four lines: a greater integration of these techniques within electronic structure+methods, improving the computational efficiency, computing more complex response functions, and tackling ever more complex materials problems with these methods. Furthermore, there is legitimate hope that these developments in many-body theories will have bearing on some of the more fundamental issues raised by some correlated materials, such as cuprates or the heavy-fermion compounds (particularly the new family of ``115 superconductors such as PuCoGa5 or CeCoIn5).

- Excited states. The quantitative treatment of excited states is a major challenge in computational materials science, both in view of its importance for spectroscopy experiments in all materials, and of its particular relevance to biological processes. Many such processes are indeed characterized by a perturbation of the electronic structure (photoexcitation, photoionization, electron transfer, redox reactions), including vision, respiration, photosynthesis, nitrogen fixation. Understanding such processes has many strategic applications (e.g.: in solar energy technologies, medicinal diagnosis, laser spectroscopy). Recently, new theoretical methods have emerged (e.g time-dependent DFT) which have considerably improved our ability to handle excited states theoretically. Much remains to be developed in this very active field on several levels (fundamental theory, computational efficiency, applications to specific materials).

- Advanced spectroscopies and 'computational spectroscopy'. The last decade has witnessed tremendous advances of many spectroscopic techniques, including: spatially resolved microscopies (e.g STM), increased bulk-sensitivity and resolution of+ photoemission spectroscopy (ARPES), ultrafast pump-probe spectroscopies, increased brightness and improved resolution in X-Ray and neutron scattering probes, and nuclear magnetic resonance.+ Computational methods and theoretical modelling (in relation with the above-mentioned excited states issue) must be brought at the stage where quantitative comparison (or prediction) with these experiments becomes possible.

- Modelling materials with complex structures. There is an ever-increasing interest in materials which possess a complex structure. Progress in materials elaboration and design (in view of a specific function) at the nano-scale is one reason. Another domain in which one is faced with materials complexity is that of biomolecules, which contain very large numbers of atoms without the crucial simplification of a periodic structure as found in solids. An example of progress is the ab initio determination of NMR chemical shifts, applied to structure determination of proteins, or modelling of iron biomineralization in ferritin cages. Furthermore, biological materials function in an environment which also requires specific modelling, often simplified by empirical force fields. Finite temperature is an additional complication, requiring again to go beyond ground-state issues. A larger scale issue is to understand self-assembly into supramolecular structures, and the dynamics of sensory systems. All these complex structural and environmental aspects require considerable efforts and improvements in our modelling abilities. Close collaboration with materials scientists, chemists or biologists is essential to assess which simplifying assumptions are legitimate.

- Materials with new or improved functionalities. Much can be gained from interdisciplinary collaborations when it comes to improving known functionalities of materials or searching for new ones. Recently for example, better treatments of strong correlation effects have allowed for a quantitative description of the electronic structure and optical properties of rare-earth based pigments, opening a path to a possible tuning of the desired color by engineering the position of the f-states (e.g by substitutions). Charge segregation in oxides, an effect first emphasized in fundamental studies of cuprates and manganites, seems directly relevant to the mechanism behind resistive memory effects, with possible applications to information storage in the long-run. Multiferroic materials are already being explored widely, and new composites also offer special functionality; for example inorganic quantum dots in an organic host for efficient solar cells or as scintillators for X-ray detectors. The programme will identify and emphasize those cases in which a joint effort bringing together understanding and design can be most productive.

Materials and experiment. The focus of the modelling and theoretical activities will be on material systems and experimental probes within the partners of the programme, covering areas that will include:

  • Oxides with novel electronic properties: cobaltates (thermoelectrics), cuprates (superconductors), ferromagnetic and multiferroic oxides+ (spintronics)
  • Rare-earth and actinide compounds: rare-earth-based pigments, and phosphors for solid-state lighting, heavy fermion compounds with novel fundamental properties.
  • Structure and modelling of biomaterials and biologically active molecules, receptor supra-molecular assembly, sensing.
  • Organicelectronic materials including crystalline, glassy, and polymeric materials.
  • Low-dimensional structures and carbon-based nanostructures.
  • Transportand optics in nanostructures, quantum dots, optical microcavities,inorganic/organic composites, and magnetic semiconductors