We develop approaches and tools to investigate and elucidate peculiar new phenomena that arise from complex many-body effects in materials and molecules. Those phenomena arise in complex experiments, such as near zero temperature for superconductivity, or are part of our every day life, such as the binding of myoglobin to oxygen.
A -TO-olbox for -S-trongly -C-orrelated -A-pproaches to -M-olecules (TOSCAM), optimized for GPUs !
TOSCAM is a highly modular package which aims at solving strongly correlated problems. The current version interfaces dynamical mean-field theory (DMFT) with linear scaling density functional theory (DFT). The package has a built-in extended Lanczos DMFT solver (ED-CPT), and allows for rapid and reliable calculations of the correlated electronic properties of molecules or other nano-scopic systems. The package is available through collaborations with our team, so please get in touch !
"Strong Correlation meets Materials Modelling: DMFT and GW in CASTEP"
The function of much modern technology is based on exploiting special physical properties of materials. This might be control of electrical current in the case of semiconductors, magnetism for data storage, the Peltier effect for solid-state refrigerators, or superconductivity for ultra high power magnets used in MRI scanners. Underpinning future refinements of such "functional materials" and development of new materials and devices lies the idea of "materials design". Computer simulation methods with the power to predict the active properties based only on the quantum-mechanical behaviour of the electrons in a particular crystal structure are a vital part of any attempt to engineer "designer materials" for future devices.
Herein we propose to extend the capability of DFT by implementing both LDA+DMFT and GW within CASTEP, the UK's premier electronic structure-based materials modelling code.
"DEFCOM: Designing Eco-Friendly and COst-efficient energy Materials"
The goal of this project is to understand and optimise the electronic and transport properties of copper-antimony-sulphide based materials, very promising compounds for cost-effective and eco-friendly energy applications. The research will build on expertise in state-of-the-art ab-initio electronic structure methods at KCL and synthesis and processing of thermoelecrics at QMUL. The research will greatly benefit from close interactions with our industrial partners Kennametal, European Thermodynamics Limited (ETL), Johnson & Matthey (JM), and BIOVIA.
"New Emergent Quantum States of Matter at High Pressure"
At ambient conditions, the light alkali metals Li, Na and K are nearly free electron (NFE) metals. But rather than becoming MORE free-electron like when compressed, these metals undergo transitions to unusual and complex structural and electronic forms as a result of density-driven changes in the interactions of the ions and electrons. While such behaviour is expected in all high-density matter, the physics is most evident in the alkali metals due to their NFE behaviour at ambient conditions, and their very high compressibilities. They thus offer a unique insight into the behaviour of all other metals at very high densities. We will exploit our team's expertise in experimental high-pressure physics to create solid and fluid alkali metals at unprecedented densities, and then determine their structural behaviour using x-ray diffraction techniques at synchrotrons, x-ray free electron lasers, and high-energy laser facilities. We will then use electronic structure and quantum-molecular-dynamics calculations to understand the physics behind the observed behaviour, and thereby develop new understanding and improved predictive capabilities in the behaviour of matter at ultra-high densities.
Take a deep breath !
Transition metal systems cover a wide-range of systems, and show a breadth of peculiar and interesting properties stemming from many-body effects in solids.
A fair understanding has been obtained in particular with the Zaanen-Sawatsky-Allen (ZSA) theory, which provides a classification of transition metal periodic solids in terms of charge transfer or Mott systems. However, right across the zoology of transition metal systems, lies an important family which is not contained within the ZSA classification, and remains poorly understood: transition metal molecules.
The most common and simplest example is haemoglobin, a metallo-porphyrin system which contains a single iron atom, and binds to oxygen, which is important for the respiration process, and carbomonoxide, which inhibits the respiration function. A consistent physical picture of the ligand binding of haemoglobin is still lacking after three decades of investigation: state-of-the-art approaches based on density functional theory predict a binding to carbomonoxide with a ten thousands times larger affinity than the binding to oxygen, obviously wrong since the latter would make respiration impossible, and also illustrates the failure and limitation of current state-of-the-art modelling approaches for transition metal ion molecules.
We recently developed a consistent approach to elucidate the mysterious properties of these molecules,
this work was published in the prestigious review PNAS
[Read more ...] .
When electron play the musical-chair game in superconductors
The question of how charges pair up in copper-oxide (“cuprates”) high-temperature superconductors has been in debate since the
discovery of high temperature superconductivity in ceramics. This stems from the very rich physics observed in the copper-oxides, and from the subtle quantum many-body effects driven by strong correlations in those materials. Indeed, the cuprates are magnetic insulators, and superconductivity is obtained by doping the materials with extra holes. Experimentally, it is observed that the strong correlations drive many different competing long-range ordered phase upon the doping mechanism, for example, magnetism, superconductivity, stripe orders, checkerboard charge density waves and others.
Theorists have been able to derive minimal models which accounts for some of the here-mentioned phases, the so-called Hubbard hamiltonian. One of the quantum states of matter however has so far resisted this minimal description: a phase where the electron goes through orbital loop currents, chasing each other and cycling in triangular plaquettes defined by a copper and two of its neighbouring oxygen atoms. This phase breaks the time-reversal symmetry, and so far has not been captured by a minimal model containing only in-plane copper and oxygen atoms.
A recent proposition by Ole Andersen suggested however that the mechanism by which electron are transferred between two oxygen atoms might involve contributions neglected so far: the transfer would involve a high energy Cu-4s orbital, neglected in the calculations, and provide a significant contribution to the direct transfer via second-order processes. In a study by TYC researcher Cedric Weber and colleagues from the University of Geneva and the University of California, Riverside, it was found that once this contribution is accounted for, the orbital current quantum phase is stabilised in a minimal Hubbard-like theory, along with superconductivity and magnetism, for reasonable range of physical parameters.
[Read more ...] .
Quantum levitation is a peculiar response of superconductors to external magnetic fields.
It is well known that BCS Superconductors expell an external magnetic field. However, type 2 superconductors, such as copper oxides, have a more peculiar response: they let part of the external magnetic field penetrate within "vortices" or "flux tubes", and are literrally pinned to the external magnetic field. This phenomena is called "quantum flux locking" or "flux pinning", as shown in this impressive video [link]
Physicists reveal quantum effects in biological oxygen transport
The news blog Nanowerk wrote a paper about our research on oxygen transport.
How quantum mechanichs help us breath
The website Inside Science wrote a vulgarisation of our work explaining how quantum mechanical effects were involved in the respiration process.
