Theoretical and experimental investigations on strongly correlated electron materials have attracted considerable attention recently, due to the desire to understand both basic many-body interactions as well to investigate the relevance of the materials to possible technological applications. The complexity of the problem of correlations makes the field intrinsically interdisciplinary, requiring an international coordination for the achievement of reliable solutions. For example very recently high quality publications in Nature and PRL on newly discovered Fe-based superconductors obtained from the ISIS spectrometers are results of international collaboration between various experimental and theory groups.  In the last six years we had a great success in organizing a theoretical and experimental magnetism meeting, which is becoming a tradition of the UK, European as well as international magnetism communities. 

Magnetism at the atomic scale obeys quantum rules. Its origins lie in the strong correlations between unpaired electrons and their localization on atomic sites. The phases and phenomena displayed by quantum dominated magnets have been found to be extremely rich and the exploitation of their potential for fundamental research on quantum many-body phenomena and as a route to discovering new states of matter is a major current research priority in the field of condensed matter physics. Furthermore, quantum magnetism is integral to the superconducting states in the cuprates, many novel phenomena in heavy fermion systems, quantum dots and nanomagnets, and is emerging as an important route for quantum information processing, tools to look at and manipulate such magnetic states. 

Magnetism at the atomic scale obeys quantum rules. Its origins lie in the strong correlations between unpaired electrons and their localization on atomic sites. The phases and phenomena displayed by quantum dominated magnets have been found to be extremely rich and the exploitation of their potential for fundamental research on quantum many-body phenomena and as a route to discovering new states of matter is a major current research priority in the field of condensed matter physics. Furthermore, quantum magnetism is integral to the superconducting states in the cuprates, many novel phenomena in heavy fermion systems, quantum dots and nanomagnets, and is emerging as an important route for quantum information processing, tools to look at and manipulate such magnetic states. 

A central challenge in field of strongly correlated electron systems is to understand, predict, and control the intricate effects of correlations in many body systems. Many materials under current investigation are classed as strongly correlated electron systems, for example, high temperature superconductors (HTSC) [1-3], frustrated and low dimensional magnets [4], quantum magnets, multiferroic manganites [5], quantum phase transitions (QPT) in heavy fermion systems [6-7] and many more. It is generally agreed that the magnetism, quantum criticality and superconductivity in d and f electrons systems are strongly interrelated. One of the most challenging tasks of condensed matter physics today is to understand the mechanism for the high temperature superconductivity in cuprates and breakdown of the Fermi-liquid ground state, so called non-Fermi-liquid behavior (NFL), in many d and f-electron systems at low temperatures. To understand these novel materials with strong interaction effects, a variety of approaches, ranging from phenomenology of experiments to numerical and analytic models, have been applied. The basic concept of theoretical condensed matter physics is about building realistic models of physical processes, often driven by experimental findings, generalising the solutions of those models to make experimental predictions, and transferring the concepts gained into other areas of research. Microscopic measurements such as neutron scattering and muon spin rotations on strongly correlated materials provide vital information on spin dynamics as well as static magnetism. Theories play an important role in understanding known phenomena and in predicting new ones, while experiments play an important role in testing the theoretical predictions as well as finding new phenomena for which new theoretical models need to be developed

Quantum Criticality and non-Fermi liquid behaviour: A quantum phase transition (QPT) is a zero temperature second order phase transition between, usually, magnetic and nonmagnetic states driven by a control parameter such as pressure, magnetic field, or composition/chemical-pressure, which regulates the amplitudes of quantum fluctuations. In a metal near the QPT there are large amplitude fluctuations in the Fermi surface area and a divergence of the quasi-particle mass. It is now generally acknowledged that these quantum fluctuations influence the physical properties of correlated electron systems even at elevated temperatures. This is because of the uncertainty principle; the energy scale of fluctuations introduces a time scale which leads to an intricate coupling of static and dynamic critical behaviour. 
Most of the concepts and models developed in this field were successfully applied to systems like CeCu6-xAux, one of the early examples exhibiting a QPT at a critical concentration xc  0.1, where long-range incommensurate antiferromagnetic order vanishes. Microscopically, however, the fate of the conduction electrons, more precisely quasi-particles, at such a QPT is largely unknown. It is in this realm that the standard model of electrons in metals fails completely and one needs new theoretical approach to understand the origin of unusual physical properties of strongly correlated electron systems. At present, there are two theoretical scenarios, neither of which is able to explain in detail the experimental observations: The more traditional Hertz-Millis-Moriya picture assumes that quasi-particles undergo singular scattering at the quantum critical point due to the abundance of low-lying magnetic fluctuations. This leads to a diverging mass for ferromagnets but also for two-dimensional antiferromagnets where singular scattering is possible on the whole Fermi surface. The other, more radical approach postulates a complete breakdown of the quasi-particles at the QCP, emphasizing their composite nature due to 4f electron-conduction electron hybridization. This would imply that the whole Fermi surface collapses, and local physics emerges (local quantum criticality). The QPT concept leads very naturally to one of the key attributes of the normal state physics, namely, that the energy scale governing spin and charge fluctuations is the temperature itself, a property labelled as E/T scaling behaviour of the dynamical susceptibility. This E/T scaling has been observed in HTSC, -Mn, as well as in NFL-heavy fermion systems. But, it is still an open debate whether or not this E/T scaling has any relation with the different possible microscopic origins of QPT/non-Fermi liquid behaviour. We are planning to have theoretical and experimental talks on this subject with a special emphasis on neutron scattering and muon studies.

Molecular Magnets: Molecular magnetism is a new and exciting area of condensed matter research. Molecular magnets are magnetic objects on the nanoscale that consist of highly symmetric clusters of typically 2-30 interacting magnetic ions. Examples are the ferric and chromic wheels (rings of antiferromagnetically interacting iron or chromium ions), more complex wheel-type structures such as the hexadecameric and tetradecameric clusters, and highly frustrated and entangled cubane-type systems. The interest in molecular magnets stems from, among other things, the fact that they can be used to develop and explore concepts of quantum magnetism in the simpler environment of zero-dimensions and with a small number of interacting spins compared to higher dimensional objects with infinite interacting spins normally investigated in condensed matter physics. Because of their intrinsic structural regularity and fundamental magnetic properties, these systems also have been proposed as natural structures to incorporate into nano-devices or as a basis for simple nano-scale magnetic objects, for example molecular motors. During the last several years, many single molecular magnets have been discovered and they are now among the most promising candidates for observing the limits between classical and quantum physics since they have a well-defined structure, spin ground state, and magnetic anisotropy. Despite their apparent simplicity the Hilbert space of these systems is in many cases still too large for exact solutions of the energy levels, hence approximations are required to develop theories and these theories can be readily tested using the results obtained through various experimental techniques, such as bulk laboratory based measurements as well as microscopic information obtained from the magnetic excitations measured using neutron scattering. Unfortunately, not all energy levels are visible to neutron scattering; transitions cannot involve a change in angular momentum larger than 1. However, the ground state of the system can be varied by tuning a magnetic field - a technique which allows many more energy levels to be measured, and quantum chaos to be studied experimentally.  We will have review type talks and short presentations on molecular and nano-magnets in the meeting.

Exotic Superconductivity including Fe-based systems: It is generally accepted that the mechanism of superconductivity in strongly correlated electron systems, such as high temperature cuprates (HTSC), heavy fermion superconductors (HFSC), layered cobaltite, the ruthenates, organic superconductors and very recently discovered Fe-based superconductors, is dramatically different than that of predicted by Bardeen-Cooper-Schrieffer (BCS) theory: according to BCS theory, the electrons team up to form pairs, known as Cooper pairs, due to virtual coupling with  phonons at low temperatures. The superconductivity in this new class of strongly correlated systems is named exotic or unconventional or anisotropic superconductivity. This includes superconductivity with exotic pairing states (mixed pairing, singlet and triplet in noncentrosymmetry compounds), low dimensionality, coexistent order parameters (superconductivity and magnetism) and quantum critical points. There is experimental evidence that the phase diagram of HTSC-cuprates, Fe-based superconductors  and heavy fermion superconductors has commonality, for example all the phase diagrams show the presence of a pseudogap as well as departure from Fermi liquid behaviour above their superconducting transition temperatures. Furthermore, antiferromagnetic spin fluctuations seem to be an essential component in pairing mechanism in all these systems. On the other hand when a superconductor lacks a crystal inversion center, the parity conservation is violated due to the asymmetric spin-orbit coupling (ASOC), and the pairing symmetry becomes nontrivial as observed in CePt3Si and UIr.
Recently many heavy fermion compounds have been discovered which exhibit coexistence of ferromagnetism and superconductivity: these two physical phenomena were previously thought to be incompatible. Superconductivity in ferromagnets must result from a different type of electron-pairing mechanism. In these materials, electrons with spins pointing in the same direction team up with each other to form Cooper pairs with one unit of spin, resulting in triplet superconductivity. In contrast, conventional superconductivity, also known as s-wave singlet superconductivity, occurs when electrons with opposite spins bind together to form Cooper pairs with zero momentum and spin. Recently, attention has been focused on the layered Sodium Cobalt Oxides NaxCoO2 that exhibit a number of remarkable properties, such as enhanced thermoelectric power (TEP) due to spin entropy and superconductivity with TC=5K. The enhanced TEP of this material makes it potentially important for technological applications and the existence of superconductivity in the hydrated compound raises a very important question about the pairing mechanism in relation to the cuprate, ruthenate and heavy fermion superconductors. A recent neutron scattering study confirms the existence of ferromagnetic correlations within the cobalt layers, but antiferromagnetic correlations perpendicular to the layers.

The mechanism of high temperature and ferromagnetic superconductors remains a challenge to theorists, and there is still no unambiguous theoretical explanation for these phenomena. Neutron scattering measurements on the HTSC compounds have revealed high energy dispersive magnetic excitations, yet it is not clear whether these excitations have any direct relation with the phenomenon of superconductivity. The excitations in YBa2Cu3O6+x and La1.875Ba0.125CuO4  have been attributed to the electronic band structure, but an alternative explanation is that the excitations are due to rigid stripe ordering and liquid-crystalline stripe ordering in La1.875Ba0.125CuO4  and YBa2Cu3O6+x respectively. To understand the mechanism of HTSC a theoretical model has been proposed based on current loops by Varma and there are some experimental evinces to support this model. Further the recent observation of FFLO (Fulde-Ferrell-Larkin-Ovchinnikov) superconductivity in some heavy fermion compounds has attracted considerable attention and it not clear whether real FFL state exists in CeCoIn5 in applied field.Fe-based superconductors: The recently discovery of superconductivity in fluorine doped RFeAsO (R=rare-earths) family with Tc=25-55K has generated great interest in the condensed matter physics community and there are huge numbers of publications in this topic since its discovery  Feb2008.  The parent compound is antiferromagnetic and superconductivity is realized by suppressing the magnetic ordering through chemical doping or applying high pressure. Understanding the nature and origin of the magnetic ordering is thus crucial in understanding the interplay between magnetism and superconductivity in the Fe-based superconductors. Further reports reveal that LiFeAs and FeSe are also show superconductivity below 18 K. It is interesting that both hole-doped Ba1-xKxFe2As2 and electron-doped BaFe2-x(Ni;Co)xAs2 exhibit superconductivity.  The critical temperatures of the RFeAsO family are exceeded only by the high-Tc cuprates, thus it is natural to ask if both materials share common interactions that are responsible for the origin of the superconductivity. The common feature seen in HTSC, Fe-based systems and HFSC is the observation of spin resonance peak and it has been found that the TC is related with the energy of the resonance peak.  In the HTSC-cuprates, while significant changes are observed in the phonon spectrum as a function of doping and temperature, the consensus is that superconductivity does not arise solely from electron-phonon coupling, as they reveal clear presence of dispersive magnetic excitations. 

We are planning to have two full sessions on exotic superconductivity with a special emphasis on Fe-based superconductors in the meeting to review the current status of research covering both experimental as well as theoretical aspects on this topic.

Frustrated and Low Dimensional Magnetism: Frustration occurs when interactions are incompatible with the topology of the space in which they act. In the case of magnetism, this occurs for example when nearest-neighbour exchange energies cannot be simultaneously minimized. This strongly reduces the magnetic ordering temperature and may even preclude conventional types of magnetic ordering. As a consequence, highly degenerate and novel ground states are formed, such as spin liquids, spin ices, resonating valence bond states, cooperative paramagnets and other unusual magnetic orderings. It also leads to unconventional low-temperature dynamics due to fluctuations between degenerate states, which can be altered by a magnetic field. Magnetic fields can be used not only to increase or decrease the degree of frustration, but also to tune the effective dimensionality in some cases. The magnetisation process of a frustrated magnet should therefore be accompanied by the appearance of new types of dynamics. Recent successes in the synthesis of novel magnetic materials with pyrochlore (corner-sharing tetrahedra) or Kagomé (corner-sharing triangles) lattices have produced a large number of inorganic and organic compounds with strong geometrical frustration. Some of these systems also have quantum spins, such as spin 1/2 Kagomé lattices, in which there is a very strong theoretical interest. Experimental studies have already produced a series of exciting and puzzling results, including the unique and as yet unexplained phase diagrams of several garnets, spinels and pyrochlores and the development of realistic theoretical models is needed to explain these.
Summary of the last meeting, July 2009:The most recent Theoretical and Experimental Magnetism Meeting (TEMM09) was held at The Cosener’s House in Abingdon, Oxfordshire on 2-3 July 2009. This was the six meeting in this series. This two-day meeting was organised by the Centre for Materials Physics and Chemistry of STFC and by the Magnetism Group of the Institute of Physics and partly funded by ISIS-Excitations Group, CCP9 and IOP neutron scattering group. The meeting attracted 78 registered participants from eight different countries. They included academics, senior researchers, post-doctoral fellows and Ph.D. students.  Considering the importance of the new discovery of FeAs-based superconductors a special session was organised on this topic. The important of computational magnetism in condensed matter physics was realised and we had a special full session on this topic. A special talk after dinner on “Discoveries that changed the world: 1932 – 1942 James Chadwick & Lise Meitner” was given by Gerry Lander (Karlsruhe and Grenoble). The meeting presented an excellent opportunity to hear and discuss with leading experts from all over the world about topics such as novel superconductivity, multiferroic materials, low-dimensional, and frustrated magnetism and quantum phase transitions. There were 29 oral presentations, out of which 11 were given by the international speakers, and 11 poster presentations. Presentations were given on both theoretical as well experimental aspects of magnetism with special emphasis on neutron scattering, muon spin rotations and x-ray scattering. Overall, the meeting was a great success, very useful and enjoyable opportunity for experimentalists to have discussions with theoreticians on various aspects of modern magnetism in a friendly environment.

Scope of the meeting in 2011-2013

The meeting will follow the tradition established by the previous meetings at Oxford and Abingdon (2003-2010) having talks on both condensed matter theories and experiments including computational magnetism. The meeting will provide an international forum for the presentation and discussion of recent developments in condensed matter theories and experiments, with a special emphasis on Fe-based superconductors and magnetic monopole as well as any newly discovered topics in this subject. Considering that molecular and nano-magnetism is a fast developing area in condensed matter physics at present, one session will be organized to give an overview of the field for the benefit of the graduate students. Presentations will consist of plenary talks, invited talks and contributed talks and there will also be a poster session.

Tentative List of Topics:

Quantum Phase Transitions and non-Fermi-liquid behaviour
Exotic Superconductivity including FeAs-based Superconductors 
Molecular and Nano-Magnetism
Orbital and Quadrupolar Ordering
Low-dimension and Frustrated Magnetism
Manganites, Multi Ferroelectricity and Nano-Magnetism
Magnetic Monopole
Computational Magnetism