Gapless Spin Liquids and Itinerant Non-Fermi-Liquids in Experiments and Models. Phase Transitions in Gauge-Matter Systems

California Institute Of Technology, Pasadena CA

Investigators

Abstract

TECHNICAL SUMMARY This award supports theoretical research and education to study fractionalized phases in spin, boson, and electron systems in 2D that have many low-energy itinerant excitations. This research is motivated by experiments on several recently discovered candidate spin-liquid materials, and from observations of non-Fermi-liquid itinerant electron states in strongly correlated materials such as the strange metal and pseudogap phases in the cuprate high temperature superconductors. The field of quantum magnetism has experienced a new era stimulated by the cuprates and by many studies of magnetic insulators with competing interactions. Quantum spin systems are prototypical examples of many-body physics; one of the conceptual developments is the possibility of spin liquid phases. A recent breakthrough is the appearance of several candidate experimental realizations which appear to be gapless spin liquids where theory is less established. A proposed state for two triangular lattice spin liquid materials has a Fermi surface of spinons, which is a kind of ?spin metal". The PI aims to determine how well these materials can be understood within the available theoretical frameworks. Having learned about such unusual states in the magnetism context, the PI will apply these ideas to understand itinerant non-Fermi-liquids of electrons. In the cuprate context, one can ask whether charge degrees of freedom can be put back into play and study the strange metal. Focusing on the itinerant charge sector, and slave-particle approaches borrowed from spin-liquid studies enable the construction of non-perturbative charge liquids. ?Bose metals.? The PI will pursue the next step, a similar program for electronic systems bringing together spin and charge sectors. An important thrust of this research is to search for such ?spin metals," ?Bose metals," and electronic non-Fermi-liquids in model systems and guide numerical studies. A promising approach is to build up towards 2D using controlled DMRG studies of candidate models on quasi-1D ladders combined with trial wavefunctions and gauge theory/bosonization analysis. The PI will also study phase transitions in statistical mechanics models with matter fields coupled to gauge fields in 2+1D. These arise as effective field theories for recently proposed unconventional quantum critical points between competing phases in quantum magnets. Establishing the nature of the transitions in such gauge theories will put further theoretical developments on firmer ground. The PI will also study phases and phase transitions in models with statistical interactions. NON-TECHNICAL SUMMARY This award supports theoretical research and education that aims to discover new magnetic states of matter, to understand their implications for unusual metallic states that appear in high temperature superconductors, and to understand exotic transformations that can occur among them that lie outside the standard theory of phase transitions. This research is motivated and stimulated by the discovery of new materials with unusual properties that challenge conventional paradigms of condensed matter physics. The emphasis of this research will be on developing theoretical concepts in the context of laboratory and computer experiments on materials and models. This is fundamental research that contributes to the intellectual foundations of our understanding of materials and new states of matter that exhibit properties and exotic phenomena that lie outside our current understanding. This is an intellectual pursuit in its own right no less fascinating than the study of the universe, but it may also lead to the discovery of new phenomena and to new device technologies.