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Search for new quantum phases in strongly-correlated quantum systems and explanation of dynamics

What kind of research is being conducted for this Priority Research Topic?
To a considerable degree, the nature of matter is determined by the behavior of electrons. Over the past half-century, considerable progress has been made in the area of electron state calculations that reveal this behavior. For semiconductors (systems that have weak electron interaction) in particular, it has become possible to make quantitative predictions of condensed matter properties to some extent, mainly through the use of the density functional approach. For systems with strong electron interaction, however, it is known that the ordinary density functional approach does not produce reliable results. In the meantime, since around 1980, many intriguing phenomena resulting from strong interaction between electrons have been discovered. Some examples are transition
metal compounds that include cuprate high-temperature superconductors, organic conductors made up of molecular crystals, and heavy electron systems known as f-electron systems.
These phenomena have a potential for new applications unlike anything seen in the semiconductor industry that experienced dramatic development during the previous century. As typical quantum many-body problems, they have also attracted considerable attention due to their potential to produce new approaches and concepts. The objective of this Priority Research Topic is to elucidate these phenomena by means of large-scale numerical calculation techniques using supercomputers. Specifically, the objective is to understand new types of quantum states and as-yet undefined quantum phases that exist in the real world. Examples of new quantum states include high-temperature superconductors that are based on new mechanisms, and various types of quantum-liquid phases that include quantum spin liquids. From the standpoint of phase transition, we are also searching for phase transitions that are not
bound by the framework of conventional Landau theory of phase transitions. The development of photo-induced phase transitions and pump-probe experimental methods that use photoelectron spectroscopy has brought considerable attention to nonequilibrium phenomena. We are working to identify the basic scientific principles for phenomena in these kinds of systems that are far removed from equilibrium.


How are large-scale numerical calculations being used in research on this topic?
What we really want to know is the behavior in the thermodynamic limit where systems are made up of many particles on the level of Avogadro's number. Since it is not possible to arrive there by means of actual calculation, we have to perform calculations for as large a number of particles as possible and to confirm convergence to the thermodynamic limit in order to ensure the reliability of the calculations. Also, as we are dealing with quantum systems, another major issue is the degree to which low temperatures can be handled. For nonequilibrium problems, how long calculations can be performed is another important issue. In addition, in simulations of the complexity of an actual material, the computational cost is increased even further. When you're trying to conduct calculations in as realistic a manner as possible, for a large size, at a low temperature, and for a long period of time, each one of these four items requires computational power that is almost equal to the entire resources of the K computer. It's crucial that you are able to devise a way to do all of these things as you pursue your research.
Currently, in terms of the number of atoms, it's now possible to handle from several hundred to several hundred thousand atoms. In terms of time, we can calculate up to about the picosecond. It's only been during the past ten years or so that techniques have been established for making quantitative estimates of the condensed matter properties of actual materials with strong electron correlation. In the sense that we're now able to make predictions based on materials that actually exist, we're approaching a very important time in history.
To use superconductors as an example, through large-scale calculations such as those possible on the K computer, we're now able to debate the stability of superconductors made of actual materials for the first time. The research that we've done recently is to use first-principles approach to calculate the magnetic properties of a group of iron-based superconductors that were discovered about four years ago. This enabled us to demonstrate that, in some materials, the strength of electron correlations changes systematically, and that it is the primary reason for the diversity of physical properties of the iron-based superconductors.

What will be the impact on society of the achievements of this Priority Research Topic research?
The achievements that we anticipate from research on this topic include quantitative prediction of the transition temperature of high-temperature superconductors, explanation of as-yet undefined superconductor mechanisms, explanation of new quantum-liquid mechanisms, and the establishment of a new theoretical foundation for quantum phase transitions, resulting in the search for and development of new functional materials.
We also hope that the study of nonequilibrium dynamics will make it possible to propose design guidelines for new functional devices that use the quantum effect such as switching elements and so on.
It goes without saying that calculating electron states, designing materials and helping to predict physical properties are issues that will have a profound effect on society.
Most of the industrial innovations achieved since the 20th century, such as semiconductor elements and devices, would not have been possible without an understanding of the microscopic properties of materials. In the sense of providing theoretical support for producing new industries, the achievements of research on this Priority Research Topic will play a crucial role in superconductor and other energy issues as well as microfabricated devices and other future industries.
Gaining an understanding of novel quantum states and new phenomena will also reveal natural structures and laws unknown heretofore. Looking back at history, we can see the profound impact of new concepts in condensed matter physics on particle physics, biology, chemistry and other fields. It is thought to be very likely that large-scale calculations using supercomputers will give us a clearer understanding of such new concepts in the future. Moreover, this research will not only make a contribution to directly relevant fields of academic study. In the sense of their cultural value as assets of humanity, the new conceptual discoveries and explanations achieved through this research
will also make a major contribution to society.